DOI: 10.1002/chem.201402402
Full Paper
& Electron Transfer
Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)–Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies Chandra B. KC,[a] Gary N. Lim,[a] Paul A. Karr,[b] and Francis D’Souza*[a] In memory of Gabriel D’Souza
Abstract: A multimodular donor–acceptor tetrad featuring a bis(zinc porphyrin)–(zinc phthalocyanine) ((ZnP–ZnP)– ZnPc) triad and bis-pyridine-functionalized fullerene was assembled by a “two-point” binding strategy, and investigated as a charge-separating photosynthetic antenna-reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis-pyridine-functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi–Hildebrand plot by using the optical data was found to be 1.17 105 m1, whereas a plot of “mole-ratio” method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6-31G* optimization, whereas the energy levels for different photochemical events was established by using data from the op-
tical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer (kENT) in the case of the triad was found to be 7.5 1011 s1 in toluene and 6.3 1011 s1 in o-dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy-transfer product, that is, singlet-excited zinc phthalocyanine to fullerene was verified from the femtosecond-transient spectral studies, both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc· + in the 850 nm range and C60· in the 1020 nm range were clearly observed. The rate of charge separation, kCS, and rate of charge recombination, kCR, for the (ZnP–ZnP)–ZnPc· + :Py2C60· radical ion pair (from the time profile of 849 nm peak) were found to be 2.20 1011 and 6.10 108 s1 in toluene, and 6.82 1011 and 1.20 109 s1 in o-dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad.
Introduction
models capable of undergoing both energy- and electrontransfer processes.[1–16] To connect the different entities of the multimodular systems, either covalent bonding or self-assembled strategies have been utilized. But in recent years, a combination of these two approaches have been successfully employed to build multimodular donor–acceptor systems. This combination approach has provided better control over monitoring the different photochemical events occurring in these systems. In building, the multimodular donor–acceptor models, porphyrins (P), and phthalocyanines (Pc) are frequently used photosensitizing electron donors,[17, 18] whereas fullerene, C60, is a common electron acceptor.[19, 20] Additionally, ferrocene, tetrathiafulvalene, BF2-chelated dipyrromethene, BF2-chelated azadipyrromethene, phenothiazine, subphthalocyanine, or perylene diimide, among others, have often been utilized as secondary electron donors or energy-funneling antenna entities.[1–22] In the systems made out of both porphyrins and phthalocyanines, the rich and extensive absorption features of these macrocycles have provided increased absorption cross-section.[17, 18]
Studies on artificial photosynthetic model compounds capable of undergoing energy migration and charge separation are key in building low-cost, light-to-electricity and light-to-fuel converting devices.[1–5] Solar energy has the potential to fulfill global energy needs in an environmentally friendly manner if efficient, economically feasible energy-conversion devices could be built.[5] Towards this, wide-ranging efforts have been applied towards developing multimodular donor–acceptor [a] C. B. KC, G. N. Lim, Prof. F. D’Souza Department of Chemistry, University of North Texas 1155 Union Circle, 305070, Denton, TX 76203–5017 (USA) Fax: 940–565–4318 E-mail: [email protected] [b] Prof. Dr. P. A. Karr Department of Physical Sciences and Mathematics Wayne State College, 111 Main Street, Wayne, Nebraska 68787 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402402. Chem. Eur. J. 2014, 20, 1 – 12
These are not the final page numbers! ÞÞ
1
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper Additionally, their complimentary absorption and emission has provided opportunity for wide-band capture and efficient use of the solar spectrum. Fullerene,[19] due to its facile reduction potentials[20] and low solvent and internal reorganization energy,[21] has been an excellent electron acceptor. Generally, fullerene in donor–acceptor systems accelerate forward electron transfer (kCS) and slow down backward electron transfer (kCR), thus, generating the much desired long-lived charge-separated states.[7–10, 21, 22] Additionally, the presence of secondary electron donors results in the generation of long-lived chargeseparated states by a hole-migration mechanism, whereas the existence of energy-funneling-antenna molecules provide better utilization of solar light in charge separation.[21, 22] Recently, multimodular systems utilizing covalently[23, 24] and noncovalently linked porphyrin and phthalocyanine have gained considerable attention due to their complimentary absorption and emission properties.[25] Selective excitation of the porphyrin entity in the covalently linked systems promote ultrafast excited-state energy transfer to phthalocyanine. In the presence of an electron acceptor, the phthalocyanine undergoes electron transfer leading into the formation of a chargeseparated state.[23, 24b] Alternatively, such porphyrin–phthalocyanine could be immobilized on a semiconductor TiO2 surface for improved solar-energy converting devices.[24a] Encouraged with these findings of covalently linked porphyrin and phthalocyanine molecules, herein, we have constructed a multimodular tetrad as a photosynthetic antenna-reaction center mimic comprised of a covalently linked bis(zinc porphyrin)-zinc phthalocyanine ((ZnP–ZnP)–ZnPc) triad coordinated to C60 functionalized with two pyridine entities (Scheme 1). Light-induced excitation transfer and electron transfer in the tetrad, probed by spectral, electrochemical, computational, and transient absorption techniques, is reported.
Scheme 1. Structures of the multimodular, bis(zinc porphyrin)–zinc phthalocyanine coordinated to fullerene by a two-point binding approach. Two possible structures of tetrads (1 and 2) involving ZnP and ZnPc binding sites are shown.
Results and Discussion Synthesis of bis(zinc porphyrin)–zinc phthalocyanine triad The synthesis of the bis(zinc porphyrin)-zinc phthalocyanine triad was carried out according to Scheme 2 based on literature method[26] with minor modifications. The details are given in the Experimental Section. Briefly, 5-(4-hydroxyphenyl)10,15,20-tris(tolualyl)porphyrin was synthesized by reacting stoichiometric amounts of 4-hydroxylbenzaldehyde, tolualdehyde, and pyrrole in propionic acid followed by chromatographic separation/purification over silica-gel column. Next, the porphyrin was reacted with 4,5-dichlorophthalonitrile in the presence of K2CO3 in dry acetone to obtain 4,5-bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin]phthalonitrile. Finally, the porphyrin derivative was reacted with 4-tert-butylphthalonitrile and ZnCl2 in dimethylaminoethanol, followed by chromatographic purification. During the course of this reaction, both phthalocyanine and porphyrin macrocycles were metalated. The compound was stored in dark prior doing spectral and transient measurements. &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
Scheme 2. Synthetic methodology adopted for (ZnP–ZnP)–ZnPc triad. DMAE = dimethylaminoethanol.
Absorption and fluorescence studies Figure 1 shows the optical absorption spectrum of the (ZnP– ZnP)–ZnPc triad along with the spectrum obtained by mixing two equivalents of zinc tetra(tolualyl)porphyrin and one equivalent of zinc tert-butyl phthalocyanine, as a control in o-dichlorobenzene (DCB). The peak intensities at the porphyrin Soret region and phthalocyanine visible-band region matched 2
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper with the spectrum recorded for the control involving a mixture of two equivalents of ZnP and one equivalent of ZnPc. For the control mixture, two emission bands at l = 600 and 650 nm of ZnP were observed, whereas the emission corresponding to ZnPc at 698 nm was almost negligent. In contrast, for the (ZnP–ZnP)–ZnPc triad, the ZnP emission bands were quenched over 80 % of the original intensity with the appearance of strong emission of ZnPc at l = 698 nm. Please note that the small peak at 616 nm (asterisk) corresponds to ZnPc absorption and not due to emission. Further, excitation spectrum of the (ZnP–ZnP)–ZnPc triad was recorded by fixing the emission monochromator to 698 nm corresponding to the emission maxima of ZnPc and scanning the excitation wavelength. As shown in Figure S1 in the Supporting Information, such spectrum showed peaks corresponding to not only ZnPc but also those of ZnP. However, no ZnP peaks were observed for the control mixture. These results demonstrate occurrence efficient singlet–singlet energy transfer from ZnP to covalently linked ZnPc.[27] The supramolecular tetrad was formed by coordinating C60Py2 to (ZnP–ZnP)–ZnPc triad in DCB. Figure 3 a shows spectral changes observed during titration. Herein, the ZnP Soret band at l = 425 nm revealed characteristic features of axial coordination with diminished intensity accompanied by a redshift.[10] For the ZnPc band located at 685 nm, slightly diminished intensity accompanied by isosbestic points at 411, 555, 619, and 703 nm were observed. A plot of mole ratio method (Figure 2 b) revealed 1:1 stoichiometry for the tetrad. The binding constant calculated from the spectral data by using Benesi–Hildebrand method[28] was found to be 1.2 105 m1 (Figure 2 c), which was over an order of magnitude higher than that reported for “single-point” metal–ligand coordination,[10] but was close to that reported for the ZnP-ZnP dyad or ZnPcZnP dyad binding to C60Py2 (Py = pyridine) through a “twopoint” metal–ligand axial binding.[34, 24b] Addition of C60Py2 to the solution of (ZnP–ZnP)–ZnPc triad quenched ZnPc emission almost quantitatively (over 98 %), whereas such changes for the weak ZnP emission were relatively small (Figure 2 b). These results collectively point out a “two-point” metal–ligand axial binding of C60Py2 to (ZnP–ZnP)–ZnPc triad resulting into the formation of (ZnP–ZnP)–ZnPc:Py2C60 multimodular supramolecular tetrad. As shown in Scheme 1, there are at least two structures for the tetrad (assuming the two pyridine entities of C60Py2 and geometric isomers are identical in the context of metal–ligand binding). That is, the first involving the ZnPc and one of the ZnP macrocycles (structure 1), whereas the other involving both ZnP entities of the triad (structure 2). The binding constants of single-point bound zinc–pyridine coordinated complexes generally fall in the same order of magnitude for both ZnP and ZnPc complexes thus making it difficult to rationalize the binding preference. Interestingly, the spectral changes in Figure 3 a clearly show involvement of ZnP by exhibiting typical spectral changes of axial binding; however, for ZnPc, the changes have been moderate. To resolve this issue of the existence of either or both structures 1 and 2, computational and electrochemical studies were found to be very useful, as dis-
Figure 1. Absorption spectrum of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) and ii) a mixture of ZnP (6.6 mm) and ZnPc (3.5 mm) in DCB.
well; however, few subtle changes were noticed. For example, the ZnPc visible bands located at l = 616 and 685 nm in the triad were redshifted in the mixture and appeared at l = 612 and 682 nm, respectively. This observation suggests occurrence of some intramolecular interactions between the macrocycles of the triad. The invariant position of porphyrin Soret and Q bands also suggests lack of direct ZnP-ZnP interactions in the triad. Figure 2 a shows the fluorescence spectrum of the (ZnP– ZnP)–ZnPc triad in DCB excited at the ZnP Soret band along
Figure 2. a) Fluorescence spectrum (lex = 425 nm) of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) and ii) a mixture of ZnP (7 mm) and ZnPc (3.5 mm) in DCB. The * is due to ZnPc absorbance at this wavelength. b) Fluorescence spectra of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) on increasing addition of C60Py2 (0.1 equiv each addition) in DCB. lex = 411 nm corresponding to one of the isosbestic points. Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
3
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper ties. These results suggest participation of both ZnPc and ZnP as electron donors in electron-transfer reactions, whereas fullerene being the electron acceptor.
