Nonlinear Polarizabilities of Symmetric and Non-symmetric Polyene and Cyanine-Like Organic Molecules

There is considerable interest in understanding the origin of the molecular nonlinear polarizabilities. Recently, the important role of bond alternation (difference in the carbon-carbon double bond and single bond lengths) in influencing the second order polarizability, β, of asymmetric conjugated dyes has been pointed out.[1] It was suggested that there is an optimal degree of alternation needed to maximize β. The connection between bond-alternation and the electronic, optical and chemical properties of conjugated molecules has also been recognized previously.[2] These studies as well as the results of prior experimental studies[3] have led us to investigate the affect of bond alternation on the third-order polarizability, γ. One aim of the present work is to gain insight into the differences between the magnitude and signs of γ for simple polyenes and cyanines. For moderate length to long polyenes, nonresonant electronic γ is positive, whereas for comparable cyanines γ is negative.