New photophysical insights on effect of gold nanoparticles on the interaction between phthalocyanine and PC70BM in solution

Accepted Manuscript New photophysical insights on effect of gold nanoparticles on the interaction between phthalocyanine and PC70BM in solution Anamika Ray, Haridas Pal, Vadivel Ramanan, Sumanta Bhattacharya PII: DOI: Reference:

S1386-1425(15)00688-5 http://dx.doi.org/10.1016/j.saa.2015.05.075 SAA 13743

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

27 March 2015 14 May 2015 24 May 2015

Please cite this article as: A. Ray, H. Pal, V. Ramanan, S. Bhattacharya, New photophysical insights on effect of gold nanoparticles on the interaction between phthalocyanine and PC70BM in solution, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.05.075

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New photophysical insights on effect of gold nanoparticles on the interaction between phthalocyanine and PC70BM in solution Anamika Ray(a), Haridas Pal(b), Vadivel Ramanan (c) and Sumanta Bhattacharya(a)* (a) The University of Burdwan, Department of Chemistry, Golapbag, Burdwan - 713 104, West Bengal, India. (b) Molecular Photochemistry Section, Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India. (c) National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai, India. *Author for correspondence; Email: [email protected] ABSTRACT Photoinduced processes in phthalocyanine/functionalized gold nanoparticles (Pc/AuNps) have been investigated by spectroscopic measurements. It is observed while AuNps reduce the magnitude of binding constant for the non-covalent complexation between [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) and H2-Pc by 15.8 times in comparison to PC70BM/ZnPc system in toluene, fluorescence of ZnPc is strongly quenched in presence of AuNp. Fluorescence lifetime determined by time-correlated single photon counting is also strongly reduced for ZnPc in presence of AuNp compared to H2-Pc. Symbolic enhancement in quantum yield of chargeseparation associated with well-defined electrostatic interaction has been confirmed for PC70BM/ZnPc supramolecule in presence of AuNp. Transient absorption measurements establish that energy transfer mechanism is operative for both PC70BM/H2-Pc and PC70BM/ZnPc supramolecules in absence and presence of AuNp in toluene. Keywords: PC70BM; H2- & ZnPc; Gold nanoparticle; binding constant; energy transfer.

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1. Introduction Phthalocyanine(Pc)/fullerene donor/acceptor conjugates have attracted immense attention in recent past due to their resemblance in the construction of photosynthetic systems and photonic devices [1,2]. Accordingly, both covalently and non-covalently linked fullerene-Pc dyads and triads have been designed for better understanding the mechanistic details of the photoinduced electron transfer process relevant to the construction of artificial photosynthetic reaction centre [3-9]. In a fullerene/Pc dyad, the role of the Pc is dual: first it is to function as an antenna and secondly, to act as a donor molecule during photo-excitation. Moreover, the supramolecular methodology offers several advantages including ease of construction, ability to monitor through-bond electron transfer and flexible structures with the choice of adopted self-assembly protocols [10]. An interesting aspect of the chemistry of fullerenes and Pcs is that they undergo spontaneous self-assembly phenomenon in solution as a result of ground state complexation [1118]. It is already well established that metal nanoparticles have size dependent optical and electrical properties [19] and in this respect, an important feature of the metal nanoparticles is the exhibition of localized surface plasmon band resonance in solution [20]. The motivation behind selecting the PC70BM (SCHEME 1) molecule as an electron acceptor comes from the work of Li et al. [21] in which they have utilized the narrow band gap of this particular molecule for enhancing the solar light harvest in the wavelength region of 350-500 nm [21]. In our present work, we have chosen two different phthalocyanine (Pc) molecule, namely, unsubstituted free-base (H2-) and zinc phthalocyanine (ZnPc) (SCHEME 1). Phthalocyanine is a unique class of molecule in porphine series. This is because of the fact that unlike free-base porphyrin, free-base Pc exhibits much higher value of binding constant (K) towards fullerene compared to metallophthalocyanine (viz., ZnPc). The present trend in K value is somewhat

