Rigidification or interaction-induced phosphorescence of organic molecules

Phosphorescent materials are mostly based on metal complexes. Metal-free organic molecules usually display phosphorescence only in a rigid matrix at 77 K. In the last few years, there has been increasing interest in the design of organic molecules displaying long-lived and highly intense room-temperature phosphorescence, an extremely difficult task since these two properties are generally conflicting. This review reports the most recent and tutorial examples of molecules that are weakly or non-phosphorescent in deaerated fluid solution and whose room temperature phosphorescence is switched on upon aggregation. The examples are divided into two classes according to the mechanism responsible for switching on phosphorescence: (i) rigidification by crystallization or by encapsulation in a polymeric matrix and (ii) interaction with other molecules of the same type (self-aggregation) or a different type by taking advantage of heavy-atom effects.