Synthesis and Characterization of Photo-Responsive Carbosilane Dendrimers

Preparation of photo-responsive carbosilane dendrimers bearing 4-phenylazo-benzonitrile units on their molecular surface has been accomplished, and their both photo and thermal behaviors have also been characterized. These functional dendrimers suggest that the apparent molecular sizes of the cis-isomers are smaller than those of the corresponding trans-isomers, since the molecular diameter of these dendrimers would be shorter on the basis of trans→cis photo-isomerization of azobenzene.


Introduction
Dendrimers are unique macromolecules with a regular, three-dimensional, spherical structure. They have attracted much attention as functional materials because they can integrate functional groups under similar conditions on their molecular surface. In our ongoing synthetic studies of carbosilane dendrimers uniformly covered with functional molecules, we have previously reported the preparation and characterization of carbosilane dendrimers carrying mesogenic groups [1] and sugar moieties [2][3][4][5][6][7].

OPEN ACCESS
Azobenzene derivatives are well-known as photo-switchable functional moieties that undergo photo-induced (trans→cis) and thermal (cis→trans) isomerization processes [8,9]. Preparation of linear polymers bearing pendant azobenzene derivatives and a dendrimer carrying an azobenzene derivative in order to investigate the photo-responsive properties has been reported [10]. It is interesting that the molecular size of the dendrimer carrying azobenzene derivatives is controlled on the basis of photo-and heat-isomerization abilities of the azobenzene unit. Therefore, these specific molecules have potential applications in conversion of photo-energy into dynamic energy as well as in drug delivery system. In this paper, we describe the preparation and characterization of a series of new photo-responsive carbosilane dendrimers 1-5 uniformly functionalized with azobenzene moieties, as shown in Figure 1.

Synthesis of Carbosilane Dendrimer Derivatives
Scheme 1 summarizes the synthetic construction of carbosilane dendrimers having photoresponsive moieties at each terminal end. A series of carbosilane dendrimers 6-9 were prepared by the method described in the literature [11]. These dendrimers were then converted into the corresponding hydroxyl derivatives 10-13 by hydroboration reactions [12]. Then 10 was treated with 6-bromohexanoyl chloride in the presence of pyridine to give 14, in which the bromine atom was next replaced by an iodine atom, giving compound 18, by the Finkelstein reaction [13][14]. Reaction of 18 with 4-(4hydroxy-phenylazo)benzonitrile (= PAB) was carried out in the presence of potassium carbonate to give 2 in good yield. Ethyl 6-[4(4-cyanophenylazo)phenoxy]-hexanoates 1, 3 and 4 were synthesized in a manner similar to that described above. In contrast, the hydrosilation of 7 with HSiMeCl 2 , subsequent allylation and introduction of an azobenzene derivative gave 5, which had a decreased number of functional groups, compared to that of 4. Scheme 1. Synthetic construction of carbosilane dendrimers having photo-responsive moieties.

Characterization of Photo-Responsive Carbosilane Dendrimers
UV spectra for 4 in THF solution both before and after irradiation (360 nm) were measured at room temperature ( Figure 2). An absorption maximum of the trans-isomer was observed at 362 nm [ Figure  2(a)], and intensity of that of the cis-isomer decreased and the peak top of the wavelength blue-shifted to 323 nm after UV irradiation [ Figure 2(b)]. However, complete transformation of the trans-isomer was not accomplished due to the heat irradiation from light source. The same experiments were performed for 1, 2, 3 and 5, and the results of the UV absorption experiments of 1, 2, 3 and 5 are summarized in Table 1.  Compounds

Analyses of the Molecular Size of the Dendrimers by GPC
It is presumed that dendrimers are globular molecules, and it was expected that the diameter of the dendrimers described in this paper would be decreased by the irradiation (360 nm) because the total length of the cis-azobenzene molecule is shorter than that of the trans-one [10]. To observe these properties, Dynamic Light Scattering (DLS) measurement is usually used. We attempted direct measurements of the dendrimers. Since the sizes of the synthesized dendrimers were smaller than the detectable limit of observation, the measurements were unfortunately not successful. We then attempted to determine the three-dimensional molecular sizes of the dendrimers by gel permeation chromatography (GPC).
Trans-isomers of 3 and 4 in THF solution were injected into the GPC apparatus, and the corresponding retention times were measured. The retention times of trans 3 and 4 were 12.87 and 11.97 min, respectively ( Table 2). The weight-average molecular weights of 3 and 4 calculated from the retention times corresponded to 6,300 and 13,600, respectively, although neither of them were in agreement with the theoretical molecular weights because of the use of linear polystyrene standards for the calibration curve.
The cis-isomers prepared by irradiation (365 nm) were analyzed in a manner similar to that described above. The retention times of the cis isomers of 3 and 4 were 13.00 and 12.09 min, respectively, and the corresponding weight-average molecular weights were 5,800 and 12,300. However, it is difficult to conclude directly that these data show the difference in molecular sizes of the trans-isomer and the cis-isomer because there are complicated interactions arising from polarities between the gel and the azobenzene unit in the GPC column. It should be noted that the differences in retention times were reproducible.

Thermal Isomerization Processes of Dendrimers
The thermal isomerization behavior of 1~5 was then investigated using 1 H-NMR experiments. The heat-isomerization process of the cis-isomers was followed by placing them in a fixed temperature NMR probe (30 °C, 35 °C, 40 °C, 45 °C and 50 °C), and the cis→trans isomerization rates were calculated from the 1 H-NMR signals in the aromatic region (trans-isomers: 6.7-8.2 ppm, cis-isomers: 6.3-7.8 ppm). The activation energies and the frequency factors were then calculated with Arrhenius plots, and the results are summarized in Table 3.
The activation energies of 2 and 3 are almost equal to that of 1, while the activation energy of 4 is significantly higher than that of 2 and 3. It is thought that the cis→trans thermal-isomerization is slightly prevented in the higher generation dendrimer because the azobenzene units in 4 would be in more crowded conditions and are piled up one upon another. In order to provide support for this notion, the activation energy of 5, which has a smaller number of azobenzene units than 4, was further measured. The cis→trans activation energy of 5 obtained was 119.7 KJ/mol, which is similar to that of 1, 2 and 3.

Crowding of the Molecular Ends of Dendrimers
When the ends of the molecules are crowded, as suggested above, the motion of the azobenzene unit would be slowed down. Therefore, investigation of the T 1 value of 1 H on the azobenzene units was carried out (Figure 3). If T 1 of 4 is smaller than that of other molecules, it would be evidence that the molecular end of 4 is crowded because a small T 1 means low mobility of the unit. The highest T 1 value of the 1 H on the azobenzene unit was found for 2, and 2, 3 and 5 had almost equal T 1 values. However, the T 1 value of 4 was lower than the values of the other molecules. These results indicate that 4 has the lowest motional freedom among the dendrimers described above.

Conclusions
Although various azobenzene-substituted dendrimers have been prepared in terms of their photoresponsiveness [15][16][17][18][19], there have been few studies on the problems generated from their conformation. In this paper, we have described the preparation of a series of carbosilane dendrimers having photo-sensitive functionalities and their isomerization process. The results of this study suggested that the densities of the molecules influence their isomerization.