Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting Cu I Compounds.

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br ( 1 ), Cbz ( 2 ), acac ( 3 ), Ph ₂ acac ( 4 ), Cp ( 5 ), and Cp* ( 6 )) with known Cu ^I complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5-15 × 10 ³ M ^-1 cm ^-1 for 1 - 6 . Furthermore, 1 - 5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λ ^em _max = 621-784 nm. Linear [Cu(Cbz)( ^Dipp CAArC)] ( 2 ) has been found to be an exceptional deep red (λ _max = 621 nm, ϕ = 0.32, τ _av = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant k _r of ca. 9 × 10 ⁵ s ^-1 , exceeding those of commercially employed Ir ^III - or Pt ^II -based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn-Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2 . To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2 , showing distinct antibunching as required for single-photon emitters.