Parent Azobenzene Research Articles

Singlet and Triplet Pathways Determine the Thermal Z/E Isomerization of an Arylazopyrazole-Based Photoswitch.

Understanding the thermal isomerization mechanism of azobenzene derivatives is essential to designing photoswitches with tunable half-lives. Herein, we employ quantum chemical calculations, nonadiabatic transition state theory, and photosensitized experiments to unravel the thermal Z/E isomerization of a heteroaromatic azoswitch, the phenylazo-1,3,5-trimethylpyrazole. In contrast to the parent azobenzene, we predict two pathways to be operative at room temperature. One is a conventional ground-state reaction occurring via inversion of the aryl group, and the other is a nonadiabatic process involving intersystem crossing to the lowest-lying triplet state and back to the ground state, accompanied by a torsional motion around the azo bond. Our results illustrate that the fastest reaction rate is not controlled by the mechanism involving the lowest activation energy, but the size of the spin-orbit couplings at the crossing between the singlet and the triplet potential energy surfaces is also determinant. It is therefore mandatory to consider all of the multiple reaction pathways in azoswitches in order to predict experimental half-lives.

In silico investigation of the photoisomerization mechanism of push-push azoderivatives

The design of novel chromophores showing specific photophysical traits is nowadays crucial for the development of new dyes and optical devices. Azobenzene derivatives comprise the largest fraction of industrial dyes and remain one of the main functions to be employed in the design of general-purpose photoactivated switches. In this context, we have investigated the optical and photoisomerization properties of two push-push azo derivatives, from a purely theoretical perspective, to outline a mechanistic landscape that can contribute to setting the grounds for the future goal of formulating dyes with specific photophysical properties. Both derivatives show downhill excited-state potential energy surfaces, hinting at fast photoactivated isomerization. In addition, both the trans and cis forms show nearly complementary absorption spectra leading to discriminatory color and differential excitation possibilities. Additionally, the reverse cis to trans ground state thermal isomerization reactions show higher energy barriers than the parent azobenzene molecule, supporting their potential use in different technological applications.

Photoswitching Properties of 5‐Methoxy‐2‐ (2‐phenyldiazenyl) Pyridine

AbstractHeteroaryl azobenzene molecular switches often have superior performance compared to their parent azobenzene derivatives. Herein, we report the efficient photoswitching of phenylazopyridine 5‐methoxy‐2‐(2‐phenyldiazenyl)pyridine using 1H NMR and UV/vis spectroscopy demonstrating >92 % Z at the photostationary state and a thermal half‐life of 6.3 days at room temperature. Kinetic studies are in good agreement with the DFT predicted isomerization barrier.

Investigating functional performance and substituent effect in modelling molecular structure, UV-visible spectra, and optical properties of D-π-A conjugated organic dye molecules: a DFT and TD-DFT study

The molecular structure, UV-visible spectra, and optical properties of D-π-A conjugated organic dye molecules (Disperse Red 1 (DR1) and Disperse Red 73 (DR73)) were analyzed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) and compared with azobenzene molecule to study the effect of donor and acceptor substituents on the molecular properties. The performance of DFT functionals is investigated using B3LYP hybrid functional and three long-range corrected functionals (CAM-B3LYP, LC-ω PBE, and ω B97XD) in conjunction with 6-31G(d,p) basis set. Using TD-DFT, we calculate the vertical excitation energies and transition dipole moment values for 100 excited states. These values were further utilized to calculate frequency dependent polarizability under sum-over-states (SOS) formalism and refractive index of these molecular systems. We observe that for azobenzene and DR1 molecules, ω B97XD predicted wavelengths corresponding to peak absorbance closest to the experimental results, while for DR73 molecule, B3LYP gave better prediction. Large polarizability response is also observed for these molecules (DR1 and DR73) in comparison to parent azobenzene structure due to charge transfer between donor and acceptor groups. For DR1 and DR73 molecules, αxx component of polarizability dominates in contrast to azobenzene where αyy dominates. The HOMO → LUMO transition during excitation contributes to the peak molecular response in simulated UV-visible spectra. The high polarizability response of selected D-π-A conjugated molecules in comparison to parent molecule suggests that these molecules are promising candidates for tailor-made photonic and optoelectronic device development. Graphical Abstract Functional and substituent effect on the optical response of D-π-A conjugated molecules modelled using DFT and TDDFT.

