Planar acetylene-expanded TTFAQ analogues

Article abstract of DOI:10.1021/ol900794j

Two planar acetylene-expanded TTFAQ analogues (8a and 8b) were synthesized through a one-pot, 4-fold Sonogashira coupling protocol. The structure and electronic properties of these compounds were investigated by single-crystal X-ray crystallography, UV-vi

Full text of DOI:10.1021/ol900794j

ORGANIC
LETTERS
2009
Vol. 11, No. 13
2736-2739
Planar Acetylene-Expanded TTFAQ
Analogues
Guang Chen, Louise Dawe, Li Wang, and Yuming Zhao*
Department of Chemistry, Memorial UniVersity of Newfoundland,
St. John’s, NL, Canada A1B 3X7
yuming@mun.ca
Received April 12, 2009
ABSTRACT
Two planar acetylene-expanded TTFAQ analogues (8a and 8b) were synthesized through a one-pot, 4-fold Sonogashira coupling protocol. The
structure and electronic properties of these compounds were investigated by single-crystal X-ray crystallography, UV-vis spectroscopy, and
cyclic voltammetry.
Tetrathiafulvalene (TTF) analogues bearing extended con-
jugated π-spacers, namely π-extended TTFs (exTTFs), are
Scheme 1.
Structures of TTFAQ and π-Extended TTFAQsa
important building blocks for organic electronic materials
and supramolecular chemistry.1 A popular strategy to design
ex-TTFs is accomplished by insertion of various conjugated
π-units in between the dithiole rings of a parent TTF structure
as illustrated in Scheme 1. Among numerous motifs explored
in the literature, the TTF analogues that are expanded through
an anthraquinoid type of spacer (generally referred to as
TTFAQs) have received considerable attention, owing to
their remarkable electron-donating ability facilitated by the
unique gaining of aromaticity at the central anthracene unit
upon two-electron oxdiation.1d,2 Further π-extended variants
of TTFAQs are less frequently addressed in the literature;
a The central cyclic π-bridges are highlighted in bold.
(1) For recent reviews on exTTFs, see: (a) Bryce, M. R. J. Mater. Chem.
2000, 10, 589. (b) Segura, J. L.; Mart´ın, N. Angew. Chem., Int. Ed. 2001,
40, 1372. (c) TTF Chemistry: Fundamentals and Application of Tetrathi-
afulValene; Yamada, J., Sugimoto, T., Eds.; Springer Verlag: Heidelberg,
2004. (d) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104, 4891. (e) Fre`re, P.; Skabara, P. J. Chem. Soc. ReV. 2005, 34, 69. (f)
Roncali, J. J. Mater. Chem. 1997, 7, 2307.
however, they afford very appealing molecular systems that
challenge the mindsets of both synthetic and theoretical
chemists.3 In 2008, our group reported a highly expanded
TTFAQ analogue 1, in which an enyne macrocycle together
with two anthraquinodimethane moieties constituted the
π-spacer.4 The central macrocyclic enyne framework of
(2) (a) Bryce, M. R.; Moore, A. J. Tetrahedron Lett. 1988, 29, 1075.
(b) Akiba, K.; Ishikawa, K.; Inamoto, N. Bull. Chem. Soc. Jpn. 1978, 51,
2674
.
10.1021/ol900794j CCC: $40.75
Published on Web 06/01/2009
 2009 American Chemical Society

