Photophysical Investigations of Triplet Harvesting Mechanisms in Metal-Free Organic Emitters

Abstract

Metal-free organic molecules exhibiting thermally activated delayed fluorescence (TADF) and/or room temperature phosphorescence (RTP) have attracted great attention due to their promising potential for applications in emerging optoelectronic technologies, including sensors, imaging, anti-counterfeiting and organic light-emitting diodes (OLEDs). TADF is a mechanism that can upconvert dark triplet states to emissive singlet excited states via reverse intersystem crossing (RISC). RTP emitters instead show a substantial high triplet formation yield along with a relatively fast triplet radiative decay rate and suppressed non-radiative deactivation pathways. Both TADF and RTP are promising and efficient approaches to harvesting the non-emissive triplet states, which can overcome the limitation of 25% internal quantum efficiency imposed by spin-statistics in OLEDs.

Although the development of metal-free organic TADF and RTP emitters has greatly progressed in recent years, various challenges still exist concerning the full understanding of the mechanisms. The photophysical properties of metal-free organic donor-acceptor-donor (D-A-D or D-A) molecules were investigated in this thesis. The aim is to investigate the roles of isomerization and molecular conformation, and the effects of energy alignment of relevant electronic excited states on the TADF and RTP mechanisms.

The work starts with the study of a series of D-A-D molecules and their structural isomers. Dual emission with contributions from both TADF and RTP is observed in some of these isomers, while others are purely fluorescent. This work highlights the similarity of these two mechanisms and the influence of D-A linkage on the intersystem crossing rates. This is followed by an investigation of the influence of molecular conformation on the TADF and RTP mechanisms, using the steric effect induced by the presence of alkyl substituents to manipulate the conformation of D-A-D emitters. The exciting topic of conformational control of RTP and TADF mechanisms in molecular crystals is then explored. Subsequent chapters address the effects of the substituents with different electron-donating, and electron-withdrawing strength on TADF emitters as a facile way to tune the electronic interactions and the energy of TADF luminescence. A clear understanding of how these effects influence the mechanisms will give clear guidelines for the design of novel and more efficient TADF and RTP emitters.