Tetrathiafulvalene

Tetrathiafulvalene (TTF) is an organosulfur compound with the formula H₂C₂S₂C=CS₂C₂H₂. It is the parent of many tetrathiafulvenes. Studies on these heterocyclic compound contributed to the development of molecular electronics, although no practical applications of TTF emerged. TTF is related to the hydrocarbon fulvalene (H₄C₄C=CC₄H₄) by replacement of four CH groups with sulfur atoms. Over 10,000 scientific publications discuss TTF and its derivatives.^[2]

Tetrathiafulvalene

Names
Preferred IUPAC name 2,2′-Bi(1,3-dithiolylidene)
Other names Δ2,2′-Bi-1,3-dithiole
Identifiers
CAS Number	31366-25-3 Y
3D model (JSmol)	Interactive image
Beilstein Reference	1282106
ChEBI	CHEBI:52444 Y
ChemSpider	89848 Y
ECHA InfoCard	100.045.979
EC Number	250-593-7
PubChem CID	99451
UNII	HY1EN16W9T Y
CompTox Dashboard (EPA)	DTXSID6067620
InChI InChI=1S/C6H4S4/c1-2-8-5(7-1)6-9-3-4-10-6/h1-4H Y Key: FHCPAXDKURNIOZ-UHFFFAOYSA-N Y InChI=1/C6H4S4/c1-2-8-5(7-1)6-9-3-4-10-6/h1-4H Key: FHCPAXDKURNIOZ-UHFFFAOYAZ
SMILES S1C=CSC1=C2SC=CS2
Properties
Chemical formula	C6H4S4
Molar mass	204.34 g·mol−1
Appearance	Yellow solid
Melting point	116 to 119 °C (241 to 246 °F; 389 to 392 K)
Boiling point	Decomposes
Solubility in water	Insoluble
Solubility in organic solvents	Soluble[vague]
Structure
Dipole moment	0 D
Hazards[1]
Occupational safety and health (OHS/OSH):
Main hazards	combustible
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H317
Precautionary statements	P261, P280, P302+P352, P333+P313, P363, P501
Related compounds
Related compounds	TCNQ Thiophene Tetracyanoethylene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Preparation

The high level of interest in TTFs spawned many syntheses of TTF and its analogues.^[3][2] Most preparations entail the coupling of cyclic C₃S₂ building blocks such as 1,3-dithiole-2-thion or the related 1,3-dithiole-2-ones. For TTF itself, the synthesis begins with the cyclic trithiocarbonate H₂C₂S₂C=S (1,3-dithiole-2-thione), which is S-methylated and then reduced to give H₂C₂S₂CH(SCH₃) (1,3-dithiole-2-yl methyl thioether), which is treated as follows:^[4]

Protonolysis of a thioether:

H₂C₂S₂CH(SCH₃) + HBF₄ → [H₂C₂S₂CH]⁺BF−4 + CH₃SH

Followed by deprotonation of the dithiolium cation with triethylamine:

2 [H₂C₂S₂CH]⁺BF−4 + 2 N(CH₂CH₃)₃ → H₂C₂S₂C=CS₂C₂H₂ + 2 [NH(CH₂CH₃)₃]⁺BF−4

Redox properties

Bulk TTF itself has unremarkable electrical properties. 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:

• its planarity, which allows π-π stacking of its oxidized derivatives,
• its high symmetry, which promotes charge delocalization, thereby minimizing coulombic repulsions, and
• 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 CH₃CN solution)

Each dithiolylidene ring in TTF has 7π electrons: 2 for each sulfur atom, 1 for each sp² carbon atom. Thus, oxidation converts each ring to an aromatic 6π-electron configuration, consequently leaving the central double bond essentially a single bond, as all π-electrons occupy ring orbitals.

History

Edge-on view of portion of crystal structure of hexamethyleneTTF/TCNQ charge transfer salt, highlighting the segregated stacking.^[5]

The salt [TTF⁺
]Cl⁻
was reported to be a semiconductor in 1972.^[6] Subsequently, the charge-transfer salt [TTF]TCNQ was shown to be a narrow band gap semiconductor.^[7] 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 (TMTSFs), and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, CAS [66946-48-3]).^[8] Several tetramethyltetrathiafulvalene salts (called Fabre salts) are of some relevance as organic superconductors.

See also

• Bechgaard salt