Kobayashi, T; Niwa, A; Haseyama, S; Takaki, K; Nagase, T; Goushi, K; Adachi, C; Naito, H; ,
A four-level model consisting of a higher triplet excited state (T-2), the lowest singlet and triplet excited states (S-1 and T-1), and the ground state was previously used to understand emission properties of a thermally activated delayed fluorescence (TADF) emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). In this report, we discuss the four-level model in more detail and apply to the other two TADF emitters, i.e., 1,2,4,5-tetrakis(carbazol-9-yl)-3,6-dicyanobenzene (4CzTPN) and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene(2CzPN), in order to determine their excited-state structures. It is suggested that in all the emitters T-2 lies between S-1 and T-1 and play an essential role in the emitting process. In 4CzTPN, phosphorescence from T-2 is clearly observed around 100 K as in 4CzIPN. Compared to the other two emitters, 2CzPN has a wider energy gap between S-1 and T-1 so that delayed fluorescence at room temperature is thought to be mixed with phosphorescence. Because of this mixing, the spectrum characteristic of phosphorescence from T-2 in 2CzPN cannot be identified. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)