Refining the Impact of Structural Variations on Photochromism and Photothermal Conversion in Viologen Crystals

Photochromic behavior in viologen-based coordination polymers (CPs) is highly sensitive to structural packing, yet a clear structure–property correlation remains insufficiently established. Here, we report a series of structurally related viologen-based CPs, in which only H2L-1 exhibits reversible photochromism under UV irradiation, associated with the formation and stabilization of viologen radical cations (V+•) under ambient conditions. The remaining analogues (H2L-2, H4LCl2-3, and H4LCl2-4, where L2– denotes N,N′-bis(3,5-bis(dicarboxylatophenyl)-4,4′-bipyridinium) are EPR silent, indicating the absence of photoinduced electron transfer. Crystallographic analyses demonstrate that subtle variations in crystallization conditions (solvent, acidity, and temperature) lead to distinct supramolecular packing modes, dominated by hydrogen bonding, halogen bonding, and π–π interactions. Notably, the shortened π–π stacking and enhanced intermolecular interactions in H2L-1 facilitate efficient charge transfer, thereby enabling its unique photochromic response. Hirshfeld surface analysis and interaction energy calculations further corroborate the critical role of π–π interactions in promoting electron migration. Importantly, the photoinduced radical generation in H2L-1 also enhances near-infrared absorption, resulting in efficient photothermal conversion, with H2L-1-UV reaching 100 °C under 1 W irradiation. These results establish a direct correlation between crystal structure, photochromic behavior, and photothermal performance, highlighting crystal engineering as an effective strategy for developing multifunctional photoresponsive materials.