Antenna-functionalized dendritic β-diketonates and europium complexes: synthetic approaches to generation growth

Six dendritic β-diketonates and their corresponding europium complexes were synthesized. These dendritic β-diketonate ligands consist of dibenzoylmethane cores, Fréchet-type poly(aryl ether) dendrons, and the carbazole-grafted peripheral functional groups. The designs of dendrimers are on the basis of high light-harvesting capability and dendron functionalization in virtue of the high extinction coefficient and carrier-injection adjustment of carbazole units. Different approaches to generation growth were utilized: the first generation europium complexes through etheral connectivity were developed via convergent synthetic approach; the second and third generation dendrons through esteral connectivity were developed by a hyperbranch core approach containing the advantages of convergent and divergent approaches. Their chemical structures were well characterized. Preliminary results show that the dendron-functionalized carbazole units not only tune the carrier-transporting capability, but also exhibit strong light-harvesting potential, resulting in a strong intense emission from the central Eu(III) ion via sensitization.     

Effect of ancillary ligands on the properties of heteroleptic green iridium dendrimers functionalized with carbazole dendrons

A series of heteroleptic green iridium dendrimers functionalized with carbazole dendrons, such as G2(pic) and G2(acac), have been synthesized, in which picolinic acid and acetylacetone are used as the ancillary ligands, respectively. Compared with the corresponding homoleptic iridium dendrimer G2 (8%), these heteroleptic ones can be prepared under mild conditions with total yields as high as 55-67%. Both the dendrimer G2(pic) and G2(acac) display bright green emissions with photoluminescence quantum yields higher than 0.80 in toluene solution. As a result, a maximum external quantum efficiency of 7.1% (21.0 cd/A) for G2(pic) and 7.7% (25.8 cd/A) for G2(acac) has been realized based on non-doped device configuration. The state-of-art performance indicates that the heteroleptic dendrimers can be promising candidates used for non-doped electrophosphorescent devices, especially when the ease of synthesis in a large scale is considered.     

Polyamido amine dendrimers functionalized with poly(phenylenevinylene) dendrons at their periphery

A series of polyamido amine (PAMAM) dendrimers (Generations 2, 3, 4, and 6) fully functionalized at their periphery with first‐ and second‐generation poly (phenylenevinylene) (PPV) dendrons have been efficiently prepared. MALDI‐TOF mass spectrometry proved to be particularly useful for the characterization of the new hybrid dendrimers as well as for the estimation of the average number of PPV dendrons attached to the surface. The optical absorption and emission properties of these systems were studied. The materials display extremely high molar extinction coefficients and emit blue light with only slightly lower fluorescence quantum yields than the corresponding free dendrons. Self‐quenching interactions between PPV units were not observed in THF. However, the luminescence properties underwent a dramatic change when toluene was used as the solvent. The lower polarity of toluene caused shrinkage of the PAMAM structure and brought the PPV chromophores closer together, leading to self‐quenching interactions and excimer formation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6409–6419, 2009     

Bipolar Heteroleptic Green Iridium Dendrimers Containing Oligocarbazole and Oxadiazole Dendrons for Bright and Efficient Nondoped Electrophosphorescent Devices

Bipolar heteroleptic green light‐emitting iridium (Ir) dendrimers G(OXD) and G(DOXD) have been designed and synthesized under mild conditions in high yields, in which the first C^N and second O^O ligands are functionalized with oligocarbazole‐ and oxadiazole‐based dendrons, respectively. To avoid affecting the optical properties of the emissive iridium core, all the functional moieties are attached to the ligands through a flexible spacer. Compared with the unipolar dendrimer G(acac ), dendrimers G(OXD) and G(DOXD) exhibit the close emission maxima of 511–512 nm and photoluminescence quantum yield of 0.39–0.40 in a solution of toluene. Moreover, on going from G(acac) to G(OXD) and G(DOXD) , we have found that the introduction of oxadiazole fragments decreases the lowest unoccupied molecular orbital (LUMO) energy levels to facilitate the electron injection and electron transporting, while their highest occupied molecular orbital (HOMO) energy levels remain unchanged. This means that, we can individually tune the HOMO and LUMO energy levels based on the heteroleptic structure to ensure the relative independence between the hole and electron in the emitting layer (EML), which is a favorable feature for bipolar optoelectronic materials. As a result, a bilayer nondoped electrophosphorescent device with G(DOXD) as the EML gives a maximum luminous efficiency of 25.5 cd A⁻¹ (η_ext: 7.4 %) and a brightness of 33 880 cd m⁻². In comparison to G(acac) (17.2 cd A⁻¹, 17 680 cd m⁻²), both the efficiency and brightness are improved by about 1.5 and 2 times, respectively. These state‐of‐the‐art performances indicate the potential of these bipolar heteroleptic iridium dendrimers as solution‐processible emitting materials for nondoped device applications.     

Segment block dendrimers via Diels-Alder cycloaddition

Segment block dendrimers consisting of polyester and polyaryl ether dendrons were synthesized using reagent free Diels-Alder cycloaddition reactions. Three generations of furan functionalized polyaryl ether dendrons were reacted with maleimide functionalized polyester dendrons of the same generation to obtain segment block dendrimers in good yields. The thermoreversible nature of these macromolecules was investigated by subjecting them to elevated temperatures in the presence of anthracene as a scavenger diene.     

Deep‐red light‐emitting phosphorescent dendrimer encapsulated tris‐[2‐benzo[b]thiophen‐2‐yl‐pyridyl] iridium (III) core for light‐emitting device applications

New deep‐red light‐emitting phosphorescent dendrimers with hole‐transporting carbazole dendrons were synthesized by reacting tris(2‐benzo[b]thiophen‐2‐yl‐pyridyl) iridium (III) complex with carbazolyl dendrons by DCC‐catalyzed esterification. The resulting first‐, second‐, and third‐generation dendrimers were found to be highly efficient as solution‐processable emitting materials and for use in host‐free electrophosphorescent light‐emitting diodes. We fabricated a host‐free dendrimer EL device with configuration ITO/PEDOT:PSS (40 nm)/dendrimer (55 nm)/BCP (10 nm)/Alq₃ (40 nm)/LiF (1 nm)/Al (100 nm) and characterized the device performance. The multilayered devices showed luminance of 561 cd/m² at 383.4 mA/cm² (12 V) for 15 , 1302 cd/m² at 321.3 mA/cm² (14 V) for 16 , and 422 cd/m² at 94.4 mA/cm² (18 V) for 17 . The third‐generation dendrimer, 17 (η_ext = 6.12% at 7.5 V), showed the highest external quantum efficiency (EQE) with an increase in the density of the light‐harvesting carbazole dendron. Three dendrimers exhibited considerably pure deep‐red emission with CIE 1931 (Commission International de L'Eclairage) chromaticity coordinates of x = 0.70, y = 0.30. The CIE coordinates remained very stable with the current density. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable materials for dendrimer light‐emitting diode (DLED) applications. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7517–7533, 2008     

Red‐Emitting Dendritic Iridium(III) Complexes for Solution Processable Phosphorescent Organic Light‐Emitting Diodes

Functionalization of a red phosphorescent iridium(III) complex core surrounded by rigid polyphenylene dendrons with a hole‐transporting triphenylamine surface allows to prevent the intermolecular aggregation‐induced emission quenching, improves charge recombination, and therefore enhances photo‐ and electroluminescence efficiencies of dendrimer in solid state. These multifunctional shape‐persistent dendrimers provide a new pathway to design highly efficient solution processable materials for phosphorescent organic light‐emitting diodes (PhOLEDs).     

Synthesis and photoluminescent properties of 1,1'-binaphthyl-based chiral phenylenevinylene dendrimers

New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6'-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1'-binaphthyl derivative (R)-1 and the appropriate dendrons (R)(2n)G(n)-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, the redox properties of the dendrimers were determined by cyclic voltammetry, showing the influence of the functional groups at the peripheral positions of the dendrimer on the redox behavior of these systems.     

