Towards combined photobiological-photochemical formation of kerosene-type biofuels: Which small 1,3-diene photodimerizes most efficiently?

A transition from fossil- to bio-based hydrocarbon fuels is required to reduce greenhouse gas emissions, yet, traditional biomass cultivation for biofuel production competes with food production and impacts negatively on biodiversity. Recently, we reported a proof-of-principle study of a two-step photobiological-photochemical approach to kerosene biofuels in which a volatile hydrocarbon (isoprene) is produced by photosynthetic cyanobacteria, followed by its photochemical dimerization into C10 hydrocarbons. Both steps can utilize solar irradiation. Here we report the triplet state (T1) sensitized photodimerization of a broader set of small 1,3-dienes to identify which structural features lead to rapid photodimerization. Neat 1,3-cyclohexadiene gave the highest yield (93%) after 24 hours of irradiation at 365 nm, followed by isoprene (66%). The long triplet lifetime of 1,3-cyclohexadiene, which is two orders of magnitude longer than those of acyclic dienes, is key to its high photoreactivity and stem from its planar T1 state structure. In contrast, while isoprene is conformationally flexible, it has both photochemical and photobiological advantages as it is the most reactive among the volatile 1,3-dienes and it can be produced by cyanobacteria. At the end, we explored the influence of solvent viscosity, diene concentration, and triplet sensitizer loading on the photodimerization, with a focus on conditions that are amenable when the dienes are produced photobiologically. Our findings should be useful for the further development of the two-step photobiological-photochemical approach to kerosene biofuels.