Reversible trans/cis photoisomerization in Langmuir-Blodgett multilayers from polyfunctional azobenzenes

Four novel, amphiphilic, N-acylaminoazobenzene derivatives containing a sulfonyl group and a β-alanine moiety were synthesized and their mono- and multilayers were prepared by the Langmuir-Blodgett (LB) technique. The opto-physical properties of the LB multilayers were investigated. Some relationship between the chemical structure of the azobenzenes and photosensitivity of their LB multilayers has been found. Reversible trans/cis photoisomerization was observed on alternate irradiation with ultraviolet and visible light of LB multilayers from azobenzenes modified by the introduction of a second aliphatic chain in the N-acylamino fragment as well as by introduction of the bulky N,N′-dicyclohexyl urea moiety.     

Systematic study on the structures and reactivity of hydrazobenzenes and azobenzenes bearing a chalcogenophosphoryl group

Hydrazobenzenes 3-5 bearing a chalcogenophosphoryl group were synthesized by palladium-catalyzed cross-coupling reactions. Their X-ray crystallographic analyses and NMR and IR spectra showed the presence of intramolecular hydrogen bonds between the N-H protons and the chalcogenophosphoryl groups. The intermolecular hydrogen bonds in phosphine oxide 3 and selenide 5 were observed in the solid state. Phosphine oxide 3, sulfide 4, and selenide 5 constructed a dimeric structure, a discrete monomeric structure, and a chain structure, respectively. As the chalcogen atom changed, the crystalline structures of the 2-chalcogenophosphorylhydrazobenzenes also changed. The hydrogen bonds affected the oxidation reactions of the hydrazobenzenes, and oxidation of hydrazobenzenes bearing a lighter chalcogen atom was more difficult. For azobenzenes bearing a chalcogenophosphoryl group, X-ray crystallographic analyses and NMR spectra showed little interaction between the azo group and the chalcogenophosphoryl groups. However, in the UV-vis spectra, the red shifts of the absorption maxima due to the n --> pi transitions indicated intramolecular interactions in the excited state, in contrast to the corresponding 4-substituted azobenzenes. In addition, photoirradiation of phosphine oxide (E)-7 gave (Z)-7, whereas that of phosphine sulfide (E)-8 and phosphine selenide (E)-9 did not give (Z)-8 and (Z)-9, suggesting that heavy chalcogen atoms quench excited states by through-space interactions. Introduction of a chalcogenophosphoryl group at the 2-position had a significant effect on the structure, spectral properties, and reactivity of hydrazobenzenes and azobenzenes. Although azobenzene (E)-10 bearing a hydroxyphosphoryl group at the 2-position did not show hydrogen bonding in the crystalline state, its optical properties and photoisomerization ratio were different from those of (E)-7.     

Site-specific functionalization on individual colloids: size control, stability, and multilayers

Individual colloidal particles are locally functionalized with nanoscale control. Here we use the particle lithography technique to mask one region of a silica or polystyrene particle (size 3.0 mum down to 170 nm), while the remaining 95% or more of the particle is coated with various sized nanocolloids. The images and data show precise and predictable control over the size of the region, with fine-tuned patch size control obtainable by changing the ionic strength of the solution. The coating on the particles remains stable even when subjected to sonication for 5 min. Both single regions and multilayer annuluses are readily formed. Particle lithography provides a general, reliable, stable, controllable, and scalable method for placing site-specific functionalizations on individual particles, opening the way to more complex particle patterning and the bottom-up assembly of more complex structures.     

Efficient preparation of nitrosoarenes for the synthesis of azobenzenes

[reaction: see text] Reaction conditions are described for the oxidation of anilines furnishing nitrosoarenes and the synthesis of unsymmetrically substituted azobenzenes. In a comparative study, the catalytic oxidation of methyl 4-aminobenzoate by hydrogen peroxide was investigated, and SeO(2) proved to be superior or equal to methyl trioxorhenium (MTO) and Na(2)WO(4), respectively. Nevertheless, the application of the inexpensive, environmentally friendly, Oxone in a biphasic system proved to be more efficient, and a variety of useful nitrosoarenes for the synthesis of azo compounds were prepared in high yield and purity on a large scale.     

Controlled synthesis and functionalization of PEGylated methacrylates bearing cyclic carbonate pendant groups

Homopolymer bearing cyclic carbonate (CC) group, ABA type triblock copolymers, and (AC)B(AC) type terpolymers with statistical arrangement of A and C monomers bearing side chain CC groups are reported here. Difunctional poly(ethylene glycol) macroinitiators (PEGMIs) were prepared from PEG of three different molecular weights. PEGMIs were subsequently used for the preparation of polymers bearing CC pendant groups from cyclic carbonate methacrylate (CCMA) under atom transfer radical polymerization to yield polymers with low polydispersity index. Homopolymer and ABA type triblock copolymers were obtained by polymerizing CCMA monomer and (AC)B(AC) type statistical terpolymers were obtained when methyl methacrylate was included as a comonomer. No polymer was obtained when styrene was used as comonomer. The cyclic carbonate groups were subjected to ring‐opening reaction with monoamine to yield side chain hydroxyurethane polymers with increased solubility and diamines to yield crosslinked insoluble materials. Changes in wettability characteristics were studied by following the water contact angle of the polymers before and after ring‐opening reaction involving the cyclic carbonate pendant group. The polymers which composed of electrolyte in the form of PEG and coordinating species in the form of pendant cyclic carbonate groups showed conductivity in the range of 2–5 × 10^?6 Scm^?1 at 23 °C after doping with lithium bis(trifluoromethane)sulfonimide as characterized by impedance spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1622–1632, 2010     

