A long-lived, highly luminescent Re(I) metal-ligand complex as a biomolecular probe

A highly luminescent rhenium (I) metal-ligand complex [Re(bcp)(CO)3(4-COOHPy)](ClO4), where bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 4-COOHPy is isonicotinic acid, has been synthesized and characterized. High quantum yields (> 0.5) and long excited-state lifetimes (0.3-10 micronseconds) in fluid solutions at room temperature were found for this complex, with remarkable emission sensitivity to microenvironment. This compound also displays highly polarized emission with a maximum anisotropy near 0.3 in the absence of rotational diffusion. This Re complex was conjugated to several biomolecules, including the proteins human serum albumin and bovine immunoglobulin G, as well as an amine-containing lipid. When bound to a protein or lipid, the decay time is near 3 microseconds and the quantum yield is approximately 0.12 in aqueous oxygenated solution at room temperature. This compound's unique spectral properties along with its conjugatability allowed us to utilize it as biomolecular probe in a variety of environments.