Trans Azobenzene Research Articles

Instantaneous temperature field induced by laser pulse in azobenzene liquid crystalline side chain polymers before alignment

Dynamic results have been worked out according to the existing theoretical model, to describe the temperature field that is established within a film of isotropic trans azobenzene liquid crystalline side chain polymers when a linearly polarized laser beam illuminates it. The pulse times adopted in calculations are 10 ns, 100 ns, 0.01 ms, 0.1 ms and 1 ms. The results with a pulse time of 1 ms, after which a temperature field is found to be formed, are discussed in detail. It has been found that (1) the rate of change in temperature will be larger when heat conductivity is smaller; (2) the temperature increases as the radius of the laser increases; (3) for a pulse time of 1 ms, the maximum temperature rise is 10.5°C and after 0.9 ms the temperature field disappears.

A novel hyperbranched polyester functionalized with azo chromophore: synthesis and photoresponsive properties

A hyperbranched polymer containing azo chromophores on the periphery has been synthesized from transesterification reaction between a hyperbranched polyester bearing peripheral hydroxyl groups and ethyl 4-{4'-[N, Ndi(hydroxyethyl)aminobutoxy]phenylazo}benzoate. The hyperbranched polyester was obtained by adding the AB2 monomers step by step to a multifunctional core molecule. Narrow molecular weight distribution, with a polydispersity index as low as 1.1, was achieved by this method. The azobenzene- functionalized hyperbranched polymer as well as the corresponding azo monomer showed typical photoresponsive behavior of an azobenzene type molecule. The trans azobenzene units of the hyperbranched polymer could be reversibly switched by UV irradiation to the cis form that recovered slowly to the trans form in the dark. However, significantly less conversion from the trans isomer to the cis isomer was observed for the azobenzenefunctionalized hyperbranched polymer at the photostationary states under the irradiation of UV light, which could be attributed to the increased steric hindrance in the hyperbranched polymer.

Ab initio study of the minimum-energy structure of trans-azobenzene

The potential energy surface (PES) of ground-state trans-azobenzene (TAB) has been explored as a function of symmetric dual C–N twist angle of the two phenyl rings about the C–N bonds in ab initio MP2/6-31+G * calculations. Two equivalent nonplanar minima have been located in which phenyl rings are lifted from the planar structure by 17.5°. The two minima are connected through a planar, transition-state structure with a potential barrier of 0.35 kcal/mol. The barrier is small enough to be overcome at room temperature although the minima hold a total of eight vibrational states below the barrier.

Orientation-dependent Magneto-Clapeyron Equation

Effects of an applied magnetic field on the phase transition temperature of anisotropic diamagnetic materials including crystals and liquid crystals were studied on the basis of a simple extension of the Clapeyron equation. The transition temperature was expressed as a function of the orientation of the material with respect to the applied field. Calculations for the melting points of the trans azobenzene crystal and a superdrawn polyethylene fiber showed that the shift of the melting point due to the orientation was of the order of 10-5–10-4 K. Larger shifts (∼ 10-3 K) were expected in the case of anisotropic-isotropic transitions in liquid crystal systems where their transition enthalpy is smaller by one to two orders than that for solid-liquid transitions.

Laser-Induced Temperature Distribution in Photo-Alignment of Azobenzene Liquid Crystalline Side Chain Polymers

A theoretical model has been worked out to describe a temperature field, which is established within a film of isotropic trans azobenzene liquid crystalline side chain (LCSC) polymers when a linearly polarized laser beam illuminates it, and contributes to the photo-induced alignment behavior of LCSC polymers at an ambient temperature below the glassy transition temperature (Tg). The distribution of the temperature field is analyzed and the mechanism that the mesogens align perpendicular to the electric vector of the light is discussed with this model. The minimum light intensity Ioff required to achieve the photo-induced phase transition of LCSC polymers at a certain temperature below Tg is calculated. A prediction of existence of an offset photo-alignment temperature Toff is made based on this theory model, while below this temperature no photo-induced phase transition can be observed when a light of certain intensity illuminates the sample.

