Photophysical, crystallographic, and electrochemical characterization of symmetric and unsymmetric self-assembled conjugated thiopheno azomethines

Article abstract of DOI:10.1021/jo070100o

Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugated compounds were synthesized by simple condensation of a stable diamino thiophene (2) with its complementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variable reactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-pot synthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiophene units was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resulting covalent conjugated connections are both reductively and hydrolytically resistant. The thermodynamically E isomer is formed uniquely for all of the azomethines synthesized, which is confirmed by crystallographic studies. These also demonstrated that the azomethine bonds and the thiophene units are highly planar and linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar to those of thiophene analogues currently used in functional devices, but with the advantage of low triplet formation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependent photophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold by intersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative means of internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene units was found. These compounds further undergo electrochemically induced oxidative cross-coupling, resulting in conjugated products that also exhibit reversible radical cation formation.

Full text of DOI:10.1021/jo070100o

Photophysical, Crystallographic, and Electrochemical
Characterization of Symmetric and Unsymmetric Self-Assembled
Conjugated Thiopheno Azomethines
Sergio Andre´s Pe´rez Guar`ın, Marie Bourgeaux, Ste´phane Dufresne, and W. G. Skene*
Department of Chemistry, PaVillon JA Bombardier, UniVersity of Montreal, CP 6128, succ. CentreVille,
Montreal, Quebec H3C 3J7, Canada
w.skene@umontreal.ca
ReceiVed January 17, 2007
Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugated
compounds were synthesized by simple condensation of a stable diamino thiophene (2) with its
complementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variable
reactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-pot
synthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiophene
units was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resulting
covalent conjugated connections are both reductively and hydrolytically resistant. The thermodynamically
E isomer is formed uniquely for all of the azomethines synthesized, which is confirmed by crystallographic
studies. These also demonstrated that the azomethine bonds and the thiophene units are highly planar
and linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar to
those of thiophene analogues currently used in functional devices, but with the advantage of low triplet
formation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependent
photophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold by
intersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative means
of internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene units
was found. These compounds further undergo electrochemically induced oxidative cross-coupling, resulting
in conjugated products that also exhibit reversible radical cation formation.
Introduction
conducting properties.1 Conjugated materials with these proper-
ties can be used in flexible light displays or low power
consumption products,2 including field effect transistors.3
Because of these potential applications, great effort has focused
on the synthesis of these and other new compounds with
Applications of conjugated materials such as organic light
emitting diodes and molecular wires are highly sought after in
technological applications due to their optical, emitting, and
10.1021/jo070100o CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/08/2007
J. Org. Chem. 2007, 72, 2631-2643
2631

Guar`ın et al.
CHART 1. Azomethines Examined and Some Representative Nitrogen-Free Derivatives
enhanced properties.4 Even though research endeavors have
concentrated on synthetic optimization, the synthesis of conju-
gated materials is unfortunately not a simple process.5,6 Con-
ventional synthetic methods require challenging and tedious
purifications to isolate the desired conjugated materials.6
Therefore, simple, high-yielding synthetic protocols for the
preparation of unsymmetrical modules with tunable physical
properties are quite rare.
