Light-responsive self-assembly towards supramolecular machines

Incorporating artificial molecular motors and switches into self-assembled systems allows transferring and amplifying the forces that these molecules generate, across increasing length scales. Interfacing the mechanics of such molecules with larger self-assembled architectures represents a viable strategy to fight against overwhelming Brownian storm and viscosity of liquid environment. Thus the focus of my research was on developing controllable light-responsive supramolecular systems by integrating molecular photoswitches into self-assembled architectures. Photo-switching was selected as an ideal strategy to alternate between assembling and non-assembling isomers reversibly, repeatedly, with an exquisite control over the timescale of the process, without accumulating chemical waste and in combination with spontaneous reversed switching. The artificial molecular photoswitches involved in this work were azobenzenes and spiropyrans. Upon irradiation with light, azobenzenes offer geometrical changes between cis and trans isomers, whereas spiropyrans provide large differences in their charged character.
Chapter 1 provides a general introduction to this thesis. Chapter 2 reviews recent progress light-responsive supramolecular tubular systems. Our motivation to work with tubular systems lies in the versatility displayed by the cellular microtubes that operate in the cell. In Chapter 3, we show how strain builds up in wholly artificial tubules, upon trans-to-cis photo-switching. The light-fueled accumulation of the strain eventually leads to the catastrophic burst of the tubules, in a mechanism that is reminiscent of the disassembly mechanism of cellular microtubules. In Chapter 4, we demonstrate a strategy that allows connecting the tubules discussed in chapter 3, with either surfaces, or with each other. In Chapter 5, we have synthesized a spiropyran-based amphiphile that self-assembles into vesicles, in water. Upon irradiation with light, merocyanine converts into the spiropyran, which leads to the transient and reversible expansion of the vesicles. In Chapter 6, we present efforts towards combining tubes with spiropyran switches, because they would promote tubular growth upon irradiation with light, when the polar protonated merocyanine converts into the spiropyran.
Overall, we have introduced a framework for the development of light-controllable nanosystems in water, with a dynamic behavior ranging from assembly, to disassembly and transient growth.