Multiredox Tetrathiafulvalene-Modified Oxide-Free Hydrogen-Terminated Si(100) Surfaces

Tetrathiafulvalene (TTF) monolayers covalently bound to oxide-free hydrogen-terminated Si(100) surfaces have been prepared from the hydrosilylation reaction involving a TTF-terminated ethyne derivative. FTIR spectroscopy characterization using similarly modified porous Si(100) substrates revealed the presence of vibration bands assigned to the immobilized TTF rings and the Si–CC– interfacial bonds. Cyclic voltammetry measurements showed the presence of two reversible one-electron systems ascribed to TTF/TTF^.+ and TTF^.+/TTF²⁺ redox couples at ca. 0.40 and 0.75 V vs SCE, respectively, which compare well with the values determined for the electroactive molecule in solution. The amount of immobilized TTF units could be varied in the range from 1.7 × 10^–10 to 5.2 × 10^–10 mol cm^–2 by diluting the TTF-terminated chains with inert <i>n</i>-decenyl chains. The highest coverage obtained for the single-component monolayer is consistent with a densely packed TTF monolayer

Controlled Grafting of Tetrathiafulvalene (TTF) Containing Diacetylenic Units on Hydrogen-Terminated Silicon Surfaces: From Redox-Active TTF Monolayer to Polymer Films

A tetrathiafulvalene (TTF)-terminated butadiyne derivative was synthesized and used for the preparation of redox-active TTF-modified hydrogen-terminated oxide-free silicon (Si–H) surfaces. TTF monolayer-modified silicon surfaces were produced when low grafting temperatures were used (typically 45 °C), whereas higher temperatures (90 °C) led to TTF polymer-modified surfaces. IR spectroscopy characterization provided evidence that TTF units bound to the surface through the formation of enyne linkers via hydrosilylation of the terminal alkyne bond. The TTF monolayers prepared at 45 °C were densely packed with a surface coverage of ca. 5.4 × 10^–10 mol of TTF per cm². For such systems, electrochemical measurements showed the redox signature of the bound TTF centers characterized by two reversible one-electron systems at ca. 0.40 and 0.73 V versus saturated calomel electrode (SCE). High values of electron-transfer rate constants were determined (>200 s^–1) and ascribed to the presence of the conjugated bridge between the attached redox-active center and the underlying silicon surface. The TTF polymer-modified surfaces prepared at 90 °C resulted from the direct grafting of polymeric structures on Si–H and/or the postattachment functionalization of the preformed TTF monolayer. Polymerization process of the TTF-terminated butadiyne derivative was also investigated in solid state by means of differential scanning calorimetry and diffuse reflectance IR spectroscopy measurements

Controlled Grafting of Tetrathiafulvalene (TTF) Containing Diacetylenic Units on Hydrogen-Terminated Silicon Surfaces: From Redox-Active TTF Monolayer to Polymer Films

A tetrathiafulvalene (TTF)-terminated butadiyne derivative was synthesized and used for the preparation of redox-active TTF-modified hydrogen-terminated oxide-free silicon (Si–H) surfaces. TTF monolayer-modified silicon surfaces were produced when low grafting temperatures were used (typically 45 °C), whereas higher temperatures (90 °C) led to TTF polymer-modified surfaces. IR spectroscopy characterization provided evidence that TTF units bound to the surface through the formation of enyne linkers via hydrosilylation of the terminal alkyne bond. The TTF monolayers prepared at 45 °C were densely packed with a surface coverage of ca. 5.4 × 10^–10 mol of TTF per cm². For such systems, electrochemical measurements showed the redox signature of the bound TTF centers characterized by two reversible one-electron systems at ca. 0.40 and 0.73 V versus saturated calomel electrode (SCE). High values of electron-transfer rate constants were determined (>200 s^–1) and ascribed to the presence of the conjugated bridge between the attached redox-active center and the underlying silicon surface. The TTF polymer-modified surfaces prepared at 90 °C resulted from the direct grafting of polymeric structures on Si–H and/or the postattachment functionalization of the preformed TTF monolayer. Polymerization process of the TTF-terminated butadiyne derivative was also investigated in solid state by means of differential scanning calorimetry and diffuse reflectance IR spectroscopy measurements

Hydrogen-Bonding Interactions in a Single-Component Molecular Conductor: a Hydroxyethyl-Substituted Radical Gold Dithiolene Complex

The anionic hydroxyethyl-substituted gold dithiolene complex [NEt₄]­[Au­(EtOH-thiazdt)₂] is synthesized and further oxidized to the neutral radical species [Au­(EtOH-thiazdt)₂]^• through electrocrystallization. Single-crystal X-ray diffraction studies highlight the existence of the two cis and trans isomers for the monoanionic complex, with involvement of the hydroxy group in intermolecular O–H···S hydrogen-bonding interactions. The neutral radical complex, [Au­(EtOH-thiazdt)₂]^•, is isostructural with its known ethyl analogue, namely, [Au­(Et-thiazdt)₂]^•. It exhibits a semiconducting behavior (σ_RT = 0.05–0.07 S cm^–1) at room temperature and ambient pressure with an activation energy of 0.14 eV. Comparison of the crystal structures and transport and magnetic properties with those of the prototypical [Au­(Et-thiazdt)₂]^• single-component conductor shows that the replacement of ethyl by a slightly bulkier hydroxyethyl substituent affects only weakly the overlap interactions, complemented here by added O–H···S hydrogen-bonding interactions

Radical or Not Radical: Compared Structures of Metal (M = Ni, Au) Bis-Dithiolene Complexes with a Thiazole Backbone

