Highly π-extended TTF analogues with a conjugated macrocyclic enyne core

Article abstract of DOI:10.1021/ol703038m

The synthesis of a class of highly π-extended tetrathiafulvalene derivatives (1a and 1b) was explored using Sonogashira macrocyclization as a key step. The solid-state structure of 1b was characterized by X-ray single crystallography, showing a substantially bent, S-shaped molecular backbone and an ordered packing geometry in a π-alkyl-alkyl-π stacking fashion. Electronic and redox properties of 1b were investigated by UV-vis absorption, fluorescence spectroscopy, and cyclic voltammetry.

Full text of DOI:10.1021/ol703038m

ORGANIC
LETTERS
2008
Vol. 10, No. 4
657-660
Highly
π-Extended TTF Analogues with
a Conjugated Macrocyclic Enyne Core
Guang Chen, Li Wang, David W. Thompson, and Yuming Zhao*
Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s,
Newfoundland A1B 3X7, Canada
yuming@mun.ca
Received December 18, 2007
ABSTRACT
The synthesis of a class of highly
π-extended tetrathiafulvalene derivatives (1a and 1b) was explored using Sonogashira macrocyclization as
a key step. The solid-state structure of 1b was characterized by X-ray single crystallography, showing a substantially bent, S-shaped molecular
backbone and an ordered packing geometry in a
UV
π
-alkyl
−
alkyl-
π
stacking fashion. Electronic and redox properties of 1b were investigated by
−
vis absorption, fluorescence spectroscopy, and cyclic voltammetry.
Synthesis of π-extended tetrathiafulvalene analogues
(exTTFs) has been an intense and active area in the research
of organic electronic materials.1 For the design of various
TTF derivatives, insertion of π-conjugated spacers such as
vinylogous and/or acene units in between the two dithiole
rings of a TTF parent structure constitutes a popular and
effective approach.1a,2 A prominent class of exTTFs derived
from this approach is the quinonoid-type exTTFs,1,2a-g which
usually exhibit significantly enhanced electron-donating and
charge-transfer properties as well as extensive applications
in molecular optoelectronic materials and devices.
systems as central π-spacers for new exTTF variants. Mart´ın
and co-workers in 2005 reported a class of exTTFs contain-
ing a unique spiroconjugated central core.3 In 1991, Bryce’s
group synthesized a series of bianthro-TTFs,4 which were
later thoroughly investigated by Guldi and co-workers.5
Nielsen6 and Diederich7 have developed a class of conjugated
enyne oligomer bridged exTTFs, while most recently a
sophisticated cyclic aromatic structure, truxene, was em-
ployed by the Mart´ın group to generate a type of truxene-
cored exTTFs.8
Among the numerous π-conjugated cyclic and acyclic
structural motifs that have been employed as spacers for
Lately, much attention has been devoted to the use of
structurally complex and highly extended π-conjugated
(3) Sand´ın, P.; Mart´ınez-Grau, A.; Sa´nchez, L.; Seoane, C.; Pou-Ame´rigo,
R.; Ort´ı, E.; Mart´ın, N. Org. Lett. 2005, 7, 295-298.
(4) Moore, A. J.; Bryce, M. R. J. Chem. Soc., Perkin Trans. 1 1991, 157.
(5) D´ıaz, M. C.; Illescas, B. M.; Mart´ın, N.; Viruela, R.; Viruela, P. M.;
Ort´ı, E.; Brede, O.; Zilbermann, I.; Guldi, D. M. Chem.sEur. J. 2004, 10,
2067.
(6) (a) Ryhding, T.; Petersen, M. Å.; Kilså, K.; Nielsen, M. B. Synlett
2007, 913. (b) Sørensen, J. K.; Vestergaard, M.; Kadziola, A.; Kilså, K.;
Nielsen, M. B. Org. Lett. 2006, 8, 1173. (c) Anderson, 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. (d)
Qvortrup, K.; Jakobsen, M. T.; Gisselbrecht, J.-P.; Boudon, C.; Jensen, F.;
Nielsen, S. B.; Nielsen, M. B. J. Mater. Chem. 2004, 14, 1768.
(7) 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. Eur. J. 2002, 8, 3601.
(1) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104, 4891. (b) Segura, J. L.; Mart´ın, N. Angew. Chem., Int. Ed. 2001, 40,
1371 and references cited therein. (c) Bryce, M. R.; Moore, A. J. Pure
Appl. Chem. 1990, 62, 473-476.
(2) For selected recent examples, see: (a) Christensen, C. A.; Batsanov,
A. S.; Bryce, M. R. J. Org. Chem. 2007, 72, 1301. (b) Christensen, C. A.;
Batsanov, A. S.; Bryce, M. R. J. Am. Chem. Soc. 2006, 128, 10484. (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.-Eur. J. 2006, 12, 2709. (d)
Hudhomme, P.; Salle´, M.; Gautier, N.; Belyasmine, A.; Gorgues, A. ArkiVoc
2006, 49. (e) Christensen, C. A.; Bryce, M. R.; Batsanov, A. S.; Becher, J.
Org. Biomol. Chem. 2003, 1, 511. (f) D´ıaz, M. C.; Illescas, B. M.; Seoane,
C.; Mart´ın, N. J. Org. Chem. 2004, 69, 4492. (g) D´ıaz, M. C.; Illescas, B.;
Mart´ın, N. Tetrahedron Lett. 2003, 44, 945. (h) Fre`re, P.; Boubekeur, K.;
Jubault, M.; Batail, P.; Gorgues, A. Eur. J. Org. Chem. 2001, 3741.
10.1021/ol703038m CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/23/2008