exTTF 1 represents the first example of a peculiar type of
radia[n]annulenes,5 the structure and properties of which have
invoked a series of questions both fundamentally and
practically important; for instance: (1) Will the central
radiaannulene π-bridge show the gaining of aromaticity
similar to that of TTFAQ upon oxidation?6 (2) Will the
incorporation of π-extension add new dimensions for control
over solid-state ordering, multistage-redox behavior, and
supramolecular stacking in solution or the solid state?1 (3)
Will enhanced electronic and photonic performances be
achievable through this new design concept? To address these
issues, synthetic access to macrocyclic enyne bridged
TTFAQ analogues is prerequisite.
Scheme 2. Synthesis of Acetylene-Expanded TTFAQs 8a and 8b
Strictly speaking, exTTF 1 does not serve as an adequate
model for the study of π-expanded TTFAQ systems, since
unlike TTFAQs and other exTTFs the two dithiole rings of
1 have no electronic communication through the long
π-spacer as a result of its nonplanar, S-shaped π-framework.4
The structural nonplanarity of 1 arises mainly from the
significant steric crowdedness among the anthraquinone and
macrocyclic enyne moieties. To circumvent this setback, we
then revised our design to a simpler motif, exTTF 2 (Scheme
1), which can be viewed as an expanded version of TTFAQ
derived from directly inserting acetylenic units into the
quinoid spacer of TTFAQ. In principle, motif 2 should afford
a better model system than exTTF 1 because its π-framework
is absent of any steric hindrance and hence capable of taking
a planar ground-state structure in favor of maximal π-con-
jugation.
The key step in the synthesis of exTTF 2 is to construct
the central macrocyclic enyne moiety. This can be ac-
complished through a one-pot, 4-fold Sonogashira cyclization
protocol developed by our group in the synthesis of exTTF
1.4 Following this strategy, two acetylene-expanded TTFAQs
8a and 8b were prepared as outlined in Scheme 2. The
synthesis began with a Wittig-Horner reaction between
ketone 4 and the ylide in situ generated by treatment of
phosphonate 3 with t-BuLi. Dithiole precursor 57 was
obtained in a modest yield and was desilylated with K2CO3
to give corresponding terminal alkyne. This intermediate
showed relatively low stability and was immediately sub-
jected to Sonogashira coupling with diiodobenzene 7a or 7b
under dilute conditions (ca. 4-6 mM). To our delight, the
two macrocyclization reactions successfully afforded the
desired products. After column chromatographic separation,
expanded TTFAQs 8a and 8b were obtained in 20% and
7% yield, respectively. Given the fact that the macrocycliza-
tion underwent four consecutive steps of cross-coupling
reactions in one pot, the yields of 8a,b are actually very
reasonable.8
(3) (a) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Ort´ı, E.; Viruela, P. M.;
Viruela, R. J. Org. Chem. 1998, 63, 1268. (b) Moore, A. J.; Bryce, M. R.
J. Chem. Soc., Perkin Trans. 1 1991, 157.
(4) Chen, G.; Wang, L.; Thompson, D. W.; Zhao, Y. Org. Lett. 2008,
10, 657.
Parallel to the one-pot approach, the synthesis of 8a and
8b through a stepwise route was also explored. As outlined
in Scheme 2, desilylation of dithiole precursor 5 followed
up by cross coupling with excess diiodobenzene 7a or 7b
afforded precursors 9a and 9b. With 9a and 9b in hand,
another iteration of intramolecular 2-fold Sonogashira cou-
pling would furnish the desired products 8a and 8b. This
synthetic route, however, was proven inefficient due to the
sluggish yield of the first coupling step. Further attempts at
macrocyclization through this approach were therefore aban-
doned.
(5) (a) Tykwinski, R. R.; Gholami, M.; Eisler, S.; Zhao, Y.; Melin, F.;
Echegoyen, L. Pure Appl. Chem. 2008, 80, 621. (b) Gholami, M.; Melin,
F.; McDonald, R.; Ferguson, M. J.; Echegoyen, L.; Tykwinski, R. R. Angew.
Chem., Int. Ed. 2007, 46, 9081. (c) Mitzel, F.; Boudon, C.; Gisselbrecht,
J.-P.; Seiler, P.; Gross, M.; Diederich, F. HelV. Chim. Acta 2004, 87, 1130.
(d) Mitzel, F.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.;
Diederich, F. Chem. Commun. 2003, 1634. (e) Gholami, M.; Chaur, M. N.;
Wilde, M.; Ferguson, M. J.; McDonald, R.; Echegoyen, L.; Tykwinski, R. R.
Chem. Commun. 2009, 3038.
(6) The macrocycle enyne framework in 1 and 2 presents a new addition
to a class of carbo-meric molecules that have attracted great interest in
terms of π-delocalization and aromaticity in neutral and ionized states. See:
(a) Lepetit, C.; Nielsen, M. B.; Diederich, F.; Chauvin, R. Chem.sEur. J.
2003, 9, 5056. (b) Lepetit, C.; Godard, C.; Chauvin, R. New. J. Chem. 2001,
25, 572. (c) Godard, C.; Lepetit, C.; Chauvin, R. Chem Commun. 2000,
1833. (d) Chauvin, R. Tetrahedron Lett. 1995, 36, 397.
The molecular structures and purity of expanded TTFAQ
analogues 8a,b and relevant precursors were unambiguously
(7) Analogues of 5 with different substituents were previously reported;
see: (a) Nielsen, M. B.; Moonen, N. N. P.; Boudon, C.; Gisselbrecht, J.-P.;
Seiler, P.; Gross, M.; Diederich, F. Chem. Commun. 2001, 1848. (b) 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.
(8) The yield of 8a (20%) corresponds to an average yield of 67% for
each individual Sonogashira coupling, while the yield of 8b (7%) corre-
sponds to 51% yield for each individual coupling.
Org. Lett., Vol. 11, No. 13, 2009
2737

