Imidazole/benzimidazole-modified cyclotriphosphazenes as highly selective fluorescent probes for Cu2+: synthesis, configurational isomers, and crystal structures

Configurational isomers (cis and trans) of imidazole-or benzimidazole-modified cyclotriphosphazenes (3a, 4a or 3b, 4b) were designed, synthesized and investigated as fluorescent probes for metal ions. The newly synthesized compounds were characterized by H-1 and P-31 NMR, and MALDI MS spectrometry. The configurations of geometric isomers were analyzed by X-ray crystallography and P-31 NMR spectroscopy on addition of CSA. The photophysical behaviour and metal ion selectivity of the compounds were investigated by UV/vis and fluorescence spectroscopy. Among the examined 20 metal ions, the fluorescence emissions of the isomer mixtures were quenched by Cu2+ together with Fe2+, Fe3+, Zn2+ and Ni2+ ions, but each individual isomer (3a,b and 4a,b) exhibited an on-off-type fluorescence response with high selectivity towards only Cu2+ with a low limit of detection ranging from 1.27 mu M to 2.04 mu M. The complex stoichiometries of 3a, b and 4a, b with Cu2+ were determined as 1 : 1 (L/M) using the method of continuous variation (Job's plot) and density functional theory (DFT) calculations; moreover the complex formation of 4a with Cu2+ was unambiguously determined by X-ray crystallographic analysis that is consistent with the results obtained by the Job's plots as well as DFT.