Even–Odd Alkyl Chain-Length Alternation Regulates Oligothiophene Crystal Structure

Abstract

Alkyl chains of varied length and steric bulk are generally appended to π-conjugated chromophores to increase solubility. These alkyl chains also regulate many aspects of the solid-state packing and, in turn, resultant electronic and optical properties of organic semiconductors, yet there remains little understanding as to how these moieties govern such characteristics. Here, we report on a series of (2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophenes) (BTTT) monomers where the single-crystal packing configurations and melting points alternate as a function of whether the alkyl side chains have an even or odd number of carbon atoms. When the alkyl chains contain an even number of carbon atoms, the molecules predominantly crystallize with the BTTT units of the stacked lamellae aligned in the same direction. In contrast, when the number of carbon atoms is odd, alternating lamellae are twisted with respect to their neighbors, i.e. the BTTT moieties in one layer are aligned in an orthogonal-like fashion to those in its neighboring lamellae, and the crystal melting points tend to be higher than those with the even-numbered alkyl chains. With density functional theory calculations and geometric analyses, the variations in crystal structure and thus the differences in physical properties are rationalized by an oscillating interlayer interaction that is a function of alkyl side chain atom count and length. This study characterizes an extreme case of an organic semiconductor crystal packing alternation to highlight the effect of even–odd assembly as an underutilized and under-reported handle for tuning the molecular packing of organic semiconductor materials.