Cis-trans isomerization of azobenzene residue

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... 

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). 

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. 

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-cis isomerization process. 

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. 

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. 

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