Metal-assembling dendrimers with a triarylamine core and their application to a dye-sensitized solar cell

A series of charge-separable and hole-transporting phenylazomethine dendrimers with a triarylamine core are prepared and evaluated for use as a charge separator in dye-sensitized solar cells (DSSCs). Triphenylamine with dendric phenylazomethine (TPA-DPA) is prepared by synthesizing up to five generations of dendrons using a convergent method. The resultant dendrimer has a rigid sphere structure similar to globular protein, with a hydrodynamic radius of 2.43 nm. Electrochemical oxidation of the TPA core reveals that the dendron units in the dendrimer have 0.35 of the attenuation factor (beta) in the electron transfer. Complexation of TPA-DPA with SnCl2 proceeds in stepwise fashion from the core to the terminal imine following the basicity gradient among imine groups in each dendron shell. DSSCs prepared by casting these dendrimers onto dye-sensitized TiO2 film exhibited a higher open-circuit voltage than the bare film through the suppression of back electron transfer. The generational growth of dendrons increases the radius of the dendrimer, resulting in a stronger association with I3- and higher open-circuit voltage with an increasing number of generations. Complexation with SnCl2 reduces the resistance of TPA-DPA and improves the fill factor. The energy conversion efficiency of the DSSC prepared using fifth-generation TPA-DPA is 21% higher than that for the bare film and, when complexed with SnCl2, provides a 34% improvement.     

Photoswitchable flexible and shape-persistent dendrimers: comparison of the interplay between a photochromic azobenzene core and dendrimer structure

Two analogous classes of dendrimers with a single azobenzene moiety at the core have been prepared. Flexible benzyl aryl ether dendrimers 1a-e were obtained in good yields by direct alkylation of diphenolic azobenzene 3 with benzyl aryl ether dendrons [G-n]-Br (n = 0-4). In rigid dendrimers 2a-e, the azobenzene configurational switch was linked to phenylacetylene dendrons through acetylenic linkages to maintain the shape-persistent nature of these dendrimers. A comparison of these two different classes of dendrimers with azobenzene cores reveals a difference in the properties of the photochromic moiety upon dendritic incorporation as well as a significant difference in the photomodulation of dendrimer properties. The E --> Z photoisomerization quantum yield decreased markedly with increasing generation for dendrimers 1a-e but only slightly for dendrimers 2a-e. However, increasing generation did not significantly alter thermal isomerization kinetics or activation barriers. The hydrodynamic volumes of azobenzene-containing dendrimers 2b-e were significantly modulated when the azobenzene unit is subjected to irradiation, while those of dendrimers 1b-e were only slightly affected.     

Molecular force field study concerning the host properties of carbosilane dendrimers

The effect of structural variations at the core of carbosilane dendrimers has been studied using Molecular Dynamics. Several derivatives of the parent dendrimer have been modeled and characterized with respect to the dimensions of the inner cavities which lie on the order of 5–15 Å. The “denseness” of the outer shell has been studied by calculating solvent accessible surfaces and excluded volumes as a function of the radius of the probe sphere. The higher generation dendrimers were found to possess dense outer shells with holes of the order of 2–3 Å. Dendron separation especially with respect to clefts in the molecule is discussed. It was found that in low generation carbosilane dendrimers the dendrons are clearly separated while at higher generations a substantial amount of interpenetration occurs. In addition, results are presented showing that carbosilane dendrimer surfaces are fractal only over a narrow range of length scales. In this range, the fractal dimensions of the surfaces are of the order of 2.1.     

Designing poly(amido amine) dendrimers containing core diversities by click chemistry of the propargyl focal point poly(amido amine) dendrons

General, fast, efficient, and inexpensive methods for the synthesis of poly (amido amine) (PAMAM) dendrimers having core diversities were elaborated. In all syntheses, the major step involved an inexpensive 1,3‐dipolar cycloaddition reaction between an alkyne and an azide in the presence of Cu(I) species, which is known as the best example of click chemistry. The propargyl‐functionalized PAMAM dendrons are obtained by the divergent approach using propargylamine as an alkyne‐focal point. Three core building blocks, 1,3,5‐tris(azidomethyl)benzene, N,N,N′,N′‐tetra(azidopropylamidoethyl)‐1,2‐diaminoethane, and 4,4′‐(3,5‐bis(azidopropyloxy)benzyloxy)bisphenyl, were designed and synthesized to serve as the azide functionalities for dendrimer growth via click reactions with the alkyne‐dendrons. These three building blocks were employed together with the propargyl‐functionalized PAMAM dendrons in a convergent strategy to synthesize three kinds of PAMAM dendrimers with different core units. This novel and pivotal strategy using an efficient click methodology provides the fast and efficient synthesis of the PAMAM dendrimers with the tailed made core units. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1083–1097, 2008     

