Single-molecule studies of light-emitting polymers: observing and manipulating polymer conformation in solution

A remarkable advance in modern science is the ability to detect the light emitted by single molecules. This achievement - the demonstration of the ultimate detection limit - lies at the heart of a new era of microscopy that is transforming our understanding of biological processes. The ability to pinpoint the exact location of a single dye molecule, quantify fluctuations in the emission of individual molecules or the possibility of stretching at will a single biopolymer are providing knowledge that before it was only possible to dream of. Whilst these techniques will continue to revolutionize biology, we now propose to apply them to give breakthroughs in materials science. The first aim of this proposal is to develop a toolbox of single-molecule techniques that offer for the first time the opportunity of observing and manipulating materials as depicted in the textbooks: molecule by molecule. We will use these techniques to make a pioneering breakthrough in linking the conformation of conjugated polymers in solution to their light-emitting properties. Understanding this relationship is crucial because a key advantage of these materials is that they can be processed into optoelectronic devices using simple deposition methods from solution.

Conjugated polymers are very promising for organic light-emitting diode (OLED) displays as well as solar cells and transistors. The display of information - on mobile phones, televisions and monitors is very important for work, communication, entertainment and learning. Advances in display technology have been dramatic and some of the most attractive are OLEDs. To meet the growing technological opportunities, it is crucial to develop a deeper understanding of how the properties of conjugated polymers relate to their conformation. The conformation is the shape of the polymer and it has a huge effect on the optical properties but it is very difficult to study because every polymer chain has a different shape. We will overcome this problem by measuring single polymers in solution, and by developing techniques to mechanically manipulate the conformation of the polymer to identify how changes in shape alter its light emission properties. This is a pioneering area because single-molecule methods for non-aqueous environments are still in their infancy and our work will allow us to understand how the solvent affects polymer conformation and light emission with an unprecedented level of detail.

Our second goal involves the novel concept in materials sciences of manipulating the polymer backbone using mechanical force whilst simultaneously monitoring its fluorescence properties. We will apply manipulation techniques to conjugated polymers for the first time. Stretching the polymer in a controlled manner will reveal the optical signature of different conformations (i.e. linear versus collapsed) enabling the emission properties to be related to the underlying shape of each individual polymer chain - information not available by any other technique.

To achieve these goals, we have put together a team of leading scientists in key areas through a collaboration between St Andrews (polymer physics & single-molecule) and Strathclyde and UCSB to assist with the synthesis of novel orthogonally functionalized conjugated polymers. The proposal also benefits from a partnership with Cambridge Display Technology Limited (CDT, UK), the leading company in polymer LED technology, that will help us to efficiently translate our findings into technological advances. Our project partners at the University of Leipzig will assist with the integration of mechanical manipulation into our single-molecule fluorescence microscopes. Our results will not only advance the field of polymer LEDs, but also other polymer optoelectronic areas such as lasers and solar cells, and the wider fields of polymer and materials science.

This proposal will enable a truly microscopic understanding of how polymer conformation in solution impacts the electronic and optical properties of conjugated polymers, across a broad range of solvents, polymer lengths, force and time scales. Plastic electronics is considered one of the key technologies in the 21st century. Worldwide markets already exceed a billion pounds and they are expected to grow at a CAGR of 36.7 percent over the 2013-2018 period (Technavio, Global OLED Display Market, Sept 2014) driven by the large-scale production of flexible, lightweight optoelectronic devices. However, for conjugated polymers to meet these demands, a much deeper understanding of their fundamental behaviour is required, as recently highlighted by the Photonics Leadership Group representing the UK Photonics industry (Source: Photonics Revolutionising our World, KTN Group, UK Photonics, 2014). Understanding conjugated polymers in solution at the single-molecule limit is crucial because OLED devices are manufactured by deposition from organic solvents. This knowledge will allow to improve the performance of photonic devices used in displays, lighting and medicine. In the long-term, advances inspired by this proposal will lead to the next generation of consumer optoelectronics and sensor technologies for healthcare and defence.

We will investigate a range of materials widely used in OLEDs and solar cells. The implications and beneficiaries of the proposal branch in several directions. First, the knowledge extracted by developing pioneering solution-based techniques for single-molecule characterization and control of polymers will impact both the UK and international academic community and will contribute to place the UK at the frontier of state-of-the-art conjugated polymer physics. Secondly, by studying polymers in technologically relevant solution-processing conditions, the outcomes of the proposal relate directly to device manufacture, expediting the link between basic research and industry. Third, polymers functionalised with reactive groups at one or both ends are unique and novel materials and the protocols used to synthesise them and attach them to surfaces and nanoparticles constitute optimized strategies that can lead to novel applications. Importantly, the proposal provides innovative molecular imaging methods that can revolutionize other areas including materials chemistry and catalysis.

Beneficiaries include the Plastic Electronics Industry which comprises around 134 companies in the UK (Source: UK Plastics Electronics Case Study 2012/2013. Plastic Electronics Leadership Group). Examples of such companies developing improved devices for applications in consumer optoelectronics, energy, healthcare and sensing include Plastic Logic, Ambicare Health, Molecular Vision, Eight19 and Cambridge Display Technology (CDT). Performing truly breakthrough research leading to large-scale manufacturing methods that can shape the polymer chains in the optimal conformation to improve device performance is essential to maintain these UK-based companies at the forefront of this highly competitive and explosively growing market. Devices with improved durability and efficiency will in turn lead to benefits for the general public through the availability of more efficient lighting solutions and lower energy displays. Indeed, through our collaboration with CDT and our own fabrication facilities, it is likely that the proposal will lead to improved fabrication procedures during the timescale of the project, and possible implementation in manufacture afterwards.

Research staff on the project will benefit from the interdisciplinary interactions between experimentalists with different backgrounds. The skills of such research collaboration will be valuable in a wide range of industrial sectors. In addition, the researchers will acquire communication and project planning skills through reporting, conferences and public engagement activities.