The one-dimensional π-electronic systems of polydiacetylene (PD) have attracted much interest of solid state and polymer scientists because PD is one of fundamental materials to understand the conductivity of organic compounds. To our knowledge, experimental studies on the band structures of PD have not been performed yet, partly because of difficulties in preparing the specimens with well-defined structures. We have recently constructed a flat array of sashlike PD (atomic sash (AS)) molecules on a surface of highly oriented pyrolytic graphite (HOPG) by the UV polymerization of a self-assembled 17,19-hexatriacontadiyne (HTDY) monolayer [1,2]. Since HOPG comprises crystallites with the axes of the honeycomb lattice oriented randomly, however, AS domains with various PD directions are produced in the reaction. Therefore, a specimen on a single crystal surface is required to investigate the anisotropic electronic structures. Molybdenum disulfide (MoS2) is a layered material similar to graphite and a single crystalline substrate is available. In this study, we have tried to obtain AS molecules on MoS2(0001) under UHV. It was found by STM observation that HTDY molecules adsorbed at 150 K form relatively unstable columnar structures above 240 K; these columns are converted into AS molecules when irradiated with UV light at room temperature. Most AS molecules exhibit AS-III structure: the zigzag planes of the all-trans alkyl chains and the PD backbone plane intersect obliquely. Since the AS-III is one of the most stable structures for an isolated AS molecule, it is suggested that the molecule-substrate interactions are weaker for MoS2(0001) than for graphite (0001).
[1] H. Ozaki, T. Funaki, Y. Mazaki, S. Masuda, Y. Harada, J. Am. Chem. Soc. 117 (1995) 5596.
[2] O. Endo, H. Ootsubo, N. Toda, M. Suhara, H. Ozaki, Y. Mazaki, J. Am. Chem. Soc. 126 (2004) 9894.
|