Time-resolved transient optical and electron paramagnetic resonance spectroscopic studies of electron donor-acceptor...

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, τ = 0.8 ps) → Franck-Condon singlet charge separation state (¹CS, τ = 7.8 ps) → ¹CS state (τ = 2.2 ns) → triplet state (³LE, τ = 16 μs). 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.