Minimizing the Electron Donor Size of Donor–Acceptor-Type Photosensitizer: Twisted Intramolecular Charge-Transfer-Induced Triplet State and Singlet Oxygen Formation
Wenbin Hu, Mingyu Liu, Xian‐Fu Zhang, Meng Shi, Mingxu Jia, Xiaofang Hu, Linlin Liu, Tan Wang
Abstract
Efficient and selective heavy atom-free singlet oxygen photosensitizers (HAFPS) are highly desirable for chemistry, pharmaceutical sciences, phototherapy, and environmental sciences. The donor–acceptor (D–A)-type HAFPS based on charge transfer/charge recombination mechanism usually requires that a photosensitizing chromophore A (such as boron dipyrromethene (BODIPY)) is attached by a large and perpendicular aryl donor D in its ground state. In this report, however, we show that much smaller non-aryl dialkylamino donors also enable BODIPY to generate singlet oxygen and excited triplet state selectively and very efficiently in nonpolar media. The mechanism is that the photoexcitation of NR2-BODIPY leads to the twisting of the NR2 so that a vertical geometry is formed, which best facilitates T1 formation via intramolecular charge transfer/charge recombination processes. This small NR2 group modification does not need any aryl (pyrenyl, anthracyl, BODIPY, naphthyl, and phenyl) to enhance the intersystem crossing; therefore the synthesis is very facile and the material is atom-economic. The synthesis contains neither halogen atoms nor heavy metal ions so that this novel strategy provides more environment- and health-friendly singlet oxygen photosensitizers for use in chemical synthesis, pharmaceutical sciences, phototherapy, environmental sciences, and physiology.