Electronic relaxations in donor–acceptor biphenyls

Luminescence and excitation anisotropy spectra of donor–acceptor biphenyls in ethanol at 77 K in conjunction with time-resolved emission are analyzed to derive the electronic relaxations in the absence of large structural and solvent relaxations. Short lifetimes and high anisotropy values of the fluorescence bands evidence a 1La/1CT mixed transition being responsible for the fluorescence and first absorption band. For the twisted derivative, 7% of the emission occurs from a 3La-type 3CT state. Employing quantum chemical calculations, a polarity-dependent energy level diagram is proposed and the effect of the twist angle on ISC is discussed.