In Situ Observation of Degradation by Ligand Substitution in Small-Molecule Phosphorescent Organic Light-Emitting Diodes

Solutions of facial-tris­(1-phenylpyrazole)­Ir­(III) (<i>fac</i>-Ir­(ppz)₃), when dissolved in either <i>tert</i>-butyl isocyanide or in solid films of 2-naphthylisocyanide, undergo replacement of a ppz ligand by the isocyanide molecules after irradiation with UV light as demonstrated by liquid chromatograph mass spectrometer analysis. Similarly, solutions of Ir­(ppz)₃ and bathophenanthroline (BPhen) in CH₂Cl₂ or acetone-<i>d</i>₆ form a brightly emissive species, [Ir­(ppz)₂(Bphen)]⁺ when irradiated with UV light as established by optical, mass, and ¹H nuclear magnetic resonance spectroscopy. Electroluminescent data from blocked organic light-emitting diode (OLED) devices demonstrate that both <i>mer</i>- and <i>fac</i>-(Ir­(ppz)₃) dissociate a ligand and coordinate a neighboring BPhen molecule when the device is operated at moderate to high current levels. These experiments offer direct evidence of the dissociation of a metal–ligand bond and subsequent ligand substitution as a degradation pathway in active OLED devices during operation and provide a route to assay in situ the stability of future dopants

Cu₄I₄ Clusters Supported by P^∧N-type Ligands: New Structures with Tunable Emission Colors

A series of Cu₄I₄ clusters (<b>1</b>–<b>5</b>) supported by two P^∧N-type ligands 2-[(di<b>R</b>phosphino)­methyl]­pyridine (<b>1</b>, R = phenyl; <b>2</b>, R = cyclohexyl; <b>3</b>, R = <i>tert</i>-butyl; <b>4</b>, R = <i>iso</i>-propyl; <b>5</b>, R = ethyl) have been synthesized. Single crystal X-ray analyses show that all five clusters adopt a rare “octahedral” geometry. The central core of the cluster consists of the copper atoms arranged in a parallelogram with μ⁴-iodides above and below the copper plane. The copper atoms on the two short edges of the parallelogram (Cu–Cu = 2.525(2)–2.630(1) Å) are bridged with μ²-iodides, whereas the long edges (Cu–Cu = 2.839(3)–3.035(2) Å) are bridged in an antiparallel fashion by the P^∧N ligands. This Cu₄I₄ geometry differs significantly from the “cubane” and “stairstep” geometries reported for other Cu₄I₄L₄ clusters. Luminescence spectra of clusters <b>3</b> and <b>4</b> display a single emission around 460 nm at room temperature that is assigned to emission from a triplet halide-to-ligand charge-transfer (³XLCT) excited state, whereas clusters <b>1</b>, <b>2</b>, and <b>5</b> also have a second band around 570 nm that is assigned to a Cu₄I₄ cluster-centered (³CC) excited state. The structural and photophysical properties of a dinuclear Cu₂I₂(P^∧N)₂ complex obtained during the sublimation of cluster <b>3</b> is also provided

Cu₄I₄ Clusters Supported by P^∧N-type Ligands: New Structures with Tunable Emission Colors