Azomethines (-NdC-) are highly attractive alternatives to
currently used conjugated materials. This is due in part to their
isoelectronic character relative to their carbon analogues.7 They
are further advantageous because of their simple synthesis
without the use of stringent reaction conditions or metal
catalysts. They are easily purified, unlike their carbon analogues,
and also exhibit high chemical and reductive resistance.8,9 The
limited number of stable diamino monomer precursors has
unfortunately hindered the development of functional materials
incorporating azomethines. Moreover, inadequate electrical/
conductive properties, poor solubility, undesired oxidative
decomposition, and irreversible radical cation formation are
problematic with current azomethines and their diamino precur-
sors, which further preclude their use as functional materials.10
The 3,4-diethyl ester 2,5-diaminothiophene (2) is a propitious
precursor for novel conjugated azomethines.11 In addition to
consisting of a thiophene moiety, which is highly desired
because of its low oxidation potential, 2 also exhibits a high
stability. This is in contrast to its unsubstituted analogue that
cannot be isolated and spontaneously decomposes under ambient
conditions. Because thiophenes have gained a wide importance
due to their spectroscopic and electrochemical properties,
azomethines derived from them would therefore possess ideal
properties and be suitable for functional materials.8,12,13
The potential advantages of azomethine-based materials
concomitant with our previous success with such compounds14
have prompted us to develop simple synthetic routes to such
materials containing only thiophene units. The main objective
is to examine the physical properties, among which are the
singlet and triplet states, and the fate of the excited states of
these new compounds. These are of particular interest given
the limited number of photophysical studies involving azo-
methines and their thiophene derivatives. The present report
describes the synthesis and characterization of unprecedented
thiopheno azomethines derived from 2 (Chart 1). Because of
the different reactivities of the amino groups between 2 and
the monoazomethine products, factors such as stoichiometry and
judicious choice of solvent are examined for selective product
formation. A one-pot modular synthesis of symmetric and
unsymmetric self-assembled conjugated azomethines consisting
of up to five thiophenes is investigated. The resulting photo-
physical, electrochemical, and band-gap properties, in addition
to crystallographic data of these unique and highly conjugated
thiopheno azomethines, are also presented.
(1) (a) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581-
2590. (b) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002,
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36.
Results and Discussion
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Funct. Mater. 2001, 11, 15-26.
Modular Synthesis. We originally sought out to synthesize
5 by a simple self-assembled approach using the stable diamino
thiophene 2 because azomethines of this type have not yet been
investigated. Additionally, we wanted to illustrate the stability
(3) Katz, H. E.; Dodabalapur, A.; Bao, Z. In Handbook of Oligo- and
Polythiophenes; Fichou, D., Ed.; Wiley-VCH Verlag GmbH: Weinheim,
Germany, 1999; pp 459-489.
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Chem. 2001, 17, 2867-2873.
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A. Macromolecules 2005, 38, 1958-1966. (b) Kiriy, N.; Bocharova, V.;
Kiriy, A.; Stamm, M.; Krebs, F. C.; Adler, H.-J. Chem. Mater. 2004, 16,
4765-4771.
(7) (a) Wang, C.; Shieh, S.; LeGoff, E.; Kanatzidis, M. G. Macromol-
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1991, 3, 878-887.
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31, 2676-2678.
(13) Skene, W. G. WO 2005073265, 2005.
(14) (a) Skene, W. G.; Dufresne, S. Org. Lett. 2004, 6, 2949-2952. (b)
Bourgeaux, M.; Pe´rez Guar`ın, S. A.; Skene, W. G. J. Mater. Chem. 2007,
17, 972-979. (c) Dufresne, S.; Bourgeaux, M.; Skene, W. G. J. Mater.
Chem. 2007, 10.1039/b616379c. (d) Bourgeaux, M.; Skene, W. G.
Macromolecules 2007, 40, 1792-1795.
(9) (a) Kucybala, Z.; Pyszka, I.; Marciniak, B.; Hug, G. L.; Paczkowski,
J. J. Chem. Soc., Perkin Trans. 2 1999, 2147-2154. (b) Skopalova´, J.;
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2001, 370, 963-969.
2632 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
TABLE 1. Product Distribution Obtained with Various Solvents
and Aldehyde Stoichiometries
SCHEME 1. General Synthetic Scheme Leading to Diamino
Thiophene 2
reagent
% product distributiona
ratio
1/2TCb
solvent
ethanol
3
4
5
6
7
2/2
1/1
0.7/0.3
0.3/0.7
2/2
52
52
24
73
35
35
71
16
48
48
76
27
40
33
17
75
83
of 2 because of its electron-withdrawing ester groups and to
demonstrate its usefulness as a precursor for new conjugated
functional materials. It was anticipated that the physical proper-
ties of the resulting azomethine would mimic those of known
carbon analogues such as 9 and 11. The thiopheno azomethines
were also anticipated to exhibit better properties than their
homologous azomethines, making them useful for functional
materials.
