# Time-Dependent Structure Assessment of Conjugated Polymer Aggregates in Solution by Single-Molecule Fluorescence Spectroscopy

## TL;DR

This paper uses single-molecule fluorescence to study how conjugated polymer aggregates change over time in solution, revealing insights into their structure and behavior.

## Contribution

The study introduces a diffusion-based single-molecule burst method to analyze aggregate size, concentration, and conformation in real time.

## Key findings

- Aggregates of P(EO-NDIT2) grow from nano- to micrometer-sized over weeks to months.
- Larger aggregates show increased fluorescence brightness and red-shifted emission.
- Aggregate size increases correlate with higher internal order and planarization of the polymer backbone.

## Abstract

Monitoring and understanding
the aggregation kinetics of n-type
polymers provide strategies to favorably control aggregation and thereby
optimize conjugated polymers ink shelf life, printability, and thin-film
properties. Here, the in situ characterization of n-type copolymer
aggregates employing ensemble absorbance and fluorescence spectroscopy
for spectral characterization, in combination with single-molecule
fluorescence spectroscopy methods to identify subcategories of aggregates,
is reported. Specifically, we utilize a diffusion-based single-molecule
burst method that resolves individual aggregates as they traverse
the observation volume, allowing us to determine the aggregate size,
concentration, and chain conformation through the statistical analysis
of single-aggregate fluorescence data. Base-stable P­(EO-NDIT2) with
branched ether-based side chains self-assembles from molecularly dissolved
chains into nano- to micrometer-sized aggregates over the course of
weeks to months, depending on the solvent used. Through spectral decomposition
and polarization-sensitive single-molecule fluorescence spectroscopy,
the aggregates were categorized by their size into small (R
h approximately 60 nm) and large (R
h approximately 300 nm) aggregates and monitored with
time. An increase in size was correlated with enhanced fluorescence
brightness and red-shifted emission as well as an increase in internal
order, as revealed by emission anisotropy. This increase in order
within the aggregates may be related to alignment of crystalline domains
and a planarization of the polymer backbone torsion.