Theoretical insights into the excited state processes of a novel fluorescent probe for thiophenol with large Stokes shift

• The direction in the proton transfer for FQOH has been investigated. • Fluorescence quenching mechanism for FQDNP has been explored. • The excited state processes for FQDNP responding to PhSH have been depicted. Based on a new designed hydroxylflavone-quinoline fluorophore, a novel fluorescent probe FQDNP for detecting thiophenol with large Stokes shift has been developed recently. Despite the excited state intramolecular proton transfer mechanism has been mentioned in the report, the detailed excited state processes for the compound FQOH produced by FQDNP reacting with PhSH are still ambiguous. Typically, in which direction will the proton of hydroxyl group transfer in the excited state? Oxygen or nitrogen atom, which one is more likely to interact with the proton in hydroxyl group by an intramolecular hydrogen bonding? In this work, we have explored the excited state processes of the probe detecting for thiophenol with the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods. The calculated photo-induced changes in the optimized geometry parameters, intramolecular hydrogen bonding energies, stretching vibration frequencies, as well as the energy barriers for FQOH imply the proton of hydroxyl group is more likely to transfer to nitrogen atom rather than to oxygen atom in the S 1 state. The calculations on the probe FQDNP reveals the fluorescence is quenched by the twisting and photoinduced electron transfer process. Our calculations provide a detailed description for the excited state processes of the probe sensing to thiophenol, which is expected to be applied in designing more effective fluorescent probes.