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Dendronized Bi-2-quinoline Ligands and Their Metal Complexes: Dendron Synthesis and Metalloassembly
Abstract
The synthesis of bis-biquinoline ligands possessing G1 and G2 Behera's amine-based dendrons with acid and ester termini was accomplished. Treatment with Cu(I) salts quantitatively generated the desired stable metal lodendrimers with a Cu(I) core.
... An important observation was made while studying Cu(I) chelation with a dendronized quinoline ligand containing terminal acid or ester groups. 54 It is important that treatment of a ligand with Cu(I) salt leads to intermolecular cross-linking with the quantitative formation of stable MCDs with a Cu(I) core. ...
The synthetic methodologies, physico-chemical peculiarities, properties, and structure of metal chelate dendrimers, star and hyperbranched polymers are considered. These compounds are subdivided into molecular, intracomplex, and macrocyclic types which in turn are grouped depending on the nature of the donor atoms (N,N-, N,О-, N,S-, О,О-, O,S-, S,S-, Р,Р-chelates, etc.). Special attention is paid to the features of the preparation of metal chelate star polymers by “arm-first”, “core-first” and click-to-chelate approaches. The main data on the synthesis, spatial structure and properties of the metal chelate hyperbranched polymers are summarized. The basic concepts and synthetic strategies leading to the different types of supramolecular metal chelate dendrimers are analyzed. The problems and future prospects of metal chelate dendrimers, star and hyperbranched polymers are outlined. The bibliography includes the papers published after 2010.
In this chapter the synthetic methodologies, physico-chemical peculiarities, properties, and spatial organization of metal chelate dendrimers as a specific sub-class of polymeric metal chelates are considered. These compounds are subdivided into molecular, intracomplex, and macrocyclic types which in turn are grouped depending on the nature of the donor atoms (N,N-, N,O-, N,S-, O,O-, O,S-, S,S-, P,P-chelates, etc.). Special attention is paid to the features of the preparation of metal chelate star polymers by “arm-first”, “core-first” and click-to-chelate approaches. The main data on the synthesis, spatial structure and properties of the metal chelates with hyperbranched dendrimers are summarized. The problems and future prospects of metal chelate dendrimers are outlined.
Synthesis of terpyridine-based building blocks has allowed the self-assembly of a nanosized metallomacrocycle with precisely positioned, peripheral metal complexes. Construction of the precursors included the assembly of a heteroleptic bisterpyridine-Ru(II) complex possessing a diiodoaryl moiety that was subsequently reacted with two equivalents of 4′-ethynyl-2,2′:6′,2′′-terpyridinevia a Pd(0)-catalyzed Sonagashira coupling, to generate the requisite monomer with two, 120°-juxtaposed metal coordination sites. Addition of one equivalent of Fe(II) to one equivalent of the bisligand afforded the metallocycle with 12 metals [6 internal Fe(II) ions and 6 external Ru(II) ions] measuring ca. 7 nm in diameter.
Several methods for the synthesis of cascade polymers (dendritic macromolecules) on adamantane have been investigated. The synthesis of two novel, branched monomers, 4-amino-4-(3-acetoxypropyl)-1,7-diacetoxyheptane and di-tert-butyl 4-amino-4-[2-(tert-butoxycarbonyl)ethyl]heptanedioate, possessing 3-fold symmetry and branches emanating from a tetrahedral carbon branch point, is described. These monomers were reacted with a monofunctional adamantane core to evaluate the reaction efficiency, ease of purification of products, and spectral characteristics.
Three phosphorescent dendrimers ( IrC1 , IrC3 , and IrF2 ) with an iridium complex core and oligocarbazole or oligofluorene substituted ligands were synthesized and characterized. The structures of the oligocarbazole were designed to maintain high triplet energy of the ligands so that phosphorescence quenching in the resulting dendrimers can be prevented, while the oligofluorene in IrF2 resulted in undesired phosphorescence quenching. Best performance was obtained from an IrC3 based electrophosphorescent light‐emitting device with a maximum luminance of 13 060 cd · m ⁻² at a driving voltage of 11.5 V and a peak current‐efficiency of 4.3 cd · A ⁻¹ at a luminance of 3 400 cd · m ⁻² , owing to its high PL efficiency, and efficient energy transfer between the iridium complex core and the ligands.
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The first examples of metallodendritic spiranes have been obtained via incorporation of single terpyridine units within each dendritic quadrant.
