Towards meso-meso-linked porphyrin arrays and meso-aryl expanded porphyrins.

Abstract

meso-meso-Linked porphyrin arrays and meso-aryl-substituted expanded porphyrins have continuously fueled my imagination for many years. In this account, my expertise in chemical research is retrospectively summarized with a particular focus on how these two novel categories of porphyrinoids were found by our group. As part of our photosynthetic model studies in collaboration with Prof. N. Mataga, the energy-gap dependence of intramolecular charge separation was examined by exploring the photoexcited dynamics of 1,4-phenylene-bridged hybrid porphyrin dimers. This study required electron-deficient porphyrins in the dimers that could serve as an electron-accepting unit towards an octaalkyl-substituted Zn(II) porphyrin donor. To this end, we employed meso-nitrated porphyrins and meso-pentafluorophenyl porphyrins. Efforts to prepare these electron-deficient porphyrins allowed us to serendipitously find both a meso-meso-linked porphyrin dimer and a series of meso-pentafluorophenyl-substituted expanded porphyrins. The meso-meso-linked Zn(II) porphyrin dimer was found as a byproduct in the nitration of 5,10-diaryl Zn(II) porphyrin with AgNO2 but became a major product in the reaction with AgPF6. This finding opened up a new path to directly linked porphyrin oligomers. The series of meso-pentafluorophenyl-substituted expanded porphyrins were prepared via BF3·OEt2-catalyzed condensation of pyrrole and pentafluorobenzaldehyde when the reaction was run at tenfold-higher substrate concentrations, as compared to the optimal conditions for the synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin. These expanded porphyrins have been shown to have attractive attributes such as flexible structures, versatile electronic states, multi-metal coordination, anion sensing, and large nonlinear optical properties. While these studies were mostly curiosity-driven, some of our work covers rather more general interests: how linearly connected molecules can be rationally synthesized and isolated in a pure and discrete form, how large π-conjugation can be realized to allow for very low energy electronic transitions, and how easily Möbius aromatic and antiaromatic molecules can be prepared.