Ring Size Determines the Conformation,Global Aromaticity and Photophysical Properties of Macrocyclic Oligofurans |
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Authors: | Or Dishi Dr. Partha Malakar Dr. Linda J. W. Shimon Prof. Sanford Ruhman Prof. Ori Gidron |
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Affiliation: | 1. Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem, 9190401 Israel;2. Chemical Research Support Unit, Weizmann Institute of Science, Rehovot, 7610001 Israel |
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Abstract: | In π-conjugated macrocycles, there is a trade-off between the global and local expression of effects such as aromaticity, with the outcome of the trade-off determined by the geometry and aromaticity of the constituent units. Compared with other aromatic rings, the aromatic character of furan is relatively small, and therefore global effects in macrocyclic furans are expected to be more pronounced. Following our introduction of macrocyclic oligofuran, we present the first synthesis of a series of π-conjugated bifuran macrocycles of various ring sizes, from trimer to hexamer, and characterize them using both computational and experimental methods. The properties of macrocyclic oligofurans change considerably with size: The smaller trimer is rigid, weakly emissive and planar as revealed by its single crystal structure, and displays global antiaromaticity. In contrast, the larger pentamer and hexamer are flexible, emissive, have non-planar structures, and exhibit local aromaticity. The results are supported by NICS and ACID calculations that indicate the global antiaromaticity of planar furan macrocycles, and by transient absorption measurements showing sharp absorption band for the trimer and only the internal conversion decay pathway. |
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Keywords: | aromaticity macrocycles oligofurans pi conjugation |
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