Synthesis of electrochemically responsive TTF-based molecular tweezers: evidence of tight intramolecular TTF pairing in solution

Article abstract of DOI:10.1016/j.tet.2007.11.110

We report the synthesis and conformational studies of TTF-containing molecular tweezers based on a 1,2,4,5-tetramethylbenzene scaffold. In the neutral form the tweezers are expected to adopt the closed conformation, while, upon oxidation, the open conformation should be preferred due to electrostatic repulsion between the oxidized TTF moieties. Cyclic voltammetry studies demonstrate electronic pairing with formation of mixed-valence [TTF]₂^{+{radical dot}} species and opening of the tweezers upon the full oxidation of the TTF groups. Variable-temperature (VT) NMR studies evidence tight intramolecular TTF pairing at low temperature. Molecular modeling studies showed clear preference for an open conformation of tweezers in a fully oxidized state.

Full text of DOI:10.1016/j.tet.2007.11.110

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1909e1917
www.elsevier.com/locate/tet
Synthesis of electrochemically responsive TTF-based molecular tweezers:
evidence of tight intramolecular TTF pairing in solution
b
a
Vladimir A. Azov a, , Rafael Gomez , Johannes Stelten
*
´
a University of Bremen, Department of Chemistry, Leobener Strasse NW 2C, D-28359 Bremen, Germany
b Universidad Complutense, Departamento de Qu´ımica Orga´nica, Avda. Complutense s/n, E-28040 Madrid, Spain
Received 10 October 2007; accepted 26 November 2007
Dedicated to Professor Fredric M. Menger on the occasion of his 70th birthday
Abstract
We report the synthesis and conformational studies of TTF-containing molecular tweezers based on a 1,2,4,5-tetramethylbenzene scaffold. In
the neutral form the tweezers are expected to adopt the closed conformation, while, upon oxidation, the open conformation should be preferred
due to electrostatic repulsion between the oxidized TTF moieties. Cyclic voltammetry studies demonstrate electronic pairing with formation of
mixed-valence [TTF]þ2 ꢀ species and opening of the tweezers upon the full oxidation of the TTF groups. Variable-temperature (VT) NMR studies
evidence tight intramolecular TTF pairing at low temperature. Molecular modeling studies showed clear preference for an open conformation of
tweezers in a fully oxidized state.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Molecular devices; Conformational analysis; NMR; Cyclic voltammetry; Tetrathiafulvalenes; Stacking
1. Introduction
Therefore, the construction of molecular systems capable of
controlled molecular-level motion has become an area of great
contemporary interest and scientific growth.
During the last several years considerable efforts have been
aimed at the design and construction of molecular switches
and molecular devices displaying positional displacement of
molecular or supramolecular components as a result of exter-
nal stimuli.1 Numerous molecular architectures displaying
rotation around a bond,2 shuffling,3 rotation in a catenane,4
coiling/decoiling,5 and other types of movements have been
reported so far. External stimuli such as light, heat, pH change,
complexation/decomplexation, and oxidation/reduction have
been used to induce molecular displacement in these systems.
Nevertheless, despite the wide variety of functional molecular
architectures already developed and investigated, there are
still many open possibilities to design and assemble molecular
machines displaying novel modes of molecular motions.
Noncyclic compounds with cavities commonly called
molecular tweezers, clips, or clefts6,7 have been already success-
fully employed as molecular hosts for guest binding in solution
as well as in solid state. Another not yet deeply exploited possi-
bility of this type of materials is to use clip-like architectures
with sufficient conformational freedom for the construction of
switching structures able to perform large-scale openeclose
movement resembling a flapping wing motion of birds or flying
insects. In a closed conformation the two tips of the tweezers
overlay each other, whereas in an open conformation the tips
open up opposite to each other.
The objective of this work is to design and synthesize a
novel red/ox responsive tweezers-type molecular architecture
capable of controllable and geometrically well-defined large-
scale molecular movement.8 For this purpose, we have chosen
tetrathiafulvalene (TTF) as an appropriate building block. One
of the advantages of TTF is the versatility of its chemistry,9
* Corresponding author. Tel.: þ49 421 218 3667; fax: þ49 421 218 3720.
