Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI 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 pi-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THE with high absorption coefficients of S-15 x 10(3) M-1 cm(-1) for 1-6. Furthermore, 1-S show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with lambda(em)(max) = 621-784 nm. Linear [Cu(Cbz)((DiPP)CAArC)] (2) has been found to be an exceptional deep red (lambda(max) = 621 nm, phi = 0.32, tau(av) = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant k(r) of ca. 9 x 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.