TD-DFT studies on Hematoporphyrin and its dimers

A theoretical study has been performed on a hematoporphyrin and its dimers which are components of Photofrin, a photosensitizer. Full geometry optimizations have been carried out using the PBEPBE functional and 6-31G(d) basis set. This combination gives better agreement with X-ray crystal data of porphyrin. Among the dimers studied, the C-C linked structure is found to have the highest stability. The predicted change of free energy (AG = - 13.9 kcal/mol) suggests that the interconversation of ester to ether would be thermodynamically favorable. The time-dependent density functional theory TDDET studies show that O-band absorption maxima undergo a less intense transition and low oscillator strength, indicating that dimers have activity when treated under higher dosage.