Blue-light molecular motor: FTIR not seeing red

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  • Published: Jun 1, 2016
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Blue-light molecular motor: FTIR not seeing red

Engendering cis trans changes

Shining blue light on a plate-like crystal suspended in water makes it continuously alternate between bending and stretching, acting as a light-controlled molecular motor. (Credit: Angew Chem/Wiley)

Shining blue light on a plate-like crystal suspended in water makes it continuously alternate between bending and stretching, acting as a light-controlled molecular motor.

According to Tomonori Ikegami, Yoshiyuki Kageyama, Kazuma Obara and Sadamu Takeda writing in the journal Angewandte Chemie, "Building a bottom-up supramolecular system to perform continuously autonomous motions will pave the way for the next generation of biomimetic mechanical systems." They have designed and constructed a non-covalent assembly of oleic acid and an azobenzene derivative that represents the first example of a square-wave limit-cycle self-oscillatory molecular motor controlled by blue light and studied with Fourier transform infrared spectroscopy (FTIR) and other techniques.

"Mechanical self-oscillation is established by successively alternating photoisomerization processes and multi-stable phase transitions," the team reports. "These results offer a fundamental strategy for creating a supramolecular motor that works progressively under the operation of molecule-based machines."

Molecular motivation

Microscopic robots and nanoscopic machines have been a long-sought goal of science fiction, but developments in chemistry, materials science and engineering are bring such systems a step closer. One aspect of research into these schemes is to emulate the evolved molecular machinery of biological systems. However, one of the biggest obstacles that blocks the road to the nanotech dream is how to transmit the motion of individual molecules or molecular clusters into structured macroscopic motion that continues as long as the system is supplied with energy. In the journal Angewandte Chemie, Takeda and Kageyama and their colleagues at Hokkaido University in Sapporo, Japan, working with JST PRESTO in Kawaguchi, have now introduced such an autonomously oscillating system.

The critical component of the team's molecular motor are the two phenyl rings on the azobenzene moiety which are bound together by an azo bridge (–N=N–). The rings can be in either a cis or trans orientation relative to each other. However, in the flat crystal, almost all of the molecules, some 99.8 percent, are in the trans configuration. When they are illuminated with blue light, however, the azo bridges are excited leading to isomerization into the cis form. This subtle change leads to a shape shift in the molecules overall, disrupting its crystal structure and inducing strain in the system. As the proportion of cis isomers increases beyond a certain percentage the overall strain on the crystal structure is sufficiently high that there is a morphological change in the crystal and the platelet bends.

Flat flip

Intriguingly, continued blue light irradiation leads to the system flexing again and reverting to its original flat state and so on. The team explains this phenomenon in terms of the fact that the cis isomer itself can be excited by the blue light and undergo a different change in its structure back to the trans configuration, although the team is not yet clear as to how and why the population of cis isomers decreases. They suggest that one explanation might be that the cis form of the molecule can more efficiently absorb the light energy but it could simply be that the crystalline phase change changes the light absorption relative to the original platelet crystal morphology. Either way, once the proportion of cis isomer to trans isomer falls below a limiting level, the original crystal structure is reformed and the bowed platelet flips back to its flat, stretched out conformation. The cycle then begins again as long as the blue light keeps shining.

While the degree of bending is only determined by the dimensions of the individual crystals, the intensity of the light influences the rate of flipping. The stronger the irradiation, the faster the shape change. The team says that such a straightforward conversion of light energy into mechanical molecular motion might one day be exploited in the design of materials that mimic the movement of cellular components that might in turn be used in a future generation of microelectromechanical systems (MEMS) or their nanoscopic counterparts.

"My study theme is 'synthesizing life', the importance of the reported work is in the creation of a dissipative structure and is considered intrinsic to the nature of life," Kageyama told SpectroscopyNOW. "To date, many dissipative self-organizing systems have been constructed using reaction-diffusion systems. In contrast, as far as I know, this work is the first clear artificial example of dissipative self-organization using the cooperation of a molecular reaction and a supramolecular structure (phase)."

Related Links

Angew Chem Int Edn 2016, online: "Dissipative and Autonomous Square-Wave Self-Oscillation of a Macroscopic Hybrid Self-Assembly under Continuous Light Irradiation"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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