Interaction Behavior between Active Hydrogen Bond Donor‐Acceptors as a Binding Decoration for Anion Recognition: Experimental Observation and Theoretical Validation

Abstract Interactions of Harmine (HM), as hydrogen bond donor with different anions as hydrogen bond acceptors were studied using spectroscopic and computational techniques. Significant gradual changes in absorption and emission spectra were observed upon addition of fluoride anions with selectivity. Wide‐ranging theoretical calculations in association with the 1 H NMR titration in CDCl 3 medium were performed to establish the binding decoration of HM with fluoride ion. Interaction energies, bond length variation, NBO charge transfer, NMR response, MEP 3D surface & 2D contour plot, UV‐Vis spectral simulation, HOMO‐LUMO energy gaps and potential energy curves (PECs) were calculated theoretically employing DFT method for ground state (S 0 ) and TD‐DFT method for first singlet excited electronic state (S 1 ) in ACN. Calculations revealed the proton transfer as ESPT mechanism [extent of proton transfer in S 0 (58 %) and S 1 (87 %)] behind the molecular interaction and spectral responses. We believe such a fluoride selective sensor like HM may find application to detect and control fluoride content in different commodity products.