Electron transfer through molecular bridges between reducible pentakis(thiophenyl)benzene subunits

Article abstract of DOI:10.1002/1521-3765(20010316)7:6<1266::AID-CHEM1266>3.0.CO;2-S

"Dimers" 3, 4 and 7, which consist of two reducible pentakis(thiophenyl)benzene subunits linked by different molecular structures, have been synthesised as model compounds for reducible molecular-wire-type synthons to represent differences in the electron

Full text of DOI:10.1002/1521-3765(20010316)7:6<1266::AID-CHEM1266>3.0.CO;2-S

FULL PAPER
Electron Transfer Through Molecular Bridges Between Reducible
Pentakis(thiophenyl)benzene Subunits
Marcel Mayor,*[a] Michael Büschel,[c] Katharina M. Fromm,[d] Jean-Marie Lehn,*[a, b]
and Jörg Daub[c]
Abstract: ªDimersº 3, 4 and 7, which
consist of two reducible pentakis(thio-
phenyl)benzene subunits linked by dif-
ferent molecular structures, have been
synthesised as model compounds for
reducible molecular-wire-type synthons
to represent differences in the electron-
transfer ability as a function of the
bridging structure. The bridging units
consist of para-divinylbenzene in 3, bis-
hydrazone in 4 and diacetylene in 7.
Their ability to transfer electrons from
one reducible subunit to the other was
investigated by electrochemical and
spectroelectrochemical methods and, in
the case of 4 and 7, the solid-state
structures support the experimental
bridge in 3 was found to completely
isolate the reducible structures (Class I
system). In contrast, the diacetylene
bridge in 7 electronically connects the
two reducible structures (Class III sys-
tem) and, thus, demonstrates its poten-
tial application as a ªmolecular wireº.
The bis-hydrazone-linked compound 4
displayed only a low level of electronic
connection between the subunits and
was only observed in the spectroelec-
trochemical investigation.
findings.
The
para-divinylbenzene
Keywords: conjugation
transfer ´ mixed-valent compounds
molecular wires polythio-
benzenes
´ electron
´
´
trical circuits or single-electron computing devices, depends
strongly on the postulate that it may be possible to move
single electrons between individual compartments.[8, 9] This
simple function, namely the transfer of electrons from one
subunit to another, has been a topic of interest for quite some
time. Since the advent of the mixed-valence chemistry in
1967,[10] numerous structures have been studied that contain
two or more active redox units, both inorganic[2] and organic,
that are connected by molecular structures; their electron
transport features have been discussed and analysed at
various levels of detail.
Introduction
Currently, a growing interest has been directed towards the
design of molecular wires[1±4] and rods,[5, 6] since such struc-
tures may serve as building blocks for nanoscale chemical
entities that are geometrically and dimensionally confined.
They may act as connectors in molecular and supramolecular
electronic and photonic devices.[7] A molecular bottom-up
approach to complex functional entities, for example, elec-
Molecular wires based on pyridinium end-groups and
carotenoid-type conjugated chains (termed caroviologens)
have already been investigated in our laboratory and have
been shown to induce electron transport through lipid vesicle
membranes.[11] The structural reorganisation of anthracene-
bridged stilbenoids by oxidation and reduction has also been
studied.[12] Most of the molecular wires based on organic
systems were, however, focused on electron-rich, oxidisable
structures, such as thiophenes,[13] nitrogen-based mixed-va-
lence systems,[14] pyrroles[15] and so forth. These oxidisable
structures loose electrons and the mobility of the holes
generated in the molecular structures was investigated.
Reducible subunits take up additional electrons in their
LUMO, which may lead to higher electronic mobility in the
connecting molecular structure.
[a] Dr. M. Mayor, Prof. J.-M. Lehn
Forschungszentrum Karlsruhe GmbH
Institut für Nanotechnologie
Postfach 3640, 76021 Karlsruhe (Germany)
Fax : (‡49)7247-82-6369
E-mail: marcel.mayor@int.fzk.de
lehn@chimie.u-strasbg.fr
[b] Prof. J.-M. Lehn
Â
ISIS, Universite Louis Pasteur
4 rue Blaise Pascal, 67000 Strasbourg (France)
[c] Dipl.-Chem. M. Büschel, Prof. J. Daub
Institut für Organische Chemie
Universität Regensburg
93040 Regensburg (Germany)
E-mail: joerg.daub@chemie.uni-regensburg.de
[d] Dr. K. M. Fromm
Â
Â
Departement de Chimie Minerale
Â
Â
Á
Analytique et Appliquee, Universte de Geneve, Sciences II
30 Quai Ernest-Ansermet, 1211 Geneva 4 (Switzerland)
E-mail: katharina.fromm@chiam.unige.ch
Poly(phenylthio)-substituted aromatic systems are well-
known for their host ± guest chemistry in the solid state[16] and
1266
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1266 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 6

1266±1272
have been shown to display remarkably low reduction poten-
tials.[17] Therefore, they appear to be promising candidates to
study the electron-transfer ability of molecular structures that
contain connected reducible subunits. They have already been
incorporated as electron-acceptor sites in a macrobicyclic
ligand,[18] in reducible rod-like molecular wires[19] and in
reducible nanosized macrocycles based on meta-diacetylene-
linked poly(phenylthio)-substituted benzenes.[20]
We chose two pentakis(thiophenyl)benzene units connect-
ed by different conjugated p systems as bridges as the model
compounds to investigate the electron-transfer ability in
molecular structures that contain reducible subunits. A
para-divinylbenzene bridge (1,4-bis(ethenyl)benzene-2',2''-
diyl) in 3, a bis-hydrazone bridge (2,3-diazabuta-1,3-dien-
1,4-diyl) in 4 and a diacetylene bridge (buta-1,3-diyne-1,4-
diyl) in 7[19] were investigated.
