Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 559–568
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Role of charge transfer interaction and the chemical physics behind effective fulleropyrrolidine/porphyrin non-covalent interaction in solution Ashis Mondal a, Kotni Santhosh b, Ajoy Bauri c, Sumanta Bhattacharya a,⇑ a b c
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India School of Chemistry, University of Hyderabad, Hyderabad, AP 500 046, India Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First time report on CT interaction
between PyC60 and porphyrin in solution. Fluorescence study elicits efficient quenching of 1 in presence of PyC60. Time-resolved emission study reveals formation of charge-separated state. DFT calculations explore electronic structure of PyC60/1 system in vacuo. Photoinduced electron transfer via T PyC60 from 1 is confirmed.
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 15 July 2013 Accepted 22 July 2013 Available online 2 August 2013 Keywords: PyC60 Monoporphyrin UV–Vis, steady state and time resolved fluorescence measurements Transient absorption study DFT and hybrid-DFT calculations
a b s t r a c t The present paper reports the photophysical insights on supramolecular interaction of a monoporphyrin derivative, namely, 1, with C60 pyrrolidine tris-acid ethyl ester (PyC60) in toluene and benzonitrile. The ground state interaction between PyC60 and 1 is facilitated through charge transfer interaction. Both UV–Vis and steady state measurements elicit almost similar magnitude of binding constant for the PyC60/1 complex in toluene and benzonitrile, viz., 6825 and 6540 dm3 mol1, respectively. Life time measurement evokes that rate of charge separation is fast in benzonitrile. Both hybrid-DFT and DFT calculations provide very good support in favor of electronic charge-separation in PyC60/1 system in vacuo. Ó 2013 Elsevier B.V. All rights reserved.
Introduction After the initial discovery in 1984 [1], the fortuitous contemporary growth of two apparently independent research lines, namely synthetic fullerene chemistry and supramolecular fullerene photochemistry, has been reciprocally beneficial and contributed to ⇑ Corresponding author. Tel.: +91 9433962777; fax: +91 342 2530452. E-mail address: [email protected] (S. Bhattacharya). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.07.054
boost activity in both fields. On the other hand, porphyrins are known to be one of the most common and versatile chromophores. Porphyrins offer a variety of desirable features such as a rigid and planar geometry, high stability, intense electronic absorption, strong fluorescence emission, and a small HOMO–LUMO energy gap [2]. For all this motivation, supramolecular architectures containing porphyrins and metalloporphyrins are particularly attractive. The molecular recognition process between fullerenes and porphyrins was recognized in a very unintentional way, prompting
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the birth of a new supramolecular recognition element [3]. The use of porphyrins and fullerenes in supramolecular chemistry is appreciated not only for the affinity that exists between the flat tetrapyrrole ring and the curved surface of the fullerene [4,5], but also for the capability that the fullerene/porphyrin assemblies possess to produce photoinduced charge-separated species mimicking the natural photosynthesis processes [6,7]. One of the main challenges in the field of supramolecular chemistry is the translation of molecular structure into function. In fact, synthetic multiporphyrin assemblies have been extensively investigated in materials science and nanotechnology and, in particular, its application to light-induced functions which attracted a great deal of interest [8–10]. The attainment of a better understanding of the dependence of photoinduced electron transfer reaction rates on the molecular structures of the donor and acceptor entities consisting fullerene/ porphyrin and fullerene/phthalocyanine systems which resulted in improving the capture and storage of solar energy are nicely described by D’Souza et al. [11]. Very recently, Torres and co-workers have demonstrated that porphyrin- and phthalocyanine/carbon nanostructure ensemble(s) may be suitably utilized for the construction of artificial photosynthetic reaction center [12]. The motivation behind selecting the PyC60 molecule as an electron acceptor comes from the pioneering work of Prato and Maggini on the synthesis and photophysical properties of such molecule [13]. Later, Sessler et al. successfully employed fulleropyrroline bearing a guanosine moiety as a recognition motif for the construction of C60-phthalocyanine (Pc) dyad system [14]. Very recently, we have got enough evidences behind effective non-covalent interaction between PyC60 and various Pc derivatives in solution [15]. The basic intention of the present work, therefore, is to find out the possibility of ground state non-covalent interaction between PyC60 and a designed monoporphyrin derivative, namely, 1 (Fig. 1), in solution having varying polarity. The motivation behind selecting monoporphyrin having coumarin unit comes from the fact that metal-free organic dye present very attractive candidate for the construction of de sensitized solar cell, with some significant advantage over the organometallic complexes. This include the ease with which they can be synthetically modified, low production cost, the absence of often scare metal elements and much higher extinction co-efficient [16]. Coumarin [17] is amongst the first organic species to demonstrate high device efficiency and having displaying good log-term ability. Various spectroscopic tools like absorption spectrophotometric, steady state fluorescence, time-resolved fluorescence and transient absorption studies are employed to find out the extent of such interaction. Quantum chemical calculations at hybrid-DFT and DFT levels of theory are performed to investigate the possibility of electronic redistribution between PyC60 and 1 during complexation. We
anticipate that the use of functionalized form of fullerene C60 to undergo non-covalent type of interaction with donor molecules having photoactive and electroactive units would certainly generate some new and interesting photophysical features in supramolecular photochemistry of fullerenes. Materials and methods PyC60 is purchased directly from Aldrich, USA and used without further purification. The monoporphyrin, 1, is obtained as a gift material from Dr. A. Bauri, BARC, India. The synthetic procedures and related spectroscopic data on this compound will be published in some other journal, very soon. UV–Vis spectroscopic grade toluene (Merck, Germany) has been used as solvent to favor non-covalent interaction between PyC60 and 1 and, at the same time, to ensure good solubility and photo-stability of the samples. UV–Vis spectral measurements are performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Emission spectra have been recorded with a Hitachi F-7000 model spectrofluorimeter. Fluorescence decay curves are measured with a HORIBA Jobin Yvon single photon counting set up employing nanoled as excitation source. PyC60 is selectively excited by 532 nm light from a Nd:YAG laser (6 ns fwhm) with 7 mJ power. For the transient absorption spectra in the visible region, a photomultiplier tube has been used as a detector for the continuous Xe-monitor light (150 W). Theoretical calculations are performed with a Pentium IV computer using SPARTAN0 06 V1.1.0 Windows version software. Results and discussions UV–Vis absorption studies The extensively conjugated aromatic chromophoric system of porphyrin generates intense bands in its absorption spectrum. The stronger and the most well-resolved absorption band of 1 is detected in the visible region (ranging from 375 to 580 nm (Fig. 2). Generally, in case of porphyrin, one intense absorption band is noticed around 400 nm due to S2 S0 transition (Soret band) and Q-absorption bands are observed in the region of 475– 650 nm due to S1 S0 transition. In our present case, 1 exhibits the Soret absorption band at 412 nm and Q-absorption peaks 503, 542 and 577 nm in toluene against the solvent as reference (Fig. 2). When the spectrum is recorded in benzonitrile, peak shift is observed in the longer wavelength region due to the increase in the polarity of the solvent. The Soret and Q-absorption peaks are shifted to 415, 504, 544 and 579 nm, respectively (Fig. 2). The first evidence in favor of ground state complexation between PyC60 and
O O N
N
Zn O
N
N
O
O Fig. 1. Structure of 1.