Cage structure helps to improve the photoisomerization efficiency of azobenzene

Azobenzene (Azo) usually cannot achieve relatively high photoisomerization efficiency because of the overlap of the n-π* absorption bands between their isomers. In this work, three Azo units were integrated into a cage (AC) and adopt a nonplanar configuration. The distortion of the Azo units in AC results in the reduction of the n-π* band overlap, and consequently increases the full E → Z and Z → E photo-conversions to 87% and 85%, respectively, which are even higher than those of the planar parent azobenzene ADA (85% of E → Z and 76% of Z → E). Further theoretical calculations demonstrate that the n-π* excitation energy gap between the two AC isomers is larger than that of ADA, which further supports the reduction of the n-π* overlap. This AC shows the highest E ↔ Z photoisomerization efficiency among the symmetrical organic covalent cages yet reported.

The Photoisomerization Pathway(s) of Push-Pull Phenylazoheteroarenes*.

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo‐based photoswitches. Their light‐induced mechanism for trans–cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). Although the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally diverse family of azoheteroarenes is unclear. Here, by means of non‐adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl‐azoheteroarenes with increasing push–pull character is unraveled. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push–pull substitution effects. Overall, the working conditions of this family of azo‐dyes is clarified and a possibility to exploit push–pull substituents to tune their photoisomerization mechanism is identified, with potential impact on their quantum yield.

Modulation of Electronic Mobility of a One-Dimensional Coordination Polymeric Molecular Wire with Light.

Metal ions often influence the photoswitching efficiency of a photochromic system. This article reports a one-dimensional polymer having cyclic azobenzenes coordinated to silver ions that are bridged by nitrates. The coordination polymer (CP-2) displays a photoresponsive behavior. The switching ability in the polymer form was faster compared to the parent azobenzene ligand without the metal ions. Azobenzenes are reported to be poorly conducting. Here, although the azobenzene ligand does not show significant electronic mobility, the coordination polymer (CP-2) displays a modest conductivity. The conductance in the cis form of the polymer is significantly higher compared to the trans form. Upon exposure to visible light, the cis form undergoes photoisomerization to the trans form with a drastic drop in the electronic mobility. The trans form can be reverted to the cis form thermally or by using UV light. Thus, this system offers a reversible control of the conductivity using light.

Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems.

The performance of molecular solar thermal energy storage systems (MOST) depends amongst others on the amount of energy stored. Azobenzenes have been investigated as high-potential materials for MOST applications. In the present study it could be shown that intermolecular attractive London dispersion interactions stabilize the (E)-isomer in bisazobenzene that is linked by different alkyl bridges. Differential scanning calorimetry (DSC) measurements revealed, that this interaction leads to an increased storage energy per azo-unit of more than 3 kcal/mol compared to the parent azobenzene. The origin of this effect has been supported by computation as well as X-ray analysis. In the solid state structure attractive London dispersion interactions between the C-H of the alkyl bridge and the π-system of the azobenzene could be clearly assigned. This concept will be highly useful in designing more effective MOST systems in the future.

Charge Transfer Driven Structural Relaxation in a Push-Pull Azobenzene Dye-Semiconductor Complex.

