In order to investigate the exact effect of stereoregular packing of head group in the side chain on the helical structure formation of polydiacetylene backbone, the larger size of bisazobenzene-substituted diacetylene monomer, 4-(4-nitrophenylazo) azobenzene-10, 12-pentacosadiynoate (BNADA) was synthesized successfully. Owing to overcrowded packing of bisazobenzene chromophores, the BNADA Langmuir-Blodgett (LB) films showed macroscopic supramolecular chirality, although BNADA molecules were achiral. Under circularly polarized UV light (CPUL) irradiation, supramolecular helix of bisazobenzene chromophores always maintained, due to the large size and lower photo-isomerization rate of bisazeo-benzene chromophores. While for polydiacetylene backbone, the helical direction of the polymer chain should be decided by the competition of the effect of stereoregular packing of bisazobenzene chromophores and the interaction between the CPUL and the diacetylene dimer. (C) 2010 Elsevier Inc. All rights reserved.