Starting with commercial pentafluorobenzaldehyde (1), the
fluorine atoms were substituted by thiophenyl sodium salt in
1,3-dimethylimidazolidin-2-one (DMI) to give the pentakis-
(thiophenyl)benzaldehyde (2) in 75% yield. Compound 2 is
the starting material for all three compounds 3, 4 and 7.
Treatment of 2 with para-xylylenebis(triphenylphosphoni-
um bromide) in presence of a base gave the para-divinylben-
zene-bridged trans,trans-1,4-bis-{[2'-(2'',3'',4'',5'',6''-pentathio-
phenyl)phenyl]ethenyl}benzene (3) in 49% yield. The trans,-
trans configuration of the bridging structure is proved by the
coupling constant of J ˆ 16 Hz between the ethenyl protons at
d ˆ 6.86 and 6.49, respectively. Compound 3 is a bright yellow
solid and soluble in organic solvents, such as dichloromethane,
chloroform, diethyl ether and toluene.
Treatment of 2 with hydrazine hydrate in acetonitrile
heated under reflux gave trans,trans-decathiophenylbenzal-
dehyde azine 4 in 32% yield as a light yellow precipitate. The
trans,trans configuration of the bridging structure was proved
by single-crystal X-ray analysis. The light yellow solid is
soluble in organic solvents, such as dichloromethane, chloro-
form, diethyl ether and toluene. Single crystals suitable for
X-ray analysis were obtained by slow diffusion of hexane into
a solution of 4 in chloroform.[21]
Herein we describe the synthesis of the model compounds
3, 4 and 7. Their electrochemical and spectroelectrochemical
properties as well as, in part, their solid-state structures, are
reported and discussed from the point of view of a possible
electron transfer through the bridging structure.
In a Corey ± Fuchs reaction sequence, the aldehyde 2 was
first converted to the dibromoolefin 5:[22] a CBr4/PPh3 solution
was added to a solution of 2 in acetonitrile heated under
reflux, and the desired dibromoolefin 5 was isolated in 69%
yield. In later experiments, the reaction was repeated in dry
dichloromethane at room temperature without changes in
yield or quality of the product. Compound 5 is a brown-yellow
solid that is very soluble in aprotic organic solvents. It is well
characterised by 1H and 13C NMR spectroscopy and FAB-MS.
Results and Discussion
Synthetic procedures: The new compounds 3 and 4, along
with the previously described compound 7,[19] each consisting
of two reducible pentakis(thiophenyl)benzenes separated by
different molecular spacers, were synthesised as shown in
Scheme 1. All new compounds were characterised by 1H and
13C NMR spectroscopy, FAB-MS and microanalysis, as well as
by single-crystal X-ray analysis for compounds 4 and 7.
‡
‡
Scheme 1. Synthesis of the model compounds 3, 4 and 7. Reaction conditions: a) DMI, PhSNa, RT; b) NaOH/CH2Cl2, Ph3P CH2(p-C6H4)CH2P Ph3 ´ 2Br ;
c) CH3CN, H2NNH2 ´ H2O, reflux; d) CH3CN, CBr4, PPh3, RT; e) THF, nBuLi, 788C, H2O, RT; f) C6H5N, CuAc2 ´ H2O, 55 8C.
Chem. Eur. J. 2001, 7, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1267 $ 17.50+.50/0
1267

M. Mayor et al.
FULL PAPER
Table 1. Cyclic voltammetry data for C6(SPh)6, 3, 4 and 7.[a]
However, several attempts to prepare a sample of 5 suitable
for microanalysis failed. All microanalyses of 5 were consis-
tently too high in carbon. Whether this error is caused by
impurities or by the burning/combustion behaviour of 5
during the analysis could not be clarified. However, the
quality of 5 was sufficient for the envisaged synthetic target
and was therefore used without further purification. The
acetylenic derivative 6 was prepared by reaction of 5 with
n-butyllithium:[22] a solution of 5 in THF was treated with a
2.5 fold excess of n-butyllithium in hexane at 788C and
quenched with aqueous ammonium chloride to give com-
pound 6 in 65% yield. 1-Ethinyl-2,3,4,5,6-pentakis(thiophen-
yl)benzene (6) is a yellow solid that is soluble in aprotic
solvents. Oxidative coupling of 6 gave compound 7:[23]
compound 6 was heated to 558C with one equivalent of
copper acetate monohydrate in pyridine to give the diacety-
lene-bridged 7 in 65% yield as an intense orange solid that is
moderately soluble in aprotic organic solvents, such as
chloroform, methylene chloride, toluene or tetrahydrofuran.
Crystallisation from hot toluene gave single crystals suitable
for X-ray analysis.
