Regulating the ESIPT processes and photophysical properties of indole derivates by introducing the appropriate functional groups

In this work, the substitution effect of different electron-donating and electron-withdrawing groups on the electron spectra and excited state intramolecular proton transfer (ESIPT) properties of the chromophore 2-(2-hydroxy-5-methylphenyl)benzo[d]oxazole-5-sulfonic acid (BOM-1) has been studied via the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. All the compounds exhibit the dual-fluorescence emission, and the fluorescence spectra of the derivatives show redshift emission compared with that of the original molecule, resulting in the larger Stokes shift as expected. The results were cross-verified by the molecular geometry, frontier molecular orbitals, infrared spectroscopy and other auxiliary calculations. These results show that the electron-withdrawing group greatly promote the occurrence of the forward ESIPT process and increase the Stokes shift. What is more special is that the Stokes shift and the energy barrier of the forward ESIPT are significantly enlarged by introducing both electron-withdrawing and electron-donating groups. The energy barrier, absorption and fluorescence spectra of luminescent molecules can be adjusted effectively by introducing appropriate functional groups. In addition, the emission spectra from the ultraviolet region (359 nm) to the orange region (601 nm) are obtained. This work would contribute to the understanding of the mechanism of ESIPT reactions and improves the luminescence properties of ESIPT molecules.