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), Ph2acac (4), Cp (5), and Cp* (6)) with known CuI 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 × 103 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 λem max = 621–784 nm. Linear [Cu­(Cbz)­(DippCAArC)] (2) has been found to be an exceptional deep red (λmax = 621 nm, ϕ = 0.32, τav = 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 IrIII- or PtII-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.

The members of a series of linear and trigonal copper(I) complexes bearing a cyclic alkyl(amino)carbene (CAAC) ligand show surprising photophysical properties compared to those of the corresponding N-heterocyclic carbene (NHC) complexes. Whereas the linear NHC complexes [CuX(NHC)] are almost non-emissive, [CuX(CAAC)] (X=Cl, Br, I) and [Cu(CAAC) ]PF show very bright emissions from their triplet excited states in the blue to green region, displaying quantum yields of up to 65 % in the solid state, even though the π-acceptor comprises only the carbene C and N atoms with no other π conjugation. [Cu(CAAC) ]PF is the fastest Cu -based triplet state emitter characterized to date, not displaying thermally activated delayed fluorescence (TADF), with an intrinsic lifetime of only 10.6 μs, that is, k =9.4×10 s , competitive with many Pt - and Ir -based emitters. In order to test the stability of such linear copper CAAC complexes in devices, some of our compounds have been applied in proof-of-principle organic light-emitting diodes (OLEDs). This case study thus demonstrates for the first time the use of CAACs as suitable π-chromophores for Cu -based phosphorescent emitters, and their implementation in OLEDs underlines the general applicability of this class of ligands in materials science.

A series of easily accessible linear N-heterocyclic carbene (NHC) copper­(I) complexes, bearing pyridine (py) and its derivatives as chromophore ligands, are barely emissive in the single-crystalline solid state. However, their powders, neat films, and dilute doped films of poly­(methyl methacrylate) (PMMA; 1–10%) show very intense blue-to-blue-green photoluminescence with remarkable quantum yields φ of up to 87% and microsecond lifetimes, indicative of triplet states being involved. These luminescence properties are similar to trigonal coordinated NHC copper­(I) bis­(pyridine) complexes, which we have also isolated and characterized with respect to their structures and photophysics. Our spectroscopic and theoretical studies provide detailed insight into the nature of the luminescence enhancing effect of the linear two-coordinated copper­(I) compounds, which is based on the formation of Cu–F interactions between the BF4 – anions and [Cu­(NHC)­(2-R-py)]+ (R = H, Me, Ph) cations. These interactions are absent in the single crystals but lead to a distorted ground-state structure in the precipitated powders or in PMMA films, giving rise to high k r. In addition, we found that our linear copper­(I) complexes exhibit mechanochromic luminescence because grinding of the single crystals leads to enhanced emission intensity. In light of the recently reported cation–anion contact-induced mechanochromic luminescence of two-coordinated copper­(I) complexes, this study supports the generality of this new mechanism for the design of mechanoresponsive phosphorescent materials.

A series of 12 lanthanide coordination polymers was synthesized from anhydrous LnCl 3 and 1,2-di(4-pyridyl)ethylene (dpe) under solvothermal conditions in either thiazole (thz) or pyridine (py). The reactions yielded ∞ 1 [Ln 2 Cl 6 (dpe) 2 (thz) 4 ]•dpe with Ln = Ce (1), Nd (2), ∞ 1 [LnCl 3 (dpe)(py) 2 ]•(dpe/py) with Ln = Gd (3), Er (4), and ∞ 1 [LnCl 3 (dpe) (thz) 2 ](dpe/thz) with Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), as well as ∞ 1 [HoCl 3 (dpe)(thz) 2 ]•thz (11) and ∞ 2 [La 2 Cl 6 (dpe) 3 (py) 2 ]•dpe ( 12). One-dimensional coordination polymers (CPs) and a two-dimensional network of five different constitutions are formed by connection of LnCl 3 units via dpe molecules. As free ligand, dpe shows an excimer effect that is reduced in the coordination polymers. In addition, dipyridylethylene proves to be a suitable sensitizer for the photoluminescence of lanthanides in the near-infrared region (NIR) only. Thereby, dpe differs from the related ligand 1,2-di(4-pyridyl)ethane. For the compounds presented, four different luminescence effects were detected: luminescence based on fluorescence of the linker dpe is observed in the visible region, whereas ligand-sensitized 4f−4f NIR emission is dominating for trivalent Nd, Er, and Yb. The Er-containing CPs show an inner-filter effect of Er 3+ , which is based on reabsorption of emission of dpe triggering the erbium NIR emission.

The high element abundance and d10 electron configuration make ZnII‐based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and ‐chemical properties of a series of dimeric and monomeric ZnII halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ‐X)2ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX2(cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo‐induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra‐long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from 3XCT/LEcAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of ZnII phosphorescent complexes.