Torsion Angle Analysis of a Thermally Activated Delayed Fluorescence Emitter in an Amorphous State Using Dynamic Nuclear Polarization Enhanced Solid-State NMR

The torsion angle between donor and acceptor segmentsof a thermallyactivated delayed fluorescence (TADF) molecule is one of the mostcritical factors in determining the performance of TADF-based organiclight-emitting diodes (OLEDs) because the torsion angle affects notonly the energy gap between the singlet and triplet but also the oscillatorstrength and spin-orbit coupling. However, the torsion angleis difficult to analyze, because organic molecules are in an amorphousstate in OLEDs. Here, we determined the torsion angle of a highlyefficient TADF emitter, DACT-II, in an amorphous state by dynamicnuclear polarization enhanced solid-state NMR measurements. From theexperimentally obtained chemical shift principal values of N-15 on carbazole, we determined the average torsion angle to be 52 & DEG;.Such quantification of the torsion angles in TADF molecules in amorphoussolids will provide deep insight into the TADF mechanism in amorphousOLEDs.