Synthesis of azo-Triazapentalenes and Study of their Photoswitching.

We report the synthesis of novel aryl-azo pyrazino-triazapentalenes through electrophilic coupling of in situ generated diazonium salts derived from ortho, ortho'-dichloro/difluoro anilines with a mesoionic pyrazino-triazapentalene (PyTAP) scaffold. The coupling reaction afforded two regioisomers, distinguished by their color, assigned as C9 (pink) and C7 (red) substituted products, based on single-crystal x-ray diffraction and NMR analysis. The photoswitching behavior of the resulting azo derivatives was investigated using UV-vis and 1H NMR spectroscopy. Three of the six compounds exhibited reversible E↔Z isomerization. The C9-substituted derivatives responded more efficiently to light irradiation than their C7 analogues, which were thermally and photochemically labile. The thermal half-lives ranged from minutes to hours and correlated with the calculated Wiberg bond indices of the central N═N bond. Contrary to trends reported in the literature, the chlorinated compounds exhibited better switching performance than the fluorinated derivatives. DFT and TD-DFT calculations provided a rationale for the observed regioselectivity of the diazo-coupling reaction, substituent-dependent absorption profiles, and photoswitching behavior.