Purely organic room-temperature phosphorescent materials for OLEDs: Molecular design, photophysics, and device strategies

Purely organic room-temperature phosphorescent (RTP) materials have emerged as a promising class of emitters for organic light-emitting diodes (OLEDs) owing to their potential for complete utilization of both singlet and triplet excitons. Compared with heavy-metal-based phosphorescent complexes containing iridium, osmium, or platinum, purely organic RTP materials offer distinct advantages, including structural tunability, low toxicity, sustainability, cost-effectiveness, and facile large-scale synthesis, making them attractive for next-generation display and lighting technologies. However, their practical application has long been constrained by the intrinsic difficulty of generating and stabilizing phosphorescence at room temperature, as purely organic compounds typically exhibit efficient phosphorescence only under cryogenic conditions or in oxygen-free environments. To address these limitations, extensive molecular engineering strategies—such as heteroatom incorporation, functional group modulation, host–guest systems, and crystallization-induced rigidification—have been developed to enhance intersystem crossing and suppress nonradiative decay. As a result, a diverse range of RTP-active small molecules, dendrimers, and polymers has been reported and successfully applied in OLEDs as either emitters or sensitizers. Notably, several of these materials have achieved impressive device efficiencies, including external quantum efficiencies (EQEs) of 24.91 % for 3,2-PIC-XT in non-doped OLEDs and 32.73 % for the dual-emissive through-space conjugated emitter 2,3-PICz-XT. In addition, the RTP dendrimer BPSAF-DCz exhibited a maximum EQE of 25.1 % in solution-processed OLEDs. Despite these advances, the efficiencies and operational stabilities of RTP-based OLEDs remain far behind those of other triplet-harvesting technologies, particularly thermally activated delayed fluorescence (TADF). Nevertheless, organic RTP systems remain of fundamental and technological importance because they provide a distinct triplet-emission mechanism, enable long-lived and dual-emissive behaviors, and offer valuable design insights for controlling triplet excitons in metal-free systems. This review provides a comprehensive overview of molecular design strategies, synthetic approaches, and structure–property relationships of purely organic RTP materials for OLED applications, summarizing key photophysical, electrochemical, and electroluminescent characteristics, and highlighting current challenges and future opportunities toward high-performance RTP-based OLEDs.