New insights into the recognition and sensing mechanism of a CN– fluorescent probe: A theoretical study

The recognition and sensing mechanism of the fluorescence probe AHAM for cyanide ions (CN^–) detection have been investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Through isomer calculations and boltzmann distribution analysis, we identified the reasonable structure for probe AHAM, which differs from the structure proposed in previous studies. We further confirmed that the reaction site between AHAM and CN^– is the hydrogen atom in the NH₂ unit, rather than the hydrogen atom in the OH unit. For the probe AHAM, the excited state results indicate that the single fluorescence emission is attributed to the excited state intramolecular proton transfer (ESIPT) process and AHAM-Keto emission. The experimentally observed weak fluorescence for AHAM is caused by the twisted intramolecular charge transfer (TICT) process of AHAM-Keto in the S₁ state. Upon the addition of CN^–, the solution exhibits turn-on fluorescence with the appearance of dual emission bands. Potential energy curves (PECs) calculations suggest that these dual emission bands correspond to emissions from AHAM-CN^–-Enol and AHAM-CN^–-Keto, respectively. The turn-on fluorescence is due to the inhibition of the TICT process. Moreover, we further investigated the ’off–on-off’ phenomenon observed during CN^– detection when TFA is added. The binding energy results illustrate that the reverse mechanism is due to the higher affinity of TFA compared to AHAM for CN^– detection.