Reflection of Molecular Twist in Unoccupied Molecular Orbitals in PTCDI Derivatives: A Density Functional Study

In this work, the structural and electronic properties of perylene tetracarboxylic diimide (PTCDI) derivative molecules have been calculated using density functional theory simulations. Here, we have obtained the equilibrium geometry for certain PTCDI derivatives and calculated their occupied and unoccupied density of states separately for molecular orbitals lying in-plane (σ type) and orthogonal to the plane (π type) of the molecules. We have also simulated the X-ray absorption spectroscopy (XAS) spectra for these molecules separately for π- and σ-type orbitals. A comparison between the unoccupied density of states and XAS data has been made because both provide a description of the molecular orbitals above the Fermi level. We have observed the presence of shallow-lying σ orbitals in twisted molecules and have obtained an almost linear relationship between the abundance of these orbitals and the degree of molecular twist. Additionally, we have shown the possibility of an experimentally viable stereoisomerism in PTCDI-C3.