A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings each equipped with two long alkoxy chains has been rationally designed. The presence of amide tethers and a chelating phenanthroline fragment connected via an ester dipole formed a new class of gelating reagents and mesomorphic materials. A few of these compounds have the tendency to form macromolecule-like aggregates through noncovalent interactions in hydrocarbon solvents and were found to exhibit thermotropic cubic mesophases. In light of the X-ray molecular structure of the methoxy ligand, an infinite network maintained by intermolecular hydrogen bonds as well as by pi-pi stacking of the phenyl subunits was evidenced. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular H-bonded network that does not persist in diluted solution. This situation is switched when the ligands are interlocked by a copper(I) cation. A strong intramolecular H-bond confirmed by X-ray diffraction of a single crystal for the methoxy case provides very stable complexes but inhibits the gelation of the solvents. Heating the complexes bearing long paraffin chains (n = 12 and 16) in the dried state leads to a self-organization into a columnar liquid-crystalline phase in which the columns are arranged along a 2D oblique symmetry as deduced from powder XRD experiments. In this case, the complexes with the appended counteranions self-assemble in a specific way to form columns. A striking observation is that the intramolecular hydrogen bond persists in the mesophase as it does in solution without any evidence of an extended network. As far as we are aware, these ligands and complexes are rare examples in which organogelation and thermotropic mesomorphic behavior could be observed in parallel with molecules bearing a chelating platform. Due to the synthetic availability of the 4-methyl-3,5-diacylaminobenzene core and the simplicity by which the chelating platforms can be graphed, this methodology represents a practical alternative to the production of functionalized organogelators and mesomorphic materials.
A series of luminescent PAMAM dendrons emanating from 8-hydroxyquinoline have been synthesized and their coordination with Zn(II) was investigated for the first time. The obtained dendritic Zn(II) complexes were soluble in common organic solvents. It was found that the luminescence intensity of G2 dendron 6 was higher than that of G1 dendron 4. Furthermore, when they were coordinated with Zn(II), red-shift was observed and the intensities of the coordinated Zn(II) complexes were higher than that of the corresponding ligands.
The 19-electron complexes [FeI(ν5-C 5R5)(ν6-arene)] (R = H, arene = C 6Me6, l,3,5-/Bu3C6H3; R = Me, arene = C6Me6, C6Et5H) are thermally stable and serve as strong single-electron reductant in a variety of stoichiometric and catalytic electron-transfer processes that are detailed in this review (prototype: [FeI(ν5-C5H 5)(ν6-C6Me6)]). They are electron-reservoirs because their redox potential is very negative and they form a redox system for which both redox forms are stable. In this sense, they are Green redox reagents, since the oxidized form is recovered after use and is even frequently catalytically used. Symmetrically, the 17-electron complex [Fe III(ν5-C5R5)(ν6- C6Me6)][SbCl6]2, the strongest organometallic single-electron oxidant with a redox potential 1 V more positive than that of its isoelectronic ferro-cenium analog, is a reservoir of electron hole.
Molecular recognition of glutarimide using 2,6-diamidopyridine units incorporated into dendritic building blocks is examined.
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.
A series of metallodendrimers, assembled by means of bis(terpyridinyl)Ru(II) connectivity on poly(propylene imine) dendrimer scaffolds, with homogeneous or heterogeneous surfaces, were prepared. Differential scanning calorimetry and thermogravimetric analysis were used to determine their thermal behavior, glass-transition temperatures, and the decomposition kinetics and temperatures; no synergy effects for these properties were observed for the heterogeneously surfaced constructs in contrast to the corresponding homogeneously coated materials, which exhibited different values depending on their surface functionalities. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1487–1495, 2004
The construction of nanometric, dendritic macromolecules by
bis(2,2′:6′,2″-terpyridine)ruthenium(ii) connectivity is
investigated. The assembly methodology, which incorporates both the
control of metal complexation sites and degree of flexibility within the
linkages, has been demonstrated.
Dinuclear ruthenium complexes containing the stable metal-metal bonded Ru 2 (CO) 4 sawhorse unit with two dendritic carboxylato bridges have been synthesized and characterized. All complexes Ru 2 (CO) 4 (O 2 CR) 2 L 2 (R) R 1 , R 2 , R 3) containing cyanobiphenyl-based poly(arylester) dendrons of first (R 1), second (R 2), and third (R 3) generation and triphenylphosphine, pyridine, or 4-picoline ligands L proved to be mesomorphic, giving rise to smectic A or smectic A and nematic phases. The supramolecular organization within the smectic A phase is governed by the nature and structure of the mesogenic units and dendritic core. Such materials are of interest for the design of catalytically active anisotropic fluids.
