Reversible light-controlled conductance switching of azobenzene-based metal/polymer nanocomposites

We present a new concept of light-controlled conductance switching based on metal/polymer nanocomposites with dissolved chromophores that do not have intrinsic current switching ability. Photoswitchable metal/PMMA nanocomposites were prepared by physical vapor deposition of Au and Pt clusters, respectively, onto spin-coated thin poly(methylmethacrylate) films doped with azo-dye molecules. High dye concentrations were achieved by functionalizing the azo groups with tails and branches, thus enhancing solubility. The composites show completely reversible optical switching of the absorption bands upon alternating irradiation with UV and blue light. We also demonstrate reversible light-controlled conductance switching. This is attributed to changes in the metal cluster separation upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress.

Photoswitching behavior of azobenzene-containing alkanethiol self-assembled monolayers on Au surfaces

The photoisomerization of self-assembled monolayers of azobenzene-containing alkanethiols, as well as of mixed monolayers of these substances with n-alkanethiol spacer molecules on Au surfaces, was studied by photoelectrochemical measurements and surface plasmon resonance spectroscopy. A strong dependence on the molecular structure of the adsorbates was found, specifically on the linker between the azobenzene moiety and the alkanethiol: while molecules with an amide group were photoinactive, those with an ether group exhibited pronounced, reversible photoisomerization in pure and mixed adlayers. Both trans-cis and cis-trans isomerization followed first-order kinetics with time constants that suggest high quantum efficiencies for these processes.

Novel loop-like aromatic compounds: a further step on the road to nanobelts and nanotubes

The synthesis and crystal structural characterization of new compounds 2–6 were accomplished. As a common synthetic methodology, the Diels–Alder reaction was applied to 9,9′,10,10′-tetradehydrodianthracene (TDDA) (7) to furnish the [12]annulenes 2 and 3 [16]annulene 6 and adduct 5.

Highly efficient reversible Z-E photoisomerization of a bridged azobenzene with visible light through resolved S(1)(n pi*) absorption bands

The reversible Z-E photoswitching properties of the (Z) and (E) isomers of the severely constrained bridged azobenzene derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) were investigated quantitatively by UV/vis absorption spectroscopy in solution in n-hexane. In contrast to normal azobenzene (AB), 1 has well separated S(1)(n pi*) absorption bands, peaking at lambda(Z) = 404 nm and lambda(E) = 490 nm. Using light at lambda = 385 nm, it was found that 1Z can be switched to 1E with very high efficiency, Gamma = 92 +/- 3%. Conversely, 1E can be switched back to 1Z using light at lambda = 520 nm with approximately 100% yield. The measured quantum yields are Phi(Z-->E) = 72 +/- 4% and Phi(E-->Z) = 50 +/- 10%. The thermal lifetime of the (E) isomer is 4.5 +/- 0.1 h at 28.5 degrees C. The observed photochromic and photoswitching properties of 1 are much more favorable than those for normal AB, making our title compound a promising candidate for interesting applications as a molecular photoswitch especially at low temperatures. The severe constraints by the ethylenic bridge apparently do not hinder but favor the Z-E photoisomerization reactions.

Synthesis of alpha-ketoester- and alpha-hydroxyester-substituted isoindazoles via the thermodynamic coarctate cyclization of ester-terminated azo-ene-yne systems

The synthesis of isoindazoles bearing alpha-ketoester and alpha-hydroxyester groups via the coarctate cyclization of ester-terminated azo-ene-yne precursors is described. Whereas previous studies on isoindazole formation have shown the reaction to proceed through a kinetic coarctate pathway, functionalization of the terminal acetylene with a methyl ester sufficiently stabilizes the carbene intermediate to make the coarctate cyclization the thermodynamic pathway. Density functional theory (DFT) computations reveal ca. 8-9 kcal mol(-1) lower energy transition states for the coarctate pathway compared to the parent system.

Chiral crystallization: freezing a rapid Cope rearrangement in the solid state

Homotropylidene and its derivatives undergo a fast Cope rearrangement. A homotropylidene system (cyclooctatetraene dimer) is reported that racemizes rapidly by such a valence isomerization in solution at room temperature. In the solid state, however, this rearrangement is frozen, and the single crystals precipitating from the racemic solution contain only one enantiomer. Since very large crystals can be grown, the racemic solution in principle can be converted into a single enantiomer by crystallization.

Mounting freestanding molecular functions onto surfaces: the platform approach

A modular system has been developed to mount molecules upright onto metal surfaces in a well controlled geometry. The approach is based on a reactive platform (triazatriangulenium salt) with an electrophilic center. Functional molecules are attached via C-C bond formation. The distance from the surface can be varied by a spacer, and the distance of the functional units from each other by the size of the platform. Self-assembly of the parent triazaangulenium salt as well as the functionalized platforms on Au(111) surfaces results in stable, hexagonally ordered adlayers.

