Z/E isomerization of poly‐para‐phenylenevinylene model compounds

The poly‐para‐phenylenevinylene (PPV) model compound 1 with cyano groups on the vinylene moieties and 2‐ethylhexyloxy chains on the central phenylene unit has been synthesized by Knoevenagel condensation of 1,4‐bis(cyanomethyl)‐2,5‐bis(2‐ethylhexyloxy)benzene and benzaldehyde. The only isomer that was obtained has been identified to have the Z,Z configuration. UV irradiation of the Z,Z isomer 1a in solution with a medium‐pressure mercury lamp gave a mixture of all three possible isomers (Z,Z, Z,E and E,E; 1a, 1b and 1c respectively). The isomers were separated by column chromatography and identified spectroscopically. Electrochemical studies and fluorescence emission spectra of 1a and 1c are reported. The substituted PPVs 2, 3 and 4 have been synthesized and their absorption and emission spectra before and after UV irradiation are compared.