Discovery of Crystallized and Weakly Coupled Aggregates of Pseudocyanine Iodide

We present a new class of pseudocyanine iodide (PIC-I) aggregates formed by freeze-induced self-assembly into layered ribbon structures. Unlike conventional PIC J-aggregates with head-to-tail dipole alignment, these ribbons adopt a complex and mixed HJ-aggregate arrangement. Unexpectedly, the aggregate ribbons exhibit intense, red-shifted fluorescence, in contrast to the typical nonemissive nature of their monomer form. At 4 K, their emission lifetimes range from ∼300 ps to ∼1 ns, substantially longer than those of J-aggregates. The combination of red-shifted emission, monomer-like absorption, and extended lifetimes reveals their mixed packing contributions. Second-order autocorrelation measurements with a Hanbury Brown and Twiss interferometer show photon bunching, providing evidence for cooperative emission from collective excitonic states─an effect not previously observed in any aggregates larger than dimers. These findings establish PIC-I HJ-aggregate ribbons as a unique platform for exploring collective photophysics in mixed aggregate molecular assemblies, suggesting potential applications in bioimaging, light-emitting devices, and sensing.