Photochemical Route Research Articles

Preface

Abstract The 19th IUPAC Conference on Physical Organic Chemistry (ICPOC-19) was held at the University of Santiago de Compostela, Santiago, Spain, 13-18 July 2008 under the local auspices of the Universities of Santiago, A Coruña, and Vigo. About 400 delegates attended ICPOC-19 from 39 countries, to participate in a scientific program comprising 11 plenary lectures, 22 invited lectures, 102 oral communications, and 224 posters. Physical organic chemistry, the study of the interrelationships between structure and reactivity in organic molecules, is a relatively young subfield of organic chemistry. At the end of the 20th century, there was a perception by some that chemists thoroughly understood organic reactivity and that there were no important problems left. This view ignores the fact that while the rigorous treatment of structure and reactivity in organic structures that is the field‚Äôs hallmark continues, physical organic chemistry has expanded to encompass other disciplines. In fact, the application of quantitative tools taken (historically) from physical chemistry to the solution of problems in mechanisms or in understanding properties has evolved to complex molecular problems, and is now being applied in studying catalysis, biochemistry, photochemistry, reactivity in the vapor phase, surface science, materials sciences, and other areas. Indeed, when considering a nice article on molecular biology, drug design, nanosystems, and catalysis, we observe that the experimental interpretation is based on a physical organic chemistry approach. This issue of Pure and Applied Chemistry contains 15 contributions corresponding to plenary and invited lectures presented at ICPOC-19: Symmetry of hydrogen bonds (C. Perrin, USA); Stabilizing reactive intermediates through site isolation (C. Copéret, France); Divalent carbon(0) compounds (G. Frenking, Germany); NMR spectroscopy and ion pairing: Measuring and understanding how ions interact (P. Pregosin, Switzerland); Photochemical routes to metal nanoparticles (J. Scaiano, Canada); Proton transfers in aromatic systems. How aromatic is the transition state? (C. Bernasconi, USA); How to predict changes in solvolysis mechanisms (H. Mayr, Germany); Kinetics and mechanism of the aminolysis of thioesters and thiocarbonates in solution (E. Castro, Chile); Understanding solvation (O. El Seoud, Brazil); Steric and electronic effects in SN2 reactions (E. Uggerud, Norway); Design of carborane molecular architectures with electronic structure computations: From endohedral and polyradical systems to multidimensional networks (J. Oliva, Spain); Mapping catalytic promiscuity in the alkaline phosphatase superfamily (F. Hollfelder, UK); DNA nucleobases properties and photoreactivity: Modeling environmental effects (L. Serrano-Andrés, Spain); Molecular organization and recognition properties of amphiphilic cyclodextrins (R. de Rossi, Argentina); Ionic liquids: Solvation ability and polarity (C. Chiappe, Italy). The conference program, as reflected both by the plenary and invited lectures as well as the oral communications, illustrates both the old and the new trends covering different research areas such as: reaction mechanisms, computational chemistry, synthetic chemistry, catalysis, gas-phase reactions, surface chemistry, molecular machines, organometallic chemistry, nanoscience, green chemistry, colloidal chemistry, supramolecular chemistry, and biochemistry. Papers presented in this issue of Pure and Applied Chemistry are representative of the different topics covered by the conference. We hope that they will serve as a stimulus for work by future generations of physical organic chemists. Luis Garcia-Rio Conference Editor

A visible light photochemical route to silver–epoxy nanocomposites by simultaneous polymerization–reduction approach

The in situ synthesis of silver–epoxy nanocomposite by visible light photoinduced processes was described. The visible light photoinitiator, camphorquinone (CQ) (2,3-bornanedione), was used to generate electron-donating radicals upon photolysis. Subsequent oxidation of these radicals to the corresponding cations in the presence of silver hexafluoroantimonate (AgSbF6) leads to the simultaneous formation of silver nanoparticles and initiation of cationic polymerization of epoxy resin, 1,3-bis(3,4-epoxycyclohexylethyl)tetramethyldisiloxane (EPOX) present in the formulation. The cured nanocomposites were characterized by FT-IR, DSC and TEM measurements.

Microtubule‐Based Gold Nanowires and Nanowire Arrays

Biological structures are attractive as templates to form nanoscale architectures for electronics because of their dimensions and the ability to interact with inorganic materials. In this study, we report the fabrication and electrical properties of microtubule (MT)-templated Au nanowires, and methods for assembling Au nanowire arrays based on these templates. The adsorption of MTs on silicon substrates is an effective means for preserving the conformation of the MT and provides a convenient platform for electrical measurements. To improve the metallization of MTs, a photochemical route for gold reduction is adapted, which leads to continuous coverage. The conductivity values measured on micrometer-long nanowires are similar to those reported for other biotemplated gold nanowires. A protocol for fabricating arrays of MT-templated gold nanowires is demonstrated.

