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.