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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)

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.

A small library of tetrasubstituted [10]cycloparaphenylene ([10]CPP) derivatives bearing alkyl, alkenyl, alkynyl and aryl substituents was constructed by a Pd-catalyzed cross-coupling reaction starting from tetratriflate [10]CPP 5 e, which was readily available in high yields on a >2 g scale. The CPP skeleton increases the reactivity of aryl triflate for oxidative addition to the Pd species, and 5 e is 10 times more reactive than its linear paraphenylene analogue, as determined by competition experiments. Theoretical calculations suggest that the accumulation of the small strain relief from each paraphenylene unit not involved in the reaction is responsible for the observed enhanced reactivity.

The transmetalation of the digold(I) complex [Au2Cl2(dcpm)] (1) (dcpm = bis(dicyclohexylphosphino)methane) with oligophenylene diboronic acids gave the triangular macrocyclic complexes [Au2(C6H4)x(dcpm)]3 (x = 3, 4, 5) with yields of over 70%. On the other hand, when the other digold(I) complex [Au2Cl2(dppm)] (1') (dcpm = bis(diphenylphosphino)methane) was used, only a negligible amount the triangular complex was obtained. The control experiments revealed that the dcpm ligand accelerated an intermolecular Au(I)–C σ-bond-exchange reaction, and that this high reversibility is the origin of the selective formation of the triangular complexes. Structural analyses and theoretical calculations indicate that dcpm ligand increases the electrophilicity of the Au atom in the complex, thus facilitating the exchange reaction, though cyclohexyl group is an electron-donating group. Furthermore, the oxidative chlorination of the macrocyclic gold complexes afforded a series of [n]cycloparaphenylenes (n = 9, 12, 15) in 78–88% isolated yields. The reaction of two different macrocyclic Au complexes gave a mixture of macrocyclic complexes incorporating different oligophenylene linkers, from which a mixture of [n]cycloparaphenylenes with various numbers of phenylene units was obtained in good yields.

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

The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.

The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.

This paper describes a new synthetic method of p-conjugated Mobius molecule by inserting an alkene or ortho-phenylene to cycloparaphenylenes. Physical properties of the obtained compounds were also studied.

This paper describes a new synthetic method of p-conjugated Mobius molecule by inserting an alkene or ortho-phenylene to cycloparaphenylenes. Physical properties of the obtained compounds were also studied.

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.

Fluorinated [6]- and [9]cycloparaphenylene (CPP) derivatives, 8F-[6]CPP and 12F-[9]CPP, were synthesized based on the previous synthesis of the parent CPPs. While the reductive aromatization conditions used in the final step of the synthesis of the parent CPPs did not work for the fluorinated compounds, the use of PBr3 and SnCl2 in acetonitrile successfully accomplished the desired transformation. The structures of F-CPPs were determined by single-crystal X-ray analysis. Photo- and electrochemical analyses and host–guest chemistry revealed the effects of the introduction of fluorine atoms.

The random synthesis of [5n]- and [7m]cycloparaphenylenes (CPPs) (n = 2–4 and m = 2–3) and the selective synthesis of [14]CPP were achieved in four and six steps, respectively, by the nickel- or platinum-mediated cyclizations of the penta- and hepta-ring units containing cis-1,4-bis(triethylsiloxy)-2,5-cyclohexadiene-1,4-diyl and the H2SnCl4-mediated aromatization of the triethylsilyl-protected cyclohexadienediol. [20]- and [21]CPPs were synthesized for the first time.

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.

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 ...

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.

Cyclic tetramers of 3,7-dibenzo[b,d]thiophene (DBT) and 3,7-dibenzo[b,d]thiophene-5,5-dioxide (DBTO), i.e., [4]cyclo-DBT ([4]CDBT) and [4]cyclo-DBTO ([4]CDBTO), respectively, are synthesized by the platinum-mediated assembly of bis-metallated DBT and DBTO and the subsequent reductive elimination of platinum. This is the first example of the synthesis of sulfur-containing cycloparaphenylene (CPP) derivatives. The structure of [4]CDBTO is unambiguously determined by the single crystal X-ray analysis. Theoretical and experimental analyses of their physical properties reveal the effect of the sulfur atom and sulfonyl group at the periphery of biphenyl units of the CPP.

[ 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.

The oxidation processes of [n]cycloparaphenylenes ([n]CPPs) (n=5-12) were systematically investigated by cyclic and rotating disk electrode voltammetry. All CPPs underwent pseudo-reversible two-electron oxidation irrespective of ring size, forming the corresponding radical cations and then dications. The results were in sharp contrast to those observed for linear oligoparaphenylenes, which only undergo one-electron oxidation. The difference in the first and second oxidation potentials in the CPP oxidation was affected by the ring size and became more significant as the decrease of CPP size. In other words, while the first oxidation from neutral CPP to the radical cation occurred faster as the size of CPP becomes smaller, the second oxidation from the radical cation to dication exhibited opposite size dependence.

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.

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.

