Changes in the Electronic Structures of a Single Sheet of Sashlike Polydiacetylene Atomic Sash upon Structural Transformations

• Published in: Journal of physical chemistry. C 2011-05, Vol.115 (19), p.9518-9525
• Main Authors: Suhara, Masanori, Ozaki, Hiroyuki, Endo, Osamu, Ishida, Toshimasa, Katagiri, Hideki, Egawa, Toru, Katouda, Michio
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp109822j

Atomic sash (AS) is a single sheet of a sashlike macromolecule comprising a column of alkyl chains bridged by a polydiacetylene (PD) chain [H. Ozaki et al. J. Am. Chem. Soc. 1995, 117, 5596–5597]. The AS is produced by the intramonolayer polymerization of 17,19-hexatriacontadiyne molecules laid flat on a graphite (0001) surface under ultrahigh vacuum. In an AS conformer initially formed at low temperature (AS-I), the PD chain and the α carbon atoms of the alkyl chains are raised higher than other carbons of methylenes in contact with the substrate; with rising temperature, the AS-I is transformed to another conformer AS-II, in which all the carbon atoms are placed in a common plane [O. Endo et al. J. Am. Chem. Soc. 2004, 126, 9894–9895]. A simplified model is constructed in this study to obtain an optimized geometric structure of the AS-I, and its electronic structures are compared with those of the AS-II. The first-principles calculations for the model under periodic boundary conditions reveal that their energies are almost the same. The optimization also tells us the existence of the third stable conformer AS-III that has been suggested on the basis of STM images. In addition, it is found that the HOMO–LUMO energy gap of the AS-I is larger than that of the AS-II, and the orbital shapes dependent on the conformation successfully explain the result. This means that the AS-I → AS-II transformation of the ideal infinite polymer is not related to the chromatic transition of bulk PDs.