Synthesis and characterization of tetrathiafulvalene-substituted di- and tetraethynylethenes with p-nitrophenyl acceptors

Article abstract of DOI:10.1021/jo802190q

(Chemical Equation Presented) Novel di- and tetraethynylethene (DEE and TEE) compounds functionalized with tetrathiafulvalene (TTF) donor groups and p-nitrophenyl acceptor groups were synthesized by palladium-catalyzed cross-coupling reactions under vario

Full text of DOI:10.1021/jo802190q

Synthesis and Characterization of Tetrathiafulvalene-Substituted
Di- and Tetraethynylethenes with p-Nitrophenyl Acceptors
Asbjørn S. Andersson, Lasse Kerndrup, Anders Ø. Madsen, Kristine Kilså, and
Mogens Brøndsted Nielsen*
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5,
DK-2100 Copenhagen Ø, Denmark
Philip R. La Porta and Ivan Biaggio
Department of Physics and Center for Optical Technologies, Lehigh UniVersity,
Bethlehem, PennsylVania 18018
mbn@kiku.dk
ReceiVed October 3, 2008
Novel di- and tetraethynylethene (DEE and TEE) compounds functionalized with tetrathiafulvalene (TTF)
donor groups and p-nitrophenyl acceptor groups were synthesized by palladium-catalyzed cross-coupling
reactions under various conditions. The molecules are strong chromophores and were investigated for
their optical properties. Placement of two TTFs and two p-nitrophenyls about a central TEE core provides
a molecule with a high third-order optical nonlinearity. The molecules experience reversible oxidations
of the TTF units, and the optical properties of the oxidized species were elucidated by spectroelectro-
chemistry. The degree of quinoid character of the p-nitrophenyl in the molecules was determined by
X-ray crystallography.
Introduction
For example, extensive conjugation in an ethynediyl-bridged
TTF dimer is apparent from its UV-vis absorption spectrum
as it is not merely a superposition of the spectra of two isolated
TTFs.5 We became interested in separating two TTF donor units
by larger acetylenic scaffolds, such as the tetraethynylethene
(TEE) unit. It was previously shown by Diederich, Gu¨nter, and
co-workers6 that donor-acceptor aryl-substituted TEEs exhibit
remarkable third-order NLO properties owing to the coplanarity
of the aryl groups. Here we describe the synthesis of new
chromophores incorporating TTF as donor unit and p-nitrophe-
nyl as acceptor unit about acetylenic cores, such as TEE or
Tetrathiafulvalene (TTF) is a good electron donor that is
oxidized reversibly in two steps, hereby gaining heteroaroma-
ticity,1 and it has for this reason found wide applications in
both materials and supramolecular chemistry.2 In recent years,
several nonlinear optical (NLO) investigations on TTF donor-
acceptor dyads have been undertaken, pioneered by Mart´ın and
co-workers,3 while the third-order NLO properties of chro-
mophores incorporating the parent TTF unit have been less
explored.4 A variety of dimeric TTFs have been reported in
which two TTF units are linked by a conjugated spacer that
may convey an intramolecular interaction between the two TTFs.
(3) Selected examples:(a) De Lucas, A. I.; Mart´ın, N.; Sa´nchez, L.; Seonae,
C.; Andreu, R.; Gar´ın, J.; Orduna, J.; Alcala´, R.; Villacampa, B. Tetrahedron
1998, 54, 4655–4662. (b) Bryce, M. R.; Green, A.; Moore, A. J.; Perepichka,
D. F.; Batsanov, A. S.; Howard, J. A. K.; Ledoux-Rak, I.; Gonza´lez, M.; Mart´ın,
N.; Segura, J. L.; Gar´ın, J.; Orduna, J.; Alcala´, R.; Villacampa, B. Eur. J. Org.
Chem. 2001, 1927–1935. (c) Insuasty, B.; Ateinza, C.; Seoane, C.; Mart´ın, N.;
Gar´ın, J.; Orduna, J.; Alcala´, R.; Villacampa, B. J. Org. Chem. 2004, 69, 6986–
6995.
(4) Hansen, J. A.; Becher, J.; Jeppesen, J. O.; Levillain, E.; Nielsen, M. B.;
Petersen, B. M.; Petersen, J. C.; S¸ahin, Y. J. Mater. Chem. 2004, 14, 179–184.
(5) Otsubo, T.; Kochi, Y.; Bitoh, A.; Ogura, F. Chem. Lett. 1994, 23, 2047–
2050.
* To whom correspondence should be addressed. Phone: +45 3532 0210.
Fax: +45 3532 0212.
(1) Nielsen, M. B.; Sauer, S. P. A. Chem. Phys. Lett. 2008, 453, 136–139,
and references cited therein.
(2) (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. (e) Special issue on molecular conductors. Batail,
P. Chem. ReV. 2004, 104, 4887–5781.
10.1021/jo802190q CCC: $40.75
Published on Web 11/18/2008
2009 American Chemical Society
J. Org. Chem. 2009, 74, 375–382 375

Andersson et al.
CHART 1
SCHEME 1. Synthesis of Donor-Acceptor trans-DEE
diethynylethene (DEE). The optical properties as well as
electrochemical and spectroelectrochemical properties are com-
pared to scaffolds incorporating dithiafulvene (DTF, i.e., TTF
“half-unit”) as donor unit, cis/trans-1 and TEE 2.7 These
previously investigated DTF compounds unfortunately suffered
from irreversible electrochemical oxidations. As eventually we
want to combine the inherent TTF redox properties with the
scaffolding power of acetylenic modules,8 we found it attrac-
tive to substitute the DTF donor groups with “intact” TTFs.
