3-[4'-(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer.

The crystallization of 3-[4'-(diethylboryl)phenyl]pyridine (1), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 °C afforded a cyclic tetramer that co-crystallized with solvent molecules. Similarly, solutions of compound 1 in toluene at 10 °C and in benzene at 8 °C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetramer was the minor component. 13 C CP/MAS NMR spectroscopy of precipitates of compound 1 suggested that precipitation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipitation from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D6 ]benzene, as determined from a van't Hoff plot, were ΔH°=-8.8 kcal mol-1 and ΔS°=-23.7 cal mol-1 K-1 , which were coincident with previously reported calculations and observations.

Electron-Transfer Properties of Phenyleneethynylene Linkers Bound to Gold via a Self-Assembled Monolayer of Molecular Tripod.

The three-point adsorption of tripod-shaped molecules enables the formation of robust self-assembled monolayers (SAMs) on solid surfaces, where the component molecules are fixed in a strictly upright orientation. In the present study, SAMs of a rigid molecular tripod consisting of an adamantane core and three CH2SH groups were employed to arrange ferrocene on a gold surface through oligo(p-phenyleneethynylene) linkers. Cyclic voltammetry of the monolayers demonstrated high surface coverage of ferrocene, yet the molecular interaction among adjacent ferrocene units was negligible. This was because of the extended intermolecular distance caused by the bulky tripod framework. The rates of electron transfer from the ferrocene to the gold surface through different linker lengths were determined by electrochemical measurements, from which the decay factor for oligo(p-phenyleneethynylene) wire was evaluated.

Diphenylcarbene Protected by Four ortho-Iodine Groups: An Unusually Persistent Triplet Carbene.

Diphenyldiazomethane with four iodine groups at the ortho positions and two tert-butyl groups at the para positions, i.e., bis(4-tert-butyl-2,6-diiodophenyl)diazomethane (1a-N2), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N2 in solution effectively generated the corresponding triplet diphenylcarbene ³1a, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at room temperature. The UV-vis spectrum of ³1a was obtained by irradiating 1a-N2 in a 2-methyltetrahydrofuran matrix at 77 K. The ESR spectrum showed no triplet carbene signals, while a radical species was observed at the anticipated temperature of the decomposition of triplet carbene ³1a. Transient absorption bands ascribable to ³1a were observed by laser flash photolysis of 1a-N2 in a degassed benzene solution and decayed very slowly with a second-order rate constant (2k/εl) of 5.5 × 10-³·s-¹. Steady-state irradiation of 1a-N2 in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a 31% yield. Triplet carbene ³1a was also trapped by either oxygen (kO2 = 6.5 × 105 M-¹·s-¹) or 1,4-cyclohexadiene (kCHD = 1.5 M-¹·s-¹) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H2. The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two ortho-iodo and two ortho-bromo groups, ³1b.

Electrochemistry of the self-assembled monolayers of dyads consisting of tripod-shaped trithiol and bithiophene on gold.

Self-assembled monolayers (SAMs) of tripod-shaped trithiols, consisting of an adamantane core with three CH2SH legs and a bithiophene group, were prepared on a Au(111) surface. Adsorption in a tripod-like fashion was supported by polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) of the SAMs, which indicated the absence of free SH groups. Cyclic voltammetry showed an irreversible cathodic wave due to reductive desorption. The SAM also showed an anodic wave due to the single-electron oxidation of the bithiophene moiety without concomitant desorption of the molecules. Although oxidation was irreversible in the absence of a protecting group, it became reversible with the introduction of a terminal phenyl group. The charge of the oxidation was one-third that of the reductive desorption, confirming a three-point adsorption. The surface coverage was ca. 50% of that expected for the anti bithiophene conformation, which suggested that an increase in the surface area per molecule had been caused by the presence of an energetically high-lying syn conformer. In accordance with this, the line shape of the oxidation wave suggested an electrostatic repulsive interaction between neighboring molecules.

Tailoring electronic states of a single molecule using adamantane-based molecular tripods.

