Conformation, molecular packing, and field effect mobility of regioregular β,β′-dihexylsexithiophene

Improved synthesis, charge carrier mobility, conformation, crystalline structure, and molecular packing of the regiochemically pure 4′,3′‴-dihexyl-2,2′;5′,2″;5″, 2‴;5‴,2′‴;5′‴,2″‴- sexithiophene (β,β′-DH6T) are reported. The sum of charge carrier mobilities of β,β′-DH6T measured by the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique was found to be ∑μ_min = 3.9 × 10^-3 cm² V ^-1 s^-1, which is comparable with the PR-TRMC mobility found for α,ω-DH6T. The field-effect mobility (FEM) of β,β′-DH6T was found to be on the order of 10^-5 cm² V^-1 s^-1, which is considerably less than the FEM of α,ω-DH6T. To understand the reason for such poor macroscopic electrical properties, the conformation and the molecular packing of β,β′-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 β,β′-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, β,β′-DH6T exhibits less dense crystalline packing than α,ω-DH6T. In contrast to the almost upright orientation of α,ω-DH6T molecules against the substrate (tilt angle about 68°), the long axis of β,β′-DH6T molecules and the surface plane form an angle of ∼20°. Thus, the crystalline structure of α,ω-DH6T "allows" the current to flow along the molecular stacks; the crystalline structure of β,β′-DH6T suppresses the charge transport.