Multiple-bridged bis-tetrathiafulvalenes: New synthetic protocols and spectroelectrochemical investigations

Article abstract of DOI:10.1021/ja000537c

Synthetic strategies for preparing dimeric tetrathiafulvalenes (TTFs) linked by either one, two, or four bridges have been developed. In particular, we report efficient few-step protocols for the preparation of face-to-face overlapped quadruple-bridged bi

Full text of DOI:10.1021/ja000537c

9486
J. Am. Chem. Soc. 2000, 122, 9486-9494
Multiple-Bridged Bis-Tetrathiafulvalenes: New Synthetic Protocols
and Spectroelectrochemical Investigations
Holger Spanggaard,† Jesper Prehn,† Mogens Brøndsted Nielsen,*,‡ Eric Levillain,*,§
Magali Allain,§ and Jan Becher*,†
Contribution from the Department of Chemistry, UniVersity of Southern Denmark (Odense UniVersity),
CampusVej 55, DK-5230 Odense M, Denmark, Laboratorium fu¨r Organische Chemie, Eidgeno¨ssische
Technische Hochschule (ETH), UniVersita¨tsstrasse 16, CH-8092 Zu¨rich, Switzerland, and Inge´nierie
Mole´culaire et Mate´riaux Organiques, CNRS UMR 6501, UniVersite´ d’Angers, 2 Bd LaVoisier,
F-49045 Angers, France
ReceiVed February 14, 2000
Abstract: Synthetic strategies for preparing dimeric tetrathiafulvalenes (TTFs) linked by either one, two, or
four bridges have been developed. In particular, we report efficient few-step protocols for the preparation of
face-to-face overlapped quadruple-bridged bis-TTFs. The ready interconversion of cis and trans TTFs in the
presence of catalytic amounts of acid was implemented in one synthetic protocol as a way to control the
isomeric outcome. The compounds were characterized by NMR spectroscopy, mass spectrometry, and elemental
analysis. Moreover, the X-ray crystal structure of the macrocycle 4b is presented and compared to semiempirical
(PM3) geometry optimizations. Cyclic voltammetry and spectroelectrochemistry were used to describe the
interactions established between two TTF units upon oxidation, that is, their ability to form mixed-valence
complexes and π-dimers either intra- or intermolecularly. The length, flexibility, and number of bridging units
in a bis-TTF, as well as the specific TTF positions being connected, determine the extent of these interactions.
Thus, rigid linkers enhance the formation of intermolecular mixed-valence complexes. For 4b, the absorption
spectrum of this mixed-valence state of TTF in solution has been recorded for the first time. Finally, preliminary
complexation experiments with different electron-deficient molecules are described.
•+
Introduction
[TTF2
]
KMV
)
(1)
(2)
[TTF][TTF•+]
Tetrathiafulvalene (TTF) is a reversible and stable two-
electron donor which has been intensively studied for more than
two decades, mainly with the aim of developing low-temperature
organic superconductors.1 The appearance of metallic conduc-
tivity in cation radical salts of TTF requires intrastack mixed-
valence (MV) interactions, whereas an integer valence generally
results in insulating materials.1,2 In solution, one-electron
oxidation of TTF leads to the formation of both MV dimers
and π-dimers. The mixed-valence state (TTF2•+) occurs when
the radical cation shares the positive charge with another not
yet oxidized TTF. π-Dimers (TTF22+) form when two radical
cations associate in a complex. These aggregate species represent
the solution counterpart of the two solid-state situations
(conductor/insulator) and can be viewed as models able to
anticipate the final stoichiometry of the material.3 The MV and
π-dimer equilibria are characterized by the following equilibrium
constants:
2+
[TTF2
]
Kπ-dim
)
[TTF•+]2
The kinetic constants of these equilibria are very fast.4 Subtract-
ing the Nernst equations for the oxidation potentials, E1 (TTF/
TTF•+) and E1′ (TTF2•+/TTF22+), affords the following relation:
KMV
RT
F
E1′ - E1 )
ln
(3)
Kπ-dim
From eq 3 it follows that when KMV ) Kπ-dim, the potentials
E1 and E1′ become identical. However, KMV > Kπ-dim implies
E1′ > E1, and two waves are expected in the voltammogram,
one for the oxidation of TTF and one for the oxidation of the
MV complex. KMV < Kπ-dim implies E1′ < E1, and only one
wave at the position of E1 will be observed.
* To whom correspondence should be addressed. Fax: (+45 66158780).
E-mail: jde@chem.sdu.dk.
† University of Southern Denmark.
Spectroelectrochemistry provides a powerful tool for analyz-
ing the π-dimer and MV formations. Indeed, the π-dimer
generated upon electrochemical oxidation usually shows an
intermolecular charge-transfer transition at a wavelength dif-
ferent from the intramolecular transition of the radical cation
unit. Thus, TTF•+ absorbs at wavelengths of maxima 430 and
‡ Eidgeno¨ssische Technische Hochschule.
§ Universite´ d’Angers.
(1) (a) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.; Geiser,
U.; Wang, H. H.; Kini, A. M.; Whangbo, M. H. Organic Superconductors
(Including Fullerenes); Prentice Hall: Englewoods Cliffs, NJ, 1992. (b)
Bryce, M. R. Chem. Soc. ReV. 1991, 20, 355. (c) Schukat, G.; Fangha¨nel,
E. Sulfur Rep. 1996, 18, 1.
(2) Torrance, J. B. Acc. Chem. Res. 1979, 12, 79.
(3) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P.
E. Phys. ReV. B 1979, 19, 730.
(4) Huchet, L.; Akoudad, S.; Levillain, E.; Roncali, J.; Emge, A.; Ba¨uerle,
P. J. Phys. Chem. B 1998, 102, 7776.
10.1021/ja000537c CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/13/2000

Bis-TetrathiafulValenes
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9487
Scheme 1. Possible Oxidation Route of a Bis-TTF (model 1)
oxidized sequentially at potentials E1 and E1′ as two one-electron
processes, forming an intermediate intramolecular MV complex.
Because of the four attachment sites, two TTFs can be
covalently linked by one, two, three, or four bridges. In this
work we have used both old and new synthetic protocols for
obtaining a series of bis-TTFs, which differ in the number, size,
and flexibility of their bridging units.
