title: Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis, Electronic Structures, Spectral Properties, and Reactivities of Tetraazaporphyrin meso-N-Oxides.
abstract: Heteroaromatic N-oxides such as pyridine and quinoline N-oxides are well studied in organic chemistry, and N-oxide formation has long been utilized for tuning the reactivities of heteroaromatics. However, the scope of aromatic N-oxidation is still restricted to relatively small azine or azole skeletons, and there has been little investigation of the photophysical/chemical effects of N-oxidation on larger heteroaromatic systems. Here, the synthesis and unique properties of new macrocyclic heteroaromatic N-oxides, tetraazaporphyrin (TAP) meso-N-oxides, are reported. N-Oxidation of TAP reduced the 18π-aromaticity of the TAP ring compared with that of the parent TAP owing to the cross-conjugated resonance structure. The optical properties of TAPs were significantly changed by N-oxidation: the N-oxides did not exhibit azaporphyrin-like but instead porphyrin-like optical properties, that is, weak Q absorption bands, strong Soret absorption bands, and weak fluorescence. These features can be explained by the near-degenerate frontier molecular orbitals resulting from N-oxide formation. Singlet oxygen quantum yields were greatly increased to almost quantitative levels by N-oxidation. The N-oxides showed near-IR-responsive photoredox properties and were suitable as both oxidants and sensitizers for oxidation reactions. Protonation of the N-oxides restored TAP-like intense Q bands and red fluorescence, offering a potential design strategy for fluorescence switches.
authors.affiliation: Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Toriumi
authors.affiliation: Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Yanagi
authors.affiliation: Elements Chemistry Laboratory, RIKEN and Advanced Elements Chemistry Research Team, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan.
authors.forename: Atsuya
authors.initials: A
authors.lastname: Muranaka
authors.affiliation: Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hashizume
authors.affiliation: Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.|Elements Chemistry Laboratory, RIKEN and Advanced Elements Chemistry Research Team, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan.
authors.forename: Masanobu
authors.initials: M
authors.lastname: Uchiyama
keywords: N-oxides; conjugation; fluorescence; porphyrinoids; singlet oxygen