Importantly, and in contrast to 1,3-cyclohexadiene, isoprene is volatile enough to escape from the cyanobacteria cells, facilitating its harvest. Yet, it is still a liquid at room temperature, which makes it facile to handle in the photochemical dimerization step. Such advantages put isoprene as the ideal 1,3-diene in our two-step photobiological-photochemical route to jet fuels. The smallest conjugated diene, 1,3-butadiene, has a lower photoreactivity and is a gas at ambient temperature (b.p. -4 °C). The lower reactivities of other linear conjugated dienes result from their higher number of methyl substituents and their radical character in the T1 state structures being placed in internal carbons, which leads to more hindered radicals and provide for hyperconjugative stabilization. Indeed, our results showed that the methyl substitution patterns affect the photosensitized dimerization of 1,3-dienes, while the large difference in the triplet lifetimes dominates the effects for cyclic 1,3-dienes.