A computational study on the photophysics of methylpheophorbide a

Pheophorbide a is a dephytylation and demetallation product of chlorophyll a isolated from plants and algae. Pheophorbide a has been used as a photosensitizer to treat microbes, cancer and multidrug resistance. Methylpheophorbide a (MPh) or its methyl ester is another photosensitizer with interesting photophysical properties such as stronger absorption at longer wavelengths compared to the absorption of porphyrins and a high singlet oxygen production quantum yield (Φ_Δ = 0.62). To gain deeper insight into the photophysics of MPh, a computational protocol was employed that allows the elucidation of the photophysical properties of methylpheophorbide a (MPh). This protocol uses Fermi's golden rule within a path integral formalism. Time-dependent density functional theory (TD-DFT) calculations at the CAM-B3LYP/def2-SVP(C-PCM) level of theory were performed. Our calculations reproduce acceptably well the vibronic structure of the Q-band of the absorption spectrum of MPh. After photoexcitation, MPh can decay to the ground state via fluorescence or it can undergo intersystem crossing. Three triplet excited states (T₁, T₂ and T₃) are found below the S₁ state with an overall spin-vibronic ISC rate constant of 6.14 × 10⁷ s^-1, in good agreement with the experimental value of 7.90 × 10⁷ s^-1. The calculated fluorescence rate is approximately five times higher than the experimental value, which can be attributed to an overestimation of the adiabatic energy of the S₁ state and to the inherent limitations of the approach employed. Consistent with the experimentally observed behavior, our calculations predict that MPh is not phosphorescent.