The spectroscopic properties and photoisomerization reactions of several (E)- and (Z)-cinnamic esters, bis cinnamic esters, and model esters and lactones in the presence and absence of Lewis acids have been investigated. The use of Lewis acids such as BF3 or EtAlCl2 results in enhanced photoisomerization efficiency and a shift in the photoequilibrium toward the thermodynamically less stable Z isomer. Enhanced E → Z photoisomerization results from selective excitation of ground-state ester–Lewis acid complexes. These complexes have been characterized by 1H NMR, ultraviolet, and fluorescence spectroscopies. The equilibrium constants for complexation are dependent upon both the electron donor strength of the ester and its conformational mobility. These factors also determine the magnitude of the red shifts in the electronic absorption spectra observed upon complexation. Enhanced E → Z photoisomerization upon complex formation is a consequence of selective excitation of the E vs. Z complex, more efficient isomerization of the excited E vs. Z complex, and larger equilibrium constants for complexation of E vs. Z esters. The photoequilibria obtained for bis cinnamic esters are highly enriched in the Z,Z and Z,E isomers in accord with independent isomerization of the two cinnamate groups; however, in the case of 1,3-trimethylenebis(cinnamate), two-bond isomerization of the E,E to Z,Z isomer is observed at low conversions.
ASJC Scopus subject areas
- Colloid and Surface Chemistry