Volume 80, Issue 3

The 12th International Symposium on Novel Aromatic Compounds (ISNA-12) was held in Awaji Island, Japan, on 22-27 July 2007. The IUPAC-sponsored symposium was organized by the local committee, Prof. Yoshito Yobe (Osaka University: chair), Prof. Masahiko Iyoda (Tokyo Metropolitan University: vice-chair), and Prof. Koichi Komatsu (Fukui University of Technology: program chair), and jointly by the Science Council of Japan and the Chemical Society of Japan (CSJ). The symposium was established by Prof. Tetsuo Nozoe in 1970 as the International Symposium on Nonbenzenoid Aromatic Compounds, and this is the third ISNA held in Japan. The aim of this symposium is to elucidate fundamental principles of synthesis and properties of novel aromatic compounds, and to extend the application of π-conjugated systems in general to a wide area of molecular science. The special topics discussed during the symposium include: - synthesis of aromatic compounds - theoretical and structural aspects of aromatic compounds - supramolecular chemistry of aromatic compounds - molecular switches and devices - functions of π-conjugated systems This symposium was attended by 447 participants from 21 countries and regions worldwide. The scientific program consisted of the Nozoe lecture presented by Prof. François Diederich following the opening ceremony, 12 plenary lectures, 21 invited lectures, 38 oral presentations, and 246 poster presentations. Nineteen articles contributed by the Nozoe lecturer and the plenary and invited lecturers appear in this issue to highlight the recent remarkable progress in this scientific field presented at ISNA-12. The next symposium, ISNA-13, will be held in Luxembourg on 19-24 July 2009 and organized by Profs. François Diederich and A. Dieter Schlüter (ETH Zürich). ISNA-14 will be organized by Prof. Michael M. Haley (University of Oregon) in the United States in 2011. Shinji Toyota Conference Editor

Our group started a research program in acetylene chemistry in 1987; since then, an intense research effort led to a fascinating journey into acetylenic scaffolding, aimed at exploring conjugative and optoelectronic properties of acetylenic chromophores. This journey included the generation of a unique molecular construction kit for acetylenic scaffolding, consisting of ( E )-1,2-diethynylethenes [DEEs, ( E )-hex-3-ene-1,5-diynes], tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-1,5-diynes), chiral 1,3-diethynylallenes (DEAs, hepta-3,4-diene-1,6-diynes), 1,4-di and 1,1,4,4-tetraethynylbutatrienes, chiral trialkynylmethanes, and 1,1,2,2-tetraethynylethanes. These building modules were subsequently applied to the synthesis of carbon-rich architectures extending into one, two, and three dimensions. They include multinanometer-long monodisperse oligomers as models for infinite acetylenic polymers, molecular switches, perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes, and an octamethoxy-substituted expanded cubane with a central C 56 core. Donor-substituted cyanoethynylethenes (CEEs) and 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) were introduced as new push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions. Dendritic multivalent CT chromophores were constructed using atom-economic, "click"-like reactions, and these systems were shown to behave as "molecular batteries", featuring exceptional electron uptake and storage capacity. The research finally led to the development of an unprecedented cascade reaction for the preparation of dendritic and oligomeric donor-acceptor (D-A) molecules. New [AB]-type oligomers become accessible in domino reactions involving repetitive sequences of [2+2] cycloadditions of tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) to polyynes, followed by retro-electrocyclizations.

After a brief introduction to the electronic structure of the three-dimensionally aromatic icosahedral closo -monocarbadodecaborate anion CB 11 H 12 - , some recent results for its permethylated version, CB 11 Me 12 - and three highly reactive electroneutral analogs are presented and discussed. These are the radical CB 11 Me 12 · , the boronium ylide CB 11 Me 11 with a naked boron vertex, and the isomeric carbonium ylide with a naked carbon vertex. These ylides are probably better viewed as unusual types of singlet borylene and carbene, respectively.

