Premelting behavior of a Langmuir-Blodgett film of dioctadecyldimethylammonium-Au(dmit)2 salt

Premelting behavior of a Langmuir-Blodgett (LB) film of dioctadecyldimethylammonium-Au(dMit)(2) (2C(18)Au(dMit)(2)) salt has been studied by means of infrared (IR) transmission spectroscopy. Two bands due to the CH2 antisymmetric stretching (v(as)(CH2)) and symmetric stretching (v(s)(CH2)) modes have been used to monitor the conformation and orientation of a hydrocarbon chain in the 2C(18)-Au(dmit)(2) LB film, and a band due to the C=S stretching mode has been employed to investigate the aggregation state of Au(dmit)(2) chromophores. The global phase mapping method in the generalized two-dimensional correlation spectroscopy (global 2D phase mapping method) has been used for examining the overlapping, shifting, and broadening of absorption components in the v(s)(CH2) band (2841-2856 cm(-1) region). Temperature-dependent IR spectral changes have revealed that there are, at least, four different ordered stages in the LB film below the order-disorder transition between 105 and 110 degreesC. (i) In the temperature range of 30-50 degreesC, the orientation and conformation of hydrocarbon chain and the aggregation state of Au(dmit)(2) chromophores do not change appreciably, in which the hydrocarbon chain contains the disordered component to some extent, compared to the most highly ordered state of the hydrocarbon chain at 60 degreesC. (ii) From 50 to 60 degreesC, the new aggregation state of Au(dmit)(2) chromophores appears, while the hydrocarbon chain changes drastically to the specific orientation where the transition moment of the v(s)(CH2) mode tends to be aligned parallel to the substrate surface, with the trans forms increasing. (iii) From 60 to 80 degreesC, the aggregation state of Au(dmit)(2) chromophores observed at 30 degreesC is almost completely convened into the new aggregation state, while the hydrocarbon chain keeps the specific orientation on average, with the gauche forms increasing. (iv) From 80 to 105 degreesC, the new aggregation state of Au(dMit)(2) chromophores remains almost unchanged until the order-disorder transition, while the hydrocarbon chain changes to the disordered state, with the gauche forms increasing. The above results have indicated that the heat treatments after the film deposition can be used for controlling the structure of the 2C(18)-Au(dMit)(2) LB film.