Enhanced host-guest interaction between [10]cycloparaphenylene ([10]CPP) and [5]CPP by cationic charges.

A dication of [5]cycloparaphenylene ([5]CPP2+) was selectively encapsulated by neutral [10]CPP to form the shortest double-layer carbon nanotube, [10]CPP⊃[5]CPP2+. While the same host-guest complex consisted of neutral CPPs, [10]CPP⊃[5]CPP, was already reported, the cationic complex showed an about 20 times higher association constant in (CDCl2)2 at 25 °C (103 mol L-1). Electrochemical and photophysical analyses and theoretical calculations suggested the partial electron transfer from [10]CPP to [5]CPP2+ in the complex, and this charge-transfer (CT) interaction is most likely the origin of the higher association constant of the dicationic complex than the neutral one.

Generation and Characterization of a Tetraradical Embedded in a Curved Cyclic Paraphenylene Unit.

Unique spin-spin (magnetic) interactions, ring-size effects on ground-state spin multiplicity, and in-plane aromaticity has been found in localized 1,3-diradicals embedded in curved benzene structures such as cycloparaphenylene (CPP). In this study, we characterized the magnetic interactions in a tetraradical consisting of two localized 1,3-diradical units connected by p-quaterphenyl within a curved CPP skeleton by electron paramagnetic resonance (EPR) spectroscopy and quantum chemical calculations. Persistent triplet species with zero-field splitting parameters similar to those of a triplet 1,3-diphenylcyclopentane-1,3-diyl diradical were observed by continuous wave (CW) or pulsed X-band EPR measurements. The quintet state derived from the ferromagnetic interaction between the two triplet diradical moieties was not detected at 20 K under glassy matrix conditions. At the B3LYP/6-31G(d) level of theory, the singlet state was lower in energy than the triplet and quintet states. These findings will aid in the development of open-shell species for material science application.

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.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity.

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

Synthesis and Properties of a Cyclohexa-2,7-anthrylene Ethynylene Derivative.

The synthesis of a cyclohexa-2,7-(4,5-diaryl)anthrylene ethynylene (1) was achieved for the first time by using 1,8-diaryl-3,6-diborylanthracene and 1,8-diaryl-3,6-diiodoanthracene as key synthetic intermediates. Macrocycle 1 possesses a planar conformation of approximately D6h symmetry, because of the triple-bond linker between the anthracene units at the 2,7-positions. It was confirmed that macrocycle 1, bearing bulky substituents at the outer peripheral positions, behaves as a monomeric form in solution without π-stacking self-association. Macrocycle 1 has an inner-cavity size that allows specific inclusion of [9]cycloparaphenylene ([9]CPP), but not [8]CPP or [10]CPP, through an aromatic edge-to-face CH-π interaction.

Selective and Gram-Scale Synthesis of [8]Cycloparaphenylene.

Selective and large-scale synthesis of [8]cycloparaphenylene (CPP) was achieved in seven steps starting from commercially available 4-bromo-4'-hydroxybiphenyl and 4,4'-dibromobiphenyl. The key unsymmetrical tetraring unit, 4-bromophenyl and 4'-bromobiphenyl-substituted cis-1,4-bis(triethylsiloxy)-2,5-cyclohexadiene-1,4-diyl (5fA), was synthesized on an ∼50 g scale by stereoselective cis-addition of 4-bromo-4'-lithiobiphenyl to 4-(4-bromophenyl)-4-hydroxy-2,5-cyclohexadien-1-one, which was synthesized on an ∼100 g scale. Platinum-mediated selective dimerization of the four-ring unit 5fB and subsequent reductive aromatization of the cyclohexadiene-diyl by H2SnCl4 gave 2 g of [8]CPP in 6.6% overall yield (10.2% on small scale).

Ultrafast Exciton Self-Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited-State Symmetry in Electron-Vibrational Coupling.

Upon photon absorption, π-conjugated organics are apt to undergo ultrafast structural reorganization via electron-vibrational coupling during non-adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self-trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited-state symmetry-dependent electron-vibrational coupling. By accessing two high-lying excited states, one-photon and two-photon allowed states, a clear discrepancy in the initial time-resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited-state symmetry on ultrafast structural reorganization and exciton self-trapping in the emerging class of π-conjugated materials is provided.

Size-Dependent Relaxation Processes of Photoexcited [ n]Cycloparaphenylenes ( n = 5-12): Significant Contribution of Internal Conversion in Smaller Rings.

[ n]Cycloparaphenylenes ([ n]CPPs; n, number of phenyl rings) have gained considerable attention because they exhibit interesting properties owing to their highly strained structure and radially oriented p orbitals. Recently, [ n]CPPs with n ≥ 5 have been synthesized, but the ring-size dependence of the deactivation processes of the excited states has not been explained particularly for smaller [ n]CPPs ( n ≤ 7). In the present study, we characterized the deactivation processes of [ n]CPPs (5 ≤ n ≤ 12) using transient absorption spectroscopy at sub-pico-, sub-nano-, nano-, and microsecond time scales. Although the fluorescence quantum yield increased with the ring size, the longest S1-state lifetime was observed with [8]CPP, and both the decrease and increase of the ring size resulted in the decrease of the lifetime. Characterization of the intersystem crossing and internal conversion processes explained unique ring-size dependence of the deactivation processes of [ n]CPPs, i.e., the enhanced radiation rate of the larger CPP and the fast internal conversion rate of smaller CPP dominate their S1-state lifetimes.

