Molecular Vibration Accelerates Charge Transfer Emission in a Highly Twisted Blue Thermally Activated Delayed Fluorescence Material

In the development of new organic light-emitting diodes, thermally activated delayed fluorescence (TADF) materials have drawn interest because of their ability to upconvert electrically generated triplet excitons into singlets. Efficient TADF requires a well-balanced large transition dipole moment (mu) between the lowest excited singlet state (S-1) and the ground state (S-0) and a small energy splitting (Delta E-ST) between S-1 and the lowest triplet state (T-1). However, a number of highly twisted donor-acceptor-type TADF molecules have been reported to exhibit high performance in OLEDs, although these molecules may sacrifice mu in exchange for a very small Delta E-ST. Here, we theoretically investigate the origin of efficient emission from a perpendicularly twisted blue emitter, MA-TA. In this system, the mu value almost vanishes in the static approximation; however, vibrational contributions increase mu considerably. Hence, we show that the dynamics of excitons have a critical role in such TADF systems.

Automated summary

Efficient organic light-emitting diodes rely on well-balanced transition dipole moments and small energy splitting between excited singlet and triplet states, with some highly twisted donor-acceptor-type TADF molecules showing high performance despite sacrificing the former for the latter. Study suggests that the dynamics of excitons play a critical role in such TADF systems.