The solid-state structures of a series of bithiazole and thiophene oligomers, as well as a series of substituted pentacenes, are rationalized in terms of "pitch and roll" inclinations from an "ideal" cofacial pi-stack. Pitch inclinations translate adjacent molecules relative to one another in the direction of the long molecular axis, whereas roll inclinations translate the molecules along the short molecular axis. Thus, moderately large pitch distortions preserve pi-pi interactions between adjacent molecules, whereas roll translations greater than 2.5 A essentially destroy pi-pi overlap between adjacent molecules. The familiar herringbone packing is characterized by large roll distortions. It is shown that thiophenes tend to exhibit large roll translations, whereas thiazoles have small roll but large pitch translations. Substituted pentacenes tend to have both moderate pitch and roll distances. The relationship of molecular packing to transport properties is discussed.

The synthesis and characterization of several co-oligomers, 5,5′-bis(4,4′-dialkyl-2,2′bithiazol-5-yl)-(3,4,3′,4′-bis(ethylenedioxy)-2,2′-dithienyl) (AT 2 A), 5,5′-bis(3,4-ethylenedioxythien-2-yl)-4,4′-dibutyl-2,2′-bithiazole (TBT), and 5,5′-bis(3,4,3′,4′-bis(ethylenedioxy)-2,2′dithien-5-yl)-4,4′-dibutyl-2,2′-bithiazole (T 2 BT 2 ) are described. The solid-state structure of AT 2 A features π-stacking with a short intermolecular distance of 3.5 Å. In thin films, the π-stacks are parallel to the film substrate as determined by X-ray diffraction. The UV-vis spectra of AT 2 A films show features that are ascribed to the effects of π-stacking. In addition, all these co-oligomers, composed of donor-acceptor moieties, show lowered optical band gaps, mainly because of the low oxidation potential imparted by incorporation of the 3,4ethylenedioxythiophene (EDOT) moiety. Cyclic voltammetry indicates that these co-oligomers can be either p-doped or n-doped, and the spectroelectrochemical measurements show that the AT 2 A films are stable upon cycling the oxidation state. All these features make these materials promising for use in electronic devices, e.g., organic thin film transistors.

It is known that macroscopic properties of colloidal suspensions are often determined by the microstructure of the particles in the suspensions, depending on the interparticle, Brownian, and hydrodynamic (if any) forces. We take electrorheological (ER) fluids as an example. By using a computer simulation and an experimental approach, we investigate the structure of ER fluids subjected to both an electric field and a shear flow. The microstructure evolution from random structure, to chains, and then to stable lamellar patterns, observed in the experiments, agrees very well with that obtained in the simulations. It is shown that the formation of such lamellar patterns originates from the difference between the dipole moment induced in the particles suspended in the ER fluids without shear and the one with shear. The results on the relaxation process of structural formation and the internal structure of layers are also presented. Thus, it seems possible to achieve various structures and hence desired macroscopic properties of colloidal suspensions by adjusting external fields and, simultaneously, a shear flow.

We have fabricated a class of colloidal electrorheological (ER) fluids, in which suspended TiO2 particles were synthesized by a sol-gel method and modified by 1,4-butyrolactone molecules with a permanent molecular dipole moment of 4.524 D. Compared with pure TiO2 ER fluids, the quasi-static yield stress of the polar- molecules-modified ER fluid is enhanced as high as 48.1 kPa when subjected to an external electric field of 5 kV/mm. Also, it possesses other attractive characters such as low current density (<14 μA/cm2) and low sedimentation. Based on a Green’s function method, we present a first-principles approach to investigate colloidal electrostatic interactions. Excellent agreement between experiment and theory has been shown for the enhancement ratio of quasi-static yield stress, which quantitatively reveals that enough polar molecules oriented within the field-directed gap between the colloidal particles can unexpectedly enhance the interactions, thus yielding the unusual enhancement. This shows a promising and flexible direction for achieving more highly active ER materials.

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