A series of Cu₄I₄ clusters (<b>1</b>–<b>5</b>) supported by two P^∧N-type ligands 2-[(di<b>R</b>phosphino)­methyl]­pyridine (<b>1</b>, R = phenyl; <b>2</b>, R = cyclohexyl; <b>3</b>, R = <i>tert</i>-butyl; <b>4</b>, R = <i>iso</i>-propyl; <b>5</b>, R = ethyl) have been synthesized. Single crystal X-ray analyses show that all five clusters adopt a rare “octahedral” geometry. The central core of the cluster consists of the copper atoms arranged in a parallelogram with μ⁴-iodides above and below the copper plane. The copper atoms on the two short edges of the parallelogram (Cu–Cu = 2.525(2)–2.630(1) Å) are bridged with μ²-iodides, whereas the long edges (Cu–Cu = 2.839(3)–3.035(2) Å) are bridged in an antiparallel fashion by the P^∧N ligands. This Cu₄I₄ geometry differs significantly from the “cubane” and “stairstep” geometries reported for other Cu₄I₄L₄ clusters. Luminescence spectra of clusters <b>3</b> and <b>4</b> display a single emission around 460 nm at room temperature that is assigned to emission from a triplet halide-to-ligand charge-transfer (³XLCT) excited state, whereas clusters <b>1</b>, <b>2</b>, and <b>5</b> also have a second band around 570 nm that is assigned to a Cu₄I₄ cluster-centered (³CC) excited state. The structural and photophysical properties of a dinuclear Cu₂I₂(P^∧N)₂ complex obtained during the sublimation of cluster <b>3</b> is also provided

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

Dependence of Phosphorescent Emitter Orientation on Deposition Technique in Doped Organic Films

Dependence of Phosphorescent Emitter Orientation on Deposition Technique in Doped Organic Film

Fine-Tuning Electronic Properties of Luminescent Pt(II) Complexes via Vertex-Differentiated Coordination of Sterically Invariant Carborane-Based Ligands

We report the synthesis of two isomeric Pt(II) complexes ligated by doubly deprotonated 1,1′-bis(<i>o</i>-carborane) (<b>bc</b>). This work provides a potential route to fine-tune the electronic properties of luminescent metal complexes by virtue of vertex-differentiated coordination chemistry of carborane-based ligands

Structural and Photophysical Studies of Phosphorescent Three-Coordinate Copper(I) Complexes Supported by an N‑Heterocyclic Carbene Ligand

A series of four neutral luminescent three-coordinate Cu­(I) complexes (IPr)­Cu­(N^∧N), where IPr is a monodentate N-heterocyclic carbene (NHC) ligand (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) and N^∧N denotes monoanionic pyridyl-azolate ligands, have been synthesized and characterized. A monomeric, three-coordinate geometry, best described as distorted trigonal planar, has been established by single-crystal X-ray analyses for three of the derivatives. In contrast to the previously reported (IPr)­Cu­(N^∧N) complexes, the compounds described here display a perpendicular orientation between the chelating N^∧N ligands and the imidazolylidene ring of the carbene ligand. The geometrical preferences revealed by X-ray crystallography correlate well with the NMR data. The conformational behavior of the complexes, investigated by variable-temperature ¹H NMR spectroscopy, indicate free rotation about the C_NHC–Cu bond in solution. The complexes display broad, featureless luminescence at both room temperature and 77 K, with emission maxima that vary between 555 and 632 nm depending on sample conditions. Luminescence quantum efficiencies of the complexes in solution (Φ ≤ 17%) increase markedly in the solid state (Φ ≤ 62%). On the basis of time-dependent density functional theory (TD-DFT) calculations and the experimental data, luminescence is assigned to phosphorescence from a metal-to-ligand charge-transfer (MLCT) triplet state admixed with ligand-centered (LC) character

Structural and Photophysical Studies of Phosphorescent Three-Coordinate Copper(I) Complexes Supported by an N‑Heterocyclic Carbene Ligand

