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.     

Mechanisms for fluorescence depolarization in dendrimers

We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule.     

Small-angle neutron scattering studies of charged carboxyl-terminated dendrimers in solutions

Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.     

Nanosized molecular propellers by cyclodehydrogenation of polyphenylene dendrimers

In a polymer analogous approach, large dendritic oligophenylenes containing benzene and tetraphenylmethane cores are transformed via oxidative cyclodehydrogenation to novel propeller-shaped molecules with large polycyclic aromatic hydrocarbon units as "blades". Structure analysis is performed by a combination of MALDI-TOF mass spectrometry, UV/vis, fluorescence, and Raman spectroscopy using solid-state sample preparation methods. These methods are also utilized to determine the degree of the cyclodehydrogenation reaction.     

Fifth-rank molecular polarization tensors

Fifth-rank molecular polarization tensors are discussed in terms of their tensor group which includes both the spatial symmetry and the permutation symmetry of the suffixes. This latter may be described by certain four-dimensional point groups, which in the nontotally symmetric cases may be projected into three-dimensional space. The study leads to a determination of the number of independent components of these tensors and a discussion of the types of phenomena which they may be expected to produce.     

Starlike dendrimers in solutions: structural properties and internal dynamics

We measured the shape and the internal dynamics of starlike dendrimers under good solvent conditions with small-angle neutron scattering and neutron spin-echo (NSE) spectroscopy, respectively. Architectural parameters such as the spacer length and generation were varied in a systematic manner. Structural changes occurring in the dendrimers as a function of these parameters are discussed, i.e., in terms of the fractal dimension and deviations of the radius of gyration from the Gaussian value. A first cumulant evaluation of the NSE spectra for each scattering vector q separately yields the length scale dependent relaxation rates. We observe a local minimum in the normalized relaxation rates Omega(q)q(3) on length scales corresponding to the overall dendrimer dimension. The dynamics is discussed within a Rouse-Zimm approach generalized to the case of starlike dendrimers of arbitrary geometry. The model allows an identification of the modes contributing to the relaxation of the dendrimer in the q and time range of the NSE experiment. The local minimum is due to collective breathing motions of (parts of) the dendrons relative to each other. Shape fluctuations are not observed.     

Kerr effect in solutions of carbosilane dendrimers with terminal mesogenic groups

First results of investigations of electro-optical properties of liquid crystalline (LC) dendrimers in solution are presented. Measurements of electric birefringence (Kerr effect) and dielectric polarization of first generation carbosilane dendrimers with different ester linked terminal mesogenic groups (cholesteryl, cyanobiphenylyl and 4-methoxyphenyl benzoate) have been carried out using dilute solutions in CCl4. The results show that the dielectric polarization is proportional to the second power of the electric field in accordance with Kerr law. The Kerr constants calculated are close to those of the low molar mass analogues of the corresponding mesogenic groups. Thus the electric birefringence of the LC dendrimer solutions is mainly determined by the electro-optical properties of their terminal mesogenic groups oriented in the electric field independently of the main chain.     

Dynamics of electronic polarization in molecular crystals

Dynamics of electronic polarization in the vicinity of charge carriers in molecular crystals is for the first time investigated here in connection with the carrier transport and intramolecular vibronic polarization. According to standard picture it has been assumed that the electronic polarization relaxation time is extremely short, as estimated from the relation τ_c = τ_d1 ≈ h/E_exc, where E_exc is the energy of the first single exciton state. In the case of anthracene (Ac) crystals, the value of τ_e is about 2 × 10⁻¹⁶ s, i.e. by several orders of magnitude shorter than a typical hopping (residence) time of charge carriers τ_h = 10⁻¹⁴ -10⁻¹³ s. It is argued that typical time of full reconstruction of the electronic polarization after individual carrier hops equals, in the slow carrier regime, approximately to t_d2 ≈ h/ΔE_exc is the width of the lowest singlet-exciton band. In Ac, this means t_d2 ≈ 0.73 × 10⁻¹⁴ s. Physical implications of this relatively high value of t_d2 in connection with carrier transport and molecular (vibronic) polarization are discussed.     

