Near-Infrared [Ir(N∧C)2(N∧N)]+ Emitters and Their Noncovalent Adducts with Human Serum Albumin: Synthesis and Photophysical and Computational Study

Near-infrared (NIR) molecular emitters based on transition-metal complexes have attracted growing attention due to their potential application for in vivo and in vitro bioimaging experiments. Their photophysical characteristics (large Stokes shift and lifetime in the microsecond domain) offer some important advantages in comparison to organic fluorophores and may provide better imaging resolution and higher sensitivity: for example, in mapping the oxygen concentration in biological objects. We have synthesized a series of [Ir(N∧C)2(N∧N)]+ complexes with emission in the NIR region (N∧C = (2-benzothienyl)phenanthridine and 6-(2-benzothienyl)phenanthridine-2-carboxylic acid; N∧N = functionalized pyridine-triazole chelates), which also display a considerable red shift of their excitation spectra to the edge of the window of transparency. The flexible protocol for the synthesis of the N∧N ligands makes possible wide variations in the peripheral ligand environment: e.g., insertion of hydrophilic carboxyl group ...