Modulation of nonradiative emission decay rate by spacer unit in donor-acceptor TADF compounds

Highly emissive thermally activated delayed fluorescence (TADF) compounds typically are constructed from twisted donor and acceptor units, usually with the help of an additional spacer unit. Such molecular structure ensures low singlet-triplet energy gap, enabling the utilization of triplet states and boosting the emission yield. Here we show that emission yield also depends on the molecular structure of spacer unit. Selecting structurally flexible biphenyl spacer unit instead of rigid fluorene was shown to selectively enhance the nonradiative decay rate of triplet states (kNRT), leading to the lowered yield of delayed fluorescence (from 0.55 to 0.43). However, more rapid kNRT led in weaker OLED external quantum efficiency (EQE) roll-off due to lowered triplet lifetime, though at the cost of lower peak efficiency. The presented work highlights the need of careful design of spacer fragments for efficient TADF emitters.