First observation of mechanochromism at the nanometer scale

A mechanically induced color transition ("mechanochromism") in poly(diacety lene) thin films has been generated at the nanometer scale using the tips o f two different scanning probe microscopes. A blue-to-red chromatic transit ion in poly(diacetylene) molecular trilayer films, polymerized fr om 10,12- pentacosadiynoic acid (poly-PCDA), was found to result from shear forces ac ting between the tip and the poly-PCDA molecules, as independently observed with near-field scanning optical microscopy and atomic force microscopy (A FM), Red domains were identified by a fluorescence emission signature. Tran sformed regions as small as 30 nm in width were observed with AFM. The irre versibly transformed domains preferentially grow along the polymer backbone direction. Significant rearrangement of poly-PCDA bilayer segments is obse rved by AFM in transformed regions. The rearrangement of these segments app ears to be a characteristic feature of the transition. To our knowledge, th is is the first observation of nanometer-scale mechanochromism in any mater ial.