Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

Abstract

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10-6 cm2·V-1·s-1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10-4 cm2·V-1·s-1. Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

Indene and Pseudoazulene Discotic Liquid Crystals: A Synthetic and Structural Study

“…For example, the self-assembly of an amphiphilic hexa-perihexabenzocoronene gave rise to a redoxactive graphitic nanotube that consisted of helical arrays of p-stacked molecules and, upon oxidation, showed electrical conductivity. [5] Liquid-crystalline discotic materials, such as side-chain-modified phthalocyanines [6] and porphyrins, as well as triphenylenes, [7] self-organize into columnar aggregates and have become of interest as potential solutionprocessed molecular electronic materials. [8] In general, columnar discotic liquid crystals consist of a rigid central core-which can be planar, pyramidal, conical or have another similar geometry-surrounded by a number of flexible groups.…”

“…The phase transition between the columnar mesophase and the new phase is clearly visible. In both phases, two relaxation processes are evident, one at around 10 6 Hz and the other at around 10 3 Hz. Both relaxations disappear upon solidification and they are thermally activated.…”

“…This find- ing led us to prepare the a,a-dicyanoindene 6, which shows polymorphism with properties similar to those displayed by the two 2,3-dichloroindenes 2 and 3. Furthermore, compound 6 was considered a suitable model structure for modification (compounds [6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

Conjugated compounds in supramolecular informational systems: A review

“…Recently, the effort of many scientists working on enhancement of the electric properties of phthalocya nine films and the efficiency of their production has been directed toward synthesis of octa substituted amphiphilic derivatives [331][332][333][334][335][336][337][338]. It was shown that such compounds can form true monolayers consisting of stacking aggregates and existing upon compression up to pressures of no more than 10-15 mN/m.…”

“…The conductivity of such films of amphiphilic phthalocyanines lies in the range of 10 -9 -10 -8 S/m -1 and can be increased by two to three orders of magni tude by electrochemical doping [337,338].…”