PAHs are made up of connected hexagonal carbon rings. They are useful to produce materials for new superconductors on the molecular scale. Also they are of astrophysical interest as a substantial fraction of the interstellar carbon is believed to be locked up in these very stable molecules. For all of these applications, a fundamental understanding of the electron distribution and its relation with topological features of PAHs is important.

The exact way in which carbon rings are attached – the topology of the molecule – seemed to play a major role here, but it was unclear how. With advanced spectroscopic experiments, scientists from Radboud University and University of Amsterdam, has shown that the topology determines how the electron distribution is linked to vibrational dynamics of the carbon skeleton.

Zigag and armchair structures

With the FELIX free electron laser at Radboud University, scientists determined the vibrational spectra of two positively charged PAH ions that consist  of five connected hexagons. Pentacene has a zigzag edge structure (Figure 1, right and Figure 2, top). The edge structure of picene is commonly referred to as armchair (Figure 1, left and Figure 2, bottom). Unexpectedly, a comparison of the IR spectra of the two PAH ions revealed  large intensity differences for the vibrations  of the two PAHs.


Koolstofringen-onderzoek-Radboud-UniversiteitFigure 1: example of an armchair-edge, and on the right a schematic example of a zigzag-edge.. Image: Radboud Universiteit

Koolstofringen-onderzoek-Radboud-UniversiteitFigure 2: pentacene with a zigzag edge structure (top) and picene with an armchair edge structure (bottom).

Coupling of electronic and nuclear motion

The (among molecular physicists) well known Born-Oppenheimer approximation constitutes a strict separation between electronic and nuclear motion. However, the described differences in the vibrational spectra of pentacene and picene show the contrary. During the first part of a vibration, one side  of the molecule has a higher electron density than the other half side.

During the second part of the vibration, the situation reverses: the electron density shifts to that side. The situation is comparable to a periodically tilting container filled with water, causing the water to slosh from one side to the other. The ‘sloshing’ of the electron density – the electron flux - enhances the absorption of infrared light at the specific frequency of the vibrating carbon atoms.

Electron flux

The current publication shows that the electron density sloshing in picene  is enhanced, whereas it largely cancels in pentacene. Calculations suggest  that this is not only the case  for picene and pentacene, but that it is an intrinsic property of PAHs with zigzag and armchair edge structures. This provides valuable insight into the electronic properties of these two classes of PAH (and graphene) topologies.