- The new technique can image the structural changes of individual molecules upon charging with unprecedented resolution.
- It can resolve minute changes in aromaticity, adsorption geometry, and bond-order relations.
Charging and discharging of a molecule is one of the most important aspects of life, as energy conversion and energy transportation depends on this process. It also plays a key role in organic photovoltaic devices and organic electronics.
Both the function and structure of a molecule change when it gets charged or discharged. Imaging these structural changes at high resolution could help researchers understand the mysterious relationship between a molecule’s charge and function, and how this charge-discharge process converts and transports energy.
Recently, an international team of researchers came up with a method that can image the structural changes of individual molecules upon charging with unprecedented resolution.
How Did They Do This?
Researchers used a 10-year-old methodology to capture the structure of molecules at atomic resolution. It involves functionalizing the tip of an atomic force microscope with one carbon monoxide molecule. A few years ago, they further improved this technique to control the molecule’s charge state.
The team combined these previous works to analyze charge states of different molecules. They charged the molecule from the tip of an atomic force microscope and then electrically isolated the molecule on a sodium chloride film to prevent the charge leakage.
Finally, they imaged the molecules with ultrahigh-resolution using functionalized carbon monoxide tip while controlling the charge.
Molecular Structures Of Different Compounds
In this study, researchers examined four model compounds with different properties and applications related to their charge-state transitions. They resolved minute changes in aromaticity, adsorption geometry, and bond-order relations.
First, they imaged azobenzene, a chemical compound containing two phenyl rings linked by N=N double bond. The two planar groups of the molecule were parallel when there was no charge. When an electron is attached (resulting in a negative charge), these planar groups tilt with respect to each other.
Next, they imaged pentacene, a polycyclic aromatic hydrocarbon composed of 5 linearly-fused benzene rings. They wanted to focus on charge-induced alterations in the strength of individual bonds.
This model compound can be manipulated in 4 different charge states – positive, neutral, negative, and doubly negative. Researchers were able to resolve which bonds within the molecule grew weaker and which grew stronger upon changing the charge. In this case, they learned how pictures captured in various charge states can be compared.
Credit: IBM Research
They then applied their methodology to an organic compound named Tetracyanoquinodimethane. It is often used as an electron acceptor and resolves bond strength and out-of-plane distortions as a charge state function.
Researchers found that the molecule stands up when it has no charge and lays down on the surface when it has a negative and double negative charge. Moreover, the high-resolution image clearly showed the increase in aromaticity of the central molecular ring [from negative to double negative charge state].
Finally, they imaged the most intriguing molecule: porphine. It is the parent compound of hemoglobin and chlorophyll. For the first time, researchers imaged alterations in aromaticity and conjugation pathway of porphine in 3 different charge states.
Overall, the study opens the avenue to investigate chemical structural changes of individual molecules for different charge states.