Stereocontrolled Synthesis and Photoisomerization Behavior of All-Cis and All-Trans Poly(<i>m</i>-phenylenevinylene)s

All-cis and all-trans isomers of poly[(arylenevinylene)-<i>alt</i>-(5-octyloxy-1,3-phenylenevinylene)]s having three kinds of arylene groups (PmPVs: arylene = <i>m</i>-phenylene (<b>1a</b>), <i>p</i>-phenylene (<b>1b</b>), 4,4′-biphenylene (<b>1c</b>)) have been synthesized in almost perfect stereoregularity by two types of palladium-catalyzed polycondensation reactions, respectively. Suzuki−Miyaura-type polycondensation of (<i>Z</i>,<i>Z</i>)-bis(2-bromoethenyl)arenes with 5-octyloxy-1,3-benzenediboronic acid pinacolate affords all-cis <b>1a</b>−<b>c</b>, whereas Hiyama-type polycondensation of (<i>E</i>,<i>E</i>)-bis(2-silylethenyl)arenes with 5-octyloxy-1,3-diiodobenzene forms all-trans <b>1a</b>−<b>c</b>. The resulting polymers with relatively short effective π-conjugation lengths interrupted by <i>m</i>-phenylene units undergo two-way photoisomerization to give PmPVs containing <i>cis</i>- and <i>trans</i>-vinylene groups in nearly 1:1 ratios irrespective of the arylene groups of staring polymers.