Resilient photoswitchable metal–organic frameworks for sunlight-induced on-demand photochromism in the solid state

Organic photoswitchable molecules have struggled in solid-state form to fulfill their remarkable potential, in terms of photoswitching performance and long-term stability when compared to their inorganic counterparts. We report the concept of non-electron deficient host's surface with optimal porosity and hydrophobicity, as a priori strategy to design photoefficient organic solid-state photochromic materials with outstanding mechanical robustness. It is realized by the nanoconfinement of photochromes in a host matrix possessing optimal porosity and hydrophobicity. The resulting photochromic nanocomposites can be prepared in multigram scale employing a one-pot reaction under ambient conditions. When exposed to a light stimulus including natural sunlight, the photoswitchable nanocomposite powder changes color promptly and reversibly, in a matter of seconds (5 s and 30 s under UV irradiation and sunlight, respectively) along with excellent photo-fatigue resistance, which are on a par with inorganic photochromes. Exemplars of commercially viable prototypes that are optically clear, comprising smart windows, complex photochromic sculptures, and self-erasing rewritable devices, were engineered by direct blending with resilient polymers. Notably, the use of high-stiffness polymers (Young's modulus > 2 GPa) is no longer considered an insurmountable challenge. Finally, photochromic films with anticounterfeiting features could be manufactured through precision printing of nanocrystals by drop-on-demand inkjet printing technology.