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McMurry coupling is one of the most useful and convenient tools for the preparation of π-conjugated molecules. However, for the synthesis of strained macrocycles containing ethynylene linkages, reduction of ethynylene to vinylene linkage sometimes took place. Especially, for the synthesis of macrocyclic π-extended thiophene hexamers using McMurry coupling of dialdehyde 1 composed of three thienylene, two ethynylene, and two formyl groups, reduction of ethynylenes to vinylenes often takes place to produce unique products in a one-pot procedure, depending on very small steric and electronic effects such as reaction temperature, amounts of titanium reagent, and substituents of thiophene hexamers. The attractive structures and functional properties of reduced thiophene hexamers have been determined using X-ray analysis and OFET measurements.

A phenyl-substituted macrocyclic π-extended thiophene hexamer 1, composed of four thienylene-ethynylene and two thienylene-vinylene units, has a solid-state structure in which π–π, CH−π, and van de...

A radical cation, generated from an extended π-conjugated thiophene 6-mer composed of four ethynylene-thienylene and two vinylene-thienylene units, was observed to form a stable three-dimensional π-dimer containing 70 π-electrons. The π-dimer prepared in solution was investigated by using magnetic circular dichroism (MCD), ESR spectroscopy, and UV-vis-NIR absorption spectroscopy. Probing the individual NIR absorption bands showed that the MCD signals can be assigned to the pseudo Faraday A term, indicating that the absorption bands are comprised of nearly degenerate electronic transitions. X-ray crystallographic analysis revealed that the π-dimer has a three-dimensional face-to-face and continuous π-conjugated donutlike structure. Analysis of the UV-vis-NIR and ESR spectra of the π-dimer in the solid state confirmed that it possesses the dimer structure. The prediction made by using TD-DFT calculations that the dimer would have a 70 π-electron diatropic nature was confirmed by using solid state 1H NMR spectroscopy.

The self-assembly and electrochromic nanofiber formation of radially π-extended tetrathiafulvalene (TTF) tetramers anchored to 1,2,4,5-tetraethynylbenzene were investigated. The tetramer with SBu-s...

We herein report a unique example of on-surface adaptive self-assembly. A pentagon-shaped macrocycle, cyclic [5]meta-phenyleneacetylene [5]CMPA, is trapped by the adaptive supramolecular network formed by an isosceles triangular molecule, alkoxy substituted dehydrobenzo[14]annulene [14]ISODBA at the liquid/graphite interface, leading to a highly ordered and large-area bicomponent self-assembled molecular network (SAMN), as revealed by scanning tunneling microscopy (STM).

Macrocyclic oligothiophenes and their π-expanded derivatives constitute versatile building blocks for the design of (supra)molecularly engineered active interfaces, owing to their structural, chemical, and optoelectronic properties. Here, it is demonstrated how resonant tunneling effect induces single molecular isomerization in a 2D crystal, self-assembled at solid-liquid interfaces under ambient conditions. Monolayers of a series of four π-expanded oligothiophene macrocycles are investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at the interface between their octanoic acid solutions and the basal plane of highly oriented pyrolytic graphite. Current-voltage characteristics confirm the donor-type character of the macrocycles, with the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO) positions consistent with time-dependent density functional theory calculations. Cyclic STS measurements show the redox isomerization from Z,Z-8T6A to its isomer E,E-8T6A occurring in the 2D crystal, due to the formation of a negatively charged species when the tunneling current is in resonance with the LUMO of the macrocycle.

A Saturn-like 1:1 complex composed of macrocyclic oligothiophene E-8T7A and C60 fullerene (C60 ) was synthesized to investigate the interaction between macrocyclic oligothiophenes and C60 in solution and the solid state. Because the Saturn-like 1:1 complex E-8T7A⋅C60 is mainly stabilized by van der Waals interactions between C60 and the sulfur atoms of the E-8T7A macrocycle, C60 is rather weakly incorporated inside the macro-ring in solution. However, in the solid state the Saturn-like 1:1 complex preferentially formed single crystals or nanostructured polymorphs. Interestingly, X-ray analysis and theoretical calculations exhibited hindered rotation of C60 in the Saturn-like complex due to interactions between C60 and the sulfur atoms. Furthermore, the photoinduced charge transfer (CT) interaction between E-8T7A and C60 in solution was investigated by using femtosecond transient absorption (TA) spectroscopy. The ultrafast TA spectral changes in the photoinduced absorption bands were attributed to the CT process in the Saturn-like structure.