O
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561
Fig. 2. UV–Vis absorption spectrum of 1 recorded in toluene (violet color) and benzonitrile (red color); [1] = 4.1 106 (M) in toluene; and [1] = 4.1 106 (M) in benzonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 comes from UV–Vis absorption spectrophotometric titration measurements. Addition of varying concentration of PyC60 solution (in toluene and benzonitrile medium) to 1 (having fixed concentration) produces remarkable change in the absorbance value of the fullerene solutions. Moreover, it is observed that new absorption peaks are appeared around 695–696 nm, the intensity of which increases gradually following the addition of increasing amount of PyC60. It should be noted at this point that neither the monoporphyrin 1 nor the PyC60 exhibit any sort of absorption spectral feature(s) in the said region of the electronic spectra. These new peaks may be ascribed as charge transfer (CT) absorption peaks. Figs. 3 and 1S(a) nicely demonstrate the CT characteristics of the PyC60/ 1 system in toluene and benzonitrile, respectively. The formation of CT absorption peak also testifies significant ground state electronic interaction between PyC60 and 1. It should be mentioned at this point that CT interaction, CT absorption band and CT emission (exciplex emission) of fullerene/porphyrin dyad systems were
first observed more than decade ago by Imahori et al. [18]. Later, ground state and excited state inter-chromophore interactions on various fullerene/porphyrin model systems are well characterized by Chukharev et al. [19]. The same group of authors have reported a detailed analysis of the absorption and emission features of a series of double-linked dyads consisting fullerene and porphyrin in 2005 [20]. Very recently, effect of halide binding on intramolecular exciplex of double-linked fullerene/zincporphyrin dyad system is explored by Lemmetyinen et al. [21]. The binding constant (K) of the PyC60/1 system, in our present case, at CT absorption peak is evaluated utilizing modified Benesi–Hildebrand (BH) [22] for cells with 1 cm optical path length as shown below,
½10 =ðDAbs:Þ ¼ ðec e1 Þ1 þ fK½PyC60 ðec e1 Þg1
ð1Þ
where DAbs. = A A0; here, A0 and A are the absorbance of 1 at the given wavelength (here CT maxima) in the absence and presence of
[1]/ΔAbs., mol.dm
-3
0.0004
0.0003
0.0002
0.0001
0.0000 7500
15000
22500 3
30000 -1
1/[PyC60], dm .mol
PyC60 + 1
Fig. 3. UV–Vis titration curve of PyC60/1 system along with the absorption spectrum of uncomplexed 1 (marked in purple color) recorded in toluene against the solvent as reference. The concentration of 1 is kept fixed at 4.1 106 mol dm3. During titration experiment, the concentration of PyC60 varies from 3.65 105 to 7.28 105 mol dm3. Inset of Fig. 1 shows modified BH plot for PyC60/1 system recorded in toluene. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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PyC60, and ec and e1 are the molar absorption coefficient of 1 at the given wavelength of measurements in the absence and presence of PyC60, respectively. Eq. (1) is valid under 1:1 approximation for 1 and PyC60 system. Excellent linear modified BH plots for the PyC60/1 system in toluene and benzonitrile are shown in inset of Fig. 3 & Fig. 1S(b), respectively. Values of K are listed in Table 1. Eq. (1) is valid under 1:1 approximation for PyC60/1 system. It should be mentioned at this point that the corrected molar extinction coefficient, i.e., (ec e1), is not quite that of the complex. The BH method [22] is an approximation that we have used many times and it gives decent answers. But the extinction coefficient is really a different one between the complex and free species that absorbs at the same wavelength.
1
PyC 60 + 1
Steady state and time resolved fluorescence studies To study the photo-induced behavior of PyC60/1 supramolecular system and the recognition motif of PyC60 towards 1, steady-state emission measurements are carried out in toluene and benzonitrile. The simple mixing of the individual components, i.e., PyC60 and 1, leads to a novel superstructure, for which we may expect that the highly fluorescent state of the singlet excited 1 is quenched by an inter-complex electron transfer to fullerene forming fullerened. It has been reported earlier that charge separation can occur from the excited singlet state of the porphyrin to fullerene in PyC60/1 non-covalent hybrid system [23]. Photophysical studies prove that in case of conformationally flexible dyads comprising fullerenes and macrocyclic receptor molecules, like porphyrin, p-stacking interactions are facilitated due to through-space interactions between these two chromophores. This has been demonstrated by quenching of 1Por fluorescence and formation of fullerene-excited states by electron transfer [24]. The steady state experiment reveals that the fluorescence of 1 is characterized by maxima at 580 and 630 nm, upon excitation at the Soret band maximum in toluene (see Fig. 4). When the steady state fluorescence experiment is performed in benzonitrile, the peak maxima shifted to 582 and 635 nm (Fig. 2S(a)). Little difference in the Stokes shift for the PyC60/1 system in toluene and benzonitrile indicates that the relaxation pathway of the photoexcited state of 1 in presence of PyC60 follows similar sort of mechanism in the said solvents. Evidence in favor of the electron-transfer deactivation obtains from the titration experiment of a toluene and benzonitrile solution of 1 with variable concentration of PyC60 in the range of 1.05 105 mol dm3 to 6.75 105 mol dm3. It has been observed that upon excitation at the Soret band maxima of 1, a PyC60 concentration dependent decrease in the intensity of the fluorescence maxima of 1 is seen in both toluene and benzonitrile. It should be mentioned at this point that a purely diffusiondriven process is ruled out, on the basis of the applied fullerene concentration. The decrease of fluorescence intensity of 1 and the shift of the 1 fluorescence, suggest a static quenching event inside the well-defined PyC60/1 supramolecular complex. On the basis of the aforementioned results, we reach the conclusions that in the PyC60/1 system, the fluorescence state of 1 is quenched by the
Fig. 4. Steady state fluorescence spectral variation of 1 (4.1 106 mol dm3) in presence of PyC60 in toluene medium; the concentration of PyC60 varies from top to bottom (as indicated in the figure) are as follows: 1.05 105, 2.10 105, 2.60 105, 3.12 105, 3.65 105, 4.15 105, 4.70 105, 5.20 105, 5.70 105, 6.25 105 and 6.75 105 mol dm3; plot of relative fluorescence intensity vs. [PyC60] for PyC60/1 system in toluene is shown in inset of Fig. 2.
addition of electron-accepting PyC60. As ground state complex formation between PyC60 and 1 is evidenced from observation of decrease in the absorbance value of the Soret absorption band of 1 in presence of fullerenes (as discussed in section ‘UV–Vis absorption studies’), let us consider the formation of a non-fluorescent 1:1 complex according to the following scheme:
PyC60 þ 1 () PyC60 =1
ð2Þ
The fluorescence intensity of the solution (i.e., 1) decreases upon addition of fullerenes. Using the relation of binding constant (K) we obtain,
K S ¼ ½PyC60 =1=½PyC60 ½1
ð3Þ
Imposing the mass conservation law, we can write
½10 ¼ ½1 þ ½PyC60
ð4Þ
where [1]0, [1] and [PyC60/1] are the initial concentrations of 1, 1 in presence of PyC60, and PyC60/1 complex, respectively. Eq. (4) can be rearranged as
½10 =½1 ¼ 1 þ ½PyC60 =½1
ð5Þ
Using the value of KS in place of [PyC60]/[1] from Eq. (3), we can write the Eq. (5) as follows
½10 =½1 ¼ 1 þ K S ½PyC60
ð6Þ
Considering the fluorescence intensities are proportional to the concentrations, Eq. (6) is expressed as
F 0 =F ¼ 1 þ K S ½PyC60
ð7Þ
where F0 is the fluorescence intensity of 1 in absence of PyC60 and F is the fluorescence intensity of 1 in presence of quencher (i.e.,
Table 1 Fluorescence lifetime of the excited singlet state of 1 (s) in absence and presence of PyC60, rate of charge separation (kCS), quantum yield for charge separation (UCS) and binding constant (K) determined by steady state UV–Vis spectroscopic (UV–Vis) and steady state fluorescence methods (fluor.) for the complex of 1 with PyC60 recorded in toluene and benzonitrile medium; Temp. 298 K. System
1 PyC60/1
Toluene
Benzonitrile
s (ns)
kCS (s1)
UCS
1.596 0.94
– 4.37 108
– 0.410
K (dm3 mol1) UV–Vis
Fluor.
– 8360
– 7210
s (ns)
kCS (s1)
UCS
K (dm3 mol1) UV–Vis
Fluores.
