Organic Crystals with Near-Infrared Amplified Spontaneous Emissions Based on 2'-Hydroxychalcone Derivatives: Subtle Structure Modification but Great Property Change.

A series of highly efficient deep red to near-infrared (NIR) emissive organic crystals 1-3 based on the structurally simple 2'-hydroxychalcone derivatives were synthesized through a simple one-step condensation reaction. Crystal 1 displays the highest quantum yield (Φf) of 0.32 among the reported organic single crystals with an emission maximum (λem) over 710 nm. Comparison between the bright emissive crystals 1-3 and the nearly nonluminous compounds 4-7 clearly gives evidence that a subtle structure modification can arouse great property changes, which is instructive in designing new high-efficiency organic luminescent materials. Notably, crystals 1-3 exhibit amplified spontaneous emissions (ASE) with extremely low thresholds. Thus, organic deep red to NIR emissive crystals with very high Φf have been obtained and are found to display the first example of NIR fluorescent crystal ASE.