Relationship between electronic structure and nonlinear optical activity of push-pull polyenes: Step towards a quantitative treatment

Mesomeric effect of substituents in stereoregular push-pull polyenes has been studied, bond length alternation being used for the monitoring of π-electron displacement along the polyene segment. Quantitative calibration of the π-donor and π-acceptor strength of various substituents has been performed with the special scale of substituents mesomeric constants. A simple quantum chemical technique for estimation of mesomeric constants has been developed. A simple additive equation describing the substituents mesomeric effect on bond length alternation in push-pull polyenes has been elaborated. Specific push-pull effect has been demonstrated to be the essential feature of push-pull polyenes. A qualitative relationship between the suggested mesomeric constants and longitudinal hyperpolarizability has been established.