Effect of positional substitution on the optical response of symmetrically disubstituted azobenzene derivatives

We have synthesized a series of symmetrically substituted p-diamidoazobenzenes, and we compare their steady-state and time-resolved responses to those of p-diaminoazobenzene and azobenzene. Our experimental and computational data indicate that the identity of the azobenzene ring substituent plays a significant role in determining the ordering of excited electronic states and the oscillator strengths for the various S-n --> S-0 transitions. We find experimentally that the symmetric disubstitution of azobenzenes also influences the S-0 isomerization surface, with the largest So barrier being found for the unsubstituted compound and the smallest barrier for p-diaminoazobenzene. We understand these data in the context of the relative electron-donating character of the para side groups.