Electrochemistry and energy-level diagram Electrochemical studies by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed to evaluate the metal–ligand binding site, the redox potentials of the tetrad, and also to evaluate the energetics of the different photochemical processes. The electrochemistry of zinc porphyrins and zinc phthalocyanines has been extensively studied in the literature.[30] Both macrocycles are known to undergo two one-electron reductions leading to the formation of p-radical anion and dianion species, and two one-electron oxidations leading to the formation of p-radical cation and dication species, respectively. Figure 5 shows a DPVs of the (ZnP–ZnP)– ZnPc triad on increasing addition of C60Py2 in o-DCB, 0.1 m (nBu4N)ClO4. Four oxidation processes located at 0.03, 0.27, 0.57, and 0.80 V versus Fc/Fc + were observed during anodic excursion, whereas three reductions at 1.50, 1.67, and 1.93 V versus Fc/Fc + were observed during cathodic excursion of the poFigure 3. a) Absorption spectral changes observed for C60Py2 (3.8 mm, 2.0 mL each additential within the potential window of the solvent. By tion) binding to (ZnP–ZnP)–ZnPc triad (3.5 mm) in DCB. b) Mole ratio plot constructed to comparing the voltammograms of pristine ZnPc, the evaluate the molecular stoichiometry (425 nm band intensity was used). c) Benesi–Hildefirst oxidation process was ascribed to the formation brand plot to calculate the binding constant. of ZnPc· + species, whereas the first reduction has been ascribed to the formation of ZnPc·. Increasing cussed below. It may be mentioned herein that addition of an addition of C60Py2 to form the tetrad revealed significant excess amount of C60Py2 beyond that is needed to form 1:1 changes. That is, the currents of the first oxidation process of complex, results in coordinating the free ZnP (or ZnPc) giving ZnPc revealed diminished currents with an anodic shift of a 1:2 complex. 32 mV, whereas the second oxidation of ZnP revealed a cathodic shift of about 20 mV suggesting participation of both ZnP and ZnPc entities in tetrad formation, as shown in Scheme 1. Computational studies Collectively, the spectral, computational, and electrochemical Figure 4 a and b show the optimized structures of (ZnP–ZnP)– results suggest involvement of both ZnPc and ZnP entities of ZnPc triad and C60Py2 at the B3LYP/6-31* level.[29] The three the (ZnP–ZnP)–ZnPc triad in supramolecular tetrad formation. macrocycles of the triad were found to be disposed in a trianThis would suggest existence of both tetrads 1 and 2 in solugular shape with tilt angles ranging between 110–1308. The tion, which is reasonable knowing that both ZnPc and ZnP macrocycle zinc–zinc distances were appropriate for C60Py2 have almost similar binding affinity for pyridine ligands. binding, more so for that involving ZnP–ZnPc binding mode. Free-energy calculations for charge-recombination (DGCR) That is, the ZnP–ZnPc distances were found to be 13.6 and and charge-separation (DGCS) processes were performed ac14.1 , whereas the ZnP–ZnP distance was 17.9 in the triad cording to the following equations based on Weller apto accommodate C60Py2, in which the NN distance between proach:[31] the two pyridine units was 10.6 . Figure 4 c and d show the structures of the tetrads involving ZnP–ZnPc and ZnP–ZnP binding to C60Py2. Both resulted in stable structures, in which DGCR ¼ ðE ox E red Þ þ DGS ð1Þ the former structure revealed slightly higher stability by 1.85 kJ mol1. The center-to-center distance between Zn and ð2Þ DGCS ¼ DE 00 ðDGCR Þ C60 for the tetrad having C60Py with connecting CH2 was about 0.5 higher than the one without CH2 fragment, in which DGCR and DGCS refer to free-energy change for charge both with a distance of 11.5 0.5 . As shown in Figure S2 in recombination and charge separation, respectively; DE00 correthe Supporting Information, the frontier HOMO and LUMO for spond to the singlet-state energy of each zinc tetrapyrrole tetrad 1 were found to be on the ZnPc and C60 entities, respec(2.04 eV for 1ZnP* and 1.84 eV for 1ZnPc*). The Eox and Ered reptively. For tetrad 2, these orbitals were on the ZnP and C60 entiresent the oxidation potential of the electron donor (ZnP and &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
4
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 4. B3LYP/6-31G* optimized structures of a) (ZnP-ZnP)-ZnPc triad, b) C60Py2, c) tetrad upon C60Py2 coordination through ZnPc-ZnP, and d) tetrad upon C60Py2 coordination through ZnP–ZnP. Structures c and d show the two possible forms of the supramolecular tetrads.
ZnPc band at 698 nm, originated from singlet–singlet excitation transfer from the 1ZnP*, revealed over 98 % quenching. This quenching could arise from the singlet–singlet energy transfer from the 1ZnPc* to C60Py2 or photoinduced electron transfer from the 1ZnPc* to C60Py2 leading to the formation of (ZnP– ZnP)–ZnPc· + :Py2C60· charge-separated state. In the absence of a spectral overlap of ZnPc fluorescence and C60Py2 absorption in the spectral region, singlet– singlet energy transfer could be ruled out as a quenching mechanism. These results are suggestive of the occurrence of sequential singlet–singlet energy transfer followed by electron transfer in the tetrads. Further femtosecond transient absorption studies were performed to secure evidence for the occurrence of energy and electron transfer events, and also to obtain kinetic information of the different photochemical events. Herein, we have employed two solvents, namely, toluene
ZnPc) and the reduction potential of the electron acceptor (C60), respectively. DGS refers to the static energy, calculated by using the “dielectric continuum model” according to the following Equation:
DGS ¼ðe2 =ð4 pe0 ÞÞ½ð1=ð2Rþ Þ þ 1=ð2R Þð1=RCC Þ=eS ð1=ð2Rþ Þ þ 1=ð2R ÞÞ=eR Þ
ð3Þ
in which R + and R are radii of the radical cation and radical anion, respectively; RCC is the center–center distances between ZnP (or ZnPc) and C60, which were evaluated from optimized structure. The values eR and eS refer to solvent dielectric constants for electrochemistry and photophysical measurements, respectively. The free-energy change (DGCS) was found to be 0.74 eV for 1ZnPc* originated, and 0.57 eV from 1ZnP* originated electron-transfer reactions in the tetrads. This indicates preference of ZnPc over ZnP as primarily electron donor in tetrad 1, whereas one of the spatially close ZnP entities acting as an electron donor in tetrad 2.
Figure 5. Differential pulse voltammograms of (ZnP–ZnP)–ZnPc triad on increasing addition of C60Py2 (0.0, 0.25, 0.75, and 1.0 equiv) in o-DCB containing 0.1 m (nBu4N)ClO4.
and dichlorobenzene, because the excited energy transfer is often efficient in nonpolar solvent, such as toluene, and electron transfer is efficient in slightly polar (o-dichlorobenzene) to polar solvents.[31] We could not employ very polar solvents, such as benzonitrile or dimethylformamide, due to the unstability of the tetrad wherein the polar solvent molecules compete with pyridine for axial binding of zinc tetrapyrroles.[10]
Photoinduced electron-transfer studies Figure 2 b shows fluorescence spectral changes of (ZnP–ZnP)– ZnPc triad on increasing addition of C60Py2, excited at l = 411 nm corresponding to one of the isosbestic points. The Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
Figure 6. Femtosecond-transient spectrum of the (ZnP)2–ZnPc triad in a) toluene and c) DCB at the indicated time intervals. Figure b and d show the time profile of the selected peaks corresponding to ZnP singlet, ZnP absorption and emission, and ZnPc emission (see text for details) in toluene and DCB.
state (time constant 1.38 ps), reflecting occurrence of ultrafast excitation transfer from 1ZnP* to ZnPc in the triad. The 1ZnPc* formed decayed with a lifetime of 1.4 ns. In DCB, considerable differences from that observed in toluene were observed (Figure 6 b). That is, the singlet deactivation of the initial 1ZnP* was much faster (time constant 0.96 ps), whereas the product of energy transfer, 1ZnPc*, formed within a time constant of about 2.0 ps. The 1ZnPc* formed decayed with a lifetime of 0.94 ns in DCB. The time constants recorded for excitation energy transfer for the present triad were similar to that reported earlier by Pereira et al.[23] and us[24] on porphyrin-phthalocyanine dyads using up conversion and femtosecond-transient measurements. In addition to the above-discussed energy transfer, additional electron transfer could also occur in the triad according to Scheme 3. Two routes could be envisioned, namely, electron transfer to the initially formed 1ZnP* from the ZnPc entity re-
Excitation of the precursor tetratolylporphyrinatozinc(II) (ZnP) by 400 nm, femtosecond laser (100 fs width) revealed instantaneous formation of singlet–singlet absorptions with maxima at 450 nm and a strong minima at 550 nm, opposite of the porphyrin ground-state absorption.[33] Additional minima at 590 and 648 nm, corresponding to singlet emission of ZnP were also observed (Figure S3 in the Supporting Information). With time, the excited singlet-state features diminished to populate the triplet state within the singlet lifetime of ZnP being 1.85 ns. Figure 6 a and c show the transient spectra of (ZnP–ZnP)– ZnPc triad at different time intervals in toluene and dichlorobenzene, respectively. In toluene, at the excitation wavelength of 400 nm, ZnP entity of the triad was largely excited. In contrast to ZnP, a short-lived porphyrin singlet–singlet peak was observed in the 450–500 nm region (time constant 1.04 ps) with the concomitant development of phthalocyanine singlet &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
6
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper triad with ZnP· and ZnPc· + entities irrespective of their origin. Free-energy calculations performed according to the earlier discussed Weller’s approach revealed reaction 2 to be slightly exothermic (DG 0.05 eV) and reaction 3 to be endothermic ((DG 0.12 eV) in DCB but not in toluene. Accordingly, spectra recorded in DCB showed weak features of electron transfer in addition to energy transfer (Figure 6 c). That is, broad band in the 600 nm region corresponding to ZnP· and 840 nm region corresponding to ZnPc· + accompanied by depletion of 900 nm band of 1ZnP* in the 900 nm region were observed. The tetrads formed by binding C60Py2 to (ZnP–ZnP)–ZnPc triad revealed occurrence of photoinduced electron transfer leading to the formation of (ZnP–ZnP)–ZnPc· + :Py2C60· and (ZnP–ZnP· + )–ZnPc:Py2C60· charge-separated states both in toluene and DCB (Figure 7); however, the spectral appearance of the former charge-separated species was much better pro-
Scheme 3. Energy- and electron-transfer (ET) events occurring in the (ZnP– ZnP)–ZnPc triad upon excitation of ZnP entity.
