A series of different 1,3-dienes were selected (1 -9, Fig. ) in order to investigate the effects of molecular structure on the photosensitized dimerization. Among these, we first focused on 1 -7. This set of compounds allows us to explore the effects of (i) increasing the number of methyl substituents, (ii) the position of the methyl substituents on the conjugated diene, i.e., substitution at the central C2/C3 atoms versus the terminal C1/C4 atoms, and (iii) acyclic versus cyclic structures. Not all dienes 1 -7 are volatile, yet with these we can assess the structural effects on triplet sensitized photodimerizations of 1,3-dienes. Noteworthy, there is a fourth dimethyl-1,3-butadiene isomer, 2-methyl-1,3-pentadiene, however, it is not readily available and it has a boiling point of 78 °C. It was therefore excluded. In a later section, we discuss the larger cycloalka-1,3-dienes 8 and 9 in their T1 states in order to probe the effect of ring-size on the twisting of the diene segments and its impact on the diene dimerization yields (vide infra). We did not consider the small 1,3-cyclopentadiene as it dimerizes thermally within hours via the Diels-Alder reaction.