An Exploration Into TADF For OLEDs: A Case Study Based On Novel Synthesised Solution-Processable Molecules

Work is presented on new molecules designed for Organic Light Emitting Diodes (OLEDs). The molecules synthesised here-in are designed to be solution processable, and display the property of Thermally Activated Delayed Fluorescence (TADF). In the process of characterising these molecules, investigation into the methodology of the field of TADF is conducted and recommendations are made for change. The recommendations presented are: a more thorough way of interpreting transient fluorescence data that does not assume that k_{RISC} << k_{ISC}; the use of the optimised T_{1} state to simulate the excited state geometry in certain circumstances; and caution about using the photoluminescence spectra at 77 K to calculate the energy of the triplet state.

The molecules synthesised are carbazole substituted DMAC-TRZ, a new 1,3-Bis(N-carbazolyl)benzene (i.e. mCp) like host, and a 10H-phenoxazine based compound with substitution para to the nitrogen (a previous synthetic challenge). Of these compounds the carbazole substituted DMAC-TRZ compounds are the focus of most of the work. Unlike DMAC-TRZ, the synthesised solution processable compounds are shown to have H-aggregate like properties when in pure film. Data is also be presented that 3 key geometries are necessary to describe the DMAC group beyond the previously described two geometries. Evidence that a transition occurs between two of these geometries in the excited state is presented based on T_{1} simulations. It is further suggested that the substitution of groups like carbazole on DMAC may prevent this transition.

Two of the compounds based on the DMAC-TRZ donor-acceptor show extremely narrow singlet-triplet energy gap. This appears to lead to the situation where cooling to 77 K has a greater effect on the rate of fluorescence, than the Reverse Inter System Crossing (RISC) rate. This can be seen as direct experimental evidence that vibronic coupling plays a part in the rate of fluorescence.