A Convenient Approach To Synthesize o‐Carborane‐Functionalized Phosphorescent Iridium(III) Complexes for Endocellular Hypoxia Imaging

The structure–property relationship of carborane‐modified iridium(III) complexes was investigated. Firstly, an efficient approach for the synthesis of o‐carborane‐containing pyridine ligands a – f in high yields was developed by utilizing stable and cheap B₁₀H₁₀(Et₄N)₂ as the starting material. By using these ligands, iridium(III) complexes I – VII were efficiently prepared. In combination with DFT calculations, the photophysical and electrochemical properties of these complexes were studied. The hydrophilic nido‐o‐carborane‐based iridium(III) complex VII showed the highest phosphorescence efficiency (abs. =0.48) among known water‐soluble homoleptic cyclometalated iridium(III) complexes and long emission lifetime (τ=1.24 μs) in aqueous solution. Both of them are sensitive to O₂, and thus endocellular hypoxia imaging of complex VII was realized by time‐resolved luminescence imaging (TRLI). This is the first example of applying TRLI in endocellular oxygen detection with a water‐soluble nido‐carborane functionalized iridium(III) complex.     

Self-host heteroleptic green iridium dendrimers: achieving efficient non-doped device performance based on a simple molecular structure

Self-host heteroleptic green iridium(III) dendrimers have been designed and easily synthesized. Through tuning the carbazole dendron density, high efficiency is achieved using these dendrimers with a simple molecular structure as the emitting layer for the non-doped organic light-emitting diodes.     

Multifunctional red phosphorescent bis-cyclometallated iridium complexes based on 2-phenyl-1,2,3-benzotriazole ligand and carbazolyl moieties

Multifunctional phosphorescent bis-cyclometallated iridium(III) complexes based on the 2-phenyl-1,2,3-benzotriazole moiety and bearing branched hole-transporting carbazole fragments were synthesized. The isolated compounds were found to be amorphous and expressed very good solubility. Introduction of flexible aliphatic chains of various lengths into the iridium complexes enabled manipulation of their glass transition temperature. The iridium complexes exhibited red phosphorescence emission at 650 nm with the lifetime of 5.7 μs and phosphorescence quantum yields of 0.22 and 0.17 in solution and solid state, respectively, at room temperature. The shielding effect of the carbazolyl moieties on the concentration quenching of phosphorescence of the iridium centers was found to result in the increased excited state lifetime and quantum yield due to the suppressed exciton migration. Non-optimized OLED devices, based on the phosphorescent bis-cyclometallated iridium(III) complex without host material were fabricated and their electroluminescence properties were evaluated.     

Investigating carbazole jacketed precursor dendrimers: sonochemical synthesis, characterization, and electrochemical crosslinking properties

Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in dendrimer chemistry was explored resulting in very facile and very fast (up to 50x) reaction times compared to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically clear and possess superior energy band gap properties making them an alternative candidate over PVK for future hole-transport layer materials in electro-optical devices.     

Janus carbosilane/phosphorhydrazone dendrimers synthesized by the ‘click’ Staudinger reaction

The first association of carbosilane dendrons (having a phosphine at the focal point) with phosphorhydrazone dendrons (having a thiophosphoryl azide at the focal point) has been successfully carried out by ‘Staudinger click’ reaction. The corresponding Janus dendrimers possess the characteristics of both components; they are oily as the carbosilane dendrons, and they can be easily functionalized as the phosphorhydrazone dendrons.     

A Novel Light‐Emitting Mixed‐Ligand Iridium(III) Complex with a Terpyridine‐Poly(ethylene glycol) Macroligand

Summary : A monoterpyridine‐poly(ethylene glycol) (mono‐tpy‐PEG) and a novel monoterpyridine‐PEG‐functionalized iridium(III ) complex were successfully synthesized and fully characterized by means of NMR, IR, and UV‐vis spectroscopy, as well as gel permeation chromatography and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. The functionalized monoterpyridine iridium(III ) complex was synthesized by a bridge‐splitting reaction of a dimeric iridium(III ) precursor complex using a chelating terpyridine ligand with a poly(ethylene glycol) tail. With this approach, a new class of light‐emitting polymeric materials revealing interesting optical properties was made avaialable.

Upon excitation of a spin‐coated film of the iridium(III ) complex prepared here, a yellow emission color (two bands in figure) was observed.     

