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

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 (BZI), 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 (k(r) ~ 7.8 × 10(5) s(−1)). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH(2)Cl(2)) at room temperature that are accompanied by a decrease in quantum yields (Φ(PL) < 23%) and radiative rate constants (k(r) < 4.0 × 10(4) s(−1)), whereas the non-radiative rate constants remain nearly constant (k(nr) ~ 1.0 × 10(5) s(−1)). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (Au(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 Au(BZI) as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).