Kinetics Of Thermal Isomerization Research Articles

π-Extended push-pull azo-pyrrole photoswitches: synthesis, solvatochromism and optical band gaps.

A new family of push-pull biphenyl-azopyrrole compounds 3b-g and 4b-d was efficiently obtained via a Suzuki cross-coupling reaction between 2-(4'-iodophenyl-azo)-N-methyl pyrrole (1a) or 3-(4'-iodophenyl-azo)-1,2,5-trimethyl pyrrole (2a) and 4'-substituted phenyl boronic acids in excellent yields. The influence of the π-biphenyl backbone and pyrrole pattern substitution was correlated with their optical properties. Solvatochromic studies via UV-visible spectrophotometry revealed that the inclusion of a 4'-nitro-biphenyl fragment favors a red-shift of the main absorption band in these azo compounds compared with their non-substituted analogues. Likewise, optical band-gaps were estimated by means of electronic absorption spectra and correlated with TD-DFT studies. The pyrrole pattern substitution and the π-conjugated backbone exhibit a clear influence on their thermal isomerization kinetics at room temperature. In all cases, biphenylazo-pyrrole compounds lead to the formation of J-type aggregates in binary MeOH : H2O solvents. Under these conditions, compounds 3b-c undergo a water-assisted cis-to-trans isomerization at room temperature.

Picosecond Switchable Azo Dyes.

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Building Strain with Large Macrocycles and Using It To Tune the Thermal Half-Lives of Hydrazone Photochromes.

Strain has been used as a tool to modulate the reactivity (e.g., mechanochemistry) and thermal isomerization kinetics of photochromic compounds. Macrocyclization is used to build-up strain in such systems, and in general the reactivity and rates increase with the decrease in macrocycle size. To ascertain the effect of strain on recently reported bistable hydrazone photoswitches, we incorporated them into macrocycles having varying aliphatic linker lengths (C3-C7), and studied their switching behavior, and effect of macrocycle size on the thermal isomerization rate. Surprisingly, while the systems with C3-C5 linkers behave as expected (i.e., the rate is faster with smaller linkers), the isomerization rate in the systems with larger aliphatic linkers (C6-C8) is enhanced up to 4 orders of magnitude. NMR spectroscopy, X-ray crystallography and DFT calculations were used to elucidate this unexpected behavior, which on the basis of our analyses results from the buildup of Pitzer (torsional), Prelog (transannular) and Baeyer (large angle) strain in the longer linkers.

Toward Fractioning of Isomers through Binding-Induced Acceleration of Azobenzene Switching.

The E/Z isomerization process of a uracil-azobenzene derivative in which the nucleobase is conjugated to a phenyldiazene tail is studied in view of its ability to form triply H-bonded complexes with a suitably complementary 2,6-diacetylamino-4-pyridine ligand. UV-vis and 1H NMR investigations of the photochemical and thermal isomerization kinetics show that the thermal Z → E interconversion is 4-fold accelerated upon formation of the H-bonded complex. DFT calculations show that the formation of triple H-bonds triggers a significant elongation of the N═N double bond, caused by an increase of its πg* antibonding character. This results in a reduction of the N═N torsional barrier and thus in accelerated thermal Z → E isomerization. Combined with light-controlled E → Z isomerization, this enables controllable fractional tuning of the two configurational isomers.

