Theoretical insights into the spectral properties of a salicylideneaniline-based fluorescence chemosensor (SB1) and its sensing mechanism for cyanide anion

The sensing mechanism of a salicylideneaniline-based fluorescence chemosensor (SB1) for the cyanide anion (CN−) has been investigated by nonadiabatic dynamics and transition state (TS) theory. The intramolecular hydrogen bonds in SB1 and the product SB1-CN are strengthened after photoexcitation, which induce the intramolecular proton transfer in 114 fs and 167 fs by the results of nonadiabatic dynamics. The fluorescence (626 nm) of SB1 emits from the proton-transfer tautomer rather than the excited state intramolecular charge transfer (ICT) state. The new appeared emission peak (523 nm) after the addition of CN− is attributed to the proton-transfer tautomer of SB1-CN, which has a large blue shift (103 nm) compared to that of SB1. The TS calculations demonstrate the occurrence of reaction from SB1to SB1-CN after overcoming an energy barrier of 10.52 kcal/mol and there is a large interaction energy (95.92 kJ/mol) between SB1 and CN−, both of which confirm that SB1 has high sensitivity and selectivity to CN−. The nucleophilic addition reaction of CN− interrupts the π-conjugation structure of SB1, resulting in an inefficient ICT, which in turn destroys the excited state intramolecular charge/proton transfer (ESICT-ESIPT) coupled system and causes the blue shift of the spectral peak. The work gives a complete explanation about ESICT-ESIPT coupling switching behavior and the sensing mechanism of SB1 for CN−.