The constructions and applications of purely organic phosphorescent material

Abstract

Phosphorescence is among the many functional features of the excited organic molecules. Organic molecules with long-lived triplet excited states enable exciton migration over long distance, which is essential in a variety of optoelectronic applications such as photovoltaics, photocatalytic reactions, display and lighting system, molecular sensing and biological imaging. In general, phosphorescence materials involve the organometallic compounds that are expensive, such as Pt, Ir and so on. Owing to the presence of metal, the phosphorescence materials is toxic and are also intrinsically unstable in the case of high-energy blue emitters. Therefore the purely organic materials that show room-temperature phosphorescence (RTP) are attractive alternatives to organometallic phosphors. The triplet excitons are not commonly generated in organic materials and are mostly consumed through radiationless processes, such as vibrational dissipation and oxygen-mediated quenching, under ambient conditions. Recently, a variety of methods from molecular design to material design to access organic room temperature phosphorescence have been explored. This review focus on the recent advancements in the field of organic phosphorescence, classifying the strategies based on material design. The outlook of organic room temperature phosphorescence is also discussed. The basic concepts for RTP process are introduced to demonstrate what’s the key factors to obtain efficient RTP. They are: (1) the promotion of both singlet-to-triplet and triplet-to-singlet intersystem crossing, and (2) suppression of the nonradiative quenching processes from the triplet to the ground state. Comparisons between RTP and delayed fluorescence are also made in the review. The triplet excited states of organic molecules can be stabilized by interactions of host molecules and guest molecules, rigid solid-crystallized structures and the chromophore embedded in polymer, which can suppress the nonradiative deactivation pathways and rates of triplet excitons by restricting the vibrational dissipation of long-lived triplets, and considerably increase the possibility of radiative decay. The directed intermolecular halogen bonding can also promote the intersystem-crossing processes by enhancing spin-orbit coupling, and improve the radiative decay. In most studies, various methods are combined to obtain phosphorescence materials of long-lived triplet excited states with high quantum yield.