Dimer 2, trimer 3, hexamer 6, octamer 8, and decamer 10 of 4,4′′-diethynyl-4′,5′-dioctyl-o-terphenyl 1 were synthesized by using copper(II) acetate-mediated Eglington coupling reaction of 1 in pyridine or pyridine–methanol. Although dimer 2 and trimer 3 were formed as major products under high dilution conditions, large oligomers such as hexamer 6, octamer 8, and decamer 10 were obtained in moderate total yield under standard Eglington conditions. The MALDI-TOF MS and GPC behavior of large oligomers exhibit the formation of giant macrocycles 3 nm diameter and 2–4 nm long in one-pot procedure.

π-Expanded oligothienylene macrocycle with a large inner cavity incorporates fullerenes such as methano[60]fullerene (C61H2) and [70]fullerene (C70) inside to form Saturn-like complexes. Although the oligothiophene macrocycle weakly interacts with fullerenes in solution, it forms stable Saturn-like fullerene complexes in the solid state. X-ray analysis of the Saturn-like complexes exhibited short contacts between the sulfur atoms of the oligothiophene macrocycle and fullerene carbons, which hinder the rotation of fullerenes. As a result, the non-covalent interaction between the oligothiophene macrocycle and fullerenes was employed in crystal structure determination of fullerenes. UV–vis–NIR spectra of the Saturn-like complexes showed weak donor–acceptor interaction between the oligothiophene macrocycle and fullerenes.

Triangular macrocyclic pentadecaphenylenes have been synthesized in moderate yields by the electron-transfer oxidation of the corresponding Lipshutz cuprates with duroquinone. Pentadecaphenylenes are very stable to light, atmospheric oxygen and prolonged heating. Despite the rigid twisted frame, pentadecaphenylenes self-assemble into various nanostructured polymorphs in the solid state. Hexaoctylpentadecaphenylene 1a acts as a gelator in benzene or toluene to form a gel, although 1a neither has a planar structure nor undergoes heteroatom substitution to assist intermolecular interaction. Furthermore, 1a incorporated fullerene C60 into its cavity to afford a fibrous 2 : 1 sandwich complex (1a–C60–1a) which showed higher gelation ability than 1a itself. Interestingly, Janus-faced 1a acts as a host of two C60 molecules, and an additional C60 was intercalated into the 1a·C60 (2 : 1) complex to form a 1 : 1 complex with an infinite chain structure like –1a–C60–1a–C60–, which enhances the gelation power of the 1a·C60 sandwich complex.

Three new macrocyclic π-extended thiophene hexamers composed of four thienylene-ethynylene and two thienylene-vinylene units with or without four alkyl substituents have been synthesized. Despite similar shape-persistent structures in solution, the alkyl substituents control the solid-state structures and morphologies. The unsubstituted hexamer exhibited a planar conformation with a theoretically predicted structure in the solid state; however, the planar hexamer with four ethyl substituents formed a closely stacked columnar crystal structure to exhibit π-π interactions. Interestingly, the hexamer with four butyl substituents adopted both planar and twisted conformations in the solid state, exhibiting polymorphism based on induced-fit stacking of molecules. Thus, the butyl-substituted hexamer produces a mixture of yellow, orange, and red single crystals from toluene/acetone, and X-ray analysis revealed six different conformations. Consequently, the small structural difference in the macrocycles causes a key effect on their functional properties in the solid state, and their morphology governs electrical conductivity and organic field-effect-transistor activity. The polymorphism of the hexamers was applied to the switching of film morphology.