'
'
Compound
Eo2 [V][b]
Eo2 [V][b]
C6(SPh)6
1.56[c]
1.39[d]
1.19[d]
1.12[e]
±
±
±
3
4
7
1.38[e]
[a] Platinum working electrode in 0.1m TBAPF6/DMF; scan rate:
‡
‡
100 mVs 1. [b] In V versus SCE (Fc /Fc couple with E8'(Fc /Fc) ˆ 0.5 V
versus SCE as internal standard), formal reduction potential E8' ˆ (Epa
‡
Epc)/2. [c] See Ref. [17]. [d] Two-electron reduction wave. [e] See Ref. [19].
only one reduction wave corresponding to two electrons at
1
1.19 Vat a scan rate of 100 mVs . A more detailed analysis
of the reduction wave showed a marginal shift of the reduction
potential to more negative values with increasing scan rate
1
(7 mV per 100 mVs ). However, the observation of a single
reduction wave indicates that the two redox centres are
independent and the bridging structure behaves rather as a
non-conjugated spacer. In the classification for mixed-valence
compounds from Robin and Day, compounds 3 and 4 with
isolated redox centres are Class I derivatives.[10]
The diacetylene-linked subunits in compound 7 display two
reversible one-electron reduction waves at a formal potential
of 1.12 and 1.38 V versus SCE, indicating that the first
reduction yields a mono-anion stabilised by delocalisation
over both electron-accepting subunits. The separation between
Electrochemical studies: The redox behaviour of compounds
3, 4 and 7, which consist of two reducible subunits connected
by a conjugated p system as a bridging unit is of particular
interest, as it will reveal the electron-transfer ability of the
different bridging structures and thereby their potential use as
ªmolecular wiresº between reducible subunits.
.
the waves of 260 mV indicates that the mono-anion 7 is
thermodynamically stabilised towards disproportionation to
give 7 and 72 by the comproportionation constant of Kc ˆ
2.55 Â 104 which suggests 7 to be a Class II derivative.[10, 24] The
diacetylene bridge connects the two subunits and facilitates active
electron transfer: it displays its ability to function as electron-
conducting spacer unit for molecular wire applications.
A more detailed analysis of the delocalisation of the first
reducing electron over the ªdimericº structures 3, 4 and 7
The electrochemical properties of compounds 3, 4 and 7
were investigated by cyclic voltammetry (CV) at room
temperature in anhydrous dimethylformamide (DMF) with
tetra-n-butylammonium
hexafluorophosphate
(0.1m
TBAPF6) as the supporting electrolyte. A three-electrode
set-up was used with a platinum disk as the working electrode,
a platinum wire as the counter-electrode and a silver wire as
the dummy reference electrode. All samples were used at a
concentration of 5 Â 10-3 m and the formal reduction potentials
((Epa ‡ Epc)/2) were determined with respect to the ferroce-
.
.
requires investigations on the single reduced species 3 , 4
.
and 7 , respectively, which were performed by spectroelec-
‡
‡
trochemical techniques.
nium/ferrocene (Fc /Fc) couple with E8'(Fc /Fc) ˆ 0.5 V
versus SCE as the internal reference. The criterion applied
for reversibility was 1.0 Æ 0.5 for the ratio of the difference
peak potential and the half-peak potential of the compound
and ferrocene in the same measurement and no shift of the
Spectroelectrochemical studies: UV/VIS/NIR spectroelectro-
chemical investigations were performed under the conditions
for electrochemistry (DMF, 0.1m TBAPF6, RT) in a cell
described elsewhere.[25]
1
half-wave potential with varying scan rates (10 ± 250 mVs ).
The relative numbers of electrons were determined by
measurements at limiting current with a rotating disk
electrode. The plateau currents were compared to the plateau
current of hexakis(para-tert-butylphenylthio)benzene as the
internal standard for a one-electron reduction, assuming
similar diffusion coefficients of the compound and the
internal standard. The values are given in Table 1 as formal
reduction potentials with respect to SCE.
The parent monomeric compound hexakis(phenylthio)ben-
zene displays a reversible one-electron reduction at 1.56 V
versus SCE under the same conditions.[17, 19] The two para-
divinylbenzene-bridged reducible subunits in compound 3
display only one reversible reduction wave corresponding to
two electrons at a formal potential of 1.39 V versus SCE.
The bis-hydrazone-linked subunits in compound 4 display
The radical anion of the parent monomeric compound
hexakis(phenylthio)benzene displays two absorption maxima
at l ˆ 420 and 590 nm and both have a bathochromic shift
with respect to the lowest energy absorption maximum of the
neutral species at l ˆ 343 nm. The uniformity of the process is
demonstrated by isosbestic points at l ˆ 313 nm and 379 nm,
and the reversibility, already observed in the CV experiment,
is confirmed by recovery of the starting spectrum after
reoxidation.
The para-divinylbenzene-bridged 3 displays only one
reversible two-electron reduction wave in the CV experiment.
The stepwise coulometric reduction to the dianion 32 is
shown by the increase in the intensity of the lowest energy
absorption at l ˆ 861 nm and a concomitant decrease of the
absorption at 331 nm from the neutral 3 (Figure 1). Isosbestic
1268
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1268 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 6

Molecular Wires
1266±1272
Figure 1. Spectroelectrochemistry of the reduction of 3 to 32 . Evolution of
the UV/Vis/NIR spectrum; potential range: 1150 to 1300 mV.
points at l ˆ 298 and 418 nm show the uniformity of the
process, while the recovered starting spectrum after reoxida-
tion documents its reversibility. As already concluded from its
CV, the two subunits are independent and, therefore, it can be
confirmed that 3 is a Class I derivative.[10]
The spectroelectrochemical investigations of bis-hydrazone
4 performed in DMF demonstrate the limited reversibility of
the reduction process within duration of the experiment
1
(30 spectra each at 480 nmmin ). The original spectrum is
not completely restored. The CV of 4 displays only one
reduction wave, which suggests that the two reducible units
are electronically independent. However, it is interesting to
note that two reductive processes can be detected in the
spectroelectrochemical experiment: the first reduction step
leads to new absorption bands at l ˆ 517, 622 and 1250 nm.
