Theoretical insights on the double ESIPT mechanism and fluorescence properties of HBIo chromophore.

2-{[3-(1H-benzoimidazol-2-yl)-2-hydroxy-5-methylbenzylidene] amino}-benzoic acid (HBIo) based on proton transfer can serve as the fluorescent probe for detecting heavy metal ions. The excited-state intramolecular proton transfer (ESIPT) reaction mechanism of the HBIo chromophore with an intramolecular asymmetric double hydrogen bond in different solvents are investigated. The reaction barrier of the ESIPT along hydrogen bond O1-H2···N3 is higher than that of ESIPT along O4-H5···N6, which indicates that the double ESIPT is a stepwise process. The time-evolving non-adiabatic excited-state dynamic simulations shows that the sequence ESIPT reactions on a time scale: the ESIPT along O1-H2···N3 is faster than the ESIPT along O4-H5···N6. The analyses of electron structure and spectra indicate that the double ESIPT couples with electron transfer, significantly enhances the fluorescence signal, thereby improving the performance of the fluorescent probe in detecting heavy metal ions.