Light-Emitting Electrochemical Cells Using Cyanine Dyes as the Active Components

Light-emitting electrochemical cells (LECs) based on cyanine molecules were prepared. High photoluminescence quantum yields were obtained for host–guest films using two cyanine dyes, reaching 27%. Sandwiching these films in between two electrodes allows for very stable near-infrared emission with a maximum radiant flux of 1.7 W m^–2 at an external quantum efficiency of 0.44%

Correlating the Lifetime and Fluorine Content of Iridium(III) Emitters in Green Light-Emitting Electrochemical Cells

In light-emitting electrochemical cells, the lifetime of the device is intrinsically linked to the stability of the phosphorescent emitter. In this study, we present a series of ionic iridium­(III) emitters based on cyclometalating phenylpyridine ligands whose fluorine substituents are varied in terms of position and number. Importantly, despite these structural modifications, the emitters exhibit virtually identical electrochemical and spectroscopic properties, which allows for proper comparison in functional devices. Quantum-chemical calculations support the properties measured in solution and suggest great similarities regarding the electronic structures of the emitters. In electroluminescent devices, the initial luminance, efficiency, and efficacy are also relatively unaffected throughout the series. However, a shorter device lifetime is obtained upon increasing the fluorine content of the emitter, which suggests drawbacks of such electron-withdrawing substituents for the design of ionic iridium­(III) emitters

Anionic Cyclometalated Iridium(III) Complexes with a Bis-Tetrazolate Ancillary Ligand for Light-Emitting Electrochemical Cells

A series of monoanionic Ir­(III) complexes (<b>2</b>–<b>4</b>) of general formula [Ir­(C^N)₂(b-trz)]­(TBA) are presented, where C^N indicates three different cyclometallating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)­pyridine; Hpqu = 2-methyl-3-phenylquinoxaline), b-trz is a bis-tetrazolate anionic N^N chelator (H₂b-trz = di­(1H-tetrazol-5-yl)­methane), and TBA = tetrabutylammonium. <b>2</b>–<b>4</b> are prepared in good yields by means of the reaction of the suitable b-trz bidentate ligand with the desired iridium­(III) precursor. The chelating nature of the ancillary ligand, thanks to an optimized structure and geometry, improves the stability of the complexes, which have been fully characterized by NMR spectroscopy and high-resolution MS, while X-ray structure determination confirmed the binding mode of the b-trz ligand. Density functional theory calculations show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are mainly localized on the metal center and the cyclometalating ligands, while the bis-tetrazolate unit does not contribute to the frontier orbitals. By comparison with selected classes of previously published cationic and anionic complexes with high ligand field and even identical cyclometallating moieties, it is shown that the HOMO–LUMO gap is similar, but the absolute energy of the frontier orbitals is remarkably higher for anionic vs cationic compounds, due to electrostatic effects. <b>2</b>–<b>4</b> exhibit reversible oxidation and reduction processes, which make them interesting candidates as active materials for light emitting electrochemical cells, along with red, green, and blue emission, thanks to the design of the C^N ligands. Photoluminescence quantum yields range from 28% (<b>4</b>, C^N = pqu, red emitter) to 83% (<b>3</b>, C^N = dfppy, blue emitter) in acetonitrile, with the latter compound reaching 95% in poly­(methyl methacrylate) (PMMA) matrix. In thin films, the photoluminescence quantum yield decreases substantially probably due to the small intersite distance between the complexes and the presence of quenching sites. In spite of this, surprisingly stable electroluminescence was observed for devices employing complex <b>2</b>, demonstrating the robustness of the anionic compounds

Nontemplate Synthesis of CH₃NH₃PbBr₃ Perovskite Nanoparticles

To date, there is no example in the literature of free, nanometer-sized, organolead halide CH₃NH₃PbBr₃ perovskites. We report here the preparation of 6 nm-sized nanoparticles of this type by a simple and fast method based on the use of an ammonium bromide with a medium-sized chain that keeps the nanoparticles dispersed in a wide range of organic solvents. These nanoparticles can be maintained stable in the solid state as well as in concentrated solutions for more than three months, without requiring a mesoporous material. This makes it possible to prepare homogeneous thin films of these nanoparticles by spin-coating on a quartz substrate. Both the colloidal solution and the thin film emit light within a narrow bandwidth of the visible spectrum and with a high quantum yield (ca. 20%); this could be advantageous in the design of optoelectronic devices

Iridium(III) Complexes with Phenyl-tetrazoles as Cyclometalating Ligands

Ir­(III) cationic complexes with cyclometalating tetrazolate ligands were prepared for the first time, following a two-step strategy based on (i) a silver-assisted cyclometalation reaction of a tetrazole derivative with IrCl₃ affording a bis-cyclometalated solvato-complex <b>P</b> ([Ir­(ptrz)₂­(CH₃CN)₂]⁺, Hptrz = 2-methyl-5-phenyl-2<i>H</i>-tetrazole); (ii) a substitution reaction with five neutral ancillary ligands to get [Ir­(ptrz)₂L]⁺, with L = 2,2′-bypiridine (<b>1</b>), 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine (<b>2</b>), 1,10-phenanthroline (<b>3</b>), and 2-(1-phenyl-1<i>H</i>-1,2,3-triazol-4-yl)­pyridine (<b>4</b>), and [Ir­(ptrz)₂L₂]⁺, with L = <i>tert</i>-butyl isocyanide (<b>5</b>). X-ray crystal structures of <b>P</b>, <b>2</b>, and <b>3</b> were solved. Electrochemical and photophysical studies, along with density functional theory calculations, allowed a comprehensive rationalization of the electronic properties of <b>1</b>–<b>5</b>. In acetonitrile at 298 K, complexes equipped with bipyridine or phenanthroline ancillary ligands (<b>1</b>–<b>3</b>) exhibit intense and structureless emission bands centered at around 540 nm, with metal-to-ligand and ligand-to-ligand charge transfer (MLCT/LLCT) character; their photoluminescence quantum yields (PLQYs) are in the range of 55–70%. By contrast, the luminescence band of <b>5</b> is weak, structured, and blue-shifted and is attributed to a ligand-centered (LC) triplet state of the tetrazolate cyclometalated ligand. The PLQY of <b>4</b> is extremely low (<0.1%) since its lowest level is a nonemissive triplet metal-centered (³MC) state. In rigid matrix at 77 K, all of the complexes exhibit intense luminescence. Ligands <b>1</b>–<b>3</b> are also strong emitters in solid matrices at room temperature (1% poly­(methyl methacrylate) matrix and neat films), with PLQYs in the range of 27–70%. Good quality films of <b>2</b> could be obtained to make light-emitting electrochemical cells that emit bright green light and exhibit a maximum luminance of 310 cd m^–2. Tetrazolate cyclometalated ligands push the emission of Ir­(III) complexes to the blue, when compared to pyrazolate or triazolate analogues. More generally, among the cationic Ir­(III) complexes without fluorine substituents on the cyclometalated ligands, <b>1</b>–<b>3</b> exhibit the highest-energy MLCT/LLCT emission bands ever reported