The long-wavelength absorption at 1250 nm may either be
attributed to a radical anion, or less likely, to a dianionic
species. Based on structural considerations, we assign this
band to a compound in which the central diazabutadiene
spacer group acquires features of an azo group. Thus, the
spacer unit is strongly reorganised in the first reduction step so
that the second reduction step is much easier and very fast.
This is in agreement with electrochemical studies and
demonstrates that an increase in the scan rate causes the
reduction wave to be slightly displaced to more negative
values. Additionally, the ratio of anodic and cathodic currents
is independent of the scan rate. The assignment to a radical
anion is further supported by the disappearance of this long-
wavelength absorption on extended reduction and leads to
new absorption bands at l ˆ 295, 409 and 765 nm. The limited
reversibility may be caused by protonation or elimination
reactions.[26] Therefore, upon reduction of 4, negative charge is
not exclusively transferred into the reducible subunits, but to
some extent into the spacer group.
Figure 2. Spectroelectrochemistry of the reduction of 7 to 72 . Top:
.
Evolution of the UV/Vis/NIR spectrum upon reduction of 7 to 7
;
potential range: 900 to 1070 mV. Bottom: Evolution of the UV/Vis/
.
NIR spectrum upon further reduction of 7 to 72 ; potential range: 1100
to 1300 mV.
.
.
Similar to 4 , the lowest energy absorption of 7 at 1310 nm
may be assigned to an intervalence charge-transfer (IV-CT)
band. The uniformity of the process is demonstrated by
isosbestic points at 298 and 457 nm, and the reversibility
already observed in the CV experiment is confirmed by
recovery of the starting spectrum after reoxidation.
On further reduction to the dianion 72 , the band at lˆ600 nm
and its shoulders, as well as the broad band at 1310 nm,
decrease, while the absorption spectrum of 72 appears with
absorptions at l ˆ 267 and 294 nm and an intense lowest
energy band at 774 nm with two shoulders at 662 and 706 nm.
Again, the isosbestic points at 317, 627 and 922 nm indicate
the uniformity of the reduction process, and the regeneration
of the starting spectrum documents its reversibility.
.
Analysis of the UV spectrum of 7 provides further insight
into the electronic coupling in the mono-reduced state. For
both Class II and III compounds, an absorption band in the
near infrared region with low intensity is expected. For a
Class II system, the electronic coupling integral H can be
calculated according to the Hush approach.[27] For Class III
systems with strong coupling, delocalisation occurs.
The two separated reduction waves of the diacetylene-
linked 7 observed by CV indicates the formation of a stable
.
radical anion 7 allowing its spectroscopic characterisation
(Figure 2). The spectroelectrochemical investigation confirms
this finding and displays, as expected for coupled subunits, two
separated reduction steps. During the first reduction process,
the absorption bands of 7 at l ˆ 346 and 407 nm vanish, while
A plot of absorbance versus wavenumber of the low-energy
.
transition at l ˆ 1310 nm of 7 leads to an unsymmetrical line
.
bands of the radical anion 7 at l ˆ 281, 600 and 1310 nm
shape with vibrational progression and an extinction coeffi-
1
appear. The 600 nm band has shoulders at 503 and 557 nm.
cient of 19500m 1 cm (Figure 3). Consequently, the band
Chem. Eur. J. 2001, 7, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1269 $ 17.50+.50/0
1269

M. Mayor et al.
FULL PAPER
.
Figure 3. Plot of absorbance (A) versus nÄ for the IT band of 7
.
shape is in agreement with either a vertical transition in a
ClassIII system with a large vibrational progression or a cut-
off ClassII IV-CT band. Recently, Nelsen has analysed
spectra of intervalence systems at the Class II/III borderline
in detail.[28] The high extinction coefficient and the vibrational
.
progression of the n ˆ 7500 cm 1 absorption of 7 account for
an almost delocalised system, which has already passed the
Class II/III borderline.
The spectra of the three doubly reduced compounds 32 , 42
and 72 resemble each other with absorptions at l ˆ 861, 765
and 774 nm, respectively. The hypsochromic shift of 72
compared to 32 is attributed to a more extended delocalisa-
tion in 32 .
Figure 4. Crystal structures of 4 (top) and 7 (bottom).
of the terminal subunits. The planes of both benzene rings
ˆ
form an angle of 728 with the plane of the C N bond, so that
Crystal structure of compounds 4 and 7: The solid-state
structures of ªdimersº 4 and 7 were determined by X-ray
diffraction and are shown in Figure 4. They offer a deeper
understanding of the ability of the reducing electrons to move
from one subunit to the other. The reducing electron will, by
definition, occupy the LUMO of the molecule which is part of
the p system. Good electronic communication between
subunits in a compound requires that their individual p
systems have a considerable overlap. Particular attention will
therefore be focused on the structural features of the p
systems of the reducible subunit and the connected bridging
structures.
only a weak conjugation of these p systems is expected for the
neutral species. This is confirmed by the CV experiments
discussed above and the observable amounts of 4 may be the
.
result of structural changes in the singly reduced species. The
alternating ªupº and ªdownº pattern (ududu) of the phenyl-
thio substituents and the range of the sulfur angles (104 ±
1068) agree with those found in the solid structure of the
parent hexakis(phenylthio)benzene.[29]
In spite of the fact that the para-divinylbenzene-bridged 3 is
a crystalline compound, several attempts to obtain suitable
single crystals of 3 failed. However, we assumed that, in
analogy to 4, the p systems of the bridging para-divinylben-
zene unit and of the terminal pentakis(phenylthio)benzene
units are more or less orthogonal and thus lead to little
overlap. The CV and spectroelectrochemical investigations
showed 3 to be a Class I compound with independent redox
centres, which confirms such an assumption.
