Electronically Excited States of Closed-Shell, Cyano-Functionalized Polycyclic Aromatic Hydrocarbon Anions

Abstract

Few anions exhibit electronically excited states, and, if they do, the one or two possible excitations typically transpire beyond the visible spectrum into the near-infrared. These few, red-shifted electronic absorption features make anions tantalizing candidates as carriers of the diffuse interstellar bands (DIBs), a series of mostly unknown, astronomically ubiquitous absorption features documented for over a century. The recent interstellar detection of benzonitrile implies that cyano-functionalized polycyclic aromatic hydrocarbon (PAH) anions may be present in space. The presently reported quantum chemical work explores the electronic properties of deprotonated benzene, naphthalene, and anthracene anions functionalized with a single cyano group. Both the absorption and emission properties of the electronically excited states are explored. The findings show that the larger anions absorption and emission energies possess both valence and dipole bound excitations in the 450–900 nm range with oscillator strengths for both types of >1×10−4. The valence and dipole bound excited state transitions will produce slightly altered substructure from one another making them appear to originate with different molecules. The known interstellar presence of related molecules, the two differing natures of the excited states for each, and the wavelength range of peaks for these cyano-functionalized PAH anions are coincident with DIB properties. Finally, the methods utilized appear to be able to predict the presence of dipole-bound excited states to within a 1.0 meV window relative to the electron binding energy.

Electronically Excited States of Potential Interstellar, Anionic Building Blocks for Astrobiological Nucleic Acids

“…Specifically, the 2hydroxynaphthalene anion 2 is 0.16 eV lower in energy than the 1-hydroxynaphthalene anion 1, and the 2-ethynylnaphthalene anion 2 is 0.01 eV is lower in energy than 1-ethynylnaphthalene anion 1. This is, again, consistent with the previous work on cyanonaphthalenes (Santaloci and Fortenberry, 2021). The reason for such behavior is that the molecule is slightly enlongated in the 2 position which increases the total spatial extent for the orbitals.…”

“…Tables 9, 10 display the relative and excitations energies for the ethynylanthracene derivatives. The relative energies follow the same type of trend reported in the previous cyanoanthracene study (Santaloci and Fortenberry, 2021) The least stable anion is the most stable radical and vice versa. Unlike hydroxyanthracene, the radical and anion do not have a significant energy difference between their isomers.…”

“…The ethynylnaphthalene radicals act differently than the hydroxynaphthalene isomers. The least stable ethynyl radicals are the most stable ethynylnaphthalene anions, again like with the −CN functionalization (Santaloci and Fortenberry, 2021), implying a trend for when the functional group is a strong electron withdrawing group. Furthermore, 1-ethynylnaphthalene radical 7 and 2-ethynylnaphthalene radical 6 are the most stable.…”

“…The augmented basis functions are placed at the COC, and the anion geometries are utilized to mimic absorption behavior because the closed-shell anion is being excited. A past study on cyano functionalized PAHs shows that the EOM-CCSD computations with the apVDZ+6s6p2d basis approach the Hartree-Fock Limit, but this level of theory still has an error range of roughly 1 meV when comparing between computed values for the DBXS energy and the eBE (Bassett and Fortenberry, 2017;Santaloci and Fortenberry, 2021). The eBE is the maximum amount of energy than can be absorbed by the anion before the electron leaves the system entirely.…”

“…However, such linear combinations of atomic orbitals to make molecular orbitals (LCAO-MO) approaches are too computationally expensive (Fortenberry and Crawford, 2011a). An alternate approach is to add even-tempered diffuse functions onto a dummy atom and keep the LCAO-MO approach for the valence orbital description (Mach et al, 2010;Fortenberry and Crawford, 2011b;Santaloci and Fortenberry, 2021). For the aug-cc-pVDZ basis set on the real atoms, this method lowers the number of basis functions by 48%, which, in return, lowers the computational cost significantly, without affecting the accuracy of the results (Fortenberry and Crawford, 2011b;Theis et al, 2015a;Morgan and Fortenberry, 2015).…”