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Blending multiple polymers together to form the so-called "high-entropy polymers (HEPs)" can generate the effects of molecular dispersion in addition to suppressing polymer phase separation. We embedded a semiconducting polymer (conjugated polymers, CPs) in an optically inert matrix composed of n polymer species and found that a molecule-level dispersion is attained in HEPs defined as n ≥ 5. In the regime of dilute CP concentrations, the photonic properties vary widely in the n = 1 matrices owing to diverse solubility parameters, but the distribution narrows with n , and the CP starts to exhibit behaviors of molecule-level dispersion at n ≥ 5, where the matrix polymers compete with each other to exert direct influences on the embedded CP. Specifically, for MEH-PPV, increasing n reduces the fluorescence redshift and spectral width from diminishing aggregation. For the rigid PFO molecules, increasing n creates a dilution effect facilitating formation of the low-energy planar β-phase. For the flexible regioregular P3HT-rr, HEPs offer well-dispersed amorphous chains highly susceptible to chain environments, thus influencing η R 's in the quasi-fixed amorphous-crystalline energy transfer landscape. The HEP effects continue for greater CP concentrations, consistent with the matrix dispersing behaviors in the dilute regime. This work demonstrates a molecular-level dispersion by HEPs, offering a method of molecular tailoring for polymer research and applications via simple mixing.

Quantum efficiencies remain a critical issue for general applications of semiconducting polymers in optoelectronics and others. In this work, we demonstrate that nearly 100% quantum efficiencies (η's) in thin solid films can be reached when the polymer molecules are mechanically stretched into molecular confinement. We selected three conjugated polymers of varied backbone stiffness and interchain coupling, prepared in both diluted and pristine states. All of the polymers when highly diluted ( c = 0.1 wt %) exhibited massive η increases after stretching to very large strains (∼300-500%) via micronecking, with the rigid polyfluorene (PFO) and semirigid MEH-PPV both manifesting η ≈ 90%, while the most flexible yet regioregular polythiophene (P3HT-rr) exhibited a 10-fold increase to ∼21%. In the pristine state, molecular aggregation and interchain coupling curtail development of the molecular confinement, but the large-strain deformation still enhances η's significantly, to ∼90% (PFO) and ∼55% (MEH-PPV) despite no increases for the crystalline P3HT-rr. Moreover, upon substitution by a bulkier side-group to reduce interchain coupling, the pristine films of polythiophene (P3EHT) exhibited a ∼3-fold increase of η after the stretching. The nearly 100% of η's in fully stretched molecules indicates that the in situ self-trapping occurring via sub-picosecond backbone interactions can be mostly responsible for energy dissipations and quite suppressible by segmental stress control. The mechanical confinement effects also indicate the fundamental role of molecular mechanics during stabilization and migration of photoexcited charges.

The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical-experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates─MSb 2 O 6 , where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb 2 O 6 shows the highest theoretical ORR activity based on the 4e - ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb 2 O 6 is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.

Due to the rather low efficiencies of conjugated polymers in solid films, their successful applications are scarce. However, recently several experiments indicated that a proper control of molecular conformations and stresses acting on the polymers may provide constructive ways to boost efficiency. Here, we report an amazingly large enhancement of photoluminescence as a consequence of strong shear forces acting on the polymer chains during nanofilm dewetting. Such sheared chains exhibited an emission probability many times higher than the nonsheared chains within a nondewetted film. This increase in emission probability was accompanied by the emergence of an additional blue-shifted emission peak, suggesting reductions in conjugation length induced by the dewetting-driven mass redistribution. Intriguingly, exciton quenching on narrow-band-gap substrates was also reduced, indicating suppression of vibronic interactions of excitons. Dewetting and related shearing processes resulting in enhanced photoluminescence efficiency are compatible with existing fabrication methods of polymer-based diodes and solar cells.

The photoluminescence (PL) of well dispersed molecules of a conjugated polymer, poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4-phenylene-vinylene] (MEH-PPV), in an optically inert matrix manifested dramatic increases when the individual molecular strands were fully stretched. The PL increase rose with stretching and may reach several folds when the mechanical strain of the matrix polymer went beyond 550%. Strong polarization effects indicate that stretching individual polymer chains was responsible for the PL enhancement. This effect was attributed to suppression of electron-phonon interactions in the stress-rigidified polymer chain segments and may be useful for efficiency-enhanced polymer-based optoelectronic devices., (© 2011 American Chemical Society)