exTTFs, conjugated enyne macrocycles are still relatively
unexplored, despite the fact that conjugated macrocycles have
been known to possess many attractive properties of both
theoretical and practical value to supramolecular, biological,
and materials science.9 To shed more light on this issue, a
new type of highly π-extended TTF analogues 1 (Figure 1)
containing organic materials. Fundamentally important knowl-
edge on the π-electron delocalization pattern and aromatic
characteristics associated with the conjugated enyne cyclic
systems10 in both neutral and oxidized (e.g., dicationic) states
can be rigorously established by both theoretical and
experimental methods. On the other hand, the unique
optoelectronic properties arising from the enyne macrocycle
unit9a,b,e and possible reactivity of Bergman-type cyclization
at the endiyne fragments11 may create new avenues for other
synthetic ventures and practical explorations.
Construction of such a complex conjugated system as 1
is not trivial and requires prudent choice of efficient synthetic
routes to prepare dithiole-acene precursors and suitable ring
closure strategies to build the enyne macrocycle. In this work,
a convergent route using iterative olefination and metal-
catalyzed cross-coupling reactions as key steps was em-
ployed. A brief retrosynthetic bond disconnection analysis
is given in Figure 1, while detailed synthetic steps are
outlined in Scheme 1. The synthesis began with a reaction
between thione 2a or 2b12 with excess anthraquinone 3 in
triethyl phosphite at 150 °C, yielding ketones 4a and 4b,
respectively. This methodology originally developed by
Bryce2a turned out to be particularly effective in the synthesis
of long-alkyl chain substituted compound 4b. Compounds
4a,b were then subjected to a Corey-Fuchs reaction to afford
dibromides 5a and 5b. A Sonogashira coupling between 5
and trimethylsilylacetylene (TMSA) under Pd/Cu catalysis
resulted in compounds 6a,b. Finally, compounds 6a and 6b
were reacted with o-diiodobenzene via a Sonogashira-type
high-dilution macrocyclization protocol, furnishing the de-
sired products 1a and 1b. Besides the cyclization product,
various acyclic oligomeric byproducts were also formed in
Figure 1. Molecular structures of compounds 1 in neutral and
dicationic states.
are introduced below, whose main structural features include
a conjugated bisbenzo enyne macrocycle core and two
dianthro units.
Incorporation of the enyne macrocyclic unit would rep-
resent a significant step in the development of novel TTF-
Scheme 1. Synthesis of exTTFs 1 and Reference Compound 8
658
Org. Lett., Vol. 10, No. 4, 2008