Convergent synthesis of platinum-acetylide dendrimers

An efficient convergent route to the main chain type of organometallic dendrimers, in which platinum moieties are linked by 1,3,5-triethynylbenzene, has been developed. The synthesis of platinum-acetylide dendrons involved the use of two types of trialkylsilyl groups for protection of the terminal acetylene. The platinum-acetylide dendrimers were prepared up to the third generation by reacting dendrons with a triplatinum core and a tetraplatinum core. Spectroscopic characterization and trace experiments by gel permeation chromatography indicated that the dendrimer molecules have no structural defects. Although a pi-conjugated system was used as the bridging ligand, electronic and fluorescence spectra suggested that the interaction among the platinum-acetylide moieties in the dendrimers was small.     

Dendrimers as Nd3+ ligands: Effect of Generation on the Efficiency of the Sensitized Lanthanide Emission

We have designed two novel dendrimers with cyclam cores with appended poly(amido amine) (PAMAM) dendrons, decorated at the periphery with four and eight dansyl chromophores, respectively. The photophysical properties of the dendrimers and their Nd³⁺ complexes have been investigated. The energy‐transfer efficiency to the lanthanide ions from these dendrimers has been studied as a function of the generation. It has been observed that an increase in the dendrimer generation as well as the number of amide units enhances the energy transfer to the lanthanide ion.     

Silole‐core phenylacetylene dendrimers and their application in detecting picric acid

Silole‐core phenylacetylene dendrimers were designed and synthesized, among them, the model compound (n = 0) and the first generation of the dendrimer (n = 1) were obtained by the reaction of 2,5‐dibromosilole with corresponding terminal alkynes, the second generation of the dendrimers (n = 2) was synthesized from 2,5‐diiodosilole. These compounds indicated the absorptions of both phenylacetylene dendrons (250–350 nm) and silole core (400–500 nm). The first generation displayed efficient energy transfer from phenylacetylene dendrons to silole core, whose energy transfer efficiency was as high as 80%. These compounds were used as chemical sensors to probe explosive, for picric acid (PA), the Stern–Volmer constants of model compound and the first generation are 7120 and 5490M^?1, respectively. J. Heterocyclic Chem., (2012).     

Luminescence properties of soluble 2,2′,7,7′‐Tetrakis(2‐(9‐hexyl‐9H‐carbazol‐3‐yl)‐vinyl)‐9,9′‐spirobifluorene‐labeled dendrimers: Photoluminescence and electroluminescence

New light emitting dendrimers were synthesized by reacting 3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzoic acid or 3,5‐bis‐[3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzyloxy]‐benzoic acid with a carbazolyl vinyl spirobifluorene moiety. A blue‐emitting core dye was encapsulated by multibenzyloxy dendrons, and two dendrimers having different densities of dendrons were prepared. Photoluminescence (PL) studies of the dendrimers demonstrated that at the higher density of benzyloxy dendrons, the featureless vibronic transitions were improved, causing lesser excimer emission. The similarity of the solution and solid emission spectra of the larger dendrimer, 10 , revealed the suppression of molecular aggregation in the solid film, which is attributed to the presence of the bulky benzyloxy dendrons. The electroluminescence spectra of multilayered devices made using 10 predominantly exhibited blue emissions; similar emission was observed in the PL spectra of its thin film. The multilayered devices made using 3 , 9 , and 10 showed luminances of 1021 cd m^?2 at 5 V, 916 cd m^?2 at 6 V, and 851 cd m^?2 at 6.5 V, respectively. The largest dendrimer, 10 , bearing a greater number of benzyloxy dendrons, exhibited a blue‐like emission with CIE 1931 chromaticity coordinates of x = 0.16 and y = 0.13, which is due to the influence of a higher shielding effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 501–514, 2008     

Single‐Crystalline Optical Microcavities from Luminescent Dendrimers

Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First‐, second‐, and third‐generation carbazole (Cz) dendrimers with a carbon‐bridged oligo(phenylenevinylene) (COPV2) core (GnCOPV2, n=1–3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2‐ and G3COPV2 align omnidirectionally. The dendrons work as light‐harvesting antennas that absorb non‐polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers.     

