First images of molecules in different charge states


Rebecca Pool

Wednesday, July 17, 2019 - 13:45
Image: Changing molecular structures of pentacene and TCNQ as they gain and lose electrons. [IBM Research]
Using atomic force microscopy, IBM researchers and collaborators from CiQUS, at the Universidade de Santiago de Compostela, and ExxonMobil, have resolved the structural changes of individual molecules upon charging 'with unprecedented resolution'.
Images reveals molecules changing shape as they gain and lose electrons.
Molecules analysed include including porphine, the parent compound of porphyrins, and as Leo Gross, Research Staff Member at the IBM Research - Zurich Laboratory, highlights, the latest results unlock some of the mysteries of molecular charge-function relationships.
As Gross writes in the IBM Research blog, energy conversion and transport in living systems relies on charging and discharging molecules.
Most important in this aspect is the family of porphyrins, comprising chlorophyll and hemoglobin, as the charge transitions of these molecules are essential for life. 
According to Gross, when a molecule gets charged, changes in both the structure and function of the molecule take place, and resolving structural changes can only improve understanding of these fundamental relationships.
The latest results build on developments from around a decade ago, when Gross and colleagues first improved AFM resolution by functionalising the tip of a low temperature AFM with a single carbon monoxide molecule.
In the years that followed, the researchers also used this method to control the charge state of molecules, examining charge transfer between weakly coupled pentacene molecules on insulating films with single-electron sensitivity and control over the atomistic details.
In their latest experiments, Gross and colleagues have combined these results, imaging molecules with ultrahigh-resolution using CO tips while at the same time controlling the charge.
To this end, the researchers biased an AFM probe tip with a voltage to charge and discharge molecules such as azobenzene and porphine from cations to anions.
As part of this work, the researchers showed that changes in adsorption geometry are resolved by investigating the well-known molecular switch, azobenzene.
The two planar groups of the molecule were parallel when the molecule was neutral but the researchers discovered that these groups tilt with respect to each other when an electron is attached, charging the molecule negatively.
The researchers then focused on charge-induced changes in the strength of individual bonds.
As Gross points out, these are small effects and he and colleagues chose a model compound (pentacene) to see if these changes could be resolved.
Analyses showed that the researchers could manipulate this model molecule in four different charge states, from positive to doubly negative.
“We resolved which bonds within the molecule grow stronger and which grow weaker when we change the charge,” says Gross. “On this model system we learned how the images taken in different charge states can be compared.”
Imaging molecules in different charge states. [IBM Research]
The researchers then applied their method to the molecule, TCNQ, which is frequently used as a charge acceptor and resolves both out-of-plane distortions and changes in bond strength as a function of charge state.
As Gross says: “Surprisingly, this molecule stands up when it’s neutral and lays down on the surface when negative or doubly negative.”
“The increased aromaticity of the central molecular ring from the negative to the doubly negative charge state [was also] observed,” he adds.
And in a final analyses, Gross and colleagues investigated, porphine, the parent compound of chlorophyll and hemoglobin.
According to the researcher, how these molecules change their conjugation pathway is controversial and it is highly important to understand their functions.
“For the first time, we could visualize changes in conjugation pathway and aromaticity of porphine in three different charge states,” he says. “With our new technique, we can increase understanding of how charge alters structure and function of molecules, which vital in so many ways, such as to photoconversion and energy transport in living organisms.”
Research is published in Science.
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