E-mail address: vazov@uni-bremen.de (V.A. Azov).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.110

1910
V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
which provides a wide range of functionalized derivatives and
paves the way to its employment for the construction of
various architectures, such as TTF-based chiral dimers,10 den-
drimers,11 cyclophanes and cages,12 and other complex struc-
tures. On the other hand, TTF is a good two-electron donor,
which undergoes aromatization upon oxidation, affording ther-
modynamically stable radical cationic and dicationic species
at relatively low oxidation potentials. This behavior has en-
abled the synthesis of donoreacceptor systems with promising
applications: in non-linear optics, as sensors, rectifiers, or even
as solar energy converters.13 Moreover, TTF has an excellent
record of application in various areas of supramolecular che-
mistry9 including the construction of various types of molecular
machines.3cee,14 All these outstanding characteristics moti-
vated us to select TTF as the red/ox active actuator in our
molecular switches.
Figure 2. Three low-energy conformers of 2.
are shown in Figure 1 to illustrate the geometrical differences
between them. Only one of the conformations (cis-boateboat)
shows a tweezers-like arrangement of the structure, with the
˚
TTF planes positioned almost parallel at ca. 6 A away from
each other, in all others the TTF units are located remotely
˚
apart from each other with distances reaching ca. 22.5 A
between the ends of the molecular backbone. The intercon-
version energy between different conformations should be
relatively small: previous experimental and theoretical estima-
tions for activation energy of boateskeweboat conformational
introversion for cycloocta-1,5-diene-like rings vary from 1.1 to
7.5 kcal/mol,17a whereas for the chaireboat the activation
energy was reported to lie within the 7.2e10.2 kcal/mol
range.17a,c
In order to estimate the conformational preferences of the
clefts 1, structure 2 containing one eight-membered dithia
ring fused with both a TTF moiety and a benzene ring has
been theoretically studied as a model compound. The semi-
empirical minimization of the three low-lying conformations
using AM1 and PM3 methods showed very low-energy differ-
ence between the boat, chair, and skew conformations all lying
within a narrow 2 kcal/mol energy range (Table 1), the boat
conformation being slightly preferential to the other two.
2. Results and discussion
2.1. Molecular design
Molecular tweezers 1 (Fig. 1) were designed to contain two
identical TTF moieties connected on their ends to the opposite
sides of a 1,2,4,5-substituted benzene scaffold via moderately
flexible eight-membered dithia rings, in a similar way to the
family of molecular tweezers first designed by Fukazawa
et al.7 on the basis of the dioxa[2.2]orthocyclophane skeleton.
Despite an extensive development of TTF chemistry, only
a few TTF-containing rigid molecular tweezers15 or structures
preferring to adopt a tweezers-like conformation in a solid
state (either neutral16a or oxidized16b,c) have been reported
so far. This molecular architecture is relatively rigid having
only a few low-energy conformations geometrically quite
different one from another. Conformational analysis of cyclo-
octa-1,5-diene-like ring systems17 (eight-membered rings with
two fixed torsion angles opposite to each other), similar to the
dithia rings of 2, indicates the existence of three low-energy
conformations: boat, chair, and skew (Fig. 2). Combinations
of these three conformations can lead to several possible
low-energy conformations for the tweezers 1, some of which
Table 1
Relative energies (kcal/mol) of the three conformations of 2
PM3
AM1
Boat
Chair
Skew
0.25
0
1.23
0
1.84
0.74
Figure 1. Selected low potential energy conformations of molecular tweezers 1 (not all possible conformations are shown). R¼SAlkyl, R1¼H, OAlkyl.

V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
1911
Thus, if only conformational preferences of the eight-
membered rings were considered, all possible combinations
of boat, chair, and skew conformations could coexist for
molecular tweezers 1. Taking into account the relatively poor
ability of ab initio computational methods, even at high-level,
to predict energies of aromatic stacking interactions,18 model-
ing the full structures with semi-empirical methods only pro-
vides a rough assessment of its geometry showing that all
possible combinations lie within a 2 kcal/mol (AM1) or
4 kcal/mol (PM3) energy window.
