Orientational Dependence of the Color and Photoconductivity of 1,4-Di-p-toluidinoanthraquinone Single Crystals

Single crystals of 1,4-di-p-toluidinoanthraquinone (also known as solvent green 3, SG3) showed different colors and photoconductivity depending on the crystal orientation. Single crystals were grown from the melt in a sandwich cell of indium tin oxide coated glass, where needle-shaped crystals of different colors (mainly blue, yellow-green, and purple) were observed. The absorption spectrum and short-circuit photocurrent of the needle-shaped crystals that appeared blue, yellow-green, and purple were measured with and without a polarizer. Both the absorption spectra and photoconductivity were dramatically different for different crystal orientations, indicating these properties were anisotropic. X-ray diffraction analysis revealed that in the crystal the SG3 molecules were slip-stacked to form a columnar structure. The polarized absorption spectra can be related to the crystal orientation of each needle-shaped crystal. The blue and yellow-green colors are attributed to an intramolecular charge-transfer (CT) transition along, respectively, the long and short molecular axes of the 9,10-anthraquinone molecular framework. The absorption corresponding to the purple color, which was not observed in solution, is proposed to be an intermolecular CT transition between neighboring molecules in the same column. The crystal orientated to show a purple color had the highest photoconductivity, because excitation of the intermolecular CT band facilitates charge separation.