0.94 0.38
– 15.20 108
– 0.588
– 5700
– 7380
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PyC60). By plotting F0/F) (relative fluorescence intensity) versus concentration of PyC60, K values are obtained for PyC60/1 complex in two different solvents (see Table 1). One typical plot expressing Eq. (7) of PyC60/1 system in toluene is shown in the inset of Fig. 4. The same plot for the PyC60/1 system in benzonitrile is shown in Fig. 2S(b). To validate the electron transfer process from 1 to PyC60 in PyC60/1 system in both the solvent, we have performed detailed nano-second time-resolved fluorescence measurements for the PyC60/1 complexation process. The time resolved fluorescence of 1 reveals a bi-exponential decay with life time (s) value of 1.595 and 0.940 ns in toluene and benzonitrile, respectively (Table 1). It is observed that upon the addition of PyC60, considerable decrease in s value of 1 takes place. The quenching of the life time is estimated to be higher in benzonitrile compared to toluene due to enhancement of solvent polarity; this finding certainly substantiates the electron transfer process. The decay curve of uncomplexed 1 in presence of PyC60 in toluene and benzonitrile are demonstrated in Figs. 3S(a) and S(b), respectively. Lifetime data of 1 in presence of PyC60 system in two different solvents are tabulated in Table 1. Binding constants and computational calculations Binding constant for the complex of 1 with PyC60 in toluene and benzonitrile are summarized in Table 1. It is observed that 1 undergoes appreciable amount of complexation with PyC60 in both the solvents studied. However, very low selectivity in binding in the K value in toluene and benzonitrile, viz., KPyC60/1(toluene)/KPyC60/1 (benzonitrile), suggests that 1 fails to discriminate PyC60 in the said solvents. It is interesting to note here that the reported K value of PyC60/1 complex is shown to exhibit higher K value compared to other host molecules like JAWS porphyrin (K = 1950 dm3 mol1) [25], thiacalixarene bisporphyrin (K = 2340 dm3 mol1) [26] and comparable to calixarene bisporphyrin (K = 4920 dm3 mol1) [26] and terphenyl porphyrin tetramer (K = 5800 dm3 mol1) reported in literature [27]. The reported K value, however, is found to exhibit much lower value than those of cyclic porphyrin dimer (K = 6.31 105 dm3 mol1) [28] and Pd bisporphyrin cleft (K = 3.70 104 dm3 mol1) [29]. Quantum chemical calculations may provide some light for the elucidation of geometric and electronic structure as well as association energy of the PyC60/1 ensemble in vacuo. In the present investigations, we have performed explicit theoretical calculations at hybrid-DFT and DFT levels of theory using Slater type of orbitals (STO) at 6-31G basis set for fullerene/1 complexes. We have employed Slater type orbitals for our calculations, which are capable of precisely predicting the optimized geometric structures with a far-less expensive treatment of electron correlation. As PyC60 molecule contains large number basis set, we have restricted our calculations by considering only the basic skeleton of such molecule and thereby, removing the alkyl group. Heat of formation (DH0f ) value for the PyC60/1 complex, in vacuo, are estimated from the difference between the total energy of the PyC60/1 complex and the sum of the individual host and guest entities separated from the optimized structure (single-point calculation). The estimated DH0f value of the PyC60/1 complex at the hybrid-DFT and DFT levels of theory are provided in Table 2. The single projection geometric structure of the PyC60/1 complex in DFT level of theory is shown in Fig. 5. Electrostatic interactions originating from the electron density at surface of the fullerene (viz., PyC60) and 1 of the PyC60/1 supramolecule are supposed to play a vital role in the interaction between PyC60 and 1. Molecular electrostatic potential (MEP) maps have been generated for PyC60 (Fig. 4S(a)), 1 (Fig. 4S(b)), and PyC60/1 system (Fig. 6) to visualize the electrostatic interactions.
Table 2 Heat of formation (DH0f ) value for PyC60/1 system in vacuo obtained from quantum chemical calculations at different levels of theory along with solvent reorganization energies (Rs) of the same system in two different solvents. Temp. 298 K. System
PyC60/1
Solvent
DH0f (kcal mol1)
Rs (eV)
Hybrid-DFT
DFT
Toluene
1.60
3.855
4.245
Benzonitrile
0.43
The MEP for 1 shows negative electrostatic potential (shown in red1) on the porphyrin ring (mostly located on the nitrogen atoms). The MEP for the PyC60 is blue–green indicating positive electrostatic potential; blue–green color of fullerene derivative corresponds to the center regions of the five- and six-membered rings. However, along the 6:6 bonds, regions of negative potentials (shown in red) can be observed. The pyrrolidine unit of PyC60 molecule contains red colored potential due to the presence of lone-pair electrons of N atom. Interestingly, in the supramolecular complex, the original blue– green color of the isolated PyC60 molecule is changed to mixture of green and bluish-green color formation. Moreover, deep red color of porphyrin is changed to reddish-yellow, indicating possibility of electron transfer between these two chromophores upon photoexcitation. One fascinating observation takes place when we visualize the electron distribution in various orbitals at different electronic states, like, HOMO, HOMO1, HOMO2, LUMO, LUMO+1 and LUMO+2 for PyC60/1 complex as evidenced from DFT calculations. It is observed that while HOMO is precisely centered on 1, the LUMO state is positioned on PyC60 molecule. This proves while former molecule is termed as donor, the later one may be attributed to electron acceptor. However, it is observed that although the HOMO1 state is located on 1, the HOMO2 state may be observed in PyC60 unit. On the contrary, all the states like LUMO+1, LUMO+2 states etc. are positioned on PyC60. This phenomenon certainly endorses the fact that upon photo-excitation, electron transfer takes place from 1 to PyC60. The pictures of various HOMOs and LUMOs of the PyC60/1 complex are visualized in Fig. 7. It could be seen that LUMO energy (ELUMO) levels of PyC60/1 complex compares well with the fullerene guest [30], while the HOMO energy (EHOMO) levels are similar to those of the uncomplexed 1 receptor. For example, in case of PyC60/1 complex, ELUMO is computed to be 3.0864 eV, which is comparable to the ELUMO of uncomplexed PyC60, i.e., 3.1502 eV. Also, the EHOMO of the same complex is estimated to be 5.0857 eV, which corroborates excellently with that of 1, viz., 5.0347 eV, obtained by DFT calculations. EHOMO and ELUMO of all the fullerene/1 complexes along with 1 are given in Table 4 (estimated from DFT calculations). It should be mentioned at this point that, electrostatic interaction is only one of the important components (and not the most important one), which can contribute to the stabilization of molecular van der Waals complex between 1 and PyC60 since both components are not charged (very small differences in electronic density distribution over the neutral molecule cannot provide strong electrostatic interaction between fullerene and Pc). This interaction is important for the complexes with charged components [31]. Together with electrostatic interaction other types of interaction like polarization interaction, p– p interaction, d–p interaction, etc. between neutral molecules also play vital role in stabilizing the complex.
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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Fig. 5. Optimized geometric structure of PyC60/1 system done by DFT calculations in vacuo.
the new solvent environment. In the present investigations, Rs of PyC60/1 complex has been estimated by applying the dielectric continuum model developed by Hauke et al. (see Eq. (8)) [33].
Rs ¼ ðe2 =4pe0 Þ½fð1=2R1 Þ þ ð1=2Rfullerene Þ ð1=RDA Þgð1=es Þ fð1=2Rfullerene Þ þ ð1=2R1 Þgð1=eR Þ
Fig. 6. MEP of PyC60/1 system done by DFT/B3LYP/6-31G calculations in vacuo.
