Functionalization of Porous Silicon with Alkenes and Alkynes via Carbocation-Mediated Hydrosilylation
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Functionalization of Porous Silicon with Alkenes and Alkynes via Carbocation-Mediated Hydrosilylation J. M. Schmeltzer, Lon A. Porter, Jr., Michael P. Stewart, Carmen M. López, and Jillian M. Buriak* Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393, USA, E-mail: [email protected] ABSTRACT Efforts to produce stable, derivatized porous silicon have yielded a number of chemical methods capable of functionalizing this interesting material with organic monolayers. Hydrideterminated porous silicon substrates react with alkenes and alkynes in the presence of dilute triphenylcarbenium salt solutions to respectively produce alkyl- and alkenyl-functionalized materials. Characterization by transmission FTIR and solid-state NMR suggests the formation of highly stable silicon-carbon bonds to yield covalently bound organic moieties. Porous silicon passivated in this fashion exhibits a greater resistance than that of the native material to chemical degradation, indicating that the organic functionalities may serve to sterically shield the nanocrystallites from nucleophiles. Hydrosilylation is proposed to proceed via hydride abstraction from the substrate followed by electrophilic attack by the subsequent species upon the alkene/alkyne, a mechanism previously hypothesized for the formation of stabilized β-silyl carbocations. The reaction is tolerant of a variety of substrate functional groups and native porous silicon surfaces but depends markedly upon the identity of the salt counteranion, among other solution parameters. INTRODUCTION The relative instability of hydride-terminated porous silicon at ambient conditions to oxidation and degradation has been the focus of much work aimed at the successful passivation and derivatization of this material for fundamental investigations and applied devices without dramatic loss of its physical and chemical properties [1, 2]. Strategies involving the replacement of hydride moieties with silicon-carbon bonds via hydrosilylation reactions of alkenes and alkynes include thermal [3], Lewis-acid mediated [4], and exciton-mediated [5] methods. It has since been found that hydrosilylation of a variety of alkenes and alkynes upon porous silicon is achieved in the presence of carbocation salts (figure 1) [6]. The chemical reactivity, stability, and characterization of product surfaces correlate closely to those prepared by other hydrosilylation techniques, indicating the formation of organic monolayers covalently bound to the silicon nanocrystallites and illustrating yet another means for the functionalization of porous silicon. EXPERIMENT Hydride-passivated porous silicon samples were prepared within a Teflon cell under ambient conditions by a previously published galvanostatic etching method employing 1:1 (v:v) 48% HF/ethanol [7]. Photoluminescent substrates derived from n-type silicon wafers (P-doped, 0.65 0.95 Ω⋅cm, (100) orientation, 1.1 cm2 exposed area) were prepared by etching under white light
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