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.
Highlights
Excited states of anions are few and far between, but they are applicable in many areas of chemistry and materials science ranging from solar energy harvesting to astrochemical observation [1,2,3,4,5]
The 1 A00 /B1 state is created from the highest occupied molecular orbital (HOMO) exciting into a π ∗ valence, particle-in-a-box (PIB) lowest unoccupied molecular orbital (LUMO)
Once a deprotonated, closed-shell cyano-functionalized polycyclic aromatic hydrocarbon (PAH) anion contains three rings, it will exhibit at least one electronically excited valence state. This is similar to inclusion of nitrogen heteroatoms in related PANH anions where similar behavior was observed for three rings [63,65]
Summary
Excited states of anions are few and far between, but they are applicable in many areas of chemistry and materials science ranging from solar energy harvesting to astrochemical observation [1,2,3,4,5]. CH2 CN− and CH2 CHO− were the first experimental examples of organic anions documented with dipole bound excited states [9,19,20,21] Excitations into these states involve promotion of a valence electron into a highly diffuse, Rydberg-like orbital and transpire in the visible and near-infrared region of the electromagnetic spectrum [22]. These properties led Sarre to examine the A 1 B1 ← X 1 A0 dipole bound excitation of CH2 CN− at 8037.78 Å for its coincidence with an astronomical absorption peak at 8037.8 ± 15 Å [23,24]. The additional cyano group should affect photophysical properties more so than simple inclusion of nitrogen heteroatoms in the PAH structure and open the door for new photophysics of anions in various astrophysical environments or for other applications of anions
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