A Theoretical Investigation of Long-Lived Room Temperature Phosphorescence in the TMB/PPT Host–Guest System

Room temperature phosphorescence (RTP) holds significant promise for applications ranging from multicolor anticounterfeiting to prolonged monitoring of short-wavelength ultraviolet (UV) radiation. However, the theoretical mechanism governing phosphorescence emission in host–guest doping systems remains incompletely understood. This work investigates the photophysical processes in an organic host–guest system comprising N,N,N′,N′-tetramethylbenzidine (TMB) and 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT). Using density functional theory (DFT), time-dependent DFT, molecular dynamics, and Marcus theory, we systematically analyzed charge transfer, energy transfer, and excited-state dynamics across varying host–guest distances. Our results demonstrate that shorter distances accelerate intersystem crossing (ISC) via dramatic ΔEST reduction and prolong RTP lifetime by suppressing nonradiative decay. Conversely, larger distances diminish spin–orbit coupling (SOC), increase singlet–triplet energy gaps, suppress ISC, and reduce persistence. Critically, extended separation does not enhance charge separation or energy transfer rates─particularly for singlet energy transfer. Thus, doping primarily enhances phosphorescence lifetime and triplet yield through strong host–guest interactions, not the long-range charge separation frequently cited in the literature. This mechanistic insight clarifies phosphorescence in doped systems and informs the design of advanced materials for flexible coatings, optical storage, and bioimaging.