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

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 (lambda) 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 pi-stacking structure, which is a promising crystal structure for high-mobility organic semiconductors. The single-crystal field-effect transistor of beta- methylthio-i4TA (beta-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 pi-stacking structure and the low lambda originating in the i4TA framework. The origins of the low lambda and the tendency to adopt the pitched pi-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 lambda, meaning that the incorporation of the thieno[2,3-b]thiophene substructure is a potential strategy to reduce lambda. On the other hand, the tendency to adopt the pitched pi-stacking structure is due to the favorable sigma*C-S-pi 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.

Automated summary

In this study, the synthesis and characterization of a previously unexplored structure, thieno[3 '',2 '':4 ',5 ']thieno[2 ',3 '-d]thieno[2,3-b]thiophene (i4TA), and its derivatives were carried out. The i4TA framework was found to have several different features compared to the commonly used building block, 4TA. The i4TA derivatives tended to crystallize into a promising π-stacking structure for high-mobility organic semiconductors.