Simultaneous Edge-on to Face-on Reorientation and 1D Alignment of Small pi-Conjugated Molecules Using Room-Temperature Mechanical Rubbing

In this study, room-temperature mechanical rubbing is used to control the 3D orientation of small -conjugated molecular systems in solution-processed polycrystalline thin films without using any alignment substrate. High absorption dichroic ratio and significant anisotropy in charge carrier mobilities (up to 130) measured in transistor configuration are obtained in rubbed organic films based on the ambipolar quinoidal quaterthiophene (QQT(CN)4). Moreover, a solvent vapor annealing treatment of the rubbed film is found to improve the optical and charge transport anisotropy due to an increased crystallinity. X-ray diffraction and atomic force microscopy measurements demonstrate that rubbing does not only lead to an excellent 1D orientation of the QQT(CN)4 molecules over large areas but also modifies the orientation of the crystals, moving molecules from an edge-on to a face-on configuration. The reasons why a mechanical alignment technique can be used at room temperature for such a polycrystalline film are rationalized, by the plastic characteristics of the QQT(CN)4 layer and the role of the flexible alkyl side chains in the molecular packing. This nearly complete conversion from edge-on to face-on orientation by mechanical treatment in polycrystalline small-molecule-based thin films opens perspectives in terms of fundamental research and practical applications in organic optoelectronics.