Synthesis of dicarboxylate oligosaccharide multilayer terminal functionality upon poly(lysine) dendrimer scaffolding

A reactive three‐layered dendrimer containing carboxyl groups was synthesized by the coupling of dicarboxylic acid and a highly reactive, two‐layered glycopeptide dendrimer. Lactose, maltose, or maltotriose was reacted with the poly(lysine) dendrimer in its third and fourth generations by reductive amination and afforded two‐layered glycolysine dendrimers. The reaction was conducted in a borate buffer (pH 9.0). ¹H NMR, ¹³C NMR, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analyses were applied for the determination of the structures of the products. When an excess amount of the oligosaccharide and a long reaction time were used, the degree of substitution increased to 1.5–2.0 against an amino group. For the preparation of highly reactive, multilayered dendrimers for an antigen carrier, C6 hydroxy groups of the oligosaccharides were selectively esterified by adipic acid and suberic acid to give 6‐O‐adipoyl oligosaccharide–poly(lysine) dendrimers and 6‐O‐suberoyl oligosaccharide–poly(lysine) dendrimers. The reactivity of these multilayered dendrimers was examined by a model reaction with phenylalanine ethyl ester. The dendrimer showed high reactivity, providing phenylalanine ethyl ester–dicarboxylate oligosaccharide–poly(lysine) dendrimers with a considerably high proportion of phenylalanine residues. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3622–3633, 2002     

Synthesis of Optically Active,X‐Shaped,Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane,Leading to Highly Emissive Circularly Polarized Luminescence

Optically active, Fréchet‐type dendrimers containing an emissive X‐shaped π‐electron system as the core unit were synthesized. Gram‐scale optical resolution and transformations of 4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were also carried out. The high‐generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X‐shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.     

Synthesis of poly(propyl ether imine) dendrimers and evaluation of their cytotoxic properties

In this paper, we report the synthesis of several poly(propyl ether imine) dendrons and dendrimers. These dendrons and dendrimers were constructed by involving an ether as the linker component and an imine as the branching component. The divergent syntheses of dendrons and dendrimers were established with the aid of two alternate Michael addition reactions and two alternate reduction reactions in a four-step iterative synthetic sequence. Dendrons up to three generations were synthesized and some of the dendrons were attached to a benzenoid core so as to obtain dendrimers up to two generations containing 12 carboxylic acids at the periphery. Divergent synthesis involving ether as the core was found to be more facile, and dendrimers up to three generations having 16 carboxylic acids at the periphery were achieved in good to excellent yields in each individual step. The adopted synthetic sequence allows us to install either alcohol, an amine, or a carboxylic acid at their peripheries. The carboxylic acid-terminated dendrons and dendrimers were evaluated as to their cytotoxic properties, and while most dendrons and dendrimers did not exhibit any measurable cytotoxicity, even up to 100 microg/mL, the second-generation dendrimer with the benzenoid core exhibited a mild toxicity at concentrations above 30 microg/mL.     

Cyclam‐Cored Dendrimers Appended with Four Dendrons of Two Different Types: Intradendrimer Energy Transfer

We have synthesized two cyclam‐cored dendrimers appended with dendrons of two different types by proper protection/deprotection of the cyclam unit. The resulting dendrimers contain six naphthyl and two dansyl units ( N6 D2 ) or two dansyl and six naphthyl units ( N2 D6 ) at the periphery. Their photophysical properties have been compared to those of a dendrimer containing 8 dansyl units ( D8 ) and a previously investigated dendrimer containing 8 naphthyl units ( N8 ). The absorption spectra are those expected on the basis of the number of chromophores, demonstrating that no ground state interaction takes place. The emission spectra of N2 D6 and N6 D2 show naphthalene localized and naphthalene excimer emission similar to those observed in the case of N8 , together with a much stronger dansyl emission with maximum at 525 nm. Addition of CF₃SO₃H to dendrimer solutions in CH₃CN/CH₂Cl₂ 1:1 (v/v) leads to protonation of the aliphatic amine units of the cyclam core at first and then of the aromatic amine of each dansyl chromophores. Cyclam can be diprotonated and this affects dansyl absorption and, most significantly, emission bands by a charge perturbation effect. Each dansyl unit is independently protonated in both dendrimers. The most interesting photophysical feature of these heterofunctionalized cyclam‐cored dendrimers is the occurrence of an intradendrimer photoinduced energy transfer from naphthyl to dansyl chromophores of two different dendrons (interdendron mechanism). The efficiency of this process is 50 % for N6 D2 and it can be increased up to 75 % upon protonation of the cyclam core and formation of N6 D2 (2H⁺). This arises from the fact that protonation of the amine units of the cyclam prevents formation of exciplexes upon naphthyl excitation, thus shutting down one of the deactivation processes of the fluorescent naphthyl excited state.     

