Conjugated oligomers containing a common central thienothiophene unit symmetrically connected to two identical thiophene oligomers were studied as model systems for a series of low bandgap organic diblock copolymers The oligothiophene side chain fragments were varied in length as a means to tune the electronic coupling between the thienothiophene and oligothiophene moieties The fragment length dependence of both the ground and excited-state electronic and structural properties of a series of diblock oligomers were investigated in detail The charge transfer character in these diblock oligomers, revealed by their optical absorption and fluorescence spectra, is responsible for their low band gap and energy gap tunability compared with their homooligomer counterparts The electronic spectra and theoretical analysis indicate a partially localized central charge in the first excited state Using experimental results and comparing them with theoretical calculations we estimate that the electronic effects from a single thienothiophene unit spreads over seven to nine adjacent units through pi-conjugation along the oligomers