Electrolyte-Controlled Self-Assembly of Water-Soluble Perylene Bisimides: A Kinetic and Stimulated Raman Study

Perylene bisimides (PBIs) are prime building blocks for functional supramolecular architectures, yet mapping their salt-induced self-assembly kinetics under physiological conditions remains challenging, due to rapid aggregation and overwhelming fluorescence. Here, we combine fluorescence stopped-flow kinetics and stimulated Raman scattering (SRS) spectroscopy to decipher the electrolyte-controlled polymerization of a spermine-functionalized PBI. We found that self-assembly obeys an anticooperative K2-K pathway, where nucleation yields rapid and salt-independent dimers. In contrast, polymer growth is strongly salt-dependent, with polymer dissociation rates decreasing as NaCl concentration increases. Crucially, chloride ions selectively stabilize the growing assemblies by drastically suppressing the monomer release, establishing a Michaelis-Menten-type saturation profile. To link these kinetic trajectories with molecular-scale structural rearrangements, SRS is used to effectively suppress the fluorescence background, revealing that the relative intensity of low-frequency Raman modes, associated with high-mass displacement and structurally delocalized in nature, are drastically reduced upon aggregation. This vibrational suppression underscores the role of packing constraints and excitonic pressure within the cofacial H-aggregates. By directly bridging macroscale kinetic pathways with microscopic vibrational dynamics, this work offers a generalizable framework to rationally design and tune water-soluble supramolecular materials for biomedical and optoelectronic applications.