SYNTHESIS, NMR RELAXATION, AND PHOTOISOMERIZATION OF AMPHIPHILIC POLYELECTROLYTES COVALENTLY TETHERED WITH AZOBENZENE MOIETIES HAVING BULKY HYDROPHOBIC SUBSTITUENTS

Amphiphilic terpolymers [consisting of 50 mol % of sodium sulfonate groups, 49.5 mol % of lauryl (La) or cyclododecyl (Cd) groups, and 0.5 mol % of a moiety of azobenzene substituted with a La group (Abz-La) or with a Cd group (Abz-Cd)] were prepared. Reference copolymers [consisting of 99.5 mol % of sodium sulfonate groups and 0.5 mol % of the Abz-La or Abz-Cd residue] were also prepared. In the terpolymers, the Abz-La and Abz-Cd residues were incorporated in the self-organized hydrophobic phases in aqueous solution, while in the reference copolymers, they are exposed to the aqueous phase. The mobility of the hydrophobic groups and azobenzene moieties in the co- and terpolymers in D2O was estimated by H-1-NMR relaxation techniques. Spin-lattice and spin-spin relaxation times indicated that the motions of the azobenzene moieties in the terpolymers were highly restricted owing to the self-organization of the hydrophobic groups inaqueous solution. In particular, the hydrophobic self-organization of the Cd groups caused more pronounced restrictions on the motions of the azobenzene moieties than the La groups did. Effects of such motional restrictions on the trans-to-cis photoisomerization and cis-to-trans thermal back-isomerization of the azobenzene moieties were investigated, finding that the former was considerably impeded, while the latter was accelerated. As predicted by the NMR relaxation times, these effects on the isomerizations were more pronounced with the Cd groups than with the La groups; i.e., motional restrictions imposed on the trans isomer and residual strains in the photogenerated cis isomer are larger in the organized Cd phase than in the La phase. These findings indicate that the Cd groups are more densely packed than are the La groups in the self-organized hydrophobic phases presumably because the Cd residue has much less conformational freedoms than does the La residue.