Thioamidation of Single-Walled Carbon Nanotubes: a New Chemical Functionalization Protocol by the Willgerodt - Kindler Reaction

A new and convenient method for covalent attachment of thioamide groups to the side-walls of carbon nanotubes (SWNT) via the Willgerodt–Kindler (WK) reaction is presented. Treating SWNTs with 4-acetylaniline and subsequently with morpholine as a secondary amine and sulfur leads to the thioamidation of SWNTs. This approach paves the way for a new chemical functionalization protocol to enhance the potential of SWNTs for further chemical reactions applicable in various fields.

Tetradehydroadamantane-1,3,5,7-di- and tetracations and their helium and hydride inclusion complexes: spherical aromaticity and evidence for a bonding interaction between carbon and helium

The unusual stability of the experimentally known 1,3-dehydro-5,7-adamantyl dication was previously explained by four-center two-electron aromaticity with three-dimensional (tetrahedral) topology. Magnetic criteria (ACID and ring-current analysis) now demonstrate that there is also a very strong contribution from hyperconjugation with all six methylene bridges. The delocalized system of electrons thus includes all valence electrons, and the structure, therefore, should rather be described as a spherically aromatic 50-electron system. The corresponding Td-symmetric tetracation with 48 electrons is antiaromatic and not a minimum structure. With a He atom or a hydride ion at the center of the cage, the tetracation is predicted to form a kinetically stable complex. Magnetic criteria demonstrate that the antiaromaticity is greatly reduced, and a bond analysis hints at bonding interactions between He (and H-) and the carbon atoms of the adamantane cage.

Homobenzene: homoaromaticity and homoantiaromaticity in cycloheptatrienes

Cycloheptatriene (C(s)) is firmly established to be a neutral homoaromatic molecule based on detailed analyses of geometric, energetic, and magnetic criteria. Substituents at the 7 (methylene) position, ranging from the electropositive BH2 to the electronegative F, favor the equatorial conformation but influence the aromaticity only to a small extent. By the same criteria, the planar transition state (C(2v)) for cycloheptatriene ring inversion is clearly antiaromatic. This is attributed to the involvement of the pseudo-2pi-electrons of the CH2 group with the 6pi-electrons of the ring to give an 8pi-electron system. Similarly, the participation of the CH2 groups into C(2v) cyclopentadiene and cyclononatetraene lead to significant 4n + 2 pi electron aromaticity. The cyclization of cycloheptatriene to norcaradiene proceeds via a highly aromatic transition structure, but norcaradiene itself is less aromatic than cycloheptatriene. An annelated cyclopropane ring does not function as effectively as a double bond in promoting cyclic electron delocalization.

Identification of flavonoids and hydroxycinnamic acids in pak choi varieties (Brassica campestris L. ssp. chinensis var. communis) by HPLC-ESI-MSn and NMR and their quantification by HPLC-DAD

Twenty-eight polyphenols (11 flavonoid derivatives and 17 hydroxycinnamic acid derivatives) were detected in different cultivars of the Chinese cabbage pak choi ( Brassica campestris L. ssp. chinensis var. communis) by HPLC-DAD-ESI-MS(n). Kaempferol was found to be the major flavonoid in pak choi, glycosylated and acylated with different compounds. Smaller amounts of isorhamnetin were also detected. A structural determination was carried out by (1)H and (13)C NMR spectroscopy for the main compound, kaempferol-3-O-hydroxyferuloylsophoroside-7-O-glucoside, for the first time. Hydroxycinnamic acid derivatives were identified as different esters of quinic acid, glycosides, and malic acid. The latter ones are described for the first time in cabbages. The content of polyphenols was determined in 11 cultivars of pak choi, with higher concentrations present in the leaf blade than in the leaf stem. Hydroxycinnamic acid esters, particularly malic acid derivatives, are present in both the leaf blade and leaf stem, whereas flavonoid levels were determined only in the leaf blade.

CuCl-Induced Formation and Migration of Isoindazolyl Carbenoids

Two azo-ene-butadiyne conjugated systems undergo CuCl-mediated cyclization to afford isoindazolyl carbenoids that could be trapped with 2,3-dimethyl-2-butene as [2+1] cycloaddition products. X-ray structure analysis of the resultant cyclopropanes showed that formal migration to the distal carbenoid isomer and subsequent trapping had occurred. The possible CuCl-induced cyclization/migration pathways were explored using density functional theory, which indicated that the reaction most likely occurred via coordination of CuCl to the distal alkyne bond.

