Spectroscopic and Theoretical Investigations on Effective and

Apr 8, 2010 - Chemistry, UniVersity of Hyderabad, Hyderabad, 500 046, AP, India, and ... state interaction between fullerenes and 1 is first evidenced...
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Spectroscopic and Theoretical Investigations on Effective and Selective Interaction of Fullerenes C60 and C70 with a Derivatized Zn-phthalocyanine: Stabilization of Charge-Recombined State by Side-On Approach of C70 A. Ray,† K. Santhosh,‡ S. Chattopadhyay,§ A. Samanta,‡ and S. Bhattacharya*,† Department of Chemistry, The UniVersity of Burdwan, Golapbag, Burdwan, 713 104, India, School of Chemistry, UniVersity of Hyderabad, Hyderabad, 500 046, AP, India, and Bio-Organic DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai, 400 085, India ReceiVed: February 10, 2010; ReVised Manuscript ReceiVed: March 12, 2010

The photophysical aspects of noncovalently linked fullerenes C60 and C70 with a designed metallophthalocyanine, namely, zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (1) have been investigated employing various spectroscopic tools such as UV-vis absorption spectrophotometry, steady state and time-resolved fluorescence, along with solution state IR measurements in toluene medium. The ground state interaction between fullerenes and 1 is first evidenced from UV-vis measurements. Binding constants (K) for the complexes of C60 and C70 with 1 are determined to be 13 235 and 27 670 dm3 · mol-1, respectively. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C60 and C70. Emission studies reveal K values of 8945 and 19 175 dm3 · mol-1 for C60/1 and C70/1 complexes, respectively. The magnitude of K suggests that 1 preferentially binds C70 in comparison to C60 as average selectivity in K of C70 over C60 is enumerated to be 2.0. Time resolved emission measurements establish that C70/1 complex is stabilized much more in comparison to C60/1 complex in terms of charge recombination. Liquid IR studies provide very good support in favor of ground state complexation between fullerenes and 1. Molecular mechanics calculations at force field model and semiempirical calculations employing third parametric method substantiate the strong binding between C70 and 1, and at the same time, determine the orientation of bound guest (here C70) within the cavity of 1. 1. Introduction In the natural photosynthesis reaction the energy transfer and electron transfer events occur between highly organized supramolecular assemblies, that is, light-harvesting antenna ensemble and photosynthetic reaction center.1 Motivated by this vision, much attention has been paid to prepare artificial photosynthetic reaction center building blocks that are capable in forming of light-induced charge separated state.2 Phthalocyanine3 (Pc), a structural analogue of azaporphyrin, exhibits numerous outstanding physicochemical properties,4,5 which makes it an appealing candidate in a variety of scientific and technological areas.6 Pc consists of versatile and stable aromatic chromophores that are very much responsible in playing a crucial role in different fields of materials science.7 This includes the application of phthalocyanines (Pcs) in the formation of liquid crystal,8 building Langmuir-Blodgett films,9 devising nonlinear optics,10 in the construction of organic solar cell,11 as active components in gas sensors,12 in devising electrochemical devices,13 producing field effect transistors,14 and in the development of photovoltaic cells.15 In this connection, metallophthalocyanines (MPcs) have also been studied extensively since the beginning of this century.16 Among all of the MPcs, ZnPc is ideally suited for the characterization of the excited state of the Pc moiety.16 It is also verified that the peripheral substituents in the Pcs influence the properties for both Pc and * To whom correspondence should be addressed. E-mail: sum_9974@ rediffmail.com. Fax: +91-342-2530452. † The University of Burdwan. § Bhabha Atomic Research Centre. ‡ University of Hyderabad.

