There has been fast-growing interest in utilizing iridium(III)polypyridine complexes as new luminescent sensors for analytes, including protons, [1] halide ions, [2] metal cations, [3] oxygen, [4] and biomolecules. [5] These complexes display changes in their emission intensities and lifetimes upon analyte binding. Although the emission maxima exhibit small shifts in some cases, the emission profiles and spectral characteristics of the luminescent probes basically remain the same. Compared to common metal-to-ligand charge-transfer (MLCT) emitters, such as the ruthenium(II)-and osmium(II)-polypyridine systems, iridium(III)-polypyridine complexes exhibit emissive states that are very sensitive to their ligands and local environment, resulting in distinct emission features. However, it appears that this behavior has not been utilized in the current array of sensors available. Whilst dual emission is not uncommon for iridium(III)-polypyridine complexes in glass at low temperature, it is very rare in fluid solutions under ambient conditions. [5b, 6] We believe that an attractive approach to the development of new iridium-(III)-based luminescent probes would be the utilization of novel complexes that display environment-responsive dualemissive properties.
Herein we report a series of novel dual-emissive cyclometalated iridium(III)-polypyridine complexes that serve as luminescent sensors for various biological receptors. The complex [Ir(ppy-CH 2 NH-C 4 H 9 ) 2 (bpy-CONH-C 2 H 5 )](PF 6 ) (1; Hppy-CH 2 NH-C 4 H 9 = 2-(4-(N-(n-butyl)aminomethyl)phenyl)pyridine; bpy-CONH-C 2 H 5 = 4-(N-(ethyl)aminocarbonyl)-4'-methyl-2,2'-bipyridine; Scheme 1) was synthesized from the reaction of the aldehyde complex [Ir(ppy-CHO) 2 (bpy-CONH-C 2 H 5 )](PF 6 ) (Hppy-CHO = 4-(2-pyridyl)benzaldehyde) with n-butylamine in refluxing methanol, followed by reduction with NaBH 3 CN. Upon irradiation, 1 exhibited intense and long-lived luminescence (Table 1).