Volume 78, Issue 4

_ The most easterly point of North America was the setting for the 11th International Symposium on Novel Aromatic Compounds (ISNA-11), which took place on 14-18 August 2005 in the picturesque harbor city of St. John's, Newfoundland and Labrador, Canada. An atypically small, but nevertheless enthusiastic, group of 135 delegates representing 17 countries and 64 academic and industrial institutions participated in ISNA-11. The scientific program consisted of 107 posters (two sessions), 26 plenary lectures, and the Nozoe lecture, which was presented by Prof. Koichi Komatsu of Kyoto University. The inaugural ISNA meeting was held in Sendai, Japan in 1970. Since then, ISNA meetings have been held at three- to four-year intervals at locations that cycle between Asia, North America, and Europe. In its infancy, the ISNA community was focused primarily on the definition and quantification of aromaticity, as well as the synthesis and study of compounds that exhibited the phenomenon, whatever it may be. Nourished by advances in synthetic methodology, analytical techniques, separation capabilities, and computational power, ISNA has blossomed over the intervening 35 years to embody a much broader and deeper set of interests. In particular, the twin issues of function and applications have emerged as integral components of ISNA's collective being. The scientific topics of ISNA-11 were selected with an eye toward balancing the traditional areas of interest with its expansion in new directions. The 15 articles that appear in this issue embody these themes, which include: the synthesis of aromatic compounds theoretically interesting aromatic molecules structural aspects of aromaticity aromatic compounds for devices fullerenes and nanotubes molecular switches and machines macrocyclic aromatic compounds Debate about the status and future direction of ISNA was actively encouraged, and the overwhelming response was that ISNA is more vibrant and relevant than ever before. It is healthy and growing in many ways. It was therefore decided to move subsequent ISNA meetings to a two-year cycle. This will commence with ISNA-12, which will be held on Awaji Island (located near Kobe) in Japan on 22-27 July 2007. Professor Yoshito Tobe (Osaka University) will serve as conference chair. The responsibility for ISNA-13 was also awarded. It will be organized by Prof. François Diederich (ETH Zürich) and will be held at a yet to be determined European location in 2009. Graham Bodwell Rik Tykwinski Conference Editors

We present the results of our systematic study on the synthesis, structures, and electronic properties of a series of cyclic π-conjugated systems with monocyclic six- to eight-membered rings as well as polycyclic benzenoid aromatics, which are fully surrounded by rigid bicyclic σ-frameworks such as bicyclo[2.2.2]octene (BCO). The study has also been extended to sulfur-containing systems such as thiophene, 1,2- and 1,4-dithiins, and oligothiophene. Such a structural modification using bicyclic frameworks is effective in elevation of the highest occupied molecular orbital (HOMO) levels of neutral π-systems and in remarkable stabilization of the corresponding cationic systems by both σ-π conjugation and kinetic effects. This has led to various new cationic π-conjugated systems such as highly stable radical cations as well as closed-shell mono- and dications, which could have never been attained by any other conventional techniques. The properties of benzene and entirely planar cyclooctatetraene (COT) possessing a marked bond fixation caused by structural modification with a strained bicyclic system (i.e., bicyclo[2.1.1]hexene, BCH) are also described.

This short review describes paths to superphanes with metal-stabilized cyclobutadiene and cyclopentadienone rings in a one-pot and stepwise fashion. The stepwise synthesis also led to a new class of belt-like systems with large rings composed of CpCo-stabilized cyclobutadiene rings connected with propane chains. Finally, our research yielded the first beltenes composed of alternating four- and eight-membered conjugated rings.

The gelling and spectroscopic properties of new soluble disubstituted acenes (anthracene, tetracene, pentacene) are reported. It is shown that the 2,3-alkoxy derivatives gel a large variety of organic solvents, a process that is related to the linear shape of the molecule. The gelation involves the formation of nanofibers through noncovalent interactions (van der Waals, π-π stacking), weaving a 3D supramolecular network or a deposit made of isolated fibers on surfaces (mica or graphite). Transport of photons through this type of nanostructure has been evidenced by the observation of an efficient energy transfer occurring from an anthracene light-harvesting matrix to a tetracene energy trap doping agent.

