This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7025--7027 7025

Cite this: Chem. Commun.,2013,49, 7025

Tetrathiafulvalene: the advent of organic metals

Nazario Martın*ab

Tetrathiafulvalene (TTF) is among the most versatile and well-known molecules which exhibits outstanding

redox properties and a remarkable electron donor character. Its first synthesis was published in a short

communication by Wudl in 1970. In this viewpoint, its synthesis and characterization are discussed and,

most importantly, the significance of TTF in the development of electrically conducting materials (organic or

synthetic metals) and its further application in molecular electronics are highlighted.

Although pristine tetrathiafulvalene (TTF) was firstly reportedby Wudl in 19701 and, simultaneously and independently, byHunig et al.2 and Coffen et al.3 in 1971, the history of TTFderivatives is considerably older. Actually, the dibenzo-fusedTTF4 was synthesized in the 1920s and dimethyl-TTF anddiphenyl-TTF derivatives were reported in the 1960s.5 However,great interest in these sulfur-containing molecules started inthe early 1970s, following the seminal paper of Wudl, whenunsubstituted TTF arose as an electron donor with outstandingredox properties.

Since then, TTF and its derivatives have probably become themost famous electron donor molecules with over ten thousandpapers published in the scientific literature on the synthesis,properties and applications of these singular molecules.6

Tetrathiafulvalene is a non-aromatic 14-p-electron system inwhich oxidation to the radical cation and dication states occurssequentially and reversibly at relatively low oxidation potentialvalues (E1/2

1 = 0.37 V and E1/22 = 0.67 V in dichloromethane vs.

the saturated calomel electrode) (Scheme 1). In contrast to theneutral TTF molecule, both the radical cation and dicationspecies are aromatic in the Huckel sense due to the 6p-electronheteroaromaticity of the 1,3-dithiolium cation and, therefore,

while TTF+� exhibits a planar D2h symmetry, TTF2+ is not planarand has a D2 symmetry, whereas neutral TTF shows a slightlyboatlike structure with C2v symmetry.7

The venture of TTF as a versatile strong electron donormolecule of interest for the preparation of electrically conduct-ing materials has its origin in the 1.5 pages pioneering ChemicalCommunications paper entitled: ‘‘Bis-1,3-dithiolium Chloride: anUnusually Stable Organic Radical Cation’’ by Wudl et al.1 In thispaper, pristine TTF was synthesized for the first time in a singlestep by deprotonation of 1,3-dithiolium hydrogen sulphate toafford TTF in ca. 50% yield. Although compounds containing theTTF core had been prepared before, the properties of the parentcompound TTF were not described. TTF was firstly described asa yellow solid; m.p. 118.5–1191, subl. 1001/0.3 mm; UV CH2C12,lmax (e) 290 (4 � 104), 310 (4 � 104), and 365 (sh); i.r. (CH2Cl2,mm) 7–95 (w), 12.55 (m), 12.8 (m), 13.6 (w), 15.5 (sh), and 15.9 (s);NMR (CDCl3, d rel. to SiMe4) 5.68 (s);8 mass spectrum (70 eV),m/z 204 (100%, parent peak), strong peaks at m/z 159, 146, 102(100%, dication of parent), 88, and 76 (CS2). Furthermore, themost significant property of TTF is also stated in this manu-script. It was reported that TTF is readily photo-oxidized in thepresence of air to a violet, water-soluble substance presumed tobe a radical cation. The bisdithiole (TTF) reacts with electrondeficient olefins such as tetracyanoethylene, dichlorodicyano-benzoquinone, etc. in analogy to other electron-rich olefins(Scheme 2).9

Most importantly, in this communication the radical cation(TTF+�Cl�) was prepared efficiently by the action of chlorine gason a carbon tetrachloride solution of TTF. It was also pointedout that a stoichiometric amount of chlorine must be employedScheme 1 TTF and its radical cation and dication species and their geometries

calculated at the B3P86/6-31G** level.

Scheme 2 Synthesis of TTF and its oxidation to the radical cation.

a Departamento de Quımica Organica, Facultad de Ciencias Quımicas, Universidad

Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain.

E-mail: nazmar@quim.ucm.es; Fax: +34 91-394-4103; Tel: +34 91-394-4227b Instituto Madrileno de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia),

Cantoblanco, 28049 Madrid, Spain

Received 10th January 2013,Accepted 12th June 2013

DOI: 10.1039/c3cc00240c

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7026 Chem. Commun., 2013, 49, 7025--7027 This journal is c The Royal Society of Chemistry 2013

to avoid further oxidation to the yellow dication (TTF2+). Inter-estingly, the ESR spectrum of the radical cation (TTF+�Cl�)(H2O–MeCN, 25 1C or EtOH, �50 1C) is a quintet, indicatingfour equivalent protons; aH = 1.26 � 0.02, g = 2.00838.