Photoexcited structural dynamics in azo-compounds may differ fundamentally whether the push-pull photochromic azo-compound is isolated or forms a heterogeneous charge transfer complex, due to a sudden oxidation of the chromophore. Herein, we use a quantum-classical self-consistent approach that incorporates nonadiabatic excited-state electronic quantum dynamics into molecular mechanics to study the photoexcited dynamics of the push-pull azo-compound para-Methyl Red in the gas phase and sensitizing the (101) anatase surface of TiO2. We find that the photoinduced S2/S0 trans-to- cis isomerization of para-Methyl Red in the gas phase occurs through a pedal-like torsion around the ϕCNNC dihedral angle, without evidence to support the inversion mechanism, likewise in the parent azobenzene molecule. However, the photoexcited structural relaxation of the charge transfer complex para-Methyl Red/TiO2 contrasts essentially with the isolated azo-compounds. Immediately after photoexcitation, the excited electron flows into the TiO2 conduction band, with an injection time constant of ≃5 fs, and no indication of isomerization is observed during the 1.5 ps simulations. Instead, a strong vibronic relaxation occurs that excites the NN stretching mode of the azo-group, which is ultimately ascribed to the NA relaxation, and delocalization, of the hole wavepacket.

Two-Photon-Induced versus One-Photon-Induced Isomerization Dynamics of a Bistable Azobenzene Derivative in Solution.

We report on a bistable azobenzene derivative with sufficiently high 2-photon absorption to induce its photochemical isomerization and measurable excited state dynamics. Broadband transient absorption spectra were recorded and compared upon 1-photon (331 nm) and 2-photon (640 nm) excitation of the S0 → S2 transition. The spectra are different at early (t ∼ 1 ps) and late (t ∼ 100 ps) time but show similar photoisomerization behavior on a 10 ps time scale. With 2-photon excitation, strong population transfer S2 → Sn occurs due to resonance absorption of a third pump photon. Subsequent internal conversion Sn → S1 results in a very hot S1 population causing extra-broadening of the transient spectra. The resonance pump absorption is common with nonlinear excitation and should be taken into account when considering photochemical applications. The 2-photon excitation cross-section σ((2)) at 640 nm was measured to be 7 GM for the specific tetra-ortho-fluorinated azobenzene derivative and 1 GM for unsubstituted parent azobenzene. The direct 2-photon induced trans-to-cis isomerization, described herein, provides an unprecedented potential for spatially addressing P-type (bistable) azobenzene photoswitches in 3D.

ChemInform Abstract: Chemoselective Deprotonative Lithiation of Azobenzenes: Reactions and Mechanisms.

ChemInformVolume 45, Issue 35 Isocyclic Compounds ChemInform Abstract: Chemoselective Deprotonative Lithiation of Azobenzenes: Reactions and Mechanisms. Thi Thanh Thuy Nguyen, Thi Thanh Thuy Nguyen Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorAnne Boussonniere, Anne Boussonniere Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorEstelle Banaszak, Estelle Banaszak Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorAnne-Sophie Castanet, Anne-Sophie Castanet Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorKim Phi Phung Nguyen, Kim Phi Phung Nguyen Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorJacques Mortier, Jacques Mortier Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this author Thi Thanh Thuy Nguyen, Thi Thanh Thuy Nguyen Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorAnne Boussonniere, Anne Boussonniere Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorEstelle Banaszak, Estelle Banaszak Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorAnne-Sophie Castanet, Anne-Sophie Castanet Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorKim Phi Phung Nguyen, Kim Phi Phung Nguyen Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this authorJacques Mortier, Jacques Mortier Inst. Mol. Materiaux Mans, CNRS, Univ. Maine, F-72085 Le Mans, Fr.Search for more papers by this author First published: 14 August 2014 https://doi.org/10.1002/chin.201435089Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume45, Issue35September 2, 2014 RelatedInformation

Chemoselective Deprotonative Lithiation of Azobenzenes: Reactions and Mechanisms

Whereas standard strong bases (n-BuLi, s-BuLi/TMEDA, n-BuLi/t-BuOK, TMPMgCl·LiCl, and LDA) reduce the N═N bond of the parent azobenzene (Y = H), aromatic H→Li permutation occurs with LTMP when a suitable director of lithiation (Y = OMe, CONEt2, F) is present in the benzene residue of the azo compound. The method allows direct access to new substituted azobenzenes.

