Time-resolved transient optical and electron paramagnetic resonance spectroscopic studies of electron donor-acceptor thermally activated delayed fluorescence emitters based on naphthalimide-phenothiazine dyads

The photophysics of naphthalimide (NI)-phenothiazine (PTZ) dyads were investigated as electron donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) emitters. Femtosecond transient absorption (fs-TA) spectra show that the photophysical processes in non-polar solvents are in singlet localized state (1LE, tau = 0.8 ps) -> Franck-Condon singlet charge separation state (1CS, tau = 7.8 ps) -> 1CS state (tau = 2.2 ns) -> triplet state (3LE, tau = 16 mu s). The 3LE state is formed via the spin-orbit charge transfer-intersystem crossing (SOCT-ISC) mechanism rather than the spin-orbit (SO)-ISC mechanism. In a polar solvent, the CS state has a much lower energy than the 3LE state; thus, the 3LE state is absent from the photophysical processes and no TADF was observed. Moreover, we found that the delayed fluorescence lifetime is related to the low-lying triplet state (3LE or 3CS states). When the 3CS state is the low-lying triplet state, the TADF lifetime is shorter than that of the 3LE state as the low-lying triplet state. In the time-resolved electron paramagnetic resonance (TREPR) spectra, both 3LE (zero field splitting parameter D = 2250 MHz, E = -150 MHz) and 3CS (D = 430 MHz, E = 0 MHz) states were observed. It is noteworthy that the electron spin polarization (ESP) phase pattern of the 3CS state was inverted at longer delay times as a consequence of the selective transition between the 3LE and 3CS states and a faster decay of one sublevel of the 3CS state. These results are strong and direct experimental evidence for the spin-vibronic coupling mechanism of TADF.