Alkyl-Parity Controlled Switching of Polar/Antipolar Organic Semiconductors

Abstract The odd-even effects are renowned as a mysterious phenomenon in broad fields of science but have never been established as an effective approach for materials engineering. We demonstrate that the parity of alkyl carbon number n can cause alternating emergence of polar/antipolar organic semiconductor crystals. This is achieved by the development of a series of polar rod-like molecules, composed of a linkage between extended π-core (head) and alkyl chains (tail), exhibiting both high layered crystallinity and well-balanced end-to-end affinity. The molecules are unidirectionally aligned to form two-dimensional array, and the eventual polar monomolecular layers present two distinct types of interlayer stacking depending on the parity of n: alternating head-to-head and tail-to-tail (antipolar) alignment in odd-n crystals, and uniform head-to-tail (polar) alignment in even-n crystals. The latter allows to obtain polar semiconductor films that considerably improve interfacial carrier transport characteristics. The findings are key for creating polarity-controlled optoelectronic materials and devices.