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unnatural as structural analogue of Zn-Pc, viz., Zinc porphyrin, always gives higher magnitude of K with both fullerenes C60 and C70 [22,23] compared to free-base porphyrin. While the Pc/AuNp systems find promising applications in photodynamic therapy of cancer [24], we can find few report related to photoinduced processes comprising AuNp and Pc having two thioacetate linkers [25]. In recent past, effect of silver (AgNp) and AuNp on the interaction of fullerenes with porphyrin have been studied in quantitative manner by our research group [26,27]. Apart from that, although there are some reports on interaction between fullerene and porphyrin in presence of silver and gold nanoparticles in recent past [21,28], there is no such investigations on non-covalent interaction between functionalized fullerene and Pc in presence of AuNp in solution. The present work reports detailed spectroscopic investigation on self-assembly phenomenon between a fullerene derivative, namely, PC70BM and Pc (both free-base and metalloPc) in presence of AuNp, for the first time. Our interest in this area of research is to see the difference in physicochemical properties between fullerene/Pc and fullerene/Pc/AuNp ensembles in solution. The results suggest considerable decrease in the value of binding constant (K) for PC70BM/H2-Pc supramolecule in presence of AuNp other than significant reduction in fluorescence lifetime of the excited singlet state (τs) of ZnPc in presence of AuNp. Photoinduced energy transfer process is established for both PC70BM/Pc and PC70BM/Pc/AuNp systems in solution. We anticipate that the present work would certainly descry interesting photophysical insights which spans from the nanoscopic to the macroscopic level across multiple length scales, and detect the change in self-assembly scenario between Pc and fullerene in fullerene/Pc/AuNp linked system.

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2. Materials and methods Both the Pc, namely, H2- and ZnPc, and the PC70BM as shown in SCHEME 1, are obtained from Aldrich, USA and used after checking the purity of the compounds through standard routine analysis like absorption spectrophotometric, ir, NMR, mass etc. UV-vis spectroscopic grade toluene (Merck, Germany) is used as solvent to favor the non-covalent interaction between fullerene and Pc and, at the same time, to ensure good solubility and photo-stability of the samples. Steady state UV-vis spectra are recorded in a Shimadzu (Japan) UV-2450 model spectrophotometer. Steady state fluorescence 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. 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). PC70BM is selectively excited by 532 nm light from a Nd:YAG laser (6 ns fwhm) with 7 mJ power. Hybrid-DFT calculations are performed in a Pentium IV computer using SPARTAN’14 Windows version software. DLS measurements have been done with Malvern Zeta Seizer instrument of Model No. NANOZS90. All the scattered photons are collected at 90° scattering angle. SEM measurements are done in a S-530 model of Hitachi, Japan instrument having IB-2 ion coater with gold coating facility. Fluorescence microscope is measured in a Leica DM1000 model (Germany) instrument.

3. Results and discussions 3.1. UV-vis and steady state fluorescence investigations It is observed that both PC70BM and Pc, viz., H2- and ZnPc, undergo spontaneous interaction at ground state in toluene as evidenced from UV-vis study; a systematic increase in the

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absorbance value of both H2-Pc (Fig. 1S) and ZnPc (Fig. 2S) take place; the values of K for PC70BM/H2-Pc and PC70BM/ZnPc systems are determined to be 128,630 and 52,545 dm3⋅mol-1, respectively. K of such systems (see Table 1) has been obtained employing Benesi-Hildebrand equation [29] (inset of Figs. 1S & 2S, respectively). The K values of PC70BM/H2-Pc and PC70BM/ZnPc systems get further support from steady state fluorescence measurements; reliable quenching curves are obtained from steady state fluorescence experiments for both PC70BM/H2Pc (Fig. 1) and PC70BM/ZnPc systems (Fig. 3S) recorded in toluene. Excellent linear plots are resulted for both the systems with correlation factor of 0.99 (inset of Figs. 1 & 3S). In fluorescence titration method, the K values are determined with the help of BH equation [29]. It is to be mentioned that the K value obtained from both UV-vis and steady state fluorescence measurements corroborate fairly well with each other. New physicochemical insight is observed when absorption spectrophometric titration measurements are carried out for PC70BM/H2-Pc and PC70BM/ZnPc systems in presence of AuNp in toluene. UV-vis measurement reveals a considerable decrease in the value of K for PC70BM/H2-Pc system in presence of AuNp by magnitude of 41,105 dm3⋅mol-1 (Table 1); however, AuNp affects little in terms of reducing the K value of PC70BM/ZnPc system in toluene, viz., 520 dm3⋅mol-1 (Table 1). The UV-vis spectra and BH plots [29] for PC70BM/H2-Pc and PC70BM/ZnPc systems are provided in Figs. 2 & 4S, respectively. UV-vis investigations, therefore, may be treated as fingerprint observation in favour of inhibition in non-covalent binding between PC70BM and Pc (i.e., both H2- and ZnPc) in presence of AuNp. Further support in this regard comes from steady state fluorescence investigations. Quenching in the intensities of fluorescence of both H2- (Fig. 5S) and ZnPc (Fig. 6S) at emission band of 690 and 675 nm, respectively, occur efficiently in presence of PC70BM; like UV-vis investigations, a sharp

