Temperature dependency of energy shift of excitonic states in a donor–acceptor type TADF molecule
Youichi Tsuchiya, Keito Mizukoshi, Masaki Saigo, Tomohiro Ryu, Keiko Kusuhara, Kiyoshi Miyata, Ken Onda, Chihaya Adachi
Abstract
In recent years, thermally activated delayed fluorescence (TADF) has attracted intense attention owing to its straightforward application to high-efficiency organic light-emitting diodes. Further, to develop high-performance TADF materials, many researchers have designed novel molecules that have a small energy gap between the lowest excited singlet and triplet states ( $$\Delta {E}_{{ST}}$$ ), and detailed analysis suggests a significant contribution of higher-lying excited states for spin flipping processes. In this study, we demonstrate a peculiar thermal behaviour of emission decay of a donor–acceptor type TADF molecule, TMCz-BO, which seems like thermal deactivation of delayed fluorescence that can be explained without a negative $$\Delta {E}_{{ST}}$$ by comprehensive kinetic analysis across various temperatures and solvents. While the activation energy has previously been treated as being temperature-independent, we stress that it should be a dynamic parameter affected by changing the solvent-solute interaction with the environmental temperature, especially in the case of a small energy gap. Thermally activated delayed fluorescence (TADF) has attracted much attention to developing high efficiency organic light-emitting diodes. Here, authors revisit the thermal behavior of a TADF material and report the temperature dependence of excitonic state energies which has not been focused so far.