Gram Scale Synthesis of Benzophenanthroline and Its Blue Phosphorescent Platinum Complex

The design, synthesis, and characterization of 12-phenyl­benzo­[<i>f</i>]­[1,7]­phenanthroline, Bzp, is reported. Its use as a fluorine-free ligand for sky blue phosphorescence is demonstrated in a cyclometalated platinum complex, BzpPtDpm. BzpPtDpm phosphoresces at the same wavelength as its analogous 4,6-difluoro­phenyl­pyridine complex at both room temperature (466 nm) and 77 K (458 nm). Finally, production of a conformationally restricted derivative of BzpPtDpm with greatly increased quantum yield (46%) validates the versatility of the synthetic route

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

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

Manipulating Triplet Yield through Control of Symmetry-Breaking Charge Transfer

The efficiency of an organic solar cell depends on the efficacy of exciton diffusion and dissociation processes, and this can be enhanced by reducing the exciton binding energy and increasing the exciton lifetime. Zinc chlorodipyrrin (ZCl) complexes exhibit reduced exciton binding energy due to ultrafast generation of intramolecular charge transfer (ICT) states via symmetry-breaking charge transfer in polar media. This Letter explores the fate of the ICT states using nanosecond transient absorption. In cyclohexane, ZCl undergoes intersystem crossing to produce triplets with ∼8 ns time constant (∼30% yield), and no ICT states are generated. However, in more polar solvents, triplets are generated within 1 ns via ICT state recombination with ∼3 times higher yield than produced via ISC. This high triplet yield in toluene (89%) and acetonitrile (76%) via ICT state recombination is a beneficial pathway to spin-protect the excited-state decay for additional charge generation from triplet excited states

Aqueous Colloidal Acene Nanoparticles: A New Platform for Studying Singlet Fission

Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films

Tandem and Triple-Junction Polymer:Nanocrystal Hybrid Solar Cells Consisting of Identical Subcells

Tandem and triple-junction polymer:nanocrystal hybrid solar cells with identical subcells based on P3HT:CdSe nanocrystal bulk heterojunctions (BHJs) are reported for the first time showing 2-fold and 3-fold increases of open-circuit voltage (<i>V</i>_OC), respectively, relative to the single-junction cell. A combination of nanocrystalline ZnO and pH-neutral PEDOT:PSS is used as the interconnecting layer, and the thicknesses of subcells are optimized with the guidance of optical simulations. As a result, the average power conversion efficiency (PCE) exhibits a significant increase from 2.0% (<i>V</i>_OC = 0.57 V) in single-junction devices to 2.7% (champion 3.1%, <i>V</i>_OC = 1.28 V) in tandem devices and 2.3% (<i>V</i>_OC = 1.98 V) in triple-junction devices

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

High-Efficiency BODIPY-Based Organic Photovoltaics

A benzannulated boron dipyrromethene (BODIPY, bDIP) molecule exhibiting strong absorption at 640 nm was synthesized. The organic dye was used in an organic solar cell as the electron donor with C₆₀ as the acceptor. The BODIPY dye demonstrated the best performance in lamellar architecture (indium tin oxide (ITO)/bDIP/C₆₀/bathocuproine/Al), giving power conversion efficiency up to 4.5% with short-circuit current (<i>J</i>_SC) of 8.7 mA/cm² and an open-circuit voltage (<i>V</i>_OC) of 0.81 V. Neutron reflectivity experiments were performed on the bilayer film to investigate the thickness dependence of <i>J</i>_SC. A 13 nm mixed layer was found to be present at the donor/acceptor interface in the bilayer device, formed when the C₆₀ was deposited onto a room temperature bDIP film. Planar-mixed heterojunction devices were fabricated to understand the extent of spontaneous mixing between the donor and acceptor materials. The native mixed region in the bilayer device was shown to most resemble 1:3 bDIP:C₆₀ layer in the structure: (ITO/bDIP/bDIP:C₆₀ blend/C₆₀/bathocuproine/Al)

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

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