A theoretical investigation on ESIPT process of a red-emitting ratiometric fluorescent probe and its fluorescent detection mechanism for cyanide anion

Excited state intramolecular proton transfer (ESIPT) process of a fluorescent probe (EP1) and its fluorescent detection mechanism for cyanide anion (CN−) have been investigated theoretically. Optimized structures indicate that the hydrogen bond (O1H2···O3) in EP1 is strengthened upon photo-excitation and the O1H2 proton in EP1-CN formed after adding CN− transfers spontaneously to O3. Potential energy curves confirm that proton transfer in EP1 is impossible because energies of the S0 and S1 states increase with the O1H2 bond length. While proton transfer in EP1-CN is unobstructed because energies of the S0 state decrease with the O1H2 bond length. Compared to EP1, the absorption and fluorescence spectra of EP1-CN are both red-shifted (87 and 41 nm) due to the large charge transfer extent. Orbital-weighted dual descriptor isosurface and condensed local nucleophilicity indices confirm that the carbon atom on the aldehyde group is the nucleophilic site of CN−. Transition state searching demonstrates that the occurrence of nucleophilic addition reaction between EP1 and CN− should overcome a reaction barrier of 14.29 kcal/mol and then get EP1-CN, which has 6.61 kcal/mol lower energy than reactants. Thus, EP1 detecting CN− is through the fluorescence variation induced by the large charge transfer extent rather than by hampering ESIPT.