Synthesis of Twisted [n]Cycloparaphenylene by Alkene Insertion.

Mono-alkene-inserted [n]cycloparaphenylenes 1 [(ene)-[n]CPP] with n=6, 8, and 10, mono-ortho-phenylene-inserted [6]CPP 2, and di-alkene-insertved [n]CPP 3 [(ene)2 -[n]CPP] with n=4, 6, and 8 were synthesized by fusing CPP precursors and alkene or ortho- phenylene groups through coupling reactions. Single-crystal X-ray diffraction analyses reveal that the strips formed by the π-surfaces of 1 and 2 exhibited a Möbius topology in the solid state. While the Möbius topology in the parent 1 and 2 in solution was lost due to the free rotation of the paraphenylene unit even at low temperatures, ene-[6]CPP 4 with eight 1-pyrrolyl groups preserved the Möbius topology even in solution. Despite a twist, 1 has in-plane conjugation and possesses a unique size dependence of the electronic properties: namely, the opposite size dependency of the HOMO-LUMO energy relative to conventional π-conjugated molecules.

Dianion and Dication of Tetracyclopentatetraphenylene as Decoupled Annulene-within-an-Annulene Models.

The dianion and dication of tetramesityl-substituted tetracyclopentatetraphenylene, a circulene consisting of alternating five- and six-membered rings, have been generated by reduction with alkali metals and oxidation with antimony(V) halides, respectively. They are theoretically predicted to adopt double annulenoid structures called annulene-within-an-annulene models in which the outer and inner conjugation circuits are significantly decoupled. The theoretical structures were experimentally proven by X-ray crystallographic analyses and the electronic configurations were supported by MCD spectra. Based on the 13 C NMR chemical shifts, negative and positive charges are shown to be located mainly at the outer periphery, indicating that the dianion and dication have delocalized 22-π and 18-π electron outer perimeters, respectively, and 8-π electron structure at the inner ring. Notably, the dianion has an open-shell character, whereas the dication has a closed-shell ground state.

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.

Radical Cation π-Dimers of Conjugated Oligomers as Molecular Wires: An Analysis Based on Nitronyl Nitroxide Spin Labels.

Nitronyl nitroxide (NN)-substituted conjugated oligomers, which were expected to self-associate in biradical cation states, were designed to analyze the capability of π-dimers as molecular wires. The oligomer moieties were composed of dithienyl-N-methylpyrrole with methoxy substituents at the inner ß-position of thiophene rings (DTP-NN. ) and its propylenedioxythiophene (ProDOT) inserted derivative (DTP-P-NN. ), or two ethylenedioxythiophene (EDOT) and two ProDOT units (E2 P2 -NN. ). Among them, chemical one-electron oxidation gave biradical cations (DTP-P).+ -NN. and (E2 P2 ).+ -NN. that formed π-dimers (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ in dichloromethane at low temperatures. ESR studies of (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ showed the presence of a relatively strong exchange interaction between two NN radicals through the radical cation π-dimer moieties. DFT calculations supported these experimental results and predicted that exchange interactions between two NN radicals were comparable or stronger than those through covalently linked quaterthiophene. Thus, the conjugated oligomer radical cation π-dimers acted as efficient molecular wires for electronic communication.

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.

Macrocyclic Oligothiophene with Stereogenic [2.2]Paracyclophane Scaffolds: Chiroptical Properties from π-Transannular Interactions.

The enantiomers of a new cyclic oligothiophene, bridged by two pseudo-ortho[2.2]paracyclophanes, were synthesized as a new class of the chiral π-conjugated system. Single-crystal X-ray diffraction analysis revealed a twisted structure for these oligothiophenes induced by a torsion of the cyclophane moieties. The embedding oligothiophenes into the inherent planar chirality provided a significant enhancement in circular dichroism (CD) spectra thanks to the large magnetic/electric transition dipole moments. In the dicationic state, an intramolecular π dimer was formed due to the strong interactions of the oligothiophenes. We further recorded unprecedented wide ranges of CD spectra in 250-1800 nm region. The transitions are reasonably described by the exciton coupling of the oligothiophenes.

