Shape of 'molecular graphene' determines electronic properties

Zigzag and armchair structures
With the FELIX free electron laser at Radboud University, Alvaro Galué 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) while 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.

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. This project was financed by NWO (the Netherlands Organisation for Scientific Research) and FOM (the Dutch Foundation for Fundamental Research on Matter).

Reference
Electron-flux infrared response to varying pi-bond topology in charged aromatic monomers, Héctor Álvaro Galué, Jos Oomens, Wybren Jan Buma, Britta Redlich, Nature Communications (2016); DOI: 10.1038/NCOMMS12633.