Results and Discussion
Synthesis. The preparation of 2,3-cyclized TTF macrocycles9
were performed according to the stepwise strategy presented in
Scheme 3. First, a dibromide (2a, b, or c) was reacted with the
monothiolate of 1 produced in situ upon treatment with one
equiv of CsOH,10 affording the monobridged compounds 3a-
c.11 Deprotection and alkylation with another equiv of 2a or b
under high-dilution conditions employing a two-syringe perfusor
pump yielded the macrocycles 4a,b12 in high yields. The tert-
butyl substituents on the phenyl group of the linkers impart
solubility of the compounds in organic solvents. It was also
possible to prepare the macrocycle 4a in two successive
deprotection/realkylation steps without intermediate workup. The
macrocycle 4c, containing phenolic bridging units, was prepared
from alkylation of the bisthiolate of 1. The two phenolic groups
offer the possibility for incorporating 4c into larger systems by
O-alkylations. Unsymmetrical macrocycles (5 and 6) were
prepared by reacting the monobridged bis-TTF 3b with either
bis(2-iodoethyl)ether or 1,2-bis(2-iodoethoxy)ethane.
Scheme 2. Possible Oxidation Route of a Bis-TTF (model 2)
2+
580 nm, whereas TTF2 absorbs around 800 nm in acetoni-
trile.4,5 For the dication TTF2+, an absorption is observed at
λmax ) 390 nm. In contrast, the absorption band of the mixed-
valence state has never been identified in solution. However, it
has been identified in polymeric films around 1800 nm, that is,
as a low-energy transition.4
To elucidate the effect on the dimerization when two identical
TTFs are covalently forced to be in close proximity, we decided
to investigate a series of dimeric TTFs linked by more or less
flexible linkers.6 When oxidizing bis-TTFs, two models (Schemes
1 and 2) can be considered, depending on whether the TTFs
are mono-oxidized in one or in two subsequent steps. In model
1 (Scheme 1), both TTFs are oxidized at the same potential,
E1. The resulting diradical cation may form intermolecular MV
complexes as well as intermolecular π-dimers. In addition,
intramolecular π-dimers may be formed if allowed to do so by
the linkers. This model is basically the same as the above model
for oxidation of TTF, with the only difference being that each
process is a two-electron process because of the two units of
TTF present. In model 2 (Scheme 2), originally proposed by
Jørgensen et al.7 for the oxidation of mono-bridged bis-TTFs
containing S(CH2)nS (n ) 1, 2) linkers,8 the two TTFs are
(9) Review on TTF macrocycles: Nielsen, M. B.; Becher, J. Liebigs
Ann. 1997, 2177.
(10) (a) Becher, J.; Lau, J.; Leriche, P.; Mørk, P.; Svenstrup, N. J. Chem.
Soc., Chem. Commun. 1994, 2715. (b) Lau, J.; Simonsen, O.; Becher, J.
Synthesis 1995, 521. (c) Simonsen, K. B.; Svenstrup, N.; Lau, J.; Simonsen,
O.; Mørk, P.; Kristensen, G. J.; Becher, J. Synthesis 1996, 3, 407. (d) Becher,
J.; Li, Z.-T.; Blanchard, P.; Svenstrup, N.; Lau, J.; Nielsen, M. B.; Leriche,
P. Pure Appl. Chem. 1997, 69, 465.
(11) Compound 3a: To a solution of 1 (1.03 g, 2.21 mmol) in dry DMF
(50 mL) was slowly added (30 min) a solution of CsOH‚H2O (0.390 g,
2.32 mmol) in dry MeOH (7 mL) under N2, whereupon the mixture was
stirred for 1 h. Then 2a (0.49 g, 1.1 mmol) in dry DMF (6 mL) was added.
The orange mixture was stirred for 30 min, after which the solvent was
removed in vacuo. Column chromatography [silica, CH2Cl2] afforded an
orange solid. Recrystallization from CHCl3/MeOH gave 3a as an orange
1
glass (1.01 g, 82%): mp 62-63 °C; H NMR (CDCl3) δ ) 7.20 (s, 1H,
ArH), 6.43 (s, 1H, ArH), 4.20 (t, 4H, J ) 6.5 Hz, OCH2), 3.16 (t, 4H, J )
6.5 Hz, SCH2), 3.02 (t, 4H, J ) 7.2 Hz, CH2CN), 2.65 (t, 4H, J ) 7.2 Hz,
SCH2) 2.43 (s, 12H, SCH3), 1.36 (s, 18H, C(CH3)3); 13C NMR (CDCl3)
δ ) 155.57, 131.82, 129.68, 125.36, 117.60, 109.00, 98.48, 66.82, 35.42,
34.42, 30.08, 19.11, 19.10, 18.59; MS(PD) m/z ) 1102 (M+). Anal. Calcd
for C40H48S16O2N2 (1101.79): C, 43.61; H, 4.39; N, 2.54; S, 46.56.
Found: C, 43.64; H, 4.36; N, 2.49; S, 46.71.
(5) Some more or less contradictory results exist in the literature with
respect to the π-dimer formation. The first electronic absorption spectrum
of the TTF radical cation was reported in water, but no concentration or
π-dimer formation data were mentioned: Wudl, F.; Smith, G. M.; Hufnagel,
E. J. J. Chem. Soc., Chem. Commun. 1970, 1453. Hu¨nig et al observed no
π-dimers of the TTF radical cation upon cooling an acetonitrile solution
until crystallization occurred, whereas dimer formation was obtained for a
dimethyldibenzo TTF derivative upon increasing the concentration or
decreasing the temperature: Hu¨nig, S.; Kiesslich, G.; Quast, H.; Scheutzow,
D. Liebigs Ann. Chem. 1973, 310. However, Torrance et al3 identified dimers
of the TTF radical cation in ethanol at 225 K. Furthermore, Huchet et al4
observed a small but evident absorption peak resulting from dimers in
acetonitrile (and in CH2Cl2/CH3CN) at room temperature when oxidizing
a relatively concentrated solution of TTF (ca. 5 mM). This absorption
disappeared at low concentration (ca. 0.5 mM) but increased on a thin film
of TTF-derivatized polythiophenes, confirming its assignment to a dimer.
(6) Review on bis-TTFs: Otsubo, T.; Aso, Y.; Takimiya, K. AdV. Mater.
1996, 8, 203.
