Perylene-3,4:9,10-bis(dicarboximide) (PDI) and its derivatives are robust organic dyes that strongly absorb visible light and display a strong tendency to self-assemble into ordered aggregates, having significant interest as photoactive materials in a wide variety of organic electronics. To better understand the nature of the electronics states produced by photoexcitation of such aggregates, the photophysics of a series of covalent, cofacially oriented, pi-stacked dimers and trimers of PDI and 1,7-bis(3',5'-di-t-butylphenoxy)perylene-3,4: 9,10-bis(dicarboximide) (PPDI) were characterized using both time-resolved absorption and fluorescence spectroscopy. The covalent linkage between the chromophores was accomplished using 9,9-dimethylxanthene spacers. Placing n-octyl groups on the imide nitrogen atoms at the end of the PDI chromophores not attached to the xanthene spacer results in PDI dimers having near optimal pi-stacking, leading to formation of a low-energy excimer-like state, while substituting the more sterically demanding 12-tricosanyl group on the imides causes deviations from the optimum that result in slower formation of an excimer-like excited state having somewhat higher energy. By comparison, PPDI dimers having terminal n-octyl imide groups have two isomers, whose photophysical properties depend on the ability of the phenoxy groups at the 1,7-positions to modify the : c stacking of the PPDI molecules. In general, disruption of optimal pi-stacking by steric interactions of the phenoxy side groups results in excimer-like states that are higher in energy. The corresponding lowest excited singlet states of the PDI and PPDI trimers are dimer-like in nature and suggest that structural distortions that accompany formation of the trimers are sufficient to confine the electronic interaction on two chromophores within these systems. This further suggests that it may be useful to build into oligomeric PDI and PPDI systems some degree of flexibility that allows the structural relaxations necessary to promote electronic interactions between multiple chromophores.