Ordering of azobenzenes by two-photon isomerization

We report on light induced orientation by two-photon isomerization of azobenzenes in films of polymer. The dynamics of isomerization and orientation by one-photon absorption and two-photon absorption (TPA) are similar, and TPA creates a degree of molecular orientation which is comparable to that achieved by single-photon isomerization, in agreement with the theoretical predictions of two-photon isomeric orientation.     

Polyelectrolytes near the rod limit

The problem of a polyelectrolyte near the rod limit is formulated incorporating both the short-range stiffness of the backbone (the “wormlike chain”) and the electrostatic interaction between segments taken as a Debye–Húckel potential. By the use of a physically valid approximation an expression is derived for the mean-square extension length of a short polyelectrolyte chain. Some remarks are made on the relationship between the electrostatic interaction and the usual excluded-volume problem for polymer solutions.     

Dynamics of Biological Polyelectrolytes

Summary: Biological polymers and structures, including proteins and DNAs, can be made in essentially monodisperse form. Proteins usually have well-defined shapes. Duplex oligonucleotides are rigid and rodlike, and longer DNAs are semiflexible coils. The DNAs also constitute a homologous series. The dynamics of both proteins and DNAs can be studied by readily available techniques such as dynamic light scattering (DLS) and fluorescence correlation spectroscopy (FCS). These systems can thus be used as model systems to elucidate elusive charge effects on the dynamics of macromolecules in solution (polyelectrolyte effects) for both rigid and semiflexible polymers. We present here as examples the results of measurements of mutual and self-diffusion coefficients dynamics of a rodlike oligonucleotide as functions of polymer concentration and the concentration of added salt (which screens the charges).     

Syntheses of new functionalized azobenzenes for potential molecular electronic devices

New non-symmetrical azobenzene derivatives have been synthesized as potential molecular electronic switching device candidates. The Oxone^® mediated oxidation of anilines provided nitroso-functionalized arenes, which were then condensed with substituted anilines to provide a series of azobenzene derivatives that could be further converted into oligo(phenylene ethynylene)s or diazonium salts. The resulting thiolacetates, thiols, or diazonium salts are capable of forming molecular layers on the surface of gold or silicon, thereby paving the way for molecular electronics testing.     

Photochemical cyclodehydrogenation of Lewis acid-conjugates of azobenzenes

Irradiation of the conjugate acids of azobenzene (1a) with Lewis acids like anhydrous AlCl₃, anhydrous SnCl₄ and anhydrous FeCl₃ in 1,2-dichloroethane results in cyclodehydrogenation to benzo[c]cinnoline (2a). The formation of benzidine (3a) along with 2a suggests that the reaction is a photochemical disproportionation. The absorption spectra of the conjugate acids in 1,2-dichloroethane reveal that inversion of the n → π* and π → π* singlet state energies occurs on complexation of the azo nitrogen with the Lewis acid. Irradiation of the Lewis acid-conjugates of 2-methylazobenzene (1b), 2,2′-dimethylazobenzene (1c), 4,4′-dimethylazobenzene (1d) and 4-chloroazobenzene (1e) also results in cyclodehydrogenation.     

Spatial confinement alters the ultrafast photoisomerization dynamics of azobenzenes

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans-azobenzene (t-Az) and several alkyl-substituted t-Az derivatives encapsulated in a water-soluble supramolecular host–guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the trans → cis photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted t-Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state trans → cis isomerization pathway for confined molecules that is not observed in solution. Analysis of the transient spectra following ππ* excitation for a series of 4-alkyl and 4,4′-dialkyl substituted t-Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distorted trans and cis structures. The variation of the dynamics due to crowding in the confined environment provides new evidence to explain the violation of Kasha''s rule for nπ* and ππ* excitation of azobenzenes based on competition between in-plane inversion and out-of-plane rotation channels.

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans-azobenzene (t-Az) and several alkyl-substituted t-Az derivatives encapsulated in a water-soluble supramolecular host–guest complex.     

Synthesis of azobenzenes: the coloured pieces of molecular materials

Azobenzenes are ubiquitous motifs very important in many areas of science. Azo compounds display crucial properties for important applications, mainly for the chemical industry. Because of their discovery, the main application of aromatic azo compounds has been their use as dyes. These compounds are excellent candidates to function as molecular switches because of their efficient cis-trans isomerization in the presence of appropriate radiation. The classical methods for the synthesis of azo compounds are the azo coupling reaction (coupling of diazonium salts with activated aromatic compounds), the Mills reaction (reaction between aromatic nitroso derivatives and anilines) and the Wallach reaction (transformation of azoxybenzenes into 4-hydroxy substituted azoderivatives in acid media). More recently, other preparative methods have been reported. This critical review covers the various synthetic methods reported on azo compounds with special emphasis on the more recent ones and their mechanistic aspects (170 references).     

