A soluble all-aromatic poly(2,5-diphenyl-1,4-phenylenevinylene) (2,5-DP-PPV) is synthesized by utilizing aromatic phosphonium and aldehyde monomers through Wittig reaction. The H-1 NMR and FTIR measurements indicate that over 50% content of cis-vinylene units exist in polymer backbone. The diphenyl-substituted benzaldehyde monomer plays an important role to enhance cis-products (Z-selectivity) in Wittig reactions. The twisted cis-segments in polymer backbone reduce the interchain interactions and enhance the solubility of such all-aromatic PPV derivative in common organic solvents. 2,5-DP-PPV exhibits good solubility in common organic solvents, such as tetrahydrofuran and chloroform. The polymer film exhibits a blue light emission (lambda(max) = 485 nm) and a very high photoluminescence efficiency of 78%. The cis-trans photo isomerization of this polymer in solution and the impact on the optical properties are also investigated. (c) 2008 Wiley Periodicals, Inc.