2.2. Synthesis and properties
Molecular tweezers 1aec (Scheme 1) were prepared by the
reaction of TTF dithiolates 3a0 and 3b0, generated in situ from
2,3-bis(2-cyanoethylthio)-tetrathiafulvalenes19 3a and 3b, re-
spectively, and tetrabromodurene derivatives 4a and 4b.20,21
Preparations were performed in a similar manner to the previ-
ously reported ones.20,22 Purification by column chromatogra-
phy in dichloromethane/petroleum ether mixtures afforded
pure products with 41e56% yields. Compounds were charac-
terized by means of NMR (1H and 13C), MS (ESI and Hi-Res
1
MALDI), and UV/vis spectroscopy. H NMR did not display
any splitting of the methylene protons of the eight-membered
dithia ring suggesting a fast conformational equilibration at
room temperature. The UV/vis spectra (CH2Cl2, 293 K) of
compounds 1aec featured typical absorption pattern for
tetrathio-substituted TTF derivatives11a,22,23 with absorption
bands at lmax ca. 300 and 330 nm, as well as a shoulder at
ca. 415 nm (Fig. 3a). All three compounds are colored solids
(orange-red or dark yellow) having high (1a) or modest (1b
and 1c) solubility in common non-polar organic solvents
(dichloromethane, toluene), and are easily oxidizable by air
when in solution.
Figure 3. (a) UV/vis spectrum of 1a (CH2Cl2, 2ꢃ10ꢁ5 M, d) and
1a4D$4SbF6L (CH2Cl2, 2ꢃ10ꢁ5 M, /); (b) spectral changes observed for
1a (CH2Cl2, 4ꢃ10ꢁ5 M) upon titration with NOSbF6 up to 2 equiv.
24,500 Mꢁ1 cmꢁ1 per each TTF unit) (Fig. 3a), similar to
5
2D$4SbFL6 with lmax 738 nm (3 22,000 Mꢁ1 cmꢁ1). At room
temperature a dichloromethane solution of 1a could be titrated
without decomposition with up to 2 equiv of NOSbF6 displaying
the development of strong absorption bands with maxima located
at 843 and at 445 nm (Fig. 3b). Both bands are characteristic of
ꢀ
tetrathio-substituted TTFþ radical cations22,24 and similar to
Compound 1a could be oxidized without decomposition by
the addition of 4 equiv of NOSbF6 (added as a solution in
acetonitrile) in degassed stabilizer-free dichloromethane
at ꢁ40 ꢂC giving a dark-blue solution. The 1a4D tetracation
(as well as the 52D dication) was characterized by NMR
(see Supplementary data, Figs. S6 and S7) and did not show
traces of decomposition upon heating up to room temperature.
The UV spectrum of 1a4D$4SbFL6 at room temperature displays
ꢀ
those of 5D , with maxima located at 868 and at 455 nm,
respectively.
2.3. Electrochemistry
Tweezer 1a has been electrochemically studied to examine
the influence of the TTFeTTF interaction on its redox and
a strong absorption band at lmax 728 nm (3 49,000 Mꢁ1 cmꢁ1
,
Scheme 1.

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V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
conformational behavior. Thus, cyclic voltammetry (CV) mea-
surements were carried out in dichloromethane solution and
using tetrabutylammonium perchlorate as the supporting elec-
trolyte, platinum as the working and counter electrode, and
Ag/Agþ as the reference electrode. The cyclic voltammograms
of 1a show the presence of two quasi-reversible electroche-
mical processes on the cathodic scan, the first one splitting into
two relatively well-defined waves at E1/2¼0.30 and 0.42 V,
and the second one at 0.66 V (Fig. 4). Tetrakis(hexylthio)tetra-
thiafulvalene 5 was also electrochemically investigated as
a reference for comparison, undergoing two 1eꢁ reversible oxi-
dation steps (E1/2¼0.34 and 0.63 V), the first leading to the
TTF radical cation and the second to the TTF dication, as it
is known from literature.9 It has been reported that when
two or more TTF moieties in the same molecule are suffi-
ciently close to each other, the first oxidation to the bis(radical
cation) is broadened or split into two steps25 as in our case.
