The advancement of photochemical processes for macromolecular tailoring has provided polymer chemists with new opportunities to construct advanced and complex polymer structures. The unique abilities of such light-mediated reactions enable establishing spatiotemporal control over the polymerization reaction as well as conducting polymerization under mild and environmentally-friendly conditions by utilizing the energy of light to drive forward chemical transformations. In this connection, it is highly desirable to utilize such photochemical processes that operate efficiently with high-wavelength, low-energy light. The use of high-energy UV light in some cases may result in undesirable side-reactions which render their applicability in biological conditions. To overcome these drawbacks, novel photochemical strategies working at higher wavelength with efficient low-energy light sources have been the focus of recent research in this area.
Recently a group of researchers from the University of New South Wales, Australia, has developed a new strategy to carry out photoinduced living radical polymerization utilizing high-wavelength, low-energy near-infrared (NIR) and far-red light sources. Shanmugam, Boyer and co-workers have reported the use of a biocatalyst, namely bacteriochlorophyll a, with light harvesting capabilities which absorbs light in the NIR and far-red regions, as efficient photoredox catalyst for reversible addition-fragmentation chain transfer (RAFT) polymerization process. Bacteriochlorophyll a is a photosynthetic bacteria which enables anoxygenic photosynthesis in dark, deep-sea waters by absorbing NIR and far-red light and utilizing sulfide, elemental sulfur, or hydrogen instead of water as an electron donor. The authors took advantage of bacteriochlorophyll a to carry out RAFT polymerization with low-energy NIR LED light sources, diminishing some undesirable side-reactions as a result of using high-energy UV and visible light to produce well-defined polymers with controlled molecular weight properties and high chain end fidelity.
In the process, bacteriochlorophyll a is used as a photoredox catalysis which facilitates photoinduced electron transfer reaction under NIR LED irradiation reducing the RAFT agent to generate initiating radicals. The propagating radicals can be deactivated by the oxidized bacteriochlorophyll a regenerating initial ground-state photocatalyst and the RAFT agent containing polymer chain. This photocatalytic cycle can be re-initiated further to grow polymer chains in a controlled manner. Furthermore, the NIR light being capable of penetrating depth was used to successfully conduct polymerization with the light source screened by a paper barrier, which reveals the potential of this process for biomedical applications where deep penetration is required without causing damage to living tissues.
Read more on this work:
Light-Regulated Polymerization under Near-Infrared/Far-Red Irradiation Catalyzed by Bacteriochlorophyll a, Angew. Chem. Int. Ed., 2015, 10.1002/anie.201510037