"Axial-bonding"-type hybrid porphyrin arrays: Synthesis, spectroscopy, electrochemistry, and singlet state properties

A series of phosphorus(V), germanium(IV), and tin(IV) porphyrin-based, "axi al-bonding"-type hybrid trimers have been readily constructed by employing a new "building-block" approach. The approach is modular in nature, and it involves simple ''inorganic" reactions such as axial bond formation of main group element containing porphyrins and insertion of metal/"metalloid" ion s into the porphyrin cavity. The architecture of these arrays is such that, while a phosphorus(V), germanium(IV), or tin(IV) complex of meso-5,10,15,2 0-(tetratolyl)porphyrin forms the basal scaffolding unit, the free-base, va nadyl, cobalt(II), nickel(II), copper(II), or zinc(II) porphyrins occupy th e two axial sites via an aryloxy bridge. Synthesis of an "all-phosphorus" a rray containing three phosphorus(V) subunits has also been accomplished. Ea ch new porphyrin array investigated in this study has been fully characteri zed by various physical methods that include mass (FAB), UV-visible, infrar ed, fluorescence, electron spin resonance (ESR), and H-1 and P-31 nuclear m agnetic resonance (NMR; 1D and 2D) spectroscopies and cyclic voltammetry. T he UV-visible and ESR spectral parameters and also the redox potential data suggest that there exists no interaction between the pi-planes of the cons tituent monomeric porphyrins in these arrays. Detailed H-1 NMR investigatio ns carried out with the trimers containing diamagnetic porphyrins reveal ch aracteristic shielding/deshielding effects for the various protons on the a xial porphyrin subunits. The ground state data, as probed by the spectrosco pic and electrochemical techniques, collectively indicate that there exists a symmetric but nonparallel disposition of the two axial porphyrins with r espect to plane of the central porphyrin. Singlet state activity of the pho toactive trimers has been probed by the steady state fluorescence method wi th selective excitation into the bands corresponding to the two constituent monomeric species. Analysis of the fluorescence emission and excitation sp ectral data suggests the occurrence of electronic energy transfer as well a s photoinduced electron transfer reactions in trimers endowed with free-bas e or zinc(II) porphyrin axial subunits. Efficiencies of the excited state p rocesses of these trimeric arrays are shown to be dependent on the type of metal/metalloid ions present in the porphyrin crevice.