Recently organic light emitting diodes (OLEDs) based on thermally-activated delayed fluorescence (TADF) materials have gained considerable attention owing to their promising device performance without using expensive rare metal elements in the emitting materials. To obtain a better device performance, new efficient TADF molecules are now rapidly developed based on the kinetics and quantum chemistry as proposed in a report of the first TADF-based OLEDs in 2009 by a group of Adachi in Kyusyu Univ. [1] For the strategy, they pointed out to reduce the energy difference (dE) between singlet (S1) and triplet (T1) states (to enhance a reverse intersystem crossing), and suppress non-radiative decay for the triplet state. Indeed, in 2012 Uoyama et al. succeeded to report a nearly 100 per cent of internal photoluminescence quantum efficiency (IPQE) by synthesizing 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), carbozolyl dicyanobenzene (CBDC) derivatives [2]. However, compared to the rapid progress of new TADF materials, fundamental understanding of the mechanism leading to such an extremely high IPQE is largely delayed.
Here we studied the excited-state dynamics of CBDC derivatives by means of pump-probe transient absorption spectroscopy (TAS). While kinetics of TADF materials have been studied by transient photoluminescence technique, direct observation of their excited state was hardly conducted. The aim of this study is to shed light on dynamical behavior of excited states of efficient TADF molecules, which may give a deeper insight into efficient TADF mechanism. CBDC derivatives can be considered as suitable candidates for such a study, since the efficiency of IPQE largely depends on the position and number of side carbozole bonding to the center dicyanobenzene moiety. TAS measurements using femtosecond (fs)- and nanosecond (ns) pulse lasers can probe directly the behavior of both S1 and T1 states from the initial stage of the excitation. By performing fs- and ns-TAS, we found a clear evidence of an intermediate state formed during intersystem crossing for CBCDs, especially for 4CzIPN. This result suggests that the excited-state dynamics of CBCDs is not simply understood from a classical simple TADF model based on S1 and T1 states. The origin of the intermediate state as well as the excited-state dynamics of CBCDs will be discussed. Ref. [1] A. Endo, et al., Adv. Mater., 21, 4802 (2009). [2] H. Uoyama, et al. Nature, 492, 234 (2012)