Thionated Perylenebisimides as Heavy‐Atom‐Free Triplet Photosensitizers: Intersystem Crossing, Electron Spin Dynamics, and Application in Photodynamic Therapy

The thionation induced intersystem crossing (ISC) in perylenebisimide (PBI) derivatives was studied. The triplet state lifetimes of the thionated PBIs are substantially shortened (0.12-0.78 µs) as compared to the unsubstituted PBI (ca. 130 µs). Moreover, there is a clear trend of shortening of the triplet excited state lifetimes and the degree of thionation of the carbonyl groups in PBI. These findings rationalize the previous report of the contradictory lower ¹O₂ quantum yields for the PBI derivatives with more carbonyl groups thionated. Time-resolved electron paramagnetic resonance (TREPR) spectral study shows that the zero field splitting parameters D (1625-1992 MHz) of the lowest triplet states of thionated PBI are larger than that of the pristine PBI chromophore (D = 1166 MHz), which is attributed to the influence of sulfur atoms. Interestingly, the electron spin polarization (ESP) pattern of the TREPR spectra is inverted at longer delay time. Importantly we observed ISC for the dianion of the thionated PBI derivatives, and photoexcited dianion species can be used as super strong reductant for photocatalysis with the advantage of long excited state lifetimes (18-85 µs), and the excited state oxidation potentials are -1.47 to -1.77 V (versus Fc/Fc⁺).