Anisotropy and Size Effects on the Optical Spectra of Polycyclic Aromatic Hydrocarbons

TL;DR

This study investigates how size and symmetry influence the optical spectra of polycyclic aromatic hydrocarbons, revealing spectral shifts and new features due to molecular size and symmetry changes.

Contribution

It provides a systematic analysis of spectral modifications in medium-sized PAHs based on size and symmetry effects, enhancing understanding of their optical properties.

Findings

Size causes a red shift in spectra and redistributes oscillator strength.

Lower symmetry introduces new optical features and splits peaks.

Broken degeneracies activate dark states in the spectra.

Abstract

The electronic and optical properties of polycyclic aromatic hydrocarbons (PAHs) present a strong dependence on their size and geometry. We tackle this issue by analyzing the spectral features of two prototypical classes of PAHs, belonging to D6h and D2h symmetry point groups and related to coronene as multifunctional seed. While the size variation induces an overall red shift of the spectra and a redistribution of the oscillator strength between the main peaks, a lower molecular symmetry is responsible for the appearance of new optical features. Along with broken molecular orbital degeneracies, optical peaks split and dark states are activated in the low-energy part of the spectrum. Supported by a systematic analysis of the composition and the character of the optical transitions, our results contribute in shedding light to the mechanisms responsible for spectral modifications in the visible and near UV absorption bands of medium-size PAHs.