Counteranion-Regulated Thermo-, Photo-, and Electrochromism in Viologen-Based Crystalline Solids

Viologen-based chromic materials are of considerable interest because their radical-mediated optical responses are highly sensitive to molecular and supramolecular environments. Herein, a viologen-derived ligand, H₂bccd [H₂bccd = 1,1'-bis((2'-carboxybiphenyl-4-yl)methyl)-4,4'-bipyridinium dichloride], was used to construct three closely related crystalline solids, Bcbbd (1), HBcbbd·Cl·2H₂O (2), and HBcbbd·I (3), under hydrothermal conditions, enabling a systematic investigation of how deprotonation degree, counterion identity, and packing mode regulate chromic behavior. Structural analyses show that 1 is a fully dechlorinated and doubly deprotonated species with a highly symmetric packing motif, whereas 2 and 3 are monodeprotonated structures with similar chain-like stacking but different counterion/guest environments. These structural variations lead to markedly different stimulus-responsive properties. Compound 2 exhibits rapid photochromism from colorless to cyan under UV irradiation, while 1 and 3 display obvious thermochromism. All three compounds show electrochromic activity, and 2 further exhibits dual electrochromism arising from stepwise redox processes. Combined crystallographic, spectroscopic, PXRD, EPR, and DFT analyses suggest that the distinct chromic behaviors originate from differences in donor-to-viologen electron transfer, halide-assisted through-space interactions, and crystal-packing-mediated radical stabilization. This work provides a useful model for understanding structure-regulated chromism in viologen-based solids.