Conformational disorder in π-conjugated cyclic systems plays a crucial role in controlling the extent of exciton delocalization in much the same way as that in linear counterparts. However, to date, there have been no detailed spectroscopic investigations on the nature of excitons in π-conjugated cyclic systems at the single-molecule level. Herein, we studied the effect of conformational disorder on the excitonic behaviors of cyclic oligothiophenes composed of 6, 8, 10, and 12 subunits (C-6T, C-8T, C-10T, and C-12T, respectively) by employing single-molecule fluorescence spectroscopy. We found that, due to the cyclic symmetry constraint which suppresses S1-S0 transition, small and rigid C-6T and C-8T exhibit extremely long fluorescence lifetimes, while short lifetimes typical of linear systems are dominant in large, flexible C-10T and C-12T. Two-dimensional correlation maps between fluorescence lifetimes and spectral positions show that, by torsional defects created through continued photoexcitation, fully delocalized cyclic excitons shrink to form acyclic excitons in the case of C-10T, while localized acyclic excitons from initial states are maintained in the case of C-12T. The distribution of linear dichroism values from C-6T to C-10T gradually broadens but narrows in C-12T, suggesting a cyclic-to-acyclic transition in excitonic nature between C-10T and C-12T.

Exciton dynamics in π-conjugated molecular systems is highly susceptible to conformational disorder. Using time-resolved and single-molecule spectroscopic techniques, the effect of chain length on the exciton dynamics in a series of linear oligothiophenes, for which the conformational disorder increased with increasing chain length, was investigated. As a result, extraordinary features of the exciton dynamics in longer-chain oligothiophene were revealed. Ultrafast fluorescence depolarization processes were observed due to exciton self-trapping in longer and bent chains. Increase in exciton delocalization during dynamic planarization processes was also observed in the linear oligothiophenes via time-resolved fluorescence spectra but was restricted in L-10T because of its considerable conformational disorder. Exciton delocalization was also unexpectedly observed in a bent chain using single-molecule fluorescence spectroscopy. Such delocalization modulates the fluorescence spectral shape by attenuating the 0-0 peak intensity. Collectively, these results provide significant insights into the exciton dynamics in conjugated polymers.

Macrocyclic 10-mesityl-1,8-diphenylanthracene dimer 4 was synthesized by using the electron-transfer oxidation of Lipshutz cuprate derived from 1,8-bis(4-bromophenyl)-10-mesityl-anthracene 7 in moderate yield. This dimer 4 is a considerably fluorescent molecule (ΦF 0.40) with high thermal, photo, and air stability. The X-ray analysis of 4 revealed a unique structure with a small inner cavity which can incorporate a small molecule or atom. 1H NMR spectra in solution and emission spectra of 4 in the solid state showed that copper(I) ion was incorporated to form a 1:1 complex 4·CuOTf, whereas silver(I) ion only weakly interacted with 4 under similar conditions.

The construction of redox-active supramolecular assemblies based on star-shaped and radially expanded tetrathiafulvalene (TTF) oligomers with divergent and extended conjugation is summarized. Star-shaped TTF oligomers easily self-aggregate with a nanophase separation to produce supramolecular structures, and their TTF units stack face-to-face to form columnar structures using the fastener effect. Based on redox-active self-organizing supramolecular structures, conducting nanoobjects are constructed by doping of TTF oligomers with oxidants after the formation of such nanostructures. Although radical cations derived from TTF oligomers strongly interact in solution to produce a mixed-valence dimer and π-dimer, it seems to be difficult to produce nanoobjects of radical cations different from those of neutral TTF oligomers. In some cases, however, radical cations form nanostructured fibers and rods by controlling the supramolecular assembly, oxidation states, and counter anions employed.

Excited-state dynamic planarization processes play a crucial role in determining exciton size in cyclic systems, as reported for π-conjugated linear oligomers. Herein, we report time-resolved fluorescence spectra and molecular dynamics simulations of π-conjugated cyclic oligothiophenes in which the number of subunits was chosen to show the size-dependent dynamic planarization in the vicinity of a ring-to-linear behavioral turning point. Analyses on the evolution of the total fluorescence intensity and the ratio between 0-1 to 0-0 vibronic bands suggest that excitons formed in a cyclic oligothiophene composed of six subunits fully delocalize over the cyclic carbon backbone, whereas those formed in larger systems fail to achieve complete delocalization. With the aid of molecular dynamics simulations, it is shown that distorted structures unfavorable for efficient exciton delocalization are more easily populated as the size of the cyclic system increases.

