Reduction of Ethynylenes to Vinylenes in a Macrocyclic π-Extended Thiophene Skeleton Under McMurry Coupling Conditions.

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.

Preparation, Spectroscopic Characterization and Theoretical Study of a Three-Dimensional Conjugated 70 π-Electron Thiophene 6-mer Radical Cation π-Dimer.

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.

Reversible Color and Shape Changes of Nanostructured Fibers of a Macrocyclic π-Extended Thiophene Hexamer Promoted by Adsorption and Desorption of Organic Vapor.

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 der Waals interactions occur. Slow addition of acetone to a solution of 1 in CS2 produces a yellow nanostructured fiber 1-A containing a 1:1.5:1 ratio of 1, acetone, and CS2. Over a 2 min period at 25 °C, 1-A gradually changes to an orange fiber 1-B containing a 1:0.5:1 ratio of 1, acetone, and CS2. On exposure to acetone vapor, 1-B regenerates 1-A (vapochromism), and removal of all solvents from 1-A and 1-B generates a red-orange fiber 1-C, which upon brief immersion in acetone/CS2 produces 1-A. Furthermore, 1-C is converted to orange yellow fiber 1-D upon exposure to acetone vapor for 1 s at 25 °C. Analysis of the horizontal and vertical profiles of the X-ray diffraction (XRD) patterns shows that removal of solvent from 1-A reversibly creates 1-B in conjunction with a shape and size change along with arching.

A Saturn-Like Complex Composed of Macrocyclic Oligothiophene and C60 Fullerene: Structure, Stability, and Photophysical Properties in Solution and the Solid State.

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.

10-Mesityl-1,8-diphenylanthracene Dimer: Synthesis, Structure, and Properties.

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.

Reversible Photoisomerization of Monolayers of π-Expanded Oligothiophene Macrocycles at Solid-Liquid Interfaces.

Self-assembled monolayers of a π-expanded oligothiophene macrocycle undergo photoisomerization between their Z,Z and E,E diastereomers at the interface between octanoic acid solutions and highly oriented pyrolytic graphite (HOPG). The switching process proceeds in situ at the solid-liquid interface and was followed by scanning tunneling microscopy (STM). Upon illumination with light at 365 nm (546 nm), a monolayer of Z,Z-8mer (E,E-8mer) photoisomerizes to the E,E-8mer (Z,Z-8mer) form with changes in 2D hexagonal packing. These findings provide insight towards the design of photoresponsive surfaces with desirable optoelectronic and structural (host-guest) properties.

Multifunctional π-expanded oligothiophene macrocycles.

This tutorial review summarizes recent progress in the design, synthesis, and multifunctional properties of fully conjugated macrocyclic π-systems. We focus on the π-expanded oligothiophene macrocycles after a short survey of macrocyclic conjugated loops and belts such as [n]cycloparaphenylenes, cyclic[n]para-phenylacetylenes, [4]cyclo-2,8-crysenylenes, and cyclo[n]thiophenes. Fully conjugated π-expanded oligothiophene macrocycles possess shape-persistent but sometimes pliable π-frames, and the electronic and optoelectronic properties of the macrocycles largely depend on the π-systems inserted into the oligothiophene macrocycles. Among them, the π-expanded oligothiophene macrocycle composed of 2,5-thienylenes, ethynylenes, and vinylenes is one of the most widely applicable macrocycles for constructing multifunctional π-systems. These π-expanded oligothiophene macrocycles from small to very large ring sizes can be prepared via a short step procedure, and their various solid state structures can be determined by X-ray analysis. Since these macrocycles have inner and outer domains, specific information concerning structural, electronic, and optical properties is expected. Furthermore, π-expanded oligothiophene macrocycles with alkyl substituents exhibit various morphologies depending on nanophase separation of molecules, and a morphological change is observed for the molecular switch.

Synthesis, structures, and photophysical properties of π-expanded oligothiophene 8-mers and their Saturn-like C60 complexes.