Pressure-dependent kinetics on the C4H7 potential energy surface and its effect on combustion model predictions

Butene (C4H8), the smallest alkene possessing both branched and straight-chain isomers, is usually an important intermediate with relatively high concentration in the combustion of hydrocarbons and oxygenated fuels. In the present study, the kinetics of thermal isomerization, decomposition and chemical activation reactions of three typical butenyl isomers, i.e. nC4H7 (CH2=CHCH2ĊH2), saxC4H7 (CH2=CHĊHCH3) and iC4H7 (CH2=C(CH3)ĊH2), were systematically investigated by theoretical calculations. High-level ab initio calculations coupled with the RRKM/master equation method were used to compute the temperature and pressure dependent rate coefficients. The results show that the existence of vinylic CC bond in iC4H7 largely impedes its decomposition rate. The transformation from iC4H7 to straight-chain C4H7 is kinetically unfavorable due to the high strain energy of the 3-membered ring structure of the isomerization transition state. Furthermore, the calculated rate coefficients were incorporated into USC Mech II and Aramco Mech 2.0 to examine the impact of our computed pressure-dependent kinetics on model predictions. Although the simulation results demonstrate limited improvement on ignition delay time and laminar flame speed of butene, substantial changes are observed for the mole fractions of important intermediate species, e.g., allene and 1,3-butediene, in butene pyrolysis. Modified Arrhenius representations of the calculated rate constants are given and should be used in combustion modeling.

Molecular switches and cages

Molecular switches can toggle between two (or more) stable states encoding different physical features. They allow complex systems to respond to changes in their environment defined by light intensity, pH, temperature or voltage. As such, molecular switches play a key role in biology and information technology and have become important components of advanced materials. Chemists have developed a large number of synthetic switches, complementing the many types found in nature. Given their prominence in biology, it is not surprisingly that photoswitches, which are actuated by ultraviolet, visible or infrared light, have been an especially productive field of study. Despite their long history, new types of synthetic photoswitches continue to emerge and “old friends”, such as azobenzenes are constantly improved and optimized. This is not only true with respect to their photophysical properties but also with regard to their synthetic accessibility. Indeed, as molecular switches are applied in ever-increasing quantities, the efficiency of their syntheses becomes a primary concern, requiring the development of new synthetic methods and strategies. The corresponding results, however, are often buried in the Supporting Information of papers that focus on functional aspects, which does not reflect the relative effort that goes into the design and synthesis of these molecules. Caged compounds are a subclass of (photo)switches that have special functional features. As opposed to “true” switches, they can only be turned ON and the abatement of their activity depends on secondary processes (such as diffusion from the active zone or, more importantly, re-uptake systems). In terms of their biological application, especially in neurobiology, they are much further developed than reversibly switchable molecules. For instance, two-photon activation, which allows for very precise localization, is fairly well established with caged compounds but in its infancy in the case of reversible photoswitches. The present Thematic Series of the Beilstein Journal of Organic Chemistry addresses the crucial role that chemistry, and particularly organic chemistry, can play in tuning the functional features of molecular switches and cages. These features include their absorption spectra, conductivity, geometry and bistability, as well as their polarity, solubility, efficacy, or catalytic activity. Photoswitches are covered extensively, ranging from diarylethenes (Pu, Wagenknecht) to dihydroazulenes/vinylheptafulvenes (Nielsen) and azobenzenes (Ruck-Braun, Hoppmann). Two reviews discuss cis-azobenzenes with rapid thermal isomerization kinetics (Velasco), and highlight the azobenzene moiety as one of the smallest light-driven molecular motors conceivable (Merino). Issues of bistability are also addressed in an account on shape-persistent, optically active macrocycles (Pasini). Molecular switches that respond to changes in pH are covered as well (Haberhauer and Aprahamian), reflecting their importance in biology and in materials science. Finally, switching is highlighted as a synthetic strategy for building advanced materials that are useful in photovoltaics (Matile). Chemists, by nature, like to control things and probably actuate macroscopic switches more often than the average person. We hope that this collection of papers will motivate some of our colleagues to also consider nanoscopic versions of switches (and cages). For those of us who deal with these molecules on a daily basis, this Thematic Series will hopefully provide further inspiration and an impetus to continue work in this fascinating field of research. Dirk Trauner Munich, June 2012

Photoisomerization and realignment of nitroazobenzene lying planar in a multilayer assembly