Two isomers of a multifunctional π-expanded macrocyclic oligothiophene 8-mer, E,E-1 and Z,Z-1, were synthesized using a McMurry coupling of a dialdehyde composed of four 2,5-thienylene and three ethynylene units under high dilution conditions. On the other hand, cyclo[8](2,5-thienylene-ethynylene) 2 was synthesized by intramolecular Sonogashira cyclization of ethynyl bromide 5. From STM measurements, both E,E-1 and Z,Z-1 formed self-assembled monolayers at the solid–liquid interface to produce porous networks, and from X-ray analyses of E,E-1 and 2, both compounds had a round shape with a honeycomb stacked structure. E,E-1 formed various fibrous polymorphs due to nanophase separation of the macrorings. E,E-1 and Z,Z-1 in solution exhibited photochromism upon irradiation with visible and UV light, respectively, and this photoisomerization was confirmed by using STM. Furthermore, amorphous films of Z,Z-1 and E,E-1 showed photoisomerization, although single crystals, fibers, and square tubes of E,E-1 remaine...

A Saturn-like 1:1 complex composed of macrocyclic oligothiophene E-8T7A and C

A series of bent π-conjugated compounds, 2a, 2b, 3a, and 3b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units with two cyano substituents on each cyclooctatetraene (COT) ring were ...

Multifunctional π-expanded macrocyclic oligothiophene 6-mer 1, as well as 9- (2) and 12-mers (3), was synthesized using a McMurry coupling reaction as the key step. The 6-mer 1 was converted to cyclo[6](2,5-thienylene-ethynylene) (4) by using a bromination-dehydrobromination procedure. From X-ray analysis, the crystal structures of nonplanar 1 and round-shaped 2 and 4 were elucidated. STM showed that 4 formed a self-assembled monolayer at the liquid/solid interface to produce a hexagonal porous network. Chemical oxidation of 1, 2, and 4 with 1 and 2 equiv of Fe(ClO4)3 produced 1(•+) and 1(2+), 2(•+) and 2(2+), and 4(•+) and 4(2+), respectively. Although oligothiophene radical cations containing β,β-disubstituted thiophenes usually do not form π-dimers, 4(•+) clearly formed a π-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 1(2+), which has 34π-electrons, exhibited a large diatropic ring current effect, whereas 34π dication 4(2+) only showed a medium diatropic ring current effect. In contrast to 1(2+) and 4(2+), 52π dication 2(2+) had biradical cationic character instead of Hückel-type cyclic conjugation. Interestingly, 6-mer 1 showed polymorphism and unusual melting point behavior due to the number of stable conformations in the solid state. Single crystals of 1 melted at 176 °C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 °C to form a lamellarly stacked microcrystalline film, which melted at 139 °C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.

We have investigated the photophysical properties of star-shaped oligothiophenes with three terthiophene arms (meta to each other, S3) or six terthiophene arms (ortho-, meta-, and para-arranged, S6) connected to an ethynylbenzene core to elucidate the relationship between their molecular structure and electronic properties by using a combination of ensemble and single-molecule spectroscopic techniques. We postulate two different conformations for molecules S3 and S6 on the basis of the X-ray structure of hexakis(5-hexyl-2-thienlyethynyl)benzene and suggest the coexistence of these conformers by using spectroscopic methods. From the steady-state spectroscopic data of compound S6, we show that the exciton is delocalized over the core structure, but that the meta-linkage in compound S3 prevents the electronic communication between the arms. However, in single-molecule spectroscopic measurements, we observed that some molecules of compound S3 showed long fluorescence lifetimes (about 1.4 ns) in the fluorescence-intensity trajectories, which indicated that π electrons were delocalized along the meta linker. Based on these observations, we suggest that the delocalized exciton is intensely sensitive towards the dihedral angle between the core and the adjacent thiophene ring, as well as to the substituted position of the terthiophene arms. Our results highlight that the fluorescence lifetimes of compounds S3 and S6 are strongly correlated with the spatial location of their excitons, which is mainly affected by their conformation, that is, whether the innermost thiophene rings are facing each other or not. More interestingly, we observed that the difference between the degrees of ring-torsional flexibility of compounds S3 and S6 results in their sharply contrasting fluorescence properties, such as a change in fluorescence intensity as a function of temperature.

