-3.17 The degree of orbital overlap between the HOMO and LUMO is controlled not only by the choice of donor and acceptor but also by the bridging unit that mediates the conjugation between the two. For example, the introduction of a methyl group or phenyl group onto the phenylene bridge affects one of the torsion angles, leading to a more twisted conformation and thus a smaller DEST, while the inclusion of more strongly electron-withdrawing groups such as nitrile or trifluoromethyl substituents also contributes to strengthening the electron-acceptor and localizing the electron density of the LUMO.