Substituent Effect on the Intermolecular Arrangements of One-Dimensional Molecular Assembly on the Si(100)-(2x1)-H Surface

The effect of methyl substitution in styrene molecules on the spatial arrangement of molecules in a one-dimensional (1-D) molecular assembly on the Si(100)-(2x1)-H surface has been studied using a scanning tunneling microscope (STM) at 300 K. Styrene molecules form well-defined 1-D molecular assemblies through a chain reaction mechanism along the dimer row direction, where the phenyl rings are separated by distances equal to that of the interdimer distance in a row and aligned parallel to each other. We observed that the substitution in a phenyl ring has no observable effect on the adsorption sites, configurations, and stacking of phenyl rings along the dimer row. In contrast, the methyl substitution at alpha site (alpha-methylstyrene) results in a 1-D assembly where the adsorption sites are similar to that of styrene but the adsorbed molecules are arranged in alternate geometrical configurations along the dimer row. In the case of beta-methylstyrene, the adsorption sites (diagonal silicon atoms in a dimer row) and the geometrical configurations of adsorbed molecules along the dimer row are different from that of styrene. These results suggest that the selective arrangement of the molecules in a 1-D assembly can be achieved by inducing a steric hindrance through substitution at specific sites of the reacting molecule.