Ab Initio Molecular Orbital and Density Functional Studies on the Stable Structures and Vibrational Properties of trans- and cis-Azobenzenes

The structure of trans-azobenzene (TAB) has been a subject of controversy in experimental and theoretical studies. To provide the theoretical basis for stable structures and vibrational properties of TAB and cis-azobenzene (CAB), we performed ab initio molecular orbital calculations based on the second-order Møller−Plesset (MP2) method and density functional theory (DFT). Only the MP2 calculation accounting for the diffuse basis set (6-31+G*) leads to the distorted structure of TAB, which is consistent with the gas-phase electron diffraction experiment. The Hartree−Fock and DFT (PW91PW91, MPW91, BP86, and B3LYP) calculations, on the other hand, result in the almost planar structure of TAB. The frequencies of normal modes of TAB and CAB are calculated most accurately by the PW91PW91/6-31+G* method followed by the BP86/6-31G* method. These results are expected to provide useful benchmark data on the accuracy of MP2 and DFT methods for describing the structural and vibrational properties of azobenzene-like molecules, which consist of weakly interacting phenyl rings.

Structures, Vibrational Frequencies, and Normal Modes of Substituted Azo Dyes: Infrared, Raman, and Density Functional Calculations

The vibrational structures of 4-nitro-azobenzene (NAB), 4-(dimethylamino)-azobenzene (DAB), and 4-nitro,4‘-(dimethylamino)-azobenzene (NDAB) are of interest due to their importance in optoelectronic applications as well as the unique isomerization mechanism involving the inversion process (at the nitrogen site). In this paper, we present the Fourier transform infrared (FTIR) and Raman spectral studies of NAB, DAB, and NDAB and also report their equilibrium structures, harmonic frequencies, and normal mode assignments, employing the hybrid Hartree−Fock/density functional (HF/DF) method with the 6-31G basis set. The results of the optimized molecular structure obtained on the basis of B3LYP with 6-31G basis set suggest a greater conjugation and π-electron delocalization for the substituted azo dyes in comparison to the parent azo molecule, viz. trans-azobenzene (TAB). It is found that the B3LYP/6-31G method is very accurate in predicting harmonic vibrational frequencies and the normal modes for the substituted azo dyes and their isotopic analogues. On the basis of the B3LYP/6-31G force field, the infrared intensities for NAB, DAB, and NDAB, and their isotopomers, are calculated and then compared with those observed experimentally. Finally, the main differences in the vibrational spectra of the substituted azobenzene derivatives are discussed from an analysis of the normal modes.

Reversible Photochromic Behaviors of Fatty Acids with Azobenzene Chromophore in Thin Film Assemblies

The mechanism of reversible photoinduced anisotropy of an azobenzene derivative was investigated by measuring optical retardation and UV/Vis dichroism. During the relaxation period, the cis-trans thermal back relaxation and orientational relaxation of azobenzene groups occur. It is believed that the orientational relaxation of trans and cis azobenzene groups to relieve the strains is a more dominant contribution to the decrease of the optical anisotropy in the film.

Photon-Gated Electron Transfer in Two-Component Self-Assembled Monolayers

The preparation and characterization of a photoactive, two-component self-assembled monolayer (SAM) consisting of 99:1 cis-p-(C6H5)NN(C6H4)O(CH2)11SH) (1) and trans-p,p-Fc(C6H4)NN(C6H4)(CH2)4SH (Fc = C5H4FeC5H5; 2) are reported. With this SAM, electron transfer between solution ferrocyanide and the Au substrate is forced to occur through mediating ferrocenyl sites in the film, resulting in a diode-like response. However, by photochemically converting large “footprint” cis-1 to the smaller footprint trans-1, the free volume within the film is increased, thereby allowing for direct electron transfer between the solution ferrocyanide and the electrode surface and a normal electrochemical response (i.e., ipa = ipc). All states of the film have been characterized by cyclic voltammetry, FTIR spectroscopy, and differential capacitance measurements. All data from these techniques are consistent with the conclusion that cis−trans azobenzene isomerization within the monolayer results in increased film porosity, which reduces the film's ability to block the access of solution redox-active species to the underlying Au electrode surface. This novel two-component structure establishes the concept for a new type of molecule-based electronic device, a “photoswitchable diode” that is capable of amplifying the signal associated with a photodriven event via an electrochemical response. With such a system, a small number of photons can release a relatively large number of electrons from the solution electron reservoir. Significantly, this system shows how one can regulate electron-transfer events involving SAMs through photochemical control over film structure and free volume.