(12) Compound 4a: Method i: To a solution of 3a (0.810 g, 0.74 mmol)
in dry DMF (45 mL) was slowly added over 30 min a solution of CsOH‚
H2O (0.270 g, 1.62 mmol) in dry MeOH (5 mL) under N2. Then the solution
was stirred for 1 h. This mixture and a solution of 2a (0.327 g, 0.75 mmol)
in dry DMF (50 mL) were simultaneously added to dry DMF (50 mL)
over 20 h by means of a two-syringe perfusor pump. The product was
precipitated using H2O (100 mL), filtered, and washed with MeOH (3 ×
20 mL). Recrystallization from toluene/MeOH afforded 4a (0.742 g, 79%)
1
as an orange powder: mp 228-229.5 °C; H NMR (CDCl3) δ ) 7.16 (s,
2H, ArH), 6.32 (s, 2H, ArH), 4.09 (t, 8H, J ) 6.6 Hz, OCH2), 3.16 (t, 8H,
J ) 6.6 Hz, SCH2), 2.44 (s, 12H, SCH3), 1.33 (s, 36H, C(CH3)3); 13C NMR
(CDCl3) δ ) 155.21, 129.49, 128.57, 127.67, 125.49, 111.61, 109.94, 97.39,
66.21, 34.85, 34.39, 30.09, 19.19; MS(PD) m/z ) 1270 (M+). Anal. Calcd
for C52H68S16O4 (1270.32): C, 49.16; H, 5.41; S, 40.38. Found: C, 49.36;
H, 5.43; S, 40.11. Method ii: To a solution of 1 (1.03 g, 2.21 mmol) in dry
DMF (50 mL) was slowly added over 30 min a solution of CsOH‚H2O
(0.390 g, 2.32 mmol) in dry MeOH (7 mL), under N2. Then the solution
was stirred for 1 h, after which 2a (0.49 g, 1.1 mmol) in dry DMF (6 mL)
was added. The solution was stirred for 30 min, and then another solution
of CsOH‚H2O (0.390 g, 2.32 mmol) in dry MeOH (5 mL) was added over
30 min. After stirring for 1 h, this mixture and a solution of 2a (0.490 g,
1.12 mmol) in dry DMF (50 mL) were simultaneously added together over
2 h. Then the solvent was removed in vacuo and the residue subjected to
column chromatography [silica, CH2Cl2/cyclohexane 1:3], yielding 4a (1.01
g, 72%) as an orange glass.
(7) Jørgensen, M.; Lerstrup, K. A.; Bechgaard, K. J. Org. Chem. 1991,
56, 5684.
(8) The ability of very short bridges to facilitate the formation of an
intramolecular MV complex has also been observed in the crystal structure
of the radical cation salt of a TTF twin donor containing one methylenedithio
bridging unit: Izuoka, A.; Kumai, R.; Sugawara, T. Chem. Lett. 1992, 285.
Oxidation of mono-bridged bis-TTF derivatives has also been studied by
Cava and co-workers: Sudmale, I. V.; Tormos, G. V.; Khodorkovsky, V.
Y.; Edzina, A. S.; Neilands, O. J.; Cava, M. P. J. Org. Chem. 1993, 58,
1355.

9488 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Spanggaard et al.
Scheme 3. Synthesis of Mono- and Dibridged Bis-TTFs
Scheme 4. Synthesis of a Macrocycle Containing Protected Thiolate Functions
To prepare macrocyclic bis-TTFs containing cyanoethyl-
protected thiolate groups in the 2,3-positions, we started out
with the differentially protected 2,3-bis(2-cyanoethylthio)-6,7-
bis[2-(4-nitrophenyl)ethylthio]tetrathiafulvalene 7 developed by
Simonsen et al.13 (Scheme 4). Adding 1 equiv of NaOMe
removes one cyanoethyl group. Two of the resulting monothio-
lates were then linked together by alkylation with the dibromide
2a. Deprotection and alkylation once more afforded the mac-
rocycle 8a, which was not isolated, but was converted into the
macrocycle 9a14 by an interchange of protecting groups. We
found that, although 1 equiv of base led to a quantitative removal
of cyanoethyl, more equivalents were necessary to remove
4-nitrophenylethyl, and the elimination product, p-nitrostyrene,
is usually difficult to remove in the purification process. For
these reasons, we decided to isolate a cyanoethyl-protected
macrocycle.
Macrocycle 9a may find wide applications in synthetic TTF
chemistry because of the possibility of its reacting further at
the four protected thiolate groups. Thus, it can be used as a
precursor for the preparation of quadruple-bridged bis-TTFs.
Removal of all four cyanoethyl groups by adding 4.5 equiv of
NaOMe, followed by the addition of 2 equiv of the dibromide
2a, resulted in formation of the belt-type15 compound 10a16 in
an excellent yield of 68% (Scheme 5, route i). This same
(13) Simonsen, K. B.; Svenstrup, N.; Lau, J.; Thorup, N.; Becher, J.
Angew. Chem., Int. Ed. Engl. 1999, 38, 1417.

Bis-TetrathiafulValenes
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9489
Scheme 5. Synthesis of a Face-to-Face Overlapped
Quadruple Bridged Bis-TTF (TTF-Belt)
Scheme 6. Intermediate in the Stepwise Synthesis of
TTF-Belt 10a
in a yield of 35% and the criss-cross overlapped macrocycle
12a as a byproduct (5%) (Scheme 5, route iii). In this one-pot
reaction,16 the formation of 12a is explained by the presence of
trans,trans intermediates that contain two or three bridging units.
Still, 10a is the predominant product. It is compelling that a
step involving eight bond-forming reactions among six separate
species results in a belt molecule in such a remarkably high
yield and that competing oligo- and polymerizations account
for a maximum of 60% of the byproducts. Thus, it seems that
the reactants, to a high degree, are preorganized for the belt-
forming reaction.
compound was synthesized earlier by less efficient and time-
consuming stepwise mono-deprotectection/mono-realkylation
steps.13
We previously reported about the construction of bicyclic
systems that employ the selective 2,7(6)-deprotection of tetrakis-
(2-cyanoethylthio)tetrathiafulvalene (11).17 This time, we inves-
tigated whether 10a could actually be prepared from sequential
deprotection/alkylation steps of 11 without intermediate puri-
fications (Scheme 5, route ii). Thus, 11 was first treated with 2
equiv of base and then 1 equiv of 2a, which yielded the
macrocyclic intermediate 13a (Scheme 6) as a mixture of cis/
(16) Compound 10a: Route i: To a solution of 9a (0.119 g, 0.0837 mmol)
in dry DMF (30 mL) was added 0.5 M NaOMe/MeOH (0.75 mL, 0.375
mmol), under N2. After stirring for 30 min, this mixture and a solution of
2a (0.0731 g, 0.168 mmol) in dry DMF (30 mL) were simultaneously added
over 4 h to dry DMF (30 mL). Then the solvent was removed in vacuo and
the residue subjected to column chromatography [silica, CH2Cl2/cyclohexane
1:3]. Recrystallization from CHCl3/MeOH afforded 10a (0.101 g, 68%) as
1
trans isomers. It was evidenced from H NMR measurements
1
a yellow powder: mp >200 °C dec; H NMR (CDCl3) δ ) 7.17 (s, 4H,
that adding catalytic amounts of trifluoroacetic acid to this
isomeric mixture caused isomerization18 to one stable isomer.
