TRIPLET PHOTOSENSITIZERS
Triplet photosensitizers are molecules or molecular complexes able to strongly absorb light, preferably in the red-infrared spectral region, and produce triplet states with high quantum yields. They find application in numerous fields ranging from biomedicine to the development photonic devices. The main goal in this field is to develop photosensitizers with optimized properties, such as low toxicity, low production costs and easy syntheisis. Fully organic materials hold great promises in this sense.
MOLECULAR MECHANISM OF PHOTODYNAMIC THERAPY
The main application of triplet photosensitizers in the biomedical field is photodynamic therapy (PTD), a light activated cancer treatment. In PTD, light absorption and triplet production from the photosensitizers triggers the formation of cytotoxic reactive oxygen species (ROS), which penetrate into the cancer cells and are able to kill them.
SOCT-ISC
Compact electron Donor-Acceptor organic dyads, in which the two moieties assume a perpendicular orientation, can effiently produce triplets through a mechanims named Spin Orbit Charge Transfer Inter System Crossing (SOCT-ISC). For instance, we have characterized the ISC efficiency of a dyad composed by a Bodipy and a Perylene unit, and applied it as a novel PTD reagent.
This simple system has a very low dark phototoxixity if compared to molecules including heavy atoms, and can induce the production of a significant amount of ROS. For more details look at this paper.
Radical Enhanced-ISC (RE-ISC)
A further method to induce ISC in heavy atom free organic dyads is that of constructing systems where a stable radical is connected to a chromophore with a relatively short linker. In this way, the total spin multiplicity of the system is that of a doublet, because of the unpaired electron localized on the radical. Upon photoexcitation triplet formation can be facilitated.
Read more: Chemistry Eur. J., 2019, 25, 15615-15627
TRIPLET-TRIPLET ANNIHILATION UP-CONVERSION
Another application of triplet photosensitizers is Triplet-Triplet Annihilation upconvertion (TTA). In this case, upon light absorption and triplet formation, energy transfer can occur among two triplet states. As the result of this process, one molecule comes back to its ground state, while the other one is promoted to a higher energy singlet state, from which emission can occur. The final outcome of this process is to upconvert a low energy photon, initially absorbed from the triplet photosensitizer, into a higer energy emitted photon.
TTA Upconversion using Bodipy sensitizers and a perylene acceptor, see J. Phys. Chem. A 2017, 121, 40, 7550-7564