Creation of Solid State Shearchromism by Shear Stress Effects

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The shear stress effects on the photochromic spiropyrans have been studied. We have used the sapphire anvil cell with the rotation mechanism and performed the in situ observations under a microscope with Raman spectroscopic measurements. The results should be led to the development of “shearchromism" and the new method for controlling chemical bonds.1. Shear stress effectsThe color of the thin film of 6-nitrospiropyran was remarkably changed from light-yellow to green under shear deformation. For the green state, the Raman spectrum could not be obtained because of the strong fluorescence. As being released stress, the color changed from green to purple and the fluorescent emission disappeared. The spectrum of the purple state was that of spiropyran with some new superimposed weak bands. These bands suggest that some spiropyran molecules are transformed to merocyanine. The chromic behavior to the green state and the strong fluorescence were observed also by the hydrostatic pressure over about 2.5GPa, which was larger than the estimated shear stress 1GPa. The shear stress gave more effective deformation to molecules than pressure.2. Photoresponsive propertiesThe thin film of 6-nitrospiropyran at ambient pressure was readily changed from yellow to purple with UV light and returned to yellow with visible light. However, the green state under shear stress remained after the irradiation of UV light and slightly faded with visible light. Spiropyrans under shear deformation less responded to the light.3. Cyclodextrin inclusion compoundThe inclusion compound of 6-nitrospiropyran in y-cyclodextrin has exhibited the chromic behavior from pink to dark red by shear stress and then to red-purple as releasing stress. Raman spectra showed that the transformation to merocyanine was induced under shear stress and that merocyanine molecule was stabilized in y-cyclodextrin. In addition, the more rapid photochromic behavior than independent spiropyran was observed.