Luminescent Tricarbonylrhenium(I) Dipyridoquinoxaline Indole Complexes as Sensitive Probes for Indole-Binding Proteins

Luminescent tricarbonylrhenium(I) dipyridoquinoxaline indole complexes [Re(N−N)(CO)3(L)](CF3SO3) (N−N = dipyrido[3,2-f:2‘,3‘-h]quinoxaline (dpq), L = N-(3-pyridoyl)tryptamine (py-3-CONHC2H4-indole) (1a), N-(N-(3-pyridoyl)-6-aminohexanoyl)tryptamine (py-3-CONHC5H10CONHC2H4-indole) (1b);
N−N = 2-(n-butylamido)dipyrido[3,2-f:2‘,3‘-h]quinoxaline (dpqa), L = py-3-CONHC2H4-indole (2a),
py-3-CONHC5H10CONHC2H4-indole (2b)) and their indole-free counterparts [Re(N−N)(CO)3(py-3-CONH-Et)](CF3SO3) (py-3-CONH-Et = N-ethyl-(3-pyridyl)formamide; N−N = dpq (1c), dpqa (2c))
have been synthesized and characterized. The crystal structure of a related complex, [Re(dpqa)(CO)3(pyridine)](PF6), has also been studied. All the complexes exhibited triplet metal-to-ligand charge-transfer
(3MLCT) (dπ(Re) → π*(N−N)) emission in fluid solutions at 298 K and in alcohol glass at 77 K. The
emission quantum yields of the complexes were reduced upon changing from CH2Cl2 to aqueous buffer.
The reduction was much more significant for the dpqa complexes than the dpq complexes due to the
hydrogen-bonding interaction of the amide substituent of the dpqa ligand with water molecules. The
interactions of these tricarbonylrhenium(I) complexes with indole-binding proteins including bovine
serum albumin and tryptophanase have been studied by emission titrations and inhibition assays,
respectively.