Solvent reorganization energy (RS) for the PyC60/1 complex The effect of solvent over electronic coupling phenomenon between PyC60 and 1 may be better understood by the estimation of solvent reorganization energy (Rs) for the PyC60/1 complex. The total reorganization energy, in general, is a sum of the two terms, i.e., inner-sphere reorganization energy (solvent-independent) R0 [32], and outer-sphere reorganization energy (solventdependent) Rs. In case of fullerene, contribution from R0, which is related to the differences in nuclear configurations between an initial and final state of the fullerene, are very small, i.e., 4.3 105 eV. This finding implies that the structure of fullerene remains very much similar in the ground and excited states, which relates to the rigidity of these spherical carbon structures. As far as Rs contribution is concerned, this is believed to be small as well. The symmetrical shape and large size of the fullerene framework requires little energy for the adjustment of an excited or reduced state to
ð8Þ
with the following parameters: radius of donor, R1 = 6.8255 Å; radius of acceptor (RPyC60) = 3.641 Å, donor–acceptor separation (RDA): RPyC60/1 = 2.354 Å; solvent dielectric constant, es (etoluene = 2.39, ebenzonitrile = 25.20). Values of Rs at two different systems are given in Table 2. It is to be mentioned here, that the solvent reorganization energies obtained in the present investigation do not corroborate well with that observed for quinone/porphyrin system [34]. The discrepancy in the value of Rs for quinone/porphyrin and PyC60/1 systems may be due to the subtle structural change in the host–guest complex which exert a large influence upon the photo-induced electron and/energy transfer process. As we are proposing for a possible electron transfer process, it is customary to estimate the driving forces for the free energies of charge-separation (DGCS) and charge-recombination (DGCR) process for the PyC60/1 supramolecular complexes. DGCR for the fullerene/1 complexes are calculated using the Weller equation [35]. In this equation, the static energy (DGS) has been calculated according to the following equation:
DGS ¼ e2 =4pe0 eR RPyC60=1
ð9Þ
here the terms e, e0 and eR refer to elementary charge, vacuum permittivity and static dielectric constant of the solvent used for rate measurements, respectively. Based on DGCR and excited state energy (E0,0) value of PyC60, the free-energy changes of the charge separation process (DGCS) have been calculated using Eq. (10) and are listed in Table 3.
DGCS ¼ ðDGCR Þ þ E0;0
ð10Þ
Table 3 reveals that the charge-separation process of 1 via the excited singlet state of PyC60, i.e., 1 PyC60 , is sufficiently exothermic in benzonitrile as compared to toluene by 2.35 eV. The faster charge recombination of the PyC60/1 complex in benzonitrile can result from one important factor: solvent reorganization energy. This af-
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HOMO
HOMO – 1
HOMO – 2
LUMO
LUMO + 1
LUMO + 2
Fig. 7. HOMOs and LUMOs of various electronic states of PyC60/1 system done by DFT calculations in vacuo.
Table 4 Comparison of three occupied and three lowest unoccupied molecular orbital levels of 1, PyC60 and, PyC60/1 system done by DFT calculations in vacuo. State
HOMO2 HOMO1 HOMO LUMO LUMO+1 LUMO+2
Energy (eV) 1
PyC60
PyC60/1
0.157 0.152 0.123 0.027 0.016 0.042
0.161 0.161 0.161 0.036 0.036 0.036
0.156 0.156 0.154 0.040 0.040 0.038
Table 3 Static energy (DGS), free energy of charge separation (DGCS), free energy of charge recombination (DGCR), free energy for radical ion pair formation (DGRIP), standard free energy change associated with the formation of a pair of separated ions from a neutral donor and acceptor (DG0) along with free energy of activation (DG) for PyC60/1 system measured in toluene and benzonitrile; Temp. 298 K. System
Solvent
DGS (eV) DGCS (eV) DGCR (eV) DG0 (eV) DG (eV)
PyC60/1 Toluene 2.57 Benzonitrile 0.24
2.65 0.303
0.685 1.645
2.65 0.30
2.82 3.10
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fects the activation energy of electron transfer. It should be mentioned at this point that although DGCR > RS for PyC60/1 complex estimated in toluene, the reverse trend is observed for then same system when the measurement is done in benzonitrile (Table 3). From this we can infer that the activation energy should be larger and the charge recombination is slower for the investigated supramolecule in toluene than that of benzonitrile. The chemical reason for this can be explained on the basis that the unpaired electron in the PyC60 anion radical is more localized in benzonitrile than that in toluene due to high value of dielectric constant of the former solvent. Transient absorption study The steady state UV–Visible spectra of both PyC60 (Fig. 5S) and 1 (Fig. 2) reveal that they do not have any appreciable absorption intensity at 532 nm. For this reason, we have predominantly excited C60 and C70 molecule by 532 nm laser light in our present investigations. It is already evidenced from the spectrum of the mixture of PyC60 with 1 in toluene and benzonitrile (Fig. 3 and Fig. 1S, respectively) that appreciable CT type interaction does not exist in the ground state where the laser experiments are performed. Figs. 8a and 9a show the triplet–triplet absorption spectrum of 1 recorded in toluene and benzonitrile, respectively at 1, 2, 5 and 10 ls intervals. Fig. 8b and 9b also demonstrate the triplet decay profile of 1 in toluene and benzonitrile, respectively. The decay time at 450 nm is estimated to be 10 ls and 33 ls, respectively. Fig. 10a nicely demonstrates the triplet–triplet absorption spectrum of PyC60 in toluene at 1, 10, 20 and 30 ls intervals. The decay profile of the
Fig. 9. (a) Triplet–triplet absorption spectrum and (b) decay profile of 1 in benzonitrile; [1] = 3.0 105 (M).
same molecule in toluene at 700 nm after 20 ls delay time is recorded and shown in Fig. 10b. The fast rise in the absorption intensity at 505 nm is attributed to be the absorption band of T PyC60 having considerable absorption intensity in this region. Triplet–triplet absorption spectrum of PyC60 in benzonitrile is demonstrated in Fig. 6S at 1, 2, 4 and 6 ls delay time. The most interesting observation from the transient absorption analysis comes from the triplet–triplet absorption spectral analysis of 1 in presence of PyC60. It is observed that a sharp decrease in the decay time of 1 takes place (as compared to its uncomplexed form) when the transient absorption spectra are recorded in benzonitrile (Fig. 7S) compared to toluene (Fig. 11). This phenomenon certainly provides very good support in favor of the formation of charge-separation state with high value of rate constant in benzonitrile (see Table 1). In both the figures, i.e., Fig. 7S and Fig. 11, the absorption band around the region between 720 and 735 nm, may be attributed to the formation of T PyC60 . Since the decay of T PyC60 remains constant on addition of 1 at different time intervals like 1, 2, 5 and 10 ls, this is clearly indicative of the fact that electron transfer reaction is taking place for the investigated supramolecule (viz., PyC60/1) in both toluene and benzonitrile. Conclusions From above discussions, the following conclusions are reached:
Fig. 8. (a) Triplet–triplet absorption spectrum and (b) decay profile of 1 in toluene; [1] = 4.3 105 (M).
(a) PyC60 forms ground state CT complex with the porphyrin derivative, 1, in both polar and non-polar solvents. (b) Efficient quenching of the fluorescence intensity of 1 in presence of PyC60 takes place.
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Fig. 10. (a) Triplet–triplet absorption spectra of PyC60 in toluene obtained after 1, 10, 20 and 30 ls time intervals and (b) decay profile of PyC60 molecule recorded in toluene at 700 nm after 20 ls delay time.
(g) Finally, we may infer that PyC60/1 model system may be of potential interest for modulating various photo-physical features comprising chromophore appended fullerene and porphyrin in near future.
Acknowledgments AM thanks the Department of Science & Technology, New Delhi for providing a research fellowship to him. Financial assistance provided by the Department of Science & Technology, New Delhi, through the Project of Ref. No. SR/S1/PC-39/2011 is also gratefully acknowledged. The authors wish to record their gratitude to Prof. Anunay Samanta for his helpful co-operations in this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.07.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 11. (a) Triplet–triplet absorption spectra of 1 in presence of PyC60 obtained by 532 nm laser photolysis in toluene in time intervals of 1.0, 2.0, 5.0 and 10.0 ls; (b) decay time profile plot for the same system observed at 450 nm.
(c) Magnitude of K for the PyC60/1 complex as estimated from UV–Vis and steady state fluorescence measurements corroborate fairly well with each other and suggest that 1 may not be selectively employed as molecular tweezers for PyC60 in solution. (d) Lifetime measurements of 1 in the absence and presence of PyC60 establish that the charge-separated state is stabilized more in polar solvent; charge-separation process of 1 via the excited singlet state of PyC60, i.e., 1 PyC60 , is sufficiently exothermic in benzonitrile as compared to toluene (Table 3). (e) Both hybrid-DFT and DFT calculations well reproduce the geometry and binding pattern of PyC60 towards 1 in forming PyC60/1 supramolecular complex. (f) Photoinduced electron transfer via T PyC60 from 1 is confirmed by observing the transient absorption spectra in toluene and benzonitrile medium.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Role of charge transfer interaction and the chemical physics behind effective fulleropyrrolidine/porphyrin non-covalent interaction in solution Ashis Mondal a, Kotni Santhosh b, Ajoy Bauri c, Sumanta Bhattacharya a,⇑ a b c
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India School of Chemistry, University of Hyderabad, Hyderabad, AP 500 046, India Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First time report on CT interaction
between PyC60 and porphyrin in solution. Fluorescence study elicits efficient quenching of 1 in presence of PyC60. Time-resolved emission study reveals formation of charge-separated state. DFT calculations explore electronic structure of PyC60/1 system in vacuo. Photoinduced electron transfer via T PyC60 from 1 is confirmed.
