Electron affinities of polycyclic aromatic hydrocarbons and electronic spectra of their mono-anions

Polycyclic aromatic hydrocarbons (PAHs) are a large class of conjugated $\pi$-electron systems of fundamental importance in many branches of chemistry. Interest in PAHs ranges from synthetic, medical and biological chemistry to environmental and interstellar chemistry. However, while ionization potentials are well known and a large number of PAH cations are well characterized from a spectroscopic point of view, electron affinities are largely unknown for many PAH systems, as well as the ground and excited state properties of the corresponding anions. We here present the electron affinities obtained in the framework of Density Functional Theory (DFT) via total energy differences for ten PAHs, ranging in size from anthracene (C$_{14}$H$_{10}$), to dicoronylene (C$_{48}$H$_{20}$); the electron affinity values obtained lie in the range 0.5-2.0~eV showing the molecules under study to be able to accept an additional electron in their LUMO $\pi^\star$ orbital. For the mono-anions we obtained the electronic absorption spectra using a TDDFT implementation in real time and real space, an approach which was proven to yield accurate results for conjugated molecules and for PAHs in particular. Concerning the low-lying excited states of $\pi^\star\gets\pi$ character occurring in the near-IR, visible and near-UV spectral range, the computed spectra are in good agreement with the available experimental data in glassy organic solids [4], yielding vertical excitation energies accurate to within a few tenths of eV, and the corresponding oscillator strengths within experimental errors, which are indeed the typical accuracy currently achievable by TDDFT. We find that PAH anions, like their parent molecules and the corresponding cations, display strong $\pi^*->\pi$ electronic transitions in the UV, a piece of information which is particularly useful since a limited amount of laboratory data is available concerning the absorption properties of PAH ions in this wavelength range