The Mechanism of the Photodimerization of [60]Fullerene Derivatives: It is Still a Charge Separation Followed by a Back Transfer!

Abstract Herein, we present a systematic study of the photochemical behavior of a set of fullerene compounds bearing various organic addends attached to the fullerene cage. It has been shown that the yield of photodimerization products strongly depends on the structure and electronic properties of the material, primarily the electron affinity (acceptor strength) of the modified fullerene core. This finding being inconsistent with the conventional [2+2]cycloaddition mechanism and pointed to an alternative pathway. This proposed pathway involves photoinduced charge separation followed by back charge transfer events leading to the efficient generation of triplet excitons, which are responsible for the formation of dimerized species. The light‐induced charge separation pathway was confirmed by electron spin resonance spectroscopy and was supported by theoretical calculations. The revealed mechanism allows one to control the photochemical behavior of the fullerene derivatives via rational structural engineering: some of the compounds were shown to be fully resistant to photodimerization, which makes them promising candidates in the development of stable organic photovoltaics. Furthermore, this approach could be used to suppress back charge transfer in donor‐acceptor blends and hinder the formation of triplet excitons which represents one a major energy loss channel in the current generation of organic solar cells.