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).