New singlet oxygen donors based on naphthalenes: synthesis, physical chemical data, and improved stability.

Singlet oxygen donors are of current interest for medical applications, but suffer from a short half-life leading to low singlet oxygen yields and problems with storage. We have synthesized more than 25 new singlet oxygen donors based on differently substituted naphthalenes in only a few steps. The influence of functional groups on the reaction rate of the photooxygenations, thermolysis, half-life, and singlet oxygen yield has been thoroughly studied. We determined various thermodynamic data and compared them with density functional calculations. Interestingly, remarkable stabilities of functional groups during the photooxygenations and stabilizing effects for some endoperoxides during the thermolysis have been found. Furthermore, we give evidence for a partly concerted and partly stepwise thermolysis mechanism leading to singlet and triplet oxygen, respectively. Our results might be interesting for "dark oxygenations" and future applications in medicine.