8864
J. Phys. Chem. B 1999, 103, 8864-8869
Photoinduced Charge Separation and Stabilization in Clusters of a Fullerene-Aniline Dyad K. George Thomas,*,†,‡ V. Biju,† D. M. Guldi,*,‡ Prashant V. Kamat,*,‡ and M. V. George*,†,‡,§ Photochemistry Research Unit, Regional Research Laboratory (CSIR), TriVandrum 695 019, India, Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556-0579, and Jawaharlal Nehru Centre for AdVanced Scientific Research, Bangalore 560 012, India ReceiVed: May 14, 1999; In Final Form: August 10, 1999
Fullerene-bridge-aniline dyad and the model fulleropyrrolidine compound form stable, optically transparent clusters in mixtures (3:1) of acetonitrile and toluene. Ground- and excited-state properties of the clusters of the dyad and the model compound are compared with their corresponding monomeric forms. Clustering of the dyad as well as the model compound exhibits a red-shifted emission maximum (λmax ∼ 738 nm) compared to their monomeric forms (λmax ∼ 714 nm). The electron transfer from the appended electron donor moiety to the parent fullerene core in the dyad cluster is evident from the decreased (∼80%) fluorescence yield. The formation of fullerene radical anion (absorption maximum at 1010 nm) with a lifetime of several hundreds of microseconds was further confirmed using nanosecond laser (337 nm) flash photolysis experiments. In contrast, the dyad molecules in their monomeric form did not yield any detectable yield of C60 radical anion following laser pulse excitation. The failure to observe any charge-transfer intermediates following laser pulse excitation, even in polar solvents such as benzonitrile or nitromethane, suggested that fast back-electrontransfer process must be operative in the monomeric dyad system. On the other hand, clustering of the fullerenebased dyads in a mixed-solvent system can provide a unique way to decrease the rate of back electron transfer, thus stabilizing the electron-transfer products.
Introduction
CHART 1
A great deal of interest has been shown in recent years to understand the photophysical and photochemical properties of fullerene-donor-based dyad systems.1,2 Of particular interest are the inter- and intramolecular photoinduced electron-transfer processes, which have potential applications in photovoltaic and optoelectronic devices.3-6 Several fullerene-based donoracceptor systems containing porphyrins,7-9 phtholocyanines,10 ruthenium complexes,11 ferrocenes,12 and anilines13,14 as donors have been synthesized for achieving photoinduced charge separation in these dyads. It has been recently demonstrated that pristine fullerenes, C60 and C70, as well as some water-soluble derivatives of C60 bearing charged functional groups form optically transparent microscopic clusters (aggregates) in mixed solvents at room temperature.2a,15-19 Instantaneous clustering in this class of molecules is mainly ascribed to the strong three-dimensional hydrophobic interactions between individual fullerene units. Desirable conditions15 for the formation of the pristine C60 clusters as well as their spectroscopic15,16 and size distribution studies15c,17 are well documented. To date no effort has been made to investigate electron-transfer processes in the clusters of fullerene-based donor-acceptor systems. Clusters of the dyad can be visualized as a self-assembled photoactive antenna system containing hydrophobic clusters of fullerene as the central core with appended donor groups (e.g., aniline moiety) as the surrounding shell. Photoinduced electron transfer in such a self-assembled core-shell system can provide useful information on the dynamics of various electron-transfer processes in clusters. †
RRL, Trivandrum, India. Notre Dame Radiation Laboratory,
[email protected]. § Jawaharlal Nehru Centre for Advanced Scientific Research. ‡
In an earlier publication, we have reported the synthesis and excited-state properties of two fulleropyrrolidine derivatives (Chart 1).14a As compared to the model compound (fulleropyrrolidine having three phenyl groups in the 1,2,5-positions), the fullerene-aniline dyad showed charge-transfer interactions in the ground and excited states. These charge-transfer interactions were dependent on the solvent polarity. Previous studies, which addressed the issue of charge separation in fullerenebased dyad systems, have reported only short-lived (∼200 ns) electron-transfer products.2d-f The quick recombination of photogenerated charge-transfer products was a limiting factor in achieving charge stabilization in these donor-acceptor systems. In the present study, we have considered an effective approach of forming clusters of donor-acceptor dyad (Chart 1) in toluene-acetonitrile solvent mixtures, for achieving charge stabilization. The results are compared with a model compound that does not contain a donor moiety (Chart 1). To the best of our knowledge this is first such report of achieving long-lived intramolecular charge-transfer products in fullerene-based dyads. Mechanistic details of the photoinduced electron transfer of dyad in homogeneous medium as well as clusters in mixed solvents have been investigated using laser flash photolysis.
