Thermocoloration and Photodiscoloration Derived from the Entanglement of Host and Guest Radicals in a Metal–Organic Framework

Color-changing materials, especially photochromic and thermochromic materials, have attracted great attention because of their potential applications in protection, decoration, display, memory, switches, photography, and so on. Herein, we show a newly designed host–guest complex system 3-BP@Cd-MOF, which exhibits multiple thermochromism and photochromism based on thermally induced host radical and light-induced guest radical dual pathways. 3-BP@Cd-MOF displayed a color change from blue to brown upon increasing the temperature from 50 to 100 °C due to the generation of more host radicals at higher temperatures. Upon light irradiation, the blue and brown complexes 3-BP@Cd-MOF-Blue and 3-BP@Cd-MOF-Brown exhibited fast and slow discoloration processes, respectively. The photobleaching phenomenon can be attributed to the generation of photoinduced guest radicals on 3-bromopropyne (3-BP) molecules, which in part quenched the host radicals. Crystal structural analysis and theoretical studies revealed that interhost–guest H-bonding interactions and interguest halogen bonding interactions are of great significance in realizing unique thermocoloration and photodiscoloration performance. 3-BP@Cd-MOF-doped polyvinylidene fluoride (PVDF) films also displayed a similar color response under heat and light stimuli, showing potential applications in over-temperature indication, information encryption, anticounterfeiting techniques, and smart temperature-controlling materials. We hope that this work can provide an alternative idea for designing thermochromic and photochromic bifunctional materials.