Photoassociation of water-soluble copolymers containing photochromic spirobenzopyran moieties

✍ Scribed by Čestmír Koňák; Ramesh C. Rathi; Pavla Kopečková; Jindřich Kopeček

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 340 KB
• Volume: 9
• Category: Article
• ISSN: 1042-7147
• DOI: 10.1002/(sici)1099-1581(1998100)9:10/11<641::aid-pat812>3.0.co;2-w

✦ Synopsis

Photoinduced random association of HPMA copolymers containing photochromic spirobenzopyran (SBP) moieties in side-chain termini have been studied by static and dynamic light-scattering and spectroscopic methods. Experimental results gave evidence of photoinduced association in solvents with different polarity (water, dioxane). The driving force of this association is electrostatic dipole±dipole interactions between zwitterionic merocyanine forms of SBP molecules formed upon UV irradiation. The intermolecular association is only possible in solvents with a low thermodynamic quality for the copolymer close to its solubility limit. An increase in solvent quality or screening by co-ions suppresses the photoassociation. The kinetics of cluster formation and dissolution were also investigated by monitoring time dependences of the intensity of scattered light, hydrodynamic size and merocyanine absorbance. A common feature of all association processes is the initial growth of cluster sizes at short irradiation times followed by a plateau after the photostationary state of SBP moieties is achieved. In contrast to hydrodynamic diameter, the static light-scattering intensity increases monotonously, reflecting increasing mass and/or concentration of clusters. The dissolution was very slow in aqueous solutions. Both the formation and dissolution of clusters are faster in dioxane solutions. The kinetics of cluster dissolution was found to be twice as slow as the relaxation time of the spontaneous photochromic conversion of SBP moieties.