Stable Radical Cations and Their π-Dimers Prepared from Ethylene- and Propylene-3,4-dioxythiophene Co-oligomers: Combined Experimental and Theoretical Investigations.

Co-oligomers composed of two 3,4-ethylenedioxythiophene (EDOT) units and two or three 3,4-propylenedioxythiophene (ProDOT) units, i.e., 2E2PEt and 2E3PEt, were newly synthesized together with the ProDOT trimer 3PMe. On the basis of cyclic voltammetry, the gaps between the first and second oxidation potentials (ΔE1-2) of 2E2PEt and 2E3PEt were found to be larger than that of the previously synthesized ProDOT tetramer 4PHex. These co-oligomers gave the fairly stable radical cations 2E2PEt•+ and 2E3PEt•+ by chemical oxidation with AgSbF6. The disproportionation of 2E2PEt•+ and 2E3PEt•+ into neutral and dicationic species, which was observed for 4PHex•+, was inhibited in accord with the larger ΔE1-2. Additionally, the formation of the π-dimers (3PMe)22+, (2E2PEt)22+, and (2E3PE)22+ was clearly observed in dichloromethane solution at low temperatures with UV-vis-NIR spectroscopy. Furthermore, the π-dimerization enthalpies of 2E2PEt•+ and 2E3PEt•+ were greater than that of 3PMe•+, suggesting the formation of fully π-contacted structures. The structures of the π-dimers were optimized at the B97D3 method, and the calculated absorption spectra of the π-dimers obtained using TD-DFT methods were in reasonable agreement with the observed ones, supporting the reliability of the calculated structures.

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.

Efficient synthesis, structure, and complexation studies of electron-donating thiacalix[n]dithienothiophene.

A series of thiacalix[n]dithiothiophenes (n=4-10) was prepared by a facile method and X-ray analysis was used to determine the molecular structures of square- (4-mer) and pentagonal-shaped macrocycles (5-mer). In the cyclic voltammograms, reversible multielectron redox processes, which are due to electronic delocalization, were observed at low oxidation potentials. The cyclic 4-mer acted as a "Janus-head" cavitand for two C60 molecules, whereas the 5- and 6-mer formed stable 1:1 complexes with C60 .

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.

Pyrrole-fused azacoronene family: the influence of replacement with dialkoxybenzenes on the optical and electronic properties in neutral and oxidized states.

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1-4 were investigated using steady-state UV-vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1-T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 1(2+) and 2(2+) was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 3(2+) and triazacoronene 4(2+) with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.

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).

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.

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.

Cyclic tetrathiophenes planarized by silicon and sulfur bridges bearing antiaromatic cyclooctatetraene core: syntheses, structures, and properties.

Cyclic tetrathiophenes 1, 2, and 3 planarized by dimethylsilyl, sulfur, and sulfone bridges bearing an antiaromatic cycloocatatetraene (COT) core were designed and synthesized to investigate the relationship among the bent angle, paratropicity, and HOMO-LUMO gap of the COT ring. The bent angles of the central COT rings of 1-3 were theoretically estimated and experimentally determined, and it was found that the planarity of the COT ring was finely adjusted in the order of 2 > 3 > 1 by using the small differences in the bond lengths between the bridging units and thiophene rings. From the comparisons of NICS values and calculated HOMO-LUMO gaps of cyclooctatetraene at various bent angles as well as the optimized structures of cyclic tetrathiophenes 1-3, similar enhancement of the paratropicity and narrowing of the HOMO-LUMO gap with decreasing bent angle of the COT rings were shown in both cyclooctatetraene and cyclic tetrathiophenes 1-3. Such predictions were experimentally proved for the first time by means of (1)H NMR and UV-vis measurements of 1-3. In comparison of the (1)H NMR chemical shifts of 1-3 with those of the corresponding precursors, upfield shifts due to a paratropic ring current in the COT ring were observed and the degree of shift increased with increasing planarity of the COT ring. Furthermore the colors of the solutions of 1 (lambda(max) = 483 nm), 2b (lambda(max) = 618 nm), and 3b (lambda(max) = 575 nm) were orange, purple, and red in CH(2)Cl(2), respectively, indicating that the HOMO-LUMO gaps of 1-3 become increasingly narrow with increasing planarity of the COT ring. Reflecting these electronic properties, CV measurements demonstrated the amphoteric redox properties of 1 and 2b, and the radical cation 1(*+), radical anion 1(*-), and dianion 1(2-) were chemically generated and successfully characterized by means of UV-vis, ESR, and NMR spectroscopies.

