Spectroscopic properties of porphyrin-like photosensitizers: insights from theory.

Electronic absorption spectra of six porphyrin-like photosensitizers, porphyrin, chlorin, bacteriochlorin, pheophytin a, porphyrazin, and texaphyrin, have been calculated within the time-dependent DFT framework (TDDFT) in conjunction with the PBE0 hybrid functional. Energetic and orbital aspects are discussed by comparing systems together so as to assess the best molecules for photodynamic therapy applications. Excitation energies and oscillator strengths are found to be in good agreement with both experimental data and previous theoretical works. In particular, whereas significant discrepancies (0.3 eV) appear for Qx bands, results become more reliable as wavelengths decrease. To elucidate the effect of the local environment, we have taken into account solvation either with explicit water molecules interacting via hydrogen bonds with the system or with a continuum model (C-PCM). The supramolecular approach does not affect spectra, while using C-PCM improves Qx and B band values and strengthens intensities significantly. In both gaseous and aqueous phases, texaphyrin, pheophytin a, and bacteriochlorin Qx bands are found in the 600-800 nm range as expected by experimental works. These data are particularly interesting in the perspective of systematic studies of other photosensitizers and should make experimentalists' works easier.