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<\/p>\nFiring femtosecond x-ray pulses at azobenzene has helped chemists study how its structure changes as it flips between isomers in solution, finding that a twisting of the carbon\u2013nitrogen bond initiates the reaction.1 The researchers behind the work say that this is the first time that their technique has been used on molecules made up of only lighter atoms. The team also suggests that their technique could be used to study other fast, light-driven reactions.<\/p>\n
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Hyotcherl Ihee<\/a> at the Korea Advanced Institute of Science and Technology describes azobenzene as \u2018the textbook molecular switch\u2019. Exposing the molecule to light causes the central nitrogen\u2013nitrogen \u00ad\u00addouble bond to rapidly switch from a trans to a cis configuration. Light-responsive materials, molecular machines and certain drugs use this feature as the isomers of azo-containing compounds often have different physical properties.<\/p>\n Despite their prevalence, the mechanism of how molecules like azobenzene change from one isomer to the other remains contentious. Researchers have suggested various theories, including rotation, inversion and a hula twist (the simultaneous rotation of the single C\u2013N bond and double N=N bond), though they have not reached a conclusion on which theory is correct.<\/p>\n \u2018The speed of the transition is not the obstacle,\u2019 says Ihee. He explains that current spectroscopic experiments mean that researchers can only infer the structure of intermediates, rather than study them directly.<\/p>\n Ihee\u2019s team has now used a technique known as time-resolved x-ray liquidography to overcome this challenge. This method involves exciting azobenzene with an ultraviolet laser, before firing femtosecond x-ray pulses from an x-ray free-electron laser at the molecule at different time points. Atoms in the molecule scatter the x-rays, which the team can then use to determine the distance between atoms and build up a picture of the molecule\u2019s structure. \u2018The data [collected] allowed us to follow, step by step, how the shape of the molecule changes on its way from trans to cis,\u2019 says Ihee.<\/p>\n \u2018We found that the reaction does not begin with a twist around the central N=N double bond [as previously suggested],\u2019 says Ihee. Instead, he describes how their analysis shows that\u2018a pedal-like motion of the azo group\u2019 initiates the reaction, changing the angle between the phenyl rings and azo group, before the N\u2013N bond rotates. This mechanism challenges that idea that the reaction happens simply through rotation or inversion.<\/p>\n \u2018This direct structural view of the [mechanism] will be very important for future developments of theoretical tools able to simulate this key class of reactions,\u2019 says Kristoffer Haldrup<\/a> at the Technical University of Denmark, who has previously used x-ray scattering experiments to study chemical dynamics. However, Haldrup says that he would have liked to see a more direct comparison between the researcher\u2019s final structural results and the data used for the analysis.<\/p>\n Aside from being the archetype of photochemistry reactions, Ihee notes that studying azobenzene \u2018is also a demanding test case for [x-ray liquidography]\u2019. Carbon, nitrogen and hydrogen \u2013 the atoms found in azobenzene \u2013 scatter x-rays weakly, meaning that the signal generated is small. As such, the technique has previously only worked for molecules containing heavy atoms, like metals.<\/p>\n