Molecular geometries and spectra of methyl pyropheophorbide-a pyrazole derivatives: a density functional theory study

Density functional theory (DFT) electronic structure calculations were performed to predict the geometries of the chlorin macrocycle core in methyl pyropheophorbide-a pyrazole derivatives (e.g. HPMPPa and PPMPPa). These compounds are new photosensitizer materials that possess the dual function activity in photodynamic therapy. We found that the geometries of the two compounds were nearly the same, and that substitution of the pyrazole moieties at C31-position of methyl pyropheophorbide-a did not have significant effects on the geometries of the chlorin macrocycle core. Excitation energies and oscillator strengths were computed by time-dependent DFT. The time-dependent DFT spectrum of HPMPPa and PPMPPa were not affected by the pyrazole substituent. To determine the hydrophilic effect of photosensitizers, we calculated the interaction energy between photosensitizer and water, and observed that the interaction energy of HPMPPa was higher than that of PPMPPa. We found that the interaction energy with specific wavelength (Q-bands) is an important factor to consider in the development of photodynamic therapy.