A team of CCI researchers led by David Vanden Bout (UT-Austin) and Colin Nuckolls (Columbia University) have recently made important advances toward the strategic design of conjugated polymers with controlled optoelectronic properties.
Figure 1. (top) Wide-field fluorescence images of MEH-PPV (a) before and (b) after solvent-vapor annealing. Each bright spot in (b) corresponds to an aggregate of MEH-PPV chains. (bottom) Polarization anisotropy histograms for (c) pristine, (d) para-terphenyl and (e) ortho-terphenyl MEH-PPV measured for 150-300 aggregates. The parameter M indicates the order of the aggregates’ nanodomains, with complete order corresponding to M = 1.
In a paper published in ACS Nano, the CCI team used a controlled solvent vapor annealing process to assemble single chains of phenylene-vinylene conjugated polymers into aggregates that were investigated spectroscopically. The conjugated polymers were modified to include structure-directing features, termed “morphons” by the CCI. By varying the type of morphon used, the ordering of nanoscale domains in the aggregates could be controlled. For example, single aggregate polarization anisotropy measurements revealed that the nanodomains of MEH-PPV aggregates containing rigid para-terphenyl groups were nearly completely aligned (ordered), whereas those in aggregates containing hinge-like ortho-terphenyl groups were nearly completely isotropic (disordered) (Figure 1).
This alignment or isotropy of the aggregates respectively facilitated or inhibited interchain coupling of MEH-PPV. The CCI team observed that the para-terphenyl MEH-PPV exhibited fluorescence spectral features similar to those of pristine MEH-PPV, indicative of extensive interchain electronic interactions. In contrast, the ortho-terphenyl MEH-PPV lacked such spectral features, indicating that the ortho-terphenyl morphons effectively prevented extensive interchain interactions (Figure 2).
Both types of morphon-included MEH-PPV chains hold promise for next-generation opto-electronic applications. Highly aligned polymer nanodomains have been demonstrated to permit long-range energy transfer, and rigid morphons such as para-terphenyl groups could be used to promote long-range alignment and thus long-range energy transfer in polymers used for plastic solar cells. Meanwhile, nanodomain disorder and minimized interchain interactions have been shown to increase electroluminescence efficiency in organic light-emitting diodes (OLEDs), so bent morphons such as ortho-terphenyl groups could be used to optimize OLED materials.
Figure 2. Representative single-molecule fluorescence spectra for (a) pristine, (b) para-terphenyl and (c) ortho-terphenyl MEH-PPV.
These results also provide exciting new insight regarding the formation of low-energy traps in polymer films. Single-chain fluorescence lifetime measurements revealed that both para- and ortho-terphenyl morphons disrupted phenylene-vinylene conjugation in MEH-PPV. These results, when considered in conjunction with the fluorescence spectra, indicate that energy trapping sites in MEH-PPV result from coupling interactions between neighboring polymer chains.
