An attractive family of cyclometalated Ir(III) dyes functionalized with tryptophan for potential neuroimaging applications

Three novel luminescent iridium(III) dyes functionalized with a tryptophan amino acid and bearing two cyclometalated 2-phenylpyridines, 2-(2,4-difluorophenyl)pyridines or 2-phenylquinolines have been prepared and well characterized. They emit at 522-561 nm with a luminescence quantum yield in the range 0.33-0.98. All the dyes are able to stain neuronal cells in rat cerebellum tissue, as evidenced by fluorescence microscopy, showing affinity for granule neurons. The complexes bearing cyclometalated 2-(2,4-difluorophenyl)pyridines or 2-phenylquinolines also have a good affinity for brain white matter. The dye with two cyclometalated 2-phenylquinolines is characterized by the best luminescence quantum yield (0.98). Besides, giving the greatest image contrast, the dye with two cyclometalated 2-phenylquinolines shows the strongest affinity for a distinct subtype of neurons found in cerebellum tissue, the purkinje neurons (as evidenced with fluorescence microscopy).