This stable isomer seems to be the cis,cis isomer, because
subsequent deprotection and alkylation steps (Scheme 5, steps
4 and 5, route ii) yielded the TTF-belt 10a16 in a yield of 45%.
If present, a trans,trans precursor should result in a mixture of
face-to-face and criss-cross overlapped macrocycles. Thus, by
performing a simple acid-induced isomerization reaction before
the final ring-closure, we are able to control the isomeric purity.
When all four thiolate functions were deprotected at once
and bridged together upon reaction with 2a, we obtained 10a
ArH), 6.68 (s, 4H, ArH), 4.32 (t, 16H, J ) 8.6 Hz, OCH2), 3.20 (m, 8H,
SCH2), 3.10 (m, 8H, SCH2), 1.32 (s, 72H, C(CH3)3); 13C NMR (CDCl3)
δ ) 155.56, 131.49, 127.15, 125.46, 118.73, 102.46, 68.00, 34.52, 33.81,
30.20; MS(FAB) m/z ) 1762 (M+). Anal. Calcd for C84H112O8S16
(1762.76): C, 57.24; H, 6.40; S, 29.10. Found: C, 57.02; H, 6.31; S, 28.81.
Route ii: To a solution of 11 (0.131 g, 0.240 mmol) in dry DMF (50 mL)
was added a solution of CsOH‚H2O (0.088 g, 0.524 mmol) in dry MeOH
(5 mL) over 20 min, under N2. After stirring for 30 min, this mixture and
a solution of 2a (0.105 g, 0.241 mmol) in dry DMF (47 mL) were
simultaneously added over 15 h to dry DMF (30 mL). The mixture was
stirred for a couple of hours, after which a few drops of a 0.1 M CF3-
COOH/DMF solution were added, which resulted in a color change from
red-orange to red-brown. Then CsOH‚H2O (0.088 g, 0.524 mmol) in dry
MeOH (5 mL) was added. After stirring for 20 min, this mixture and a
solution of 2a (0.107 g, 0.245 mmol) in dry DMF were simultaneously
added over 4 h to dry DMF (30 mL). After stirring for 30 min, the mixture
was concentrated in vacuo. Column chromatography [silica, (i) CH2Cl2/
petroleum ether 1:1; (ii) CH2Cl2/petroleum ether 3:1] followed by recrys-
tallization from CHCl3/MeOH afforded 10a (0.095 g, 45%) as a yellow
powder. Route iii: To a solution of 11 (0.121 g, 0.222 mmol) in dry DMF
(40 mL) was added CsOH‚H2O (0.169 g, 1.01 mmol) in dry MeOH (6
mL), under N2. After stirring for 30 min, this mixture and a solution of 2a
(0.197 g, 0.452 mmol) in dry DMF (50 mL) were simultaneously added
over 17 h to dry DMF (30 mL). The mixture was stirred for a couple of
hours and then concentrated in vacuo. The residue was subjected to column
chromatography [silica, (i) CH2Cl2/cyclohexane 1: 4; (ii) CH2Cl2/cyclo-
hexane 1:2]. The first orange fraction was recrystallized from CHCl3/MeOH
to yield 10a (0.0685 g, 35%) as a yellow powder. The second orange fraction
was recrystallized from CHCl3/MeOH to yield 12a (0.009 g, 5%) as a yellow
powder. 12a: mp >200 °C; 1H NMR (CDCl3) δ ) 7.13 (s, 4H, ArH), 7.05
(s, 4H, ArH), 4.31 (t, 16H, J ) 4.1 Hz, OCH2), 3.26 (t, 16H, J ) 1.9 Hz,
SCH2), 1.30 (s, 72H, C(CH3)3); MS(FAB) m/z )1762 (M+).
(14) Compound 9a: To a solution of 7 (0.259 g, 0.351 mmol) in dry
DMF (30 mL) was added 0.5 M NaOMe/MeOH (0.75 mL, 38 mmol) over
20 min, under N2. The solution was stirred for 20 min, after which 2a
(0.0766 g, 0.176 mmol) in dry DMF (40 mL) was added. The mixture was
stirred for 10 min, after which 0.5 M NaOMe (0.75 mL, 0.38 mmol) was
added. After stirring for 10 min, this mixture and a solution of 2a (0.0763
g, 0.175 mmol) in dry DMF (40 mL) were simultaneously added over 1 h
to dry DMF (30 mL). The mixture was stirred for 10 min, after which 0.5
M NaOMe/MeOH (24 mL, 12 mmol) was added. After stirring for 10 min,
3-bromopropionitrile (6 mL, 74 mmol) in DMF (20 mL) was added. After
stirring for another 10 min, the solvent was removed in vacuo and the residue
subjected to column chromatography [silica, (i) CH2Cl2/petroleum ether
3:1, (ii) CH2Cl2]. Recrystallization from CHCl3/MeOH yielded 9a (0.114
g, 46%) as a yellow powder: mp 203-204 °C; 1H NMR (CDCl3) δ )
7.19 (s, 2H, ArH), 6.61 (s, 2H, ArH), 4.28 (t, 8H, J ) 7.5 Hz, OCH2),
3.21-3.08 (2 × t, 16H, SCH2), 2.71 (t, 8H, J ) 6.7 Hz, CH2CN), 1.35 (s,
36H, C(CH3)3); 13C NMR (CDCl3) δ ) 155.54, 131.20, 128.96, 126.80,
125.41, 117.56, 101.80, 67.38, 34.50, 33.99, 31.30, 30.82, 30.19, 18.92;
MS (FAB) m/z ) 1426 (M+). Anal. Calcd for C60H72N4O4S16‚0.5CH2Cl2
(1468.68): C, 49.48; H, 5.01; N, 3.81. Found: C, 49.51; H, 4.94; N, 3.83.
(15) Alternative procedures for preparing TTF-belts: (a) Adam, M.;
Enkelmann, V.; Ra¨der, H.-J.; Ro¨hrich, J.; Mu¨llen, K. Angew. Chem., Int.