The syntheses rely on modifications of the Sonogashira pal-
ladium-catalyzed coupling reaction9 between aryl/vinylic halides
and terminal alkynes and on ready access to the p-nitrophenyl-
based vinylic bromide building blocks trans-3, 4, and 5 recently
developed by us (Chart 1).7
Results and Discussion
Synthesis. The vinylic bromide trans-37 was treated with
ethynyltetrathiafulvalene 65 in a Sonogashira cross-coupling
reaction to give trans-7 in good yield (Scheme 1). To prepare
the corresponding cis compound, the dibromide 47 was con-
verted to cis-3 in a stereoselective hydrogenolysis10 using
Bu3SnH and cat. [Pd(PPh3)4] (Scheme 2). This compound was
subsequently subjected to a Sonogashira cross-coupling reaction
with 6, which furnished cis-7. The alternative one-pot reaction
(4 to cis-7) gave a lower yield compared to that of the stepwise
SCHEME 2. Synthesis of Donor-Acceptor cis-DEE
(6) (a) Bosshard, C.; Spreiter, R.; Gu¨nter, P.; Tykwinski, R. R.; Schreiber,
M.; Diederich, F. AdV. Mater. 1996, 8, 231–234. (b) Spreiter, R.; Bosshard, C.;
Kno¨pfle, G.; Gu¨nter, P.; Tykwinski, R. R.; Schreiber, M.; Diederich, F. J. Phys.
Chem. B 1998, 102, 4451–4465. (c) Gubler, U.; Spreiter, R.; Bosshard, C.;
Gu¨nter, P.; Tykwinski, R. R.; Diederich, F. Appl. Phys. Lett. 1998, 73, 2396–
2398. (d) Gubler, U.; Bosshard, C. AdV. Polym. Sci. 2002, 158, 123–191. (e)
Buresˇ, F.; Schweizer, W. B.; May, J. C.; Boudon, C.; Gisselbrecht, J.-P.; Gross,
M.; Biaggio, I.; Diederich, F. Chem.-Eur. J. 2007, 13, 5378–5387.
(7) (a) Qvortrup, K.; Andersson, A. S.; Mayer, J.-P.; Jepsen, A. S.; Nielsen,
M. B. Synlett 2004, 2818–2820. (b) Andersson, A. S.; Qvortrup, K.; Torbensen,
E. R.; Mayer, J.-P.; Gisselbrecht, J.-P.; Boudon, C.; Gross, M.; Kadziola, A.;
Kilså, K.; Nielsen, M. B. Eur. J. Org. Chem. 2005, 3660–3671.
(8) For reviews on acetylenic scaffolding with DEEs and TEEs, see: (a)
Tykwinski, R. R.; Diederich, F. Liebigs Ann./Recl. 1997, 649–661. (b) Diederich,
F. Chem. Commun. 2001, 219–227. (c) Tykwinski, R. R.; Zhao, Y. M. Synlett
2002, 1939–1953. (d) Nielsen, M. B.; Diederich, F. Chem. Rec. 2002, 2, 189–
198. (e) Nielsen, M. B.; Diederich, F. Synlett 2002, 544–552. (f) Fallis, A. G.
Synlett 2004, 2249–2267. (g) Nielsen, M. B.; Diederich, F. Chem. ReV. 2005,
105, 1837–1867.
(9) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467–
4470.
(10) (a) Uenishi, J.; Kawahama, R.; Yonemitsu, O.; Tsuji, J. J. Org. Chem.
1996, 61, 5716–5717. (b) Uenishi, J.; Kawahama, R.; Yonemitsu, O.; Tsuji, J.
J. Org. Chem. 1998, 63, 8965–8975.
process. Although the two stereoisomers have nonidentical
proton chemical shift values, a verification of the proposed
stereochemistry was not possible based only on the H NMR
1
376 J. Org. Chem. Vol. 74, No. 1, 2009

TTF-Substituted Di- and Tetraethynylethenes
SCHEME 3. Synthesis of Donor-Acceptor TEE
SCHEME 4. Synthesis of TTF-TEE
hence we decided to use the catalyst system, [Pd(PhCN)2Cl2]/
t-Bu3P/CuI, developed by Hundertmark et al.,14 which allows
coupling of aryl bromides with a wide variety of terminal
acetylenes at rt. By using this catalyst system in combination
with ultrasonification at 30 °C for 5 h we obtained the TTF-
substituted TEE 12 in a yield of 31%. Substituting [Pd(PhCN)2-
Cl2]/t-Bu3P with the commercially available [Pd(t-Bu3P)2]
improved the yield to 40% and reduced the reaction time to 30
min employing ultrasound conditions. Promotion of the Sono-
gashira cross-coupling reaction using ultrasound has attracted
considerable interest in recent years.7,15 In particular, we have
found this method useful for Sonogashira reactions that produce
compounds of limited stability and that hence would not tolerate
conditions of conventional or microwave heating.7
Finally, a cross-coupling reaction was performed between the
vinylic dibromide 1316 and the alkyne 6 (Scheme 4), which
furnished TTF-TEE 14 lacking the acceptor substituents. The
rather low yield of this 2-fold coupling reaction may in part be
ascribed to the lower reactivity of the vinylic dibromide in
the absence of the electron-withdrawing p-nitrophenyl groups.
