Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

Abstract

Triplet exciton harvesting is crucial in organic light emitting diodes (OLEDs) because the triplet states produced by electron and hole recombination account for up to 75% of the total excitons, whereas the singlet states account for 25%. However, the triplet state of organic molecules is usually either non-emissive at room temperature or gives very long phosphorescence lifetimes, both of which are detrimental to OLEDs. Thermally activated delayed fluorescence (TADF) emitters are one answer to this challenge. Typical TADF emitters are based on an electron donor–acceptor structure motif, and the low-lying states include the charge transfer singlet (¹CT) state and triplet (³CT) state, and a closely-lying localized triplet (³LE) state. Although many efficient TADF emitters have been developed for OLEDs, the underpinning photophysical processes of TADF, for instance, the charge separation, the forward intersystem crossing (ISC) and the reverse ISC (rISC), and the coupling of the excited states of these emitters, are far from clear. Herein, we introduce recent developments in the study of the photophysical processes of TADF emitters using the time-resolved electron paramagnetic resonance (TREPR) spectroscopy method. This spectral tool supplies unique information on the dynamics of the transient paramagnetic species involved in the TADF processes, for instance, the ³CT and the ³LE states, as well as the ISC mechanisms. Physical insights have been obtained with TREPR spectra on the TADF mechanism, such as simultaneous observation of the ³CT and ³LE states, vibrational and spin-vibronic coupling mediated ISC, and the electronic configuration and spatial delocalization of the ³CT and ³LE states’ wave functions. Factors beneficial to TADF obtained via theoretical computations are also briefly introduced.