Theoretical electron affinities of PAHs and electronic absorption spectra of their mono-anions

Abstract

Abstract. We present theoretical electron affinities, calculated as total energy differences, for a large sample of polycyclic aromatic hydrocarbons (PAHs), ranging in size from azulene (C 10 H 8 ) to dicoronylene (C 48 H 20 ). For 20 out of 22 molecules under study we obtained electron affinity values in the range 0.4-2.0 eV, showing them to be able to accept an additional electron in their LUMO π orbital. For the mono-anions we computed the absolute photo-absorption cross-sections up to the vacuum ultraviolet (VUV) using an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which has already been proven to yield accurate results for neutral and cationic PAHs. Comparison with available experimental data hints that this is the case for mono-anions as well. We find that PAH anions, like their parent molecules and the corresponding cations, display strong π * ← π electronic transitions in the UV. The present results provide a quantitative foundation to estimate the fraction of specific PAHs which can be singly negatively charged in various interstellar environments, to simulate their photophysics in detail and to evaluate their contribution to the interstellar extinction curve.

Theoretical evaluation of PAH dication properties

“…In our previous studies (Malloci et al 2004(Malloci et al , 2005, we calculated the photo-absorption cross-section σ(E) of neutral and singlyionised PAHs using the real-time real-space TD-DFT implementation of the octopus computer code (Marques et al 2003). …”

“…Even if PAHs and related species containing from less than 50 to more than 200 carbon atoms are expected to be present in the ISM (Boulanger 1999), we restricted ourselves here to species containing up to a maximum of 66 carbon atoms, since computational costs scale steeply with dimensions. Following our previous work on neutral, cationic, and anionic PAHs (Malloci et al 2004(Malloci et al , 2005, we computed the absolute photo-absorption cross-section for the doubly-ionised species. For these same molecules we also evaluated the IR spectral properties, extending the sample of specific PAH ++ for which they were previously available (Ellinger et al 1999;Bauschlicher & Bakes 2000;Pauzat & Ellinger 2002).…”

Gas phase adiabatic electron affinities of cyclopenta-fused polycyclic aromatic hydrocarbons

“…strong oxidants. In addition, the AEAs of CP-PAHs warrant further investigations of the properties of their radical anions in connection with their possible occurrence in the interstellar medium [6] and as an aid for the identification of bio-active representatives [7,11,12]. …”

“…This stems from their identity as sub-structures of fullerenes and carbon nanotubes [1,2], their potential to show unusual (photo)physical properties, e.g. facile (consecutive) one-electron reduction processes [3], anomalous fluorescence [4] and deviant aromatic character [5], and, their alleged presence (in charged form) in the interstellar medium [6]. In addition, CP-PAHs were found to possess bio-activity [7].…”

Polycyclic Aromatic Hydrocarbons and the Extinction Curve

“…Fig. 2), Malloci and co-workers (Malloci et al 2005(Malloci et al , 2007a(Malloci et al , 2007b proceeded to compute photo-absorption spectra of a large number of PAHs over the whole wavelength range relevant for interstellar extinction, and made their results publicly available on a web-based database 1 . This made possible a systematic study of the effect of hydrogenation and charge state.…”