Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis, Electronic Structures, Spectral Properties, and Reactivities of Tetraazaporphyrin meso-N-Oxides

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Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis, Electronic Structures, Spectral Properties, and Reactivities of Tetraazaporphyrin meso-N-Oxides

Atsuya Muranaka, Shunsuke Yanagi, Naoyuki Toriumi, Daisuke Hashizume, Masanobu Uchiyama

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. Macrocyclic heteroaromatic N-oxides: Tetraazaporphyrin (TAP) meso-N-oxides are synthesized by direct oxidation of TAPs. The introduction of an oxygen atom significantly alters the physicochemical properties: the N-oxides exhibit weaker red/near-IR absorption and fluorescence, whereas singlet oxygen quantum yields are greatly enhanced. The N-oxides show near-IR-responsive photoredox properties, and protonation of the N-oxides restores TAP-like intense red absorption and fluorescence.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701300