  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.