We report the synthesis and electrophosphorescent behavior of a series of novel iridium complex materials (Complexes A–F), which are composed of ligands bearing polyphenylphenyl dendron groups and acetylacetonate. Yellow to saturated red organic light-emitting diodes (OLEDs) based on these newly developed Ir complexes were fabricated through solution process by doping the complex materials into polyvinyl carbazole (PVK)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) matrices. The emission wavelengths of the materials could be effectively tuned from 549nm to 640nm by changing the conjugation of the ligands either through incorporating additional aromatic segment (e.g. phenyl or fluorenyl group) onto the basic dendron ligand or fusing two of the phenyl rings on the polyphenylphenyl dendron group. High performance devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) (50nm)/PVK:PBD (40%):Ir complex (6%) (70nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (12nm)/Alq3 (20nm)/Mg:Ag (150nm) have been demonstrated. For example, when Complex B was used as the emissive layer, maximum current efficiency of 34.0cd/A and external quantum efficiency of 10.3% have been achieved. When 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) was used as the block layer, the efficiencies can be further improved to 46.3cd/A and 13.9%, respectively. These solution processed OLED devices demonstrated quite stable EL efficiencies over a large range of current density, which indicated that triplet–triplet annihilation in electrophosphorescence could be effectively suppressed by incorporation of the polyphenylphenyl dendron structure into iridium complexes.
The synthesis of β-cyclodextrin-based dendrimers 3 and 5 is described; using phenolphthalein as a UV–VIS active probe, it is shown that the binding cavities of these modified receptors retain their molecular recognition properties and can be employed as polyfunctional monomers in self-assembly.
The synthesis of new α-diimine ligands containing zero and first generation dendritic wedges is described. These ligands were reacted with (COD)PdCl2 to give the corresponding palladium α-diimine complexes. The resulting palladium complexes are active in the oligomerisation of ethene and the activity was compared to a related nickel complex.
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% (21.0cd/A) for G2(pic) and 7.7% (25.8cd/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.
The cyclopentadienyl(β-diketiminato)titanium and zirconium chlorides (η5-C5H5)MCl2(CH(C(NC6H4-4-OR)CH3)2) (M=Ti (4-dend), Zr (5-dend)), where R corresponds to the first generation carbosilane dendron (dendritic wedge) Si(CH2CH2SiMePh2)3, have been synthesised. After activation with methylaluminoxane, the activity of 4-dend and 5-dend as catalysts for ethylene polymerisation has been determined and compared with that of the non-dedritic counterpart (η5-C5H5)MCl2(CH(C(NC6H5)CH3)2) (M=Ti (4), Zr (5)).
2,2′:6′,2″-Terpyridine end-functionalized dendrimers were prepared utilizing chlorinated siloxane dendrimer and 6-hydroxyhexa-4′-(2,2′:6′,2″-terpyridine) ether. The terpyridine groups on dendrimers accepted the ruthenium chloride ions on the mild condition that produced the paramagnetic ruthenium complex Gn-mTPY-RuCl3. The continual addition of 2,2′:6′,2″-terpyridine to the ruthenium–terpyridine complex on the dendritic periphery progressed to bis(2,2′:6′,2″-terpyridine)ruthenium(II) complex Gn-m[TPY-Ru-TPY] (n=1, m=4; n=2, m=8; n=3, m=16) which revealed diamagnetic property.
Complexes [Ir(C^N)2(G1-bpy)]PF6, where C^N is a cyclometallating ligand derived from 2-(2'-thienyl)pyridine and 2-phenylpyridine, and G1-bpy is a dendritic bipyridine ligand of the first generation, 4,4'-bis[3?,5?-bis(benzyloxy)phenylethyl]-2,2'-bipyridine, were prepared and characterized by 1H NMR, electronic absorption, and emission spectroscopy. The polyether dendritic substituents exert a “ soft” effect on the spectral and luminescence properties of the complexes, manifested as slight destabilization of the electronically excited charge-transfer state involving the bipyridine ligand, as compared to the model complexes [Ir(C^N)2(bpy)]PF6.
Facile alkoxylation of 4′-chloro-2,2′ :6′,2″-terpyridine with the mono- and tri-hydroxylic cascade (dendritic) building blocks, 4-(3-hydroxypropyl)-4-(3-benzyloxypropyl)-1,7-bis(benzyloxy)heptane 4 and 4-amino-4-(3-hydroxypropyl)heptane-1,7-diol 3, has allowed the synthesis of a dodecaruthenium macromolecule 11 employing ligand–metal–ligand connectivity.
