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

Abstract

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.

Comparison of resistance to protein adsorption and stability of thin films derived from α-hepta-(ethylene glycol) methyl ω-undecenyl ether on HSi(111) and HSi(100) surfaces

“…H-Si(100) and atomically flat H-Si(111) surfaces were prepared similarly to the reported procedure [47][48][49]. Briefly, both (100) and (111) oriented, single-sided polished silicon wafers were cut into pieces of ∼1 × 1 cm 2 .…”

Thin films derived from giant, tripod-shaped oligophenylenes end-capped with triallylsilyl groups on hydrogen-terminated Si(111) surfaces

“…Atomically flat H-Si(111) surfaces were prepared according to the reported procedures [10]. Briefly, the Si(111) wafers were cleaned with NH 4 OH/H 2 O 2 /H 2 O (v/v 1:1:4) at 80 • C for 20 min, thoroughly washed with Millipore water, etched in 10% buffer-HF (Transene) for 10 min and then in 40% NH 4 F (Riedel-deHaen) for 10 min under N 2 purge, and dried immediately with a stream of N 2 .…”

“…Therefore, it is highly desirable to develop molecular building blocks with a large, surface-reactive footprint that can selectively bind to and cover the nanospots. To this end, we have recently studied thin films derived from a series of carbosilane dendrons with a bromophenyl group at the focal point and many surface-reactive groups at the periphery [9][10][11][12][13]. The bromophenyl groups in these films serve as handles for attaching conjugated molecules via palladiumcatalyzed coupling reactions.…”

“…Unfortunately, the yields of these reactions were low (30-50%) especially for the films derived from the large (high generation) dendrons, most likely due to the misorientation of some of the bromophenyl groups. During the kinetically controlled chemisorption process, these highly flexible dendrons could randomly adopt orientations where the bromophenyl groups were tilted toward the surfaces or even buried inside the films, thus preventing the sterically demanding coupling reactions [9][10][11][12][13].…”

“…The molecule 1 is composed of three heptaphenylene legs joined at a Si atom with a bromophenyl group, forming a rigid, tripod-shaped framework. Each tripod leg is end-capped with a triallylsilyl group for chemisorption on H-Si(111) through formation of multiple Si-C bonds by surface hydrosilylation [10,[15][16][17][18]. The footprint of the tripod is about 5 nm across [19], much larger than the carbosilane dendron adsorbates that are less than 3 nm across [9][10][11][12][13].…”