Polymorphic coumarinopyrone with four fluorescent colors: A case of switching of solid-state luminescence by controlling the torsion angel between the donor and the fluorophore

Polymorphic luminogens not only provide ideal models to study the relationship between packing patterns and emission properties, but also facilitate the design of new high performance luminogens. Intermolecular interactions, such as π-π stacking, hydrogen bonding are widely involved to explain the relationships between crystals and emissions, while intramolecular effects, especially the torsion angels between the donor and the fluorophore have not been fully recognized. In this paper, a new coumarin derivative coumarinopyrone (CP) was synthesized and four crystals (CP-G1, CP-G2, CP-R and CP-O) were obtained by recrystallizing. These four crystals with different stacking modes or molecular conformations elicited bright fluorescent emissions varied from 520 nm to 620 nm. The relationship of fluorescent emissions with the intra-/intermolecular interactions was revealed through single crystals analysis. Interestingly, crystals CP-O (emission at 580 nm) and CP-R (emission at 620 nm) had nearly identical stacking mode, but differentiated at tiny torsion angel between the diethylamino donor and the coumarin fluorophore. It may provide crystal-level evidence to explain the effect of the rotation of one donor on fluorescence properties. The emissions of CP could be reversibly switched among blue and orange through recrystallizing or solvent fuming. CP-G2 and CP-O exhibited fluorescence response to NH3, and CP-O was further expected to be a food spoilage sensor.