10.1021/jp9915679 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/04/1999
Clusters of a Fullerene-Aniline Dyad
J. Phys. Chem. B, Vol. 103, No. 42, 1999 8865
TABLE 1: Absorption and Emission Properties of Monomers and Clusters of the Dyad and Model Compound compound
solventa
extinction coefficient (470 nm) (M-1 cm-1)
λmax(em) (nm)
relative emission intensityb
model compd dyad model compd dyad
toluene toluene toluene-acetonitrile (1:7) toluene-acetonitrile (1:7)
0.40 × 104 0.24 × 104 2.96 × 104 1.86 × 104
714 713 737 738
c c 100 19
a Solvent ratios are based on volume/volume (v/v). b Emission intensities (at 738 nm) were measured by matching the optical density (0.35) of clusters. c The absolute fluorescence quantum yield (77 K) of the model compound12,13b and dyad14 were reported as 6 × 10-4 and 4.2 × 10-4, respectively.
Figure 1. (A) Effect of addition of acetonitrile on the absorption spectra in a toluene solution of the dyad (30 µM): Acetonitrile (v/v) (a) 80%, (b) 75%, and (c) 60%. Inset shows the absorption spectrum of dyad (30 µM) in toluene. (B) Absorption spectra of the dyad at different concentrations in a mixture (3:1) of acetonitrile and toluene: (a) 68 µM; (b) 51 µM; (c) 34 µM; (d) 17 µM.
Results and Discussion Characteristics of the Fullerene Clusters. Both the dyad and model compound possess a strong absorption in the UV with an extended tail absorption in the visible region. In nonpolar solvents such as toluene, the absorbance of both these compounds follows the Beer-Lambert law up to a concentration of 50 µM, thus ruling out the possibility of aggregate formation. On the other hand, the presence of polar solvents such as acetonitrile induces clustering of fullerene moieties. The cluster formation of the dyad and model compound was probed by monitoring their absorption spectra in mixtures of acetonitrile and toluene (Figure 1A) and as a function of substrate concentration (Figure 1B). Such a clustering phenomenon is facilitated by the fact that the hydrophobic nature of the fullerene moiety decreases the solubility, with increasing fraction of polar solvent. Stable clusters of C60 and C70 were prepared earlier using a variety of different methods.15-19 In the present study, we have adopted a “fast addition method”15a-c for the preparation of the clusters by injecting toluene solutions of dyad (or model compound) into acetonitrile. The dyad (and model compound) molecules exist as monomers in acetonitrile-toluene mixtures
containing 0-60% (v/v) acetonitrile. However, sudden changes in the absorption properties were noted as the content of acetonitrile in the solvent mixture was increased beyond 60%. Solvent-induced clustering was evident, for both these compounds, in acetonitrile-toluene mixtures containing more than 75% (v/v) acetonitrile. Figure 1A shows the effect of varying the acetonitrile content (60-80% (v/v)), on the absorption spectrum of the dyad (at a constant substrate concentration of 30 µM). Upon increase of the acetonitrile content, the spectra in the 400-750 nm region were transformed into a featureless and broad absorption (note that, at high acetonitrile content, the light scattering is also a contributing factor). Parallel to these changes, a significant increase in molar extinction coefficients (Table 1) in acetonitrile-toluene solvent mixture was noticed. These optically transparent clusters are quite stable at room temperature, particularly in 75% (v/v) acetonitrile-toluene mixtures, and can be readily reverted to their monomeric analogue when diluted with toluene. Spectral properties of the monomer and cluster forms of the dyad and model compound are summarized in Table 1. The absorption spectra of dyad in 75% (v/v) acetonitriletoluene mixture recorded at different dyad concentrations are shown in Figure 1B. The changes in optical density observed at different concentrations of dyad were nonlinear, thus indicating the cluster formation in the mixed solvents. The absorption spectra of the dyad and the model compound in toluene possess a sharp band at 430 nm, which is characteristic of [6,6]-closed fullerene monoadducts, and a weak band around 700 nm (inset of Figure 1A). The latter absorption band corresponds to the spin-allowed 0-0 transition from the singlet ground state to the singlet excited state. Assignments have been made to the specific transitions throughout the UV-Vis, including the 430 and 700 nm bands, of a few monofunctionalized fullerene derivatives (methanofullerenes).20 In the case of the clusters of the dyad (and model compound), these specific transitions are not observed and may possibly be merged inside the broad envelope. The clusters of the dyads were further characterized using the dynamic light scattering method. In a mixture (3:1) of acetonitrile-toluene, the size distribution of clusters was found to be narrow (120-200 nm), with a mean diameter of 175 nm (Figure 2). The sizes of these clusters are similar to those reported for the C60 clusters at different experimental conditions.15c,17 Such a large cluster diameter indicates that each cluster is composed of several hundreds of fullerene moieties. Yet, the suspension remains optically transparent and stable for spectroscopic measurements. Emission Characteristics of the Dyad Clusters. The fluorescence and phosphorescence spectra of monofunctionalized pyrrolidine derivatives of fullerene have been examined in detail.2a,13 At room temperature, they exhibit a characteristic 0-0 emission band, which is associated with a spin-allowed transition between the lowest vibrational state of the singlet excited state and the singlet ground state. The maximum of this
