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

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...