Temperature dependency of energy shift of excitonic states in a donor–acceptor type TADF molecule

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:msub><mml:mrow><mml:mi>E</mml:mi></mml:mrow> <mml:mrow><mml:mi>S</mml:mi> <mml:mi>T</mml:mi></mml:mrow> </mml:msub> </mml:math> ), 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:msub><mml:mrow><mml:mi>E</mml:mi></mml:mrow> <mml:mrow><mml:mi>S</mml:mi> <mml:mi>T</mml:mi></mml:mrow> </mml:msub> </mml:math> 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.