An optical phase transition in thin films of highly conducting charge-transfer salts: silver and copper 2,5-dimethyl-N,N′-dicyanoquinonediimine

Thin films of higly conducting 2,5-dimethyl-N,N′-dicyanoquinonediimine (2,5-DM-DCNQI) radical-anion salts have been prepared by solution growth and vacuum deposition on various substrates. Unlike the parent electron acceptor tetracyanoquinodimethane (TCNQ), 2,5-DM-DCNQI did not react with copper metal in the solid state. After successive depositions of copper metal or copper(I) iodide and 2,5-DM-DCNQI to produce a layered structure, exposure to acetonitrile vapor of the yellow, double-layered film induced charge transfer from copper to 2,5-DM-DCNQI to give a blue or blue-black film. We measured the optical properties of these metal 2,5-DM-DCNQI films by FT-IR and UV-Vis spectroscopy and imaged the surface using scanning tunneling microscopy (STM). We also observed a laser-induced optical phase transition in these films as well as in films of other related TCNQ radical-anion salts. This switching threshold study has shown that 2,5-DM-DCNQI salts switch at a threshold less than half that of the analogous TCNQ salts.