isopropanol
10
11
7
7
10
3
3
17
8
11
2
1/1
0.3/0.7
0.7/0.3
2/0
5
1
0/2
2/2
1/1
0.3/0.7
0.7/0.3
2/0
0/2
2/2
79
43
52
29
68
21
<1
n-butanol
56
48
71
32
100
<1
The key diamino thiophene (2) precursor required for the
condensation with the thiophene aldehydes was synthesized
according to a modified Gewald’s reaction (Scheme 1).11,26 This
compound is particularly interesting because its electron-
withdrawing esters reduce the amine’s reactivity. The additional
effect of the ester groups is the increased stability of 2 such
that it can be obtained as a solid and can be handled without
any special inert requirements. Conversely, its unsubstituted
analogue cannot be isolated, and it spontaneously decomposes
once exposed to ambient conditions. The deactivating esters also
decrease the amine nucleophilicity, making its condensation with
an aldehyde more difficult. Such is the reason for failed coupling
reactions to afford any significant amount of the bisazomethines
5-7 under standard azeotrope distillation in anhydrous solvents
such as toluene, THF, or with other typical mild dehydration
methods. These mild conditions provide only the monoazome-
thines 3 or 4 regardless of the reaction conditions and the
number of aldehyde equivalents used. Highly polar solvents such
as DMF and high reaction temperatures are required to displace
the equilibrium as to favor the second condensation and to afford
the bisazomethines 5-8. The azomethine bond of 3 and 4 has
a strong electron-withdrawing character that exerts a combined
effect with the ester withdrawing groups to further deactivate
the primary amine. Stringent conditions are consequently
required to activate the amine and to allow the second
condensation to proceed for bisazomethine formation. Thus, a
method for selective product formation is possible because of
the inherent different amine reactivity, which can be exploited
by using different solvents and stoichiometry.
100
49
50
22
68
19
30
DMSO
neatc
51
50
78
32
20
35
36
1/1
0.3/0.7
0.7/0.3
2/2
20
15
21
17
64
20
3
1/1
2/0d
0/2d
46
54
a Amounts determined by HPLC after reacting at 70 °C for 24 h.
b Number of equivalents of 1 and 2-thiophene carboxaldehyde (2TC) relative
to 2. c Reaction done in the absence of solvent and heating to 50 °C for 10
min with a catalytic amount of trifluoroacetic acid. d Bisazomethines are
quantitatively formed upon heating the neat mixture for an additional 5
min at 60 °C.
Such a selective product formation was tested with various
solvents and with different aldehyde stoichiometries. Alcohol
solvents were preferred because of their ability to be made
readily anhydrous and their high volatility allowing for easy
product isolation, unlike DMF and DMSO. Moreover, such
properties displace the equilibrium to favor azomethine forma-
tion. With ethanol, 3 and 4 were quantitatively formed by
refluxing either with 1 equiv of the corresponding aldehyde with
2 or with an excess of aldehyde at room temperature. Only a
small amount of the bisazomethine (<1%) was observed when
refluxing for more than 1 week with an excess of the corre-
sponding aldehyde. The monoazomethines were quantitatively
obtained in high purity upon removal of the solvent when using
1 equiv of aldehyde. Interestingly, the resulting product crystal-
lizes as large block-like crystals by simple removal of the solvent
after the reaction is completed. The crystals obtained are of high
quality and are suitable for X-ray analysis without additional
recrystallization efforts. The simple crystallization of the
compounds indicates the high degree of crystallinity of these
thiophene azomethines.25
(15) Becker, R. S.; Seixas de Melo, J.; Mac¸anita, A. L.; Elisei, F. J.
Phys. Chem. 1996, 100, 18683-18695.
(16) Seixas de Melo, J.; Elisei, F.; Becker, R. S. J. Chem. Phys. 2002,
117, 4428-4435.
(17) Seixas de Melo, J.; Burrows, H. D.; Svensson, M.; Andersson, M.
R.; Monkman, A. P. J. Chem. Phys. 2003, 118, 1550-1556.
(18) Seixas de Melo, J.; Silva, L. M.; Arnaut, L. G.; Becker, R. S. J.
Chem. Phys. 1999, 111, 5427-5433.
(19) Seixas de Melo, J.; Elisei, F.; Gartner, C.; Aloisi, G. G.; Becker, R.
