Acidichromism and chiroptical switch based on the self-assembly of a cyanine dye on the PLGA/PAH LbL film

The self-assembly and functionalization of a cyanine dye (3,3′-disulfopropyl-9-methylselenacarbocyanine, SeCy) on the layer-by-layer (LbL) assembled poly(L-glutamic acid) (PLGA) film was investigated. PLGA was assembled into an LbL film through the alternative deposition with poly(allylamine hydrochloride) (PAH). The cyanine dye was self-assembled on this film through loading from the aqueous solution. When an appropriate aqueous solution of the cyanine dye (0.1 mM, pH = 3.5) was used, the cyanine dye could be easily assembled on the PLGA/PAH film and the dye aggregated and stacked in a helical way in the film. Induced circular dichroism (ICD) of the achiral dye was observed for such dye-loaded LbL films. Upon exposing the dye loaded film to HCl gas, the color of the film changed and the ICD of the dye disappeared. Interestingly, both the color and ICD could be recovered again by subsequently exposing the film to water vapor. The process was repeated more than 20 times. Thus, both acidichromism and a chiroptical switch were realized in the complex film. A mechanism related to the cooperative loading of the dye into the LbL film, dye aggregation, conformational change of PLGA and their relationship with the supramolecular chirality of the film was discussed. The work provided an easy way to realize the chiroptical switch in the film using a supramolecular control over the dye assembly.