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A Directly Fused Tetrameric Porphyrin Sheet and ItsAnomalous Electronic Properties That Arise from the PlanarCyclooctatetraene Core

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dcterms:abstract

Oxidation of a directly meso−meso linked cyclic porphyrin tetramer 2 gave a porphyrin sheet 3.The symmetric square structure of 3 is indicated by its simple 1H NMR spectrum that exhibits only twosignals for the porphyrin β-protons. The absorption spectrum of 3 displays characteristic Soret-like broadbands and weak Q-bands, and its magnetic circular dichroism (MCD) spectrum exhibits a negative FaradayA term at the 762 nm band as a rare case, indicating the absorption as a transition from a nondegeneratelevel to a degenerate level. A slightly longer S1-state (1.1 ps) and smaller TPA cross section (2750 GM)than a tetrameric linear porphyrin tape also indicate its unique electronic properties. The porphyrin sheet3 forms stable 1:2 complexes with guest molecules G1 and G2, whose 1H NMR spectra exhibit remarkabledownfield shifts for the guest protons that are located just above the cyclooctatetraene (COT) core of 3,whereas the imidazolyl protons bound to the zinc(II) porphyrin local cores are observed at slightly upfieldpositions. These results have been qualitatively accounted for in terms of the presence of a strong paratropicring current around the COT core that propagates through the whole π-electronic network of 3, hencecompeting with and canceling the weak diatropic ring currents of the local zinc(II) porphyrins. This explanationwas supported by DFT calculation performed at the GIAO-B3LYP/6-31G* level, which indicated large positiveNICS values within the COT core and small NICS values within the local zinc(II) porphyrins.

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