MPc.17-19 The above features of Pc or MPc stimulate us to employ it as a promising candidate for the construction of a supramolecular recognition element. On the other hand, fullerenes C60 and C70 exhibit a number of interesting electronic20 and photophysical21 properties. They can successively accept six electrons, which makes them suitable candidates in building donor-acceptor model compounds for the study of electron transfer processes.22,23 Fullerenes are also employed as suitable building blocks for the development of multicomponent supramolecular systems because of their threedimensional structure24 and energetically favorable first reduction potentials.25 Low reorganization energy26,27 of fullerenes is very much responsible for the acceleration of the photoinduced charge separation and deceleration of charge recombination.28 Considering all of the above-mentioned properties of both fullerenes and Pcs, we can anticipate that the use of Pc in forming fullerene-based donor-acceptor ensembles will certainly create a new dimension in noncovalent chemistry of fullerenes. So far, several studies have already been performed to examine the photophysical behavior of a covalently linked fullerenephthalocyanine dyad.29-32 However, in view of the abovementioned applications of the fullerene/Pc covalent assembly, it is very much desirable to explore various types of fullerene/ Pc noncovalent interaction after selecting designed Pc as a suitable host molecule or recognition element. Studies on noncovalently linked fullerene/phthalocyanine composite, however, are rare. Very recently, our research group substantiates the role of electrostatic interaction in fullerene/Pc noncovalent assembly by choosing very simple unsubstituted free-base, that is, H2- and Zn-Pc molecules.33 The fullerene/phthalocyanine

10.1021/jp1012818  2010 American Chemical Society Published on Web 04/08/2010

Interaction of Fullerenes with Zn-Phthalocyanine

Figure 1. Structure of 1.

noncovalent interaction is also validated in terms of the complexation process of a C60 derivative, namely, tert-butyl(1,2-methanofullerene)-61-carboxylate with H2- and Zn-Pc in toluene.34 Very recently, Abe et al. have nicely demonstrated the photoanodic characteristics of an organic p/n bilayer of fullerene (C60, n-type semiconductor) and metal-free phthalocyanine (H2Pc, p-type semiconductor) in water phase.35 However, an interesting scenario may arise if we take substituted Pc or MPc in which the substituted groups (preferably nonpolar in nature) will form an octopus-like embrace surrounding the Pc or MPc. In this way we can anticipate some different type of binding motif between fullerenes and Pc or fullerenes and MPc. The goal of our present investigations is to study the interaction between fullerenes, namely, C60 and C70, with a derivatized Zn-Pc molecule, namely, 1 (Figure 1) employing various spectroscopic tools such as UV-vis absorption spectroscopic technique, steady state and time-resolved fluorescence, and solution IR measurements in toluene medium. Molecular mechanics calculations at force field model (MMMF) and semiempirical calculations at third parametric level (PM3) interpret very nicely the stability differences between C60 and C70 complexes of 1. We anticipate that the substituted Zn-Pc molecule may impart some novel photophysical characteristics that would be very helpful in interpreting the binding motif of fullerene toward phthalocyanine in fullerene/phthalocyanine noncovalent interactions.

