Time-Dependent Density Functional Theory Investigation of Excited State Intramolecular Proton Transfer in Tris(2-hydroxyphenyl)triazasumanene

Previously, our group reported dual-emission spectra for tris(2-hydroxyphenyl)triazasumanene (OHPhTAS), comprising three OH···N-type intramolecular hydrogen bonds from three phenolic rings connected to the nitrogen-doped buckybowl skeleton, corresponding to the excited state intramolecular proton transfer (ESIPT) in the solid state. However, the dual emission is not observed in a nonpolar solution. In this study, the mechanism and multiplicity of potentially photoinduced dynamic ESIPT were investigated both in ground (S₀) and in excited states (S₁) by time-dependent density functional theory calculations. Different pathways, concerted and stepwise (single, double, or triple) PT processes, are considered. The calculated vertical emission energies (S₁ → S₀ states) and adiabatic total energies at S₀ and S₁ states of OHPhTAS and its tautomers revealed that a single PT, trienol (EEE) → monoketo (KEE), is the main contribution in OHPhTAS with an ultrasmall PT energy barrier. The nonradiative decay of OHPhTAS was analyzed by the potential energy curve (PEC) at the S₁ state along EEE* to KEE*. The results indicated that nonradiative decay was prohibited in the solid state but significantly stabilized in nonpolar solutions. The nonradiative routes in the solution state were confirmed by the minimum energy crossing point of the T₁/S₀ pathway, wherein the dihedral angle φ between the phenolic ring and pyridine moiety on the buckybowl structure relaxed to 123°.