Organic solid-state photochromism using porous scaffolds

When exposed to light, organic photochromic molecules undergo a fast and reversible change in their electronic structure, resulting in a shift in colour. Solid-state composites of these photochromes are more resistant to environmental factors and better suited for commercial settings than their solution forms. However, solid-state matrices frequently impose geometric limitations on the photoisomerization of these compounds, reducing their photoswitching efficiency. This fundamental constraint considerably impedes the use of organic photochromes in real-world applications. A promising approach to preserving the photoswitching behaviour of organic photochromes in the solid state is to incorporate the molecules within a robust porous scaffold featuring precisely designed pores, such as a metal–organic framework, covalent organic framework, porous organic polymer or metal–organic cage. The physicochemical properties of these scaffolds — such as pore size and structure, hydrophobicity and electronic character — determine the photoswitching efficiency of the integrated photochromes and, thus, the photoresponsive behaviour of the material. There is, however, a dearth of understanding about which features of a porous matrix yield efficient solid photoswitchable materials, given a particular organic photochrome. In this Review, we address the outstanding challenges limiting solid-state photochromic materials based on organic photoswitches. We present design principles for identifying the optimal porous scaffolds for high-efficiency photochromic materials and conclude with the future opportunities of these materials.