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Bis(subphthalocyanine)–azaBODIPY triad for ultrafast photochemical processes† Habtom B. Gobeze, Venugopal Bandi and Francis D’Souza* Multi-modular supramolecular systems capable of undergoing photoinduced energy and electron transfer are of paramount importance to design light-to-energy and light-to-fuel converting devices. Often, this has been achieved by linking two or more photo-active or redox-active entities with complementary spectral and photochemical properties. In the present study, we report a new triad made out of two entities of subphthalocyanine covalently linked to BF2-chelated azadipyrromethene ((SubPc)2–azaBODIPY). The triad was fully characterized by spectral, computational, electrochemical and photochemical techniques. The B3LYP/ 6-31G* calculations revealed a structure wherein the donor, SubPc, and the acceptor, azaBODIPY, were well separated with no steric crowding. The different redox states were established from the differential pulse voltammetry studies and the data were used to estimate free-energy change associated with charge separation. Such calculations revealed the charge separation from either the 1SubPc* or 1azaBODIPY* to be thermodynamically feasible for yielding the (SubPc)SubPc +–azaBODIPY radical ion-pair. Steady-state fluorescence studies revealed quantitative quenching of 1SubPc* in the triad and solvent dependent quenching of 1azaBODIPY* indicating participation of both fluorophores in promoting photochemical events. In nonpolar toluene, singlet–singlet energy transfer from the 1SubPc* to azaBODIPY was observed, while
Received 19th June 2014, Accepted 17th July 2014
in polar benzonitrile, evidence of energy transfer was feeble. Femtosecond laser flash photolysis studies
DOI: 10.1039/c4cp02707h
provided concrete evidence for the occurrence of ultrafast photoinduced electron transfer by providing spectral proof for the formation of the (SubPc)SubPc +–azaBODIPY
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charge separated state. The charge
recombination followed populating the 3azaBODIPY* prior to returning to the ground state.
Introduction In recent years, research on multi-modular supramolecular systems containing two or more chromophores/sensitizers has gathered attention from the scientific community due to their potential applications ranging from light energy harvesting, sensors to optoelectronic devices.1–8 Investigating their photophysical/ photochemical properties is of great importance as they are reliant on the functional sensitizer molecules. Among such sensitizers, boron(III) subphthalocyanines (SubPcs), the lowest homologs of phthalocyanines, are a class of interesting molecules because of their unique structural and photochemical properties.9 The 14p-electron conjugated SubPcs exhibit intense fluorescence and nonlinear properties owing to the bowl-shaped structure. SubPcs have recently been used as functional materials in applications such as building supramolecular architectures, organic photovoltaics, organic light emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and chemical sensors.10,11 Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-5017, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: 1H NMR of compound 1a and the triad, mass spectrum of the triad, femtosecond transient absorption spectra of SubPc and azaBODIPY in benzonitrile. See DOI: 10.1039/c4cp02707h
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Another class of compounds that have gathered much attention are BF2-chelated dipyrromethene (BODIPY) and BF2-chealted azadipyrromethene (azaBODIPY).12,13 These dyes are known for their high absorption coefficients, high fluorescence quantum yields, long excited state lifetimes and good solubility in organic solvents. Additionally, their redox and optical properties can be tuned by macrocycle peripheral modifications. Generally, azaBODIPYs absorb and emit in the red and near-infrared (NIR) regions compared to BODIPY dyes. Consequently, azaBODIPYs have been lately used in several applications including photodynamic therapy and in building red/NIR light harvesting capable donor–acceptor systems.14–17 By combining these two classes of molecules, it is possible to generate a new class of photoactive materials capable of undergoing photoinduced energy and electron transfer. This has been verified by covalently linking SubPc and BODIPY sensitizers in a few cases.18 However, such type of donor–acceptor conjugates have not been built using azaBODIPY, in spite of azaBODIPY’s red/NIR light capturing and emitting properties.14–17 This has been accomplished in the present study, wherein azaBODIPY is functionalized with two entities of SubPc (see Scheme 1). As shown here, efficient photoinduced electron transfer from the 1 azaBODIPY* within the triad has been witnessed by
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(n-Bu4N)ClO4, used in electrochemical studies was obtained from Fluka Chemicals (Ronkonkoma, NY). The synthesis of BF2-chelated azadipyrromethene, azaBODIPY, used as the control compound, and the precursor, the BF2 chelate of [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol2-ylidene]amine (1b), is given elsewhere.16a Synthesis of the (SubPc)2–azaBODIPY triad
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Scheme 2 provides the methodology employed for the synthesis of the (SubPc)2–azaBODIPY triad. The details are given below. Synthesis of chloro[2,9,16-tri(4-tert-butyl)subphthalocyaninato] boron(III), SubPc, 1a19 Scheme 1 Structure of the (SubPc)2–azaBODIPY triad synthesized and investigated in the present study.
femtosecond laser flash photolysis measurements wherein spectral proof for formation of the (SubPc)SubPc +–azaBODIPY charge separated state is secured. Interestingly, in nonpolar toluene, competing singlet–singlet energy transfer from the 1SubPc* to azaBODIPY was also observed.
Experimental section
4-tert-Butyl phthalonitrile (500 mg, 2.74 mmol) was kept in a 50 ml RB flask under nitrogen for 30 min. Then 5 ml of BCl3 (1 M solution in p-xylene) was added and refluxed for 2 hours. After cooling the reaction mixture to room temperature and flushing with nitrogen, the solution was evaporated and the crude compound was purified by column chromatography on silica gel with CH2Cl2 : ethyl acetate (4 : 1) to give a dark red colored solid compound 1a: yield 334 mg (61%); 1H NMR (400 MHz, CDCl3) d = 8.93–8.88 (d, 3H), 8.83–8.74 (t, 3H), 8.02–7.98 (d, 3H), 1.56–1.50 (s, 27H). UV/Vis lmax in benzonitrile = 571 nm and in toluene = 568 nm.
Chemicals and materials
Synthesis of the (SubPc)2–azaBODIPY triad (1)
All the reagents were obtained from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents utilized in the syntheses were from Fischer Chemicals (Plano, TX). Tetra-n-butylammonium perchlorate,
Compounds 1b (20 mg, 0.037 mmol) and 1a (60 mg, 0.1 mmol) were dissolved in 6 ml of toluene in a small RB flask and stirred under nitrogen for 15 min and then TEA was added (0.120 ml,
Scheme 2
Synthetic methodology adopted for the (SubPc)2–azaBODIPY triad.
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0.86 mmol) and refluxed for 48 hours under nitrogen. Then the reaction mixture was cooled to room temperature and flushed with nitrogen for 30 min. After diluting the reaction mixture with dichloromethane, it was washed with 1 M HCl followed by deionized water and the organic layer was dried over Na2SO4 and the solvent was evaporated. The residue was purified by column chromatography on silica gel with CH2Cl2 : ethyl acetate (4.5 : 0.5) to give a dark pink colored solid compound of the triad 1: yield 10 mg (16%); 1H NMR (400 MHz, CDCl3) d = 8.90–8.85 (d, 6H), 8.78–8.72 (t, 6H), 7.98–7.94 (d, 6H), 7.90–7.84 (d, 4H), 7.46–7.42 (d, 4H), 7.38–7.32 (m, 10H), 6.76 (s, 2H), 1.60–1.54 (s, 54H). 13C NMR (400 MHz, CDCl3) d = 131.2(m), 130.5, 129.2, 128.7, 128.4,128, 127.7, 121.8, 118.4, 35.8, 31.8, 31.2, 30.8, 29.8. Mass: C104H92B3F2N15O2 calcd: 1654.78, found 1636.9(M+ F), 1654.7 (M+), 1677.7(M+ + Na+). (See Fig. S1–S3 in ESI† for 1H NMR and mass spectral results). UV/Vis lmax in benzonitrile = 572 nm and 700 nm, and in toluene = 568 nm and 695 nm. Instrumentation Spectral measurements. The UV-visible and near-IR spectral measurements were carried out using a Shimadzu 2550 UV-Vis spectrophotometer or Jasco V-670 spectrophotometer. The steadystate fluorescence emission was monitored by using a Varian (Cary Eclipse) Fluorescence Spectrophotometer or a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with PMT (for UV-visible) and InGaAs (for near-IR) detectors. A right angle detection method was used for fluorescence measurements at room temperature. All the solutions were purged with nitrogen gas prior to spectral measurements. The 1H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. The lifetimes were measured with the time correlated single photon counting (TCSPC) lifetime option with nano-LED excitation sources on the Nanolog. The computational calculations were performed by the B3LYP/6-31G* method using the GAUSSIAN 03 software package.20 The HOMO and LUMO orbitals were generated using the GaussView program. Electrochemistry. Differential pulse voltammetry was performed on a Princeton Applied Research potentiostat/galvanostat Model 263A using a three electrode system. A platinum button electrode was used as the working electrode, while a platinum wire served as the counter electrode and an Ag/Ag+ electrode was used as the reference electrode. The ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged with nitrogen gas prior to electrochemical and spectral measurements. Femtosecond transient absorption spectral measurements. Femtosecond transient absorption spectroscopy experiments were performed using an ultrafast femtosecond laser source (Libra) by Coherent incorporating a diode-pumped, mode-locked Ti:Sapphire laser (Vitesse) and a diode-pumped intra-cavity 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 a femtosecond harmonic generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses was derived from the fundamental output of Libra (compressed output 1.45 W, pulse
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width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into the harmonic generator which produces the second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, while 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 using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed toluene at 298 K.