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decrease in the vale of K is observed (Table 1). An excellent liner correlation is obtained as evidenced from fluorescence BH plot [29] (inset of Figs. 5S & 6S, respectively). Table 1 reveals that AuNp reduces the magnitude of K for PC70BM/H2-Pc complex significantly compared to that of PC70BM/ZnPc system estimated in toluene. However, the relative binding strengths of the two PC70BM/ZnPc and PC70BM/ZnPc/AuNp is not very convincing since the differences in K values for these two taking into consideration the error limits nearly overlap. This can be interpreted on the basis of the fact that the rigidity of ZnPc skeleton renders AuNp to cover its πsurface region available for supramolecular complexation with PC70BM; however, flexibility of H2-Pc molecule allows AuNp to cover the π-surface area of Pc in such manner which would create considerable impact in terms of reducing the strength of intermolecular interaction between PC70BM and H2-Pc. The rigidity of H2-Pc and ZnPc, however, may be explained on the basis of the point group symmetry of these molecules. Fig. 7S clearly indicates that the highest order rotational axis of ZnPc is identified as the C4 axis passing through the central Zn atom at right angles to the plane of the molecule. However, in case of H2-Pc, three numbers of C2 rotation operations may be identified, two about the axes of the plane of the molecule and one perpendicular to them. Therefore, due to the absence of C4 rotational axis, H2-Pc is supposed to be more flexible compared to ZnPc. Noteworthy is that, fullerene C70, a structural isomer of PC70BM, is reported to exhibit ~1.7 and 2.3 [14] times lower magnitude of K with H2- and ZnPc in comparison to PC70BM/H2-Pc and PC70BM/ZnPc systems, respectively, reported in Table 1.

3.2. Time-resolved fluorescence and measurement of quantum yield Time-resolved fluorescence investigations evoke new photophysical insights in absence and presence of AuNp. In absence of AuNp and PC70 BM, the photo-excited states of H2- and ZnPc

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show bi- and monoexponential decay pathway with τs value of 5.86 and 3.57 ns, respectively. In presence of AuNp, although there is slight change in the lifetime value of H2-Pc, a sharp reduction in the value of τs is observed for ZnPc (see Table 1). The lifetime value of H2-Pc and H2-Pc/AuNp systems appear to be the same, except for a slightly faster decay for later compared to that of uncomplexed H2-Pc. It is thus assumed that 96% of the H2-Pc molecules are assembled in such a way, that they are completely quenched due to the interactions with the AuNp or between the H2-Pc. Moreover, ZnPc decays bi-exponentially in presence of AuNp. Estimation of rate constant of charge-separation of the excited singlet (kCSs) and quantum yield of chargeseparation for the excited singlet (ϕCSs) [30] shows ~44 and ~14 times increase in the value of said parameters, respectively, for ZnPc compared to H2-Pc in presence of AuNp. Interestingly, there is no further enhancement in the value of ϕCSs for ZnPc when the same parameter is measured with reference to PC70BM/AuNp ensemble. On the contrary, a large extent of increase (~ 22.0 times) in the value of ϕCSs is observed for H2-Pc in presence of PC70BM/AuNp ensemble in comparison to H2-Pc/AuNp mixture. This phenomenon clearly envisages that AuNp significantly alters the mechanistic pathway responsible behind the photoexcited decay of H2and ZnPc. Variation in the value of τs for both H2- and ZnPc in presence of AuNp, PC70BM and PC70BM/AuNp systems are demonstrated in Figs. 8S & 3, respectively. The lifetime experiment, therefore, suggests considerable enhancement in the electrostatic interaction between ZnPc and PC70BM in presence of AuNp while dispersive forces associated with π-π interaction dictates the binding phenomenon between PC70BM and H2-Pc in presence of AuNp. To gain further insight into photophysical characteristics of H2- and ZnPc in absence and presence of AuNp, we have measured quantum yield of fluorescence (Φf), rate constant of energy transfer (kEnT) [31] and radiative rate constant (Kf) (reported in Table 2) [32,33]. Table 2

