Regulating pi-conjugated pathways using a photochromic 1,2-dithienylcyclopentene

Linear pi-conjugation is reversibly re-routed by irradiation of a photochromic bis(terthiophene).     

Electrochemically induced formation of organomercury compounds

A procedure for the alkylation of metallic mercury by (dimethylglyoximato)butylcobalt(III) electrochemically generated in the presence of butyl bromide is proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 309–312, February, 1994.     

Synthesis of photochromic 1,2-dihetarylethene using regioselective acylation of thienopyrroles

[formula: see text] The influence of catalysts, acid chlorides, and solvents in the acylation of methyl 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylate was studied. Conditions for the regioselective acylation processes were found. Thienopyrrole-based photochrome was synthesized for the first time.     

New photochromic 1,2-diarylperfluorocyclopentenes bearing unsymmetrical six-membered aryl units

A new class of unsymmetrical photochromic diarylethenes based on the six-membered naphthalene-benzene and naphthalene-pyridine backbones has been firstly developed and their properties have been discussed. The two different six-membered aryl moieties were connected directly to the central cyclopentene ring and available to participate in the photoinduced cyclization reaction. The three diarylethenes exhibit distinctly different photochromism by photoirradiation in solution: they turn red, yellow, and orange upon photocyclization, which may be resulted from the different substituent and six-membered aryl moiety effects. Compared with the analog bearing a benzene unit, diarylethene bearing a pyridine moiety has a shorter absorption maximum and a smaller fluorescent quantum yield, and it shows a dual-addressable photo-switch by photoirradiation and acid/base stimulation.     

Electrochemically induced growth of zinc oxide.

The electrolytically induced precipitation of zinc oxide from zinc nitrate solution on gold surfaces in the presence of water-soluble polymers was examined for reaction times between 0.5 and 600 seconds. Regardless of the additive, polycrystalline films of zinc oxide have formed after 30 seconds, but polymeric additives dramatically change the morphology of the ZnO films. Amperometric analysis and fitting the diffusion reduced the current density-time curve according to Avrami kinetics and it reveals that polymers bearing methacrylic acid groups result in spherical growth whereas such with sulfonic acid groups lead to a platelike growth of crystallites. Without additive prisms grow predominantly in one dimension. These findings are confirmed also by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis.     

Electrochemically induced electron transfer through molecular bridges

In this Opinion, we address some of the most important results obtained electrochemically in the area of intramolecular electron transfer (ET). The focus is on freely diffusing molecular systems in which a donor D and an acceptor A are separated by a well-defined bridge B (D-B-A systems). B can be a saturated spacer, a delocalized bridge, or the more complex peptide backbones. As to the acceptors, the selected examples encompass species that can be charged reversibly but a special emphasis is on ETs associated with the concerted cleavage of a sigma bond (dissociative ETs). Our goal is to showcase the essential background, the most appropriate electrochemical tools and methodologies, and a series of selected examples where molecular electrochemistry has provided invaluable information on the mechanisms of intramolecular ET and electronic communication through bridges.     

Electrochemically induced motion in copper complexes of tetraferrocene-cyclam

The tetra(ferrocene)-cyclam macrocycle 1,4,8,11-tetra(ferrocenylmethyl)-1,4,8,11-tetraazacyclotetradecane (L) and two corresponding Cu^I and Cu^II complexes have been prepared and characterized from spectroscopic and electrochemical studies (cyclic voltammetry and rotating disk electrode experiments). Both complexes exist under two energetically distinct geometries which differ mainly from the relative positionning of the ferrocenylmethyl groups above or below the macrocyclic plane and it is shown that a reversible change from one geometry to the other can be electrochemically induced following a copper-centered electron transfer. The equilibrium constants, mechanisms and kinetic parameters of these rearrangements have been evaluated using electrochemical simulation. Presented at the 3rd Chianti Electrochemistry Meeting, July 3.–9., 2004, Certosa di Pontignano, Italy     

Electrochemically induced Favorsky rearrangement of alkyl benzyl ketones

Electrolysis of alkyl benzyl ketones in MeOH in an undivided electrolyzer in the presence of the NaI—NaOH mediator system induces the process similar to the Favorsky rearrangement to produce arylalkanecarboxylates in 80—90% yield (per substance) and with the 50—55% current efficiency.     

Electrochemically induced sol-gel deposition of zirconia thin films

A novel electrochemical method for deposition of ZrO(2) thin films is described. The films, 50-600 nm thick, were obtained by applying moderate positive or negative potentials (+2.5 V to -1.5 V versus SHE) on conducting surfaces immersed in a 2-propanol solution of zirconium tetra-n-propoxide [Zr(OPr)(4)] in the presence of minute quantities of water (water/monomer molar ratios in the range of 10(-5) to 10(-1)), which was the limiting reagent. Oxidative electrochemical formation of solvated H(+) and reductive formation of OH(-) catalyze the hydrolysis and condensation of the metal alkoxide precursor. The magnitude of the applied potential and its duration provide a convenient way of controlling the film thickness. The films consist of an amorphous phase, as revealed by XRD measurements. The effects of different parameters, such as the applied potential and its duration, the amount of added water and the current-time characteristics, were studied. A mechanism for the electrodeposition of the zirconia films which is in accordance with our findings is proposed.     

