Cyclophanes have been firmly entrenched as a distinct class of compounds for well over half a century. The two main factors that have kept this field of chemistry going so strongly for such a long time are tremendous structural diversity and the interesting behaviour that is often observed. Although a very large number cyclophanes has been reported, only a very small proportion of them contain polycyclic aromatic systems that can be thought of as "large", i.e. with ≥4 rings. This Review puts the spotlight on such cyclophanes, illuminating both the chemistry that was used to synthesize them and what was learned from studying them. Context for the main body is provided by the careful consideration of the anatomy of a cyclophane and the classification of general synthetic approaches. The subsequent sections cover eleven different PAHs and are organized primarily according to increasing size of the aromatic system, starting with pyrene (C16, the only large polycyclic aromatic system to have been incorporated into numerous cyclophanes) and ending with hexabenzo[bc,ef,hi,kl,no,qr]coronene (C42).

Dedicated to Professor Reginald Mitchell on the occasion of his 65th birthdayCyclacenes, for example, [12]cyclacene (1; Figure 1), and cyclophenacenes, for example, cyclo [12]phenacene (2), are aromatic belts that map onto the surfaces of various fullerenes and single-walled carbon nanotubes (SWCNTs).[1] Indeed, they can be viewed as the shortest possible open-ended (uncapped) zig-zag and armchair single-walled carbon nanotubes, respectively.[1e] Although interest in the synthesis of aromatic belts, especially cyclacenes, predates the discovery of the fullerenes [2] and SWCNTs [3] by decades, [4] the discovery of SWCNTs and their homology to aromatic belts has engendered more intense synthetic and theoretical interest, not only in cyclacenes [5] and cyclophenacenes, [6] but also in related aromatic/conjugated belts. [7] Of particular interest to us are the Vögtle belts, [8] for example, 3, which are armchair SWCNT segments having a pyrenoid or rylenoid motif, depending upon ones perspective.A recurring theme in the numerous unsuccessful attempts to synthesize aromatic belts is the inability to aromatize partially saturated belt precursors. The interplay of two major energetic factors, strain and aromaticity, is at the heart of the problem.[9] The generation of a nonplanar aromatic system from a non-or partially aromatic precursor is inherently disfavored by the build-up of strain, but favored by the aromatic stabilization energy (ASE) of the aromatic system being formed. Increasing the distortion from planarity causes the strain energy to become larger and the ASE to become smaller. As a result, strain ultimately becomes dominant and aromatization, by standard methods such as the elimination of water, can become unfavorable. For example, the addition of water (2 equiv) to Schlüters "double-stranded cycle" has been calculated to be exothermic by 42.2 kcal mol À1 .[7b]Therefore, if any approach to aromatic belts is ultimately going to be successful it will not only have to target a relatively stable structural motif, but also rely upon methodology that is capable of generating highly nonplanar aromatic systems under relatively mild conditions. The valence isomerization/dehydrogenation (VID) reaction of [2.2]metacyclophane-1,9-diene (4) to give pyrene 6 is such a method (Scheme 1 a), [10] and it has been used for the synthesis of [n](2,7)pyrenophanes containing nonplanar

A series of 1,n-dioxa[n](2,7)pyrenophanes (n = 7-12) with increasingly nonplanar pyrene moieties was synthesized by a 9-10 step sequence starting from 5-hydroxyisophthalic acid. The crystal structure of each member of this series was determined crystallographically. Several spectroscopic properties were found to vary with the extent of the nonplanarity of the pyrene unit. The way in which the distortion from planarity of the pyrene system influences its pi-electron delocalization was investigated by using two quantitative descriptors of aromaticity based on geometry (HOMA) and magnetism (magnetic susceptibility and NICS). Both methods suggest that the aromaticity of the pyrene moiety is diminished only slightly upon increasing the bend angle theta from 0 degrees to 109.2 degrees.