Figure 1. Single-crystal structure of compound 8b: (A) front view of ORTEP plot, (B) side view of ORTEP plot, (C) side view of crystal
packing diagram, and (D) front view of crystal packing diagram. Ellipsolid probability at 30% level. Note that solvent CHCl3 molecules are
present in the ORTEP plots.
characterized by NMR, MS, and IR analyses (see the
Supporting Information). Compound 8b shows much better
solubility in common organic solvents than 8a due to the
presence of solubilizing decyloxy chains. Single crystals of
8b were successfully grown by slow evaporation of the
solution of 8b in CHCl3/CH3CN (4:1, v/v) in the presence
of Bu4NBF4 at ca. 4 °C. With these crystals, the solid-state
structure properties of 8b were elucidated by means of single-
crystal X-ray crystallography.The single-crystal structure of
8b is depicted in Figure 1A,B, in which a coplanarity of
the central enyne macrocycle with adjacent phenyl and
dithiole rings is clearly seen. Of note is that the central enyne
macrocycle of 8b has geometric parameters similar to those
observed in the crystal structure of exTTF 1.4 A closer
between the mean planes of two adjacent molecules is
measured at 3.729 Å, and the close proximity between the
donor (dithiole) and acceptor (enyne ring) is suggestive of
intermolecular charge-transfer interactions.10 Apparently, the
planar framework of 8b is key to the formation of efficient
π-π stacking in the solid state, which is expected to add
benefit for electronic and optoelectronic applications.11
The electronic properties of TTFAQ analogues 8a and 8b
were investigated by UV-vis and fluorescence spectroscopy.
Figure 2A gives the normalized UV-vis absorption spectra
for compounds 8a, 8b, and precursor 5 measured in CHCl3.
Compound 8a shows two characteristic low-energy absorp-
tion bands at 480 and 453 nm, which are nearly identical to
those of 8b at 483 and 457 nm. These absorptions are
substantially red-shifted in comparison to the maximum
absorption band (λmax) of dithiole precursor 5, corroborating
the presence of significant electronic interactions between
the dithiole rings and the central macrocyclic enyne units in
8a and 8b. The fluorescence spectra for 8a and 8b are shown
in Figure 2B, where both compounds exhibit a similar broad
and structureless emission profile. The maximum emission
wavelength (λem) of decyloxy chain attached exTTF 8b
appears at 510 nm, which is slightly blueshifted relative to
that of 8a at 522 nm.
comparison of the two sets of crystallographic data, however,
9
shows that the bond length alternation (BLA) index (δBLA
)
of 8b (0.155 Å) is slightly shorter than that of exTTF 1 (δBLA
) 0.165 Å). The variation in BLA suggests that the cyclic
enyne segment in 8b possesses a higher degree of π-delo-
calization than exTTF 1, resulting from stronger electronic
interactions between the dithiole rings and the central enyne
macrocycle.
A very intriguing aspect of the solid-state structure of 8b
lies in its crystal packing geometry, which stands in contrast
to that of nonplanar exTTF 1.4 As shown in Figure 1C,D,
the molecules of 8b pack in a columnar fashion in the crystal,
where the electron-rich dithiole rings directly overlap with
the relatively electron-deficient macrocyclic enyne units,
affording a slipped face-to-face stacking. The distance
Electrochemical redox behavior of compound 8b was
studied by cyclic voltammetry (CV), whereas efforts to
characterize 8a by CV failed due to limited solubility. Figure
3 shows the cyclic voltammograms of 8b determined in
(10) The electrostatic potential surface for 8a was calculated at the
B3LYP/6-31G(d) level of theory using the Spartan’06 program. The drawing
in the graphical abstract shows that the highest electron density is
concentrated at the dithiole rings, while the lowest electron density at the
central enyne macrocycle unit.
(9) δBLA is determined based on the formula δBLA ) Rs - Rm, where Rs
and Rm denote the average bond distances for C-C single bonds and
multiple bonds on the macrocyclic enyne framework. The crystallographic
data for 1 and 8b can be found at Cambridge Crystallographic Data Centre
(CCDC). Deposition numbers: 680314 and 726951.
(11) Yamada, J.-i.; Akutsu, H.; Nishikawa, H.; Kikuchi, K. Chem. ReV.
2004, 104, 5057.
2738
Org. Lett., Vol. 11, No. 13, 2009