The solid-state structure of 7 shows that the molecule
consists of two pentakis(thiophenyl)benzene units linked by a
diacetylene bridge with a crystallographic inversion centre in
the middle of the C8 C8' central bond (Figure 4). The two
central benzene cores of the reducible subunits are separated
by 9.4 Š (centre to centre) and are in parallel planes
separated by 0.75 Š. The bond length in the diacetylene
The solid-state structure of 4 shows that the molecule
consists of two reducible pentakis(thiophenyl)benzene sub-
ˆ
ˆ
units covalently linked by a trans,trans-CH N-N CH bridge
with a crystallographic inversion centre in the middle of the
N N bond. The two reducible subunits are separated (centre
to centre) by 8.7 Š and the central benzene cores are in
parallel planes which are separated by 3.8 Š. The bond
ˆ
ˆ
lengths in the bridging unit (C6C7 N1 N1' C7'C6') are
1.488 Š (C6 C7), 1.254 Š (C7 N1) and 1.421 Š (N1 N1').
The bridging angles of 120.48 (C6-C7-N1) and 111.18 (C7-N1-
N1') are as expected for a benzaldehyde-azine structure. Of
particular interest, however, is the torsion angle of ꢀ728
ˆ
ꢁ
ꢁ
between the C N double bond and the central benzene plane
bridge (C6 C7 C8 C8' C7' C6') are 1.436 (C6 C7), 1.202
1270
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1270 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 6

Molecular Wires
1266±1272
toluene. The organic layer was separated and dried over MgSO4.
Evaporation of the solvent yielded 2 (9.49 g, 14.67 mmol, 75%) as a yellow
solid. M.p. 1228C; IR (KBr): nÄ ˆ 3051 (w), 1704 (s), 1579 (m), 1476 (s), 1438
(s), 1288 (w), 1164 (w), 1080 (m), 1023 (m), 999 (w), 929 (w), 735 (s), 698
(m), 687 (s), 469 cm 1 (w); 1H NMR (CDCl3): d ˆ 6.85 ± 7.35 (m, 25H), 9.87
(s, 1H); 13C NMR (CDCl3): d ˆ 126.32, 126.39, 126.83, 128.19, 128.48,
128.85, 128.96, 129.26, 136.32, 136.94, 137.23, 140.91, 146.57, 147.56, 150.72,
190.08; FAB-MS: m/z (%): 650 (8), 649 (21), 648 (46), 647 (75), 646 (100)
(C7 C8) and 1.372 Š (C8 C8'), and the bridge angles of 175.5
(C6-C7-C8) and 178.28 (C7-C8-C8') deviate only slightly from
linearity. The conformation of 7 shows the phenylthio groups
in para and ortho positions to the diacetylene bridge point
to the same side of the central benzene core to give an
unexpected (udddu) pattern. However, this does not disturb
the sulfur angles, which are in the expected range (104 ± 1068).
The overlap between the p system of the diacetylene bridging
structure and the p system of the reducible subunits should be
independent of the torsion angle of one subunit relative to the
other. Even though both central benzene rings of 7 are in
parallel planes, it is probably not a requirement for the
electronic interaction between them. Evidently, the cylindri-
cal acetylenic p system bears the advantage of this ªrotational
freedomº. A stronger overlap would be expected for a
conformationally optimised double-bond connector and in
fact, the already mentioned bathochromic shift of the lowest
energy absorption of the doubly reduced para-divinylben-
zene-bridged structure 32 , in contrast to the doubly reduced
diacetylene-bridged structure 72 , may be caused by such an
effect in the doubly reduced form. Other redox centres
connected by anthracene-bridged stilbenoids have been
reported to display such conformational changes upon
oxidation.[12]
‡
[M] ; elemental analysis calcd (%) for C37H26OS5: C 68.70, H 4.05; found:
C 68.81, H 4.07.
trans,trans-1,4-Bis-{[2'-(2'',3'',4'',5'',6''-pentathiophenyl)phenyl]ethenyl}
benzene (3): A 50% NaOH solution (4 mL) was added to pentathiophenyl
benzaldehyde (0.21 g, 323 mmol) and para-xylylenebis(triphenylphospho-
nium bromide) (0.13 g, 163 mmol) in CH2Cl2 (8 mL). The colour changed
immediately from yellow to red. After 20 h the reaction mixture was
poured into water and extracted with CH2Cl2. Column chromatography
(silica, benzene) yielded 3 (0.13 g, 79 mmol, 49%) as light yellow solid. Rf
(silica, xylene): 0.70; m.p. 768C; 1H NMR (CDCl3): d ˆ 6.95 ± 7.5 (m, 54H),
6.86 (d, J ˆ 16 Hz, 2H), 6.49 (d, J ˆ 16 Hz, 2H); 13C NMR (CDCl3): d ˆ
149.32, 149.21, 147.49, 146.09, 142.64, 138.13, 137.60, 136.29, 134.39, 128.93,
128.35, 128.16, 127.99, 126.58, 126.02; FAB-MS: m/z (%): 1367 (20), 1366
‡
(36), 1365 (69), 1364 (89), 1363 (100, molecular mass), 1362 (61) [M] , 1275
(33), 1274 (45), 1273 (50), 1272 (35); elemental analysis calcd (%) for
C82H58S10: C 72.21, H 4.29; found: C 72.36, H 4.44.
