Theoretical study on the effect of naphthalene modifications on the ESIPT mechanism and photophysical properties of benzimidazoles

Recently, the researchers found that the large ring benzimidazoles chromophores are prone to deprotonation to form double ion holes, which can strongly bind with organic cations. The findings apply well in the field of cation detection, however, the investigations of monomolecular excited state hydrogen bonding kinetic properties are still lacking. In this paper, we selected six structures RBI (1a, 1b, 1c) and HBI (2a, 2b, 2c) with the naphthalene modifications to analyze the effect on the excited state intramolecular proton transfer (ESIPT) and fluorescence phenomena with quantum chemical calculation. Firstly, the geometry was fully optimized, the hydrogen bonding parameters and infrared vibrational frequencies were calculated to illustrate the more pronounced effect of naphthalene modifications on excited-state hydrogen bonding properties. Secondly, analyses of absorption and emission spectra indicated that molecular photophysical behaviors depends on the naphthalene modifications. Moreover, the analyses of electronic structures quantitatively revealed that different fluorescence properties are attributed to the effect of naphthalene modification on intramolecular charge transfer (ICT). Finally, through establishing potential energy curves and simulating the time-resolved excited-state dynamics trajectories of ESIPT, the reactive activation energies were accurately calculated and the probability of the reaction occurring was determined. After a series of theoretical analyses, it was found that when the naphthalene is modified directly above the molecule, the degree of ICT is greater, and the inhibition of the ESIPT reaction is smaller, which further affects the fluorescence properties of the molecules, and is better applied to the detection of cations and other aspects.