Absorption, luminescence, resonance Raman, and resonance coherent anti-Stokes Raman spectroscopy on unsymmetrically substituted diacetylene single crystals with color zones

We have analyzed the color zones in single crystals of the diacetylene derivative 6-(p-toluenesulfonyloxy)-2,4-hexadiynyl-p-fluorobenzenesulfonate by means of various optical spectroscopical methods in order to derive a plausible model for the origin of this type of chromism. By applying absorption, luminescence, resonance Raman, and coherent anti-Stokes Raman spectroscopy we obtained sufficient information for an understanding of the nature of the color zones. With the help of a nearly free-electron model calculation and a theoretical model for side group torsions, we found that a disturbance of the π-electron conjugation by a defect hindrance on side groups is the proper reason for the observed chromism. Growth defects in {111} crystal-growth sectors with a Burgers vector in the [010] direction cause the shape of the color zones. We found, particularly, that resonance coherent anti-Stokes Raman spectroscopy proves to be a valuable tool for the investigation of electronic and structural properties of the diacetylene crystals.