Preparation of symmetric and asymmetric aromatic azo compounds from aromatic amines or nitro compounds using supported gold catalysts

This protocol describes the aerobic oxidation of aromatic anilines to aromatic azo compounds using gold (Au) nanoparticles supported on TiO(2) as a catalyst. Yields above 98% are achieved under a few bars of oxygen pressure. It should be noted that the use of stoichiometric amounts of environmentally unfriendly reagents, e.g., transition metals and nitrites, commonly used in current syntheses of azo compounds, is avoided using this approach. The protocol is illustrated with the synthesis of parent azobenzene from aniline, and this reaction takes 22 h. Au on TiO(2) can also be used as a hydrogenation catalyst, making it possible to prepare azo compounds directly from nitroaromatics through a two-step (hydrogenation followed by aerobic oxidation), one-pot, one-catalyst reaction. In addition, the catalytic process is efficient for the synthesis of symmetric and a range of asymmetric aromatic azo compounds from the mixtures of two anilines substituted with electron-donor and electron-acceptor substituents.

Cis–trans isomerization mechanism of 4-aminoazobenzene in the S 0 and S 1 states: A CASSCF and DFT study

The isomerization mechanism of the ground-state and the first single excited-state (S 0 and S 1) for 4-aminoazobenzene (4-AAB) was studied on the basis of the DFT-B3LYP and CASSCF calculations. The results indicate that the thermal cis– trans ( E ↔ Z) isomerization of 4-AAB mainly concerns the inversion mechanism, while the photoisomerization on the S 1 state involves the rotational pathway that is characterized by the S 1/S 0 conical intersection with a twisting structure. By means of the investigation on the geometrical parameters, it was found that the relaxation modes, which determine the decay dynamics of the excited states, are different for the E- and Z-isomers upon excited in the S 1 state. Moreover, the S 1 potential energy surface of 4-AAB is found to be similar to that of parent azobenzene. During the decay process of excited E-isomer through the rotation of the NN bond, however, the activation energy barrier of 4-AAB is lower than that of azobenzene, which suggests that the corresponding decay lifetime of S 1 excited E-4-AAB is smaller than that of E-azobenzene.

Formation and manipulation of discrete supramolecular azobenzene assemblies

Azobenzene derivatives were deposited onto a Au(111) surface and studied by scanning tunneling microscopy. The symmetry of the parent azobenzene was broken by introducing tert-butyl groups which are known to decouple the molecular core from the substrate, on the one end, and carboxylic acid groups which direct and stabilize the supramolecular assembly structure on the surface by intermolecular hydrogen bonding, on the other end. As a consequence of the interacting COOH groups, the molecules assemble on the surface either in extended, polymeric chains and/or in discrete, hexameric rosettes. The high stability of the rosette structure is proven experimentally by controlled lateral displacement on the surface without breaking the non-covalent interactions. Although switching attempts were not successful, the approach herein should facilitate the construction of well-defined multi-switch arrays.

Photoisomerization of Disperse Red 1 Studied with Transient Absorption Spectroscopy and Quantum Chemical Calculations

The photoisomerization of the push-pull substituted azo dye Disperse Red 1 is studied using femtosecond time-resolved absorption spectroscopy and other spectroscopic and computational techniques. In comparison with azobenzene, the pipi* state is more stabilized by the effects of push-pull substitution than the npi* state, but the latter is probably still the lowest in energy. This conclusion is based on the kinetics, anisotropy of the excited state absorption spectrum, the spectra of the ground states, and quantum chemical calculations. The S(1)(npi*) state is formed from the initially excited pipi* state in <0.2 ps, and decays to the ground state with time constants of 0.9 ps in toluene, 0.5 ps in acetonitrile, and 1.4 ps in ethylene glycol. Thermal isomerization transforms the Z isomer produced to the more stable E isomer with time constants of 29 s (toluene), 28 ms (acetonitrile), and 2.7 ms (ethylene glycol). The pathway of photoisomerization is likely to be rotation about the N=N bond. Quantum chemical calculations indicate that along the inversion pathway ground and excited state energy surfaces remain well separated, whereas rotation leads to a region where conical intersections can occur. For the ground-state Z to E isomerization, conclusive evidence is lacking, but inversion is more probably the favored pathway in the push-pull substituted systems than in the parent azobenzene.

