A mixed film of 10,12-pentacosadiynoic acid (DA) and an amphiphilic cationic hemicyanine (SP) in the ratio 1:1 has been fabricated. This mixed film has been proved to be completely homogeneous due to the good compatibility between both molecules, i.e., hydrophobic regions sized and polar regions of opposite charge. This adequate balance between the vertical sections of the hydrophobic and hydrophilic groups allows the formation of an ion pair between them. In this mixed film, DA molecules organize in monolayer instead of trilayer as occurs for pure DA film. Surface pressure measurements (isotherms and compression−expansion cycles), Brewster angle microscopy, reflection spectroscopy, and PM-IRRAS prove this structure. Using BAM to reveal the structure of this mixed film, circular domains with internal anisotropy because the ordered alignment of hemicyanine groups (strong self-aggregation) are observed. Under UV irradiation a new polyacetylene (PDA) form was fabricated in a homogeneous mixed monolayer despite of the distance measured between the DA molecules by the presence of SP dye. The polymerization takes place only in the circular domains. Under expansion the domains forming during compression and after UV irradiation form a nanocomposite stable material with partially elastic outer region.

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Polydiacetylene (PDA) and its derivatives are promising materials for applications in a vast number of fields, from organic electronics to biosensing. PDA is obtained through polymerization of diacetylene (DA) monomers, typically using UV irradiation. DA polymerization is a 1-4 addition reaction with both initiation and growth steps with topochemical control, leading to the "blue" polymer form as primary reaction product in bulk and at interfaces. Herein, the diacetylene monomer 10,12-pentacosadiynoic acid (DA) and the amphiphilic cationic N,N'-dioctadecylthiapentacarbocyanine (OTCC) have been used to build a mixed Langmuir monolayer. The presence of OTCC imposes a monolayer supramolecular structure instead of the typical trilayer of pure DA. Surface pressure, Brewster angle microscopy, and UV-vis reflection spectroscopy measurements, as well as computer simulations, have been used to assess in detail the supramolecular structure of the DA:OTCC Langmuir monolayer. Our experimental results indicate that the DA and OTCC molecules are sequentially arranged, with the two OTCC alkyl chains acting as spacing diacetylene units. Despite this configuration is expected to prevent photopolymerization of DA, the polymerization takes place without phase segregation, thus exclusively leading to the red polydiacetylene form. We propose a simple model for the initial formation of the "blue" or "red" PDA forms as a function of the relative orientation of the DA units. The structural insights and the proposed model concerning the supramolecular structure of the "blue" and "red" forms of the PDA are aimed at the understanding of the relation between the molecular and macroscopical features of PDAs.

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