Temperature-induced neutral-ionic transition in tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ)

Experimental evidence is presented that indicates the occurrence of a temperature-induced neutral-ionic transition occurring at ${\mathit{T}}_{\mathit{c}}$≃205 K in the mixed-stack organic charge-transfer crystal, 3,3’,5,5’-tetramethylbenzidine-7,7,8,8-tetracyanoquinodimethane (TMB-TCNQ). The transition is characterized by a large first-order-like jump in the magnetic susceptibility and a dimeric distortion of the stacks similar to those observed in tetrathiafulvalene-chloranil (TTF-CA). However, contrary to the case in TTF-CA the molecular ionicity jump at ${\mathit{T}}_{\mathit{c}}$ is comparatively small in TMB-TCNQ. The nature of this transition observed in TMB-TCNQ is discussed drawing comparisons with that seen in TTF-CA.