Luminescent compounds change emission colors when poked or scratched

July 14, 2016
Organic crystalline materials with gold atoms emit changing colors from blue to orange upon stimulation.
Changes in colors emitted through mechanochromism: a) when the blue crystal in emission was ground at the center, the color turned yellow; and b) a round filter paper thinly coated with blue mechanochromic molecules in emission -- the yellowish letters "Au" appeared after the surface was scratched with a spatula. (Image: Hokkaido University)
In a phenomenon known as mechanochromism, certain solid and liquid crystalline materials change their photoluminescence properties upon mechanical stimulation, such as grinding, ball-milling, and crushing. Although such compounds have attracted much attention with hopes of various applications, up until now it has been thought difficult to synthesize mechanochromic compounds with desired emission properties and behaviors, because each molecule would emit a different color. Now, scientists at Hokkaido University (Sapporo, Japan) have synthesized 48 organic compounds containing gold atoms, or gold(I) isocyanide complexes, 28 of which were found to be mechanochromic, emitting changing colors such as blue, green, yellow, and orange upon stimulation.1
Original emission color (left in each rectangle) and changed emission color upon mechanical stimulation (right in each rectangle) for various gold(I) isocyanide complexes. (Image: Hokkaido University)

With the aim of unraveling the mechanism behind the color changes, the researchers conducted detailed analyses of the crystal structures of almost all the molecules in the studied compounds. They found various formations of molecular arrangements, which, they concluded, is key to understanding the changes in their photoluminescence properties.

The finding could be applied to develop a sensor for detecting minute forces that occur in a several-nanometer segment in cells -- currently regarded as a difficult task. "If such a sensor is developed, it will contribute to identifying the mechanism for cellular movements and such," says Hajime Ito, one of the researchers.



1. Seki T. et al., Journal of the American Chemical Society, May 10, 2016; doi: 10.1021/jacs.6b02409

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