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.