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 remained unchanged under similar conditions. E,E-1 with a 12.5-14.7 Å inner cavity incorporated fullerene C60 in the cavity in solution and the solid state to produce a Saturn-like complex, whose structure was determined by X-ray analysis. 2 also formed a Saturn-like complex with C60 in the solid state. These Saturn-like complexes are stabilized by van der Waals interactions between the sulfur atoms of 8-mer and C60. The complexes exhibited charge-transfer interactions in the solid state. Like E,E-1, Saturn-like complex E,E-1⊃C60 formed small cube and fiber structures depending on the solvent used, whereas those of Saturn-like complex 2⊃C60 were limited due to the rigidity of the macroring of 2.

Bent π-conjugated systems composed of three-dimensional benzoannulenes.

This article describes bent π-conjugated systems composed of alternating o-phenylene and Z-vinylene units. all-Z-[n]Benzo[4n]annulenes are higher homologues of dibenzocyclooctatetraene (DBCOT) with a concave π system, and attempts were made to convert [20]- and [24]annulenes having partial belt structures of fullerenes and carbon nanotubes into [10]- and [12]phenacenes. A bent π-conjugated system composed of two DBCOT units showed dynamic syn-anti equilibrium in solution and behaved as dynamic molecular tweezers (DMTs). The syn isomers of the DMTs formed blue charge-transfer complexes with DDQ in solution, and this complexation and color change were applied to thermochromism. Furthermore, DMTs having two CN groups on each COT ring exhibited crystalline-state emission in the presence of solvent molecules, although almost no emission was observed in solution or the solid state. Based on this crystalline-state emission, a vapochromic system was established. The multifunctional properties of DMTs caused by the flexibility of the nonplanar π system are summarized.

Excited-State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring-to-Linear Excitonic Behavioral Turning Point.

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.

Star-shaped tetrathiafulvalene oligomers towards the construction of conducting supramolecular assembly.

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.

Multifunctional π-expanded macrocyclic oligothiophene 6-mers and related macrocyclic oligomers.

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 ß,ß-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.

Steric control in the π-dimerization of oligothiophene radical cations annelated with bicyclo[2.2.2]octene units.

A Two series of oligothiophenes 2(nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a-c, annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π-dimerization and the structure-property relationships of the π-dimers of oligothiophene radical cations. Their radical-cation salts were prepared through chemical one-electron oxidation by using nitrosonium hexafluoroantimonate. From variable-temperature electron spin resonance and electronic absorption measurements, the π-dimerization capability was found to vary among the members of the 2(nT)(+)(·)SbF6(-) series and 3(+)(·)SbF6(-) series of compounds. To examine these results, density functional theory (DFT) calculations at the M06-2X/6-31G(d) level were conducted for the π-dimers. This level of theory was found to successfully reproduce the previously reported X-ray structure of (2(3T))2(2+) having a bent π-dimer structure with cis-cis conformations. The absorption bands obtained by time-dependent DFT calculations for the π-dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π-dimerization were divided into four factors: 1) SOMO-SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain-length dependence are discussed in detail.

Solvent-induced crystalline-state emission and multichromism of a bent π-surface system composed of dibenzocyclooctatetraene units.

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).

Structure-dependent electronic nature of star-shaped oligothiophenes, probed by ensemble and single-molecule spectroscopy.

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.

Effect of substituents on the structure, stability, and π-dimerization of dithienylpyrrole radical cations.

A series of 2,5-di(2-thienyl)-N-methylpyrrole derivatives 1a-1d with methylthio end-capping groups and electron-donating substituents at the 3-position of the thiophene rings was synthesized, and the effects of the substituents on the structure, stability, and π-dimerization ability of the radical cation were investigated using UV-vis-NIR and electron spin resonance spectra and density functional theory (DFT) calculations. Among the electron-donating methyl, methoxy, and methylthio substituents, the methoxy derivative 1c gave the most stable radical cation, which persisted in dichloromethane at room temperature under nitrogen for several hours without any apparent decomposition. In addition, 1c(•+) had the largest π-dimerization enthalpy among 1a(•+)-1d(•+). DFT calculations with the M06-2X method revealed that methyl and methylthio derivatives 1b(•+) and 1d(•+) as well as 1c(•+) adopt a cis-cis conformation, in contrast to the trans-trans conformer of unsubstituted 1a(•+), while the π-dimers of all of these compounds were shown to have a cis-cis conformation. On the basis of further detailed analyses, the preformed cis-cis conformation and the weaker intramolecular and intermolecular steric repulsions were considered to explain why 1c(•+) has the largest π-dimerization enthalpy.

