Spotlights - American Chemical Society

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Spotlights pubs.acs.org/JPCL

Spotlights: Volume 7, Issue 22

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MOLECULAR WATER LILIES: ORIENTING SINGLE MOLECULES IN A POLYMER FILM BY SOLVENT VAPOR ANNEALING

ENTROPY AND DISORDER ENABLE CHARGE SEPARATION IN ORGANIC SOLAR CELLS Organic solar cells are a dynamic field not only because they promise cheap and flexible devices but also because they advance the frontiers of the fundamental physics of transport in disordered systems. Arguably the most important question about organic solar cells is why they generate electric current at all; the mechanism behind the often very efficient separation of electrons from holes at donor−acceptor interfaces remains an open question in organic solar cell research. Hood and Kassal (10.1021/acs.jpclett.6b02178) tackle this question in their Letter. They show that the usually neglected entropy of charge separation can exceed the Coulomb binding energy, especially in the presence of disorder. Thus, the free energy of separation can be negative, meaning that the charges are not actually bound at all. The authors’ findings provide a general conceptual framework for understanding charge separation in organic solar cells, challenge the conventional view of strongly bound chargetransfer states, and provide alternative design guidelines for improved organic solar cells.

Organic light-emitting diodes (OLEDs) are appealing in part because they use less energy than conventional LEDs while creating brighter light. OLEDs consist of layers of thin films, and the microscopic alignment of fluorophores in the active layer is crucial to its functioning. Research is needed to fully understand this microscopic alignment in order to ultimately control it; this control is key because the transition dipole orientation determines the significance of efficiency losses to waveguide modes in these devices. With this goal in mind, Würsch et al. (10.1021/acs.jpclett.6b02119) used a shapepersistent macrocyclic molecule as a fluorescent probe to measure molecular orientation by exploiting the unique polarization anisotropy characteristics. The authors demonstrate nanoscale control of orientation and positioning of these molecules by solvent vapor annealing. During the annealing process, the molecules were free to diffuse and accumulate at the polymer/air interface, where they adopted a flat orientation with respect to the surface, akin to water lilies lying flat on the surface of a pond. Their findings could also be significant not only in the field of OLED research but also in the area of singlemolecule spectroscopy, where luminescent molecules are often embedded in a polymer film. Knowledge of the position and orientation of single molecules may clarify spectroscopic observables, in particular, the spatial localization in superresolution microscopy techniques.

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WHAT CAN WE LEARN ABOUT CHOLESTEROL’S TRANSMEMBRANE DISTRIBUTION BASED ON CHOLESTEROL-INDUCED CHANGES IN MEMBRANE DIPOLE POTENTIAL? It has been shown that cholesterol molecules are vital for regulating a variety of cell membrane properties (e.g., in-plane structural organization, fluidity, and lateral dynamics), but the transmembrane distribution of cholesterol has remained a matter of debate. In their Letter, Falkovich et al. (10.1021/ acs.jpclett.6b02123) describe their study of the distribution of cholesterol molecules across cell membranes. The authors considered the electric properties of membranes and questioned how changes in cholesterol’s transmembrane distribution might influence the membrane’s dipole potential. Using microsecond-long atomistic molecular dynamics simulations, they probed the impact of cholesterol on the dipole potential in asymmetric lipid membranes and found that the potential changes in a cholesterol-dependent manner. They found that moving cholesterol from the extracellular to the cytosolic leaflet increased the dipole potential on the cytosolic side and vice versa. These computational results, being insensitive to the choice of a lipid force field, are in line with a limited number of experimental works previously reported. The findings provide greater insight into the transmembrane distribution of cholesterol molecules in cell membranes, one of the biggest puzzles in membrane biology.

REALISTIC SURFACE DESCRIPTIONS OF HETEROMETALLIC INTERFACES: THE CASE OF TIWC COATED IN NOBLE METALS

Noble metals have been used in many surface catalysis studies, both computational and experimental, because of their unique surface catalytic properties. Several surface architectures have been devised with the aim of reducing noble metal loadings, and core−shell materials have shown the greatest promise. Despite many computational advances, a method is still needed to accurately describe the electronic properties of shell topologies. In their Letter, Hendon et al. (10.1021/ acs.jpclett.6b02293) employ density functional theory to develop a computational method via simulated annealing to obtain realistic surface topology descriptions at the heterometallic junction. After validating the model, they applied the process to other metals and core topologies to guide the experimental search for more active, stable, and selective catalysts with significantly reduced noble metal loadings. This general procedure is hoped to pave the way for rational design of next-generation heterometallic catalysts. Moreover, the findings could be of interest not only to the catalysis community but also to the broader range of chemists, physicists, and engineers who use these materials in reactors. © 2016 American Chemical Society

Published: November 17, 2016 4763

DOI: 10.1021/acs.jpclett.6b02609 J. Phys. Chem. Lett. 2016, 7, 4763−4763