Electron Transfer Reactions in the Excited Singlet States of Dimethyl Substituted Phenol−2-Nitrofluorene Systems: Evidence for the Marcus Inverted Region and Concurrent Occurrence of Energy Transfer Processes

Abstract

Studies were made on the nature of photoinduced electron transfer (ET) reactions within the electron donor (D) (3,5-dimethylphenol, 35DMP; 2,6-dimethylphenol, 26DMP; 3,5-dimethylanisole, 35DMA; 2,5-dimethylanisole, 25DMA) and acceptor (A) 2-nitrofluorene (2NF) in both highly polar acetonitrile (ACN) and nonpolar cyclohexane (CH) solvents at 296 K by electronic absorption, steady state, and time-resolved, in the nanosecond time domain, spectroscopic methods. No ground state charge transfer (CT) complex was found for the present D−A pairs. Large fluorescence quenching rates (∼1012 dm3 mol-1 s-1) were observed in both CH and ACN solvents. Evidence for concurrent occurrence of Förster's type singlet−singlet energy transfer process along with ET was found. No static quenching was observed. Radiative energy transfer was found to play an insignificant role within the present D−A systems. Occurrence of highly exothermic outer-sphere type ET reaction within the Marcus inverted region was inferred. In nonpolar CH a contact exciplex of CT nature was observed, whereas in the highly polar ACN environment the anionic radical of the sterically hindered phenol 2,6-DMP was found as final product. At 77 K occurrences of both Förster's type singlet−singlet and Dexter's type triplet−triplet energy transfer processes were inferred within the present D−A systems from steady state and time-resolved spectroscopic studies.