Excited-state intramolecular proton transfer in non-fused five- and fused six-membered ring pyrrole-pyridine hydrogen bond systems

Abstract

Abstract The non-fused five-membered ring structure of 2-(1H-pyrrol-2-yl)pyridine (5-HB) is more flexible than the fused six-membered ring structure of 10,11,12,13-tetrahydro-9H-quinolino[8,7-a]carbazole (6-HB) to form the intramolecular hydrogen bond between pyrrole and pyridine, while the six-membered ring structure possesses more favorable distance and orientation between pyrrole and pyridine. In the present work, we carried out the electronic structure calculations and nonadiabatic dynamics simulations to gain insight into the effect of different structure feature on the excited-state intramolecular proton transfer (ESIPT) processes of 5-HB and 6-HB. The geometric parameters, IR vibrational spectra, bond critical point (BCP) parameters and reduced density gradient (RDG) show that the intramolecular hydrogen bond of 6-HB is much stronger than that of 5-HB. The 6-HB has a lower kinetic stability and a higher chemical activity than 5-HB through the analysis of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The pyridine N1 atom of 6-HB shows much stronger ability than that of 5-HB to attract the hydrogen proton based on the natural population analysis (NPA). The analysis of the potential energy curves and time evolution of selected bond distances indicate that the ESIPT process of fused six-membered ring structure is easier and faster than the non-fused five-membered ring structure. Our findings might be useful for the design of relevant ESIPT systems.