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 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 (ΦPL > 80%) and radiative rate constants (kr ~ 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 (ΦPL < 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. An organic light-emitting diode (OLED) fabricated using AuBZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).
Highlights
Luminescent, neutral two-coordinate, linear d10 metal complexes of coinage metals i.e., Cu(I), Ag(I), Au(I) have recently been reported (Di et al, 2017; Romanov et al, 2018, 2019; Hamze et al, 2019a,b; Shi et al, 2019)
We modified the literature procedure, using excess triethyl orthoformate [HC(OEt)3] and distilling the excess off during the reaction, which increases the yield of the reaction to 76%
Similar to MCAAC and MMAC complexes, the synthesis of MBZI complexes starts with addition of Ag2O to the benzoimidazolium salt or CuCl to BZI carbene generated in situ with base to form the respective benzoimidazole silver(I) or copper(I) chloride complexes
Summary
Neutral two-coordinate, linear d10 metal complexes of coinage metals i.e., Cu(I), Ag(I), Au(I) have recently been reported (Di et al, 2017; Romanov et al, 2018, 2019; Hamze et al, 2019a,b; Shi et al, 2019). The MBZI complexes have structures, redox potentials and photoluminescent efficiencies ( PL = 0.8–1.0) similar to the MCAAC analogs, but different excited-state dynamics.
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