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

Abstract

Reactions of the main-group cation Tl(I) with anions of 2,5-derivatives of TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have led to the isolation of a family of unprecedented semiconducting main-group-metal-organic frameworks, namely, [Tl(TCNQX(2))], (X = H, Cl, Br, I). A comparison of single-crystal and powder X-ray diffraction data revealed the existence of a third polymorph of the previously reported material Tl(TCNQ)] and two distinct polymorphs of [Tl(TCNQCl(2))], whereas only one phase was identified for [Tl(TCNQBr(2))] and [Tl(TCNQI(2))]. These new results are described in the context of the structures of other known binary metal-TCNQ frameworks that display a variety of coordination environments for the central cation, namely, four-, six-, and eight-coordinate, and different arrangements of the adjacent TCNQ radicals-parallel versus perpendicular-in the stacked columns. The halogen substituents affect the structures and the properties of these compounds, owing to both steric and electronic effects as evidenced by the semiconducting properties of crystals of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))], which correlate well with the distances of adjacent TCNQ radicals in the columns. 1D infinite Hückel model simulations of the band structures of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))] were conducted with and without consideration of the Tl(I) cations, the results of which indicate that the charge mobility does not strictly occur in one dimension. The modulations of the band structures with various assumptions of the energy difference (Δ) between the Tl(I) 6s orbital and the TCNQ LUMO orbital were calculated and are discussed in light of the observed properties.

Electroactive Organic Building Blocks for the Chemical Design of Functional Porous Frameworks (MOFs and COFs) in Electronics

“…[103] In this context,several examples of electrically conductive frameworks have been achievedu sing TCNQ ligands,s uch as the well-known [Cu(TCNQ)], [104] which possesses two structural phases:p hase I( s = 10 À1 Scm À1 )i sm uch more conductive than phaseII( s = 10 À5 Scm À1 )b ecause of the presence of delocalized stacks of mixed-valence TCNQ moieties. [49] In as imilarw ay,e lectrical conductivity in [Tl(TCNQX 2 )] [105] systems (s = 10 À2 -10 À3 Scm À1 )s trongly depends on the arrangement and distances of the stacked radicalanion TCNQ moieties within the different crystalline polymorphs (Figure 7). Notably, permanent porosity wasn ot reported for any of these frameworks.…”

Electrically Conductive Porous Metal–Organic Frameworks

“…[64] Ideally,this approach can give rise to band-like transport, as in TTF-TCNQ itself and other organic metals.S table organic radicals are the best candidates for ligands because they provide free charge carriers,t hereby improving charge density.Alarge body of literature exists on the use of this approach to design conductive coordination polymers or molecular conductors, [65,66] many of which display conductivity values in excess of 1Scm À1 . [67][68][69][70][71] To our knowledge,however,n one of these exhibit permanent porosity.…”

MOF-based electronic and opto-electronic devices

“…Electrical conductivity has also been extensively studied in various TCNQ-based charge transfer complexes with Cu, Ag, alkali metals, as well as organic molecules such as tetrathiafulvalene (TTF), all of which assemble into columnar stacks of A + D À with TCNQ interplanar distances of B3.5 Å. [77][78][79] Conduction in these materials is through the p-stacked TCNQ molecules, rather than through the metalligand bond, and values 410 3 S cm À1 at room temperature have been reported. 78,79 Guided by the knowledge gained from the extensive studies of conducting organic and coordination polymers, a number of groups recently demonstrated promising strategies for realizing conducting MOFs with permanent porosity.…”