Synthesis of bithiophenesilane dendrimer of the first generation

The synthesis of tris{5′-[methylbis(2-thienyl)silyl]2,2′-bithienyl-5-yl} methylsilane, a first-generation bithiophenesilane dendrimer, is described. The conditions of effective formation of methyltrithienylsilane were found; methyltris(5-bromo-2-thienyl)silane and a number of other monofunctional derivatives of methyltrithienylsilane were synthesized for the first time. The advantages and drawbacks of the Suzuki and Kumada reactions for the formation of bithienyl fragments in the synthesis of oligothienylsilane dendrimers are discussed.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 673–679, March, 2005.     

Redox-controlled interaction of biferrocenyl-terminated dendrimers with beta-cyclodextrin molecular printboards

This paper describes the synthesis and electrochemistry of biferrocenyl-terminated dendrimers and their beta-cyclodextrin (beta-CD) inclusion complexes in aqueous solution and at surfaces. Three generations of poly(propylene imine) (PPI) dendrimers, decorated with 4, 8, and 16 biferrocenyl (BFc) units, respectively, were synthesized. A water-soluble BFc derivative forms stable inclusion complexes with beta-CD. The intrinsic binding constant is K(i)=2.5 x 10(4) M(-1). The BFc dendrimers were solubilized in water by complexation of the end groups with beta-CD, resulting in large water-soluble supramolecular assemblies. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) showed that all the end groups are complexed to beta-CD. Adsorption of the dendrimers at self-assembled monolayers (SAMs) of heptathioether-functionalized beta-CD on gold ("molecular printboards") resulted in stable monolayers of the dendrimers due to the formation of multivalent host-guest interactions between the BFc end groups of the dendrimers and the immobilized beta-CD molecules. The number of interacting end groups is 3, 4, and 4 for dendrimer generations 1, 2, and 3, respectively. The complexation of BFc to beta-CD is sensitive to the oxidation state of the BFc unit. Oxidation of neutral BFc-Fe(2) ((II,II)) to the cationic, mixed-valence biferrocenium BFc-Fe(2) ((II,III)+) resulted in dissociation of the host-guest complexes. Scan-rate-dependent CV and DPV analyses of the dendrimer-beta-CD assemblies immobilized at the beta-CD host surface and in solution revealed that the dendrimers are oxidized in three steps. First, the surface-beta-CD-bound BFc moieties are oxidized to the mixed-valence state, Fe(2) ((II,III)+), followed by the oxidation of the non-surface-interacting BFc groups to the Fe(2) ((II,III)+) state. The third step involves the oxidation of all the BFc moieties to the Fe(2) ((III,III)2+) state.     

Non-linear dielectric polarization of water solutions in dioxane

The dielectric polarization and its change in an external electric field were measured for dioxane—water diluted solutions up to 0.3 molar fraction of water. The results were found to be qualitatively similar to those previously observed for dioxane—alcohols mixtures.     

Hydrodynamic properties of rigid pyridine-containing poly(phenylene) dendrimers in solutions

The hydrodynamic properties of pyridine-containing polyphenylene dendrimers of the third and fourth generations in chloroform are studied by photon correlation spectroscopy and viscometry. It has been demonstrated that the hydrodynamic characteristics of these macromolecules in dilute solutions are similar to those of nondraining spheres. The hydrodynamic radius of these dendrimers is shown to be proportional to their molecular mass to a power of 1/3. It has been established that the macromolecules of the dendrimers under examination in solutions conserve the conformation and size over a wide temperature range. The detailed analysis of hydrodynamic data allowed a conclusion concerning an extremely low content of the polymer inside the equivalent sphere for the above dendrimers in solutions. The compounds of interest may be referred to as rigid dendritic systems.     

Viscoelastic properties of dendrimers in the melt from nonequlibrium molecular dynamics

The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight.     

