Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene

Abstract

The structural, photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (<b>BZI</b>), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow spectra (full width at half maximum = 44 nm, 2,300 cm^-1) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (Φ_PL > 80%) and radiative rate constants (<i>k</i> _<i>r</i> ~ 7.8 × 10⁵ s^-1). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH₂Cl₂) at room temperature that are accompanied by a decrease in quantum yields (Φ_PL < 23%) and radiative rate constants (<i>k</i> _<i>r</i> < 4.0 × 10⁴ s^-1), whereas the non-radiative rate constants remain nearly constant (<i>k</i> _<i>nr</i> ~ 1.0 × 10⁵ s^-1). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (<b>Au</b> ^BZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm^-1. An organic light-emitting diode (OLED) fabricated using <b>Au</b> ^BZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).