Detailed photophysical measurements of intramolecular charge transfer (ICT) states have been made both in solution and solid state. Temperature dependent time resolved emission, delayed emission and photoinduced absorption are used to map the energy levels involved in molecule decay, and through detailed kinetic modelling of the thermally activated processes observed, true electron exchange energies and other energy barriers of the systems determined with the real states involved in the reversed intersystem crossing mechanism elucidated.
For specific donor acceptor molecules, the CT singlet and local triplet states (of donor or acceptor) are found to be the lowest lying excited states of the molecule with very small energy barrier between them kT. In these cases the decay kinetics of the molecules become significantly different to normal molecules, and the effect of rapid recycling between CT singlet and local triplet states is observed which gives rise to the true triplet harvesting mechanism in TADF. Using a series of different TADF emitters we will show how the energy level ordering effects or does not effect TADF and how ultimate OLED performance is dictated by energy level ordering, from 5% to 22% external quantum efficiency. From this understanding, we are able to define three criterion for TADF in different molecules and these will be discussed.
Here, we report our investigations on the development of novel, metal-free, organic materials able to harvest triplet states using efficient thermal activated delayed fluorescence (TADF), and dual fluorescence-phosphorescence emissions at room temperature (RT-DFP). These materials show enormous potential in different technological applications, including the development of materials for oxygen, and temperature sensing, optical power limiters, bio-imaging, and in organic light emitting diodes (OLEDs).
TADF, also known as E-type delayed fluorescence, has gained very rapid interest as a mechanism to improve efficiencies in OLEDs, due to the possibility of harvesting approx. 100% of the excitons formed from charge recombination, without requiring the use of expensive and scarce materials such as iridium or platinum. As TADF, the observation of RT-DFP in pure organic materials has also potential for many technological applications, and in particular in sensing applications. Recently, organic materials with long lived triplet photo-induced absorption were used to develop optical power limiters for low light levels. Magnetic modulation over visible room temperature phosphorescence using weak magnetic fields was also reported. Moreover, RT-DFP in principle, can be also used as a way to harvest triplet states in OLEDs and produce light directly from both singlet and triplet states, which may allow the design of metal free organic white emitters for lighting applications.
In this talk, the complex and rich photophysics of materials showing very efficient TADF and RT-DFP is discussed in detail, showing how a simple change on the molecular structure allows switching-off the strong TADF and opening the channel for efficient RT-DFP to be observed. The role of the energy ordering of electronic states on the efficiency of both mechanisms is also discussed, giving clear guidelines for the design of new emitters, and opening the way for TADF and RT-DFP to be explored in technological applications.
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