Aminonaphthalimide-based dipodal imidazolium/triazole receptors for fluorescent sensing of nucleoside polyphosphates

Novel aminonaphthalimide-based two-armed imidazolium/triazole receptors were synthesized to compare their fluorescent responses towards nucleoside polyphosphates with those of three-armed podands described recently. Our two constitutional isomer ligands comprised one imidazolium and an additional 1,2,3-triazole moiety in the binding site exhibited ‘turn-on’ fluorescence enhancements in HEPES buffer (pH 7.5) upon addition of ATP, GTP and UTP. The sensitivity of sensing was found to strongly depend on the order of heterocyclic subunits in the arms, when the imidazolium group was displaced to the vicinity of the naphthalimide marker significantly higher sensitivity was achieved. In aqueous solutions, however, the association constants were considered to be unreliable due to the occurrence of some colloidal precipitate (unvisible to the naked eye) that could be detected only by light scattering. The spectroscopic measurements repeated in HEPES–acetonitrile mixed solvent affording stable solutions gave association constants log K = 6.3–6.5 for ATP, GTP and UTP alike (1:2 ligand–triphosphate stoichiometry) retaining the selectivity over di-and monophosphates.

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