Selective Synthesis and Crystal Structure of [10]Cycloparaphenylene

[10]Cycloparaphenylene ([10]CPP) was selectively synthesized in four steps in 13% overall yield from commercially available 4,4′-diiodobiphenyl by using mono-I–Sn exchange, Sn–Pt transmetalation, I–Pd exchange, and subsequent oxidative coupling reactions. The single-crystal X-ray structure of [10]CPP is described

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

Radical cations and dications of [<i>n</i>]­cyclo-<i>p</i>-phenylenes ([<i>n</i>]­CPPs, <i>n</i> = 5, 6, 10, and 12), which are the models of those of linear oligo-<i>p</i>-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, [<i>n</i>]­CPP^•+, and dications, [<i>n</i>]­CPP²⁺, exhibited remarkable batho­chromic shifts in their UV–vis–NIR absorption bands, suggesting that [<i>n</i>]­CPP^•+ and larger [<i>n</i>]­CPP²⁺ 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 [<i>n</i>]­CPP^•+ and [<i>n</i>]­CPP²⁺, respectively. In [<i>n</i>]­CPP^•+, the spins and charges were equally and fully delocalized over the <i>p</i>-phenylene rings of the CPPs, as noted by ESR. ¹H NMR revealed that the hydrogen of [<i>n</i>]­CPP²⁺ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [<i>n</i>]­CPP²⁺. The single resonances observed in all [<i>n</i>]­CPP²⁺ 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 <i>anti</i>-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 ¹H 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 [<i>n</i>]­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 [4<i>n</i>]­annulenes

In-Plane Aromaticity in Cycloparaphenylene Dications: A Magnetic Circular Dichroism and Theoretical Study

The electronic structures of [8]­cyclo­para­phenylene dication ([8]­CPP²⁺) 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]­CPP²⁺ shows in-plane aromaticity with a (4<i>n</i> + 2) π-electron system (<i>n</i> = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]­CPP²⁺ but also all [<i>n</i>]­CPP (<i>n</i> = 5–10) dications and dianions exhibit in-plane aromaticity

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 <i>anti</i>-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 ¹H 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 [<i>n</i>]­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 [4<i>n</i>]­annulenes

Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules

Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions

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 [<i>n</i>]­cycloparaphenylene dications ([<i>n</i>]­CPP²⁺, <i>n</i> = 5–9), which have unique in-plane aromaticity. The fluorescence peak positions of the [<i>n</i>]­CPP²⁺s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with <i>n</i> > 5. The fluorescence quantum yield of [6]­CPP²⁺ was the highest among the [<i>n</i>]­CPP²⁺s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [<i>n</i>]­CPP²⁺s were smaller than those of neutral [<i>n</i>]­CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [<i>n</i>]­CPP²⁺s undergo smaller structural changes upon S₀–S₁ transition than [<i>n</i>]­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 [<i>n</i>]­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

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

Synthesis of Concentrated Polymer Brushes via Surface-Initiated Organotellurium-Mediated Living Radical Polymerization

An organotellurium chain transfer agent (CTA) bearing a triethoxysilyl group at one end and a 2-methyltellanyl-2-methylpropionate group at the other was prepared and immobilized on the surface of a silicon wafer and silica nanoparticle (SiP). Surface-initiated organotellurium-mediated living radical polymerization (SI-TERP) from the immobilized CTA in the presence of nonimmobilized (free) organotellurium CTA was examined. Concentrated polymer brushes (CPBs) having surface occupancies above 0.1 were prepared by polymerization of various monomers, including styrene, methyl methacrylate, butyl acrylate, <i>N</i>-isopropyl acrylamide, <i>N</i>-vinyl pyrrolidone (NVP), and <i>N</i>-vinyl carbazole (NVC). All CPBs were formed in a controlled manner, with number-average molecular weights close to the theoretical values and low polydispersity indices (<1.41). Structurally well-controlled CPBs comprising unconjugated monomers, NVP and NVC, were prepared for the first time. Atomic force microscopy and transmission electron microscopy analyses of the CPBs revealed the highly stretched and anisotropic structure of the grafted polymer chain in a good solvent

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 <i>cis</i>-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 <i>n</i>-type active layer in bulk heterojunction photovoltaics