Redox Switchable Photonic Materials Based on Organoimido-Polyoxometalate/Cyclodextrin Host-Guest Complexes

Lead Research Organisation: University of East Anglia
Department Name: Chemistry

Abstract

Photonic materials interact with light in useful and interesting ways. They enable its manipulation, and conversion into other forms of energy. One important class of photonic materials are non-linear optical (NLO) materials, which can be used to manipulate and adjust the properties of laser light beams. For example, they are used to make green lasers by second harmonic generation (SHG) from an infra-red source, and in electro-optic (EO) modulators that transfer digital electronic signals into fibre-optic telecommunications.

At present, most commercial NLO materials are simple inorganic salts. These are inexpensive, durable and ideal for simple SHG applications. However, in telecommunications and computing they suffer from slow speed, as their responses originate from displacement of (relatively heavy) ions in response to the electric field of light. Molecular organic and metal-organic materials promise faster responses, because they arise from displacement of lighter, faster electrons, and also rational property tuning and the possibility of rapid property switching (i.e. on/off for optical or electrooptical transistors). But it is difficult to obtain molecules combining high NLO activity with adequate transparency and photostability, and adding the ability to reversibly switch between on/off states is a still greater challenge. Recently, we discovered a promising new class of molecular NLO materials based on polyoxometalates (POMs) - a type of molecular metal oxide cluster - connected to organic groups. These POM-based chromophores (POMophores) obtain high NLO coefficients from materials with small, stable organic groups and excellent transparency, and show redox properties that could be used to switch the NLO response.

The next stage, addressed in this project, is to assemble POMophores into bulk materials that can be used in devices - specifically EO modulators and transistors. To do this, we must find a way to align all of the POMophores so that they point in the same direction and give a net NLO effect. This is challenging, as methods for controlled assembly of POM-based materials are currently very limited, and to achieve the goal we will develop a new approach where we first trap the POMophore in a molecular container. The molecular containers are designed in such a way that they form a film where the desired molecular orientation is forced on the POMophore. In addition to organising the POMophores to give bulk NLO properties, the containers will also protect them from degradation when we investigate redox-switching of the NLO response.

POMs offer many other properties beyond non-linear optics - for example many POM clusters are excellent catalysts or photocatalysts due to their ability to rapidly accept and transfer electrons, some have magnetic and/or luminescence properties introduced by incorporating suitable heterometals into the POM framework, and POMs have also demonstrated anti-viral activity. Therefore, we expect that other areas of chemistry and materials science will benefit from methods enabling their encapsulation and control over their positioning on the nanoscale. Possibilities could include selective catalysis, solar energy conversion, memory devices, and even targeting of biologically/medicinally active POM species for therapeutic interventions. This project will lay the groundwork necessary for such developments, as well as potentially producing the new, high performance bulk NLO materials needed for future telecommunications and computing.

Planned Impact

I anticipate impact in three main areas:

1. Economic and Social Impact: This project is in the general area of photonic materials. The UK photonics industry is one of the success stories of our economy post-2008: it is in the global top 5, now worth >£12.9 bn, is growing at 5.3% p.a., and employs 65,000 people who have 3x the UK average productivity per worker. Continuing this trajectory requires development of new and innovative materials, as we will do in this proposal focused on new non-linear optical materials. The goal is to produce a basis for high performance electroptic (EO) modulators, used to convert digital electronic signals into optical signals, and eventually, electroptic transistors that could be a basis for optical data processing.
These could facilitate a move from electronic to electro-optical and all optical data transmission and processing, increasing the speed, and lowering the power consumption of ICT. As our demand for computing power and telecommunications speed is seemingly insatiable, and ca. 10% of global electricity consumption is now attributed to ICT, the potential economic benefit of improved technology is clear. The materials we study are also potentially relevant to optical power limiters and imaging agents, another source of economic impact. The project also supports the UK economy by training highly skilled workers - the PDRA will gain skills in synthesis and measurement that are widely marketable in academia and beyond. Moreover, forming individuals with high level problem solving and other transferable skills benefits society in numerous ways, beyond specific scientific knowledge.

2. Environmental Impact: Significant, positive environmental impact will emerge from this project if it can contribute to development of materials for optical telecommunications and computing. This is because the current rate of increase of ICT use is not sustainable without technological change: while the power consumption of our devices (computers,phones, smart TVs) may seem small compared to household heating, lighting and transport, the power consumption of networks and data centres is massive. The global energy footprint of ICT comfortably exceeds that of aviation, and it has been suggested that watching one hour of streamed video consumes more electricity in remote networks than two refrigerators do in a year. As optical telecommunications and computing promise inherently higher bandwidth and lower energy consumption, they can facilitate a continued and sustainable ICT revolution.

3. Social and Cultural Impact - through fundamental science: This project involves development of new methods, new materials and measurement of their properties, so it clearly impacts on fundamental science. The POM nanostructure assembly methods I propose will be completely new, and thus could influence science for some time - they (encapsulation by a molecular container), and their proposed application, are also attractive to lay audiences of all ages and potentially an excellent vehicle for outreach.

Publications

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