Carbosilane Dendrons Research Articles

Step-Growth Synthesis and Interfacial Friction Properties of Surface Dendron Coatings

We report a new procedure for synthesizing self-assembled two-tiered coatings comprised of a tethered carbosilane dendron canopy layer over an alkylsilane sublayer. The sublayer is anchored to a silicon substrate, and the areal density of the canopy can be controlled by changing the concentration of reactive vinyl groups on the sublayer surface. The structure of the dendron layer is characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS), water contact angle, and ellipsometry. Steric hindrance due to close packing of functional groups has a strong effect on the reaction efficiency, and dendron structure can be obtained only in canopies with low areal densities. Lateral force microscopy (LFM) is used to study interfacial friction and adhesion properties of the coatings. Structural variations in the canopy layer are found to play a crucial role in interfacial friction properties of these coatings.

Carbosilane Dendrons Functionalized at Their Focal Point

AbstractThe Si–Ph bond of PhSi[(CH2)3SiMe2Bz]3 (5) is cleaved with triflic acid to give TfOSi[(CH2)3SiMe2Bz]3, which, in turn, reacts with triethylammonium chloride or potassium cyclopentadienide to give, respectively, ClSi[(CH2)3SiMe2Bz]3 (8) and (C5H5)Si[(CH2)3SiMe2Bz]3 (10). This strategy can be applied to the post‐growth incorporation of nucleophiles to the focal point of carbosilane dendritic wedges. In this way, cyclopentadiene‐functionalized dendritic wedges of second and third generation C5H5‐Gn‐[(CH2)3SiMe2Bz]x (n = 2, x = 9, 11; n = 3, x = 27, 12) have been obtained starting from Ph‐Gn‐[(CH2)3SiMe2Bz]x (6, 7). The metallocenes [{(BzMe2SiCH2CH2CH2)3SiC5H4}2MCl2] (M = Ti, 14; Zr, 15) have also been obtained from 10 and their catalytic behavior in ethylene and propylene polymerization, using MAO as a cocatalyst, has been studied and compared to that of related non‐dendritic complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Atomic Force Microscopy Studies of Mesoscopic Membranous Bubbles on Monolayers Derived from SiCl3-Terminated Carbosilane Dendrons on Mica

Monolayers of dendrimers were prepared on mica by spin-coating of the second generation carbosilane dendrons with 9 SiCl(3) periphery groups. AFM images of the films showed the presence of soft yet robust, dome-shaped features with a base diameter of 100-2000 nm. The apparent height of the features, ranging from 10 to 200 nm, rapidly reduced under increasing compression force, eventually to the same value ( approximately 2.5 nm) corresponding to a bilayer of the flattened dendrons. The change in shape of the features in response to the compression force from the AFM tip was fully reversible, indicating that the features were robust. The contrast of the features in the tapping mode AFM (TMAFM) phase images flipped at a setpoint ratio of approximately 0.55. In contrast to the reported amplitude vs displacement (A/z) curves for compliant materials, A/z curves of the features showed that the reduction of amplitude was larger than the tip displacement as if the cantilever tip were repelled by the soft features. This result cautions the use of amplitude/phase vs displacement (APD) curves for interpreting TMAFM images and for optimizing conditions for TMAFM imaging of very soft and "sticky" surfaces. On the basis of the AFM studies, we believe that the dome-shaped features are membranous air bubbles. The membranes of the bubbles were probably composed of a bilayer of the dendron molecules bound through the peripheral silanol groups. The bilayer could be formed by self-assembly of the molecules on top of the air bubbles entrapped at the monolayer/solution interface during spin-coating.