The binding of the host H1 (N,N‘-bis(6-pivalamidopyrid-2-yl)-3,5-pyridinedicarboxamide) to three different ruthenium polypyridine complexes with an attached barbituric acid and barbital moieties (RuG1, RuG2, RuG3) (where G1 = 5-[4-(4‘-methyl)-2,2‘-bipyridylidene]-2,4,6-(1H,3H,5H)-pyrimidinetrione, G2 = 5-[4-(4‘-methyl)-2,2‘-bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione, and G3 = 5-ethyl, 5-[4-(4‘-methyl)-2,2‘-bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione) and Ru = (4,4‘-di-tert-butyl-bpy)2Ru (bpy = 2,2‘-bipyridine) has been studied in chlorinated solvents by NMR and fluorescence titrations. Significant binding was only observed between H1 and the RuG2 series, while steric hindrance significantly diminished binding between H1 and RuG1 or RuG3. The high binding constant for RuG2 was related to the presence of the enolate form of the barbituric acid guest which forms strong H-bonds with the complementary host H1. For the organic barbituric acid and barbital guests, the keto and enol bind only weakly to H1 (K 102 M-1); binding is further increased in the presence of base to generate the enolate. In contrast, formation of the RuG2 enolate occurs upon binding to H1 without any additional base. The ruthenium polypyridine cation (compared to the organic barbituric acid derivatives) facilitates ionization of the enol to enolate thus producing a better complementary H-bonding site between the guest and host. Molecular mechanics calculations confirmed the experimental observatons that the enolate has the highest binding constant to the Host H1, while the corresponding enol form has the weakest binding.
The concept of dendritic networks via four bis(2,2‘:6‘,2‘ ‘-terpyridine)ruthenium(II) connectivities was utilized to create “dendritic methane”-type metallomacromolecules. In addition, two structurally isomeric metallodendrimers (16 and 17) were designed, synthesized, and characterized. These two isomers were spectrally alike and displayed very similar physical and chemical properties; however, the internal density and void regions of these molecules were differentiated by cyclic voltammetry. The effects of the bulkiness of internal and external dendritic branches on the terpyridine−ruthenium complexes are described. A similar synthetic strategy allowed the preparation of two generations of neutral metallomacromolecules (33 and 36) possessing no external counterions. The benefits of these internally charge-balanced, neutral species are described, and the effects of increasing dendritic branching on their electrochemical behavior are detailed.
This study explored practical applications of the unique green fluorescence of terbium(III)-cored poly(benzyl ether) dendrimer complexes for luminescence polymeric materials. At first, to maximize the fluorescence ability, we examined the influence of the isomerism in the dendron skeleton, both at the focal point and the hyperbranch repeating unit. Next, a 4-vinylphenyl group was successfully introduced at the terminal of the dendron subunit for radical polymerization. A 3,4-dioxybenzoate moiety for the focal point of the dendron gave stronger fluorescence than the usual 3,5-isomer; however, the authentic 3,5-dioxybenzyl ether moiety appeared the best hyperbranch repeating unit for the fluorescence ability. The terminal vinyl groups enabled the copolymerization of the terbium(III)-cored dendrimer complex with N-isopropylacrylamide, resulting in a green-fluorescent, clear hydrogel. The hydrogel showed a gel phase transition at 33−35 °C to exhibit temperature-induced switching in the opacity.
The synthesis of novel dendrimers with various functional cores, including those of the porphyrin macrocycle, has been a very active area of research. Phthalcyanine (Pc), a close relative of porphyrin, and its derivatives form one of the most studied classes of functional organic materials. The author briefly summarizes recent research showing that Pc-containing dendrimers possess the ability to form clear, robust glassy solids that appear to be indefinitely stable towards crystallization and that contain a high concentration of Pc functionality.
Metallodendrimers built around a [Ru(dpp)3]2+-type core (dpp = 4,7-diphenyl-1,10-phenanthroline) were prepared containing peripheral phenyl moieties. The convergent synthesis of the ligands was accomplished by coupling dendritic branches with a focal amino function to the chelating phenanthroline precursor under the formation of sulfonamide linkages. Complexation of ruthenium ions afforded the corresponding metallodendrimers with up to 24 peripheral phenyl units in the case of the largest dendritic structure. The absorption spectra and luminescence properties of the four new dendrimers are reported. The dendritic effect is clearly visible, going from zero to second generation, as demonstrated by an elongation in the excited-state lifetime in aerated acetonitrile and improved emission quantum yields relative to the reference complex containing a [Ru(dpp)3]2+ core. Interestingly, the use of rigid and conjugated ruthenium-based cores results, for all dendritic structures, in luminescence lifetimes that are several microseconds long in deaerated solutions, even at room temperature.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
A new series of dendrimers was assembled through formation of homo- and heteroleptic RuII complexes with [2,2′: 6′,2″]terpyridine ligands bearing hydrophilic and hydrophobic dendrons, with the aim to develop amphiphilic vectors for potential use in gene delivery (Scheme 1). The synthesis started with the preparation of the 4′-(3,5-dihalo-4-methoxyphenyl)-[2,2′: 6′,2″]terpyridine ligands 1a,bvia the Kröhnke pyridine synthesis (Scheme 2), followed by attachment of dendrons 10a–10f (Fig. 2) by Sonogashira cross-coupling to give the dendritic ligands 11–16 (Schemes 3 and 4). Ligands were subsequently introduced into the coordination sphere of RuIII to give the stable intermediates [Ru(11)Cl3] (24; Scheme 7) and [Ru(14)Cl3] (27; Scheme 8). These were transformed under reductive conditions into the heteroleptic complexes [Ru(11)(13)](PF6)2 (25) and [Ru(13)(14)](PF6)2 (29). Removal of the (tert-butoxy)carbonyl (Boc) protecting groups in 25 and 29 then gave the desired amphiphilic dendrimers 26 (Scheme 7) and 30 (Scheme 8) with branchings of generations 0 and 1. Complex formation was analyzed by high-resolution matrix-assisted laser-desorption-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry (HR-MALDI-FT-ICR-MS), which provided spectra featuring unique fragment-ion series and perfectly resolved isotope distribution patterns (Figs. 4 and 5). The preparation of homo- and heteroleptic complexes with terpyridine ligands bearing generation-2 dendrons failed due to steric hindrance by the bulky wedges.
