Novel Luminescent Tricarbonylrhenium(I) Polypyridine Tyramine-Derived Dipicolylamine Complexes as Sensors for Zinc(II) and Cadmium(II) Ions

Three luminescent tricarbonylrhenium(I) polypyridine complexes containing a tyramine-derived 2,2'-dipicolylamine (DPAT) unit, [Re((NN)-N-boolean AND)(CO)(3)(py-TU-DPAT)](CF3SO3) (py-TU-DPAF = 3-(2-(4-hydroxy-3-(2,2'-dipicolyl amethyl)phenyl)ethylthioureidyl)pyridine; (NN)-N-boolean AND=1,10-phenanthroline (phen) (1 a), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me-4-phen) (2a), 4,7-diphenyl-1,10-phenanthroline (Ph-2-phen) (3a)), and their DPAT-free counterparts, [Re((NN)-N-boolean AND)(CO)(3)(py-TU-Et)](CF3SO3) (py-TU-Et = 3-(ethylthioureidyl)pyridine; (NN)-N-boolean AND = phen (1b), Me-4-phen (2b), Ph-2-phen (3b)), have been synthesized and characterized. Their electrochemical and photophysical properties have been studied. Upon photoexcitation, all the complexes exhibited triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (d pi(Re) -> pi* - ((NN)-N-boolean AND)) emission in fluid solutions at 298 K and in low-temperature alcohol glass. The DPAT complexes showed lower emission quantum yields and shorter emission lifetimes compared to those of the DPAT-free analogues, indicative of the quenching properties of the appended DPAT unit. The DPAT complexes also exhibited pH-dependent emission. with their emission intensities at pH < 3 being ca. 40 fold higher than those at pH > 11. These complexes displayed emission enhancement and lifetime elongation in the presence of zinc(II) and cadmium(II) ions, The cellular uptake of all the complexes by human cervix epithelioid carcinoma (HeLa) cells has been examined by ICP-MS. We have investigated the cytotoxicity of the complexes by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay, and the results revealed that all the complexes were more cytotoxic than cisplatin. Furthermore, the cellular uptake of complexes 3a and 3b and the intracellular ion-binding, properties of the former complex have been studied by laser-scanning confocal microscopy.