Tailoring Emission Lifetime for Dopant-Induced Phosphorescence

Despite phosphorescent materials having found wide application in optoelectronic and biological areas, the mechanistic understanding of ultralong room temperature phosphorescence (RTP) from metal-free emitters is underdeveloped. Recently, an increasing number of reports have suggested that RTP from purely organic phosphorescent materials requires the presence of trace impurities or dopants in the bulk emitter. Contributing to a better understanding of mechanistic aspects, we demonstrate that the RTP lifetime is directly proportional to intermolecular charge transfer (CT) properties of the crystalline bulk emitter. Given that the CT efficiency is contingent on crystal packing arrangement, variations of interemitter distances in the crystal lattice by modulating sterics of the emitter core structure represent an efficient means to manipulate the dopant-induced RTP lifetime. The versatility of the "steric modulation" approach has been demonstrated in two different dopant/emitter systems. A 38-fold increase (from 21 to 775 ms) was observed in the RTP lifetime of benzo[b]carbazole-doped carbazole-containing materials.