The influence of conformation and energy gaps on optical transition moments in donor–acceptor biphenyls Original Research Article

Transition moments of absorption, Ma, and fluorescence, Mf, of three donor–acceptor biphenyls (I–III) are compared with those obtained by quantum chemical calculations to verify the previous conclusions of photoinduced twisting relaxations. The configurational analysis in terms of a state interaction model (analysis of electronic structure using reference states) reveals that existing experimental formalisms are insufficient to quantitatively explain the remarkably strong coupling of the electron transfer state (1ET) with 1La and S0. Using AM1/CI optimized geometries, excited state structural relaxations other than twisting (e.g. bond-length shortening) are shown to lead to a better π-conjugation connected with increased Mf values. On the other hand, ZINDO/S-CI calculations show that solvent polarity induced energy gap dependent S1 decoupling from 1La reduces Mf especially for highly twisted rotamers. Nevertheless, in the experimental range of energy gaps between 1La and 1ET of around 10 000 cm−1, the polarity induced decrease of Mf plays a minor role. Due to this result, the experimental Mf values in polar solvents are indeed consistent with a photoinduced relaxation of II towards planarity and of III towards perpendicularity.