Competition and regulation between triplet phosphorescence and singlet oxygen generation efficiency

Abstract

The utilization of triplet state energy is important in numerous applications, including triplet phosphorescence emission and triplet photosensitizers in photodynamic therapy. This study revealed the competition between phosphorescence emission and energy transfer from the triplet state of metalloporphyrins to surrounding oxygen. Imidazole was used as the triplet protector to regulate the interaction between the triplet state and surrounding oxygen, based on a series of phosphorescent metalloporphyrin-based photosensitizers. With increasing imidazole concentration in the metalloporphyrin solution, phosphorescence intensity increased, whereas the efficiency of singlet oxygen generation decreased. For example, the phosphorescence quantum yields for gadolinium-hematoporphyrin monomethyl ether (Gd-HMME) after dialysis (against imidazole), undialyzed Gd-HMME, and undialyzed Gd-HMME with additional imidazole were 0.02%, 1.40%, and 1.60%, respectively, while the singlet oxygen quantum yields were 49%, 40%, and 19%, respectively. The same phenomenon was also observed for other porphyrins coordinated to Gd ions and other metal-ion-containing metalloporphyrins. Our results provide a strategy for achieving balanced phosphorescence and photosensitivity from metalloporphyrins to design multifunctional materials.