CHAIN-LENGTH DEPENDENCE OF THE 3RD-ORDER POLARIZABILITY OF DISUBSTITUTED POLYENES - EFFECTS OF END-GROUPS AND CONJUGATION LENGTH

Various alpha,omega-disubstituted polyenes of increasing length and bearing various donor and acceptor end groups have been synthesized in order to investigate the cabin-length dependence of their second-order hyperpolarizability gamma. Both symmetric polyenes bearing electron-withdrawing or electron-donating end groups and asymmetric donor-acceptor (push-pull) polyenes have been studied for the same donor/acceptor couple. Values of gamma have been obtained through the electric field induced second-harmonic generation (EFISH) and the third-harmonic generation (THG) experimental techniques. In each series of homologous compounds, the lengthening of the polyenic conjugated path results in a sharp increase in gamma values. This behavior can be modeled by gamma = kn(a) relationships with respect to the number of n of conjugated double bonds of the polyenic chain. The exponent value (a) depends on the substitution pattern. The steepest increase of gamma is obtained when both end groups are electron donors. Very large values of gamma are found for the longest symmetric polyenes bearing strong electron donors. No saturation effect has been noticed within the molecule size range investigated (up to 40 angstrom).