Results and discussion Fig. 1 shows the absorption spectrum of the (SubPc)2–azaBODIPY triad along with that of the control compounds SubPc and azaBODIPY in toluene. The absorption bands of pristine SubPc were located at 303 and 568 nm while those of pristine azaBODIPY were located at 304, 478 and 652 nm. Interestingly, in the triad, absorption peaks corresponding to SubPc revealed little or no spectral shifts, that is, the main peak was located at 568 nm; however, the peak corresponding to azaBODIPY was red-shifted by 43 nm and appeared at 695 nm. However, another control experiment performed using hydroxy functionalized azaBODIPY, viz., BF2 chelate of [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl][5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine (1b), revealed such a red-shift suggesting that SubPcs are not directly responsible for the observed spectral changes.16a Similar spectral trends were also observed in o-dichlorobenzene (DCB) and benzonitrile (PhCN) solvents. For example, in benzonitrile, the main peaks of SubPc and azaBODIPY bands were located at 571 and 656 nm, respectively, while these bands in the triad were located at 572 and 700 nm. The 4–5 nm red-shift of absorption bands in PhCN (3–4 nm in DCB) compared to that in toluene is consistent with the trend expected for general solvent effects.
Fig. 1 Normalized absorption spectrum of SubPc (black), azaBODIPY (blue) and the (SubPc)2–azaBODIPY triad (red) in toluene.
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Fig. 2 shows the fluorescence and excitation spectra of the triad and the control compounds in toluene and benzonitrile. SubPc revealed a strong emission at 584 nm with a shoulder band at 620 nm in toluene while this band was slightly redshifted and appeared at 590 nm with the shoulder band at 627 nm in PhCN. Pristine azaBODIPY also revealed a strong emission at 682 nm in toluene and at 686 nm in benzonitrile. However, for compound 1b this emission was located at 726 nm in PhCN, consistent with the earlier discussed red-shifts.16 When excited at the peak maxima corresponding to SubPc, the (SubPc)2–azaBODIPY triad in toluene revealed drastic quenching (495%) with the appearance of a new band at 726 nm corresponding to the emission of azaBODIPY (Fig. 2a). Interestingly, in PhCN, no such new peak of azaBODIPY was observed although the fluorescence intensity of SubPc was found to be quenched over 96% (Fig. 2b). These results indicate the occurrence of singlet– singlet energy transfer from the 1SubPc* to azaBODIPY to populate 1 azaBODIPY* in toluene but not in benzonitrile. In order to confirm such energy transfer, the excitation spectrum of the triad was recorded by setting the emission monochromator wavelength to correspond to the emission maxima of azaBODIPY in the triad while scanning the excitation wavelength from 300 to 750 nm. Fig. 2c shows the excitation spectrum in which peaks corresponding to both azaBODIPY and SubPc are observed, confirming the occurrence of excitation energy transfer.21
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Further, the intensity ratio of the SubPc to azaBODIPY visible bands was measured to estimate the energy transfer efficiency. For the excitation spectrum shown in Fig. 2c this ratio was found to be 0.46 which compared with an intensity ratio of 1.77 for the absorption spectrum shown in Fig. 1. This resulted in about 26% energy transfer efficiency in the triad in toluene. Further, the effect of appended SubPc units on azaBODIPY fluorescence in the triad was also investigated. As shown in Fig. 3a, the fluorescence intensity of the directly excited azaBODIPY was 25% quenched when compared to the emission intensity of pristine azaBODIPY in toluene. However, as shown in Fig. 3b, the azaBODIPY emission was quenched over 98% in benzonitrile compared to the intensity of pristine azaBODIPY. This was also true in DCB, where over 96% of quenching of azaBODIPY fluorescence was observed. In the absence of energy transfer (emission of azaBODIPY at a lower energy than that of SubPc) as a quenching mechanism, electron transfer from 1azaBODIPY* could be envisioned as a possible cause. In order to establish this possibility, computational, electrochemical and photochemical studies were performed, as discussed below. Electrochemical and computational studies Fig. 4 shows differential pulse voltammograms (DPV) of the triad along with those of the control compounds in DCB containing 0.1 M (t-Bu4N)ClO4. In agreement with the earlier
Fig. 2 Fluorescence spectra of SubPc (red) and the (SubPc)2–azaBODIPY triad (black) in (a) toluene and (b) PhCN at the excitation wavelength of SubPc intense visible band maxima. The concentrations were held at 10 mM. (c) Excitation spectrum of the (SubPc)2–azaBODIPY triad (10 mM) in toluene. The emission monochromator was set to 726 nm corresponding to azaBODIPY emission and the excitation monochromator wavelength was scanned from 300 to 750 nm.
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Fig. 4 Differential pulse voltammograms (DPVs) corresponding to both oxidation and reduction processes of the (SubPc)2–azaBODIPY triad (red), SubPc (blue) and azaBODIPY (magenta) in o-dichlorobenzene containing 0.1 M (t-Bu4N)ClO4. Scan rate = 5 mV s 1, pulse width = 0.25 s, pulse height = 0.025 V.
Fig. 3 Fluorescence spectra of azaBODIPY (blue) and the (SubPc)2– azaBODIPY triad (red) in (a) toluene and (b) PhCN at the excitation wavelength of azaBODIPY band maxima. The concentrations were held at 10 mM.
published results,16 the first one-electron oxidation of azaBODIPY was located at 0.81 V vs. Fc/Fc+ while the first two reductions were located at 0.86 and 1.62 V vs. Fc/Fc+. The first oxidation and the first reduction of SubPc were located at 0.57 and 1.55 V vs. Fc/Fc+, respectively. Interestingly, in the triad, the first azaBODIPY centered reduction was located at 0.92 and the second reduction, an overlap of the first reduction of SubPc and the second reduction of azaBODIPY, was located at 1.61 V, while the first SubPc centered oxidation was located at 0.55 V and the first oxidation of azaBODIPY appeared at 0.64 V as a shoulder to the main SubPc oxidation peak. The small cathodic and anodic shifts suggest some intramolecular interactions between the entities. It is also important to note that the currents for the SubPc based redox processes were almost twice as much as that of azaBODIPY due to the presence of two SubPc entities in the triad. Fig. 5a shows the optimized structure of the (SubPc)2– azaBODIPY triad at the B3LYP/6-31G* level. In the energy minimized structure, the two SubPc units were linked through the convex surface side of the molecule leaving the bowl shape
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of SubPc structure intact. In agreement with the earlier reported X-ray structure,16 the azaBODIPY segment was almost flat with tilted phenyl rings on the macrocycle periphery. All of the three boron atoms of the triad assumed roughly a tetrahedral geometry, and formed a V-shape with an angle of 861, and B(SubPc)– B(azaBODIPY) distances of 7.88 and 7.42 Å, respectively. The two SubPc entities were well separated with a boron-to-boron distance of 10.5 Å revealing no steric hindrance. As shown in Fig. 5b, the highest occupied molecular orbital (HOMO) was located on the azaBODIPY entity while the HOMO 1 was on one of the SubPc entities. The lowest unoccupied molecular orbital (LUMO) was on the azaBODIPY. At this computation level and by comparison with
Fig. 5 (a) B3LYP/6-31G* optimized structure of the (SubPc)2–azaBODIPY triad. (b) Frontier HOMO 1, HOMO and LUMO of the optimized structure.
