Chemistry Seminar Series: Rational Design of TADF- emitters: from classical to multiple resonance

Biographical Information

Dr. Evgeny A. Mostovich is an assistant professor from Novosibirsk State University, Russia. He is one of the leading specialist in the field of organic synthesis and organic electronics. His scientific research is related to new organic semiconductor materials for optoelectronic applications – organic light-emitting diodes, field-effect transistors, light-emitting transistors, organic lasers with optical and electrical pumping, energy and information storage devices, etc.

Abstract

Harvesting nearly 100% of electrogenerated excitons in OLED is possible by using phosphorescent or TADF emitters. Nevertheless, phosphorescent materials possess several disadvantages such as high cost (due to using noble metals complexes), board emission spectra, and rather long triplet lifetime originating from metal-to-ligand charge transfer (MLCT) emission character. In 2012, Adachi’s research group reported the first state-of-the-art emitter based on thermally activated delayed fluorescence (TADF) realizing 100% reverse intersystem crossing from the lowest triplet excited state (T1) to the lowest singlet one (S1) using simple aromatic compounds

However, classical TADF-emitters suffer from broad emission spectra due to physical separation of Donor and Acceptor moieties and charge transfer (CT) nature of the excited states. In addition, twisted structure used for achieving small energy splitting between S₁ and T₁ (ΔE_ST) leads to short lifetimes and low oscillator strengths (and consequently to low rate constants of fluorescence). In 2016, Hatakeyama reported new groundbreaking TADF-emitter design principle, which is based on combination of highly rigid heteroatomic molecular skeleton with multiple resonance effect (MR, e.g. alternation of electron density in the molecule)². That allows achieving narrowband emission with a full width at half maximum (FWHM) of less than 30 nm without using Donor-Acceptor structure. Moreover, MR-TADF emitters can be used as terminal emitters in TADF-assisted hyperfluorescence OLED devices, which makes these molecules promising candidates for OLED applications.

Here we discuss rational design principles of TADF emitters from classical Donor-Acceptor systems to more complicated MR-TADF using quantum mechanical modelling, synthetic approaches and materials study using time resolved emission spectroscopy.