The synthesis, crystal structure, and characterization of a silicon-silicon triply bonded species, disilyne with two bis[bis(trimethylsilyl)methyl]isopropylsilyl substituents, are described. The nature of the sp-hybridized silicon atoms is discussed from the viewpoint of spectroscopic results and theoretical calculations. The reactivity of the disilyne with alkali metals, t BuLi, and π-bonded compounds such as cis - and trans -2-butenes and phenylacetylene is also described.

Reactions of {(C 6 F 5 )Pt[S(CH 2 CH 2 -) 2 ](μ-Cl)} 2 and R 3 P yield the bis(phosphine) species trans -(C 6 F 5 )(R 3 P) 2 PtCl [R = Et ( Pt'Cl ), Ph, ( p -CF 3 C 6 H 4 ) 3 P; 88-81 %]. Additions of Pt'Cl and H(C≡C) n H ( n = 1, 2; HNEt 2 , 20 mol % CuI) give Pt'C 2 H (37 %, plus Pt'I , 16 %) and Pt'C 4 H (88 %). Homocoupling of Pt'C 4 H under Hay conditions (O 2 , CuCl, TMEDA, acetone) gives Pt'C 8 Pt' (85 %), but Pt'C 2 H affords only traces of Pt'C 4 Pt' . However, condensation of Pt'C 4 H and Pt'Cl (HNEt 2 , 20 mol % CuI) yields Pt'C 4 Pt' (97 %). Hay heterocouplings of Pt'C 4 H or trans -( p -tol)(Ph 3 P) 2 Pt(C≡C) 2 H ( Pt*C 4 H ) and excess HC≡CSiEt 3 give Pt'C 6 SiEt 3 (76 %) or Pt*C 6 SiEt 3 (89 %). The latter and wet n -Bu 4 N + F - react to yield labile Pt*C 6 H (60 %). Hay homocouplings of Pt*C 4 H and Pt*C 6 H give Pt*C 8 Pt* (64 %) and Pt*C 12 Pt* (64 %). Reaction of trans -(C 6 F 5 )( p -tol 3 P) 2 PtCl ( PtCl ) and HC≡CH (HNEt 2 , 20 mol % CuI) yields only traces of PtC 2 H . However, an analogous reaction with HC≡CSiMe 3 gives PtC 2 SiMe 3 (75 %), which upon treatment with silica yields PtC 2 H (77 %). An analogous coupling of trans -(C 6 F 5 )(Ph 3 P) 2 PtCl with H(C≡C) 2 H gives trans -(C 6 F 5 )(Ph 3 P) 2 Pt(C≡C) 2 H (34 %). Advantages and disadvantages of the various trans -(Ar)(R 3 P) 2 Pt end-groups are analyzed.

By employing the furan annulation protocol, a new series of furan-containing teraryl [n.2]cyclophenes ( n = 2-6 and 12) are prepared. These cyclophenes exhibit charge-transfer character in the absorption spectra and unusually large Stokes shifts in the emission spectra. They have neither particularly strong electron-donating moieties nor electron-withdrawing groups, but exhibit unusual second-order nonlinear optical (NLO) properties. The π-systems in teraryl system and in the bridging double bond are highly twisted. Interaction between these twisted π-systems may account for the significant enhancement in hyperpolarizability. Thiophene analog behaved similarly. The five-membered heteroaromatic rings may not only serve as electron donors, but also may accommodate the appropriate geometry to enable the interactions between the oligoaryl systems and the double bond leading to unusual photophysical and NLO properties.

The synthesis of mechanically interlocked molecular compounds has advanced by leaps and bounds since the early days of statistical methods and covalent-directing strategies. Template-directed synthesis has emerged as the method of choice for the construction of increasingly complex and functional [2]catenanes and [2]rotaxanes. In particular, mechanically interlocked molecules employing π-donating and π-accepting recognition units have been produced with remarkable efficiencies and show great promise in technologies as diverse as molecular electronics and drug delivery.

