Luminescent Applications of Re(I) and Ir(IІІ) Complexes for Cellular Imaging

Luminescent transition metal complexes are receiving great attention for use as bioimaging agents, in organic light-emitting devices (OLEDs), solar cells and sensors. They possess biological stability, low toxicity, large Stokes shifts, and long luminescence lifetimes. A new series of rhenium(І) and iridium(III), complexes incorporating polyamines were synthesized to use as dyes for cellular imaging in particular to replacement organic dyes such as FM1-43. Different probes were used in this study: click chemistry was used to prepare ancillary (N^N) ligands such as pyridyl triazoles and cyclometallated ligand (C^N) such as phenyltriazoles, whereas Stille coupling was used to synthesise ancillary ligand type (bipy) and cyclometallated type (phpy). In addition, Sonagashira coupling is used to synthesise ligands substituted with electron withdrawing groups in order to tune the emission of both complexes of rhenium (І) and iridium(ІІІ). Pyridine and triazole monodentate axial ligands were prepared by using reductive amination reactions containing either alkyl chains or polyamines. Their complexes with rhenium(І) and iridium(ІІІ) were synthesised using microwave methods and characterized by NMR spectroscopy and mass spectrometry and purified by HPLC. Luminescence properties, lifetime and quantum yield were studied in water solution at room temperature to investigate the effect of substituents in both the ancillary and cyclomelated ligands. Rhenium(І) and iridium(ІІІ) complexes substituted with electron withdrawing groups show broad emission in the green region with long lifetime and quantum yield. Rhenium(І) complexes bearing inverse pyridyltriazoles exhibit a high quantum and lifetime compared to corresponding complexes containing regular pyridyltriazoles. Rhenium(І) complexes containing triazole as a monodentate ligand exhibit better photophysical properties compared with analogoues pyridine complexes. It was found that substituent effects may tune and alter the optical and electronic properties of the complexes; their luminescence properties were studied in order to select which complexes would be suitable for use in fluorescence microscopy to image synaptic vesicles. Lipophilicity studies confirmed that the complexes substituted with long alkyl chains have high logP values and this would be promising to use as a bioimaging probe.