An Activatable Triplet Sensitizer Based on Triplet Electron Transfer and Its Application for Triplet-Triplet Annihilation Upconversion.

Activatable triplet photosensitization refers to a photosentization process which can be turned on/off easily by external stimulus. Activatable triplet photosensitizations are normally achieved by interfering with the singlet excited state before the intersystem cross process (ISC), i.e., the formation process of triplet states of sensitizer. To achieve novel activatable triplet photosensitization, a disulfide-bridged porphyrin zinc(II) dyad (ZnPor-S-S-ZnPor) is prepared. Although fast ISC can be conducted in this dyad, an extremely low efficiency is obtained when employing this dyad as a triplet donor in triplet-triplet annihilation upconversion (TTA-UC) for sensitizing perylene. This is because of the presence of electron transfer from the triplet state of the porphyrin zinc(II) unit to the disulfide bond, which quickly quenches the triplet state of the porphyrin zinc(II) unit. This electron transfer process can be stopped by the cleavage of the disulfide bond in the presence of thiol, and TTA-UC efficiency can be enhanced significantly. Our result demonstrates for the first time that the disulfide bond can act as not only an easy cleavage linker but also a triplet electron acceptor. Furthermore, quenching the triplet states of sensitizer by triplet electron transfer provides an alternative protocol for designing activatable triplet sensitizers except controlling the singlet excited state before the ISC process.