Organic Light-emitting Diodes (OLEDs) have moved quickly from basic design concept to commercial displays and have gained popularity on the basis of the image quality possible, due in part to the better blacks than are possible with liquid crystal displays (LCDs). However, the technology is still relatively new and currently, the displays remain stubbornly more expensive than LED-backlit LCDs.
In its simplest form, emission from OLEDs required the formation of an excited state through injected of charge such that an electron is injected into the LUMO while a hole is injected into the HOMO. Light is emitted as the excited state relaxes non-radiatively. Because the electron and hole are injected independently of one another, then the excited states can be either singlet (S1) or triplet (T1) in nature and it can be shown statistically that these states form in the ratio three T1 states for every S1 state (75:25). Selection rules dictate that S1–T1 interconversion is spin-forbidden so that for purely organic molecules, emission normally occurs only from the S1 excited state meaning that 75% of the energy used in creating the T1 excited states is lost. However, the use of certain complexes of heavy transition metals, most notably iridium(III), gets round this problem as there is extensive spin-orbit coupling which changes the spin description and rapidly converts all excited states into the T1 state from which emission can now occur. These iridium complexes are now components of commercially available OLED displays.
However, there is still a concern relating both to availability and price of iridium (a precious metal), hence there has been interest in other approaches to efficient, metal-free luminophores.
Thermally Activated Delayed Fluorescence (TADF)
TADF is a hot and vibrant area of research that offers the possibility of metal-free luminophores suitable for application in OLED displays.
The major difference is that the energies of the S1 and T1 excited states are so close in energy that there is thermally promoted interconversion so that all excited states are harvested and all emission is from the singlet manifold via delayed fluorescence.
In simple terms, the design principles require a ’donor’ pi-system and an ’acceptor’ pi-system that are not in conjugation in order to ensure a very small separation between S1 and T1. There are various ways in which this orthogonality can be realised on the basis of chemical functionality and also steric strain and preliminary studies have identified some routes of potential interest. Further, with significant expertise in the group around liquid crystals, it will be interesting to explore the useful additionality that LC properties may confer and then address the significant design challenge of combining structural orthogonality (needed for emission) with molecular anisotropy (needed for LC behaviour).
The work will be based in synthetic chemistry in which area the student will gain significant experience, complemented by gaining expertise in the normal methods of chemical characterisation (NMR spectroscopy, mass spectrometry, single-crystal X-ray analysis). Target compounds will then be studied by various photophysical techniques with ambient measurements made here in York, while low-temperature and device measurements will be made in collaboration with groups in the UK and beyond.
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