INTRAMOLECULAR CHARGE-TRANSFER AND ENHANCED QUADRATIC OPTICAL NONLINEARITIES IN PUSH-PULL POLYENES

Push-pull polyenes, which have an electron-donating group (D) and an e lectron-withdrawing group (A) grafted on opposite ends of a conjugated polyenic chain, are of particular interest as model compounds for lon g-distance intramolecular charge transfer (ICT), as well as potent non -linear optical chromophores. Several series of push-pull polyenes of increasing length, combining aromatic donor moieties and various accep tor groups, have been prepared and studied. Their linear and non-linea r optical properties have been investigated by performing electro-opti cal absorption measurements (EOAM) and electric-field-induced second-h armonic generation (EFISH) experiments in solution. Each molecule show s a broad and intense ICT absorption band in the visible associated wi th an increase in the dipole moment (Delta mu). Lengthening the polyen ic chain linking the D and A groups results in a bathochromic shift of the ICT absorption band and induces a linear increase in the excited state dipole. In contrast, the ground state dipole remains roughly con stant. As a result, the longest molecules exhibit huge Delta mu. value s (up to 42 D) as well as markedly enhanced quadratic hyperpolarizabil ities (beta). In addition, the nature of the end groups has been found to influence strongly both the ICT and optical non-linearities: large r beta and Delta mu values, as well as steeper length dependences, are obtained with push-pull phenylpolyenes bearing strong accepters, (C) 1997 Elsevier Science S.A.