Conducting films based on single-component molecular metals

Abstract

We demonstrate that single component molecular metals can be used as conductive inks for printed electronics. The resistance is 0.3 kΩ/sq, in a Ni complex, one order of magnitude better than commercial carbon based conductive inks.Conductive inks are particularly attractive for electronic devices using printing or other solution processing technology such as flexible displays, photovoltaics or sensors integrated in textiles or paper . Moreover, functionalisation of these materials is difficult, hindering a more selective interface 4 . Even in conducting polymers, the best strategy to modify their properties is mixing compounds, making it difficult to control the reproduci bility and anticipate the final effect 5 . Small organic molecules have proven to be easier to control by strategic chemical modification, both the electronic properties and chemical sensitivity 6 . That is the case, for instance, of the perylenediimides, used as organic semiconductor in field-effect transistors. The introduction of electronegative groups, such as halogens, on bay positions of the core aromatic scaffold can modify the energy levels with direct consequences on the transport and electrochemical properties, whereas additional side groups in the imide positions lead to changes in solubility and fluorescence 7 . To reach metallic conduction, an appropriate combination of molecular order and chemical interactions are necessary for producing charge carriers drift with minimal scattering. Defects and steric constrains, seriously affect the transport mechanism and, as a result, only a few materials have been developed 8 .Single component molecular metals (SCMM) are a recent class of molecular conductors composed of a single neutral molecule, with a transition metal bisdithiolene core, offering the simplicity of only one molecule to be engineered and processed. Conductivity as high as 400 S/cm and even superconductivity has been reported, both measured in singlecrystal 9 . In compressed polycrystalline powder samples values decrease substantially due to grain-boundary effect, yet, some tetrathiafulvalenedithiolate compounds were able to achieve 200 S/cm 10 . Even if these materials present low solubility, a trimethylenetetrathiafulvalenedithiolate derivative was successfully used as a conductive film 11 , whereas a thiophenedithiolene derivative was used as flexible metallic films for piezo-resistive sensors. However, these were achieved either as a highly viscose mixture that becomes rapidly rigid, or as a bilayer composite film prepared in situ by a modified reticulated doping technique 12 , which are incompatible with printing process. Moreover, similar approaches for compounds with more extended chemical structure and higher conductivity were restricted by their lower solubility.In this communication, we report the fabrication of a highly conductive and durable SCMM films using a solution process method compatible with printing technology. To the best of our knowledge, this is the first evidence of using a single component m...

Thiophene-Fused Nickel Dithiolenes: A Synthetic Scaffold for Highly Delocalized π-Electron Systems

“…In contrast, 6c exhibits a minor tetrahedral distortion of the nickel coordination geometry due to a small twist (∼7°) along the long axis of the complex. Such small tetrahedral distortions have been previously observed in various structures of NiTDT and are typical of square-planar metal dithiolenes . In addition to tetrahedral distortion, 6c also exhibits twisting about the interannular bonds to the two external thiophenes (∼6–12°).…”

“…Such ring fusion increases molecule planarity and enhances orbital overlap, thus maximizing electron delocalization into the organic ligands. Many fused-ring dithiolene systems have been studied, including the fusion of benzene, − functionalized benzene, − thiophene, ,,− pyridine, quinoxaline, , and other heterocyclic systems. − The incorporation of such fused-ring systems allows synthetic modification of the resulting materials via substituents and heteroatoms on the fused-ring units, allowing a great deal of diversity in the resulting dithiolene systems and the potential to finely tune both the electronic structures and solid-state arrangements. In addition, fused benzene analogues containing additional disulfide pairs have also allowed the generation of extended one- and two-dimensional dithiolene-based coordination polymers. , …”

“…In order to maintain a linear structure and to maximize conjugation along the molecular backbone, the known metal 2,3-thiophenedithiolene ,,− scaffold (Figure ) was chosen as the initial core, from which the addition of aryl groups to the exterior thiophene α positions would produce complexes with more extended π systems. Initial success in the production of a thiophene-extended nickel thiophenedithiolene complex (Figure ) was previously reported, and this current report now presents a general approach to the production of such aryl-extended nickel thiophenedithiolene complexes as a potential new class of molecular materials.…”

The quest for single component molecular metals within neutral transition metal complexes

“…It is worthwhile to mention that [Ni(dtdt) 2 ] films showed related properties to commercial carbon-based conductive inks. 62 The copper analogues [Cu(dtdt) 2 ] and [Cu(a-tdt) 2 ] were also prepared showing electric conductivities, ranging between 0.10-0.40 S cm À1 , however with a semiconducting behaviour. 63 The methyl substituted [M(a-tdt) 2 ] analogues, [Au(a-mtdt) 2 ] and [Ni(a-mtdt) 2 ] were also prepared and studied.…”

Gold and Nickel Extended Thiophenic-TTF Bisdithiolene Complexes

“…Besides enlarging the SCMM family of compounds, the characterization of these salts allowed the study of steric effects on electric conductivity when compared with the transport properties of related non-substituted complexes, [M(α-tdt) 2 ] 0 and [M(dtdt) 2 ] 0 (α-tdt = thiophenetetrathiafulvalenedithiolate, dtdt = dihydrothiophenetetrathiafulvalenedithiolate; M = Au and Ni) [ 17 ]. SCMM are promising candidates as components in electronic devices, being able to be processed using solution techniques they can be used, for example as conductive inks [ 18 ].…”