Reversible Luminescent Switching in an Organic Cocrystal: Multi-Stimuli-Induced Crystal-to-Crystal Phase Transformation.

A luminescent cocrystal system is reported to undergo crystal-to-crystal phase transformation from yellow-emitting polymorph I to green-emitting polymorph II, triggered by THF fuming or heating, and the green emission can recover to the initial yellow emission by grinding. The established spectroscopic and crystallographic analyses demonstrate that the phase transition occurred and benefits from the combined effect of similar molecular arrange sequence and unique alteration of intermolecular interactions from halogen/hydrogen bonds in I to π-π stacking in II. Furthermore, I and II exhibit red-shift emission under hydrostatic pressure. The emission of I and II shows a red-shift and recovers towards the initial emission upon acid-base fuming. This is a rare example of reversible luminescent switching of cocrystal based upon crystal-to-crystal phase transition, and provides an alternative strategy to develop multi-stimuli responsive materials.