A didecyloxy-substituted poly(phenylenedicyanovinylene), DiCN-PPV, has been synthesized. The dicyano-substituted vinylene units exist in both trans and cis (similar to 65:35) conformations as determined by H-1 NMR analysis, and cannot be converted to all trans due to the presence of a thermodynamic equilibrium of the two conformations, in contrast to the vinylene units in regular PPVs. The unusually high cis content makes this polymer highly amorphous, very soluble in organic solvent, and highly fluorescent in the solid state with an estimated quantum yield up to 0.34, four times more fluorescent than its chloroform solution. The LUMO and HOMO energies of the new polymer were measured by cyclovoltammetry. The cyano groups in DiCN-PPV brings a decrease in LIJMO energy by 0.79 eV, and makes the polymer more stable to intense white light (>20 times as strong as the sunlight) than poly(2,5-didecyloxy-1,4-phenylenevinylene), C10O-PPV, by more than 2 orders of magnitude. The excellent photochemical stability and high fluorescence quantum yield in the solid state make DiCN-PPV a good candidate for outdoor fluorescent applications such as remote optical sensing.