S. J. Phys. Chem. A 2000, 104, 6907-6911.
(20) Scaiano, J. C. CRC Handbook of Organic Photochemistry; CRC
Press: Boca Raton, FL, 1989.
(21) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15,
1-250.
Unlike with ethanol, the double condensation to afford the
bisazomethines 5 or 6 was possible with either isopropanol (IPA)
or n-butanol (BuOH) along with 2 equiv of the corresponding
aldehyde. The polarity differences and higher boiling points of
these two solvents are sufficiently different from ethanol as to
overcome the decreased amino reactivity of the monoazo-
methine. The double condensation to afford the bisazomethine
is also possible in the absence of solvent. Pure bisazomethines
were quantitatively obtained without purification by heating 2
equiv of the corresponding aldehyde at 50 °C for 15 min without
any solvent.
(22) (a) Andrews, L. J.; Deroulede, A.; Linschltz, H. J. Phys. Chem.
1978, 82, 2304-2309. (b) Murphy, R. S.; Moorlag, C. P.; Green, W. H.;
Bohne, C. J. Photochem. Photobiol., A 1997, 110, 123-129.
(23) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(24) Skene, W. G. Polym. Prepr. 2004, 45, 252-253.
(25) Skene, W. G.; Dufresne, S.; Trefz, T.; Simard, M. Acta Crystallogr.
2006, E62, o2382-o2384.
(26) (a) Skene, W. G. PCT Int. Appl., 2005. (b) Bourgeaux, M.;
Vomsheid, S.; Skene, W. G. Synth. Commun. 2007, in press.
J. Org. Chem, Vol. 72, No. 7, 2007 2633

Guar`ın et al.
SCHEME 2. Modular Synthesis of Symmetric and Unsymmetric Conjugated Thiophene Compounds via Solvent and
Stoichiometry Control in the Presence of Trifluoroacetic Acid Catalysta
a (i) Refluxing ethanol with either 1 equiv or an excess of aldehyde, (ii) n-butanol or isopropanol with 1 equiv of aldehyde at 60 °C, (iii) n-butanol or
isopropanol with 1 equiv of the corresponding aldehyde at 60 °C, (iv) n-butanol or isopropanol with 2 equiv of aldehyde at 60 °C, and (v) 2 equiv of
aldehyde heating at 50 °C without solvent.
The reactivity differences of the amino group in the mono-
azomethine and that of 2 were further examined for selective
product formation by judicious choice of solvent and reagent
stoichiometry. The relative reactivities of 1 and 2-thiophene
carboxaldehyde (2TC) toward 2 were first examined by reacting
varying amounts of the aldehydes in different solvents and by
examining the product distribution. A statistical product distri-
bution consisting of monoazomethine, bisazomethine, and
starting reagents is expected with 1 equiv of aldehyde in the
absence of any product control. The resulting product distribu-
tion in Table 1 confirms that both aldehydes exhibit the same
reactivity, because the product distribution parallels that of the
aldehyde reagent ratios. The results further demonstrate that the
condensation of 2 with an aldehyde forms quantitatively the
monoazomethine before it undergoes the second condensation
to afford to bisazomethine. Bisazomethine formation is only
possible when IPA is used as the solvent. Formation of the
bisazomethines was also possible with BuOH by further adding
1 equiv of aldehyde to the reaction medium followed by heating.
The exclusive monoazomethine formation observed in the
various solvents is a result of the amine reactivity differences
between 2 and its corresponding monoazomethines. Such
reactivity differences could be exploited to provide selective
product formation in a stepwise/one-pot fashion by controlling
parameters such as stoichiometry or solvent choice. Exclusive
product formation of either the monoazomethines or the
bisazomethines could ultimately be modulated via these pa-
rameters according to Scheme 2. To verify whether such a one-
pot selective product formation was possible, we examined the
use of different solvents in a sequential order along with various
aldehyde amounts. The monoazomethine 3 was used as a
benchmark because it could be quantitatively obtained from
211,13,27 in ethanol with a stoichiometric amount of 2TC.