Ed. Engl. 1992, 31, 309. (b) Matsuo, K.; Takimiya, K.; Aso, Y.; Otsubo,
T.; Ogura, F. Chem. Lett. 1995, 523.
(17) (a) Li, Z.-T.; Stein, P. C.; Svenstrup, N.; Lund, K. H.; Becher, J.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2524. (b) Li, Z.-T.; Stein, P. C.;
Becher, J.; Jensen, D.; Mørk, P.; Svenstrup, N. Chem. Eur. J. 1996, 2, 624.
(c) Li, Z.-T.; Becher, J. Chem. Commun. 1996, 639. (d) Nielsen, M. B.; Li,
Z.-T.; Becher, J. J. Mater. Chem. 1997, 7, 1175.
(18) For studies on acid-catalyzed isomerization of TTF, see: Souizi,
A.; Robert, A. J. Org. Chem. 1987, 52, 1610.

9490 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Spanggaard et al.
Figure 1. X-ray crystal structure of 4b.
Scheme 7. Formation of Belts vs Bis-Crowns, the Linker
Size Determining the Course of the Reaction
Figure 2. Stereodrawing of the packing diagram of 4b showing some
of the intermolecular contacts: a ) distance(S8-S8) ) 3.431(5) Å,
b ) distance(S5-S5) ) 3.585(7) Å.
Figure 3. PM3 geometry optimization of 4b, starting from a
U-conformation.
Using the stepwise strategy (route ii) we were able to obtain
the bis(ethylene)glycol bridged TTF-belt 14 in a yield of 20%
(Scheme 7). However, when extending the length and flexibility
of the bridge by using 1,2-bis(2-iodoethoxy)ethane as the
alkylating reagent, only the TTF-biscrown 15 was obtained, and
no TTF-belt was isolated.
V-shaped conformation. From other starting geometries, we
obtained S- and V-shaped structures that possess about the same
energy ((7 kcal mol-1), taking into account the large number
of atoms. Thus, none of these conformations allows intramo-
lecular π-π interactions between the two TTFs.
Cyclic Voltammetry. The electrochemical behavior of the
bis-TTFs showed that the voltammograms, composed of two
oxidation waves, depended on the length and flexibility of the
bridge. Indeed, a broadening or a splitting of the first oxidation
wave was observed when the flexibility of the bridge decreased.
Thin-layer cyclic voltammetry (TLCV)21 revealed that this first
oxidation involves either a one-step two-electron process or two
one-electron processes per one unit of bis-TTF. The second
oxidation and final step is a two-electron process leading to
two fully oxidized TTFs. The TLCV of 4b revealed two one-
electron processes and one two-electron process. The splitting
of the first wave may be accounted for by either of the above
models (1 or 2). In the case of a model 1 mechanism, the
electron ratio obtained by TLCV corresponded to processes
involving two, two, and four electrons, respectively, per two
units of 4b. In the next section, we will show that this
mechanism is the operating one for the oxidation of 4b and
that it originates from the rigidity of the linkers. Cyclic
voltammetry data were collected, together with spectroelectro-
chemical data (vide infra), in Table 1.
X-ray Crystallography and Semiempirical Calculations.
X-ray analysis19 of 4b showed that the two TTF units adopted
an S-type conformation in the solid-state, preventing intramo-
lecular interactions between the TTFs (Figure 1). In this
conformation, the steric interactions between the two bulky tert-
butyl substituents seem to be minimized. As shown in Figure
2, the molecules adopt close intermolecular S-S contacts shorter
than the sum of the van der Waals radii (3.7 Å) along the three
directions of space. Each molecule has 12 S-S contacts, varying
between 3.431(5) and 3.585(7) Å, with adjacent molecules.
Semiempirical PM3 geometry optimizations of 4b were
carried out from different starting geometries employing the
Gaussian 98 program package.20 The first optimization was
started from a U-type conformation with an interplanar distance
of about 5 Å (Figure 3). Optimization disrupts this U-form by
forcing the two TTFs away from each other into a more
(19) Single crystals of 4b were obtained after recrystallisation from CH2-
Cl2/EtOH with a few drops of C6H5Cl at 276 K. A yellow single crystal of
4b was selected by optical examination. X-ray data were collected at 294
K on an Enraf Nonius Mach 3 four circles diffractometer equipped with a
graphite monochromator utilizing Mo KR radiation (λ ) 0.71073 Å). The
structure was solved by direct methods (SIR) using MolEN package
programs [Crystal Structure Analysis: Molecular Enraf-Nonius (MolEN),
1990, Delft Instruments X-ray Diffraction, B. V. Rontnenweg 1, 2624 BD
Delft, The Netherlands] and was refined on F by the full-matrix least-squares
method using anisotropic thermal parameters for S and isotropic ones for
C. No absorption corrections were made. The H atoms were included in
the calculation without refinement.
Spectroelectrochemistry. First, we investigated the spec-
troelectrochemistry22 of tetramethylthiotetrathiafulvalene (TMT-
TTF), the monomeric unit of the synthesized bis-TTFs. One-
electron oxidation of TMT-TTF in dichloromethane led to the
appearance of an absorption band with a maximum around 470
(21) Thin-layer cyclic voltammetry was performed using a Pt electrode
of area 0.0314 cm2; thin-layer conditions were close to 20 µm. A scan rate
of 5 mV s-1 was used.
(20) M. J. Frisch et al. Gaussian 98, Revision A.7; Gaussian, Inc.:
Pittsburgh, PA, 1998.

Bis-TetrathiafulValenes
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9491
Table 1. Spectroelectrochemical Data in CH2Cl2 Containing 0.4 M Bu4NPF6
radical cationc
max (nm)
π-dimer
max (nm)
MV complex
max (nm)
dicationc
compd
E1 (V)a
E1′ (V)a
E2 (V)a
Eapp (V)b
λ
λ
λ
Eapp (V)b
λmax (nm)
TMT-TTF
0.52
0.55
0.54
0.56d
0.48
0.47
0.53
0.73
0.86
0.87
0.87
0.88
0.92
0.84
1.00
1.16
0.67
0.67
0.67
0.67
0.65
0.64
0.66
0.81
470
460
460
470
470
440
460
470
840
810
790
810
0.91
0.91
0.91
0.91
0.91
0.89
1.11
1.17
660
650
650
640
650
630
540
670
3a
4a
3b
4b
16
14
10a
0.64
820
2300
770, 1000
660
740c
a Half-wave potentials (vs Ag/AgCl) obtained by cyclic voltammetry. Concentration of TMT-TTF, 5 mM; all other compounds, 1.25 mM. b Applied
potential at which the absorption spectrum was measured and the maxima evaluated. c Per each unit of TTF. d Broad.
nm, which can be assigned to an intramolecular transition of
the radical cation, TMT-TTF•+.23 Moreover, formation of the
π-dimer (TMT-TTF)2 results in an intermolecular charge-
fact that no precipitation was observed in the chemical oxidation
experiment makes it unlikely to occur during the spectroelec-
trochemical experiment. Moreover, any precipitation should
result in an adsorption phenomenum (shape of the wave) in the
cyclic voltammogram, which was not the case. Our choice of
using CH2Cl2 as solvent was, first of all, based on the solubility
of the bis-TTFs, because these were not soluble in CH3CN.