A convenient combinatorial-style route for the incorporation of multiple, differing functional groups, in a controllable ratio, onto a dendritic poly(propylene imine) scaffolding is described. Attachment of the functionality is accomplished by the connective formation of bis(2,2':6',2-terpyridine)Ru(II) complexes via reaction of a terpyridine-modified dendritic surface with 1-->3 branched monomers each possessing a focal terpyridine moiety. This synthetic approach produces a heterogeneous surface coating that is compared and contrasted to that of analogous homogeneous surfaced constructs. UV-vis absorption and TGA data for the metallodendrimers are also reported.
This paper reports the synthesis of a polymerizable dppe-derivative with eight pendent styrenyl units linked to the dppe core through four tyrosine residues. This diphosphine ligand was utilized in the synthesis of a series of P2PdX2 complexes (X2=(R)-BINOL, (S)-BINOL, Cl2 and π-1,3-Ph2-allyl+). The compounds were used as comonomers for the synthesis of porous organic polymers (poly EDMA). Molecular imprinting effects on X2-ligand removal were investigated. Overall, the presence of a cavity of significant size was found to be beneficial to the rate of the allylic alkylation reaction, however, chiral BINOL shaped cavities did not influence the enantio-selectivity of the reaction.
Intermolecular interactions play a crucial role in the performance of organic light-emitting diodes (OLEDs). Here we report the photophysical and electroluminescence properties of a fac-tris(2-phenylpyridyl)iridium(III) cored dendrimer in which highly branched biphenyl dendrons are used to control the intermolecular interactions. The presence of fluorene surface groups improves the solubility and enhances the efficiency of photoluminescence, especially in the solid state. The emission peak of the dendrimer is around 530 nm with a PL quantum yield of 76% in solution and 25% in a film. The photophysical properties of this dendrimer are compared with a similar dendrimer with the same structure but without the fluorene surface groups. Dendrimer LEDs (DLEDs) are prepared using each dendrimer as a phosphorescent emitter blended in a 4,4′-bis(N-carbazolyl)biphenyl host. Device performance is improved significantly by the incorporation of an electron-transporting layer of 1,3,5-tris(2- N-phenylbenzimidazolyl)benzene. A peak external quantum efficiency of 10% (38 CdA–1) for the dendrimer without surface groups and 13% (49.8 CdA–1) for the dendrimer with fluorene surface groups is achieved in the bilayer LEDs.
The synthesis and characterization of two new phosphorescent cationic iridium(III) cyclometalated diimine complexes with formula [Ir(L)2(N-N)]+(PF6–) (HL = (9,9-diethyl-7-pyridinylfluoren-2-yl)diphenylamine); N-N = 4,4′-dimethyl-2,2′-bipyridine (1), 4,7-dimethyl-1,10-phenanthroline (2)) are reported. Both complexes are coordinated by cyclometalated ligands consisting of hole-transporting diphenylamino (DPA)- and fluorene-based 2-phenylpyridine moieties. Structural information on these heteroleptic complexes has been obtained by using an X-ray diffraction study of complex 2. Complexes 1 and 2 are morphologically and thermally stable ionic solids and are good yellow phosphors at room temperature with relatively short lifetimes in both solution and solid phases. These robust iridium complexes can be thermally vacuum-sublimed and used as phosphorescent dyes for the fabrication of high-efficiency organic light-emitting diodes (OLEDs). These devices doped with 5 wt % 1 can produce efficient electrophosphorescence with a maximum brightness of up to 15 610 cd m–2 and a peak external quantum efficiency of ca. 7 % photons per electron that corresponds to a luminance efficiency of ca. 20 cd A–1 and a power efficiency of ca. 19 lm W–1. These results show that charged iridium(III) materials are useful alternative electrophosphors for use in evaporated devices in order to realize highly efficient doped OLEDs.
