THE PEDAL MOTIONS IN CRYSTALS

Pedal Motion in Crystals

We are very familiar with the movements of pedals in bicycles which helps in rotational movements of tires and thereby the linear movement of the vehicle. Can you imagine pedal movements in crystals?

It is observed in the molecules like stilbenes, azobenzenes, bis-(triarylmethyl)peroxide, spiropyrans, polyenes, etc. undergo pedal motion in their crystals at suitable conditions. Research scholars who were working on derivatives of these compounds or related compounds can study these effects if they have the required instrumental facilities.

The pedal effect is observed due to the disorientation of the molecules in the crystal lattice. In these disordered sites, the molecules can adopt two conformations which are dynamic. The interconversion between the two conformers in the crystals can approximate to a 2-fold rotation about the longest axis of the molecules; for example, a pair of benzene rings moves like the pedals of a bicycle. The conformational interconversions take place if the conformers have a low energy barrier and resulted in pedal motion. If the energy barrier is high, the pedal motion does not show any disordering in the crystal structure due to the insignificant population of the minor conformer. The pedal motion is difficult to detect and, therefore, easy to overlook and occur only in crystals that have an orientational disorder or a large void around the molecules.

The pedal motion is a key process of photoreaction such as photodimerization, photoisomerization, and photochromism, in crystals that have wide technological applications. The isomerization in butadienes through the pedal motion is reminiscent of the photoisomerization in the visual pigment rhodopsin. The photocycle of photoactive yellow protein (PYP) emphasizes the potential importance of the pedal motion in proteins. It is also suggested that molecular motions in crystalline solids have potential uses in fabricating nanoscale devices.

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The pedal motion can be identified by NMR spectroscopy, measurements of heat capacity, X-ray diffraction analyses at different temperatures and molecular mechanics calculations.

For more read

1. Harada J and Ogawa K, Pedal motion in crystals, Chem. Soc. Rev., 2009, 38, 2244–2252. org/10.1039/B813850H

2. Harada J and Ogawa K, Invisible but Common Motion in Organic Crystals:  A Pedal Motion in Stilbenes and Azobenzenes, J. Am. Chem. Soc. 2001, 123, 44, 10884-10888. org/10.1021/ja011197d

3. Sharma M K and Bharadwaj P K, A Dynamic Open Framework Exhibiting Guest- and/or Temperature-Induced Bicycle-Pedal Motion in Single-Crystal to Single-Crystal Transformation, Inorg. Chem. 2011, 50, 1889–1897. 10.1021/ic102305v

4. Hutchins K M, et. al., Unlocking pedal motion of the azo group: three- and unexpected eight-component hydrogen-bonded assemblies in co-crystals based on isosteric resorcinols, Supramolecular Chemistry, 2018, 30(5-6), 533-539. org/10.1080/10610278.2018.1435884

5. Quentin J, et. al., Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals, Molecules, 2020, 25, 907; 10.3390/molecules25040907

6. Dale B L, et. al., Investigation of structure and dynamics in a photochromic molecular crystal by NMR crystallography, MRC, 2019, 576(5), 230-242. org/10.1002/mrc.4805

7. Vande Velde C M L, Thermodynamic parameters of the pedal motion in the crystal structures of two bromomethylated azobenzenes, Cryst Eng Comm, 2015, 17, 5751-5756. 10.1039/c5ce00905g