Ladder-type π-conjugated skeletons are promising building units for new materials for organic electronics, wherein their annelated coplanar structures enhance the π-conjugation, leading to a set of desirable properties such as intense fluorescence and high carrier mobility. As a new entry into this class of skeletons, we now disclose two series of ladder molecules, i.e., silicon- and carbon-bridged oligo( p -phenylenevinylene)s and thiophene- and selenophene-based heteroacenes. The former ladder molecules contain the silole substructure in which the silicon bridges affect the electronic structure and thus photophysical properties through the σ*-π* conjugation. The latter heteroacenes can be recognized as the heteroatom analogs of acenes and may be promising materials for organic thin film transistors. These two series of molecules have been synthesized based on newly developed intramolecular cyclizations of acetylenic compounds.

A variety of aryleneethynylenes are synthesized by double elimination reaction of β-substituted sulfones. The acetylenic bond is formed from readily available aromatic aldehydes and benzylic sulfones. A sequence of aldol reaction, trapping of the aldolates with a leaving group, and eliminations proceeds in one pot. The utility of this protocol is exemplified by synthesis of dihalo diphenylacetylenes, 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene, and double-helical aromatic acetylenes.

The unique geometrical features and structural potential of the centropolyindanes, a complete family of novel, 3D polycyclic aromatic hydrocarbons, are discussed with respect to the inherent orthogonality of their arene units. Thus, the largest member of the family, centrohexaindane, a topologically nonplanar hydrocarbon, is presented as a "Cartesian hexabenzene", because each of its six benzene units is stretched into one of the six directions of the Cartesian space. This feature is discussed on the basis of the X-ray crystal structures of centrohexaindane and two lower members of the centropolyindane family, viz. the parent tribenzotriquinacenes. Recent progress in multiple functionalization and extension of the indane wings of selected centropolyindanes is reported, including several highly efficient six- and eight-fold C-C cross-coupling reactions. Some particular centropolyindane derivatives are presented, such as the first twelve-fold functionalized centrohexaindane and a tribenzotriquinacene bearing three mutually orthogonal phenanthroline groupings at its molecular periphery. Challenges to further extend the arene peripheries of the tribenzotriquinacenes and fenestrindanes to give, eventually, graphite cuttings bearing a central bowl- or saddle-shaped center are outlined, as is the hypothetical generation of a "giant" nanocube consisting of eight covalently bound tribenzotriquinacene units. Along these lines, our recent discovery of a related, solid-state supramolecular cube, containing eight molecules of a particular tribromotrinitrotribenzotriquinacene of the same absolute configuration, is presented for the first time.

The application of 3,3'-diphenyl-2'2-bithiophene as a helical scaffold capable of electrochemical polymerization to yield the corresponding polythiophene is reported. One unique feature of this monomer is its theoretically predicted (DFT) ability to mimic redox-stimulated contraction and expansion. This ability, coupled with traditional electromechanical actuation properties of bulk, redox-active conjugated polymers (CPs), yields a polymeric "molecular muscle" capable of both contraction and expansion.

The formation of various macrocyclic cyclophanes via ring-closing olefin metathesis is possible through the use of a pendant pentafluorobenzyl ester group. A quadrupolar interaction between the cyclophane core and the auxiliary is proposed to act as a gearing element facilitating cyclization. The development of these noncovalent interactions as gearing elements as well as the investigation of the effect of the site of metathesis upon the macrocyclization process is described.

The title journey is undertaken at the levels of both theory and experiment. Since 1983, homoaromaticity has been shown to play at most a minor role in the stability of Scott's [N]pericyclyne hydrocarbons - the first ring carbo -mers of cycloalkanes. This statement has been systematically refined for N = 3-6 by using both classical theoretical tools and newly designed tools based on electron localization function (ELF) analysis. The compatibility of the [5]- and [6]-pericyclyne cores with vertex functionalities was established by the synthesis of 20 oxy ( carbo -cyclitol) derivatives. The stereoisomeric resolution of two of them has been achieved. Hexaoxy-[6]pericyclynes are actually potential precursors of the carbo -benzenes. Criteria based on density functional theory (DFT) calculations (magnetic, energetic, structural/"electronic") show that the aromaticity of carbo -[N]annulenic species is comparable to that of their parent molecule. This has been challenged by the synthesis of several novel carbo -benzenic molecules with various substitution patterns. The theory-experiment interplay is pursued by considering ring carbo -mers of other conjugated ring systems such as radialenes. The second carbo -mers (butadiyndiyl-expanded rings) of [3]radialene and benzene are also envisioned. Hexaphenyl- carbo 2 -benzene has been observed by 1 H and UV-vis spectroscopy.

