Synthesis and characterization of cruciform-conjugated molecules based on tetrathiafulvalene

Article abstract of DOI:10.1021/jo702735d

(Chemical Equation Presented) Stepwise acetylenic scaffolding provides cruciform-conjugated molecules with vertically disposed π-systems, one being an extended tetrathiafulvene (TTF) unit. Two synthetic routes for a cruciform dimer incorporating a total o

Full text of DOI:10.1021/jo702735d

Synthesis and Characterization of Cruciform-Conjugated Molecules
Based on Tetrathiafulvalene
Mikkel Vestergaard, Karsten Jennum, Jakob Kryger Sørensen, Kristine Kilsa˚, and
Mogens Brøndsted Nielsen*
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5,
DK-2100 Copenhagen Ø, Denmark
mbn@kiku.dk
ReceiVed January 2, 2008
Stepwise acetylenic scaffolding provides cruciform-conjugated molecules with vertically disposed
π-systems, one being an extended tetrathiafulvene (TTF) unit. Two synthetic routes for a cruciform dimer
incorporating a total of two extended TTFs was developed, employing either an oxidative Hay coupling
reaction as the final dimerization step or as one of the initial steps. Both routes take advantage of ready
access to new mono- and diethynylbenzene building blocks incorporating a variety of other functionalitites.
The redox and optical properties of the cruciform molecules were investigated by cyclic and differential
pulse voltammetries and UV-vis absorption spectroscopy. The access to multiple redox states renders
the molecules attractive candidates as wires, or possibly transistors, for molecular electronics. Generation
of a quinoid structure by oxidation is supported by density functional theory calculations.
Introduction
(TTF) donor together with an oligo(phenyleneethynylene) OPE3
backbone.6 TTF exhibits two reversible one-electron oxidation
steps and has for this reason found wide applications in both
materials and supramolecular chemistry.7 The conjugation
Cruciform molecules containing two perpendicularly disposed
π-systems are attractive as molecular wires and switches for
molecular electronics and advanced materials1 because of their
multiple linear and cross-conjugated pathways for electron
delocalization. Recent examples of cruciform-conjugated mol-
ecules include bis-phenyloxazole/terphenyl systems,2 several
oligo(phenylenevinylene) and oligo(phenyleneethynylene) struc-
tures,3 functionalized tetraethynylethenes,4 and acetylenic scaf-
folds of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene.5
We recently devised an efficient synthesis of the cruciform
molecule 1 incorporating the redox-active tetrathiafulvalene
(3) (a) Wilson, J. N.; Josowicz, M.; Wang, Y.; Bunz, U. H. F. Chem.
Commun. 2003, 2962-2963. (b) Wilson, J. N.; Bunz, U. H. F. J. Am. Chem.
Soc. 2005, 127, 4124-4125. (c) Marsden, J. A.; Miller, J. J.; Shirtcliff, L.
D.; Haley, M. M. J. Am. Chem. Soc. 2005, 127, 2464-2476. (d) Spitler, E.
R.; Shirtcliff, L. D.; Haley, M. M. J. Org. Chem. 2007, 72, 86-96. (e)
McGrier, P. L.; Solntsev, K. M.; Scho¨nhaber, J.; Brombosz, S. M.; Tolbert,
L. M.; Bunz, U. H. F. Chem. Commun. 2007, 2127-2129. (f) Gerhardt,
W. W.; Zucchero, A. J.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem.s
Eur. J. 2007, 13, 4467-4474. (g) Grunder, S.; Huber, R.; Horhoiu,
Gonza´lez, M. T.; Scho¨nenberger, C.; Calame, M.; Mayor, M. J. Org. Chem.
2007, 72, 8337-8344. (h) Zhou, N.; Wang, L.; Thompson, D. W.; Zhao,
Y. Tetrahedron Lett. 2007, 48, 3563-3567. (i) Błaszczyk, A.; Fischer, M.;
von Ha¨nisch, C.; Mayor, M. Eur. J. Org. Chem. 2007, 2630-2642.
(4) (a) Tykwinski, R. R.; Schreiber, M.; Carlo´n, R. P.; Diederich, F.;
Gramlich, V. HelV. Chim. Acta 1996, 79, 2249-2280. (b) Nielsen, M. B.;
Diederich, F. Chem. ReV. 2005, 105, 1837-1867.
(1) (a) Gholami, M.; Tykwinski, R. R. Chem. ReV. 2006, 106, 4997-
5027. (b) Galbrecht, F.; Bu¨nnagel, T. W.; Bilge, A.; Scherf, U.; Farrell, T.
In Functional Organic Molecules; Mu¨ller, T. J. J., Bunz, U. H. F., Eds.;
Wiley-VCH: Weinheim, 2007; pp 83-118. (c) Weibel, N.; Grunder, S.;
Mayor, M. Org. Biomol. Chem. 2007, 5, 2343-2353.
(2) Klare, J. E.; Tulevski, G. S.; Sugo, K.; de Picciotto, A.; White, K.
A.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 6030-6031.
(5) Chen, G.; Zhao, Y. Tetrahedron Lett. 2006, 47, 5069-5073.
(6) Sørensen, J. K.; Vestergaard, M.; Kadziola, A.; Kilsa˚, K.; Nielsen,
M. B. Org. Lett. 2006, 8, 1173-1176.
10.1021/jo702735d CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/20/2008
J. Org. Chem. 2008, 73, 3175-3183
3175

Vestergaard et al.