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the earlier discussed electrochemical results, HOMO 1 on the SubPc could be considered as an electron donor while LUMO on azaBODIPY could be considered as an electron acceptor. The free energy change for charge separation (DGCS) from the singlet excited states of SubPc (E0–0 = 2.15 eV) and azaBODIPY (E0–0 = 1.79 eV) within the triad was calculated using spectroscopic, computational and electrochemistry data following the Rehm– Weller approach, according to eqn (1)–(3).22
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DGCR = Eox DGCS = DE00
Ered + DGS
(1)
( DGCR)
(2)
where DE00 and DGS correspond to the energy of the singlet excited state of the sensitizers and electrostatic energy, respectively. The Eox and Ered represent the oxidation potential of the electron donor (SubPc) and the reduction potential of the electron acceptor (azaBODIPY), respectively. DGS refers to the static energy, calculated by using the ‘dielectric continuum model’ according to the following equation: DGS =
(e2/(4pe0))[(1/(2R+) + 1/(2R ) + 1/(2R ))/eR)]
(1/RCC)/eS
(1/(2R+) (3)
where R+ and R are radii of the radical cation and radical anion, respectively; RCC is the center–center distance between
the donor and the acceptor, evaluated from the optimized structure. The symbols eR and eS refer to solvent dielectric constants for electrochemistry and photophysical measurements, respectively. The free-energy calculations revealed the formation of the (SubPc)SubPc +–azaBODIPY charge separated state to be exothermic from both 1SubPc* and 1azaBODIPY*. The calculated DG values were found to be 0.87 and 0.57 eV, respectively, from 1SubPc* and 1azaBODIPY* to yield the (SubPc)SubPc +– azaBODIPY charge separated state. In order to gather evidence of charge separation and evaluate kinetics of these processes, femtosecond transient absorption spectral studies were performed, as discussed below.
Femtosecond transient absorption studies First, femtosecond transient spectra of precursors, azaBODIPY and SubPc, were recorded in toluene and benzonitrile. As shown in Fig. 6a and Fig. S4a in ESI,† at the excitation wavelength of 400 nm of 100 fs laser pulses, azaBODIPY revealed instantaneous formation of a singlet excited state with peak maxima at 515 nm and minima at 660 nm, opposite of the ground state absorption. These observations are consistent with singlet state azaBODIPY formation with an energy ESinglet = 1.79 eV.16 With time, this signal recovered as shown in Fig. 6b and Fig. S4b in ESI,†
Fig. 6 Femtosecond transient absorption spectra of (a) azaBODIPY and (c) SubPc in toluene. The samples were excited with 400 nm femtosecond (pulse width = 100 fs) laser pulses. (b) Time profile of the 660 nm band of azaBODIPY.
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according to the lifetime of azaBODIPY being 1.78 ns in toluene and 1.64 ns in benzonitrile, as determined from the timecorrelated single photon counting method (TCSPC). Upon laser excitation, SubPc revealed rapid formation of transient species, which exhibited maxima in the 700–750 nm range and minima at 515 and 570 nm, tracking the ground state absorption maxima shown in Fig. 1. The lifetime of SubPc determined from the TCSPC method was found to be 3.29 ns and 2.97 ns, respectively, in toluene and benzonitrile. These attributes agreed well with the SubPc singlet excited state with an energy ESinglet = 2.15 eV. The recovery of the minima led to a new band at 477 nm corresponding to the triplet state of SubPc (ETriplet = 1.45 eV).23 Fig. 7 shows the transient absorption spectra of the (SubPc)2– azaBODIPY triad in both toluene and benzonitrile. In both the solvents, spectral features characteristic of the formation of the (SubPc)SubPc +–azaBODIPY charge separated state were observed. That is, transient bands corresponding to the generation of SubPc + in the 775 nm range10e,f and azaBODIPY in the 450 and 820 nm range16a were observed. The evidence for the singlet–singlet energy transfer from 1SubPc* in toluene was rather weak perhaps due to a less efficient process and a smaller fraction of SubPc undergoing excitation compared to azaBODIPY at the excitation wavelength of 400 nm. In the triad, the recovery of the SubPc peak at 570 nm and azaBODIPY at 700 nm was rather rapid compared to the
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pristine samples. With time, the peaks corresponding to the (SubPc)SubPc +–azaBODIPY charge separated state decayed with the appearance of a new band in the 590 nm range corresponding to the formation of triplet excited azaBODIPY. The rate constants of charge separation (kCS) and charge recombination (kCR) were evaluated by monitoring the rise and decay of the 775 and 820 nm bands. However, the time constant for the growth of the radical species was a few hundred femtoseconds close to the time-resolution of the instrument. In contrast, the time constant for decay of the transient bands corresponding to charge recombination was reasonable as shown in Fig. 7b and d. The kCR evaluated from the decay was found to be 5.9 1010 s 1 in toluene and 7.3 1010 s 1 in benzonitrile, revealing ultrafast charge separation and charge recombination. Fig. 8 summarizes the photochemical events of the (SubPc)2– azaBODIPY triad deduced from the present study. Selective excitation of SubPc leads to the formation of 1SubPc* that can undergo either energy transfer to azaBODIPY (EnT) in competition with charge separation in nonpolar toluene or charge separation leading to the formation of the (SubPc)SubPc +–azaBODIPY radical ionpair in polar benzonitrile (CS-1). Population of 3SubPc* from the initial 1SubPc* was not observed, suggesting inefficient intersystem crossing of 1SubPc* in the triad. Similarly, direct excitation of azaBODIPY or the energy transfer product of 1SubPc* (in toluene)
Fig. 7 Femtosecond transient absorption spectra of the (SubPc)2–azaBODIPY triad in (a) toluene and (c) benzonitrile. The samples were excited with 400 nm femtosecond (pulse width = 100 fs) laser pulses. (b) and (d) Time profile of the 820 nm band of azaBODIPY used to evaluate the charge recombination rates.
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Notes and references
Fig. 8 Energy level diagram showing different photochemical events in the triad. The second SubPc unit of the triad is omitted for simplicity. Solid arrow – major photochemical event, dashed arrow – minor photochemical event.
populates 1azaBODIPY* that could undergo charge separation leading to the (SubPc)SubPc +–azaBODIPY radical ion-pair (CS-2) or populate the 3azaBODIPY* via intersystem crossing. The latter process seems to be inefficient since at the earlier time scales spectral features corresponding to only the radical ion-pair were observed. The (SubPc)SubPc +–azaBODIPY radical ion-pair could undergo charge recombination to yield the ground state (SubPc)2-azaBODIPY; however, the low-lying triplet state of azaBODIPY (ETriplet B 1.0 eV) seems to play a role as spectral features of 3azaBODIPY* during the charge recombination process were observed. That is, charge recombination via populating 3 azaBODIPY* prior to returning to the ground state was observed.
Summary In summary, the present study demonstrates the synthesis and characterization of a supramolecular triad comprised of two entities of subphthalocyanine and a BF2-chelated azadipyrromethene entity. The spectral, computational, electrochemical and steady-state fluorescence studies established the role of each entity of the triad upon photoexcitation, in addition to their complementary absorption and emission properties. Importantly, formation of the (SubPc)SubPc +–azaBODIPY charge separated state from either the 1SubPc* or 1azaBODIPY* was established. In nonpolar toluene, singlet–singlet energy transfer from the 1SubPc* to azaBODIPY competed with electron transfer. Femtosecond transient absorption studies of the triad were confirmative of charge separation and the measured time constants of this process revealed ultrafast charge separation. Further, the charge recombination followed populating the triplet excited state of 3azaBODIPY* prior to returning to the ground state. Further studies along this line are in progress in our laboratory.
Acknowledgements This work was financially supported by the National Science Foundation (Grant No. 1110942 to FD).