Adsorption structures and electronic states of molecular tripods, having a Br atom (BATT) and a ferrocene derivative (Ferrocene-ATT) at the head part of the adamantane-based trithiolate, adsorbed on Au(111) have been investigated using low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that BATT and Ferrocene-ATT form self-assembled monolayers (SAMs), and their orderings are identical to one another, which suggests that the adsorption structure of adamantane-based molecular tripods is independent of the type of functional substituent attached to the head part. The electronic states originated from the ferrocene group were confirmed in the STS spectrum of Ferrocene-ATT whereas those are absent in the BATT spectrum. We note that the ferrocene part has few interactions with the Au substrate owing not only to the upright geometry of Ferrocene-ATT but also to the insulative properties of the adamantane base. The STS mapping revealed the spatial distribution of the electronic state of Ferrocene-ATT.

Ideal redox behavior of the high-density self-assembled monolayer of a molecular tripod on a Au(111) surface with a terminal ferrocene group.

A dyad consisting of a tripod-shaped trithiol with an adamantane core and a terminal ferrocenyl group linked through ap-phenyleneethynylene bridge was synthesized. The trithiol formed a stable self-assembled monolayer (SAM) on Au(111), wherein each molecule is bound to the surface by three-point adsorption using all sulfur atoms, with confirmation by PM-IRRAS and XPS analyses. Cyclic voltammetry of the SAM showed a line shape typical of an ideal adsorbed system, that is, a monolayer with negligible electrostatic interaction among the terminal ferrocenyl groups. Thus, a rare SAM was achieved, in which the component molecules were isolated from adjacent molecules without the coadsorption of nonelectroactive molecules.

Allyl radical nature of a phenylcyclopentadienyl radical annelated with two homoadamantene frameworks.

1-Phenylcyclopentadiene fused with two homoadamantene units at the 2,3- and 4,5-positions (4) was deprotonated with KH to lead to the quantitative formation of the corresponding cyclopentadienyl (Cp) anion 3(-). This anion was oxidized by AgCl to afford an orange crystalline solid consisting of Cp radical 3(•) and cyclopentadiene 4. The ESR spectrum in hexane exhibited approximately 15 lines, demonstrating that the two homoadamantene frameworks were equivalent and that the C1-C2(Ph)-C3 moiety of the five-membered ring formed a symmetrical allyl-like radical in agreement with the prediction by DFT calculation. The reaction of the Cp radical 3(•) with an oxygen molecule in the presence of Ag(+)SbF(6)(-) afforded the SbF(6)(-) salt of a phenylpyrylium ion annelated with two homoadamantene frameworks (8(+)SbF(6)(-)).

Generation and reaction of a phenyl-substituted cyclopentadienyl cation annelated with two homoadamantene frameworks.

The generation of the title cation 2(+) and its reaction under solvolytic and non-nucleophilic conditions were investigated. When the precursor chlorocyclopentadiene 5 was reacted with silica gel that contained water or with anhydrous MeOH, the corresponding 5-hydroxy- and 5-methoxycyclopentadienes (7 and 6) were produced in 68 and 81% yields, respectively. This indicates that 2(+) is formed as an intermediate under solvolytic conditions and persists without any rearrangement of the homoadamantane frameworks, at least during the period before capture by the nucleophile. On the other hand, the abstraction of a chloride ion from 5 by Ag(+) in the absence of a nucleophile at -78 degrees C resulted in the quantitative formation of allyl cation 8(+), incorporated in a bicyclo[3.1.0]hexane framework, via the Wagner-Meerwein rearrangement of a homoadamantane framework. Cation 8(+) was isolated as the SbF6(-) salt, and its structure was determined by X-ray crystallography. Quenching this cation with MeOH afforded a methyl ether 14, with a cyclopentadiene structure retained but one of the homoadamantane frameworks had undergone a structural change by a further Wagner-Meerwein rearrangement.

NMR studies on monofunctionalized fullerenyl cation and anion encapsulating a H2 molecule.

(1)H NMR chemical shifts of molecular hydrogen encapsulated in dichloromethyl-C(60) cation and (1-octynyl)-C(60) anion were studied to clarify the difference in magnetic shielding effects inside the fullerene cages. The signals of the H(2) molecule inside both cationic and anionic C(60) cages appeared at lower fields than those of the neutral counterparts. These results were interpreted based on the results of NICS calculations. It was demonstrated that the H(2) molecule inside the C(60) cages can serve as an excellent NMR probe of aromaticity for both cationic and anionic C(60) derivatives.

3-(Dimethylboryl)pyridine: synthesis, structure, and remarkable steric effects in scrambling reactions.

