Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization?

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H^+) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C═N double bond, leading to isomerization. Treating Z-H^+ with base (K_(2)CO_3) yields a mixture of E and “metastable” Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E_T) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H^+) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone–azo tautomerization followed by rotation around a C–N single bond, as opposed to the more common rotation mechanism around the C═N double bond

Xylene isomerization over zeolite catalysts

Paraxylene is used as a raw material for the production of the synthetic fibers. The production of paraxylene can be obtained by isomerization of xylene using zeolites as catalyst. At equilibrium, the mixture of xylene contains 24% para, 24% ortho and 52% of metaxylene. The objective of this experiment is to access the effectiveness of several zeolite catalysts for the isomerization of xylene. The study was carried out using micro reactor packed with zeolite (0.5g). In this work, the activity and selectivity of the catalyst in the isomerization of xylene depend on the type of zeolite used. HZSM-5 catalyst gives higher activity and selectivity over other type of zeolite

Kinetic Monte Carlo Simulations for Birefringence Relaxation of Photo-Switchable Molecules on a Surface

Recent experiments have demonstrated that in a dense monolayer of photo-switchable dye Methyl-Red molecules the relaxation of an initial birefringence follows a power-law decay, typical for glass-like dynamics. The slow relaxation can efficiently be controlled and accelerated by illuminating the monolayer with circularly polarized light, which induces trans-cis isomerization cycles. To elucidate the microscopic mechanism, we develop a two-dimensional molecular model in which the trans and cis isomers are represented by straight and bent needles, respectively. As in the experimental system, the needles are allowed to rotate and to form overlaps but they cannot translate. The out-of-equilibrium rotational dynamics of the needles is generated using kinetic Monte Carlo simulations. We demonstrate that, in a regime of high density and low temperature, the power-law relaxation can be traced to the formation of spatio-temporal correlations in the rotational dy- namics, i.e., dynamic heterogeneity. We also show that the nearly isotropic cis isomers can prevent dynamic heterogeneity from forming in the monolayer and that the relaxation then becomes exponential.Comment: 14 pages, 12 figure

Reversible low-light induced photoswitching of crowned spiropyran-DO3A complexed with gadolinium(III) ions.

Photoswitchable spiropyran has been conjugated to the crowned ring system DO3A, which improves its solubility in dipolar and polar media and stabilizes the merocyanine isomer. Adding the lanthanide ion gadolinium(III) to the macrocyclic ring system leads to a photoresponsive magnetic resonance imaging contrast agent that displays an increased spin-lattice relaxation time (T₁) upon visible light stimulation. In this work, the photoresponse of this photochromic molecule to weak light illumination using blue and green light emitting diodes was investigated, simulating the emission spectra from bioluminescent enzymes. Photon emission rate of the light emitting diodes was changed, from 1.75 × 10¹⁶ photons·s⁻¹ to 2.37 × 10¹² photons·s⁻¹. We observed a consistent visible light-induced isomerization of the merocyanine to the spiropyran form with photon fluxes as low as 2.37 × 10¹² photons·s⁻¹ resulting in a relaxivity change of the compound. This demonstrates the potential for use of the described imaging probes in low light level applications such as sensing bioluminescence enzyme activity. The isomerization behavior of gadolinium(III)-ion complexed and non-complexed spiropyran-DO3A was analyzed in water and ethanol solution in response to low light illumination and compared to the emitted photon emission rate from over-expressed Gaussia princeps luciferase

