Ultrafast photophysics of an orange–red thermally activated delayed fluorescence emitter: the role of external structural restraint

Abstract

The application of thermally activated delay fluorescence (TADF) emitters in the orange–red regime usually suffers from the fast non-radiative decay of emissive singlet states (k^S_NR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although k^S_NR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the k^S_NR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and k^S_NR. Spectroscopic and theoretical investigations indicated that S₁/S₀ ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic–vibrational coupling through the Huang–Rhys factor, while k^S_NR is largely affected by the reorganization energy of the promoting modes. By restraining S₁/S₀ ES-SR in doping films, the k^S_NR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S₁/S₀ ES-SR, promoting modes and k^S_NR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.