1
Moreover, it has a characteristic H NMR spectrum that lends
itself to easy monitoring of the reaction progress via the large
upfield shift of the imine proton as seen in Figure 1. Upon
complete formation of 3 in ethanol, the solvent was removed
whereupon the purity and its quantitative formation were verified
by NMR. Isopropanol was then added along with 1 equiv of
the corresponding aldehyde to afford either the symmetric (5)
or the unsymmetric (7) bisazomethine upon heating. The same
two-step/one-pot procedure also afforded 6 and 7 from 4 with
the corresponding aldehydes. The symmetric bisazomethines
were also formed from 2 with 2 equiv of aldehyde in isopropanol
with heating. Because the corresponding monoazomethine is
exclusively formed regardless of the solvent used, we exploited
this for the one-pot synthesis of 7 in IPA. The use of 1 equiv
of aldehyde in IPA afforded exclusively either the monoazo-
methine 3 or 4, depending on the aldehyde selected. One
equivalent of the other corresponding aldehyde was then added
directly to the reaction mixture to afford the unsymmetric
bisazomethine with heating. Therefore, selective formation of
either symmetric or unsymmetric conjugated compounds ranging
from 1 to 5 thiophene units can be had exclusively in a one-pot
method according to Scheme 2.
All compounds were easily identified by their characteristic
1
imine peak via H NMR in the range of 8.2-8.8 ppm as seen
in Figure 1. Interestingly, only one imine singlet was observed
for all of the azomethines studied, implying the exclusive
formation of one isomer. Only in the case of 7 were two imine
(27) Skene, W. G.; Trefz, T. PMSE Prepr. 2004, 91, 326-327.
2634 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
FIGURE 1. 1H NMR spectra of thiopheno azomethines in deuterated
acetone: A ) 3, B ) 4, C ) 5, D ) 6, and E ) 7.
FIGURE 2. Normalized ground-state absorption spectra of 1 (O), 4
(9), and 7 (0) measured in anhydrous acetonitrile. Inset: Shift in
absorbance maxima versus the number of atoms in the conjugated
framework for compounds 1-5.
peaks detected because it possesses two nonequivalent imines.
Absolute assignment to either the E or the Z isomer for the
products is, however, not possible by NMR and can only be
had by X-ray crystal structure analysis (vide infra).
azomethines.29 The energy gap (∆E) reported in Table 2 for
the azomethines is lower than that for some representative
analogues 9-11. This is a result of the energy levels undergoing
a pronounced stabilization from the heteroconjugated bond and
the electron-withdrawing ester groups. The latter perturb the
LUMO level, which affect the spectroscopic and electrochemical
properties. The fluorescence spectra also show a bathochromic
shift that correlates with the enhanced degree of conjugation
because of the increased stabilization that occurs with increasing
the number of thiophenes. The lack of structured emission
observed in the fluorescence spectra suggests there is no defined
order in the singlet excited and ground states and is consistent
with other oligothiophenes.
Accurate band-gap energies can be obtained from the absorp-
tion onset because little difference between the absorbances in
solution and in thin films was observed. Thin films are known
to increase the planarity of twisted molecules, leading to
increased conjugation concomitant with an absorption batho-
chromic shift. The absence of spectroscopic shift supports the
crystallographic data (vide infra) that the thiopheno azomethines
are linear and planar both in solution and in thin films. These
structural effects contribute to the high delocalization and to
the high degree of conjugation exhibited by the azomethines.
This is further supported by the observed band-gap narrowing
that occurs with increasing the number of thiophenes in the
compounds examined. The resulting effect is band-gaps that
are lower than those of their nitrogen-free derivatives, as seen
in Table 2.
It is generally understood that azomethine formation is
reversible, and, as a result, the products can be easily hydrolyzed.