All bent out of shape: The solvent of crystallization effectively enhances the emission of flexible, bent, π-conjugated molecules in the crystalline state owing to control of the packing of the molecules in the structure. Multichromism such as thermochromism and vapochromism also arises from the solvent-controlled packing. This crystalline-state emission is attributable to the flexibility of cyclooctatetraene units of the bent π-conjugated molecules in the solid state (see figure).

Multifunctional dodecadehydrotris(tetrathiafulvaleno)[18]annulenes 1b–1d, which have six peripheral butylthio substituents or six ester substituents, were synthesized together with the correspondin...

Intramolecular interactions of face-to-face tetrathiafulvalenes (TTFs) in 1,8-bis(tetrathiafulvalenyl)naphthalene frameworks were investigated in neutral and cationic states. From X-ray analysis an...

Terthiophene-appended gold nanoparticles were prepared by the reduction of AuCl 4 − (C 8 H 17 ) 4 N + with NaBH 4 in the presence of bis[2,5-di(3-hexylthiophen-2-yl)thiophene-3-carboxyloxyhexanyl]disulfide. A hexagonal self-assembly of particles with gold core diameters (1.9±0.1 nm) was detected by high-angle annular dark-field scanning transmission electron microscopy. The electric conductivity of the iodine-doped film was 9.1×10 −6 S cm −1 , which was ascribable to the terthiophene-based inter-ligand π–π interactions. The Au/terthiophene hybrid spin-coated film consisted of a highly three-dimensional assembled structure of terthiophenes, as inferred from grazing-incidence small-angle X-ray scattering, indicating that such monodispersed and small-sized gold nanoparticles can serve as a template for this organization. In this study, a gold nanoparticle-templated assembly of oligothiophenes has been fabricated for proposing a method to develop tailor-made organizations of π-conjugated oligomers.

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.

Large two-dimensional (2D) carbon-rich molecules with defined sizes, shapes, and compositions have attracted considerable attention because of their one-dimensional (1D) stacking in solution, supramolecular 2D crystallinity on surfaces, and optical properties both in solution and in the solid state. In particular, planar cyclic p frames with large inner cavities and long alkyl or alkoxy side chains have been used for the construction of either supramolecular architectures or weakly bound host–guest complexes by the nanoscopic phase separation among the p frame, alkyl moiety, and inner sphere. Thus, large 2D planar condensed aromatics without inner cavities tend to form crystals, liquid crystals, and nanostructures through fairly strong p–p and van der Waals interactions, and the self-association is an enthalpically driven process that is entropically disfavored. In a similar manner, shape-persistent cyclic conjugated molecules with large inner cavities should assemble into supramolecular structures with reduced dimensionality, that is, 1D and 2D nanostructures. However, our preliminary experiments show that their thermodynamic parameters are sometimes different from those of large 2D planar condensed aromatics. We have recently developed a new synthetic method to prepare cyclic oligophenylenes by electrontransfer oxidation of Lipshutz cuprates and reported the synthesis, photophysical properties, and supramolecular architectures of cyclic nonaphenylenes 1a–c (Figure 1). Next, we converted 1a–c into the corresponding triphenylene trimers 2a–c by dehydrogenative bond formation to examine their optical properties and self-assembling behavior. Although 1a–c could be converted into triphenylene trimers 2a–c by using dehydrogenative cyclization, 2a has poor solubility in common organic solvents. Therefore, the optical properties and the formation of nanostructures of 2b,c were investigated and compared with those of 1b,c from a viewpoint of the structure–property relationship. In the case of 1b, fine colorless crystals with high solid-state emission were obtained owing to its nonplanar oligophenylene structure. As for 1c, a fluorescent fibrous material precipitated from a mixture of diisopropyl ether (IPE)/methanol (1:1). The fibers of 1c formed a sheet structure perpendicular to the fiber direction, and the intermolecular distance between the phenyl rings at the apex position was determined to be 4.3 ; this indicated no p–p stacking interactions and a Lego-like interlocking structure. As triphenylene trimers 2b,c have large shape-persistent p frames with long alkyl and alkoxy side chains, 2b,c should show a nanoscopic phase separation and amphiphilicity and, hence, stronger supramolecular self-aggregation. Herein, we report the emission properties and self-aggregation behavior of 2b,c as well as the formation of a dendritic supramolecular architecture comprised of a charge-transfer (CT) complex of 2b and 2,4,7-trinitrofluorenone (TNF). As reported previously, 1b,c were synthesized by using electron-transfer oxidation of Lipshutz cuprates 4b,c prepared from 3b,c in a one-pot procedure (Scheme 1). Although the yields of 1b,c were not high, they were obtained as the major components of the soluble products. Under dehydrogenative cyclization conditions, 1b was treated with CuACHTUNGTRENNUNG(OTf)2 and AlCl3 in CS2 to produce 2b (73%), and 1c was treated with FeCl3 in nitromethane/dichloromethane (3/ [a] Y. Hanai, Dr. M. J. Rahman, Dr. J. Yamakawa, Dr. M. Takase, Prof. T. Nishinaga, Prof. M. Iyoda Department of Chemistry, Graduate School of Science and Engineering Tokyo Metropolitan University Hachioji, Tokyo 192-0397 (Japan) Fax: (+81)42-677-2525 E-mail : iyoda@tmu.ac.jp [b] Dr. M. Hasegawa Department of Chemistry School of Science Kitasato University Sagamihara, Kanagawa 252-0373 (Japan) [c] Dr. K. Kamada Research Institute for Ubiquitous Energy Devices National Institute of Advanced Industrial Science and Technology (AIST) AIST Kansai Center, Ikeda, Osaka 563-8577 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100336.

