Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

Abstract

Anomalous induced magnetic effects were observed in a directly fused square-planar porphyrin sheet 1, in that the protons above the center of the tetraporphyrin core were characteristically shifted downfield in the 1H NMR spectrum. These observations suggest a rare paratropic ring-current effect around the planar cyclooctatetraene (COT) core of 1. To examine the spatial distribution of the induced magnetic effect, face-to-face dimeric complexes of porphyrin sheet 1 with bipyridyl-type guest molecules (G1-G3) were prepared, which provided complexation-induced shifts (CISs) of the guest molecules as a neat experimental guide to the distance dependence of the induced magnetic effects in 1. Nucleus-independent chemical shift (NICS) values of 1 were calculated by varying the distance of the probe from the plane of 1. Whereas a simple bell-type profile was estimated for the complex (1)2-(G1)4, the distance profiles of the CIS became increasingly flat for (1)2-(G2)4 and (1)2-(G3)4. Finally, we investigated the paratropic ring-current effect just above the COT core of the complex 1-(G4)2, which agrees well with the theoretically estimated distance-dependent induced magnetic effect. Consequently, both experimental and theoretical studies on the complexes of porphyrin sheets with guest molecules revealed for the first time a unique distance dependence of the paratropic ring current.

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…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]. 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].…”

Theoretical study of two‐dimensionally fused zinc porphyrins: DFT calculations

“…1) [2,3]. Although porphyrins are well known to play a central role in many biological processes such as sunlight absorption and electron transfer during the initial photochemical step of photosynthesis [4], the possibilities of TA n have been suggested recently for applications in electroconductive nanowires [5,6] and nonlinear optics [7].…”

Transport properties of two‐dimensionally fused zinc porphyrins from linear‐response approach

“…Within the standard phonon-mediated BCS mechanism, we obtained a weak superconducting EPC constant k sc ¼ 0.35 calculated using the superconducting or so called Eliashberg spectral function a 2 sc F(x) and a low superconducting transition temperature T c % 0.5 K estimated using Allen-Dynes equation [2]. Although the extremely large-sized ZnP n sheets have not been synthesized yet, the square-fused tetrameric zinc oligoporphyrin with the planar structure, ZnP 4 , was synthesized and characterized [3,4]. We think that ZnP 1 is not only for a fundamental interest as an analogy to the 2D-graphene but also expected new electronic properties induced by the central metals including their axial coordinations.…”