MOLECULAR MACHINES 2-D MATERIALS
CREDIT: J. AM. CHEM. SOC.
Thin perovskite shines white Perovskites are famous for harvesting light in emerging solar cells, but the inexpensive and easy-to-make semiconductors could also provide a simple path to light-emitting devices. Commercial white light-emitting diodes are made either by combining different colored LEDs or using LEDs that excite phosphor coatings to produce a white glow. Researchers have previously developed a few two-dimensional perovskite crystals that can emit white light without help, but scientists have yet to use them to build a working LED. Mercouri G. Kanatzidis, Lingling Mao, and coworkers at Northwestern University have outlined perovskite crystal design considerations that could help change that. The team developed three new lead-bromide materials using different cationic amine “spacers.” In each, lead-bromide octahedra join to form two-dimensional surfaces. These surfaces stack into layers with the organic molecules sandwiched between. Two of the team’s perovskite crystals are flat, but the one that uses the smallest cation—2-(dimethylamino)ethylamine or DMEN—takes on a buckled, egg-carton structure. This perovskite also emits light over a broad spectrum of wavelengths, resulting in a white light source with a glow akin to that of a fluorescent bulb, the team reports (J. Amer. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01312). Edward H. (Ted) Sargent, a photovoltaic materials researcher at the University of Toronto, comments that the work is promising for perovskite LEDs. It provides a “powerful new degree of freedom: engineering perovskites via the length and shape of organic ligand,” he adds. “This is an inorganic synthetic chemist’s dream,” Kanatzidis says of the versatility of these 2-D perovskites. “The system allows you to make changes and gives you new results every time.”—MATT
DAVENPORT
Nanomachines wind and unwind polymers Light-powered molecular machines work in tandem to contract and expand material Combining two types of light-activated molecular machines, chemists in France have created a system that winds and unwinds polymer chains, resulting in a material that contracts and expands, depending upon the wavelength of light that shines upon it. The system, developed by the University of Strasbourg’s Nicolas Giuseppone and coworkers, could lead to new types of actuators and artificial muscles. A couple years ago, Giuseppone’s group tethered light-activated motor molecules to polymer chains and observed that they could wind up the polymer with ultraviolet light and make the macroscopic material contract (Nat. Nanotech. 2015, DOI: 10.1038/ nnano.2014.315). But once the polymer was shrunk, nothing more could be done. So the chemists went back to the drawing board and added a light-activated modulator molecule to
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the system. This modulator unwinds the polymer when exposed to white light and re-expands the material (Nat. Nanotech. 2017, DOI: 10.1038/nnano.2017.28). “Now we have a way to reset the system,” Giuseppone explains. “We can, in principle, make an actuator at any scale that can go forward and backward, fueled only by light.” Chenfeng Ke, an expert in molecular machines at Dartmouth College, likens the system to molecular clockwork: The two molecular machines act cooperatively to keep the system in motion. From a practical standpoint, Ke says, Giuseppone’s system is probably not ready for commercial use, “but the design is very elegant, and it paves the way for scientists to think about how to come up with strategies for building these types of multicomponent systems,” he adds.—BETHANY HALFORD
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The combination of a motor molecule and a modulator Winding molecule wind and unwind polymer chains, respectively, in Giuseppone’s system.
Visible Ring opening
Unwinding MARCH 27, 2017 | CEN.ACS.ORG | C&EN
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