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reveals that AuNp acts as an efficient quencher towards ZnPc as Kf value of such molecule is decreased by ~6.0 times compared to only 1.0 in case of H2-Pc. Table 2 also explores that no significant quenching in the value of Φf takes place for H2-Pc in presence of AuNp; extent of decrease in the value of Φf is more pronounced in case of PC70BM/ZnPc complex in presence of AuNp. Order of kEnT, therefore, signifies that static quenching mechanism is operative for the investigated supramolecules in presence of AuNp.

3.3. Theoretical calculations Hybrid-DFT calculations for the investigated supramolecules (Fig. 4) are performed in absence of AuNp in vacuo. Trend in the value of heat of formation (∆Hf0) for the complexes of PC70BM with H2- and ZnPc corroborates fairly well with the value of K reported in Table 1. Hybrid-DFT calculations reveal while HOMO and LUMO are located in the Pc (both H2- and ZnPc) and PC70BM moiety, respectively, at ground state, changes in the position of such electronic states take place at excited state (Figs. 5 & 6). Molecular electrostatic potential maps (MEP) have been generated for PC70BM/H2-Pc and PC70BM/ZnPc complexes in vacuo (Fig. 7). Fig. 7 clearly indicates strong propensity of electrostatic interaction between PC70BM and ZnPc during supramolecular interaction. DFT calculations provide some light for elucidation of energy transfer mechanism obtained in our present work. It is observed that although LUMO energy levels of H2- and ZnPc remain almost degenerate, HOMO energy level of ZnPc remains more close to PC70BM than that of H2-Pc (Fig. 9S). Therefore, probability of energy transfer would be higher in case of PC70BM/ZnPc system. Typical Jablosnski diagram showing the energy transfer process in PC70BM/ZnPc system is pictorially represented in SCHEME 2.

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3.4. Particle size measurements Particle size analysis through dynamic light scattering (DLS) measurements have been done to shade light into the nature of intermolecular interaction persists between functionalized fullerene and Pc in presence of AuNp. The size of AuNp is determined to be in the region of 3.122 to 4.849 nm (see supporting information) as we procured AuNp having size of 3-6 nm (Aldrich, USA, catalogue no. 54349). New physicochemical feature is observed when DLS measurements are performed for H2-Pc/AuNp and ZnPc/AuNp system in toluene. It is observed that in case of former system, the size of the nanoaprticle is increased considerably, i.e., ~20.95 nm (Fig. 8). However, no such significant change in the size of AuNp is noted in presence of ZnPc, e.g., 3.577 nm (Fig. 9). The large difference in size before and after mixing with AuNp corresponds to the thickness of such nanoparticles over the surface of phthalocyanines. Furthermore, the DLS measurements find that the particle size of AuNp in PC70BM/H2-Pc systems becomes 21.05 nm (Fig. 10S) providing strong support in favour of the large decrease in the value of K for such system. However, in case of PC70BM/ZnPc ensemble mixture, moderate enhancement in the size of AuNp is observed, i.e., 13.55 nm (Fig. 11S). This may be the first example to track the occurrence of non-covalent interaction between PC70BM and Pc in presence of AuNp and subsequently, its effect over strength of binding by particle size analysis.

3.5. Scanning electron microscope measurements Scanning electron microscope (SEM) measurements of H2- (Fig. 12S(a)) and ZnPc (Fig. 12S(b)) in presence of AuNp reveal formation of uneven sized particles ranging from smaller to larger dimension, viz. 3 to 25 nm, respectively. However, the SEM image of H2-Pc + PC70BM + AuNp system obtained from drop-casted films of the clusters on a stab made of copper indicates

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formation of ill-defined packed cluster; formidable extent of increase in the particle size is observed along the long axis (Fig. 13S(a)). In case of ZnPc + PC70BM + AuNp system, however, no such scenario is observed; instead, molecular cluster having sharp edge is formed (Fig. 13S(b)). The above results clearly demonstrate that dispersion of AuNp over the surface of H2and ZnPc control the shape and the size of the PC70BM/H2-Pc/AuNp and PC70BM/ZnPc/AuNp system in solution.

