The precipitate and the coacervate are two aggregated states in the polyelectrolyte complexes (PECs). The precipitate‐to‐coacervate transition and glass transition in PECs have been widely reported in the past. In many cases, the two phenomena are studied independently, although both of them are apparently affected by water and small ions. Here, utilizing a PEC system consisting of poly(acrylic acid) (PAA) and a cationic bolaamphiphile (DBON), we explore the states of PECs as a function of salt, temperature, and the molecular weight of PAAs. By a combination of microscopic observation, time‐resolved fluorescence measurements, and differential scanning calorimetry, we identify salt/temperature driven precipitate‐to‐coacervate transitions of the complexes. The thermally induced morphology transformation from the precipitate to coacervate occurs around the glass transition temperature, indicating a strong correlation between the two processes. As the molecular weight of the PAA increases, the thermal transition temperature becomes higher. This finding offers new insights on the mechanistic interactions that dictate the aggregated states of PECs. Based on the photothermal effect of DBON, we also develop a UV light‐induced strategy to mediate the precipitate‐to‐coacervate transition, providing a fantastic platform to create functional PEC materials.

Structural information at an assembled molecule/ solid interface has been limited, due in part to the obscure NMR signals for the atoms co-ordinated to the solid surface. Here, we report an effective detection of the surface composition of the ligands on gold nanoparticles (AuNPs) by 1 H solid-state NMR under fast magic angle spinning. An investigation of the confinement effect of the ligand shell was successfully performed by multiple pulse dipolar filter NMR. We demonstrate that the affinity of surface species on AuNPs is affected not only by interfacial chemical bonding but also by physical confinement, leading to a new fundamental insight into the structure-property correlation for major interfacial behaviors, such as ligand exchange reaction and curvature effect on properties. The concept of physical confinement and the characterization method developed in this work is probably applicable to all ligand/nanoparticle interfaces and thus can provide a clue to the design and execution of the manipulating strategy for nanocomposites.