trans,trans-Decathiophenyl benzaldehyde azine (4): Pentathiophenyl benz-
aldehyde (0.532 g, 822 mmol) and hydrazine hydrate (0.016 g, 328 mmol)
were refluxed in CH3CN (12 mL) for 16 h. The light yellow precipitate was
collected by filtration, washed with cold CH3CN and dried under high
vacuum to yield 4 (0.135 g, 105 mmol, 32%) as a light yellow solid. Rf (silica,
toluene/hexane 1:2): 0.31; m.p. 2018C; 1H NMR (CDCl3): d ˆ 8.15 (s, 2H),
6.85 ± 7.2 (m, 50H); 13C NMR (CDCl3): d ˆ 159.25, 149.24, 147.93, 144.59,
142.13, 137.60, 137.52, 136.92, 129.09, 128.94, 128.87, 128.61, 128.37, 128.10,
126.37, 126.21, 126.13; FAB-MS: m/z (%): 1293 (22), 1292 (44), 1291 (75),
Conclusions
‡
1290 (79), 1289 (100) [M] , 1202 (34), 1201 (64), 1200 (71), 1199 (93), 1181
The present results indicate the suitability of diacetylene
connections as active electron-transfer linkers between mo-
lecular entities, while the para-divinylbenzene and the bis-
hydrazone bridges probably require additional structural
conditions for efficient electron transfer. The potential of
the diacetylene connection is illustrated, to some extent, by
the rapidly growing field of acetylene ªscaffoldingº.[30, 31]
Furthermore, they confirm the potential of poly(thiophen-
yl)-substituted aromatic compounds to act as reducible
subunits in molecular structures and devices. This is the case
for reducible molecular rigid rods,[19] reducible molecular
macrocycles[20] and reducible macrobicyclic ligands[18] that
contain such groups.
(34), 1180 (35), 1179 (47); elemental analysis calcd (%) for C74H52N2S10
C 68.91, H 4.06; found: C 68.83, H 4.15.
:
1-(2',2'-Dibromoethenyl)-2,3,4,5,6-pentathiophenyl benzene (5): Penta-
thiophenyl benzaldehyde (0.839 g, 1.30 mmol) was heated under reflux in
dry CH3CN (30 mL).
A solution of carbon tetrabromide (1.29 g,
3.89 mmol) and triphenyl phosphine (2.04 g, 7.78 mmol) in dry CH3CN
(10 mL) was added over a period of 5 min. After heating under reflux for
20 min, filtration and evaporation of the solvent gave a yellow solid as a
crude product. Column chromatography (silica, toluene/hexane 1:1)
yielded 5 (0.72 g, 0.897 mmol, 69%) as a yellow dye. IR (film): nÄ ˆ 3071
(w), 1579 (m), 1476 (s), 1438 (s), 1323 (w), 1286 (w), 1080 (m), 1023 (m), 999
1
(w), 837 (m), 736 (s), 698 (s), 686 (s), 646 (m), 481 cm (m); 1H NMR
(CDCl3): d ˆ 7.15 ± 7.52 (m, 25H), 6.70 (s, 1H); 13C NMR (CDCl3): d ˆ
93.98, 125.92, 126.50, 127.71, 127.86, 128.01, 128.78, 128.92, 129.15, 136.13,
136.76, 137.46, 143.70, 145.72, 146.28, 147.16; FAB-MS: m/z (%): 805 (11),
‡
804 (28), 803 (21), 802 (40), 801 (14), 800 (18), [M] , 725 (10), 724 (28), 723
‡
(43), 722 (100), 721 (36), 720 (77) [M Br] .
1-Ethinyl-2,3,4,5,6-pentathiophenyl benzene (6): Compound
5 (0.10 g,
Experimental Section
0.125 mmol) was dissolved in absolute THF (10 mL) and cooled to
788C. n-Butyllithium (1.6m in hexane, 0.2 mL, 0.32 mmol) was added
dropwise. The mixture was stirred for 1 h under 708C, allowed to warm to
room temperature and then stirred at this temperature for 1 h at. A
saturated NaCl solution (40 mL) was added, and the mixture extracted with
toluene (3 Â 10 mL). The combined organic layers were dried over MgSO4.
Column chromatography (silica, toluene/hexane 1:2) yielded 6 (0.052 g,
0.08 mmol, 65%) as a yellow dye. IR (film): nÄ ˆ 3285 (w), 2956 (w), 1582
(m), 1477 (s), 1437 (m), 1082 (m), 1024 (m), 735 (s), 687 cm 1 (s); 1H NMR
(CDCl3): d ˆ 3.58 (s, 1H), 6.9 ± 7.4 (m, 25H); 13C NMR (CDCl3): d ˆ 80.37,
88.35, 125.54, 125.76, 125.88, 126.72, 127.69, 128.15, 128.77, 128.96, 129.88,
137.34, 138.02, 138.41, 140.21, 145.14, 149.22, 153.81; FAB-MS: m/z (%): 645
General: All reaction vessels were flame-dried in an nitrogen atmosphere.
