Photoisomerization of azobenzene residue

THE PHOTOISOMERIZATION 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... 

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). 

Figure 2. Relative quantum yields in the trans-cis photoisomerization of azobenzene residues in the backbone of a polyamide ( ) and its low molecular weight analog (o) in mixtures of unlabeled polyamide and dimethyl sulfoxide.

Even when the azobenzene chromophores are incorporated into the polymer backbone, the dipole moment increase of azobenzene residues by photoirradiation can also induce a change in polymer chain conformation. The solution viscosity of poly(dimethylsiloxane) containing azobenzene residues in the main chain decreased upon ultraviolet irradiation, and the effect was attributed to the trans to cis photoisomerization [9]. The photodecrease of the viscosity depended on the polarity of the solvent. It was 24% in non-polar heptane, but negligible in polar dichloroethane. 

Sung et al. measured, by repeated laser pulse irradiation, the trans to cis photoisomerization of azobenzene chiomoidiores incorporated at three specific sites on the polystyrene chain the chain center (C-PS), a chain end (E-P or a side group (S-PS). Typical changes in trans content during pulse irradiation are shown in Fig. 14.Sungetal. considered that the microenvironment oftheazobaizene residue... 

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. 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

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. 

The photoresponsive behavior of a y-CD capped with an azobenzene moiety is very different from that of the fi-CD analog. Photoisomerization of the trans-AE regioisomer 129 led to a photostationary state containing only 47% cis isomer, which is much lower than that obtained with azobenzene as a free species or as a pendant (> 80%). This indicates that a stereochemical difficulty exists for trans to cis conversion. Since the cis photoisomer requires a distance of 6 A between the 4,4 sulfur atoms and the A and E glucose residues are located 9 A apart, it was concluded that the y-CD shape changes from a circle to an ellipse and that the cis isomer is... 

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... 

Paik and Morawetz [21] found that methacrylate or styrene copolymers bearing small numbers of azobenzene or azonaphthalene residues underwent somewhat slower photoisomerization in the glassy state than in solution. The photostationary states were similar in bulk and solution. In analogous polyamides and polyesters, photoisomerization was drastically reduced in bulk. 

Hov ver, Ekenbach considered it due to the restricted flucUiation of free volume. To diminate such unstaUe cis azobenzene molecules in the polymer, we prq>ared a cis-rich sample cast into film from solution imder continuous UV li t irradiation. Its thermal isomerization reaction ( in Fig. 13) almost obeys first-or kinetics, supxnrting Hk idea that the existence of residual strain in the ds-azobenzme film sample prepared by photoisomerization is responsible for the anomalous deviation from first-order kinetics. 

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