Abstract
N-Heterocyclic carbene (NHC) cyclometalated gold(III) complexes remain very scarce and therefore their photophysical properties remain currently underexplored. Moreover, gold(III) complexes emitting in the blue region of the electromagnetic spectrum are rare. In this work, a series of four phosphorescent gold(III) complexes was investigated bearing four different NHC monocyclometalated (C<^>C*)-type ligands and a dianionic (N<^>N)-type ancillary ligand ((N<^>N)= 5,5'-(propane-2,2-diyl)bis(3-(trifluoromethyl)-1H-pyrazole) (mepzH$_2$)). The complexes exhibit strong phosphorescence when doped in poly(methyl methacrylate) (PMMA) at room temperature, which were systematically tuned from sky-blue [$\lambda_{PL}$= 456 nm, CIE coordinates: (0.20, 034)] to green [$\lambda_{PL}$= 516 nm, CIE coordinates: (0.31, 0.54)] by varying the monocyclometalated (C^C*) ligand framework. The complexes revealed high quantum efficiencies ($\phi_{PL}$) of up to 43 % and excited-state lifetimes ($\tau_0$) between 15-266 μs. The radiative rate constant values found for these complexes ($k_{r} = 10^{3} - 10^{4} s^{-1}$) are the highest found in comparison to previously known best-performing monocyclometalated gold(III) complexes. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these complexes further lend support to the excited-state nature of these complexes. The calculations showed a significant contribution of the gold(III) metal center in the lowest unoccupied molecular orbitals (LUMOs) of up to 18 %, which was found to be unique for this class of cyclometalated gold(III) complexes. Additionally, organic light-emitting diodes (OLEDs) were fabricated by using a solution process to provide the first insight into the electroluminescent (EL) properties of this new class of gold(III) complexes.
Malmberg, Robert