Modern transition-metal-catalyzed cross-coupling reactions, especially of the Kumada, Heck, and Suzuki types, have provided facile access to various bridge-annelated [2.2]paracyclophane derivatives including trifoliaphane and its tribenzo analog. The reduction of trifoliaphane with potassium metal in [D 8 ]-THF led to hexakis[ p -benzyl]benzene anion, which could efficiently be trapped with various electrophiles. Highly efficient multifold Suzuki couplings were performed with hexabromobenzene, octabromonaphthalene, and hexabromotriphenylene. The obtained hexa- and octaalkenylarene derivatives disclose interesting molecular shapes. Eventually, a newly developed cascade coupling of bromoarenes containing peri -positioned C-H bonds has led to indeno-annelated polycyclic aromatic hydrocarbons including indenocorannulene as well as the tetrakis- and tris( tert -butylindeno)pyrene which disclose very interesting spectroscopic properties. In particular, tetrakis( tert -butylindeno)pyrene self-assembles in solution by way of π-stacking, and it can be reduced with potassium metal to a stable dianion, a readily dimerizing trianion radical, and a tetraanion.

To explore the supramolecular properties of the hollow space of carbon nanotubes, the belt-shaped conjugated systems with sizable and deep cavities are intriguing molecules. Recently, we found that the "carbon nanorings", cyclic [6]paraphenyleneacetylene ([6]CPPA), and the related compounds involving two naphthylene rings, form stable inclusion complexes with fullerenes in solution as well as in the solid state. The stability of these complexes correlates well with the van der Waals (VDW) contact between the host and guest. Thus, carbon nanorings having a deep cavity should form considerably stable complexes with fullerenes. In this context, we have designed a new carbon nanoring, cyclic [6](1,4)naphthyleneacetylene. The host forms considerably stable inclusion complexes with fullerenes and can act as the best fluorescence sensor for fullerenes among all the known hosts. The NMR spectra reveal the formation of two major conformational isomers, in which five or six naphthylene rings surround a guest molecule to gain wide contact area. The high affinity between C 60 and naphthylene rings similar to an ion-dipolar interaction plays an important role in the novel guest-induced conformation fixation.

Fullerene-mixed peroxides C 60 (O)(OO t Bu) 4 and C 60 (OO t Bu) 6 are effective precursors for cage-opened fullerene derivatives. Lewis acids induce the heterolysis of peroxo O-O bond in C 60 (O)(OO t Bu) 4 . Subsequent Hock-type rearrangement affords both [5,6]- and [6,6]-fullerene hemiketals - oxahomofullerene. Photolysis of C 60 (OO t Bu) 6 results in the homolysis of O-O and C-O bonds, affording fullerene diketone as the major product. Spectroscopic data and single-crystal analysis confirmed the cage-opening reactions.

In this paper, we report on our ongoing progress on the synthesis and study of branched molecular architectures with a conjugated scaffold. Specifically, new conjugated dendrons incorporating 1,2,4-triethynyl-phenyl units have been developed and used as building blocks for the synthesis of isomeric branched conjugated systems and light-harvesting dendrimers.

Dicyano- and tricyano-substituted aromatic angular building blocks were systematically complexed with silver triflate, and their structures were determined by means of single-crystal X-ray diffraction. The molecular assembly of 1,3,5-tris(cyanomethyl)-2,4,6-triethylbenzene with silver triflate from benzene resulted in a layered structure with distorted square pyramidal silver sites. The structure resulting from the complexation of 1,3,5-tris(cyanomethyl)-2,4,6-trimethylbenzene with silver triflate is dependent on the solvent of crystallization. From benzene or toluene, reaction of 1,3,5-tris(cyanomethyl)-2,4,6-trimethylbenzene with silver triflate yielded a porous, channel-containing, solvated structure, but from acetone the resulting material was a network solid containing no solvent. Complexation of 1,4-bis(cyanomethyl)-2,3,5,6-tetraethylbenzene and 1,4-bis(cyanomethyl)-2,3,5,6-tetramethylbenzene with silver triflate resulted in network solids where the triflate anions were strongly coordinated to the silver.

Shape-persistent conjugated macrocycles are challenging synthetic targets with interesting properties and potential for a number of applications. In contrast to the standard kinetic approaches to macrocycles, reversible Schiff-base condensation offers a one-step, thermodynamic route to large macrocycles. These macrocycles, which feature "salen"-type coordination environments and an interior that resembles a crown-ether have interesting properties themselves and are novel substrates for supramolecular chemistry. This short review article places conjugated [3+3] Schiff-base macrocycles in the context of shape-persistent macrocycles and discusses their synthesis. In addition, specific examples of supramolecular chemistry with these macrocycles highlight their unique behavior when compared to conjugated carbon-based macrocycles and flexible crown-ethers.