A selective, practical, and large-scale synthesis of [6]cycloparaphenylene, the second smallest cycloparaphenylene to be synthesized, was achieved in nine steps starting from commercially available 1, 4-dibromobenzene and 4′-bromobiphenyl-4-ol. The key intermediate, cis-1, 4-(4-bromophenyl)-1, 4-bis(triethylsiloxy)cyclohexa-2, 5-diene, was prepared on a large scale (>20 g) and was selectively dimerized to form a cyclic precursor of [6]cycloparaphenylene by platinum-mediated assembly and subsequent reductive elimination. Deprotection of the triethylsilyl group and subsequent tetrachlorostannic acid (H2SnCl4)-mediated reductive aromatization gave [6]cycloparaphenylene in 23% overall yield from the commercially available substrates on gram scale.

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.

[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.

Cycloparaphenylenes (CPPs), a class of hoop-shaped conjugated macrocycles, have attracted the attention of researchers in various fields because of their interesting properties. Although their properties in the singlet-excited state have been reported, there is no systematic information on the triplet-excited state. In the present study, the properties of triplet-excited [n]CPP (n = 8-12, where n denotes the number of phenyl rings) were comprehensively investigated. The phosphorescence peak shifted to the shorter-wavelength side with increasing ring size of the CPPs, indicating smaller triplet energy for smaller CPPs. It was found that the triplet energy of a smaller CPP is even smaller than those of poly(p-phenylene)s, indicating that small hoop-shaped conjugated macrocycles are effective in realizing low-band gap materials. By applying laser flash photolysis, the Tn-T1 absorption spectra of CPPs were obtained, from which the size-dependence of energy levels of higher triplet states were also determined. Generation of singlet oxygen by the energy transfer from a triplet-excited CPP was confirmed. The generation yield became smaller with increasing size of the CPP in accordance with the triplet yield. From these observations, the size-dependence of the deactivation pathways is explained.

A cyclic tetramer of pyrene, [4]cyclo-2,7-pyrenylene ([4]CPY), was synthesized from pyrene in six steps and 18% overall yield by the platinum-mediated assembly of pyrene units and subsequent reductive elimination of platinum. The structures of the two key intermediates were unambiguously determined by X-ray crystallographic analysis. DFT calculations showed that the topology of the frontier orbitals in [4]CPY was essentially the same as those in [8]cycloparaphenylene ([8]CPP), and that all the pyrene units were fully conjugated. The electrochemical analyses proved the electronic properties of [4]CPY to be similar to those of [8]CPP. The results are in sharp contrast to those obtained for the corresponding linear oligomers of pyrene in which each pyrene unit was electronically isolated. The results clearly show a novel effect of the cyclic structure on cyclic π-conjugated molecules.

This article describes the most recent developments in the synthesis of three-dimensional π-conjugated molecules and the elucidation of their properties made by our research group. Various cycloparaphenylenes (CPPs) of different sizes and a cage-like 3D molecule were synthesized based on the platinum-mediated assembly of π-units and subsequent reductive elimination of platinum. The assembly of π-units by this method mimics the self-assembly process for the formation of supramolecular ligand-metal complexes with 3D cages and polyhedral structures. Furthermore, reductive elimination of platinum successfully took place with high efficiency, despite the high strain energy of the target molecule. Several size-dependent physical properties of CPPs, namely the photophysical, redox, and host-guest chemistries, were also clarified. These results are of use for a molecular-level understanding of CNT physical properties as well as fullerene peapods. Theoretical and electrochemical studies suggest that small CPPs and their derivatives should be excellent lead compounds for molecular electronics.

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, N-isopropyl acrylamide, N-vinyl pyrrolidone (NVP), and N-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 (

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.

Generation of carbanions from organostibines and organobismuthines through heteroatom-metal exchange reactions was examined from synthetic and mechanistic viewpoints. The exchange reaction proceeded spontaneously upon treatment with various organometallic reagents, such as alkyl lithiums, tetraalkyl zincates, and alkyl magnesium halides to afford the corresponding carbanions quantitatively. Due to the high reactivity of these heteroatom compounds, the exchange reactions took place exclusively even in the presence of various polar functional groups, which potentially react with organometallic species. The advantage of this method was exemplified by the end-group transformation of living polymers that bear these heteroatom species at the ω-polymer end, prepared by using organostibine and bismuthine-mediated living radical polymerizations. Various polymers that bear polar functional groups and acidic hydrogen-for example, poly(methyl methacrylate), poly(butyl acrylate), poly(N-isopropyl acrylamide), and poly(2-hydroxyethyl methacrylate)-could be used in the exchange reactions, and subsequent trapping with electrophiles afforded the corresponding polymers with controlled molecular weights, molecular weight distributions, and end-group functionalities. Competition experiments showed that organostibines and organobismuthines were among the most reactive heteroatom compounds towards organometallic reagents and that their high reactivity was responsible for the high chemoselectivity in the exchange reaction.

