3.1 Basic Concept
Summarizing the previous chapters, the following main conditions must be met when creating a new system based on azobenzene that undergoes a dramatic geometrical change upon photoisomerization.
• Incorporation of the responsive moiety in macromolecular scaffolds
• Integration of a high number of switchable elements
• Construction of rigid architectures
• Integration of anisotropic characteristics
The rigid-rod polymers depicted in Figure 16 offer an excellent starting point for the new light-responsive system since all the aforementioned requirements are already met.[51] Furthermore, for a similar polymer design exhibiting less solubilizing groups, the publication describes the formation of spherical aggregates in non-polar organic solution. Aiming to gain full control over the assembly process, exchanging the hydrophobic design for an amphiphilic one should allow the formation of cylindrical micelles in aqueous medium (Figure 17).
Figure 17. Conceptual illustration of the targeted light-responsive system based on an amphiphilic rigid-rod polymer that dramatically changes size and shape upon photoisomerization of the azobenzene units, the formation of cylindrical micelles is disturbed by the bended structure of the polymers Z state leading to a complete disappearance of the aggregates at best, R and R’ denote hydrophilic and hydrophobic groups, respectively.
In this case, photoinduced shrinking of the single polymer chains upon E→Z isomerization should efficiently disassemble the micelles. Assuming that the linear structure of the rigid polymers would
be necessary to form micelles of cylindrical shape, a potential cooperative effect might be observed as soon as the number of straight polymer chains is below the critical micelle concentration (CMC). Additionally, the CMC should rise upon isomerization due to the higher polarity of Z azobenzene and the accompanied better solubility in water.
3.2 Molecular Design and Retrosynthesis
The targeted polymer comprises an aromatic backbone with side chains of hydrophilic (head group) and hydrophobic (tail) nature. Normally, the size of these substituents and their occupied volume is highly important when striving for specific micelle geometries. The rigid linear structure of the azo macromolecule, however, determines a cylindrical aggregation shape since assembling into spherical objects would require unfavored bending of the rods under the assumption that polar and non-polar groups can only aggregate among similar moieties. Nonetheless, the hydrophilic outer shell of micellar aggregates is larger in comparison to the lipophilic interior space when considering the circular cross section of the cylinder. Hence, the hydrophilic groups should be attached to the azobenzene aromatic rings which occupy more space than the single phenyl linker.
Typical lipophilic substituents for the formation of micelles and vesicles are alkyl chains of different size in which especially dodecyl chains have been proven suitable in numerous publications. Furthermore, this group is already present in the original polymer design providing a starting point for synthetic considerations. However, modifications are required in order for both C12-chains to point in the same direction, hence facilitating the aggregation. On the polar side, many water solubilizing moieties are available with glyme chains, ammonium ions, and sulfonates being the most popular. The former holds the advantage of no charge which simplifies synthesis and avoids influence of counter ions on micelle formation. The hydrophilic and hydrophobic groups are attached in ortho position with respect to the bond between the azobenzene and the phenyl linker inducing a twist angle necessary for electronic decoupling and thus efficient photoisomerization leading to high PSS compositions.
Figure 18. Final design of the polymer potentially forming cylindrical micelles in aqueous medium, two dodecyl chains at the phenyl linker and two polar groups, i.e. triglyme derivative, sulfate, ammonium ion, at the azobenzene constitute the amphiphilic properties, the triglyme derivative is the first target since there is no charge involved facilitating the synthesis and the micelle formation due to the absence of counter ions.
The final design is depicted in Figure 18. The phenyl linker bears two dodecyl chains pointing in the same direction that are attached in the form of alkoxides at benzylic position. The azobenzene unit has oxygens in two meta positions that can be easily substituted by different polar groups via etherification. The triglyme derivative is the first target due to the reasons mentioned above.
Two pathways can be followed for the synthesis of the polymer, i.e. the Suzuki polycondensation approach and the polyazo coupling approach (Figure 19).
Figure 19. Two retrosynthetic pathways can be followed towards the amphiphilic polymer, i.e. the Suzuki polycondensation approach (top) and the polyazo coupling approach (bottom); while synthesis of both monomers for the Suzuki reaction is easier, material of high purity is needed to meet the exact 1:1 stoichiometry; the azo coupling require only one monomer which is more difficult to make due to a higher degree of functionalization.
In the former case, the polymer backbone is dissected between the aromatic rings yielding two synthetic equivalents that are cross-coupled by a Suzuki reaction. The advantage of that method is the usually easier accessibility of the monomers since they are not as highly functionalized as a monomer exhibiting all substituents at once. The drawback, however, is the requirement of ultrapure materials because exact stoichiometry is of great importance when striving for products of high molecular weight in an A2 + B2 polymerization. Even slight deviations from a 1:1 ratio can result in significantly smaller macromolecules. In contrast, the second approach displays opposite characteristics, i.e. stoichiometry is negligible as only one monomer exists, however, the combination of all functional groups in one molecule renders synthesis more difficult. The strategy of the polyazo coupling was followed already by Wiktorowicz et al. with good results employing Red-Al as reductive agent.[52]
First, the Suzuki polycondensation approach is followed since the easier synthesis gives potentially faster access to polymeric material.