Amphiphilic tetrathiafulvalene (TTF) having both long alkylthio chains and pyridylethynyl groups was synthesised, and its nanostructure and redox properties were investigated. The bis(pyridylethynyl)TTF formed 1D flat and helical tapes. Since these tapes were of micrometre size, the conductivities of neutral and I2-doped tapes could be measured by directly attaching gold wires to the tape. After doping with I2, the colour of the tape changed from purple to black, and the tape behaved as a semiconductor up to 2.6 × 10− 4 S cm− 1.

4,5-Dialkyl-1,2-bis(5-tropolonylethynyl)benzenes 5a and 5b were synthesized, and dioctyl derivative 5a was converted into the corresponding trinuclear palladium(II) macrocycle 1a by reacting it wit...

Combined DFT calculations and UV-vis-NIR, ESR, and SQUID measurements revealed that the ground-state electronic structure of a linear π-conjugated oligomer dication composed of two pyrrole and six or seven thiophene rings and methylthio end-capping units is dominated by a singlet biradical character.

The Diels–Alder reaction of 1,2-dihydropyridine derivatives (1-phenoxycarbonyl-1,2-dihydropyridine 1 or 1-methoxycarbonyl-1,2-dihydropyridine 4) with N-acryloyl (1S)-2,10-camphorsultam (1S)-2 {or N-acryloyl (1R)-2,10-camphorsultam (1R)-2} in the presence of Lewis acid, such as titanium tetrachloride, zirconium tetrachloride, and hafnium tetrachloride afforded the endo-cycloaddition product, 2-azabicyclo[2.2.2]octane derivatives in good yields with excellent diastereoselectivity. The absolute stereochemistry assignment of the endo-cycloaddition product (1S)-5a starting from N-acryloyl (1S)-2,10-camphorsultam (1S)-2 has been established to be (1S,4R,7S) and the reaction mechanism was proposed.

A series of thienylene-ethynylene-tetrathiafulvalene oligomers 1–6 have been synthesized by the Sonogashira reaction of iodo-tetrathiafulvalene derivatives with thienylethynes. The X-ray structure of 4,5-bis(2-thienylethynyl)tetrathiafulvalene (1b) revealed a planar structure of the molecule. The electronic spectra of neutral oligomers 1–6 suggest a partial intramolecular charge transfer between the TTF and thienylethyne units. The cyclic voltammetric measurements of 1–4 showed multi-redox processes.