SCHEME 1. Generation of Quinoid Structure upon Two-Electron Oxidation
pathway along the OPE moeity of 1 is expected to change from
linearly to cross-conjugated upon oxidation of the vertically
appended TTF as the dication should take a quinoid structure
at the central benzene ring (Scheme 1). This change in
conjugation pathway along the OPE is assumed to be ac-
companied by a change in the conductivity (on/off). We imagine
that this switching could be accomplished by tuning a gate
potential in a three-electrode field-effect transistor in which the
two thiol end-groups of the OPE unit is connected to source
and drain electrodes made of gold. Hummelen and co-workers8
recently employed the same underlying principle of “conjugation
pathway switching” in the design of an anthraquinone redox-
controlled wire. Indeed, numerous reports have revealed the
inefficiency of cross conjugation for electron delocalization in
acyclic molecules, for example, in dendralenes,9 expanded
dendralenes,10 and dithiafulvene oligomers.11 In a synthesis-
driven approach, we aim at developing efficient synthetic
protocols for oligomeric TTF-based, redox-active cruciforms.
It should be mentioned that other interesting combinations of
TTF and OPE (or derivatives thereof) in which the TTF unit is
part of the lateral backbone have been reported.12
identical trimethylsilyl protected terminal acetylenes. Here, we
report an efficient route to the unsymmetrical compound 3
containing two different trialkylsilyl end-groups to allow for
stepwise desilylation and hence stepwise acetylenic scaffolding.
We have employed this compound for the synthesis of the cruci-
form dimer 4. Alternatively, this dimer can be prepared by a
four-fold Wittig reaction of the tetraaldehyde 5. The synthetic
routes to both compounds 3 and 4 provide a selection of inter-
mediate building blocks that are useful for acetylenic scaffolding
in general. It is also noteworthy that we have previously in our
work on acetylenic TTF scaffolding experienced considerable
instability of the intermediate terminal acetylenes.13 This
instability has to a large extent been overcome in the modules
2 and 3. In addition to synthetic protocols, the optical and
electrochemical properties of the new cruciform molecules are
presented. We have also subjected the dication formed upon
oxidation to a computational study to elucidate the degree of
quinoid character and hence the rationale behind the design.
We shall briefly mention that examples of molecules incor-
porating a total of four dithiafulvene units about a central conju-
gated core, such as compound 4, are rather limited in the litera-
The previously reported6 synthesis of compound 1 was
achieved from the diacetylenic building block 2 containing two
ture and include structures based on anthraquinone cores,14
a
[4]radialene core,15 or a single benzene core.16 OPEs function-
alized with redox-active fullerenes have been reported,17 but
here the fullerenes are isolated from the OPE via nonconjugated
linkers.
(7) (a) Bryce, M. R. AdV. Mater. 1999, 11, 11-23. (b) Nielsen, M. B.;
Lomholt, C.; Becher, J. Chem. Soc. ReV. 2000, 29, 153-164. (c) Bryce,
M. R. J. Mater. Chem. 2000, 10, 589-598. (d) Segura, J. L.; Mart´ın, N.
Angew. Chem., Int. Ed. 2001, 40, 1372-1409.
(8) van Dijk, E. H.; Myles, D. J. T.; van der Veen, M. H.; Hummelen,
J. C. Org. Lett. 2006, 8, 2333-2336.
(9) (a) Brain, P. T.; Smart, B. A.; Robertson, H. E.; Davis, M. J.; Rankin,
D. W. H.; Henry, W. J.; Gosney, I. J. Org. Chem. 1997, 62, 2767-2773.
(b) Fielder, S.; Rowan, D. D.; Sherburn, M. S. Angew. Chem., Int. Ed.
2000, 39, 4331-4333.
3176 J. Org. Chem., Vol. 73, No. 8, 2008

Cruciform-Conjugated Molecules Based on TetrathiafulValene
SCHEME 2
Results and Discussion
monoalkynylated product 8 in a fair yield of 52% after careful
column chromatographic workup. Next, a second cross-coupling
reaction with triisopropylsilylacetylene gave the differentially
protected diacetylenic product 9 in quantitative yield. The ethyl
ester groups were reduced with diisobutylaluminium hydride
(DIBAL-H) to provide the diol 10. This compound was then
oxidized by pyridinium chlorochromate (PCC) affording the
dialdehyde 11. A two-fold Wittig reaction with the phoshonium
salt 12 and triethylamine as base gave the extended TTF 3. A
selective desilylation of the SiMe3 group by K2CO3 in MeOH/
THF followed by an oxidative Hay coupling reaction20 gave
the dimer target 4, albeit in modest yield.
The second route to the dimer 4 starts with a Hay dimerization
reaction as presented in Scheme 3. First, compound 9 was
selectively desilylated to the terminal acetylene 13. Under these
reactions conditions, the ethyl ester groups were transesterified
to methyl ester groups. Compound 13 was then oxidatively
homocoupled to form the butadiyne product 14 that was
subsequently treated with DIBAL-H to furnish the tetraol 15.
A four-fold oxidation using PCC gave the tetraaldehyde 5. A
final four-fold Wittig reaction with the phosphonium salt 12
and triethylamine ultimately gave the dimer 4.
