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 (similar to 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 (phi(PL) > 80%) and radiative rate constants (k(r) similar to 7.8 x 10(5) 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 (phi(PL) < 23%) and radiative rate constants (k(r) < 4.0 x 10(4) s(-1)), whereas the non-radiative rate constants remain nearly constant (k(nr) similar to 1.0 x 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).