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

Abstract

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters <b>SFST</b> and <b>SFOT</b> 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 <b>SFST</b> and <b>SFOT</b> have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. <b>SFOT</b> 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 <b>SFST</b> introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor-acceptor distance. <b>SFST</b> thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of <b>SFST</b>, i.e., 12.5%, is much lower than that of <b>SFOT</b>. The chirality of these two spiro emitters results in circularly polarized luminescence. Because <b>SFST</b> has a more distorted molecular architecture than <b>SFOT</b>, the luminescence dissymmetry factor (|<i>g</i>_lum|) of circularly polarized luminescence of one enantiomer of the former, namely, either <b>(<i>S</i>)-SFST</b> or <b>(<i>R</i>)-SFST</b>, is almost twice that of <b>(<i>S</i>)-SFOT</b>/<b>(<i>R</i>)-SFOT</b>. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with <i>g</i>_EL of 1.30 × 10^-3 and 1.0 × 10^-3 for <b>(<i>S</i>)-SFST</b> and <b>(<i>S</i>)-SFOT</b>, respectively.