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

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 S 1 /S 0 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 S 1 /S 0 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 S 1 /S 0 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.