Osuka Atsuhiro
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
Chem Rec. 2015 Feb;15(1):143-59. doi: 10.1002/tcr.201402050. Epub 2014 Oct 15.
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.
中位-中位连接的卟啉阵列和中位芳基取代的扩展卟啉多年来一直激发着我的想象力。在这篇综述中，我回顾性地总结了自己在化学研究方面的专业知识，特别关注我们团队是如何发现这两类新型卟啉化合物的。作为我们与N. Mataga教授合作进行的光合模型研究的一部分，通过探索1,4-亚苯基桥连的混合卟啉二聚体的光激发动力学，研究了分子内电荷分离的能隙依赖性。这项研究需要二聚体中具有缺电子卟啉，其可作为八烷基取代的Zn(II)卟啉供体的电子接受单元。为此，我们使用了中位硝化卟啉和中位五氟苯基卟啉。制备这些缺电子卟啉的过程使我们意外地发现了一个中位-中位连接的卟啉二聚体和一系列中位五氟苯基取代的扩展卟啉。中位-中位连接的Zn(II)卟啉二聚体是在用AgNO2硝化5,10-二芳基Zn(II)卟啉时作为副产物发现的，但在与AgPF6的反应中成为主要产物。这一发现为直接连接的卟啉低聚物开辟了一条新途径。与合成5,10,15,20-四(五氟苯基)卟啉的最佳条件相比，当反应在十倍高的底物浓度下进行时，通过BF3·OEt2催化吡咯与五氟苯甲醛的缩合反应制备了一系列中位五氟苯基取代的扩展卟啉。这些扩展卟啉已被证明具有吸引人的特性，如灵活的结构、多样的电子态、多金属配位、阴离子传感和大的非线性光学性质。虽然这些研究大多是出于好奇心驱动，但我们的一些工作涉及更广泛的兴趣：如何合理地合成并以纯净和离散的形式分离线性连接的分子，如何实现大的π共轭以允许非常低能量的电子跃迁，以及如何容易地制备莫比乌斯芳香和反芳香分子。