Surface and biocidal activity of some synthesized metallo azobenzene isothiouronium salts

A novel series of azobenzene isothiouronium salts of different alkyl chains (propyl, hexyl and dodecyl) were synthesized by reaction of 4-((4-methylphenyl)azo)phenol with 1,3-dibromopropane, 1,6-dibromohexane and 1,12-dibromododecane, respectively. These salts were reacted with copper (II) halide to give their corresponding metallo complexes. The surface tension measurements for the synthesized compounds show that the metallo complexes have adsorption and micellization better than that of the parent azobenzene isothiouronium salts. The pathogenic Gram-negative bacteria, Gram-positive bacteria, fungi and yeast were used to determine the biocidal activity of these compounds using gradient plate technique. The results indicate that the copper complexes of the synthesized azobenzene isothiouronium salts have a relatively better biocidal activity than the parent salts.

Surface plasmon resonance of azobenzene‐incorporated polyelectrolyte thin films as an H+ indicator

AbstractAn azo‐dye‐incorporated polyionene was spin‐coated as thin films on silver evaporated glass substrates. The films were used in conjunction with surface plasmon resonance (SPR) spectroscopy to examine the utility of combining these techniques for optical H+‐ion sensing. A method suggested by Kano and Kawata was used to enhance the sensitivity of the classical SPR measurements toward absorption detection. With the exposure of the composite polyionene films to HCl vapor, changes in the reflectivity signal were obtained in proportion to the concentration of H+ ions and according to the analyzing wavelength. The transformation of the parent azobenzene into the azohydrazone polar form determined the displacement of the absorption band and was the basis for the absorption‐enhanced SPR measurements. The difference in the SPR signals for the neutral sample and that exposed to interaction with HCl was most demonstrated for the wavelength at which the sample spectrum changed from being nonabsorbing to absorbing. The results show that chromophoric polyionenes are good materials for the production of sensitive substrates and can be combined with SPR spectroscopy for the optical sensing of aggressive gaseous species. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1459–1464, 2002

Kinetics of photochromic reactions in a 10-membered dibenzoazo crown ether

The paper reports on results of measurements of the kinetics of the photochromic reaction in a member of the series of azobenzene-containing crown ethers. The 10-member crown (referred to as O2N2) undergoes a reversible photo- and thermally driven isomerisation, similarly as parent azobenzene. It is expected that constraints imposed by the crown on conformations of azobenzene-containing crown ethers may modify the energetics of the isomerisation process. The experiments were performed in pMMA matrices. The isomerisation was monitored spectrophotometrically. The trans – cis isomerisation was driven photochemically ( λ =365 or 500 nm) and the reverse cis – trans isomerisation was driven by the excitation in the visible (413 nm) or occurred spontaneously in the dark. The pre-exponential factor of the latter process amounts to ca. 6×10 11 s −1 and the activation energy is equal to ca. 118 kJ/mol, being higher than in other azobenzene-containing crown ethers and in parent azobenzene. The stability of the cis isomer at room temperature is much higher than that in other crown ethers, the extrapolated ambient-temperature time constant being of the order of several years. The stability of the cis form of O2N2 in the darkness, combined with a good reversibility is a promising feature for future applications of the material in optical data storage.

Liquid Crystalline Cholesteryl Azo Dyes

Abstract Compounds of the formula 3(β)-cholestery where X=O or NH, Ar=1,4-phenylene or 3-hydroxy-1,4-phenylene, and Ar'=phenyl, 4′-cyanophenyl, 4′-nitrophenyl, or 4′-N,N-diethylaminophenyl were prepared which have broad liquid crystalline ranges as determined by differential scanning calorimetry. Changes were found in the characteristics of the parent azobenzene compounds following carbonate or carbamate formation. The pitch and order parameters of the title compounds were determined in several nematic hosts. In addition, a method for assessing the alignment of the cholesteryl moiety in liquid crystalline hosts is presented.