A complete series of dianionic, monoanionic, and neutral dithiolene complexes formulated as [Ni­(Et-thiazdt)₂]^<i>n</i>, with <i>n</i> = −2, −1, 0, and Et-thiazdt: <i>N</i>-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate, is prepared using an optimized procedure described earlier for the N–Me derivatives. Electrochemical and spectroscopic properties confirm the electron-rich character of the Et-thiazdt dithiolate ligand. The three complexes are structurally characterized by single-crystal X-ray diffraction. The paramagnetic anionic complex [Ni­(Et-thiazdt)₂]⁻¹, as Ph₄P⁺ salt, exhibits side-by-side lateral interactions leading to a Heisenberg spin chain behavior. The solid-state structure of the neutral, diamagnetic [Ni­(Et-thiazdt)₂]⁰ complex shows a face-to-face organization with a large longitudinal shift, at variance with the structure of its radical and neutral gold dithiolene analogue described earlier and formulated as [Au­(Et-thiazdt)₂]^•. Comparison of the two structures, and those of the other few structurally characterized pairs of Ni/Au dithiolene complexes, demonstrates the important role played by overlap interactions between gold dithiolene radical species. Despite its closed-shell character, the neutral nickel complex [Ni­(Et-thiazdt)₂]⁰ exhibits a semiconducting behavior with a room-temperature conductivity σ_RT ≈ 0.014 S cm^–1

Anisotropic Chemical Pressure Effects in Single-Component Molecular Metals Based on Radical Dithiolene and Diselenolene Gold Complexes

On the basis of the reported radical neutral complex [Au­(Et-thiazdt)₂] (Et-thiazdt = <i>N</i>-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate), a series of single-component conductors derived from [Au­(Et-thiazdt)₂], also noted as [AuS₄(S)₂], has been developed, by replacing the outer sulfur atoms of the thiazoline-2-thione rings by oxygen atoms and/or by replacing the coordinating sulfur atoms by selenium atoms toward the corresponding diselenolene complexes. Comparison of the X-ray crystal structures and transport properties of the four isostructural complexes, noted as [AuS₄(S)₂], [AuS₄(O)₂], [AuSe₄(S)₂], and [AuSe₄(O)₂], shows that the oxygen substitution on the outer thiazoline ring actually decreases the conductivity by a factor of 100, despite a contracted unit cell volume reflecting a positive chemical pressure effect. On the other hand, the S/Se substitution increases the conductivity by a factor of 100, and the pressure needed to transform these semiconductors into the metallic state is shifted from 13 kbar in [AuS₄(S)₂] to only ≈6 kbar in [AuSe₄(S)₂]. Analysis of unit cell evolutions and ab initio band structure calculations demonstrates the strongly anisotropic nature of this chemical pressure effect and provides an explanation for the observed changes in conductivity. The greater sensitivity of these neutral single-component conductors to external pressure, as compared with “classical” radical salts, is also highlighted

Atropisomerism in a 10-Membered Ring with Multiple Chirality Axes: (3<i>Z</i>,9<i>Z</i>)‑1,2,5,8-Dithiadiazecine-6,7(5<i>H</i>,8<i>H</i>)‑dione Series

For the first time, chirality in (3<i>Z</i>,9<i>Z</i>)-1,2,5,8-dithiadiazecine-6,7­(5<i>H</i>,8<i>H</i>)-dione series was recognized. Enantiomers of the 4,9-dimethyl-5,8-diphenyl analogue were isolated at room temperature, and their thermal stability was determined. X-ray crystallography confirmed the occurrence of a pair of enantiomers in the crystal. Absolute configurations were assigned by comparing experimental and calculated vibrational/electronic circular dichroism spectra of isolated enantiomers. A distorted tesseract (four-dimensional hypercube) was used to visualize the calculated enantiomerization process, which requires the rotation around four chirality axes. Conformers of higher energy as well as several concurrent pathways of similar energies were revealed

Variable Magnetic Interactions between S = 1/2 Cation Radical Salts of Functionalizable Electron-Rich Dithiolene and Diselenolene Cp₂Mo Complexes

A series of Cp₂Mo­(dithiolene) and Cp₂Mo­(diselenolene) complexes containing N-alkyl-1,3-thiazoline-2-thione-4,5-dithiolate ligand (R-thiazdt, R = Me, Et, CH₂CH₂OH) and N-alkyl-1,3-thiazoline-2-thione-4,5-diselenolate ligand (R-thiazds, R = Me, Et) have been synthesized. These heteroleptic molybdenum complexes have been characterized by electrochemistry, spectroelectrochemistry, and single crystal X-ray diffraction. They act as very good electron donor complexes with a first oxidation potential 200 mV lower than in the prototypical Cp₂Mo­(dmit) complex and exhibit almost planar MoS₂C₂ (or MoSe₂C₂) metallacycles. All five complexes formed charge transfer salts with a weak (TCNQ) and a strong acceptor (TCNQF₄), affording ten different charge-transfer salts, all with 1:1 stoichiometry. Crystal structure determinations show that the S/Se substitution in the metallacycle systematically affords isostructural salts, while the Cp₂Mo­(R-thiazdt) complexes with R equals ethyl or CH₂CH₂OH can adopt different structures, depending on the involvement of the hydroxyl group into intra- or intermolecular hydrogen bonding interactions. Magnetic susceptibility data of the salts are correlated with their structural organization, demonstrating that a face-to-face organization of the Me-thiazdt (or Me-thiazds) ligand favors a strong antiferromagnetic interaction, while the bulkier R = Et or R = CH₂CH₂OH substituents can completely suppress such intermolecular interactions, with the added contribution of hydrogen bonding to the solid state organization