Here, we describe the optical and thermal isomerization kinetics, and photoinduced realignment properties of 2-nitro-4-methoxy-azobenzene tethered to an azacrown-type multi-cation (HNA) or a linear cationic poly(methacrylamide) (PNA) in solution and in a multilayered film. The thermal back-isomerization of HNA in solution followed ‘zeroth-order’ kinetics, whereas that of PNA confirmed to a first-order exponential function. UV/vis spectrometry tilt angle analysis of the azobenzenes in a multilayer film (composed of HNA/ι-carrageenan (CAG) bilayers) is lying relatively flat (θ=23.2±1.3°, with respect to the substrate surface). It was found that the orientation of the azobenzenes in the multilayer films significantly influenced the realignment of the transition moment of the photochromic moieties (with regard to the polarization of the irradiation light). The order parameter S (which gives an indication of the dichroic level), induced on the (HNA/CAG) multilayer film was found to be 20 times larger than that observed for the (PNA/CAG) film, in which the chromophores were isotropically distributed.

Adhesion of Photon-Driven Molecular Motors to Surfaces via 1,3-Dipolar Cycloadditions: Effect of Interfacial Interactions on Molecular Motion

We report the attachment of altitudinal light-driven molecular motors to surfaces using 1,3-dipolar cycloaddition reactions. Molecular motors were designed containing azide or alkyne groups for attachment to alkyne- or azide-modified surfaces. Surface attachment was characterized by UV-vis, IR, XPS, and ellipsometry measurements. Surface-bound motors were found to undergo photochemical and thermal isomerizations consistent with unidirectional rotation in solution. Confinement at a surface was found to reduce the rate of the thermal isomerization process. The rate of thermal isomerization was also dependent on the surface coverage of the motors. In solution, changes in the UV-vis signal that accompany thermal isomerization can be fit with a single monoexponential decay. In contrast, thermal isomerization of the surface-bound motors does not follow a single monoexponential decay and was found to fit a biexponential decay. Both one- and two-legged motors were attached to surfaces. The kinetics of thermal isomerization was not affected by the valency of attachment, indicating that the changes in kinetics from solution to surface systems are related to interactions between the surface-bound motors.

Effects of Solvent Viscosity and Polarity on the Isomerization of Azobenzene

Isomerization of azobenzene has been studied for many years, but some aspects are still unclear or controversial. This work provides further insight and clarifies the role that environmental parameters play in determining the reaction kinetics of azobenzene derivatives in solution. Detailed spectroscopic measurements of the photoisomerization and thermal isomerization kinetics in mixtures of various solvents and polystyrene have been carried out. The results indicate a strong dependence of both photoisomerization and thermal isomerization rates on the polarity, but not on the viscosity of the solvent.

The kinetics of thermal isomerization of β-pinene and a mixture of β- and α-pinenes in supercritical ethanol

Thermal isomerization of β-pinene and equimolar mixture of α- and β-pinenes in supercritical ethanol was studied experimentally. The reactivity of structurally similar α- and β-pinenes were compared in the same supercritical solvent. It was shown that both pinenes undergo thermal transformations independently of one another upon co-thermolysis in supercritical (SC) ethanol. Conversion of α-pinene yields monocyclic limonene as the main product, and β-pinene gives acyclic β-myrcene. This study allowed developing the first kinetic model of β-pinene thermolysis in SC ethanol.

Photoswitchable flexible and shape-persistent dendrimers: comparison of the interplay between a photochromic azobenzene core and dendrimer structure.

