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Thiophene-based materials play an important role in the growing field of organic semiconductors due to their good environmental stability and electronic properties. To realize the goal of rational design of electronic materials with improved performance, systematic studies that explore the relationship between molecular structure and optical and solid-state properties are needed. Three such studies form the basis of this thesis. In order to study the effect of linkage regiochemistry on optical and solid-state properties of oligothiophenes, a series of beta-linked oligothiophenes was synthesized. UV-visible absorption and fluorescence spectroscopies were used to contrast their behavior with the alpha-linked analogs. It was found that the introduction of the beta-linkage between rings disrupts conjugation as evidenced by a blue shift of the longest wavelength of absorption and emission relative to the alpha-linked counterparts. However, in the solid state the beta-linked oligothiophenes studied present pi-pi interactions, an arrangement which is predicted to be favorable for device application such as organic thin film transistors (OTFT). The effect of position and degree of sulfur oxidation on the fundamental properties of a series of planarized oligothiophene S,S-dioxides was also studied. The optical data reveal a red shift in the longest wavelength of absorption relative to the non-oxidized analogs that is indicative of a reduced HOMO-LUMO gap. The position of oxidation rather than the extent of oxidation is the most critical factor in controlling electronic properties. In the solid-state, some of the oligothiophene S,S-dioxides studied present pi-pi interactions which are completely absent in the non-oxidized analog. The effect of the introduction of conjugated spacers between dialkylated-thieno[3,2- b]thiophene units on molecular conformation and thus conjugation was investigated by the synthesis of 3,6-dinonylthieno[3,2-b]thiophene-based polymers containing 2,2'-bithiophene, thieno[3,2-b]thiophene, and ethynylene units. The introduction of these spacing units leads to dramatically improved planarity and conjugation as demonstrated by a red shift in the longest wavelength of absorption relative to poly(3,6-dinonylthieno[3,2-b ]thiophene). This suggests that dialkylated-thieno[3,2-b ]thiophene can be used as a building block for semiconducting polymers but that a spacer unit is required to obtain a more planar backbone conformation.