Exploring a Dicationic Benzochromene with Dual photochromic and Electrochromic Response

This study reports the synthesis and characterization of the first dicationic 3,3diaryl-3H-benzo[f]chromene, substituted with pyridinium moieties at the C-8 position and the para-position of one of the C-3 aryl groups. The dicationic benzochromene and its precursors exhibit good photochromism with strong photocolourability in MeCN solutions. The neutral bispyridyl-substituted precursor transitions from colourless to orange (δPSS 452 nm) in solution upon UV irradiation, with a half-life (t1/2) of 812 s. Upon UV irradiation, the paleyellow dicationic benzochromene develops a deep orange hue in solution, accompanied by the formation of a broad absorption band with a shoulder around 460 nm, tailing to ca. 550 nm. The resulting photomerocyanine exhibits a shorter half-life (t1/2 = 522 s) compared to its neutral precursor. Spectroelectrochemical studies show that both the chromene form and the derived photomerocyanine of the dicationic benzochromene are electrochemically active, with reduction processes occurring at the pyridinium moieties. Upon applying a negative potential (−1.15 V to −1.35 V vs. Ag/AgCl) to the chromene form, a distinct strong absorption band at 520 nm emerges, indicative of the formation of a delocalized δ-radical species. The combined photochromic and electrochromic properties of the dicationic benzochromene highlight its distinct dual-functional behaviour, with different responses to both light and electrochemical stimuli.