Volume 84, Issue 4

The 14th International Symposium on Novel Aromatic Compounds (ISNA-14), held in Eugene, OR, USA from 24 to 29 July 2011, dealt with broad themes: molecular design, geometry, and function, realized through the hands of chemists. Aromatic compounds underlie these themes in the same way that stone and steel underlie architecture. Indeed, the ISNA conferences have been central to the development of the architectural approach to chemistry. The 256 ISNA-14 participants came from around the globe and enjoyed 62 talks, 148 posters, and a fine social program. The Nozoe Lecture, delivered by Prof. Peter Bäuerle of the University of Ulm, initiated an avalanche of outstanding science that lasted five days. The participation of many first-time attendees and seasoned ISNA veterans demonstrated the continuing vitality of the ISNA series and bodes well for ISNA-15, to be held in Taipei, Taiwan from 28 July to 2 August 2013. The University of Oregon was a delightful venue for the conference. Excursions to the ocean and to vineyards provided opportunities to meet old friends, make new ones, and see this lovely corner of the world. And, lo and behold, it did not rain! This issue of Pure and Applied Chemistry is a microcosm of ISNA-14, reflecting the thoughts, trends, scientific style, and problems addressed. The compilation of papers is synergistic and tells us more than each story taken separately—it tells us what chemists are thinking about now. I hope this issue might today pique the curiosity and creativity of a new investigator or might tomorrow reveal the key role played by novel aromatic compounds in the development of chemistry. Benjamin T. King Conference Co-chair

This contribution describes two different approaches aimed at the synthesis of monolayer molecular sheets with internal order, or, in other words, 2D polymers. First, an interfacial strategy is presented in which terpyridene-based hexafunctional monomers spread at the air/water interface are connected to one another with the help of metal salts. While this approach leads to micrometer-sized monolayer sheets that are mechanically stable enough to be spanned over several micrometer-sized holes, their internal structure could not yet be proven. The second approach rests upon solid-state photochemistry, and properly designed monomers are covalently connected with one another while being held in layered geometries owing to crystal packing. Exfoliation to single sheets can be achieved, and molecular structure is supported by a Raman spectroscopic analysis. We consider this the first case of a rational synthesis of 2D polymers and briefly touch on the impact this may have.

This report details the advances in synthetic strategies toward arylene–ethynylene macrocycles (AEMs). After a brief description of traditional methods, we summarize recent advances based on dynamic covalent chemistry (DCC) whereby a highly active and functional group tolerant alkyne metathesis catalyst yields scalable quantities of AEMs under thermodynamic controlled reaction conditions.

Double-stranded polymeric ladderphanes are obtained by ring-opening metathesis polymerization (ROMP) of bisnorbornene derivatives by the first generation of Grubbs catalyst (G-I). A range of two- and three-dimensional organic and organometallic linkers are used to connect two norbornene units. The structures of these double-stranded polymers are proved by spectroscopic means and scanning tunneling microscopic (STM) images. Hydrolytic cleavages of these ladderphanes give the corresponding single-stranded polymers with the same degree of polymerization and polydispersity as those of the double-stranded counterparts. Helical polymeric ladderphanes are also synthesized similarly when chiral linkers are used. Strong intereactions between adjacent linkers have been revealed by their physical properties in these polymers. Chemical modification of ladderphanes is achieved by bis-dihydroxylation, diimide reduction of double bonds, and electrochemical oxidation of linkers. Unsymmetrical ladderphanes with well-defined lengths and narrow dispersity are also obtained by replication and by sequential polymerization.

Benzannulated enyne-allenes bearing an aryl substituent at both the alkynyl and allenyl termini are excellent precursors of 5-aryl-11 H -benzo[ b ]fluorenyl derivatives having the carbon frameworks of fullerene fragments. The mildness of the reaction conditions and the availability of several synthetic methods for benzannulated enyne-allenes allow the design of new synthetic pathways leading to bowl- and basket-shaped fullerene fragments. Specifically, a bowl-shaped C 28 H 18 hydrocarbon bearing a 27-carbon framework of C 60 and a basket-shaped C 56 H 40 hydrocarbon bearing a 54-carbon framework of C 60 were synthesized. The interior 30-carbon core of the C 56 H 40 hydrocarbon can be regarded as a partially hydrogenated [5,5]circulene, a C 30 H 12 semibuckminsterfullerene. In addition, a basket-shaped C 56 H 38 hydrocarbon was also constructed to serve as a precursor toward an end-cap template for (6,6) carbon nanotubes.

A bottom-up synthesis of structurally uniform carbon nanotubes (CNTs) and graphenes is recognized as one of the greatest challenges of primary importance in nanocarbon science. This paper highlights our efforts to address these challenges since 2005. These endeavors have led to (i) modular, size-selective, and scalable synthesis of [ n ]cycloparapheneylenes (CPPs), the shortest segment of armchair CNTs, (ii) design and synthesis of the shortest segment of chiral CNTs, and (iii) efficient synthesis of carbon nanosheets through catalytic C–H bond arylation of polycyclic aromatic hydrocarbons (PAHs). We consider these works as a possible first step toward a controlled synthesis of structurally uniform CNTs and nanographenes.

