Synthesis and Photophysical Properties of Laterally Asymmetric Digold(I) Alkynyls and Triazolyl: Ancillary Ligand and Organic Functionality Dictate Excited-State Dynamics

Four
new laterally asymmetric dinuclear gold­(I) complexes made
from a new chromophoric ligand (Au-DiBTF0–3) have been characterized. Changing the organic ancillary
ligand from a phosphine ((Au-DiBTF0 (PMe3)
and Au-DiBTF1 (PCy3)) to an N-heterocyclic
carbene (Au-DiBTF2) with an alkynyl linking moiety or
modifying the alkynyl linkage to a triazolyl moiety with the ancillary
ligand being an N-heterocyclic carbene (Au-DiBTF3) impacts
the ground- and excited-state properties of these systems. All four
complexes exhibit structured absorption and emission spectra with
weak phosphorescence. A bathochromic shift is observed in both the
ground-state absorption and luminescence spectra as the series varies
from Au-DiBTF0 to Au-DiBTF3. The dinuclear
alkynyl complexes (Au-DiBTF0–2) display
decreased fluorescence lifetimes and fluorescence quantum yields along
with more efficient intersystem crossing when the capping ligand is
changed from a trialkylphosphine to an N-heterocyclic carbene. This
change results in comparable rates of radiative decay and intersystem
crossing and negligible rates of nonradiative decay. Changing the
π-bridging moiety (Au-DiBTF3) results in a diminished
fluorescence quantum yield, shorter fluorescence lifetime, and increased
intersystem quantum yield, resulting in faster intersystem crossing
accompanied by slower radiative decay and more efficient nonradiative
decay relative to the alkynyl-bridged complexes. Density functional
theory calculations are in accord with the observed photophysics,
with nearly identical S1-to-T2 energy gaps for
the dinuclear alkynyl complexes (Au-DiBTF0 and -2) and a smaller energy gap for Au-DiBTF3. Experimentally, Au-DiBTF3 has the highest rate constant and quantum yield
of intersystem crossing of the new gold­(I) organometallics.