Intramolecular charge transfer and enhanced quadratic optical non-linearities in push pull polyenes

Abstract

Push-pull polyenes, which have an electron-donating group (D) and an electron-withdrawing group (A) grafted on opposite ends of a conjugated polyenic chain, are of particular interest as model compounds for long-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 acceptor groups, have been prepared and studied. Their linear and non-linear optical properties have been investigated by performing electro-optical absorption measurements (FOAM) and electric-field-induced second-harmonic generation (EFISH) experiments in solution. Each molecule shows a broad and intense ICT absorption band in the visible associated with an increase in the dipole moment (Δμ). Lengthening the polyenic 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 constant. As a result, the longest molecules exhibit huge Δμ values (up to 42 D) as well as markedly enhanced quadratic hyperpolarizabilities (β). In addition, the nature of the end groups has been found to influence strongly both the ICT and optical non-linearities: larger β and Δ μ values, as well as steeper length dependences, are obtained with push-pull phenylpolyenes bearing strong acceptors.