A facile method for the synthesis of 3-(dimethylboryl)pyridine (1a) is described. Compound 1a assembles into a rigid cyclic tetramer stabilized via intermolecular boron-nitrogen coordination bonds both in the crystalline state and in solution. The outstanding structural feature of 1a, as compared with previously reported 3-(diethylboryl)pyridine (2a) (which adopts a cone conformation), is that the tetramer of 1a adopts a 1,2-alternate conformation. To investigate the effect of substituents at the boron atom on the stabilities of the oligomers, scrambling experiments of the component molecules using 1, 2, and 3-(di-n-butylboryl)pyridines 3 were carried out. Although heating at 80-90 degrees C for 20 h was required to attain the equilibrium of the scrambling reactions when the component molecules of the tetramers were 2 or 3, the scrambling in 1 proceeded under relatively mild conditions (60 degrees C, 3 h). This difference in reaction conditions required for 1, as compared to conditions required for 2 or 3, could not be explained solely by the stabilities based on bond lengths or THC. It appears that whereas only an S(N)1-type pathway may be involved in the scrambling of 2 or 3, both S(N)1- and S(N)2-type mechanisms operate simultaneously during scrambling reactions of 1 or an intermediate mechanism between S(N)1 and S(N)2 operates, which was supported by kinetic studies and calculations using model compounds.

Hierarchical chiral framework based on a rigid adamantane tripod on Au(111).

We have investigated the tripod-shaped bromo adamantane trithiol (BATT) molecule on Au(111) using scanning tunneling microscopy (STM) at 4.7 K. Adsorption of BATT leads to formation of highly ordered self-assembled monolayers (SAMs) with three-point contacts on Au(111). The structure of these SAMs has been found to have a two-tiered hierarchical chiral organization. The self-assembly of achiral monomers produces chiral trimers, which then act as the building blocks for chiral hexagonal supermolecules. SAMs begin to form from the racemic mixture of assembled molecules in ribbon-shaped islands, followed by the transformation to enantiomeric domains when SAM layers develop two-dimensionally across hcp domains. Such a chiral phase transition at the two-dimensional domain can arise from a subtle balance between molecule-substrate and intermolecular interactions. Two structural factors, the S atom (stabilization) and the methylene groups (chirality) located just above the S atom, induce the chiral ordering of BATT on Au(111).

Rigid molecular tripod with an adamantane framework and thiol legs. Synthesis and observation of an ordered monolayer on Au111.

Tripod-shaped trithiols 1-3, containing CH2SH groups at the three bridgehead positions of the adamantane framework and a halogen-containing group [Br (1), p-BrC6H4 (2), or p-IC6H4 (3)] at the fourth bridgehead, were synthesized, and self-assembled monolayers (SAMs) were prepared on atomically flat Au111 surfaces. The three-point chemisorption of these tripods was confirmed by polarization modulation infrared reflection absorption spectroscopy, which showed the absence of a S-H stretching band. Scanning tunneling microscopy of the SAM of 1 exhibited a hexagonal arrangement of the adsorbed molecule with a lattice constant of 8.7 angstroms. A unidirectionally oriented, head-to-tail array of 1, which allows the close approach of neighboring molecules, is proposed as a reasonable model of the two-dimensional crystal, where the adsorbed sulfur atoms form a quasi-(radical3 x radical3)R30 degrees lattice. The charge of the electrochemical reductive desorption of the SAM of 1 was in good agreement with the expected surface coverage, while the SAMs of 2 and 3 showed somewhat less (ca. 70%) charge. The large negative reduction peak potentials, observed for the SAM of 1, are taken to indicate a tight anchoring of this tripod by three sulfur atoms.

Study of photoinduced electron transfer between [60]fullerene and proton-sponge by laser flash photolysis: addition effects of organic acid.

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.

MALDI-Fourier transform mass spectrometric and theoretical studies of donor-acceptor and donor-bridge-acceptor fullerenes.

Fourier-transform ion cyclotron resonance (FTICR) mass spectrometric studies have been performed on donor-acceptor and donor-bridge acceptor fullerene-based systems. Matrix-assisted laser desorption/ionization (MALDI) was used for ion production; both the positive and negative ion modes were utilized. In addition, collision-induced dissociation (CID) experiments were carried out to study the movement of the charge (electron or hole) upon fragmentation. The experiments are complemented by ab initio theoretical calculations yielding both molecular orbital energies and electron density distributions. It was found that the theoretical electron density map predicted the experimentally observed fragmentation correctly in every case. Both the calculations and the MS experiments may be useful in studying these and related donor-acceptor systems in view of their use for charge separation and eventually, solar energy production.

