Photochromic hollow shells: photoisomerization of azobenzene polyionene in solution, in multilayer assemblies on planar and spherical surfaces

Based on the layer-by-layer electrostatic deposition, photochromic hollow shells consisting of an alternating multilayer system were prepared by using an anionic polyelectrolyte ι-Carrageenan, CAG and a cationic polyionene that contains rigid azobenzene chromophores separated by flexible spacer chains, i.e. poly(((4,4′-bis(6-dimethylammonio)hexyl)oxy)azobenzene bromide), PAZ-6. The formation of the hollow shells was investigated by means of confocal layer scanning microscopy and AFM. No significant spectral changes were found to occur in comparison with the behavior of PAZ-6 in an aqueous solution, when PAZ-6 was adsorbed forming the alternating multilayer assemblies (MA) on the hollow shells. However, in the case of MA on a planar substrate, a strong red-shift of the maximum absorbance from 347 to 373 nm was observed. The photochemical isomerization of azobenzene in hollow wall was compared with those in an aqueous solution and in MA on a planar substrate. The E–Z–E photoisomerization cycle of PAZ-6 in the hollow shells and in the solution leads to a bathochromically shifted and higher ππ* absorbance compared with those of the initial state, revealing the conversion of H-aggregation species to nonaggregated E-isomers by the isomerization cycle.