Different ESIPT Mechanisms for Angular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Abstract

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms of angular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S1 state, which may facilitate the ESIPT process. Particularly, in toluene, the S1‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.