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1 (a) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40; (b) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2009, 42, 1890. 2 M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910. 3 (a) D. M. Guldi, G. M. A. Rahman, V. Sgobba and C. Ehli, Chem. Soc. Rev., 2006, 35, 471; (b) D. I. Schuster, K. Li and D. M. Guldi, C. R. Chim., 2006, 9, 892; (c) G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 6768. 4 (a) A. Satake and Y. Kobuke, Org. Biomol. Chem., 2007, ´. Herranz and N. Martin, 5, 1679; (b) J. L. Delgado, M. A J. Mater. Chem., 2008, 18, 1417. 5 (a) J. L. Sessler, C. M. Lawrence and J. Jayawickramarajah, Chem. Soc. Rev., 2007, 36, 314; (b) S. Fukuzumi, K. Ohkubo, F. D’Souza and J. L. Sessler, Chem. Commun., 2012, 48, 9801. 6 (a) T. Umeyama and H. Imahori, Energy Environ. Sci., 2008, 1, 120; (b) T. Hasobe, Phys. Chem. Chem. Phys., 2010, 12, 44; (c) H. Imahori, T. Umeyama, K. Kei and T. Yuta, Chem. Commun., 2012, 48, 4032. 7 (a) M. E. El-Khouly, O. Ito, P. M. Smith and F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79; (b) F. D’Souza and O. Ito, Chem. Commun., 2009, 4913; (c) F. D’Souza and O. Ito, in Multiporphyrin Array: Fundamentals and Applications, ed. D. Kim, Pan Stanford Publishing, Singapore, 2012, ch. 8, pp. 389–437; (d) F. D’Souza and O. Ito, Chem. Soc. Rev., 2012, 41, 86; (e) F. D’Souza and O. Ito, Molecules, 2012, 17, 5816; ( f ) C. A. Wijesinghe, M. E. El-Khouly, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2013, 19, 9629; (g) F. D’Souza, A. S. D. Sandanayaka and O. Ito, in Organic Nanomaterials, ed. T. Torres and G. Bottari, Wiley-VCH, Weinheim, 2013, pp. 187–204. 8 (a) S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609; (b) S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283; (c) S. Fukuzumi and T. Kojima, J. Mater. Chem., 2008, 18, 1427; (d) S. Fukuzumi, T. Honda, K. Ohkubo and T. Kojima, Dalton Trans., 2009, 3880; (e) S. Fukuzumi and K. Ohkubo, J. Mater. Chem., 2012, 22, 4575. 9 (a) C. G. Claessens, D. Gonzalez-Rodrı`guez and T. Torres, Chem. Rev., 2002, 102, 835; (b) N. Kobayashi, Bull. Chem. Soc. Jpn., 2002, 75, 1; (c) J. Mack and n. Kobayashi, Chem. Rev., 2011, 111, 281; (d) M. E. El-Khouly and S. Fukuzumi, J. Porphyrins Phthalocyanines, 2011, 15, 111; (e) C. G. Claessens, D. GonzalezRodriguez, M. S. Rodriguez-Morgade, A. Medina and T. Torres, Chem. Rev., 2014, 114, 2192 and references cited therein. ´lez-Rodrı`guez, T. Torres, D. M. Guldi, J. Rivera 10 (a) D. Gonza ´lezand L. Echegoyen, Org. Lett., 2002, 4, 335; (b) D. Gonza Rodrı`guez, T. Torres, D. M. Guldi, J. Rivera, M. A. Herranz and L. Echegoyen, J. Am. Chem. Soc., 2004, 126, 6301; ´lez-Rodrı`guez, T. Torres, M. A. Herranz, (c) D. Gonza L. Echegoyen, E. Carbonell and D. M. Guldi, Chem. – Eur. ´lez-Rodrı`guez, E. Carbonell, J., 2008, 14, 7670; (d) D. Gonza D. M. Guldi and T. Torres, Angew. Chem., Int. Ed., 2009, ´lez-Rodrı`guez, E. Carbonell, G. de 48, 8032; (e) D. Gonza Miguel Rojas, C. A. Castellanos, D. M. Guldi and T. Torres, J. Am. Chem. Soc., 2010, 132, 16488; ( f ) C. Romero-Nieto,
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Published on 18 July 2014. Downloaded by Monash University on 24/10/2014 23:14:02.
11
12
13
14
15
16
J. Guilleme, J. Fernandez-Ariza, M. S. Rodrı`quez-Morgade, ´lez-Rodrı`guez, T. Torres and D. M. Guldi, Org. Lett., D. Gonza ´nchez-Molina, C. G. Claessens, 2012, 14, 5656; ( g) I. Sa B. Grimm, D. M. Guldi and T. Torres, Chem. Sci., 2013, 4, 1338. (a) K. L. Mutolo, E. I. Mayo, b. P. Rand, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2006, 128, 8108; (b) M. E. El-Khouly, S. H. Shim, Y. Araki, O. Ito and K.-Y. Kay, J. Phys. Chem. B, 2008, 112, 3910; (c) M. E. El-Khouly, D. K. Ju, K.-Y. Kay, F. Dsouza and S. Fukuzumi, Chem. – Eur. J., 2010, 16, 6193; (d) R. Pandey, A. A. Gunawan, K. A. Mkhoyan and R. J. Holmes, Adv. Funct. Mater., 2012, 22, 617; (e) M. E. El-Khouly, J.-H. Kim, J.-H. Kim, K.-Y. Kay and S. Fukuzumi, J. Phys. Chem. C, 2012, 116, 19709; ( f ) C. B. KC, G. N. Lim, M. E. Zandler and F. D’Souza, Org. Lett., 2013, 15, 4612. (a) A. Treibs and F. H. Kreuzer, Liebigs Ann. Chem., 1968, 718, 208; (b) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891. (a) A. Gorman, J. Killoran, C. O’Shea, T. Kenna, W. M. Gallagher and D. F. O’Shea, J. Am. Chem. Soc., 2004, 126, 10619; (b) M. J. Hall, S. O. McDonnell, J. Killoran and D. F. O’Shea, J. Org. Chem., 2005, 70, 5571; (c) M. Tasior and D. F. O’Shea, Bioconjugate Chem., 2010, 21, 1130. (a) S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne, W. M. Gallagher and D. F. O’Shea, J. Am. Chem. Soc., 2005, 127, 16360; (b) A. Palma, M. Tasior, D. O. Frimannsson, T. T. Vu, R. Meallet-Renault and D. F. O’Shea, Org. Lett., 2009, 11, 3638; (c) J. Murtagh, D. O. Frimannsson and D. F. O’Shea, Org. Lett., 2009, 11, 5386. (a) M. Yuan, X. Yin, H. Zheng, C. Ouyang, Z. Zuo, H. Liu and Y. Li, Chem. – Asian J., 2009, 4, 707; (b) P. A. Bouit, K. Kamada, P. Feneyrou, G. Bergine, L. Toupet, O. Maury and C. Andraud, Adv. Mater., 2009, 21, 1151; (c) S. Y. Leblebici, L. Catane, D. E. Barclay, T. Olson, T. L. Chen and B. Ma, ACS Appl. Mater. Interfaces, 2011, 3, 4469; (d) K. Flavin, K. Lawrence, J. Bartelmess, M. Tasior, C. Navio, C. Bittencourt, D. F. O’Shea, D. M. Guldi and S. Giordani, ACS Nano, 2011, 5, 1198; (e) K. Flavin, I. Kopfa, J. Murtagh, M. Grossi, D. F. O’Shea and S. Giordani, Supramol. Chem., 2012, 24, 23. (a) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, M. Supur, S. Fukuzumi and F. D’Souza, J. Phys. Chem. A, 2011, 115, 9810; (b) M. E. El-Khouly, A. N. Amin, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2012, 18, 5239; (c) A. N. Amin, E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. Commun., 2012, 48, 206; (d) F. D’Souza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler and S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 654; (e) V. Bandi, K. Ohkubo, S. Fukuzumi and F. D’Souza, Chem. Commun., 2013, 49, 2867; ( f ) V. Bandi, M. E. El-Khouly,
18728 | Phys. Chem. Chem. Phys., 2014, 16, 18720--18728
Paper
17
18
19
20
21
22 23
V. N. Nesterov, P. A. Karr, S. Fukuzumi and F. D’Souza, J. Phys. Chem. C, 2013, 117, 5638; (g) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2013, 19, 7221; (h) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D’Souza, J. Phys. Chem. C, 2014, 118, 2321; (i) M. E. El-Khouly, S. Fukuzumi and F. D’Souza, ChemPhysChem, 2014, 15, 30. (a) Y. Kataoka, Y. Shibata and H. Tamiaki, Chem. Lett., 2010, 39, 953; (b) W.-J. Shi, R. Menting, E. A. Ermilov, P. C. Lo, B. Roeder and D. K. P. Ng, Chem. Commun., 2013, 49, 5277; (c) W.-J. Shi, M. E. El-Khouly, K. Ohkubo, S. Fukuzumi and D. K. P. Ng, Chem. – Eur. J., 2013, 19, 11332. (a) J.-Y. Liu, H.-S. Yeung, W. Xu, X. Li and D. K. P. Ng, Org. Lett., 2008, 10, 5421; (b) S. Cetindere, B. Cosut, S. Yesilot, M. Durmus and A. Kilic, Dyes Pigm., 2014, 101, 234. (a) A. Meller and A. Ossko, Monatsh. Chem., 1972, 103, 150; ´lez-Rodrı`guez, B. del Rey, (b) C. G. Claessens, D. Gonza T. Torres, G. Mark, H.-P. Schuchmann, C. von Sonntag, J. G. MacDonald and R. S. Nohr, Eur. J. Org. Chem., 2003, 2547. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 03, Gaussian, Inc., Pittsburgh PA, 2003. (a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, Singapore, 3rd edn, 2006; (b) E. Maligaspe, T. Kumpulainen, H. Lemmetyinen, N. V. Tkachenko, N. K. Subbaiyan, M. E. Zandler and F. D’Souza, J. Phys. Chem. A, 2010, 114, 268; (c) E. Maligaspe, T. Kumpulainen, N. K. Subbaiyan, M. E. Zandler, H. Lemmetyinen, N. V. Tkachenko and F. D’Souza, Phys. Chem. Chem. Phys., 2010, 12, 7434; (d) R. Jacobs, K. Stranius, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler and F. D’Souza, Inorg. Chem., 2012, 51, 3656. D. Rehm and A. Weller, Isr. J. Chem., 1970, 7, 259. A. Medina, C. G. Claessens, G. M. A. Rahman, A. M. Lamsabhi, O. Mo, M. Yanez, D. M. Guldi and T. Torres, Chem. Commun., 2008, 1759.