A series of four neutral luminescent three-coordinate Cu­(I) complexes (IPr)­Cu­(N^∧N), where IPr is a monodentate N-heterocyclic carbene (NHC) ligand (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) and N^∧N denotes monoanionic pyridyl-azolate ligands, have been synthesized and characterized. A monomeric, three-coordinate geometry, best described as distorted trigonal planar, has been established by single-crystal X-ray analyses for three of the derivatives. In contrast to the previously reported (IPr)­Cu­(N^∧N) complexes, the compounds described here display a perpendicular orientation between the chelating N^∧N ligands and the imidazolylidene ring of the carbene ligand. The geometrical preferences revealed by X-ray crystallography correlate well with the NMR data. The conformational behavior of the complexes, investigated by variable-temperature ¹H NMR spectroscopy, indicate free rotation about the C_NHC–Cu bond in solution. The complexes display broad, featureless luminescence at both room temperature and 77 K, with emission maxima that vary between 555 and 632 nm depending on sample conditions. Luminescence quantum efficiencies of the complexes in solution (Φ ≤ 17%) increase markedly in the solid state (Φ ≤ 62%). On the basis of time-dependent density functional theory (TD-DFT) calculations and the experimental data, luminescence is assigned to phosphorescence from a metal-to-ligand charge-transfer (MLCT) triplet state admixed with ligand-centered (LC) character

Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores

Boron dipyrromethene (BODIPY) derivatives have found widespread utility as chromophores in fluorescent applications, but little is known about the photophysical properties of pyridine-based BODIPY analogues, dipyridylmethene dyes. Indeed, it has been reported that boron difluoride dipyridylmethene (DIPYR) is nonemissive, and that derivatives of DIPYR have modest, if any, luminescence. In this report, we explore this little-touched area of chemical space and investigate the photophysical properties of three simple DIPYR dyes: boron dipyridylmethene, boron diquinolylmethene, and boron diisoquinolylmethene. The three dyes strongly absorb in the blue-green part of the spectrum (λ_em = 450–520 nm, ε = 2.9–11 × 10⁴ M^–1 cm^–1) and display green fluorescence with high quantum yields (Φ_PL = 0.2, 0.8, and 0.8, respectively). Key photophysical properties in these systems were evaluated using a combination of TD-DFT and extended multiconfigurational quasidegenerate second-order perturbation theory (XMCQDPT2) methods and compared to experimental results, revealing that high quantum yields of the quinoline and isoquinoline derivatives are a result of the relative reordering of S₁ and T₂ state energies upon benzannulation of the parent structure. The intense absorption and high emission efficiency of the benzannulated derivatives make these compounds an intriguing class of dyes for further derivatization

Photophysical Properties of Cyclometalated Pt(II) Complexes: Counterintuitive Blue Shift in Emission with an Expanded Ligand π System

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C^∧N)­Pt­(O^∧O) complexes (ppy)­Pt­(dpm) (<b>1</b>), (ppy)­Pt­(acac) (<b>1</b>′), and (bzq)­Pt­(dpm) (<b>2</b>) and newly synthesized (dbq)­Pt­(dpm) (<b>3</b>) (C^∧N = 2-phenylpyridine (ppy), benzo­[<i>h</i>]­quinoline (bzq), dibenzo­[<i>f</i>,<i>h</i>]­quinoline (dbq); O^∧O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds <b>1</b>, <b>1</b>′, <b>2</b>, and <b>3</b> were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C^∧N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (<i>E</i>_0–0 = 479 nm, <b>1</b>; <i>E</i>_0–0 = 495 nm, <b>2</b>; <i>E</i>_0–0 = 470 nm, <b>3</b>) with disparate radiative rate constants (<i>k</i>_r = 1.4 × 10⁵ s^–1, <b>1</b>; <i>k</i>_r = 0.10 × 10⁵ s^–1, <b>2</b>; <i>k</i>_r = 2.6 × 10⁵ s^–1, <b>3</b>). Zero-field splittings of the triplet states (Δ<i>E</i>_III–I = 11.5 cm^–1, <b>1</b>′; Δ<i>E</i>_III–I < 2 cm^–1, <b>2</b>; Δ<i>E</i>_III–I = 46.5 cm^–1, <b>3</b>) were determined using high resolution spectra recorded in Shpol’skii matrices. The fact that the <i>E</i>_0–0 energies do not correspond to the extent of π-conjugation in the aromatic C^∧N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C^∧N ligands in the triplet state alter the spin–orbit coupling in the complexes