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

A long-lived triplet charge separated state (3CS state lifetime: 0.56 mu s) wasobserved in acompactelectron donor-acceptor dyad with electron donor phenothiazine (PTZ)and acceptor anthraquinone (AQ) directly connected by a single C-N bond (AQ-PTZ). The1CS state energy (2.0 eV in cyclohexane) is lower than those of the3AQ (2.7 eV) or the3PTZstate (2.6 eV). By oxidation of the PTZ unit, thus increasing of the1CS state energy (2.7 eV incyclohexane), thermally activated delayedfluorescence (TADF) was observed [Tau= 17.7 ns(99.9%)/1.5 mu s (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectraconfirm the electron spin multiplicity of the3CS state, and the zero-field-splitting (ZFS)parameters|D|and|E|are 48.2 mT and 11.2 mT, respectively. These results are useful for designof compact electron donor-acceptor dyads to access the long-lived3CS state and study theTADF mechanism.