Ion Interactions with a New Ditopic Naphthalimide-Based Receptor: A Photophysical, NMR and Theoretical (DFT) Study

Abstract

A new multi-component chemosensor system comprising a naphthalimide moiety as fluorophore is designed and developed to investigate receptor–analyte binding interactions in the presence of metal and non-metal ions. A dimethylamino moiety is utilized as receptor for metal ions and a thiourea receptor, having acidic protons, for binding anions. The system is characterized by conventional analytical methods. The absorption and fluorescence spectra of the system consist of a broad band typical for an intramolecular charge transfer (ICT). The effects of various metal-ion additives on the spectral behavior of the present sensor system are examined in acetonitrile. It is found that among the metal ions studied, alkali/alkaline earth-metal ions and transition-metal ions modulate the absorption and fluorescence spectra of the system. As an additional feature, the anion signaling behavior of the system in acetonitrile is studied. A decrease in fluorescence efficiency of the system is observed upon addition of fluoride and acetate anions. Fluorescence quenching is most effective in the case of fluoride ions. This is attributed to the enhancement of the photoinduced electron transfer from the anion receptor to the fluorophore moiety. Hydrogen-bond interactions between the acidic NH protons of the thiourea moiety and the F⁻ anions are primarily attributed to the fluoride-selective signaling behavior. Interestingly, a negative cooperativity for the binding event is observed when the interactions of the system are studied in the presence of both Zn²⁺ and F⁻ ions. NMR spectroscopy and theoretical calculations are also carried out to better understand the receptor–analyte binding.