X-ray Crystallographic Analysis. Single crystals of cis-3
were obtained by slow evaporation of a CH2Cl2/heptane solution
at rt. The structure obtained by X-ray crystallographic analysis
is shown in Figure 1a. Unfortunately, we did not manage to
obtain crystals of cis-7 that were suitable for X-ray analysis.
However, crystals of trans-7 suitable for X-ray crystallographic
analyses were grown by slow evaporation of a CDCl3 solution
at rt. The X-ray crystal structure of trans-7 confirms the
postulated isomer designation (Figure 1b).
spectra. Yet, the IR spectrum of trans-7 showed a strong
absorption at 933 cm-1 which was not present in the spectrum
of cis-7. This band was assigned to a C-H out-of-plane twisting
mode characteristic of a trans substituted ethene. Conclusive
evidence for the isomer designations was obtained from an X-
ray crystal structure of trans-7 (vide infra).
Since the solubility of the previously reported DTF-func-
tionalized TEE 2 in organic solvents was rather low,7 we decided
to incorporate hexyl substituents at the TTF units in the
analogous TTF-functionalized TEE. The synthesis is revealed
in Scheme 3. Monolithiation of the known 4,5-dihexylTTF 811
was accomplished using LDA in THF at -78 °C and subsequent
trapping with 1,2-diiodoethane afforded 9 in 78% yield. Next,
a Sonogashira cross-coupling reaction with trimethylsilylacety-
lene gave 10 in 62% yield. This relatively low yield was due to
a competing deiodination reaction producing 4,5-dihexylTTF
in almost 30% (isolated yield). This side reaction has been
reported previously and seems to be more pronounced for
electron-rich electrophiles.11,12 Desilylation provided the ter-
minal alkyne 11. Attempted coupling of dibromide 5 with 11
using the [Pd(PPh3)2Cl2]/CuI catalyst system failed to provide
the target donor-acceptor compound 12. Many cross-couplings
are enhanced by using bulky, electron-rich phosphines,13 and
All the atoms of trans-7 are nearly coplanar, except that the
central double bond is twisted ca. 23° in respect to the plane of
the rest of the molecule. The crystal packing diagram (Figure
(12) (a) Nguyen, P.; Yuan, Z.; Agocs, L.; Lesley, G.; Marder, T. B. Inorg.
Chim. Acta 1994, 220, 289–296. (b) Tretyakov, E. V.; Knight, D. W.; Vasilevsky,
S. F. J. Chem. Soc., Perkin Trans. 1 1999, 3713–3720. (c) Chou, M.-Y.; Mandal,
A. B.; Leung, M.-k. J. Org. Chem. 2002, 67, 1501–1505. (d) Zeidler, J. Collect.
Czech. Chem. Commun. 2004, 69, 1610–1630. (e) Nierle, J.; Barth, D.; Kuck,
D. Eur. J. Org. Chem. 2004, 867–872.
(13) For a review on electron-rich phosphines in organic synthesis, see:
Valentine, D. H., Jr.; Hillhouse, J. H. Synthesis 2003, 2437–2460.
(14) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett.
2000, 2, 1729–1731.
(15) (a) Palimkar, S. S.; Kumar, P. H.; Lahoti, R. L.; Srinivasan, K. V.
Tetrahedron 2006, 62, 5109–5115. (b) Fu, N.; Zhang, Y.; Yang, D.; Chen, B.;
Wu, X. Catalysis Comm. 2008, 9, 976–979.
(11) Andersson, A. S.; Kilså, K.; Hassenkam, T.; Gisselbrecht, J.-P.; Boudon,
C.; Gross, M.; Nielsen, M. B.; Diederich, F. Chem.–Eur. J. 2006, 12, 8451–
8459.
(16) Lange, T.; van Loon, J.-D.; Tykwinski, R. R.; Schreiber, M.; Diederich,
F. Synthesis 1996, 537–550.
(17) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
J. Org. Chem. Vol. 74, No. 1, 2009 377

Andersson et al.
FIGURE 1. Ortep-style plots of the X-ray crystal structures of cis-3 (a) and trans-7 (b) (generated using the program Platon17).
CHART 2
comparison, the same quinoid character was found for the
p-nitrophenyl ring of p-NO2C4H6C≡CC6H4NH2 (δr ) 0.011 Å).
Compound 15,20 with a single triple bond between the DTF-
unit and the p-nitrophenyl ring, exhibits a δr value of 0.027 Å,
indicating a larger contribution of the quinoid structure in the
ground-state compared with the other molecules (Chart 2).
Electronic Absorption Spectroscopy. UV–vis absorption
data for the new compounds cis-7, trans-7, 12, and 14 are
collected in Table 1 together with literature data for cis-1, trans-
1, and 2.7b
FIGURE 2. Crystal packing of trans-7 (generated using the program
PyMol18).
The longest-wavelength absorption for TTF-TEE 12 extends
beyond 850 nm and is significantly red-shifted relative to that
of cis-7 and trans-7 (Figure 4). This decrease in the HOMO-
LUMO gap is ascribed to the more extended conjugated TEE
system compared with the DEE system. The strongest absorption
band in trans-7 exhibits approximately twice the molar absorp-
tivity than that of the corresponding absorption band of cis-7;
a trend that was also observed for cis-1 and trans-1.7b A large
red-shift of the longest-wavelength absorption is seen when
comparing the TTF-substituted compounds cis-7, trans-7 and
12 with the corresponding DTF-substituted compounds cis-1,
trans-1, and 2. These red-shifts are explained by the stronger
donor character of TTF compared to that of DTF, and hence a
higher energy HOMO, which decreases the HOMO-LUMO
gap. Interestingly, the CT absorption bands in the TTF
compounds have much smaller molar absorptivities compared
to the absorption bands at higher energy. This is in contrast
with the finding from the DTF-substituted compounds, for which
the CT bands exhibit roughly the same molar absorptivity as
FIGURE 3. Definition of bond lengths for calculation of quinoid
character (δr).
