Red-emitting heteroleptic iridium(III) complexes: photophysical and cell labeling study

Two red-emitting heteroleptic iridium(III) complexes (Ir-p and Ir-q) were synthesized and their photophysical and biological properties were analyzed. After their structures have been confirmed by several techniques, such as ¹H NMR, ¹³C NMR, FT-IR, UV-Vis, and MALDI TOF analyses, their luminescence behavior was investigated in ethanol and PBS (physiological medium, pH ~ 7.4) solutions. Emission spectra of both complexes are dominated by ³MLCT states at room temperature in ethanolic solution, but at 77 K the Ir-q exhibits the ³LC as the dominant emission state. The Ir-q complex shows one of the highest emission quantum yields, 11.5%, for a red emitter based on iridium(III) complexes in aerated PBS solution, with color coordinates (x;y) of (0.712;0.286). Moreover, both complexes display high potential to be used as a biological marker with excitation wavelengths above 400 nm, high water solubility (Ir-p 1838 μmol L^-1, Ir-q 7601 μmol L^-1), and distinct emission wavelengths from the biological autofluorescence. Their cytotoxicity was analyzed in CHO-k1 cells by MTT assays, and the IC₅₀ was estimated as being higher than 131 μmol L^-1 for Ir-p, and higher than 116 μmol L^-1 for Ir-q. Concentrations above 70% of viability were used to perform cell imaging by confocal and fluorescence microscopies and the results suggest that the complexes were internalized by the cell membrane and they are staining the cytoplasm region.