Abstract
In this work, N,N’-bis(salicylidene)-(2-(3′,4′-diaminophenyl)benzothiazole) (named as “BTS”) system was studied about its excited-state intramolecular proton transfer (ESIPT) process. The analyses about reduced density gradient (RDG) reveal the formation of two intramolecular hydrogen bonds in BTS system. Bond lengths and angles, infrared (IR) vibrations as well as frontier molecular orbitals (MOs) using TDDFT method indicate that the strength of hydrogen bond should be enhanced in the S1 state. Particularly, hydrogen bond O1–H2···N3 undergoes larger variations compared with O4–H5···N6, which infers that hydrogen bond O1–H2···N3 may play a decisive role in the ESIPT process of BTS system. Given the two hydrogen bonds of BTS molecule, two types of potential energy curves have been constructed, which confirms that only single proton transfer process occurs due to lower energy barrier along with O1–H2···N3 rather than O4–H5···N6. This work not only presents a reasonable explanation for previous experiment, but also clarifies the specific ESIPT mechanism for BTS system.
Highlights
As one of the most fundamental weak interaction, intra- as well as inter- molecular hydrogen bond is illocal in nature[1,2,3]
At B3LYP/TZVP theoretical level, the structural optimizations of the four molecular configurations were performed with DFT/TDDFT methods
The excited-state intramolecular proton transfer (ESIPT) process of BTS was studied with TDDFT method at the IEFPCM/B3LYP/TZVP theory level
Summary
As one of the most fundamental weak interaction, intra- as well as inter- molecular hydrogen bond is illocal in nature[1,2,3]. Upon photoexcitation, excited-state intra- (inter-) molecular proton transfer (ESIPT) process is the initial step of many photobiological and photochemical reactions, which is crucial in nature. Since BTS owns two intramolecular hydrogen bonds, it is possible for the molecule to undergo ESIPT process along with one or two hydrogen bonds In this previous work[40], Duarte and co-workers did not pay much attention to how the ESIPT process takes place in S1 state and whether single or double proton transfer process happens is not clear up to now. The TDDFT method has been recognized as a quite useful tool in the theoretical research of hydrogen-bonded systems[45,46,47,48,49,50,51,52]
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