Ultralong Room Temperature Phosphorescence of the Host in Host–Guest Doped Systems

ABSTRACT Room temperature phosphorescence (RTP) materials with long luminescence lifetimes and tunable properties are highly desirable for applications in optoelectronics, information encryption, and bioimaging. Although host–guest doping is an effective strategy to achieve high‐performance RTP in most systems, the RTP originates from the guest through host confinement and charge/energy transfer. Activating RTP from the host remains a significant challenge due to inefficient triplet exciton manipulation. Here, we successfully achieved ultralong host RTP (224 ms) in the host–guest doped systems with a controlled triplet state energy gap (Δ E T ) of 0.36–0.44 eV, which facilitates an endothermic reverse triplet–triplet energy transfer (rTTET) process from the guest to the host, and thereby promotes the repopulation of host triplet excitons. This process establishes a dynamic thermal equilibrium between forward triplet–triplet energy transfer (TTET) and rTTET, leading to thermally activated delayed phosphorescence from the host. As a result, the lifetime of the host's RTP is extended by 1120‐fold (from 0.2 to 224 ms). More importantly, the photoluminescence properties of the host's RTP can be systematically and predictably tuned by varying the Δ E T , temperature, and doping ratio. This work provides a mechanistic insight and paradigm into the design of RTP materials through intermolecular exciton dynamics.