Significant structural relaxations of excited [n]cycloparaphenylene dications (n = 5-9).

Hoop-shaped macrocycles such as cycloparaphenylenes ([n]CPPs, where n denotes the number of phenylene rings) have attracted considerable attention in recent years because of their interesting properties arising from the highly strained aromatic structure and radially oriented p-orbitals. While the radical cation and dication states of [n]CPPs have been characterized, there is no information available about their excited states, which are expected to exhibit enhanced redox properties. In this study, we investigated the S1 state of [n]CPP2+ by transient absorption measurements in the visible and near-IR regions. The energy of the transient absorption peak exhibited a linear relationship with the reciprocal of the repeating unit, which indicated that the distribution of the excited state expanded with the size of the ring. In addition, smaller CPP2+s showed longer excited state lifetimes. Theoretical calculations suggested that there was a substantial structural relaxation of the smaller CPP2+s accompanying the changes in the charge distribution. Therefore, it was concluded that the smaller Franck-Condon factor resulting from the considerable structural change and larger S1 energy were responsible for the longer S1 state lifetime of smaller CPP2+s.

Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications.

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [ n]cycloparaphenylene dications ([ n]CPP2+, n = 5-9), which have unique in-plane aromaticity. The fluorescence peak positions of the [ n]CPP2+s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP2+ was the highest among the [ n]CPP2+s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [ n]CPP2+s were smaller than those of neutral [ n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [ n]CPP2+s undergo smaller structural changes upon S0-S1 transition than [ n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S0-S1 transition of smaller [ n]CPP2+s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP2+ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap.

Strain-Induced Double Carbon-Carbon Bond Activations of Cycloparaphenylenes by a Platinum Complex: Application to the Synthesis of Cyclic Diketones.

The carbon-carbon (C-C) bond activation of [n]cycloparaphenylenes ([n]CPPs) by a transition-metal complex is herein reported. The Pt0 complex Pt(PPh3 )4 regioselectively cleaves two C-C σ bonds of [5] CPP and [6]CPP to give cyclic dinuclear platinum complexes in high yields. Theoretical calculations reveal that the relief of ring strain drives the reaction. The cyclic complex was further transformed into a cyclic diketone by using a CO insertion reaction.

Gram-Scale Syntheses and Conductivities of [10]Cycloparaphenylene and Its Tetraalkoxy Derivatives.

[10]Cycloparaphenylene ([10]CPP) and its tetraalkoxy derivatives were synthesized on the gram scale in 7 steps starting from 1,4-benzoquinone or 2,5-dialkoxy-1,4-benzoquinone. The key steps involve the highly cis-selective bis-addition of 4-bromo-4'-lithiobiphenyl to the quinones to produce a five-ring unit containing cyclohexa-1,4-diene-3,6-diol moiety, the platinum-mediated dimerization of the five-ring unit, and the H2SnCl4-mediated reductive aromatization of cyclohexadienediol. The tetraalkoxy substituents increased the solubility of [10]CPP in common organic solvents. The carrier-transport properties of thin films of [10]CPP and its derivatives were measured for the first time and indicated that [10]CPP derivatives could rival phenyl-C61-butyric acid methyl ester, which is used widely as an n-type active layer in bulk heterojunction photovoltaics.

Bromination of Cycloparaphenylenes: Strain-Induced Site-Selective Bis-Addition and Its Application for Late-Stage Functionalization.

Bromination of [n]cycloparaphenylenes (CPPs) is herein reported. Small [n]CPPs (n<8) underwent a bis-bromine addition reaction with high site selectively to produce tetrabromo adducts in moderate to excellent yields. Theoretical calculations revealed that thermodynamic stability dictates both the reactivity and site selectivity of the reaction. The addition product was further converted into the octabromo product by a FeBr3 -catalyzed site-selective bromination reaction. The tetra- and octabromine adducts were then transformed into mono- to tetrabromo CPPs, which were further converted into several CPP derivatives. Therefore, bromination and subsequent transformations provide a path for late-stage functionalization of CPPs.

Synthesis and Characterization of [n]CPP (n = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization.

Radical cations and dications of [n]cyclo-p-phenylenes ([n]CPPs, n = 5, 6, 10, and 12), which are the models of those of linear oligo-p-phenylenes without a terminus, were synthesized as hexafluoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF6 or SbF5. The radical cations, [n]CPP(•+), and dications, [n]CPP(2+), exhibited remarkable bathochromic shifts in their UV-vis-NIR absorption bands, suggesting that [n]CPP(•+) and larger [n]CPP(2+) exhibit longer polyene character than the shorter analogues. The larger bathochromic shift was consistent with the narrower HOMO-SOMO and HOMO-LUMO gaps in larger [n]CPP(•+) and [n]CPP(2+), respectively. In [n]CPP(•+), the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. (1)H NMR revealed that the hydrogen of [n]CPP(2+) shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [n]CPP(2+). The single resonances observed in all [n]CPP(2+) strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]CPP by VT-NMR experiment and theoretical calculation.

Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops.

Three N-substituted tetracyclo(2,7-carbazole)s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring anti-carbazoles, which can be used as covalently bonded "methane probes". These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = -2.70 ppm in (1)H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo(9-methyl-2,7-carbazole) and [n]cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]CPP to [7]CPP generate deshielding cones, which are typical of antiaromatic [4n]annulenes.

Regioselective Synthesis and Characterization of Multinuclear Convex-Bound Ruthenium-[n]Cycloparaphenylene (n = 5 and 6) Complexes.

Mono- and multinuclear complexes of ruthenium and [n]cycloparaphenylene (CPP, n = 5 and 6) were synthesized in excellent yields through ligand exchange of the cationic complex [(Cp)Ru(CH3CN)3](PF6) with CPP. In the multinuclear complexes, ruthenium selectively coordinated to alternate paraphenylene units to give bis- and tris-coordinated Ru complexes for [5] and [6]CPPs, respectively. Single-crystal X-ray analysis revealed the Ru was coordinated with η(6)-hapticity on the convex surface of CPP.

In-plane aromaticity in cycloparaphenylene dications: a magnetic circular dichroism and theoretical study.

The electronic structures of [8]cycloparaphenylene dication ([8]CPP(2+)) and radical cation ([8]CPP(•+)) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV-vis-NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]CPP(2+) shows in-plane aromaticity with a (4n + 2) π-electron system (n = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]CPP(2+) but also all [n]CPP (n = 5-10) dications and dianions exhibit in-plane aromaticity.

Practical synthesis of [n]cycloparaphenylenes (n = 5, 7-12) by H2SnCl4-mediated aromatization of 1,4-dihydroxycyclo-2,5-diene precursors.

Cyclic precursors of cycloparaphenylenes (CPPs) containing 1,4-dihydroxy-2,5-cyclohexadien-1,4-diyl units are prepared by modifying a synthetic method developed by Jasti and co-workers for the synthesis of corresponding 1,4-dimethoxy derivatives. Reductive aromatization of the diyl moieties by SnCl2/2 HCl takes place under mild conditions and affords the CPPs in good yields, incorporating 5 or 7-12 phenylene units. Highly strained [5]CPP is synthesized in greater than 0.3 g scale. (119)Sn NMR spectroscopy clarifies the in situ formation of an ate complex, H2SnCl4, upon mixing a 2:1 ratio of HCl and SnCl2, which serves as a highly active reducing agent under nearly neutral conditions. When more than 2 equivalents of HCl, in relation to SnCl2, are used, acid-catalyzed decomposition of the CPP precursors takes place. The stoichiometry of HCl and SnCl2 is critical in achieving the desired aromatization reaction of highly strained CPP precursors.

Ligand-Controlled Synthesis of [3]- and [4]Cyclo-9,9-dimethyl-2,7-fluorenes through Triangle- and Square-Shaped Platinum Intermediates.

The syntheses of [3]- and [4]cyclo-9,9-dimethyl-2,7-fluorenes ([3] and [4]CFRs), cyclic trimer, and tetramers of 9,9-dimethyl-2,7-fluorene (FR), respectively, were achieved by the platinum-mediated assembly of FR units and subsequent reductive elimination of platinum. A triangle-shaped tris-platinum complex and a square-shaped tetra-platinum complex were obtained by changing the platinum ligand. The structure of the triangle complex was unambiguously determined by X-ray crystallographic analysis. Reductive elimination of each complex gave [3] and [4]CFRs. Two rotamers of [3]CFR were sufficiently stable at room temperature and were separated by chromatography. The physical properties of the CFRs were also investigated theoretically and experimentally.

Radical ions of cyclopyrenylene: comparison of spectral properties with cycloparaphenylene.

Hoop-shaped π-conjugated molecules have attracted much attention. In this study, the radical ions of [4]cyclo-2,7-pyrenylene ([4]CPY), a cyclic tetramer of pyrene, and [4]cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]CHPY) were investigated using radiation chemical methods, namely, γ-ray radiolysis and pulse radiolysis. The absorption spectra of the radical ions of [4]CPY and [4]CHPY showed clear peaks in the near-IR and UV-vis regions similar to those of [8]cycloparaphenylene ([8]CPP). Theoretical calculations using time-dependent density functional theory provided reasonable assignments of the observed absorption bands. It was indicated that the C4-C5 and C9-C10 ethylene bonds of [4]CHPY do not contribute to the electronic transitions, resulting in absorption spectra similar to those of [8]CPP. On the other hand, it was confirmed that the allowed electronic transitions of the radical ions of [4]CPY are different from those of the radical ions of [4]CHPY and [8]CPP.