Theoretical insights into the excited-state process of 4-tert-butyl-2-(5-(5-tert-butyl-2-methoxyphenyl)thiazolo[5,4-d]thiazol-2-yl)-phenol

In this paper, we theoretically explore the motivation and behaviors of the excited-state intramolecular proton transfer (ESIPT) reaction for a novel white organic light-emitting diode (WOLED) material 4-tert-butyl-2-(5-(5-tert-butyl-2-methoxyphenyl)thiazolo[5,4-d]thiazol-2-yl)-phenol (t-MTTH). The “atoms in molecules” (AIM) method is adopted to verify the formation and existence of the hydrogen bond OH···N. By analyzing the excited-state hydrogen bonding behaviors via changes in the chemical bonding and infrared (IR) vibrational spectra, we confirm that the intramolecular hydrogen bond OH···N should be getting strengthened in the first excited state in four kinds of solvents, thus revealing the tendency of ESIPT reaction. Further, the role of charge-transfer interaction is addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. Also, the electron distribution confirms the ESIPT tendency once again. The scanned and optimized potential energy curves according to variational OH coordinate in the solvents demonstrate that the proton transfer reaction should occur in the S ₁ state, and the potential energy barriers along with ESIPT direction support this reaction. Based on the excited-state behaviors reported in this work, the experimental spectral phenomenon has been reasonably explained.