Supramolecular assemblies (1,2) of organic materials represents an exciting perspective for the future trend in device applications, such as sensors, memories, optoelectronic device and semiconductive junctions as well. Among the innovative organic materials the metallophthalocyanines (MPc), represents one of the most interesting class of synthetic compounds due to their electronic, optical, chemical and structural properties (3,4). MPc are organic semiconductors and the understanding of their supramolecular assemblies, namely solid state arrangements, affect strongly the device properties, i.e. the solid state properties. In this work, a copper MPc (CuPc) was considered and then processed by the Langmuir-Blodgett technique (LB) (5) in order to obtain supramolecular assemblies aimed to sensors and optoeletronic devices.
Langmuir films of CuPc were obtained at the air/liquid interface by proper selection of the subphase and then characterized by the [pi]-A isotherms and by the Brewster Angle Microscopy (BAM) (6). This microscopy allowed one to reveal directly the 2D-3D transformations of the film which occur during its formation as well as to image its morphology. The simultaneous recording of [pi]-A isotherm permitted one to speculate about the molecular arrangement at the air/water interface. The analysis underlined that the CuPc formed stable Langmuir films (area variation less than 1 %/min). Defects and domains were mainly imaged at surface pressures lower than 5-10 mN/m, whereas uniform Langmuir films were visible at higher surface pressures. 3D structures started to appear exceding the surface pressure of 20-25 mN/m suggesting the beginning of an aggregating process induced by the molecular organization or a collapse phenomenon of the film at the air/water interface. Langmuir films were transferred onto a solid support by the LB vertical dipping and by the Langmuir-Schaefer methods (7). Quartz Crystal Microbalance, UV/Vis spectroscopic, and electro-optical measurements provided evidence concerning the molecular packing degree, the chemical structure and the optoelectronic properties of CuPc molecules when arranged in a supramolecular assembly respectively. A BAM analysis on deposited films underlined the possibility of fabricate of uniform films as well as to confirm that the 2D-3D trasformations, occuring at the air/water interface, were irreversible. Supramolecular assemblies were fabricated by depositing alternating layers of CuPc and cadmium arachidate and then structurally characterized by X-ray synchrotron radiation. X-ray curves revealed several Bragg reflections, allowing one to suggest the structural model of the realized assemblies. Measurements at different temperatures showed structural reorganizatios in the layers.
References
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