Because the CVs of the bis-TTFs did not show any adsorption
phenomena either, we can exclude that precipitation of their
radical cations occurs within the concentration range of the
experiments.
2+
transfer absorption band at λmax ) 840 nm. The ratio between
the inter- and intramolecular absorptions did not change
significantly in the concentration range 0.5-2.5 mM, which
means that the equilibrium constant for the dimerization is very
high in CH2Cl2. From experiments in which the radical cation
was generated by chemical oxidation (NOPF4) in CH2Cl2, no
concentration dependence (measured by UV-vis) was observed
between 10-7 and 10-3 M. The same results were observed in
CH3CN. In this concentration range, the radical cation was
soluble, and no precipitation was observed. Under these
conditions, the equilibrium constant can be estimated to be g109
M-1, which corresponds to a ratio of [(TMT-TTF)22+]/[TMT-
TTF•+] that is close to 10 at 10-7 M. Consequently, the
dimerization kinetic constant must be higher than 109 s-1M-1
in order to be coherent with the electrochemical data, that is,
two reversible oxidation processes. Indeed, it was not possible
to observe an ESR signal in an ESR spectroelectrochemical
experiment,24 because of the very small concentration of
unassociated radical cations.25 This observation, together with
earlier studies,4 makes the interpretation of the band at 840 nm
as an intermolecular interaction in a dimer complex reliable.
Moreover, the intrinsic radical cation absorption band at 470
nm must originate mainly from dimerized radical cations. The
Further oxidation generates the dication TMT-TTF2+, which
shows an absorption band around λmax ) 660 nm as the only
remaining spectral feature.26 The emergence of the band in the
oxidation progress, after the disappearance of the intra- and
intermolecular radical cation absorptions, makes us very con-
fident about its assignment, even though it is shifted significantly
from the value reported for the dication transition of TTF2+ in
acetonitrile (λmax ) 390 nm).4 Delocalization of the positive
charges in TMT-TTF2+ onto the SMe groups is a possible and
reasonable explanation for this difference.
The spectroelectrochemical data for 3a and 4a appeared very
similar (Table 1). For both compounds, the ratio between the
intrinsic radical cation (referring to one TTF monomer unit)
absorption and the π-dimer absorption was almost unaltered
upon dilution (concentration range, 0.3-1.4 mM). This con-
centration independence suggests that the π-dimers,to a high
extent, are formed intramolecularly according to a model 1
oxidation. For two TTF radical cations to interact, a distance
of about 3.5 Å is required.27 Indeed, the flexibility of the linkers
facilitates the formation of such intramolecular complexes,
which involve less Coulombic repulsion than do tetracationic
intermolecular (3a)24+/(4a)24+ complexes. The fact that only one
wave for the two-electron oxidation generating the diradicals
(3a)2+ and (4a)2+ is observed in the voltammograms of 3a and
4a (E1′ e E1) reflects that the formation of π-dimers is favored
relative to the formation of MV complexes. The ability of bis-
TTFs containing flexible linkers to form intramolecular π-dimers
was earlier observed in the crystal structures of divalent cation
salts of 2,3-cyclized bis-TTF macrocycles containing S(CH2-
CH2O)nCH2CH2S (n ) 1, 2) bridges.28 Moreover, spectroelec-
trochemical experiments in solution have revealed intramolecular
aggregation of TTF radical cations in a (TTF)21-glycol dendrimer
in which flexible polyether spacers allow TTFs in close spatial
contact to interact.29
(22) Spectroelectrochemical experiments were performed in a cell made
of Teflon. A 2-mm-diameter stationary platinum disk, polished to a mirror
finish, was used as the working electrode. As the counter electrode, a
platinum wire was employed. All experiments were performed at an optical
path length close to 200 µm. A Lambda 19 Perkin-Elmer spectrophotometer
was employed. The current was provided by an EGG PAR 273 potentiostat;
a scan rate of 1.25 mV s-1 was used. HPLC-grade dichloromethane was
used as the solvent and n-Bu4NPF6 (0.4 M) was used as the supporting
electrolyte in all of the experiments. For more details, see: Gaillard, F.;
Levillain, E. J. Electroanal. Chem. 1995, 398, 77.
(23) Earlier studies of the TMT-TTF radical cation in acetonitrile have
revealed the following absorption maxima: (i) 461, 366, 329, and 262 nm.
Moses, P. R.; Chambers, J. Q. J. Am. Chem. Soc. 1974, 96, 945. (ii) 850,
560 (sh), 480 (sh), 454, and 261 nm. Kreicberga, J.; Neilands, O. Zh. Org.
Khim. 1985, 21, 2009. We were able to reproduce about the same values
as reported by Kreicberga and Neilands in acetonitrile by chemical oxidation
(NOPF4) of TMT-TTF at room temperature. In our opinion, these absorp-
tions should be assigned to either intermolecular or intramolecular absorp-
tions within the π-dimer. See main text for discussion. The dimerization of
TMT-TTF radical cations is strong in both CH2Cl2 and CH3CN. This
enhanced ability of TMT-TTF relative to TTF to form dimers is probably
a result of the four extra sulfur atoms which can facilitate dimer formation
by favorable S-S interactions.
(24) ESR experiments were run on a Bruker ESP 300 spectrometer driven
by the Bruker ESP 1600 computer program. Using this same equipment,
similar experiments probing dimer formation of tetrathienylenevinylenes
were carried out earlier: Levillain, E.; Roncali, J. J. Am. Chem. Soc. 1999,
121, 8760.
(26) For the TMT-TTF dication Moses and Chambers23 report the
following absorption maxima in acetonitrile: 710, 488, and 456 nm. By
chemical oxidation (NOPF4) of TMT-TTF in acetonitrile we obtained almost
the same low-energy absorption (λmax ) 706 nm), which is in good
agreement with the spectroelectrochemical result, whereas neither of our
experiments substantiate the two high-energy absorptions.
