rISC - the game of strategic molecular design for high efficiency OLEDs

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Lighting and displays form essential parts of our daily lives and consume approximately 20% of the electricity used worldwide. It has been shown that lights visible from space at night time are an effective measure of a country's economic development, while energy capture through the development of materials that absorb more sunlight is at the heart of attempts to reduce our reliance upon unsustainable fuel sources. In the context of generating light, Organic Light Emitting Diodes (OLEDs) are a rapidly expanding technology which are highly appealing owing to their potential high energy efficiency and ability to be printed roll-to-roll on a plastic substrate. However, in terms of efficiency, initial attempts to implement OLEDs based upon purely organic materials were restricted by the type of excited state which emits the light. Indeed, upon electrical excitation 25% of the emitting molecules are in emissive singlet excited state, whilst 75% are in non-emissive triplet excited states. However, conventional organic materials cannot emit from the triplet excited states. The proposal consists of developing a new class of OLEDs that exploits thermally activated delayed fluorescence (TADF). Here a triplet state is thermally activated to become iso-energetic with a singlet excited state, enabling efficient rISC into a radiative singlet manifold. One of the major challenges that must be overcome in TADF materials is that the use of pure-organic materials means that the often weak spin-orbit coupling between the singlet and triplet states can lead to rISC rates that extend into the millisecond range. This causes poor roll-off in device efficiency at higher current densities. However, through a new conceptual design of TADF molecules, we have shown that it is possible to achieve both a reverse intersystem crossing (rISC) rate > 1E7 s-1 and a unity photoluminescence quantum yield (PLQY), a combination previously considered untenable.In this proposal we will combine detailed synthetic, computational and photophysical studies to develop approaches for exploiting steric hindrance and non-covalent interactions to exert finer conformational control of the excited state dynamics to enhance functional properties. This is performed in conjunction with detailed studies to establish a deeper understanding of the excited states and their geometries formed by charge recombination. This proposal will deliver new understanding about the emission processes in TADF OLEDs and how to enhance the rISC rate to beyond 1E8 s-1 whilst retaining PLQY ~ 1. Achieving these outcomes will have a major disruptive impact on the OLED industry and ensure that the UK remains in the lucrative OLED materials supply chain.