Modeling Room-Temperature Phosphorescence in Organic Materials: A Spin–Vibronic Hamiltonian Approach

Room-temperature phosphorescence (RTP) in purely organic systems is a rare yet highly desirable phenomenon for applications in sensing, imaging, and anti-counterfeiting. Unlike fluorescence, which arises from spin-allowed transitions, RTP originates from spin-forbidden triplet-to-singlet transitions and typically requires both enhanced intersystem crossing (ISC) and protection from non-radiative decay. Despite its importance, the microscopic mechanisms underlying RTP remain elusive. To simulate the fluorescence and phosphorescence spectra of organic materials, we employ the Frenkel–Holstein Hamiltonian that explicitly includes both singlet and triplet excitons in the basis set, and a spin-relaxation term that mixes states with different multiplicities and exchange interactions. Distinct transfer rates are assigned to each manifold: singlet excitons transfer via both Förster and Dexter mechanisms, whereas triplet excitons transfer exclusively through Dexter coupling. The mixing of singlet/triplet manifolds induced by spin orbit coupling (SOC) results in a triplet-dominated absorption band that borrows oscillator strength from the singlet transition. The intensity of this band scales directly with SOC strength and inversely with the singlet–triplet energy gap. Upon incorporating vibronic coupling, the vibronic A₁/A₂ intensity ratio of the triplet- and singlet-dominated peaks depend on the signs of the Dexter and net singlet couplings, respectively, consistent with reported vibronic progression rules for excitonic aggregates. This theoretical framework elucidates the key spectroscopic signatures of coupled singlet–triplet excitons and provides a pathway for modeling RTP to rationalize efficient phosphorescence in purely organic materials at room temperature.