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Dye-sensitized solar cells based on organic dyes have attracted considerable attention in recent years. These dyes present important differences in geometry and electronic structure, and diverse mechanisms for electronic injection into semiconductor conduction band have been reported. In this work, the electronic structure and the optical response of several dyes, free and bound to TiO₂, have been analyzed and compared using time-dependent density functional theory (TD-DFT) performing calculations both in real time and frequency domains. The aim is to illustrate which aspects of each sensitizer produce different electron injection mechanisms. From the analysis it is concluded that for adsorbed NKX-2311, a coumarin based dye, the photoexcitation occurs toward an orbital mainly localized on the molecule, so this molecule is classified as a type I sensitizer, which follows an indirect mechanism for electron …