Controlling the optical properties of conjugated materials, especially their bandgaps, is critical to nearly all of applications of these materials. The most prevalent strategy involves changes to the structures of conjugated backbones, while side chains are generally reserved for imparting solubility. This paper, using a series of donor-acceptor conjugated oligo- and poly-(arylene-ethynylene)s that have terephthalate units as the electron-deficient unit, demonstrates examples of how the structures of side chains that are not formally part of the conjugated backbone can have significant effects on bandgaps of these materials. In organic solution, changing alkoxy substituents on the terephthalate unit yields changes in absorbance onsets of, in some cases, greater than 20 nm; the position of absorbance spectra of these materials correlates with the Taft sigma* values of the ester alkoxy groups, consistent with the side chains inductively altering the electron-accepting nature of the terephthalate ring. This structure-property relationship persists in the solid state. These results indicate that synthetically simple side-chain substitutions of formally nonconjugated groups may be useful in rational design of the optoelectronic properties of conjugated materials in both solution and the solid state.