Luminescent cyclometalated iridium(III) dipyridoquinoxaline indole complexes as biological probes

Four luminescent cyclometalated iridium(III) dipyridoquinoxaline complexes appended with an indole moiety [Ir(N^C)₂(N^N)] (PF₆) (HN^C = 2-phenylpyridine, Hppy; N^N = 2-(N-(2-(indole-3-acetamido)ethyl) aminocarbonyl)dipyrido[3,2-f:2′,3′-h]quinoxaline, dpqC2indole (1a), N^N = 2-(N-(6-(indole-3-acetamido)hexyl)aminocarbonyl)dipyrido[3,2-f:2′, 3′-h]quinoxaline, dpqC6indole (1b); HN^C = 7,8-benzoquinoline, Hbzq, N^N = dpqC2indole (2a), N^N = dpqC6indole (2b)) have been synthesized and characterized. Upon irradiation, all the complexes displayed moderately intense and long-lived luminescence under ambient conditions and in 77 K glass. On the basis of the photophysical data, the emission of the complexes has been assigned to an excited state of triplet metal-to-ligand charge-transfer ( ³MLCT) ((dπ(Ir) → π*(N^N)) character. Cyclic voltammetric studies revealed indole-based and iridium-based oxidations at ca. +1.10 V and +1.24 V vs. SCE, respectively, and ligand-based reductions at ca. -1.07 to -2.29 V vs. SCE. The interactions of the complexes with an indole-binding protein, bovine serum albumin (BSA), have been examined by emission titrations. © 2010 Science China Press and Springer-Verlag Berlin Heidelberg.