Interestingly, a varying degree of selectivity in the photodimerizations is observed in the chromatograms since some dienes produced a larger number of different dimer isomers than others (Fig. -S4, ESI). In the case of 5, the large number of dimer isomers in the products can be traced back to the different isomers in the starting material. Considering that we are interested in the total yield of dimers, we do not assess the specific structures of each of the dimers produced. Earlier studies have explored the structure of dimers in detail for some of the 1,3-dienes, and found that photosensitized dimerization of isoprene, 1,3-cyclohexadiene, and 2,3-dimethyl-1,3-butadiene were observed as, respectively, seven, three and seven different dimers. The compositions of these product mixtures (dimer isomers) were strongly dependent on the photosensitizers employed and on the distribution between the available diene conformers in the S0 state. Interestingly, the various 1,3-dienes exhibit a wide range in the yields of their dimers, spanning from 93% down to values below 9%. Among the acyclic dienes that can be used neat (2 -6), 2 displayed the highest yield, followed by 3 and 4, while the lowest was obtained for 5 and 6 (both ~9%). At this point, it can be noted that 5 exhibits also a low quantum yield of sensitized Z/E-photoisomerization. The 1,3-butadiene photodimerization could only be run in hexane solution (15 wt.% as commercially available), and the observed yield was merely 6.5%. Now, when 2 and 5 (the most and the least reactive acyclic diene, respectively) were run in hexane under the same conditions as those of 1, the yield of dimers of 2 was still higher (18%) than that of 1, but that of 5 decreased from 9% to 1% (Table , footnote e). These results show that 5 is much less reactive than 1, and together with 6, the least reactive of the acyclic 1,3-dienes. As 2 still gives the highest yield among our selected small acyclic 1,3-dienes, it becomes apparent that 2 has advantages over 1 from a photochemical point of view, besides that it already can be produced photobiologically. A potential benefit of 1 could be that it should evaporate more easily from the cells than 2.