Photoinduced processes in zinc porphyrin-C 60 dyad (ZnP-C 60 ) in different organic solvents have been investigated by fluorescence lifetime measurements and pico-and nanosecond time-resolved transient absorption spectroscopies. Irrespective of the solvent polarity, the charge-separated state (ZnP •+ -C 60 •-) is formed via photoinduced electron transfer from the excited singlet state of the porphyrin to the C 60 moiety. However, the resulting charge-separated state decays to different energy states depending on the energy level of the chargeseparated state relative to the singlet and triplet excited states of the C 60 moiety. In nonpolar solvents such as benzene ( s ) 2.28), the charge-separated state undergoes charge recombination to yield the C 60 singlet excited state, followed by intersystem crossing to the C 60 triplet excited state, since the energy level of the chargeseparated state is higher than that of the C 60 singlet excited state (1.75 eV). More polar solvents such as anisole ( s ) 4.33) render the energy level of the charge-separated state lower than the C 60 singlet excited state, resulting in the direct formation of the C 60 triplet excited state (1.50 eV) from the charge-separated state, formed by the photoinduced charge separation from the porphyrin to the C 60 singlet excited state as well as from the porphyrin excited singlet state to the C 60 . In polar solvents such as benzonitrile ( s ) 25.2), where the energy level of the charge-separated state (1.38 eV) is low compared with the C 60 triplet excited state, the charge-separated state, produced upon excitation of the both chromophores, decays directly to the ground state. Such solvent dependence of charge recombination processes in ZnP-C 60 can be rationalized by small reorganization energies of porphyrins and fullerenes in electron-transfer processes.

Spectroscopic, redox, and photochemical behavior of self-assembled donor−acceptor dyads formed by axial coordination of zinc tetraphenylporphyrin, (TPP)Zn, and fulleropyrrolidine bearing either pyridine or imidazole coordinating ligands were investigated. The UV−vis, 1H NMR, and ESI-mass spectral studies, as well as computational studies, revealed supramolecular 1:1 dyad formation between the electron donor [(TPP)Zn] and the electron acceptor, fulleropyrrolidine entities. The determined formation constant K values followed the order o-pyridyl ≪ m-pyridyl ≃ p-pyridyl ≪ N-phenyl imidazole entities of the fulleropyrrolidine. The evaluated thermodynamic parameters revealed stable complexation with complex dissociation enthalpies ranging between 26 and 32 kJ mol-1. The 1H NMR studies revealed axial coordination of the pyridine or imidazole ligands to the central zinc of (TPP)Zn, while the ESI-Mass spectral studies performed in CH2Cl2 matrix revealed the expected molecular ion peak of the self-assembled dyads. The geometric and electronic structures of the dyads were probed using ab initio B3LYP/3-21G(*) methods. Such studies revealed stable complexation between (TPP)Zn and fulleropyrrolidine entities. The majority of the highest occupied frontier molecular orbital (HOMO) was found to be located on the (TPP)Zn entity, while the lowest unoccupied molecular orbital (LUMO) was found to be entirely on the fullerene entity. The redox behavior of the isolated self-assembled dyads was investigated in o-dichlorobenzene, 0.1 (TBA)ClO4. A total of seven one-electron redox processes corresponding to the oxidation and reduction of zinc porphyrin ring, and the reduction of fullerene entities were observed within the accessible potential window of the solvent. These electrochemical results suggest weak interactions between the constituents in the ground state. The excited-state electron-transfer reactions were monitored by both steady-state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene, upon coordination of either the pyridine or imidazole entities of fulleropyrrolidine to (TPP)Zn, the main quenching pathway involved charge separation from the singlet excited (TPP)Zn to the C60 moiety. The calculated rate of charge separation was found to range between 107 and 1010 s-1 depending upon the axial ligand (pyridine or imidazole) of the fulleropyrrolidine. However, in a coordinating solvent like benzonitrile, intermolecular electron transfer predominantly takes place mainly from the triplet excited (TPP)Zn to the C60 moiety. The present studies also revealed little or no quenching of the singlet excited fulleropyrrolidine upon coordination of (TPP)Zn.

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