Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Mobius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.

Expanded Porphyrins: Intriguing Structures, Electronic Properties, and Reactivities

One of the most important objectives in materials, chemical, and physical sciences is the creation of large conjugated macrocycles with well-defined shapes, since such molecules are not only theoretically and experimentally interesting but also have potential applications in nanotechnology. Fully unsaturated macrocycles are regarded as models for infinitely conjugated π systems with inner cavities, and exhibit unusual optical and magnetic behavior. Macrocycles have interior and exterior sites, and site-specific substitution at both or either site can afford attractive structures, such as 1D, 2D, and 3D supramolecular nanostructures. These nanostructures could be controlled through the use of π-extended large macrocycles by a bottom-up strategy. Numerous shape-persistent π-conjugated macrocycles have been synthesized, but only a few are on the nanoscale. This Review focuses on nanosized π-conjugated macrocycles (>1 nm diameter) and giant macrocycles (>2 nm diameter), and summarizes their syntheses and properties.

Conjugated macrocycles: concepts and applications.

The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Mobius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

Since the discovery of its facile synthesis in 2001, meso-aryl-substituted expanded porphyrins have been developed as a new class of azaannulenes in light of their facile redox interconversions, conformational flexibilities involving flipping of the constitutional pyrroles, rich metal coordination behaviors, unprecedented chemical reactivities, effective platforms to realize versatile electronic states including Mobius aromatic and antiaromatic species, and abilities to stabilize organic radicals. In this Review, the syntheses, structures, and optical, electronic, and magnetic properties of meso-aryl-substituted expanded porphyrins and their metal complexes have been updated with a particular focus on the relationship between “aromaticity and molecular twist (molecular topology)”. While the importance of the interplay of these two characteristics has been long recognized from the theoretical viewpoint, meso-aryl-substituted expanded porphyrins offered solid experimental evidence to provide Mobius aromatic...

Chemistry of meso-Aryl-Substituted Expanded Porphyrins: Aromaticity and Molecular Twist.

meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by 1H, 13C, and 19F 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 °C exist largely as an equilibrium among several rapidly interconverting twisted Mobius 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 Mobius-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 °C in THF, these rapid interconversions among Mobius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species h...

Unambiguous identification of Möbius aromaticity for meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1).

At room temperature, meso-hexaaryl-substituted [28]hexaphyrins(1.1.1.1.1.1) in solution exist largely as an equilibrium between planar antiaromatic and distorted Mobius aromatic conformers. As the temperature decreases, the molecular structure changes into the distorted Mobius topology that commonly occurs in [28]hexaphyrins, which gives rise to longer excited singlet and triplet state lifetimes than planar antiaromatic [28]hexaphyrins. Temperature-dependent two-photon absorption measurements of [28]hexaphyrin indicate that the degree of aromaticity of Mobius [28]hexaphyrin is large, comparable to that of Huckel aromatic planar [26]hexaphyrin. Through our spectroscopic investigations, we have demonstrated that a subtle balance between the strains induced by the size of the [28]hexaphyrin macrocyclic ring and the energy stabilization contributed by π-electron delocalization in the formation of distorted Mobius [28]hexaphyrin leads to the molecular structure change into the Mobius topology as the temperatur...

Shigeki Mori

Papers

Unambiguous identification of Möbius aromaticity for meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1).

Temperature-dependent conformational change of meso-hexakis(pentafluorophenyl) [28]Hexaphyrins(1.1.1.1.1.1) into Möbius structures.