3.6. Fluorescence microscope measurements In fluorescence microscope experiments, after exposure of 275 millisecond, H2-Pc shows very good blue fluorescence (Figure 10(a)). Following the addition of AuNp, fluorescence intensity of H2-Pc gets reduced and larger exposure time (295 millisecond) is needed to get reliable image (Fig. 10(b)). PC70BM reduces the fluorescence intensity of H2-Pc at a larger extent compared to AuNp (Fig. 10(c)) with much larger exposure time (806.3 millisecond). In case of PC70BM/H2Pc/AuNp ensemble, no further change in the fluorescence intensity as well as exposure time is observed (Fig. 10(d)). However, extraordinary feature is observed when the fluorescence microscope experiment is performed with ZnPc in absence and presence of AuNp. It is observed upon exposure of 205 millisecond, ZnPc shows intense blue fluorescence (Fig. 11(a)). However, in presence of AuNp, there is noteworthy reduction in the intensity of fluorescence intensity of ZnPc; much larger exposure time (1.4 second) is needed for getting reliable fluorescent image (Fig. 11(b)). It should be mentioned at this point that PC70BM fails to reduce the intensity of fluorescence for ZnPc when nanoparticles are absent in solution. As a result of this, very good fluorescence microscope image is obtained with less exposure time (500 millisecond, Fig. 11(c)). The fluorescence microscope image of PC70BM/ZnPc/AuNp ensemble (Fig. 11(d)) shows

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preeminent feature with dark greenish-blue fluorescence having very large exposure time (3.70 seconds). The point to be noted here is that Surface Plasmon Resonance of AuNp is observed at ~520 nm due to absorption enhancement.

3.7. Transient absorption measurements From the steady state UV-visible spectra of H2- and ZnPc measured in toluene (Figs. 1S & 2S), it is clearly evidenced that both the free-base and metalloPc do not have any appreciable absorption intensity at 532 nm. Therefore, probability of formation of TZnPc* or TH2-Pc* states as a result of direct excitation of ZnPc and H2-Pc, is small. For this reason, we have predominantly excited PC70BM molecule by 532 nm laser light in our present investigations (Fig. 12). Upon 532 nm laser exposure on PC70BM in the presence of H2- (14S) and ZnPc (Fig. 15S) in toluene, the transient absorption band of TPC70BM* was observed at 700 nm; however, absorption band due to H2-(/Zn)Pc.+ and PC70BM.- are not detected in the long wavelength region of the visible spectra. The negative absorbance at the shorter wavelength region (~670 nm) may be due to the depletion of H2-(/Zn)Pc since the decay of TPC70BM* is accelerated on addition of both H2- and ZnPc, which clearly proves that reaction other than electron transfer takes place. In presence of AuNp, no such alteration in the mechanism of energy transfer takes place. The transient absorption spectra of the PC70BM + H2-Pc + AuNp and PC70BM + ZnPc + AuNp systems are shown in Figs. 16S & 17S, respectively. In toluene, the energy levels for the radical ion-pair states are significantly raised. As a direct consequence, the possibility of electron transfer mechanism is shut off, while the intriguing energy transfer route is activated. The latter is confirmed spectroscopically through transient absorption analysis. As a result of this, triplettriplet energy transformation from the triplet state of PC70BM substantiates the absence of radical

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ion-pair formation for the investigated supramolecules. Moreover, the bleaching of phthalocyanine Q-absorption band in PC70BM/Pc supramolecules, together with the fast recovery of the AuNp surface plasmon band in PC70BM/Pc/AuNp system substantiates the energy transfer mechanism for the investigated supramolecules in present work. First order decay rate constant and corresponding lifetime of the transient species at different absorption peaks, viz., 420 nm, 520 nm, 580 nm and 700 nm, obtained during laser flash photolysis experiment are given in Table 3.