Preparation, Characterization, and Heck Reaction of Siloxane Films Derived from Carbosilane Dendrons with a Bromophenyl Group at the Focal Point and up to 27 SiCl3 Groups at the Periphery

Multidentate organosiloxane thin films were prepared on SiO2/Si surfaces by solution phase deposition of carbosilane dendrons containing a bromophenyl group at the focal point and 3 (Br-G0), 9 (Br-G1), and 27 (Br-G2) SiCl3 groups at the periphery. The films were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). The results indicated that about six Br-G0 molecules covered the same surface area as three Br-G1 molecules and one Br-G2 molecule. Hence, the density of the bromophenyl groups in the films could be defined by the size (generation) of the dendron adsorbates. We also demonstrated that the bromophenyl groups on the film surfaces could serve as a handle for attaching conjugated molecules via formation of C-C bonds. Thus, upon treatment of the films with 4-fluorostyrene under Heck reaction conditions, XPS analysis showed that about 90, 66, and 51% of the bromine atoms in the films prepared from Br-G0, Br-G1, and Br-G2 were consumed, and 94, 82, and 58% of the consumed bromine atoms were replaced by fluorostyryl groups. The remaining bromophenyl groups were probably not accessible to the reactants because of their unfavorable orientation. The overall yields for the surface Heck reaction were estimated to be 84, 54, and 30% for the films prepared from Br-G0, Br-G1, and Br-G2, respectively.

Dendritic β-diketiminato titanium and zirconium complexes: synthesis and ethylene polymerisation

The cyclopentadienyl(β-diketiminato)titanium and zirconium chlorides (η 5-C 5H 5)MCl 2(CH(C(NC 6H 4-4-OR)CH 3) 2) (M = Ti ( 4-dend), Zr ( 5-dend)), where R corresponds to the first generation carbosilane dendron (dendritic wedge) Si(CH 2CH 2SiMePh 2) 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 (η 5-C 5H 5)MCl 2(CH(C(NC 6H 5)CH 3) 2) (M = Ti ( 4), Zr ( 5)).

Preparation and Heck Reaction of Multidentate Carbosilane Films Derived from Focally Functionalized and Allyl-Terminated Dendrons on Hydrogen-Terminated Silicon(111) Surfaces

Multidentate carbosilane films were prepared by thermally induced hydrosilylation of allyl-terminated carbosilane dendrons of generations 0, 1, and 2 (G0-G2) on hydrogen-terminated silicon(111) surfaces. The dendron molecules contain three (G0), nine (G1), and twenty-seven (G2) allyl groups at the periphery, and a bromophenyl functional group at the focal point. The dendron films were characterized by contact-angle goniometry, ellipsometry, Fourier transform infrared spectroscopy in the attenuated total reflection mode, and X-ray photoelectron spectroscopy (XPS). Upon hydroboration of the remaining allyl groups in the films, the percentage of the introduced boron atoms in the films were measured by XPS. The results indicate the presence of roughly 20%, 27%, and 46% of unreacted allyl groups in the G0, G1, and G2 films, respectively. The mechanistic aspects of the chemisorption of these dendron molecules on H-Si(111) surfaces are discussed. XPS studies indicate that seven G0 molecules cover approximately the same area on the substrate as three G1 molecules and one G2 molecule. After treatment of the G0, G1, and G2 films with 4-fluorostyrene under the Heck reaction conditions, the XPS studies indicate that about 84%, 71%, and 55% of the Br atoms were consumed, yielding the replacement of ca. 58-70% of the reacted Br atoms by the fluorostyryl groups. The remaining bromophenyl groups were inactive toward the Heck reaction, probably due to their disfavorable position/orientation in the films.

Wettabilities of Organosiloxane Thin Films Derived from SiCl3-Terminated Carbosilane Dendrons on Mica

We prepared submonolayers, monolayers, and multilayers of organosiloxanes derived from the second to fourth generation (G2−G4) SiCl3-terminated carbosilane dendrons on mica by spin-casting. The surface properties of these films, such as wettabilities, surface tensions, works of adhesion with various liquids, pore size, and surface coverage, were investigated by means of contact-angle measurements. All G2−G4 films exhibit high hysteresis of ∼20°, probably due to molecular scale chemical heterogeneity and surface roughness. The surface tensions of the G2−G4 monolayers are similar: γS (total) = ∼29 mJ/m2, γSd (dispersive) = ∼24 mJ/m2, and γSp (polar) = ∼5 mJ/m2. However, only γSd (∼21 mJ/m2) contributes to the surface tensions of the multilayers, suggesting that the underlying polar mica substrate is significantly screened by the dendron multilayers. Works of adhesion (WSL) for water, glycerol, dimethylformamide, methylene iodide, squalane, and hexadecane on the films were measured. The multilayers exhibit a lower WSLd (dispersive) than the monolayers (difference of ∼3 mJ/m2), consistent with a decrease of γSd from monolayers to multilayers. Furthermore, both the average pore size and the surface coverage of the G2−G4 monolayers were estimated to be 0.31−1.1 nm2 and ∼77%, respectively.

