Abstract:
Unlike conventional fluorescence or phosphorescence materials, next generation organic lightemitting materials can harvest both the first excited singlet (S1) and first excited triplet (T1)
states for light emission with a nearly 100% of emission efficiency. In particular, materials
showing thermally activated delayed fluorescence (TADF), so-called TADF materials or TADF
molecules, have attracted considerable attention because TADF materials can be consisted of
only light elements, such as H, C, and N, having a potential to reduce a production cost of
organic light-emitting diodes (OLEDs). Currently, a synthesis of highly efficient of TADF materials is a serious demand for the development of OLEDs. A lack of understand of the TADF
emission mechanism is the bottleneck of the synthesis of TADF molecules. TADF, which is a
radiative transition from S1 states produced via thermally up-conversion of T1 state, is generally
known to rely on the energy difference between S1 and T1. Therefore, control of the energy difference enhances the TADF emission efficiency. However, the actual rate cannot be determined
by the energy difference only. This fact suggests an importance of excited-state dynamics for
the TADF emission mechanism. Ultrafast time-resolved spectroscopies are powerful tools to
study excited-state dynamics of materials. Especially, transient absorption spectroscopies can
directly detect time behaviour of both S and T state. In this talk, I give a general introduction
of TADF materials and concept of thermal up-conversion of T state. Then, using our ultrafast
spectroscopic techniques, I show our recent results of the detail emission mechanism of TADF.
Our results aid in designing highly efficient TADF materials.
