Photophysical Properties of Conformationally Distorted Metal-Free Porphyrins. Investigation into the Deactivation Mechanisms of the Lowest Excited 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 photophysical behavior, and the distinctions between the two classes correlate well with the conformations of the molecules. Normal porphyrins qualitatively and quantitatively follow the deactivation pathways traditionally reported for planar metal-free porphyrins. In contrast, the out-of-plane distortion in a series of sterically-crowded porphyrins results in unusual optical properties and enhanced radiationless decay of the ¹(π, π∗) excited state. In particular, macrocycle distortions increase the rates of both the internal conversion and intersystem crossing decay pathways. Enhanced internal conversion of ¹(π, π∗) to the ground state in the perturbed porphyrins is interpreted as arising from an enhanced Franck-Condon factor associated with a structural reorganization in the excited state. Enhanced intersystem crossing from ¹(π, π∗) most likely arises from increased spin-orbit coupling caused by the nonplanarity of the macrocycle. These results demonstrate that structural perturbations of porphyrin macrocycles, imposed by peripheral substitution 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.