PHOTOPHYSICAL PROPERTIES OF CONFORMATIONALLY DISTORTED METAL-FREE PORPHYRINS - INVESTIGATION INTO THE DEACTIVATION MECHANISMS OF THE LOWESTEXCITED SINGLET-STATE

Time-resolved and steady-state optical data are presented for a series of substituted free-base porphyrins. The porphyrins are grouped into ''normal'' and ''perturbed'' categories based on observed photophysica l behavior, and the distinctions between the two classes correlate wel l with the conformations of the molecules. Normal porphyrins qualitati vely and quantitatively follow the deactivation pathways traditionally reported for planar metal-free porphyrins. In contrast, the out-of-pl ane distortion in a series of sterically-crowded porphyrins results in unusual optical properties and enhanced radiationless decay of the (1 )(pi,pi) excited state. In particular, macrocycle distortions increas e the rates of both the internal conversion and intersystem crossing d ecay pathways, Enhanced internal conversion of (1)(pi,pi) to the grou nd state in the perturbed porphyrins is interpreted as arising from an enhanced Franck-Condon factor associated with a structural reorganiza tion in the excited state. Enhanced intersystem crossing from (1)(pi,p i>) most likely arises from increased spin-orbit coupling caused by t he nonplanarity of the macrocycle. These results demonstrate that stru ctural perturbations of porphyrin macrocycles, imposed by peripheral s ubstitution in vitro or the protein environment in vivo, can result in significant changes in electronic properties, including the rates and yields of the fundamental excited state deactivation processes.