https://doi.org/10.1016/j.mtsust.2022.100149
“Spiropyran (SP) is also another group of photochromic materials with two orthogonal π-networks bonded by a central tetrahedral carbon (Cspiro). When exposing to UV irradiation, a 2π + 2π + 2π pericyclic transition occurs that the Cspiro-O bind in the colorless spiropyran is cleaved. This results in a ring-open colored zwitterionic merocyanine (MC) with an increasing molecular length from 12.2 Å to 14.0 Å and an extension of π electron delocalization, which also goes along with a conversion from a hydrophobic form to hydrophilic form. Upon visible light or thermally at an ambient temperature, the merocyanine isomers return to closed colorless SP form [35].
However, the photoswitching reactions of spiropyran predominantly occur in solution, with only a few show photo-activity in the solid state because of the low thermal stability of zwitterionic MC form. To overcome this MC-SP spontaneous thermal interconversion in the solid state, integrating photochromic SP molecules into MOFs materials has been accomplished because the necessary steric degree of freedom is achieved via separating the single SP molecules [10]. The first SP based MOF was reported by Guangshan and coworkers [36] via non-covalent impregnation of SP molecules. Furthermore, when light and thermal relaxation were applied, the BSP/JUC-120 film exhibited high reversibility of photo-isomerization with the long-term stability of the open merocyanine configuration due to the pore confinement effect [36].”