Mp. Schwartz et al., Interaction of pi-conjugated organic molecules with pi-bonded semiconductor surfaces: Structure, selectivity, and mechanistic implications, J AM CHEM S, 122(35), 2000, pp. 8529-8538
The (001) surface of silicon contains pairs of atoms that are held together
with a strong sigma bond and a weak pi bond. The interaction of styrene wi
th the Si(001) surface has been investigated as a model system for understa
nding the interaction of conjugated pi-electron systems to pi-bonded semico
nductor surfaces. Scanning tunneling microscopy images show one primary bon
ding configuration, slightly off-center from the middle of a dimer row. Inf
rared spectra using isotopically labeled styrene establish that attachment
occurs in a highly selective way, bonding through the external vinyl group
and leaving the aromatic ring almost completely unperturbed. Ab initio calc
ulations reveal that the interaction between the pi electrons of the vinyl
group of styrene and the electron-deficient end of a Si=Si dimer is strongl
y attractive. It is proposed that this attraction facilitates a low-symmetr
y interaction between the surface dimers and the vinyl group, leading to a
highly selective reaction pathway for which Woodward-Hoffmann rules do not
apply. The implications for selective attachment of other conjugated pi-ele
ctron systems to other pi-bonded semiconductor surfaces are discussed.