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 15 July 2013 Accepted 22 July 2013 Available online 2 August 2013 Keywords: PyC60 Monoporphyrin UV–Vis, steady state and time resolved fluorescence measurements Transient absorption study DFT and hybrid-DFT calculations
a b s t r a c t The present paper reports the photophysical insights on supramolecular interaction of a monoporphyrin derivative, namely, 1, with C60 pyrrolidine tris-acid ethyl ester (PyC60) in toluene and benzonitrile. The ground state interaction between PyC60 and 1 is facilitated through charge transfer interaction. Both UV–Vis and steady state measurements elicit almost similar magnitude of binding constant for the PyC60/1 complex in toluene and benzonitrile, viz., 6825 and 6540 dm3 mol1, respectively. Life time measurement evokes that rate of charge separation is fast in benzonitrile. Both hybrid-DFT and DFT calculations provide very good support in favor of electronic charge-separation in PyC60/1 system in vacuo. Ó 2013 Elsevier B.V. All rights reserved.
Introduction After the initial discovery in 1984 [1], the fortuitous contemporary growth of two apparently independent research lines, namely synthetic fullerene chemistry and supramolecular fullerene photochemistry, has been reciprocally beneficial and contributed to ⇑ Corresponding author. Tel.: +91 9433962777; fax: +91 342 2530452. E-mail address: [email protected] (S. Bhattacharya). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.07.054
boost activity in both fields. On the other hand, porphyrins are known to be one of the most common and versatile chromophores. Porphyrins offer a variety of desirable features such as a rigid and planar geometry, high stability, intense electronic absorption, strong fluorescence emission, and a small HOMO–LUMO energy gap [2]. For all this motivation, supramolecular architectures containing porphyrins and metalloporphyrins are particularly attractive. The molecular recognition process between fullerenes and porphyrins was recognized in a very unintentional way, prompting
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the birth of a new supramolecular recognition element [3]. The use of porphyrins and fullerenes in supramolecular chemistry is appreciated not only for the affinity that exists between the flat tetrapyrrole ring and the curved surface of the fullerene [4,5], but also for the capability that the fullerene/porphyrin assemblies possess to produce photoinduced charge-separated species mimicking the natural photosynthesis processes [6,7]. One of the main challenges in the field of supramolecular chemistry is the translation of molecular structure into function. In fact, synthetic multiporphyrin assemblies have been extensively investigated in materials science and nanotechnology and, in particular, its application to light-induced functions which attracted a great deal of interest [8–10]. The attainment of a better understanding of the dependence of photoinduced electron transfer reaction rates on the molecular structures of the donor and acceptor entities consisting fullerene/ porphyrin and fullerene/phthalocyanine systems which resulted in improving the capture and storage of solar energy are nicely described by D’Souza et al. [11]. Very recently, Torres and co-workers have demonstrated that porphyrin- and phthalocyanine/carbon nanostructure ensemble(s) may be suitably utilized for the construction of artificial photosynthetic reaction center [12]. The motivation behind selecting the PyC60 molecule as an electron acceptor comes from the pioneering work of Prato and Maggini on the synthesis and photophysical properties of such molecule [13]. Later, Sessler et al. successfully employed fulleropyrroline bearing a guanosine moiety as a recognition motif for the construction of C60-phthalocyanine (Pc) dyad system [14]. Very recently, we have got enough evidences behind effective non-covalent interaction between PyC60 and various Pc derivatives in solution [15]. The basic intention of the present work, therefore, is to find out the possibility of ground state non-covalent interaction between PyC60 and a designed monoporphyrin derivative, namely, 1 (Fig. 1), in solution having varying polarity. The motivation behind selecting monoporphyrin having coumarin unit comes from the fact that metal-free organic dye present very attractive candidate for the construction of de sensitized solar cell, with some significant advantage over the organometallic complexes. This include the ease with which they can be synthetically modified, low production cost, the absence of often scare metal elements and much higher extinction co-efficient [16]. Coumarin [17] is amongst the first organic species to demonstrate high device efficiency and having displaying good log-term ability. Various spectroscopic tools like absorption spectrophotometric, steady state fluorescence, time-resolved fluorescence and transient absorption studies are employed to find out the extent of such interaction. Quantum chemical calculations at hybrid-DFT and DFT levels of theory are performed to investigate the possibility of electronic redistribution between PyC60 and 1 during complexation. We
anticipate that the use of functionalized form of fullerene C60 to undergo non-covalent type of interaction with donor molecules having photoactive and electroactive units would certainly generate some new and interesting photophysical features in supramolecular photochemistry of fullerenes. Materials and methods PyC60 is purchased directly from Aldrich, USA and used without further purification. The monoporphyrin, 1, is obtained as a gift material from Dr. A. Bauri, BARC, India. The synthetic procedures and related spectroscopic data on this compound will be published in some other journal, very soon. UV–Vis spectroscopic grade toluene (Merck, Germany) has been used as solvent to favor non-covalent interaction between PyC60 and 1 and, at the same time, to ensure good solubility and photo-stability of the samples. UV–Vis spectral measurements are performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Emission spectra have been recorded with a Hitachi F-7000 model spectrofluorimeter. Fluorescence decay curves are measured with a HORIBA Jobin Yvon single photon counting set up employing nanoled as excitation source. PyC60 is selectively excited by 532 nm light from a Nd:YAG laser (6 ns fwhm) with 7 mJ power. For the transient absorption spectra in the visible region, a photomultiplier tube has been used as a detector for the continuous Xe-monitor light (150 W). Theoretical calculations are performed with a Pentium IV computer using SPARTAN0 06 V1.1.0 Windows version software. Results and discussions UV–Vis absorption studies The extensively conjugated aromatic chromophoric system of porphyrin generates intense bands in its absorption spectrum. The stronger and the most well-resolved absorption band of 1 is detected in the visible region (ranging from 375 to 580 nm (Fig. 2). Generally, in case of porphyrin, one intense absorption band is noticed around 400 nm due to S2 S0 transition (Soret band) and Q-absorption bands are observed in the region of 475– 650 nm due to S1 S0 transition. In our present case, 1 exhibits the Soret absorption band at 412 nm and Q-absorption peaks 503, 542 and 577 nm in toluene against the solvent as reference (Fig. 2). When the spectrum is recorded in benzonitrile, peak shift is observed in the longer wavelength region due to the increase in the polarity of the solvent. The Soret and Q-absorption peaks are shifted to 415, 504, 544 and 579 nm, respectively (Fig. 2). The first evidence in favor of ground state complexation between PyC60 and
O O N
N
Zn O
N
N
O
O Fig. 1. Structure of 1.
