Subpicosecond transient absorption of donor-acceptor biphenyls. Intramolecular control of the excited state charge transfer processes by a weak electronic coupling
Résumé
The photoinduced intramolecular charge transfer processes of three differently twisted 4-(dimethylamino)-4‘-cyanobiphenyl derivatives (I−III) have been investigated using time-resolved transient absorption and gain spectroscopy in the subpicosecond range. Independent of twist angle and solvent polarity, the kinetics and spectral evolutions after excitation clearly reveal a precursor−sater relationship for the electron transfer from a less emissive state of mixed 1Lb/CT character to a highly emissive charge transfer (1CT) state. Beside the occurrence of dual fluorescence gain, two transient absorption bands for the 1CT state and one for the precursor state (1FC) are observed. All bands are assigned to electronic transitions and correlated for all solvents and compounds. The band intensities are discussed with solvent polarity and twist angle controlled mixing between the charge transfer state 1CT and the higher lying 1Lb and 1La states. In acetonitrile, the transient spectra of the pretwisted donor−acceptor biphenyl III, in contrast to the planar I and II, can be approximated by the sum of cation and anion spectra of the subunits demonstrating decoupled moieties. The kinetics of the CT processes are not dominated by solvation dynamics alone. As an example, in acetonitrile, (τl = 0.2 ps, τs < 1 ps) the kinetics are slower than 2.5 ps. The involvement of a weak electronic coupling matrix element is favored as a source for the intramolecular control of the CT reactions. Furthermore, for the strongly twisted biphenyl derivative III, a secondary intramolecular process to a more relaxed species (CTR) occurs after the initial CT step, in agreement with fluorescence studies.