Stable organic open-shell systems have attracted much attention in the field of molecule-based magnetism. We have been exploring novel stable neutral radicals based on a phenalenyl system known as an odd-alternant hydrocarbon π-radical with a highly spin-delocalized nature. Recently, we have designed and synthesized novel oxophenalenoxyl neutral radical systems possessing two oxygen atoms on the phenalenyl skeleton. These systems are unique in comprising some topological isomers depending on the positions of oxygen substituents on the phenalenyl skeleton. The isomers exhibit different topological symmetries of spin density distributions ( spin topological symmetry control ). In addition, two-stage one-electron reductions of these systems give the corresponding radical dianions, which show remarkably different topological symmetries of a spin-delocalized nature from those of the neutral radical systems ( redox-based spin diversity ). In this paper, we discuss the unique spin-delocalized nature of 3-, 4-, and 6-oxophenalenoxyl systems in view of the topological symmetry and redox ability, emphasizing the results from the radical dianion of 4-oxophenalenoxyl system from both experimental and theoretical sides.

This report describes the synthetic strategies toward and optoelectronic properties of substructures of the non-natural, planar carbon networks graphyne and graphdiyne, which are based on the dehydrobenzo[12]annulene and dehydrobenzo[18]annulene framework, respectively.

A strategy for the synthesis of cyclophenacene-type aromatic belts (or armchair nanotube segments) that relies upon a valence isomerization/dehydrogenation reaction is described, and progress toward achieving this goal is presented.

The longer C-C bond than the standard (1.54 Å) is so weakened that it is cleaved easily, as found in the parent hexaphenylethane (HPE). However, the compounds with an ultralong C-C bond (1.75 Å) can be isolated as stable solids when the bond-dissociated species does not undergo any reactions other than bond reformation. This is the central point in designing the highly strained HPEs, which were obtained by two-electron reduction of the corresponding dications. Steric repulsion of "front strain" is the major factor to expand the central C-C bond of HPEs. During the detailed examination of the ultralong C-C bond, the authors discovered the intriguing phenomenon of "expandability": the C-C bond length can be altered over a wide range by applying only a small amount of energy (1 kcal mol -1 ) supplied by crystal packing force. This observation indicates that the much longer C-C bond than the shortest nonbonded contact (1.80 Å) will be realized under the rational molecular design concept.

Photochromic compounds reversibly change not only the absorption spectra but also their geometrical and electronic structures. This principle can be applied for the photoswitching of the physical properties of the molecular materials. In particular, photoswitching of the flow of information through the molecule is interesting because information processing using molecular devices is attracting interest in the molecular electronics field. The photoswitchings of the magnetic exchange interaction and the electrical conductance using photochromic diarylethene are described.

The dianions of tetraphenylsilole and tetraphenylgermole behave as aromatic species, as shown by the equalization of C-C bond lengths in these rings and by nucleus-independent chemical shift (NICS) calculations. Similarly, the dianions of 9-silafluorene and 9-germafluorene are aromatic. In the latter species, the heteroatom five-membered rings are more aromatic than the attached benzene rings. Also to be recognized as aromatically stabilized compounds are the stable unsaturated diiminocarbenes, diiminosilylenes, and diiminogermylenes. Evidence for the aromatic delocalization in these molecules comes from NMR, NICS calculations, Raman spectroscopy, isodesmic molecular orbital (MO) calculations, and calculated isomerization stabilization energies. From the presently available evidence, the aromaticity appears to alternate, with C > Ge > Si, going down group 14.

The supramolecular aggregation properties of new prototypes of artificial amphiphiles consisting of fullerene or calixarene building blocks serving as platforms for the connection of hydrophobic and dendritic hydrophilic groups are reported. Very water-soluble monoadducts of C 60 carrying a defined number of negative charges at a given pH were hybridized with cationic porphyrins including cyctochrome C and investigated with respect to photoinduced electron transfer. Fullerene adducts with dendritic deprotected sugar addends self-assemble to supramolecular sugar balls in water with a narrow size distribution of the micelles of around 4 nm. Various examples of cone- or T-shaped amphiphiles involving calixarene and fullerene cores self-organize to the first examples of shape-persistent micelles whose supramolecular arrangement in water has been determined with unprecedented precision based on the analysis of cryo-transmission electron microscopy (TEM) investigations.

