ELECTRON-TRANSFER REACTIONS IN THE EXCITED SINGLET-STATES OF DIMETHYLSUBSTITUTED PHENOL-2-NITROFLUORENE SYSTEMS - EVIDENCE FOR THE MARCUS INVERTED REGION AND CONCURRENT OCCURRENCE OF ENERGY-TRANSFER PROCESSES

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-dimethylanis ole, 25DMA) and acceptor (A) 2-nitrofluorene (2NF) in bath highly pola r acetonitrile (ACN) and nonpolar cyclohexane (CH) solvents at 296 K b y electronic absorption, steady state, and time-resolved, in the nanos econd time domain, spectroscopic methods. No ground state charge trans fer (CT) complex was found for the present D-A pairs. Large fluorescen ce quenching rates (similar to 10(12) dm(3) mol(-1) s(-1)) were observ ed in both CH and ACN solvents. Evidence for concurrent occurrence of Forster's s type singlet-singlet energy transfer process along with ET was found. No static quenching was observed. Radiative energy transfe r was found to play an insignificant role within the present D-A syste ms. Occurrence of highly exothermic outer-sphere type ET reaction with in the Marcus inverted region was inferred. In nonpolar CH a contact e xciplex of CT nature was observed, whereas in the highly polar ACN env ironment the anionic radical of the sterically hindered phenol 2,6-DMP was found as final product. At 77 K occurrences of both Forster's typ e singlet-singlet and Dexter's type triplet-triplet energy transfer pr ocesses were inferred within the present D-A systems from steady state and time-resolved spectroscopic studies.