4. Conclusions The effect of gold nanoparticle over self-assembly phenomenon between PC70BM and Pc is investigated in solution for the first time. It is observed that AuNps reduce the magnitude of binding between PC70BM and H2-Pc significantly compared to that of PC70BM/ZnPc system. Both DLS and SEM measurements clearly demonstrate that dispersion of AuNp over the surface of H2- and ZnPc control the shape and the size of the PC70BM/H2-Pc/AuNp and PC70BM/ZnPc/AuNp systems in solution. Estimation of kCSs and ϕCSs establish that AuNps control the mechanistic pathway behind the photoexcited decay of H2- and ZnPc during noncovalent interaction with PC70BM. The bleaching of the phthalocyanine Q-band in Pc/AuNP together with the faster recovery of the gold nanoparticle surface plasmon band bleach indicates energy transfer from the photoexcited gold nanoparticles to the attached phthalocyanine (Figs. 15 & 16) [25]. Such information would be very much helpful for better understanding the fundamental properties of various other supramolecular recognition elements. Studies to further understand this phenomenon are in progress.

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Supporting Information Available. UV-vis spectral variation of H2-Pc and ZnPc in presence of PC70BM, fluorescence spectral variation of ZnPc in presence of PC70BM, UV-vis spectral variation of ZnPc in presence of PC70BM and AuNp, Fluorescence spectral variation of H2-Pc in presence of PC70BM and AuNp, Fluorescence spectral variation of H2- and ZnPc in presence of PC70BM and AuNp, ZnPc and H2-Pc molecules have been orientated on the xy plane, perpendicular to the z axis, Time-resolved fluorescence decay profile of H2-Pc in presence of PC70BM, AuNp, and PC70BM + AuNp mixture, HOMO and LUMO energy levels of PC70BM, H2-Pc and ZnPc, at different electronic states done by DFT/B3LYP/6-31G* calculations in vacuo, Size analysis of AuNp having size of 3-6 nm, Particle size distribution analysis of AuNp in presence of PC70BM/H2-Pc system recorded in toluene at 298K, Particle size distribution analysis of AuNp in presence of PC70BM/ZnPc system recorded in toluene at 298K, SEM images of H2- and ZnPc in presence of AuNp recorded in toluene at 298K and SEM images of H2-Pc and ZnPc in presence of PC70BM + AuNp recorded in toluene at 298K, Transient absorption

analysis

of

PC70BM/H2-Pc,

PC70BM/ZnPc,

PC70BM/H2-Pc/AuNp

and

PC70BM/ZnPc/AuNp systems recorded in toluene are gives as Figures 1S-17S, respectively along with respective data in tabular form. Figures 1S-17S and data in tabular form are gives as supplementary materials. This material is available free of charge via the Internet.

5. Acknowledgements We are thankful to Prof. P. K. Ghosh, Dr. Srijata Mitra and Dr. S. Chakraborty, USIC, BU, India for their helpful cooperations in fluorescence microscope and SEM measurements, respectively. We gratefully acknowledge the kind cooperations of Dr. Selvaraju of National Centre for Ultrafast Processes, University of Madras, India for transient absorption analysis. AR

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thanks CSIR, New Delhi for providing Research Associate to her. Grant-In-Aid through CSIR, New Delhi funded Research Project of Sanction No. 01(2711)/13/EMR-II is also greatly acknowledged.

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28. S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger. Nat. Photonics 3 (2009) 297-302. 29. H. A. Benesi, J. H. Hildebrand. J. Am. Chem. Soc. 71 (1949) 2703-2707. 30. F. D’Souza, R. Chitta, S. Gadde, M. E. Zandler, A. L. McCarty, A. S. D. Sandanayaka, Y. Araki, O. Ito. Chem.-Eur. J. 11 (2005) 4416-4428. 31. J. N. Clifford, T. Gu, J. –F. Nierengarten, N. Armaroli. Photochem. Photobiol. Sci. 5 (2006) 1165-1172. 32. Principles of Fluorescence Spectroscopy, J. R. Lakowicz, 3rd Ed. 2006, XXVI, p. 1-980, Springer. 33. Molecular Fluorescence: Principles and Applications, B. Valeur, 2002, p. 1-387, WileyVCH.

