Solid-state packing of conjugated oligomers: from pi-stacks to the herringbone structure

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.