Azo-polysiloxanes

Modified Azo-Polysiloxanes for Complex Photo-Sensible Supramolecular Systems... 

Another possibility for azo-polysiloxane applications is to obtain photo-sensible micelles [15-18]. The interest for this application is explained by the possibility to use polymeric micellar aggregates for controlled release of substances such as drugs [19, 20]. There are few reports on the use of light as an external stimulus for small amphiphilic molecules by incorporahug the azobeuzeue chromophore iuto surfactant [16] or for light-responsive micellar aggregates formed by amphiphilic block copolymers [17,18]. 

The azo-polysiloxanes were obtained in a two-step reaction, starting from a polysiloxane containing chlorobenzyl groups iu the side-chaiu. Iu the first step, the polysiloxane was modified [21] with 4-hydroxyazobenzene (50-75% substitution degree) and, in the second one, the uiueacted chlorobenzyl groups were substituted with nucleobases or douor/acceptor groups. 

In the second step (Scheme 1), 0.5 g azo-polysiloxane was dissolved in 7ml DMSO under stirring the necessary quantity of nucleobase (as a function of the imposed substitution degree) and -0.1 g K COj were added and then the reaction mixture was heated 9 h at 55°C (under nitrogen atmosphere). The polymer was precipitated in methanol and washed 3 times with methanol to eliminate the unreacted products and dried under vacuum. In the case of donor/acceptor groups, 0.5 g azo-polysiloxane was dissolved in 7 ml DMSO under stirring the necessary quantity of sodium phenoxide and 0.1 g Bu NHSO were added and then the reaction mixture was heated 4-5 h at 80°C. The polymer purification was similar. 

The amphiphilic polymers were obtained, too, in a two step reaction, in agreement with Scheme 1. The azo-polysiloxane obtained in a first step (as previously described) and containing unreacted chorobenzyl groups (40%) was quatemized with triethyl and tributylamines in solution (CH Cl ) at 30°C, using reaction times situated between 5 and 7 h. 

The first investigated azo-polysiloxane contains nucleobases in the side chain. This azo-polymer class presents a potential interest in the fields of optoelectronic or biomolecnle immobilization and nanomanipulation. The optoelectronic applications are the consequence of the azo-materials capacity to generate surface relief grating (SRG) under UV irradiation. In spite of the fact that the SRG problem is stndied by many research gronps, nntil now, the mechanism of the surface... 

Structuration is not completely elucidated. It is well known that essentially for the surface structuration is the trans-cis isomerization process of the azo-groups, under UV-VIS irradiation. As a consequence, a very careful study regarding the pho-tochromic behavior of the azo-polysiloxanes was effectuated. The photochromic response at the light stimuli was evaluated in different conditions trans-cis isomerization under UV irradiation cis-trans relaxation in the presence of the natural visible light cis-trans relaxation only thermally activated (in the dark). It must be underlined that there are no reported studies in the literature concerning the cis-trans relaxation process in the presence of natural visible light, a very important aspect in our opinion regarding the elucidation of SRG formation mechanism. 

Fig. 1 H-NMR spectrum corresponding to an azo-polysiloxane modified with adenine...

Figure 10 presents the kinetic trans-cis photoisomerization process, under UV irradiation in the solid state, hi this case, significant differences appear between samples behaviour, as a function of the nucleobase chemical structures. It is interesting to note that, in the case of azo-polysiloxane substituted with adeiune (sample 2 -Table 1), the behaviours in the solid state and in solution are similar. That means that the polysiloxane chain flexibility, combined with the amorphous polymer ordering assure enough free volume for the trans-cis isomerization process. 

A very interesting behaviour is obtained for the azo-polysiloxane modified with adenine (Sample 2 - Table 1). In spite of the fact that the trans-cis isomerization process, as a result of the UV irradiation, is very fast in the solid state (similar to the behaviour in solution), the relaxation takes place in two steps, the recuperation rate in the first step being only 15 %. This behaviour can be explained by some associations that can take place between the cA-azobenzenic groups and adenine. The same curve profile, but less evident, is obtained for the azo-polysiloxane modified with cytosine... 

Sample 5 - Table 1) which has a chemical structure similar to that of adenine. A faster relaxation process is obtained in the case of the azo-polysiloxane modified with thymine, this behaviour being in agreement with the idea of the H-bonding formation that can create a 3D-physical network which forces the azobenzenic groups to relax faster. 

Figure 12 presents the cis-trans relaxation process in the dark (only thermal activation) which is much slower that in the presence of natural visible light. For all the systems, an important percentage of relaxation takes place in the first 24-30 h after this period, the relaxation rate decreases dramatically. As in the case of the natural visible light activated relaxation process, the dark relaxation processes of the azo-polysiloxanes containing adenine and cytosine are very similar. 

Concerning the photochromic behaviour, the situation is different if we compare this polymer group with the azo-polysiloxanes modified with nucleobases, especially concerning the relaxation process. For the photochromic behaviour in solution and in the solid state, there are some differences concerning the maximum conversion degree of the azo-groups in the cA-form (about 15-20%). Figs. 14 and 15 present the photochromic response under UV irradiation of the Samples 6-8 in solution and in the solid state, respectively. 

One can observe that the response rate in solntion to the UV irradiation is faster for the systems containing naphthalene and anthracene nnits, comparatively with the p-nidophenol containing azo-polysiloxane. In the solid state, the differences concerning the response rate diminish, bnt the maximnm CM-gronp conversion degree is sitnated only at abont 52%. 

One can underline that, in the case of the azo-polysiloxanes containing donor/acceptor groups, the relaxation process is total, no associations between cw-azobenzene and donor/acceptor groups (that can disturb the relaxation process) being evidenced. 

Fig. 17 Plot of the Ij/Ij ratio as a function of the azo-polysiloxane concentration corresponding to Samples 9 and 10...

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