2) shows a short contact between one of the vinyl hydrogen
atoms and an oxygen atom of the nitro group of a neighbor
molecule (2.56 Å). Molecules of trans-7 form pseudodimers,
in which the molecules stack with an antiparallel molecular
dipole orientation and an intermolecular distance of 3.64 Å.
The bond length alternation in the ground-state of the
p-nitrophenyl ring can be expressed by the quinoid character
of the ring defined as in Figure 3.19
Interestingly, no change in quinoid character was found upon
replacing the DTF-donor (trans-1, δr ) 0.011 Å) with the
stronger electron donor TTF (trans-7, δr ) 0.011 Å). For
(18) DeLano, W. L. The PyMOL Molecular Graphics System; DeLano
Scientific: San Carlos, CA, 2002.
(19) Dehu, C.; Meyers, F.; Bredas, J. L. J. Am. Chem. Soc. 1993, 115, 6198–
6206.
(20) 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.
378 J. Org. Chem. Vol. 74, No. 1, 2009

TTF-Substituted Di- and Tetraethynylethenes
a
TABLE 1. UV–vis Absorption Maxima (λmax) and Molar Absorptivities (E) in CHCl3
compd
λ
max [nm] (ꢀ [M-1 cm-1])
cis-7
trans-7
12
303 (27700)
325 (sh, 24600)
362 (sh, 17400)
349 (42300)
433 (30800)
364 (25100)
423 (12600)
424 (29800)
343 (sh, 33400)
497 (br, 2700)
490 (br, 5900)
661 (br, 8400)
377 (24800)
300 (sh, 51600)
301 (32900)
305 (sh, 16800)
305 (sh, 16400)
301 (29900)
336 (61500)
322 (33200)
330 (20600)
325 (19300)
327 (34600)
14
520 (br, 6100)
491 (35400)
cis-1b
trans-1b
2b
381 (23000)
525 (sh, 29000)
a sh ) shoulder; br ) broad. b Reference 7b.
CHART 3
FIGURE 4. UV–vis spectra of acetylenic TTF chromophores in CHCl3.
the higher energy transitions. It is also noteworthy that
compound 14 devoid of the p-nitrophenyl acceptor groups
exhibits a significant CT absorption band, albeit blue-shifted
relative to that of 12. This observation agrees with previous
findings that the TEE core is by itself a good electron acceptor.21
Thus, tetrakis(trimethylsilylethynyl)ethene (16) is reduced at
–1.96 V vs Fc+/Fc.21
TABLE 2. Cyclic Voltammetry Data Measured in CH2Cl2 + 0.15 M
Bu4NPF6a
Compd
E01 [V]b
E02 [V]b
Nonlinear Optical Properties. The third-order optical po-
larizability of 12 was determined by measuring the third-order
susceptibility of a solution of the compound in chloroform at
various concentrations. We used Degenerate Four Wave Mixing
with 1-ps long pulses at a wavelength of λ ) 1500 nm. The
rotational average of the third-order polarizability of 12 was
found to be γrot ) 9 ( 3 × 10-48 m5V-2. This value can be
compared to related TEE molecules incorporating instead ani-
lino donor groups,6 which were measured by third-harmonic
generation from a wavelength of 1.9 µm. The best molecule
(17) was found to have a third-order polarizability γrot ) 14.8
( 0.7 × 10-48 m5V-2, while TEE 18 incorporating the same
11.6 ( 0.6 × 10-48 m5V-2 (Chart 3).6d Thus, compound 12
exhibits NLO properties that resemble those of related arylated
TEEs,6b and the high γ-value indicates that larger acetylenic
TEE-TTF scaffolds are worthwhile to pursue in the future. As
TTF is a particularly useful π-donor molecule for intermolecular
host-guest systems based on donor-acceptor interactions,2b
larger supramolecular assemblies based on such scaffolds are
also interesting targets. In addition, it should be noted that the
three reversible redox states of TTF (0, +1, +2) makes it a
particularly attractive unit in redox-controlled NLO devices.
Indeed, compound 12 exhibits reversible electrochemistry (vide
infra) in contrast to the previously investigated TEE-extended
TTF 2.
cis-7
trans-7
10
12
14
+0.13
+0.01
- 0.07
0.00
+0.04
+0.02
+0.61
+0.52
+0.45
+0.48
+0.45c
+0.55
19
a All potentials versus Fc+/Fc. E0 ) (Epc + Epa)/2, where Epc and Epa
are the cathodic and anodic peak potentials, respectively. Index 1 and 2
refers to the first and second reversible oxidation steps, respectively.
c Cathodic peak is enhanced, presumably owing to adsorption.