sulting into the formation of (ZnP–ZnP·)2–ZnPc· + charge-separated state (reaction 2 in Scheme 3), and electron transfer from the energy-transfer product (reaction 1 in Scheme 1), 1ZnPc* to one of the ZnP entities to result into the formation of (ZnP– ZnP·)2–ZnPc· + charge-separated state (reaction 3 in Scheme 3). It is important to note that both the electron-transfer routes result into the formation of same radical ion pair, that is, the
Figure 7. Femtosecond-transient spectrum of the tetrad (ZnP–ZnP)–ZnPc:Py2C60 formed by binding of C60Py2 to (ZnP–ZnP)–ZnPc triad in a) toluene and c) DCB at the indicated time intervals. The b and d show the time profile of the ZnPc· + and Py2C60· transient bands monitored at l = 850 and 1020 nm in the corresponding solvent. Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
7
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper nounced than the latter due to the appearance of transient bands corresponding to ZnPc· + at 840 nm and Py2C60· at 1020 nm, sufficiently far from bands of other transient species. As shown in Figure 7 a, in toluene upon excitation by using 400 nm femtosecond laser, the singlet-excited ZnP formed in the l = 450–500 nm region deactivated to populate the singlet-excited state of ZnPc by excitation transfer as was witnessed from the emission of 1ZnPc* at l = 682 nm. Recovery of this signal led to subsequent formation of ZnPc· + and Py2C60· as was evidenced from their corresponding transient bands at l = 850 and 1020 nm, respectively. Similar results were obtained in dichlorobenzene, as shown in Figure 7 b; however, the spectral features in the l = 850–1100 nm range were rather broad suggesting exciplex formation, similar to that reported for ZnPc-C60 and ZnP-C60 dyads in the literature.[32] Nevertheless, the present results clearly show formation of (ZnP–ZnP)– ZnPc· + :Py2C60· mainly from the intermediate energy-transfer product, 1ZnPc*. This was also supported by the time profiles of ZnPc· + and Py2C60·, shown in right-hand panels. The time constants for both radical-cation and radical-anion derived from the monoexponential decay fits were almost same for a given solvent. This suggests that the spectral features arise from tetrad 1 having ZnPc and C60 closer than in tetrad 2. Although thermodynamically less favored, electron transfer from 1 ZnP* cannot be excluded, especially in tetrad 2 due to close proximity of the donor and acceptor entities. In this event, the ZnP· + band expected in the 650 nm range seem be buried under the strong negative-emission band of 1ZnPc* at l = 682 nm. The rate of charge separation, kCS, and rate of charge recombination, kCR, for the (ZnP–ZnP)–ZnPc· + :Py2C60· radicalion pair (from the time profile of 849 nm peak) were found to be 2.20 1011 and 6.10 108 s1 in toluene, and 6.82 1011 and 1.20 109 s1 in dichlorobenzene, respectively. The different photochemical events occurring in the tetrad 1 in DCB are summarized in an energy-level diagram in Figure 8. A similar diagram could be envisioned in toluene with the energy of radical-ion pair being higher by approximately 0.17 eV than that in DCB due to lower polarity of toluene.[33] The energy levels were calculated primarily from the earlier discussed spectral and redox data. At the excitation wavelength of 400 nm, the ZnP entity of (ZnP–ZnP)– ZnPc:Py2C60 of tetrad 1 mainly gets excited to populate the singlet-excited state. The 1ZnP* thus formed can deactivate either by undergoing intersystem crossing to populate the triplet-excited state of ZnP, or electron transfer to result into the formation of (ZnP–ZnP· + )2-ZnPc:Py2C60· radical-ion pair, or energy transfer to ZnPc to produce 1ZnPc*. These processes are in competition with any radiative (fluorescence) and nonradiative decay of 1ZnP*. Both steady-state fluorescence and transient spectral studies on the (ZnP–ZnP)–ZnPc triad demonstrated occurrence of ultrafast, efficient excited-energy transfer from 1ZnP* to ZnPc as the main photochemical event. Further, the newly formed (ZnP–ZnP)–1ZnPc*:Py2C60 undergoes electron transfer to give (ZnP–ZnP)–ZnPc· + :Py2C60· radical-ion pair. This radical-ion pair persists for about 2–3 ns, depending upon the solvent polarity, prior returning to the ground state. A hole shift from ZnPc· + to ZnP within the tetrad generating (ZnP– &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
Figure 8. Energy-level diagram showing the photochemical events occurring in the tetrad 1 (ZnP–ZnP)–ZnPc:Py2C60 in DCB. Solid arrows represent major photochemical events and dashed arrows show minor photochemical events.
ZnP· + )–ZnPc:Py2C60· is an uphill process, thus, the transient spectral revealed peaks of ZnPc· + and Py2C60· radical ions. Upon formation of (ZnP–ZnP· + )–ZnPc:Py2C60·, characteristic transient band of ZnP· + around 650 nm was expected due the presence strong ZnPc emission band in the spectral region. The kinetic data revealed faster charge separation and relatively slow rate of charge recombination, with the expected solvent-polarity trends.
Conclusion A photosynthetic antenna-reaction center mimicking supramolecular tetrad capable of undergoing sequential photoinduced energy transfer followed by electron transfer has been newly assembled and characterized. The tetrad featured two energy-harvesting zinc porphyrin entities, and a zinc phthalocyanine and a fullerene as electron-donor and -acceptor entities. The spectral, electrochemical, and computational studies suggested binding of the bis-pyridine-functionalized fullerene to the triad with a binding constant of 1.17 105 m1, whereas leaving one of the zinc macrocycles uncomplexed. Excitation of the free zinc porphyrin of the triad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine with a rate constant close to the femtosecond transient setup. Photoinduced electron transfer from the singlet-excited zinc phthalocyanine to fullerene was witnessed both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc· + in the 850 nm range and C60· in the 1020 nm range, and not that of ZnP· + in the 650 nm range were observed, clearly establishing (ZnP-ZnP)-ZnPc· + :Py2C60· radical-ion-pair formation. The rate of charge separation, kCS, and rate of charge recombination, kCR, were found to be 6.70 1011 and 8
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper 6.48 108 s1 in toluene, and 1.99 1011 and 1.98 109 s1 in dichlorobenzene, respectively. These results reveal ultrafast energy transfer followed by charge separation in the multimodular tetrad.
Synthesis of (ZnP–ZnP)2–ZnPc triad 5-(4-Hydroxyphenyl)-10,15,20-tris(tolualyl)porphyrin (1) 4-Hydroxybenzaldehyde (500 mg, 4.09 mmol), p-tolualdehyde (1.44 mL, 12.28 mmol), and pyrrole (1.14 mL, 16.36 mmol) were kept in a round-bottom (RB) flask (500 mL), and then heated at reflux with 100 mL of propionic acid for 4 h. After cooling the mixture to RT, the solvent was evaporated, and mixture was purified by using silica column. The desired compound was eluted by CHCl3/CH3OH (90:10) as a second fraction. Yield: 13 %. 1H NMR (CDCl3, 400 MHz): d = 2.70 (s, 2 H, pyrrole H), 2.75 (s, 9 H, CH3), 7.20 (d, 2 H, Ar-H), 7.55 (d, 6 H, Ar-H), 8.05 (d, 2 H, Ar-H), 8.15 (d, 6 H, Ar-H), 8.85 ppm (br, 8 H, Ar-H).
Experimental Section Spectral measurements The UV/Vis spectral measurements were carried out with a Shimadzu Model 2550 double monochromator UV/Vis spectrophotometer. The fluorescence emission was monitored by using a Varian Eclipse spectrometer. A right-angle detection method was used. The 1 H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Cyclic voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer by using a three-electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements by using argon gas. The computational calculations were performed by DFT B3LYP/6–31G* methods with Gaussian 09 software package.[29]
4,5-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin] phthalonitrile (2) 5-(4-Hydroxyphenyl)-10,15,20-tris(tolualyl)porphyrin (1; 450 mg, 0.66 mmol), 4,5-dichlorophthalonitrile (33 mg, 0.16 mmol), and K2CO3 (639 mg, 4.62 mmol) were kept in a RB (100 mL) flask under nitrogen for 20 min then heated at reflux in dry acetone (50 mL) for 48 h. After cooling to RT, the mixture was filtered, and solvent was evaporated. Obtained crude product was purified by using silica column, and the desired compound was eluted by 100 % CH2Cl2. Yield: 42 %. 1H NMR (CDCl3, 400 MHz): d = 2.82 (s, 4 H, pyrrole H), 3.22 (s, 18 H, CH3), 7.18 (d, 4 H, Ar-H), 7.52 (m, 12 H, Ar-H), 7.79 (s, 2 H, Ar-H), 7.95 (d, 4 H, Ar-H), 8.05 (m, 12 H, Ar-H), 8.80 ppm (br, 16 H, Ar-H).
Laser-flash photolysis
2,3-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrinato zinc (II)]9,16,23 tert-butyl phthalocyaninatozinc(II) (3)
Femtosecond-transient absorption-spectroscopy experiments were performed by using an ultrafast femtosecond laser source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. The fundamental output of the laser (95 %) was introduced into harmonic generator, which produced second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, whereas the rest of the output was used for generation of white-light continuum. In the present study, the second harmonic 400 nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed by using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed toluene at 298 K.
4,5-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin] phthalonitrile (2; 90 mg, 0.06 mmol), 4-tert-butylphthalonitrile (101 mg, 0.55 mmol), and ZnCl2 (375 mg, 2.75 mmol) were kept in a RB flask (100 mL) for 20 min under nitrogen. Then, the mixture was heated at reflux with 2-dimethylaminoethanol (4 mL) for 18 h under dark condition. After cooling the mixture to RT, the solvent was evaporated, and thus obtained green crude product was purified by using silica column. The desired compound was eluted by CHCl3/CH3OH (90:10). Yield: 18 %. 1H NMR (CDCl3, 400 MHz): d = 1.25–1.85 (m, 27 H, tBu), 3.10 (s, 18 H, CH3), 7.10 (m, 4 H, Ar-H), 7.50 (br, 12 H, ArH), 7.80 (m, 4 H, Ar-H), 8.10 (br., 14 H, Ar-H), 8.75 (br., 9 H, Ar-H), 9.20 ppm (br, 16 H, Ar-H); MS (MALDI-TOF): m/z calcd 2214.64 [M] + , found: 2296.30 [M + 2 CH3OH + H2O] + .
Acknowledgements This work was supported by the National Science Foundation (Grant No. 1110942 to F.D.). The computational work was completed by utilizing the Holland Computing Center of the University of Nebraska.
Chemicals Buckminsterfullerene, C60 (+ 99.95 %), was from SES Research (Houston, TX). All the reagents were from Aldrich Chemicals (Milwaukee, WI), whereas the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (nBu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. The synthesis of C60Py2 is given elsewhere.[34] Bis(porphyrin)–phthalocyanine triad was synthesized according to literature methods with few modifications.[26] Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
Keywords: charge separation · electron transfer · fullerenes · multimodular tetrads · porphyrins · zinc tetrapyrrole [1] a) Light-Harvesting Antennas in Photosynthesis (Eds.: B. R. Green, W. W. Parson), Kluwer, Dordrecht, 2003; b) Handbook of Photosynthesis, 2nd ed. (Ed.: M. Pessarakli), CRC Press LLC, Boca Raton, 2005; c) Photosynthetic Light Harvesting (Eds.: R. Cogdell, C. Mullineaux), Springer, Dordrecht, 2008.
