Effect of Thionation of the Carbonyl Groups in Naphthalimide‐Phenoxazine Electron Donor–Acceptor Dyads on the Excited‐ State Dynamics: Transient Optical and Electron Paramagnetic Resonance Spectral Studies

We performed thionation of the electron acceptor naphthalimide (NI) moiety in donor-acceptor (D-A) dyads in a series of thio-substituted D-A dyads. Thionation significantly enhances the electron-accepting ability of the chromophore, facilitating the formation of low-lying charge-separated (CS) states. Femtosecond transient absorption spectra revealed intersystem crossing (ISC) rate constants of S-NI-PXZ (k_ISC = 3.8 × 10¹¹ s^-1) and DS-NI-PXZ (k_ISC = 2.4 × 10¹¹ s^-1), which are interestingly independent of the number of sulfur atoms. Generally, fluorescence quenching was observed due to the rapid ISC process. Native NI-PXZ exhibits thermally activated delayed fluorescence (TADF). Thionation increased the singlet-triplet energy gap, making the TADF vanish. In n-hexane, nanosecond transient absorption spectroscopy detected the triplet lifetimes shortened from 11.6 µs of NI-PXZ to 0.66 µs for S-NI-PXZ and 0.25 µs for DS-NI-PXZ. Time-resolved electron paramagnetic resonance revealed the large zero-field splitting D value (4200-9800 MHz) of the triplet state of the thionated derivatives, which is due to the much larger SOC matrix elements of triplet state of DS-NI-PXZ (ca. 135 cm^-1) and DS-NI (ca. 136 cm^-1) than the nonthionated NI (3.1 cm^-1). Thionation of the carbonyl groups in chromophore is a promising method for design of triplet photosensitizers to achieve fast ISC.