Short-chain basket handle porphyrins: singlet and triplet excited state properties

Abstract

The excited state properties of the title compounds and their Zn²⁺ derivatives, determined by fluorescence and photoexcited tripled ESR methods, are reported. Fluorescence spectral studies indicate that the basket handle porphyrins are less fluorescent than the parent meso-tetraphenyl porphyrin (H₂TPP). The combined effects of intersystem crossing and internal conversion account for this difference. Estimated excited state potentials suggest that the cross trans linked isomer (PSI) is a better electron donor in the first singlet excited state. Zero-field splitting parameters D and E of the three isomers evaluated from photoexcited triplet ESR show small but significant differences attributed to distortion of the porphyrin plane caused by introduction of a short bridging group. The electron spin polarization (ESP) pattern remains the same as in H₂TPP, indicating the usual spin-orbit coupling mechanism for intersystem crossing.