Design strategies for thermally activated delayed fluorescence materials : donor modification and heavy atom effects

The design of novel luminophores with delayed emission is of crucial importance to the field of optoelectronics. In particular, all-organic thermally activated delayed fluorescence (TADF) emitters have gained prominence in recent years due to their ability to harvest triplet excitons for light emission without the use of precious metals. TADF emitters have found applications in organic light emitting diodes, bioimaging probes, stimuli-responsive materials, and as photosensitizers for light-driven reactions. In the design of these emitters, the balance of parameters such as photoluminescence quantum yield, spectral width, photostability, and delayed emission yield must be carefully considered and controlled by molecular design. The study of molecular design principles and structure-property relationships in novel emitters is therefore of paramount importance for advancing the utility of TADF materials. Molecular rigidification is a known design principle that leads to improved brightness, photostability, and colour purity due to the suppression of non-radiative decay pathways. In addition, the incorporation of heavy atoms into these systems can promote spin-forbidden processes such as intersystem crossing and reverse intersystem crossing, which are fundamental to their photophysics. This thesis explores the application of rigidified azatriangulene donor groups with triazine and napthalimide acceptor groups for the design and synthesis of TADF materials. These donors employ dimethylmethylene bridges as rotational locks to exploit the benefits of molecular rigidification for the photoluminescence quantum yield and multi-photon absorption properties of the resultant emitters. Donor tunability is explored in these systems by exchanging bridging carbon atoms for sulfur or oxygen heteroatoms, giving sulfidoazatriangulene or oxoazatriangulene donors, respectively, and achieving emission colours across the visible spectrum from blue to orange-red. The bridging heteroatoms are shown to alter the electron-donating ability and the thermal stability of the donors, providing insight into the molecular design principles for this class of emitters. Additionally, the sulfide-bridged donors can be oxidized to sulfoxide or sulfone derivatives, to tune their electron density and resultant emission properties. Finally, the incorporation of heavy halogen atoms into the host matrix of glassy organic dots containing TADF dopants is used to enhance delayed emission processes in the dopant as a generalizable strategy.