Uncovered Effects of thieno[2,3-b]thiophene Substructure in a Tetrathienoacene Backbone: Reorganization Energy and Intermolecular Interaction

Abstract

Although tetrathienoacene, thieno[2″,3″:4′,5′]thieno[2′,3′-d]thieno[3,2-b]thiophene (4TA), has been used as a representative building block for organic semiconductors and polymers in the last two decades, its structural isomer, thieno[3″,2″:4′,5′]thieno[2′,3′-d]thieno[2,3-b]thiophene (i4TA), is still an unexplored structure. We are interested in the i4TA framework as a potential building block for organic semiconductors because the theoretically calculated reorganization energy (λ) of i4TA (149 meV) is significantly lower than that of 4TA (325 meV). In this study, we performed the synthesis and characterization of parent i4TA and its methylthio derivatives and found that several features of i4TA derivatives differ from those of 4TA. Parent i4TA and its derivatives tend to crystallize into the pitched π-stacking structure, which is a promising crystal structure for high-mobility organic semiconductors. The single-crystal field-effect transistor of β-methylthio-i4TA (β-MT-i4TA) showed typical transistor behaviors including as high as 3.5 cm2 V–1 s–1 hole mobility and band-like transport. This is understood as an outcome of the pitched π-stacking structure and the low λ originating in the i4TA framework. The origins of the low λ and the tendency to adopt the pitched π-stacking structure were investigated by theoretical analyses. The cross-conjugation path resulting from the thieno[2,3-b]thiophene substructure in the i4TA framework plays a pivotal role in reducing λ, meaning that the incorporation of the thieno[2,3-b]thiophene substructure is a potential strategy to reduce λ. On the other hand, the tendency to adopt the pitched π-stacking structure is due to the favorable σ*C–S–π intermolecular interaction of the i4TA framework, and the intermolecular interaction is also regarded as an outcome of the thieno[2,3-b]thiophene substructure. On the basis of these experimental and theoretical results, we conclude that the i4TA framework and the thieno[2,3-b]thiophene substructure are promising but overlooked structures as building blocks for organic semiconductors.