Insight into the excited-state intramolecular double-proton transfer of the 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol: one-step or stepwise mechanism?

• Published in: Theoretical chemistry accounts 2017-05, Vol.136 (5), p.1-7, Article 62
• Main Authors: Lu, Meiheng, Yang, Yunfan, Chu, Tianshu
• Format: Article
• Language: English
• Subjects: Atomic/Molecular Structure and Spectra; Chemical bonds; Chemistry; Chemistry and Materials Science; Density functional theory; Excitation; Excitation spectra; Hydrogen bonds; Infrared analysis; Infrared spectra; Inorganic Chemistry; Molecular orbitals; Molecular structure; Organic Chemistry; Physical Chemistry; Potential energy; Protons; Regular Article; Theoretical and Computational Chemistry; Two dimensional analysis; Vibrational spectra
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-017-2088-9

The excited-state double-proton transfer mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol has been theoretically investigated based on the methods of density functional theory and time-dependent density functional theory. Geometric structure comparison and infrared vibrational spectra analysis confirm that the strengthening of the intramolecular hydrogen bond in the first excited state ( S 1 ) has facilitated the proton transfer process. The frontier molecular orbitals analysis illustrates that the nature of the hydrogen bond enhancement lies in the charge redistribution upon photo-excitation, which is further confirmed by NBO charge analysis quantitatively. The reduced dimensionality (two-dimensional) potential energy surfaces have been scanned for both the ground state ( S 0 ) and the first electronic excited state to reveal the mechanism and pathway of the two protons being transferred. Compared with the concerted path of proton transfer, the stepwise path with one proton being transferred first and then a second one followed exhibits a relatively mild energy barrier and seems to be more rational and favorable.