Spin-localized cyclopentadienyl radical annelated with homoadamantene frameworks: isolation, X-ray crystal structure, and ESR characterization.

A cyclopentadienyl radical annelated with two homoadamantene frameworks 1 was synthesized and isolated in stable, crystalline form by the single-electron oxidation of the corresponding cyclopentadienyl anion. The X-ray structure clearly demonstrated distinct bond alternation in the cyclopentadienyl ring, suggesting that radical 1 has the characteristics of a spin-localized 2,4-cyclopentadien-1-yl radical. The two homoadamantene frameworks are nonequivalent in crystals at 100 K, while ESR spectra indicated that they are equivalent in toluene at room temperature due to rapid changes in conformation.

Generation of fullerenyl cation (EtO)2P+(OH)CH2-C60+ from RC60-H and from RC60-C60R (R = CH2P(O)(OEt)2).

A novel fullerenyl cation (EtO)2P+(OH)CH2-C60+ was generated by simply dissolving the monofunctionalized hydrofullerene RC60-H or singly bonded dimer RC60-C60R (R = CH2P(O)(OEt)2) in oxidizing acids such as H2SO4 and FSO3H. The cation was also formed in CH2Cl2 by one-electron oxidation with aminium radical cation and was used for further functionalization of C60.

Electron-transfer-induced substitution of alkylated C60 chlorides with proton sponge.

A series of alkylated C(60) chlorides 1,4-RC(60)Cl (1) were found to undergo nucleophilic substitution with 1,8-bis(dimethylamino)naphthalene (2), affording 1,4-RC(60)Ar [3, Ar = 4,5-bis(dimethylamino)-1-naphthyl] in good yields. An S(RN)1 mechanism, initiated by a single-electron transfer from 2 to 1, is proposed on the basis of the enhanced rates compared with the rate of the S(N)1 reaction of 1 with anisole. The involvement of free radicals in the reaction is supported by the formation of a small amount of dimer RC(60)-C(60)R (4) as a byproduct. The enhanced ability of C(60) chlorides 1 to accept an electron, attributable to the inductive effect of the directly attached chlorine atom, was demonstrated by its reduction potential and calculated LUMO energy.

Synthesis and properties of novel oligothiophenes surrounded by bicyclo[2.2.2]octene frameworks.

Novel oligothiophenes surrounded by bicyclo[2.2.2]octene (abbreviated as BCO) frameworks ranging from dimer to hexamer, 1(nT) (n = 2, 3, 4, 6), were prepared, and their structures and electronic properties were investigated. Dimer 1(2T) was synthesized by oxidative coupling of the 2-lithiated monomer generated from 4,5-BCO-annelated 2-bromothiophene 8 with CuCl2 in 76% yield. Trimer 1(3T) and tetramer 1(4T) were synthesized by Stille coupling of 2,5-dibromo-3,4-BCO-annelated thiophene 4 and of the 5,5'-dibromo derivative of bis(3,4-BCO-thiophen-2-yl) 10 with 2-stannylated 4,5-BCO-annelated thiophene 9 in 41% and 46% yield, respectively. Hexamer 1(6T) was synthesized by oxidative coupling of terthiophene 12, tris-annelated with BCO units, in 81% yield. X-ray crystallographic studies showed that the thiophene rings in 1(2T) and 1(3T) are rotated around the inter-ring C-C bond(s) with the C=C-C=C dihedral angles of -174.3(5) degrees for 1(2T) and -149.7(3) degrees and 34.4(3) degrees for 1(3T). In the crystal structures of 1(2T) and 1(3T), no pi-stacking was observed as expected from the steric effect of the BCO units. Theoretical calculations for 1(2T) and 1(3T) at the B3LYP/6-31G(d) level indicated that the annelation with BCO units either at the 2,3- or 3,4-positions of thiophene rings raises both the KS HOMO and LUMO levels. In the electronic absorption spectra of 1, the longest wavelength absorption band corresponding to the pi-pi transition is bathochromically shifted with the increase in absorption intensity as the number of thiophene rings increases, and the absorption of the polythiophene 1 with infinite length was predicted to be 419 nm. The cyclic voltammetry of 1 in CH2Cl2 at -78 degrees C (2T) or at room temperature (3T, 4T, 6T) showed two reversible oxidation waves, indicating that the radical cation and dication of 1 are stable under these conditions.