Photo-excitation energy transfer between a titanium(iv)–porphyrin complex and oxygen molecule

Abstract

The Ti-TPyP reagent, i.e. an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)-porphyrinato]titanium(IV) complex, denoted as TiO(tpyp), was developed as a highly sensitive and specific reagent for detecting hydrogen peroxide. In our previous paper, the photosensitizing effect of the TiO(tpyp) complex on the detection of hydrogen peroxide using this reagent was explored based on the experimental findings. In the present paper, computational analysis was carried out for the molecular level elucidation of the energy transfer mechanism between the photo-excited TiO(tpyp) complex and triplet oxygen to form singlet oxygen under aqueous acidic conditions. The protonated form of the TiO(tpyp) complex, i.e. [TiO(tpypH₄)]⁴⁺ complex, was found to be the entity responsible for the energy transfer to triplet oxygen. The interaction sites between the [TiO(tpypH₄)]⁴⁺ complex with triplet oxygen were then discussed based on their molecular orbital characteristics and the potential energy changes accompanying the approach of triplet oxygen molecule Σ³O₂ to the [TiO(tpypH₄)]⁴⁺ complex. The obtained results show that the C–N portion of the porphyrin ring plays a predominant role in the energy transfer from the photo-excited porphyrin to Σ³O₂. Σ³O₂ is then converted to Δ¹O₂ through Σ¹O₂. The Δ¹O₂ molecule reaches its stable state at the distance of 3.21 Å from the C–N portion of the porphyrin ring with its stabilization energy of 0.99 kcal mol⁻¹. The stabilization energy for the Δ¹O₂ molecule thus formed is small, implying no stable bond formation between the Δ¹O₂ molecule and the [TiO(tpypH₄)]⁴⁺ complex and that it moves away from the C–N portion immediately after the energy transfer from the photo-excited porphyrin.