J. Phys. Chem. A, Vol. 114, No. 17, 2010 5545 model proposed by Gouterman et al.37-39 The absorption spectrum of 1 measured in toluene medium is shown in Figure 2. Compound 1 is characterized by the formation of a distinct absorption band at 677 nm. Electron donating ability and absorption peak position can be tuned substantially by introducing appropriate substituents in the peripheral position of Pc or MPc. In our present investigations, the Q-band exhibits ∼6 nm red shift after introducing eight electron donating octyloxy groups in the peripheral position of 1 in comparison to H2-Pc. According to Gouterman’s four orbital model,40 this red shift in Pc is originated from the narrow band energy gap caused by an increase in highest occupied molecular orbitals (HOMO) energy level due to the presence of eight electron donating octyloxy groups. Evidence in favor of ground state electronic interaction between fullerene and 1 first comes from UV-vis absorption measurements. It is observed that appreciable color change takes place from magenta to deep blue and wine red to olive green for C60/1 and C70/1 complexes (in toluene medium), respectively. However, we failed to locate the charge transfer absorption bands in the visible region. Formidable support in favor of ground state interaction between fullerenes and 1 comes from the UV-vis titration experiment. The titration experiment is performed by keeping the concentration of 1 fixed at 1.87 × 10-6 and 3.90 × 10-7 mol · dm-3 for C60/1 and C70/1 systems, respectively, and varying the concentration of fullerenes (8.0 × 10-6 to 5.52 × 10-5 mol · dm-3). It is observed that the value of absorbance at 407 nm for C60/1 (Figure 3) and at 472 nm for C70/1 (Figure 1S) systems, are increased systematically following the gradual addition of fullerene solutions. The 407 and 472 nm absorption bands for C60 and C70 resulted from allowed and forbidden transitions, respectively.41-43 For fullerenes, the absorptions between 190 and 410 nm are due to allowed 1 T1u-1Ag transitions, whereas those between 410 and 620 nm are due to orbital forbidden singlet-singlet transitions.41,42 It should be mentioned at this point that these later absorptions in the visible region are responsible for the purple color of C60 and the deep wine red color of C70. K values of the fullerene/1 systems have been estimated in accordance with the BenesiHildebrand (BH) equation44 for cells with 1 cm optical path length as shown below.

2. Materials and Methods 1 is purchased from Aldrich, USA. C60 and C70 are collected from Sigma-Aldrich, USA. UV-vis spectroscopic grade toluene (Merck, Germany) is used as solvent to favor the intermolecular noncovalent interaction between fullerene and 1, as well as to provide good solubility and photostability of the samples. UV-vis spectral measurements have been performed on a Shimadzu UV-2450 model spectrophotometer fitted with TB85 Peltier controlled thermo-bath using quartz cell with 1 cm optical path length. Steady state emission spectra are recorded with a Hitachi F-4500 model spectrofluorimeter. Fluorescence decay experiments have been measured with a HORIBA Jobin Yvon single-photon counting setup employing Nanoled as excitation source. Liquid IR measurements are performed with a JASCO FT/IR-5300 model spectrometer. Theoretical calculations are performed on a Pentium IV computer using SPARTAN′06 V1.1.0 Windows version software. 3. Results and Discussion 3.1. UV-vis Absorption Studies. It is already established that Pcs are well characterized by their well resolved electronic absorption bands in the region of 330-700 nm and marked by formation of intense blue-green color.36 For MPc, generation of Q (i.e., S1rS0) and B (i.e., S2rS0) bands follows the orbital

[A]0[D]0/d/ ) ([D]0/d/) + (1/Kε/)

(1)

Here [A]0 and [D]0 are the initial concentrations of the acceptor (i.e., fullerenes C60 and C70) and donor (1) solutions in toluene, respectively; d/ is the corrected absorbance of the fullerene/1 mixture at the wavelength of measurement (i.e., 407 nm C60/1 and 472 nm for C70/1 mixtures) recorded against the same concentration of 1 as reference. The quantity ε/ means the corrected molar absorptivity of the complex. K is the binding constant of the fullerene/1 complex. Equation 1 is valid under 1:1 approximation for 1 and fullerene systems. It should be mentioned at this point that the corrected molar extinction coefficient, ε/, is not quite that of the complex. The BH method44 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. Excellent linear BH plots are obtained for both systems studied in the present investigations. Typical BH plots for the C60/1 and C70/1 systems are shown in Figures 3b and 2S, respectively. Values of K are listed in Table 1. Ground state interactions between fullerenes (C60 and C70) and 1 get credence from liquid IR measurements in toluene

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Figure 2. UV-vis absorption spectrum of 1 (5.56 × 10-6 mol · dm-3) recorded in toluene medium.

