Rhenium(i) polypyridine complexes functionalized with a diaminoaromatic moiety as phosphorescent sensors for nitric oxide

Abstract

A series of rhenium(I) polypyridine complexes functionalized with a diaminoaromatic moiety has been developed as phosphorescent sensors for nitric oxide (NO). The diamine complexes [Re(N⁁N)(CO)₃(py-diamine)](CF₃SO₃) [py-diamine = 3,4-diaminopyridine; N⁁N = 1,10-phenanthroline (phen) (1a), 2,9-dimethyl-1,10-phenanthroline (Me₂-phen) (2a), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me₄-phen) (3a), 4,7-diphenyl-1,10-phenanthroline (Ph₂-phen) (4a)] were synthesized and characterized. These complexes were only weakly emissive due to the diaminoaromatic moiety that quenches the ³MLCT [dπ(Re) → π*(N⁁N)] emission by photoinduced electron transfer (PET). However, in the presence of NO, these diamine complexes were converted to the triazole derivatives [Re(N⁁N)(CO)₃(py-triazole)](CF₃SO₃) [py-triazole = 1H-1,2,3-triazolo[4,5,c]pyridine; N⁁N = phen (1b), Me₂-phen (2b), Me₄-phen (3b), Ph₂-phen (4b)], which revealed intense emission upon excitation. The emission of the complexes was independent to pH under neutral and basic conditions (pH > 6). The cytotoxicity and cellular uptake properties of these complexes were studied by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay and ICP-MS, respectively. The potential application of these complexes as intracellular NO sensors was also investigated.