Photoinduced intramolecular charge transfer in planar vs. twisted donor-acceptor terphenyls

Photoinduced intramolecular charge transfer (ICT) was studied in 4-N,N-dibutylamino-4''-cyano-terphenyl and its conformationally more planar and more twisted analogues. Semi-empirical calculations (CNDO/S-CI) suggest that the twisted intramolecular charge transfer (TICT) states (A(2)* with the dimethylanilino group as the electron donor (D) perpendicular to the planar cyano diphenyl moiety (acceptor, A), A(3)* with the dimethylamino diphenyl group (D) perpendicular to the benzonitrile group (A) and A(2,3)* with the three phenyl groups successively perpendicular such that D and A are parallel but linked by a phenyl group in a perpendicular relative conformation) are sufficiently low lying to become the lowest excited states in polar solvents because of their high dipole moments. Theoretical considerations suggest radiative properties for the TICT states A,* and A,* because of the large orbital coefficient at the single bond link and the possibility of strong coupling by deviation from perpendicularity. In contrast, non-radiative properties are suggested for the TICT state A(2,3)*, because D and A are separated by a neutral phenyl ring. The excited state dipole moments were obtained from the solvatochromic behaviour of the fluorescence. The decrease in the fluorescence quantum yields and the shortening of the fluorescence lifetimes in polar solvents (observed only for the sterically hindered compound) were analysed. The effect of a preferred non-planar ground state conformation has been accounted for by a different TICT state population and/or a modification of the TICT state emissive properties. (C) 1997 Elsevier Science S.A.