Circularly Polarized Thermally Activated Delayed Fluorescence Emitters in Through-Space Charge Transfer on Asymmetric Spiro Skeletons.

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters and are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in and have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor-acceptor distance. thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of , i.e., 12.5%, is much lower than that of . The chirality of these two spiro emitters results in circularly polarized luminescence. Because has a more distorted molecular architecture than , the luminescence dissymmetry factor (||) of circularly polarized luminescence of one enantiomer of the former, namely, either or , is almost twice that of /. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with of 1.30 × 10 and 1.0 × 10 for and , respectively.