Mode‐Selective C═C and C─N Vibrational Coupling Modulates Nonradiative Decay in Regioisomeric Organic Phosphorescent Emission

Structural isomerization has emerged as an effective strategy for improving the performance of organic room‐temperature phosphorescent materials, but reported explanations are mainly attributed to differences in molecular packing, spin‐orbit coupling, excited‐state electronic configurations, etc. The intrinsic vibrational differences encoded in the molecular structures of isomers, which may fundamentally govern triplet‐state excitons’ nonradiative behaviors, remain largely unexplored. Here, we construct five phosphorescent molecular structural isomerization pairs based on both purely hydrocarbon and heteroatom‐containing aromatic compounds, elucidating how regioisomerization redistributes local aromatic character and skeletal rigidity, thereby selectively suppressing the dominant dissipative vibrational modes (C═C and C─N) in the triplet state. This selective vibrational restriction effectively reduces nonradiative decay loss and ultimately determines the efficiency and lifetime of triplet excitons. The representative regioisomeric phosphor further demonstrates long‐lived bioimaging capability with an extended in vivo imaging window, highlighting the practical relevance of this design strategy. This work identifies dominant vibrational coordinates as the primary origin of isomer‐dependent RTP behaviors and establishes a vibrationally resolved mechanistic framework for excited‐state relaxation engineering.