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

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 (<i>R</i> _h approximately 60 nm) and large (<i>R</i> _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.