Thiosquaraine rotaxanes: synthesis, dynamic structure, and oxygen photosensitization

Thiosquaraine dyes have sulfur atoms instead of oxygens on the central squaraine core, and they are powerful singlet oxygen photosensitizers. Stability studies show that they are susceptible to attack by nucleophiles. This problem was circumvented by preparing a mechanically interlocked thiosquaraine rotaxane. NMR studies of the rotaxane indicate an unusual dynamic molecular structure due to a nonsymmetrical coconformation. Upon irradiation with red light, the thiosquaraine rotaxane generates the same amount of singlet oxygen as the known photosensitizer methylene blue.