This fact suggests an electronic stabilization of the first-
formed radical cation by the p electrons of the neighboring
TTF moiety leading to a reduction of the potential of the first
1eꢁ oxidation step. Furthermore, the second 1eꢁ oxidation
step becomes more difficult as a result of the proximity of
the first-formed radical cation, thus explaining the cathodic
shift of this process (i.e., the second oxidation is more difficult
due to the proximity of the firstly-formed radical cation).
Therefore, we can conclude that the first two overlapping
waves in the voltammograms of 1a can be assigned to the
1eꢁ sequential formation of 1a mono(radical cation) and bis-
(radical cation), respectively. The reversible second two-elec-
tron process leading to the formation of the dication indicates
that the two chromophores are at larger distance and no longer
in electronic communication,26 whereas the first two 1eꢁ oxi-
dations of 1a indicate that these processes proceed when
both TTF units are in close proximity to each other. Thus,
closed conformation of the molecular tweezer 1a is likely pre-
ferred at least in the mono-oxidized state and cyclic voltam-
metry gives in this way an indication of the conformational
transition to an open conformation due to Coulombic repulsion
between the two doubly charged TTF moieties.
2.4. Variable-temperature NMR (VT-NMR)
NMR of 1a at room temperature (Fig. 5a) displayed
a unique singlet for the eCH2e groups (Hc) located between
the benzene ring and the TTF moiety, suggesting fast confor-
mational equilibration of the two eight-membered rings at
room temperature. To estimate the energy barriers for confor-
mation interconversion of 1a, VT-NMR experiments were
conducted at low temperature in CD2Cl2 solution. The exper-
iments showed that conformational switching froze on an
NMR timescale at about 200 K. The resonance of the Hc methyl-
ene group at 4.34 ppm split into an AB quartet, whereas the
Hf (alkylthio-methylene group) triplet at 2.74 ppm broke up
into two broadened multiplets when temperature reached
203 K (Fig. 5a). The splitting of alkylthio-methylene group
Hf was also confirmed by low-temperature COSY experiments
(Fig. 5b), which displayed strong cross-peaks for two broad-
ened multiplets at 2.45 and 2.78 ppm. The inequivalence of
the two protons of the alkylthio-methylene group at low tem-
perature indicates that the free rotation of the thioalkyl chain is
frozen suggesting a stacking of the TTF moieties. Thus, the
splitting patterns of Hc and Hf protons confirm the preferential
alignment of 1a in the closed tweezers-like conformation with
tight contacts between the TTF flaps at low temperature. NMR
spectra at low temperatures did not display the presence of any
other conformations of 1a, testifying that the closed conforma-
tion is the most energetically preferred one. By monitoring the
signals of Hc and Hf (coalesced at 209 and 211 K, respec-
tively) the activation free enthalpy DGz for the conformational
equilibration of 1a was estimated as 10.1ꢄ0.5 kcal/mol using
methods of dynamic NMR spectroscopy.27 Taking into ac-
count that conformational averaging can proceed through
boateskeweboat17a conformational introversion of the eight-
membered ring with the activation energy of the process lying
in the range from 1.1 to 7.5 kcal/mol,17 we can deduce that the
attractive TTFeTTF interaction increases the activation free
enthalpy by 2.6e9 kcal/mol. It is reasonable to assume that
this raise of activation free enthalpy DGz largely represents
the magnitude of the pairing interactions between the two
TTF units in CD2Cl2. More precise experimental and theoreti-
cal investigations of the conformational processes of the flexi-
ble structural elementdeight-membered ringdare in progress
now to increase the accuracy of the assessment.
Unlike as for pure 1a, VT-NMR scans of 1a4D$4SbF6L in
CD2Cl2 displayed a sharp singlet for the Hc methylene
protonsdan evidence for fast conformational equilibration
on the NMR timescale (see Supplementary data, Fig. S8) up
to 213 K.