Resonant Electron Tunneling Induces Isomerization of π-Expanded Oligothiophene Macrocycles in a 2D Crystal.

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.

Structural, optical, and electronic properties of a series of 3,4-propylenedioxythiophene oligomers in neutral and various oxidation States.

A series of 3,4-propylenedioxythiophene (ProDOT) oligomers (nP(Hex)) with dihexyl side chains and methylthio end-capping units was synthesized as a model of poly(3,4-alkylenedioxythiophene)s. The slope of the linear relationship between the energy of the absorption maxima of nP(Hex) in the neutral states and the reciprocal of the number of monomer units (1/n) was found to be comparable to that of 3,4-ethylenedioxythiophene (EDOT) oligomers, suggesting that both the ProDOT and the EDOT oligomers have a similar effective conjugation. In cyclic voltammetry measurements, both the first and second oxidation waves and the third and fourth waves were shown to merge into one peak with increasing chain length. The stepwise chemical oxidations of nP(Hex) with SbCl(5) in CH(2)Cl(2) at room temperature gave their stable cationic species in various oxidation states, and it was found that only the radical cations (polarons) have an obvious absorption band in the visible region. Interestingly, when the absorption spectra of tetramer radical cation 4P(Hex)(+·) were measured at low temperatures, reversible disproportionation into dication 4P(Hex)(2+) and neutral species 4P(Hex) was observed in addition to π-dimer formation. Furthermore, the radical cations of the longer oligomers showed only the disproportionation reaction. From the comparisons of the results of experiments and the theoretical calculations of the dications, 6P(Hex)(2+) was found to have a closed-shell nature, and only a weak singlet biradical character appeared even in longer oligomers 10P(Hex)(2+) and 12P(Hex)(2+). Overall, the electron-donating dioxy substituents are considered to stabilize high p-doping levels with closed-shell dication (bipolaron) structures in poly(3,4-alkylenedioxythiophene)s, which enables the transparency properties of the polymers.

Probing coherence in synthetic cyclic light-harvesting pigments.

A series of π-extended cyclic thiophene oligomers of 12, 18, 24, and 30 repeat units have been studied using methods of ultrafast time-resolved absorption, fluorescence upconversion, and three-pulse photon echo. These measurements were conducted in order to examine the structure-function relationships that may affect the coherence between chromophores within the organic macrocycles. Our results indicate that an initial delocalized state can be seen upon excitation of the cyclic thiophenes. Anisotropy measurements show that this delocalized state decays on an ultrafast time scale and is followed by the presence of incoherent hopping. From the use of a phenomenological model, we conclude that our ultrafast anisotropy decay measurements suggest that the system does not reside in the Förster regime and coherence within the system must be considered. Three-pulse photon echo peak shift experiments reveal a clear dependence of initial peak shift with ring size, indicating a weaker coupling to the bath (and stronger intramolecular interactions) as the ring size is increased. Our results suggest that the initial delocalized state increases with ring size to distances (and number of chromophores) comparable to the natural light-harvesting system.

Conducting supramolecular nanofibers and nanorods.

Recent progress in the study of electroconducting nanomaterials such as nanofibers, nanorods and other nanostructures based on the supramolecular self-assembly of hexabenzocoronenes, oligo(thiophene)s, tetrathiafulvalenes and perylene-3,4,9,10-tetracarboxylic diimides is described in this tutorial review. Conducting nanofibers and nanorods are constructed by doping pi-donors or pi-acceptors with oxidants or reductants before/after the formation of such nanostructures; however, some nanofibers show electric conductivity without any doping in the neutral state. Although cation radicals and anion radicals seem to be difficult to form nanofibers and nanorods, a limited number of cation radicals produce conducting nanofibers. For nanofibers and nanorods composed of weak pi-donors and pi-acceptors, their conductivities are measured by time-resolved microwave conductivity techniques.