Diaryl-Hemiindigo and Aza-Diarylethene based Multi-State Photoswitches for Smart Materials Generation

This thesis is inevitably tied to multi-stimuli responsive, multi-state molecular photoswitches and their applications in materials science. Therefore, two new structural classes of molecular photoswitches have been developed: diaryl-hemiindigos (diaryl-HIs) and zwitterionic aza-diarylethenes (aza-DAEs). Both structural classes were investigated in solution and advanced materials applications. Diaryl-HIs consist of one indigo-half and a stilbene-moiety carrying an additional aryl-substituent. They are synthesized by a new, modular, high yielding and robust three- to four-step counting route. Diaryl-HIs are green and red light responsive and more bathochromically shifted in absorption than comparable monoaryl-HIs or their mono- or diaryl-substituted sulfur analogues. Strong photochromism, color contrasts and high isomer enrichment between states, as well as high thermal isomerization barriers that can be modulated upon acid addition make diaryl-HIs potent multi-stimuli responsive molecular tools in solution and polymers. Acid and base addition allows a four-state switching cycle including a chemical fueling step, populating a metastable state. The first and second generation of zwitterionic aza-DAEs are comprised of a central cyclopentene core, non-symmetrically substituted at the double bond with one pyrimidine- and one thiophene-derivative. Upon light irradiation, a 6π-aza-electrocyclization reaction allows the reversible formation of a C=N bond building up a positively charged bicyclic aromatic ring in conjugation to a negatively charged thiolate-unit forming the zwitterion. Depending on the substituents, the zwitterions can either isomerize photoinduced and/or thermally to the open isomer again. Acid addition can serve as a trigger to modulate thermal stabilities of the zwitterions by protonating the thiolate. Additionally, the zwitterions can perform a photoinduced double bond isomerization in (first generation) or without the presence of acid (second generation). This enables either a four- or three-state photoswitching cycle. Zwitterionic aza-DAEs have proven to be potent photochromes in polystyrene, leading to a reversible 3D-relief imprint into the polymer surface upon light irradiation. Additionally, zwitterionic aza-DAEs are able to photoinitiate radical polymerization of methyl acrylate (MA), even in the presence of diaryl-HI, acting as photoswitch after the polymerization process.