Laser photoisomerization of methyl α-arylcinnamates; effect of chloro substitution

An investigation of the 308 nm laser photochemistry of methyl α-phenylcinnamate (1a) and a series of p-chloro-substituted derivatives is reported. Dilute solutions of (E)-1a or (Z)-1a undergo photoisomerization, but also form ring closure products analogous to those observed during the photolysis of cis-stilbene. The photostationary state (PSS) compositions (E/Z) were determined by analytical high performance liquid chromatography (HPLC). The PSS values are not dependent on the laser power or the presence of oxygen. However, they are unusually rich in the E isomer compared with the PSS of alkyl cinnamates (over 70% E compared with about 50% E for alkyl cinnamates). p-Chloro substitution on the α ring causes a small increase in the E content of the PSS, whereas p-chloro substitution on the β ring leads to a dramatic reduction in the E content of the PSS (28% E). Possible explanations for these results are discussed.