Halocuprate(I) Zigzag Chain Structures with N-Methylated DABCO Cations – Bright Metal-Centered Luminescence and Thermally Activated Color Shifts

Two compounds 1,4-dimethyl-1,4-diazoniabicyclo[2.2.2]octane catena-tetra-μ-halo-dicuprate(I) with DABCOMe2 Cu2X4 (1: X = Br, 2: X = I) were synthesized by hydrothermal reaction of copper(I) halides with the corresponding 1,4-diazoniabicyclo[2.2.2]octane (DABCO) dihydrohalides in an acetonitrile/methanol mixture. Both compounds crystallize monoclinically, 1 with a = 9.169(4) Å, b = 10.916(6) Å, c = 15.349(6) Å, β = 93.93(2)°, V = 1533(1) Å3, Z = 4, space group P21/n (no. 14) and 2 with a = 15.826(9) Å, b = 9.476(5) Å, c = 22.90(2) Å, β = 90.56(5)°, V = 3434(5) Å3, Z = 8, space group P21 (no. 4), respectively (lattice constants refined from powder diffraction data measured at 293 K). The cations in both compounds are formed by in situ N-methylation of DABCOH22+ cations by methanol in a SN2 reaction. Both compounds contain an anionic copper(I) halide chain structure consisting of trans edge-sharing CuX4 tetrahedra. The chains are strongly kinked at every 2nd junction thus forming a zigzag structure. The shortest halide-halide distances are observed between the halide ions of adjacent tetrahedra which are approaching each other due to the kinking. This structure type shows a specific luminescence behavior. Under optical excitation, the compounds exhibit yellow (1) and green (2) emission with photoluminescence quantum yields of ΦPL = 52 and 4%, respectively, at ambient temperature. According to DFT and TDDFT calculations, the emission is assigned to be a phosphorescence essentially involving a metal centered transition between the HOMO consisting mainly of copper 3d and halide p orbitals and the LUMO consisting mainly of copper 4s and 4p orbitals. The temperature dependence of the emission spectra, decay times, and quantum yields has been investigated in detail, especially for 1. From the resulting trends it can be concluded that the emission for T ≤ 100 K stems from energetically lower lying copper halide segments. Such segments represent the structural motif of the halocuprate(I) chains. With increasing temperature energetically higher lying segments are populated which also emit, but open the pathway for thermally activated energy transfer to quenching defects

Photophysical characterizations of OLED relevant Cu(I) complexes exhibiting thermally activated delayed fluorescence (TADF)

In this thesis, different classes of OLED relevant Cu(I) complexes as well as one Ag(I) complex were investigated especially with regard to their photophysical properties. Hereby, the main focus was to establish a relationship between the molecular structure of the compounds and their emission properties. To achieve this, for each compound class different, systematically varied substances were studied. Hereby, dinuclear halide-bridged Cu(I) and Ag(I) complexes with aminophosphine and diphosphine ligands as well as mononuclear Cu(I) complexes with three or four coordinations were in the focus of the investigations. To obtain insight into the electronic structures, the emission behavior of the substances was studied in a wide temperature range between T = 1.3 K and 300 K. In addition, for some compounds also the influence of the surrounding matrix environment on the emission behavior was investigated. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed to gain a more profound insight into the electronic states that determine the emission behavior of the compounds. The performed measurements allowed to develop a detailed understanding for the photophysical properties, such as emission decay time, emission color, and emission quantum yield and how these properties can be changed by defined chemical modifications. In addition, for most of the studied substances the occurrence of a thermally activated delayed fluorescence (TADF) could be proven. Furthermore, the mechanisms that control the occurrence of a TADF could be elucidated. Through this, valuable insights could be gained for future developments of novel, efficient emitters for OLED applications based on Cu(I) and Ag(I) complexes

A new class of deep-blue emitting Cu(I) compounds – effects of counter ions on the emission behavior

Three deep blue emitting Cu(I) compounds, [Cu(PPh3)tpym]PF6, [Cu(PPh3)tpym]BF4, and [Cu(PPh3)tpym]BPh4 (tpym = tris(2-pyridyl)methane, PPh3 = triphenylphosphine) featuring the tripodally coordinating tpym and the monodentate PPh3 ligands were studied with regard to their structural and photophysical properties. The compounds only differ in their respective counter ions which have a strong impact on the emission properties of the powder samples. For example, the emission quantum yield can be significantly increased for the neat material from less than 10% to more than 40% by exchanging BPh4− with PF6−. These effects can be linked to different molecular packings which depend on the counter ion. In agreement with these results, it was found that the emission properties also strongly depend on the surrounding matrix environment which was elucidated by investigating photophysical properties of the compounds as powders, doped into a polymer matrix, and dissolved in a fluid solution, respectively. The observed differences in the emission behavior can be explained by different and pronounced distortions that occur in the excited state. These distortions are also displayed by density functional theory (DFT) calculations

A new class of luminescent Cu(I) complexes with tripodal ligands – TADF emitters for the yellow to red color range

A new class of emissive and neutral Cu(I) compounds with tripodal ligands is presented. The complexes were characterized chemically, computationally, and photophysically. Under ambient conditions, the powders of the compounds exhibit yellow to red emission with quantum yields ranging from about 5% to 35%. The emission represents a thermally activated delayed fluorescence (TADF) combined with a short-lived phosphorescence which represents a rare situation and is a consequence of high spin–orbit coupling (SOC). In the series of the investigated compounds the non-radiative rates increase with decreasing emission energy according to the energy gap law while the radiative rate is almost constant. Furthermore, a well-fit linear dependence between the experimental emission energies and the transition energies calculated by DFT and TD-DFT methods could be established, thus supporting the applicability of these computational methods also to Cu(I) complexes

Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet–Triplet Splitting in Brightly Emitting and Sublimable Cu(I) Compounds

Photophysical properties of two highly emissive three-coordinate Cu­(I) complexes, (IPr)­Cu­(py₂-BMe₂) (<b>1</b>) and (Bzl-3,5Me)­Cu­(py₂-BMe₂) (<b>2</b>), with two different N-heterocyclic (NHC) ligands were investigated in detail (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene; Bzl-3,5Me = 1,3-bis­(3,5-dimethylphenyl)-1<i>H</i>-benzo­[<i>d</i>]­imidazol-2-ylidene; py₂-BMe₂ = di­(2-pyridyl)­dimethylborate). The compounds exhibit remarkably high emission quantum yields of more than 70% in the powder phase. Despite similar chemical structures of both complexes, only compound <b>1</b> exhibits thermally activated delayed blue fluorescence (TADF), whereas compound <b>2</b> shows a pure, yellow phosphorescence. This behavior is related to the torsion angles between the two ligands. Changing this angle has a huge impact on the energy splitting between the first excited singlet state S₁ and triplet state T₁ and therefore on the TADF properties. In addition, it was found that, in both compounds, spin–orbit coupling (SOC) is particularly effective compared to other Cu­(I) complexes. This is reflected in short emission decay times of the triplet states of only 34 μs (<b>1</b>) and 21 μs (<b>2</b>), respectively, as well as in the zero-field splittings of the triplet states amounting to 4 cm^–1 (0.5 meV) for <b>1</b> and 5 cm^–1 (0.6 meV) for <b>2</b>. Accordingly, at ambient temperature, compound <b>1</b> exhibits <i>two</i> radiative decay paths which are thermally equilibrated: one via the S₁ state as TADF path (62%) and one via the T₁ state as phosphorescence path (38%). Thus, if this material is applied in an organic light-emitting diode, the generated excitons are harvested mainly in the singlet state, but to a significant portion also in the triplet state. This novel mechanism based on two separate radiative decay paths reduces the overall emission decay time distinctly

Synthesis and electronic properties of π-extended flavins

Flavin derivatives with an extended π-conjugation were synthesized in moderate to good yields from aryl bromides via a Buchwald–Hartwig palladium catalyzed amination protocol, followed by condensation of the corresponding aromatic amines with violuric acid. The electronic properties of the new compounds were investigated by absorption and emission spectroscopy, cyclic voltammetry, density functional theory (DFT) and time dependent density functional theory (TDDFT). The compounds absorb up to 550 nm and show strong luminescence. The photoluminescence quantum yields ϕPL measured in dichloromethane reach 80% and in PMMA (poly(methyl methacrylate)) 77%, respectively, at ambient temperature. The electrochemical redox behaviour of π-extended flavins follows the mechanism previously described for the parent flavin

Dinuclear Cu(I) Complex with Combined Bright TADF and Phosphorescence. Zero-Field Splitting and Spin–Lattice Relaxation Effects of the Triplet State

The three-fold bridged dinuclear Cu(I) complex Cu-2(mu-I)(2)(1N-n-butyl-5-diphenyl-phosphino-1,2,4-triazole)(3), Cu2I2(PN)(3), shows bright thermally activated delayed fluorescence (TADF) as well as phosphorescence at ambient temperature with a total quantum yield of 85% at an emission decay time of 7 mu s. The singlet (S1)-triplet (T-1) energy gap is as small as only 430 cm(-1) (53 meV). Spin-orbit coupling induces a short-lived phosphorescence with a decay time of 52 mu s (T = 77 K) and a distinct zero-field splitting (ZFS) of T-1 into substates by similar to 2.5 cm(-1) (0.3 meV). Below T approximate to 10 K, effects of spin-lattice relaxation (SLR) are observed and agree with the size of ZFS. According to the combined phosphorescence and TADF, the overall emission decay time is reduced by similar to 13% as compared to the TADF-only process. The compound may potentially be applied in solution-processed OLEDs, exploiting both the singlet and triplet harvesting mechanisms

Thermally Activated Delayed Fluorescence (TADF) and Enhancing Photoluminescence Quantum Yields of [Cu^I(diimine)(diphosphine)]⁺ ComplexesPhotophysical, Structural, and Computational Studies

The complexes [Cu­(I)­(POP)­(dmbpy)]­[BF₄] (<b>1</b>) and [Cu­(I)­(POP)­(tmbpy)]­[BF₄] (<b>2</b>) (dmbpy = 4,4′-dimethyl-2,2′-bipyridyl; tmbpy = 4,4′,6,6′-tetramethyl-2,2′-bipyridyl; POP = bis­[2-(diphenylphosphino)-phenyl]­ether) have been studied in a wide temperature range by steady-state and time-resolved emission spectroscopy in fluid solution, frozen solution, and as solid powders. Emission quantum yields of up to 74% were observed for <b>2</b> in a rigid matrix (powder), substantially higher than for <b>1</b> of around 9% under the same conditions. Importantly, it was found that the emission of <b>2</b> at ambient temperature represents a thermally activated delayed fluorescence (TADF) which renders the compound to be a good candidate for singlet harvesting in OLEDs. The role of steric constraints within the complexes, in particular their influences on the emission quantum yields, were investigated by hybrid-DFT calculations for the excited triplet state of <b>1</b> and <b>2</b> while manipulating the torsion angle between the bipyridyl and POP ligands. Both complexes showed similar flexibility within a ±10° range of the torsion angle; however, <b>2</b> appeared limited to this range, whereas <b>1</b> could be further twisted with little energy demand. It is concluded that a restricted flexibility leads to a reduction of nonradiative deactivation and thus an increase of emission quantum yield