2, RIKEN, Wako, Saitama, Japan
3, Sumitomo Chemical Co. Ltd., Tsukuba, Ibaraki, Japan
Molecular orientation of organic semiconductors is crucial for its performance of their device applications such as OFET. Many orientation processes including wet ones have been investigated and the oriented growth on an alignment layer is one of the most convenient method as it has been utilized in manufacturing liquid crystal displays for a long time. The first author has been fascinated by an poly(tetrafluoroethylene)（PTFE）alignment layer since he saw a letter by Wittmann and Smith in 1991.1 Although the unique non-sticking and poor wetting properties of PTFE is rather undesirable for applications, its two remarkable features are quite important: one is a wide variety of the materials oriented on it and the other is some cases (linear conjugated molecules such as bisazo dyes2,3, a bisethyne3, or a bisazomethine4) where an extremely high degree of orientation (exceeding 0.9 of an uniaxial order) can be achieved. Hence, the purpose of this presentation is to show its unique orientation mechanism we found, thus being a theoretical model for designing a novel alignment layer for organic electronics.
We have been investigating the mechanism by a molecular dynamics (MD) simulation and have just concluded that a negatively charged shallow “atomic groove” between two adjacent PTFE chains orients linear molecules by trapping them along the the groove.3 It is formed by the weak helix of the chain and its charge is due to fluorine atoms, thus the mechanism being quite unique as the other properties of PTFE. The dynamics simulates adsorption of each single molecule on a surface consisting of eight C32F66 chains uniaxially aligned in a plane. The orientational order was estimated from the dichroism in their polarized absorption spectra and correlate well with those estimated from the calculations. This atomic groove effect was first discovered in 20025 and furthermore some modification of the F charge recently demonstrated the significant contribution of its negative charge. Recent progress in computing ability enabled us to simulate the difference in the orientation by tiny changes in a molecule end. Such accuracy ensures the validation of the conclusion. If you can design an alignment layer showing such negatively charged atomic grooves on its surface, it will have the two features of aligned PTFE.
This work is supported by JSPS-KAKENHI(JP17K4996).
 J. C. Wittmann, P.Smith., Nature, 352, 414 (1991);  T. Tanaka, et al., Langmuir, 17, 2197 (2001);  T. Tanaka, M. Ishitobi, Chem. Lett., accepted (2017);  T. Tanaka, et al., Chem. Lett., 40, 1170(2011);  T. Tanaka, M. Ishitobi, J. Phys. Chem. B, 106, 564 (2002).