Long-Lived Triplet Charge Separated State and Thermally Activated Delayed Fluorescence in a Compact Orthogonal Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyad.

A long-lived triplet charge separated state (CS state lifetime: 0.56 μs) was observed in a electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond (). The CS state energy (2.0 eV in cyclohexane) is lower than those of the AQ (2.7 eV) or the PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the CS state energy (2.7 eV in cyclohexane), thermally activated delayed fluorescence (TADF) was observed [τ = 17.7 ns (99.9%)/1.5 μs (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectra confirm the electron spin multiplicity of the CS state, and the zero-field-splitting (ZFS) parameters || and || are 48.2 mT and 11.2 mT, respectively. These results are useful for design of compact electron donor-acceptor dyads to access the long-lived CS state and study the TADF mechanism.