17

Table 1. Binding Constants (K, dm3⋅mol-1) Determined by Absorption Spectrophotometric and Steady State Fluorescence Techniques, Lifetime of the Excited Singlet (τs), Rate Constant of Charge-Separation of the Excited Singlet (kCSs) and Quantum Yield of Charge-Separation for the Excited Singlet (ϕCSs) for the Complexes of PC70BM with H2- and ZnPc in Absence and Presence of AuNp Recorded in Toluene at 298K Along with the Heat of Formation (∆Hf0) Value of PC70BM/H2-Pc and PC70BM/ZnPc Systems Done by Hybrid-DFT Calculations in vacuo. K, dm3⋅mol-1

System

τs, ns

kCSs, sec-1

ϕCSs

∆Hf0, kcal⋅⋅mol-1

UV-Vis

Fluorescence

H2-Pc

-

-

5.86

-

-

-

H2-Pc/AuNp

-

-

5.64

6.4 × 106

0.036

-

PC70BM/H2-Pc

128,630

126,130

1.25

6.3 × 108

0.78

4.7

±

±

6,430

6,300

PC70BM/H2-

87,525

80,300

1.21

6.5 × 108

0.79

-

Pc/AuNp

±

±

4,375

4,015

ZnPc

-

-

3.57

-

-

-

ZnPc/AuNp

-

-

1.77

2.85×108

0.504

-

PC70BM/ZnPc

52,545

60,320

3.26

2.7 × 107

0.087

12.2

PC70BM/ZnPc/AuNp

52,025

57,425

1.76

2.88 × 108 0.507

18

-

Table 2. Quantum Yield of Fluorescence (Φf), Rate Constant of Energy Transfer (kEnT) and Radiative Rate Constant (kf) for the Complexes of PC70BM with H2- and ZnPc in Absence and Presence of AuNp Recorded in Toluene at 298K. System

Φf

kEnT, sec-1

Kf, sec-1

H2-Pc

0.055

―

9.40 × 106

H2-Pc/AuNp

0.050

1.70 × 107

8.85 × 106

PC70BM/H2-Pc

0.010

7.70 × 108

8.0 × 106

PC70BM/H2-Pc/AuNp

9.20 × 10-3

8.50 × 108

7.60 × 106

ZnPc

0.12

―

3.35 × 107

ZnPc/AuNp

0.010

3.10 × 109

5.65 × 106

PC70BM/ZnPc

1.42 × 10-3

2.55 × 1010

4.35 × 105

PC70BM/ZnPc/AuNp

1.55 × 10-3

4.35 × 1010

8.80 × 105

19

Table 3. First Order Decay Rate Constant (k) and Corresponding Lifetime of the Transient species (τ) Obtained at Different Absorption Peaks During Laser Flash Photolysis Experiment of PC70BM, PC70BM/H2-Pc, PC70BM/H2-Pc/AuNp, PC70BM/ZnPc and PC70BM/H2-Pc/AuNp Determined In Nitrogen Saturated Toluene. System

420 nm k (s-1) × 104

520 nm τ

580 nm

k (s-1) × 104 τ (µs)

700 nm

k (s-1) ×

τ

k (s-1) ×

τ

104

(µs)

104

(µs)

