Radiative Exciplexes of 1,2,4,5-Tetracyanobenzene with Sterically Hindered Alkylbenzenes

Abstract

The formation of radiative exciplexes of a series of electron donor−acceptor pairs from 1,2,4,5-tetracyanobenzene (TCB) and sterically hindered substituted benzenes is reported for the first time. The dependence of radiative and nonradiative electron transfer on the driving force and separation distance in the exciplexes is explored in nonpolar solvents. It is found that exciplexes of donor−acceptor pairs of hindered and unhindered donors with similar structures and electron-transfer properties give almost the same fluorescence quantum yields. Isotope effects of solvents on return electron-transfer rate constants of the exciplexes described above are found to be undetectable. This implies that the exciplexes are approximately two-component systems without the structural involvement of a solvent. Linear relationships between nonradiative return electron-transfer rate constants, log(k-et) and the driving force, −ΔG-et°, are observed for exciplexes from both sterically unhindered and hindered donors. On the basis of all of these results, we conclude that there is no sterically controlled changeover from an inner-sphere to an outer-sphere mechanism in the present systems.