Molecular hyperpolarizabilities of barbituric acid and cyclobutene-1,2-dione derivatives. Electronic and steric effects

Abstract

A series of merocyamne dyes containing various donors and barbituric acid and cyclobutene-1,2-dione moieties as the acceptors have been synthesized and their first-order hyperpolarizabilities β were determined. The β values of barbituric acid derivatives increase as the strength of the donor is increased from 4-dimethylaminophenyl to trimethylindolinyl to benzothiazolinyl, apparently due to the gradual decrease in the bond length alternation (BLA) from a large positive value to an optimum one by a stronger donor. In contrast, the β values for the cyclobutene-1,2-dione derivatives decrease with the same variation of the donors even though the cyclobutene-1,2-dione is a poorer acceptor than the barbituric acid moiety. The results have been attributed to the electron-donating ability of the donors and the increased distortion of the chromophores from planarity.