Bipyridine Derivatives at a Solid/Liquid Interface: Effects of the Number and Length of Peripheral Alkyl Chains

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Abstract
Bipyridine derivatives (bpys) with various number and length of peripheral alkyl chains (with carbon numbers of n = 11−17) were synthesized, and their self-assembled monolayers were observed by scanning tunneling microscopy (STM) at a 1-phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. The effects of the number, the substitution position, and the length of alkyl chains on the two-dimensional structures were systematically studied. Bpys substituted by a single alkyl chain in the p-position on each side adopted an almost linear form with zigzag-type alignment of the π-conjugated unit, whereas, in the case of m-substitutution, the bpys showed Z-shaped morphology with interdigitated alkyl chains. In both cases, no odd−even alkyl chain length effects were observed. The bpys with double alkyl chains at m- and p-positions displayed odd−even alkyl chain effects, suggesting that the formation of two-dimensional structure is dominated by the interactions between alkyl chains. Bpys with triple alkyl chains at o-, m-, and p-positions also showed odd−even alkyl chain effects, but only for the higher number of carbon atoms in the alkyl chain unit (n = 14−17). These results indicate that concerted intermolecular interactions of the alkyl chain unit introduce the odd−even chain length effect on the self-assembled two-dimensional structure. After coordination of PdCl2, odd−even effects were quenched, and bpys were converged into the same lamellar structure, in which the molecules are almost linear. All the structural differences due to the odd−even alkyl chain length effect were explained in terms of intermolecular and molecule−substrate interactions.

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