Isomerization of azobenzene residues

D. Tabak and H. Morawetz, Rates of conformational transitions in solutions of randomly coiled polymers. III. Cis-trans isomerization of azobenzene residues in the backbone of polyamides, Macromolecules 3, 403 410 (1970). 

More recently, a similar photochromic behavior was observed by Eisenbach32,33) for the cis-trans isomerization of azobenzene residues attached as side groups to copolymers when examined below their glass transition temperature. These copolymers were obtained by copolymerization of the monomers 4-methacryloylaminoazobenzene (TV) and 4-(3-methaeryloyloxybutylcarbonylamino)azobenzene (V) with various alkyl acrylates and methacrylates. 

THERMAL CIS-TRANS ISOMERIZATION OF AZOBENZENE RESIDUES IN THE BACKBONE OF POLYMER CHAINS... 

Both the hindered rotation around the amide bond and the thermal cis-trans isomerization of azobenzene residues are characterized by high energy barriers in the neighborhood of 20 kcal/ mole. Some years ago, Malkin and Fischer studied the photochemical isomerization of azobenzene and found that the quantum yield is temperature-dependent they interpreted their data as reflecting an energy barrier of 2-3 kcal/mole between the excited cis and trans species. A comparison of the quantum yields for the photoisomerization of azobenzene residues in the backbone of polymer chains and in low molecular weight analogs should, therefore, indicate whether conformational transitions characterized by low activation... 

On alternate irradiation of UV and visible light, the viscosity was changed as much as 60%, as shown in Fig. 3. The content of the rra 5-azobenzene form estimated from the absorption spectrum was related to the viscosity on alternate irradiation with UV and visible light. The similarity in behavior suggests that the photodecrease and photoincrease in viscosity are directly correlated with the isomerization of azobenzene residues in the polymer backbone. The isomerization from trans- to c/5-form kinks the polymer chain, resulting in compact conformation. The compact conformation returns to the initial extended conformation either thermally or by visible light irradiation. 

A comparison of the photochemical behavior of azobenzene residues In the backbone and the side chains of polymers Is of special Interest since the short lifetime of the excited state makes It more difficult for the chain to undergo long-range conformational changes favoring the trans-cls Isomerization process. 

The experimental results revealed no significant difference in the rate of photoisomerization of azobenzene residues in the backbone of polyamides and in low molecular weight analogous azobenzene derivatives when both were studied in dilute solution.28 However, while the photochemical reactivity of the small species was relatively insensitive to the concentration of added polymer, the quantum yield for the photoisomerization of the azobenzene residues in the polymer backbone dropped precipitously with increasing concentration. In a glassy polymer film containing 8% DMSO plasticizer, the quantum yield for the isomerization of the polymer was reduced by a factor of 2500 while it was reduced only by a factor of 5 for the small molecule (Figure 3). 

The photoisomerization of trans-azobenzene to the cis form and the thermally activated back reaction can be followed conveniently by UV spectroscopy. We used these processes to compare the behavior of azobenzene residues in the backbones of polymer chains and their low molecular weight analogs. This approach has the advantage over the NMR technique in that we are not limited to solutions of high fluidity, but can study also very viscous and even glassy systems. Data obtained on the dark reaction indicated again no difference between the rates of azobenzene residues in polyamide backbones and in small molecules (8). The isomerization rate of the polymer remained unchanged even when the solution concentration was raised to the point where the molecular chains were heavily intertwined. 

Here the quantum yield of a small number of azobenzene residues built into the backbone of a polyamide was compared with the quantum yield of an analogous low molecular weight azobenzene derivative in a series of polyamide - DMSO mixtures. As expected, the quantum yield for the azobenzene residues in the polymer backbones was very small in systems containing a high polymer concentration, since here conformational transitions of the polymer chains would necessarily be cooperative, while the photoisomerization of the small molecule was relatively insensitive to the polymer concentration. However, in highly dilute solution there was absolutely no difference between the photoisomerization efficiency of azobenzene residues built into the polymer chains and the small azobenzene derivatives. This identity in the behavior of the polymer and its analog in dilute solution shows that the isomerization of azobenzenes in the chain backbone can be accomplished without a simultaneous hindered rotation in another portion of the chain. 

The E-Z isomerization of an azobenzene unit was employed in an approach towards photocontrol of the chiral recognition event in a membrane.1581 To this end, (4-(phenyl-azo)phenyl carbamate residues were attached to carbamate-protected glucose units of cellulose and amylose. The photomodulation of the chiral recognition was explained by a change in the ordering of the polymer, leading to a change in solubility. 

Figure 4 Isomerization of the azobenzene moiety in the side chain of the residue upon photoirradiation of oligonucleotide 15 (A) and schematic illustration of photoregulation of the formation and dissociation of a DNA duplex (B).

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The large difference in the response times for optical absorption and light scattering suggests a two step mechanism for the photostimulated unfolding process. During the isomerization of the backbone azobenzene residues, the total chain conformation maintains in the initial compact conformation. After the isomerization is completed, the conformation relaxes to a more stable extended conformation in 1 ms. 

The transition state is then still in equilibrium with the ground state and incorporation of the azobenzene residue Into a polymer chain has no effect on Its rate of isomerization. 

In contrast to the first-order kinetics of the conventional azobenzene isomerization in solution, a slight deviation from first-order kinetics was observed even in aqueous solutions for polyelectrolytic azobenzene polymers substituted with hydrophobic alkyl residues (14, Fig. 4).43 In an aqueous solution of the amphiphilic polysulfonates containing a small amount of azobenzene moiety, the extent of E-to-Z photoisomerization under UV irradiation was reduced as a result of the compartmentalization of the azo chromophores in a hydrophobic core, which resulted in motion of their restriction. 

Also, when copolymers of polystyrene and 4-(methacryloyl-amino)azobenzene containing 2.2-6.5% of the latter are irradiated in a cyclohexane soluticMi with 15 flashes of 347 nm of Ught. The polymeric chains contract [221], This occurs at a high rate per second as a consequence of isomerization. At a later stage, several hundred seconds after the flash, there is evidence of polymer aggregation and precipitation [221], In addition, when azobenzene residues are introduced into the main chain of poly(dimethylsiloxane), reversible solution viscosity changes can be obtained by irradiation with ultraviolet light [221],... 

FIGURE 5.5.3 Control of cell adhesion on photoresponsive surfaces, (a) Access control of GRGDS peptides for biospecific cell adhesion by photoinduced cis-trans Isomerization of terminal azobenzene residues in grafted polymers (54). 

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

Azobenzene derivatives may be photoisomeriz-ed from the stable trans form to the cis isomer and the reverse reaction takes place at a convenient rate by a thermal mechanism in the neighborhood of ambient temperatures. Since the isomerization is accompanied by a large shift in the ultraviolet spectrum, the kinetics of both the photochemical and the thermal process may be followed by UV spectroscopy. We have utilized this technique for comparing rate constants of the thermal reaction, 27 and the photochemical quantum yields S in polymers containing azobenzene residues and in their low molecular weight analogs. 

Thus, any deviation from first-order kinetics will indicate that the azobenzene residues in the system under study isomerize at different rates. Such a dispersion of the rate constant for hindered rotation was postulated by other investigators. Cochran et al reached this conclusion on the basis of the molecular weight dependence of the relaxation times observed with polystyrene by sound absorption studies. Valeur and MonnerieS subjected a solution of polystyrene with anthracene residues incorporated into the chain backbone to a nanosecond light flash and found that the time-dependence of the anthracene fluorescence deviated from a simple exponential decay this was ascribed to differences in the local conformational mobility of the chain. 

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