Multi-Switchable Nano-Objects Based On Photoacids

Addressing climate change remains a significant global challenge, emphasizing the need for more efficient utilization of renewable energy, with a particular focus on maximizing the potential of solar energy. To fully exhaust all possibilities and the potential of solar energy it is necessary to also design new light-responsive structures, which respond to light in different ways. In this thesis, a new type of switchable nano-objects is assembled from various photoacid molecules in form of disulfonated naphthol derivates with generation four (G4) poly(amido amine) dendrimers as model polyelectrolytes. Well-defined nanoparticles are formed spontaneously under light exclusion with hydrodynamic radii ranging from 80 nm ≤ RH ≤ 360 nm in aqueous solution. Due to the unique property of photoacids to undergo an excited-state dissociation of the photoacidic hydroxyl group, the assemblies can be switched through irradiation. This leads to a change in size and shape of the self-assembled nano-objects in solution. The structural characterization of the nano-assemblies is studied with dynamic light scattering (DLS), static light scattering (SLS), small angle neutron scattering (SANS) and atomic force microscopy (AFM). While the thermodynamics of the interactions are investigated using isothermal titration calorimetry (ITC) and UV/Vis spectroscopy reveals molecular characteristics, the charges of the assemblies are characterized with ζ-potential measurements. By adding another triggerable molecule, it is possible to form multi-responsive supramolecular structures. Therefore, different types of molecules have been combined. First, the dianionic azo dye Acid Yellow 38 (AY38), which exhibits a trans-cis isomerization upon UV-irradiation, directly forms ternary assemblies with a photoacid and the dendrimer. Since the azo dye can be triggered with a different wavelength than the photoacid, a multi-responsive system with different switching possibilities of assembly size and shape is formed. The stability of these nano-assemblies can be understood by the different interaction strength of the cis and trans form of AY38 with the polyelectrolyte. As a further type of triggerable molecule, we choose a hydroxyflavylium molecule. It is known for its network of light-dependent reactions. By combination with the photoacid, two main cycles are possible responding to light irradiation and pH-changes. The reversibility of the switching cycles and structures can be controlled by the component ratios. Lastly, the photoacid is combined with a water-soluble spirooxazine without any polyelectrolyte. Despite the anionic character of both molecules, nano-assemblies are formed directly though dipole interactions. Characteristic ring opening reactions of spirooxazines lead to structural changes due to varying π-π and dipole interaction strength of the isomers. Overall, this thesis establishes a new concept of using light responsive molecules for supramolecular assembly controlling the shape and size of the resulting nano-objects. The thesis further emphasizes the importance of induced secondary interactions, like hydrogen bonds and π-π interactions, and their fundamental role in self-assembly. Not only are photoacids interesting for multi-switchable assemblies but also for future applications such as switchable hybrid nano-structures.