Spherulitic
crystallization in Langmuir–Blodgett (LB) films
of the ditetradecyldimethylammonium-Au(dmit)2 [2C14N+Me2-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 °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 °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 2C14N+Me2-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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