Solvent-Controlled Doublet Emission of an Organometallic Gold(I) Complex with a Polychlorinated Diphenyl(4-pyridyl)methyl Radical Ligand: Dual Fluorescence and Enhanced Emission Efficiency.

A paramagnetic, luminescent organometallic gold(I) complex Au(CF)(PyBTM), where PyBTM is a photostable fluorescent polychlorinated diphenyl(4-pyridyl)methyl radical, was prepared, and its crystal and electronic structures and magnetic and optical properties were investigated. Magnetic studies using electron spin resonance spectroscopy and a superconducting quantum interference device magnetometer indicated the existence of S = / spin per molecule, with the spin density distributed mainly on the PyBTM ligand. The complex exhibited fluorescence in CHCl with emission peak wavelength (λ) of 619 nm and the absolute fluorescence quantum yield (ϕ) of 0.04, confirming that Au(CF)(PyBTM) is the first luminescent organometallic complex with a coordinated luminescent radical. Solvent-dependent unique luminescent characteristics were observed in halogenated solvents (CCl, CHCl, CHCl, and ClCHCHCl). ϕ decreased, and λ shifted to longer wavelengths as the polarity (dielectric constant) of the solvent increased. Notably, the complex in CCl displayed fluorescence with ϕ = 0.23, which was quite high in radicals, while showed dual fluorescence in CHCl and ClCHCHCl with lifetimes of around 1 and 7 ns for two emissive components. Density functional theory (DFT) and time-dependent (TD)-DFT calculations indicated that the fluorescence occurred from an interligand charge transfer (CT) excited state in CCl, in which the CF and PyBTM moieties acted as electron donor and acceptor, respectively, while the fluorescence was centered at the PyBTM ligand in the other three solvents. This method, i.e., the formation of an interligand CT state, to enhance ϕ is distinctly different from the methods reported previously. The present study revealed that a coordination bond is available for forming emissive CT excited states that lead to high ϕ, providing a novel method with greater capability for realizing highly emissive radicals.