Ring Rigid‐Flexible Regulatory Strategy for Modulating Organic Room Temperature Phosphorescence

ABSTRACT Organic room‐temperature phosphorescence (RTP) represents a distinctive photophysical phenomenon intrinsically linked to the interplay between intersystem crossing (ISC) efficiency and spin‐orbit coupling (SOC). Consequently, precise modulation of these fundamental properties is essential for achieving desirable RTP characteristics. Molecular geometry can influence the electronic factors associated with ISC and SOC. Nevertheless, systematic investigations aimed at elucidating and strategically manipulating molecular configurations to tailor RTP properties remain insufficiently explored. Herein, we present a rigid‐flexible regulation strategy: two pairs of novel annulene derivatives ( C 6 ‐BNH and C 6 ‐BNPM, C 8 ‐BNH and C 8 ‐BNPM) with distinct molecular configurations were designed and synthesized. Interestingly, the non‐coplanar flexible structures of C 8 ‐BNH and C 8 ‐BNPM exhibited desirable phosphorescence properties, whereas their rigid planar counterparts ( C 6 ‐BNH and C 6 ‐BNPM) displayed only very weak RTP that was barely perceptible to the naked eye. Structure–photophysics relationships were examined using photoluminescence measurements, crystal structures, and theoretical calculations. The results suggest that the C 8 compounds undergo changes in molecular geometry, intermolecular contacts, and aromaticity‐related descriptors between the ground and excited states. These observations, together with the calculated SOC matrix elements, are consistent with a greater ISC propensity in the C 8 series and may contribute to its RTP behavior. This work presents an effective molecular configuration modulation approach for RTP material development, advancing research in the exciting yet underexplored field of organic RTP.