Our synthesis starts from the readily available ditriflate 618
(Scheme 2). Treatment of this compound with an excess of
trimethylsilylacetylene under Sonogashira cross-coupling condi-
tions19 furnished the diacetylenic product 7.6 Using slightly less
than one equivalent of the acetylene provided conveniently the
(10) Burri, E.; Diederich, F.; Nielsen, M. B. HelV. Chim. Acta 2002, 85,
2169-2182.
(11) Nielsen, M. B.; Petersen, J. C.; Thorup, N.; Jessing, M.; Andersson,
A. S.; Jepsen, A. S.; Gisselbrecht, J.-P.; Boudon, C.; Gross, M. J. Mater.
Chem. 2005, 15, 2599-2605.
(12) (a) Wang, C.; Batsanov, A. S.; Bryce, M. R. Chem. Commun. 2004,
578-579. (b) Wang, E.; Li, H.; Hu, W.; Zhu, D. J. Polym. Science, Part
A: Polymer Chem. 2006, 44, 2707-2713.
(13) (a) Nielsen, M. B.; Utesch, N. F.; Moonen, N. N. P.; Boudon, C.;
Gisselbrecht, J.-P.; Concilio, S.; Piotto, S. P.; Seiler, P.; Gu¨nter, P.; Gross,
M.; Diederich, F. Chem.sEur. J. 2002, 8, 3601-3613. (b) Nielsen, M. B.
Synlett 2003, 1423-1426.
(14) (a) Mart´ın, N.; Pe´rez, I.; Sa´nchez, L.; Seoane, C. J. Org. Chem.
1997, 62, 870-877. (b) D´ıaz, M. C.; Illescas, B. M.; Seoane, C.; Mart´ın,
N. J. Org. Chem. 2004, 69, 4492-4499. (c) D´ıaz, M. C.; Illescas, B. M.;
Mart´ın, N.; Perepichka, I. F.; Bryce, M. R.; Levillain, E.; Viruela, R.; Ort´ı,
E.; Chem.sEur. J. 2006, 12, 2709-2721. (d) Ryhding, T.; Petersen, M.
Å.; Kilsa˚, K.; Nielsen, M. B. Synlett 2007, 6, 913-916.
(15) Misaki, Y.; Matsumura, Y.; Sugamoto, T.; Yoshida, Z. Tetrahedron
Lett. 1989, 30, 5289-5292.
Compound 8 also served as a precursor for other useful
acetylenic building blocks as shown in Scheme 4. Reduction
with DIBAL-H gave the diol 16 that was then oxidized to the
dialdehyde 17. Two-fold Wittig reaction subsequently gave the
extended TTF 18 incorporating both a reactive triflate group
and a protected terminal acetylene group.
(16) Salle, M.; Belyasmine, A.; Gorgues, A.; Jubault, M.; Soyer, N.
Tetrahedron Lett. 1991, 32, 2897-2900.
(17) de la Cruz, J. L. D.; Hahn, U.; Nierengarten, J.-F. Tetrahedron Lett.
2006, 47, 3715-3718.
(18) Zhang, Q.; Shi, C.; Zhang, H.-R.; Wang, K. K. J. Org. Chem. 2000,
65, 7977-7983.
(19) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467-4470.
(20) Hay, A. S. J. Org. Chem. 1962, 27, 3320-3321.
J. Org. Chem, Vol. 73, No. 8, 2008 3177

Vestergaard et al.
SCHEME 3
fulvene ring (Scheme 1). The third oxidation is presumably
oxidation of the central backbone. A nonaromatic quinoid
structure of the dication may facilitate this oxidation of the
backbone (vide supra).
Cyclic voltammetry of the dimer 4 revealed two broad waves.
However, DPV revealed a shoulder to the first oxidation step
(inset in Figure 1). Somewhat surprisingly, the oxidations of 4
are anodically shifted relative to those of the monomer 3
although the charge can be delocalized over a larger conjugated
area. A similar decrease in donor strength has nevertheless been
observed for other large acetylenic scaffolds of TTF.21
Absorption Spectroscopy. The absorption spectra of very
dilute solutions of compounds 3 and 4 in CHCl3 are revealed
in Figure 2. The monomer 3 exhibits absorption maxima at λmax
311, 405, and 422 nm with molar absorptivities (ꢀ) of 42 300,
38 000, and 37 900 M-1cm-1, respectively. The dimer 4 is a
very strong chromophore basically in the entire region from
ca. 300 to 500 nm with broad absorption maxima: λmax (ꢀ) )
331 nm (66 000 M-1cm-1), 386 nm (72 600 M-1cm-1), 437
(shoulder) nm. Accordingly, the low-energy absorption of 4 is
red-shifted relative to that of 3, which is explained by the
extended conjugation in 4.
