Multiscale Simulations Reveal the Mechanism of Host‐Induced Room‐Temperature Phosphorescence in Multiple Resonance Emitters

Abstract Multiple resonance (MR) emitters typically exhibit narrowband thermally activated delayed fluorescence (TADF), yet their recently discovered host‐assisted room‐temperature phosphorescence (RTP) remains unclear. Herein, the origin of this luminescence switching is elucidated through a combined theoretical and experimental study of a prototypical MR emitter, quinolino[3,2,1‐de]acridine‐5,9‐dione (QAO). While QAO displays TADF in 9,9′‐(1,3‐phenyl)di‐9H‐carbazole ( m CP) film but switches to RTP in benzophenone (BP) crystal. Multiscale simulations reveal that BP‐induced conformational distortion triggers a conversion from S ₁ (π, π ^* ) to S ₁ (n, π ^* ). This change i) introduces a dipole‐forbidden transition, leading to >300‐fold reduction in radiative decay rate; ii) and largely enhances the spin–orbit coupling of S ₁ → T ₁ , accelerating the intersystem crossing (ISC) rate by five orders of magnitude, with the aid of enhanced vibronic coupling. Additionally, the singlet‐triplet energy gap shows a tiny change with values of ≈0.2 eV, supporting the reverse ISC for triplet exciton harvesting. As a result, the bright TADF of QAO in m CP film is converted to RTP in BP crystal, with theoretical predictions showing excellent agreement with experimental emission spectra and RTP lifetimes. These findings provide fundamental insights into the molecular design of high‐performance MR‐based RTP materials, paving the way for next‐generation organic optoelectronic applications.