2.5. Molecular modeling of the oxidized tweezers
To explain the conformational behavior of the oxidized
tweezers 14D (R, R1¼H) additional molecular modeling stud-
ies were performed. Despite semi-empirical optimization did
Figure 4. Comparative CV of the tweezers 1a and tetrakis(hexylthio)tetrathia-
fulvalene 5 (CH2Cl2, 20 ꢂC, 0.1 M Bu4NPF6, scan rate 200 mV/s).

V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
1913
Figure 6. Comparison between closed conformation of 1 and several stable
open conformations of 14D (R, R1¼H). Geometry minimization was per-
formed using PM3 method.
which have one or both of the eight-membered rings in the
boat conformation and thus relatively short TTFeTTF dis-
tances, were driven to one of the stable conformations during
geometry optimization. Electrostatic repulsion is the stabiliz-
ing force for the open conformations of 14D as it is clearly
seen (Table 2) from the energy difference between anti- and
˚
syn-chairechair conformations: a reduction of ca. 0.7 A in
the distance between the charged TTF moieties costs about
2.5 kcal/mol in energy. Thus, several low-lying conformations
for the tetracation 14D were theoretically predicted. Intercon-
version between them, that can proceed through relatively
low-barrier transition states, proceeds fast on the NMR time-
scale at 193 K and higher temperatures and leads to averaging
of Hc and Hf protons.
The preferential conformation of the dication 12D in solu-
tion can only be speculated about. Although initially it was
claimed that cation radicals of TTF exist mostly in dimerized
ꢀ
form (TTFþ )2 forming so called p-dimers,25c it was reported
later24a that dimerization of tetrathioalkyl radical cations could
not be detected in solution. Dimerization of non-substituted
ꢀ
TTFþ radical cations was observed at low temperatures by
ESR24a as well as in solid state by X-ray crystallography,16b,c
displaying fairly short average interplanar distances of
˚
3.32e3.38 A. Spatial preorganization of TTF groups in 1 is
likely to facilitate their stacking even in solution, where elec-
trostatic repulsion between the charged moieties cannot be
compensated by the adjacent counter ions. Future ESR mea-
surements are likely to clarify conformational preferences of
the oxidized states.
Table 2
Relative energies and average TTFeTTF distancesa of the low-energy confor-
Figure 5. (a) Variable-temperature 1H NMR spectra of tweezer 1a (CD2Cl2,
2 mM) showing the transition from the dynamically flexible structure
(293 K) to the tightly closed conformation (193 K); (b) low-temperature
COSY spectrum of 1a (193 K, 360 MHz, CD2Cl2).
mations of 14D (R, R1¼H)
PM3
AM1
Energy,
kcal/mol
Distance,a
˚
A
Energy,
kcal/mol
Distance,
˚
A
not show significant energy preference for any of the confor-
mations of 1, for 14D only the conformations with large dis-
˚
tance between charged TTF moieties (>15 A) would be
energetically stable (Fig. 6): chairechair (both syn- and
anti-), skeweskew (both syn- and anti-rotational), and
chaireskew all lying within 2.5 kcal/mol (AM1) or 4 kcal/
mol (PM3) energy range (Table 2). Other conformations,
anti-Chairechair
syn-Chairechair
Chaireskew
syn-Skeweskew
anti-Skeweskew
a
0
16.01
15.22
16.05
16.43
16.43
0
15.70
15.07
15.74
16.05
16.04
2.52
2.36
3.65
3.75
2.45
0.54
0.17
0.23
Distance measured between the centers of the inner double bonds of TTF
moieties.

1914
V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
Another open question is how energetically strong can the
chromatography (FC) was carried out using silica gel 60
from Fluka 230e440 mesh (particle size 36e70 mm).
stroke of the opening motion be if we consider the opening
motion from the closed to one of the open conformations
upon oxidation to tetracation. Rough estimations using the
PM3 method showed that the energy of the stroke arises to
ca. 70 kcal/mol, if the minimized closed conformation of 1
is considered as a starting pointdan energy amount almost
equal to the average strength of a carbonecarbon bond.
