Unraveling the Electronic Structure of Azolehemiporphyrazines: Direct Spectroscopic Observation of Magnetic Dipole Allowed Nature of the Lowest π–π Transition of 20π-Electron Porphyrinoids

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2014-06, Vol.118 (25), p.4415-4424
• Main Authors: Muranaka, Atsuya, Ohira, Shino, Toriumi, Naoyuki, Hirayama, Machiko, Kyotani, Fumiko, Mori, Yukie, Hashizume, Daisuke, Uchiyama, Masanobu
• Format: Article
• Language: English
• Subjects: Azo Compounds - chemistry; Azoles - chemistry; Density functional theory; Electrons; Magnetic dipoles; Magnetic Resonance Spectroscopy; Mathematical analysis; Molecular Structure; Physical chemistry; Porphyrins - chemistry; Progressions; Quantum Theory; Ring currents; Spectroscopy; X-rays
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp5001557

Hemiporphyrazines are a large family of phthalocyanine analogues in which two isoindoline units are replaced by other rings. Here we report unambiguous identification of 20π-electron structure of triazolehemiporphyrazines (1, 2) and thiazolehemiporphyrazine (3) by means of X-ray analysis, various spectroscopic methods, and density functional theory (DFT) calculations. The hemiporphyrazines were compared in detail with dibenzotetraazaporphyrin (4), a structurally related 18π-electron molecule. X-ray analysis revealed that tetrakis(2,6-dimethylphenyloxy)triazolehemiporphyrazine (1b) adopted planar geometry in the solid state. A weak absorption band with a pronounced vibronic progression, observed for all the hemiporphyrazines, was attributed to the lowest π–π* transition with the electric-dipole-forbidden nature. In the case of intrinsically chiral vanadyl triazolehemiporphyrazine (2), a large dissymmetry (g) factor was detected for the CD signal corresponding to the lowest π–π* transition with the magnetic-dipole-allowed nature. Molecular orbital analysis and NICS calculations showed that the azolehemiporphyrazines have a 20π-electron system with a weak paratropic ring current.