Finally, in this work the redox reversibility of the radicalcation species (TTF+�) was also discussed provided that chemicalreduction with sodium hydrogen bisulphite allowed recoveringthe neutral species (TTF).

Soon afterwards, the discovery of the first organic conduc-tors based on tetrathiafulvalene (TTF+�Cl�) in 1972 (ref. 10) andmetal TTF–TCNQ in 1973 (ref. 11) made tetrathiafulvalene oneof the most studied and well-known heterocyclic systems, andone of the most important molecular building blocks for thedevelopment of the so-called electrically conducting materials(also known as organic metals or synthetic metals) as well as thefurther and challenging development of molecular electronics.

TTF stacks in the solid state when combined with p-electronacceptors to form charge transfer complexes (CTCs). Many ofthe so formed CTCs possess interesting conducting and mag-netic properties. The most famous and relevant CTC, being thefirst organic metal, was formed by TTF and the strong electronacceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) with a1 : 1 stoichiometry (Fig. 1). The electrical conductivity deter-mined for this CTC showed typical metallic behaviour atT > 54 K. Interestingly, TTF and TCNQ molecules form segre-gated molecular stacks in the complex, resulting in stronglyanisotropic conducting properties, with a Peierls transition(from metal to insulator) at lower temperature (T o 54 K).

Just a few years ago a most intriguing finding was reported:when placed next to each other, the crystals of TTF and TCNQstick together and, surprisingly, form a conducting strip about2 nm across at their interface. The current flows between thesurfaces and is confined to the two layers of molecules incontact, which requires high quality surfaces (Fig. 2).12

The excitement aroused in the scientific community by therealization that organic compounds were able to conductelectricity, similarly to metals, led to a huge synthetic effortfor the preparation of new TTF derivatives, namely by modifyingthe substitution pattern at the periphery of the molecule or byreplacing the S atom by a different calcogen (Se, Te).13

A breakthrough in the field of electrically conducting mate-rials was the use of the tetraselenafulvalene (TSF) derivativetetramethyl-TSF (TMTSF) which led to the first organic

superconductors (Tc = 1–2 K) of formula (TMTSF)2X (X = PF6�,

AsF6�, TaF6

�, NbF6�, ReO4

� and ClO4�) firstly reported by

Bechgaard in 1980 (ref. 14) and, since then, known asBechgaard’s salts.

Interestingly, only the perchlorate salt resulted to be asuperconductor at ambient pressure, whereas the othersrequired the application of high pressure (5–12 kbar) to thecrystals to get superconducting properties. X-Ray structuredetermination showed that the ClO4

� anions are located withinchannels in the stacked structure with short and stabilizingO� � �H contacts.

Since then, and during the last three decades, a great varietyof new tetrachalcogenofulvalene-based superconductors havebeen synthesised, thus improving the field of electrically con-ducting materials. Among them, the most relevant have beenthe salts of bis-ethylenedithio-tetrathiafulvalene (BEDT-TTF) offormula (BEDT-TTF)2X which showed superconducting behav-ior at Tc = 11.6 K in k-(BEDTTTF)2[Cu(CN)2]Br (ref. 15) andTc = 12.8 K in k-(BEDT-TTF)2[Cu(CN)2]Cl under an appliedpressure of 0.3 kbar.16

The chemical structures of these salts are organized in akappa-packing motif of orthogonal dimers of BEDT-TTF molec-ules and a chain network of the respective anions. Therefore,they are structurally different from the planar stacks of TTF orTMTSF units found in the aforementioned TTF–TCNQ complexor (TMTSF)2X salts. So far, all attempts carried out to improvethe critical temperature Tc by modifying the BEDT-TTF molec-ule, either by using O or Se atoms or by changing the substitu-tion pattern at the periphery, have been unsuccessful. Theincrease of Tc for the phase transition into the superconductingstate in organic materials has only been achieved more recentlyin doped cubic and hexagonal C60 fullerene crystals (namelywith alkaline and alkaline-earth metals) such as RbTl2C60

which exhibits one of the highest values of the Tc of 48 Kreported so far.17

Despite the above important scientific findings, whichwould be more than enough for a molecule to be consideredat the top in chemistry, TTF and its derivatives offer new and insome cases little-exploited possibilities at the molecular,macromolecular and supramolecular levels. Actually, the inter-est in the TTF molecule goes beyond the field of materialschemistry and, in this regard, in addition to the covalentFig. 1 Segregated stacks of TTF and TCNQ (from CCDC).