Computational Study. To investigate further the possibility
of forming a quinoid character upon removing two electrons
from 3, we subjected the related compound 19 (Figure 3) devoid
of the CO2Me substituents to a computational study using the
Gaussian 03 program package.22 First, geometry-optimizations
were performed at the B3LYP/6-31G(d) level for both the
neutral and dication structures. A frequency analysis was
performed in each case to ascertain that a real minimum had
been obtained (i.e., no imaginary vibrational frequencies). The
obtained structure of the neutral compound has each dithiaful-
(21) Andersson, A. S.; Qvortrup, K.; Torbensen, E. R.; Mayer, J.-P.;
Gisselbrecht, J.-P.; Boudon, C.; Gross, M. Kadziola, A.; Kilsa˚, K.; Nielsen,
M. B., Eur. J. Org. Chem. 2005, 3660-3671.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003.
Electrochemistry. The redox properties of the cruciform
TTFs were studied by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV). The data are collected in Table 1
and voltammograms are shown in Figure 1. According to CV,
compound 3 exhibits three separated and quasireversible oxida-
tions. The first two oxidations of 3 are interpreted as one-
electron oxidations originating from oxidation of each dithia-
SCHEME 4
3178 J. Org. Chem., Vol. 73, No. 8, 2008

Cruciform-Conjugated Molecules Based on TetrathiafulValene
TABLE 1. Potentialsa Obtained from Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) Data
CVb
DPV
1
2
3
1
2
3
Eox
Eox
Eox
Eox
Eox
Eox
1
2
3
compound
(Epa1; Epc
)
(Epa2; Epc
)
(Epa3; Epc
)
V vs
V vs
V vs
V vs Fc+/Fc
0.56 (0.61; 0.51)
0.61 (br) (0.66; 0.56)
V vs Fc+/Fc
0.67 (0.70; 0.64)
0.96 (br) (1.03; 0.89)
V vs Fc+/Fc
0.93 (0.98; 0.88)
Fc+/Fc
0.52
Fc+/Fc
0.63
Fc+/Fc
0.90
3
4
0.53
0.63 (sh)
0.91
a All the potentials were determined in CH2Cl2 using Ag/Ag+ as a reference electrode, Pt as the counter electrode, and glassy carbon as the working
b
electrode. Supporting electrolyte: 0.1M NBu4PF6. Eox is the half-wave potential; Epa and Epc are the anodic and cathodic peak potentials. Scan rate 100
mV s-1
.
FIGURE 1. CV of 3 (dotted line) and 4 (solid line) in CH2Cl2 with
0.1 M Bu4NPF6. Inset shows DPV of 4 with arrows marking the three
oxidation peaks.
FIGURE 3. Bond lengths obtained by geometry-optimization (B3LYP/
6-31G(d)) and quinoid characters along the vertical and parallel
directions of the neutral and dicationic compounds.
FIGURE 4. Calculated vertical ionization energies (B3LYP/6-311++G-
(2d,p)).
FIGURE 2. Absorption spectra of 3 (2.8 × 10-5 M) and 4 (1.5 ×
10-5 M) in CHCl3.
respectively, and that of benzene is 9.28 eV, which is in good
agreement with an experimental value23 of 9.24 eV. It transpires
that the quinoid compound 21, resembling the central part of
vene ring rotated by an angle of 15° relative to the plane defined
by the central benzene ring, while the three rings are maintained
in the same plane in the structure of the dication (deviation of
only 0.07°). From the bond lengths, the quinoid characters along
3
2+, has a significantly smaller ionization energy than that of
compound 20 containing an intact benzene ring. Both com-
pounds are much stronger electron donors than benzene.
A dimer structure (22) devoid of both the ester and triiso-
propylsilyl substituents was also geometry-optimized at the
B3LYP/6-31G(d) level (Figure 5). Again the two dithiafulvene
rings are slightly distorted from the plane defined by each
benzene ring (13-14°). In addition, the two units are rotated
relative to each other; thus, the two benzene rings are rotated
by ca. 30° relative to each other. The frontier orbitals of both
19 and 22 are shown in Figure 6. The highest occupied
molecular orbital (HOMO) of the dimer is mainly located at
the two extended TTF units, while the lowest unoccupied
molecular orbital (LUMO) is mainly located at the central
phenyleneethynylene backbone. This orthogonal orientation of
the HOMO and LUMO relative to each other supports the idea
behind the cruciform design. Electron transport in a field-effect
transistor can occur through a LUMO, which is favorably
the backbone of the molecule (δr ) and vertically to the
||
backbone (δr , Figure 3) were calculated. The quinoid character
vertically to the wire increases significantly from 0.017 in the
neutral compound to 0.074 in the dication. In accordance hereto,
the length of the fulvene C-C bond (a) increases from 1.357
to 1.418 Å. The quinoid character along the wire decreases upon
oxidation. Only minor changes are observed for the C-CtC
bond (f, g) lengths. The quinoid structure of 192+ with a cross-
conjugated pathway along the wire supports the idea as stated
above of exploring these molecules as transistors for molecular
electronics.
The electrochemical experiments reveal that compound 3
undergoes a third oxidation step, which we ascribe to oxidation
of the central backbone. Disruption of the aromatic backbone
by the formation of a quinoid structure may facilitate this third
oxidation. Thus, we find by DFT calculations (B3LYP/6-
311++G(2d,p)) that the vertical ionization energies of the model
compounds 20 and 21 (Figure 4) are 7.94 and 7.58 eV,
(23) Nemeth, G. I.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1993,
215, 151-155 and references cited therein.
J. Org. Chem, Vol. 73, No. 8, 2008 3179

Vestergaard et al.
FIGURE 5. Optimized structure of dimer 22 (B3LYP/6-31G(d)).
aligned along the wire in the neutral molecule. Upon two-
electron oxidation, the depicted HOMO becomes the new
orthogonally disposed LUMO, now unfavorably aligned.