9
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper [16] a) R. Chitta, K. Ohkubo, M. Tasior, N. K. Subbaiyan, M. E. Zandler, D. T. Gryko, S. Fukuzumi, F. D’Souza, J. Am. Chem. Soc. 2008, 130, 14263; b) F. D’Souza, E. Maligaspe, K. Ohkubo, M. E. Zandler, N. K. Subbaiyan, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131, 8787; c) F. D’Souza, N. K. Subbaiyan, Y. Xie, J. P. Hill, K. Ariga, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131, 16138; d) C. A. Wijesinghe, M. E. El-Khouly, N. K. Subbaiyan, M. Supur, M. E. Zandler, K. Ohkubo, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2011, 17, 3147; e) M. E. El-Khouly, C. A. Wijesinghe, V. N. Nesterov, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2012, 18, 13844; f) C. K. C. Bikram, N. K. Subbaiyan, F. D’Souza, J. Phys. Chem. C 2012, 116, 11964; g) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2013, 19, 7221; h) C. A. Wijesinghe, M. E. El-Khouly, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2013, 19, 9629; i) C. B. KC, G. N. Lim, M. E. Zandler, F. D’Souza, Org. Lett. 2013, 15, 4612; j) F. D’Souza, S. Gadde, S. Islam, C. A. Wijesinghe, A. L. Schumacher, M. E. Zandler, Y. Araki, O. Ito, J. Phys. Chem. A 2007, 111, 8552; k) M. E. El-Khouly, S. Fukuzumi, F. D’Souza, ChemPhysChem 2014, 15, 30. [17] The Porphyrin Handbook, Vols. 1 – 20 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, San Diego, 2000. [18] a) Phthalocyanine: Properties and Applications (Eds.: C. C. Leznoff, A. B. P. Lever), VCH, Weinheim, 1993; b) C. G. Claessens, D. Gonzlez-Rodrguez, T. Torres, Chem. Rev. 2002, 102, 835. [19] a) H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162; b) W. Kr tschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman, Nature 1990, 347, 354; c) R. E. Smalley, Angew. Chem. Int. Ed. Engl. 1997, 36, 1594; d) Fullerenes, Chemistry, Physics and Technology (Eds.: K. M. Kadish, R. S. Ruoff), Wiley-Interscience, New York, 2000. [20] Q. Xie, E. Perez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 1992, 114, 3978. [21] a) H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 2607; b) H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. P. Luo, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 6617; c) H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2002, 124, 5165; d) H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito, H. Yamada, S. Fukuzumi, Chem. Eur. J. 2004, 10, 3184; e) D. M. Guldi, H. Imahori, K. Tamaki, Y. Kashiwagi, H. Yamada, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A 2004, 108, 541. [22] a) H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki, Y. Sakata, F. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 100; b) M. Fujitsuka, O. Ito, H. Imahori, K. Yamada, H. Yamada, Y. Sakata, Chem. Lett. 1999, 721; c) F. D’Souza, R. Chitta, S. Gadde, D. M. S. Islam, A. L. Shumacher, M. E. Zandler, Y. Araki, O. Ito, J. Phys. Chem. B 2006, 110, 25240; d) A. Y. Smirnov, L. G. Mourokh, P. K. Ghosh, F. Nori, J. Phys. Chem. C 2009, 113, 21218; e) S. H. Lee, A. G. Larsen, K. Ohkubo, Z. O. Cai, J. R. Reimers, S. Fukuzumi, M. J. Crossley, Chem. Sci. 2012, 3, 257. [23] a) A. M. V. M. Pereira, A. R. M. Soares, A. Hausmann, M. G. M. S. Neves, A. C. Tome, A. M. S. Silva, J. A. S. Cavaleiro, D. M. Guldi, T. Torres, Chem. Phys. Chem. Phys. 2011, 13, 11858; b) A. M. V. M. Pereira, A. Hausmann, J. P. C. Tome, O. Trukhina, M. Urbani, M. G. P. M. S. Neves, J. A. S. Cavaleiro, D. M. Guldi, T. Torres, Chem. Eur. J. 2012, 18, 3210. [24] a) C. B. KC, K. Stranius, P. D’Souza, N. K. Subbaiyan, H. Lemmetyinen, N. V. Tkachenko, F. D’Souza, J. Phys. Chem. C 2013, 117, 763; b) C. B. KC, K. Ohkubo, P. A. Karr, S. Fukuzumi, F. D’Souza, Chem. Commun. 2013, 49, 7614. [25] a) F. Ito, Y. Ishibashi, S. R. Khan, H. Miyasaka, K. Kameyama, M. Morisue, A. Satake, K. Ogawa, Y. Kobuke, J. Phys. Chem. A 2006, 110, 12734; b) E. Maligaspe, T. Kumpulainen, H. Lemmetyinen, N. V. Tkachenko, N. K. Subbaiyan, M. E. Zandler, F. D’Souza, J. Phys. Chem. A 2010, 114, 268; c) K. Stranius, R. Jacobs, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler, F. D’Souza, J. Porphyrins Phthalocyanines 2010, 14, 948; d) R. Jacobs, K. Stranius, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler, F. D’Souza, Inorg. Chem. 2012, 51, 3656. [26] Z. Zhao, T. Nyokong, M. D. Maree, Dalton Trans. 2005, 3732. [27] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, Heidelberg, 2006. [28] H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71, 2703. [29] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
[2] a) Photochemical Conversion and Storage of Solar Energy (Ed.: J. S. Connolly), Academic Press, New York, 1981; b) Molecular Level Artificial Photosynthetic Materials (Ed. G. J. Meyer), Wiley, Hoboken, 1997. [3] a) D. Gust, T. A. Moore in The Porphyrin Handbook, Vol. 8 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, Burlington, 2000, pp. 153 – 190; b) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40; c) S. D. Straight, G. Kodis, Y. Terazono, M. Hambourger, T. A. Moore, A. L. Moore, D. Gust, Nature Nanotechnol. 2008, 3, 280; d) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009, 42, 1890; e) D. Gust, T. A. Moore, A. L. Moore, Faraday Discuss. 2012, 155, 9. [4] a) Energy Harvesting Materials (Ed.: D. L. Andrews), World Scientific, Singapore, 2005; b) S. Gnes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324; c) N. Armaroli, V. Balzani, Angew. Chem. 2007, 119, 52; Angew. Chem. Int. Ed. 2007, 46, 52; d) H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, 1809. [5] a) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729; b) P. V. Kamat, J. Phys. Chem. C 2007, 111, 2834; c) S. Fukuzumi, Eur. J. Inorg. Chem. 2008, 1351. [6] a) M.-J. Blanco, M. Consuelo Jimenez, J.-C. Chambron, V. Heitz, M. Linke, J.-P. Sauvage, Chem. Soc. Rev. 1999, 28, 293; b) X. Dang, X, J. T. Hupp, J. Photochem. Photobiol. A 2001, 143, 251; c) V. Balzani, A, Credi, M. Venturi, ChemSusChem 2008, 1, 26; d) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910; e) G. Renger, Curr. Sci. 2010, 98, 1305; f) K. Kalyanasundaram, M. Graetzel, Curr. Opin. Biotechnol. 2010, 21, 298; g) D. G. Nocera, Inorg. Chem. 2009, 48, 10001. [7] a) D. M. Guldi, A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006, 35, 471; b) J. N. Clifford, G. Accorsi, F. Cardinali, J. F. Nierengarten, N. Armaroli, C. R. Chemie. 2006, 9, 1005; c) N. Martin, L. Sanchez, M. A. Herranz, B. Illesca, D. M. Guldi, Acc. Chem. Res. 2007, 40, 1015; d) G. Bottari, G. de La Torre, D. M. Guldi, T. Torres, Chem. Rev. 2010, 110, 6768; e) D. M. Guldi, Chem. Commun. 2011, 47, 606. [8] a) D. Holten, D. F. Bocian, J. S. Lindsey, Acc. Chem. Res. 2002, 35, 57; b) J. S. Lindsey, D. F. Bocian, Acc. Chem. Res. 2011, 44, 638. [9] a) H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 2004, 14, 525; b) S. Fukuzumi, Pure Appl. Chem. 2007, 79, 981; c) S. Fukuzumi, Phys. Chem. Chem. Phys. 2008, 10, 2283; d) S. Fukuzumi, T. Kojima, J. Mater. Chem. 2008, 18, 1427; e) S. Fukuzumi, T. Honda, K. Ohkubo, T. Kojima, Dalton Trans. 2009, 3880; f) S. Fukuzumi, K. Ohkubo, J. Mater. Chem. 2012, 22, 4575. [10] a) M. E. El-Khouly, O. Ito, P. M. Smith, F. D’Souza, J. Photochem. Photobiol. C 2004, 5, 79; b) F. D’Souza, O. Ito, Coord. Chem. Rev. 2005, 249, 1410; c) R. Chitta, F. D’Souza, J. Mater. Chem. 2008, 18, 1440; d) F. D’Souza, O. Ito, Chem. Commun. 2009, 4913; e) F. D’Souza, A. S. D. Sandanayaka, O. Ito, J. Phys. Chem. Lett. 2010, 1, 2586; f) F. D’Souza, O. Ito, Chem. Soc. Rev. 2012, 41, 86; g) O. Ito, F. D’Souza, Molecules 2012, 17, 5816; h) F. D’Souza, O. Ito in Multiporphyrin Array: Fundamentals and Applications (Ed.: D. Kim), Pan Stanford Publishing, Singapore, 2012, Chapter 8, pp. 389 – 437. [11] a) D. I. Schuster, K. Li, D. M. Guldi, C. R. Chim. 2006, 9, 892; b) R. Ziessel, A. Harriman, Chem. Commun. 2011, 47, 611; c) A. C. Benniston, A. Harriman, Coord. Chem. Rev. 2008, 252, 2528. [12] a) N. K. Subbaiyan, C. A. Wijesinghe, F. D’Souza, J. Am. Chem. Soc. 2009, 131, 14646; b) H. Imahori, T. Umeyama, K. Kei, T. Yuta, Chem. Commun. 2012, 48, 4032; c) P. V. Kamat, G. C. Schatz, J. Phys. Chem. C 2009, 113, 15473; d) T. Hasobe, Phys. Chem. Chem. Phys. 2010, 12, 44. [13] a) J. L. Sessler, C. M. Lawrence, J. Jayawickramarajah, Chem. Soc. Rev. 2007, 36, 314 – 325; b) E. M. Prez, N. Martn, Chem. Soc. Rev. 2008, 37, 1512; c) D. Gonzalez-Rodriguez, E. Carbonell, D. M. Guldi, Angew. Chem. Int. Ed. 2009, 48, 8032. [14] a) M. R. Wasielewski, Chem. Rev. 1992, 92, 435; b) A. Osuka, N. Mataga, T. Okada, Pure Appl. Chem. 1997, 69, 797; c) L. Flamigni, F. Barigelletti, N. Armaroli, J. P. Collin, I. M. Dixon, J. P. Sauvage, J. A. G. Williams, Coord. Chem. Rev. 1999, 190 – 192, 671; d) F. Diederich, M. Gomez-Lopez, Chem. Rev. Soc. 1999, 28, 263. [15] a) A. Takai, M. Chkounda, A. Eggenspiller, C. P. Gros, M. Lachkar, J.-M. Barbe, S. Fukuzumi, J. Am. Chem. Soc. 2010, 132, 4477; b) J.-Y. Liu, M. E. El-Khouly, S. Fukuzumi, D. K. P. Ng, Chem. Asian J. 2011, 6, 174; c) J.-Y. Liu, M. E. El-Khouly, S. Fukuzumi, D. K. P. Ng, Chem. Eur. J. 2011, 17, 1605; d) S. Fukuzumi, K. Ohkubo, F. D’Souza, J. L. Sessler, Chem. Commun. 2012, 48, 9801; e) K. Ohkubo, Y. Kawashima, S. Fukuzumi, Chem. Commun. 2012, 48, 4314.
&
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
10
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc, Wallingford CT. 2010. [30] K. M. Kadish, E. V. Caemelbecke, G. Royal, in The Porphyrin Handbook (Eds. K. M. Kadish, K. M., Smith, R. Guilard, R.), Vol. 8, Academic Press, San Diego, 2000, Chapter 55. [31] a) D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259; b) N. Mataga, H. Miyasaka in Electron Transfer (Eds.: J. Jortner, M. Bixon), Wiley, Hoboken, 1999, Part 2, pp. 431 – 496.
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
[32] a) H. Lemmetyinen, N. V. Tkachenko, A. Efimov, M. Niemi, J. Phys. Chem. C 2009, 113, 11475; b) M. Niemi, N. V. Tkachenko, A. Efimov, L. Alexander, K. Ohkubo, S. Fukuzumi, H. Lemmetyinen, J. Phys. Chem. A 2008, 112, 6884; c) F. D’Souza, E. Maligaspe, P. A. Karr, A. L. Schumacher, M. El Ojaimi, C. P. Gros, J.-M. Barbe, K. Ohkubo, S. Fukuzumi, Chem. Eur. J. 2008, 14, 674; d) N. V. Tkachenko, H. Lemmetyinen, J. Sonoda, K. Ohkubo, T. Sato, H. Imahori, S. Fukuzumi, J. Phys. Chem. A 2003, 107, 8834. [33] S. K. Das, B. Song, A. Mahler, V. N. Nesterov, A. K. Wilson, O. Ito, F. D’Souza, J. Phys. Chem. C. 2014, 118, 3994. [34] F. D’Souza, S. Gadde, M. E. Zandler, M. Itou, Y. Araki, O. Ito, Chem. Commun. 2004, 2276.
Received: February 27, 2014 Published online on && &&, 0000
11
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Electron Transfer
Artificial photosynthesis: Occurrence of ultrafast energy transfer (EnT) and electron transfer leading to the formation of a charge-separated (CS) state is demonstrated in a new donor–acceptor supramolecular tetrad comprising a bis(zinc porphyrin)–(zinc phthalocyanine) linked to fullerene as a photosynthetic antenna-reaction center mimic (see scheme).
