The use of alkylsulfanyl and alkylsulfonyl side chains are demonstrated to be a useful synthetic strategy for tuning the electronic properties of organic semiconductors, as shown in thienothiophene vinylene polymers. By changing the oxidation state of sulfanyl to sulfonyl, we lower the HOMO and LUMO energy levels of our substituted polymers, as well as enhance their fluorescence. Fine-tuning of the energy levels was achieved by combining sulfanyl and sulfonyl substituted thienothiophene monomers through random polymerization, yielding polymers with low-band gaps (1.5 eV) yet benefiting from a structurally uniform conjugated backbone. The effects of these functional side chains are presented through DFT calculations, UV-vis, fluorescence, and electrochemical measurements, as well as crystallographic analysis of a sulfanyl-substituted oligomer. The semiconducting properties of the new polymers are studied in OFET and OPV devices.