Electronic Structure and Triplet–Triplet Energy Transfer in Artificial Photosynthetic Antennas

Three Pd(II) phthalocyanine–carotenoid dyads featuring chromophores linked by amide bonds were prepared in order to investigate the rate of triplet–triplet (T-T) energy transfer from the tetrapyrrole to the covalently attached carotenoid as a function of the number of conjugated double bonds in the carotenoid. Carotenoids having 9, 10 and 11 conjugated double bonds were studied. Transient absorption measurements show that intersystem crossing in the Pd(II) phthalocyanine takes place in 10 ps in each case and that T-T energy transfer occurs in 126, 81 and 132 ps in the dyads bearing 9, 10 and 11 double bond carotenoids, respectively. To identify the origin of this variation in T-T energy transfer rates, density functional theory (DFT) was used to calculate the T-T electronic coupling in the three dyads. According to the calculations, the primary reason for the observed T-T energy transfer trend is larger T-T electronic coupling between the tetrapyrrole and the 10-double bond carotenoid. A methyl group adjacent to the amide linker that connects the Pd(II) phthalocyanine and the carotenoid in the 9 and 11-double bond carotenoids is absent in the 10-double bond carotenoid, and this difference alters its electronic structure to increase the coupling

Artificial Photosynthesis: An Approach for a Sustainable Future

International audienceThe energy needs of humankind has experimented a sharp increase since the beginning of the Anthropocene due to a large increase in population and the evolution of our society’s lifestyle. Recent projections suggest that it will likely lead to a major crisis due to environmental issues associated with the increasing use of fossil fuel as major energy source, as well as due to a rapid dwindling of the classical and easily accessible fossil-fuels stocks. These issues require a quick response if the lifestyle adopted by our societies shall be sustained. Several solutions have been envisioned to tackle these problems, of which, the development of Artificial Photosynthetic systems is one of the most appealing. The field of artificial photosynthesis takes Nature itself as a source of inspiration, to propose alternative energy harvesting and storage strategies. This field of research not only aims at mimicking the main processes that permitted photosynthetic organisms to thrive and become the most successful autotrophs on earth, but as well at improving and optimizing these processes using synthetic materials. In this chapter, the underlying mechanisms that enable photosynthetic organisms to convert (and store) solar energy into a directly usable chemical energy will be discussed. Then it will be explained how these concepts can be extended to artificial systems and ultimately used to our own benefit