Excited-State Intramolecular Proton Transfer: A TD-DFT Study of the Delayed Fluorescence Mechanism in Triquinolonobenzene

Abstract The interplay between excited-state intramolecular proton transfer (ESIPT) and thermally activated delayed fluorescence (TADF) was investigated in the prototypical emitter triquinolonobenzene (TQB), using angular-quinacridone (a-QD), which exhibits analogous ESIPT behavior but lacks TADF, as a reference system. Excited-state transition-state optimizations reveal that ESIPT is essentially barrierless in both molecules. Owing to the negligible spin–orbit coupling (SOC), intersystem crossing (ISC) is ineffective prior to or during the proton-transfer process. Following ESIPT to the keto tautomer (TQB-TB), however, efficient ISC channels become available, with the calculated S1 → T1 and S1 → T2 rate constants reaching 1.10 × 108 and 1.37 × 108 s–1, respectively. More importantly, the exceptionally small S1–T2 energy gap (0.01 eV) enables highly efficient reverse ISC, resulting in a T2 → S1 rate constant of 9.70 × 107 s–1, whereas the direct T1 → S1 rISC remains very slow (1.18 × 102 s–1) owing to the much larger S1–T1 gap. Analysis of the vibronic coupling mechanism shows that Herzberg–Teller contributions dominate most ISC and reverse ISC processes, whereas the Franck–Condon contribution remains indispensable for the S1 ↔ T2 channels, accounting for 41.82% (S1 → T2) and 26.10% (T2 → S1) of the corresponding rates. These results establish a comprehensive mechanistic picture in which ESIPT promotes efficient TADF by activating favorable singlet–triplet interconversion pathways in the keto tautomer, providing molecular-level insight into the design principles of ESIPT-assisted TADF emitters.