Isomerization dynamics of a buckled nanobeam

We analyze the dynamics of a model of a nanobeam under compression. The model is a two mode truncation of the Euler-Bernoulli beam equation subject to compressive stress. We consider parameter regimes where the first mode is unstable and the second mode can be either stable or unstable, and the remaining modes (neglected) are always stable. Material parameters used correspond to silicon. The two mode model Hamiltonian is the sum of a (diagonal) kinetic energy term and a potential energy term. The form of the potential energy function suggests an analogy with isomerisation reactions in chemistry. We therefore study the dynamics of the buckled beam using the conceptual framework established for the theory of isomerisation reactions. When the second mode is stable the potential energy surface has an index one saddle and when the second mode is unstable the potential energy surface has an index two saddle and two index one saddles. Symmetry of the system allows us to construct a phase space dividing surface between the two "isomers" (buckled states). The energy range is sufficiently wide that we can treat the effects of the index one and index two saddles in a unified fashion. We have computed reactive fluxes, mean gap times and reactant phase space volumes for three stress values at several different energies. In all cases the phase space volume swept out by isomerizing trajectories is considerably less than the reactant density of states, proving that the dynamics is highly nonergodic. The associated gap time distributions consist of one or more `pulses' of trajectories. Computation of the reactive flux correlation function shows no sign of a plateau region; rather, the flux exhibits oscillatory decay, indicating that, for the 2-mode model in the physical regime considered, a rate constant for isomerization does not exist.Comment: 42 pages, 6 figure

Photoswitchable catalysis by a nanozyme mediated by a lightsensitive cofactor

The activity of a gold nanoparticle-based catalyst can be reversibly up- and down-regulated by light. Light is used to switch a small molecule between cis- and trans-isomers, which inhibits the catalytic activity of the nanoparticles to different extent. The system is functional in aqueous buffer, which paves the way for integrating the system in biological networks

Utilizing Imogolite Nanotubes as a Tunable Catalytic Material for the Selective Isomerization of Glucose to Fructose

The isomerization of glucose to fructose is an important step in the conversion of biomass to valuable fuels and chemicals. A key challenge for the isomerization reaction is achieving high selectivity towards fructose using recyclable and inexpensive catalysts. Imogolite is a single-walled aluminosilicate nanotube characterized by surface areas of 200-400 m2/g and pore widths near 1 nm. In this study, imogolite nanotubes are used as a heterogeneous catalyst for the isomerization of glucose to fructose. Catalytic testing demonstrates the catalytic activity of imogolite for the isomerization of glucose to fructose. Imogolite is a highly tunable structure and can be modified through substitution of Si with Ge or through functionalization of methyl groups to the inner surface. These modifications change the surface properties of the nanotubes and enable tuning of the catalytic performance. Aluminosilicate imogolite is the most active material for the conversion of glucose. Conversion of glucose of 30% and selectivity for fructose of 45% is achieved using aluminosilicate imogolite. Modification of imogolite with germanium or methyl groups decreases the conversion, but increases the selectivity. Generally, the selectivity for fructose decreases as the conversion of glucose increases. Interestingly, the imogolite nanotubes have comparable catalytic selectivity at similar conversion as base catalyzed reactions. Catalyst recycling experiments revealed that organic content accumulates on the nanotubes that results in a minor reduction in conversion while maintaining similar catalytic selectivity. Overall, imogolite nanotubes demonstrate an active and tunable catalytic platform for the isomerization of glucose to fructose.American Chemical Society Petroleum Research Fund (ACS-PRF 55946-DNI5)National Science Foundation (NSF CBET 1605037; 1653587 and NSF CBET REU 1645126)Ohio State University Institute of Materials Research (OSU IMR FG0138)The Undergraduate Research Office and Office of ResearchA one-year embargo was granted for this item.Academic Major: Chemical Engineerin

Concentration dependence of thermal isomerization process of methyl orange in ethanol

The thermal isomerization (TI) rates of methyl orange (MO) and 4-dimethylaminoazobenzene (DMAAB) in ethanol (EtOH) are measured. Usually TI rates of azobenzene dyes are known to be concentration independent. However, the TI rate of MO showed a concentration dependence whereas that of DMAAB did not. The TI rate of DMAAB in EtOH became larger by the addition of alkali halide. This phenomenon is caused mainly by the interaction between DMAAB and cation. MO is a derivative of DMAAB in which one end of the azobenzene is substituted by a SO3-Na+ group. The interaction with the dissociated Na+ ion is considered to be an origin of the concentration dependence of the TI rate of MO