Mechanistic Insights into Room-Temperature Phosphorescence in a 1,4-Diiodotetrafluorobenzene-Phenanthrene Cocrystal

Abstract

Room-temperature phosphorescent (RTP) organic materials are attracting increasing interest for applications in optoelectronics, sensing, photomedicine, bioimaging, and OLED technologies. Metal-free organic emitters are particularly appealing due to their low toxicity, tunable photophysics, and reduced cost compared with organometallic systems. Recent work by Abe et al. (Adv. Mater. 2024, 36, 2211160) demonstrated that cocrystals composed of 1,4-diiodotetrafluorobenzene (DITFB) and phenanthrene (Phen) exhibit efficient RTP, yet the microscopic mechanisms enabling this behaviour remain unclear. Here, we investigate the radiative and non-radiative excited-state processes in the Phen-DITFB system using an embedded multiscale approach to elucidate the factors governing RTP in organic cocrystals. Our results show that cocrystallisation profoundly reshapes the excited-state landscape relative to the isolated molecules. Aggregation increases the density of triplet states near the lowest singlet excited state (S1), creating multiple energetically accessible ISC channels (T4 -T10). Spin-orbit couplings are simultaneously enhanced through both intermolecular and, in specific cases, intramolecular charge-transfer contributions. The resulting triplet manifold displays a diversity of electronic characters: several states are localised on the DITFB units, enabling intermolecular charge-transfer-assisted S1→Tn transitions, while T9 shows intramolecular electron reorganisation on Phen that further strengthens the SOC. This interplay between localisation, CT character, and orbital composition produces highly efficient ISC pathways, as confirmed by the computed rate constants. Collectively, these findings provide a detailed mechanistic picture of RTP in the Phen-DITFB cocrystal. Cocrystallisation enhances RTP by increasing the density of accessible triplet states, strengthening SOC through CT-mediated interactions, and suppressing competing non-radiative decay pathways. This study highlights how molecular electronic structure and supramolecular organisation act synergistically to enable efficient RTP in purely organic systems.