Beyond direct excitation: advancing photochromism via triplet sensitization
Triplet sensitization, inspired in part by the natural management of triplet-state energy in photosynthetic systems, has emerged as a transformative strategy for overcoming the intrinsic photophysical limitations of photochromic systems driven by direct excitation, including rapid fatigue, inefficient photoconversion, and the stringent requirement for high-energy ultraviolet light. By exploiting triplet excited states and Dexter-type triplet–triplet energy transfer (TET), this strategy enables red-shifted activation and improved switching performance under milder irradiation conditions. This review summarizes recent advances in triplet-sensitized photochromism across two mechanistic platforms, E/Z isomerization (azobenzenes and overcrowded-alkene molecular motors) and electrocyclization (diarylethenes), examining the full range of triplet sensitizers employed to date, from metalloporphyrins, organic chromophores, and semiconductor quantum dots to metal-to-ligand charge-transfer (MLCT) and charge-transfer complexes (CTCs). Finally, we examine the key challenges of directional control, structural organization, and efficient long-wavelength sensitization, while discuss emerging strategies that may promote triplet-sensitized photochromism as a versatile platform for next-generation photoresponsive materials and light-controlled biomedicine.