Iron pentacarbonyl assisted photochemical route to 2,5- and 2,6-divinyl-substituted 1,4-benzoquinones from 1-ene-3-ynes

Photochemical reaction between the enynes, ( Z)-1-methoxybut-1-ene-3-yne, 1 or isopropenyl acetylene, 2 with CO in presence of Fe(CO) 5 yields the 2,6- and 2,5-divinyl-substituted 1,4-benzoquinones: 2,6-bis{( Z)-2-methoxyvinyl}-1,4-benzoquinone ( 3, 42%), 2,5-bis{( Z)-2-methoxyvinyl}-1,4-benzoquinone ( 4, 31.5%), [{η 2,η 2:2,6-di(prop-1-en-2-yl)-1,4-benzoquinone}tricarbonyliron] ( 5, 45%), and {η 2,η 2:2,5-di(prop-1-en-2-yl)-1,4-benzoquinone}tricarbonyliron] ( 6, 65%).

Photochemical Synthesis of Electrically Conductive CdS Nanowires on DNA Scaffolds

Electrically conductive CdS nanowires are fabricated via a photochemical synthesis route using DNA as a scaffold. The nanowires are found to be several micrometers long and are either 40 – 50 nm in diameter or 70 – 80 in diameter when synthesized using solution-suspended or immobilized DNA, respectively. Various characterization methods showed that the wires consist of one continuous phase and exhibit Ohmic behavior similar to electrodes. The one-step process described herein does not perturb the conformation of the DNA chain and may be extended to the synthesis of other semiconducting composite nanoparticles.

Synthesis and evaluation of substituted benzoisoquinolinones as potent inhibitors of Chk1 kinase

From HTS lead 1, a novel benzoisoquinolinone class of ATP-competitive Chk1 inhibitors was devised and synthesized via a photochemical route. Using X-ray crystallography as a guide, potency was rapidly enhanced through the installation of a tethered basic amine designed to interact with an acidic residue (Glu91) in the enzyme pocket. Further SAR was explored at the solvent front and near to the H1 pocket and resulted in the discovery of low MW, sub-nanomolar inhibitors of Chk1.

The Photochemical Cyclodehydrogenation Route to Polycyclic Azaarenes

This paper reports an investigation of photochemical cyclodehydrogenation for the preparation of polycyclic azaarenes. Various naphthoquinolines and naphthoisoquinolines were obtained from 2-, 3-, and 4-[2-(1- and 2-naphthyl)vinyl]pyridine. 4-[2-(3-Phenanthryl)vinyl]pyridine gave pyreno[1,10- HIJ]isoquinoline. An efficient preparation of (2-arylvinyl)pyridines is described.

Determination of oxygen permeability of polymers using in-situ photo-generated heptacene

Heptacene generated in-situ in a polymer matrix using a photochemical route undergoes oxidation with diffused oxygen leading to the formation of endoperoxides. We have found the high reactivity of heptacene, a linear poly(acene), toward molecular oxygen can be used to determine oxygen permeability of polymer films. The rate of disappearance of heptacene, due to oxidation, was found to be directly related to the oxygen permeability of the polymer. A simple spectroscopic method to estimate oxygen permeability of polymer films by monitoring the visible/near infra-red absorption of heptacene is described.

Synthesis of spiropyrans: H-abstractions in 3-cycloalkenyloxybenzopyrans

A photochemical route for the synthesis of some benzopyronospiropyrans from 2-furyl-3-cycloalkenyloxybenzopyrones involving H-abstraction is reported. How a methyl group on the furyl ring affects the product formation is also investigated.

Identification of flurbiprofen and its photoproducts in methanol by gas chromatography–mass spectrometry

A sample of 10 mM flurbiprofen in methanol (or ethanol) was photoirradiated with sixteen 8 W low-pressure quartz mercury lamps irradiated at 306 nm in a Panchum PR-2000 photochemical reactor. In total, four major photoproducts derived from each sample were observed from the HPLC chromatogram. The photoproducts were separated and their structures elucidated by various spectroscopic methods. Alternatively, using GC-MS, 11 major photoproducts were observed. A reaction scheme of flurbiprofen in methanol is proposed: the photochemical reaction routes occur mainly via esterification and decarboxylation, followed by oxidation with singlet oxygen to produce a ketone, alcohols and other derivatives.