Synthesis and characterization of 2,2‐bis(methylol)propionic acid dendrimers with different cores and terminal groups

Three sets of aliphatic polyester dendrimers based on 2,2‐bis(methylol)propionic acid (bis‐MPA) were synthesized. Two of the sets had benzylidene terminal groups and either a trimethylolpropane or triphenolic core moiety. The last set had acetonide terminal groups and a triphenolic core moiety. Benzylidene‐[G#1]‐anhydride and acetonide‐[G#1]‐anhydride were used as the reactive building blocks in the construction of all dendrimers. The large excess of building blocks used in the coupling reactions initially resulted in considerable material loss. This waste was eliminated through the development of a recycling method. ¹H and ¹³C NMR and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) analysis were used to verify the purity of all compounds. Size exclusion chromatography (SEC) was used, as well as MALDI‐TOF, for molecular weight determinations. The SEC measurements were conducted with a universal calibration method and an online right‐angle laser light scattering detector. Measured dendrimer molecular weights were close to their theoretical molar masses. Observations were also made of the hydrodynamic radius and intrinsic viscosity for the different dendrimers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1758–1767, 2004     

Size and branching effects on the fluorescence of benzylic dendrimers possessing one apigenin fluorophore at the core

Different generations of dendrimers incorporating one fluorescent core of apigenin and three Fréchet benzylic dendrons have been prepared. The chief geometric features of these dendrimers have been obtained by Molecular Dynamics simulations. These computational data suggest that the asphericities of dendrimers belonging to the third and fourth generations are considerably larger than those associated with lower radii of gyration. Fluorescence spectra of high generation dendrimers evolve along time and quantum yields show an appreciable lowering for the fourth generation dendrimer. All these data suggest aggregation phenomena and lower quantum yields for nonspheric dendrimers in solution.     

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 electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers containing a 4,4′-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both the first and second redox processes shift to less-negative values as the dendrimer generation increases.     

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     

Characterization of dendrimer properties by capillary electrophoresis and their use as pseudostationary phases

The general properties of dendrimers and in particular their electrolytic characteristics that are relevant in electrokinetic separations, are described. In order to confirm theoretical considerations on commercial dendrimer charge and hydrodynamic radius, several capillary zone electrophoresis (CZE) experiments were performed. Electrophoretic mobilities measured at different pH values indicated a sensible increase of dendrimer hydrodynamic radius at pH values lower than 2.5. This was probably due to the Coulombic repulsion of charged amine groups of the inner dendrimer shells. The principal reasons that should address the use of dendrimers as pseudostationary phases in micellar electrokinetic chromatography (MEKC) are discussed. Moreover, a survey of different separations performed utilizing dendrimers in MEKC as well as of several future plausible uses of various classes of dendrimers is presented.     

Hydrodynamic, conformational, and electrooptical properties of cylindrical dendrimers with dendron ionic addition in solutions

Cylindrical dendrimers of first, second, and third generations with side dendrons based on L-aspartic acid which are attached to poly(styrenesulfonic acid) chains via ionic bonds are studied by molecular hydrodynamic, optical, and electrooptical measurements. Macromolecules of the said dendrimers demonstrate significant kinetic rigidity in external electric and hydrodynamic fields simultaneously with moderate equilibrium chain rigidity. The comparative analysis of the experimental data on various cylindrical dendrimers with the polyvinyl backbone and L-aspartic acid-based dendrons is performed. It has been shown that, in solvents not disturbing intramolecular hydrogen bonds between side dendrons, the conformational and dynamic properties of various cylindrical dendrimers are similar. The molecular characteristics of cylindrical dendrimers in non-dissociating solvents are insignificantly affected by the mode of dendron attachment to the backbone (covalent or ionic binding).