N,N′-dicyanoquinone diimines (DCNQIs): unique acceptors for conducting materials

Abstract

Replacement of the classical acceptor TCNQ with (substituted) DCNQIs opens new routes to organic conducting materials. The radical anion salts [2,5-R¹,R²-DCNQI]₂M (R¹,R²= Hal, Me, MeO; M = Li, Na, K, NH₄, Cu, Ag, Tl) all belong to the same space group l4_1/a or related ones (C2/c or P4/n); therefore, the correlation between molecular size and electronic effects of the substituents and bulk properties (conductivity, susceptibility, etc.) can be evaluated. Only the copper salts display metallic conductivity, some of which undergo a phase transition (T_{M → 1}) to a semiconductor on cooling. For this transition a threshold coordination angle N–Cu–N is found, by changing which an unprecedentedly large isotope effect upon deuteriation of [2,5-Me₂DCNQI]₂ can be induced. Upon the transition T_{M → 1} the conductivity decreased by a factor of 10⁷–10⁸.