Using Click Chemistry to Tune the Properties and the Fluorescence Response Mechanism of Structurally Similar Probes for Metal Ions

Four cyclam-based fluorescent molecular probes were prepared using click chemistry and investigated to determine the effect of the triazole connectivity and fluorophore substitution on the photophysical properties and metal-ion response mechanism. The fluorescence of the ligands is turned on in the presence of Zn-II but varies in intensity depending on the solvent; the highest signal changes are found in MeCN. Complexation with Cu-II leads to fluorescence quenching, but only in the aqueous solvent, in which the triazole is involved in coordination of the metal. The length of the pendant triazole arm influences the ligand field around pentacoordinate Cu-II to yield a distorted square-planar coordination geometry in the ligands with an ethylene linker. The quantum yield (QY) of the emission intensity can be tuned by the triazole substitution, thereby resulting in increases in QY of more than one order of magnitude (ligand 1 versus 3). Changing the fluorophore opens up alternative radiative processes, thus leading to phosphorescence-like behavior in coumarin probe 2. Steady-state fluorescence emission spectra of the free ligands and their respective Cu-II complexes in a glass environment reveal two different inherent fluorescence quenching mechanisms. In the free ligands an electron-transfer deactivates the excited chromophore, whereas in the Cu-II complexes it is energy transfer that results in complete quenching of the emission of the ligands.