Electrochemically induced chemomechanical bending of bilayered ion-exchange membranes

A bilayered ion-exchange membrane consisting of poly(styrene-co-4-vinylpyridinium ion) (anion-exchange membrane) and a gel-like mixture of poly(vinyl alcohol) and poly(acrylic acid) (cation-exchange membrane) was prepared. The bilayered membrane strip, one end of which is fixed, is placed between two carbon electrodes in a cell which contains KCl solution of 0.01 mol kg⁻¹ molality. If the electric field is applied from the cation-exchange layer side to the anion-exchange layer side, the cation-exchange layer becomes the K⁻ form and is elongated. On the contrary, if the electric field is reversed, dissociation of water into H⁺ and OH⁻ occurs at the interface between the two layers. Consequently, the carboxyl group becomes the acid form, resulting in contraction of the layer. The anion-exchange layer, which contains a strong base group, becomes either the Cl form or the OH form with changing the electric field, but there is no detectable change in volume. Thus, if the electric field applied to the membrane is periodically reversed, periodical bending of the bilayered membrane is observed.     

Electrochemically induced Henry reaction of nitromethane and carbonyl compounds

The electrolysis of carbonyl compounds and nitromethane in methanol or in the mixture of methanol and DMF in an undivided cell results in the formation of corresponding β-nitroalcohols in 60-75% yields. Thus, the simple electrocatalytic system can produce under mild conditions an electrochemically induced Henry reaction.     

Electrochemically driven stereoselective approach to syn-1,2-diol derivatives from vinylarenes and DMF

We have developed an electrochemically driven strategy for the stereoselective synthesis of protected syn-1,2-diols from vinylarenes with N,N-dimethylformamide (DMF). The newly developed system obviates the need for transition metal catalysts or external oxidizing agents, thus providing an operationally simple and efficient route to an array of protected syn-1,2-diols in a single step. This reaction proceeds via an electrooxidation of olefin, followed by a nucleophilic attack of DMF. Subsequent oxidation and nucleophilic capture of the generated carbocation with a trifluoroacetate ion is proposed, which gives rise predominantly to a syn-diastereoselectivity upon the second nucleophilic attack of DMF.

An electrochemical method that provides an operationally simple synthesis of masked syn-1,2-diols from styrenes and DMF has reported. The TFA ion is engaged in the formation of a key intermediate, which gives rise predominantly to syn-selectivity.     

Laser induced photochromic mechanism of 3-phenyl-3-[1,2-dimethylindol-3-YL]-3H-naphtho[2,1-B]pyran

The photochromic process of 3-phenyl-3-[1,2-dimethylindol-3-yl]-3H-naphtho[2,1-b]pyran [I] has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. Both excited singlet state and triplet state are involved in the photocoloration process. The decay kinetics of photoproducts are also studied. The maximum absorption wavelength and lifetime of the transient species are solvent dependent.     

Studies on the photochromic mechanism of 1,2-dimethyl-5-phenyl-3-pyrryl succinic anhydride

Photochromism of 1,2-dimethyl-5-phenyl-3-pyrryl fulgide (compound I) has been investigated in various solvents. The photocyclization process of I has been examined by means of nanosecond time resolved absorption spectroscopy; a new transient species was found on the nanosecond time scale. Oxygen can severely quench the new transient species. The photocyclization process is shown to be complicated. Both the excited singlet state and the excited triplet state are involved in the photochromic process.     

Electrochemically induced chemical reactions: Kinetics of competition with electron transfer

Acid—base or electrophile—nucleophile chemical reactions can be induced by electrochemical means in the case where the reaction becomes faster at the +1e (or ?1e) level than at the starting level. This is typically the case for SRN1 aromatic nucleophilic substitution. When bond cleavage occurs at the +1e (or ?1e) level, a competing route may be opened by the electroactivity of the cleaved species being higher than that of the substrate. Electron transfer at the electrode (ECE) or in the solution (DISP) thus appears as a possible side-reaction decreasing the efficiency of the electrochemical inducement. The kinetics of this competition is investigated in the context of cyclic voltammetry. The kinetic characteristics are shown to be dramatically different in the ECE and the DISP cases providing an example of the operational significance of the distinction between these two modes of electron transfer. Diagnostic criteria and procedures for rate constant determination are discussed. An experimental illustration of the role of the various operational parameters substrate and nucleophile concentrations, sweep rate, is given. It involves the electrochemically catalyzed aromatic nucleophilic substitution of 2-chloroquinolin by benzenethiolate in liquid ammonia.