Photophysical properties of free and metallated meso-substituted tetrabenzotriazaporphyrin from density functional theory investigation

• Photophysical properties of tetrabenzotriazaporphyrins are computed.
• The electronic spectra show a Q band falling in the so-called therapeutic window.
• Two excited triplet states below the first excited singlet one have been found.
• Spin–Orbit matrix elements have been computed.
• Zn(II) complex is able to activate the oxygen molecule.

Density functional theory and its Time-dependent extension have been herein employed to elucidate the photophysical properties of a meso-substituted tetrabenzotriazaporphyrin and its magnesium and zinc complexes. Absorption electronic spectra, singlet-triplet energy gaps and spin–orbit matrix elements, have been provided. The investigated compounds have been found to possess two excited triplet states (T1 and T2) lying below the energy of the excited singlet one (S1). The Spin–Orbit matrix elements for both the radiationless S1 → T1 and S1 → T2 couplings have been thus computed. On the basis of our results, the Zn(II) complex shows i) a singlet-triplet energy gap higher than that required to generate the cytotoxic singlet oxygen species, ii) a significant coupling between S1 and T2 states and, accordingly, it could be proposed as a potential photosensitizer in photodynamic therapy.

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