b
two well-resolved reversible oxidations in CH2Cl2 (Table 2,
Figure 5). For compound 14 some adsorption of the oxidized
species seems to occur at the electrode as judged from the
appearance of the CV. For both 12 and 14, each oxidation step
is ascribed to two electrons and hence the two TTFs act as
independent redox centers. The first oxidation of 14 is anodically
shifted relative to the TTF reference compound 19, while the
second oxidation step occurs more readily. Both oxidation steps
for cis-7 are anodically shifted relative to those of trans-7. In
cis-7 the electron-withdrawing p-nitrophenyl-substituent is
forced closer in space to the TTF moiety than in trans-7, and
this interaction may possibly account for the more difficult
oxidation of cis-7 relative to that of trans-7. The redox potentials
of the parent ethynylTTF 19 are very similar to those of trans-
7, which seems to indicate that the through-bond interaction
alone of the donor and acceptor is not enough to alter the
oxidation potentials (Chart 4). This conclusion is in agreement
with the small ground-state quinoid character ascertained by
X-ray crystallography as well as by frontier orbital calculations
(vide infra).
p-nitrophenyl acceptor groups as 12 was found to have γrot
)
Electrochemistry. Cyclic voltammetry (CV) data show that
the novel TTF compounds cis/trans-7, 10, and 12 experience
(21) Gisselbrecht, J.-P.; Moonen, N. N. P.; Boudon, C.; Nielsen, M. B.;
Diederich, F.; Gross, M. Eur. J. Org. Chem. 2004, 2959–2972.
J. Org. Chem. Vol. 74, No. 1, 2009 379

Andersson et al.
TABLE 3. Low-Energy Absorption Maxima (λmax) of Oxidized
Species in CH2Cl2 (+0.15 M Bu4NPF6)a
Compd
λmax [nm]
[cis-7]+
[cis-7]2+
[trans-7]+
[trans-7]2+
122+
426, 618
550 (br)b
409, 626
528 (br)
438, 678
550 (br)
430, 580
390
124+
c
TTF+
c
TTF2+
a br ) broad. b Spectral evolution did not show fully isosbestic
behavior for this oxidation process. c Reference 22.
Computational Study. Compound 12 devoid of the hexyl
substituents was subjected to a computational study at the
B3LYP/6-31G(d) level using the Gaussian03 program pack-
age.23 The optimized structure deviates slightly from planarity.
The frontier orbitals are shown in Figure 7. Both the HOMO-1
and HOMO are located almost entirely at the TTF units, while
the LUMO and LUMO+1 include both the nitrophenyl groups
and the central TEE spacer and in the case of the LUMO also
a part of the TTFs. The location of the HOMO at the donor
groups agrees well with the relatively small change in electro-
chemical properties induced by the TEE unit and contrasts the
appearance of the HOMO of 2 that was found previously to
include both the DTFs and the TEE spacer.7b
FIGURE 5. Cyclic voltammograms measured in CH2Cl2 + 0.15 M
Bu4NPF6. All potentials vs Fc+/Fc.
CHART 4
Owing to the electron-donating effect of the hexyl substituents
both oxidation steps of 10 are lowered by ca. 0.1 V relative to
those of 19. The oxidation steps for TEE 12 are anodically
shifted relative to those of the corresponding ethynylTTF 10.
Spectroelectrochemistry of TEE 12 is shown in Figure 6. For
the first oxidation step (electrolysis at +0.10 V versus Fc+/Fc),
absorption bands at λmax ) 438 and 678 nm are observed and
attributed to the dication 122+. These evolving absorption bands
are assigned to intrinsic absorptions of the two TTF radical
cations. The spectral evolution shows isosbestic points (not
shown), which suggest that disproportionation does not take
place; that is, mixed oxidative states are not observed. The
second oxidation (electrolysis at +0.58 V versus Fc+/Fc),
generating the tetracation 124+, is accompanied by a decrease
in the 678-nm absorption, while a broad absorption around 550
nm appears. For comparison, the parent TTF radical cation
exhibits absorptions at 430 and 580 nm, while the dication
absorbs at 390 nm.22 The lowest energy absorption maxima are
collected in Table 3 together with spectroelectrochemical data
of cis/trans-7. The extended conjugation results in significantly
red-shifted absorption maxima of the oxidized species of the
acetylenic TTF scaffolds relative to those of the parent TTF.
Conclusions
Palladium-catalyzed cross-coupling reactions have allowed
the synthesis of DEE/TEE-TTFs incorporating p-nitrophenyl
acceptor groups. These chromophores exhibit charge-transfer
absorptions that are red-shifted relative to those of previously
investigated DEE/TEE-DTFs. TEE compound 12 incorporating
two geminal TTF groups and two geminal p-nitrophenyl groups
exhibits a high third-order optical nonlinearity that is comparable
to that of related arylated TEEs, which makes it an attractive
candidate for future device fabrication. It is noteworthy that this
compound exhibits two reversible two-electron oxidations. These
reversible redox properties were not experienced by the previ-
ously investigated TEE-DTF 2 incorporating two DTFs instead
of two TTFs. The π-electrons of the DTF units in 2 are
considerably delocalized onto the TEE unit; this delocalization
does not seem to be as significant for the π-electrons of the
TTF units in 12 (and neither for those in cis/trans-7) as judged
from both cyclic voltammetry data and the appearance of the
HOMO and HOMO-1. The optical properties of the cationic
species were investigated in detail by spectroelectrochemistry,
(22) Huchet, L.; Akoudad, S.; Levillain, E.; Roncali, J.; Emge, A.; Ba¨uerle,
P. J. Phys. Chem. B 1998, 102, 7776–7781.
(23) 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.; Bakken, V.; 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.: Wallingford, CT, 2004.
FIGURE 6. UV–vis spectroelectrochemistry of 12 in CH2Cl2 (+ 0.15
M Bu4NPF6). Electrolysis at +0.10 V vs Fc+/Fc generates 122+, and
electrolysis at +0.58 V vs Fc+/Fc generates 124+
.
