Competing geometric and electronic effects in adsorption of phenylenediamine structural isomers on the Ge(100)-2 × 1 surface

The adsorption behavior of ortho-, meta-, and para-phenylenediamine (PD) structural isomers (o-, m- and p-PD) at the Ge(100)-2 × 1 surface has been studied and compared with aniline. Our results indicate that at room temperature, aniline and all PD isomers adsorb on Ge(100)-2 × 1 via NH dissociation, with no dative-bonded adducts observed. For each PD isomer on Ge(100)-2 × 1, a fraction of the adducts are adsorbed through both amino groups. This fraction is greatest for m-PD, somewhat lower for o-PD, and significantly lower for p-PD. The observed trend in reactivity is ascribed to a combination of geometric and electronic effects. Our results indicate that geometrically, the ortho isomer is best suited to surface attachment through both amino groups, while electronically, the aromatic nature of the benzene spacer between isomer amino groups renders dual NH dissociation most favorable for the meta isomer. The results elucidate the competing role of electronic and geometric effects in this adsorption system.