Theoretical and Experimental Considerations of the Direct Photo-Isomerization and Chemistry of 1,3-Pentadiene

Potential energy curves as a function of the angle of twist around single and double bonds have been calculated for piperylene (1,3-pentadiene). Also, direct-excitation cis→trans photoisomerization and trans→methycyclobutene photochemistry have been investigated as a function of the temperature. The theoretical analysis strongly indicates that the cyclobutene is formed photochemically from the trans-s-cis isomer which is created thermally from the trans-homer in the ground state (not by photoisomerization). The temperature-dependency data support this conclusion. The theoretical data also indicate that there is a small barrier to photoisomerization around the double bond in the first excited singlet (and triplet) state. The temperature-dependency data also support this conclusion. The potential energy curves are in harmony with the known emission (none exists), photoisomerization, and internal-conversion data for the cis- and trans-piperylenes. The emission properties of model cyclic diene steroids are compared with those expected from linear dienes, and the emission results of the former are interpreted within the theoretical framework developed.