(µs) PC70BM

5.9

17

5.9

17

7.7

13

7.3

14

PC70BM/H2-Pc

6.2

16

5

20

6.2

16

1.0

97

PC70BM/H2-

6.9

14

3.4

29

6.1

16

3.8

26

PC70BM/ZnPc

7.0

14

3.1

33

5.4

19

5.8

17

PC70BM/ZnPc/

4.1

24

3.5

29

5.7

18

5.5

18

Pc/AuNp

AuNp

20

Figure captions Fig. 1. Steady state fluorescence spectral variation of H2-Pc (3.65 × 10-6 mol.dm-3) in presence of PC70BM {(6.3 – 59.5) × 10-6 mol⋅dm-3} recorded in toluene at 298K. λex = 342 nm; λem = 690 nm. Fig. 2. UV-vis spectral variation of H2-Pc (1.30 × 10-5 mol.dm-3) in presence of varying concentration of PC70BM (1.15 × 10-5 to 5.15 × 10-5 mol.dm-3) and AuNp recorded in toluene at 298K; inset of Fig. 2 indicates BH plot of the same system done in toluene. The concentration of H2-Pc is kept fixed throughout the concentration. Fig. 3. Time-resolved fluorescence decay profile of ZnPc (2.25 × 10-6 mol·dm-3) in presence of PC70BM (3.0 × 10-5 mol·dm-3), AuNp, and PC70BM (3.0 × 10-5 mol·dm-3) + AuNp mixture recorded in toluene at 298K. In Fig. 3, blue, magenta, red, cyan green, black and green colours represent blank ZnPc, ZnPc + PC70BM, ZnPc + AuNp, ZnPc + PC70BM + AuNp, instrument response function and fit, respectively; λex = 375 nm; λ em = 675 nm. Fig. 4. Hybrid-DFT (B3LYP/6-31G*) optimized geometric structure of (a) PC70BM/H2-Pc and (b) PC70BM/ZnPc complexes done in vacuo. Fig. 5. Pictorial representations of HOMOs and LUMOs of PC70BM/H2-Pc complex at its various electronic states done by Hybrid-DFT (B3LYP/6-31G*) calculations in vacuo. Fig. 6. Pictorial representations of HOMOs and LUMOs of PC70BM/ZnPc complex at its various electronic states done by Hybrid-DFT (B3LYP/6-31G*) calculations in vacuo. Fig. 7. MEP of (a) PC70BM/H2-Pc and (b) PC70BM/ZnPc complexes done by Hybrid-DFT calculations (B3LYP/6-31G*) in vacuo. Fig. 8. Particle size distribution analysis of AuNp in presence of H2-Pc (3.08 × 10-6 mol. dm-3) recorded in toluene at 298K.

21

Fig. 9. Particle size distribution analysis of AuNp in presence of ZnPc (3.80 × 10-6 mol. dm-3) recorded in toluene at 298K. Fig. 10. Photographs obtained from fluorescence microscope measurements for (a) blank H2-Pc (5.90 × 10-6 mol. dm-3), (b) H2-Pc (5.90 × 10-6 mol. dm-3) + AuNp; (c) H2-Pc (5.90 × 10-6 mol. dm-3) + PC70BM (6.75 × 10-5 mol.dm-3) mixture; and (d) H2-Pc (5.90 × 10-6 mol. dm-3) + PC70BM (6.75 × 10-5 mol.dm-3) mixture in presence of AuNp. Fig. 11. Photographs obtained from fluorescence microscope measurements for (a) uncomplexed ZnPc (5.55 × 10-6 mol.dm-3), (b) ZnPc (5.55 × 10-6 mol.dm-3)/AuNp system; (c) ZnPc (5.55 × 106

3

mol.dm-3)/PC70BM (6.75 × 10-5 mol.dm-3) system; and (d) ZnPc (5.55 × 10-6 mol.dm)/PC70BM (6.75 × 10-5 mol.dm-3)/AuNp system.

Fig. 12. (a) Transient absorption analysis of PC70BM recorded in toluene; (b) decay time profile plot of PC70BM observed at 700 nm.

22

2.5

2000

2.0

1600

F0/(F0-F)

Fluorescence intensity, a.u

1800

1400 1200 1000

1.5 1.0 0.5

800

0.0 0.0

600

5.0x10

400

4

1.0x10 3

5

1/[PC70BM], dm .mol

200

1.5x10

5

-1

0 660

670

680

690

700

710

720

730

740

750

Wavelength, nm

Fig. 1 2

2.5 2.0

A0/∆A

1.6

A b s.

1.2

1.5 1.0 0.5 0.0

0.8

4

4

4

4

2.0x10 4.0x10 6.0x10 8.0x10 1.0x10 3

5

-1

1/[PC70BM], dm .mol

0.4 0 300

400

500 600 Wavelength, nm

Fig. 2

23

700

800

Fig. 3

(a)

(b) Fig. 4

24

Fig. 5

25

Fig. 6

26

Fig. 7

4

Intensity (%)

3

2

1

0 0

20

40

Size (nm)

Fig. 8

Fig. 9

27

60

80

100

(a)

(b)

(c)

(d)

Fig. 10

(a)

(b)

(c)

(d)

Fig. 11

28

(a) 0.040 0.035 0.030 0.025

decay at 700nm decay tim e 14 µ s

∆Abs

0.020 0.015 0.010 0.005 0.000 -0.005 0

10

20

Time, µ s

(b) Fig. 12

29

30

40

50

SCHEME 1

SCHEME 2

30

Scientific Highlights:
Fullerene- Pc-AuNp supramolecular interaction is established in solution;
Considerable decrease in binding for PC70BM/H2-Pc system in presence of AuNp;
Efficient quenching of the photo-excited singlet state of Pc in presence of AuNp;
Transient absorption studies envisage energy transfer mechanism.

31

Graphical Abstract:

32