Core carbo-mer of an Extended Tetrathiafulvalene: Redox-Controlled Reversible Conversion to a carbo-Benzenic Dication.

carbo-Benzene is an aromatic molecule devised by inserting C2 units within each C-C bond of the benzene molecule. By integrating the corresponding carbo-quinoid core as bridging unit in a π-extended tetrathiafulvalene (exTTF), it is shown that a carbo-benzene ring can be reversibly formed by electrochemical reduction or oxidation. The so-called carbo-exTTF molecule was thus experimentally prepared and studied by UV-visible absorption spectroscopy and cyclic voltammetry, as well as by X-ray crystallography and by scanning tunneling microscopy (STM) on a surface of highly oriented pyrolytic graphite (HOPG). The molecule and its oxidized and reduced forms were subjected to a computational study at the density functional theory (DFT) level, supporting carbo-aromaticity as a driving force for the formation of the dication, radical cation, and radical anion. By allowing co-planarity of the dithiolylidene rings and carbo-quinoidal core, carbo-exTTFs present a promising new class of redox-active systems.

Redox-Modulations of Photophysical and Single-molecule Magnet Properties in Ytterbium Complexes Involving Extended-TTF Triads.

The reaction between the 2,2'-benzene-1,4-diylbis(6-hydroxy-4,7-di-tert-butyl-1,3-benzodithiol-2-ylium-5-olate triad (H2SQ) and the metallo-precursor [Yb(hfac)3]2H2O led to the formation of a dinuclear coordination complex of formula [Yb2(hfac)6(H2SQ)]0.5CH2Cl2 (H2SQ-Yb). After chemical oxidation of H2SQ in 2,2'-cyclohexa-2,5-diene-1,4-diylidenebis(4,7-di-tert-butyl-1,3-benzodithiole-5,6-dione (Q), the latter triad reacted with the [Yb(hfac)3]2H2O precursor to give the dinuclear complex of formula [Yb2(hfac)6(Q)] (Q-Yb). Both dinuclear compounds have been characterized by X-ray diffraction, DFT optimized structure and electronic absorption spectra. They behaved as field-induced Single-Molecule Magnets (SMMs) nevertheless the chemical oxidation of the semiquinone to quinone moieties accelerated by a factor of five the relaxation time of the magnetization of Q-Yb compared to the one for H2SQ-Yb. The H2SQ triad efficiently sensitized the YbIII luminescence while the chemical oxidation of H2SQ into Q induced strong modification of the absorption properties and thus a quenching of the YbIII luminescence for Q-Yb. In other words, both magnetic modulation and luminescence quenching are reached by the oxidation of the protonated semiquinone into quinone.

Protonated paramagnetic redox forms of di-o-quinone bridged with p-phenylene-extended TTF: A EPR spectroscopy study.

The chemical oxidation and reduction processes of deprotonated, direduced o-quinone-exTTF-o-quinone in protic solvents were studied by EPR spectroscopy. The formation of relatively stable paramagnetic protonated redox forms of the parent triad was very surprising. The character of spin-density distribution in the semiquinone-quinone and semiquinone-catechol redox forms indicates that the p-phenylene-extended tetrathiafulvalene connector provides a quite effective electronic communication channel between dioxolene coordination sites. It was found that the deprotonated, direduced o-quinone-exTTF-o-quinone is capable to reduction of the metal copper in solution. The radical anion species formed in this reaction exists in solution as a solvent-separated ion pair with a copper cation. A character of spin-density distribution in a radical anion species leads to the conclusion that the ligand corresponds to type III of the Robin-Day classification.

The Advent of Electrically Conducting Double-Helical Metal-Organic Frameworks Featuring Butterfly-Shaped Electron-Rich π-Extended Tetrathiafulvalene Ligands.

To diversify metal-organic framework (MOF) structures beyond traditional Euclidean geometries and to create new charge-delocalization pathways beneficial for electrical conductivity, we constructed a novel double-helical MOF (dhMOF) by introducing a new butterfly-shaped electron-rich π-extended tetrathiafulvalene ligand equipped with four benzoate groups (ExTTFTB). The face-to-face oriented convex ExTTFTB ligands connected by Zn2(COO)4 paddlewheel nodes formed ovoid cavities suitable for guest encapsulation, while π-π-interaction between the ExTTFTB ligands of neighboring strands helped create new charge-delocalization pathways in iodine-mediated partially oxidized dhMOF. Iodine vapor diffusion led to oxidation of half of the ExTTFTB ligands in each double-helical strand to ExTTFTB•+ radical cations, which putatively formed intermolecular ExTTFTB/ExTTFTB•+ π-donor/acceptor charge-transfer chains with the neutral ExTTFTB ligands of an adjacent strand, creating supramolecular wire-like charge-delocalization pathways along the helix seams. In consequence, the electrical conductivity of dhMOF surged from 10-8 S/m up to 10-4 S/m range after iodine treatment. Thus, the introduction of the electron-rich ExTTFTB ligand with a distinctly convex π-surface not only afforded a novel double-helical MOF architecture featuring ovoid cavities and unique charge-delocalization pathways but also, more importantly, delivered a new tool and design strategy for future development of electrically conducting stimuli-responsive MOFs.