Bridging the Gap between Porphyrins and Porphycenes: Substituent-Position-Sensitive Tautomerism and Photophysics in meso-Diphenyloctaethylporphyrins

Abstract

henylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH???N distances. It resembles porphycene, a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 which are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate 3 orders of magnitude lower than in porphycene, but 3-4 orders of magnitude higher than in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10- diphenylporphyrin (2) exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen bonding strength as in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ?HOMO . ?LUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.