THz absorption measurement and calculation of organic thermally activated delayed fluorescence materials

Summary form only given. The research of organic semiconductor materials are increasingly investigated for organic light emitting diode (OLED) and the other organic light emitting devices. Recently, a novel scheme of the fluorescence system, “thermally activated delayed fluorescence (TADF)” has been reported with new several molecular designs that shows very high internal electroluminescence quantum yield of almost 100 % [1, 2]. In this scheme, the reversive inter system crossing from T1 state to S1 state can be activated by a heating corresponding to the energy around 30-300 meV. The energy region is almost corresponds to the photon energy of FIR and terahertz light. On the other hand, a ring-cavity-based terahertz parametric oscillators (ring-TPOs) [3] have been developed in photon energy region of 4.8-48 meV (1-4 THz in frequency). In this report, the TADF molecules' optical absorption properties were studied in frequency region of 0.6-1.6 THz by using a ring-TPO because the single frequency irradiation will be needed for the interaction study. To discuss the belonging of the experimental absorption from the powder state of TADF molecule, density functional theory (DFT)-based calculations were also performed for comparison. It revealed that each absorption property originated from the intramolecular motions of the electron donor and acceptor units. To the best of our knowledge, terahertz spectroscopy has never been attempted to investigate the structural properties of TADF materials [4].