Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with the DZVP/DZVP2 all-electron mixed basis sets are used to study the related energies, structures, frontier molecular orbitals and UV-Vis spectra for [M(Tp)(PPh3)(Cl)(L)](M = Ru and Fe; L = C3H4N2 and Cl3H11N). The related energies between the singlet state (low-spin) and nonet state (high-spin) for these complexes are reported. Because of the low related energies, these complexes are expected to be in the singlet state (low-spin). The calculated structural parameters for complexes 1 and 3 (Ru-based) are in very good agreement with the experimental values, and the geometries of complexes 2 and 4 (Fe-based) have been studied as well. The metal-ligand bond distances for the Fe-based complexes are predicted to be slightly shorter than those of the Ru-based complexes due to the small spatial extent of the 3d wave functions of the Fe atom. These complexes display a HOMO of metal d and pi(Cl) orbitals in character, and the LUMO is contributed by metal d and pi*(PPh3) or pi*(C13H11N) orbitals. The HOMO-LUMO energy gap (Delta EL-H) can be reduced by a pi-electron rich ligand (such as C13H11N) and a low electronegativity metal atom (such as Fe). A pi-electron rich ligand (such as C13H11N) can increase the electron accepting ability, which leads to more electrons being pumped into the pi*(PPh3) and pi*(C13H11N) orbitals and results in a red-shift and intensity-enhanced absorption in the UV-Vis spectrum. The UV-Vis absorption intensity can be enhanced by solvent (such as CH3OH) as well as resulting in a blue-shift, which suggests that it is due to the polarizability and dielectric strength of the solvent. Owing to the low electronegativity of the Fe atom, a red-shift occurs in the UV-Vis spectra for complexes 2 and 4. The primary absorption features for complexes 1 and 3 are attributed to MLCT/LLCT transitions; on the other hand, a MLCT transition results in the primary absorption features for complexes 2 and 4. Our results show that Ru can be replaced by the inexpensive Fe as the photo-sensitizer. In addition, these Ru- and Fe-based complexes are good candidates for photo-sensitizers in DSSCs due to rich absorption bands and strong absorption intensities in the visible region.