Structural, Photophysical, and Magnetic Circular Dichroism Studies of Three Rigidified meso‐Pentafluorophenyl‐Substituted Hexaphyrin Analogues

• Published in: Chemistry : a European journal 2017-05, Vol.23 (27), p.6682-6692
• Main Authors: Ishida, Masatoshi, Furuyama, Taniyuki, Lim, Jong Min, Lee, Sangsu, Zhang, Zhan, Ghosh, Sudip K., Lynch, Vincent M., Lee, Chang‐Hee, Kobayashi, Nagao, Kim, Dongho, Sessler, Jonathan L.
• Format: Article
• Language: English
• Subjects: Absorption; aromaticity; Chemistry; Conjugation; Dichroism; electrochemistry; Electron states; Electronic structure; expanded porphyrinoids; magnetic circular dichroism; Photon absorption; photophysics; porphyrinoids; Protonation; Substitutes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201700759

Detailed electronic, structural, photophysical, and redox studies of a series of meso‐pentafluorophenyl‐substituted hexaphyrins, namely amethyrin (1), rosarin (2), and rubyrin (3), are described. In prior work, it was found that the electronic states of the antiaromatic hexapyrrolic macrocycle, [24]rosarin 2, could be modified by exposure to several Brønsted acids (e.g., HCl, HBr and HI) to produce either one‐ and two‐electron reduced species, or both. In an effort to gain further insights into the reactivity of hexaphyrins possessing different π‐conjugation pathways, the β‐dodecamethyl‐substituted [24]amethyrin 1 was prepared and its electronic structure was analyzed along with that of the o‐phenylene‐bridged [26]rubyrin 3 and rosarin 2 The [4n] and [4n+2] π‐conjugated formulations of 2 and 3, respectively, were inferred from steady‐state, fs‐transient absorption and two photon absorption measurements. Similar photophysical analyses lead to the conclusion that 1 is best considered as nonaromatic or weakly antiaromatic. Magnetic circular dichroism (MCD) spectroscopic analyses of hexaphyrins 1 and 3, as well as comparisons to 2, and theoretical perimeter MO diagram analyses provided support for the electronic assignments. In contrast to what was found for 2, simple protonation of 1 and 3 by halohydric acids did not induce an evident, redox‐based change in the electronic structure of the macrocycle. Protonation was explored as a means of altering the inherent electronic states of three congeneric expanded porphyrinoids, specifically three rigidified hexaphyrin analogues possessing different numbers of meso‐carbon linkages. As inferred from a suite of spectroscopic studies, only the [24]rosarin, but not its analogues [24]amethyrin or [26]rubyrin, proved redox active under conditions of simple protonation (see figure).