medium. IR studies prove that the -NH proton of uncomplexed 1 (3028.51 cm-1, Figure 3S) suffers a considerable amount of shift in the presence of C60 (3030.44 cm-1, Figure 4S) and C70 (3400.81 cm-1, Figure 5S). All these phenomena suggest that 1 effectively forms host-guest complexes with fullerenes C60 and C70. Larger amount of shift in the stretching frequency for the C70/1 complex compared to C60/1 complex, namely, ∼370 cm-1, provides very good support in favor of larger value of K in the case of the former system. 3.2. Steady State and Time Resolved Fluorescence Investigation. The luminescence behavior of 1 is characterized by the appearance of fluorescence maxima at 686 nm and two humps at 714 and 758 nm in toluene medium. With the gradual addition of variable concentration of fullerene to the solution of 1, the emission intensity of 1 decreases substantially. The decrease in intensity of fluorescence for C70/1 complex (Figure 4a) is found to be much greater compared to that of C60/1 complex (Figure 6Sa). This observation indicates greater binding between C70 and 1 in comparison to that between C60 and 1. It should be mentioned at this point that a purely diffusion-driven process is ruled out, on the basis of the applied fullerene concentration. On the basis of the aforementioned results, we reach the conclusion that in the fullerene/1 complexes, the excited singlet state of 1 is quenched by the addition of electronaccepting C60 and C70. For fullerene/phthalocyanine supramolecular system, the formation of fullerene•-/phthalocyanine•+ is already reported45,46 due to the predominant character of electrostatic interaction behind the formation of such a supramolecular ensemble. However, in our present investigations, the role of dispersive forces associated with π-π interaction can not be ignored as selectivity in the K value between C70 and C60 complexes of 1 is estimated to be ∼2.0. From these observations we can anticipate that the fullerene core gets close contact with the phthalocyanine molecule. K values of the fullerene/1 complexes are determined according to modified BH equation44 and are listed in Table 1. Fluorescence BH plots for the C60/1 and C70/1 systems are shown in Figures 6Sb and 4b, respectively.

Apart from steady state fluorescence experiments, we have performed detailed nanosecond time-resolved fluorescence measurements for all the fullerene/1 complexes in toluene medium. The titration experiment has been carried out at a fixed concentration of 1 and variable concentration of C60 and C70. The time-resolved fluorescence of the phthalocyanine 1 reveals a single exponential decay with a lifetime of 3.8 ns (Figure 5i). It is observed that upon the gradual addition of fullerenes C60 and C70, there is practically no change in the lifetime compared to the uncomplexed 1 and monoexponential decay is followed (Figures 5ii and 5iii). A plot of τ0/τ versus [fullerene] should give a straight line parallel to the x-axis, that is, [fullerene]. Here, τ0 is the lifetime of 1 in absence of quencher, namely, fullerene; τ is the lifetime of 1 in presence of quencher; and [Q] is the quencher concentration. In our present case, excellent linear plots having very small slope values have been obtained for C60/1 and C70/1 systems. Variation of τ0/τ versus concentration of the quencher for the fullerene/1 systems are shown in Figure 7S. Lifetime data of C60/1 and C70/1 systems are tabulated in Table 1S. From the lifetime experiment, the rate constant for charge separation (kCS) is calculated using the equation:

kCS ) (1/τ)complex - (1/τ)ref

(1)

where τcomplex and τref are the lifetime of the fullerene/1 complexes and 1, respectively. The quantum yield of the chargeseparated state (ΦCS) is determined according to the equation:

ΦCS ) [(1/τ)complex - (1/τ)ref]/(1/τ)complex

(2)

The estimated values of kCS and ΦCS are summarized in Table 2. Table 2 indicates efficient charge separations for the presently investigated supramolecules. A similar sort of phenomenon is already observed for other fullerene/phthalocyanine supramo-

Interaction of Fullerenes with Zn-Phthalocyanine

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Figure 3. (a) UV-vis titration experiment of C60/1 complex recorded in toluene medium against the pristine donor solution kept at constant concentration of 1.87 × 10-6 mol · dm-3; (i) indicates the spectrum of uncomplexed C60 having concentration of 5.50 × 10-6 mol · dm-3; the concentration of C60 increases (from bottom to top) in the C60/1 mixture are given as follows (ii) 1.65 × 10-5, (iii) 2.20 × 10-5, (iv) 2.75 × 10-5, (v) 4.40 × 10-5, and (vi) 5.52 × 10-5 mol · dm-3; (b) BH plot for the C60/1 system recorded in toluene medium.

