Tetrathiafulvalene

Tetrathiafulvalene is a organosulfur compound with the formula (H₂C₂S₂C)₂. Studies on this heterocyclic compound contributed to the development of molecular electronics. TTF is related to the hydrocarbon fulvalene, (C₅H₄)₂, by replacement of four CH groups with sulfur atoms. Over 10,000 scientific publications discuss TTF and its derivatives.^[1]

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Preparation

The high level of interest in TTF’s has spawned the development of many syntheses of TTF and its analogues.^[1] Most preparations entail the coupling of cyclic C₃S₂ building blocks such as 1,3-dithiole-2-thiones or the related 1,3-dithiole-2-ones. For TTF itself, the synthesis begins with the trithiocarbonate H₂C₂S₂CS, which is S-methylated and then reduced to give H₂C₂S₂CH(SCH₃), which is treated as follows:^[2]

H₂C₂S₂CH(SCH₃) + HBF₄ → [H₂C₂S₂CH⁺]BF₄^- + HSCH₃

2 [H₂C₂S₂CH⁺]BF₄^- + 2 Et₃N → (H₂C₂S₂C)₂ + 2 Et₃NHBF₄

Redox properties

Bulk TTF itself has unremarkable electrical properties (as do most organic compounds). Distinctive properties are, however, associated with salts of its oxidized derivatives, such as salts derived from TTF⁺.

The high electrical conductivity of TTF salts can be attributed to the following features of TTF: (i) its planarity, which allows π-π stacking of its oxidized derivatives, (ii) its high symmetry, which promotes charge delocalization, thereby minimizing coulombic repulsions, and (iii) its ability to undergo oxidation at mild potentials to give a stable radical cation. Electrochemical measurements show that TTF can be oxidized twice reversibly:

TTF → TTF⁺ + e^- E = 0.34 V

TTF⁺ → TTF²⁺ + e^- E = 0.78 V (vs. Ag/AgCl in MeCN solution).

Each dithiolylidene ring in TTF has 7π electrons: 2e for each sulfur atom, 1e for each sp² carbon atom. Thus, oxidation converts each ring to an aromatic 6π-electron configuration.

History

Wudl et al. first demonstrated that the salt [TTF⁺]Cl^- was a semiconductor.^[3] Subsequently, Ferraris et al. showed that the charge-transfer salt [TTF]TCNQ is a narrow band gap semi-conductor.^[4] X-ray diffraction studies of [TTF][TCNQ] revealed stacks of partially oxidized TTF molecules adjacent to anionic stacks of TCNQ molecules. This “segregated stack” motif was unexpected and is responsible for the distinctive electrical properties, i.e. high and anisotropic electrical conductivity. Since these early discoveries, numerous analogues of TTF have been prepared. Well studied analogues include tetramethyltetrathiafulvalene (Me₄TTF), tetramethylselenafulvalenes (TMTSF’s), and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, CAS [66946-48-3]).^[5]

See also

Bechgaard salt

References

1. ^ ^{a b} Bendikov, M; Wudl, F; Perepichka, D. F. “Tetrathiafulvalenes, Oligoacenenes, and Their Buckminsterfullerene Derivatives: The Brick and Mortar of Organic Electronics” Chemical Reviews 2004, volume 104, 4891-4945.
2. ^ Wudl, F.; Kaplan, M. L. "2,2'Bi-1,3-dithiolylidene (Tetrathiafulvalene, TTF) and Its Radical Cation Derivatives" Inorganic Syntheses, 1979, volume XIX, pp. 27-30. ISBN 0-471-04542-X.
3. ^ Wudl, F.; Wobschall, D.; Hufnagel, E. J. “Electrical Conductivity by the Bis(1,3-dithiole)-bis(1,3-dithiolium) System” Journal of the American Chemical Society 1972, volume 94, pp 670-672; DOI: 10.1021/ja00757a079
4. ^ Ferraris, J.; Cowan, D. O.; Walatka, V. V., Jr.; Perlstein, J. H. “Electron transfer in a new highly conducting donor-acceptor complex” Journal of the American Chemical Society 1973, volume 95, 948.DOI: 10.1021/ja00784a066
5. ^ Larsen, J.; Lenoir, C. “2,2'-Bi-5,6-Dihydro-1,3-Dithiolo[4,5-b][1,4]dithiinylidene (BEDT-TTF)” Organic Syntheses, Collected Volume 9, p.72 (1998).

Further reading

• Rovira, C “Bis(ethylenethio)tetrathiafulvalene (BET-TTF) and Related Dissymmetrical Electron Donors: From the Molecule to Functional Molecular Materials and Devices (OFETs)” Chemical Reviews 2004, volume 104, 5289-5317.
• Iyoda, M; Hasegawa, M; Miyake, Y “Bi-TTF, Bis-TTF, and Related TTF Oligomers” Chemical Reviews 2004, volume 104, 5085-5113.
• Frere, P.; Skabara, P. J. “Salts of Extended Tetrathiafulvalene analogues: relationships Between Molecular Structure, Electrochemical Properties and Solid State Organization” Chemical Society Reviews 2005, volume 34, 69-98.
• Gorgues, Alain; Hudhomme, Pietrick; Salle, Marc. Highly Functionalized Tetrathiafulvalenes: Riding along the Synthetic Trail from Electrophilic Alkynes. Chemical Reviews 2004, volume 104, 5151-5184.