8866 J. Phys. Chem. B, Vol. 103, No. 42, 1999
Thomas et al. SCHEME 1
Figure 2. Particle size distribution of the dyad cluster in a tolueneacetonitrile (3:1) mixture.
Figure 3. (A) Normalized emission spectra of the model compound (30 µM) in (a) toluene and (b) 12.5% (v/v) toluene-acetonitrile. (B) Emission spectra of the optically matched solutions of the dyad and the model compound: (a) dyad; (b) model compound in toluene (absorbance of 0.35 at the excitation wavelength (470 nm)).
emission band is typically at 710 nm, accompanied by a shoulder around 790 nm. The emission shoulder has been attributed to the transition to a higher vibrational level of the singlet ground state. We investigated the effect of clustering on the emission properties of both dyad and model compound. In general, clustering leads to a bathochromic shift in their emission maximum for all the compounds under investigation (Table 1). As a representative example, the normalized emission spectra of the monomeric and clustered forms of the model compound are compared in Figure 3A. Clusters of both compounds under investigation exhibit emission in the same spectral region (λmax ∼ 740 nm). On the basis of the electron-donating ability of aniline and the electron-accepting properties of the fullerene core, we expect the electron transfer to proceed within the dyad cluster, thus directly influencing the emission properties. The emission properties of the dyad and model compound were compared using optically matched solutions (absorbance at 470 nm, the wavelength of excitation, was adjusted to 0.35), and their relative fluorescence yields are summarized in Table 1. A substantial decrease in the fluorescence quantum yield observed
for clusters of dyad (Figure 3B) supports the hypothesis that the intramolecular electron transfer directly competes with the radiative deactivation of the excited singlet state. As compared to toluene, the increased polarity of an acetonitrile-toluene mixture thermodynamically favors such an electron transfer. An earlier study14a of the dependence of fluorescence quantum yield of dyad monomers on the solvent polarity has shown that nearly 60% fluorescence quenching is achieved in a mixture (1:1) of acetonitrile and toluene. In the present study formation of dyad clusters leads to a large bathochromic shift in emission maximum (∼25 nm) accompanied by a substantial decrease in the fluorescence quantum yield (Table 1). An illustration of self-assembled clusters of the dyad molecules is shown in Scheme 1 (please note that each cluster unit contains several hundreds of dyad molecules and the scheme indicates a simplified representation). The transfer of an electron from the aniline moiety to the covalently linked fullerene moiety, thus, is quite efficient as indicated from the quenched singlet excited state. The obvious question would then arise is whether the transferred electron remains on the parent dyad or can hop around to adjacent fullerene moieties. Dynamics of Photoinduced Charge Transfer in Dyad Clusters. The dynamics of photoinduced electron transfer in clusters of the dyad was further probed using picosecond and nanosecond laser flash photolysis experiments. In neat solvents such as toluene, fullerenes and functionalized fullerenes exhibit a broad absorption around 920 nm, following the UV or visible excitation. This absorption band is a characteristic spectral feature of the S1-Sn transition of the singlet excited state. The excited singlet state behavior of the fullerene clusters in acetonitrile-toluene mixture was significantly different than that observed in toluene. The difference absorption spectra recorded immediately after the 532 nm laser pulse (pulse width 18 ps) excitation for the clusters in deoxygenated acetonitrile-toluene mixtures do not exhibit any prominent absorption bands in the 700-960 nm region (Figure 4). However, with increasing time, the cluster solutions of dyad as well as model compound exhibit an absorption growth throughout the UV-Vis-NIR region. Time-resolved transient absorption spectra shown in Figure 4 indicate the growth of such a broad absorption band, which continues up to several nanoseconds. Close packing of the fullerene moieties in these clusters facilitates excited-state interactions, which possibly induce excited-state charge-transfer interactions. Such excited state interactions are likely to dominate in the case of the dyad in which fullerene and aniline moieties participate in a photoinduced charge transfer. The spectral changes noted for the dyad cluster are different from those of the triplet excited state, but they are similar to those
Clusters of a Fullerene-Aniline Dyad
Figure 4. Difference absorption spectra recorded after 532 nm laser pulse excitation of the 30 µM dyad in 12.5% (v/v) toluene-acetonitrile.
