Trapping in dendrimers and regular hyperbranched polymers

Dendrimers and regular hyperbranched polymers are two classic families of macromolecules, which can be modeled by Cayley trees and Vicsek fractals, respectively. In this paper, we study the trapping problem in Cayley trees and Vicsek fractals with different underlying geometries, focusing on a particular case with a perfect trap located at the central node. For both networks, we derive the exact analytic formulas in terms of the network size for the average trapping time (ATT)-the average of node-to-trap mean first-passage time over the whole networks. The obtained closed-form solutions show that for both Cayley trees and Vicsek fractals, the ATT display quite different scalings with various system sizes, which implies that the underlying structure plays a key role on the efficiency of trapping in polymer networks. Moreover, the dissimilar scalings of ATT may allow to differentiate readily between dendrimers and hyperbranched polymers.     

Electron transport and excitations in graphene

Graphene is a two-dimensional, single atomic layer carbon structure with many unique physical and chemical properties. In this article, I will focus on electron transport through graphene, both incoherent and coherent, the control of the current through external electric fields, and the realization of high frequency transistors based on graphene. I will also discuss briefly graphene.’s optical properties and how they can be used to create wide wavelength range, ultra-fast photodetectors. Finally, I will present results on the field-induced opening of a tunable band-gap in bi-layer graphene.     

Properties of optical excitations in polymers

We consider the influence of the nonconservation effects on the properties of elementary excitations induced by an electromagnetic field in the polymer or chain sytstem. The hamiltonian is derived which conserves the number of elementary excitations (quasi-particles). The harmonic energy spectrum of the elementary excitations arising in the system is determined.     

Femtosecond excitation energy transport in triarylamine dendrimers

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.     

Coherent exciton transport in dendrimers and continuous-time quantum walks

We model coherent exciton transport in dendrimers by continuous-time quantum walks. For dendrimers up to the second generation the coherent transport shows perfect recurrences when the initial excitation starts at the central node. For larger dendrimers, the recurrence ceases to be perfect, a fact which resembles results for discrete quantum carpets. Moreover, depending on the initial excitation site, we find that the coherent transport to certain nodes of the dendrimer has a very low probability. When the initial excitation starts from the central node, the problem can be mapped onto a line which simplifies the computational effort. Furthermore, the long time average of the quantum mechanical transition probabilities between pairs of nodes shows characteristic patterns and allows us to classify the nodes into clusters with identical limiting probabilities. For the (space) average of the quantum mechanical probability to be still or to be again at the initial site, we obtain, based on the Cauchy-Schwarz inequality, a simple lower bound which depends only on the eigenvalue spectrum of the Hamiltonian.     

Synthesis and optical and electrochemical properties of glucose-cored ferrocenyl dendrimers

Triazole-based ferrocenyl glycoconjugates 1, 2, and 3 were synthesized by regiospecific copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) of azidoferrocenyl derivatives with glucose pentaacetylide. Higher generation ferrocenyl glycoconjugates form the stable ferrocenium cation and exhibit increased light harvesting property as revealed from cyclic voltammogram studies and ultraviolet–visible spectrum respectively due to the presence of more ferrocenyl and triazolyl units than the lower generation dendrimer.     

Facile synthesis and optical properties of bithiophenesilane monodendrons and dendrimers

Three generations of bithiophenesilane monodendrons and dendrimers consisting of 3-45 2,2'-bithiene-5,5'-diyl units were synthesized by means of effective coupling reactions between the corresponding bithienyllithium derivatives and chlorosilanes. These compounds show efficient photoluminescence in the violet-blue region, the quantum yield of which is 5-15 times higher than that for the parent bithiophene or bithiophenesilanes.     

Silsesquioxane-based nanocomposite dendrimers with photo-luminescent and charge transport properties

The synthesis and characterization of octavinylsilsesquioxane (OVS)-based nanocomposite dendrimers with luminescent and charge transport properties are reported. The nanocomposite dendrimers were prepared in high yield using mild Heck chemistry of mono-haloaromatic compounds with the peripheral vinylsilane groups of OVS. Attachment of 2-naphthalene, 2-(9,9-dimethyl)fluorene, and 2-(4-phenyl)-5-(1-naphthyl)-1,3,4-oxadiazole resulted in materials with blue-violet emission (360-380 nm) and photo-luminescent quantum efficiencies (PLQEs) from 1 to 26%. Blue-green emission was observed for attachment of 1-pyrene, 9-anthracene, and N1-(4-phenyl)-N1,N4,N4-triphenylbenzene-1,4-diamine with PLQEs ranging from 23 to 50%. Despite the planar characteristics of the organic dendrons, the nanocomposite dendrimers are completely amorphous and have high glass transition temperatures (Tg) ranging from 115 to 186 degrees C with decomposition temperatures (Td) exceeding 450 degrees C. Matrix-assisted laser desorption ionization-time of flight shows that unlike traditional Heck chemistry, haloaromatic compounds are adding twice across the vinylsilane groups. Finally, organic light emitting diodes using the aromatic amine-based dendrimer as hole injection layers show 55% improvement in device efficiency over traditional materials (5.16 vs. 3.24 cd A(-1)) with brightness levels exceeding 40,000 cd m(-2).     