To create novel π-conjugated compounds, we constructed various molecular architectures from anthracene units and acetylene linkers. Several cyclic oligomers ranging from dimers to dodecamers were synthesized by macrocyclization of acyclic precursors with metal-catalyzed coupling reactions. The structures, dynamic behavior, and spectroscopic features were greatly influenced by the number of anthracene units and the combination of building units and linkers. Optically active and circular dichroism (CD)-active enantiomers of some chiral cyclic oligomers were resolved by chiral high-performance liquid chromatography (HPLC). Conformational analysis of hexamers and higher oligomers was performed with the aid of density functional theory (DFT) calculations. Acyclic oligomers underwent reversible folding–unfolding processes via photochemical and thermal reactions. These results suggest that transannular π–π interactions between anthracene units are important factors in controlling the structural and spectroscopic properties and functions of π-conjugated compounds. The scope and perspectives of this molecular design are discussed on the basis of previous studies.

The synthesis of 3-hexylthiophene-based oligothiophenes with a length of approximately 10 nm bearing anchor units at both terminal positions has been accomplished. In addition, we have designed and synthesized completely encapsulated oligothiophenes to investigate single-molecule conductance. Their properties are evaluated by UV–vis absorption spectra and cyclic voltammetry (CV) measurements. The conductance of a single-molecule junction for thiol-introduced oligothiophenes was measured by 10-nm-scale nanogap gold electrodes or modified scanning tunneling microscope (STM) techniques.

This article describes a concept for designing fullerene-based electron-accepting materials to obtain high open-circuit voltage ( V OC ) in organic thin-film photovoltaic devices without an accompanying decrease in short-circuit current density. The keys to this concept are (1) reducing the size of the fullerene π-conjugated system to realize high V OC and (2) shortening the inter-fullerene distance in the solid-state packing structure to achieve high short-circuit current density ( J SC ), which is made possible by well-designed supramolecular organization or a small organic addend. In this article, two representative examples are discussed. One is 1,4-bis(silylmethyl)[60]fullerene (SIMEF), which forms a columnar fullerene-core array for high electron mobility and undergoes thermal crystallization for good phase separation with the electron-donating material. The other is a 56π-electron fullerene derivative bearing a dihydromethano group, the smallest carbon addend, which does not disrupt fullerene–fullerene contact in the solid state.

A number of indolocarbazole-based anion receptors were prepared and their anion-binding behaviors were characterized in solution and in the solid state. First, chain-length-dependent binding affinities of chloride ion were revealed using a series of indolocarbazoles that consisted of one to four indolocarbazole units. The binding affinities were steadily enhanced from monomer to dimer, then to trimer by Gibb’s free energy (–∆∆ G ) = 2.4 ± 0.1 kcal/mol, and then nearly saturated. Second, a water-soluble trimer folded to generate an internal helical cavity with six convergent NHs, wherein small halides bound in water in the order of Cl – (65 M –1 ) > F – (46 M –1 ), and Br – (19 M –1 ). Third, X-ray crystal structures clearly proved helical folding of a trimer in the presence of sulfate ion, in which left- and right-handed helices stacked alternatively. It was also shown that the selectivity of anion binding could be varied by the modification of the spacer groups connecting indolocarbazole units. Finally, we prepared chiral indolocabazole dimers that adopted helical structures by intramolecular hydrogen bonds and displayed complete inversion of the helical sense upon anion binding. The dimers gave characteristic optical readouts in a reversible manner according to chemical stimuli, thus functioning as chirooptical molecular switches.

Intramolecular hydrogen bonding in oligomeric aromatic amides may induce the backbones to adopt folded, zigzag, or other extended conformations, depending on the positions of the amide units and the hydrogen-bonding sites on the aromatic rings. This article summarizes our efforts in exploring the applications of this family of preorganized oligomers in supramolecular and materials chemistry. Several series of preorganized frameworks or foldamers have been developed as efficient acyclic receptors for binding both neutral and ionic guests. The backbones have been modified with discrete functional groups from the ends or the side chains. The resulting molecules have been applied for designing new molecular tweezers, assembling ordered supramolecular architectures (organogels and vesicles), and directing the formation of complicated macrocyclic and capsular systems. When the hydrogen-bonded folded segments are connected to n -butyl methacrylate copolymers as cross-links, the resulting copolymers display unique reversible mechanical properties owing to the breaking and recovering of the intramolecular hydrogen bonds. When the folded segments are incorporated into the dumbbell components of the donor–acceptor interaction-based pseudo[2]rotaxanes and [2]rotaxanes, they are capable of tuning the slippage/deslippage and switching of the ring component between the discrete “stations” in their dumbbell component.

