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    Spherulitic Crystallization in Langmuir-Blodgett Films of the Ditetradecyldimethylammonium-Au(dmit)(2) Salt

    Year: 2020

    Journal: Langmuir, Volume 36, 1-Jan, page 554–562

    Authors: Miura, Yasuhiro F.; Akiyama, Hironari; Sugimoto, Naoki; Akagi, Yoshiya; Aoyama, Tetsuya; Shiroishi, Hidenobu; Takahashi, Mitsuo

    Organizations: Ministry of Education, Culture, Sports, Science, and Technology (MEXT)Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT); JSPS KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [22560011, 15K04655]

    Spherulitic crystallization in Langmuir-Blodgett (LB) films of the ditetradecyldimethylammonium-Au-(dmit)(2) [2C(14)N(+)Me(2)-Au(dmit)(2)] salt has been characterized by polarized light microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) analyses. Analyses by differential scanning calorimetry (DSC) for the bulk crystals indicate that annealing in the temperature range of 58-100 degrees C may be appropriate to improve the order in the LB film. The polarized light microscopy measurement further revealed that a spherulite structure was formed after the film was annealed at 80 degrees C for 60 min. FT-IR spectroscopy measurements demonstrated that the order of the principal hydrocarbon chains was improved and that the rotation of CH3 groups was hindered by the annealing. Out-of-plane XRD analyses revealed that the d-spacing of the 2C(14)N(+)Me(2)-Au(dmit)(2) LB film changed from 3.1 to 2.5 nm upon annealing. We hypothesize that a layered structure with interdigitated hydrocarbon chains, which is equivalent to or close to that of the corresponding bulk crystal, has been realized in the LB film by annealing. We consider that two different kinds of one-dimensional (1D) interactions along the a-axis are the driving forces to realize the spherulite structure in the LB system; one is an extended 1D contact due to the pronounced interdigitation of the alkyl chains of the ammonium ion, which plays a role similar to that of the folding of chains in the lamellar structures of polymers, and the other is a 1D extended sulfur sulfur contact between Au(dmit)(2) dimer pairs. So far, spherulite formation in LB films has been reported almost exclusively for polymerized materials. Here, we demonstrate that a spherulite texture can also be formed in LB films based on nonpolymerized materials via the interdigitation of hydrocarbon chains, leading to a new well-ordered state.
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