Ultra-fast, ultra-small and ultra-dilute: an integrated understanding of conjugated polymers in solution across spatial and temporal scales

Conjugated polymers are an important class of organic (carbon-based) semiconductor. They combine novel semiconducting electronic properties with simple fabrication of devices from solution. They enable the field of plastic electronics including flexible lighting, displays, solar cells and electronics. Their combination of flexibility, ability to interconvert electricity and light, and simple fabrication is very unusual and enables many applications.

One of the most interesting features of these materials is that they are both semiconductors and soluble. This means they can be dissolved to make a solution and deposited by simple processes such as ink jet printing to make working electronic and optoelectronic devices (such as light sources and solar cells). The behaviour of conjugated polymers in solution is very complicated because each polymer consists of a chain of atoms that is ultra-small and flexible, and so can fold in a different way. A particular shape of the polymer is known as its conformation. The conformation of the polymer is not static and can change at ultra-fast timescales in solution. The properties of the material then depend on the conformations of the constituent polymer chains, and the structure of films of the material are strongly influenced by the conformation of the polymers in the solution used to make the film.

The aim of this proposal is to achieve a breakthrough in our understanding of conjugated polymers in solution by developing and applying new measurement techniques. Remarkably it is now possible to study individual molecules, one at a time to see how they are different from each other. In contrast to these ultra-dilute conditions, most experiments just measure average properties. The difference between these approaches is huge - like the difference between knowing the average height of people is 165 cm and the actual height of every person. We have made the first measurements in the world of single conjugated polymers in solution, demonstrating the feasibility of the approach. We now aim to transform our understanding of conjugated polymers in solution by studying individual molecules over a very wide range of timescales and length scales. In addition, we will study the interactions between polymers and how they form aggregates in solution, which are also known to impact the conformation of the polymer and the properties of the material.

This work will involve developing measurements of exceptional spatial and temporal resolution. By bridging the gap between ultra-dilute single-molecule methods in solution, ultra-fast pump-probe and ultra-small super-resolution imaging, the proposal will deliver a set of 'first of its kind techniques' that will give unprecedented insights into CP function. The proposal will provide, for the first time, a measurement of the structural heterogeneity and dynamics of CP chains in solution and a direct correlation between each conformation and its photophysical properties. This basic knowledge will lay the foundations for improved conjugated polymers and devices and empower a broad range of applications across material sciences.

Knowledge impact: conjugated polymers have the potential to generate new applications like flexible lighting, circuits and sensors that can be integrated into our homes, vehicles, clothes and even our bodies. However, to meet these technological demands, scientists need to be able to model structure-property relationships at the molecular level so that it is possible to design systems with more controlled, and reproducible, properties (i.e., conformation, self-assembly). To date, a direct quantitative measure of the structure-property relationship at the level of single chains in solution, or at an interface, has not been realized. This is mostly due to the lack of appropriate experimental methods across the vast range of relevant temporal and spatial scales. As a result, polymer shape and flexibility in solution has been assumed rather than demonstrated.

This proposal will enable a mode of predictive science for organic electronics by providing information that until now has been inaccessible to the conjugated polymers community- for example (i) how many conformational states are available for a given conjugated polymer in solution, (ii) how fast can a polymer fluctuate between these shapes and what controls the energy barrier for the transition, (iii) what are the mechanistic details driving the interaction between polymer chains in solution and what controls the timescale of the process? (iv) what parameters define the morphology of the aggregates formed? How do these multiple process/parameters impact optical properties? Accessing these questions will impact companies working on organic electronics and other academics across material sciences. In addition to technical advances enabling fundamental knowledge, the chemical strategies and hetero-functional organic dyes delivered by the proposal can lead to novel applications in biotechnology, sensing and photonics.

Economic impact: The list of potential applications of plastic electronics is endless and the total sector is expecting a growth from $16 billion today to $76 billion by 2023 (KTN Magazine, UK Plastic Electronics). According to a UK Plastic Electronics Leadership Group's sector study, more than 134 UK companies and 33 universities are involved in plastic electronics R&D, providing direct employment for over 2500 individuals and a revenue of £234m. Examples of these companies include Cambridge Display Technology, Plastic Logic, Ambicare Health, Molecular Vision and others. The proposal will contribute to maintain the leading role that UK holds in some aspects of plastic electronics (i.e. organic light emitting diodes) and it will improve economic competitiveness of these UK companies by using new insights to rationalise the interpretation of polymer structure in solution and how to use this to tailor solution-based deposition methods. Control of conformation is expected to lead to devices with improved performance, resistance to air and durability that will benefit the general public by leading to more efficient, low-energy consumption lighting solutions, sensors and displays.

People and training: The project will provide interdisciplinary training in skills spanning polymer and organic chemistry, optical microscopy and time-resolved spectroscopy. Core to our approach is embedding chemical and physics competence in both PDRAs and potential PhDs working in the project. The expertise gained will be unique and very valuable in a wide range of academic and industrial sectors.

Dissemination and education: The project will impact on society through outreach activities aimed at explaining the physics at extremely small and ultrafast scales and how gaining knowledge at the microscopic level can help catalyze technological innovation and the improvement of existing ones. We will focus on specific delivery of impact via The Edinburgh Science Festival (April each year) and National Science Week (March each year) as a route to reach a large and young audience.