Solvent Effects upon the Thermal Cis-Trans Isomerization and Charge-Transfer Absorption of 4-(Diethylamino)-4'-nitroazobenzene

Article abstract of DOI:10.1021/jo00165a005

A comprehensive study of the solvent effects on the rate of the thermal cis-trans isomerization and the position of the charge-transfer absorption of the trans isomer of 4-(diethylamino)-4'-nitroazobenzene (DENAB) is presented.The isomerization rates, the activation parameters, and the absorption maxima have been measured in a variety of protic and aprotic solvents.The data are analyzed and interpreted by using the Taft and Kamlet multiparameter approach and the pyrene fluorescence parameter P_y.The results of the correlations suggest that the isomerization reaction is accelerated in hydrogen-bonding solvents relative to aprotic solvents of similar polarity due to a hydrogen-bonding interaction between the solvent and the nitro group of the dye.Correlation of the absorption maxima with the Taft-Kamlet ?^* parameter shows a similar dependence in protic and aprotic media.It is apparent that the absorption process is not affected by hydrogen bonding as strongly as the rate of isomerization.A linear correlation of the rate of isomerization with the position of the CT absorption in different solvents is suggestive of the fact that a similar degree of charge transfer exists in the transition state for the isomerization reaction and in the Frank-Condon excited state.