In a recent study, it has been shown that organic photovoltaic (OPV) solar cells consisting of polymers with certain
stoichiometric ratios of alkyl thiophene:thieno[3,4-<i>b</i>]thiophene monomeric units in random sequences, when combined
with [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM), may have potentials for creating more efficient devices. Such a
potential enhancement is mainly due to the light harvesting in most of the visible and near infrared region by these low
band-gap polymers. However, very little is known about the photoinduced energy/electron transfer and transport within
these copolymers. It is important to understand both the ultrafast interactions between these two monomeric units when
they are linked in the copolymers and their interactions with the electron acceptor PCBM in order to determine the
transport mechanisms in these systems, and then to create the architectures that optimize electronic transport properties.
Therefore, three oligomer molecules have been synthesized to model the local interactions in the copolymers, each of
which consists of a thieno[3,4-<i>b</i>] thiophene derivative at its center linked with two alkyl oligothiophene side units. The
alkyl oligothiophene units for the three molecules are 2, 4, or 8 units in length. By performing transient absorption and
fluorescence upconversion measurements, the nature of the early exciton diffusion and energy transfer between these
different units is elucidated.