Metallodendritic catalysts combine the advantages of homogeneous and heterogeneous catalysts: they are soluble and perfectly well defined on the molecular level, and yet they can be recovered by precipitation, ultra-filtration or ultra-centrifugation (as biomolecules) and recycled several times. In this article, we summarize our recent work in this field with examples operating under ambient conditions in metathesis, Pd-catalyzed Sonogashira coupling, redox catalysis of nitrate and nitrite cathodic reduction to ammonia and various oxidation reactions by H2O2 catalyzed by polyoxometallates. The dendritic effects on the catalytic efficiencies are scrutinized, i.e., the comparison of the metallodentritic catalysts with their monomeric models and among the dendrimer generations. It is concluded that metallostars or low-generation metallodendrimers usually are optimized catalysts in terms of efficiency and recovery/re-use.
Several synthetic strategies have been explored to prepare dendrimers having the [Ru(bpy)3]2+ complex as their core (bpy = 2,2′-bipyridine). Dendritic ligands have been synthesized by attaching branches in the 4,4′-positions of bpy. The largest dendritic bipyridine ligand contains 54 peripherical methylester units. Four RuII dendritic complexes have been prepared. Their absorption and emission spectra are very similar to those of the unsubstituted parent RuII–bipyridine complexes. The large dendritic complexes, however, exhibit a more intense emission and a longer excited-state lifetime than [Ru(bpy)3]2+ in aerated solutions. This is due to the shielding effect of the dendrimer branches on the Ru–bipyridine core, which limits the quenching effect of molecular oxygen. For the largest dendritic complex, which contains 54 peripherical methylester units, the excited-state lifetime in aerated acetonitrile solution is longer than 1 μs, and the rate constant for dioxygen quenching is twelve times smaller than for [Ru(bpy)3]2+.
Eu3+-, Tb3+- and Er3+-cored dendrimer complexes were prepared by self-assembly of three fluorinated dendrons, each with a carboxylate anion focal point, around the lanthanide ion. Energy transfer from the peripheral fluorinated phenyl moieties of the dendrons to the lanthanide cation was evidenced spectroscopically for Eu3+- and Tb3+-cored dendrimer complexes in solution. The excitation of perfluorinated aromatic groups was found to decay with ca. 0.7 ns and a longer decay time 10–13 ns was related to the coordination at the Ln3+ focal point. Luminescence from the lanthanide core decays with lifetime in the range 1–1.5 ms over a wide concentration range (μM–mM), similar to the luminescence decay time of the corresponding acetate ion complexes in D2O. The main quenching mechanism of the lanthanide emission appears to be due to vibrations among surrounding C–H bonds of the intermediate shell of the flexible dendrimer scaffold. Antenna effect and energy harvesting from the surface of the dendrimer and transfer to the core was the main mechanism for luminescecnce in the dendrimer complexes with lanthanide cations.
A study was conducted to demonstrate the derivation of dendrimers from 1→3 branching motifs. A three-atom distance was needed between the branch point and the reactive chemical center to circumvent retardation of these chemical transformations. Triol was treated with the chloroacetic acid, esterified (MeOH,H+ to produce polyester reduced (LAH) and transformed (TsCl) to the corresponding tris(tosylate). The simplest 1→3 branched monomer TRIS had been used for simple amidation where it readily transformed esters to the corresponding amide by a facile two-step procedure, transesterification followed by rearrangement. This ester triol transformation was also demonstrated in the conversion of numerous initially nondendritic hydrophobic materials into more hydrophilic compounds, such as neutral, cyclophanedodecaalcohol.
Here, the charge transporting properties of a family of highly phosphorescent iridium(III) complex-cored carbazole dendrimers designed to have improved charge transport by incorporating carbazole units into the dendrons are studied. Firstly, the effect of the dendrimer generation and the role of dendron for materials with one dendron per ligand of the core are considered. It is shown, in contrast to previously reported light-emitting dendrimers, that in this case the carbazolyl-based dendrons have an active role in charge transport. Next, the effect on the charge transport of attaching two dendrons per ligand to the dendrimer core is explored. In this latter case, for the so called "double dendron" material a highly non-dispersive charge transport behavior is observed, together with a time-of-flight mobility of the order of 10-3cm2V-1s-1. Furthermore the lowest energetic disorder parameter (σ) ever reported for a solution-processed conjugated organic material is found, σ < 20 meV.
