Photo-responsive molecules in combination with self-assembled structures have been widely used in devising soft functional materials. Especially photoisomerizable azobenzene molecules incorporated into supramolecular structures or liquid crystalline systems, which enable the control of order and amplifying the molecular photo-response to a macroscopic scale, have triggered great interest. On the other hand, larger-scale self-assemblies based on, e.g., block copolymer or colloidal structures may significantly modify light propagation in the self-assembled materials, leading to photonic properties and structural colors. The main focus of this thesis lies in this intriguing interplay between light and self-assembly shown by these photo-controlled and photonic systems. Publication I studies the effect of self-assembly on the photoresponse and isomerization kinetics of azobenzenes in hydrogen-bonded polymeric complexes. These complexes of hydroxyazobenzene derivatives are used to demonstrate a drastic increase in the thermal cis-trans isomerization rate upon a transition from a disordered low-azobenzene-content state to a high-concentration state with lamellar smectic-like order. Comparison between disordered and ordered high-concentration states shows that the former exhibits significantly slower kinetics, pinpointing the importance of cooperativity created by the molecular order and alignment. In Publication II the amorphous hydrogen-bonded complexes of hydroxyazobenzenes are used to demonstrate how thermal isomerization kinetics of azobenzenes can be exploited in optical sensing of hydrogen-bonding gas vapors, especially relative humidity. The thermal isomerization of different complexes is shown to exhibit exponential changes with huge dynamic range, high reproducibility and tunability in the presence of water and ethanol. The use as a sensor is further demonstrated. Using isomerization kinetics for sensing is potentially opening new avenues for using azobenzene molecules and their supramolecular complexes in practical applications. Publication III investigates halogen-bonded supramolecular liquid crystals containing azobenzene moieties. These complexes of stilbazole and fluorinated azobenzene derivatives are shown to systematically form liquid crystals held together by the highly directional halogen bonding. The photoisomerization-induced LC-isotropic transition is studied in detail, and the cis-isomer content required for the transition is determined. In Publication IV a generic method is developed for using the self-assembly of block copolymer micelles with a quaternizable block to create photonic fluids and 3D photonic crystals. For this, reaching micelles with narrow size distributions and highly swollen coronae is critical. The observed crystallization demonstrates that these qualities are reached. The micelles spontaneously assemble in up to millimeter-sized crystal lattices. In addition, the photonic crystals result in extremely narrow reflection peaks ranging over the visible region, have high tunability through changes in concentration and salinity, and form reversibly.
- Ikkala, Olli, Supervisor
- Priimagi, Arri, Advisor, External person
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- self-assembly, photo-response, azobenene, block copolymer, micelle, photonic crystal