Inorganic aerosol formation and growth in the Earth's lower and upper atmosphere

This chapter describes the photo-chemical production of aerosol particles in two very different regions of the atmosphere: iodine oxide particles in the marine boundary layer (MBL), and meteoric smoke particles that form in the upper mesosphere from the ablation of interplanetary dust. These two systems are surprisingly analogous - the source of the condensable inorganic vapours is external to the atmosphere, being injected into the atmosphere from the ocean or from space - and the particles are formed by homogeneous nucleation. The purpose of the chapter is to describe a laboratory and modelling study to understand at a fundamental level how the nucleation and growth of the particles occurs. Iodine oxide particles were produced from the photo-oxidation of gaseous I 2 with O 3 , which is most likely the primary photo-chemical route to produce the bursts of new particles observed in the MBL at seaweed-rich coastal locations. The captured particles were observed to be fractal-like (i.e., with open or non-compact structures), and to be composed of the stable oxide I 2 O 5 . Meteoric smoke analogues of iron oxide, silicon oxide, and iron silicate composition were similarly formed from the photo-oxidation of iron- and silicon-containing gas-phase precursors in the presence of O 3 . Imaging of the iron-containing particles showed them to be extended, fractal aggregates. For each system, models were developed to elucidate the growth kinetics of the particles and to characterise them in terms of standard fractal parameters. I 2 O 5 particles were found to have a fractal dimension (D f ) value of 2.5 at long growth times, consistent with a particle-cluster diffusion-limited aggregation (DLA) mechanism, whereas smoke analogues had lower D f values (1.75) which appear to result from a magnetic aggregation process.

The Dynamics of Self-Trapped Beams of Incoherent White Light in a Free-Radical Photopolymerizable Medium

Detailed experimental studies of the dynamics of self-trapped beams of white light (400-800 nm) in a photosensitive organosiloxane medium are presented. Self-trapped white light beams with similar spatial profiles formed in the organosiloxane at intensities ranging across an order of magnitude (2.7-22.0 W.cm-2). Beam-profiling measurements showed that these spatially and temporally incoherent wave packets propagate without diffracting (broadening) by initiating free-radical polymerization of methacrylate groups and corresponding refractive index changes in the organosiloxane medium. Analyses of their temporal evolution showed that the intensity-dependent behavior of self-trapped white light is similar to that of self-trapped laser light despite the extreme differences in their phase structure and chromaticity; the self-trapped incoherent beams even show the complementary oscillations of width and intensity that is characteristic of self-trapped coherent light. Furthermore, the dynamics of the self-trapped white light beams was found to be strongly correlated to the kinetics of free-radical polymerization and corresponding rates of refractive index changes in the organosiloxane. These studies provide accessible photochemical routes to self-trapped incoherent wave packets, which are extremely difficult to generate in conventional nonlinear optical media that owe their responses to higher-order dielectric susceptibility tensors. This could enable the experimental verification of theoretical models developed for the nonlinear propagation of white light and stimulate research into more complex self-trapping phenomena such as the interactions of self-trapped incoherent beams and spontaneous pattern formation due to modulation instability in a uniform incoherent optical field. These findings also carry potential for the development of self-induced waveguide, optical solder and interconnect technology for incoherent light emitted by incandescent sources or LEDs.

Photochemical Route to the Synthesis of Thiolane 1-Oxides

The preparation of thiolane 1-oxide 10 by photochemically mediated rearrangement of sulfoxide 9 in high yield is described. The efficient construction of quaternary carbon centers underscores the utility of this methodology in organic synthesis. Further manipulation of photoadduct 10 leads to the formation of diverse heterocyclic structures.

Practical Synthesis of Au Nanowires via a Simple Photochemical Route

E-mail: j1song@sunchon.ack.rReceived November 16, 2005Key Words : Au Na, nowires Ph, otochemistryThe synthesis and characterization of nanosized materialsis an important issue in basic science research as well as intechnological applications, since their optical, electronic,magnetic, and catalytic properties differ greatly from thebukl maetraisl Th. e properites of nanomaterials are crtcalii lydependent not only on particle size but on particle shape aswell.