Two analogous classes of dendrimers with a single azobenzene moiety at the core have been prepared. Flexible benzyl aryl ether dendrimers 1a-e were obtained in good yields by direct alkylation of diphenolic azobenzene 3 with benzyl aryl ether dendrons [G-n]-Br (n = 0-4). In rigid dendrimers 2a-e, the azobenzene configurational switch was linked to phenylacetylene dendrons through acetylenic linkages to maintain the shape-persistent nature of these dendrimers. A comparison of these two different classes of dendrimers with azobenzene cores reveals a difference in the properties of the photochromic moiety upon dendritic incorporation as well as a significant difference in the photomodulation of dendrimer properties. The E --> Z photoisomerization quantum yield decreased markedly with increasing generation for dendrimers 1a-e but only slightly for dendrimers 2a-e. However, increasing generation did not significantly alter thermal isomerization kinetics or activation barriers. The hydrodynamic volumes of azobenzene-containing dendrimers 2b-e were significantly modulated when the azobenzene unit is subjected to irradiation, while those of dendrimers 1b-e were only slightly affected.

Spontaneous Thiono-Thiolo Isomerization of Thiono-Phosphates. Does it Appear without Impurities?

Abstract Thiono-thiolo isomerization of 0-alkyl esters of thiophosphoric acids to corresponding S-alkyl isomers has been extensively studied because of high biological acivity of both esters. Such an isomerization proceeding without other reactants initially added is called thermal isomerization. Kinetic studies of this process allow to distinguish between reaction pathways of methyl esters A2P(S)OMe leading only to S-methyl esters and of diesters AP(S)(OMe)2, which give also other products. It is well known that the isomerization can be effected by organic bases (amines, phosphines), corresponding alkylonium thiophosphates, and protic acids. We decided to investigate the kinetics of thermal isomerization in solutions, to examine the influence of impurities of reactants or solvents for thiophosphates studied up to now appear to be not thoroughly purified. O,O-Diphenyl-0-methylthiophosphate, 5,5-dimethyl-2-methoxy-2-thio-l,3,2-dioxaphos-phosphorinane and O,O-dimethyl-O-(4-nitro)phenylthiophosphate as model...

Fourier transform infrared spectroscopic study of polyacetylene, 2. Thermal isomerization kinetics and mechanism

AbstractThermal isomerization kinetics of high Z (>90%) rare‐earth polyacetyleneIUPAC name: poly(vinylene). was studied by Fourier transform infrared spectrophotometry in the temperature range of 354 to 443 K. The isomerization kinetics do not follow any simple kinetic reaction order; the apparent activation energy was determined to be 48,1 kJ·mol−1. The Z to E isomerization in polyacetylene is discussed in terms of a diradical two‐step mechanism.

ChemInform Abstract: THERMAL ISOMERIZATION OF 3,3‐DIHALO‐1,2‐DIPHENYLAZIRIDINES

AbstractDie Isomerisierung der Dihalogenaziridine (I) in Tetrachlorkohlenstoff oder Tetrachlorethylen liefert die Imidoylhalogenide (II), aus denen durch Hydrolyse die Anilide (III) gebildet werden.

Thermal isomerization of 3,3-dihalo-1,2-diphenylaziridines

The kinetics of thermal isomerization of 3,3-dihalo-1,2-diphenylaziridines containing fluorine, chlorine, or bromine atoms in the ring were studied, and the activation parameters of the reaction were determined. A concerted mechanism for the formation of imidoyl halides from haloaziridines that includes conrotatory opening of the aziridine ring with halogen migration of the 1,2 type is proposed on the basis of the data obtained.

An electron spin resonance study of the kinetics of thermal isomerization of bitrophenyl (Bi-2,4,6-cycloheptatrien-1-yl)

An electron spin resonance study of the kinetics of thermal isomerization of bitrophenyl (Bi-2,4,6-cycloheptatrien-1-yl)

The Kinetics of Thermal Isomerization of Methylcyclopropane

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe Kinetics of Thermal Isomerization of MethylcyclopropaneJohn P. ChesickCite this: J. Am. Chem. Soc. 1960, 82, 13, 3277–3285Publication Date (Print):July 1, 1960Publication History Published online1 May 2002Published inissue 1 July 1960https://doi.org/10.1021/ja01498a011RIGHTS & PERMISSIONSArticle Views106Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts