Time-Resolved EPR Study of the Photophysics and Photochemistry of 1-(3-(Methoxycarbonyl)propyl)-1-phenyl[6.6]C_(61)

Abstract

Time-resolved (TR) EPR was used to study the photophysics and photochemistry of 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6.6]C_(61) (M1). The CW TREPR spectra of M1 in the photoexcited triplet state, frozen in a rigid matrix and in liquid solution at room temperature, were compared with those of ^3C_(60). The introduction of the substituent on C_(60) has a striking effect on the spectra of the triplets, which is attributed to the lifting of the orbital degeneracy by the reduction in symmetry. Fourier transform (FT) EPR was used in an investigation of electron-transfer reactions in liquid solutions mediated by ^3M1. Of particular interest was the system of M1/chloranil (CA)/ perylene (Pe). Photoexcitation of M1 is found to lead to the formation of the chloranil anion radical and the perylene cation radical. From the chemically induced dynamic electron polarization (CIDEP) patterns in the FTEPR spectra and the dependence of the reaction kinetics on reactant concentrations, it was deduced that CA^- is formed by two competing pathways following photoexcitation of M1:  (1) direct electron transfer from ^3M1 to CA followed by electron transfer from Pe to M1^+ and (2) energy transfer from ^3M1 to Pe followed by oxidative quenching of ^3Pe by CA. In both pathways, M1 acts as a light-energy harvester and mediator of electron-transfer reactions from Pe to CA without itself being consumed in the process, that is, as a photocatalyst. It is found that the functionalization of C_(60) makes its triplet state a worse electron donor and acceptor, but it has no significant effect on the triplet energy transfer reaction.

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