Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo[g,p]chrysene (DBC), that substitution position (i.e., meta vs para ) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. In particular, incorporation of four methoxy groups in parent DBC at meta positions similarly impacts the energies of phenanthrene-like HOMO and biphenyl-like HOMO-1, while their incorporation at para position swaps energetic ordering of HOMO and HOMO-1. We demonstrate that a straightforward analysis of FMOs provides valuable insight toward the rational design of novel PAHs with tailored redox properties.