ELECTRONIC STATES OF PORPHYCENES

Polarized UV-visible absorption, emission, and magnetic circular dichroism of a series of porphycenes are reported and interpreted in terms of the classical perimeter model as well as semiempirical CNDO/S, INDO/S, and PPP calculations. Differences relative to porphyrins follow readily from their different topology. Two results are particularly striking: (i) unlike the soft MCD chromophores, porphyrins, the porphycenes are negative-hard chromophores and provide a clear example of the competition between the mu+ contributions and the mu- contributions to the B terms of the Soret bands and (ii) fluorescence polarization results for free-base porphycene suggest the existence of a fast intramolecular proton-transfer process in the first singlet excited state, which calls for a closer examination by time-resolved methods.