Self‐Assembly Behavior of Diacetylenic Acid Molecules upon Vapor Deposition: Odd–Even Effect on the Film Morphology

A series of linear carboxylic acids containing diacetylenic units at different positions along the chain (C12H25(C≡C)2(CH2)nCOOH, n=7–11) were vacuum‐deposited on clean silica substrates. The morphologies of the initial films after UV irradiation were studied. A clear odd–even effect on the morphology of the initial film was observed in that, depending on the spacer length between the diacetylenic unit and carboxyl head group, rings or dendrites of acid dimer layers were obtained. A molecular dynamic simulation of the aggregation process suggests that two competing intermolecular interactions and thus aggregation directions are involved and modulated by the odd or even carbon chain length. Further modulation of the interaction by substitution of a phenyl group at the terminus of the chain or by changing the carboxyl head group to an amidobenzoic acid head group led to a similar odd–even effect but with different dimensions or trends, which can be rationalized similarly. These results give the opportunity to create aligned conjugated polymer chains of different dimensions through self‐assembly for applications in molecular/organic electronics. Vacuum‐deposited thin films of three series of diacetylenic acids exhibit distinctly different morphologies, depending on whether the spacer chain length is an odd or even number of carbons (odd–even effect). The carboxyl head groups form cyclic dimers, the orientation of which relative to the trans‐zigzag plane of the hydrocarbon chain appears to be different when the spacer chain length is an odd or even number, which determines the direction of aggregation during crystal growth giving unique ring‐type or two‐dimensional dendrite‐type morphologies.