Atomic force microscopy and theoretical investigation of the lifted-up conformation of polydiacetylene on a graphite substrate

The structure of a single polydiacetylene compound on a graphite substrate was investigated using atomic force microscopy (AFM). The linear conjugated polydiacetylenes were obtained through chain polymerization of a monomolecular layer of diacetylene compound on a graphite substrate under ultraviolet light irradiation. AFM observations revealed that the polydiacetylenes were imaged higher than the unpolymerized monomer rows. This result supports the 'lifted-up' conformation model, in which the polydiacetylene backbone is geometrically raised. To investigate why the polymer backbone is lifted, we also carried out first-principles density-functional calculations in the local density approximation. These calculations suggested that the steric hindrance between the alkyl side-chains of the monomers and the oligomer caused the lifted-up conformation.