Nonplanarity and its implications on triplet state characteristics of basket handle porphyrins

Abstract

The influence of non-planarity in the porphyrin ring on the triplet state characteristics of free base and Zn(II) derivatives of basket handle porphyrins is investigated by triplet ESR. The deviation of the zero-field splitting parameter, D, from planar H₂TPP or ZnTPP is dependent on the degree of non-planarity and increases as the non-planarity increases. The electron spin polarization (ESP) pattern for most derivatives remains the same as observed for H₂TPP or ZnTPP. However, MSICI₈, PSIBr₈, ZnMSIICI₈ and Znbutyl II show different ESP patterns, suggesting a redirection of the spin sublevel activity in these isomers. The ESP pattern for PSIBr₈ is temperature dependent (eeaeaa at −160°C and eeeaaa at −106°C). The calculated structure using molecular mechanics reveals maximum deformation in these isomers which in turn governs the population and decay of the triplet sublevels. The structures indicate a different orientation of bridging phenyl groups in PSIBr₈ and MSICl₈ with respect to the porphyrin plane.