A Multistimuli Responsive Crystalline Cd(II)-Viologen Coordination Polymer with Single-Crystal-Single-Crystal Transformation

It is necessary to develop stable and fast multi-stimuli responsive materials due to the growing demand in our daily life. In this work, a new viologen-based Cd-complex (1) exhibits multiple thermochromic and photochromic behaviors through 10 states with 7 colors. For example, it responds to both Cu K alpha/Mo K alpha X-ray sources and UV dual light quickly with a color change from colorless to dark blue (1X) (Cu K alpha/Mo K alpha X-ray sources) and cyan (1-UV) (UV light), respectively. Interestingly, it exhibits a three-step coloration phenomenon when heated, which is unprecedented in viologen compounds. Crystal 1 undergoes a color change to pink, blue, and brown under 130, 180, and 240 degrees C, respectively. In addition, upon fumigation, both 1P and 1Q undergo a decoloration process to colorless (1K) and yellow (1T), respectively. Four more states (1P, 1K, 1T, and 1O) obtained via dehydration-hydration treatment are all photochromic. More importantly, via single-crystal-single-crystal transformation (SC-SC), the photochromic and thermochromic behaviors of 1 were investigated from the molecular level, which is also rather rare for thermochromic species. The detailed electron donor and the pathways for electron transfer were clearly given according to the results of crystal structure. The colorful states upon external stimuli may be attributed to the multiple pathways for electron transfer.

Automated summary

The study synthesized a novel viologen-based Cd-complex demonstrating multiple thermochromic and photochromic responses, including unique color changes under heating and UV light exposure. Through single-crystal-single-crystal transformation, the behavior of the material was studied at the molecular level, providing insights into the pathways for electron transfer and resulting in various colorful states upon external stimuli.