Synthesis of azo‐Triazapentalenes and Study of their Photoswitching

ABSTRACT 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 1 H 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.