Synthesis and optical properties of conjugated dendrimers with unsymmetrical branching

An improved synthetic approach to conjugated monodendrons with unsymmetrical branching structures is reported. Dendrimers containing two or three such conjugated monodendrons are synthesized and their optical properties are studied. Such dendrimers exhibit broad absorptions and very high fluorescence quantum yields, making them promising candidates for applications in molecular-based photonics.     

Synthesis and optical properties of polyphenylene dendrimers based on perylenes

A series of polyphenylene-dendronized perylenes have been synthesized, and their physical and mesoscopic properties have been investigated. The attached polyphenylene dendrons have significant effects on the physical properties of the perylenes. They increase the solubility of perylenes in common organic solvents, suppress significantly the aggregation of the perylene core, and lead to red-shifted absorption and emission. The polyphenylene dendrons give rise to a strong absorption band in the UV region and exhibit efficient intramolecular energy transfer to the perylene moiety. The functionalization of perylenes with polyphenylene dendrons allows the preparation of films by spin-coating.     

Synthesis, characterization and optical response of polyene-cored stilbenoid dendrimers

The synthesis of first- and second-generation dendrimers bearing phenylenevinylene chromophores within the dendritic branches (stilbenoid dendrimers) and polyenes (3 and 5 double bonds) as cores is described. A preliminary study of the optical properties of the resulting compounds was conducted by UV/vis and fluorescence spectroscopy.     

Branched polyphenylenes and phenylene dendrimers: NMR and optical studies

Two branched polyphenylenes with 1,3,5-triphenylbenzene as branching centers were synthesized together with a family of phenylene cyclotrimers as model compounds. On the basis of the NMR analysis, specifically ¹H NMR, ¹³C NMR and 2D heteronuclear correlation experiments (HSQC and HMBC) of model compounds, the huge number of overlapping signals in the polymer spectra are attributed to aromatic protons and carbon atoms of the branched phenylene structure. The comparison with absorption spectra of linear model compounds clearly shows that the polymer optical properties depend on the length of the segments between the branching cores. This result strongly supports the proposed NMR assignment proving that the combination of the two techniques is a powerful tool for unveiling complex branched structures.     

Dynamics and thermodynamics of water in PAMAM dendrimers at subnanosecond time scales

Atomistic molecular dynamics simulations are used to study generation 5 polyamidoamine (PAMAM) dendrimers immersed in a bath of water. We interpret the results in terms of three classes of water: buried water well inside of the dendrimer surface, surface water associated with the dendrimer-water interface, and bulk water well outside of the dendrimer. We studied the dynamic and thermodynamic properties of the water at three pH values: high pH with none of the primary or tertiary amines protonated, intermediate pH with only the primary amines protonated, and low pH with all amines protonated. For all pH values we find that both buried and surface water exhibit two relaxation times: a fast relaxation ( approximately 1 ps) corresponding to the libration motion of the water and a slow ( approximately 20 ps) diffusional component related to the escaping of water from one domain to another. In contrast for bulk water the fast relaxation is approximately 0.4 ps while the slow relaxation is approximately 14 ps. These results are similar to those found in biological systems, where the fast relaxation is found to be approximately 1 ps while the slow relaxation ranges from 20 to 1000 ps. We used the 2PT MD method to extract the vibrational (power) spectrum and found substantial differences for the three classes of water. The translational diffusion coefficient for buried water is 11-33% (depending on pH) of the bulk value while the surface water is about 80%. The change in rotational diffusion is quite similar: 21-45% of the bulk value for buried water and 80% for surface water. This shows that translational and rotational dynamics of water are affected by the PAMAM-water interactions as well as due to the confinement in the interior of the dendrimer. We find that the reduction of translational or rotational diffusion is accompanied by a blue shift of the corresponding libration motions ( approximately 10 cm(-1) for translation, approximately 35 cm(-1) for rotation), indicating higher local force constants for these motions. These effects are most pronounced for the lowest pH, probably because of the increased rigidity caused by the internal charges. From the vibrational density of states we also calculate the enthalpies and entropies of the various waters. We find that water molecules are enthalpically favored near the PAMAM dendrimer: energy for surface water is approximately 0.1 kcal/mol lower to that in the bulk, and approximately 0.5-0.9 kcal/mol lower for buried water. In contrast, we find that both the buried and surface water are entropically unfavored: buried water is 0.9-2.2 kcal/mol lower than the bulk while the surface water is 0.1-0.2 kcal/mol lower. The net result is a thermodynamically unfavored state of the water surrounding the PAMAM dendrimer: 0.4-1.3 kcal/mol higher for buried water and 0.1-0.2 kcal/mol for surface water. This excess free energy of the surface and buried waters is released when the PAMAM dendrimer binds to DNA or metal ions, providing an extra driving force.     

