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 unit at different positions along the chain (C 12 H 25 (C≡C) 2 (CH 2 ) n COOH, n = 7-11) were vacuum-deposited on clean silicon substrate. Morphologies of the initial films after UV-irradiation were studied. A clear odd-even effect on the morphology of 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 amidobenzoic acid head group led to similar odd-even effect but of different dimension or trend, which can be rationalized similarly. These results would open the opportunity to create aligned conjugated polymer chains of different dimensions through self-assembly for applications in molecular/organic electronics.