Protonation behavior of thiaporphyrin and thiabenzoporphyrin

• Macrocyclic current pathways of a thiaporphyrin were altered by two-step protonation.
• X-ray structures of a various kind of monoprotonated thiabenzoporphyrin were all flat.
• Diprotonated N4- and N3S-porphyrins adopted a saddled distortion.
• The monoprotonated thiabenzoporphyrin was observed in the wider acid strength range.

The protonation behavior of bicyclo[2.2.2]octadiene (BCOD)-fused thiaporphyrin and thiabenzoporphyrin was examined by titration with trifluoroacetic acid. Neutral, monorotonated, and diprotonated species of thiaporphyrins were easily detected by UV–vis spectral measurement. The first (K1) and second (K2) equilibrium constants were found to be K1 = 520 and K2 ≈ 7.1 for BCOD-fused thiaporphyrin and K1 = 1.4 × 104 and K2 ≈ 1.4 for thiabenzoporphyrin. The difference between K1 and K2 values of thiaporphyrins is larger than that observed for porphyrins (N4-porphyrins). The main contributing conjugation pathways for thiaporphyrins in both protonated species were investigated by NICS and AICD quantum calculations using their atomic geometries obtained by X-ray structural analysis. The 18π-conjugation pathways of the neutral, monoprotonated, and diprotonated species thiaporphyrins were found to contain 18, 19, and 20 atoms, respectively. Results of 1H NMR studies also support the major contributing pathways.

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