How an azobenzene molecule is switched by light

HIMS chemists obtain fundamental insight in operation of popular molecular switch

7 January 2015

The relatively simple azobenzene molecule enables the tuning of material properties by using light. Although azobenzene has ample applications in areas ranging from photopharmacology to molecular nanotechnology, its operational mechanism has remained rather elusive. Researchers from the Molecular Photonics group of the Van 't Hoff Institute for Molecular Sciences (HIMS) have now unravelled the azobenzene switching mechanism.

Their research, performed in cooperation with the University of Bologna, provides the first experimental benchmark for theoretical calculations on isolated azobenzene light absorption. The results were published 6 januari in Nature Communications.

Switchable properties 

Changing materials properties by means of external stimuli is an important spearhead in current materials research. It offers, for example, the possibility to release or activate drugs at a preferred time and location. Chemists also work on materials with switchable catalytic or optical properties.

The switching operation can, amongst others, be triggered by temperature, electricity or light. The use of light has many advantages. It is safe (non-invasive), selective (targeting only one molecular component in the material) and it enables very fast and very local switching.

Rotation along a double bond

Since many years the azobenzene molecule has been one of the enabling components in 'light switchable' materials, due to its remarkable property that its structure can be changed upon interaction with light. 

azobenzene cis-trans isomerisation (English)

The azobenzene molecule consists of two identical parts that can adopt different positions with respect to each other, leading to the so-called trans- and cis-isomers. The nice thing about azobenzene is that these two isomers can reversibly be transformed into each other by light. This proceeds in a process referred to as trans-cis isomerisation, in which the two parts appear to rotate along the central double bond (shown in blue).

The conformational change affects not only the spatial structure but also the electronic configuration of azobenzene and thus its chemical activity. This change is reversible: a second light pulse can bring the azobenzene molecule back to its original conformation.

High-resolution laser spectroscopy

Despite the huge popularity of azobenzene as a molecular light switch, the mechanism behind the structural changes was not at all clear. One of the main problems in this respect was the impossibility of studying an azobenzene molecule under isolated conditions. This is particularly important as many experiments indicate that the properties of the molecule depend to a critical extent on its environment.

Scientists of the Molecular Photonics group at the Van 't Hoff Institute for Molecular Sciences (University of Amsterdam) now report the first study of isolated azobenzene molecules. Using advanced multi-colour laser spectroscopic techniques with unprecedented sensitivity they were able to determine the forces that act on the atoms after the molecule has absorbed light.

Their findings have led to a thorough understanding of the crucial role of molecular vibrations in enabling the light-induced structural changes of azobenzene.

prof. dr. W.J. (Wybren Jan) Buma

Professor Wybren Jan Buma. Photo by Jan Willem Steenmeijer.

Forbidden transitions

Research leader Wybren Jan Buma, professor in Molecular Spectroscopy, explains: "Absorption of light brings molecules into a so-called electronically excited state. In this process the laws of quantum mechanics determine which states can be excited easily and which states cannot. For symmetrical molecules such as azobenzene this means that certain transitions are in fact completely 'forbidden'."

"This leads to the rather remarkable conclusion that bringing azobenzene into the ‘switchable’ state by light absorption is in fact not allowed. It turns out that the molecule can only be brought in the ‘switchable’ state when it is vibrating. In that case the molecule loses its symmetry because of the moving atoms, Buma explains, and the ‘switchable’ state is no longer 'forbidden'.”

Experimental benchmark

The advanced laser spectroscopy of the Amsterdam researchers enabled them to 'zoom in' on vibrating azobenzene molecules, to determine how their vibration enables light absorption and to establish how the molecular structure changes upon light absorption. With this Buma and co-workers provide an experimental benchmark for theoretical calculations regarding azobenzene light absorption.

Torsion out of plane

Azobenzene vibrating out-of-plane (torsion).

Buma: "We were able to establish that light absorption takes place most effectively in the case of a torsional vibration of the double bond between the nitrogen atoms. This is an out-of-plane distortion. On the other hand we also observed inversion of the nitrogen atoms, which is an in-plane distortion. In the past researchers have published all kinds of scenarios for the trans-cis isomerisation pathway, which were always somewhere in between these two extremes."

Buma adds that in the past many experimental results were accompanied by theoretical studies that appeared to support the presented interpretations. "Any real experimental benchmark in the form of studies on isolated molecules was absent. Our data now provide this benchmark and thereby define the 'playing field' in these matters."

Inversion in plane

Azobenzene vibrating in-plane (inversion).

According to Buma a lot of puzzling results now fall into place. For instance, it is now possible to understand the results of various experiments on non-isolated azobenzenes that until now seemed to lead to contradictory results. 

Better switches

The research was part of the PhD research of Eric Tan, who graduated 25 November last year. Tan determined that isomerisation of isolated azobenzene molecules proceeds an order of magnitude slower than what was previously assumed. This means that the influence of the molecular environment is much stronger than thought earlier.

Tan: "We now are able to understand how to control the azobenzene switch through its environment. We know the influence of the solvent, we can predict the effect of adding molecular groups to the azobenzene, and we understand the effect of restricting the molecular movement - which is the case in a solid. All this enables the rational development of better molecular switches tuned to specific applications”.

Publication

Eric M.M. Tan, Saeed Amirjalayer, Szymon Smolarek, Alexander Vdovin, Francesco Zerbetto & Wybren Jan Buma: Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy, Nat. Commun. 6, 5860 (2015), DOI: 10.1038/ncomms6860 

The research was funded bij the Netherlands Organisation for Scientific Research NWO. 

The United Nations has proclaimed 2015 as the International Year of Light and Light-based Technologies (IYL 2015).The research above illustrates the prominent role of light in scientific research. More information on IYL 2015 can be found at the Dutch and International websites.

Logo International Year of Light

Published by  HIMS