Equilibrium, photophysical and photochemical examination of anionic lanthanum(iii) mono- and bisporphyrins: the effects of the out-of-plane structure

Abstract

Lanthanum(III) ion forms kinetically labile complexes with the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (H2TSPP 4-), the compositions and formation constants of which significantly depend on the presence of potential axial ligands (at 0.01 M). Deviating from the chloride ion, acetate coordinating to the metal center hinders the formation of bisporphyrin complex. In these 10 lanthanum(III) complexes, the metal center, due to its large ionic radius (103.2 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. Metalation significantly decreases the quantum yield of the fluorescence from the S1 excited state. Quantum chemical calculations (DFT) confirm the considerable OOP 15 displacement of the La(III) center (about 120 pm in the monoporphyrin complexes). The monoporphyrins display efficient fluorescence (0.03), while the bisporphyrin does not emit. Differing from the normal (in-plane) metalloporphyrins, excitation of these lanthanum(III) porphyrins leads to an irreversible ligandto-metal charge transfer (LMCT) followed by the opening of the porphyrin ring, which is also typical of OOP complexes. Dissociation releasing free-base porphyrin can also be observed upon irradiation of the 20 monoporphyrin in acetate solution, while in the presence of chloride ions interconversions of the monoand bisporphyrins may also take place beside the irreversible photoredox reaction.

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

“…The effect of the porphyrin concentration was also investigated: the overall photochemical quantum yields monotonously increased with the concentration. This suggests that the excited complex molecules can undergo bimolecular, thermal (redox) reaction with the species in ground state [9].…”

“…The presence of bidentate, O-donor acetate ion enhances the coordination of the first porphyrin ligand, due to its trans effect, but it hinders the connection of a further porphyrin, i.e. the formation of bisporphyrins, because it remains on the lanthanide(III) ion in axial position [9,22]. Insertion of these larger metal ions into the porphyrin cavity can be spectrophotometrically followed due to the redshifts of UVVis, intraligand ππ* absorption bands (Fig.…”

“…The equilibria between the complexes are varied by excitation, i.e. the significant parts of the overall photochemical quantum yields originate from these photoinduced transformations of complexes to each other in this photostationary state [9]. Light accelerates these reactions by orders of magnitude, compared to their very low rate in dark at room temperature [22].…”

“…Furthermore, lanthanide(III) porphyrins may be useful in the photocatalytic cleavage of water because the metal center, reduced due to the photoinduced charge transfer from ligand to metal (LMCT) [1,2,17,18], has a negative redox potential large enough to produce hydrogen (except Eu 3+ , Fig. 2) [9]. In this work, we studied the photophysical and primary photochemical properties of the complexes between a water-soluble, anionic porphyrin, the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (H2TSPP 4-, abbreviated as H2P 4-) and early lanthanide(III) ions (Ln=La,Ce,Nd,Sm).…”

“…1) [3][4][5][6]. Lanthanide(III) complexes are the model compounds of metallo-oligoporphyrins, in which the π-π interaction depends on the distance between the macrocycles, strongly influenced by the size of the metal center [7][8][9]. Therefore, lanthanide(III) ions offer good opportunities to examine this dependence due to the fine tuning of the out-of-plane distances, utilizing the wellknown lanthanide contraction: the radii of Ln 3+ ions monotonously decrease from 116 pm to 98 pm (at coordination number 8) with the increase of atomic numbers within the lanthanide series (Fig.…”