Role of Solvent and Effect of Substituent on Azobenzene Isomerization by Using Room-Temperature Ionic Liquids as Reaction Media (original) (raw)
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BMIM PF 6 (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf 2 N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF 6 − and Tf 2 N −. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF 6 and in BMIM Tf 2 N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF 6 − and Tf 2 N − anions during the cis-trans isomerization.
The Journal of Physical Chemistry A, 2009
Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis f trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono-and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis f trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push-pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined values. Also, trends in the resulting rate constants are correct. Other computed properties such as activation entropies and thus preexponential rate factors are in only moderate agreement with experiment.
Angewandte Chemie International Edition, 2008
Reversible trans-cis photoisomerization of azobenzene and its derivatives has been extensively studied and exploited as a photoswitch in numerous molecular systems and functional materials. [1] Thermally activated cis-trans isomerization is also well understood, the rate of which depends on the substituents on the azobenzene and the environment surrounding the chromophore. [1, Little attention has been paid to the effect of an electric field on the isomerization of azobenzene. Fujishima et al. first reported an electrochemical process inducing the conversion of cis-azobenzene to transazobenzene. With amphiphilic azobenzene derivatives forming a Langmuir-Blodgett (LB) monolayer film on an electrode, cis-azobenzene formed on UV irradiation can be reduced to hydrazobenzene (two-electron reduction in aqueous solution), which is then reoxidized to trans-azobenzene. This coupled photochemical and electrochemical isomerization was suggested for high-density information storage. Later, the same group found that cis-azobenzene in such an LB monolayer could return to the trans isomer by an electrostatic process involving no redox reaction. Since the azobenzene derivative was deposited on the working electrode dipped in an electrolyte solution, cis-trans isomerization was attributed to the interaction of azobenzene with the high electric field on the order of 10 3 V mm À1 present in the electrical double layer. However, the underlying mechanism remains unknown, since even with a high field of 10 3 V mm À1 , direct interaction with azobenzene, the trans and cis isomers of which have different dipole moments (Dm % 3 D for unsubstituted azobenzene) cannot be responsible for a drastic lowering of the cis-trans isomerization barrier. More recently, reversible cis-trans isomerization of individual azobenzene molecules on a metal surface was achieved by scanning tunneling microscopy (STM), first with tunneling electrons and then by means of the electric field at the STM junction. In the latter case, the measured threshold voltage for cis-trans isomerization was also in the range of 10 3 V mm À1 . The effect of high electric fields on the isomerization of other chromophores was also reported. Here we report the astonishing finding that with an azobenzene derivative dissolved in liquids or liquid crystals, a low static electric field applied on the mixture between two conductive electrodes can induce fast cis-trans isomerization. In benzonitrile, for instance, the rate constant of cis-trans isomerization under an external field strength of 1 V mm À1 could be six orders of magnitude faster than thermal isomerization in the absence of the field. We show that this drastic electric-field-induced effect could have important implications for doped liquid crystals using azobenzene as a photoswitch to control the electrooptical properties.
Liquid Crystals, 2008
Imidazolium-substituted azobenzene liquid crystal derivatives were synthesised and their chemical structures were determined by 1 H NMR, 13 C NMR and UV spectroscopy and elemental analysis. The liquid crystalline properties pf the new compounds were investigated by differential scanning calorimetry, polarising optical microscopy and powder X-ray diffraction. The monomer DC 10 was found to exhibit a monolayer smectic C (SmC) phase with schlieren texture on both heating and cooling. The effects of ionic interactions as well as the length of the alkyl chains on the mesophase are discussed. The results indicate that ionic interactions between imidazolium groups are the driving force for the formation of the monolayer SmC phase, the thermal stability of which is enhanced.
Physical chemistry chemical physics : PCCP, 2015
The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an equimolar mixture of benzene and dimethylimidazolium triflimide [C1C1im][NTf2] is that in the former the benzene moieties are tied to the imidazolium ring, while in the latter they move independently. We use femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and molecular simulations to explore some properties of these two systems. The Kerr spectra show small differences in the spectral densities; the simulations also show very similar environments for both the imidazolium rings and the phenyl or benzene parts of the molecules. The low frequency vibrational densities of states are also similar in the model systems. In order to perform the simulations we developed a model for the [BzC1im](+) cation and found that the barriers to rotation of the two parts of the molecule are low.
Liquid Crystals, 2015
Two new groups of azobenzene ester derivatives were synthesised: alkyl 4-[4-(nonyloxy)phenyl]diazenyl]benzoates and 4-[4-(nonyloxy)phenyl]diazenyl]phenyl alkanoates. All 35 presented homologues are mesogenic. Moreover, some of the above-mentioned compounds exhibit rich liquid-crystalline polymorphism likewise tetramorphism. During this investigation by the use of polarising optical microscopy, differential scanning calorimetry and X-Ray studies, six types of mesophases were detected: nematic, smectics (A, C, I, F) and G. Furthermore, due to the presence of the photosensitive azo moiety, the E-Z isomerisation reaction is possible. This process, which is initiated by the UV irradiation, causes significant changes in the UV-Vis absorption spectra of investigated compounds. However, the photoisomerisation is a reversible process and in the dark the thermal relaxation of Z isomer takes place. Based on the achieved data, the kinetic constants of the isomerisation and relaxation processes were calculated. It shows that conversion of the ester bond makes some changes in the optical properties. The shift of about 7 nm of the absorbance maximum was observed. Surprisingly, the inversion of the ester group has significant influence on the liquid-crystalline polymorphism replacing one mesophase (for benzoates) into four (for alkanoates).