Peculiar photoinduced properties of water-soluble, early lanthanide(III) porphyrins

Abstract

Water-soluble, early lanthanide(III) mono-and bisporphyrin complexes possess very similar UV-Vis absorption, photophysical and photochemical properties, as a consequence of a special type of aggregation, through the peripheral substituents. In the absence of the bidentate, O-donor acetate ligand, bisporphyrin can form too, which has slightly redshifted and broadened absorption bands, compared to those of the monoporphyrin. Also the bisporphyrin displays a blueshifted and less intense fluorescence, related to the free-base porphyrin. The formation of complexes and the transformation between the mono-and bisporphyrins are very slow reactions in dark at room temperature. These reactions are accelerated by the photolysis of the system; which are considerable by-processes of the photoredox degradation. Depending on the wavelength of irradiation, two types of photoproducts can appear: during the photolysis at the Soret-band, a radical type intermediate can be observed, which disappears in dark. However, during the irradiation at the Q-bands, a new photoproduct appears, which is stable in dark and undetectable in the case of post-transition metal ions' out-of-plane porphyrin complexes.

Visible light-driven photophysics and photochemistry of water-soluble metalloporphyrins

“…The irradiation wavelength does not significantly influence the overall quantum yield as it was found for the cerium(III) porphyrins [38,40]. However, the type of the photoproducts considerably depends on the excitation energy.…”

“…Notably, porphyrins are able to efficiently sensitize the near-infrared luminescence of lanthanide ions, which can be widely applied from telecommunication to biomedical optical imaging [34,35]. Besides, lanthanide(III) porphyrins may be applied for photocatalytic hydrogen production from water because their reduced metal centers (Ln(II)) formed in photoinduced LMCT processes (typical for OOP complexes [36,37]) have considerably high negative redox potentials (except for europium) [38]. In this section, we survey the photophysical and primary photochemical properties of the complexes between a water-soluble, anionic porphyrin, the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TSPP 4-) and three early lanthanide(III) ions (Ln = La, Ce, Nd).…”

“…It is partly the consequence of the strong Ln(III)-H2O bond. Accordingly, a harder Odonor ligand, such as acetate, can hinder the coordination of the second porphyrin to the metal ion [34,35,38,39]. In the absence of acetate, bisporphyrin (Ln3TSPP2 3-) can form too, which has slightly redshifted and broadened absorption bands, compared to those of the monoporphyrin (Fig.…”

“…In the lanthanide(III) porphyrins, the out-of-plane location of the metal center promotes two types of photochemical reactions, which are not characteristic for the in-plane complexes [3,38]: 1) photoinduced demetalation (without any charge transfer), due to the lability of the complex; 2) oxidative degradation of the porphyrin ring, initialized by an irreversible, photochemical LMCT reaction. The reduction of the metal center increases its size (along with the decrease of its charge density), promoting the split of the coordinative bonds.…”

“…The overall photochemical quantum yields monotonously increased with the concentration [9,38,40]. This phenomenon suggests that a bimolecular excited state (excimer formation), or dark reactions following the excitations, or the primary photochemical steps involve groundstate lanthanide(III) porphyrin complexes.…”