Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution

Abstract

Abstract The photophysics of a red-emitting push–pull triarylamine compound (fvin) comprising a triphenylamino electron donor core and a dicyanovinylene electron acceptor group is investigated by steady-state absorption and emission and femtosecond time-resolved absorption spectroscopy in room-temperature n -hexane, toluene, ethanol, and acetonitrile solvents. Fvin is strongly fluorescent in apolar solvents upon excitation of the S 0 → S 1 intramolecular charge transfer (ICT) transition, but hardly emissive in polar solvents. Time-resolved spectra reveal a strong dependence of the excited-state dynamics on both the solvent polarity and viscosity. Unlike n -hexane solutions where the fluorescent ICT excited state remains weakly solvated and keeps a structure close to that of the Franck–Condon level, significant stabilization of the ICT state operates in toluene, leading to a relaxed fluorescent ICT′ form with stronger charge transfer character. This was featured by a notable dynamic Stokes shift of the stimulated emission (SE) band occurring in 15 ps. However the similarly high fluorescence quantum yield and lifetime in n -hexane and toluene excludes any large structural distortion on going from ICT to ICT′. In acetonitrile and ethanol, the SE shift dynamics is enhanced and clearly biphasic. The first step can be ascribed to an ICT → ICT′ conformational relaxation as in toluene. The second step, accompanied by a drastic drop of the SE intensity and a strong evolution of the excited-state absorption spectrum (time-constant 1.5 ps in acetonitrile, 9 ps in ethanol), corresponds to the appearance of a new ICT″ state with enhanced charge localization and separation, most probably associated with a rotation of the phenyldicyanovinylene group. This large-amplitude distortion is followed by fast internal conversion to the ground state (9 ps in acetonitrile, 21 ps in ethanol), accounting for the almost nonemissive character of fvin in polar solvents.