380 J. Org. Chem. Vol. 74, No. 1, 2009

TTF-Substituted Di- and Tetraethynylethenes
FIGURE 7. Frontier orbitals of 12 (devoid of the hexyl substituents) (B3LYP/6-31G(d)).
cis-1-Bromo-4-(4-nitrophenyl)but-1-en-3-yne (cis-3). To a
which revealed significantly red-shifted absorption maxima of
the oxidized species relative to those of the parent TTF. All in
all, the incorporation of TTF rather than DTF is an important
structural modification, and larger acetylenic scaffolds based
on TEE-TTFs are attractive targets to pursue in the future.
solution of the dibromide 4 (415 mg, 1.25 mmol) in toluene (10
mL) cooled to 0 °C was added [Pd(PPh3)4] (72 mg, 0.063 mmol)
followed by Bu3SnH (0.35 mL, 1.3 mmol), and the mixture was
stirred at 0 °C for 2.5 h. Heptane (75 mL) was added and the organic
phase washed with H2O (2 × 50 mL), dried (MgSO4), concentrated
in vacuo, and removal of volatile stannane byproducts was effected
by oil pump vacuum (ca. 0.1 mmHg) overnight. Column chroma-
tography (SiO2, EtOAc/heptane 1:19) afforded cis-3 (184 mg, 58%)
as an off-white solid. mp 87-88 °C. 1H NMR (300 MHz, CDCl3):
δ ) 6.55 (d, 7.6 Hz, 1 H), 6.78 (d, 7.6 Hz, 1 H), 7.63 (d, 9.1 Hz,
2 H), 8.19 (d, 9.1 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ )
90.0, 94.6, 115.0, 120.4, 123.6, 129.5, 132.4, 147.3. IR (KBr): ν˜
) 3436 (br), 3081 (m), 2200 (w), 1636 (w), 1595 (s), 1576 (m),
1513 (vs), 1490 (m), 1338 (vs), 1319 (s), 1285 (m), 1171 (w), 1107
(s), 980 (w), 853 (vs), 772 (w), 749 (m), 730 (s), 685 (m), 578
(m), 521 (m) cm-1. MS(GC): m/z (%) ) 251 [M+, 100%], 253
(98%). HR-MS(FAB): m/z ) 251.9675 [M + H+] (calcd for
C10H7NO2Br: 251.9660).
cis-4-[6-(4-Nitrophenyl)hex-3-ene-1,5-diynyl]tetrathiaful-
valene (cis-7). To an argon-degassed solution of cis-3 (25 mg, 99
µmol), 6 (35 mg, 153 µmol) and i-Pr2NH (0.1 mL) in toluene (2
mL) were added [Pd(t-Bu3P)2] (3.5 mg, 7 µmol) and CuI (1.9 mg,
10 µmol) and the mixture was stirred at rt for 2 h. Then, the reaction
mixture was filtered through a short plug of silica (SiO2, CH2Cl2/
cyclohexane 1:1). Column chromatography (SiO2, CHCl2/heptane,
1:2) afforded cis-7 (29 mg, 73%) as a red solid. mp 147-148 °C.
1H NMR (300 MHz, CDCl3): δ ) 6.16 (s, 2 H), 6.40 (s, 1 H), 6.41
(s, 1 H), 6.64 (s, 1 H), 7.74 (d, 8.8 Hz, 2 H), 8.29 (d, 8.8 Hz, 2H).
13C NMR (75 MHz, CD2Cl2): δ ) 89.6, 92.0, 92.1, 96.5, 107.7,
114.7, 116.0, 119.5, 119.7, 120.2, 120.9, 124.2, 127.1, 130.1, 133.2,
147.9. IR (KBr): ν˜ ) 3436 (br), 3073 (m), 2925, (w), 2169 (m),
1592 (s), 1510 (vs), 1489 (w), 1338 (vs), 1308 (w), 1286 (w), 1275
(w), 1255 (vw), 1228 (w), 1169 (w), 1107 (m), 1087 (m), 894 (w),
853 (s), 822 (w), 797 (m), 779(w), 747 (s), 685 (w), 660 (m), 644
(m), 513 (w) cm-1. MS(FAB): m/z ) 399 [M+]. HR-MS(FAB):
m/z ) 398.9527 [M+] (calcd for C18H9NO2S4: 398.9516). Anal.
calcd for C18H9NO2S4: C, 54.11; H, 2.27; N, 3.51. Found: C, 53.84;
H, 2.20; N, 3.36.
Experimental Section
General Methods. All reactions were carried out under an
atmosphere of Ar or N2 by applying a positive pressure of the
protecting gas. Deactivation of silica was done by stirring the silica
with Et3N/heptane 1:9, packing the column and finally flushing with
heptane. 13C NMR data were calibrated using δ(CDCl3) ) 77.0
ppm and δ(CD2Cl2) ) 54.0 ppm.
Nonlinear Optics. The third-order susceptibility of compound
12 dissolved in chloroform was determined by degenerate four wave
mixing (DFWM) at a wavelength of 1500 nm and using a 1-kHz
pulsed laser (pulse duration 1 ps). The DFWM signal for varying
concentrations of the solution was measured along with an identical
cuvette of chloroform for reference, and the signals where calibrated
using a fused silica reference with a third-order susceptibility of
1.9 × 10-22 m2V-2
.
Electrochemistry and UV-Vis Absorption Spectroscopy.