Direct Evidence for the Creation of Micropores in UV-Irradiated Poly(Methyl Methacrylate)

ABSTRACTFilms of PMMA containing azobenzene were prepared by casting from polymer solutions containing the probe or by exposing pre-irradiated films to the vapor of azobenzene. These materials were irradiated at 335 nm and the amount of trans azobenzene which had been photoisomerized was monitored spectrophotometrically. The largest amount of photoisomerization was observed for the pre-irradiated samples which had been penetrated by azobenzene from the vapor, indicating that the created free volume allowed the isomerization to proceed more easily.

The triplet state of azobenzene

The quenching rate constants ( k q) for triplet energy transfer from aromatic hydrocarbons to trans and cis azobenzene have been measured by flash kinetic spectrophotometry. For both isomers the plot log k qversus E T of the donors shows a “non-classical” behaviour. A model of the energy-transfer process, which accounts for the experimental k q values, allows the energy of the lowest triplet state of both isomers to be determined.

X-ray crystal structure of azoxybenzene oxotetrachlorotungsten(VI), (C 6H 5N) 2OW VIOCl 4

The crystal and molecular structure of azoxybenzene oxotetrachlorotungsten(VI) has been determined from X-ray diffractometer data. The structure was solved by Patterson and Fourier methods and refined by least-squares techniques to R = 0.058 for 1869 independent reflections. The crystals are monoclinic, space group P2 1/ c, with Z = 4, in a unit cell of dimensions: a = 8.314(3), b = 15.100(5), c = 12.901(7) Å, α = 95.31(5)°. The azoxybenzene residue, the structure of which resembles that of free trans azobenzene, is linked to the tungsten atom through its oxygen atom. The coordination at the metal (two oxygen atoms and four chlorine atoms) corresponds to a distorted octahedron. This distortion is very similar to those observed in similar tungsten compounds. There is a intramolecular C⋯O distance of 2.77 Å between two atoms four bonds apart, of the azoxybenzene residue.

Substituent Effects on the Bands Near 1600 cm−1in The Ir Spectra of Azobenzene and Some Substituted Azobenzenes

Abstract The mean positions and apparent extinction coefficients of the ring stretching bands near 1600 cm−1 in the IR spectra of trans azobenzene and some substituted trans azobenzenes are reported. These results are discussed in relation to values reported for substituted benzenes.

Linear elution adsorption chromatography : X. Electronic and steric effects in hetero-aromatic solutes. Separation of the di-aza-aromatics, quinones and related solutes on alumina

Hetero-aromatic solutes in which the hetero group is strongly adsorbing and forms part of the aromatic ring ( e.g. pyridine, fluorenone) are markedly affected in their adsorption on alumina by intramolecular electronic and steric effects. Crowding of the hetero group by adjacent solute groups in these compounds results in strong reduction of the hetero group adsorption energy. The presence of electron withdrawing substituents in the derivatives of the fluorenones, quinones, di-aza-aromatics and pyridines also reduces the adsorption energy of the hetero group; because the nitrogen atom in the aza-aromatics and the carbonyl group in the quinones are strongly electron withdrawing, the di-aza-aromatics are generally no more strongly adsorbed than corresponding mono-aza-aromatics, while the quinones are about as strongly adsorbed as corresponding mono-carbonyl compounds. The presence of two strongly adsorbing solute groups in adjacent positions, however, as in the ortho quinones and pyridazine derivatives, strongly increases solute adsorption energy, because both solute groupscan simultaneously bond with a single strong adsorbent site. Similar factors appear to determine the relative separability of the cis and trans azobenzene isomers. Using the theory of intramolecular electronic effects derived elsewhere from the study of organic reaction rates and equilibria, it is possible to quantitatively calculate the changes in solute adsorption energy which occur in the pyridines, di-aza-aromatics, fluorenones and quinones as a result of substituent electronic interactions. Electronic effects in other solute types are also examined theoretically, and concluded to be generally much smaller, with the exception of certain cases where the solute may function as either an acid or base. Less strongly adsorbing hetero-aromatic solutes ( e.g. thiophenes, furans) show similar, but smaller responses to substituent steric and electronic interactions.