Impurity-Induced Phosphorescence in Purely Organic Materials

Metal-free or purely organic phosphorescent materials (phosphors) with ultralong emission lifetimes have emerged as an important class of multifunctional materials with a wide array of applications. However, the mechanistic understanding of phosphorescence in purely organic emitters apparently lags behind practical accomplishments, and the existing knowledge has not been summarized properly. For example, despite the impurity effect on phosphorescence from metal-free organic emitters having been well-recognized for over a century, recent studies have rediscovered the critical role of impurities in the phosphorescence. Hence, this review aims at bridging the apparent knowledge gap by integrating prior understanding with the latest findings on the role of impurities in phosphorescence. In addition, purification methods of phosphors, methodologies to test for impurity-induced emission, and mechanistic considerations for impurity-induced phosphorescence are highlighted.