We present an ab initio study of the electronic properties of styrene molecules adsorbed on the dimerized Si(100) surface, ranging from the single molecule to the full monolayer (ML). The adsorption mechanism primarily involves the vinyl group via a [2+2] cycloaddition process that leads to the formation of covalent SiC bonds and a local surface derelaxation, while it leaves the phenyl group almost unperturbed. The investigation of the functionalized surface as a function of the coverage (e.g., 0.5–1 ML) and of the substrate reconstruction reveals two major effects. The first results from Si dimer-vinyl interaction and concerns the controlled variation of the energy band gap of the interface. The second is associated to phenyl-phenyl interactions, which give rise to a regular pattern of electronic wires at surface, stemming from the π-π coupling. These findings suggest a rationale for tailoring the nanopatterning of the surface in a controlled way.