This work is funded by EPSRC
The interest in organic molecules exhibiting thermally activated delayed fluorescence (TADF) has been reinvigorated in recent years owing to their potential to be exploited as emitters in highly efficient purely organic light emitting diodes (OLEDs). However, designing new molecules that exhibit efficient TADF is a non-trivial task because they would appear to require the optimisation of a number of contrasting properties. For example these molecules must exhibit rapid conversion between the singlet and triplet manifolds without the use of heavy elements to enhance spin–orbit coupling. They should also display a large fluorescence rate, but simultaneously a small energy gap between low lying singlet and triplet states. Consequently to achieve systematic material design, a detailed understanding of the fundamental factors influencing the photophysical behaviour of TADF emitters is essential.
Initial work in this area by the group of Prof. Adachi focused upon minimising the energy gap between low lying singlet and triplet states so that thermal energy could drive the triplet->singlet conversion required. This was achieved using charge transfer states. We have demonstrated that the mechanism for efficient triplet harvesting is a so-called spin-vibronic mechanism. Here, efficient reverse intersystem crossing (rISC) does not only occur between two charge transfer states (the previously accepted view in the field), but also requires coupling to another excited state, of different electronic character. This coupling is activated by specific vibrational degrees of freedom and therefore understanding how a molecule moves in the excited state is crucial.
The interest in organic molecules exhibiting thermally activated delayed fluorescence (TADF) has been reinvigorated in recent years owing to their potential to be exploited as emitters in highly efficient purely organic light emitting diodes (OLEDs). However, designing new molecules that exhibit efficient TADF is a non-trivial task because they would appear to require the optimisation of a number of contrasting properties. For example these molecules must exhibit rapid conversion between the singlet and triplet manifolds without the use of heavy elements to enhance spin–orbit coupling. They should also display a large fluorescence rate, but simultaneously a small energy gap between low lying singlet and triplet states. Consequently to achieve systematic material design, a detailed understanding of the fundamental factors influencing the photophysical behaviour of TADF emitters is essential.
Initial work in this area by the group of Prof. Adachi focused upon minimising the energy gap between low lying singlet and triplet states so that thermal energy could drive the triplet->singlet conversion required. This was achieved using charge transfer states. We have demonstrated that the mechanism for efficient triplet harvesting is a so-called spin-vibronic mechanism. Here, efficient reverse intersystem crossing (rISC) does not only occur between two charge transfer states (the previously accepted view in the field), but also requires coupling to another excited state, of different electronic character. This coupling is activated by specific vibrational degrees of freedom and therefore understanding how a molecule moves in the excited state is crucial.
The nature of the spin-vibronic mechanism can be exploited to propose new perspectives for molecular design, which reduces the effective activation energy of TADF, without reducing the radiative rate (See figure above). This has recently be exploited experimentally by Noda, Nakanotani, Adachi leading to improved device performance.
The importance of states with different character (e.g. charge transfer and locally excited states) means that its not only one energy gap which is now important (Figure above left). But it also opens the possibility for host tuning, i.e. exploiting the polarity of the host environment to drive alignment of the energy levels required for TADF. Throughout all considerations about the role of the host, it is important to remember the key factors affecting the effect of solid state solvation on the energetic and emission properties of TADF molecules (Figure above middle).
The importance of particular vibrational degrees of freedom identified a key tension in design of TADF emitters. While vibrational modes are responsible for promoting triplet harvesting, they are also responsible for modifying radiative and non-radiative decay and emission width. The latter being especially important in the context of colour purity in OLEDs. Consequently, understanding the synergy between modes driving TADF and emission width is crucial for molecular design.
Recently, in collaboration with the Prof. Monkman and Prof. Bryce, the group has exploited the fundamental
understanding of the many competing processes in TADF to achieve a new design of TADF emitters
based upon a rigid triazatruxene multi-donor core and very stable dibenzothiophene-S,S-dioxide acceptors (Figure above right). This emitter exhibits excellent device performance, >30% EQE and low efficiency roll off at higher current densities, due to a triplet harvesting mechanism, which for the first time is faster than Ir based metal-organic complexes!
A video summary of our work in this area can also be found below:
The importance of particular vibrational degrees of freedom identified a key tension in design of TADF emitters. While vibrational modes are responsible for promoting triplet harvesting, they are also responsible for modifying radiative and non-radiative decay and emission width. The latter being especially important in the context of colour purity in OLEDs. Consequently, understanding the synergy between modes driving TADF and emission width is crucial for molecular design.
Recently, in collaboration with the Prof. Monkman and Prof. Bryce, the group has exploited the fundamental
understanding of the many competing processes in TADF to achieve a new design of TADF emitters
based upon a rigid triazatruxene multi-donor core and very stable dibenzothiophene-S,S-dioxide acceptors (Figure above right). This emitter exhibits excellent device performance, >30% EQE and low efficiency roll off at higher current densities, due to a triplet harvesting mechanism, which for the first time is faster than Ir based metal-organic complexes!
A video summary of our work in this area can also be found below: