Charge-transfer Complex - Electrical Conductivity

Electrical Conductivity

In 1954 researchers at Bell Laboratories and elsewhere reported charge-transfer complexes with resistivities as low as 8 ohms·cm in combinations of perylene with iodine or bromine. In 1962, the well-known acceptor tetracyanoquinodimethane (TCNQ) was reported. Tetrathiafulvalene (TTF) was synthesized in 1970 and found to be a strong electron donor. In 1973 it was discovered that a combination of these components formed a strong charge-transfer complex, henceforth referred to as TTF-TCNQ. The complex is formed in solution and may be crystallized into a well-formed crystalline solid. The solid shows almost metallic electrical conductance and was the first discovered purely organic conductor. In a TTF-TCNQ crystal, TTF and TCNQ molecules are arranged independently in separate parallel-aligned stacks and an electron transfer occurs from donor (TTF) to acceptor (TCNQ) stacks. Hence, electrons and electron holes are separated and concentrated in the stacks and can traverse in a one-dimensional direction along the TCNQ and TTF columns, respectively, when an electric potential is applied to the ends of a crystal in the stack direction.

The first organic molecule that forms a superconductor was discovered in 1980. Tetramethyl-tetraselenafulvalene-phosphorus hexafluoride (TMTSF₂PF₆), a semi-conductor at ambient conditions, shows superconductivity at low temperature (critical temperature) and high pressure: 0.9 K and 12 kbar. Since 1980, many organic superconductors have been synthesized, and the critical temperature has been raised to over 100 K as of 2001. Unfortunately, critical current densities in these complexes are very small.