Design strategy for thermally activated delayed fluorescence materials with multiple resonance effect

Abstract

Since the initial report on multiple resonance thermally activated delayed fluorescence (MR-TADF) molecules, their narrow band emissions, high quantum yields and other characteristics have consistently fueled research interest in the realm of organic electronics, particularly organic light emitting diodes (OLED). These molecules swiftly ascended to the forefront of research, serving as a pivotal focus, giving rise to numerous high-performance devices and meticulously crafted molecules. Devices featuring MR-TADF molecules as the luminescent core continually redefine our comprehension of OLED, with some employing hyperfluorescence technology attaining peak performance in specific photochromic domains today. Presently, with the escalating demand for ultra-high-resolution displays, the international telecommunication union (ITU) has unveiled the next generation color gamut standard, BT.2020. This standard delineates the broadest display color gamut, mandating monochromatic primary color wavelengths of 467, 532, and 630 nm, constituting an exceptionally extensive color gamut. Simultaneously, achieving high-resolution displays with such an expansive color gamut imposes unprecedentedly stringent requirements on the color purity of device elements. Consequently, it imposes a formidable color purity target for display technology. In the past, traditional fluorescent materials struggled to meet these demands. The advent of BT.2020, however, has presented new opportunities for the advancement of MR-TADF molecules, leading to a surge in popularity in this field. In recent years, with copious research and practical applications, the MR-TADF molecular family has undergone rapid evolution. Nevertheless, discussions and summaries primarily centered on the field's development, with limited focus on molecular design strategies. This deficiency hinders adequate reference for researchers entering the field. Consequently, this article expounds upon the design principles of select MR-TADF molecules reported in the past three years. It delves into aspects such as the X-π-X principle, fast reverse intersystem crossing processes, narrow-band emission, and high oscillator strength. Additionally, it posits future design directions, including the incorporation of non-traditional structures into the MR-TADF domain. Finally, the article offers suggestions for the prospective development and industrialization of the MR-TADF field.