Such a significant energy output can make 1a a valuable
switching building block for the construction of functional
dynamic molecular architectures.
4.2. Synthetic procedures
4.2.1. Synthesis of compounds 1, general procedure
TTF derivative 3 (0.4 mmol) was dissolved in dry DMF
(8 mL) and degassed by a freezeepumpethaw cycle; then
CsOH (0.84 mmol, 1.68 mL) was added as 0.5 M solution in
MeOH at 0 ꢂC. The mixture was allowed to warm to room tem-
perature and stirred for 30 min, turning from orange to dark
brown-red in color. Compound 4 (0.2 mmol) was dissolved in
dry THF (4 mL), the solution was degassed by a freezee
pumpethaw cycle. Both solutions were simultaneously added
in 3e4 equal portions to a degassed DMF/THF 2:1 mixture
(15 mL) with intervals of 10 min while vigorously stirring.
The reaction mixture turned orange-yellow, sometimes a pre-
cipitate formed. The mixture was allowed to stir for additional
30 min, and then was evaporated to dryness, the residue was
redissolved in CH2Cl2, and the solution was washed with water
and brine, dried (Na2SO4), and evaporated. The residue was
purified by flash chromatography on silica gel.
3. Conclusions
In conclusion, we have presented new molecular tweezers
containing two TTF units and capable of reversible large-scale
switching between two profoundly different conformations
upon oxidation/reduction. The new functional molecular ar-
chitecture is a perspective scaffold for the construction of elec-
trochemically responsive molecular machines and, after proper
modification, also of tweezers-type molecular receptors. The
preorganization of TTF moieties in these systems gives a
unique opportunity to investigate the TTFeTTF dimerization
process in solution and estimate the TTFeTTF pairing energy.
The system also opens an opportunity for further studies
of TTFeTTF interactions in the mixed-valence dimers
4.2.2. Compound 1a
Chromatography (CH2Cl2/PE 3:1) afforded an orange-red
product in 56% yield. Mp: 102e104 ꢂC. Rf¼0.38 (SiO2;
1
CH2Cl2/PE 2:3). H NMR (200 MHz, CDCl3, 25 ꢂC): d 0.87
ꢀ
ꢀ
[(TTF)2]þ and cation dimers (p-dimers) ([TTF]þ )2 using
(12H, t, J¼6.6 Hz, Hg), 0.96 (6H, t, J¼7.0 Hz, Hk), 1.22e1.65
(44H, m), 1.88 (4H, quint., J¼7.2 Hz, Hi), 2.74 (8H, t, J¼
7.0 Hz, Hf), 4.13 (4H, t, J¼6.8 Hz, Hh), 4.32 (8H, s, Hc). 13C
NMR (50 MHz, CDCl3, 25 ꢂC): d 14.23, 14.37, 22.73, 22.90,
25.85, 28.43, 29.87, 30.43, 31.52, 31.97, 32.22, 36.51, 77.42
(Cc), 109.20, 115.66, 127.63 (Ce), 129.50 (Cd), 130.32 (Cb),
152.19 (Ca). UV/vis (CD2Cl2): lmax (3) 307 (31,300), 331
(28,800), 410 (sh., 3800) nm. HRMS (MALDI, DHB): calcd
for C58H86O2S16 [Mþ] 1326.2157, found 1326.2097. NMR
assignment was performed on the basis of HMBC spectrum
(see Supplementary data, Fig. S2).
spectroelectrochemical and ESR methods.
