PHASE-CHANGE OF 3,6-DIPHENYLPYRROLO[3,4-C]PYRROLE-1,4-DITHIONES FOR OPTICAL STORAGE-SYSTEMS

'Diketopyrrolopyrroles' (DPP) are industrially important red pigments which resist bleaching. Thionation (replacement of the O- by the S-atom; the title compound (DTPP)) brings about an intense near-IR absorption in the solid state. Because of this, DTPP has recently attracted attention as a material for laser printers and optical information-storage systems based on GaAsAl laser diodes. Three crystal modifications I, II, and III are known to exist for DTPP. Among these, only modification III exhibits an intense near-IR absorption (860 nm), whereas the absorption band appears only at 690-710 nm in modifications I and II. The near-IR absorption is due to interplanar pi-pi interactions of the alternating N-atom of one DTPP molecule and thiocarbonyl C-atom of the neighboring molecule along the stacking axis. The formation or blocking of the pi-pi interaction ('phase change') by means of optical energy can be applied to an optical information storage system. The optical disk (structure: substratelhydrazone/DTPP/Al) exhibits a reflectivity change from cn. 30 to 45% on writing with a power of ca. 9 mW at 780 nm.