Photoisomerization of fluorinated 1,3-dimethyl-5-propenyl uracils

We investigated the photo-induced cis/trans isomerization of double bonds in fluorinated derivatives of 1,3-dimethyl-5-propenyl uracils. When 1,3-dimethyl-5-(E-1,2,3,3,3-pentafluoropropenyl)uracil was irradiated by light λ > 300 nm, we obtained the photostationary state involving equilibration with 1,3-dimethyl-5-(Z-1,2,3,3,3-pentafluoropropenyl)uracil where the more sterically congested isomer Z was the major product (E:Z = 1:1.5). Irradiation of Z isomer as the starting material also led to the identical photostationary state. In the case of 1,3-dimethyl-5-(1,3,3,3-tetrafluoropropenyl)uracil the more sterically congested isomer E was even more abundant (Z:E = 1:7.5). The results of DFT calculations suggest that photo-induced cis/trans isomerization of these compounds can run through the formation of allylmethylene biradical species, which are the transition states produced during photoexcitation. Preference of the formation of more sterically hindered isomers in photostationary state one can rationalize on the basis of the difference in oscillator strength and the role of equilibration between conformers s-cis/s-trans.