Photoresponsive supramolecular polymer films : comparison of the hydrogen and ionic bonding strategies

Supramolecular non-covalently bonded azobenzene-containing complexes are easy-to-prepare alternatives to covalently bonded azopolymers as photoresponsive materials. The rapid and reversible nanoscale photoisomerization of azobenzenes enables effective external control of the host material's properties and they can give rise to large-scale motions, such as macroscopic mass transport with polarized illumination. The light-induced phenomena in azomaterials offer great potential in various areas ranging from photonics to biology. Photoresponsive supramolecular materials often hydrogen bond between a passive polymer and a photoactive small molecule. However, ionic bonding between oppositely-charged components is also a versatile supramolecular approach to design azobenzene-containing systems. In this M.Sc. thesis, supramolecular assemblies of photopassive poly(4-vinylpyridine) (P4VP) and its quaternized derivative (P4VPMe) are hydrogen-bonded (H-bonded) and ionically bonded (i-bonded), respectively, with analogous photoactive small molecules. The small molecules studied are 4-hydroxy-4-dimethylaminoazobenzene (azoOH) and methyl orange (MO), respectively, where the OH functionality of azoOH is replaced by a sulfonate group in MO. These photoactive complexes are studied with the following objectives: 1. To understand the effect of the bonding type on the photoinduced properties of azocomplexes, in particular their molecular orientation and their surface relief grating (SRG) diffraction efficiency (DE). 2. To determine the role of the polymer molecular weight (5.2, 50, and 200 kg/mol) on the photosensitivity of the azocomplexes.  The complexation between components is first confirmed using static infrared (IR) spectroscopy and 1H NMR. UV-visible spectroscopy studies under illumination of linearly polarized 488-nm laser light of spin-coated thin films confirms the absence of phase separation in both series of complexes and reveals a similar minimum percentage of cis isomers (14% vs. 20% for azoOH and MO, respectively).  Polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS) is then used to investigate the impact of the supramolecular interaction type and the polymer molecular weight (MW) on the molecular-level photoinduced orientation (<P2>) of the azocomplexes using linearly polarized 488-nm light. Then, their photoinduced macroscopic-scale motion that produces surface relief gratings (SRG) is investigated using an interference pattern of circularly polarized 488-nm light.  We find that i-bonded complexes respond to light more strongly than their analogous  H-bonded complexes, both in terms of <P2> and DE values. The PM-IRSAS results also show a much larger contribution of angular redistribution over angular hole burning for the MO-based complexes. In addition, the host polymer MW does not impact the molecular photo-orientation, while it does affect the SRG inscription efficiency. The low MW complex shows a higher DE than the two higher MW complexes. We can conclude that chain entanglement plays a more important role than the glass transition temperature for phototransport in these systems.  The M.Sc. thesis provides fundamental knowledge of the effect of the supramolecular interaction type and strength on the photosensitivity of azocomplexes. This knowledge contributes to practical guidelines for the design of efficient supramolecular photoresponsive polymers.

Keyphrases: Supramolecular Functionalization, Azobenzene, Azopolymer Complex, Photo-Orientation, Photoisomerization, Photoinduced Surface Relief Grating