Photoinduced Intramolecular Charge Transfer in a Series of Differently Twisted Donor−Acceptor Biphenyls As Revealed by Fluorescence

Abstract

This photophysical study addresses the general question of how electron transfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less π-conjugation. The effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N,N-dimethylamino-4‘-cyanobiphenyls are investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited state are analyzed by comparison with the absorption spectra and interannular twist angle (φ)-dependent CNDO/S calculations. Independent of the twist angle φ and solvent polarity, the first excited singlet state of these donor−acceptor (D−A) biphenyls (I−III) is an emissive intramolecular 1CT state of the 1La-type transferring charge from the dimethylaminobenzene (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D−A fluorene I, the flexible D−A biphenyl II shows only a weak dependence of the fluorescence radiative rate constants kf (0.4−0.6 ns-1) on the solvent polarity, consistent with a planarization in the excited state of II. In contrast, the strongly pretwisted biphenyl III behaves similarly to I and II only in nonpolar solvents (〈kf〉 = 0.3 ns-1, indicating partial excited-state relaxation toward planarity), whereas with increasing polarity the mean radiative rate 〈kf〉 decreases down to 0.03 ns-1. A fast equilibrium between a more planar and a more twisted rotamer distribution in the 1CT state of III explains the appearance for III of additional photophysical effects such as (a) strong decrease of the radiative rates with increasing polarity, (b) two long (>200 ps) fluorescence lifetimes with precursor−successor relation, and (c) excited-state quenching by protic solvents.