Synthesis and solution studies of silver(I)-assembled porphyrin coordination cages

Abstract

The synthesis and the characterization of two porphyrin coordination cages are reported. The design of the cage formation is based on the coordination of silver(I) ions to the pyridyl units of 3-pyridyl appended porphyrins. (1)H/(109)Ag NMR spectroscopy, and diffusion-ordered spectroscopy (DOSY) experiments demonstrate that both the free base porphyrin 2H-TPyP and the Zn-porphyrin Zn-TPyP form the closed cages, [Ag4(2H-TPyP)2](4+) and [Ag4(Zn-TPyP)2](4+), respectively, upon addition of two equivalents of Ag(+). The complexation processes are characterized in details by means of absorption and emission spectroscopy in diluted CH2Cl2 solutions. The data are discussed in the frame of the point-dipole exciton coupling theory; the two porphyrin monomers, in fact, experience a rigid face-to-face geometry in the cages and a weak inter-porphyrin exciton coupling. An intermediate species is observed, for Zn-TPyP, in a porphyrin/Ag(+) stoichiometric ratio of about 1:0.5 and is tentatively ascribed to an oblique open form. The occurrence of a photoinduced electron-transfer reaction within the cages is excluded on the basis of the experimental outcomes and thermodynamic evaluations. Photophysical experiments evidence different reactivities of singlet and triplet excited states in the assemblies. A lower fluorescence quantum yield and triplet formation is discussed in relation to the constrained geometry of the complexes. Unusually long triplet excited state lifetimes are measured for the assemblies.

Keywords: UV/Vis spectroscopy; coordination compounds; photophysics; porphyrins; self-assembly; silver.