Fluorene vs. Spirobifluorene: Effect of the π‐System on TADF Properties

There are many options to design a molecular structure that could result in thermally activated delayed fluorescence (TADF). One promising strategy is to use the donor‐π‐acceptor motive where an electron‐donating unit is linked to an electron‐acceptor via an aryl moiety like phenyl. While this approach is widely used and well understood, the performance of the chromophores can be limited by different energy loss pathways, e.g. internal conversion or by π‐stacking. To circumvent these problems rigid structures with sterically demanding substituents are applied. In this work, we designed two TADF emitters based on phenothiazine and nitrile linked via spiro‐9,9’‐bi[fluorene] or 9,9‐dimethylfluorene and compared the effect of the linker on the physical properties of the dyes. This work emphasizes the importance of careful design of conjugated spacer for efficient TADF emitters.