The substituent effect on the antimony atom in organostibine-mediated degenerative transfer living radical polymerization has been studied. 2-Diphenylstibanyl-2-methylpropionitrile (1a) was synthesized as an organostibine chain transfer agent (CTA), and its effect was compared with the previously reported dimethylstibanyl analogue 1b. Both CTAs successfully polymerized butyl acrylate (BA) in a highly controlled manner giving poly(butyl acrylate)s possessing number average molecular weights close to the theoretical values determined by BA/1 ratios and narrow molecular weight distribution. The controllability of styrene polymerization by 1a was slightly decreased but the decrease is still at an acceptable level. However, 1a did not control the polymerization of methyl methacrylate (MMA). The results are in sharp contrast that 1b can highly control the polymerization of styrene and MMA. Kinetic studies using diphenylstibanyl-substituted polystyrene macro-CTA 2 in styrene polymerization revealed that the degenerative chain transfer reaction of the diphenylstibanyl group takes place faster than that of the dimethylstibanyl group, suggesting that the reactivity of the diphenylstibanyl group is not the origin of the low control. Chain extension experiments revealed the formation of a considerable amount of dead polymers when 1a was used as a CTA, and the frequent occurrence of termination reaction is the most probable origin of the low control. The addition of tetraphenyldistibine significantly increased the control of styrene and MMA polymerization, and structurally well-defined polystyrene and poly(methyl methacrylate) were obtained by applying a binary system consisting from 1a and tetraphenyldistibine. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:307–315, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20681

The reaction of poly(methyl methacrylate) (PMMA), poly( i -butyl methacrylate), poly(2-hydroxyethyl methacrylate), and polymethacrylonitrile bearing organotellurium, organostibine, and organobismuthine ω-living polymer end groups with 2,2,6,6-tetramethylpiperidine 1-oxy (TEMPO) under thermal or photochemical conditions gave the corresponding ω-vinylidene functionalized polymethacrylates and polymethacrylonitrile with high end group fidelity. Treatment of the PMMA with ethyl 2-[(tributylstannyl)methyl]acrylate also gave a PMMA bearing the same ω-vinylidene end functionality. 1 H NMR, GPC, MALDI TOF MS, and thermogravimetric analyses revealed the highly controlled and defined structure of poly(alkyl methacrylate)s and polymethacrylonitrile in terms of molecular weight, molecular weight distribution, and the ω-polymer end structure.

Diphenyl(2,6-dimesitylphenylthio)bismuthine (1a) serves as an excellent cocatalyst in organobismuthine-mediated living radical polymerization (BIRP). Both low and high molecular weight polystyrenes and poly(butyl acrylate)s (PBAs) with controlled molecular weights and low polydispersity indexes (PDIs) were synthesized by the addition of a catalytic amount of 1a to an organobismuthine chain-transfer agent, methyl 2-dimethylbismuthanyl-2-methylpropionate (3). The number-average molecular weight (M(n)) of the resulting polymers increases linearly with the monomer/3 ratio. Structurally well-defined polystyrenes with M(n)'s in the range from 1.0 x 10(4) to 2.0 x 10(5) and PDIs of 1.07-1.15 as well as PBAs with M(n)'s in the range from 1.2 x 10(4) to 2.8 x 10(6) and PDIs of 1.06-1.43 were successfully prepared under mild thermal conditions. Control experiments suggested that 1a reversibly reacts with the polymer-end radical to generate an organobismuthine dormant species and 2,6-dimesitylphenylthiyl radical (2a). This reaction avoids the occurrence of chain termination reactions involving the polymer-end radicals and avoids undesired loss of the bismuthanyl polymer end group. The bulky 2,6-dimesitylphenyl group attached to the sulfur atom may prevent the addition of thiyl radicals to the vinyl monomers to generate new polymer chains.

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.

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.

Title compounds containing dihydroxycyclohexadiendiyl units are prepared by a modified synthetic method developed by Jasti and co-workers for the synthesis of corresponding dimethoxy derivatives.

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.

The selective synthesis of [6]-, [8]-, and [10]cycloparaphenylenes (CPPs) was achieved by a new synthetic route involving Ni(0)-mediated coupling of bis(para-haloaryl)dinuclear arylplatinum complex...

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.

This article describes the most recent developments in the synthesis of three-dimensional π-conjugated molecules and the elucidation of their properties made by our research group. Various cycloparaphenylenes (CPPs) of different sizes and a cage-like 3D molecule were synthesized based on the platinum-mediated assembly of π-units and subsequent reductive elimination of platinum. The assembly of π-units by this method mimics the self-assembly process for the formation of supramolecular ligand-metal complexes with 3D cages and polyhedral structures. Furthermore, reductive elimination of platinum successfully took place with high efficiency, despite the high strain energy of the target molecule. Several size-dependent physical properties of CPPs, namely the photophysical, redox, and host-guest chemistries, were also clarified. These results are of use for a molecular-level understanding of CNT physical properties as well as fullerene peapods. Theoretical and electrochemical studies suggest that small CPPs and their derivatives should be excellent lead compounds for molecular electronics.