Reactions were carried out under nitrogen with commercial reagents
without further purification. THF: distilled over Na. CH3CN: distilled over
CaH2. Column chromatography (CC): commercial-grade solvents, dis-
tilled; silica gel: Geduran SI60 from Merck. Thin-layer chromatography
(TLC): Macherey-Nagel pre-coated TLC plates SILG-50UV254, visual-
isation by UV. Melting points: Büchi B-540, uncorrected. IR: Perkin ±
1
Elmer FT-IR1600. H NMR: Bruker AC200 (200 MHz); d relative to the
solvent signal: CDCl3: dH ˆ 7.26. 13C NMR: Bruker AC200 (50 MHz); d
relative to the solvent signal: CDCl3: dC ˆ 77.00. The mass spectra were
Â
performed at the Laboratoire de Spectrometrie de Masse, Strasbourg
‡
(26), 644 (49), 643 (99), [M] , 642 (89), 641 (100), 565 (15), 457 (13), 456
(France).
(32), 454 (16), 424 (23), 422 (19), 379 (17), 378 (38), 377 (22), 348 (35), 347
(52), 346 (89), 327 (16), 316 (32), 315 (17), 314 (40); elemental analysis calcd
(%) for C38H26S5: C 70.99, H 4.08; found: C 70.88, H 4.22.
Pentathiophenyl benzaldehyde (2): Thiophenyl sodium salt (18 g,
136.20 mmol) was dissolved in 1,3-dimethylimidazolidin-2-one (DMI;
100 mL) and pentafluorobenzaldehyde (3.81 g, 19.43 mmol) was added.
The solution immediately turned orange and was stirred for 12 h at room
temperature, poured into a saturated NaCl solution and extracted with
1,4-Di(pentathiophenyl)phenyl-buta-1,3-diyne (7): Compound 6 (0.115 g,
0.178 mmol) and copper(ii) acetate monohydrate (0.039 g, 0.197 mmol,
Chem. Eur. J. 2001, 7, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1271 $ 17.50+.50/0
1271

M. Mayor et al.
FULL PAPER
1.1 equiv) were heated to 558C in pyridine (5 mL). After 1 h the reaction
mixture was poured onto ice water and extracted with toluene. The organic
layer was washed successively with 25% AcOH, 1m NaHCO3 and saturated
NaCl, and dried over MgSO4. Evaporation of the solvent yielded a crude
product (0.139 g). Crystallisation from CHCl3/hexane gave 7 (65%) as
orange crystals. M.p. 1868C; IR (KBr): nÄ ˆ 1581 (m), 1477 (s), 1438 (m),
Bonvoisin, J.-P. Launay, M. Van der Auweraer, F. C. de Schryver, J.
Phys. Chem. 1994, 98, 5052 ± 5057.
[15] A. F. Diaz, K. K. Kanazawa, J. P. Gardini, J. Chem. Soc. Chem.
Commun. 1979, 635.
[16] Comprehensive Supramolecular Chemistry, Vol. 6 (Eds.: J. L. Atwood,
J. E. D. Davis, D. D. MacNicol, F. Vögtle, G. A. Downing, J.-M. Lehn),
Elsevier, Oxford, 1996, pp. 421 ± 464.
[17] J. H. R. Tucker, M. Gingras, H. Brand, J.-M. Lehn, J. Chem. Soc.
Perkin Trans. 2 1997, 1303 ± 1307.
1
1311 (w), 1079 (m), 1023 (m), 999 (w), 741 (m), 732 (s), 697 (m), 686 cm
(m); 1H NMR (CDCl3): d ˆ 6.8 ± 7.3 (m, 50H); 13C NMR (CDCl3): d ˆ
83.01, 86.62, 126.23, 126.31, 126.52, 128.18, 128.40, 129.01, 129.28, 133.78,
136.48, 137.44, 137.80, 146.51, 146.59, 148.43; FAB-MS: m/z (%): 1287 (20),
[18] M. Mayor, J.-M. Lehn, Helv. Chim. Acta 1997, 80, 2277 ± 2285.
[19] M. Mayor, J.-M. Lehn, K. M. Fromm, D. Fenske, Angew. Chem. 1997,
109, 2468 ± 2471; Angew. Chem. Int. Ed. Engl. 1997, 36, 2370 ± 2372.
[20] M. Mayor, J.-M. Lehn, J. Am. Chem. Soc. 1999, 121, 11231 ± 11232.
[21] Single-crystal data for 4: C74H52N2S10, M ˆ 1289.78 gmol 1, triclinic,
‡
1286 (41), 1285 (68), 1284 (88), 1283 (100), [M] , 1282 (66), 1281 (29), 1207
(29), 1206 (30), 1205 (28), 643 (50), 642 (44), 641 (83); elemental analysis
calcd (%) for C76H50S10: C 71.10, H 3.93; found: C 71.30, H 4.07.
