Theoretical insights into the ultrafast excited-state intramolecular proton transfer (ESIPT) mechanism in a series of amide-based N[sbnd]H⋯N hydrogen-bonding compounds

Abstract

Excited-state intramolecular proton transfer (ESIPT) has many important potential applications. In this paper, the geometries of a series of amide-based NH⋯N hydrogen-bonding compounds in their ground S0 states and first excited singlet S1 states were optimized with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches. Both topological analysis and noncovalent interactions analysis show strong intramolecular hydrogen-bonds in the studied five systems and the electron density function ρ(r) exhibits good linear relationship with the distance of H⋯N2. The potential energy curves of the S0 and S1 states were scanned by using of DFT and TD-DFT to elucidate the ESIPT process. It reveals that all the systems considered here can undergo an ultrafast ESIPT reaction, with energy barrier of less than 0.05 eV, giving rise to the single fluorescence emission from the proton-transfer tautomer. It is also found that the shorter the hydrogen bond in the normal-form S1 state, the easier the ESIPT takes place.