  Functionalized polcyclic aromatic hydrocarbons (PAHs) are an interesting
class of molecules in which the electronic state of the graphene-like
hydrocarbon part is tuned by the functional group. Searching for new types of
donor and acceptor molecules, a set of new PAHs has recently been investigated
experimentally using ultraviolet photoelectron spectroscopy (UPS). In this
work, the electronic structure of the PAHs is studied numerically with the help
of B3LYP hybrid density functionals. Using the DELTA-SCF method, electron
binding energies have been determined which affirm, specify and complement the
UPS data. Symmetry properties of molecular orbitals are analyzed for a
categorization and an estimate of the related signal strength. While SIGMA-like
orbitals are difficult to detect in UPS spectra of condensed film, calculation
provides a detailed insight into the hidden parts of the electronic structure
of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**)
was used to calculate electron affinity and LUMO eigenvalues. The calculated
electron affinity (EA) provides a classification of the donor/acceptor
properties of the studied molecules. Coronene-hexaone shows a high EA,
comparable to TCNQ, which is a well-known classical acceptor. Calculated
HOMO-LUMO gaps using the related eigenvalues have a good agreement with the
experimental lowest excitation energies. TD-DFT also accurately predicts the
measured optical gap.
