Spectroscopic and TD-DFT studies on the turn-off fluorescent chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone for the selective recognition of fluoride and copper ions

The copper and fluoride ions sensing mechanism of a chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone (AntCy) was investigated via colorimetric, fluorescence, electrochemical and NMR titration studies. Detailed investigations on their sensing mechanism was done using DFT and TD-DFT studies. 1H NMR titration shows deprotonation of NH protons by fluoride ion is a prominent step in naked eye recognition. The Gibbs free energy of overall sensing reaction has moderate transition barrier with 18.19 kcal mol−1. Using the vibrational frequency analysis, all the local minima of ground state and excited state were confirmed. Due to the small size and strong electronegativity of fluoride, an intramolecular hydrogen bonding interaction with N(3)–H, which is closer to anthracene moiety was found to be preferentially formed. The excited state proton transfer mechanism was further confirmed with donor–acceptor interactions using Natural Bond Orbital (NBO) analysis and Potential Energy Surface (PES) analysis. The ICT mechanism for copper ion sensing was also confirmed with TD-DFT calculations.

A schematic representation of sensing mode of AntCy towards copper and fluoride ions. The fluoride ion forms a hydrogen bond with NH proton near to the anthracene moiety. The optimised N–H bond length in excited state proton transfer is at 1.00 Å, which is confirmed by PES and NBO studies.Figure optionsDownload as PowerPoint slide