TABLE 1: Binding Constants (K) Estimated by UV-Vis and Steady State Fluorescence Measurements for the Complexes of 1 with C60 and C70 along with the Selectivity in K between C70 and C60 (KC70/C60)a K (dm3 · mol-1)

K70/K60

system

UV-vis

fluorescence

UV-vis

fluorescence

C60/1 C70/1

13 235 27 670

8945 19 175

2.10

2.15

a

T ) 298 K.

lecular complexes.47,48 The values of both kCS and ΦCS are higher for C60/1 complex compared to C70/1 complex, which warrants estimation of some more physicochemical parameters for enabling us to explain this interesting feature. For this reason, we have determined the value of free energy of charge separation (∆GCS) and charge recombination (∆GCR) for the fullerene/1 complexes in our present investigations. The most significant

and interesting aspect of the present investigations is that the C70/1 complex is stabilized much more in comparison to the C60/1 complex in terms of charge recombination. This is quite expected as the value of K is much higher in the case of the C70/1 complex. We may anticipate that strong π-π interactions may arise between C70 and 1, which is quite unnatural in fullerene/Pc interactions where predominantly electrostatic interaction control the binding motif between these supramolecular entities.33,34 Thus, it is very much essential to elucidate all the stereoscopic structures of the fullerene/1 complexes in our present investigations by the help of suitable quantum chemical calculations. 3.3. Binding Constants and Theoretical Calculations. Table 1 lists the K values of various fullerene/1 complexes. It is observed that for all the complexes studied, C70 exhibits a two times higher selectivity value of K in comparison to C60. Such a stabilization of the C70/1 supramolecular complex can

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Figure 4. (a) Steady state fluorescence spectral variation of 1 (3.46 × 10-6 mol · dm-3) in the presence of C70 in toluene medium; the concentrations of C70 from top to bottom are as follows: 0, 7.27 × 10-6, 9.09 × 10-6, 10.92 × 10-6, 12.73 × 10-6, 14.55 × 10-6, 16.37 × 10-6, and 18.18 × 10-6 mol · dm-3; (b) BH fluorescence plot of the C70/1 system in toluene medium.

be attributed due to the presence of additional intermolecular interaction between the graphitic 6:6 plane of C70 and the flat π-region of 1 macrocycle. Primarily, the attractive interactions between C70 and 1 is driven by the presence of dispersive forces associated with π-π interactions. The most concrete evidence of the above statement is illustrated by the side-on rather than end-on binding of C70 with 1. Both MMMF and PM3 calculations well reproduce the above feature regarding orientation of bound guest (here C60 and C70) with the plane of the 1 receptor. For example, in the case of C70/1 complex, the side-on interaction of C70 with 1 generates enthalpies of formation (∆Hf°) value of -2.070 kJ · mol-1, whereas ∆Hf° is determined to be -0.135 kJ · mol-1 in its end-on orientation employing PM3

calculations. Thus, C70/1 complex gains ∼1.935 kJ · mol-1 of extra stabilization energy when it approaches the cavity of 1 in a side-on manner rather than in its end-on orientation. It is already well established that in fullerene/porphyrin complexes, the 6:6 ring-juncture bond of the C70, rather than 6:5 ringjuncture bond, lies closest to the porphyrin plane,49-51 as the 6:6 “double” bonds of C70 are more electron-rich than 6:5 “single” bonds. Thus, in our present work, the equatorial face of C70 is centered over the electropositive center of the Pc plane, which can be viewed as an enhancement in van der Waals interaction due to availability of greater surface area favoring strong π-π interactions. Stereoscopic structures for all the fullerene/1 complexes done by PM3 calculations are visualized