The First Organosiloxane Thin Films Derived from SiCl3-Terminated Dendrons. Thickness-Dependent Nano- and Mesoscopic Structures of the Films Deposited on Mica by Spin-Coating

This article describes a new type of organosiloxane thin films derived from SiCl3-terminated carbosilane dendrons of the second, third, and fourth generations, containing 9, 27, and 81 SiCl3 terminal groups. The films were deposited on mica surfaces by spin-coating from solutions of the dendrons with a series of concentrations. The resulting submonolayer to multilayer films were investigated with atomic force microscopy (AFM). AFM studies showed that the morphology of the films was highly dependent upon the generation of the dendrons and the film thickness. Mesoscopic ring, disk, or hole structures were observed. These structures were composed of nanoparticles with sizes corresponding to one dendron molecule or the cluster of a few laterally bound molecules. At submonolayer coverage, the molecules tended to flatten and spread out on mica surfaces. Significantly, this study showed that molecularly flat monolayers could be obtained with the dendrons containing up to 81 SiCl3 terminal groups. Prior to curing, the dendron films could be shaved by an AFM tip, while the films became robust after curing at elevated temperatures, indicating the formation of a strong siloxane network among the dendron molecules. Direct AFM observations indicated that the ring structures were formed from isolated droplets during the evaporation of the liquid thin films. The mechanisms for the formation of the observed film morphology are discussed, based on the structure and properties of the dendrons and the effect of the evaporation process.

Insulated Molecular Wires: Dendritic Encapsulation of Poly(triacetylene) Oligomers, Attempted Dendritic Stabilization of Novel Poly(pentaacetylene) Oligomers, and an Organometallic Approach to Dendritic Rods

Multinanometer-long end-capped poly(triacetylene) (PTA) and poly(pentaacetylene) (PPA) oligomers with dendritic side chains were synthesized as insulated molecular wires. PTA Oligomers with laterally appended Fréchet-type dendrons of first to third generation were prepared by attaching the dendrons (8, 13, and 17, respectively, Scheme 1) to (E)-enediyne 18 by a Mitsunobu reaction and subsequent Glaser-Hay oligomerization under end-capping conditions (Scheme 2). Whereas first-generation oligomers up to the pentamer were isolated (1a – e), for reasons of steric overcrowding, only oligomers up to the trimer (2a – c) were formed at the second-generation level, and only the end-capped monomer and dimer (3a, b) were isolated at the third-generation level. By repetitive sequences of hydrosilylation (with the Karstedt catalyst), followed by allylation or vinylation, a series of carbosilane dendrons were also prepared (Schemes 3 and 4). Attachment of the second-generation wedge 40 to (E)-enediyne 18, followed by deprotection and subsequent end-capping Hay oligomerization, provided PTA oligomers 4a – d with lateral carbosilane dendrons (Scheme 5). UV/VIS Studies (Figs. 5 – 10) demonstrated that the insulating dendritic layers did not alter the electronic characteristics of the PTA backbone, even at the higher-generation levels. Despite distortion from planarity due to the bulky dendritic wedges, no loss of π-electron conjugation along the PTA backbone was detected. A surprising (E)→(Z) isomerization of the diethynylethene (DEE) core in the third generation derivative 3a was observed, possibly photosensitized by the bulky Fréchet-type dendritic wedge. Electrochemical investigations by steady-state voltammetry and cyclic voltammetry showed that the first reduction potential of the PTA oligomer with Fréchet-type dendrons is shifted to more negative values as the dendritic coverage increases. With compounds 5a – c, the first oligomers with a poly(pentaacetylene) backbone were obtained by oxidative Hay oligomerization under end-capping conditions (Scheme 6). The synthesis of dendritic PPA oligomers by oxidative coupling of (E)-enetetrayne 60 under end-capping conditions provided oligomers 61a – d, which were formed as mixtures of stereoisomers due to unexpected thermal (E)→(Z) isomerization (Scheme 8). In another novel approach towards dendritic encapsulation of molecular wires with a Pt-bridged tetraethynylethene (TEE) oligomeric backbone, the trans-dichloroplatinum(II) complex trans-67 with dendritic phosphane ligands (Fig. 14) was coupled under Hagihara conditions to mono-deprotected 69 under formation of the extended monomer 65 (Scheme 12). Again, an unexpected thermal (E)→(Z) isomerization, possibly induced by steric strain between TEE moieties and dendritic phosphane ligands in the unstable complex, led to the isolation of 65 as an isomeric mixture only.