C. B. KC, G. N. Lim, P. A. Karr, F. D’Souza* && – && Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)– Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies
&
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
12
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
& Electron Transfer
Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)–Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies Chandra B. KC,[a] Gary N. Lim,[a] Paul A. Karr,[b] and Francis D’Souza*[a] In memory of Gabriel D’Souza
Abstract: A multimodular donor–acceptor tetrad featuring a bis(zinc porphyrin)–(zinc phthalocyanine) ((ZnP–ZnP)– ZnPc) triad and bis-pyridine-functionalized fullerene was assembled by a “two-point” binding strategy, and investigated as a charge-separating photosynthetic antenna-reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis-pyridine-functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi–Hildebrand plot by using the optical data was found to be 1.17 105 m1, whereas a plot of “mole-ratio” method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6-31G* optimization, whereas the energy levels for different photochemical events was established by using data from the op-
tical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer (kENT) in the case of the triad was found to be 7.5 1011 s1 in toluene and 6.3 1011 s1 in o-dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy-transfer product, that is, singlet-excited zinc phthalocyanine to fullerene was verified from the femtosecond-transient spectral studies, both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc· + in the 850 nm range and C60· in the 1020 nm range were clearly observed. The rate of charge separation, kCS, and rate of charge recombination, kCR, for the (ZnP–ZnP)–ZnPc· + :Py2C60· radical ion pair (from the time profile of 849 nm peak) were found to be 2.20 1011 and 6.10 108 s1 in toluene, and 6.82 1011 and 1.20 109 s1 in o-dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad.
Introduction
models capable of undergoing both energy- and electrontransfer processes.[1–16] To connect the different entities of the multimodular systems, either covalent bonding or self-assembled strategies have been utilized. But in recent years, a combination of these two approaches have been successfully employed to build multimodular donor–acceptor systems. This combination approach has provided better control over monitoring the different photochemical events occurring in these systems. In building, the multimodular donor–acceptor models, porphyrins (P), and phthalocyanines (Pc) are frequently used photosensitizing electron donors,[17, 18] whereas fullerene, C60, is a common electron acceptor.[19, 20] Additionally, ferrocene, tetrathiafulvalene, BF2-chelated dipyrromethene, BF2-chelated azadipyrromethene, phenothiazine, subphthalocyanine, or perylene diimide, among others, have often been utilized as secondary electron donors or energy-funneling antenna entities.[1–22] In the systems made out of both porphyrins and phthalocyanines, the rich and extensive absorption features of these macrocycles have provided increased absorption cross-section.[17, 18]
Studies on artificial photosynthetic model compounds capable of undergoing energy migration and charge separation are key in building low-cost, light-to-electricity and light-to-fuel converting devices.[1–5] Solar energy has the potential to fulfill global energy needs in an environmentally friendly manner if efficient, economically feasible energy-conversion devices could be built.[5] Towards this, wide-ranging efforts have been applied towards developing multimodular donor–acceptor [a] C. B. KC, G. N. Lim, Prof. F. D’Souza Department of Chemistry, University of North Texas 1155 Union Circle, 305070, Denton, TX 76203–5017 (USA) Fax: 940–565–4318 E-mail: [email protected] [b] Prof. Dr. P. A. Karr Department of Physical Sciences and Mathematics Wayne State College, 111 Main Street, Wayne, Nebraska 68787 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402402. Chem. Eur. J. 2014, 20, 1 – 12
These are not the final page numbers! ÞÞ
1
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper Additionally, their complimentary absorption and emission has provided opportunity for wide-band capture and efficient use of the solar spectrum. Fullerene,[19] due to its facile reduction potentials[20] and low solvent and internal reorganization energy,[21] has been an excellent electron acceptor. Generally, fullerene in donor–acceptor systems accelerate forward electron transfer (kCS) and slow down backward electron transfer (kCR), thus, generating the much desired long-lived charge-separated states.[7–10, 21, 22] Additionally, the presence of secondary electron donors results in the generation of long-lived chargeseparated states by a hole-migration mechanism, whereas the existence of energy-funneling-antenna molecules provide better utilization of solar light in charge separation.[21, 22] Recently, multimodular systems utilizing covalently[23, 24] and noncovalently linked porphyrin and phthalocyanine have gained considerable attention due to their complimentary absorption and emission properties.[25] Selective excitation of the porphyrin entity in the covalently linked systems promote ultrafast excited-state energy transfer to phthalocyanine. In the presence of an electron acceptor, the phthalocyanine undergoes electron transfer leading into the formation of a chargeseparated state.[23, 24b] Alternatively, such porphyrin–phthalocyanine could be immobilized on a semiconductor TiO2 surface for improved solar-energy converting devices.[24a] Encouraged with these findings of covalently linked porphyrin and phthalocyanine molecules, herein, we have constructed a multimodular tetrad as a photosynthetic antenna-reaction center mimic comprised of a covalently linked bis(zinc porphyrin)-zinc phthalocyanine ((ZnP–ZnP)–ZnPc) triad coordinated to C60 functionalized with two pyridine entities (Scheme 1). Light-induced excitation transfer and electron transfer in the tetrad, probed by spectral, electrochemical, computational, and transient absorption techniques, is reported.
Scheme 1. Structures of the multimodular, bis(zinc porphyrin)–zinc phthalocyanine coordinated to fullerene by a two-point binding approach. Two possible structures of tetrads (1 and 2) involving ZnP and ZnPc binding sites are shown.
Results and Discussion Synthesis of bis(zinc porphyrin)–zinc phthalocyanine triad The synthesis of the bis(zinc porphyrin)-zinc phthalocyanine triad was carried out according to Scheme 2 based on literature method[26] with minor modifications. The details are given in the Experimental Section. Briefly, 5-(4-hydroxyphenyl)10,15,20-tris(tolualyl)porphyrin was synthesized by reacting stoichiometric amounts of 4-hydroxylbenzaldehyde, tolualdehyde, and pyrrole in propionic acid followed by chromatographic separation/purification over silica-gel column. Next, the porphyrin was reacted with 4,5-dichlorophthalonitrile in the presence of K2CO3 in dry acetone to obtain 4,5-bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin]phthalonitrile. Finally, the porphyrin derivative was reacted with 4-tert-butylphthalonitrile and ZnCl2 in dimethylaminoethanol, followed by chromatographic purification. During the course of this reaction, both phthalocyanine and porphyrin macrocycles were metalated. The compound was stored in dark prior doing spectral and transient measurements. &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
Scheme 2. Synthetic methodology adopted for (ZnP–ZnP)–ZnPc triad. DMAE = dimethylaminoethanol.
Absorption and fluorescence studies Figure 1 shows the optical absorption spectrum of the (ZnP– ZnP)–ZnPc triad along with the spectrum obtained by mixing two equivalents of zinc tetra(tolualyl)porphyrin and one equivalent of zinc tert-butyl phthalocyanine, as a control in o-dichlorobenzene (DCB). The peak intensities at the porphyrin Soret region and phthalocyanine visible-band region matched 2
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper with the spectrum recorded for the control involving a mixture of two equivalents of ZnP and one equivalent of ZnPc. For the control mixture, two emission bands at l = 600 and 650 nm of ZnP were observed, whereas the emission corresponding to ZnPc at 698 nm was almost negligent. In contrast, for the (ZnP–ZnP)–ZnPc triad, the ZnP emission bands were quenched over 80 % of the original intensity with the appearance of strong emission of ZnPc at l = 698 nm. Please note that the small peak at 616 nm (asterisk) corresponds to ZnPc absorption and not due to emission. Further, excitation spectrum of the (ZnP–ZnP)–ZnPc triad was recorded by fixing the emission monochromator to 698 nm corresponding to the emission maxima of ZnPc and scanning the excitation wavelength. As shown in Figure S1 in the Supporting Information, such spectrum showed peaks corresponding to not only ZnPc but also those of ZnP. However, no ZnP peaks were observed for the control mixture. These results demonstrate occurrence efficient singlet–singlet energy transfer from ZnP to covalently linked ZnPc.[27] The supramolecular tetrad was formed by coordinating C60Py2 to (ZnP–ZnP)–ZnPc triad in DCB. Figure 3 a shows spectral changes observed during titration. Herein, the ZnP Soret band at l = 425 nm revealed characteristic features of axial coordination with diminished intensity accompanied by a redshift.[10] For the ZnPc band located at 685 nm, slightly diminished intensity accompanied by isosbestic points at 411, 555, 619, and 703 nm were observed. A plot of mole ratio method (Figure 2 b) revealed 1:1 stoichiometry for the tetrad. The binding constant calculated from the spectral data by using Benesi–Hildebrand method[28] was found to be 1.2 105 m1 (Figure 2 c), which was over an order of magnitude higher than that reported for “single-point” metal–ligand coordination,[10] but was close to that reported for the ZnP-ZnP dyad or ZnPcZnP dyad binding to C60Py2 (Py = pyridine) through a “twopoint” metal–ligand axial binding.[34, 24b] Addition of C60Py2 to the solution of (ZnP–ZnP)–ZnPc triad quenched ZnPc emission almost quantitatively (over 98 %), whereas such changes for the weak ZnP emission were relatively small (Figure 2 b). These results collectively point out a “two-point” metal–ligand axial binding of C60Py2 to (ZnP–ZnP)–ZnPc triad resulting into the formation of (ZnP–ZnP)–ZnPc:Py2C60 multimodular supramolecular tetrad. As shown in Scheme 1, there are at least two structures for the tetrad (assuming the two pyridine entities of C60Py2 and geometric isomers are identical in the context of metal–ligand binding). That is, the first involving the ZnPc and one of the ZnP macrocycles (structure 1), whereas the other involving both ZnP entities of the triad (structure 2). The binding constants of single-point bound zinc–pyridine coordinated complexes generally fall in the same order of magnitude for both ZnP and ZnPc complexes thus making it difficult to rationalize the binding preference. Interestingly, the spectral changes in Figure 3 a clearly show involvement of ZnP by exhibiting typical spectral changes of axial binding; however, for ZnPc, the changes have been moderate. To resolve this issue of the existence of either or both structures 1 and 2, computational and electrochemical studies were found to be very useful, as dis-
Figure 1. Absorption spectrum of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) and ii) a mixture of ZnP (6.6 mm) and ZnPc (3.5 mm) in DCB.
well; however, few subtle changes were noticed. For example, the ZnPc visible bands located at l = 616 and 685 nm in the triad were redshifted in the mixture and appeared at l = 612 and 682 nm, respectively. This observation suggests occurrence of some intramolecular interactions between the macrocycles of the triad. The invariant position of porphyrin Soret and Q bands also suggests lack of direct ZnP-ZnP interactions in the triad. Figure 2 a shows the fluorescence spectrum of the (ZnP– ZnP)–ZnPc triad in DCB excited at the ZnP Soret band along
Figure 2. a) Fluorescence spectrum (lex = 425 nm) of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) and ii) a mixture of ZnP (7 mm) and ZnPc (3.5 mm) in DCB. The * is due to ZnPc absorbance at this wavelength. b) Fluorescence spectra of i) (ZnP–ZnP)–ZnPc triad (3.5 mm) on increasing addition of C60Py2 (0.1 equiv each addition) in DCB. lex = 411 nm corresponding to one of the isosbestic points. Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
3
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper ties. These results suggest participation of both ZnPc and ZnP as electron donors in electron-transfer reactions, whereas fullerene being the electron acceptor.
