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

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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.