(25) In contradiction to this experiment, Moses and Chambers23 measured
an ESR signal in their studies of the TMT-TTF radical cation. However, in
light of their deviating spectral data, we question if their solution did, in
fact, contain the pure and fully mono-oxidized TMT-TTF.
(27) Phillips, T. E.; Kistenmacher, T. J.; Ferraris, J. P.; Cowan, D. O. J.
Chem. Soc., Chem. Commun. 1973, 471.

9492 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Spanggaard et al.
Figure 4. Spectroelectrochemical data for 4b (1 mM in CH2Cl2
containing 0.4 M Bu4NPF6).
Figure 6. Experimental cyclic voltammogram of 4b and calculated
voltammograms using models 1 and 2 at 0.1 V s-1. For the adjustment,
the voltammograms obtained for the experimental scan rates (5, 2, 1,
0.5, 0.2, and 0.1 V s-1) at room temperature are simultaneously taken
into account. The reaction rates, kf, kπ-dim, and kf, kMV, and the
intermolecular π-dimer equilibria have been taken to be arbitrary
constants and equal to 109 M-1 s-1 and 0.1 M-1, respectively. The
Figure 5. Absorption spectra (at 0.65 V) of 4b at (a) 1.25 mV and (b)
0.25 mM, in CH2Cl2 containing 0.4 M Bu4NPF6. Note that the spectrum
in (b) has been multiplied by a factor of 3. (Radical cation refers to
one unit of TTF.)
diffusion coefficient, D, was 6.7 × 10-6 cm2 s-1
.
The monobridged compound 3b showed a broadening of the
first oxidation wave in the voltammogram. For the related
macrocycle 4b, a significant splitting of the first wave was
observed, which can be explained by a strong propensity to form
MV complexes. An absorption band at λmax ) 2300 nm (Figure
4), unambiguously assigned to the MV state, developed during
the first oxidation wave and disappeared when the oxidation
proceeded. It is noteworthy that this band is the first spectral
evidence in solution for this state. A gradual increase of the
π-dimer absorption was observed during the oxidation, as finally
the MV complex was also converted to the π-dimer. When the
sample (1.25 mM) was diluted five times, the absorption band
arising from the π-dimers was almost halved relative to the
intrinsic radical cation (referring to one TTF monomer unit)
absorption (Figure 5). This concentration dependence implies
the presence of intermolecular π-dimers (4b)24+, which are
formed in favor of intramolecular π-dimers because of the
rigidity of the linkers. It also confirms that this band was
assigned correctly in the above discussion. The intrinsic radical
cation absorption is now originating from radical cation units
2+
not only assembled in π-dimers, that is, from (4b)24+, (4b)2
,
and possibly 4b2+ (being in fast exchange with each another
on the time scale of the experiment). In other words, an
interplanar distance between two TTF units of about 3.5 Å is
difficult to achieve intramolecularly, in accordance with the
X-ray crystal structure and semiempirical calculations on the
neutral molecule (vide supra). However, whether the intermo-
lecular π-dimers are formed in solution between V- or S-shaped
conformations is difficult to rationalize from the solid- and gas-
phase studies of neutral 4b. Since an intermolecular π-dimer,
(4b)24+, involves a total of 4 positive charges, it is suddenly
less favorable than the formation of intermolecular MV com-
plexes. Hence, KMV > Kπ-dim, implying E1′ > E1 (eq 3), and
two waves are seen in the voltammogram. The difference E1′
- E1 is ca. 0.16 V, resulting in a ratio of KMV/Kπ-dim of ca.
500.
From these experiments, we concluded that the MV state
occurs intermolecularly and, consequently, model 2, taking into
account an intramolecular MV, can be excluded. To raise further
support for the model 1 mechanism for the oxidation of 4b, we
carried out theoretical simulations (simulation and fit) of the
electrochemical behavior of 4b (Figure 6). Comparison to the
experiment shows qualitatively good agreement.
(28) (a) Akutagawa, T.; Abe, Y.; Nezu, Y.; Nakamura, T.; Kataoka, M.;
Yamanaka, A.; Inoue, K.; Inabe, T.; Christensen, C. A.; Becher, J. Inorganic
Chem. 1998, 37, 2330. (b) Akutagawa, T.; Abe, Y.; Hasegawa, T.;
Nakamura, T.; Inabe, T.; Christensen, C. A.; Becher, J. Chem. Lett. 2000,
132.
(29) Christensen, C. A.; Goldenberg, L. M.; Bryce, M. R.; Becher, J.
Chem. Commun. 1998, 509.
Spectra of 3b at high wavelengths did not reveal the
absorption band that results from the MV complex, but to judge

Bis-TetrathiafulValenes
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9493
Table 2. Differential Pulse Data (vs Ag/AgCl) in CH2Cl2
Containing 0.1 M Bu4NPF6
compd
E1 (V)
E1′ (V)
E2 (V)
3b
4b
5
0.55
0.44
0.46
0.55
0.63
0.63
0.60
0.63
0.91
0.90
0.88
0.91
6
from the small separation between E1 and E1′ in the voltam-
mogram (broad wave), its concentration is much smaller than
that of the MV complex of 4b. However, employing differential
pulse voltammetry,30 it was possible to resolve the first oxidation
into two sharp peaks (Table 2). Additionally, compounds 5 and
6 showed a splitting of the first redox potential as a result of
MV formation. The splitting is more evident for 5 than for 6,
which can be explained by the difference in the length and,
hence, the flexibility of the polyether bridge. Thus, the potentials
of 3b and 6 are almost identical, which means that the extra
bridge present in 6 is too flexible to make any difference. In
contrast, macrocycle 5 obtains the same large splitting between
E1 and E1′ as macrocycle 4b.
For comparison, the 2,7(6)-bridged macrocycle 16 was
investigated.17d The arrangement of the two TTFs in this
macrocycle resulted in two very intense absorption bands at
maxima of about 770 and 1000 nm upon two-electron oxidation
that forms 162+. The intensities of these bands do not decrease
relative to the radical cation (referring to one TTF monomer
unit) absorption at λmax ) 440 nm upon dilution (1.25-0.25
mM). Thus, the two TTFs are in an ideal arrangement for
interacting intramolecularly. The high-energy absorption
(λmax ) 770 nm) may be assigned to an intramolecular transition
of the diradical, whereas the low-energy absorption (λmax ) 1000
nm) may arise from intramolecular π-dimers, the formation of
which is made possible by the flexible polyether linkers, in
accordance with the above discussion.
