Dramatically Different Conductivity Properties of Metal–Organic Framework Polymorphs of Tl(TCNQ): An Unexpected Room‐Temperature Crystal‐to‐Crystal Phase Transition

Abstract

The synthesis and fabrication of nanoscale materials for new types of electronic and magnetic devices is a central theme in materials science research in this second decade of the 21st century. Given that conventional storage materials are estimated to approach their miniaturization limit by 2016, [1] heightened efforts are being directed at the design and synthesis of new types of bistable nanoscale materials, including those capable of undergoing a change from low to high resistance under the application of an electric field. Such nonvolatile memory devices are capable of operating at increased speeds and require less energy than conventional memory devices. Among the materials being investigated for resistance-based memory are materials that contain organic components and whose properties are influenced by magnetic or electric fields. [2] Materials that respond to the application of an electric field or changes in light, pressure, or temperature are being sought for incorporation into electronic devices with ultrafast operating speeds. [3,4] Examples of molecule-based materials that exhibit fascinating properties are the spin-crossover complex [Fe(picolylamine) 3 Cl 2 (C 2 H 5 OH)], [5,6] the neutralionic transition system TTF-chloranil (TTF = tetrathiafulvalene), [7][8][9] the metallo-organic conductor Cu(DM-DCNQI) 2 , [10][11][12][13][14][15][16] (DM-DCNQI = dimethyl-N,N'-dicyanoquinonediimine) and the salt (EDO-TTF) 2 PF 6 , [17,18] (EDO-TTF = ethylenedioxytetrathiafulvalene). These materials provide compelling evidence for the contention that molecular solids may eventually be useful in device applications.In terms of electric-field-induced behavior, the most extensively studied examples are the organocyanide-based materials Cu(TCNQ) (TCNQ = 7,7,8,, which exhibits reversible switching from a highresistance state to a conducting state promoted by the application of an electric field or upon irradiation, [19][20][21] and the current-driven conductor K(TCNQ) salt.[22] The latter material is a key member of the binary series of alkali-metal salts of TCNQ that behave as so-called "Mott insulators" at high temperatures, in which the fully reduced radical anions are arranged in columns with evenly spaced TCNQ units. At lower temperatures, these "soft" materials undergo a phase transition in which the TCNQ units are brought into close proximity as a result of p dimerization. The electrons are then trapped in the dimers, the conductivity drops, and the materials pass into the spin-Peierls insulating state.An approach that we have adopted for discovering conducting TCNQ phases is to capitalize on the rich chemistry of alkali metals while circumventing some issues that hinder their conductivity. In this vein, thallium is an interesting element, since it can behave as a pseudo-alkali metal. In contrast to other Group 13 elements, Tl prefers the 1 + oxidation state (although Tl 3+ is known), and many similarities between the chemistry of alkali-metal ions and Tl + have been noted.[23] The electronegativity of Tl (2.04)...

Conducting Organic Frameworks Based on a Main‐Group Metal and Organocyanide Radicals

“…[Tl(TCNQ)] phase III ( 5 ) : Crystals of a new phase of the material were unexpectedly obtained during an attempt to prepare a bulk sample of [Tl(TCNQ)] phase I. When the reaction between TlPF 6 and (Bu 4 N) + TCNQ − is performed in dry acetonitrile, the result is a dark‐purple powder for which the powder XRD pattern is very similar to the simulated spectrum obtained from the single‐crystal data of [Tl(TCNQ)] phase I 23. Surprisingly, however, unlike phase I the new material does not undergo a phase transition when suspended in water or methanol for several days.…”

“…To put these new results into perspective, it is instructive to examine the diversity of the structures of the 1:1 [M(TCNQ)] materials (excluding the alkali metals) depicted in Figure 5 12. 20, 23, 27 The particular architecture is characterized by the coordination number of the metal cations and the orientation motif and stacking of the TCNQ radicals A four‐coordinate environment is observed for the Cu I and Ag I materials, with a perpendicular arrangement of the TCNQ ligands for [Cu(TCNQ)] phase I and [Ag(TCNQ)] phase II. In contrast, [Cu(TCNQ)] phase II and [Cu(TCNQX 2 )] (X=Cl, Br) adopt a parallel packing mode for the TCNQ moiety with different relative positions of adjacent columns (non‐coplanar versus coplanar).…”

“…With these details in mind, we prepared the compound [Tl(TCNQ)], which was found to exist as two polymorphs with dramatically different conducting properties. Moreover, a surprising unprecedented room‐temperature crystal‐to‐crystal phase transition was observed to occur between the polymorphs in a humid environment 23…”

“…Given these aforementioned fascinating results, we undertook the syntheses of thallium(I) materials with the 2,5‐subsituted derivatives, TCNQX 2 (X=Cl, Br, I). Herein we report the new semiconductors [Tl(TCNQX 2 )] along with their structures and a theoretical analysis and comparisons to the related TCNQ phases, which have been published, in part, in a previous communication 23…”

Spin state switching in iron coordination compounds

“…The vast majority of their applications are related to pore sizes, shapes, flexibility and structures/environments. Particularly interesting are those PCPs where the solid-state properties of the framework (magnetism [417], luminescence [418], conductivity [419–420], charge transport [421], etc.) change upon sorption/desorption of guest molecules.…”