Dendrimer Films Research Articles

Photocontrol of the Supramolecular Chirality Imposed by Stereocenters in Liquid Crystalline Azodendrimers

A series of liquid crystalline dendrimers have been synthesized by peripheral functionalization of PPI dendrimers (generations having 16 or 32 functional groups) with photochromic azobenzene units that contain stereocenters at the terminal alkyl chains. Complete functionalization was checked by NMR and MALDI-TOF mass spectrometry. The mesomorphic properties were analyzed by optical microscopy, DSC, X-ray diffraction and AFM. The dendrimers exhibit a smectic phase that can be frozen at room temperature depending on the thermal history of the sample. Dendrimers did not exhibit a CD response in solution. However, chiroptical properties were detected in films and these were assigned to a supramolecular chirality arising from aggregation of azobenzene units and imposed by the stereocenters. The chiroptical properties depend to a small extent on the thermal history of the sample and can be modulated with light. For example, irradiation with UV light erases the CD signal of the films due to trans−cis isomerization, but this eventually reverts to the initial CD signal by cis−trans back isomerization and aggregation. Irradiation with 488 nm circular polarized light (CPL) modulates the CD signal depending on the handedness of the light and generation of dendrimeric core. Irradiation with l-CPL reinforces the supramolecular chirality imposed by stereocenters in both dendrimers. However, irradiation with r-CPL switches the CD signal in films of the generation dendrimer having 16 functional groups, G2-AZO-C4(S), but this does not occur in films of the higher generation dendrimer, G3-AZO-C4(S).

Effects of solution processing and thermal annealing on the phosphorescence of iridium(III) complex-cored dendrimer films

We present evidence that the photoluminescence (PL) spectrum, PL kinetics and PL efficiency of solution-cast films of a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)-cored dendrimer are not affected by the choice of solvent and thermal annealing. In contrast, when the dendrimer is dispersed into a 4,4′-bis( N-carbazolyl)biphenyl (CBP) host, thermal annealing induces phase separation at a micrometer scale, which results in incomplete energy transfer from CBP to the dendrimer. The insensitivity of the dendrimer film morphology and PL properties to the casting conditions and thermal treatment shows the advantage of host-free electrophosphorescent dendrimers for the further development of light-emitting displays, whilst blended systems are potentially more suitable for photovoltaic applications.

Solid-State Dendrimer Sensors: Probing the Diffusion of an Explosive Analogue Using Neutron Reflectometry

Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 +/- 30 A) and thick (750 +/- 50 A) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by approximately 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.

Electron-transfer mediation on poly-aryl dendrimer-modified electrodes

Electrochemical properties of a dendrimer-modified electrode that was prepared by immobilization of ferrocenyl-terminated dendrimers on a poly-phenyl acetate anchoring layer were investigated in CH 2Cl 2. The anchoring layer was made by electro-grafting of the corresponding diazonium salt on a glassy carbon surface. The method allows the fabrication of a robust interface where the properties of the dendrimers are well-preserved. Moreover, the control of the layer properties as the permeation of molecules from the solution to the surface could be tuned up from only limited to totally blocked through the electrochemical conditions used during the electro-grafting of the anchoring layer. Detailed investigations performed with cyclic voltammetry and on different types of layers show that the modified electrode catalyses the oxidation of redox substrates. The process depends on the standard potential of the redox couple compared to that of the adsorbed dendrimer molecules. Experiments indicate that the electron exchange with molecules in solution takes place mainly at the dendrimer film–solution interface as the dendrimers inside the film permit the charge-transfer through the modified film to the carbon substrate. The interest of using robust electrode dendrimer relies on the possibility of large structural variations allowing the careful introduction of specific properties in the layer.

Exciton migration and quenching in poly(propylene imine) dendrimers

Exciton migration between chromophore groups of the poly(propylene imine) dendrimer in chloroform solution and in solid state has been investigated by means of the time-resolved fluorescence measurements. Fluorescence decay kinetics, dynamic band shift and the depolarization rate have been analyzed. Exciton migration in a single dendrimer was found to be slow in comparison with temperature-dependent chromophore reorientation time of 150–600 ps. In a solid state chromophore groups form collective excitonic states responsible for the dendrimer film fluorescence. Exciton migration and localization to the lowest energy sites within the distributed density of states take place on a subnanosecond–nanosecond time scale.

