Ion Pairs of Charged Porphyrins: Ordered Arrangement and Radical-Pair Formation

Abstract

π-Electronic ions with appropriate geometries and peripheral substituents provide assemblies through the interactions between charged building subunits, resulting in fascinating electronic properties. Structures and properties of the assemblies can be controlled by the combined positively and negatively charged species in the assemblies.[1,2] In fact, π-electronic ion pairs comprising porphyrin-based π-electronic anions[3] have exhibited characteristic assembling modes via i π– i π interactions and resulting electronic properties such as solid-state absorption, which was correlated with the arrangement of constituent charged π-systems, and photoinduced electron transfer.[4] On the other hand, ion pairs of porphyrin–AuIII complexes as π-electronic cations, prepared with the combination of various anions including π-electronic anions, formed assemblies as crystals and thermotropic liquid crystals, whose ionic components were highly organized by i π– i π interactions (mainly electrostatic and dispersion forces).[5] Among various combinations of these porphyrin cations and anions, the “activated” ion pair of meso-EWG (electron-withdrawing group)-substituted cation and meso-EDG (electron-donating group)-substituted anion exhibited the electron transfer in the steady state according to solvent polarity, resulting in the production of the radical pair. The ESR in frozen toluene revealed the formation of a heterodiradical in a close stacking structure by the antiferromagnetic dipolar interaction and temperature-dependent spin transfer behavior.[6,7][1] Recent reviews: (a) Haketa, Y. et al. Mol. Syst. Des. Eng. 2020, 5, 757; (b) Yamasumi, K. et al. Bull. Chem. Soc. Jpn. 2021, 94, 2252. [2] Recent reports on pyrrole-based π-electronic molecules: (a) Watanabe, Y. et al. Chem. Eur. J. 2020, 26, 6767; (b) Haketa, Y. et al. J. Am. Chem. Soc. 2020, 142, 16420; (c) Kuno, A. et al. Chem. Eur. J. 2021, 27, 10068; (d) Fujita, M. et al. Chem. Commun. 2022, 58, 9870. [3] (a) Sasano, Y. et al. Dalton Trans. 2017, 46, 8924; (b) Sasano, Y. et al. Chem. Eur. J. 2019, 25, 6712. [4] Sasano, Y.; Tanaka, H. et al. Chem. Sci. 2021, 12, 9645. [5] (a) Haketa, H. et al. iScience 2019, 14, 241; (b) Tanaka, H. et al. Chem. Asian J. 2019, 14, 2129. [6] Tanaka, H. et al. J. Am. Chem. Soc. 2022, 144, 21710. [7] The details of excited-state radical pairs: Tanaka, H. et al. to be submitted.