Selenacalix[4]selenophene: Synthesis, Structure, and Gel Formation of Cyclic Selenoether of Selenophene.

A selenacalix[4]selenophene derivative, a cyclic selenophene oligomer linked with divalent selenium atoms, was successfully synthesized via a one-pot cyclization reaction using 2,5-dibromo-3,4-bis(4-t-butylphenyl)selenophene and (Bu3Sn)2Se. The X-ray analysis revealed an annular geometry consisting of two sets of vertical and horizontal selenophene pairs. The observed geometry was preserved by intermolecular Se···Se and Se···π interactions, leading to a one-dimensional array. This self-assembled network results in a supramolecular gel, despite the absence of any hydrogen bonding sites.

Dynamic molecular tweezers composed of dibenzocyclooctatetraene units: synthesis, properties, and thermochromism in host-guest complexes.

Novel dynamic molecular tweezers (DMTs) 3 a, 3 b, 4 a, 4 b, and 5 b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecular structure shows rigid syn and anti forms in an equilibrium mixture in solution. The syn and anti conformers can be observed by NMR. The isomerization barriers of 3 a, 3 b, 4 a, 4 b, and 5 b are in the range of 16.5-21.3 kcal mol(-1), depending on steric repulsion between substituents of the COT rings and protons of the central benzene ring. These DMTs form complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 1,2,4,5-tetracyano-benzene (TCNB) in solution and in the solid state. The binding abilities of these DMTs increase with electron-donating substituents on COT, which increase the electron densities of the cavity of the syn form, as supported by theoretical calculations. In addition, elongation of the terminal alkoxy chains of the DMTs was found to cause the enhancement of van der Waals contact with guest molecules. Therefore, 5 b, which has CH(2)OMe groups on the COT rings and longer ethoxy groups on the terminal benzene rings, showed the highest electron density of the cavity and hence the highest binding ability with the electron-deficient guest molecules. Interestingly, solutions of 3 b, 4 b, and 5 b show thermochromism in the presence of DDQ. A solution of 3 b or 4 b with DDQ in CHCl(3) is green due to charge-transfer interaction at room temperature and the color changes from green to yellow upon heating to 60 degrees C and from green to blue upon cooling to -40 degrees C, whereas the high complexation ability of 5 b with DDQ only shows a change in the shade of blue.

Magnetic alignment in solid state and temperature hysteresis in aqueous tetrahydrofuran solution for tetrathiafulvaleno[18]annulenes.

Tris(tetrathiafulvaleno)dodecadehydro[18]annulene-hexaesters have a multi-functionality that is very sensitive to small differences in the ester side-chain. Self-aggregation of the [18]annulenes in amphiphilic media such as THF-H(2)O (v/v, 1:1) either produce a fibrous structure or result in temperature hysteresis of the color and (1)H NMR signals. This temperature hysteresis in solution is due to both strong self-aggregation behavior and unique cluster formation in a binary solution of THF and water.

Computational and experimental studies on the index of antiaromaticity for 4n π-systems (n ≥ 2).

A linear correlation between nucleus-independent chemical shift (NICS) and relative hardness for various sizes of antiaromatic 4n π-systems (n ≥ 2) was revealed with HF computations using the gauge-including atomic orbital (GIAO) and time-dependent (TD) methods. Based on this finding, a method to evaluate the relative hardness of 4n π-systems (n ≥ 2) as a new experimental measure of antiaromaticity has been developed.