"Front Matter". In: Organosilanes in Radical Chemistry - Index of

References 215
islands were observed (see above). It is also worth mentioning that, although
there is clear preference for lines to grow along the edge of a dimer row,
occasionally double lines are observed suggesting that hydrogen atom abstraction
across a dimer row is also possible (separation of silicon atoms within a
dimer is 2.34 A ˚ ). Abstraction across a single dimer has not been observed and is
likely to be a prohibitively strained process (separation of silicon atoms on
either edge is 5.34 A ˚ ) [46].
8.5.5 SOME EXAMPLES OF TAILORED EXPERIMENTS ON
MONOLAYERS
Organic monolayers bonded to silicon in order to be of significant utility often
require chemical functionality other than the methyl groups that terminate the
most easily prepared monolayers of alkyl chains. The use of a, v-derivatized
long-chain hydrocarbons has been a successful methodology, although not all
functional groups are compatible for this purpose. For example, the reaction of
Si(111) with 11-bromo-1-undecene has been used to prepare the anchor
w (CH2) 11Br on the surface. Then, end-grafted polysilanes have been synthesized
by reaction of polysilane having a silyl anion at the terminus with the
bromoalkyl group [69]. This end-graft technique, gave the opportunity for
direct AFM observation of polysilane single-molecule structures and allowed
the physical properties of isolated polysilane chains to be measured. Another
example is the reaction of Si(111) with ethyl undecylenate to afford the anchor
w (CH2) 10C(O)OEt on the surface. Then, the ester groups were converted into
di(2-thienyl)carbinol moieties by reaction with 2-thienyllithium and subsequently
photoanodically oxidized in the presence of thiophene to yield a
strongly adherent and smooth conducting film [70]. It is also worth mentioning
that monolayers prepared from 1-octadecene on Si(111) were found to undergo
chlorination exclusively on the terminal methyl group, when exposed to Cl2
with illumination at 350 nm [71]. This unusual high selectivity of radical chlorination
demonstrates a strong steric effect due to the densely packed array of
alkyl chains that favors attack at the chain ends. Other free radical reactions are
expected to be convenient ways to functionalize methyl-terminated alkyl monolayers.
8.6 REFERENCES
1. West, R., J. Organomet. Chem., 1986, 300, 327.
2. Miller, R.D., and Michl, J., Chem. Rev., 1989, 89, 1359.
3. West, R. In The Chemistry of Organic Silicon Compounds, Volume 3, Z. Rappoport
and Y. Apeloig (Eds), Wiley, Chichester, 2001, pp. 541–563.
4. Koe, J.R., and Fujiki, M., Silicon Chem., 2002, 1, 77.
5. Harrod, J.F., Prog. Catal., 1992, 147.