Light-Off Salient Effect: Thermal Phase Transitions of Molecular Crystals Controlled by Photoirradiation

The ways that molecular crystals respond to various stimuli have attracted much research attention, thus expanding the functional diversity of their response behavior. For example, ever since the reports of luminescent mechanochromic compounds emerged and began to increase in number, a unique class of such materials has been reported, i.e., luminescent mechanochromic materials based on a single-crystal-to-single-crystal phase transition or crystal bending. An elaborate strategy would be required to diversify further the known array of stimulus responses, and such findings would expand the materials applications of molecular crystals. Here, we achieved a light-off salient effect by effectively combining two external stimuli. Benzodifuranone 1 has three distinct polymorphs, and their thermal phase transitions can be modulated using photoirradiation. Two of the polymorphs, i.e., 1A and 1B, undergo a thermal phase transition to high-temperature polymorph 1X. Upon cooling, 1X transforms back to the 1A phase. Notably, the thermal phase-transition temperature can be reduced by approximately 30 °C under illumination by ultraviolet light. Leveraging this strategy to control the phase transition of 1 using two external stimuli, we demonstrated a light-off salient effect.