Highly branched green phosphorescent tris-cyclometalated iridium(III) complexes for solution-processed organic light-emitting diodes

A series benzoimidazole-based dendritric complexes of iridium dendrimers containing Fréchet-type dendrons with peripheral fluorenyl surface groups have been synthesized. These iridium dendrimers are green-emitting with high phosphorescence quantum yield, and can be spin-coated as films of good quality. From cyclic voltammograms (CV), high onset potentials at 1.42–1.58 V due to the peripheral fluorene group were observed. Device from a second generation dendrimer 17 with structure of ITO/PEDOT:PSS/CBP: 20 wt% 17/TPBI/LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene): polystyrenesulfonate and CBP = bis( N-carbazolyl)biphenyl) has the best performance: maximum external quantum efficiency of 13.58% and maximum current efficiency of 45.7 cd/A. Space-charge-limited current (SCLC) flow technique was used to measure the mobility of charge carriers in the blend films of the compounds in CBP. Blend films of higher generation dendrimers have lower hole mobility, albeit with higher device efficiencies.

Photocarrier Transport Property of Phenylene-vinylene Dendrimer Studied by Time-of-Flight Method

The photocarrier transport property of the three-generation dendrimer, which is composed of phenylene-vinylene as the dendron group and triphenylamine as the surface group G3D3-PV(TPA), was studied by the time of flight (TOF) method using a sandwich-type cell of indium-tin-oxide (ITO)/G3D3-PV(TPA)/Mg-Al. The photocurrent decay shows that hole transport occurs stronger than electron transport, and dispersive transport is observed in the dendrimer film. The mobility at 25 °C exhibits negative electric field dependence, and the intrinsic mobility at zero electric field is 7.1 × 10−5 cm2/(V.s). This is faster than that of linear poly(p-phenylene-vinylene). Besides this, the result is one order smaller than that of poly(3-hexylthiophene), but faster than those of poly(3-dodecylthiophene) and poly(3-octadecylthiophene). The result indicates that one of the important parameters for the carrier transport is the average distance between π-conjugated molecules. The fact that the carrier hopping of the G3D3-PV(TPA) dendrimer occurs effectively implies the structure would be a disk shape since the phenylene-vinylene dendron sites of the different dendrimer molecule are estimated to be close to each other.

Localized surface plasmon resonance coupling in Au nanoparticles/phosphorus dendrimer multilayer thin films fabricated by layer-by-layer self-assembly method

Multilayer thin films of anionic gold nanoparticles (AuNPs) and cationic phosphorus dendrimers were deposited on 3-(diethoxymethyl-silyl)propylamine (3-APDMES)-coated substrates using layer-by-layer (LbL) assembly driven by electrostatic interactions. The growth of Au/dendrimer multilayers composed of AuNPs with diameters of ∼3 nm and ∼16 nm and dendrimer with ∼2 nm diameter was monitored by UV-vis spectroscopy. The relative amounts of AuNPs and dendrimers in the multilayer films were calculated using a quartz crystal microbalance. The Au-containing multilayers have two surface plasmon bands at ∼530 nm and ∼600 nm, where the latter exhibits a red shift upon increasing the areal density of AuNPs as well as increasing the layer number. The localized surface plasmon resonance (LSPR) band of the hybrid films can be tuned by adding NaCl to the dendrimer solution or by removing the organic matrix. These results demonstrate that the near-field coupling between the LSPR bands of neighboring Au layers is responsible for the controlled absorption behavior. Au mesoporous films after removing dendrimers show LSPR sensing properties for alcohols with different refractive indices in the range 1.33–1.41. A linear relationship was obtained between the LSPR peak wavelength and the refractive index of the surrounding medium.

Dual functions of a new n-type conjugated dendrimer: light-emitting material and additive for polymer electroluminescent devices

We demonstrate a novel light-emitting diode (LED) of a graded bilayer structure that comprises poly(N-vinylcarbazole) (PVK) with good hole transport ability as the energy donor and a new distyrylanthracene-triazine-based dendrimer with enhanced electron transport ability as the light-emitting molecule. The device contains a graded bilayer structure of the PVK film covered with the dendrimer film prepared by sequential spin-casting of the dendrimer layer from a solvent that only swells the PVK layer. The bilayer device demonstrated a significantly enhanced electoluminescence quantum efficiency compared with the dendrimer single layer device or the PVK : dendrimer blend device with optimized composition. We also prepared composite LEDs with an MEH-PPV : emissive dendrimer blend. By doping the electron-deficient MEH-PPV layer with a small amount of the distyrylanthracene-triazine-based dendrimer, we could not only enhance the device performance but also depress the long-wavelength emission of MEH-PPV.