We describe the preperation of a dendrimer that is solution-processible and contains 2-ethylhexyloxy surface groups, biphenyl-based dendroms and a fac-tris[2-(2,4-difluorophenyl)pyridyl]iridium(III) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III) complex, bis[2-(2,4-difluorophenyl)pyridyl]picolinate iridium(III) (FIrpic). To avoid the change in emission color that would arisefrom attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by seperating them with an ethane linkage. Bilayer devices containing a light-emitting layer comprised of 30.wt-% blend of the dendrimer in 1,3-bis(N-carbazolyl)benzene (mCP) and a 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene electron-transport layer have external quantum and power efficiencies, respectively, of 10.4% and 11 lm W-1 at 100 cd m(-2) and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4% corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out-coupling of light from the device.
A simple convergent procedure has been developed for the preparation of solution processable phosphorescent dendrimers with biphenyl-based dendrons and fac-tris(2-phenylpyridyl)iridium(III) cores. We found that the attachment point and branching of the dendrons are important for controlling the color of the light emission. Photolumineseence excitation measurements showed that energy could be transferred efficiently from the dendrons to the core. Solution photoluminescence quantum yield (PLQY) measurements of the dendrimers were of order 70%, showing that the attachment of the dendron did not decrease the luminescence efficiency of the core iridium complex. The PLQYs of the neat dendrimer films increased with generation with the second-generation dendrimer having a neat film PLQY of 31%, 1(1)/(2) times higher than the first-generation dendrimers and almost 3 times that of the nondendritic iridium complex, demonstrating the power of the dendrimer architecture to control intermolecular interactions. Electrochemical experiments showed that charge was injected directly into the core of the dendrimers.
A simple convergent procedure has been developed for the formation of sterically encumbered phosphorescent dendrimers. The procedure is demonstrated with the preparation of a first-generation dendrimer composed of a fac-tris(2-phenylpyridyl)iridium(III) core and three dendrons. Each dendron is comprised of a branching phenyl unit with a further four phenyl groups attached. The lack of surface groups on the dendrons was found to reduce solubility and also reduced the level of control over the intermolecular interactions of the emissive and electroactive core in films. The 6-fold decrease in photoluminescence quantum yield in going from solution (69%) to the solid state (11%) showed that there were strong intermolecular interactions of the emissive cores in the solid state. Single-layer devices with the dendrimer blended with 4,4'-bis(N-carbazolyl)biphenyl showed an external quantum efficiency of 1.7% (5.4 cd/A) at 100 cd/m(2) and 11.4 V, giving a power efficiency of 1.5 lm/W.
A scientific review informed about the functions and applications of dendrimers resulting from supramolecular and physical properties. The review focused on the importance of the purities of dendrimers that potentially varied from one family to the other and significantly influencing the adequate achievement of the functions. It focused on the powerful concepts of dendrimer chemistry in terms of functions and potential applications. The findings revealed that dendrons bonded to polymers were called dendronized polymers and they disclosed properties relevant to those of dendrimers. Branched or hyperbranched polymers were found to be useful alternatives to dendrimers that had significant commercial view and showed closely related properties for functions. Local dendrimer dynamics, such as local motion were also compared to supercooled liquids and linear polymers, including glass transition aspects.
In this Concept article, we summarize and discuss recent reports on dendritic molecular electrochromic batteries. Giant dendrimers containing 3(n+2) terminal tethers (n = generation number) and terminated by first-raw late-transition-metal metallocenes, permethyl metallocenes and other sandwich complexes were shown to be redox robust. Indeed, they can be oxidized and reduced without decomposition and exist under two stable oxidation states (Fe(III/II), Co(III/II)). Thus, a pre-determined number of electrons (up to 14,000) per dendrimer can be exchanged. Cyclic voltammetry showed a remarkable complete reversibility even up to 14,000 Fe and Co termini in metallodendrimers, indicating fast electron hoping among the redox sites and between dendrimers on a carbon surface covered by arylcarboxylate groups. The dendrimer sizes were measured by dynamic light scattering in solution and by AFM (subsequent to flattening in the condensed state also indicating that these metallodendrimers aggregate to form discrete nanoparticles of dendrimers, as atoms do). The metallodendrimer size varies considerably between the two redox forms due to tether extension of the cationic dendrimers upon oxidation, and a breathing mechanism was shown by atomic and electric force microscopy (AFM and EFM). When the redox potential is very negative, the reduced form is an electron-reservoir system that can deliver a large number of electrons per dendrimer to various reducible substrates. These systems are thus potential dendritic molecular batteries with two different colors for the two redox forms (electrochromic behavior).
