Taming Intramolecular Charge Transfer via Positional Isomerism for Tunable Blue Room‐Temperature Phosphorescence

Comprehensive Summary Harnessing intramolecular charge transfer (ICT) is a potent strategy for developing high‐performance organic room‐temperature phosphorescence (RTP) materials. However, profound excited‐state structural relaxation typically induces significant emission red shift, fundamentally impeding the realization of blue RTP. Herein, we present a spatial engineering strategy to finely modulate the degree of ICT through positional isomerism without altering the core molecular scaffold. By systematically tuning methoxy substitution ( p ‐OMe, m ‐OMe, and o ‐OMe), we discovered that a unique intramolecular O···N non‐covalent interaction in o ‐OMe effectively locks a highly twisted conformation, thereby moderating ICT strength. Theoretical calculations demonstrate a progressive decrease in triplet‐state root‐mean‐square deviation (RMSD) values from p ‐OMe to o ‐OMe, confirming the successful suppression of excited‐state geometric relaxation. Consequently, the emission undergoes a progressive blue shift from green (500 nm, p ‐OMe) to blue RTP (473 nm, o ‐OMe) within a PMMA matrix. The decisive role of this O···N conformational lock is further validated through host‐generality experiments and a rationally designed methyl‐substituted control. Leveraging these efficient triplet excitons as Förster resonance energy transfer (FRET) donors, a full‐color afterglow library was established for advanced information encryption. This study highlights that precise non‐covalent modulation is an elegant and efficient strategy for overcoming spectral red shift in ICT‐type blue RTP systems.