O
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561
Fig. 2. UV–Vis absorption spectrum of 1 recorded in toluene (violet color) and benzonitrile (red color); [1] = 4.1 106 (M) in toluene; and [1] = 4.1 106 (M) in benzonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1 comes from UV–Vis absorption spectrophotometric titration measurements. Addition of varying concentration of PyC60 solution (in toluene and benzonitrile medium) to 1 (having fixed concentration) produces remarkable change in the absorbance value of the fullerene solutions. Moreover, it is observed that new absorption peaks are appeared around 695–696 nm, the intensity of which increases gradually following the addition of increasing amount of PyC60. It should be noted at this point that neither the monoporphyrin 1 nor the PyC60 exhibit any sort of absorption spectral feature(s) in the said region of the electronic spectra. These new peaks may be ascribed as charge transfer (CT) absorption peaks. Figs. 3 and 1S(a) nicely demonstrate the CT characteristics of the PyC60/ 1 system in toluene and benzonitrile, respectively. The formation of CT absorption peak also testifies significant ground state electronic interaction between PyC60 and 1. It should be mentioned at this point that CT interaction, CT absorption band and CT emission (exciplex emission) of fullerene/porphyrin dyad systems were
first observed more than decade ago by Imahori et al. [18]. Later, ground state and excited state inter-chromophore interactions on various fullerene/porphyrin model systems are well characterized by Chukharev et al. [19]. The same group of authors have reported a detailed analysis of the absorption and emission features of a series of double-linked dyads consisting fullerene and porphyrin in 2005 [20]. Very recently, effect of halide binding on intramolecular exciplex of double-linked fullerene/zincporphyrin dyad system is explored by Lemmetyinen et al. [21]. The binding constant (K) of the PyC60/1 system, in our present case, at CT absorption peak is evaluated utilizing modified Benesi–Hildebrand (BH) [22] for cells with 1 cm optical path length as shown below,
½10 =ðDAbs:Þ ¼ ðec e1 Þ1 þ fK½PyC60 ðec e1 Þg1
ð1Þ
where DAbs. = A A0; here, A0 and A are the absorbance of 1 at the given wavelength (here CT maxima) in the absence and presence of
[1]/ΔAbs., mol.dm
-3
0.0004
0.0003
0.0002
0.0001
0.0000 7500
15000
22500 3
30000 -1
1/[PyC60], dm .mol
PyC60 + 1
Fig. 3. UV–Vis titration curve of PyC60/1 system along with the absorption spectrum of uncomplexed 1 (marked in purple color) recorded in toluene against the solvent as reference. The concentration of 1 is kept fixed at 4.1 106 mol dm3. During titration experiment, the concentration of PyC60 varies from 3.65 105 to 7.28 105 mol dm3. Inset of Fig. 1 shows modified BH plot for PyC60/1 system recorded in toluene. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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PyC60, and ec and e1 are the molar absorption coefficient of 1 at the given wavelength of measurements in the absence and presence of PyC60, respectively. Eq. (1) is valid under 1:1 approximation for 1 and PyC60 system. Excellent linear modified BH plots for the PyC60/1 system in toluene and benzonitrile are shown in inset of Fig. 3 & Fig. 1S(b), respectively. Values of K are listed in Table 1. Eq. (1) is valid under 1:1 approximation for PyC60/1 system. It should be mentioned at this point that the corrected molar extinction coefficient, i.e., (ec e1), is not quite that of the complex. The BH method [22] is an approximation that we have used many times and it gives decent answers. But the extinction coefficient is really a different one between the complex and free species that absorbs at the same wavelength.
1
PyC 60 + 1
Steady state and time resolved fluorescence studies To study the photo-induced behavior of PyC60/1 supramolecular system and the recognition motif of PyC60 towards 1, steady-state emission measurements are carried out in toluene and benzonitrile. The simple mixing of the individual components, i.e., PyC60 and 1, leads to a novel superstructure, for which we may expect that the highly fluorescent state of the singlet excited 1 is quenched by an inter-complex electron transfer to fullerene forming fullerened. It has been reported earlier that charge separation can occur from the excited singlet state of the porphyrin to fullerene in PyC60/1 non-covalent hybrid system [23]. Photophysical studies prove that in case of conformationally flexible dyads comprising fullerenes and macrocyclic receptor molecules, like porphyrin, p-stacking interactions are facilitated due to through-space interactions between these two chromophores. This has been demonstrated by quenching of 1Por fluorescence and formation of fullerene-excited states by electron transfer [24]. The steady state experiment reveals that the fluorescence of 1 is characterized by maxima at 580 and 630 nm, upon excitation at the Soret band maximum in toluene (see Fig. 4). When the steady state fluorescence experiment is performed in benzonitrile, the peak maxima shifted to 582 and 635 nm (Fig. 2S(a)). Little difference in the Stokes shift for the PyC60/1 system in toluene and benzonitrile indicates that the relaxation pathway of the photoexcited state of 1 in presence of PyC60 follows similar sort of mechanism in the said solvents. Evidence in favor of the electron-transfer deactivation obtains from the titration experiment of a toluene and benzonitrile solution of 1 with variable concentration of PyC60 in the range of 1.05 105 mol dm3 to 6.75 105 mol dm3. It has been observed that upon excitation at the Soret band maxima of 1, a PyC60 concentration dependent decrease in the intensity of the fluorescence maxima of 1 is seen in both toluene and benzonitrile. It should be mentioned at this point that a purely diffusiondriven process is ruled out, on the basis of the applied fullerene concentration. The decrease of fluorescence intensity of 1 and the shift of the 1 fluorescence, suggest a static quenching event inside the well-defined PyC60/1 supramolecular complex. On the basis of the aforementioned results, we reach the conclusions that in the PyC60/1 system, the fluorescence state of 1 is quenched by the
Fig. 4. Steady state fluorescence spectral variation of 1 (4.1 106 mol dm3) in presence of PyC60 in toluene medium; the concentration of PyC60 varies from top to bottom (as indicated in the figure) are as follows: 1.05 105, 2.10 105, 2.60 105, 3.12 105, 3.65 105, 4.15 105, 4.70 105, 5.20 105, 5.70 105, 6.25 105 and 6.75 105 mol dm3; plot of relative fluorescence intensity vs. [PyC60] for PyC60/1 system in toluene is shown in inset of Fig. 2.
addition of electron-accepting PyC60. As ground state complex formation between PyC60 and 1 is evidenced from observation of decrease in the absorbance value of the Soret absorption band of 1 in presence of fullerenes (as discussed in section ‘UV–Vis absorption studies’), let us consider the formation of a non-fluorescent 1:1 complex according to the following scheme:
PyC60 þ 1 () PyC60 =1
ð2Þ
The fluorescence intensity of the solution (i.e., 1) decreases upon addition of fullerenes. Using the relation of binding constant (K) we obtain,
K S ¼ ½PyC60 =1=½PyC60 ½1
ð3Þ
Imposing the mass conservation law, we can write
½10 ¼ ½1 þ ½PyC60
ð4Þ
where [1]0, [1] and [PyC60/1] are the initial concentrations of 1, 1 in presence of PyC60, and PyC60/1 complex, respectively. Eq. (4) can be rearranged as
½10 =½1 ¼ 1 þ ½PyC60 =½1
ð5Þ
Using the value of KS in place of [PyC60]/[1] from Eq. (3), we can write the Eq. (5) as follows
½10 =½1 ¼ 1 þ K S ½PyC60
ð6Þ
Considering the fluorescence intensities are proportional to the concentrations, Eq. (6) is expressed as
F 0 =F ¼ 1 þ K S ½PyC60
ð7Þ
where F0 is the fluorescence intensity of 1 in absence of PyC60 and F is the fluorescence intensity of 1 in presence of quencher (i.e.,
Table 1 Fluorescence lifetime of the excited singlet state of 1 (s) in absence and presence of PyC60, rate of charge separation (kCS), quantum yield for charge separation (UCS) and binding constant (K) determined by steady state UV–Vis spectroscopic (UV–Vis) and steady state fluorescence methods (fluor.) for the complex of 1 with PyC60 recorded in toluene and benzonitrile medium; Temp. 298 K. System
1 PyC60/1
Toluene
Benzonitrile
s (ns)
kCS (s1)
UCS
1.596 0.94
– 4.37 108
– 0.410
K (dm3 mol1) UV–Vis
Fluor.
– 8360
– 7210
s (ns)
kCS (s1)
UCS
K (dm3 mol1) UV–Vis
Fluores.