Thickness Dependence of the Fluorescence Lifetime in Films of Bisfluorene-Cored Dendrimers

The effect of film thickness on fluorescence lifetime, quantum yield, and exciton diffusivity is studied in first-generation bisfluorene-cored dendrimers with E-stilbenyl and biphenyl-based dendrons. A decrease of the fluorescence lifetime and quantum yield is observed in films thinner than 50 nm spin-coated on fused silica substrates. The radiative decay of the singlet excited-state and singlet exciton diffusion rates are independent of the film thickness within a 20% uncertainty. The fluorescence lifetime of dendrimers dispersed in an inert host is also independent of film thickness. The nonradiative decay rate increases by a factor of 4 in thin films of dendrimers with first-generation E-stilbenyl dendrons. The same effect is observed on different glass and sapphire substrates, and it is much weaker in the dendrimer with first-generation biphenyl dendrons, which allows us to rule out quenching by substrate. The data can be explained by long-range (dipole−dipole) energy transfer to quenching sites concentrated at the surface of thin films. The results contribute to understanding of the thin film photophysics and will help development of light-emitting devices.

Voltammetric studies of dendrimer multilayers: Layer-by-layer assembly of metal-peptide dendrimers multilayers

Multilayer films of metal-peptide dendrimers having a ferrocene core were constructed on gold surfaces functionalized with mercaptoundecanoic acid using a layer-by-layer approach for film construction. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to monitor the redox activity and stability of these films. The multilayer films showed good stability and exhibited direct electron transfer with the underlying electrode. The specific active surface concentration can be varied by the number of layers, the nature of the dendrimer, and the nature of metal ions used. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Synthesis and incorporation in Langmuir films of oligophenylenevinylene dendrimers bearing a polar head group and different dendritic poly(benzyl ether) branches

Oligophenylenevinylene (OPV) units containing a hydroxyl polar head group and different dendritic poly(benzyl ether) branches bearing aliphatic chains (n-propyl and n-dodecyl) have been synthesized. The obtained dendrimers were characterized by 1H and 13C NMR, FTIR and absorption spectroscopies. Optical properties of the dendrimers have been studied in toluene and dichloromethane solutions. Changes in the maximum absorption wavelength were observed for the dendrimers OPV core. Increasing the dendron branches generation gave rise to a hypsochromic shift of the maximum absorption band of the OPV core. The ability of all dendrimers to form Langmuir films at the air–water interface was also investigated; the obtained films were characterized by surface pressure versus molecular area isotherms, and by compression–expansion cycles (hysteresis curves). All dendrimers were able to form stable Langmuir monolayers in the air–water interface, and exhibited a good reversible behavior upon successive compression/expansion cycles. The terminal alkyl chains length of the dendritic branches has a remarkable influence on the packing of highest generation dendrimers in the monolayers. These dendrimers can be promising prospects for the preparation of Langmuir–Blodgett films, in view of optical and electronic potential applications.

Influence of the dendron chemical structure on the photophysical properties of bisfluorene-cored dendrimers

A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications.

Ionic Liquid-Based Preparation of Cellulose−Dendrimer Films as Solid Supports for Enzyme Immobilization

Surface-active cellulose films for covalent attachment of bioactive moieties were achieved by codissolution of cellulose with polyamidoamine (PAMAM) dendrimers in an ionic liquid followed by regeneration of the composite as a film. Different generations of PAMAM were used for the formation of cellulose-dendrimer composites, as well as films with the dendrimer covalently bonded to the cellulose by means of the linker 1,3-phenylene diisocyanate. Surface characterization, thermal stability, and utility for immobilization of laccase were determined. The presence of the dendrimer amino groups was confirmed by detailed characterization of the films' surfaces. These modified films exhibit acceptable thermal stability, comparable to that of other regenerated cellulose films, but the number of active functional groups on the surface is much smaller than the theoretical amount expected. Films made with 1,3-phenylene diisocyanate as linker for covalently bound cellulose and dendrimers exhibit a better performance for immobilization of laccase than those prepared by simple mixing of the cellulose and dendrimer. In general, a linear correspondence between the dendrimer generation within the films and the specific activity of immobilized laccase in such films was not observed.

Transport properties of field-effect transistor with Langmuir-Blodgett films of C60 dendrimer and estimation of impurity levels

Field-effect transistor (FET) device has been fabricated with Langmuir-Blodgett films of C60 dendrimer. The device showed n-channel normally off characteristics with the field-effect mobility of 2.7×10−3cm2V−1s−1 at 300K, whose value is twice as high as that (1.4×10−3cm2V−1s−1) for the FET with spin-coated films of C60 dendrimer. This originates from the formation of ordered π-conduction network of C60 moieties. From the temperature dependence of field-effect mobility, a structural phase transition has been observed at around 300K. Furthermore, the density of states for impurity levels was estimated in the Langmuir-Blodgett films.

