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 AuI(C6F5)­(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 = 1/2 spin
per molecule, with the spin density distributed mainly on the PyBTM
ligand. The complex exhibited fluorescence in CHCl3 with
emission peak wavelength (λem) of 619 nm and the
absolute fluorescence quantum yield (ϕem) of 0.04,
confirming that AuI(C6F5)­(PyBTM)
is the first luminescent organometallic complex with a coordinated
luminescent radical. Solvent-dependent unique luminescent characteristics
were observed in halogenated solvents (CCl4, CHCl3, CH2Cl2, and ClCH2CH2Cl). ϕem decreased, and λem shifted
to longer wavelengths as the polarity (dielectric constant) of the
solvent increased. Notably, the complex in CCl4 displayed
fluorescence with ϕem = 0.23, which was quite high
in radicals, while showed dual fluorescence in CH2Cl2 and ClCH2CH2Cl 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 CCl4, in which the C6F5 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 ϕem 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 ϕem, providing a novel method with
greater capability for realizing highly emissive radicals.