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-DiBTF0to 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-T2energy gaps for the dinuclear alkynyl complexes (Au-DiBTF0and -2) and a smaller energy gap for Au-DiBTF3. Experimentally, Au-DiBTF3has the highest rate constant and quantum yield of intersystem crossing of the new gold(I) organometallics.