Hydrogen-bonding induced alternating thin films of dendrimer and block copolymer micelle

The hydrogen-bonding induced alternating multilayer thin films of dendrimers and block copolymer micelles were demonstrated. The block copolymer micelles derived from amphiphilic poly(2-ethyl-2-oxazoline)-block-poly(e-carprolactone) (PEtOz-PCL) in aqueous phase have a core-shell structure with a mean hydrodynamic diameter of 26 nm. The hydrogen bonding between the PEtOz outer shell of micelle and the carboxyl unit of poly(amidoamine) dendrimer of generation 4.5 (PAMAM-4.5G) at pH 3 was utilized as a driving force for the layerby-layer alternating deposition. The multilayer thin film was fabricated on the poly(methyl methacrylate) (PMMA) thin film spin-coated on silicon wafer or glass substrate by the alternate dipping of PEtOz-PCL micelles and PAMAM dendrimers in aqueous solution at pH 3. The formation of multilayer thin film was characterized by using ellipsometry, UV-vis spectroscopy, and atomic force microscopy. The PEtOz outer shell of PEtOz-PCL micelle provided the pH-responsive hydrogen bonding sites with peripheral carboxylic acids of PAMAM dendrimer. The multilayer thin film was reversibly removed after dipping in aqueous solution at pH ≥ 5.6 due to dissociation of the hydrogen bonding between PEtOz shell of PEtOz-PCL micelle and peripheral carboxyl units of PAMAM dendrimer.

Electrochemical preparation of copper–dendrimer nanocomposites: picomolar detection of Cu2+ ions

The present work describes, for the first time, in situ electrochemical preparation of dendrimer-encapsulated Cu nanoparticles using a self-assembled monolayer of fourth-generation amine-terminated polyamidoamine (PAMAM) dendrimer as the template. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) studies of the modified surface confirmed the presence of Cu nanoparticles entrapped in dendrimer film. Au electrode modified with a monolayer of the dendrimer enables preconcentration and subsequent voltammetric detection of Cu(2+) at picomolar concentrations. Further, Cu nanoparticles in the dendrimer monolayer could be electrochemically derivatised to Cu hexacyanoferrate, which exhibits specific crystal planes, unlike the random distribution of crystal planes in bulk-formed Cu hexacyanoferrate, which is another catalytically active material for sensor applications.

Dendrimer films as matrices for electrochemical fabrication of novel gold/palladium bimetallic nanostructures

Electrodeposition of novel Au/Pd bimetallic nanostructures with dendrimer films as matrices has been reported. The dendrimers exhibited highly open structures arising from protonation of amines and this made them have good penetrability for solvent molecules. The unique properties of dendrimers obviously affected the morphologies and compositions of deposited bimetallic nanostructures compared with those from unmodified surfaces. Field-emitted scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy were used to characterize these nanostructures. The results showed that these bimetallic nanostructures were consisted of many small nanoparticles. Effects of ratios of KAuCl 4 and K 2PdCl 6, electrodeposition time and layer number of assembled dendrimers were thoroughly investigated. At last, electrochemical properties of the bimetallic nanostructures were also studied.

Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers

A study of the amplified spontaneous emission (ASE) properties of three bisfluorene-cored dendrimers in the solid state is reported. The results show that the dendron type has a strong impact on the photoluminescence quantum yield and affects the ASE threshold, the optical gain, and loss coefficients. Optically pumped distributed feedback lasers operating in the blue spectral region were fabricated by spin coating the dendrimer films on top of a two-dimensional corrugated fused silica substrate. A best lasing threshold of 4.5μJ∕cm2 and a slope efficiency of 8.3% were obtained, which demonstrate the high potential of these materials for laser applications.

Bioactive multilayer thin films of charged N, N-disubstituted hydrazine phosphorus dendrimers fabricated by layer-by-layer self-assembly

Charged N, N-disubstituted hydrazine phosphorus-containing dendrimers are deposited either as alternate all-dendrimers multilayers or alternating with linear polymers on 3-mercaptopropionic acid or 3-aminopropyldimethylethoxysilane coated surfaces via electrostatic layer-by-layer self-assembly. The behavior of the film formation is investigated by surface plasmon resonance spectroscopy and ellipsometry. Fetal cortical rat neurons were cultured on the dendrimer films in order to investigate the influence of the surface charge of the outermost layer on their adhesion and maturation. It was found that neurons attached preferentially and matured slightly faster on film surfaces terminated with positively charged dendrimers than on negatively charged surfaces.