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This Feature Article overviews a new class of π-conjugated materials - macrocyclic furans. Starting from their synthesis, we review their unique structural, optical and electronic properties, chemical reactivity, and potential application as synthons. Finally, we discuss the study of oligofuran macrocycles as a model system for exploring the concept of global aromaticity and the size limitation of Hückel's rule in neutral macrocycles.
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Aromaticity can be assigned by Hückel's rule, which predicts that planar rings with delocalized (4n + 2) π-electrons are aromatic, whereas those with 4n π-electrons are antiaromatic. However, for neutral rings, the maximal value of "n" to which Hückel's rule applies remains unknown. Large macrocycles exhibiting global ring current can serve as models for addressing this question, but the global ring current are often overshadowed in these molecules by the local ring current of the constituent units. Here, we present a series of furan-acetylene macrocycles, ranging from the pentamer to octamer, whose neutral states display alternating contributions from global aromatic and antiaromatic ring currents. We find that the odd-membered macrocycles display global aromatic characteristics, whereas the even-membered macrocycles display contributions from globally antiaromatic ring current. These factors are expressed electronically (oxidation potentials), optically (emission spectra), and magnetically (chemical shifts), and DFT calculations predict global ring current alternations up to 54 π-electrons.
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Applying sequential Diels-Alder cycloaddition and deoxygenation to small π-conjugated furan macrocycles fully converts them to 1,4-naphthalophanes with either ethylene or acetylene spacers, depending on the reaction conditions. 1,4-Napthalenophane tetraene exhibits a 1,3-alternating conformation in the solid state, inclusion of solvent molecules within the macrocycle, and low reduction potentials.
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Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of -0.23 vs. Fc/Fc+ , and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30πe- aromatic state, as indicated by the formation of a strong diatropic current observed in the 1 Hâ NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.
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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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Conformación MolecularRESUMEN
We report the first π-conjugated macrocyclic system with an oligofuran backbone. The calculated HOMO-LUMO gap is similar to that of the corresponding linear polymer, indicating a remarkable electron delocalization. The X-ray structure reveals a planar conformation, in contrast to the twisted conformation of macrocyclic oligothiophenes. The intermolecular π-π stacking distance is extremely small (3.17 Å), indicating very strong interactions. The macrocycle forms large π-aggregates in solution and shows a tendency toward highly ordered multilayer adsorption at the solid-liquid interface. The face-on orientation of molecules explains the higher hole mobility observed in the out-of-plane direction.
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Macrocyclic oligothiophenes ( nCT) have been extensively explored for their unique optical and electronic properties, while their oxygen-containing analogues, macrocyclic oligofurans ( nCF), are not known. In this work, macrocyclic oligofurans bearing 5-16 units (5CF-16CF) are studied theoretically at the B3LYP/6-311G(d) level. We find that small macrocycles (6CF-8CF) exhibit planar or nearly planar geometries, low strain energies, low HOMO-LUMO gaps, and strong π-conjugation as also reflected in their Raman spectra. These findings are in sharp contrast to macrocyclic oligothiophenes of the same size, which are distorted from planarity. Additionally, small macrocyclic furans display significantly lower reorganization energies and ionization potentials compared with their thiophene analogues. Overall, the observed properties highlight the potential of macrocyclic oligofurans to function as p-type organic electronic materials.
