A Fluorescent 1,3-Diaminonaphthalimide Conjugate of Calix[4]arene for Sensitive and Selective Detection of Trinitrophenol: Spectroscopy, Microscopy, and Computational Studies, and Its Applicability using Cellulose Strips

A new fluorescent 1,3-diaminonaphthalimide conjugate of calix[4]arene receptor (R) was synthesized and characterized. The receptor displays good selectivity towards trinitrophenol (TNP) over other explosive aromatic- and aliphatic-nitro compounds by exhibiting changes in its fluorescence emission. Receptor-coated cellulose paper strips are equally effective in terms of their selective detection of TNP over other aromatic- and aliphatic-nitro compounds. When used in solution or on cellulose paper strips, R can detect up to submicromolar concentration of TNP by exhibiting changes in its fluorescence emission and in its supramolecular structure upon interaction. Interestingly, the microscopy features of R, TNP, and {R+TNP} are quite distinct, indicating the interactions present between R and TNP, as studied by using AFM and TEM. Computationally modeled complexes of receptor with TNP and TNT show enormous difference in their interaction energies in the favor of TNP by showing the host-guest interaction of cation ⋯ anion type in the presence of TNP but not TNT. This is because the receptor adopts an "arms-open"-type structure in the case of the TNP complex, whereas it adopts an "arms-closed"-type structure in the presence of TNT. Both the experimental and the computational studies reveal that the receptor selectively binds to TNP over TNT. Thus, R-coated Whatman No.1 filter paper strips provide easy, rapid, and economical detection of trace amounts of TNP both by visual and spectral measurement. Open arms for TNP: A new fluorescent 1,3-diaminonaphthalimide conjugate of a calix[4]arene receptor (R) was synthesized and characterized. The receptor displays good selectivity towards trinitrophenol (TNP) over other explosive aromatic and aliphatic nitro compounds, like trinitrotoluene (TNT). R can detect up to submicromolar concentrations of TNP by exhibiting changes in its fluorescence emission and in its supramolecular structure upon interaction (see figure).