0.94 0.38
– 15.20 108
– 0.588
– 5700
– 7380
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PyC60). By plotting F0/F) (relative fluorescence intensity) versus concentration of PyC60, K values are obtained for PyC60/1 complex in two different solvents (see Table 1). One typical plot expressing Eq. (7) of PyC60/1 system in toluene is shown in the inset of Fig. 4. The same plot for the PyC60/1 system in benzonitrile is shown in Fig. 2S(b). To validate the electron transfer process from 1 to PyC60 in PyC60/1 system in both the solvent, we have performed detailed nano-second time-resolved fluorescence measurements for the PyC60/1 complexation process. The time resolved fluorescence of 1 reveals a bi-exponential decay with life time (s) value of 1.595 and 0.940 ns in toluene and benzonitrile, respectively (Table 1). It is observed that upon the addition of PyC60, considerable decrease in s value of 1 takes place. The quenching of the life time is estimated to be higher in benzonitrile compared to toluene due to enhancement of solvent polarity; this finding certainly substantiates the electron transfer process. The decay curve of uncomplexed 1 in presence of PyC60 in toluene and benzonitrile are demonstrated in Figs. 3S(a) and S(b), respectively. Lifetime data of 1 in presence of PyC60 system in two different solvents are tabulated in Table 1. Binding constants and computational calculations Binding constant for the complex of 1 with PyC60 in toluene and benzonitrile are summarized in Table 1. It is observed that 1 undergoes appreciable amount of complexation with PyC60 in both the solvents studied. However, very low selectivity in binding in the K value in toluene and benzonitrile, viz., KPyC60/1(toluene)/KPyC60/1 (benzonitrile), suggests that 1 fails to discriminate PyC60 in the said solvents. It is interesting to note here that the reported K value of PyC60/1 complex is shown to exhibit higher K value compared to other host molecules like JAWS porphyrin (K = 1950 dm3 mol1) [25], thiacalixarene bisporphyrin (K = 2340 dm3 mol1) [26] and comparable to calixarene bisporphyrin (K = 4920 dm3 mol1) [26] and terphenyl porphyrin tetramer (K = 5800 dm3 mol1) reported in literature [27]. The reported K value, however, is found to exhibit much lower value than those of cyclic porphyrin dimer (K = 6.31 105 dm3 mol1) [28] and Pd bisporphyrin cleft (K = 3.70 104 dm3 mol1) [29]. Quantum chemical calculations may provide some light for the elucidation of geometric and electronic structure as well as association energy of the PyC60/1 ensemble in vacuo. In the present investigations, we have performed explicit theoretical calculations at hybrid-DFT and DFT levels of theory using Slater type of orbitals (STO) at 6-31G basis set for fullerene/1 complexes. We have employed Slater type orbitals for our calculations, which are capable of precisely predicting the optimized geometric structures with a far-less expensive treatment of electron correlation. As PyC60 molecule contains large number basis set, we have restricted our calculations by considering only the basic skeleton of such molecule and thereby, removing the alkyl group. Heat of formation (DH0f ) value for the PyC60/1 complex, in vacuo, are estimated from the difference between the total energy of the PyC60/1 complex and the sum of the individual host and guest entities separated from the optimized structure (single-point calculation). The estimated DH0f value of the PyC60/1 complex at the hybrid-DFT and DFT levels of theory are provided in Table 2. The single projection geometric structure of the PyC60/1 complex in DFT level of theory is shown in Fig. 5. Electrostatic interactions originating from the electron density at surface of the fullerene (viz., PyC60) and 1 of the PyC60/1 supramolecule are supposed to play a vital role in the interaction between PyC60 and 1. Molecular electrostatic potential (MEP) maps have been generated for PyC60 (Fig. 4S(a)), 1 (Fig. 4S(b)), and PyC60/1 system (Fig. 6) to visualize the electrostatic interactions.
Table 2 Heat of formation (DH0f ) value for PyC60/1 system in vacuo obtained from quantum chemical calculations at different levels of theory along with solvent reorganization energies (Rs) of the same system in two different solvents. Temp. 298 K. System
PyC60/1
Solvent
DH0f (kcal mol1)
Rs (eV)
Hybrid-DFT
DFT
Toluene
1.60
3.855
4.245
Benzonitrile
0.43
The MEP for 1 shows negative electrostatic potential (shown in red1) on the porphyrin ring (mostly located on the nitrogen atoms). The MEP for the PyC60 is blue–green indicating positive electrostatic potential; blue–green color of fullerene derivative corresponds to the center regions of the five- and six-membered rings. However, along the 6:6 bonds, regions of negative potentials (shown in red) can be observed. The pyrrolidine unit of PyC60 molecule contains red colored potential due to the presence of lone-pair electrons of N atom. Interestingly, in the supramolecular complex, the original blue– green color of the isolated PyC60 molecule is changed to mixture of green and bluish-green color formation. Moreover, deep red color of porphyrin is changed to reddish-yellow, indicating possibility of electron transfer between these two chromophores upon photoexcitation. One fascinating observation takes place when we visualize the electron distribution in various orbitals at different electronic states, like, HOMO, HOMO1, HOMO2, LUMO, LUMO+1 and LUMO+2 for PyC60/1 complex as evidenced from DFT calculations. It is observed that while HOMO is precisely centered on 1, the LUMO state is positioned on PyC60 molecule. This proves while former molecule is termed as donor, the later one may be attributed to electron acceptor. However, it is observed that although the HOMO1 state is located on 1, the HOMO2 state may be observed in PyC60 unit. On the contrary, all the states like LUMO+1, LUMO+2 states etc. are positioned on PyC60. This phenomenon certainly endorses the fact that upon photo-excitation, electron transfer takes place from 1 to PyC60. The pictures of various HOMOs and LUMOs of the PyC60/1 complex are visualized in Fig. 7. It could be seen that LUMO energy (ELUMO) levels of PyC60/1 complex compares well with the fullerene guest [30], while the HOMO energy (EHOMO) levels are similar to those of the uncomplexed 1 receptor. For example, in case of PyC60/1 complex, ELUMO is computed to be 3.0864 eV, which is comparable to the ELUMO of uncomplexed PyC60, i.e., 3.1502 eV. Also, the EHOMO of the same complex is estimated to be 5.0857 eV, which corroborates excellently with that of 1, viz., 5.0347 eV, obtained by DFT calculations. EHOMO and ELUMO of all the fullerene/1 complexes along with 1 are given in Table 4 (estimated from DFT calculations). It should be mentioned at this point that, electrostatic interaction is only one of the important components (and not the most important one), which can contribute to the stabilization of molecular van der Waals complex between 1 and PyC60 since both components are not charged (very small differences in electronic density distribution over the neutral molecule cannot provide strong electrostatic interaction between fullerene and Pc). This interaction is important for the complexes with charged components [31]. Together with electrostatic interaction other types of interaction like polarization interaction, p– p interaction, d–p interaction, etc. between neutral molecules also play vital role in stabilizing the complex.
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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Fig. 5. Optimized geometric structure of PyC60/1 system done by DFT calculations in vacuo.
the new solvent environment. In the present investigations, Rs of PyC60/1 complex has been estimated by applying the dielectric continuum model developed by Hauke et al. (see Eq. (8)) [33].
Rs ¼ ðe2 =4pe0 Þ½fð1=2R1 Þ þ ð1=2Rfullerene Þ ð1=RDA Þgð1=es Þ fð1=2Rfullerene Þ þ ð1=2R1 Þgð1=eR Þ
Fig. 6. MEP of PyC60/1 system done by DFT/B3LYP/6-31G calculations in vacuo.
