Solvent-Polarity-Dependent Excited-State Behavior and Thermally Active Delayed Fluorescence for Triquinolonobenzene

The triple hydrogen-bonded triquinolonobenzene (TQB) molecule is investigated for its excited-state dynamics and proton transfer (ESIPT) mechanism in different solvents in this work. Through insights into electrostatic potential surface (EPS), reduced density gradient, and isosurfaces of gradient, we confirm that three intramolecular hydrogen bonds are formed for the TQB molecule. Exploring geometrical parameters involved in hydrogen bonds, infrared (IR) vibrational spectra, and bond energy via atoms in molecules (AIM) analyses, it could be verified that hydrogen bonds are strengthened in the first (S₁) excited state. Based on comparing the energy gaps among frontier molecular orbitals (MOs) in four aprotic solvents, we predict that the ESIPT reaction of TQB could be facilitated with the increase of solvent polarity. Comparing the relationship among all the stable configurations and simulating potential energy surfaces (PESs), we present that the ESIPT process of the TQB system could be controlled through solvent polarity. Given the thermally active delayed fluorescence (TADF) process of the ESIPT product in the S₁ state via reverse intersystem crossing (RISC), we verify that polar solvents suppress the TADF process to some extent. We speculate that the moderate polar solvents can facilitate high efficiency photoluminescence for the TQB system, which endows the TQB system with a kind of compelling optoelectronic and biological material.