Cis photoisomerization

Sundstrdm V and Gillbro T 1985 Dynamics of trans-cis photoisomerization of stiibene in hydrocarbon solutions Ber. Bunsenges Phys. Chem. 89 222-6... 

Spectral Transparence Starting from 230 nm 4-Hydroxy-Azobenzene Presence of a Thermally and Photochemically Isomerizable Azo Group Trans-Cis Photoisomerization Photochromism ... 

The remainder of the present article is divided into two parts. The first one reviews the main points of our combined theoretical-experimental approach. The second one reports an application of it to the study of the mechanism and dynamics of trans-cis photoisomerization of bisdimethylaminopentamethine cyanine (BMPC). ... 

Figure 9. Schematic reprsentation of a classical trajectory moving on the Si and So energy surfaces of the H2—(CH) -NHt trans cis photoisomerization, starting near the planar Franck-Condon geometry. The geometric coordinates are (a) torsion of the C2—C3 and C3 C4 bonds and (b) asymmetric stretching coupled with pyramidalization. Both Si and So intersect at a conical intersection (Si/S0 Cl) located near the minimum of the Si surface (Min-C ) where the C2C3C4N5 torsion angle is 104°. [Reproduced with permission from [87], Copyright 2000 Amercian Chemical Society],...

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

The results of differential scanning calorimetry(DSC) indicate the change in aggregation state. The trans micelle showed a main endothermic peak at 14 2°C(A H =1.0 kcal/mol), corresponding to a gel-liquid crystal phase transition, whereas the transition temperature for the cis micelle appeared at 11.9°C( AH = 0.8 kcal/mol). This is unequivocal evidence that the trans-cis photoisomerization is a sufficient perturbation to alter the state of molecular aggregation. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

Figure 5. Read-out intensity change(I.j./Iq) and absorbance change (At/A0) owing to trans - cis photoisomerization of BMAB in 5CB liquid crystal at 34°C. [BMAB] = 4.9 mol%(1) and 3.0 mol%(2).

McGrath and Junge [36] reported a photoresponsive poly(aryl ether) dendrimer with azobenzene as the dendrimer core. These dendrimers exhibited reversible trans to cis photoisomerization by irradiation at 350 nm. The authors proposed the use of this type of dendrimer as novel photoswitchable transport vectors. This is based on the expected ability of dendrimers to encapsulate or eject small molecules reversibly upon light perturbation. 

Figure 8.11. Diffuse reflectance absorption spectra of a strongly fluorescent sample (1,6-diphenylhexatriene adsorbed on porous alumina) (a) conventional measurement w ith monochromatic irradiation and detection via an integrating sphere (b) measurement in a fluorimeter with two monochromators. Reaction spectra during Irons - cis photoisomerization are also given (adapted from Ref. 26).

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

Other y-lactone containing systems that have been studied include derivatives of 3-oxabicyclo[3.1.0]hexan-2-one173 and 3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3//)-one (213), which on irradiation in methanol undergoes a trans-cis photoisomerization followed by a photochemically induced trans esterification to give the coumarin (214).174... 

All the chromophores are thermally stable with decomposition temperatures varying from 230 to 263°C. Thienyl azodyes linked to a polyester matrix were obtained for optical storage material, where the trans-cis photoisomerization was used at this purpose (07MI4477). 

Reddy M, Reddy V, Srinivas U, Reddy M, Rao V. Regioselective E(trans)-Z(cis) photoisomerization in napthyldiene derivatives. Proc Indian Acad Sci (Chem Sci) 2002 114(6) 603-609. 

Azobebzene undergoes trans-cis photoisomerization and this photoswitchable behavior can be used for the photoregulation of various molecular phenomena. A simple example is on-off photoregulation of ester hydrolysis by using an azoben-... 

Azobenzene with two (3-CD units (117) was prepared and found that it undergoes trans to cis photoisomerization with 66% cis at the photostationary state [107], The cis form returns to the original trans form with a half-life of 55 hr at 25°C. The azobenzene derivative was expected to bind one large molecule by the hands of two CD units, but so far no data were reported on this behavior. 

A mixture of an azobenzene derivative and an FLC (Figure 2), in which the concentration of the azobenzene guest was 3 mol%, was prepared in the surface-stabilized state in a very thin LC cell. Then the mixture was irradiated with light at 366 nm to cause trans-cis photoisomerization of the azobenzene guest molecule. It... 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

Of course, stability and formation of HFP-azosulfonyl-Lys complexes is less favored on going from pure HFP to HFP/MeOH or HFP/DCE solvent mixtures. At appropriate and critical solvent compositions, the formation of the electrostatic complexes described above might be favored or inhibited by the electronic situation of the azo moieties, which differs depending on whether they are in the apolar, conjugated trans form, or in the more polar, unconjugated cis form. In other words, the trans-cis photoisomerization of the azo units, which is the primary photochemical event, does not seem to be sufficient to induce appreciable variations of the back-... 

A sample of poly(L-ornithine) containing 48 mol% azo units was found to adopt the a-helix structure in HFP/water = 1/1. In this solvent mixture, however, irradiation at 360 nm followed by irradiation at 460 nm produced the trans-cis photoisomerization of the azo moieties, but did not induce any change of the backbone conformation. When the surfactant sodium dodecyl sulfate was added to the HFP/water solvent mixture, the CD spectrum displayed an intense side chain CD couplet and a negative band at about 225 nm which was assigned to the presence of a P-structure. The CD bands were almost completely abolished upon trans-cis photoisomerization. 

The azo-modified, elastin-like polypeptide XIV illustrated in Scheme 9 exhibits a so-called inverse temperature transition" that is, the compound gives cross-linked gels that remain swollen in water at temperature below 25 °C but deswell and contract upon a rise of temperature. The trans-cis photoisomerization of the azo units, obtained through alternating irradiation at 350 and 450 nm, permits photomodulation of the inverse temperature transition.[S9] The result indicates that attachment of a small proportion of azobenzene chromophores is sufficient to render inverse temperature transition of elastin-like polypeptides photoresponsive, and provides a route to protein-based polymeric materials capable of photomechanical transduction. 

Sekkat et al. 93 have investigated the photobehavior under polarized light of spin-coated films obtained from a hairy-rod poly(L-glutamate) with azobenzene in the side chains (Scheme 11, compound XIX). It is known 96,971 that, in the presence of a linearly polarized pump light beam, azo molecules experience a cycle of photoisomerization reactions and align themselves perpendicularly to the pump beam polarization direction. For the above polypeptide it was shown that, in the trans cis photoisomerization cycle, the created cis state is aligned perpendicularly to the polarization of the pump beam. 95 ... 

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