meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by (1)H, (13)C, and (19)F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25 degrees C exist largely as an equilibrium among several rapidly interconverting twisted Möbius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take either planar or Möbius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100 degrees C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Möbius conformation. Detailed analyses of the solid-state Möbius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic pi-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Möbius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.

Recently, expanded porphyrins have come to the forefront in the research field of aromaticity, and been recognized as the most appropriate molecular system to study both Hückel and Möbius aromaticity because their molecular topologies can be easily changed and controlled by various methods. Along with this advantage, many efforts have been devoted to the exploration of the aromaticity-molecular topology relationship based on electronic structures in expanded porphyrins so that further insight into the aromaticity--a very attractive field for chemists--can be provided. In this tutorial review, we describe the recent developments of various topology-controlled expanded porphyrins and their photophysical properties, in conjunction with the topological transformation between Hückel and Möbius aromaticity by various conformational control methods, such as synthetic methods, temperature control, and protonation.

Round the twist: Metalation of [36]octaphyrin 1 provided the Möbius aromatic Pd2 complex 3 as well as the Hückel antiaromatic Pd2 complex 2. This method can be applied to other expanded porphyrins and Group 10 metal ions. The aromatic/antiaromatic character was supported by NMR spectrscopy, NICS calculation, and two‐photon absorption measurements.

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.

Two synthetic methods of meso-aryl-substituted subporphyrins have been developed by means of the reaction of pyridine-tri-N-pyrrolylborane with a series of aryl aldehydes. One method relies on the condensation under Adler conditions with chloroacetic acid in refluxing 1,2-dichlorobenzene to afford subporphyrins in 1.1-3.2%, and the other is a two-step reaction consisting of the initial treatment of the two substrates with trifluoroacetic acid at 0 degrees C followed by air-oxidation in refluxing 1,2-dichlorobenzene to provide subporphyrins in up to 5.6% yield. 1H NMR studies indicate that phenyl and sterically unhindered substituents at the meso position of subporphyrins rotate rather freely even at -90 degrees C, whereas the rotation of meso-2,4,6-trimethoxyphenyl substituents is strictly prohibited even at 130 degrees C. The structures of six subporphyins have been revealed by X-ray crystallographic analysis to be all cone-shaped tripyrrolic macrocycles. Dihedral angles of meso-phenyl and sterically unhindered aryl substituents to the subporphyrinic core are rather small (38.3-55.7 degrees ) compared to those of porphyrin analogues, whereas those of meso-2,4,6-trimethoxy-substituted subporphyrins are large (68.7-75.7 degrees ). These rotational features of the meso-aryl substituents lead to their large influences on the electronic properties of subporphyrins, as seen for 4-nitrophenyl-substituted subporphyrin 14e that exhibits perturbed absorption and fluorescence spectra, depending upon solvents. Large solvent-polarity dependence of the fluorescence of 14e suggests the charge-transfer character for its excited state. Electrochemical and theoretical studies are performed to understand the electronic properties. Overall, meso-aryl-substituted subporphyrins are promising chromophores in future functional devices.

Aromaticity is a key concept in chemistry, dating back to Faraday's discovery of benzene in 1825 and Kekulé's famous alternating-double-bond structure of 1865. In 1858, the Möbius strip was discovered by Möbius and Listing. The Hückel rules for predicting aromaticity, stating that [4n + 2] π electrons result in an aromatic system, work for planar molecules. Although molecules with Möbius geometry are not found in nature, chemists have tried to synthesize such molecules since the first theoretical prediction by Heilbronner in 1964 and the prediction of Möbius aromaticity for suitable compounds with [4n] π electrons. However, Möbius-aromatic molecules have proved difficult to synthesize, and sometimes even to identify. Here we summarize recent contributions of several research groups that have succeeded in synthesizing Möbius-type molecules. The results of this survey lead us to suggest that the generation of Möbius topologies in expanded porphyrins is easier than hitherto appreciated.

A new set of spectroscopic tools is proposed that may be used to distinguish antiaromatic compounds from their corresponding aromatic congeners. This prediction is based on a detailed analysis of the optical and photophysical properties of a matched set of expanded porphyrins. In particular, the antiaromatic porphyrinoids having [4n] π-electrons within their conjugation pathway exhibit distinct photophysical features that differ dramatically from what is observed for the corresponding aromatic congeners. The clear diagnostic differences seen between the antiaromatic and aromatic compounds leads us to propose that the spectroscopic methods detailed in this Perspective could emerge as general tools that may be used to characterize the electronic characteristics of complex systems for which a number of potential electronic states can be envisioned on the basis of simple line formulas or analyses of π-electron populations. W ith a storied history dating back to the days of early interest in benzene, the concept of aromaticity continues to excite the imagination of chemists, even as its importance has become increasingly apparent in fields as diverse of biomedicine and materials science. Not surprisingly, therefore, considerable effort has been devoted to exploring and understanding aromaticity. As a consequence, aromaticity has come to be defined in terms of five key experimental parameters, namely, energetics, structure, reactivity, magnetism, and spectroscopic features. 1 While the chemical and physical properties of many canonical aromatic systems have been investigated in detail, a complete understanding of the underlying structure-property relationships, including H€ uckel's [4n þ 2] rule and other types of quantitative indices of aromaticity, 2-7 still remains elusive. 8,9 This is even more true in the case of antiaromaticity.

A strong correlation among calculated Nucleus-Independent Chemical Shift (NICS) values, molecular planarity, and the observed two-photon absorption (TPA) values was found for a series of closely matched expanded porphyrins. The expanded porphyrins in question consisted of [26]hexaphyrin, [28]hexaphyrin, rubyrin, amethyrin, cyclo[6]pyrrole, cyclo[7]pyrrole, and cyclo[8]pyrrole containing 22, 24, 26, 28, and 30 pi-electrons. Two of the systems, [28]hexaphyrin and amethyrin, were considered to be antiaromatic as judged from a simple application of Hückel's [4n + 2] rule. These systems displayed positive NICS(0) values (+43.5 and +17.1 ppm, respectively) and gave rise to TPA values of 2600 and 3100 GM, respectively. By contrast, a set of congeners containing 22, 26, and 30 pi-electrons (cyclo[n]pyrrole, n = 6, 7, and 8, respectively) were characterized by a linear correlation between the NICS and TPA values. In the case of the oligopyrrolic macrocycles containing 26 pi-electron systems, a further correlation between the molecular structure and various markers associated with aromaticity was seen. In particular, a decrease in the excited state lifetimes and an increase in the TPA values were seen as the flexibility of the systems increased. Based on the findings presented here, it is proposed that various readily measurable optical properties, including the two-photon absorption cross-section, can provide a quantitative experimental measure of aromaticity in macrocyclic pi-conjugated systems.