Currently, there is a great interest towards developing organic semiconductors for use in solar cells and lighting displays. Derivatives of one of the most important chromophores, diketopyrrolopyrrole (DPP), are commonly employed as the active material in field-effect transistors, as they exhibit high hole mobilities. The intramolecular structure of 2T-DPP-2T with four thiophene units(T) is classified as a donor-acceptor-donor (DAD) chromophore, where the bithiophene units are donors and the DPP unit is the acceptor. The absorption spectrum of the aggregated form of a polymer based on the 2T-DPP-2T repeat units in 1,1,2,2-tetrachloroethane solution (TCE) was measured by Janssen et. al. The spectrum is red-shifted relative to a unaggregated polymer, which is an identifying feature of a J-aggregate. In addition, the ratio of the first two vibronic peaks decreases substantially in going from the unaggregated phase to the aggregate, which is an identifying feature of an H-aggregate. These contradicting behaviors were also observed by Punzi et. al. for an aggregate of the 2T-DPP-2T chromophore. Such behavior cannot be explained by the classical Frenkel-Holstein model. One challenge has been that the intermolecular charge transfer (ICT) plays an important role in the absorption and emission spectrum in the molecular aggregates of DPP. The bulk of this thesis has been to expand the Frenkel-CT-Hosltein model to include intramolecular and intermolecular charge transfer. The model accounts unusual red-shifted H-aggregates observed in the experiments. The experimental spectra of two different DPP-based chromophores are successfully reproduced with our theoretical model. Furthermore, based on perturbative expression for ICT coupling, an effective Frenkel Holstein (EFH) model is proposed and employed to successfully simulate the absorption and emission spectrum of DPP4T aggregates, as long as charge-transfer coupling is smaller than the energy gap between the Frenkel- and ICT excitations. The emission spectrum of DPP4T is also successfully reproduced by this new model, including the temperature dependence. / Chemistry