Electronic spectra of push-pull 4-phenylaminoazobenzene derivatives

A series of push-pull type 4-phenylaminoazobenzene derivatives bearing an electron-withdrawing 4'-substituent were probed by electronic spectra in solution. The visible absorption maxima of these azobenzenes were correlated with the solvent parameters through the McRae theory as well as the solvent donor numbers. While the absorption spectra of these neutral species were solvent-dependent those of protonated species were almost solvent independent. On the other hand. the absorption maxima and the rates of thermal cis-to-trans isomerization in a given solvent were correlated with Hammett constants. These results are discussed with the help of semi-empirical molecular-orbital calculations and compared with previously published data on related compounds.