Push-pull polyenes and carotenoids with enhanced quadratic nonlinear optical susceptibilities

Abstract

A wide choice of push-pull polyenes and carotenoids of increasing length (up to 30 angstrom) and bearing various donor and acceptor end groups has been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability (beta) . (beta) measurements have been performed using the electric field induced second harmonic generation (EFISH) technique. In each series of homologous compounds, the lengthening of the conjugation path results in a pronounced increase in (mu) (beta) values. This behavior leads to very large static (mu) (beta) (0) values for the longest molecules and can be modeled by (mu) (beta) (0) equals kn a relationships with respect to the number n of double bonds in the polyenic chain. The exponent value (a) was found to depend on the end groups. The replacement of a double bond by a triple bond in the middle of the polyenic chain results in a blue shift of the charge transfer absorption band and leads to smaller (mu) (beta) (0) values. However, this phenomenon is modulated by the end groups.