4. Experimental section
4.1. General
Reagent grade chemicals and solvents were used without
further purification unless otherwise stated. All reactions
were carried out under Ar or N2 atmosphere. 1H and 13C
NMR spectra were recorded with Bruker Avance DPX-200
and Bruker Avance WB-360 spectrometers. Chemical shifts
(d) are reported in parts per million (ppm) downfield from
tetramethylsilane using the residual solvent peak as internal
reference: CDCl3 (7.26 ppm for 1H, 77.23 ppm for 13C),
CD2Cl2 (5.32 ppm for 1H). Variable-temperature NMR
(VT-NMR) measurements were performed on Bruker Avance
WB-360 using internal temperature reference. ESI-MS spectra
were measured on Bruker Esquire LC spectrometer using
direct injection method and MALDI spectra were measured
on PerSeptive Biosystems Voyager DE Pro spectrometer with
2,5-dihydroxybenzoic acid used as a matrix. UV/vis spectra
were recorded with Varian Cary 50 Conc spectrophotometer;
oxidative titrations of TTF derivatives were performed in
stabilizer-free degassed dichloromethane by the addition of
0.01 or 0.02 M NOSbF6 solution in acetonitrile. Melting
points (mp) were obtained with an electrothermal capillary
melting-point apparatus and were not corrected. Thin layer
chromatography (TLC) was performed on silica gel aluminum
cards from Fluka with F-254 fluorescent indicator. Flash
4.2.3. Compound 1b
Chromatography (CH2Cl2) afforded an orange-red product in
41% yield. Mp: 220e223 ꢂC (decomp.). Rf¼0.71 (SiO2; CH2Cl2/
1
PE 2:1). H NMR (200 MHz, CDCl3, 25 ꢂC): d 0.87 (12H, t,
J¼6.8 Hz), 1.24e1.65 (32H, m), 2.75 (8H, t, J¼7.0 Hz), 4.27
13
(8H, s), 7.07 (2H, s). C NMR (90 MHz, CDCl3, 25 ꢂC):
d 14.20, 22.77, 28.45, 29.92, 31.54, 36.60, 38.10, 127.79,
129.77, 132.91, 135.25. UV/vis (CD2Cl2): lmax (3) 297
(29,900), 336 (30,300), 410 (sh., 4800) nm. HRMS (MALDI,
DHB): calcd for C46H62S16 [Mþ] 1126.0384, found 1126.0488.
4.2.4. Compound 1c
Chromatography (CH2Cl2/PE 3:1) afforded a dark yellow
product in 42% yield. Mp: 236e239 ꢂC (decomp.). Rf¼0.2
1
(SiO2; CH2Cl2/PE 1:1). H NMR (200 MHz, CDCl3, 25 ꢂC):
d 0.96 (6H, t, J¼7.0 Hz), 1.32e1.62 (12H, m), 1.89 (4H,
13
quint., J¼7.2 Hz), 2.35 (12H, s), 4.16 (4H, t, J¼6.8 Hz),
4.32 (8H, s). C NMR (50 MHz, CDCl3, 25 ꢂC): d 14.40,

V.A. Azov et al. / Tetrahedron 64 (2008) 1909e1917
1915
19.29, 22.93, 25.89, 30.46, 32.01, 77.43, 110.03, 115.43,
127.32, 129.23, 130.26, 152.28. UV/vis (CD2Cl2): lmax (3)
305 (29,100), 328 (25,000), 410 (sh., 3900) nm. HRMS
(MALDI, DHB): calcd for C38H46O2S16 [Mþ] 1045.9028,
found 1045.8758.
populated degenerate two-site uncoupled system and Eq. 2
was used for a coalescing AB system with an exchange
between A and B protons:27
DGzc ¼ aT½9:972 þ logðT=dnÞꢅ
ð1Þ
ð2Þ
h
ꢀ
ꢃi
ꢁ
ꢂ
4.2.5. 2,3,6,7-Tetrakis(hexylthio)tetrathiafulvalene 525g
Orange-red crystalline solid. Mp: 26e28 ꢂC. 1H NMR
(200 MHz, CD2Cl2, 25 ꢂC): d 0.89 (12H, t, J¼6.6 Hz),
1.22e1.47 (24H, m), 1.63 (8H, quint., J¼7.4 Hz), 2.82 (8H,
t, J¼7.4 Hz). UV/vis (CD2Cl2): lmax (3) 311 (15,600), 332
(15,700) nm.
