Spectroscopic, electrochemical, and photochemical studies of self-assembled via axial coordination zinc porphyrin-fulleropyrrolidine dyads

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, H-1 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 much less than m-pyridyl similar or equal to p-pyridyl much less than 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 H-1 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 C-60 moiety. The calculated rate of charge separation was found to range between 10(7) and 10(10) 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 C-60 moiety. The present studies also revealed little or no quenching of the singlet excited fulleropyrrolidine upon coordination of (TPP)Zn.