Effects of structural factors on the pi-dimerization and/or disproportionation of the cation radical of extended TTF containing thiophene-based pi-conjugated spacers

Delocalization tunable by ligand substitution in [L2Al]n− complexes highlights a mechanism for strong electronic coupling

Organo-aluminum mixed-valent complexes combine properties of both organic and transition element mixed-valent compounds. This supports delocalized electronic structures that are structurally and electronically tunable.

Non-covalent bridging of bithiophenes through chalcogen bonding grips

In this work, chalcogen functionalized dithiophenes, equipped on both extremities with chalcogen-bonding recognition heterocycles, have been prepared following two synthetic pathways.

Understanding Color Tuning and Reversible Oxidation of Conjugated Azomethines

With the aim of achieving reversible oxidation and color tuning, the effect of the central aromatic on the spectroscopic, electrochemical, and spectrochemical properties of a series of electrochromic azomethine triads was investigated. The absorption of the alkylated thiophene derivatives was blue-shifted relative to their unalkylated counterparts when the central aromatic was either a bi- or terthiophene. It was further found that the alkylated thiophene derivatives had larger Stokes shifts than their unsubstituted counterparts. Theoretical calculations demonstrated that the torsion angles of these alkylated cores with respect to the flanking azomethines were responsible for the spectroscopic effects. While the electrochemical oxidation potential of the triads varied by only 100 mV, the reversibility of their anodic process was contingent on the central aromatic. The absorption of the electrochemically produced state red-shifted between 165 and 280 nm from its corresponding neutral state, leading to perceived color changes between orange and blue. Reversible color changes were chemically mediated with ferric chloride/hydrazine. The absorption of the chemically oxidized state shifted between 155 and 220 nm from the corresponding neutral state, contingent on the central aromatic. The palette of perceived colors that was possible with oxidation included orange, yellow, blue, and gray.

Diindenothienoacene–tetrathiafulvalene redox systems

The redox properties and cation associations of extended tetrathiafulvalenes with diindenothienoacene cores are strongly dependent on the number and regioisomeric structural orientation of the central thiophene units.

Porphyrins fused with strongly electron-donating 1,3-dithiol-2-ylidene moieties: redox control by metal cation complexation and anion binding

A new class of redox-active free base and metalloporphyrins fused with the 1,3-dithiol-2-ylidene subunits present in tetrathiafulvalene, termed MTTFP (M = H2, Cu, Ni, Zn), have been prepared and characterized. The strong electron-donating properties of MTTFP were probed by electrochemical measurement and demonstrated that oxidation potentials can be tuned by metalation of the free base form, H2TTFP. X-ray crystal structures of H2TTFP, ZnTTFP, and CuTTFP revealed that a severe saddle-shape distortion was observed with the dithiole rings bent out of the plane toward one another in the neutral form. In contrast, the structure of the two-electron oxidized species (CuTFFP(2+)) is planar, corresponding to a change from a nonaromatic to aromatic structure upon oxidation. A relatively large two-photon absorption (TPA) cross-section value of H2TTFP(2+) (1200 GM) was obtained for the free base compound, a value that is much higher than those typically seen for porphyrins (<100 GM). Augmented TPA values for the metal complexes were also seen. The strong electron-donating ability of ZnTTFP was further enhanced by binding of Cl(-) and Br(-) as revealed by thermal electron-transfer between ZnTTFP and Li(+)-encapsulated C60 (Li(+)@C60) in benzonitrile, which was "switched on" by the addition of either Cl(-) or Br(-) (as the tetrabutylammonium salts). The X-ray crystal structure of Cl(-)-bound ZnTTFP was determined and provided support for the strong binding between the Cl(-) anion and the Zn(2+) cation present in ZnTTFP.

Synthesis and radical coupling of pyridine-bridged π-extended tetrathiafulvalene (TTF)-type donors and push–pull analogues

A new family of pi-extended TTF analogues (3a-c) and D-pi-A chromophores (5a-c), in which the electroactive units (1,3-dithiole rings and 2,2-dicyanovinyl groups) are connected through a pyridine bridge with a meta substitution pattern, is reported. The redox behavior of these compounds has been investigated by cyclic voltammetry and theoretical calculations performed at the B3P86/6-31G** level. Unlike many pi-extended TTF derivatives, the 1,3-dithiole rings in compounds 3a-c do not behave independently and two oxidation processes are observed with an anodic separation ranging from 50 to 150 mV. Calculations show that electrons are equally extracted from both dithiole rings. A biradical structure is predicted for the dication state due to the near-degeneracy of the HOMO and HOMO - 1 orbitals. The presence of both donor (D) and acceptor (A) fragments in conjugates results in irreversible oxidation and reduction processes associated with the 1,3-dithiole ring and with the 2,2-dicyanovinyl moiety, respectively. An electrochemical-chemical-electrochemical (ECE) process takes place for all the compounds reported. The chemical process implies the dimerization of the radical cation for compounds 5 and the oligomerization of the biradical dication for compounds 3. The ECE process therefore generates new neutral dimeric (5) or oligomeric (3) species that incorporate the TTF vinylogue core.

Persistent ? radical cations: self-association and its steric control in the condensed phase

Pi Radical cations, which are highly reactive in general, can be made persistently stable by appropriate structural modification with heteroatoms, pi-conjugated systems, and alkyl substituents. Many of these pi radical cations undergo self-association in the condensed phase. The steric control of such self-association of stabilized pi radical cations is the subject of the present article. Such an association can result in the formation of pi- and/or sigma-dimers. The pi-dimerization in particular is now considered as an important intermolecular interaction for model studies of a charge-transport phenomenon in positively doped conducting polymers. On the other hand, the intermolecular interactions can be suppressed when the pi-system is modified with sterically demanding structural units, for example, by annelation with bicycloalkene frameworks. This structural modification not only brings about unusual stabilization of the radical cations but provides valuable information on the electronic structure/properties of the positively charged pi-systems in a segregated state.

Salts of extended tetrathiafulvalene analogues: relationships between molecular structure, electrochemical properties and solid state organisation

By considering the structures of many salts derived from extended TTF analogues, relationships between the molecular architecture of the donors with their electrochemical properties and their stacking mode in the salts are presented in this critical review. Three categories of donors corresponding to their extension modes have been considered. Firstly, for linearly extended TTFs the crucial role of the spacer in modifying the electrochemical properties and the packing mode in the salts is presented. Secondly, bidimentional extension of the donors obtained by linking several dithiafulvenyl units on a TTF core led to materials with increased dimensionality. Finally, the last class corresponds to the fusion, directly or across a benzene ring, of TTF frameworks. The former are the base of many salts with metallic behaviour. (148 references.).