) [M + H+]. Anal. Calcd for C18H21O7SSi: C, 46.34; H, 4.54.
Found: C, 46.52; H, 4.47.
Compound 9. The triflate 8 (1.00 g, 2.14 mmol), Pd(PPh3)-
Cl2 (0.045 g, 0.06 mmol), and CuI (0.025 g, 0.13 mmol) were
dissolved in dry, argon-degassed THF (30 mL). Then degassed
NEt3 (10 mL) and triisopropylsilylacetylene (0.0.96 mL, 4.292
mmol) were added, and the reaction mixture was stirred for 48
h at rt. Then the reaction mixture was poured into CH2Cl2 (250
mL) and washed with saturated aqueous NH4Cl (2 × 150 mL).
The organic phase was dried (MgSO4), filtered, and evaporated
to dryness under reduced pressure to give a brown oil. Column
chromatography (SiO2, EtOAc/heptane 1:4) gave the product 9
Conclusions
Cruciform-conjugated molecules based on extended TTFs
were prepared from a variety of acetylenic precursors. In
particular, the synthetic protocols benefit from ready access to
aromatic building blocks containing both triflate and ethynyl
functional groups allowing for stepwise acetylenic scaffolding
via metal-catalyzed cross-coupling reactions. The cruciform
TTFs are stable and undergo quasireversible oxidations, which
makes the structural motifs interesting as potential wires for
molecular electronics. According to a computational study,
generation of a quinoid structure upon oxidation seems likely
and substantiates the design behind these molecules. The
potential strength of the cruciform design for molecular
electronics applications also manifests itself in the orthogonally
disposed HOMO and LUMO.
Previously, it was shown6 that end-capping with thiol groups
to allow for adhesion of the molecules between gold electrodes
is readily performed. Yet, the sulfurs present in the dithiaful-
venes may on the other hand complicate a proper orientation
of the cruciforms between gold electrodes. Substitution of the
methoxycarbonyl ester substituents by more bulky groups,
hereby rendering the heterocyclic sulfur atoms inaccessible, may
solve this problem in the further refinement of these first
prototype molecules.
1
(1.053 g, 99%) as a clear oil. H NMR (300 MHz, CDCl3): δ
) 0.27 (s, 9H), 1.11-1.14 (m, 21H), 1.36-1.44 (m, 6H), 4.36-
4.45 (m, 4H), 8.00 (s, 1H), 8.03 (s, 1H). 13C NMR (75 MHz,
CDCl3): δ ) -0.1, 11.4, 14.4 (×2; i.e., two signals), 18.8,
61.9 (×2), 99.9, 102.3, 102.5, 103.9, 122.5, 123.0, 134.9, 135.4,
136.1, 136.5, 165.3, 165.5. MS (FAB): m/z ) 499.3 [M + H+].
Anal. Calcd for C28H42O4Si2: C, 67.42; H, 8.49. Found: C,
67.50; H, 8.67.
Compound 10. The diester 9 (1.04 g, 2.08 mmol) was
dissolved in dry CH2Cl2 (9 mL), and the mixture was cooled to
0 °C. Then DIBAL-H (16.7 mL, 16.7 mmol, 1M in hexane)
was added during 10 min. After stirring for 3.5 h at 0 °C, the
mixture was poured into aqueous NH4Cl (110 mL). Then CH2-
Cl2 (110 mL) was added, and the mixture was stirred for 10
min. The resulting gel was passed through a short column of
Celite. The layer was washed with CH2Cl2 (200 mL), and the
filtrate was dried (MgSO4) and concentrated in vacuo. The
residue was subjected to column chromatography (SiO2, CH2-
Cl2) to provide the product 10 (0.707 g, 82%) as a colorless oil
Experimental Section
1
that solidified upon standing. H NMR (300 MHz, CDCl3): δ
Compound 8. Diethyl-2,5-bis(trifluoromethylsulfonyloxy)-
terephthalate 6 (5.00 g, 9.65 mmol), Pd(PPh3)2Cl2 (0.20 g, 0.29
mmol), and CuI (0.10 g, 0.53 mmol) were dissolved in dry,
argon-degasssed THF (25 mL). Then argon-degassed NEt3 (10
mL) and trimethylsilylacetylene (1.13 mL, 8.20 mmol) were
added. After stirring for 24 h at rt, the reaction mixture was
poured into CH2Cl2 (250 mL) and washed with saturated
aqueous ammonium chloride (2 × 150 mL) and brine (100 mL).
The organic phase was dried (MgSO4), filtered, and evaporated
to dryness under reduced pressure to give a red-brown semisolid.
Column chromatography (SiO2, EtOAc/heptane 1:9) gave the
product 8 as an oil, which slowly solidified upon standing (1.990
g, 52%). 1H NMR (300 MHz, CDCl3): δ ) 0.28 (s, 9H), 1.40-
1.55 (m, 6H), 4.40-4.50 (m, 4H), 7.79 (s, 1H), 8.23 (s, 1H).
