The phase transition of sashlike polydiacetylene (PD) atomic sash in an extrathin (0.4 nm) vapor-deposited monolayer on a graphite (0001) surface has been observed by in-situ scanning tunneling microscopy (STM). The conformation of the atomic sash composed of a row of lying alkyl chains stitched up with a PD chain is different around the crossing points of the alkyl and PD chains in the two phases, and hence the contrast of the STM image is different in the middle of the sashes. The difference is associated with the color transition of bulk PDs.

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When diacetylene RÀCtCÀCtCÀR 0 molecules are arranged in a bulk crystal, a LangmuirÀBlodgett (LB) film, or a selfassembled monolayer (SAM) etc. and irradiated with UV light, 1,4-polymerization takes place topochemically to yield a polydiacetylene (PD) (dCRÀCtCÀCR 0 d) n . 1À4 The π-conjugated systems have attracted attention on account of the electrical and optical properties. 5,6 Though a correlation between the isomerization and color of PDs had been a controversial issue, the color transition is correlated to a change in the conjugation length 7 or/ and that in the conformation of the substituents R and R 0 containing methylene sequences. 8,9 For such fundamental studies, the selection of simple substituents is crucial to elucidate the PD electronic and geometric structures interrelated closely.Most theoretical studies 10 are limited to unsubstituted PDs (R = R 0 = H), which are difficult to obtain in a laboratory owing to the extreme reactivity of the monomer, although there are a few first-principles theoretical studies on PDs with realistic substituents. 11,12 An alternative to the hydrogen is a n-alkyl group: one might assume that polyalkadiyne retains simplicity to directly link the real system with the theoretical consideration.To our knowledge, however, poly(2,4-hexadiyne) (R = R 0 = CH 3 ) is probably the only substituted PD for which the first-principles calculations have been reported. 10,11 From the experimental point of view, on the contrary, the aggregates of diacetylene molecules having alkyl groups are usually prepared by wet techniques. This means that R 0 inevitably contains a polar group at the end and complicates the intermolecular interactions and the molecular behavior upon polymerization. In addition, if the PD chains of an ordinary LB film 3 or a SAM 4 are buried in the close assembly of "standing" alkyl chains, they are hardly probed by extraordinarily surface-sensitive techniques such as Penning ionization electron spectroscopy (PIES) 13,14 and scanning probe microscopy. 15,16 For the direct observation of the local electronic and geometric structures, lying PD chains exposed outside the film are required, and atomic sash (AS) on a graphite (0001) surface 17À22 is appropriate for this purpose. ABSTRACT: 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, 5596À5597]. 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 R 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, 9894À9895]. A simplified model is constructed in this study ...

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