Supramolecular organization of highly conducting organic thin films by the Langmuir-Blodgett technique

Abstract

The optimization of organic materials in semiconductor devices or as electrical conductors in devices is closely related to the optimization of electroactive molecular structures which can self-assemble into highly ordered solid films when cast from solution. This supramolecular optimization problem is dissected in the present feature article by focusing on self-assembly of thin films at the interface between water and air (Langmuir films). Recent advances towards fabrication of highly conducting Langmuir-Blodgett (LB) films is reviewed. Owing to new accurate structural methods, including synchrotron X-ray diffraction and scanning probe microscopy (in particular atomic force microscopy, AFM), the steps in the assembly process of Langmuir-Blodgett films can now be studied in unprecedented detail. This is exemplified for the electroactive molecular surfactant 2-octadecylsulfanyl-p-benzoquinone, and for amphiphilic derivatives of regioregular polythiophene. In the first case control over the packing of the electroactive part of the molecule is lost because the alkyl chains dominate the packing. For amphiphilic regioregular polythiophene the ability to π stack is the important feature that controls the assembly process leading to highly conducting polycrystalline films (σ≈100 S cm ^–1 ). Comparisons of prototype systems such as disks, boards and rods dressed with alkyl chains show that in cases where π stacking is allowed to dominate over alkyl chain packing the structures formed at the air-water interface resemble bulk structures of similar molecules. Based on these analogies the link between the LB-assembly process and other solution based methods for film fabrication (such as spin coating) is discussed.