Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene
Rasha Hamze, Muazzam Idris, Daniel Sylvinson Muthiah Ravinson, Moon Chul Jung, Ralf Haiges, Peter I. Djurovich, Mark E. Thompson
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 (BZI), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow (full width at half maximum = 44 nm, 2300 cm-1) and vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (PLQY > 80%) and radiative rate constants (kr ~ 7.8×105 s-1). The luminescence is solvatochromic, undergoing red-shifts in a polar solvent (CH2Cl2) at room temperature that are accompanied by a decrease in quantum yields (PL < 23%) and radiative rate constants (kr < 4.0×104 s-1), whereas the non-radiative rate constants remain nearly constant (knr ~ 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 (AuBZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm 1. Organic light-emitting diodes (OLEDs) fabricated using AuBZI as a luminescent dopant have an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).