Intramolecular charge transfer versus intersystem crossing: The way toward super-high photothermal efficiency by thionation

Abstract

Development of organic molecules with high photothermal conversion efficiency (PCE) in the near-infrared (NIR) window is a vital yet challenging topic in the field of photothermal therapy. A series of thionated perylenediimides (PDI-4CHA-S) derivatives have been synthesized by replacing carboxylic oxygens in imides with sulfur atoms, providing chances to unravel the effect of thionation on the photophysical properties toward photothermal agents (PTAs) with high PCE. In the presence of the electron-donating substituents at the bay-position, sulfur atoms participate the intramolecular charge transfer (ICT), which induces remarkable redshift in the absorption maximum. Meanwhile, the ICT together with the heavy atoms effect of sulfur atoms promotes the intersystem crossing (ISC), resulting in the generation of the active oxygen species (ROS) upon photoexcitation. Interestingly, the quantum yield of ROS decrease with increasing the number of S-substitutes, which is the resultant of the attenuated ICT in the case of multiple S-substitution. Overall, the promotion in ISC due to thionation is negligible compared to the quenching of the radiation transition and enhancement of the absorption coefficient caused by ICT, hence the energy of the excited PDI-4CHA-S molecules is depleted mainly in the form of heat, leading to the ultrahigh PCE of 88.0% under 808 nm laser irradiation. In particular, the trithionated PDI-4CHA-3S performs surprisingly high PCE in the second near-infrared (NIR-II) window (1064 nm). The superior photothermal effect and excellent photostability of the thionated PDI derivative provide them promising photothermal agents for NIR photothermal therapy.