Diogen Laertije - Životi i mišljenja istaknutih filozofa

The vast majority of polyhedral assemblies prepared by combining organic bent ligands and “photophysically innocent” palladium­(II) metal ions are nonemissive. We report here a simple strategy to switch on the luminescence properties of a polyhedral assembly by combining a thermally activated delayed fluorescence (TADF) organic emitter based on a dipyridylcarbazole ligand scaffold with Pd²⁺ ions, giving rise to a luminescent Pd₆L₁₂ molecular cube. The assembly is capable of encapsulating within its cavity up to three molecules per cage of fluorescein, in its neutral lactone form, and up to two molecules of Rose Bengal in its dianionic quinoidal form. Photoinduced electron transfer (PeT) between the photoactive cage and the encapsulated Fluorescein and photoinduced energy transfer (PET) from the cage to encapsulated Rose Bengal have been observed by steady-state and time-resolved emission spectroscopy

Panchromic Cationic Iridium(III) Complexes

We report the synthesis, X-ray structures, and optoelectronic characterization of two cationic iridium complexes bearing bis­[(4-methoxyphenyl)­imino]­acenaphthene ligands. These complexes exhibited panchromic absorption extending as far as 800 nm, making them of interest for solar-energy-harvesting applications

Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands

Herein we report the synthesis of 4-aryl-1-benzyl-1<i>H</i>-1,2,3-triazoles (atl), made via “Click chemistry” and their incorporation as cyclometallating ligands into new heteroleptic iridium(III) complexes containing diimine (N^∧N) ancillary ligands 2,2′-bipyridine (bpy) and 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine (dtBubpy). Depending on decoration, these complexes emit from the yellow to sky blue in acetonitrile (ACN) solution at room temperature (RT). Their emission energies are slightly blue-shifted and their photoluminescent quantum efficiencies are markedly higher (between 25 and 80%) than analogous (C^∧N)₂Ir(N^∧N)⁺ type complexes, where C^∧N is a decorated 2-phenylpyridinato ligand. This increased brilliance is in part due to the presence of the benzyl groups, which act to sterically shield the iridium metal center. X-ray crystallographic analyses of two of the atl complexes corroborate this assertion. Their electrochemistry is reversible, thus making these complexes amenable for inclusion in light-emitting electrochemical cells (LEECs). A parallel computational investigation supports the experimental findings and demonstrates that for all complexes included in this study, the highest occupied molecular orbital (HOMO) is located on both the aryl fragment of the atl ligands and the iridium metal while the lowest unoccupied molecular orbital (LUMO) is located essentially exclusively on the ancillary ligand

The Blue Problem: OLED Stability and Degradation Mechanisms

OLED technology has revolutionized the display industry and is promising for lighting. Despite its maturity, there remain outstanding device and materials challenges to address. Particularly, achieving stable and highly efficient blue OLEDs is still proving to be difficult; the vast array of degradation mechanisms at play, coupled with the precise balance of device parameters needed for blue high-performance OLEDs, creates a unique set of challenges in the quest for a suitably stable yet high-performance device. Here, we discuss recent progress in the understanding of device degradation pathways and provide an overview of possible strategies to increase device lifetimes without a significant efficiency trade-off. Only careful consideration of all variables that go into OLED development, from the choice of materials to a deep understanding of which degradation mechanisms need to be suppressed for the particular structure, can lead to a meaningful positive change toward commercializable blue devices

Mono- and Dinuclear Cationic Iridium(III) Complexes Bearing a 2,5-Dipyridylpyrazine (2,5-dpp) Ligand

The synthesis, X-ray structures, photophysical, and electrochemical characterization of mono- (<b>1</b>) and dinuclear (<b>2</b>) cationic iridium­(III) complexes bearing a 2,5-dipyridylpyrazine (2,5-dpp) ancillary ligand are reported. Upon the complexation of a first equivalent of iridium, the photoluminescence shifts markedly into the deep red (λ_em = 710 nm, Φ_PL = 0.9%) compared to other cationic iridium complexes such as [Ir­(ppy)₂(bpy)]­PF₆. With the coordination of a second equivalent of iridium, room temperature luminescence is completely quenched. Both <b>1</b> and <b>2</b> are luminescent at low temperatures but with distinct excited state decay kinetics; the emission of <b>2</b> is significantly red-shifted compared to <b>1</b>. Emission both at 298 and 77 K results from a mixed charge-transfer state. Density functional theory (DFT) calculations and electrochemical behavior point to an electronic communication between the two iridium complexes

Mono- and Dinuclear Cationic Iridium(III) Complexes Bearing a 2,5-Dipyridylpyrazine (2,5-dpp) Ligand

