Structures of 17,19-Hexatriacontadiyne Monolayers on Au(111) Studied by Infrared Reflection Absorption Spectroscopy and Scanning Tunneling Microscopy Osamu Endo, ² Taro Furuta, ² Hiroyuki Ozaki,* ,² Masashi Sonoyama, ‡ and Yasuhiro Mazaki § Department of Organic and Polymer Materials Chemistry, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan, Department of Applied Physics, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Chemistry, School of Science, Kitasato UniVersity, Sagamihara, Kanagawa 228-8555, Japan ReceiVed: September 13, 2005; In Final Form: March 6, 2006 The aggregation and reaction of 17,19-hexatriacontadiyne molecules are studied on a Au(111) surface. The molecular orientation and arrangement are elucidated by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). A vapor-deposited monolayer and a multilayered film formed by adsorption from the solution provide IRA spectra with bands due to the antisymmetric and symmetric stretching of methylenes in the gauche conformation. After the adsorbed film is rinsed with the solvent, however, the spectrum loses the gauche bands and is characterized by the enhanced C-H distal and C-H proximal stretching bands, which means that all-trans molecules are laid flat. Only STM images for the rinsed film display columnar structures on the herringbones of the reconstructed Au(111) surface; the alkyl chain direction is found to be parallel to the Au atom row. The results indicate that an ordered monolayer is formed first at the liquid-solid interface, and then, disordered overlayers with the gauche conformation are grown but removed by a rinse. Upon exposure to UV light, thus obtained monomer columns are converted into oligomers with flexible backbones and an increased gauche population in the alkyl chains, which resemble red phase polydiacetylenes in LB films. Introduction The self-assembly of chain hydrocarbons lying on metal surfaces is one of the intriguing phenomena in surface and interface chemistry. The very early study of low-energy electron diffraction showed that short n-alkanes lie flat to form a monolayer with columnar (or lamellar) structures on Pt(111) under an ultrahigh vacuum (UHV). 1 Nowadays, the columnar structures are displayed more directly for n-alkanes and the related compounds with diverse lengths and functional groups by scanning tunneling microscopy (STM), mostly in monolayers at liquid-graphite (0001) and liquid-Au(111) interfaces. 2-14 The elucidation of the monolayer structures enables us to expect the practical use of the inherent functionalities of the monolayers for applications including electronic devices, catalysts, and nanomaterials. We have planed such a development using an alkadiyne monolayer. A column of lying 17,19-hexatriacontadiyne (HTDY; C 16 H 33 C≡CC≡CC 16 H 33 ) molecules self-assembled in a mono- layer on a graphite (0001) surface is converted into a single sheet of sashlike polydiacetylene (PD) [atomic sash (AS)] upon exposure to UV light (Figure 1). 15-18 PDs having one- dimensional π electronic systems can be endowed with conduc- tive properties and exhibit chromatic transition (blue to red or vice versa) induced by various stimuli including temperature, voltage, light, ligands, and pH. 19-25 The chromatic transition is accompanied by changes in the electronic structure, effective conjugation length, and conductivity. For the AS molecules with lying PDs on graphite (0001), both the blue and the red phases have been detected by Raman spectroscopy. 26 In addition, the phase transition of the AS between two structures with different contrasts in the STM images, which may also have some relation to the chromatic transition of the PDs, has been revealed recently. 18 To understand the mechanism of polymerization in each HTDY column as well as that of the AS transition, we must make a thorough investigation on the structure of the HTDY monolayer. The orientation and arrangement of HTDY molecules, the internal structure of the AS, and changes in the electronic structures during the intramonolayer polymerization were confirmed by Penning ionization electron spectroscopy 15,16 and * To whom correspondence should be addressed. E-mail: hiroyuki@ cc.tuat.ac.jp. ² Tokyo University of Agriculture and Technology. ‡ Nagoya University. § Kitasato University. Figure 1. Models for (a) the arrangement of HTDY molecules laid with their carbon zigzag planes parallel to the surface (flat-on orientation) in a monolayer and (b) the structure of a single sheet of a sashlike polydiacetylene (AS) formed by the intramonolayer photopo- lymerization of HTDY molecules in each column of part a. 15-18 Note that part b is for one of the AS conformers (AS-II), in which all of the carbon atoms of the polydiacetylene chain and the alkyl chains are held in a common plane. 18 13100 J. Phys. Chem. B 2006, 110, 13100-13106 10.1021/jp055161+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006