Molecular Photonic Breadboards

New manufacturing methods are required if we are to live sustainably on the earth. In the electronics industry there is enormous interest in the possibility of manufacturing devices using organic materials: they can be manufactured sustainably from earth-abundant resources at energy costs that are typically significantly less than those associated with the production of equivalent inorganic materials. Electronic devices based on organic components are now readily available in the high street. For example, organic light-emitting diodes are used to produce the displays used in some high-end TV sets and in smartphones (e.g. iPhone X). However, a fundamental problem prevents the realisation of the full potential of organic materials in electronic devices. When light is absorbed by molecular semiconductors, it causes the creation of excitons - pairs of opposite charges - that carry excitation through the device. However, the excitons in organic materials recombine and cancel themselves out extremely rapidly - they can only move short distances through the material. This fundamental obstacle limits the application of organic materials in consumer electronics and also in many other areas of technology - in quantum communications, photocatalysis and sensor technologies.

We propose an entirely new approach to solving this problem that is based on combining molecular designs inspired by photosynthetic mechanisms with nanostructured materials to produce surprising and intriguing quantum optical effects that mix the properties of light and matter.

On breadboards, threaded mounts hold optical components relative to one another so that rays of light can be directed through an optical system. This proposal also aims to design breadboards, but of a very different kind. The smallest components will be single chromophores (light absorbing molecules), held at fixed arrangements in space by minimal building blocks called antenna complexes, whose structures are inspired by those of proteins involved in photosynthesis. Antenna complexes are designed and made from scratch using synthetic biology and chemistry so that transfer of energy can be controlled by programming the antenna structure. Instead of using threaded mounts, we will organise these components by attachment to reactive chemical groups formed on solid surfaces by nanolithography. In these excitonic films, we will develop design rules for efficient long-range transport.

In conventional breadboards, light travels in straight lines between components. However, we will use the phenomenon of strong light-matter coupling to achieve entirely different types of energy transfer. In strong coupling, a localised plasmon resonance (an light mode confined to the surface of a nanoparticle) is hybridised with molecular excitons to create new states called plexcitons that combine the properties of light and matter. We will create plexcitonic complexes, in each of which an array of as many as a thousand chromophores is strongly coupled to a plasmon mode. In these plexcitonic complexes, the coupling is collective - all the chromophores couple to the plasmon simultaneously, and so the rules of energy transfer are completely re-written. Energy is no longer transferred via a series of linear hopping steps (as it is in organic semiconductors), but is delocalised instantaneously across the entire structure - many orders of magnitude further than is possible in conventional organic semiconductors. By designing these plexcitonic complexes from scratch we aim to create entirely new properties. The resulting materials are fully programmable from the scale of single chromophores to macroscopic structures.

By combining biologically-inspired design with strong light-matter coupling we will create many new kinds of functional structures, including new medical sensors, 'plexcitonic circuits', and quantum optical films suitable for many applications, using low-cost, environmentally benign methods.

Wealth creation

Much of the work described in this proposal is fundamental in nature: it aims to establish a new field of investigation based on the use of synthetic biological approaches to control quantum optical phenomena. However, it is firmly directed towards possible applications and molecular photonic breadboards (MPB) are a generic approach to the control of optical energy transfer that may be used to address many technological problems.

One important area of application is in optoelectronics. The short exciton diffusion lengths (10 nm) of polymer semiconductors are a fundamental barrier to their commercial exploitation. If we succeed in achieving efficient long-range transport of excitons in our molecular photonic breadboards, this will be transformational for many areas. Thus it will be important to explore the utility of MPB for device design. In particular, we will examine their suitability for solar energy capture. To this end we have made the design of polymer scaffolds an integral element of our programme. Although biomolecules give precise structural control, polymers are a much more suitable scaffold for large-scale device production. The approaches we will develop will be based on inexpensive synthesis carried out in aqueous conditions. There is extensive expertise in Sheffield to help us design prototypical solar cells. These measurements will be carried out in the lifetime of the project to explore the potential for early translation.

Society

The work described here may have significant impact on healthcare. A key element of the programme is the design of membrane systems for the measurement of exciton dynamics. These membranes offer enormous potential for the measurement of membrane processes (currently membrane proteins are the target for ca. 50% of drugs). The integration of strong plasmon-exciton coupling into optical biosensors is potentially transformative, offering fast quantitative analysis on a simple, inexpensive platform suitable for adoption in clinical chemistry laboratories with minimal infrastructure. We will explore the clinical utility of such approaches for cancer diagnosis with collaborators in the Sheffield Dental Hospital.

Energy supply problems are of economic significance because they impact the UK's international competitiveness, and they also impact on quality of life. The development of cost-effective renewable energy sources is vital to the UK economy, to the well-being of its citizens, and to the reduction of long-term harm caused by climate change. The goal of this proposal is to provide design principles to facilitate the development of improved, low-cost polymer photovoltaic devices. Achieving this goal would impact in a positive way on quality of life in the UK. For very similar reasons, it would also impact positively on international development: in the world's poorest nations the need for inexpensive sources of renewable energy is even greater, and often, the need to mitigate the effects of climate change is the greatest there too.

People

We will train 10 PDRAs in cross-disciplinary fields relevant to important emerging technologies. We will build an exciting, vibrant programme of research designed to attract the most able young scientists, and will help them to stretch themselves. They will form an enthusiastic multi-disciplinary team working to meet highly ambitious technical goals and targets. There will necessarily be a strong emphasis on collaboration and team-work. The multidisciplinary nature of this project will ensure that all hired personnel will become 'multi-lingual' in various branches of science. This rare and enriching experience will broaden their horizons, whether they choose to take up employment within industry or follow an academic career path. We will build a programme of outreach activities to take our excitement, and insights from the programme, into the wider community to inspire others.