Electronically Excited States of Anisotropically Extended Singly-Deprotonated PAH Anions

Abstract

Polycyclic aromatic hydrocarbons (PAHs) play a significant role in the chemistry of the interstellar medium (ISM) as well as in hydrocarbon combustion. These molecules can have high levels of diversity with the inclusion of heteroatoms and the addition or removal of hydrogens to form charged or radical species. There is an abundance of data on the cationic forms of these molecules, but there have been many fewer studies on the anionic species. The present study focuses on the anionic forms of deprotonated PAHs. It has been shown in previous work that PAHs containing nitrogen heteroatoms (PANHs) have the ability to form valence excited states giving anions electronic absorption features. This work analyzes how the isoelectronic pure PAHs behave under similar structural constructions. Singly deprotonated forms of benzene, naphthalene, anthracene, and tetracene classes are examined. None of the neutral-radicals possess dipole moments large enough to support dipole-bound excited states in their corresponding closed-shell anions. Even though the PANH anion derivatives support valence excited states for three-ringed structures, it is not until four-ringed structures of the pure PAH anion derivatives that valence excited states are exhibited. However, anisotropically extended PAHs larger than tetracene will likely exhibit valence excited states. The relative energies for the anion isomers are very small for all of the systems in this study.

Excited State Trends in Bidirectionally Expanded Closed-Shell PAH and PANH Anions

“…The latter case is referred to as "apVDZ +4s" and has shown use as an efficient means of describing candidate dipole-bound excited states or, more accurately stated, near-continuum states whether dipole-bound or true continuum states. 1,2,27 These efficiencies are needed for molecules as large as the analyzed PAHs. Even so, the largest molecules studied here, such as olympicene, overtax the hardware requiring that these "+4s" orbitals be placed on the smaller cc-pVDZ basis set to create the utilized "pVDZ+4s" set.…”

“…The higher-symmetry H1 isomer is not the lowest energy isomer in either case, but it induces the largest dipole moment at 1.06 D. The anionic relative energies vary only by 0.066 eV, an order of magnitude less than those in tetracene at 0.276 eV 2 and less than those in pyrenidene at 0.498 eV. 1 Table 2 contains the vertical excitation energies for the even-numbered carbon atom PAHs.…”

“…The valence excitation energies are just above those of the eBE determined from the coincident nearcontinuum states, and the next-larger tetracene anion derivative isomers exhibit valence excited states. 2 Simply bending anthracene into phenanthrene appears to change this notably.…”

“…The analysis of such deprotonated molecules has been done with much success for small molecules 14,15,18,19,26,27 as well as PAHs and their nitrogenated, heteroatom counterparts, PANHs. 1,2,26 PAHs are well-studied chemical compounds that have applications across the chemical spectrum. They are known carcinogens, hold promise in materials chemistry, and are likely of great significance in the chemistry of astrophysical environments.…”

Interstellar dehydrogenated PAH anions: vibrational spectra

“…However, as a preliminary study, we have limited our calculation to molecules with the lowest possible spin-multiplicity. This is because the sample molecules that we have considered exhibit the lowest energy with the lowest possible spinmultiplicity corresponding to the most stable configuration to endure in the interstellar domain (Bauschlicher & Ricca 2013;Theis et al 2015;Fortenberry et al 2016). For example, for C24H −1 11 , lowest lying state is a singlet state.…”