Å
space group P1 (No. 2), a ˆ 9.614(2), b ˆ 10.578(2), c ˆ 17.080(3) Š,
Vˆ 1570.8(5) Š3, Z ˆ 1, 1calcd ˆ 1.363 gcm 3, F(000) ˆ 670, Tˆ 200 K,
l ˆ 0.71073 Š, m(MoKa) ˆ 0.397 mm 1, 6.08 < 2q < 58.548, 9040 reflec-
tions of which 7615 unique and 7613 observed, 467 parameters refined,
GOF(on F 2) ˆ 1.038, R1 ˆ S j Fo Fc j /SFo ˆ 0.0495, wR2 ˆ 0.1348 for
I > 2s and R1 ˆ 0.0742, wR2 ˆ 0.1534 for all data. Crystals of 4 were
measured on an Enraf ± Nonius four-circle diffractometer at 200 K.
The data collection was best for a crystal of 0.9 Â 0.4 Â 0.2 mm and an
absorption correction by analytical integration. The structure was
solved with direct methods and refined by full-matrix least-squares on
F 2 with the SHELX-97 package. All heavy atoms were refined
anisotropically. Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-147302. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223336-033; e-mail: deposit@ccdc.cam.
ac.uk)..
Acknowledgements
A Ph.D. fellowship of the ªStudienstiftung des Deutschen Volkesº for M.B.
is gratefully acknowledged.
[1] J. M. Tour, Chem. Rev. 1996, 96, 573 ± 553, and references therein.
[2] M. D. Ward, Chem. Soc. Rev. 1995, 121 ± 134, and references therein.
[3] M. A. Ratner, B. Davis, M. Kemp, V. Mujica, A. Roitberg, S. Yaliraki
in Molecular Electronics: Science and Technology; Annals of the New
York Academy of Science, (Eds.: A. Aviram, M. Ratner), 1998,
pp. 22 ± 37.
[4] A. Harriman, R. Ziessel, Chem. Commun. 1996, 1707 ± 1716.
[5] P. F. H. Schwab, M. D. Levin, J. Michl, Chem. Rev. 1999, 99, 1863 ±
1933, and references therein.
[6] F. Barigeletti, L. Flamigni, J.-P. Collin, J.-P. Sauvage, Chem. Commun.
1997, 333 ± 338.
[7] J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives,
VCH, Weinheim, 1995, Chapter 8.
[22] E. J. Corey, P. L. Fuchs, Tetrahedron Lett. 1972, 36, 3769 ± 3772.
[23] A. S. Hay, J. Org. Chem. 1960, 25, 1275.
[24] C. Creutz, Prog. Inorg. Chem. 1983, 30, 1 ± 71.
[25] M. Büschel, C. Stadler, C. Lambert, M. Beck, J. Daub, J. Electroanal.
Chem. 2000, 484, 24 ± 32, and references therein.
[8] Molecular Electronics (Eds.: J. Jortner, M. Ratner), Blackwell,
Oxford, 1997.
[9] E. G. Emberly, G. Kirczenow in Molecular Electronics: Science and
Technology; Annals of the New York Academy of Science (Eds.: A.
Aviram, M. Ratner), 1998, pp. 54 ± 67.
[10] M. Robin, P. Day, Adv. Inorg. Radiochem. 1967, 10, 247 ± 422.
[11] T. S. Arrhenius, M. Blanchard-Desce, M. Dvolaitzky, J.-M. Lehn, J.
[26] B. Soucaze-Guillous, H. Lund, J. Electroanal. Chem. 1997, 423, 109 ±
114.
[27] Assuming that the adiabatic energy surface for thermal electron
transfer (ET) is obtained from a two-state classical model between
diabatic surfaces; see: N. S. Hush, Prog. Inorg. Chem. 1967, 8, 391 ±
444; N. S. Hush, Electrochim. Acta 1968, 13, 1005 ± 1023; N. S. Hush,
Coord. Chem. Rev. 1985, 64, 135 ± 157.
[28] S. F. Nelsen, Chem. Eur. J. 2000, 6, 581 ± 588, and references therein.
[29] A. D. U. Hardy, D. D. MacNicol, D. R. Wilson, J. Chem. Soc. Perkin
Trans. 2 1979, 1011 ± 1019.
Ã
Malthete, Proc. Natl. Acad. Sci. USA 1986, 83, 5355 ± 5359; L. B. Š.
Johansson, M. Blanchard-Desce, M. Almgren, J.-M. Lehn, J. Phys.
Chem. 1989, 93, 6751 ± 6754; S. I. Kugimiya, T. Lazrak, M. Blanchard-
Desce, J.-M. Lehn, J. Chem. Soc. Chem. Commun. 1991, 1179 ± 1182.
[12] A. Knorr, J. Daub, Angew. Chem. 1997, 109, 2926 ± 2929; Angew.
Chem. Int. Ed. Engl. 1997, 36, 2817 ± 2819.
[13] ªSulfur-Containing Oligomersº, P. Bäuerle in Electronic Materials:
The Oligomer Approach (Eds. K. Müllen, G. Wegner), VCH, Wein-
heim, 1998, Chapter 2.
[30] F. Diederich in Modern Acetylene Chemistry (Eds.: P. J. Stang, F.
Diederich), VCH, Weinheim, 1995, 443 ± 471.
[31] U. H. F. Bunz, Y. Rubin, Y. Tobe, Chem. Soc. Rev. 1999, 28, 107 ± 119.
[14] Examples include: S. F. Nelsen, H. Q. Tran, M. A. Nagy, J. Am. Chem.
Soc. 1998, 120, 298 ± 304; C. Lambert, G. Nöll, Angew. Chem. 1998,
110, 2239 ± 2242; Angew. Chem. Int. Ed. 1998, 37, 2107 ± 2110; J.
Received: July 18, 2000 [F2607]
1272
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0706-1272 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 6