Synthesis and characterization of organometallic complexes for biological imaging

Luminescent transition metal complexes are of interest for biological imaging applications due to their unique photophysical properties including large Stokes shift, long fluorescence lifetimes, and tunable emission. Altering ligand structure is a useful way to manipulate the charge transfer (CT) and mixed character states that give these complexes their photophysical properties. A series of organometallic complexes containing ligands with externally facing nitrogens has been prepared and synthesized. Complexes of the form Pt (II)(tbbpy)(C2-py), Re(CO)3(N^N), Ir(C^N)2(N^N), and Ir(C^N)2(N^O) have been studied for their potential application as biological imaging agents. The goal of this project is to compare the properties of these complexes to analogs where the external nitrogen has bonded to an oxygen to assess their potential as ratiometric hypoxia (low oxygen concentration) probes. Two cyclometalated iridium complexes, Ir(ppy)2(5-Me-pz-COO) and Ir(dfppy)2(5-Me-pz-COO) were synthesized along with their N-oxide congeners. It was found that the presence of an N-oxide caused a 140 mV cathodic shift in the reduction potential. For Ir(ppy)2(5Me-pz-COO), the N-oxide caused a 20 nm red-shift in emission and a significant quenching of emission intensity. These data show promising results for the ability of the N-oxide to tune the properties of the complexes. This report details the successful synthesis of several additional complexes that show potential for this application