Squaraine rotaxane endoperoxides (SREPs) are storable chemiluminescent compounds that undergo a clean cycloreversion reaction that releases singlet oxygen and emits near-infrared light when warmed to body temperature. This study examined the effect of solvent on SREP chemiluminescence intensity and found that acidic alcohols, such as 2,2,2-trifluoroethanol, α-(trifluoromethyl)benzyl alcohol, and 1,1,1,3,3,3-hexafluoroisopropanol, greatly increased chemiluminescence. In contrast, aprotic solvents, such as trifluoroethylmethyl ether, had no effect. The interlocked rotaxane structure was necessary as no chemiluminescence was observed when the experiments were conducted with samples containing a mixture of the two non-interlocked components (squaraine thread and macrocycle endoperoxide). Spectroscopic analyses of the enhanced SREP chemiluminescent reactions showed a mixture of products. In addition to the expected squaraine rotaxane product caused by cycloreversion of the endoperoxide, a diol derivative was isolated. The results are consistent with an endoperoxide O–O bond cleavage process that is promoted by the hydrogen bonding solvent and produces light emission from a squaraine excited state.
Optical Imaging of Bacterial Infection in Living Mice Using Deep-Red Fluorescent Squaraine Rotaxane Probes