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

Abstract

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries has 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), Ph₂acac (4), Cp (5), 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 350-550 nm in THF with high absorption coefficients of 5-15·10³ M^-1cm^-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)(^DippCAArC)] (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. 9·10⁵ s^-1, exceeding those of commercially employed Ir^III- or Pt^II-based emitters. Time-resolved transient absorption and fluorescence up-conversion 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 non-classical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct anti-bunching as required for single photon emitters.