The systematic trends in structural and electronic properties of perylenediimide (PDI)-derived dye molecules have been investigated by DFT calculations based on the projector-augmented wave (PAW) method including gradient-corrected exchange-correlation effects. Time-dependent density functional theory (TDDFT) calculations have been performed to study the visible absorbance activity of these complexes. The effect of different ligands and halogen atoms attached to PDI were studied to characterize the light-harvesting properties. The atomic size and electronegativity of the halogen were observed to alter the relaxed molecular geometries, which in turn influenced the electronic behavior of the dye molecules. The ground-state molecular structure of isolated dye molecules studied in this work depends on both the halogen atom and the carboxylic acid groups. DFT calculations revealed that the carboxylic acid ligands did not play an important role in changing the HOMO-LUMO gap of the sensitizer. However, they serve as an anchor between the PDI and substrate TiO2 surface of the solar cell or photocatalyst. A commercially available dye sensitizer, ruthenium bipyridine [Ru(bpy)(3)](2+) (RuBpy), was also studied for electronic and structural properties in order to make a comparison with PDI derivatives for light-harvesting properties. Results of this work suggest that fluorinated, chlorinated, brominated, and iodinated PDI compounds can be useful as sensitizers in solar cells and in artificial photosynthesis.