Nearly Isotropic Conjugated Polymer Aggregates with Efficient Local Exciton Diffusion

Abstract

To bridge the gap between single-molecule and bulk thin film studies of organic semiconductors and understand the influence of molecular structure on photophysical properties across scales, we prepared and characterized aggregates containing a few to more than a thousand single chains via well-controlled solvent vapor swelling of an inert polymeric matrix containing the conjugated polymer poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The single chains were initially immobilized in conformations with few interchain contacts, which can support exciton funneling on the single-molecule level. We show that aggregates prepared via solvent swelling and templated on such molecules result in largely isotropic aggregates. Analysis of aggregate polarization anisotropy, photoluminescence transient decay, and photoluminescence spectra as a function of aggregate size suggests that in these aggregates exciton diffusion is locally enhanced upon initial aggregation but does not reach extended length scales with further aggregation and annealing.

Effect of Thermal Annealing on Conformation of MEH-PPV Chains in Polymer Matrix: Coexistence of H- and J-Aggregates

“…MEH-PPV chains were considered to be either isolated or slightly aggregated in solid solution of 1/100 weight ratio [ 28 ]. PL 0–0 and 0–1 transitions in blend films share similar energies with those in neat film, indicating the formation of aggregation [ 36 ], or a single polymer chain is coiled to form stacked segments of different chromophores [ 24 ]. For extended conformations of single polymer chains, such energy transfer processes between chromophores are less likely to happen; therefore, the additional emission peak at ~550 nm (this is “bluer” than the PL 0–0 transition of solution) should be due to the emission of MEH-PPV with extended conformation, described as oligomer-like emission [ 37 , 38 , 39 , 40 ].…”

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

“…Conformational changes of the conjugated polymer chain not only influence exciton formation and delocalization effects considered in section , but also influence long-range exciton energy transfer, which is well documented. − First, torsional conformations of a polymer chain lead to torsional defects arising from rotation around the single bonds on the backbone of a conjugated molecule and thus decrease the effective conjugation length of the molecule and limit its ability to shuttle the charge and electronic energy. Second, polymer intrachain disorder and intermolecular packing results in varying concentration of chain–chain contacts through which the exciton can migrate.…”

Spatial and Spectral Super-resolution Imaging for Characterizing Multichromophoric Systems

“…We note that given the initial concentration of MEH-PPV molecules and the degree and time of swelling, as described in the Experimental Details section, previous work suggests that typical MEH-PPV aggregates in this study are composed of fewer than 40 single molecules and extend less than 100 × 40 nm. 15,34 Twentythree features with two-or three-step photobleaching were studied. In all cases, sectioning was performed via STaSI; the characteristics of the final surviving emitter were determined through localization imaging and averaging the spectra of all frames associated with the final section, and the characteristics of the first and/or second emitters to photobleach were determined via afSHRImP subtractions for spatial information and spectral averaging and subtraction for spectral information, both as described above.…”