Two-dimensional solid-state topochemical reactions of 10,12-pentacosadiyn-1-ol adsorbed on graphite

The photoinduced topochemical reaction of the diacetylene (DA) compound 10,12-pentacosadiyn-1-ol in the two-dimensional (2D) crystal phase adsorbed on graphite was examined by scanning tunneling microscopy (STM). The reaction efficiency and the structure of the generated polydiacetylene depend on its polymorphic forms, i.e., the "herringbone" or the "parallel" monomer arrangements. The reaction efficiency of the herringbone arrangement is lower than that of the parallel arrangement because the distance R between the probable reactant acetylenic carbon atoms of the herringbone arrangement is longer than that of the parallel arrangement. However, the fact that polydiacetylenes form from the herringbone arrangement (R = 0.58 nm) is contrary to the geometric criteria for the polymerization of three-dimensional (3D) crystals (the reaction was only observed if R < 0.4 nm). The polymerization criterion for the 2D phase differs from that of the 3D phase. In addition, the STM cross-section profiles of the polydiacetylenes reveal that the "lifted-up" and "in-plane" conformations form from the parallel and herringbone arrangements, respectively.