π-Expanded oligothienylene macrocycle with a large inner cavity incorporates fullerenes such as methano[60]fullerene (C61H2) and [70]fullerene (C70) inside to form Saturn-like complexes.[1] Although the oligothiophene macrocycle weakly interacts with fullerenes in solution, it forms stable Saturn-like fullerene complexes in the solid state. X-ray analysis of the Saturn-like complexes exhibited short contacts between the sulfur atoms of the oligothiophene macrocycle and fullerene carbons, which hinder the rotation of fullerenes. As a result, the non-covalent interaction between the oligothiophene macrocycle and fullerenes was employed in crystal structure determination of fullerenes. UV–vis–NIR spectra of the Saturn-like complexes showed weak donor–acceptor interaction between the oligothiophene macrocycle and fullerenes. [1] M. Iyoda, H. Shimizu, S. Aoyagi, H. Okada, B. Zhou, Y. Matsuo, Can. J. Chem. 95 (2017) 315-319.

Octithiophene sterically segregated by annelation with bicyclo[2.2.2]octene (BCO) units was synthesized to study the unimolecular properties of longer oligothiophene dications. For the preparation of such longer oligomers, a new route for the synthesis of the monomer annelated with BCO unit at the 3,4-positions of thiophene ring was developed. Attempted synthesis of octithiophene 1(8T) fully annelated with BCO units was hampered by low solubility of the product, and octithiophene 2(8T) having octyl groups instead of the BCO units at the second rings from the both ends of 1(8T) was synthesized to solve the solubility problem. Neutral 2(8T) has a lower planarity due to the steric repulsion between the octyl substituents and the neighboring thiophene units. However, dication 2(8T)2+, which was obtained as a stable salt by chemical two-electron oxidation with NO+SbF6–, has a planar quinoid structure, as judged from a linear correlation between the inverse chain length and the absorption energy for 1(nT)2+ (n = 3,4,6) and 2(8T)2+. Based on the comparison with the calculated absorption spectra and the result of ESR inactive properties, 2(8T)2+ appears to have a singlet ground state with open-shell biradical character rather than a closed-shell singlet structure.

Fully conjugated π-expanded macrocyclic oligothiophenes with 24π to 180π electron systems have been synthesized using a modified McMurry coupling reaction as the key step. For the synthesis of cyclo[n](2,5-thienylene-ethynylene)s composed of thiophenes and acetylenes, bromination-dehydrobromination and double elimination procedure were employed. X-ray analyses of macrocyclic oligothiophenes with 24π, 60π, and 72π systems revealed unique molecular and packing structures, reflecting planar cyclic frameworks with medium to large inner cavities. All the giant macrocycles exhibit multistep reversible redox behavior with a fairly low first-oxidation potentials, reflecting their cyclic conjugation. The doping of macrocycles with iodine forms semiconductors owing to their π-donor properties and π–π stacking ability. Interestingly, 90π, 108π, and 120π systems self-aggregate in the solid state to form red nanofibers or petal structure, whereas 150π to 180π systems form red nanoparticles. The structures of fibrous aggregates and particles have been established by scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses. Furthermore, two-photon properties of 72π, 108π, 144π, and 180π systems show that the increasing π-conjugation leads to an increase in the two-photon absorption cross-section with magnitudes as high as 100 000 GM in the visible spectral region.

Cyclooctatetraene (COT), the first 4nπ-electron system to be studied, adopts an inherently nonplanar tub-shaped geometry of D2d symmetry with alternating single and double bonds, and hence behaves as a nonaromatic polyene rather than an antiaromatic compound. Recently, however, considerable 8π-antiaromatic paratropicity has been shown to be generated in planar COT rings even with the bond alternated D4h structure. In this review, we highlight recent theoretical and experimental studies on the antiaromaticity of hypothetical and actual planar COT. In addition, theoretically predicted triplet aromaticity and stacked aromaticity of planar COT are also briefly described.