The symmetry of molecular polarization tensors

Polarization tensors are discussed in terms of their intrinsic symmetry group which is a direct product of the point group and the subgroup of the permutation group relevant to the experiment. The study of these latter groups is simplified by use of the isomorphism with certain point groups and permutations of suffixes can be visualized by rotations and reflections of the vertices of various objects in space. The approach unites the previous treatments and provides a means of constructing the bases for the irreducible tensor components. The difficulties introduced by Laplace's equation are explained and the information obtainable from induced birefringence experiments (Kerr and Cotton–Mouton effects) discussed for various systems.     

Designer dendrimers: branched oligosulfonimides with controllable molecular architectures

The synthesis of "designer" dendrimers and dendrons with sulfonimide units at every branching point is reported. The synthesis is based on a series of (regio)selective functionalization reactions of amines and sulfonamides allowing precise control of the dendrimers' shape, the number of branches in each generation, and their peripheral decoration with functional groups. In principle, structurally different branches can be incorporated at any position within the dendrimer structure at will. Structurally perfect symmetrical and two-faced "Janus"-type dendrimers, as well as dendrimers and dendrons with intended interstices were synthesized on a preparative scale and fully characterized. Oligosulfonimide dendrons of various generations bearing an aryl bromide functional group at their focal points were attached to a p-phenylene core with the aid of Suzuki cross-coupling reactions resulting in dendrimers with photoactive terphenyl cores. The structure and the high purity of all dendritic sulfonimides were confirmed by means of (1)H and (13)C NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. The utility of MALDI-TOF mass spectrometry for the analytical characterization of these dendrimers was evaluated in comparison to electrospray ionization. Two model branched oligosulfonimides were characterized in the solid state by single-crystal X-ray analysis. Reaction selectivities and conformation of sulfonimide branching points were rationalized by DFT calculations.     

Interfacial polarization phenomena in organic molecular films

Electrostatic phenomena occurring at the interface between metal/organic and organic/organic materials are discussed from the viewpoint of dielectrics physics. Focusing on two important origins of surface polarization phenomena, orientational ordering of polar molecules and displacement of excess charges at the interface, surface polarization phenomena of organic thin films are discussed. To define the orientational order of polar molecules, orientational order parameters are introduced, and surface polarization due to the alignment of dipoles is expressed. The generation of Maxwell displacement current (MDC) and optical second harmonic generation (SHG) that are specific for surface organic monomolecular films are discussed, and some experimental evidence are shown. As an extension of the concept of surface Fermi level introduced to discuss the electrostatic phenomena due to electron transfer at the interface between metal-organic insulators, the surface Fermi level is extended to the discussion on the electrostatic phenomena of organic semiconductor materials on metals. In this paper, some experimental evidence of surface polarization originating from polar molecules and displacement of excess charges are shown. After that, with consideration of these surface phenomena, single electron tunneling of organic films are briefly discussed in association with surface polarization phenomena.     

Palladium catalysis using dendrimers: molecular catalysts versus nanoparticles

Dendritic Pd catalysts, dendrimer-stabilized Pd nanoparticle (PdNP) catalysts, and their comparison and combined use for carbon–carbon coupling reactions are discussed with emphasis on the research carried out in the author’s laboratory during the last decade. Multinuclear star-shaped catalysts rather than dendritic catalysts can reach the efficiency of the best monometallic catalysts, whereas PdNPs stabilized by dendrimers can react with turnover numbers close to 10⁶ and bring useful mechanistic indications. In both areas, leaching issues are examined. Finally, results of the literature in asymmetric Pd catalysis by chiral dendrimers and Pd nanoparticles stabilized by chiral ligands are also reviewed, revealing the importance of the dendritic and molecular ligand design and the role of leaching Pd atoms.     

Nonequilibrium nuclear polarization in the photolysis of aniline solutions

Conclusions Negative polarization of the protons of aniline and other compounds is observed upon the photoexcitation of aniline solutions. This effect is a consequence of optical nuclear polarization in short-lived donor-acceptor complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1890–1892, August, 1987.