Interaction of Fullerenes with Zn-Phthalocyanine

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Figure 5. Time-resolved fluorescence decay profiles of (i) uncomplexed 1 (1.0 × 10-5 mol · dm-3), (ii) C60 (42.85 × 10-5 mol · dm-3) + 1 (1.0 × 10-5 mol · dm-3) mixture, and (iii) mixture of C70 (21.43 × 10-5 mol · dm-3) with 1 (1.0 × 10-5 mol · dm-3) recorded in toluene medium, and indicated by red, blue, and pink color, respectively. The instrument response function is also shown.

TABLE 2: Rate Constants of Charge Separation (kCS), Quantum Yield for Charge Separation (ΦCS), Free Energies of Charge Separation (∆GCS), and Charge Recombination (∆GCR) for the Complexes of 1 with C60 and C70 Recorded in Toluene Mediuma system

kCS (sec-1)

ΦCS

∆GCS (eV)

∆GCR (eV)

C60/1 C70/1

1.0 × 10 5.0 × 106

0.034 0.017

-4.95 -4.77

-8.10 -8.31

a

7

T ) 298 K.

TABLE 3: Enthalpies of Formation (∆Hf°) Value for the C60 and C70 Complexes of 1 Done by MMMF and PM3 Methods ∆Hf° (kJ · mol-1) system

MMMF

PM3

C60/1 C70/1

-174.740 -472.160 (end-on) -484.720 (side-on)

-1.480 -0.135 (end-on) -2.070 (side-on)

in Figure 6. As is evident, the C70/1 complex exhibits a much lower value of ∆Hf° in comparison to the C60/1 complex revealed by both MMMF and PM3 calculations. This is because the electron density around the C60 surface remains symmetrical due to its truncated icosahedron structure having point group of Ih22 and also from the fact that in donor/acceptor type host/ guest interaction, C70 has the upper hand due to its higher value of electron affinity compared to C60.52 MMMF calculations reproduce well the above feature in terms of measured bond distances between the fullerenes plane and 1. Thus, the distance between the Zn atom of 1 and the nearest carbon atom of C60 (DZn1-C147(C60)) is calculated to be 3.772 Å, whereas the nearest distance between the Zn atom of 1 and C70 (DZn1-C133(C70)) is found to be 3.637 Å, only. From this we can infer that C70 approaches the cavity of 1 at a closer distance compared to C60. Similarly, the bond No. 348 of C60 is positioned at a distance of 3.702 Å, which is far greater in comparison to C70 (bond No. 322) with 1, that is, 3.591 Å. 4. Conclusions Summarizing the results of the present investigations, we reach the following conclusions: Both C60 and C70 undergo

Figure 6. Stereoscopic structures of (a) C60/1, (b) C70/1 (in side-on orientation of C70), and (c) C70/1 (in end-on orientation of C70) systems, done in vacuo employing semiempirical PM3 calculations.

effective ground state complexation with a designed Pc derivative, namely, 1, in toluene medium. It is observed that the intensity of the absorption maxima at 407 and 472 nm for C60 and C70 solutions, respectively, increase systematically with the increasing concentration of the acceptor. This phenomenon has been utilized nicely to elucidate the values of K for the complexes of fullerenes with 1 in our present investigations. Steady state fluorescence studies reveal electron transfer from the excited fluorophore, that is, 1, to the fullerenes entities for all the investigated supramolecular complexes. Steady state fluorescence quenching studies also enable us to estimate the values of K of the complexes of 1 with both C60 and C70. Inspections of K values reveal that C70 exhibits moderate value of selectivity toward 1 in toluene medium. Both UV-vis and steady state fluorescence measurements corroborate fairly well