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Bis(subphthalocyanine)–azaBODIPY triad for ultrafast photochemical processes† Habtom B. Gobeze, Venugopal Bandi and Francis D’Souza* Multi-modular supramolecular systems capable of undergoing photoinduced energy and electron transfer are of paramount importance to design light-to-energy and light-to-fuel converting devices. Often, this has been achieved by linking two or more photo-active or redox-active entities with complementary spectral and photochemical properties. In the present study, we report a new triad made out of two entities of subphthalocyanine covalently linked to BF2-chelated azadipyrromethene ((SubPc)2–azaBODIPY). The triad was fully characterized by spectral, computational, electrochemical and photochemical techniques. The B3LYP/ 6-31G* calculations revealed a structure wherein the donor, SubPc, and the acceptor, azaBODIPY, were well separated with no steric crowding. The different redox states were established from the differential pulse voltammetry studies and the data were used to estimate free-energy change associated with charge separation. Such calculations revealed the charge separation from either the 1SubPc* or 1azaBODIPY* to be thermodynamically feasible for yielding the (SubPc)SubPc +–azaBODIPY radical ion-pair. Steady-state fluorescence studies revealed quantitative quenching of 1SubPc* in the triad and solvent dependent quenching of 1azaBODIPY* indicating participation of both fluorophores in promoting photochemical events. In nonpolar toluene, singlet–singlet energy transfer from the 1SubPc* to azaBODIPY was observed, while
Received 19th June 2014, Accepted 17th July 2014
in polar benzonitrile, evidence of energy transfer was feeble. Femtosecond laser flash photolysis studies
DOI: 10.1039/c4cp02707h
provided concrete evidence for the occurrence of ultrafast photoinduced electron transfer by providing spectral proof for the formation of the (SubPc)SubPc +–azaBODIPY
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charge separated state. The charge
recombination followed populating the 3azaBODIPY* prior to returning to the ground state.
Introduction In recent years, research on multi-modular supramolecular systems containing two or more chromophores/sensitizers has gathered attention from the scientific community due to their potential applications ranging from light energy harvesting, sensors to optoelectronic devices.1–8 Investigating their photophysical/ photochemical properties is of great importance as they are reliant on the functional sensitizer molecules. Among such sensitizers, boron(III) subphthalocyanines (SubPcs), the lowest homologs of phthalocyanines, are a class of interesting molecules because of their unique structural and photochemical properties.9 The 14p-electron conjugated SubPcs exhibit intense fluorescence and nonlinear properties owing to the bowl-shaped structure. SubPcs have recently been used as functional materials in applications such as building supramolecular architectures, organic photovoltaics, organic light emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and chemical sensors.10,11 Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-5017, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: 1H NMR of compound 1a and the triad, mass spectrum of the triad, femtosecond transient absorption spectra of SubPc and azaBODIPY in benzonitrile. See DOI: 10.1039/c4cp02707h
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Another class of compounds that have gathered much attention are BF2-chelated dipyrromethene (BODIPY) and BF2-chealted azadipyrromethene (azaBODIPY).12,13 These dyes are known for their high absorption coefficients, high fluorescence quantum yields, long excited state lifetimes and good solubility in organic solvents. Additionally, their redox and optical properties can be tuned by macrocycle peripheral modifications. Generally, azaBODIPYs absorb and emit in the red and near-infrared (NIR) regions compared to BODIPY dyes. Consequently, azaBODIPYs have been lately used in several applications including photodynamic therapy and in building red/NIR light harvesting capable donor–acceptor systems.14–17 By combining these two classes of molecules, it is possible to generate a new class of photoactive materials capable of undergoing photoinduced energy and electron transfer. This has been verified by covalently linking SubPc and BODIPY sensitizers in a few cases.18 However, such type of donor–acceptor conjugates have not been built using azaBODIPY, in spite of azaBODIPY’s red/NIR light capturing and emitting properties.14–17 This has been accomplished in the present study, wherein azaBODIPY is functionalized with two entities of SubPc (see Scheme 1). As shown here, efficient photoinduced electron transfer from the 1 azaBODIPY* within the triad has been witnessed by
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PCCP
(n-Bu4N)ClO4, used in electrochemical studies was obtained from Fluka Chemicals (Ronkonkoma, NY). The synthesis of BF2-chelated azadipyrromethene, azaBODIPY, used as the control compound, and the precursor, the BF2 chelate of [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol2-ylidene]amine (1b), is given elsewhere.16a Synthesis of the (SubPc)2–azaBODIPY triad
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Scheme 2 provides the methodology employed for the synthesis of the (SubPc)2–azaBODIPY triad. The details are given below. Synthesis of chloro[2,9,16-tri(4-tert-butyl)subphthalocyaninato] boron(III), SubPc, 1a19 Scheme 1 Structure of the (SubPc)2–azaBODIPY triad synthesized and investigated in the present study.
femtosecond laser flash photolysis measurements wherein spectral proof for formation of the (SubPc)SubPc +–azaBODIPY charge separated state is secured. Interestingly, in nonpolar toluene, competing singlet–singlet energy transfer from the 1SubPc* to azaBODIPY was also observed.
Experimental section
4-tert-Butyl phthalonitrile (500 mg, 2.74 mmol) was kept in a 50 ml RB flask under nitrogen for 30 min. Then 5 ml of BCl3 (1 M solution in p-xylene) was added and refluxed for 2 hours. After cooling the reaction mixture to room temperature and flushing with nitrogen, the solution was evaporated and the crude compound was purified by column chromatography on silica gel with CH2Cl2 : ethyl acetate (4 : 1) to give a dark red colored solid compound 1a: yield 334 mg (61%); 1H NMR (400 MHz, CDCl3) d = 8.93–8.88 (d, 3H), 8.83–8.74 (t, 3H), 8.02–7.98 (d, 3H), 1.56–1.50 (s, 27H). UV/Vis lmax in benzonitrile = 571 nm and in toluene = 568 nm.
Chemicals and materials
Synthesis of the (SubPc)2–azaBODIPY triad (1)
All the reagents were obtained from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents utilized in the syntheses were from Fischer Chemicals (Plano, TX). Tetra-n-butylammonium perchlorate,
Compounds 1b (20 mg, 0.037 mmol) and 1a (60 mg, 0.1 mmol) were dissolved in 6 ml of toluene in a small RB flask and stirred under nitrogen for 15 min and then TEA was added (0.120 ml,
Scheme 2
Synthetic methodology adopted for the (SubPc)2–azaBODIPY triad.
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PCCP
0.86 mmol) and refluxed for 48 hours under nitrogen. Then the reaction mixture was cooled to room temperature and flushed with nitrogen for 30 min. After diluting the reaction mixture with dichloromethane, it was washed with 1 M HCl followed by deionized water and the organic layer was dried over Na2SO4 and the solvent was evaporated. The residue was purified by column chromatography on silica gel with CH2Cl2 : ethyl acetate (4.5 : 0.5) to give a dark pink colored solid compound of the triad 1: yield 10 mg (16%); 1H NMR (400 MHz, CDCl3) d = 8.90–8.85 (d, 6H), 8.78–8.72 (t, 6H), 7.98–7.94 (d, 6H), 7.90–7.84 (d, 4H), 7.46–7.42 (d, 4H), 7.38–7.32 (m, 10H), 6.76 (s, 2H), 1.60–1.54 (s, 54H). 13C NMR (400 MHz, CDCl3) d = 131.2(m), 130.5, 129.2, 128.7, 128.4,128, 127.7, 121.8, 118.4, 35.8, 31.8, 31.2, 30.8, 29.8. Mass: C104H92B3F2N15O2 calcd: 1654.78, found 1636.9(M+ F), 1654.7 (M+), 1677.7(M+ + Na+). (See Fig. S1–S3 in ESI† for 1H NMR and mass spectral results). UV/Vis lmax in benzonitrile = 572 nm and 700 nm, and in toluene = 568 nm and 695 nm. Instrumentation Spectral measurements. The UV-visible and near-IR spectral measurements were carried out using a Shimadzu 2550 UV-Vis spectrophotometer or Jasco V-670 spectrophotometer. The steadystate fluorescence emission was monitored by using a Varian (Cary Eclipse) Fluorescence Spectrophotometer or a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with PMT (for UV-visible) and InGaAs (for near-IR) detectors. A right angle detection method was used for fluorescence measurements at room temperature. All the solutions were purged with nitrogen gas prior to spectral measurements. The 1H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. The lifetimes were measured with the time correlated single photon counting (TCSPC) lifetime option with nano-LED excitation sources on the Nanolog. The computational calculations were performed by the B3LYP/6-31G* method using the GAUSSIAN 03 software package.20 The HOMO and LUMO orbitals were generated using the GaussView program. Electrochemistry. Differential pulse voltammetry was performed on a Princeton Applied Research potentiostat/galvanostat Model 263A using a three electrode system. A platinum button electrode was used as the working electrode, while a platinum wire served as the counter electrode and an Ag/Ag+ electrode was used as the reference electrode. The ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged with nitrogen gas prior to electrochemical and spectral measurements. Femtosecond transient absorption spectral measurements. Femtosecond transient absorption spectroscopy experiments were performed using an ultrafast femtosecond laser source (Libra) by Coherent incorporating a diode-pumped, mode-locked Ti:Sapphire laser (Vitesse) and a diode-pumped intra-cavity 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 a femtosecond harmonic generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses was derived from the fundamental output of Libra (compressed output 1.45 W, pulse
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width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into the harmonic generator which produces the second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, while 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 using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed toluene at 298 K.
