Identifying Different Spin Mixing Channels Occurring in Charge-Transfer States

Charge-transfer (CT) states play a critical role in harvesting nonemissive triplets realized by triplet-to-singlet conversion through spin mixing to generate thermally activated delayed fluorescence (TADF) by providing a spin-orbital coupling (SOC) and small energy difference (ΔEST). This paper discusses the spin mixing directly identified by magneto-photoluminescence (magneto-PL) for intramolecular and intermolecular CT states based on a synthesized donor-acceptor-donor (D-A-D) type molecule DMTD-Cz. The PL spectra indicate that this D-A-D molecule shows intramolecular CT states - only in solutions - but both intramolecular and intermolecular CT states in solid-state thin films, allowing separate identification of the spin mixing occurring in intramolecular and intermolecular CT states using magneto-PL measurements. We found that intramolecular and intermolecular CT states exhibit negligible and appreciable magneto-PL signals up to 900 mT at room temperature, respectively. Simultaneously, the intramolecular and intermolecular CT states show negligible and appreciable delayed fluorescence, respectively, in thin films. These results provide the direct observation that the SOC generates trivial and vital spin mixing within intramolecular and intermolecular CT states. This indicates that the triplets in intermolecular CT states can be harvested by using a spin mixing mechanism while the triplets in intramolecular CT states should be harvested by using a spin conserving mechanism through localized triplet excitons toward developing efficient organic light-emitting materials.