Solvent effect on excited-state intramolecular proton transfer process based on 'naked' diazaborepins

Excited-state intramolecular proton transfer (ESIPT) process based on 'naked' diazaborepins (NDABs) by modification on the parent structure (NDAB-H) is confirmed by experiment and theory calculation. An emission peak is detected at longer wavelength with large Stokes shift in toluene, and a new emission shoulder gradually appeared at short wavelength as the increase of solvent polarity, indicating the ESIPT process is tuned by solvent. Herein, the solvent effect and the mechanism of the ESIPT reaction are further investigated by DFT calculation. Without considering the effect of proton in solvent, seven aprotic solvents with different dielectric constant from DMSO (46.8) to cyclohexane (1.18) are chosen. Typical bond lengths, angles, electron distribution and transfer on frontier molecular orbital and thermodynamics analysis are discussed in different solvents for comparison. This work proved that the ESIPT process is inhibited as the increase of the solvent polarity that is in good agreement with the experimental date.