Results and discussion Fig. 1 shows the absorption spectrum of the (SubPc)2–azaBODIPY triad along with that of the control compounds SubPc and azaBODIPY in toluene. The absorption bands of pristine SubPc were located at 303 and 568 nm while those of pristine azaBODIPY were located at 304, 478 and 652 nm. Interestingly, in the triad, absorption peaks corresponding to SubPc revealed little or no spectral shifts, that is, the main peak was located at 568 nm; however, the peak corresponding to azaBODIPY was red-shifted by 43 nm and appeared at 695 nm. However, another control experiment performed using hydroxy functionalized azaBODIPY, viz., BF2 chelate of [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl][5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine (1b), revealed such a red-shift suggesting that SubPcs are not directly responsible for the observed spectral changes.16a Similar spectral trends were also observed in o-dichlorobenzene (DCB) and benzonitrile (PhCN) solvents. For example, in benzonitrile, the main peaks of SubPc and azaBODIPY bands were located at 571 and 656 nm, respectively, while these bands in the triad were located at 572 and 700 nm. The 4–5 nm red-shift of absorption bands in PhCN (3–4 nm in DCB) compared to that in toluene is consistent with the trend expected for general solvent effects.
Fig. 1 Normalized absorption spectrum of SubPc (black), azaBODIPY (blue) and the (SubPc)2–azaBODIPY triad (red) in toluene.
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Fig. 2 shows the fluorescence and excitation spectra of the triad and the control compounds in toluene and benzonitrile. SubPc revealed a strong emission at 584 nm with a shoulder band at 620 nm in toluene while this band was slightly redshifted and appeared at 590 nm with the shoulder band at 627 nm in PhCN. Pristine azaBODIPY also revealed a strong emission at 682 nm in toluene and at 686 nm in benzonitrile. However, for compound 1b this emission was located at 726 nm in PhCN, consistent with the earlier discussed red-shifts.16 When excited at the peak maxima corresponding to SubPc, the (SubPc)2–azaBODIPY triad in toluene revealed drastic quenching (495%) with the appearance of a new band at 726 nm corresponding to the emission of azaBODIPY (Fig. 2a). Interestingly, in PhCN, no such new peak of azaBODIPY was observed although the fluorescence intensity of SubPc was found to be quenched over 96% (Fig. 2b). These results indicate the occurrence of singlet– singlet energy transfer from the 1SubPc* to azaBODIPY to populate 1 azaBODIPY* in toluene but not in benzonitrile. In order to confirm such energy transfer, the excitation spectrum of the triad was recorded by setting the emission monochromator wavelength to correspond to the emission maxima of azaBODIPY in the triad while scanning the excitation wavelength from 300 to 750 nm. Fig. 2c shows the excitation spectrum in which peaks corresponding to both azaBODIPY and SubPc are observed, confirming the occurrence of excitation energy transfer.21
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Further, the intensity ratio of the SubPc to azaBODIPY visible bands was measured to estimate the energy transfer efficiency. For the excitation spectrum shown in Fig. 2c this ratio was found to be 0.46 which compared with an intensity ratio of 1.77 for the absorption spectrum shown in Fig. 1. This resulted in about 26% energy transfer efficiency in the triad in toluene. Further, the effect of appended SubPc units on azaBODIPY fluorescence in the triad was also investigated. As shown in Fig. 3a, the fluorescence intensity of the directly excited azaBODIPY was 25% quenched when compared to the emission intensity of pristine azaBODIPY in toluene. However, as shown in Fig. 3b, the azaBODIPY emission was quenched over 98% in benzonitrile compared to the intensity of pristine azaBODIPY. This was also true in DCB, where over 96% of quenching of azaBODIPY fluorescence was observed. In the absence of energy transfer (emission of azaBODIPY at a lower energy than that of SubPc) as a quenching mechanism, electron transfer from 1azaBODIPY* could be envisioned as a possible cause. In order to establish this possibility, computational, electrochemical and photochemical studies were performed, as discussed below. Electrochemical and computational studies Fig. 4 shows differential pulse voltammograms (DPV) of the triad along with those of the control compounds in DCB containing 0.1 M (t-Bu4N)ClO4. In agreement with the earlier
Fig. 2 Fluorescence spectra of SubPc (red) and the (SubPc)2–azaBODIPY triad (black) in (a) toluene and (b) PhCN at the excitation wavelength of SubPc intense visible band maxima. The concentrations were held at 10 mM. (c) Excitation spectrum of the (SubPc)2–azaBODIPY triad (10 mM) in toluene. The emission monochromator was set to 726 nm corresponding to azaBODIPY emission and the excitation monochromator wavelength was scanned from 300 to 750 nm.
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Fig. 4 Differential pulse voltammograms (DPVs) corresponding to both oxidation and reduction processes of the (SubPc)2–azaBODIPY triad (red), SubPc (blue) and azaBODIPY (magenta) in o-dichlorobenzene containing 0.1 M (t-Bu4N)ClO4. Scan rate = 5 mV s 1, pulse width = 0.25 s, pulse height = 0.025 V.
Fig. 3 Fluorescence spectra of azaBODIPY (blue) and the (SubPc)2– azaBODIPY triad (red) in (a) toluene and (b) PhCN at the excitation wavelength of azaBODIPY band maxima. The concentrations were held at 10 mM.
published results,16 the first one-electron oxidation of azaBODIPY was located at 0.81 V vs. Fc/Fc+ while the first two reductions were located at 0.86 and 1.62 V vs. Fc/Fc+. The first oxidation and the first reduction of SubPc were located at 0.57 and 1.55 V vs. Fc/Fc+, respectively. Interestingly, in the triad, the first azaBODIPY centered reduction was located at 0.92 and the second reduction, an overlap of the first reduction of SubPc and the second reduction of azaBODIPY, was located at 1.61 V, while the first SubPc centered oxidation was located at 0.55 V and the first oxidation of azaBODIPY appeared at 0.64 V as a shoulder to the main SubPc oxidation peak. The small cathodic and anodic shifts suggest some intramolecular interactions between the entities. It is also important to note that the currents for the SubPc based redox processes were almost twice as much as that of azaBODIPY due to the presence of two SubPc entities in the triad. Fig. 5a shows the optimized structure of the (SubPc)2– azaBODIPY triad at the B3LYP/6-31G* level. In the energy minimized structure, the two SubPc units were linked through the convex surface side of the molecule leaving the bowl shape
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of SubPc structure intact. In agreement with the earlier reported X-ray structure,16 the azaBODIPY segment was almost flat with tilted phenyl rings on the macrocycle periphery. All of the three boron atoms of the triad assumed roughly a tetrahedral geometry, and formed a V-shape with an angle of 861, and B(SubPc)– B(azaBODIPY) distances of 7.88 and 7.42 Å, respectively. The two SubPc entities were well separated with a boron-to-boron distance of 10.5 Å revealing no steric hindrance. As shown in Fig. 5b, the highest occupied molecular orbital (HOMO) was located on the azaBODIPY entity while the HOMO 1 was on one of the SubPc entities. The lowest unoccupied molecular orbital (LUMO) was on the azaBODIPY. At this computation level and by comparison with
Fig. 5 (a) B3LYP/6-31G* optimized structure of the (SubPc)2–azaBODIPY triad. (b) Frontier HOMO 1, HOMO and LUMO of the optimized structure.
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the earlier discussed electrochemical results, HOMO 1 on the SubPc could be considered as an electron donor while LUMO on azaBODIPY could be considered as an electron acceptor. The free energy change for charge separation (DGCS) from the singlet excited states of SubPc (E0–0 = 2.15 eV) and azaBODIPY (E0–0 = 1.79 eV) within the triad was calculated using spectroscopic, computational and electrochemistry data following the Rehm– Weller approach, according to eqn (1)–(3).22
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DGCR = Eox DGCS = DE00
Ered + DGS
(1)
( DGCR)
(2)
where DE00 and DGS correspond to the energy of the singlet excited state of the sensitizers and electrostatic energy, respectively. The Eox and Ered represent the oxidation potential of the electron donor (SubPc) and the reduction potential of the electron acceptor (azaBODIPY), respectively. DGS refers to the static energy, calculated by using the ‘dielectric continuum model’ according to the following equation: DGS =
(e2/(4pe0))[(1/(2R+) + 1/(2R ) + 1/(2R ))/eR)]
(1/RCC)/eS
(1/(2R+) (3)
where R+ and R are radii of the radical cation and radical anion, respectively; RCC is the center–center distance between
the donor and the acceptor, evaluated from the optimized structure. The symbols eR and eS refer to solvent dielectric constants for electrochemistry and photophysical measurements, respectively. The free-energy calculations revealed the formation of the (SubPc)SubPc +–azaBODIPY charge separated state to be exothermic from both 1SubPc* and 1azaBODIPY*. The calculated DG values were found to be 0.87 and 0.57 eV, respectively, from 1SubPc* and 1azaBODIPY* to yield the (SubPc)SubPc +– azaBODIPY charge separated state. In order to gather evidence of charge separation and evaluate kinetics of these processes, femtosecond transient absorption spectral studies were performed, as discussed below.