Electrochemistry and energy-level diagram Electrochemical studies by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed to evaluate the metal–ligand binding site, the redox potentials of the tetrad, and also to evaluate the energetics of the different photochemical processes. The electrochemistry of zinc porphyrins and zinc phthalocyanines has been extensively studied in the literature.[30] Both macrocycles are known to undergo two one-electron reductions leading to the formation of p-radical anion and dianion species, and two one-electron oxidations leading to the formation of p-radical cation and dication species, respectively. Figure 5 shows a DPVs of the (ZnP–ZnP)– ZnPc triad on increasing addition of C60Py2 in o-DCB, 0.1 m (nBu4N)ClO4. Four oxidation processes located at 0.03, 0.27, 0.57, and 0.80 V versus Fc/Fc + were observed during anodic excursion, whereas three reductions at 1.50, 1.67, and 1.93 V versus Fc/Fc + were observed during cathodic excursion of the poFigure 3. a) Absorption spectral changes observed for C60Py2 (3.8 mm, 2.0 mL each additential within the potential window of the solvent. By tion) binding to (ZnP–ZnP)–ZnPc triad (3.5 mm) in DCB. b) Mole ratio plot constructed to comparing the voltammograms of pristine ZnPc, the evaluate the molecular stoichiometry (425 nm band intensity was used). c) Benesi–Hildefirst oxidation process was ascribed to the formation brand plot to calculate the binding constant. of ZnPc· + species, whereas the first reduction has been ascribed to the formation of ZnPc·. Increasing cussed below. It may be mentioned herein that addition of an addition of C60Py2 to form the tetrad revealed significant excess amount of C60Py2 beyond that is needed to form 1:1 changes. That is, the currents of the first oxidation process of complex, results in coordinating the free ZnP (or ZnPc) giving ZnPc revealed diminished currents with an anodic shift of a 1:2 complex. 32 mV, whereas the second oxidation of ZnP revealed a cathodic shift of about 20 mV suggesting participation of both ZnP and ZnPc entities in tetrad formation, as shown in Scheme 1. Computational studies Collectively, the spectral, computational, and electrochemical Figure 4 a and b show the optimized structures of (ZnP–ZnP)– results suggest involvement of both ZnPc and ZnP entities of ZnPc triad and C60Py2 at the B3LYP/6-31* level.[29] The three the (ZnP–ZnP)–ZnPc triad in supramolecular tetrad formation. macrocycles of the triad were found to be disposed in a trianThis would suggest existence of both tetrads 1 and 2 in solugular shape with tilt angles ranging between 110–1308. The tion, which is reasonable knowing that both ZnPc and ZnP macrocycle zinc–zinc distances were appropriate for C60Py2 have almost similar binding affinity for pyridine ligands. binding, more so for that involving ZnP–ZnPc binding mode. Free-energy calculations for charge-recombination (DGCR) That is, the ZnP–ZnPc distances were found to be 13.6 and and charge-separation (DGCS) processes were performed ac14.1 , whereas the ZnP–ZnP distance was 17.9 in the triad cording to the following equations based on Weller apto accommodate C60Py2, in which the NN distance between proach:[31] the two pyridine units was 10.6 . Figure 4 c and d show the structures of the tetrads involving ZnP–ZnPc and ZnP–ZnP binding to C60Py2. Both resulted in stable structures, in which DGCR ¼ ðE ox E red Þ þ DGS ð1Þ the former structure revealed slightly higher stability by 1.85 kJ mol1. The center-to-center distance between Zn and ð2Þ DGCS ¼ DE 00 ðDGCR Þ C60 for the tetrad having C60Py with connecting CH2 was about 0.5 higher than the one without CH2 fragment, in which DGCR and DGCS refer to free-energy change for charge both with a distance of 11.5 0.5 . As shown in Figure S2 in recombination and charge separation, respectively; DE00 correthe Supporting Information, the frontier HOMO and LUMO for spond to the singlet-state energy of each zinc tetrapyrrole tetrad 1 were found to be on the ZnPc and C60 entities, respec(2.04 eV for 1ZnP* and 1.84 eV for 1ZnPc*). The Eox and Ered reptively. For tetrad 2, these orbitals were on the ZnP and C60 entiresent the oxidation potential of the electron donor (ZnP and &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
4
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
Figure 4. B3LYP/6-31G* optimized structures of a) (ZnP-ZnP)-ZnPc triad, b) C60Py2, c) tetrad upon C60Py2 coordination through ZnPc-ZnP, and d) tetrad upon C60Py2 coordination through ZnP–ZnP. Structures c and d show the two possible forms of the supramolecular tetrads.
ZnPc band at 698 nm, originated from singlet–singlet excitation transfer from the 1ZnP*, revealed over 98 % quenching. This quenching could arise from the singlet–singlet energy transfer from the 1ZnPc* to C60Py2 or photoinduced electron transfer from the 1ZnPc* to C60Py2 leading to the formation of (ZnP– ZnP)–ZnPc· + :Py2C60· charge-separated state. In the absence of a spectral overlap of ZnPc fluorescence and C60Py2 absorption in the spectral region, singlet– singlet energy transfer could be ruled out as a quenching mechanism. These results are suggestive of the occurrence of sequential singlet–singlet energy transfer followed by electron transfer in the tetrads. Further femtosecond transient absorption studies were performed to secure evidence for the occurrence of energy and electron transfer events, and also to obtain kinetic information of the different photochemical events. Herein, we have employed two solvents, namely, toluene
ZnPc) and the reduction potential of the electron acceptor (C60), respectively. DGS refers to the static energy, calculated by using the “dielectric continuum model” according to the following Equation:
DGS ¼ðe2 =ð4 pe0 ÞÞ½ð1=ð2Rþ Þ þ 1=ð2R Þð1=RCC Þ=eS ð1=ð2Rþ Þ þ 1=ð2R ÞÞ=eR Þ
ð3Þ
in which R + and R are radii of the radical cation and radical anion, respectively; RCC is the center–center distances between ZnP (or ZnPc) and C60, which were evaluated from optimized structure. The values eR and eS refer to solvent dielectric constants for electrochemistry and photophysical measurements, respectively. The free-energy change (DGCS) was found to be 0.74 eV for 1ZnPc* originated, and 0.57 eV from 1ZnP* originated electron-transfer reactions in the tetrads. This indicates preference of ZnPc over ZnP as primarily electron donor in tetrad 1, whereas one of the spatially close ZnP entities acting as an electron donor in tetrad 2.
Figure 5. Differential pulse voltammograms of (ZnP–ZnP)–ZnPc triad on increasing addition of C60Py2 (0.0, 0.25, 0.75, and 1.0 equiv) in o-DCB containing 0.1 m (nBu4N)ClO4.
and dichlorobenzene, because the excited energy transfer is often efficient in nonpolar solvent, such as toluene, and electron transfer is efficient in slightly polar (o-dichlorobenzene) to polar solvents.[31] We could not employ very polar solvents, such as benzonitrile or dimethylformamide, due to the unstability of the tetrad wherein the polar solvent molecules compete with pyridine for axial binding of zinc tetrapyrroles.[10]
Photoinduced electron-transfer studies Figure 2 b shows fluorescence spectral changes of (ZnP–ZnP)– ZnPc triad on increasing addition of C60Py2, excited at l = 411 nm corresponding to one of the isosbestic points. The Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
Figure 6. Femtosecond-transient spectrum of the (ZnP)2–ZnPc triad in a) toluene and c) DCB at the indicated time intervals. Figure b and d show the time profile of the selected peaks corresponding to ZnP singlet, ZnP absorption and emission, and ZnPc emission (see text for details) in toluene and DCB.
state (time constant 1.38 ps), reflecting occurrence of ultrafast excitation transfer from 1ZnP* to ZnPc in the triad. The 1ZnPc* formed decayed with a lifetime of 1.4 ns. In DCB, considerable differences from that observed in toluene were observed (Figure 6 b). That is, the singlet deactivation of the initial 1ZnP* was much faster (time constant 0.96 ps), whereas the product of energy transfer, 1ZnPc*, formed within a time constant of about 2.0 ps. The 1ZnPc* formed decayed with a lifetime of 0.94 ns in DCB. The time constants recorded for excitation energy transfer for the present triad were similar to that reported earlier by Pereira et al.[23] and us[24] on porphyrin-phthalocyanine dyads using up conversion and femtosecond-transient measurements. In addition to the above-discussed energy transfer, additional electron transfer could also occur in the triad according to Scheme 3. Two routes could be envisioned, namely, electron transfer to the initially formed 1ZnP* from the ZnPc entity re-
Excitation of the precursor tetratolylporphyrinatozinc(II) (ZnP) by 400 nm, femtosecond laser (100 fs width) revealed instantaneous formation of singlet–singlet absorptions with maxima at 450 nm and a strong minima at 550 nm, opposite of the porphyrin ground-state absorption.[33] Additional minima at 590 and 648 nm, corresponding to singlet emission of ZnP were also observed (Figure S3 in the Supporting Information). With time, the excited singlet-state features diminished to populate the triplet state within the singlet lifetime of ZnP being 1.85 ns. Figure 6 a and c show the transient spectra of (ZnP–ZnP)– ZnPc triad at different time intervals in toluene and dichlorobenzene, respectively. In toluene, at the excitation wavelength of 400 nm, ZnP entity of the triad was largely excited. In contrast to ZnP, a short-lived porphyrin singlet–singlet peak was observed in the 450–500 nm region (time constant 1.04 ps) with the concomitant development of phthalocyanine singlet &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
6
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper triad with ZnP· and ZnPc· + entities irrespective of their origin. Free-energy calculations performed according to the earlier discussed Weller’s approach revealed reaction 2 to be slightly exothermic (DG 0.05 eV) and reaction 3 to be endothermic ((DG 0.12 eV) in DCB but not in toluene. Accordingly, spectra recorded in DCB showed weak features of electron transfer in addition to energy transfer (Figure 6 c). That is, broad band in the 600 nm region corresponding to ZnP· and 840 nm region corresponding to ZnPc· + accompanied by depletion of 900 nm band of 1ZnP* in the 900 nm region were observed. The tetrads formed by binding C60Py2 to (ZnP–ZnP)–ZnPc triad revealed occurrence of photoinduced electron transfer leading to the formation of (ZnP–ZnP)–ZnPc· + :Py2C60· and (ZnP–ZnP· + )–ZnPc:Py2C60· charge-separated states both in toluene and DCB (Figure 7); however, the spectral appearance of the former charge-separated species was much better pro-
Scheme 3. Energy- and electron-transfer (ET) events occurring in the (ZnP– ZnP)–ZnPc triad upon excitation of ZnP entity.