Catalytic enantioselective reactions driven by photoinduced electron transfer

Photoinduced electron transfer is an essential step in the conversion of solar energy into chemical energy in photosystems I and II (ref. 1), and is also frequently used by chemists to build complex molecules from simple precursors. During this process, light absorption generates molecules in excited electronic states that are susceptible to accepting or donating electrons. But although the excited states are straightforward to generate, their short lifetimes makes it challenging to control electron transfer and subsequent product formation-particularly if enantiopure products are desired. Control strategies developed so far use hydrogen bonding, to embed photochemical substrates in chiral environments and to render photochemical reactions enantioselective through the use of rigid chiral complexing agents. To go beyond such stoichiometric chiral information transmission, catalytic turnover is required. Here we present a catalytic photoinduced electron transfer reaction that proceeds with considerable turnover and high enantioselectivity. By using an electron accepting chiral organocatalyst that enforces a chiral environment on the substrate through hydrogen bonding, we obtain the product in significant enantiomeric excess (up to 70%) and in yields reaching 64%. This performance suggests that photochemical routes to chiral compounds may find use in general asymmetric synthesis.

A surfactant-assisted photochemical route to single crystalline HgS nanotubes

A surfactant-assisted photochemical method has been developed to prepare tube structural single crystalline mercury sulfide (α-HgS). The HgS nanotubes have well-defined structure with the wall thickness of 20–30 nm. The as-prepared HgS nanotubes were characterized by employing techniques including X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy. The characterizations of the products in various stages exhibited the probable reaction process could be divided into three steps in sequence: the formation of the nascent poly(vinylpyrrolidone) (PVP)-coated HgS particles, their assembly and crystal growth. It was found that PVP served as dispersant and stabilizer in the formation of the homogeneous nascent HgS particles and more importantly, the direct UV photocrosslinking of PVP was exploited to induce the assembly of the PVP-coated particles and the growth to tube structure.

The Photo-Dehydro-Diels-Alder Reaction: An Efficient Route to Naphthalenes

The photo-dehydro-Diels-Alder reaction (PDDA), a new photochemical route to naphthalenes is presented. The [4+2] cycloaddition takes place between 3-arylynones and arylacetylenes, in which these moieties may be located in two molecules (intermolecular PDDA, 1) or in the same molecule (intramolecular PDDA, 5 and 9). Especially the latter approach is attractive from a preparative point of view and permits a straightforward access to highly functionalized naphthalenes. The irradiation of unsymmetrical reactants 9 provides, in contrast to symmetrical reactants 5, two isomeric naphthalenes. We found that the regioselectivity can be easily influenced by suitably located substituents in the aromatic rings and by blocking undesired positions. Notably, the PDDA may be used for the preparation of binaphthyls, as shown by the formation of 14 and 16. The mechanism of the PDDA was elucidated by spectroscopic analyses and theoretical (DFT) calculations.

Sono‐ and Photochemical Routes for the Formation of Highly Dispersed Gold Nanoclusters in Mesoporous Titania Films

AbstractA sono‐ and photochemical approach has been developed to incorporate highly dispersed gold nanoclusters into mesoporous TiO2 films. The first step involves the sonication of a TiO2 film immersed in a gold chloride solution. This effectively removes the air trapped in the porous film matrix and drives the gold chloride into the pore channels, leading to a homogeneous adsorption of ionic Au in the TiO2 mesoporous matrix. The second step takes advantage of the photocatalytic property of TiO2 to reduce the adsorbed Au ions to Au0. As the gold nanoclusters thus produced are stabilized by the TiO2 mesonetwork, no organic capping molecules are required. Highly dispersed Au/TiO2 nanoheterojunction arrays can be obtained using this interesting approach.

Photochemical synthesis of Bi 2Se 3 nanosphere and nanorods

A photochemical reaction route was developed to synthesize bismuth selenide (Bi 2Se 3) nanoparticles and nanorods. In an aqueous solution, bismuth nitrate (Bi(NO 3) 3) reacted with sodium selenosufate in the presence of reducing agent and complexing agent, and the dispersed Bi 2Se 3 nanoparticles were obtained with the average size of 35 nm. The nanorods were prepared via an alumina template route in the same solution. We carried out experiments to study the formation mechanism in the morphology control of the products and found that series of factors played an effective role including the irradiation time, pH value, the reducing agents and the species of complexing agents. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM).

Polymer stabilized silver nanoparticles: A photochemical synthesis route

This study describes a novel and convenient way for the preparation of polymer stabilized colloidal silver by an ultra-violet irradiation technique. Methoxypolyethylene glycol (MPEG) generates free radicals in presence of ultra-violet radiation and acts as the reducing agent towards the silver ion. MPEG also serves as a stabilizer of the silver particles formed.