On the mechanism of the 1,2-cycloaddition of azobenzenes to ketenimines

The 1,2-cycloaddition of unsymmetrical azobenzenes has been found to be stereoselective and the rate is essentially solvent independent. These observations lead to the postulation of a near concerted process which encounters a steric effect after alignment.     

Functionalized Conjugated Polyelectrolytes for Biological Sensing and Imaging

Conjugated polyelectrolytes (CPEs) are macromolecules with highly delocalized π‐conjugated backbones and charged side chains, which are unique types of active materials, with wide applications in optoelectronics, sensing, imaging, and therapy. By attaching specific groups (e.g., recognition elements, magnetic resonance (MR) contrast agents, gene carriers, and drugs) to the side chain or backbone of CPEs, functionalized CPEs have been developed and used for specific biological applications. In this account, we summarize the recent progress of functionalized CPEs with respect to their synthesis and biomedical applications. Future perspectives are also discussed at the end.     

Light driven reactions of single physisorbed azobenzenes

We present a successful attempt of decoupling a dye molecule from a metallic surface via physisorption for enabling direct photoisomerization. Effective switching between the isomers is possible by exposure to UV light via the rotation pathway.     

Aggregation phenomena in polyelectrolyte multilayers made from polyelectrolytes bearing bulky functional,hydrophobic fragments

The functionalization of polyelectrolyte multilayers often implies the use of bulky functional fragments, attached to a standard polyelectrolyte matrix. Despite of the high density of non-charged, often hydrophobic substituents, regular film growth by sequential adsorption proceeds easily when an appropriate polyelectrolyte counter ion is chosen. However, the functional fragments may cluster or aggregate. This complication is particularly evident when using chromophores and fluorophores as bulky pendant groups. Attention has to be paid to this phenomenon for the design of functional polyelectrolyte films, as aggregation may modify crucially the properties. The use of charged spacer groups does not necessarily suppress the aggregation of functional side groups. Still, clustering and aggregation depend on the detailed system employed, and are not obligatory. In the case of cationic poly(acrylamide)s labeled with naphthalene and pyrene fluorophores, for instance, the polymers form intramolecular hydrophobic associates in solution, as indicated by strong excimer formation. But the polymers can undergo a conformational rearrangement upon adsorption so that they are decoiled in the adsorbed films. Analogous observations are made for polyanions bearing mesogenic biphenyls fragments. In contrast, polycations functionalized with the dye coumarin 343 show little aggregation in solution, but a marked aggregation in the ESA films.     

光响应性偶氮聚电解质自组装研究

将3种侧链带有不同偶氮生色团的聚电解质,用逐层浸渍的方法使其组装成多层膜.利用原子力显微镜(AFM)研究了自组装膜的表面形貌以及随着组装层数增加的变化.详细研究了在紫外光的照射下,薄膜内偶氮生色团由反式到顺式构型的光响应转变,以及在暗室中由顺式回复到反式构型的回复特性.与自组装膜的光响应性和相应的旋涂膜进行比较发现,其中的PNAPE/PDAC自组装膜没有明显的光色效应,而PEAPE/PDAC,PPAPE/PDAC具有明显的光色效应.在暗室中,随生色团的结构不同和在不同的凝集态中,由顺式回复到反式构型的速率明显不同.     

Conjugated Polyelectrolytes as New Platforms for Drug Screening

In recent years, conjugated polyelectrolytes (CPEs) have attracted increasing attention for their applications in highly sensitive biosensors by taking advantage of their unique optical amplification properties. In comparison to previous applications tailored for highly sensitive biomacromolecule detection, this Focus Review highlights recent research efforts in the development of water‐soluble CPEs as a new class of optical platforms for the screening of potential drugs. Three types of biological targets for the search of small‐molecule active drugs are described: nucleic acids, enzymes, and RNA–protein complexes. Future research directions for drug screening based on CPEs are also presented.     

Gel Permeation Chromatography of Polyelectrolytes

Abstract

The mechanisms are discussed which control the GPC elution of ionic solutes, both the polyelectrolytes and the low molecular salts. The processes involved are quite general and valid in organic and in aqueous solvents. The conclusion is that gel permeation chromatography is a powerfull method to characterize poly-electrolytes; and it is shown that the correct data on molecular weight distribution can be obtained when the ionic content in the eluent is larger than 5.10^?2 M and when the concentration injected is lower than the critical overlapping concentration. The interpretation of chromatograms can be performed using the universal calibration and a viscosimetric detector.     

Photoinduced isothermal phase transitions of liquid-crystalline macrocyclic azobenzenes

Liquid-crystalline macrocyclic compounds, tethered by two or three azobenzenes bearing alkoxy side chains, exhibit isothermal phase transitions from liquid-crystal to isotropic as well as from crystal to isotropic phases upon light irradiation due to the drastic conformational change of their macrocyclic backbone.