Better without doping: Use of a dendron with a high carbazole density around an iridium core has been found to improve the performance of nondoped electrophosphorescent devices more effectively than the use of higher-generation dendrimers. A promising efficiency as high as 45.7 cd A-1 (13.4 %) together with a high luminance is obtained for 6 B-G1 (see picture). These values are very close to those of the corresponding doped device.
Two orthogonal non-covalent binding sites, namely metal-ligand complexation of Ru and bipyridine and intermolecular hydrogen bonding, facilitate the self-assembly of a new type of supramolecular dendrimers.
Dendritic catalysts and dendrimers in catalysis were discussed. Dendrimers can combine the advantages of homogeneous and heterogeneous catalysis. Results showed that it is possible to remove the catalyst from the reaction medium after a reaction carried out in the presence of a metallodendritic catalyst.
Photodynamic therapy (PDT) is a promising therapeutic modality for treatment of solid tumors. In this study, third-generation aryl ether dendrimer porphyrins (DPs) with either 32 quaternary ammonium groups (32(+)DPZn) or 32 carboxylic groups (32(-)DPZn) were evaluated as a novel, supramolecular class of photosensitizers for PDT. DPs showed a different cell-association profile depending on the positive or negative charge on the periphery, and both DPs eventually localized in membrane-limited organelles. In contrast, protoporphyrin IX (PIX), which is a hydrophobic and relatively low molecular weight photosensitizer used as a control in this study, diffused through the cytoplasm except the nucleus. Confocal fluorescent imaging using organelle-specific dyes indicated that PIX induced severe photodamage to disrupt membranes and intracellular organelles, including the plasma membrane, mitochondrion, and lysosome. On the other hand, cells treated with DPs kept the characteristic fluorescent pattern of such organelles even after photoirradiation. However, notably 32(+)DPZn achieved remarkably higher (1)O(2)-induced cytotoxicity against LLC cells than PIX. Furthermore, both dendrimer porphyrins had far lower dark toxicity as compared with PIX, demonstrating their highly selective photosensitizing effect in combination with a reduced systemic toxicity.
Routes for the syntheses of isomeric, zwitterionic, bisterpyridine-Ru(II)-based macromolecules are described. Access to these novel architectures is facilitated by the construction of terpyridine-modified, 1-->3 C-branched, ester-terminated building blocks. Constitutional isomers result from the interchangable placement of methyl and tert-butyl ester groups on both the branched framework near the Ru(II) centers and the termini of the branched construct. Water solubility is imparted to each isomer through selective transformation of the tert-butyl esters to their corresponding carboxylates. Along with the standard characterization techniques, electrochemical and spectroscopic data also support the structural formation.
The synthesis, photophysical and nonlinear optical properties of several new multi-octupolar tris(bipyridine) ruthenium complexes are reported. The preparation on these complexes is based on the initial construction of multipodal 4,4'-dialkylaminostyryl-2,2'-bipyridine ligands (DAAS-bpy). Thermally stable polyimides featuring octupolar ruthenium trisbipyridyl complexes have been readily obtained by a polycondensation reaction. The controlled coordination strategy of dipodal and tripodal bipyridines to ruthenium(II) has also been successfully used to build bimetallic, trimetallic as well as the first metallodendrimer made of seven metallo-octupoles. These polymetallic species exhibit very intense absorption bands in the visible and long-lived luminescence. The quadratic NLO-susceptibilities beta of these macromolecules have been characterized by harmonic light scattering at 1.91 microm and compared with those of the corresponding monometallic species. The NLO studies clearly demonstrates a quasi-supramolecular ordering in the metallodendrimer.
Several types of six-armed, metal tris(bipyridine) core dendrimers were synthesized. Bis-4,4'-alkoxy bipyridine dendrons were prepared and employed to make tris(bipyridine) dendrimers. Although the ruthenium-centered and iron-centered dendrimers displayed quasi-reversible cyclic voltammetry, the analogous cobalt-centered complex did not. The synthesis of 4,4'-disubstituted bipyridines containing -CH(2)OR groups proceeded in low yield. The reactions of the dicarbanion of 4,4'-dimethyl bipyridine prepared with LDA and mesylate, triflate, and bromide groups were found to result in no or poor yields of carbon-carbon bond formation. Use of KDA in place of LDA resulted in much higher yields of dendritic bipyridines.
2,2':6',2''-Terpyridine (terpy) ligands and their iron(II) and cobalt(II) complexes bearing first and second generation Frechet-type dendritic wedges have been prepared and structurally characterised. The rotational freedom within the dendritic wedge results in a great diversity in conformational space. Structural studies are reported on first generation species and their metal complexes, and the variety of inter- and intramolecular interactions which dominate the conformation of the dendrimer and the packing of the molecules (ions) in the lattice are discussed.