<title>Molecular substitution and nonlinear optical properties of chromophores</title>

Abstract

Different factors controlling the linear and nonlinear optical (NLO) properties are discussed for conjugated organic materials having linear chain or ladder structure. It is shown that one-dimensional conjugated molecules reveal similar values of hyperpolarizability as molecules with ladder structures having the same geometric length. After normalizing the value of third order nonlinear response to the number of pi-electrons involved, the chain molecules show significantly higher values. The introduction of heteroatoms into the conjugated part of molecules or linking pendent groups to them are efficient means to tailor not only linear but also nonlinear optical properties. One example of heterosubstitution is the introduction of nitrogen into the conjugated main chain which leads to a change of molecular geometry compared to pure hydrocarbon molecules which is connected with a change of absorption and nonlinear optical properties. So azomethine dyes have a blue shifted absorption and lower NLO susceptibility than stilbene dyes. This can be explained by a non-planar structure of the azomethine molecules reducing the overlap of pi-orbitals. A second important modification of optical properties can be reached by partial fluorination of the molecules or by replacing usual acceptors like nitril- or nitrogroups by a trifluoromethyl group. The second order hyperpolarizabilities beta of different fluorine-containing chromophores were determined by solvatochromic measurements. The investigated fluorine-containing chromophores show beta- values as high as standard NLO-dyes like disperse red I but different linear optical properties.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.