J. Phys. Chem. B, Vol. 103, No. 42, 1999 8867
Figure 6. Difference absorption spectra (near-infrared region) recorded after 337 nm laser pulse excitation of dyad (30 µM) in a deoxygenated toluene-acetonitrile (1:3) mixture. The inset shows the enlarged view of difference absorption spectra of the dyad recorded 10 µs after excitation.
Figure 5. Difference absorption spectra (visible region) recorded after 337 nm laser pulse excitation of dyad (30 µM) in a deoxygenated toluene-acetonitrile (1:3) mixture.
reported for a charge-separated radical pair in a rigidly spaced fullerene-aniline system.13a These results, thus, indicate the possibility of an intramolecular electron transfer between the singlet excited fullerene and the aniline. The broad nature of the 700-800 nm spectral band prevented us from resolving the contribution that arises from the charge-separated pair and the triplet excited state. In an earlier study we have characterized the triplet excited state of the dyad (λmax at 700 nm) using flash photolysis, and the radical anion spectrum was generated by pulse radiolysis (λmax at 1010 nm).14a If indeed the photoinduced charge separation is feasible in the dyad, we should be able to probe the charge-transfer process by monitoring the radical anion spectrum of C60. The time-resolved absorption spectra recorded in the visible and NIR regions, following the 337 nm laser pulse excitation (pulse width 2 ns) of the clusters of dyad in deoxygenated acetonitrile-toluene (3:1) mixtures, are shown in Figures 5 and 6, respectively. The nanosecond laser flash photolysis experiments provided a means to characterize longlived transients in the photochemical reaction. The difference absorption spectra recorded 1 µs after laser flash excitation exhibit two maxima, one around 700 nm and the other around 1010 nm. The absorption band in the 700 nm region decays completely in 10 µs. On the other hand, the absorption band in the NIR region (1010 nm) exhibits a relatively long lifetime of several hundred microseconds (inset of Figure 6). The fact that the lifetimes of these two absorption bands are widely different indicates the presence of two different transient species in the difference absorption spectrum. The absorption-time profiles of the dyad cluster in deoxygenated toluene-acetonitrile (1:3) recorded at 700 and 1010 nm are shown in Figure 7. The 700 nm transient exhibits a monoexponential decay with a lifetime of about 1.8 µs, and
Figure 7. Absorption-time profiles recorded at (a) 700 and (b) 1010 nm following 337 nm laser pulse excitation of the dyad (30 µM) in a deoxygenated toluene-acetonitrile (1:3) mixture.
