Self-assembled 2D patterns of structural isomers in isobutenyl compounds revealed by STM at solid/liquid interface

Abstract

The 2D patterns of structural isomers in isobutenyl compounds possessing long alkyl tails were systematically investigated by using scanning tunneling microscopy at solid/liquid interface. The alkyl chains with carbon numbers of 18 and 19 were linked with either amide or ester bonds at different positions of naphthalene unit. In addition to the 2,3-substituted isobutenyl compounds of our previous studies, the 2,6- and heterogeneously substituted 2,3–2,6-isobutenyl compounds were investigated in the present study. In the 2,6-substituted amide compounds (2,6-NCn), the intermolecular hydrogen bonds lead to the columnar structure without the interdigitation of alkyl chains, whereas 2,6-substituted ester compounds (2,6-OCn) showed the 2D structure with interdigitated alkyl chains. In the 2,3–2,6-substituted compounds containing amide linked alkyl chains (2,3-2,6-NCn), the alkyl chain unit exhibited a herring bone-like arrangement possibly due to the defined structural framework based on a intramolecular hydrogen bond. In contrast, flexibility of 2,3-2,6-OCn allowed to form the 2D structure with parallel alignment of the naphthalene unit and interdigitation of alkyl chains. The rearranged structural isomers of isobutenyl compounds could be obtained by the thermal reaction of tandem Claisen rearrangement (TCR). The different orientations of naphthalene units in the 2,6-NCnTCR were found depending on the alkyl chain length in order to avoide the steric repulsion between alkyl chains, namely odd-even effect of alkyl chains was emerged. In the case of 2,6-OCnTCR, odd-even effect was projected as presence or absence of alkyl chain interdigitation. The alkyl chain length had no influence on the 2D structures of 2,3-2,6-NCnTCR, whereas 2D structures of 2,3-2,6-OCnTCR showed slight difference in the orientation of naphthalene units depending on the alkyl chain length. These results suggest that a wide variety of 2D structures can be formed by structural isomers with different alkyl chain length, and that the TCR can be used for tuning the emergence of odd-even effect.