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 solv ent green 3, SG3) showed different colors and photoconductivity depend ing on the crystal orientation. Single crystals were grown from the me lt in a sandwich cell of indium tin oxide coated glass, where needle-s haped crystals of different colors (mainly blue, yellow-green, and pur ple) were observed. The absorption spectrum and short-circuit photocur rent of the needle-shaped crystals that appeared blue, yellow-green, a nd purple were measured with and without a polarizer. Both the absorpt ion spectra and photoconductivity were dramatically different for diff erent crystal orientations, indicating these properties were anisotrop ic. X-ray diffraction analysis revealed that in the crystal the SG3 mo lecules were slip-stacked to form a columnar structure. The polarized absorption spectra can be related to the crystal orientation of each n eedle-shaped crystal. The blue and yellow-green colors are attributed to an intramolecular charge-transfer (CT) transition along, respective ly, the long and short molecular axes of the 9,10-anthraquinone molecu lar 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 cryst al orientated to show a purple color had the highest photoconductivity , because excitation of the intermolecular CT band facilitates charge separation.