Lanthanide-Porphyrin Hybrids: from Layered Structures to Metal–Organic Frameworks with Photophysical Properties

Rare-earth layered hydroxides with
intercalated tetrasulfonated porphyrins and corresponding to the chemical
formula Ln2(OH)4.7(Por)0.33·2H2O (Ln = Eu3+, Tb3+; Por = 5,10,15,20-tetrakis­(4-sulfonatophenyl)­porphyrin
(TPPS) and PdTPPS) have been prepared to investigate their photophysical
properties. A slight variation of the synthetic procedure led to the
metal–organic framework (MOF) assembled from a distorted octahedral
oxometalate clusters [Eu6(μ6-O)­(μ3-OH)8(H2O)14]8+. These secondary building units (SBUs) are linked together by six
distorted porphyrin units. During activation, the original SBU loses
not only water molecules from the coordination sphere but also the
central μ6-O atom. The loss of the central atom results
in the distortion of the octahedral [Eu6(μ6-O)­(μ3-OH)8(H2O)14]8+ SBU into a trigonal antiprismatic [Eu6(μ3-OH)8(H2O)2]10+ SBU with two μ3-OH groups nearly in plane with
the europium atoms and the reduction of pores to approximately 2 ×
3 Å. As a result, the MOF has no accessible porosity. This transformation
was thoroughly characterized by means of single-crystal X-ray crystallographic
analysis of both phases. Solid-state photophysical investigations
suggest that the MOF material is fluorescent; however, in contrast
to the prepared layered hydroxides, the as-prepared MOF is an effective
sensitizer of singlet oxygen, O2(1Δg), with a relatively long lifetime of 23 ± 1 μs.
The transition is also accompanied by variation in photophysical properties
of the coordinated TPPS. The alteration of the fluorescence properties
and of the O2(1Δg) lifetime
presents an opportunity for preparation of MOFs with oxygen-sensing
ability or with oxidation potential toward organic molecules by O2(1Δg).