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with each other in terms of selectivity in the value of binding constant. The free-energy calculations on charge recombination evoke that electron transfer from the excited 1 to the fullerenes in the fullerene/1 complexes is an unlikely process. At the same time, it establishes that side-on interaction of C70 with 1 is very feasible in our present investigations. The geometry and electronic structures of the supramolecular complexes are visualized by both MMMF and PM3 calculations. Large negative ∆Hf° value for the C70/1 complex and larger value of the same thermo chemical parameter for the C70/1 complex indicates that, other than the electrostatic mechanism, dispersive forces associated with π-π interactions also play a vital role in forming a strong ground state noncovalent complex between C70 and 1. Finally, we can say that the results emanating from the present investigations will open up new possibilities for modulating the photophysical characteristics of functionalized fullerene/Pcs host-guest systems. Acknowledgment. A.R. acknowledges The University of Burdwan for providing a junior research fellowship to her through the state-funded project of the Government of West Bengal, India. This work is financially supported by the Department of Science and Technology, New Delhi, India, through Fast Track scheme of Sanction No. SR/FTP/CS-22/ 2007. The authors also wish to record their gratitude to the Editor and the learned reviewers for making valuable comments. Supporting Information Available: Lifetime data of 1 in absence and presence of varying concentrations of C60 and C70 at 298K, UV-vis titration experiment of C60/1 complex recorded in toluene medium against the pristine donor solution, BH plot of C60/1 system in toluene medium, IR spectrum of 1 in absence and presence of C60 and C70, steady state quenching experiment of C60/1 system in terms of respective fluorescence spectrum of 1 in absence and presence of C60, BH fluorescence plot of C60/1 system and variation of relative lifetime of 1 in absence and presence of fullerenes C60 and C70 in toluene medium are given as Table 1S and Figures 1S-7S, respectively. Table 1S and Figures 1S-7S are provided as Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: New York, 1993. (b) Molecular Mechanisms of Photosynthesis; Blankenship, R. E., Ed.; Blackwell Science: 2002. (2) D’Souza, F.; Chitta, R.; Ohkubo, K.; Tasior, M.; Subbaiyan, N. K.; Zandler, M. E.; Rogacki, M. K.; Gryko, D. T.; Fukuzumi, S J. Am. Chem. Soc. 2008, 130, 14263. (3) (a) The Porphyrin Hand Book; Kadish, K. M.; Smith, K. M.; Guillard, R.; Eds.; Academic Press: San Diego, 2003, Vols 15-20. (b) Hanack, M.; Heckmann, H.; Polly, R. In Methods in Organic Chemistry; Schaumann, E., Ed.; Thieme: Stuttgart, 1998; Vol. E 9d, p 717. (c) de la Torre, G.; Nicolau, M.; Torres, T. In Phthalocyanines: Synthesis, Supramolecular Organization and Physical Properties; Nalwa, H. S., Ed.; Academic Press: New York, 2001. (d) de La. Torre, G; Vazquez, P.; Agullo-Lopez, F.; Torres, T. J. Mater. Chem 1998, 8, 1671. (e) de la Torre, G.; Va´zquez, P.; Agullo′-Lo′pez, F.; Torres, T. Chem. ReV. 2004, 104, 3723. (4) Claessens, C. G.; Hahn, U.; Torres, T. Chem. Record 2008, 8, 75. (5) Gonza´lez-Rodrı´guez, D.; Bottari, G. J. Porphyrins Phthalocyanines 2009, 13, 624. (6) Guldi, D. M.; Zilbermann, I.; Gouloumis, A.; Va′zquez, P.; Torres, T. J. Phys. Chem. B 2004, 108, 18485. (7) De, L. T. G.; Claessens, C. G.; Torres, T. Chem. Commun. 2007, 2000. (8) Makhseed, S.; Bumajdad, A.; Ghanem, B.; Msayib, K.; McKeown, N. B. Tetrahedron Lett. 2004, 45, 4865.

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