Cyclic voltammetry was measured using a platinum working
electrode and a Pt wire counter electrode. All potentials are
expressed relative to that of Fc+/Fc and were measured in CH2Cl2
with 0.15 M Bu4NPF6 as supporting electrolyte; scan rate 0.1 V
s-1. All measured potentials are uncorrected for ohmic drop.
Spectroelectrochemical experiments were performed in a 1-mm
absorption cuvette (Quartz), the counter electrode was separated
from the solution by a glass frit, and a Pt grid (mesh 400) was
used as working electrode. Setting the potential at ca. 0.1 V more
oxidative value than the peak potentials found from cyclic volta-
mmetry, the UV/vis spectra of the neutral and cationic species were
recorded on a UV-vis spectrophotometer. The same spectropho-
tometer was used to determine the molar absorptivities of the neutral
species (1-cm path length cuvette).
trans-4-[6-(4-Nitrophenyl)hex-3-ene-1,5-diynyl]tetrathiaful-
valene (trans-7). To a solution of the bromide trans-3 (43 mg,
0.17 mmol) in argon-degassed toluene (1.5 mL) was added
[Pd(PPh3)4] (10 mg, 8.6 µmol), and the mixture was stirred at rt
for 0.5 h. Then i-Pr2NH (0.3 mL) was added followed by the
terminal alkyne 6 (42 mg, 0.18 mmol) in degassed THF (1 mL).
The reaction mixture was degassed with argon for a few minutes,
whereupon CuI (7 mg, 37 µmol) was added. After stirring for 3 h
at rt, the mixture was filtered through a short plug of silica (SiO2,
CH2Cl2). Column chromatography (SiO2, CH2Cl2/cyclohexane, 1:3)
afforded trans-7 (56 mg, 82%) as a red solid. mp 207-209 °C. 1H
NMR (300 MHz, CDCl3): δ ) 6.25 (s, 1 H), 6.26 (s, 1 H), 6.33 (s,
1 H), 6.34 (s, 1 H), 6.57 (s, 1 H), 7.58 (d, 8.8 Hz, 2 H), 8.20 (d,
8.8 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ ) 87.2, 91.8, 92.7,
93.6, 107.5, 113.9, 115.5, 118.8, 119.1, 120.6, 121.5, 123.7, 126.5,
129.5, 132.3, 147.2. IR (KBr): ν˜ ) 3436 (br), 3073 (w), 2167 (m),
1595 (m), 1583 (m), 1510 (vs), 1489 (w), 1339 (vs), 1309 (m),
1285 (w), 1254 (w), 1224 (w), 1173 (w), 1109 (m), 933 (s), 853
4,5-Dihexyl-4′-iodotetrathiafulvalene (9). To a solution of 4,5-
dihexylTTF 8 (534 mg, 1.43 mmol) in THF (25 mL) cooled to
-78 °C was added freshly prepared LDA (2.64 mL, 0.65 M in THF,
1.72 mmol) dropwise and the mixture was stirred for 2 h at low
temperature. Freshly purified 1,2-diiodoethane (484 g, 1.72 mmol)
was added in one portion and the mixture stirred for 1 h and then
warmed to 0 °C, quenched by addition of saturated, aqueous
Na2S2O3 (2 mL) and concentrated in vacuo. Et2O (100 mL) was
added and the organic layer washed with H2O (2 × 100 mL), dried
(MgSO4) and concentrated in vacuo. Column chromatography
(deactivated SiO2, Et2O/pentane 2:98) afforded 9 as an orange oil
(558 mg, 78%). 1H NMR (300 MHz, CDCl3): δ ) 0.88 (t, 6.6 Hz,
6 H), 1.2-1.4 (m, 12 H), 1.42-1.51 (m, 4 H), 2.33 (t, 7.9 Hz, 4
H), 6.38 (s, 1 H). 13C NMR (75 MHz, CDCl3): δ ) 14.0, 22.5,
28.7 (two overlapping), 29.6, 31.5, 63.9, 109.1, 112.0, 124.3, 128.7
(x 2). MS(FAB): m/z ) 498 [M+]. HR-MS(EI): m/z ) 498.0032
[M+] (calcd for C18H27IS4: 498.0040).
(s), 829 (w), 797 (w), 777(m), 748 (m), 686 (w), 649 (m) cm-1
.
4,5-Dihexyl-4′-(trimethylsilylethynyl)tetrathiafulvalene (10).
A solution of iodide 9 (219 mg, 0.439 mmol) in Et3N (5 mL)
was degassed vigorously with argon, whereupon trimethylsilyl-
MS(FAB): m/z ) 399 [M+]. HR-MS(FAB): m/z ) 398.9518 [M+]
(calcd for C18H9NO2S4: 398.9516).
J. Org. Chem. Vol. 74, No. 1, 2009 381

Andersson et al.
acetylene (93 µL, 0.64 mmol), [Pd(PPh3)2Cl2] (15 mg, 21 µmol)
and CuI (8 mg, 42 µmol mmol) were added. The reaction mixture
was stirred for 2 h at rt and then heptane (50 mL) was added. The
reaction mixture was filtered, and the filtrate washed with H2O (2
× 50 mL), dried (MgSO4) and concentrated in vacuo. Column
chromatography (deactivated SiO2, heptane) afforded 10 as an
orange oil (128 mg, 62%). 1H NMR (300 MHz, CDCl3): δ ) 0.19
(s, 9 H), 0.88 (t, 6.6 Hz, 6 H), 1.2-1.4 (m, 12 H), 1.41-1.52 (m,
4 H), 2.33 (t, 7.9 Hz, 4 H), 6.48 (s, 1 H). 13C NMR (75 MHz,
CDCl3): δ ) -0.4, 14.0, 22.5, 28.7 (two overlapping), 29.6, 31.5,
95.2, 99.5, 106.4, 112.1, 116.0, 125.8, 128.5, 128.8. MS(FAB): m/z
) 468 [M+]. HR-MS(FAB): m/z ) 468.1474 [M+] (calcd for
C23H36S4Si: 468.1469).