13C NMR (75 MHz, CDCl3): δ ) -0.2, 14.2, 14.2, 62.4, 62.9,
100.7, 103.9, 112.4/116.7/120.9/125.2 (-CF3), 123.9, 124.8,
127.5, 137.4, 138.9, 146.7, 162.7, 164.1. MS (FAB) m/z: 467.1
) 0.26 (s, 9H), 1.13 (m, 21H), 2.10 (m, 2H), 4.79 (d, J ) 5.9
Hz, 2H), 4.81 (d, J ) 5.9 Hz), 7.54 (s, 2H). 13C NMR (75 MHz,
CDCl3): δ ) 0.0, 11.4, 18.8, 63.5, 63.6, 98.4, 101.7, 102.2,
104.0, 121.4, 121.8, 131.0, 131.3, 142.2 (two overlapping; i.e.,
one signal). MS (FAB): m/z ) 415.3 [M + H+]. Anal. Calcd
for C24H38O2Si2: C, 69.50; H, 9.24. Found: C, 69.16; H, 9.25.
Compound 11. To a solution of the diol 10 (0.667 g, 1.61
mmol) in dry CH2Cl2 (45 mL) was added molecular sieves (4
Å, 0.7 g) and Celite (0.65 g). The mixture was stirred for 5
min, whereupon PCC (1.28 g, 5.93 mmol) was added. After
stirring for 3.5 h, the mixture was filtered through a short column
(SiO2, CH2Cl2), providing the product 11 (0.639 g, 97%) as a
1
yellow analytically pure oil. H NMR (300 MHz, CDCl3): δ
) 0.29 (s, 9H), 1.14 (m, 21H), 8.07 (s, 1H), 8.08 (s, 1H), 10.54
(s, 1H), 10.59 (s, 1H). 13C NMR (75 MHz, CDCl3): δ (ppm)
) -0.2, 11.4, 18.8, 98.8, 100.7, 103.0, 105.8, 126.3, 126.6,
3180 J. Org. Chem., Vol. 73, No. 8, 2008

Cruciform-Conjugated Molecules Based on TetrathiafulValene
FIGURE 6. Frontier orbitals calculated at B3LYP/6-31G(d) level.
132.6, 132.7, 138.7, 138.8, 190.5, 190.7. MS (FAB): m/z )
411.3 [M + H+]. Anal. Calcd for C24H34O2Si2: C, 70.19; H,
8.34. Found: C, 70.16; H, 8.46.
(m), 2865 (m), 2149 (w), 1734 (s), 1586 (s), 1470 (w), 1434
(m), 1400 (w), 1258 (s), 1094 (m), 1028 (m), 996 (w), 869 (m),
846 (m), 760 (m), 679 (w) cm-1. 1H NMR (300 MHz, CDCl3):
δ ) 0.29 (s, 9H), 1.16 (m, 21H), 3.85 (s, 12H), 6.84 (s, 1H),
6.84 (s, 1H), 7.38 (s, 1H), 7.42 (s, 1H). 13C NMR (75 MHz,
CDCl3): δ ) 0.1, 11.5, 18.9, 53.4, 99.3, 102.6, 102.8, 104.7,
112.7, 112.8, 121.5, 122.1, 128.5, 129.3, 129.8, 131.5, 133.3,
133.5, 135.1, 135.2, 159.9, 160.0, 160.2 (two overlapping; i.e.,
one signal); Two signals overlapping. MS (FAB): m/z ) 814
[M+]. Anal. Calcd for C38H46O8S4Si2: C, 55.99; H, 5.69.
Found: C, 56.00; H, 5.63.
Compound 3. Compounds 11 (0.311 g, 0.76 mmol) and 12
(0.974 g, 1.92 mmol) were dissolved in dry MeCN (5 mL) and
THF (15 mL), whereupon NEt3 (0.37 mL, 2.56 mmol) was
dropwise added. After stirring for 3 h, the mixture was diluted
with CH2Cl2 (45 mL), washed with H2O (3 × 25 mL), dried
(MgSO4), and concentrated in vacuo. Column chromatography
(SiO2, CH2Cl2) gave the product 3 (0.460 g, 81%) as a red solid.
Mp 134-137 °C. IR (KBr): ν ) 3439 (w), 3013 (w), 2953
J. Org. Chem, Vol. 73, No. 8, 2008 3181

Vestergaard et al.
Compound 4. Method 1: To a solution of compound 3
(0.113 g, 0.138 mmol) in THF (4 mL) and MeOH (15 mL)
was added K2CO3 (0.027 g, 0.20 mmol). After stirring for 30
min, the mixture was diluted with Et2O (100 mL), washed with
H2O (50 mL) and brine (50 mL), dried (MgSO4), and concen-
trated in vacuo. The residue was redissolved in CH2Cl2 (15 mL),
and then Hay catalyst (10.0 mL) was added [Hay catalyst: CuCl
(0.26 g) and TMEDA (0.32 g) in CH2Cl2 (10 mL)]. After stirring
for 2 h under air, more CH2Cl2 (20 mL) was added, and
the mixture was stirred under air overnight. Then CH2Cl2
(100 mL) was added, and the mixture was washed with H2O
(50 mL) and brine (50 mL). The organic phase was dried
(MgSO4) and concentrated in vacuo. Column chromatography
(SiO2, CH2Cl2) gave the product 4 (0.021 g, 19%) as a red
solid.
