Researchers Chance upon ABBA Flaws in Conjugated Polymers

A team of researchers headed by Professor Giovanni Costantini from the University of Warwick has produced the first ever detailed pictures of the structure of conjugated polymers.

STM image showing how the detail of the structure of C14DPPF-F can be seen. The polymer backbones appear as bright rows and the alkyl side chains are seen as darker rows perpendicular to the backbones. (Image credit: University of Warwick)

Although these polymers have been in demand for their ability to conduct electricity, to date, they can be also depicted as highly camera shy since their structure could not be determined by any means. The new method enables scientists to not only determine their structure but also clearly observe it with their own eyes.

Conjugated polymers have the ability to conduct electricity since they are a chain of conjugated molecules where electrons can freely move owing to their overlapping electron p-orbitals. In effect, they are exceptional molecular wires. Furthermore, they are similar to semiconductor materials, since they have energy gaps, allowing them to be used for photovoltaic (organic solar cells) and electronic (plastic electronics) applications.

Usually, modern functional polymers are copolymers; in other words, they are formed of a (perfectly regular) series of different monomers. The order of these monomers plays a vital role in their optoelectronic characteristics that can be specifically impaired by errors in the way the monomers actually link up in a chain to make the polymer (so-called polymerization errors that occur when these materials are synthesized).

Yet, existing analytical techniques have not proved effective in detecting the type and precise position of these errors. Mass spectrometry cannot be applied for this purpose since shorter polymer chains are more probably ionized and hence are overrepresented in the spectra.

Costantini and his colleagues have put forward and executed a totally innovative strategy to overcome this basic analytical challenge. The fundamental concept is exceptionally simple, but simultaneously transformative: the polymers should be deposited onto a surface and imaged with the help of high-resolution scanning tunneling microscopy (STM).

This strategy effectively achieves one of the visionary hypotheses by Richard Feynman in his famous 1959 speech “There’s Plenty of Room at the Bottom,” where he stated that in the future, “it would be very easy to make an analysis of any complicated chemical substance; all one would have to do would be to look at it and see where the atoms are.”

Although the atomic-scale resolution of STM is perfect for this goal, the challenge is in the fact that first, it is important that the chains of polymer molecules are deposited without any damage in vacuum onto atomically clean and flat surfaces. The regular technique for achieving this is to heat the molecular material until it sublimes; however, for larger molecules similar to polymers, this effectively melts the structure to be investigated.

Thus, the researchers have selected an innovative technique in which a cloud of the polymer is sprayed into a vacuum chamber through a sequence of tiny openings, enabling a single ordered layer to be deposited onto a surface that completely represents the original polymer sample. Splendidly resolved pictures were produced by STM of these layers, evidently showing sub-monomer details of the conjugated polymers.

The scientists headed by Professor Giovanni Costantini from the University of Warwick with collaborators from Imperial, Cambridge, and Liverpool have reported the outcomes of the study in a paper titled “Sequencing conjugated polymers by eye” published in the Science Advances journal on June 15, 2018.

The exceptional detail of the high-resolution STM images of the structure of conjugated polymers taken by the researchers can help with quality control as well as fine-tuning of polymer design; they can also be used as something similar to an intellectual property (IP) passport photo for polymers. The researchers propose that such clear and precise images could assist synthetic researchers to accurately demonstrate the design they desire to legally protect by drastically enhancing the information available to support an application for IP protection.

In their study, the scientists have demonstrated the robustness of the innovative method by investigating the conjugated polymer: “Poly Tetradecyl-diketopyrrolopyrrole-furan-co-furan.” This is a conjugated polymer of the DPP-based class that at present exhibits certain ideal performances in optoelectronic devices.

This material is highly effective if its polymer chains are formed in an interspersed series of one large “A” monomer and a smaller “B” monomer. Yet, defects can occur during the production and hinder that perfect sequence, thus also impairing its fascinating conducting and light-gathering properties. Thus far, the hypotheses were that this largely happens when two of the larger “A” monomers directly join together in a BAAB sequence.

When these defects occur, voids or gaps are formed in the assembly of the conjugated polymer with respect to those errors in the chain. The University of Warwick-headed researchers were successful in using their innovative visualization method to evidently demonstrate all these gaps and then to zoom in further onto the polymer chains, exactly spotting each of the faulty monomer sequences. Thereby, to their astonishment, they discovered not the anticipated BAAB defects but ABBA flaws.

Professor Giovanni Costantini, a physicist in the University of Warwick’s Department of Chemistry, stated that “This new capability to image conjugated polymers with sub-monomeric spatial resolution, allow us, for the first time, to sequence a polymeric material by simply looking at it. Some of the first images we produced using this technique were so detailed that when the researchers who synthesised the polymers first saw them, their overjoyed impression reminded me of how new parents react to the first ultrasound scans of their babies.”

Besides representing a significant technical breakthrough, this new technique of combining vacuum electrospray deposition with high-resolution scanning tunnelling microscopy also has the potential to revolutionise the analytical capabilities in the application-relevant field of conjugated polymers where other currently available techniques are extremely limited.”

I am particularly grateful to the University of Warwick which directly funded that the purchase of the electrospray deposition equipment that was crucial to making this significant technical breakthrough.”

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