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Mind your PQPs: 1D nanostructures were fabricated by the self‐assembly of amphiphilic centrally charged discotic 9‐alkyl‐2‐phenylbenzo[8,9]quinolizino[4,5,6,7‐fed]phenanthridinylium (PQP) salts. Increasing the alkyl chain length of the PQP salts resulted in nanoscaled lamellar structures, whereas changing the counterion of the PQP salts from Cl− to BF4− led to a change from nanoribbons to helices and tubes.

A new strategy for the synthesis of Buckybowls is presented and initial attempts to implement it are reported. This involves annulation of further rings onto polycyclic aromatic systems that prefer to be planar but have been "pre-bent" by the installation of a tether. Pyrenophane 2b reacts with TCNE and PTAD to give 1:1 and 1:2 adducts, respectively. The less strained pyrenophane 2c is unreactive toward TCNE but gives a 1:2 adduct with PTAD. Attempted electrophilic aromatic brominations of pyrenophane 2e under a variety of conditions were unsuccessful, as were attempts to brominate cyclophanediene 1c, the direct synthetic precursor of 2c. Tether cleavage and addition reactions occurred rather than substitution. In an effort to circumvent tether cleavage problems, [7]-, [8]- and [9](2,7)pyrenophanes 22b-d were prepared. However, attempted bromination and Friedel-Crafts acylations failed. Evidence for the fleeting existence of [6](2,7)pyrenophane 22a was also obtained. Comparison of structural data (X-ray and AM1 calculations) for the pyrenophanes 22a-d with their 1,n-dioxa analogues 2a-d indicates that the nature of the tether has a strong effect on the degree of bend in the pyrene moiety and this led to the identification of trioxapyrenophane 28 as the next target in the quest for increasingly bent pyrenes.

We report here the photophysical, electrochemical, and electrogenerated chemiluminescent studies of four new pyrenophanes. Comparison with the cyclic voltammogram obtained using the planar parent compound, pyrene, showed these bent-pyrene-containing cyclophanes have a shift in the thermodynamic reduction potentials to more negative values. AM1 semiempirical calculations support such findings in which the LUMO energy increased with the degree of nonplanarity. The HOMO energy, however, remained essentially unchanged for all the compounds in this study. The nonplanarity in these compounds resulted in a substantial decrease in fluorescence quantum yield as well as a slight red shift in both the absorption and emission spectra. In addition, unlike pyrene, no excimer emission was evident in the fluorescence spectra even at concentrations as high as 1 mM. However, the ECL spectra of all four pyrenophanes, produced by generating the anion radical in the presence of benzoyl peroxide as a coreactant, showed broad “excimer-like” emission in addition to the monomer emission observed at shorter wavelengths.

A benzene ring nestled into the concave face of a bent pyrene characterizes the title compound in the crystal. The synthesis of this compound was accomplished using the valence isomerization−dehydrogenation (VID) method and marks the official launch of our journey en route to aromatic belts first proposed by Prof. Vo 1gtle.We have recently developed a general route to the [n](2,7)pyrenophanes 3 1 and several analogues containing oxygen atoms in the bridge. [2][3][4] The lynchpin of this strategy is the conversion of a tethered syn-[2.2]metacyclophanediene system 1 to the corresponding pyrenophane 3 by a valence isomerization-dehydrogenation (VID) protocol (Scheme 1). Noteworthy features of this strategy are the mild conditions (reflux in benzene) required to generate the severely distorted polycyclic aromatic hydrocarbon (PAH) moieties during the key step and the availability of gram quantities of the pyrenophanes using this approach.With reliable access to bent pyrenes, the application of the VID methodology to the challenging goal of preparing aromatic belts consisting of a repeating pyrene motif, i.e., 4 (Figure 1) can now be targeted realistically. The 5-fold symmetric belt 4a manifests itself in the equators of the fullerenes D 5h C 70 5 and D 5d C 80 , 6 whereas the 6-fold sym-