Compound 15. The tetraester 14 (0.100 g, 0.126 mmol) was
dissolved in dry THF (7 mL) and cooled to 0 °C. DIBAL-H
(1.40 mL, 1.40 mmol, 1 M in hexane) was added over a period
of 5 min. After stirring for 3 h at 0 °C, the reaction mixture
was allowed to warm to rt. After stirring for another 24 h, H2O
(2 mL) was added, and the mixture was poured into CH2Cl2
(100 mL). The organic phase was washed with 0.08 M HCl (2
× 50 mL), brine (50 mL), dried (MgSO4), filtered, and
evaporated to dryness under reduced pressure to give the product
1
15 (0.073 g, 85%) as slightly yellow crystals. H NMR (300
MHz, CDCl3): δ ) 1.13 (m, 42H), 1.94 (t, J ) 6.5 Hz, 2H),
2.14 (t, J ) 6.5 Hz, 2H), 4.83 (d, J ) 6.5 Hz, 4H), 4.86 (d, J
) 6.5 Hz, 4H), 7.62 (s, 2H), 7.64 (s, 2H). 13C NMR (75 MHz,
CDCl3): δ ) 11.5, 18.9, 63.2, 63.5, 79.7, 81.0, 99.6, 103.9,
119.9, 122.8, 131.5, 131.7, 142.5, 143.2. HR-MS (FAB): m/z
) 682.3891 [M+] calcd for C42H58O4Si2, 682.3874.
Method 2: The tetraaldehyde 5 (0.050 g, 0.075 mmol) and
phosphonium salt 12 (0.188 g, 0.370 mmol) were dissolved in
a mixture of dry THF (10 mL) and dry MeCN (3 mL),
whereupon NEt3 (0.072 mL, 0.520 mmol) was added. After
stirring for 3 h at rt, the reaction mixture was poured into CH2-
Cl2 (150 mL). The mixture was washed with water (2 × 50
mL) and brine (50 mL), dried (MgSO4), filtered, and evaporated
to dryness under reduced pressure. Column chromatography
(SiO2, CH2Cl2) gave the product 4 as a red solid (0.102 g, 92%).
Mp 207-210 °C dec. 1H NMR (300 MHz, CDCl3): δ ) 1.16
(m, 42H), 3.86-3.88 (3× s, 24H), 6.87 (s, 2H), 6.96 (s, 2H),
7.48 (s, 2H), 7.50 (s, 2H). 13C NMR (75 MHz, CDCl3): δ )
11.5, 18.9, 53.5 (×2; i.e., two signals), 53.6 (two overlapping;
i.e., one signal), 80.5, 82.0, 100.3, 104.6, 112.0, 112.3, 119.8,
123.0, 129.4, 129.5, 129.6, 132.0, 134.1, 134.9, 135.1, 136.1,
159.7 (two overlapping), 160.1, 160.2; two signals overlapping.
MS (FAB): m/z ) 1481.5 [M+]. Anal. Calcd. for C70H74O16S8-
Si2: C, 56.65; H, 5.03. Found: C, 56.56; H, 4.94.
Compound 5. The tetraol 15 (0.071 g, 0.104 mmol) was
dissolved in dry CH2Cl2 (15 mL), whereupon molecular sieves
(4 Å), Celite (1 g), and PCC (0.157 g, 0.728 mmol) were
added. After stirring for 3 h at rt, the mixture was filtered
through a dry column (SiO2), and the column was washed
with additional CH2Cl2 (100 mL). The combined organic
fractions were evaporated to dryness under reduced pressure to
1
give the product 5 (0.060 g, 86%) as a white solid. H NMR
(300 HMz, CDCl3): δ ) 1.14-1.16 (m, 42H), 8.12 (s, 2H),
8.20 (s, 2H), 10.51 (s, 2H), 10.60 (s, 2H). 13C NMR (75,
CDCl3): δ ) 11.4, 18.8, 79.4, 81.7, 100.5, 104.7, 123.5, 127.8,
133.6, 133.8, 138.9, 139.9, 189.4, 190.0. MS (FAB): m/z )
675.3 [M+]. Anal. Calcd for C42H50O4Si2: C, 74.73; H, 7.47.
Found: C, 74.59; 7.73.
Compound 16. The triflate 8 (1.00 g, 2.14 mmol) was
dissolved in dry CH2Cl2 (6 mL) and cooled to -78 °C.
DIBAL-H (1 M in hexane, 9.65 mL, 9.65 mmol) was added
over a period of 5 min. After stirring for 45 min at -78 °C, the
solution was allowed to warm to rt, and after another 2 h it was
poured into CH2Cl2 (450 mL). The combined organic phase was
washed with 0.08 M HCl (100 mL) and saturated aqueous NH4-
Cl (100 mL). The organic phase was dried (MgSO4), filtered,
and evaporated to dryness under reduced pressure to give the
product 16 as a white solid (0.619 g, 75%). Mp 89-92 °C. 1H
NMR (300 MHz, CDCl3): δ ) 0.26 (s, 9H), 1.89 (t, J ) 6.2
Hz, 1H), 2.10 (t, J ) 6.2 Hz, 1H), 4.77 (d, J ) 6.2 Hz, 2H),
4.84 (d, J ) 6.2 Hz, 2H), 7.42 (s, 1H), 7.70 (s, 1H). 13C NMR
(75 MHz, CDCl3): δ ) 0.0, 59.3, 62.9, 100.4, 102.2, 119.7,
120.8, 121.5, 132.6, 133.8, 145.4, 146.7. MS (FAB): m/z )
383.2 [M+]. Anal. Calcd for C14H17F3O5SSi: C, 43.97; H, 4.48.