Fig. 2 Crystals of TTF (yellow) and TCNQ (red) (from ref. 12).

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7025--7027 7027

chemistry, TTF is an important building block in supramolec-ular chemistry, crystal engineering, and in systems able tooperate as machines.6 Furthermore, TTF has been used as acatalyst for radical-polar crossover reactions, thus mimickingthe samarium iodide chemistry.18

Actually, in recent years, a variety of new concepts have beenapplied to TTF and its derivatives. Thus, chemical sensors andredox-switchable ligands have been prepared from TTF whilemolecular shuttles, molecular switches and logic gates havebeen prepared from TTF-containing rotaxanes and catenanes.19

A large synthetic effort has been devoted to the preparation ofthe so-called organic ferromagnets, many of which are derivedfrom TTF.6

TTF has also played a prominent role in molecular electronics.Since 1974, when Aviram and Ratner proposed in a theoreticalpaper that rectification of electrical current through a singleDonor–spacer–Acceptor (D–s–A) molecule could be possible(Fig. 3),20 a great synthetic effort has been undertaken to preparea variety of molecules in which a strong donor moiety iscovalently attached to a strong acceptor moiety through acovalent, saturated bridge. Eventually, this concept became areality when new D–s–A molecules allowed the preparation of thefirst confirmed unimolecular rectifier.21

It has also been confirmed that TTF can display efficientnonlinear optic (NLO) responses in the second and thirdharmonic generation as well as a good thermal stability. Thesefindings can be combined with the redox ability of TTF as anexternal stimulus to provide a promising strategy for themolecular engineering of switchable NLO materials.22

The design and synthesis of organic molecules with twophoto and redox chromophores, namely an electroactive donorand an acceptor fragment connected through a spacer, thatdisplay photoinduced charge separation is an important topicin chemistry. These molecules can be used as artificial photo-synthetic systems to transform sunlight into chemical energy.In this regard, fullerenes have been shown to have uniquephotophysical properties for the preparation of photovoltaicdevices.23 The first fullerene endowed with TTF was reported in1996 showing remarkable photophysical properties and leadingto photoinduced charge-separated (CS) states with remarkablelifetimes. Since then, a wide variety of C60-TTF have beenprepared. In contrast to other known electron donor molecules,TTF and its derivatives are non-aromatic molecules which uponoxidation form the 1,3-dithiolium cation which possessesaromatic character (Scheme 1). This gain of aromaticity informing the radical cation and dication species of TTF in theoxidation process is an important improvement for increasingthe stabilization of the charge-separated state in the search forefficient photovoltaic systems.24

In summary, this pioneering work by Wudl with 248 cita-tions (Scifinder) represents one of the important and outstand-ing landmarks in chemistry. As a result, TTF and its derivativeshave gained a prominent place in science in their own right.

Notes and references1 F. Wudl, G. M. Smith and E. J. Hufnagel, J. Chem. Soc., Chem.

Commun., 1970, 1453–1454.2 S. Hunig, G. Kiesslich, D. Sceutzow, R. Zhrandik and P. Carsky, Int. J.

Sulfur Chem., Part C, 1971, 109–122.3 D. L. Coffen, J. Q. Chambers, D. R. Williams, P. E. Garrett and

N. D. Canfield, J. Am. Chem. Soc., 1971, 93, 2258–2269.4 W. R. H. Hurtley and S. Smiles, J. Chem. Soc., 1926, 2263–2270.5 H. Prinzbach, H. Berger and A. Luttringhaus, Angew. Chem., Int. Ed.

Engl., 1965, 4, 435–439.6 Many review papers have been devoted to TTF and its derivatives.

Just to name a few published in the last few years, see:(a) M. R. Bryce, J. Mater. Chem., 2000, 10, 589–598; (b) J. L. Seguraand N. Martın, Angew. Chem., Int. Ed., 2001, 40, 1372–1409;(c) M. Bendikov, F. Wudl and D. F. Perepichka, Chem. Rev., 2004,104, 4891–4945. See also: (d) P. Batail (Ed.), Special issue onMolecular Conductors, Chem. Rev., 2004, 104, 4887–5781.

7 (a) C. Katan, J. Phys. Chem. A, 1999, 103, 1407–1413; (b) I. Hargittai,J. Brunvoll, M. Kolonits and V. Khodorkovsky, J. Mol. Struct., 1994,317, 273–277.

