Formation, photophysical and photochemical properties of water-soluble bismuth(III) porphyrins: The role of the charge and structure

Bismuth(III) ion forms kinetically labile complexes with the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (H2TSPP4−) and the 5,10,15,20-tetrakis(1-methyl-4-pyridinium)porphyrin cation (H2TMPyP4+), the formation constants of which are 3.54 × 104 M−1 and 1.62 × 103 M−1, respectively (at pH = 6). In these complexes, the metal center, due to its large ionic radius (103 pm), is located out of the ligand plane, distorting it. Accordingly, the absorption and fluorescence spectra of these coordination compounds display special properties characteristic of the so-called sitting-atop (SAT) or out-of-plane (OOP) porphyrin complexes. The shifts of the absorption bands upon metalation indicate that the structural distortion results in stronger perturbation on the S2- than on the S1-states of the porphyrin ligand. Metalation significantly decreases the lifetime and the quantum yield of the fluorescence from the S1 excited state. The relatively rare S2-fluorescence can also be detected for these metalloporphyrins. Quantum chemical calculations (DFT and TDDFT) confirm the considerable OOP displacement of the Bi(III) center (88 pm) and the typical tendencies of the band-shifts. Differing from the normal (in-plane) metalloporphyrins, excitation of these bismuth(III) porphyrins leads to an irreversible ligand-to-metal charge transfer (LMCT) followed by the opening of the porphyrin ring, which is also typical of SAT complexes. The quantum yields of this photoinduced redox reaction are significantly higher for the anionic than for the cationic complex, due to the stronger Lewis-basicity of H2TSPP4−. In the mechanism of the reaction between these free-base porphyrins and bismuth(III) ions formation of a longer-lived intermediate was observed. In the case of the cationic porphyrin also the photochemistry of this intermediate could be studied; deviating from the photoinduced behavior of the final product metalloporphyrins, excitation of this intermediate results in predominantly the dissociation to the initial porphyrin ligand and metal ion, indicating a relatively weak coordination bond and significantly distorted structure.