Ionicity Phase Diagram of Trifluoromethyl-TCNQ (CF3TCNQ) Charge-Transfer Solids

Abstract

Abstract A series of charge-transfer (CT) solids of trifluoromethyl-7,7,8,8-tetracyanoquinodimethane (CF3TCNQ) with various electron donor molecules were prepared and their IR and UV–vis–near-IR spectra and electrical conductivity were measured. The information was applied to produce an ionicity phase diagram of CF3TCNQ CT solids. A boundary for ionicity of CF3TCNQ was found in combination with donor molecules of dibenzo[c,d]phenothiazine, diaminodurene, or dibenzotetrathiafulvalene (DBTTF). With stronger donors than DBTTF, the CF3TCNQ molecules were fully ionized and acted as a counter anion. No conductors with partially charged CF3TCNQ species were obtained. Besides the conventional 1:1 fully ionic insulators with segregated stacks, tetramethyl-TTF·CF3TCNQ·CH3CN and bis(methylthio)ethylenedithio-TTF·CF3TCNQ had fully ionic alternating stacks of DDAA units and showed Frenkel triplet excitons. (BEDO-TTF)2(CF3TCNQ) [BEDO-TTF: bis(ethylenedioxy)-TTF] consisted of a mixed-valence segregated stack of donor molecules and completely ionized acceptor molecules, and showed metallic behavior down to 1.8 K even in a compressed pellet sample. Langmuir–Blodgett films composed of (BEDO-TTF)2(CF3TCNQ) and matrix (arachidic acid) showed a conductivity of 3–6 S cm−1 at room temperature and a nearly temperature-independent conductivity down to 80 K. Semiconducting (TMTSF)2(CF3TCNQ) (TMTSF: tetramethyltetraselenafulvalene) had one-dimensional segregated stacks of dimerized TMTSF molecules separated by completely ionized CF3TCNQ, the molecular plane of which was arranged parallel to the TMTSF column. The ionicity phase diagram of the CF3TCNQ CT solids, i.e., a plot of the first CT transition energy vs. donor strength, clearly discriminated these different kinds of CT solids and will be utilized for the prediction and design of the functional CT solids.