Photoinduced intramolecular charge transfer in a series of differently twisted donor - Acceptor biphenyls as revealed by fluorescence

This photophysical study addresses the general question of how electron tra nsfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less pi-conjugation. The effects of pho toinduced intramolecular charge transfer on the electronic and molecular pr operties of a series of differently twisted 4-N,N-dimethylamino-4'-cyanobip henyls an investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited s tare are analyzed by comparison with the absorption spectra and interannula r twist angle (phi)-dependont CNDO/S calculations. Independent of the twist angle phi and solvent polarity, the first excited singlet state of these d onor-acceptor (D-A) biphenyls (I-III) is an emissive intramolecular (CT)-C- 1 state of the L-1(a)-type transferring charge from the dimethylaminobenzen e (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D-B fluorene I, the flexible D-A biphenyl LT shows only a weak dependence of t he fluorescence radiative rate constants k(f) (0.4-0.6 ns(-1)) on the solve nt 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 ([k(f)] = 0.3 ns(-1), indicating partial exci ted-state relaxation toward planarity), whereas with increasing polarity th e mean radiative rate (kt) decreases down to 0.03 ns(-1). A fast equilibriu m between a more planar and a more twisted rotamer distribution in the (CT) -C-1 state of III explains the appearance for III of additional photophysic al effects such as (a) strong decrease of the radiative rates with increasi ng polarity, (b) two long (>200 ps) fluorescence Lifetimes with precursor-s uccessor relation, and (c) excited-state quenching by protic solvents.