Highly Anisotropic Conjugated Polymer Aggregates: Preparation and Quantification of Physical and Optical Anisotropy

Abstract

Controlling morphological order of conjugated polymers over mesoscopic and microscopic scales could yield critical improvements in the performance of organic electronics.Here, we utilize a multimodal apparatus allowing for controlled solvent vapor annealing and simultaneous wide-field epifluorescence microscopy to demonstrate bottom-up growth of morphologically ordered anisotropic aggregates prepared from single poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) chains, with length scales controllable from tens of nanometers to several micrometers. Preparation of micrometer-scale fiber aggregates that interconnect to form spanning networks is also demonstrated. We quantify aggregate physical and optical anisotropy, degree of quenching, and exciton diffusion characteristics as a function of aggregate size. The demonstration of controlled preparation of highly anisotropic aggregates provides a path for controlled postprocessing of organic thin films at length scales relevant to the operation of devices.

Optical Microscopic Techniques for Synthetic Polymer Characterization

“…In recent studies, Yang et al employed measurements of luminescence intensity and optical anisotropy to follow the formation of MEH-PPV aggregates in PMMA films under solvent vapor annealing conditions . In this study, the polymer was spin-cast from PMMA solutions in toluene, having different MEH-PPV concentrations.…”

In Situ Optical Imaging of the Growth of Conjugated Polymer Aggregates

“…0.5 n m ) containing 6 wt % host poly(methyl methacrylate) (PMMA) matrix on a QCM sensor and a glass coverslip and placed in the SVA chamber. An acetone–chloroform solvent mixture with liquid volume ratio of 50:50 % was employed to swell the films . We performed SVA for about 55 min to generate and grow aggregates, during which we carefully controlled the extent of film swelling to maintain a constant thickness (Supporting Information, Figure S1a).…”

“…We modeled a situation in which two aggregates form a minimal contact, a situation that would result in the initial characteristics of each aggregate being nearly the same before and after coalescence. Such a pair of aggregates may be effectively modeled by two equivalent polarization ellipsoids,– each with transition dipoles ( μ x , μ y , μ z ) and an aspect ratio α =| μ z |/| μ x | (| μ x |=| μ y |), present together within a diffraction‐limited spot. Through comparison of aggregate volume and M distribution in experiments and simulation, aggregates before coalescence are determined to have average aspect ratio α =2.7 (Supporting Information, Text 6 and Figure S5).…”

“…The M values of the early aggregates show a single narrow Gaussian distribution peaked at M ≈0.82 (Figure d, upper), revealing that the morphology of aggregates in the initial growth stages is highly anisotropic. Given that single‐molecule MEH‐PPV molecules have been shown to adopt highly ordered anisotropic conformations after SVA as evidenced by values of M >0.9, the anisotropic morphology of the early aggregates suggests that initial aggregation processes preserve the morphological anisotropy templated by the single chain building blocks. The M distribution for the final aggregates shifts towards lower M values and broadens, displaying a population build‐up at M ≈0.45 (Figure d, lower).…”

Real‐Time Spectral Evolution of Interchain Coupling and Assembling during Solvent Vapor Annealing of Dispersed Conjugated Polymers

“…Single‐molecule fluorescence spectroscopy and imaging methods can be applied to monitor SVA dynamics at the level of single conjugated polymer (CP) chains and single aggregates. [ 21–26 ] Single CP molecules tend to change conformation by unfolding from globules to extended chains when exposed to solvent vapor. For single CP aggregates, the chain‐stacking rearrangement process substantially affects interchain coupling strength; this process can be observed by measuring the fluorescence spectral shift.…”