8 The proton resonance of TTF2+ is downfield-shifted to 9.51 ppm;P. R. Ashton, V. Balzani, J. Becher, A. Credi, M. C. T. Fyfe,G. Mattersteig, S. Menzer, M. B. Nielsen, F. M. Raymo, J. F. Stoddart,M. Venturi and D. J. Williams, J. Am. Chem. Soc., 1999, 121, 3951–3957.

9 (a) N. Wiberg, Angew. Chem., Int. Ed. Engl., 1968, 7, 766;(b) R. W. Hoffmann, Angew. Chem., Int. Ed. Engl., 1968, 7, 754–765.

10 F. Wudl, D. Wobschall and E. J. Hufnagel, J. Am. Chem. Soc., 1972,94, 670–672.

11 J. Ferraris, D. O. Cowan, V. V. Walatka Jr. and J. H. Perlstein, J. Am.Chem. Soc., 1973, 95, 948–949.

12 H. Alves, A. S. Molinari, H. Xie and A. F. Morpurgo, Nature Mater.,2008, 7, 574–580.

13 (a) M. R. Bryce, J. Mater. Chem., 1995, 5, 1481–1496; (b) G. Schukatand E. Fanghanel, Sulfur Rep., 1996, 18, 1–12; (c) J. Garın, Adv.Heterocycl. Chem., 1995, 62, 249–304.

14 K. Bechgaard, C. S. Jacobsen, K. Mortensen, H. J. Pederson andN. Thorup, Solid State Commun., 1980, 33, 1119–1125.

15 A. M. Kini, U. Geiser, H. Wang, K. D. Carlson, J. M. Williams,W. K. Kwok, K. G. Vandervoot, J. E. Thompson, D. L. Stupka, D. Jungand M.-H. Whangbo, Inorg. Chem., 1990, 29, 2555–2557.

16 J. M. Williams, A. M. Kini, H. Wang, K. D. Carlson, U. Geiser,L. K. Montgomery, G. J. Pyrka, D. M. Watkins, J. M. Kommers,S. J. Oryschuk, A. V. Striebycrouch, W. K. Kwok, J. E. Schirber,D. L. Overmeyer, D. Jung and M.-H. Whangbo, Inorg. Chem., 1990,29, 3262–3265.

17 V. M. Loktev, E. A. Pashitskiı, R. Shekhter and M. Jonson, Low Temp.Phys., 2002, 28, 821–829.

18 (a) J. A. Murphy and S. J. Roome, J. Chem. Soc. Perkin Trans. 1, 1995,1349–1353; (b) L. F. Tietze and U. Beifuss, Angew. Chem., Int. Ed.Engl., 1993, 32, 131–163.

19 C. P. Collier, E. W. Wong, M. Belohradsky, F. M. Raymo, J. F. Stoddart,P. J. Kuekes, R. S. Williams and J. R. Heath, Science, 1999, 285, 391–394.

20 A. Aviram and M. Ratner, Chem. Phys. Lett., 1974, 9, 2271–2275.21 R. M. Metzger, B. Chen, U. Hopfner, M. V. Lakshmikantham,

D. Villaume, T. Kawai, X. Wu, H. Tachibana, T. V. Hughes,H. Sakurai, J. W. Baldwin, C. Hosch, M. P. Cava, L. Brehmer andG. J. Ashwell, J. Am. Chem. Soc., 1997, 119, 10455–10466.

22 (a) A. I. de Lucas, N. Martın, L. Sanchez, C. Seoane, R. Andreu,J. Garın, J. Orduna, R. Alcala and B. Villacampa, Tetrahedron, 1998,54, 4655; (b) M. Gonzalez, J. L. Segura, C. Seoane, N. Martın, J. Garın,J. Orduna, B. Villacampa, R. Alcala, V. Hernandez and J. T. Lopez-Navarrete, J. Org. Chem., 2001, 66, 8872–8882.

23 J. L. Delgado, P. A. Bouit, S. Filippone, M. A. Herranz and N. Martin,Chem. Commun., 2010, 46, 4853–4865.

24 (a) N. Martın, L. Sanchez, M. A. Herranz, B. Illescas and D. M. Guldi,Acc. Chem. Res., 2007, 40, 1015–1024; (b) F. G. Brunetti, J. L. Lopez,C. Atienza and N. Martın, J. Mater. Chem., 2012, 22, 4188–4205.

Fig. 3 Molecular rectifier proposed by Aviram and Ratner.

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