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(crylic acid) derivatives [PAA-TPEx (x = 0.01-0.05)] provide a fluorescent Ca2+ sensor, where aggregation-induced emission (ALE) 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-TPE0.01 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-TPE0.01, Ca2+ and Mg2+ induced polymer-chain folding associated with ATE 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+.