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

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 (ΔE_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 ΔE_T, temperature, and doping ratio. This work provides a mechanistic insight and paradigm into the design of RTP materials through intermolecular exciton dynamics.