Heptacyclic spiro-TADF emitters with efficient long-wavelength emission via through-space interactions

Efficient thermally activated delayed fluorescence (TADF) emitters can be developed using intramolecular through-space charge transfer (TSCT) in spiro compounds featuring a donor-σ-acceptor (D-σ-A) configuration. However, traditional TSCT TADF molecules often struggle with weak charge transfer, making long-wavelength emission challenging. In this study, a series of efficient TSCT TADF molecules with curved structures was synthesized by incorporating heptacyclic spiro structures with various donors. Although the spiro-carbon atom disrupts through-bond conjugation, the curved molecular configuration enhances intramolecular through-space interactions. These interactions, including intramolecular carbon-carbon and hydrogen-involved interactions, increase molecular rigidity and boost photoluminescence quantum yield up to 98%. The robust intramolecular TSCT allows precise tuning of long-wavelength emissions from yellow to red (556–647 nm) in dilute toluene solutions. Organic light-emitting diodes (OLEDs) based on these molecules exhibit efficient long-wavelength electroluminescence from yellow-green to red (544–626 nm), achieving a maximum external quantum efficiency of up to 22.1%. This represents the first TSCT TADF system with D-σ-A configuration capable of producing yellow, orange, and red electroluminescence, offering new strategies for developing efficient red and near-infrared TSCT TADF emitters.