Femtosecond transient absorption studies First, femtosecond transient spectra of precursors, azaBODIPY and SubPc, were recorded in toluene and benzonitrile. As shown in Fig. 6a and Fig. S4a in ESI,† at the excitation wavelength of 400 nm of 100 fs laser pulses, azaBODIPY revealed instantaneous formation of a singlet excited state with peak maxima at 515 nm and minima at 660 nm, opposite of the ground state absorption. These observations are consistent with singlet state azaBODIPY formation with an energy ESinglet = 1.79 eV.16 With time, this signal recovered as shown in Fig. 6b and Fig. S4b in ESI,†
Fig. 6 Femtosecond transient absorption spectra of (a) azaBODIPY and (c) SubPc in toluene. The samples were excited with 400 nm femtosecond (pulse width = 100 fs) laser pulses. (b) Time profile of the 660 nm band of azaBODIPY.
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according to the lifetime of azaBODIPY being 1.78 ns in toluene and 1.64 ns in benzonitrile, as determined from the timecorrelated single photon counting method (TCSPC). Upon laser excitation, SubPc revealed rapid formation of transient species, which exhibited maxima in the 700–750 nm range and minima at 515 and 570 nm, tracking the ground state absorption maxima shown in Fig. 1. The lifetime of SubPc determined from the TCSPC method was found to be 3.29 ns and 2.97 ns, respectively, in toluene and benzonitrile. These attributes agreed well with the SubPc singlet excited state with an energy ESinglet = 2.15 eV. The recovery of the minima led to a new band at 477 nm corresponding to the triplet state of SubPc (ETriplet = 1.45 eV).23 Fig. 7 shows the transient absorption spectra of the (SubPc)2– azaBODIPY triad in both toluene and benzonitrile. In both the solvents, spectral features characteristic of the formation of the (SubPc)SubPc +–azaBODIPY charge separated state were observed. That is, transient bands corresponding to the generation of SubPc + in the 775 nm range10e,f and azaBODIPY in the 450 and 820 nm range16a were observed. The evidence for the singlet–singlet energy transfer from 1SubPc* in toluene was rather weak perhaps due to a less efficient process and a smaller fraction of SubPc undergoing excitation compared to azaBODIPY at the excitation wavelength of 400 nm. In the triad, the recovery of the SubPc peak at 570 nm and azaBODIPY at 700 nm was rather rapid compared to the
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pristine samples. With time, the peaks corresponding to the (SubPc)SubPc +–azaBODIPY charge separated state decayed with the appearance of a new band in the 590 nm range corresponding to the formation of triplet excited azaBODIPY. The rate constants of charge separation (kCS) and charge recombination (kCR) were evaluated by monitoring the rise and decay of the 775 and 820 nm bands. However, the time constant for the growth of the radical species was a few hundred femtoseconds close to the time-resolution of the instrument. In contrast, the time constant for decay of the transient bands corresponding to charge recombination was reasonable as shown in Fig. 7b and d. The kCR evaluated from the decay was found to be 5.9 1010 s 1 in toluene and 7.3 1010 s 1 in benzonitrile, revealing ultrafast charge separation and charge recombination. Fig. 8 summarizes the photochemical events of the (SubPc)2– azaBODIPY triad deduced from the present study. Selective excitation of SubPc leads to the formation of 1SubPc* that can undergo either energy transfer to azaBODIPY (EnT) in competition with charge separation in nonpolar toluene or charge separation leading to the formation of the (SubPc)SubPc +–azaBODIPY radical ionpair in polar benzonitrile (CS-1). Population of 3SubPc* from the initial 1SubPc* was not observed, suggesting inefficient intersystem crossing of 1SubPc* in the triad. Similarly, direct excitation of azaBODIPY or the energy transfer product of 1SubPc* (in toluene)
Fig. 7 Femtosecond transient absorption spectra of the (SubPc)2–azaBODIPY triad in (a) toluene and (c) benzonitrile. The samples were excited with 400 nm femtosecond (pulse width = 100 fs) laser pulses. (b) and (d) Time profile of the 820 nm band of azaBODIPY used to evaluate the charge recombination rates.
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Notes and references
Fig. 8 Energy level diagram showing different photochemical events in the triad. The second SubPc unit of the triad is omitted for simplicity. Solid arrow – major photochemical event, dashed arrow – minor photochemical event.
populates 1azaBODIPY* that could undergo charge separation leading to the (SubPc)SubPc +–azaBODIPY radical ion-pair (CS-2) or populate the 3azaBODIPY* via intersystem crossing. The latter process seems to be inefficient since at the earlier time scales spectral features corresponding to only the radical ion-pair were observed. The (SubPc)SubPc +–azaBODIPY radical ion-pair could undergo charge recombination to yield the ground state (SubPc)2-azaBODIPY; however, the low-lying triplet state of azaBODIPY (ETriplet B 1.0 eV) seems to play a role as spectral features of 3azaBODIPY* during the charge recombination process were observed. That is, charge recombination via populating 3 azaBODIPY* prior to returning to the ground state was observed.
Summary In summary, the present study demonstrates the synthesis and characterization of a supramolecular triad comprised of two entities of subphthalocyanine and a BF2-chelated azadipyrromethene entity. The spectral, computational, electrochemical and steady-state fluorescence studies established the role of each entity of the triad upon photoexcitation, in addition to their complementary absorption and emission properties. Importantly, formation of the (SubPc)SubPc +–azaBODIPY charge separated state from either the 1SubPc* or 1azaBODIPY* was established. In nonpolar toluene, singlet–singlet energy transfer from the 1SubPc* to azaBODIPY competed with electron transfer. Femtosecond transient absorption studies of the triad were confirmative of charge separation and the measured time constants of this process revealed ultrafast charge separation. Further, the charge recombination followed populating the triplet excited state of 3azaBODIPY* prior to returning to the ground state. Further studies along this line are in progress in our laboratory.
Acknowledgements This work was financially supported by the National Science Foundation (Grant No. 1110942 to FD).