4-Ethynyl-4′,5′-dihexyltetrathiafulvalene (11). To a solution
of 10 (128 mg, 0.273 mmol) in THF (15 mL) and MeOH (3 mL)
was added K2CO3 (40 mg, 0.289 mmol) and the mixture was stirred
for 30 min at rt. Heptane (50 mL) was added and the organic phase
was washed with H2O (25 mL) and brine (25 mL), dried (MgSO4)
and concentrated in vacuo to give the terminal alkyne 11 (107 mg,
99%) as an orange, instable oil. 1H NMR (300 MHz, CDCl3): δ )
0.89 (t, 6.6 Hz, 6 H), 1.2-1.4 (m, 12 H), 1.41-1.54 (m, 4 H),
2.33 (t, 7.9 Hz, 4 H), 3.21 (s, 1 H), 6.56 (s, 1 H). MS(GC): m/z )
396 [M+].
28.8, 29.7 (×2), 31.5, 90.7, 91.0, 92.3, 98.1, 104.7, 114.6, 114.9,
115.5, 118.3, 123.8, 128.7 (x 2), 129.0, 129.1, 132.9, 147.7. IR
(KBr): ν˜ ) 3450 (br), 3096 (w), 2955, (s), 2925 (s), 2854 (s), 2167
(s), 1591 (s), 1519 (vs), 1459 (m), 1439 (m), 1372 (w), 1341 (vs),
1287 (w), 1265 (w), 1173 (w), 1105 (m), 959 (w), 852 (s), 835
(m), 779(m), 748 (s), 686 (m) 529 (w) cm-1. MS(FAB): m/z )
1106 [M+]. Anal. calcd for C58H62N2O4S8: C, 62.89; H, 5.64; N,
2.53. Found: C, 62.76; H, 5.68; N, 2.47.
4-[6-(4-Triisopropylsilyl)-4-(4-triisopropylsilylethynyl)-3-
(tetrathiafulvalenylethynyl)hex-3-ene-1,5-diynyl]tetrathiaful-
valene (14). To a solution of dibromide 13 (100 mg, 0.183 mmol)
and terminal alkyne 6 (117 mg, 0.512 mmol) in argon-degassed
toluene/THF (1:1, 6 mL) and i-Pr2NH (72 µL) were added [Pd(t-
Bu3P)2] (13 mg, 26 µmol) and CuI (9.8 mg, 51 µmol). After stirring
for 24 h, the reaction mixture was filtered through a short plug of
silica (SiO2, CH2Cl2/heptane 1:1) and concentrated. The residue
was subjected to column chromatography (SiO2, EtOAc/heptane
1:6) to afford 14 (37.5 mg, 24%) as a dark violet solid. mp 240-241
°C (dec). 1H NMR (300 MHz, CDCl3): δ ) 1.12 (m, 42 H), 6.32/
6.33 (2s, 4 H), 6.58 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ )
11.2, 18.7, 89.6, 90.2, 103.5, 104.3, 107.8, 113.6, 114.9, 115.5,
118.5, 118.7, 119.2, 126.9. HR-MS(ES): m/z ) 840.1041 [M+]
(calcd. for C40H48S8Si2: 840.1060).
4-[6-(4-Nitrophenyl)-4-(4-nitrophenylethynyl)-3-(tetrathiaful-
valenylethynyl)hex-3-ene-1,5-diynyl]tetrathiafulvalene (12). To
a solution of dibromide 5 (25 mg, 53 µmol) and 11 (70 mg, 176
µmol) in argon-degassed toluene/THF (1:1, 1 mL) and i-Pr2NH
(40 µL) were added [Pd(t-Bu3P)2] (2.3 mg, 4.5 µmol) and CuI (1
mg, 5 µmol) and the mixture was subjected to ultrasonification at
rt for 30 min. Then, the reaction mixture was filtered through a
short plug of neutral alumina (CH2Cl2/cyclohexane 1:1) and
concentrated to the point of precipitation. The crude product was
filtered and washed with cyclohexane to afford 12 (23 mg, 40%)
Acknowledgment. We thank University of Copenhagen and
the Danish Research Agency for financial support (grant #2111-
04-0018). We are grateful for the valuable help from Mr.
Flemming Hansen in the collection of the X-ray data.
Supporting Information Available: NMR spectra of all new
compounds, X-ray crystallographic data for cis-3 and trans-7
(cif files), and atom coordinates and energies obtained from
computational study on compound 12 (without hexyl substitu-
ents). This material is available free of charge via the Internet
at http://pubs.acs.org.
1
as a dark green solid. mp >250 °C (dec). H NMR (300 MHz,
CDCl3): δ ) 0.89 (t, 6.2 Hz, 12 H), 1.2-1.4 (m, 24 H), 1.4-1.6
(m, 8 H), 2.37 (m, 8 H), 6.73 (s, 2 H), 7.79 (d, 8.8 Hz, 4 H), 8.28
(d, 8.8 Hz, 4 H). 13C NMR (75 MHz, CDCl3): δ ) 14.0, 22.6,
JO802190Q
382 J. Org. Chem. Vol. 74, No. 1, 2009