1=2
DGzc ¼ aT 9:972 þ log T= dn2 þ 6JA2B
T is the temperature in Kelvin, dn [Hz] the chemical shift
between two coalescing resonances in the absence of ex-
change, J is a coupling constant for AB system, and
a¼4.575ꢃ10ꢁ3 for DGzc in kcal/mol. Coalescing Hf broadened
multiplets (for assignment see Fig. 5a) were treated as broad-
ened singlets using Eq. 1, whereas for coalescing AB system
of Hc Eq. 2 was used. Activation energies obtained using Hf
and Hc laid within 0.3 kcal/mol.
4.2.6. Oxidation of TTF derivatives, general procedure
A solution of the corresponding TTF derivative (1e2 mM)
in CD2Cl2 was degassed by a freezeepumpethaw cycle,
cooled to ꢁ40 ꢂC, and a 0.1 M solution of NOSbF6 in
CD3CN (2 equiv for each TTF unit) was added dropwise.
After 30 min the solution was degassed one more time to
remove traces of NO and sealed in an NMR tube.
4.5. Molecular modeling
Molecular modeling was performed using HyperChem
Release 7.0 for Windows (HyperCube, Inc.). Conformational
searches were done using MMþ force field using analogues
with O-substituted eight-membered rings due to inadequate
parametrization for sulfur atoms in the force field. Geometry
of low-lying conformations was further optimized after the
substitution of O-atoms with S-atoms using semi-empirical
AM1 and PM3 (RHF) methods.
4.2.7. Compound 14þ$4SbF6ꢁ
Deep blue in CD2Cl2 solution. 1H NMR (360 MHz, CD2Cl2,
25 ꢂC): d 0.90 (12H, t, J¼7.0 Hz, Hg), 0.97 (6H, t, J¼7.2 Hz,
Hk), 1.32e1.61 (36H, m), 1.87e1.96 (12H, m), 3.45
(8H, t, J¼7.2 Hz, Hf), 3.89 (4H, t, J¼6.4 Hz, Hh), 4.74
(8H, s, Hc). UV/vis (CD2Cl2): lmax (3) 307 (24,000), 728
(49,000) nm.
Acknowledgements
4.2.8. 2,3,6,7-Tetrakis(hexylthio)tetrathiafulvalene, bis-
hexafluoroantimonate 52þ$2SbF6ꢁ
Deep blue in CD2Cl2 solution. 1H NMR (200 MHz,
CD2Cl2, 25 ꢂC): d 0.90 (12H, t, J¼6.8 Hz), 1.26e1.58 (24H,
m), 1.90 (8H, quint., J¼7.4 Hz), 3.42 (8H, t, J¼7.0 Hz).
UV/vis (CD2Cl2): lmax (3) 748 (22,000) nm.
The work was financially supported from a grant of Fonds
der Chemischen Industrie (V.A.A.). R.G. is indebted to the
´
‘Programa Ramon y Cajal’ for support. Help of Dr. Thomas
Dulcks and Ms. D. Kemken (MS) is gratefully acknowledged.
¨
We also thank Prof. J. L. Segura and Prof. W. -D. Stohrer for
helpful discussions.
4.3. Cyclic voltammetry
Supplementary data
Experiments were performed with a computer controlled
Autolab potentiostat in a three-electrode single-compartment
cell (5 mL). The platinum working electrode consisted of a
platinum wire sealed in a soft glass tube with a surface of A¼
0.785 mm2, which was polished down to 0.5 mm with Buehler
polishing paste prior to use in order to obtain reproducible sur-
faces. The counter electrode consisted of a platinum wire and
the reference electrode was an Ag/AgCl secondary electrode.
For the measurements, concentrations of 5ꢃ10ꢁ3 mol Lꢁ1 of
the electroactive species were used in freshly distilled and
deaerated dichloromethane (Lichrosolv, Merck) and 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6, Fluka),
which was twice recrystallized from ethanol and dried under
vacuum prior to use.
Supplementary data (1H and 13C NMR and ESI-MS of the
1
new compounds, H NMR of 1a4D$4SbFL6 and 52D$2SbFL6
as well as VT-NMR of 1a4D$4SbFL6 ) associated with this
article can be found in the online version, at doi:10.1016/
j.tet.2007.11.110.
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