A Directly Fused Tetrameric Porphyrin Sheet and Its Anomalous Electronic Properties That Arise from the Planar Cyclooctatetraene Core

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 two signals for the porphyrin beta-protons. The absorption spectrum of 3 displays characteristic Soret-like broad bands and weak Q-bands, and its magnetic circular dichroism (MCD) spectrum exhibits a negative Faraday A term at the 762 nm band as a rare case, indicating the absorption as a transition from a nondegenerate level 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 sheet 3 forms stable 1:2 complexes with guest molecules G1 and G2, whose 1H NMR spectra exhibit remarkable downfield 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 upfield positions. These results have been qualitatively accounted for in terms of the presence of a strong paratropic ring current around the COT core that propagates through the whole pi-electronic network of 3, hence competing with and cancelling the weak diatropic ring currents of the local zinc(II) porphyrins. This explanation was supported by DFT calculation performed at the GIAO-B3LYP/6-31G level, which indicated large positive NICS values within the COT core and small NICS values within the local zinc(II) porphyrins.

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…The distance-dependent NICS at the center of the COT core for 17(Zn) was also performed [86], which was shown to decrease monotonously with increasing distance as shown in the case of cyclobutadiene [87]. Furthermore, the complex of 17(Zn) with 1,4-bis(1-methylimidazol-2-ylethynyl)benzene or 5,15-bis(1-methylimidazol-2-yl)-10,20-dihexylporphyrin experimentally proved the paratropic ring current of the central COT core [84]. However, in spite of the large difference between the NICS values of 17(Zn) and 18, the absorption spectra of 17 and 18 is quite similar [85], suggesting that the HOMO-LUMO gaps of 17 and 18 are almost identical.…”

“…Directly fused tetrameric porphyrin sheets 17 and their free base analogue 18 (Figure 12), which are also calculated to have a planar COT core, have been synthesized [84,85]. The directly meso-meso linked cyclic porphyrin tetramer which is the precursor for 17(Zn), was synthesized through a stepwise coupling reaction sequence.…”

“…Free base sheet 18 was obtained by demetallation of 17 [85]. As for the paratropicity of the central COT ring, NICS calculations were performed at the GIAO/B3LYP/6-31G*//B3LYP/6-31G* level (LANL2DZ for zinc atom) and the values were +61.7 for 17(Zn) and +21.7 for 18 (29.2 for D 4h COT at the same level) [84]. The distance-dependent NICS at the center of the COT core for 17(Zn) was also performed [86], which was shown to decrease monotonously with increasing distance as shown in the case of cyclobutadiene [87].…”

Directly Linked Corrole Oligomers via Facile Oxidative 3–3′ Coupling Reaction

“…6H contains a small amount of unidentified by-products judging from the 1 H NMR spectrum. The metalations of 3H and 4H with Co(OAc) 2 were then performed in pyridine to afford 3Co¢py 6 and 4Co¢py 8 , respectively, which gave well-resolved sharp 1 H NMR spectra in benzene-d 6 . The single crystals of 3Co¢py 6 were formed from a solution of pyridine and n-heptane.…”

“…8 As such, this simple synthetic method that allows a direct porphyrinporphyrin connection has been demonstrated quite useful for construction of novel multiporphyrinic architectures of attractive properties and functions.…”