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 (beta,beta'-DH6T) are reported. The sum of charge carrier mobilities of beta,beta'-DH6T measured by the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique was found to be Sigmamu(min) = 3.9 x 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'-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'-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'-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'-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'-DH6T molecules and the surface plane form an angle of similar to20degrees. Thus, the crystalline structure of alpha,omega-DH6T "allows" the current to flow along the molecular stacks; the crystalline structure of beta,beta'-DH6T suppresses the charge transport.
Chemistry of Materials, 2004, Vol 16, Issue 23, p. 4757-4764