# Discovery of Crystallized and Weakly Coupled Aggregates of Pseudocyanine Iodide

## TL;DR

Scientists discovered a new type of PIC-I aggregate with unique light-emitting properties, which could be useful in imaging and lighting technologies.

## Contribution

The discovery of PIC-I HJ-aggregate ribbons with cooperative emission from collective excitonic states is novel.

## Key findings

- PIC-I aggregates form layered ribbons with mixed HJ-aggregate structures and red-shifted fluorescence.
- These aggregates show photon bunching, indicating cooperative emission from collective excitonic states.
- They have emission lifetimes up to 1 ns, much longer than conventional J-aggregates.

## Abstract

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.