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Luminescent Liquid Crystals


   Department of Chemistry

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  Prof D W Bruce  Applications accepted all year round  Self-Funded PhD Students Only

About the Project

Background

Organic light-emitting diodes (OLEDs) represent a highly competitive, flat-panel display technology that is increasingly penetrating the market once dominated by liquid crystal displays. OLEDs work by generating an excited state (exciton) of a particular luminophore by in effect injecting an electron from an electrode into the LUMO and a hole from an electrode into the HOMO. Light is emitted when the exciton returns to the ground state. Because of the way the process works, 25% of the excited states generated are singlet in nature, while 75% are triplet and with simple organic luminophores, this is problematic as emission from the triplet state is forbidden formally and so most of the input energy is wasted. There are two main approaches to the issue. First is to design luminophores capable of efficient spin-orbit coupling, which would get round the spin problem and allow emission from the triplet state. This is most effectively carried out in complexes of heavy transition metals and indeed complexes of iridium are used commercially. Alternatively, if a material can be designed in which the singlet and triplet excited states are very close in energy, then it is possible to observed so-called thermally activated delayed fluorescence (TADF) in which the singlet state is re-populated from the triplet state leading to the observation of prompt fluorescence from the singlet state and delayed fluorescence from the re-populated singlet state. We are interested in both approaches.

Objectives

However, the twist we bring is to modify the luminophores so that they also show liquid crystal (LC) properties. The reasons for this are related (i) to the use of the organised LC to promote more efficient charge migration in the device and (ii) to the possibility for creating alignment that would lead to either linear or circularly polarised light emission. In general, the design of emissive, metal-containing LCs is a little more straightforward and we have experience using iridium(III), platinum(II) and gold(III). However, there remain significant challenges to be addressed and overcome. The design of LC-TADF materials is a little more challenging, but in 2021 we reported the first example of such a material and we have other examples being prepared for publication. This is a very new area and one ripe for exploitation as we seek to understand how different molecular types that represent the different mechanisms for TADF may be made compatible with liquid crystallinity.

Experimental Approach

These projects have their base in synthetic chemistry, but once prepared then it is necessary to determine the liquid-crystalline properties, the photophysical response and the device properties. We are not equipped to do all of this work and so we collaborate with a group in China for the device work and groups in Durham and in Germany for aspects of the photophysics.

Novelty

LC-OLED materials are still uncommon and to date we have the only published work on LC-TADF. There is a great deal to do…

Training

The project includes: synthetic chemistry in which area the student will gain significant expertise, complemented by gaining expertise in the normal methods of chemical characterisation (NMR spectroscopy, mass spectrometry, single-crystal X-ray analysis); liquid crystal characterisation using optical microscopy, calorimetry and small-angle X-ray scattering (SAXS); photophysical characterisation making steady-state and emission lifetime measurements; device characterisation with the possibility, pandemic allowing, to make your own devices in the Chinese lab. As such, the training provided is extremely broad.

All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/cdts/   

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. The Department strives to provide a working environment which allows all staff and students to contribute fully, to flourish, and to excel: https://www.york.ac.uk/chemistry/ed/ .


Funding Notes

This project is open to students who can fund their own studies or who have been awarded a scholarship separate from this project. The Chemistry Department at York is pleased to offer Wild Fund Scholarships to new students who will pay tuition fees at the overseas rate. Scholarships are competitive and awarded based on academic ability and financial need. For further information see: https://www.york.ac.uk/chemistry/postgraduate/research/funding/wild-fund-scholarships/

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