Evolution of 2, 3′-bipyridine class of cyclometalating ligands as efficient phosphorescent iridium(III) emitters for applications in organic light emitting diodes

Nowadays, the design and development of novel phosphorescent iridium(III) complexes for various optoelectronic applications is a well-recognized area of research. The fascinating photophysical properties of iridium(III) compounds are strongly influenced by the spin-orbit coupling exerted by the iridium(III) core, usually resulting in intense emissions with short excited-state lifetimes, which can be precisely controlled with the aid of molecular engineering of the chelating ligand. This review focuses on the recent developments and state of the art knowledge on phosphorescent iridium(III) compounds, especially on heteroleptic complexes derived from 2,3′-bipyridine class of cyclometalating and ancillary ligands, highlighting the excited state phenomenon behind their emission behavior.     

Multifunctional Supramolecular Dendrimers with an s‐Triazine Ring as the Central Core: Liquid Crystalline,Fluorescence and Photoconductive Properties

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s‐triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X‐ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated     

Synthesis of Fréchet type dendritic benzyl propargyl ether and Fréchet type triazole dendrimer

Fréchet type dendritic benzyl propargyl ethers were synthesized by the reaction of propargyl bromide with the corresponding Fréchet type dendritic benzyl alcohol. A propargyl focal point functionalized dendrons were applied for the construction of symmetric and unsymmetric dendrimers containing 1,2,3-triazole rings as connectors via click chemistry with a tripodal azide core or a azide focal point functionalized Fréchet type dendrons.     

Coumarin-cored carbazole dendrimers as solution-processed non-doped green emitters for electroluminescent devices

A series of green-emitting thiophenyl coumarin-cored carbazole dendrimers containing carbazole dendrons up to the third generation as substituent were synthesized and characterization. Their optical, thermal, electrochemical, and electroluminescent properties as non-doped solution-processed light-emitters for OLEDs were investigated. By incorporating carbazole dendrons in the molecule, we are able to reduce the crystallization and retain the high emissive ability of a planar thiophenyl coumarin fluorescent core in the solid state as well as improve the thermal stability of the material. These dendrimers showed a bright-green fluorescence and can form morphologically stable amorphous thin films with glass-transition temperatures as high as 285 °C. Simple structured solution-processed OLEDs using these materials as hole-transporting non-doped emitters and BCP as a buffer layer emit a stable green electroluminescence (λ_EL=502–526 nm) with high luminance efficiencies (up to 7.92 cd/A at 7.39 mA/cm²) and high green color purity (CIE=0.26, 0.62, which are close to the pure green color).     

Self-host yellow iridium dendrimers based on carbazole dendrons: synthesis,characterization and application in solution-processed organic light-emitting diodes

On the basis of different generation carbazole dendrons, a series of self-host yellow Ir dendrimers (Y-G0, Y-G1 and Y-G2) have been successfully synthesized and characterized in detail. It is found that the peripheral dendrons can effectively reduce the intermolecular interactions between emissive Ir cores, as verified by the increased photoluminescence quantum yields and film lifetimes. Among these dendrimers, Y-G2 bearing the second generation dendrons shows the best non-doped device performance, revealing a peak luminous efficiency of 20.2 cd/A. The value is nearly twice that of Y-G0 without any dendrons, which could be further improved to 32.1 cd/A by dispersing Y-G2 into a host matrix. We believe that this work will shed light on the development of highly efficient yellow phosphorescent dendrimers with a self-host strategy.     

Attachment of an aromatic dendritic macromolecule to gold surfaces

Surface immobilization of dendrons and dendrimers presents an exciting opportunity for creating a wide variety of functionalized polymeric architectures suitable for the immobilization of biomolecules. Dendritic molecules contain multifunctional groups that can be efficiently modified to control the properties of the resulting polymers. We are developing strategies to generate a highly functionalized surface using multifunctional and rigid dendrons immobilized onto different substrates. In this paper, electrochemical methods and scanning probe microscopy were used to explore the immobilization of a dendritic macromolecule (3,5-bis(3,5-dinitrobenzoylamino)benzoic acid) or (D-NO₂) onto gold electrodes. D-NO₂ adsorbs spontaneously by dipping the metal surface in dendron solution and also via grafting of cystamine covalent attached to gold electrode. Reduction of this layer generates the hydroxylamine product. The resulting redox-active layer exhibits a well-behaved redox response for the adsorbed nitroso/hydroxylamine couple.