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1 (a) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40; (b) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2009, 42, 1890. 2 M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910. 3 (a) D. M. Guldi, G. M. A. Rahman, V. Sgobba and C. Ehli, Chem. Soc. Rev., 2006, 35, 471; (b) D. I. Schuster, K. Li and D. M. Guldi, C. R. Chim., 2006, 9, 892; (c) G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 6768. 4 (a) A. Satake and Y. Kobuke, Org. Biomol. Chem., 2007, ´. Herranz and N. Martin, 5, 1679; (b) J. L. Delgado, M. A J. Mater. Chem., 2008, 18, 1417. 5 (a) J. L. Sessler, C. M. Lawrence and J. Jayawickramarajah, Chem. Soc. Rev., 2007, 36, 314; (b) S. Fukuzumi, K. Ohkubo, F. D’Souza and J. L. Sessler, Chem. Commun., 2012, 48, 9801. 6 (a) T. Umeyama and H. Imahori, Energy Environ. Sci., 2008, 1, 120; (b) T. Hasobe, Phys. Chem. Chem. Phys., 2010, 12, 44; (c) H. Imahori, T. Umeyama, K. Kei and T. Yuta, Chem. Commun., 2012, 48, 4032. 7 (a) M. E. El-Khouly, O. Ito, P. M. Smith and F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79; (b) F. D’Souza and O. Ito, Chem. Commun., 2009, 4913; (c) F. D’Souza and O. Ito, in Multiporphyrin Array: Fundamentals and Applications, ed. D. Kim, Pan Stanford Publishing, Singapore, 2012, ch. 8, pp. 389–437; (d) F. D’Souza and O. Ito, Chem. Soc. Rev., 2012, 41, 86; (e) F. D’Souza and O. Ito, Molecules, 2012, 17, 5816; ( f ) C. A. Wijesinghe, M. E. El-Khouly, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2013, 19, 9629; (g) F. D’Souza, A. S. D. Sandanayaka and O. Ito, in Organic Nanomaterials, ed. T. Torres and G. Bottari, Wiley-VCH, Weinheim, 2013, pp. 187–204. 8 (a) S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609; (b) S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283; (c) S. Fukuzumi and T. Kojima, J. Mater. Chem., 2008, 18, 1427; (d) S. Fukuzumi, T. Honda, K. Ohkubo and T. Kojima, Dalton Trans., 2009, 3880; (e) S. Fukuzumi and K. Ohkubo, J. Mater. Chem., 2012, 22, 4575. 9 (a) C. G. Claessens, D. Gonzalez-Rodrı`guez and T. Torres, Chem. Rev., 2002, 102, 835; (b) N. Kobayashi, Bull. Chem. Soc. Jpn., 2002, 75, 1; (c) J. Mack and n. Kobayashi, Chem. Rev., 2011, 111, 281; (d) M. E. El-Khouly and S. Fukuzumi, J. Porphyrins Phthalocyanines, 2011, 15, 111; (e) C. G. Claessens, D. GonzalezRodriguez, M. S. Rodriguez-Morgade, A. Medina and T. Torres, Chem. Rev., 2014, 114, 2192 and references cited therein. ´lez-Rodrı`guez, T. Torres, D. M. Guldi, J. Rivera 10 (a) D. Gonza ´lezand L. Echegoyen, Org. Lett., 2002, 4, 335; (b) D. Gonza Rodrı`guez, T. Torres, D. M. Guldi, J. Rivera, M. A. Herranz and L. Echegoyen, J. Am. Chem. Soc., 2004, 126, 6301; ´lez-Rodrı`guez, T. Torres, M. A. Herranz, (c) D. Gonza L. Echegoyen, E. Carbonell and D. M. Guldi, Chem. – Eur. ´lez-Rodrı`guez, E. Carbonell, J., 2008, 14, 7670; (d) D. Gonza D. M. Guldi and T. Torres, Angew. Chem., Int. Ed., 2009, ´lez-Rodrı`guez, E. Carbonell, G. de 48, 8032; (e) D. Gonza Miguel Rojas, C. A. Castellanos, D. M. Guldi and T. Torres, J. Am. Chem. Soc., 2010, 132, 16488; ( f ) C. Romero-Nieto,
Phys. Chem. Chem. Phys., 2014, 16, 18720--18728 | 18727
View Article Online
PCCP
Published on 18 July 2014. Downloaded by Monash University on 24/10/2014 23:14:02.
11
12
13
14
15
16
J. Guilleme, J. Fernandez-Ariza, M. S. Rodrı`quez-Morgade, ´lez-Rodrı`guez, T. Torres and D. M. Guldi, Org. Lett., D. Gonza ´nchez-Molina, C. G. Claessens, 2012, 14, 5656; ( g) I. Sa B. Grimm, D. M. Guldi and T. Torres, Chem. Sci., 2013, 4, 1338. (a) K. L. Mutolo, E. I. Mayo, b. P. Rand, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2006, 128, 8108; (b) M. E. El-Khouly, S. H. Shim, Y. Araki, O. Ito and K.-Y. Kay, J. Phys. Chem. B, 2008, 112, 3910; (c) M. E. El-Khouly, D. K. Ju, K.-Y. Kay, F. Dsouza and S. Fukuzumi, Chem. – Eur. J., 2010, 16, 6193; (d) R. Pandey, A. A. Gunawan, K. A. Mkhoyan and R. J. Holmes, Adv. Funct. Mater., 2012, 22, 617; (e) M. E. El-Khouly, J.-H. Kim, J.-H. Kim, K.-Y. Kay and S. Fukuzumi, J. Phys. Chem. C, 2012, 116, 19709; ( f ) C. B. KC, G. N. Lim, M. E. Zandler and F. D’Souza, Org. Lett., 2013, 15, 4612. (a) A. Treibs and F. H. Kreuzer, Liebigs Ann. Chem., 1968, 718, 208; (b) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891. (a) A. Gorman, J. Killoran, C. O’Shea, T. Kenna, W. M. Gallagher and D. F. O’Shea, J. Am. Chem. Soc., 2004, 126, 10619; (b) M. J. Hall, S. O. McDonnell, J. Killoran and D. F. O’Shea, J. Org. Chem., 2005, 70, 5571; (c) M. Tasior and D. F. O’Shea, Bioconjugate Chem., 2010, 21, 1130. (a) S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne, W. M. Gallagher and D. F. O’Shea, J. Am. Chem. Soc., 2005, 127, 16360; (b) A. Palma, M. Tasior, D. O. Frimannsson, T. T. Vu, R. Meallet-Renault and D. F. O’Shea, Org. Lett., 2009, 11, 3638; (c) J. Murtagh, D. O. Frimannsson and D. F. O’Shea, Org. Lett., 2009, 11, 5386. (a) M. Yuan, X. Yin, H. Zheng, C. Ouyang, Z. Zuo, H. Liu and Y. Li, Chem. – Asian J., 2009, 4, 707; (b) P. A. Bouit, K. Kamada, P. Feneyrou, G. Bergine, L. Toupet, O. Maury and C. Andraud, Adv. Mater., 2009, 21, 1151; (c) S. Y. Leblebici, L. Catane, D. E. Barclay, T. Olson, T. L. Chen and B. Ma, ACS Appl. Mater. Interfaces, 2011, 3, 4469; (d) K. Flavin, K. Lawrence, J. Bartelmess, M. Tasior, C. Navio, C. Bittencourt, D. F. O’Shea, D. M. Guldi and S. Giordani, ACS Nano, 2011, 5, 1198; (e) K. Flavin, I. Kopfa, J. Murtagh, M. Grossi, D. F. O’Shea and S. Giordani, Supramol. Chem., 2012, 24, 23. (a) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, M. Supur, S. Fukuzumi and F. D’Souza, J. Phys. Chem. A, 2011, 115, 9810; (b) M. E. El-Khouly, A. N. Amin, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2012, 18, 5239; (c) A. N. Amin, E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. Commun., 2012, 48, 206; (d) F. D’Souza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler and S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 654; (e) V. Bandi, K. Ohkubo, S. Fukuzumi and F. D’Souza, Chem. Commun., 2013, 49, 2867; ( f ) V. Bandi, M. E. El-Khouly,
18728 | Phys. Chem. Chem. Phys., 2014, 16, 18720--18728
Paper
17
18
19
20
21
22 23
V. N. Nesterov, P. A. Karr, S. Fukuzumi and F. D’Souza, J. Phys. Chem. C, 2013, 117, 5638; (g) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem. – Eur. J., 2013, 19, 7221; (h) V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D’Souza, J. Phys. Chem. C, 2014, 118, 2321; (i) M. E. El-Khouly, S. Fukuzumi and F. D’Souza, ChemPhysChem, 2014, 15, 30. (a) Y. Kataoka, Y. Shibata and H. Tamiaki, Chem. Lett., 2010, 39, 953; (b) W.-J. Shi, R. Menting, E. A. Ermilov, P. C. Lo, B. Roeder and D. K. P. Ng, Chem. Commun., 2013, 49, 5277; (c) W.-J. Shi, M. E. El-Khouly, K. Ohkubo, S. Fukuzumi and D. K. P. Ng, Chem. – Eur. J., 2013, 19, 11332. (a) J.-Y. Liu, H.-S. Yeung, W. Xu, X. Li and D. K. P. Ng, Org. Lett., 2008, 10, 5421; (b) S. Cetindere, B. Cosut, S. Yesilot, M. Durmus and A. Kilic, Dyes Pigm., 2014, 101, 234. (a) A. Meller and A. Ossko, Monatsh. Chem., 1972, 103, 150; ´lez-Rodrı`guez, B. del Rey, (b) C. G. Claessens, D. Gonza T. Torres, G. Mark, H.-P. Schuchmann, C. von Sonntag, J. G. MacDonald and R. S. Nohr, Eur. J. Org. Chem., 2003, 2547. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 03, Gaussian, Inc., Pittsburgh PA, 2003. (a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, Singapore, 3rd edn, 2006; (b) E. Maligaspe, T. Kumpulainen, H. Lemmetyinen, N. V. Tkachenko, N. K. Subbaiyan, M. E. Zandler and F. D’Souza, J. Phys. Chem. A, 2010, 114, 268; (c) E. Maligaspe, T. Kumpulainen, N. K. Subbaiyan, M. E. Zandler, H. Lemmetyinen, N. V. Tkachenko and F. D’Souza, Phys. Chem. Chem. Phys., 2010, 12, 7434; (d) R. Jacobs, K. Stranius, E. Maligaspe, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler and F. D’Souza, Inorg. Chem., 2012, 51, 3656. D. Rehm and A. Weller, Isr. J. Chem., 1970, 7, 259. A. Medina, C. G. Claessens, G. M. A. Rahman, A. M. Lamsabhi, O. Mo, M. Yanez, D. M. Guldi and T. Torres, Chem. Commun., 2008, 1759.
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