The syntheses of perfluoroalkyl-annelated conjugated units: hexafluorocyclopenta[ c ]thiophene, 4,4-difluoro-4 H -cyclopenta[2,1- b :3,4- b ']dithiophene, and 6,6,12,12-tetrafluoroindeno[3,2- b ]fluorine, and these-containing oligomers have been accomplished. The annelation of the perfluoroalkyl groups effectively lowers the lowest unoccupied molecular orbitral (LUMO) energy levels without disrupting the effective conjugation of the backbones, which was unambiguously clarified by spectroscopic and electrochemical measurements as well as X-ray analysis. The perfluoroalkyl-annelated oligothiophenes exhibited n-type semiconducting behavior with field-effect electron mobility up to 0.018 cm 2 V -1 s -1 .

Indole-based synthetic receptors which bind anions by multiple hydrogen bonds in organic solvents have been prepared. The biindole scaffold bearing two good hydrogen bond donors of indole NHs has been used as a molecular building block to construct the receptors. Incorporation of more hydrogen bond donors or acceptors increases the binding strength and selectivity toward a specific ion. Two macrocycles having different sizes of the internal cavities have been also synthesized, and their binding properties have been compared. Two macrocycles display distinct binding modes with polyatomic anions such as azide, as unequivocally proven by X-ray crystal structures. Finally, a series of oligoindoles containing four, six, and eight indoles have been prepared by Sonogashira reactions. The oligoindoles fold into helical structures upon binding with chloride. The binding affinities of the oligoindoles with chloride increase with increasing of the chain length, which provides an additional evidence for helical folding of the complexes.

Functional π-electron systems can be synthesized in a diversity-oriented fashion by applying multicomponent and domino reactions. Based upon alkyne activation by Sonogashira coupling, reactive triple or double bonds become the key functionality for sequential and consecutive synthesis of heterocycles, such as β-amino vinyl nitrothiophenes, δ-amino propenylidene indolones, indolizines, furans, pyrroles, annelated 2-amino pyridines, and spirocyclic benzofuranones and indolones. All these chromophores and fluorophores possess peculiar properties such as nonlinear optical (NLO) activity, pH-sensitivity, distinct solid-state fluorescence, or large Stokes shifts.

During the past two decades, shape-persistent conjugated macrocycles with a broad spectrum of attributes and topologies have been synthesized. This includes macrocycles with remarkable electronic, optical, and supramolecular properties, as well as intriguing frameworks. Expanded radialenes are a class of conjugated shape-persistent macrocycles that arise from the formal insertion of acetylene units into a radialene framework. A related class of macrocycles, the expanded radiaannulenes, contains both endo- and exocyclic vinylene and vinylidene segments, respectively, and accordingly exhibits properties intermediate between radialenes and annulenes. Enyne building blocks have been developed that are suitable for forming a macrocyclic framework through a step-wise sequence of Pd-catalyzed cross-coupling reactions. This "building-block" approach allows us to explore a range of molecular architectures that will ultimately provide for an understanding of π-delocalization in these compounds. The synthesis and structural characterization of the first members of this new class of expanded radialenes and radiaannulenes are described.

Building supramolecular architectures with well-defined shapes and functions is of great importance in materials science, nanochemistry, and biomimetic chemistry. In recent years, we have devoted much effort to the construction of well-defined supramolecular structures through noncovalent forces such as hydrogen bonding, π-stacking, metal-ligand bonds, and hydrophilic and hydrophobic interactions, with the aid of functional building blocks. The morphologies and their physical properties were studied, and new methods for the construction of one-dimensional nanoscale structures have been developed. In this review, we summarize our recent studies on the design and synthesis of the supramolecular systems, as well as the physical properties of nanoscale structures.

We present our strategy to control the rotation in a molecular rotary motor and the synthesis of a family of ruthenium complexes designed to perform such a task. The molecules have a piano-stool structure with a "stator" meant to be grafted on an oxide surface, and a "rotor" bearing redox-active ferrocene groups, so that addressing the molecule with nanoelectrodes would trigger rotation. The rigidity of the structure is allowed by the use of aromatic building blocks both in the stator and in the rotor fragments.