Solvent reorganization energy (RS) for the PyC60/1 complex The effect of solvent over electronic coupling phenomenon between PyC60 and 1 may be better understood by the estimation of solvent reorganization energy (Rs) for the PyC60/1 complex. The total reorganization energy, in general, is a sum of the two terms, i.e., inner-sphere reorganization energy (solvent-independent) R0 [32], and outer-sphere reorganization energy (solventdependent) Rs. In case of fullerene, contribution from R0, which is related to the differences in nuclear configurations between an initial and final state of the fullerene, are very small, i.e., 4.3 105 eV. This finding implies that the structure of fullerene remains very much similar in the ground and excited states, which relates to the rigidity of these spherical carbon structures. As far as Rs contribution is concerned, this is believed to be small as well. The symmetrical shape and large size of the fullerene framework requires little energy for the adjustment of an excited or reduced state to
ð8Þ
with the following parameters: radius of donor, R1 = 6.8255 Å; radius of acceptor (RPyC60) = 3.641 Å, donor–acceptor separation (RDA): RPyC60/1 = 2.354 Å; solvent dielectric constant, es (etoluene = 2.39, ebenzonitrile = 25.20). Values of Rs at two different systems are given in Table 2. It is to be mentioned here, that the solvent reorganization energies obtained in the present investigation do not corroborate well with that observed for quinone/porphyrin system [34]. The discrepancy in the value of Rs for quinone/porphyrin and PyC60/1 systems may be due to the subtle structural change in the host–guest complex which exert a large influence upon the photo-induced electron and/energy transfer process. As we are proposing for a possible electron transfer process, it is customary to estimate the driving forces for the free energies of charge-separation (DGCS) and charge-recombination (DGCR) process for the PyC60/1 supramolecular complexes. DGCR for the fullerene/1 complexes are calculated using the Weller equation [35]. In this equation, the static energy (DGS) has been calculated according to the following equation:
DGS ¼ e2 =4pe0 eR RPyC60=1
ð9Þ
here the terms e, e0 and eR refer to elementary charge, vacuum permittivity and static dielectric constant of the solvent used for rate measurements, respectively. Based on DGCR and excited state energy (E0,0) value of PyC60, the free-energy changes of the charge separation process (DGCS) have been calculated using Eq. (10) and are listed in Table 3.
DGCS ¼ ðDGCR Þ þ E0;0
ð10Þ
Table 3 reveals that the charge-separation process of 1 via the excited singlet state of PyC60, i.e., 1 PyC60 , is sufficiently exothermic in benzonitrile as compared to toluene by 2.35 eV. The faster charge recombination of the PyC60/1 complex in benzonitrile can result from one important factor: solvent reorganization energy. This af-
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HOMO
HOMO – 1
HOMO – 2
LUMO
LUMO + 1
LUMO + 2
Fig. 7. HOMOs and LUMOs of various electronic states of PyC60/1 system done by DFT calculations in vacuo.
Table 4 Comparison of three occupied and three lowest unoccupied molecular orbital levels of 1, PyC60 and, PyC60/1 system done by DFT calculations in vacuo. State
HOMO2 HOMO1 HOMO LUMO LUMO+1 LUMO+2
Energy (eV) 1
PyC60
PyC60/1
0.157 0.152 0.123 0.027 0.016 0.042
0.161 0.161 0.161 0.036 0.036 0.036
0.156 0.156 0.154 0.040 0.040 0.038
Table 3 Static energy (DGS), free energy of charge separation (DGCS), free energy of charge recombination (DGCR), free energy for radical ion pair formation (DGRIP), standard free energy change associated with the formation of a pair of separated ions from a neutral donor and acceptor (DG0) along with free energy of activation (DG) for PyC60/1 system measured in toluene and benzonitrile; Temp. 298 K. System
Solvent
DGS (eV) DGCS (eV) DGCR (eV) DG0 (eV) DG (eV)
PyC60/1 Toluene 2.57 Benzonitrile 0.24
2.65 0.303
0.685 1.645
2.65 0.30
2.82 3.10
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fects the activation energy of electron transfer. It should be mentioned at this point that although DGCR > RS for PyC60/1 complex estimated in toluene, the reverse trend is observed for then same system when the measurement is done in benzonitrile (Table 3). From this we can infer that the activation energy should be larger and the charge recombination is slower for the investigated supramolecule in toluene than that of benzonitrile. The chemical reason for this can be explained on the basis that the unpaired electron in the PyC60 anion radical is more localized in benzonitrile than that in toluene due to high value of dielectric constant of the former solvent. Transient absorption study The steady state UV–Visible spectra of both PyC60 (Fig. 5S) and 1 (Fig. 2) reveal that they do not have any appreciable absorption intensity at 532 nm. For this reason, we have predominantly excited C60 and C70 molecule by 532 nm laser light in our present investigations. It is already evidenced from the spectrum of the mixture of PyC60 with 1 in toluene and benzonitrile (Fig. 3 and Fig. 1S, respectively) that appreciable CT type interaction does not exist in the ground state where the laser experiments are performed. Figs. 8a and 9a show the triplet–triplet absorption spectrum of 1 recorded in toluene and benzonitrile, respectively at 1, 2, 5 and 10 ls intervals. Fig. 8b and 9b also demonstrate the triplet decay profile of 1 in toluene and benzonitrile, respectively. The decay time at 450 nm is estimated to be 10 ls and 33 ls, respectively. Fig. 10a nicely demonstrates the triplet–triplet absorption spectrum of PyC60 in toluene at 1, 10, 20 and 30 ls intervals. The decay profile of the
Fig. 9. (a) Triplet–triplet absorption spectrum and (b) decay profile of 1 in benzonitrile; [1] = 3.0 105 (M).
same molecule in toluene at 700 nm after 20 ls delay time is recorded and shown in Fig. 10b. The fast rise in the absorption intensity at 505 nm is attributed to be the absorption band of T PyC60 having considerable absorption intensity in this region. Triplet–triplet absorption spectrum of PyC60 in benzonitrile is demonstrated in Fig. 6S at 1, 2, 4 and 6 ls delay time. The most interesting observation from the transient absorption analysis comes from the triplet–triplet absorption spectral analysis of 1 in presence of PyC60. It is observed that a sharp decrease in the decay time of 1 takes place (as compared to its uncomplexed form) when the transient absorption spectra are recorded in benzonitrile (Fig. 7S) compared to toluene (Fig. 11). This phenomenon certainly provides very good support in favor of the formation of charge-separation state with high value of rate constant in benzonitrile (see Table 1). In both the figures, i.e., Fig. 7S and Fig. 11, the absorption band around the region between 720 and 735 nm, may be attributed to the formation of T PyC60 . Since the decay of T PyC60 remains constant on addition of 1 at different time intervals like 1, 2, 5 and 10 ls, this is clearly indicative of the fact that electron transfer reaction is taking place for the investigated supramolecule (viz., PyC60/1) in both toluene and benzonitrile. Conclusions From above discussions, the following conclusions are reached:
Fig. 8. (a) Triplet–triplet absorption spectrum and (b) decay profile of 1 in toluene; [1] = 4.3 105 (M).
(a) PyC60 forms ground state CT complex with the porphyrin derivative, 1, in both polar and non-polar solvents. (b) Efficient quenching of the fluorescence intensity of 1 in presence of PyC60 takes place.
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Fig. 10. (a) Triplet–triplet absorption spectra of PyC60 in toluene obtained after 1, 10, 20 and 30 ls time intervals and (b) decay profile of PyC60 molecule recorded in toluene at 700 nm after 20 ls delay time.
(g) Finally, we may infer that PyC60/1 model system may be of potential interest for modulating various photo-physical features comprising chromophore appended fullerene and porphyrin in near future.
Acknowledgments AM thanks the Department of Science & Technology, New Delhi for providing a research fellowship to him. Financial assistance provided by the Department of Science & Technology, New Delhi, through the Project of Ref. No. SR/S1/PC-39/2011 is also gratefully acknowledged. The authors wish to record their gratitude to Prof. Anunay Samanta for his helpful co-operations in this work. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.07.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 11. (a) Triplet–triplet absorption spectra of 1 in presence of PyC60 obtained by 532 nm laser photolysis in toluene in time intervals of 1.0, 2.0, 5.0 and 10.0 ls; (b) decay time profile plot for the same system observed at 450 nm.
(c) Magnitude of K for the PyC60/1 complex as estimated from UV–Vis and steady state fluorescence measurements corroborate fairly well with each other and suggest that 1 may not be selectively employed as molecular tweezers for PyC60 in solution. (d) Lifetime measurements of 1 in the absence and presence of PyC60 establish that the charge-separated state is stabilized more in polar solvent; charge-separation process of 1 via the excited singlet state of PyC60, i.e., 1 PyC60 , is sufficiently exothermic in benzonitrile as compared to toluene (Table 3). (e) Both hybrid-DFT and DFT calculations well reproduce the geometry and binding pattern of PyC60 towards 1 in forming PyC60/1 supramolecular complex. (f) Photoinduced electron transfer via T PyC60 from 1 is confirmed by observing the transient absorption spectra in toluene and benzonitrile medium.
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