Electronic spectra of push-pull 4-phenylaminoazobenzene derivatives

A series of push-pull type 4-phenylaminoazobenzene derivatives bearing an e lectron-withdrawing 4'-substituent were probed by electronic spectra in sol ution. The visible absorption maxima of these azobenzenes were correlated w ith the solvent parameters through the McRae theory as well as the solvent donor numbers. While the absorption spectra of these neutral species were s olvent-dependent those of protonated species were almost solvent independen t. On the other hand. the absorption maxima and the rates of thermal cis-to -trans isomerization in a given solvent were correlated with Hammett consta nts. These results are discussed with the help of semi-empirical molecular- orbital calculations and compared with previously published data on related compounds.