the 1010 nm transient exhibits a biexponential decay with lifetimes of 1.8 and 60 µs. The short-lived transient represents the triplet excited state (λmax at 700 nm) and the long-lived transient arises from the fullerene radical anion (λmax at 1010 nm). From these decay traces we obtain the lifetimes of the triplet excited state as 1.8 µs and the radical anion as 60 µs. On the basis of the steady-state fluorescence and timeresolved transient absorption studies, we can summarize the photochemical events as illustrated in Scheme 1. Excitation of dyad clusters results in the population of a singlet excited state, and two competing processes deactivate the singlet excited state, viz., intersystem crossing to the triplet excited state and the electron transfer from the aniline donor group to the photoexcited fullerene. We further probed the charge-recombination kinetics as a function of the dyad cluster concentration. Figure 8 shows the absorption-time decay profile at 1010 nm, recorded in deoxygenated toluene (trace a) and in deoxygenated acetonitrile-toluene, at different dyad concentrations (traces b-d). The lifetime of the fast component in the kinetic trace (traces b-d), viz., the triplet excited state, remains almost constant (∼1.8 µs) at the different dyad cluster concentrations (30-60 µM). Interestingly, the lifetime of radical anion shows a variance on the concentration of dyad cluster. When the concentration of the dyad cluster was increased to 60 µM, a decrease in the lifetime of the C60 radical anion (45 µs) is observed (trace d; Figure 8). Precipitation of the clusters at higher concentrations
8868 J. Phys. Chem. B, Vol. 103, No. 42, 1999
Figure 8. Absorption-time decay profile at 1010 nm, following 337 nm laser pulse excitation: (a) 30 µM dyad in deoxygenated toluene; (b) 30 µM; (c) 45 µM; (d) 60 µM dyad in deoxygenated acetonitriletoluene (3:1).
limited our ability to probe the contribution of inter- or intracluster interactions in the back electron transfer. The absorption spectrum of the dyad clusters remain unchanged after the laser flash photolysis and emission experiments and can be reverted to their monomeric analogue upon addition of toluene. An interesting finding of the present investigation is the ability of the dyad clusters to stabilize the electron-transfer products. Most of the previous studies in fullerene-based dyads have shown only a limited success in achieving efficient charge separation lasting for only a few hundreds of nanoseconds.2d Fast recombination between the two counterparts has often been cited as the major limiting factor in achieving charge stabilization. Since it is necessary to have well-stabilized charge-transfer products for the harvesting of light energy, researchers often employ a heterogeneous media. For example, intermolecular photoinduced electron-transfer products of fullerene and electron donors live significantly longer in polymeric6a,b and micellar21 systems. In the present study, the self-induced aggregation of dyad in the mixed solvent provides a microheterogeneous environment that facilitates charge separation for several hundred microseconds. The close network of fullerene moieties in the cluster facilitates the hopping of electrons from the parent fullerene to the adjacent molecule, thus increasing the spatial distance between the charge separated pair. An important parameter that promotes the charge separationrecombination processes is the polarity of the medium. The solvent mixture used in the present study is highly polar in nature (dielectric constant of CH3CN is 38.0). To distinguish between the effect of polarity and clustering, we investigated the effect of solvent polarity on the photophysical properties of dyad monomer. Solvents of wide range of polarity, namely toluene ( )2.38), benzonitrile ( ) 25.2), and nitrobenzene ( ) 34.8), were selected for these sets of experiments. It may be noted that the dyad molecule does not aggregate in these solvents, in the concentration range of our study (∼30 µM). Time-resolved absorption spectra of the dyad in toluene as well as benzonitrile were recorded following the 337 nm laser pulse excitation. The experiments using nitrobenzene as solvent were carried out using 532 nm laser pulse excitation (YAG laser), so that any direct excitation of the solvent was avoided. Time-resolved absorption spectra (NIR region), recorded 2.5 µs after 337 nm laser pulse excitation of the dyad in toluene, are shown Figure 9. The absorption-time profile at 700 and
Thomas et al.
Figure 9. Difference absorption spectra (near-infrared region) recorded after 337 nm laser pulse excitation of dyad (30 µM) in deoxygenated toluene. The inset shows the difference absorption spectra (near-infrared region) recorded after 337 nm laser pulse excitation of dyad (30 µM) in deoxygenated benzonitrile.
