Shear-induced mechanochromism in polydiacetylene monolayers

We use atomic force microscopy to actuate and characterize the nanoscale "m echanochromism" of polydiacetylene monolayers on atomically-flat silicon ox ide substrates. We find explicit evidence that the irreversible blue-to-red transformation is caused by shear forces exerted normal to the polydiacety lene polymer backbone. The anisotropic probe-induced transformation is char acterized by a significant change in the tilt orientation of the side chain s with respect to the surface normal. We discuss preliminary molecular dyna mics simulations and electronic structure calculations on twelve-unit polyd iacetylene oligomers that allow us to correlate the transformation with bon d-angle changes in the conjugated polymer backbone.