'Made to order' crystal opens new door in optics

November 29, 2001

MINNEAPOLIS / ST. PAUL--It's not quite a magic crystal, but it assembles itself just as its "master" bids and even performs the trick of doubling the frequency of light. University of Minnesota researchers have created an organic crystal lattice that forms in exact predetermined architecture--a rare feat--and exhibits polarity, which is as difficult as coaxing magnets to line up with all the north poles facing the same way. The technology opens the door to new types of "polar" crystals, many of which can change red light to green or blue light and may be used to build shorter wavelength lasers. Other potential uses include optical switching and drug purification. The research will be published in the Nov. 30 issue of Science.

On the molecular level, a crystal lattice is a series of chambers where the "floor," "ceiling" and "pillars" are all held in place by weak chemical attractions. Inside the chambers are "guest" molecules that perform some function. In order for the crystal to work, all the chambers must be the same and all the guest molecules must be aligned in the same direction. If some guests are standing heads up and others are upside down or sideways, the material as a whole will have no sense of direction--no polarity.

"It's usually difficult to get molecules to go the way you want because the forces [pulling them into position] are so weak," said Michael Ward, professor of chemical engineering and materials science and first author of the Science paper. "What we're doing is crystal engineering, which means designing solid-state structures based on molecules by looking at the molecules themselves and asking how they'll guide themselves into a 3-D crystal lattice."

The new crystals double the frequency of light, a property often associated with polar crystals. Although the intensity of light that exits the crystal is lessened, the phenomenon means that, for example, green or blue lasers may be fashioned using red light. Blue lasers have been difficult to fabricate, said Ward, but some made from inorganic materials do exist. They are desirable for telecommunications because information can be transmitted faster at higher frequencies and shorter wavelengths.

Related crystal lattices can also be used to separate molecules with the same composition but different structure. In particular, they may have the potential for separating drugs that exist in two forms, like right- and left-handed gloves. The need for such separation and purity of a certain handedness is exemplified by thalidomide; one form alleviates morning sickness, the other causes birth defects.

In optical switching, the crystals would work as transistors to pass information.

A crystal lattice is like a hotel for its guest molecules. Unlike hotels for people, however, the guests take their places at the same time the lattice assembles itself. The floors and ceilings are identical sheets consisting of a patchwork of two different types of molecules, with "pillars"--a third type of molecule--stretching between.

Such lattices are symmetrical, and a guest molecule would have no idea which way to orient itself within the framework of sheets and pillars. In a polar crystal, the guest molecules are polar, which means that one end is relatively negative in charge and the other end relatively positive. A row of polar guests would tend to line up in alternating orientation, so that positive ends are bracketed by negative ends and vice versa. Thus, their charges would cancel each other out and the crystal would be nonpolar.

But Ward and his colleagues used pillars shaped like bananas. These molecules can only fit between the sheets if they line up in one direction, i.e. ))) or (((, but not both. This in turn forces all the guests inside the lattice to line up in the same direction, and the material exhibits polarity.

"Now that we know how to do this, we can try other 'pillar' molecules and other 'guests' with the same basic framework," said Ward. "What makes this unique is, it's so amenable to modifications."

Working with Ward on the project were K. Travis Holman and Adam Pivovar. The work was supported by the National Science Foundation, and Holman was partially supported by the National Science and Engineering Research Council of Canada.

Michael Ward, Chemical Engineering and Materials Science Dept., (612) 625-3062, wardx004@umn.edu

Deane Morrison, University News Service, (612) 624-2346, morri029@umn.edu

University of Minnesota

Related Molecules Articles from Brightsurf:

Finally, a way to see molecules 'wobble'
Researchers at the University of Rochester and the Fresnel Institute in France have found a way to visualize those molecules in even greater detail, showing their position and orientation in 3D, and even how they wobble and oscillate.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

How molecules self-assemble into superstructures
Most technical functional units are built bit by bit according to a well-designed construction plan.

Breaking down stubborn molecules
Seawater is more than just saltwater. The ocean is a veritable soup of chemicals.

Shaping the rings of molecules
Canadian chemists discover a natural process to control the shape of 'macrocycles,' molecules of large rings of atoms, for use in pharmaceuticals and electronics.

The mysterious movement of water molecules
Water is all around us and essential for life. Nevertheless, research into its behaviour at the atomic level -- above all how it interacts with surfaces -- is thin on the ground.

Spectroscopy: A fine sense for molecules
Scientists at the Laboratory for Attosecond Physics have developed a unique laser technology for the analysis of the molecular composition of biological samples.

Looking at the good vibes of molecules
Label-free dynamic detection of biomolecules is a major challenge in live-cell microscopy.

Colliding molecules and antiparticles
A study by Marcos Barp and Felipe Arretche from Brazil published in EPJ D shows a model of the interaction between positrons and simple molecules that is in good agreement with experimental results.

Read More: Molecules News and Molecules Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.