The solid-state photochromism of viologen derivatives offers significant potential for the development of optical and multifunctional materials; however, the structural factors governing their color fading behavior remain poorly understood. In this study, we synthesized and characterized four viologen-based ionic compounds featuring different cationic substituents (H or tert-butyl) and counter anions (Br⁻ or p-toluenesulfonate, TsO⁻) to systematically investigate the correlation between crystal structure and the thermal fading rate of photoinduced color changes. Upon UV irradiation, all compounds exhibited characteristic coloration due to the formation of viologen radical cations, followed by thermal decolorization in the dark. The fading rate was evaluated using time-dependent diffuse reflectance UV-vis spectroscopy. Structural analyses via single-crystal X-ray diffraction revealed that, in bromide salts, shorter donor–acceptor (D–A) distances between the bromide anion and viologen cation corresponded to faster fading rates, consistent with a distance-dependent electron back-transfer mechanism. In contrast, for TsO⁻ salts, one compound exhibited faster fading despite a longer D–A distance. Crystal packing analysis and DFT calculations indicated that this behavior stems from a π–π stacking-mediated electron transfer pathway between the viologen cation and the aromatic TsO⁻ anion. These findings highlight that, while ionic D–A distances dominate in systems with atomic anions, supramolecular interactions such as π–π stacking significantly influence photochromic fading in systems with molecular anions. This work provides new insights into structure–property relationships in solid-state photochromic systems and contributes to the rational design of viologen-based materials with tunable optical responses.
Hirai et al. (Sat,) studied this question.