Found: C, 44.22; H, 4.51.
Compound 13. Compound 9 (0.563 g, 1.13 mmol) and K2-
CO3 (0.138 g, 1.00 mmol) were dissolved in THF (20 mL),
whereupon MeOH (10 mL) was added. After stirring for 8 h at
rt under an argon atmosphere, the mixture was poured into CH2-
Cl2 (150 mL). The organic phase was washed with water (3 ×
100 mL), brine (75 mL), dried (MgSO4), filtered, and evaporated
to dryness under reduced pressure to give the product (0.360 g,
1
80%) as a yellow solid. Mp 78-79 °C. H NMR (300 MHz,
CDCl3): δ ) 1.15 (m, 21H), 3.50 (s, 1H), 3.92 (s, 3H), 3.96
(s, 3H), 8.08 (s, 1H), 8.10 (s, 1H). 13C NMR (75 MHz,
CDCl3): δ ) 11.4, 18.8, 52.8, 81.0, 84.7, 100.3, 103.6, 122.0,
123.5, 134.8, 135.3, 136.6, 136.7, 165.4, 165.7. MS (FAB): m/z
) 399.1 [M + H+]. Anal. Calcd for C23H30O4Si: C, 69.31; H,
7.59. Found: C, 69.34; H, 7.66.
Compound 17. A mixture of the diol 16 (0.550 g, 1.44
mmol), PCC (1.085 g, 5.03 mmol), Celite (2.0 g), and molecular
sieves (4 Å) in dry CH2Cl2 (40 mL) was stirred for 3 h at rt.
Then the reaction mixture was filtered through a short, dry silica
gel column, and the column was washed with additional
CH2Cl2 (150 mL). Concentration in vacuo gave the product 17
as an oil, which slowly solidified upon standing (0.506 g,
93%). Mp 49-52 °C. 1H NMR (300 MHz, CDCl3): δ ) 0.30
(s, 9H), 7.88 (s, 1H), 8.17 (s, 1H), 10.28 (s, 1H), 10.53 (s, 1H).
13C NMR (75 MHz, CDCl3): δ ) -0.3, 97.2, 107.1, 120.9,
121.2, 127.2, 131.6, 136.3, 140.7, 148.9, 185.3, 189.2. MS
(FAB): m/z ) 379.0266 [M + H+] calcd for C14H14F3O5SSi2,
379.0283.
Compound 14. Compound 13 (0.635 g, 1.49 mmol) was
dissolved in CH2Cl2 (50 mL). Then Hay catalyst (1.5 mL) was
added, and the mixture was vigorously stirred under air for 14
h. The mixture was diluted with CH2Cl2 (100 mL), and the
organic phase was washed with water (2 × 100 mL), brine (100
mL), dried (MgSO4), filtered, and evaporated to dryness under
reduced pressure to a red foam. Column chromatography (SiO2,
EtOAc/heptane 1:9) gave the product 14 (0.430 g, 68%) as
1
slightly yellow crystals. Mp 153-55 °C. H NMR (300 MHz,
CDCl3): δ ) 1.15 (m, 42H), 3.93 (s, 6H), 3.99 (s, 6H), 8.13
(s, 2H), 8.15 (s, 2H). 13C NMR (75 MHz, CDCl3): δ ) 11.4,
18.8, 52.8, 53.0, 80.9, 81.7, 101.2, 103.7, 121.6, 123.9, 134.9,
135.4, 137.0 (×2; i.e., two signals), 165.0, 165.5. MS (FAB):
m/z ) 795.6 [M + H+]. Anal. Calcd for C46H58O8Si2: C, 69.49;
H, 7.35. Found: C, 69.78; H, 7.57.
Compound 18. The dialdehyde 17 (0.449 g, 1.19 mmol) and
phosphonium salt 12 (1.507 g, 2.97 mmol) were dissolved in a
3182 J. Org. Chem., Vol. 73, No. 8, 2008

Cruciform-Conjugated Molecules Based on TetrathiafulValene
mixture of dry THF (30 mL) and dry MeCN (10 mL). Then
NEt3 (0.578 mL, 4.15 mmol) was added. After stirring for 12
h at rt, the mixture was evaporated to dryness under reduced
pressure. The residue was redissolved in CH2Cl2 (200 mL) and
washed with water (2 × 100 mL) and brine (100 mL), dried
(MgSO4), filtered, and evaporated to dryness under reduced
pressure. Column chromatography (SiO2, CH2Cl2) gave the
(FAB): m/z ) 782.2 [M+]. Anal. Calcd for C28H25F3O11S5Si:
C, 42.96; H, 3.22. Found: C, 42.92; H, 3.19.
Acknowledgment. The Danish Research Agency is ac-
knowledged for financial support (Grant 2111-04-0018).
Supporting Information Available: General experimental
methods; NMR spectra of compounds 3-5, 8-11, 13-18; tables
of atom coordinates and absolute energies of 19-22 and benzene.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
product as a red solid (0.826 g, 89%). Mp 182-186 °C. H
NMR (300 MHz, CDCl3): δ ) 1.54 (s, 9H), 3.87-3.88 (2× s,
12H), 6.43 (s, 1H), 6.88 (s, 1H), 7.24 (s, 1H), 7.52 (s, 1H). MS
JO702735D
J. Org. Chem, Vol. 73, No. 8, 2008 3183