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

The optimization of organic materials in semiconductor devices or as electr ical conductors in devices is closely related to the optimization of electr oactive molecular structures which can self-assemble into highly ordered so lid films when cast from solution. This supramolecular optimization problem is dissected in the present feature article by focusing on self-assembly o f thin films at the interface between water and air (Langmuir films). Recen t advances towards fabrication of highly conducting Langmuir-Blodgett (LB) films is reviewed. Owing to new accurate structural methods, including sync hrotron X-ray diffraction and scanning probe microscopy (in particular atom ic force microscopy, AFM), the steps in the assembly process of Langmuir-Bl odgett films can now be studied in unprecedented detail. This is exemplifie d for the electroactive molecular surfactant 2-octadecylsulfanyl-p-benzoqui none, and for amphiphilic derivatives of regioregular polythiophene. In the first case control over the packing of the electroactive part of the molec ule is lost because the alkyl chains dominate the packing. For amphiphilic regioregular polythiophene the ability to pi stack is the important feature that controls the assembly process leading to highly conducting polycrysta lline films (sigma approximate to 100 S cm(-1)). Comparisons of prototype s ystems such as disks, boards and rods dressed with alkyl chains show that i n cases where pi stacking is allowed to dominate over alkyl chain packing t he structures formed at the air-water interface resemble bulk structures of similar molecules. Based on these analogies the link between the LB-assemb ly process and other solution based methods for film fabrication (such as s pin coating) is discussed.