Synthesis and cubic nonlinear optical behavior of phenyl and ferrocenyl-ended resorcinarene-based dendrimers

Dendrimers were synthesized with phenyl and ferrocenyl-ended groups joined by vinyl moieties. All the dendrons used for dendrimers synthesis had showed trans configuration. This configuration as well as the ‘cone’ conformation of the resorcinarenes was preserved in the dendrimers, as it was shown by ¹H NMR spectroscopy. The chemical structure and purity of the synthesized dendrimers were confirmed by ¹H and ¹³C NMR, FAB+, MALDI-TOF, electrospray mass spectra, and elemental analysis. Cubic nonlinear optical behavior of this first generation of resorcinarene dendrimers was studied. The χ⁽³⁾ values estimated from the THG Maker-fringe technique for the phenyl and ferrocenyl-ended resorcinarene dendrimers dispersed in thin solid films are of the order of 10⁻¹³ and 10⁻¹² esu, respectively.     

Control of electron transport using redox‐active core dendrimers

We are constructing a model system to elucidate the molecular structure‐property relationships for attenuation of electron transfer (e.g. electron encapsulation). This information is relevant in bio‐electron transfer schemes and in emerging molecular electronics schemes such as storage of information using individual molecules. Our system consists of an inorganic cluster surrounded by dendritic ligands which act as an organic coating. Although the electrochemical and photophysical properties of a variety of metal clusters have been established, very little has been described on the chemistry on metal clusters.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Energy transport in peptide helices: a comparison between high- and low-energy excitations

Energy transport in a short helical peptide in chloroform solution is studied by time-resolved femtosecond spectroscopy and accompanying nonequilibrium molecular dynamics (MD) simulations. In particular, the heat transport after excitation of an azobenzene chromophore attached to one terminus of the helix with 3 eV (UV) photons is compared to the excitation of a peptide C=O oscillator with 0.2 eV (IR) photons. The heat in the helix is detected at various distances from the heat source as a function of time by employing vibrational pump-probe spectroscopy. As a result, the carbonyl oscillators at different positions along the helix act as local thermometers. The experiments show that heat transport through the peptide after excitation with low-energy photons is at least 4 times faster than after UV excitation. On the other hand, the heat transport obtained by nonequilibrium MD simulations is largely insensitive to the kind of excitation. The calculations agree well with the experimental results for the low-frequency case; however, they give a factor of 5 too fast energy transport for the high-energy case. Employing instantaneous normal mode calculations of the MD trajectories, a simple harmonic model of heat transport is adopted, which shows that the heat diffusivity decreases significantly at temperatures initially reached by high-energy excitation. This finding suggests that the photoinduced energy gets trapped, if it is deposited in high amounts. The various competing mechanisms, such as vibrational T(1) relaxation, resonant transfer between excitonic states, cascading down relaxation, and low-frequency mode transfer, are discussed in detail.     

Trapping efficiency of polyurethane foam and amberlite XAD-2 for various organochlorine compounds

The trapping efficiency of polyurethane foam (PUF) and Amberlite XAD-2 have been investigated for various OCPs. Atmospheres containing different concentrations of 16 OCPs have been generated and trapped in the solid adsorbent cartridges. Conditions such as the adsorption by the cartridges, the extraction of the compounds, the evaporation of the final solution, the breakthrough value and the analysis of the compounds by GC/ECD have been studied. The adsorption efficiency has been better on PUF than on Amberlite XAD-2. Recovery values ranging from 80% for p-p TDE to 105% for heptachlor are obtained, most of them being higher than 90%. The relative standard deviation values for the compounds investigated are less than 5% for a total sample amount of 0.2 g of each compound.     

Optical energy transport and interactions between the excitations in a coumarin-perylene bisimide dendrimer

Energy transfer properties of novel coumarin-perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate approximately 10 ps(-1)) and efficient (quantum yield approximately 99.5%) donor-acceptor energy transfer is observed. The random distributions of donor-acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor-acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump-pump-probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities.