Size and shape effects in aligned PAHs

In the EUROPAH, we are approaching the end of the project. Some ESRs will continue their PhD afterwards, while others (like me) will defend their theses in 2020. So, time to summarize the work that has been done.

Together with some colleagues from LCAR and LCPQ labs in Toulouse, we have been mostly working on a special class of PAH species called polyacenes.

Polyacenes are molecules that are composed of aligned carbonaceous hexagons (benzene-like) with hydrogens attached to the edge carbons.

Sometimes they are also called linear PAHs (see picture below). Polyacenes are of interest for astronomy due to the assumed abundance of PAHs in the interstellar medium. Moreover, crystal properties of some polyacenes led to the development of modern OLED (Organic Light-Emitting Diod) screens. Notably, naphthalene (the smallest polyacene) is well known as the main ingredient of mothballs.

Tetracene (18 carbon and 12 hydrogen atoms) is a polyacene with 4 carbonaceous hexagons and with the so-called zigzag-shaped edges (see schematic drawing at the bottom of the picture).

Here in Toulouse, we have developed a module within the deMon-Nano code that allows to simulate the motion of molecules following an excitation by light (e.g. laser). Dynamics of the atomic nucleus is simulated as in classical mechanics, i.e. via the Newton’s second law F = ma, and the motion of electrons is described on a quantum level of theory by the time-dependent Schrödinger equation. Such a mixed quantum-classical approach is computationally cheaper than a fully quantum dynamics and it is usually a good approximation since nuclei are heavy and slowly moving particles compared to electrons.

First, polyacenes with 2 to 7 carbonaceous hexagons have been investigated and an interesting effect has been observed, namely that the computed relaxation times show an order-of-magnitude alternation with an increasing size of the molecule. Our interpretation of the underlying mechanisms has been published in the Physical Chemistry Chemical Physics journal and included in the themed collection 2019 PCCP HOT Articles by the Editorial Board.

Inspired by some laboratory experiments, we decided to compare the dynamics of two PAHs that have the same number of carbon and hydrogen atoms, but which are arranged in a slightly different way. The tetracene (see above) and chrysene (see below) isomers have been chosen as a test case due to certain similarities in their absorption spectra and notably different shapes of edges. We have observed an ultrafast electronic relaxation in chrysene and compared it with tetracene. More details can be found in our article in The Journal of Chemical Physics.

Chrysene (18 carbon and 12 hydrogen atoms) is an isomer of tetracene, but it is not a polyacene. It has the so-called armchair-shaped edges (see schematic drawing at the bottom of the picture).

Featured image reproduced from our article by permission of the PCCP Owner Societies.

This post was written by Evgeny Posenitskiy, who was pursuing his PhD at the Université Paul Sabatier in Toulouse.