Efficient blue organic luminescent materials are highly desired because they are one of the fundamental elements of the three primary colors in organic light-emitting diodes (OLEDs).Organic fluorescence molecules with blue emission can be readily designed, which are employed as the first-generation luminescent materials in OLEDs. However, such kind of devices can make use of only 25% electro-generated excitons under eletrical excitation, leading to low external quantum efficiency with the upper limit of 5%–7.5%.Afterwards, the second-generation noble-metal-containing phosphorescence materials were developed, which can reach unity exciton utilization by converting singlet excitons to triplet excitons via intersystem crossing on the basis of heavy-atom induced large spin-orbit coupling (SOC). But because of the intrinsic metal to ligand charge transfer characteristics, pure blue emissions can hardly be achieved in most phosphorescence materials, and the long lifetimes of triplet excitons result in poor stability of these materials applied in OLEDs. The third generation thermally activated delayed fluorescence (TADF) materials can improve the stability of devices while ensuring high exciton utilization, and their metal free pure organic properties also meet the requirements of green environmental protection and economy. However, most of the high efficiency TADF materials reported so far still lack short wavelength blue light and deep blue light materials. The main reason is that the strong intramolecular charge transfer (ICT) property of TADF molecules will lead to the red shift of the light emission wavelength. In this work, we take xanthone as the core, carbazole and its derivatives as the electron donor, adjust the ICT properties of the molecule by changing the twisted angle of the intramolecular donor acceptor, and prepare TADF materials from sky blue light to deep blue light, with a solid-state luminous quantum efficiency of 94%. Using these materials, efficient blue and deep blue OLED devices can be prepared. The emission peaks of the devices are at 462 nm and 442 nm, and the external quantum efficiencies are 33.7% and 22.2%, respectively. This work provides a new molecular design strategy for the design of efficient blue light delayed fluorescent materials.
Ph. D. candidate Jinke Chen in our group is the first author of this article, and this work has been published on CCS. Chemistry (DOI: 10.31635/ccschem.022.202202196).