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

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.