this reaction as characterized by MALDI-TOF MS analysis.
The yield of 1a was trace, and complete isolation of 1a from
concomitant byproducts by flash column chromatography
was not tenable. The low yield and difficulty encountered
in purification of 1a could be a consequence of the poor
solubility of the methyl-substituted exTTF 1a. For n-decyl
chain decorated exTTF 1b, the solubility was dramatically
increased, and a better yield of 26% was achieved in the
macrocyclization step. Compound 1b was thoroughly purified
by silica flash column chromatography and then fully
characterized by IR, 1H and 13C NMR, and MS analyses. In
addition to exTTFs 1a,b, a reference compound 8 was also
prepared for comparison purposes via cross-coupling desi-
lylated 6b with excess iodobenzene under Pd/Cu catalysis.
Interestingly, when a 1:1 molar ratio of desilylated 6b and
iodobenzene was applied in this coupling reaction, a sig-
nificant amount of homo-cross-coupled byproduct 7 was also
produced, which can be viewed as well as an exTTF variant
extended by acene and acyclic cross-conjugated oligo(enyne)
π-spacers.6,7
planar conformation, while the two dithole rings are in a
nearly perpendicular orientation versus the central cyclic
enyne plane. The interplanar angle between the two planes
of adjacent dithiole and anthracene units is measured at 15.5°,
while the angle between the dithiole ring and central cyclic
enyne plane is at 79.3°. These angles are in line with those
bend parameters observed in the crystal structures of quino-
nedimethane-type exTTFs.2a,e
In the crystal lattice, two molecules of 1b are closely
positioned in a manner as depicted in Figure 2b. The chain
length of n-decyl groups in 1b and the central bisbenzo-
cyclic enyne moiety effect a π-alkyl-alkyl-π intermolecular
stacking,2e interlocking the two molecules orthogonally with
respect to one another.
Electronic properties of 1b and its “half-structure”,
compound 8, were investigated by UV-vis and fluorescence
spectroscopic methods. Comparison of the properties of 1b
and 8 has disclosed essential information with respect to
structure-property relationship. Figure 3a shows the UV-
Single crystals of 1b were grown through slow diffusion
of MeOH into a CHCl3 solution of 1b at 4 °C. The crystal
structure of 1b was then investigated by X-ray crystal-
lographic analysis. Two structurally similar polymorphs were
observed in the single crystal of 1b. Figure 2a shows the
Figure 3. (a) UV-vis absorption spectra of 1b and 8 measured in
CHCl3. (b) Fluorescence spectra of 1b (λex ) 531 nm, Φ ) 0.13)
and 8 (λex ) 447 nm, Φ ) 0.064) measured in degassed CHCl3.
Figure 2. (a) ORTEP drawing of compound 1b at the 50%
probability level (n-decyl chains were removed for clarity). (b)
Solid-state packing of two molecules of 1b in the unit cell. Selected
bond lengths (Å): C23-C24 1.361(5), C24-C37 1.474(6), C32-
C37 1.410(5), C31-C32 1.478(7), C31-C38 1.374(7), C38-C40
1.431(5), C40-C41 1.189(5), C41-C42 1.439(5), C42-C47 1.429-
(7). Selected bond angles (deg): C25-C24-C37 114.9(3), C30-
C31-C32 113.8(4), C39-C38-C40 113.9(4), C38-C40-C41
174.3(6), C40-C41-C42 175.2(5), C41-C42-C47 121.1(4).
vis spectra of 1b and 8. Compound 1b exhibits three
significant absorption bands at 528, 419, and 387 nm,
respectively, while reference 8 shows only two absorption
(9) (a) Gholami, M.; Tykwinski, R. R. Chem. ReV. 2006, 106, 4997. (b)
Kar, M.; Basak, A. Chem. ReV. 2007, 107, 2861. (c) Nielsen, M. B.;
Diederich, F. Chem. ReV. 2005, 105, 1837. (d) Iyoda, M.; Kuwatani, Y.;
Yamagata, S.; Nakamura, N.; Todaka, M.; Yamamoto, G. Org. Lett. 2004,
6, 4667. (e) Zhao, T.; Liu, Z.; Song, Y.; Xu, W.; Zhang, D.; Zhu, D. J.
Org. Chem. 2006, 71, 7422.
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572-580. (b) Maurette, L.; Godard, C.; Frau, S.; Lepetit, C.; Soleilhavoup,
M.; Chauvin, R. Chem.sEur. J. 2001, 7, 1165. (c) Matzger, A. J.; Vollhardt,
K. P. C. Tetrahedron Lett. 1998, 39, 6791. (d) Chauvin, R. Tetrahedron
Lett. 1995, 36, 397-400.
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Tykwinski, R. R.; Scott, L. T.; Hopf, H.; Rabinovitz, M. J. Am. Chem.
Soc. 2006, 128, 4703. (b) Rawat, S.; Zaleski, J. M. Synlett 2004, 393.
ORTEP plot highlighting the geometric features of the
conjugated molecular skeletons for one of the polymorphs.
Compound 1b displays a substantially bent “S”-shaped
structure, in which the central macrocylic enyne assumes a
(8) Pe´rez, E. M.; Sierra, M.; Sa´nchez, L.; Torres, M. R.; Viruela, R.;
Viruela, P. M.; Ort´ı, E.; Mart´ın, N. Angew. Chem., Int. Ed. 2007, 46, 1847.
Org. Lett., Vol. 10, No. 4, 2008
659

