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Formation, photophysics and photochemistry of water-soluble cadmium(II) porphyrins : the effects of bromination and the axial hydroxo ligand

In slightly alkaline solution (pH 8) Cd(II) ion and 5, 10, 15, 20-tetrakis(4-sulfonatophenyl) porphyrin (H2P4−) form a kinetically labile complex (HOCdP5−), in which the metal center is located out of the ligand plane, due to the effects of the axial hydroxo ligand and the relatively large radius of Cd2+. Both acidification and irradiation at the Soret-band can result in the dissociation of the axial ligand, reducing the out-of-plane distance of the metal center and the distortion of the macrocycle (CdP4−). Besides, irradiation of both types of metalloporphyrins promotes an irreversible ligand-to-metal charge transfer leading to the oxidative degradation of the coordinated porphyrin at both the Soret- and the Q-bands. Under the same conditions, in the case of the octabromo derivative of this water-soluble porphyrin (H2BrP4−), the distorted structure accelerates the formation of the corresponding complex with cadmium(II) compared to its reaction with the parent, unbrominated ligand. The structure of this porphyrin (HOCdBrP5−), similarly to the free base and CdBrP4− (formed via acidification of irradiation of HOCdBrP5−), strongly distorted by the Br substituents significantly affects the characteristic features of the absorption and emission spectra, red-shifting the position of the main bands of these porphyrins compared to those of the unbrominated species. Also the emission quantum yields and lifetimes are dramatically diminished by bromination. Deviating from the unbrominated species, photodegradation of the brominated derivatives proved to be very oxygen sensitive. Besides, CdBrP4− is transformed into a new porphyrin derivative upon both Soret- and Q-band irradiation. DFT calculations of the geometrical structures and the absorption bands show good correlations with the observed photophysical and photochemical properties, due to the drastic distortions of the macrocyclic ligand.

water-soluble porphyrin; out-of-plane; cadmium(II) ion; photochemistry; bromination; charge transfer

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