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Nano-Scale Mechanochromism |
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Polydiacetylenes (PDAs) exhibit a blue-to-red color transition that can be stimulated by heat, chemical or biological attachment, or by mechanical stress (mechanochromism). Our experiments were conducted using a PDA trilayer film formed from 10,12-pentacosadiynoic acid (PCDA). Each layer consists of pendant methylene side chains attached to a polymerized diacetylene backbone that is oriented parallel to the substrate (see below). Optical absorption takes place within the p-conjugated polymer backbone. Although not fully understood, it is believed that the color transition results from molecular conformational changes, such as side chain ordering and orientation, which impart stresses that lead to different backbone configurations.
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Below we present the first ever nano-scale images of mechanochromism. The movies consist of a series of contact mode AFM topographic images. During imaging, shear forces between the tip & sample rearrange PDA molecules, causing them to transform into the red state. This region exhibits distinct topographic changes, and is strongly fluorescent, as observed with fluorescence microscopy and NSOM (see further below). The movies show that the growth occurs preferentially along the backbone (vertical) direction.
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Click the icon to view movie #1. Scan size: 1 µm. |
Each movie repeats 3 times. The fast-scan direction is horizontal, the slow scan direction is vertical. In both movies, the polymer backbone chains are oriented in the vertical direction, so the tip is scanning perpendicalar to the chain direction. The movies are in QuickTime format. If the movie doesn't start automatically, click the button on the lower left to play. |
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Click the icon to view movie #2. Scan size: 4 µm. |
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Below we show a blue PCDA sample investigated with the NSOM/IFM instrument. NSOM shear force topography (left) and simultaneous fluorescence (right) images (2.4x2.4 µm2) reveal tip-induced mechanochromism. In Scan 1 (top), no fluorescence is seen over the flat PCDA region. In the subsequent Scan 2, topographic changes are created, and fluorescence emission is produced within the imaging region. The topographic features display alignment along the polymer backbone direction (arrow).
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Special thanks to B.S. Swartzentruber for movie editing. All images copyright © Sandia National Laboratories. Images may not be used without permission. |
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