Interaction of π-Conjugated Organic Molecules with π-Bonded Semiconductor Surfaces:  Structure, Selectivity, and Mechanistic Implications

The (001) surface of silicon contains pairs of atoms that are held together with a strong σ bond and a weak π bond. The interaction of styrene with the Si(001) surface has been investigated as a model system for understanding the interaction of conjugated π-electron systems to π-bonded semiconductor surfaces. Scanning tunneling microscopy images show one primary bonding configuration, slightly off-center from the middle of a dimer row. Infrared 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 calculations reveal that the interaction between the π electrons of the vinyl group of styrene and the electron-deficient end of a SiSi dimer is strongly attractive. It is proposed that this attraction facilitates a low-symmetry 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 π-electron systems to other π-bonded semiconductor surfaces are discussed.