sulting into the formation of (ZnP–ZnP·)2–ZnPc· + charge-separated state (reaction 2 in Scheme 3), and electron transfer from the energy-transfer product (reaction 1 in Scheme 1), 1ZnPc* to one of the ZnP entities to result into the formation of (ZnP– ZnP·)2–ZnPc· + charge-separated state (reaction 3 in Scheme 3). It is important to note that both the electron-transfer routes result into the formation of same radical ion pair, that is, the
Figure 7. Femtosecond-transient spectrum of the tetrad (ZnP–ZnP)–ZnPc:Py2C60 formed by binding of C60Py2 to (ZnP–ZnP)–ZnPc triad in a) toluene and c) DCB at the indicated time intervals. The b and d show the time profile of the ZnPc· + and Py2C60· transient bands monitored at l = 850 and 1020 nm in the corresponding solvent. Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
7
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper nounced than the latter due to the appearance of transient bands corresponding to ZnPc· + at 840 nm and Py2C60· at 1020 nm, sufficiently far from bands of other transient species. As shown in Figure 7 a, in toluene upon excitation by using 400 nm femtosecond laser, the singlet-excited ZnP formed in the l = 450–500 nm region deactivated to populate the singlet-excited state of ZnPc by excitation transfer as was witnessed from the emission of 1ZnPc* at l = 682 nm. Recovery of this signal led to subsequent formation of ZnPc· + and Py2C60· as was evidenced from their corresponding transient bands at l = 850 and 1020 nm, respectively. Similar results were obtained in dichlorobenzene, as shown in Figure 7 b; however, the spectral features in the l = 850–1100 nm range were rather broad suggesting exciplex formation, similar to that reported for ZnPc-C60 and ZnP-C60 dyads in the literature.[32] Nevertheless, the present results clearly show formation of (ZnP–ZnP)– ZnPc· + :Py2C60· mainly from the intermediate energy-transfer product, 1ZnPc*. This was also supported by the time profiles of ZnPc· + and Py2C60·, shown in right-hand panels. The time constants for both radical-cation and radical-anion derived from the monoexponential decay fits were almost same for a given solvent. This suggests that the spectral features arise from tetrad 1 having ZnPc and C60 closer than in tetrad 2. Although thermodynamically less favored, electron transfer from 1 ZnP* cannot be excluded, especially in tetrad 2 due to close proximity of the donor and acceptor entities. In this event, the ZnP· + band expected in the 650 nm range seem be buried under the strong negative-emission band of 1ZnPc* at l = 682 nm. The rate of charge separation, kCS, and rate of charge recombination, kCR, for the (ZnP–ZnP)–ZnPc· + :Py2C60· radicalion pair (from the time profile of 849 nm peak) were found to be 2.20 1011 and 6.10 108 s1 in toluene, and 6.82 1011 and 1.20 109 s1 in dichlorobenzene, respectively. The different photochemical events occurring in the tetrad 1 in DCB are summarized in an energy-level diagram in Figure 8. A similar diagram could be envisioned in toluene with the energy of radical-ion pair being higher by approximately 0.17 eV than that in DCB due to lower polarity of toluene.[33] The energy levels were calculated primarily from the earlier discussed spectral and redox data. At the excitation wavelength of 400 nm, the ZnP entity of (ZnP–ZnP)– ZnPc:Py2C60 of tetrad 1 mainly gets excited to populate the singlet-excited state. The 1ZnP* thus formed can deactivate either by undergoing intersystem crossing to populate the triplet-excited state of ZnP, or electron transfer to result into the formation of (ZnP–ZnP· + )2-ZnPc:Py2C60· radical-ion pair, or energy transfer to ZnPc to produce 1ZnPc*. These processes are in competition with any radiative (fluorescence) and nonradiative decay of 1ZnP*. Both steady-state fluorescence and transient spectral studies on the (ZnP–ZnP)–ZnPc triad demonstrated occurrence of ultrafast, efficient excited-energy transfer from 1ZnP* to ZnPc as the main photochemical event. Further, the newly formed (ZnP–ZnP)–1ZnPc*:Py2C60 undergoes electron transfer to give (ZnP–ZnP)–ZnPc· + :Py2C60· radical-ion pair. This radical-ion pair persists for about 2–3 ns, depending upon the solvent polarity, prior returning to the ground state. A hole shift from ZnPc· + to ZnP within the tetrad generating (ZnP– &
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
Figure 8. Energy-level diagram showing the photochemical events occurring in the tetrad 1 (ZnP–ZnP)–ZnPc:Py2C60 in DCB. Solid arrows represent major photochemical events and dashed arrows show minor photochemical events.
ZnP· + )–ZnPc:Py2C60· is an uphill process, thus, the transient spectral revealed peaks of ZnPc· + and Py2C60· radical ions. Upon formation of (ZnP–ZnP· + )–ZnPc:Py2C60·, characteristic transient band of ZnP· + around 650 nm was expected due the presence strong ZnPc emission band in the spectral region. The kinetic data revealed faster charge separation and relatively slow rate of charge recombination, with the expected solvent-polarity trends.
Conclusion A photosynthetic antenna-reaction center mimicking supramolecular tetrad capable of undergoing sequential photoinduced energy transfer followed by electron transfer has been newly assembled and characterized. The tetrad featured two energy-harvesting zinc porphyrin entities, and a zinc phthalocyanine and a fullerene as electron-donor and -acceptor entities. The spectral, electrochemical, and computational studies suggested binding of the bis-pyridine-functionalized fullerene to the triad with a binding constant of 1.17 105 m1, whereas leaving one of the zinc macrocycles uncomplexed. Excitation of the free zinc porphyrin of the triad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine with a rate constant close to the femtosecond transient setup. Photoinduced electron transfer from the singlet-excited zinc phthalocyanine to fullerene was witnessed both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc· + in the 850 nm range and C60· in the 1020 nm range, and not that of ZnP· + in the 650 nm range were observed, clearly establishing (ZnP-ZnP)-ZnPc· + :Py2C60· radical-ion-pair formation. The rate of charge separation, kCS, and rate of charge recombination, kCR, were found to be 6.70 1011 and 8
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper 6.48 108 s1 in toluene, and 1.99 1011 and 1.98 109 s1 in dichlorobenzene, respectively. These results reveal ultrafast energy transfer followed by charge separation in the multimodular tetrad.
Synthesis of (ZnP–ZnP)2–ZnPc triad 5-(4-Hydroxyphenyl)-10,15,20-tris(tolualyl)porphyrin (1) 4-Hydroxybenzaldehyde (500 mg, 4.09 mmol), p-tolualdehyde (1.44 mL, 12.28 mmol), and pyrrole (1.14 mL, 16.36 mmol) were kept in a round-bottom (RB) flask (500 mL), and then heated at reflux with 100 mL of propionic acid for 4 h. After cooling the mixture to RT, the solvent was evaporated, and mixture was purified by using silica column. The desired compound was eluted by CHCl3/CH3OH (90:10) as a second fraction. Yield: 13 %. 1H NMR (CDCl3, 400 MHz): d = 2.70 (s, 2 H, pyrrole H), 2.75 (s, 9 H, CH3), 7.20 (d, 2 H, Ar-H), 7.55 (d, 6 H, Ar-H), 8.05 (d, 2 H, Ar-H), 8.15 (d, 6 H, Ar-H), 8.85 ppm (br, 8 H, Ar-H).
Experimental Section Spectral measurements The UV/Vis spectral measurements were carried out with a Shimadzu Model 2550 double monochromator UV/Vis spectrophotometer. The fluorescence emission was monitored by using a Varian Eclipse spectrometer. A right-angle detection method was used. The 1 H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Cyclic voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer by using a three-electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements by using argon gas. The computational calculations were performed by DFT B3LYP/6–31G* methods with Gaussian 09 software package.[29]
4,5-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin] phthalonitrile (2) 5-(4-Hydroxyphenyl)-10,15,20-tris(tolualyl)porphyrin (1; 450 mg, 0.66 mmol), 4,5-dichlorophthalonitrile (33 mg, 0.16 mmol), and K2CO3 (639 mg, 4.62 mmol) were kept in a RB (100 mL) flask under nitrogen for 20 min then heated at reflux in dry acetone (50 mL) for 48 h. After cooling to RT, the mixture was filtered, and solvent was evaporated. Obtained crude product was purified by using silica column, and the desired compound was eluted by 100 % CH2Cl2. Yield: 42 %. 1H NMR (CDCl3, 400 MHz): d = 2.82 (s, 4 H, pyrrole H), 3.22 (s, 18 H, CH3), 7.18 (d, 4 H, Ar-H), 7.52 (m, 12 H, Ar-H), 7.79 (s, 2 H, Ar-H), 7.95 (d, 4 H, Ar-H), 8.05 (m, 12 H, Ar-H), 8.80 ppm (br, 16 H, Ar-H).
Laser-flash photolysis
2,3-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrinato zinc (II)]9,16,23 tert-butyl phthalocyaninatozinc(II) (3)
Femtosecond-transient absorption-spectroscopy experiments were performed by using an ultrafast femtosecond laser source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. The fundamental output of the laser (95 %) was introduced into harmonic generator, which produced second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, whereas the rest of the output was used for generation of white-light continuum. In the present study, the second harmonic 400 nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed by using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed toluene at 298 K.
4,5-Bis[5-phenoxy-10,15,20-tris(tolualyl)porphyrin] phthalonitrile (2; 90 mg, 0.06 mmol), 4-tert-butylphthalonitrile (101 mg, 0.55 mmol), and ZnCl2 (375 mg, 2.75 mmol) were kept in a RB flask (100 mL) for 20 min under nitrogen. Then, the mixture was heated at reflux with 2-dimethylaminoethanol (4 mL) for 18 h under dark condition. After cooling the mixture to RT, the solvent was evaporated, and thus obtained green crude product was purified by using silica column. The desired compound was eluted by CHCl3/CH3OH (90:10). Yield: 18 %. 1H NMR (CDCl3, 400 MHz): d = 1.25–1.85 (m, 27 H, tBu), 3.10 (s, 18 H, CH3), 7.10 (m, 4 H, Ar-H), 7.50 (br, 12 H, ArH), 7.80 (m, 4 H, Ar-H), 8.10 (br., 14 H, Ar-H), 8.75 (br., 9 H, Ar-H), 9.20 ppm (br, 16 H, Ar-H); MS (MALDI-TOF): m/z calcd 2214.64 [M] + , found: 2296.30 [M + 2 CH3OH + H2O] + .
Acknowledgements This work was supported by the National Science Foundation (Grant No. 1110942 to F.D.). The computational work was completed by utilizing the Holland Computing Center of the University of Nebraska.
Chemicals Buckminsterfullerene, C60 (+ 99.95 %), was from SES Research (Houston, TX). All the reagents were from Aldrich Chemicals (Milwaukee, WI), whereas the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (nBu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. The synthesis of C60Py2 is given elsewhere.[34] Bis(porphyrin)–phthalocyanine triad was synthesized according to literature methods with few modifications.[26] Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
Keywords: charge separation · electron transfer · fullerenes · multimodular tetrads · porphyrins · zinc tetrapyrrole [1] a) Light-Harvesting Antennas in Photosynthesis (Eds.: B. R. Green, W. W. Parson), Kluwer, Dordrecht, 2003; b) Handbook of Photosynthesis, 2nd ed. (Ed.: M. Pessarakli), CRC Press LLC, Boca Raton, 2005; c) Photosynthetic Light Harvesting (Eds.: R. Cogdell, C. Mullineaux), Springer, Dordrecht, 2008.
