CHAIN-LENGTH DEPENDENCE OF THE QUADRATIC HYPERPOLARIZABILITY OF PUSH-PULL POLYENES AND CAROTENOIDS - EFFECT OF END-GROUPS AND CONJUGATION PATH

Numerous push-pull polyenes and carotenoids of increasing length and b earing various donor and acceptor end groups have been synthesized in order to investigate the chain-length dependence of their quadratic hy perpolarizability beta. The variation of beta was studied using the el ectric-field-induced second-harmonic (EFISH) generation technique. In each series of homologous compounds, the lengthening of the polyenic c onjugation path results in a sharp increase in beta with the number n of double bonds of the polyenic chain. This behaviour leads to very la rge values of beta for the longest molecules whose length reaches 25 a ngstrom. A steeper increase of beta was observed when the end groups a re weaker electron donors and electron acceptors. Moreover, beta = kn( a) relationships have been established, the a exponent depending on th e end groups. Finally, the effect of the introduction of a triple bond in the middle of the polyenic linker has been studied. The replacemen t of a double bond by a triple bond in the conjugation path leads to s maller beta and to a blue shift of the charge transfer absorption band . However, this phenomenon is modulated by the nature of the end group s. Therefore, in some cases, the presence of the triple bond may advan tageously result in a pin of transparency with almost no decrease in b eta.