Mesoscopic Structural Aspects of Ca²⁺-Triggered Polymer Chain Folding of a Tetraphenylethene-Appended Poly(acrylic acid) in Relation to Its Aggregation-Induced Emission Behavior

We recently reported that tetraphenylethene-appended poly­(acrylic acid) derivatives [PAA-TPE_x (x = 0.01–0.05)] provide a fluorescent Ca²⁺ sensor, where aggregation-induced emission (AIE) of the TPE pendants occurs in conjunction with Ca²⁺-triggered polymer-chain folding. On the basis of dynamic and static light-scattering data, here we discuss the hydrodynamic radius and molar mass of PAA-TPE_0.01 in the presence of Ca²⁺, Mg²⁺, or Na⁺ at various concentrations and elucidate the origin of the Ca²⁺ selectivity. In contrast to Na⁺, which exclusively triggered nonfluorescent interpolymer aggregation of PAA-TPE_0.01, Ca²⁺ and Mg²⁺ induced polymer-chain folding associated with AIE from the TPE pendants. Importantly, Ca²⁺ caused polymer-chain folding more effectively than Mg²⁺. Consequently, polymer aggregates formed in the presence of Ca²⁺ possessed a much higher inner density than those formed in the presence of Mg²⁺, leading to a more pronounced AIE behavior and, in turn, the Ca²⁺ ion selectivity over Mg²⁺.