Objective: The main objective of this research work was to give insight from theory in interpreting electronic absorption spectra of tetrapyrrolic macrocycles bearing carboxylic acid groups: protoporphyrin IX, pheophorbide a and its 1-hydroxyethyl derivatives for application in photodynamic therapy.Methods: All calculations were carried out by using the Gaussian 03W version 6.0. Electronic excitation energies and oscillator strengths were computed as vertical excitations from the minima of the ground state structures by using ZINDO and TD-DFT approach in vacuo. The simulated spectra were obtained by using the GaussSum 2.2.0 program.Results: The results showed that chlorine compounds (pheophorbide a and its 1-hydroxyethyl derivative) display the red-most absorption (Qx) at longer wavelengths and their absorption were stronger than porphyrin compounds (protoporphyrin IX and its 1-hydroxyethyl derivative). On the other hand, the 1-hydroxyethyl derivatives were not able to red-shift the absorption compared to the parent compounds.Conclusion: The chlorin compounds bearing carboxylic acid groups were, however, more promising candidates to be utilized in PDT compared to the corresponding porphyrin compounds.Keywords: Absorption spectra, 1-Hydroxyethyl derivative, Photodynamic therapy, Pheophorbide a, Protoporphyrin IX, TD-DFT, ZIND
