Conformation, Molecular Packing, and Field Effect Mobility of Regioregular (beta),(beta)Dihexylsexithiophene

Improved synthesis, charge carrier mobility, conformation, crystalline structure, and molecular packing of the regiochemically pure 4ʹ,3ʹ ʹʹ ʹ-dihexyl-2,2ʹ;5ʹ,2";5" 2"ʹ;5"ʹ,2"";5"",2ʹ""-sexithiophene ((beta),(beta)ʹ-DH6T) are reported. The sum of charge carrier mobilities of (beta),(beta)ʹ-DH6T measured by the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique was found to be (sigma)(mu)min = 3.9 × 10^-3 cm^2 V^-1 s^-1, which is comparable with the PR-TRMC mobility found for (alpha),(omega)-DH6T. The field-effect mobility (FEM) of (beta),(beta)ʹ-DH6T was found to be on the order of 10^-5 cm^2 V^-1 s^-1, which is considerably less than the FEM of (alpha),(omega)-DH6T. To understand the reason for such poor macroscopic electrical properties, the conformation and the molecular packing of (beta),(beta)ʹ-DH6T were systematically studied by means of UV-vis spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray techniques. Absorption spectra of a (beta),(beta)ʹ-DH6T spin-cast film indicate the planar conformation of the aromatic backbone. SEM and AFM reveal the formation of an ordered lamellar phase. As a single-crystal X-ray study shows, (beta),(beta)ʹ-DH6T exhibits less dense crystalline packing than (alpha),(omega)-DH6T. In contrast to the almost upright orientation of (alpha),(omega)-DH6T molecules against the substrate (tilt angle about 68°), the long axis of (beta),(beta)ʹ-DH6T molecules and the surface plane form an angle of ~20°. Thus, the crystalline structure of (alpha) , (omega)-DH6T "allows" the current to flow along the molecular stacks; the crystalline structure of (beta),(beta)ʹDH6T suppresses the charge transport.