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 ( 3 CS state lifetime: 0.56 μs) was observed in a compact electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond ( AQ-PTZ ). The 1 CS state energy (2.0 eV in cyclohexane) is lower than those of the 3 AQ (2.7 eV) or the 3 PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the 1 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 3 CS 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 design of compact electron donor-acceptor dyads to access the long-lived 3 CS state and study the TADF mechanism.