Experimental and Theoretical Characterization of the Valence Isomerization of Bi-2H-azirin-2-yls to Diazabenzenes

3,4-diazidocyclobutenes 16 were prepared from the corresponding dihalides. Some of these diazides, such as parent compound 16 d and phenyl-substituted derivatives 16 c,f, underwent spontaneous stereoselective electrocyclic ring opening below room temperature, whereas the tetraalkyl derivatives of 16 had to be heated to force the same reaction. In most cases, the resulting 1,4-diazidobuta-1,3-dienes 8 were isolated to study their photochemical transformation into bi-2H-azirin-2-yls 9 via intermediate mono-azirines 17. Except for starting materials with a low number of substituents such as 9 d and 9 f, title compounds 9 underwent a thermal valence isomerization which led exclusively to pyridazines 18 at surprisingly low temperatures. Based on quantum-chemical calculations for the parent bi-2H-azirinyl-2-yl 9 d at the UB3LYP/6-31+G(d) and MR-MP2/TZV(2df,2p) levels, the valence isomerization process is best explained by simultaneous homolytic cleavage of both C--N single bonds of 9 to generate energetically favorable N,N' diradicals 26, which cyclize to 18. The theoretical studies indicate also that one stereoisomer of 9, namely, the rac compound, should undergo valence isomerization more easily than the other, which is in conformity with different rates of these rearrangement reactions found experimentally. For the tetramethyl-bi-2H-azirin-2-yls 9 g, which are better models for the experimentally studied compounds, simultaneous homolytic cleavage of both C--N single bonds is also predicted by the calculations, although the intermediate diradicals 26 g are significantly higher in energy than those of the parent system 9 d.

Phane Properties of [2.2]Paracyclophane/Dehydrobenzoannulene Hybrids

A series of [2.2]paracyclophane/dehydrobenzo[14]annulene (PC/DBA) hybrids (hydrocarbons 5, 6, 9, 10 b, and 10 c), [2.2]paracyclophane/dehydro[14]annulene (PC/DA) hybrids (7 and 8) and suitable model systems (11, 12, and 33) has been synthesized. Comparison of the electronic absorption spectra in each series of compounds provides further insight into the global communication between the decks in the [2.2]paracyclophane unit.

Synthesis and Properties of the First Möbius Annulenes

Heilbronner in 1964 predicted that annulenes with ".. a planar perimeter of N=4r AO's, which would yield an open shell configuration when occupied by 4r electrons, can be twisted into a closed shell Möbius strip perimeter without loss in pi electron energy". We have been able to synthesize the first [4n]annulene with such a Möbius topology and now present further Möbius isomers and the details of their preparation as stable compounds. To address the question whether the twist in the pi system has an effect on the properties we systematically investigate energy, geometry and magnetic parameters of a large number isomers of [16]annulenes. The Möbius twisted annulenes are consistently more aromatic than the non-twisted isomers. This is true for the parent as well as our benzoannelated systems. Our results are in contrast to those published recently by C. Castro, W. L. Karney, P. von R. Schleyer et al.

Biscyclization Reactions in Butadiyne- and Ethyne-Linked Triazenes and Diazenes: Concerted versus Stepwise Coarctate Cyclizations

A series of alkyne-linked bis-2H-indazoles has been prepared by the double cyclization of ethyne- or butadiyne-linked phenyltriazene or phenyldiazene moieties. Even though there are two five-membered ring cyclizations and several triple bond shifts involved, the reactions proceed rapidly under neutral conditions with mild heating, affording the heterocycles in excellent yields. DFT calculations, in agreement with experimental observations, indicate that the reactions: (1) occur via a very short-lived carbene intermediate, (2) are concerted via an asymmetrical transition state, or (3) are even synchronous, with as many as 16 bonds that are made or broken simultaneously. The biscyclizations presented herein strikingly illustrate the concept of coarctate reactions, the stabilization of transition states by coarctate Möbius aromaticity, the ethynologation principle, and the stereochemical rules.

The existence of secondary orbital interactions

B3LYP/6-311+G** (and MP2/6-311+G**) computations, performed for a series of Diels-Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope-TSs are favored energetically over the respective exo-TSs. Likewise, the computed magnetic properties (nucleus-independent chemical shifts and magnetic susceptibililties) of the endo- (as well as the Cope) TS's reveal their greater electron delocalization and greater aromaticity than the exo-TS's. However, Woodward and Hoffmann's original example is an exception: their endo-TS model, involving the DA reaction of a syn- with an anti-butadiene (BD), actually is disfavored energetically over the corresponding exo-TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo-TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Stein's rule of "maximum accumulation of double bonds:" both the dienophile and the diene should have syn conformations. Plots along the IRC's show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo-TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo-TSs are only slightly lower in energy than the respective exo-TSs, the magnetic properties of the endo-TS's are significantly exalted over those for the exo-TS's and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with pi-character and the diene pi MOs.