Contributions of a Higher Triplet Excited State to the Emission Properties of a Thermally Activated Delayed-Fluorescence Emitter

Thermally activated delayed-fluorescence emitters attract considerable attention for highly efficient, low-cost organic light-emitting diodes that do not rely on heavy metals. The decay mechanism of excited states in these emitters is generally understood in terms of intersystem crossings between the lowest singlet and triplet excited states. However, the authors show that the second-lowest triplet excited state also plays an important role, and that the singlet-triplet energy gap cannot be determined accurately without considering it.