Ligands incorporating a tetraazamacrocycle receptor, a 'click'-derived
triazole and a 1,8-naphthalimide fluorophore have proven utility as probes for
metal ions. Three new
cyclam-based
molecular probes are reported, in which a piperidinyl group has been introduced at the 4-position of the
naphthalimide fluorophore. These compounds have been synthesized using the
copper(I)-catalyzed
azide-
alkyne Huisgen cycloaddition and their photophysical properties studied in detail. The alkylamino group induces the expected red-shift in absorption and emission spectra relative to the simple
naphthalimide derivatives and gives rise to extended fluorescence lifetimes in aqueous
buffer. The photophysical properties of these systems are shown to be highly
solvent-dependent. Screening the fluorescence responses of the new conjugates to a wide variety of
metal ions reveals significant and selective fluorescence quenching in the presence of
copper(II), yet no fluorescence enhancement with
zinc(II) as observed previously for the simple
naphthalimide derivatives. Reasons for this different behaviour are proposed. Cytotoxicity testing shows that these new
cyclam-
triazole-
dye conjugates display little or no toxicity against either DLD-1 colon
carcinoma cells or MDA-MB-231
breast carcinoma cells, suggesting a potential role for these and related systems in
biological sensing applications.