Structural Odd–Even Effect Impacting the Dimensionality of Transport in BTBT‐CnOH Organic Field Effect Transistors

The synthesis and characterization of a series of [1]benzothieno[3,2‐b][1]benzothiophene (BTBT) molecules disubstituted by hydroxy aliphatic chains in positions 2 and 7 (BTBT‐CnOH), where the intralayer molecular stacking alternates between a classical and an inverted herringbone mode as a function of whether the alkyl sides chains have an even or an odd number of carbon atoms are reported. This odd–even effect does not only affect the interlayer distance of the lamellar structures and the melting points, but also the electronic properties. The BTBT‐CnOH odd series develops a classical herringbone pattern with edge‐to‐edge S⋯S interaction chains linked together by face‐to‐edge S⋯S interaction chains with 2D mobility. However, the even series has only edge‐to‐edge interactions in an inverted herringbone organization and thus only a 1D conducting character. These two types of herringbone patterns have different field effect transistor characteristics and mobilities, those of the odd members being systematically higher than their even neighbors. This is the first example of an odd–even effect impacting the electronic properties of an organic semiconductor. An unprecedented odd–even alternation in the relative orientation of π‐conjugated cores in the structures of organic semiconductors as a function of the number of methylene spacers in the side alkyl chains changes the dimensionality of carrier transport from 1D to 2D.