An electrooptical memory effect is observed with solid thin films of the photoconductor zinc-octakis(beta-decoxyethyl) porphyrin (ZnODEP) sandwiched between two optically transparent electrodes. Upon irradiation with the simultaneous application of an electric field, electron-hole pairs are generated and separated within the photoconductive layer. These electron-hole pairs become "frozen" within the films when the irradiation is interrupted. These trapped charges can be released by irradiation of the cell, resulting in a transient short-circuit photocurrent. No cross talk between adjacent memory elements separated by approximately 0.2 micrometer (a density of 3 gigabits per square centimeter) was detected. The charge storage system is robust and nonvolatile. The response time for the write-read beam is in the subnanosecond range, and no refreshing is required for long-term retention of trapped charges.

The compound hexa-(3,7,11,15-tetramethylhexadecanyl)hexa-peri-hexabenzocoronene (HBC-C4/16), soluble in most organic solvents such as chloroform and THF, was synthesized. This compound shows a stable discotic hexagonal columnar mesophase between −36 and 231 °C. Symmetrical sandwich cells of ITO/HBC-C4/16/ITO (∼1 μm thick) were prepared by capillary filling by the molten state for the studies of their optical and optoelectronic properties. Two distinct textures, one very bright and the other gray, were clearly seen in thin films of HBC-C4/16 between two crossed polarizers in an optical microscope at room temperature. Short-circuit photocurrents (I sc) were obtained with such cells, and the gray areas generated a much higher short circuit photocurrent compared to that of the bright spots. This photocurrent difference was attributed to different orientations of the HBC-C4/16 molecular columns relative to the ITO electrode surface. The charge trapping and detrapping effects found earlier with other materials, such as zinc porphyrins (ZnODEP), and of potential use in optoelectronic data storage were also observed with these cells.

Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance. Metal insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (approximately 100 microm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10(-13) A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.

Chong-yang Liu was born in Shaanxi, China, and educated at Northwestern University, the Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, and the Hahn-Meitner-Institut fu ¨r Kernforschung Berlin, Germany, where he carried out research with Chen Yixuan, Li Wenzhao, Wang Hongli, and H. Tributsch, respectively. He was a Postdoctoral Fellow with Allen J. Bard and is presently a Research Scientist Associate at the University of Texas at Austin. Allen J. Bard holds the Hackerman-Welch Regents' Chair in Chemistry at the University of Texas at Austin. His principle research interests are in the application of electrochemical techniques to the study of chemical problems.