Insulated Molecular Wires: Dendritic Encapsulation of Poly(triacetylene) Oligomers, Attempted Dendritic Stabilization of Novel Poly(pentaacetylene) Oligomers, and an Organometallic Approach to Dendritic Rods

Multinanometer-long end-capped poly(triacetylene) (PTA) and poly(pentaacetylene) (PPA) oligomers with dendritic side chains were synthesized as insulated molecular wires. PTA Oligomers with laterally appended Fréchet-type dendrons of first to third generation were prepared by attaching the dendrons (8, 13, and 17, respectively, Scheme 1) to (E)-enediyne 18 by a Mitsunobu reaction and subsequent Glaser-Hay oligomerization under end-capping conditions (Scheme 2). Whereas first-generation oligomers up to the pentamer were isolated (1a – e), for reasons of steric overcrowding, only oligomers up to the trimer (2a – c) were formed at the second-generation level, and only the end-capped monomer and dimer (3a, b) were isolated at the third-generation level. By repetitive sequences of hydrosilylation (with the Karstedt catalyst), followed by allylation or vinylation, a series of carbosilane dendrons were also prepared (Schemes 3 and 4). Attachment of the second-generation wedge 40 to (E)-enediyne 18, followed by deprotection and subsequent end-capping Hay oligomerization, provided PTA oligomers 4a – d with lateral carbosilane dendrons (Scheme 5). UV/VIS Studies (Figs. 5 – 10) demonstrated that the insulating dendritic layers did not alter the electronic characteristics of the PTA backbone, even at the higher-generation levels. Despite distortion from planarity due to the bulky dendritic wedges, no loss of π-electron conjugation along the PTA backbone was detected. A surprising (E)→(Z) isomerization of the diethynylethene (DEE) core in the third generation derivative 3a was observed, possibly photosensitized by the bulky Fréchet-type dendritic wedge. Electrochemical investigations by steady-state voltammetry and cyclic voltammetry showed that the first reduction potential of the PTA oligomer with Fréchet-type dendrons is shifted to more negative values as the dendritic coverage increases. With compounds 5a – c, the first oligomers with a poly(pentaacetylene) backbone were obtained by oxidative Hay oligomerization under end-capping conditions (Scheme 6). The synthesis of dendritic PPA oligomers by oxidative coupling of (E)-enetetrayne 60 under end-capping conditions provided oligomers 61a – d, which were formed as mixtures of stereoisomers due to unexpected thermal (E)→(Z) isomerization (Scheme 8). In another novel approach towards dendritic encapsulation of molecular wires with a Pt-bridged tetraethynylethene (TEE) oligomeric backbone, the trans-dichloroplatinum(II) complex trans-67 with dendritic phosphane ligands (Fig. 14) was coupled under Hagihara conditions to mono-deprotected 69 under formation of the extended monomer 65 (Scheme 12). Again, an unexpected thermal (E)→(Z) isomerization, possibly induced by steric strain between TEE moieties and dendritic phosphane ligands in the unstable complex, led to the isolation of 65 as an isomeric mixture only.