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.

Preparation
The high level of interest in TTF’s has spawned the development of many syntheses of TTF and its analogues. 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:
 * 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 electrical conductivity of TTF salts can be attributed to the following features of TTF: (i) its planarity, which allows &pi;-&pi; 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 &rarr; TTF+ + e-    E = 0.34 V
 * TTF+ &rarr; TTF2+ + e-   E = 0.78 V (vs. Ag/AgCl in MeCN solution).

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

History
Wudl et al. first demonstrated that the salt [TTF+]Cl- was a semiconductor. Subsequently, Ferraris et al. showed that the charge-transfer salt [TTF]TCNQ is a narrow band gap semi-conductor. 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]).