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

Area of Science:

• Physical Chemistry
• Astrochemistry
• Quantum Chemistry

Background:

• Certain anions possess intrinsic excited states, independent of external influences.
• These states arise from the stabilization of valence orbitals, where electron binding energy exceeds valence excitation energy.
• Closed-shell anions are prime candidates due to their inherent stability from spin-paired ground states.

Purpose of the Study:

• To determine the critical structural features in deprotonated polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing PAHs (PANHs) that lead to valence excited states.
• To establish a general trend for the occurrence of these excited states in PAHs/PANHs of varying sizes and dimensionalities.

Main Methods:

• Theoretical analysis of electronic structure and stability of closed-shell anions.
• Investigation of valence accepting orbital stabilization and electron binding energies.
• Establishing a correlation between molecular size (number of six-membered rings) and the onset of valence excited states.

Main Results:

• A general trend for the emergence of valence excited states in PAHs/PANHs is established, applicable to various sizes and dimensions.
• Valence excited states are present in PAHs/PANHs once they comprise seven six-membered rings.
• For most PAH/PANH classes, this critical number of rings is closer to four.

Conclusions:

• Deprotonated, closed-shell PAHs and PANHs can exhibit intrinsic valence excited states.
• The size and structure of PAHs/PANHs dictate the presence of these excited states, with a general threshold identified.
• Despite potentially weak absorption, the abundance of PAHs in astronomical environments suggests their significant contribution to observed spectra.