Study of the thermally-activated delayed fluorescence (TADF) mechanism of phenothiazine–dibenzothiophene-S,S-dioxide electron donor–acceptor dyads using steady-state and time-resolved optical and electron paramagnetic resonance spectroscopies

We studied a series of thermally activated delayed fluorescence (TADF) emitters based on phenothiazine-dibenzothiophene-S,S-dioxide (PTZ-DTO) electron donor-acceptor (D-A) dyads, using femtosecond/nanosecond transient absorption (fs/ns-TA) spectroscopy and pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy. In the analyzed compounds, a Se atom replaced the S atom in the phenothiazine (PTZ) unit to study the heavy atom effect on the reverse intersystem crossing (RISC). Moreover, oxidation of the PTZ to sulfoxide and sulfone was also used to tune the energy of the charge separated (CS) state, while keeping the other factors (3LE state energy, LE = locally excited, and electronic coupling between the donor and acceptor) intact to large extent. Fs-TA spectra show that charge separation occurs rapidly (ca. 7.6 ps) in non oxidized compunds, while for the compounds with the PTZ unit oxidized the CS is slightly slower (10.9 ps). Ns-TA spectroscopy demonstrated the coexistence of 3CS and 3LE states for the non oxidized dyads and the absence of heavy atom effect on RISC. The oxidation of the PTZ unit increased the CS state energy, so that only the 3LE state remains observed. All our results demonstrated that RISC rate constants are not enhaced in the presence of heavy atoms for the studied dyads. TREPR spectra show the presence of the 3LE state, and that the triplet state is formed most likely via spin-orbit charge transfer intersystem crossing (SOCT-ISC) because the electron spin polarization (ESP) phase pattern is (e, a, e, a, e, a). These studies are useful for an in-depth understanding of the photophysics of the electron D-A TADF emitters.