Solvent-Mediated Phase Propagation Governs Excitonic Heterogeneity in Chiral Fibrils.

Phase propagation and phase separation in biomolecular condensates have emerged as key concepts for understanding the nonequilibrium evolution of functional assemblies, yet achieving analogous processes in soft molecular aggregates remains challenging. Here, we directly visualize phase propagation between coexisting J-type coupled and excimer-emitting domains within individual crystalline fibrils of a chiral perylenediimide dimer (RCy-PDI2) using fluorescence lifetime imaging microscopy (FLIM). FLIM analysis revealed heterogeneous distribution of short (τ blue ≈ 1.5 ns) and long (τ green ≈ 8 ns) fluorescence lifetime components in a single aggregate fibril. Time-dependent FLIM measurements facilitated the understanding of phase propagation and shrinkage-induced fibril fracture in the aggregate fibrils. Steady-state and chiroptical measurements of (RCy-PDI2) aggregate fibrils indicated the presence of J-type aggregate formation and excimer emission. Three-dimensional electron diffraction (3D ED) reveals a toluene-intercalated structure enforcing a V-shaped geometry (interplanar angle: 26.3°), enabling strong Coulombic coupling (J c = -55 to -73 meV) consistent with J-type interactions. Toluene diffusion and detrapping drive structural relaxation, converting J-type domains into excimer-emitting regions within the same fibril, establishing a direct structural basis for excitonic state selection. This work correlates excitonic heterogeneity with phase propagation, expanding our understanding of the role of solvent diffusion and molecular stacking in the development of functional supramolecular materials.