“Axial-Bonding”-Type Hybrid Porphyrin Arrays: Synthesis, Spectroscopy, Electrochemistry, and Singlet State Properties

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Abstract
A series of phosphorus(V), germanium(IV), and tin(IV) porphyrin-based, "axial-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" ions into the porphyrin cavity. The architecture of these arrays is such that, while a phosphorus(V), germanium(IV), or tin(IV) complex of meso-5,10,15,20-(tetratolyl)porphyrin forms the basal scaffolding unit, the free-base, vanadyl, cobalt(II), nickel(II), copper(II), or zinc(II) porphyrins occupy the two axial sites via an aryloxy bridge. Synthesis of an "all-phosphorus" array containing three phosphorus(V) subunits has also been accomplished. Each new porphyrin array investigated in this study has been fully characterized by various physical methods that include mass (FAB), UV-visible, infrared, fluorescence, electron spin resonance (ESR), and (1)H and (31)P nuclear magnetic resonance (NMR; 1D and 2D) spectroscopies and cyclic voltammetry. The UV-visible and ESR spectral parameters and also the redox potential data suggest that there exists no interaction between the pi-planes of the constituent monomeric porphyrins in these arrays. Detailed (1)H NMR investigations carried out with the trimers containing diamagnetic porphyrins reveal characteristic shielding/deshielding effects for the various protons on the axial porphyrin subunits. The ground state data, as probed by the spectroscopic and electrochemical techniques, collectively indicate that there exists a symmetric but nonparallel disposition of the two axial porphyrins with respect to plane of the central porphyrin. Singlet state activity of the photoactive trimers has been probed by the steady state fluorescence method with selective excitation into the bands corresponding to the two constituent monomeric species. Analysis of the fluorescence emission and excitation spectral data suggests the occurrence of electronic energy transfer as well as photoinduced electron transfer reactions in trimers endowed with free-base or zinc(II) porphyrin axial subunits. Efficiencies of the excited state processes of these trimeric arrays are shown to be dependent on the type of metal/metalloid ions present in the porphyrin crevice.