is reckoned quasi-reversible and its origin is tentatively
assigned to a simultaneous two-electron oxidation at the
dithiole rings of 8b. Of note is that the value of the first
oxidation potential (Epa1) is similar to those reported for
TTFAQs,12 but much lower than that of nonplanar exTTF 1
(Epa ) 0.98 V).4 This result suggests that the acetylene
1
expanded macrocyclic bridge in 8b mediates electronic
communication between the two dithiole rings to an extent
that is comparable to the quinoid bridge in a typical TTFAQ.
When scanned from -0.5 to +0.9 V, a second anodic current
2
peak emerges at Epa ) 0.76 V, which is associated with a
relatively sharp cathodic current peak at Epc2 ) 0.65 V. When
the switching potentials are expanded from -0.5 to +1.2
V, two anodic peaks at 0.64 and 0.76 V along with one broad
cathodic peak at 0.62 V are observed in the voltammogram.
In the scan range of -0.5 to +1.5 V, a third anodic peak at
1.32 V appears, while there is no noticeable cathodic current
observed in the voltammogram. The irreversible pattern here
may be rationalized by an EC mechanism, where an
electrochemically promoted reaction13 swiftly follows up the
oxidation of 8b. Differential pulse voltammetric analysis (see
the inset of Figure 3) also corroborates that 8b undergoes
three distinct oxidation processes at 0.59, 0.71, and 1.13 V,
respectively. Investigations on the detailed electrochemical
mechanisms are currently underway, and the results will be
reported in due course.
Figure 2. (A) Normalized UV-vis absorption spectra for 8a, 8b,
and 5 measured in CHCl3. (B) Normalized fluorescence spectra for
8a and 8b measured in CHCl3 (λex ) 400 nm).
In conclusion, we have presented the synthesis and
characterization of the first examples of planar acetylene-
expanded TTFAQ analogues. It is worth remarking that the
one-pot Sonogashira cyclization protocol was again proven
to be very efficient in constructing shape-persistent macro-
cyclic enyne structures. These planar acetylene-expanded
TTFAQ analogues, in view of their efficient solid-state
packing and electrochemical activities, may deliver useful
applications in TTF based organic electronic materials and
devices.
varied ranges of potential scan at 0 °C. In the scan range of
-0.5 to +0.72 V, the voltammogram shows a pair of redox
waves at Epa ) 0.67 V and Epc ) 0.53 V. Given the fact
that the separation between the two peaks is 0.11 V and the
peak positions are scan rate dependent, this redox wave pair
1
1
Acknowledgment. We thank NSERC Canada, Canada
Foundation for Innovation (CFI), and Industrial Research and
Innovation Fund (IRIF) for funding.
Supporting Information Available: Synthetic procedures
and spectroscopic characterization for all new compounds
and crystallographic data for 8b. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL900794J
Figure 3. Cyclic voltammograms of 8b measured in varied potential
(12) Christensen, C. A.; Batsanov, A. S.; Bryce, M. R. J. Org. Chem.
2007, 72, 1301.
scan windows at 0 °C: solvent, CH2Cl2; electrolyte, Bu4NBF4 (0.1
M); working electrode, glassy carbon; counter electrode, Pt;
reference electrode, Ag/AgCl; scan rate, 0.5 V/s. Inset: differential
pulse voltammogram of 8b measured at 0 °C.
(13) An electrochemically induced 1,5-cyclization of the endiyne
segments at the central macrocycle is speculated to be involved. See:
Ramkumar, D.; Kalpana, M.; Varghese, B.; Sankararaman, S. J. Org. Chem.
1996, 61, 2247.
Org. Lett., Vol. 11, No. 13, 2009
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