An ambipolar field-effect transistor (FET) based on tetracyanoquarterthienoquinoid (TCT 4 Q) was constructed. When a set of source, drain, and gate voltages were applied to a thin film of TCT 4 Q at temperatures lower than 150 K, both positive and negative carriers were trapped and frozen even after removal of the gate voltage. The frozen carriers worked as a floating gate with the gradient by creating a PN(NP) junction through the injection of oppositely charged “mobile” carriers. The device exhibited a distinct rectifying effect when an alternating current (50 < f <500 mHz) was applied through the source and drain electrodes. Moreover, the function of the molecular device is programmable and erasable.

A series of symmetrical D-π-A-π-D molecules comprised of benzo[1,2- d ;4,5- d' ]bisoxazole (BBO) and various arylenevinylenes was synthesized via a Knoevenagel condensation of 2,6-dimethyl BBO and the corresponding aryl aldehydes. The resulting compounds had energy levels that were easily modulated and broad absorptions in the visible spectrum. They also demonstrated high fluorescence quantum yields in solution. The solvatochromism of several derivatives was examined in a number of solvents, and it was found that the emission of the triphenylamine derivative varied by almost 100 nm, depending on the polarity of the solvent. Collectively, these results indicate that the optical and electronic properties of benzobisoxazoles are readily tuned through the choice of aryl co-monomer.

Although tetrathiafulvalene (TTF) and its derivatives have been extensively studied as important organic electronic materials over the past half century, tetrathiafulvalene vinyl-ogues (TTFVs) still remain a relatively underdeveloped branch in the family of TTF derivatives. Our recent work has investigated the synthesis and characterization of a class of diphenyl-substituted TTFVs carrying alkynyl functionality. The unique conformational and redox properties of such TTFV derivatives along with the versatile chemistry enabled by acetylenic groups (e.g., metal-catalyzed coupling and click reactions) have led us to a variety of functional molecular architectures ranging from oligoynes, polymers, and molecular tweezers, to macrocycles. Property studies of these new TTFV-based molecular materials point to appealing applications in molecular electronics and optoelectronics.

In this paper, we report our ongoing progress in the preparation of fullerene-azide or fullerene-alkyne building blocks, allowing their further chemical transformation under the copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction conditions.

We describe the synthesis, characterization, and fabrication of bioelectronic materials derived from small oligopeptides containing π-conjugated subunits directly embedded into peptide backbones. The rapid incorporation of π-conjugated subunits directly into peptide backbones was accomplished through an on-resin dimerization procedure recently explored in our research. We investigated these peptides’ abilities to self-assemble in aqueous environments into 1D nanostructures with intimate π–π interactions. Finally, we discuss a simple processing method to develop aligned networks of 1D π-conjugated peptide nanostructures where the resulting electronic and photophysical properties are altered as a direct impact of the aligned π-conjugated network.

Polycyclic aromatic hydrocarbons (PAHs) are in a class of functional organic compounds with increasing importance in organic electronics. Their tunable photophysical properties and typically strong intermolecular associations make them ideal materials in applications where control of charge mobility is essential. Hexa- peri -hexabenzocoronene (HBC) is a disc-shaped PAH that self-associates into columnar stacks through strong π–π interactions. By decorating the periphery of the HBC molecule with various substituents, a range of properties and functions can be obtained including solution processability, liquid crystallinity, and semiconductivity. In this review article, the synthesis, properties, and functions of HBC derivatives are presented with focus on work published in the last five years.

Application of topological analysis and graph theory to benzenoid hydrocarbons leads to the determination of fundamentals of aromaticity: the Hückel rule and the Clar rule. The approach, based on a treatment of the adjacency matrix, allows resonance energy (RE)-like characteristics to be estimated with quite good accuracy, and magnetic aromaticity indices to be derived for both the individual rings and the whole molecules. It also allows an effective approach for interpreting ring current formation in molecules when exposed to an external magnetic field. The transformation of the perturbation matrix into a form describing the canonical structures allows their gradation and determination of their stabilizing/destabilizing character.

Bowl-to-bowl inversion is one of the characteristic behaviors for some flexible open-end molecular bowls consisting of polycyclic aromatic hydrocarbons with benzene rings fused by imbedded five-membered rings (π bowls). This intriguing dynamics was studied with sumanene, which is the smallest C 3v symmetric fragment of fullerenes. In this article, our ongoing research on the bowl-to-bowl inversion of sumanene, its benzylic anion species, hexasubstituted derivatives, mononaphthosumanene, and [CpRu( η 6 -sumenene)] + complex is summarized. Estimation based on density functional theory (DFT) calculation is also described.

Antiaromatic dications and dianions are available through oxidation or reduction of unsaturated precursors as well as through ionization of diols. Their experimental accessibility allows them to be used to evaluate the effectiveness of theoretical treatments of antiaromaticity such as the nucleus-independent chemical shift (NICS). Because measures of antiaromaticity are generally larger in magnitude than corresponding measures of aromaticity, they demonstrate relationships between different measures of aromaticity–antiaromaticity, such as those based on magnetic and energetic properties, more effectively than do aromatic measures. Antiaromatic species are more sensitive probes of delocalization than are aromatic species, giving the possibility of observation of subtle effects. Antiaromatic species possess a small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap, giving rise to species with diradical character.