1010 nm (a representative example is shown in trace a of Figure 8) are compared. A single-exponential decay (τ )17.0 µs) was observed at both these wavelengths. The electron transfer from aniline to the singlet state of fullerene is thermodynamically favorable (∆G ) -18.2 kcal mol-1) in polar solvents.22 However, the nanosecond time-resolved absorption spectra recorded in solvents of high dielectric constant like benzonitrile (inset of Figure 9) and nitrobenzene did not show any detectable absorbance in the 1010 nm region, thus ruling out the presence of any charge-separated radical pair at our monitoring time scale (100 ns). The fast charge recombination process seems to be the reason for not observing the charge-separated state in the nanosecond time scale in neat solvents. The polarity of solvent thus plays a major role in promoting both the forward- as well as back-electron-transfer processes. By comparing the photochemical behavior of dyad in neat solvents and toluene-acetonitrile mixtures (1:3), one can infer two major conclusions: (i) The aniline-fullerene dyad molecules undergo fast intramolecular electron transfer in polar solvents, and (ii) charge stabilization is seen only in solvent mixtures that favor aggregation or clustering of dyad molecules. It is evident that the dyad molecules undergo a spontaneous self-assembly in mixed solvents in such a way that the hydrophobic fullerene moieties come together leaving the polar aniline moiety away. The self-assembled geometry of cluster framework helps the initially transferred electron to leave the parent fullerene molecule and hop to the adjacent fullerene molecule. Continued hopping of the electron from one fullerene moiety into the other increases the distance of separation of radical pair and in turn retards the charge recombination (Scheme 1). The transferred electrons thus get fully delocalized within the fullerene cluster network, while the positive charges may be localized at the aniline moiety. The results presented here demonstrate a novel method for charge stabilization, by taking advantage of the clustering behavior of C60. Conclusions Fullerene-aniline dyad, as well as the model fulleropyrrolidine chosen in this study, form optically transparent microscopic clusters in mixtures (3:1) of acetonitrile and toluene at room temperature. The manifold increase in molar extinction coefficients in the visible region makes the cluster dyads a promising candidate for the design of solar energy harvesting devices. On the basis of the steady-state and time-resolved studies, it is further concluded that the charge stabilization processes are highly efficient in the clusters of the dyads
Clusters of a Fullerene-Aniline Dyad compared to homogeneous medium. The remarkable stability of the charge-separated intermediates in the clustered dyads is attributed to the hopping of the electrons between the fullerene molecules. The results of the photophysical studies in the clusters illustrate a promising model for circumventing the back-electrontransfer processes in the donor-bridge-acceptor systems. Experimental Section Methods. The UV-visible spectra were recorded on a Shimadzu 2100 or GBC 918 spectrophotometer. Dynamic light scattering studies were carried out using a Coulter model N4 plus particle size analyzer. The emission spectra were recorded on a Spex-Fluorolog F112-X equipped with a 450 W xenon lamp and a Hamamatsu R928 photomultiplier tube. The excitation and emission slits were 1 and 4, respectively. A 570 nm long pass filter was placed before the emission monochromator in order to eliminate the interference from the solvent. Solvent spectra were recorded in each case and subtracted. Quantum yields of fluorescence were measured by a relative method using optically dilute solutions (absorbance adjusted to 0.35 at 470 nm in the case of clusters). The fullerene-aniline dyad14a and the model compound23 were synthesized by the reported procedure. Laser Flash Photolysis. Picosecond laser flash photolysis experiments were performed using 355 nm laser pulses from a mode-locked, Q-switched Quantel YG-501 DP Nd:YAG laser system (output 1.5 mJ/pulse, pulse width ∼18 ps).24a,b The white continuum picosecond probe pulse was generated by passing the fundamental output through a D2O/H2O mixture. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.), interfaced with an IBMAT computer. Time zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported in this study are at room temperature (297 K) and have an experimental error of (10%. The deaerated dye solution was continuously flowed through the sample cell during the measurements. Nanosecond laser flash photolysis experiments were performed with a Laser Photonics PRA/model UV-24 nitrogen laser system (337 nm, 2 ns pulse width, 2-4 mJ/pulse) with front face excitation geometry. A typical experiment consisted of a series of 2-3 replicate shots per single measurement. The average signal was processed with an LSI-11 microprocessor interfaced with a VAX computer. Details of the experimental setup can be found elsewhere.24c Acknowledgment. The authors thank the Council of Scientific and Industrial Research, Government of India, and the Office of Basic Energy Science of the Department of the Energy for financial support of this work. This is contribution No. RRLT-PRU 98 from RRL, Trivandrum, India, and NDRL 4083 from the Notre Dame Radiation Laboratory. M.V.G. also acknowledges the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India, for their financial support. References and Notes (1) For a definition of dyad molecules, see: Balzani V.; Scandola F. In Supramolecular Photochemistry; Ellis Horwood: New York, 1991; Chapter 12. (2) For some recent reviews on fullerene-link-donor systems, see: (a) Sun, Y.-P. In Molecular and Supramolecular Photochemistry, Vol. 1,
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