The synthesis, X-ray structures, photophysical, and electrochemical characterization of mono- (<b>1</b>) and dinuclear (<b>2</b>) cationic iridium­(III) complexes bearing a 2,5-dipyridylpyrazine (2,5-dpp) ancillary ligand are reported. Upon the complexation of a first equivalent of iridium, the photoluminescence shifts markedly into the deep red (λ_em = 710 nm, Φ_PL = 0.9%) compared to other cationic iridium complexes such as [Ir­(ppy)₂(bpy)]­PF₆. With the coordination of a second equivalent of iridium, room temperature luminescence is completely quenched. Both <b>1</b> and <b>2</b> are luminescent at low temperatures but with distinct excited state decay kinetics; the emission of <b>2</b> is significantly red-shifted compared to <b>1</b>. Emission both at 298 and 77 K results from a mixed charge-transfer state. Density functional theory (DFT) calculations and electrochemical behavior point to an electronic communication between the two iridium complexes

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha’s rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp3 C-linked D–A compounds due to the arrest of intramolecular electronic communication between two triplet states (T1H and T1L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp2 C-linked D–A counterparts, RTP from one locally excited T1 state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP

Cationic Platinum(II) Complexes Bearing Aryl-BIAN Ligands: Synthesis and Structural and Optoelectronic Characterization

Five cationic platinum­(II) complexes bearing a 2-(3′-substituted aryl)­pyridine cyclometalating ligand (C^∧N) and a neutral Ar-BIAN ligand have been synthesized: [Pt­(ppy)­(PhBIAN)]­PF₆ (<b>1</b>), [Pt­(3Fppy)­(PhBIAN)]­PF₆ (<b>2</b>), [Pt­(3MeOppy)­(PhBIAN)]­PF₆ (<b>3</b>), [Pt­(3MeOppy)­(4-FPhBIAN)]­PF₆ (<b>4</b>), [Pt­(ppy)­(4-MeOPhBIAN)]­PF₆ (<b>5</b>). All complexes have been characterized by NMR spectroscopy and mass spectrometry. Complexes <b>2</b> and <b>3</b> have been characterized by X-ray crystallography. Structure–property relationships were established from UV–visible spectroscopy and cyclic voltammetry studies. Interestingly, we found that when both the C^∧N and the Aryl-BIAN ligands contained electron-donating MeO groups the absorption spectrum for the platinum complex extended out to 650 nm. The electrochemical studies of these complexes established that they are electronically compatible dye molecules for dye-sensitized solar cells

Cationic Platinum(II) Complexes Bearing Aryl-BIAN Ligands: Synthesis and Structural and Optoelectronic Characterization

Five cationic platinum­(II) complexes bearing a 2-(3′-substituted aryl)­pyridine cyclometalating ligand (C^∧N) and a neutral Ar-BIAN ligand have been synthesized: [Pt­(ppy)­(PhBIAN)]­PF₆ (<b>1</b>), [Pt­(3Fppy)­(PhBIAN)]­PF₆ (<b>2</b>), [Pt­(3MeOppy)­(PhBIAN)]­PF₆ (<b>3</b>), [Pt­(3MeOppy)­(4-FPhBIAN)]­PF₆ (<b>4</b>), [Pt­(ppy)­(4-MeOPhBIAN)]­PF₆ (<b>5</b>). All complexes have been characterized by NMR spectroscopy and mass spectrometry. Complexes <b>2</b> and <b>3</b> have been characterized by X-ray crystallography. Structure–property relationships were established from UV–visible spectroscopy and cyclic voltammetry studies. Interestingly, we found that when both the C^∧N and the Aryl-BIAN ligands contained electron-donating MeO groups the absorption spectrum for the platinum complex extended out to 650 nm. The electrochemical studies of these complexes established that they are electronically compatible dye molecules for dye-sensitized solar cells

Green Phosphorescence and Electroluminescence of Sulfur Pentafluoride-Functionalized Cationic Iridium(III) Complexes

We report on four cationic iridium­(III) complexes [Ir­(C^N)₂(d<i>t</i>Bubpy)]­(PF₆) that have sulfur pentafluoride-modified 1-phenylpyrazole and 2-phenylpyridine cyclometalating (C^N) ligands (d<i>t</i>Bubpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridyl). Three of the complexes were characterized by single-crystal X-ray structure analysis. In cyclic voltammetry, the complexes undergo reversible oxidation of iridium­(III) and irreversible reduction of the SF₅ group. They emit bright green phosphorescence in acetonitrile solution and in thin films at room temperature, with emission maxima in the range of 482–519 nm and photoluminescence quantum yields of up to 79%. The electron-withdrawing sulfur pentafluoride group on the cyclometalating ligands increases the oxidation potential and the redox gap and blue-shifts the phosphorescence of the iridium complexes more so than the commonly employed fluoro and trifluoromethyl groups. The irreversible reduction of the SF₅ group may be a problem in organic electronics; for example, the complexes do not exhibit electroluminescence in light-emitting electrochemical cells (LEECs). Nevertheless, the complexes exhibit green to yellow-green electroluminescence in doped multilayer organic light-emitting diodes (OLEDs) with emission maxima ranging from 501 nm to 520 nm and with an external quantum efficiency (EQE) of up to 1.7% in solution-processed devices