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Stabilization of High Electrical Conductivity in Organic Charge Transfer Salts

IP.com Disclosure Number: IPCOM000085749D
Original Publication Date: 1976-May-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 38K

Publishing Venue

IBM

Related People

Engler, EM: AUTHOR [+3]

Abstract

Organic charge-transfer salts such as TTF-TCNQ(2) and TSeF-TCNQ display room temperature DC electrical conductivities of 10/2/-10/3/(ohm-cm)/-1/and exhibit metallic-like behavior over wide temperature ranges. At low temperatures the conductivity typically increases 10-30 times the room temperature value to a peak value, at which these materials undergo a phase transition from a metallic to semiconductor state.

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Stabilization of High Electrical Conductivity in Organic Charge Transfer Salts

Organic charge-transfer salts such as TTF-TCNQ(2) and TSeF-TCNQ display room temperature DC electrical conductivities of 10/2/-10/3/(ohm-cm)/- 1/and exhibit metallic-like behavior over wide temperature ranges. At low temperatures the conductivity typically increases 10-30 times the room temperature value to a peak value, at which these materials undergo a phase transition from a metallic to semiconductor state.

The turning off of the high conductivity at low temperature is considered to occur due to the special structural characteristics of these materials. The crystal structure of these metallic-like, charge-transfer salts consists of separate columns or stacks of the planar donor and acceptor molecules with very short intrastack separations, permitting overlap of their pi-molecular orbitals. Due to the directionality of the pi-electron wave functions, overlap is strongest along the stacking axis, but minor in other crystallographic directions. These materials are therefore highly anisotropic and pseudo one-dimensional in character.

One-dimensional metals are predicted to be unstable to lattice distortions due to the Peierls instability. The Peierls instability arises from the fact that the one- dimensional metal at low enough temperatures, is unstable toward a soft mode structural transition driven by the divergent response of the electron gas at q = 2k(F).

A means is described for stabilizing high conductivity in organic charge- transfer salts by designing into the constituent organic donors and acceptors higher dimensionality, so as to suppress the phase transition associated with the Peierls instability. Appropriate illustrative examples of this approach are outlined below.

By employing specifically designed, rigid carbon skeletons, donors or acceptors can be forced to occupy perpendicular positions. Two possible "orientation substrates" are the allene and the spiro(4.41) systems shown below:

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Planer donors or acceptors would be locked perpendicular when attached to these substrates. Some examples of these two-dimensional molecules are shown as follows:

(Image Omitted)

The perpendicularly-locked molecules must be combined with integral ratios of monomeric acceptor, and then reacted with the appropriate donor salt to yield t...