bands at 446 and 347 nm. The lowest-energy absorption band
of 1b, relative to that of its half-structure 8, has substantially
red-shifted by ca. 3482 cm-1, indicating that the HOMO-
LUMO gap of 1b has been substantially reduced due to the
extensive π-delocalization at the central enyne cyclic con-
jugated unit.
Table 1. Redox Potential Data of Compounds 1b and 8 (vs
Ag/AgCl)
oxidation (V)
Epa
reduction (V)
entry
Epc
Epa
Epc
The fluorescent properties of 1b and 8 are delineated in
Figure 3b. The maximum emission peak (λem) of 1b appears
at 628 nm, a wavelength red-shifted by 44 nm (1200 cm-1)
versus that of 8 (584 nm). This observation is consistent with
the UV-vis results which show that exTTF 1b has smaller
HOMO-LUMO gap than does 8. The fluorescence quantum
yield of 1b (Φ ) 0.13) is about double that of its
half-structure 8 (Φ ) 0.064), which is counterintuitive with
respect to expectations based on energy gap laws. Detailed
Franck-Condon line-shape analyses as a function of tem-
perature13 are currently underway to characterize the lowest
lying emitting state.
1b
8
0.98, 1.32
1.01, 1.25
0.73, 1.10
0.16, 1.02
-0.23
-0.26
In the oxidation region, two irreversible redox wave pairs
are discernible in both compounds. The redox potentials are
very similar except for the first cathodic peak potential. For
compound 8, the value has dramatically shifted to 0.16 V
relative to that of exTTF 1b at 0.73 V. The resemblance of
1b and 8 in oxidation profile indicates that exTTF 1b retains
an electron-donating ability quite similar to its half-structure
8. The loss of TTF-like electron donating ability in 1b can
be presumably attributed to the substantial energy cost
required to planarize its highly bent structure upon oxidation.
In summary, we have developed a new synthetic entry to
a class of dianthro-enyne macrocycle based exTTF deriva-
tives. Insertion of macrocyclic enyne π-spacer into TTF has
brought about interesting solid-state packing and electronic
absorption/emission properties to exTTF 1b, but rendered it
a poorer electron-donor than those exTTFs with relatively
shorter π-spacers.2-8 Nevertheless, this work represents the
first experimental exploration of such type of exTTFs, and
will serve useful guidance in further quest for new molecular
hybrids made up of TTF and conjugated cyclic oligo(enyne)
building blocks.
Electrochemical redox properties of 1b and 8 were
characterized by cyclic voltammetry (see Figure 4 and Table
Acknowledgment. We acknowledge the support from the
Natural Sciences and Engineering Research Council of
Canada (NSERC), Canadian Foundation for Innovation
(CFI), and Memorial University of Newfoundland. Ms. Julie
Collins and Ms. Louise Dawe of Memorial University are
acknowledged for assistance in X-ray structural analysis, and
Dr. Chris Flinn of Memorial University is acknowledged for
very helpful discussions.
Figure 4. Cyclic voltammograms of 1b and 8 as measured in
CHCl3/CH3CN (4:1, v/v) at room temperature. Bu4NBF4 (0.1 M)
as the supporting electrolyte, glassy carbon as the working electrode,
Pt wire as the counter electrode, and Ag/AgCl as the reference.
Scan rate: 0.1 V s-1
.
1). Scanning cathodically, 1b and 8 both show an irreversible
cathodic peak current at similar potentials. The origin of this
current peak is not clear and waits for further investigations.
Supporting Information Available: Experimental pro-
cedures, synthetic and spectroscopic data for new com-
pounds, and a CIF file for 1b. This material is available free
of charge via the Internet at http://pubs.acs.org.
(12) (a) Moore, A. J.; Bryce, M. R. Tetrahedron Lett. 1992, 33, 1373.
(b) Chen, G.; Zhao, Y. Tetrahedron Lett. 2006, 47, 5069.
(13) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439.
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