9
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper [16] a) R. Chitta, K. Ohkubo, M. Tasior, N. K. Subbaiyan, M. E. Zandler, D. T. Gryko, S. Fukuzumi, F. D’Souza, J. Am. Chem. Soc. 2008, 130, 14263; b) F. D’Souza, E. Maligaspe, K. Ohkubo, M. E. Zandler, N. K. Subbaiyan, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131, 8787; c) F. D’Souza, N. K. Subbaiyan, Y. Xie, J. P. Hill, K. Ariga, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 2009, 131, 16138; d) C. A. Wijesinghe, M. E. El-Khouly, N. K. Subbaiyan, M. Supur, M. E. Zandler, K. Ohkubo, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2011, 17, 3147; e) M. E. El-Khouly, C. A. Wijesinghe, V. N. Nesterov, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2012, 18, 13844; f) C. K. C. Bikram, N. K. Subbaiyan, F. D’Souza, J. Phys. Chem. C 2012, 116, 11964; g) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2013, 19, 7221; h) C. A. Wijesinghe, M. E. El-Khouly, M. E. Zandler, S. Fukuzumi, F. D’Souza, Chem. Eur. J. 2013, 19, 9629; i) C. B. KC, G. N. Lim, M. E. Zandler, F. D’Souza, Org. Lett. 2013, 15, 4612; j) F. D’Souza, S. Gadde, S. Islam, C. A. Wijesinghe, A. L. Schumacher, M. E. Zandler, Y. Araki, O. Ito, J. Phys. Chem. A 2007, 111, 8552; k) M. E. El-Khouly, S. Fukuzumi, F. D’Souza, ChemPhysChem 2014, 15, 30. [17] The Porphyrin Handbook, Vols. 1 – 20 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, San Diego, 2000. [18] a) Phthalocyanine: Properties and Applications (Eds.: C. C. Leznoff, A. B. P. Lever), VCH, Weinheim, 1993; b) C. G. Claessens, D. Gonzlez-Rodrguez, T. Torres, Chem. Rev. 2002, 102, 835. [19] a) H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162; b) W. Kr tschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman, Nature 1990, 347, 354; c) R. E. Smalley, Angew. Chem. Int. Ed. Engl. 1997, 36, 1594; d) Fullerenes, Chemistry, Physics and Technology (Eds.: K. M. Kadish, R. S. Ruoff), Wiley-Interscience, New York, 2000. [20] Q. Xie, E. Perez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 1992, 114, 3978. [21] a) H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 2607; b) H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. P. Luo, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 6617; c) H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2002, 124, 5165; d) H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito, H. Yamada, S. Fukuzumi, Chem. Eur. J. 2004, 10, 3184; e) D. M. Guldi, H. Imahori, K. Tamaki, Y. Kashiwagi, H. Yamada, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A 2004, 108, 541. [22] a) H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki, Y. Sakata, F. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 100; b) M. Fujitsuka, O. Ito, H. Imahori, K. Yamada, H. Yamada, Y. Sakata, Chem. Lett. 1999, 721; c) F. D’Souza, R. Chitta, S. Gadde, D. M. S. Islam, A. L. Shumacher, M. E. Zandler, Y. Araki, O. Ito, J. Phys. Chem. B 2006, 110, 25240; d) A. Y. Smirnov, L. G. Mourokh, P. K. Ghosh, F. Nori, J. Phys. Chem. C 2009, 113, 21218; e) S. H. Lee, A. G. Larsen, K. Ohkubo, Z. O. Cai, J. R. Reimers, S. Fukuzumi, M. J. Crossley, Chem. Sci. 2012, 3, 257. [23] a) A. M. V. M. Pereira, A. R. M. Soares, A. Hausmann, M. G. M. S. Neves, A. C. Tome, A. M. S. Silva, J. A. S. Cavaleiro, D. M. Guldi, T. Torres, Chem. Phys. Chem. Phys. 2011, 13, 11858; b) A. M. V. M. Pereira, A. Hausmann, J. P. C. Tome, O. Trukhina, M. Urbani, M. G. P. M. S. Neves, J. A. S. Cavaleiro, D. M. Guldi, T. Torres, Chem. Eur. J. 2012, 18, 3210. [24] a) C. B. KC, K. Stranius, P. D’Souza, N. K. Subbaiyan, H. Lemmetyinen, N. V. Tkachenko, F. D’Souza, J. Phys. Chem. C 2013, 117, 763; b) C. B. KC, K. Ohkubo, P. A. Karr, S. Fukuzumi, F. D’Souza, Chem. Commun. 2013, 49, 7614. [25] a) F. Ito, Y. Ishibashi, S. R. Khan, H. Miyasaka, K. Kameyama, M. Morisue, A. Satake, K. Ogawa, Y. Kobuke, J. Phys. Chem. A 2006, 110, 12734; b) E. Maligaspe, T. Kumpulainen, H. Lemmetyinen, N. V. Tkachenko, N. K. Subbaiyan, M. E. Zandler, F. D’Souza, J. Phys. Chem. A 2010, 114, 268; c) K. Stranius, R. Jacobs, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler, F. D’Souza, J. Porphyrins Phthalocyanines 2010, 14, 948; d) R. Jacobs, K. Stranius, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler, F. D’Souza, Inorg. Chem. 2012, 51, 3656. [26] Z. Zhao, T. Nyokong, M. D. Maree, Dalton Trans. 2005, 3732. [27] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, Heidelberg, 2006. [28] H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71, 2703. [29] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
[2] a) Photochemical Conversion and Storage of Solar Energy (Ed.: J. S. Connolly), Academic Press, New York, 1981; b) Molecular Level Artificial Photosynthetic Materials (Ed. G. J. Meyer), Wiley, Hoboken, 1997. [3] a) D. Gust, T. A. Moore in The Porphyrin Handbook, Vol. 8 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, Burlington, 2000, pp. 153 – 190; b) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40; c) S. D. Straight, G. Kodis, Y. Terazono, M. Hambourger, T. A. Moore, A. L. Moore, D. Gust, Nature Nanotechnol. 2008, 3, 280; d) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009, 42, 1890; e) D. Gust, T. A. Moore, A. L. Moore, Faraday Discuss. 2012, 155, 9. [4] a) Energy Harvesting Materials (Ed.: D. L. Andrews), World Scientific, Singapore, 2005; b) S. Gnes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324; c) N. Armaroli, V. Balzani, Angew. Chem. 2007, 119, 52; Angew. Chem. Int. Ed. 2007, 46, 52; d) H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, 1809. [5] a) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729; b) P. V. Kamat, J. Phys. Chem. C 2007, 111, 2834; c) S. Fukuzumi, Eur. J. Inorg. Chem. 2008, 1351. [6] a) M.-J. Blanco, M. Consuelo Jimenez, J.-C. Chambron, V. Heitz, M. Linke, J.-P. Sauvage, Chem. Soc. Rev. 1999, 28, 293; b) X. Dang, X, J. T. Hupp, J. Photochem. Photobiol. A 2001, 143, 251; c) V. Balzani, A, Credi, M. Venturi, ChemSusChem 2008, 1, 26; d) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910; e) G. Renger, Curr. Sci. 2010, 98, 1305; f) K. Kalyanasundaram, M. Graetzel, Curr. Opin. Biotechnol. 2010, 21, 298; g) D. G. Nocera, Inorg. Chem. 2009, 48, 10001. [7] a) D. M. Guldi, A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006, 35, 471; b) J. N. Clifford, G. Accorsi, F. Cardinali, J. F. Nierengarten, N. Armaroli, C. R. Chemie. 2006, 9, 1005; c) N. Martin, L. Sanchez, M. A. Herranz, B. Illesca, D. M. Guldi, Acc. Chem. Res. 2007, 40, 1015; d) G. Bottari, G. de La Torre, D. M. Guldi, T. Torres, Chem. Rev. 2010, 110, 6768; e) D. M. Guldi, Chem. Commun. 2011, 47, 606. [8] a) D. Holten, D. F. Bocian, J. S. Lindsey, Acc. Chem. Res. 2002, 35, 57; b) J. S. Lindsey, D. F. Bocian, Acc. Chem. Res. 2011, 44, 638. [9] a) H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 2004, 14, 525; b) S. Fukuzumi, Pure Appl. Chem. 2007, 79, 981; c) S. Fukuzumi, Phys. Chem. Chem. Phys. 2008, 10, 2283; d) S. Fukuzumi, T. Kojima, J. Mater. Chem. 2008, 18, 1427; e) S. Fukuzumi, T. Honda, K. Ohkubo, T. Kojima, Dalton Trans. 2009, 3880; f) S. Fukuzumi, K. Ohkubo, J. Mater. Chem. 2012, 22, 4575. [10] a) M. E. El-Khouly, O. Ito, P. M. Smith, F. D’Souza, J. Photochem. Photobiol. C 2004, 5, 79; b) F. D’Souza, O. Ito, Coord. Chem. Rev. 2005, 249, 1410; c) R. Chitta, F. D’Souza, J. Mater. Chem. 2008, 18, 1440; d) F. D’Souza, O. Ito, Chem. Commun. 2009, 4913; e) F. D’Souza, A. S. D. Sandanayaka, O. Ito, J. Phys. Chem. Lett. 2010, 1, 2586; f) F. D’Souza, O. Ito, Chem. Soc. Rev. 2012, 41, 86; g) O. Ito, F. D’Souza, Molecules 2012, 17, 5816; h) F. D’Souza, O. Ito in Multiporphyrin Array: Fundamentals and Applications (Ed.: D. Kim), Pan Stanford Publishing, Singapore, 2012, Chapter 8, pp. 389 – 437. [11] a) D. I. Schuster, K. Li, D. M. Guldi, C. R. Chim. 2006, 9, 892; b) R. Ziessel, A. Harriman, Chem. Commun. 2011, 47, 611; c) A. C. Benniston, A. Harriman, Coord. Chem. Rev. 2008, 252, 2528. [12] a) N. K. Subbaiyan, C. A. Wijesinghe, F. D’Souza, J. Am. Chem. Soc. 2009, 131, 14646; b) H. Imahori, T. Umeyama, K. Kei, T. Yuta, Chem. Commun. 2012, 48, 4032; c) P. V. Kamat, G. C. Schatz, J. Phys. Chem. C 2009, 113, 15473; d) T. Hasobe, Phys. Chem. Chem. Phys. 2010, 12, 44. [13] a) J. L. Sessler, C. M. Lawrence, J. Jayawickramarajah, Chem. Soc. Rev. 2007, 36, 314 – 325; b) E. M. Prez, N. Martn, Chem. Soc. Rev. 2008, 37, 1512; c) D. Gonzalez-Rodriguez, E. Carbonell, D. M. Guldi, Angew. Chem. Int. Ed. 2009, 48, 8032. [14] a) M. R. Wasielewski, Chem. Rev. 1992, 92, 435; b) A. Osuka, N. Mataga, T. Okada, Pure Appl. Chem. 1997, 69, 797; c) L. Flamigni, F. Barigelletti, N. Armaroli, J. P. Collin, I. M. Dixon, J. P. Sauvage, J. A. G. Williams, Coord. Chem. Rev. 1999, 190 – 192, 671; d) F. Diederich, M. Gomez-Lopez, Chem. Rev. Soc. 1999, 28, 263. [15] a) A. Takai, M. Chkounda, A. Eggenspiller, C. P. Gros, M. Lachkar, J.-M. Barbe, S. Fukuzumi, J. Am. Chem. Soc. 2010, 132, 4477; b) J.-Y. Liu, M. E. El-Khouly, S. Fukuzumi, D. K. P. Ng, Chem. Asian J. 2011, 6, 174; c) J.-Y. Liu, M. E. El-Khouly, S. Fukuzumi, D. K. P. Ng, Chem. Eur. J. 2011, 17, 1605; d) S. Fukuzumi, K. Ohkubo, F. D’Souza, J. L. Sessler, Chem. Commun. 2012, 48, 9801; e) K. Ohkubo, Y. Kawashima, S. Fukuzumi, Chem. Commun. 2012, 48, 4314.
&
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
10
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc, Wallingford CT. 2010. [30] K. M. Kadish, E. V. Caemelbecke, G. Royal, in The Porphyrin Handbook (Eds. K. M. Kadish, K. M., Smith, R. Guilard, R.), Vol. 8, Academic Press, San Diego, 2000, Chapter 55. [31] a) D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259; b) N. Mataga, H. Miyasaka in Electron Transfer (Eds.: J. Jortner, M. Bixon), Wiley, Hoboken, 1999, Part 2, pp. 431 – 496.
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
These are not the final page numbers! ÞÞ
[32] a) H. Lemmetyinen, N. V. Tkachenko, A. Efimov, M. Niemi, J. Phys. Chem. C 2009, 113, 11475; b) M. Niemi, N. V. Tkachenko, A. Efimov, L. Alexander, K. Ohkubo, S. Fukuzumi, H. Lemmetyinen, J. Phys. Chem. A 2008, 112, 6884; c) F. D’Souza, E. Maligaspe, P. A. Karr, A. L. Schumacher, M. El Ojaimi, C. P. Gros, J.-M. Barbe, K. Ohkubo, S. Fukuzumi, Chem. Eur. J. 2008, 14, 674; d) N. V. Tkachenko, H. Lemmetyinen, J. Sonoda, K. Ohkubo, T. Sato, H. Imahori, S. Fukuzumi, J. Phys. Chem. A 2003, 107, 8834. [33] S. K. Das, B. Song, A. Mahler, V. N. Nesterov, A. K. Wilson, O. Ito, F. D’Souza, J. Phys. Chem. C. 2014, 118, 3994. [34] F. D’Souza, S. Gadde, M. E. Zandler, M. Itou, Y. Araki, O. Ito, Chem. Commun. 2004, 2276.
Received: February 27, 2014 Published online on && &&, 0000
11
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Electron Transfer
Artificial photosynthesis: Occurrence of ultrafast energy transfer (EnT) and electron transfer leading to the formation of a charge-separated (CS) state is demonstrated in a new donor–acceptor supramolecular tetrad comprising a bis(zinc porphyrin)–(zinc phthalocyanine) linked to fullerene as a photosynthetic antenna-reaction center mimic (see scheme).
C. B. KC, G. N. Lim, P. A. Karr, F. D’Souza* && – && Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)– Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies
&
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
12
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
