Unlocking the Room Temperature Phosphorescence through Halogen Engineering in Carbazole Dimer.

Room-temperature phosphorescence (RTP) in metal-free organic materials offers immense potential for advanced optoelectronic applications. However, the rational design and fine-tuning of RTP remain challenging due to the complex correlation between molecular structure and photophysical processes. Herein, we explored the impact of intersystem crossing (ISC), spin-orbit coupling (SOC), and halogen interactions to promote RTP in crystalline brominated carbazole dimer (BrCz-D). In contrast, the unsubstituted analogue, carbazole dimer (Cz-D) exhibits thermally activated delayed fluorescence (TADF) under ambient conditions. Femtosecond transient absorption (fsTA) spectroscopy measurements confirmed the population of triplet manifolds in both dimers. Bromine substitution significantly enhances spin-orbit coupling (VSOC = 14.94 cm⁻¹), enabling efficient ISC and robust RTP in BrCz-D. Enhanced RTP in crystalline BrCz-D is attributed to unique halogen interactions, including Br···Br, C···Br, and H···Br, within the crystal lattice. Such halogen interactions are negligible in the solution state, accounting for the lack of RTP under ambient conditions. The present work highlights the critical role of SOC and halogen bonding in achieving efficient RTP for designing high-performance organic phosphorescent materials through crystallochemistry.