Figure 7. Absorption spectra in CH2Cl2 of mixtures of DDQ (3.1 ×
10-4 M) with (a) TMT-TTF (6.2 × 10-4 M), (b) 4a (3.1 × 10-4 M),
and (c) 10a (3.1 × 10-4 M).
results in a gradual increase in the energy of this transition. For
the related compound 10a, a very broad π-dimer absorption band
is observed at λmax ) 740 nm. No splitting of the first wave in
the voltammograms of 10a and 14 occurs, signaling that MV
complexes are not formed to any significant extent, either intra-
or intermolecularly. Moreover, the confinement of two TTFs
in a quadruple-bridged arrangement increases the redox poten-
tials of 10a relative to those of 4a, that is, electrostatic repulsion
between the two TTF mono- or dications is not so easily
opposed by conformational changes. For comparison, a strong
propensity to form an intramolecular MV state was earlier
observed for a criss-cross, overlapped, quadruple-bridged bis-
TTF.31a A splitting of the second redox potential for criss-
crossed, overlapped bis-TTFs was shown to depend highly on
the actual linker size.31
Complexation Studies. Taking advantage of the electron
donating abilities of TTF, we carried out preliminary complex-
ation experiments with different acceptors. However, the strong
acceptors 7,7,8,8-tetracyanoethylene-p-quinodimethane (TCNQ)
and tetracyanoethylene (TCNE) were found to show no sig-
nificant complexation ability with either 3a or 4a, according to
1H NMR experiments in CDCl3. In contrast, 2,3-dichloro-5,6-
dicyano-p-benzoquinone (DDQ) forms a charge-transfer (CT)
complex with 4a with an absorption at λmax ) 780 nm in CH2-
Cl2 (Figure 7). For the complex of TMT-TTF with DDQ, the
CT absorption band appeared at a significantly higher wave-
length of λmax ) 850 nm. The difference in absorption maximum
may result from the formation of a 1:1 inclusion complex
between 4a and DDQ in which the two TTFs encapsulate one
unit of DDQ. A similar inclusion complex between DDQ and
a dimeric bisethylenedithiotetrathiafulvalene (BEDT-TTF) was
earlier reported by Sugawara and co-workers.32 To our surprise,
the TTF-belt 10a showed no complexation at all, that is, no
complexes are formed in which DDQ is associated either at the
outside (alongside) of 10a or in its cavity. If such outside
interactions are neglected for the complexes between 4a and
DDQ, an association constant for the anticipated 1:1 inclusion
complex of Ka ) 160 M-1 (CDCl3, 303K) is obtained from 1H
NMR dilution experiments employing the chemical shift change
of the aryl proton pointing into the cavity. Further work is in
The spectrum of the TTF-belt 14 shows an intense, almost
concentration-independent absorption band (relative to the
intrinsic radical cation absorption in the concentration range
0.25-2.5 mM) at a low wavelength of λmax ) 660 nm which
may be assigned to either an intramolecular transition of the
diradical or to an intramolecular π-dimer. The face-to-face
arrangement of the two TTFs results in a high-energy absorption
of the tetracation 144+ at λmax ) 540 nm. Thus, proceeding from
TMT-TTF (660 nm) to a dibridged bis-TTF (630-650 nm) and,
finally, to a more rigid quadruple-bridged bis-TTF (540 nm)
(31) (a) Nielsen, M. B.; Thorup, N.; Becher, J. J. Chem. Soc., Perkin
Trans. 1 1998, 1305. (b) Tanabe, J.; Kudo, T.; Okamato, M.; Kawada, Y.;
Ono, G.; Izuoka, A.; Sugawara, T. Chem. Lett. 1995, 579. (c) Otsubo, T.;
Aso, Y.; Takimiya, K. AdV. Mater. 1996, 8, 203. (d) Takimiya, K.; Imamura,
K.; Shibata, Y.; Aso, Y.; Aso, F.; Ogura, F.; Otsubo, T. J. Org. Chem.
1997, 62, 5567.
(30) Differential pulse voltammetry was performed using an Autolab,
PGSTAT10 potentiostat (ECO Chemie BV) with n-Bu4NPF6 as the
supporting electrolyte. Counter and working electrodes were made of Pt,
and the reference electrode was Ag/AgCl.
(32) Tachikawa, T.; Izuoka, A.; Sugawara, T. J. Chem. Soc., Chem.
Commun. 1993, 1227.

9494 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Spanggaard et al.
progress to shed more light on the structures of these donor-
to originate from intermolecular (4b)22+ complexes, according
to measurements at various concentrations and according to
cyclic voltammogram simulations. In the presence of more
flexible linkers, intramolecular association of radical cations to
π-dimers suppresses the formation of MV complexes. These
findings are important for both the future design of organic
conductors based on TTF as well as supramolecular systems
that rely on the different and interconvertible redox states of
TTF.33
Complexation studies with the electron acceptor DDQ have
been carried out. Although the macrocycle 4a forms a donor-
acceptor complex with DDQ, as does TMT-TTF, no complex-
ation is observed for the TTF-belt 10a, according to UV-vis
spectroscopy.
acceptor complexes.
Conclusions
We have developed simple and high-yielding synthetic
procedures for preparing mono-, bis-, and quadruple-bridged
dimeric TTFs. The remarkably high yields obtained from any
of three synthetic routes of the quadruple-bridged belt molecule
10a seem to result from preorganization of the reactants. Thus,
from simple starting materials, 10a can be prepared in an overall
yield ranging from 30 to 45%. One route takes advantage of
the well-known ability of TTF to isomerize in the presence of
catalytic amounts of acid. This isomerization reaction is now
implemented into the synthetic protocol and can be considered
as a standard method to control the isomeric purity in such
reactions.
Cyclic voltammetry and spectroelectrochemical measurements
clearly demonstrate that both the nature of the linkers and the
way in which they are connecting the two TTFs are of
importance for controlling the redox potentials and the formation
of MV and π-dimers in solution. Upon oxidation of the rigid
macrocycle 4b, MV complexes were generated to such a high
extent that we were able to measure its absorption wavelength
in solution (λmax ) 2300 nm). The MV absorption of 4b seems
Supporting Information Available: Full experimental
procedures; tables of bond distances and angles, positional
parameters, and general displacement parameters for 4b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA000537C
(33) Recent review of TTF in supramolecular chemistry: Nielsen, M.
B.; Lomholt, C.; Becher, J. Chem. Soc. ReV. 2000, 29, 153.