Because of the high degree of conjugation exhibited by 3-8
due to the azomethine bond, these compounds are expected to
be relatively stable, and such is the case. This is supported by
the lack of noticeable product decomposition under prolonged
ambient humid conditions that would otherwise hydrolyze other
azomethines. The azomethine stability is also evidenced by the
lack of reaction with common reducing reagents such as
NaBH3CN, NaBH4, high-pressure H2/Pd, and even DIBAL,
using standard protocols.14 No apparent reduction of the
azomethine bond was observed after refluxing in the presence
of DIBAL for 16 h. Furthermore, the thiopheno azomethines
have a high degree of resistance to oxidative decomposition
unlike their carbon analogues.28 The stability of the thiopheno
azomethines is further evidenced through their resistance to
photodecomposition and photoisomerization with intense laser
irradiation at 355 nm. The condensation between 2 and aromatic
aldehydes therefore leads to strong self-assembled bonds that
can be considered as molecular “snaps”. The bond can be
snapped together with mild dehydration and can only be
unsnapped with harsh conditions such as refluxing in concen-
trated sulfuric acid. Similarly to the azomethine robustness, 2
is also resistant, evidenced by the lack of observed acid-
catalyzed transesterification of the thiopheno esters, which
otherwise occurs with other esters under the experimental
protocols employed.
Spectroscopic Properties. The absorption spectra show the
degree of conjugation of the adducts, resulting from the
azomethine bond, and they are comparable to their nitrogen-
free derivatives. The linear trend observed for the reciprocal
number of atoms along the conjugated framework versus the
absorption shift shown in Figure 2 supports the conjugated
nature of the azomethine bond. There is a resulting increase in
stabilization with each additional thiophene ring, consistent with
a planar and rigid π-conjugation. The absorption and emission
spectra provide information relating to the energy differences
between the excited and ground states for the thiophene
The excited-state properties of the unique thiopheno azo-
methines were determined by steady-state and time-resolved
fluorescence. The measured fluorescence lifetimes are consistent
with a unimolecular deactivation process. Because low fluo-
rescence quantum yields were measured (Table 2) along with
low radiative rate decay constants (kf) for all of the compounds
relative to bithiophene, the excited-state deactivation modes must
occur by nonradiative channels such as internal conversion (IC)
or intersystem crossing (ISC) to the triplet state. The latter results
(29) Van Der Looy, J. F. A.; Thys, G. J. H.; Dieltiens, P. E. M.; De
Schrijver, D.; Van Alsenoy, C.; Geise, H. J. Tetrahedron 1997, 53, 15069-
15084.
(28) Horowitz, G. AdV. Mater. 1998, 10, 365-377.
J. Org. Chem, Vol. 72, No. 7, 2007 2635

Guar`ın et al.
TABLE 2. Photophysical Data of Thiopheno Azomethines and Some Nitrogen-Free Derivatives Measured in Anhydrous Acetonitrile
e
λabs
ꢀmax
(M-1 cm-1
λem
∆E
Eg
Φfl
τfl
kr
knr
λphos
τph
ET
g
h
compound
(nm)a
)
(nm)b
(eV)c
(eV)d
(10-2
)
(ns)f
(10-6 s-1
)
(10-8 s-1
)
(nm)i
(ms)j
(eV)k
1
2
3
4
5
6
7
8
350
305
400
466
440
492
470
440
351
413
423
21 850
9500
425
335
480
541
534
592
581
529
405
479
445
3.2
3.7
2.9
2.5
2.6
2.2
2.2
2.6
3.1
2.7
2.5
3.1
2.3
3.8
0.9
13.5
14.0
6.2
13.2
5.8
7.2
3.1
0.2
0.9
26
2.8
0.2
0.1
0.2
0.7
0.5
0.2
450
367
11
441
521
691
700
703
709
705
703
653
720
4.5
11.3
6.4
6.2
6.3
6.5
4.7
5.9
0.06
0.02
2.8
2.4
1.8
1.8
1.8
1.7
1.8
1.8
1.9
1.7
0.7
0.7
1.6
0.8
1.7
1.4
4.9
46
19 000
25 500
27 300
31 700
30 300
27 100
24 200
42 700
30 000
2.6
2.3
2.3
1.9
2.1
2.3
3.2
2.4
2.8
0.29
0.06
0.28
0.42
0.38
0.40
9.0
9l
10l
11m
33
7.4
a Absorption. b Fluorescence. c Absolute HOMO-LUMO difference. d Spectroscopic band-gap. e Fluorescence quantum yield relative to bithiophene.19
f Fluorescence lifetime. g Radiative rate constant kr ) φfl/τf. h Nonradiative rate constant knr ) kr(1 - φfl)/φfl. i Phosphorescence maximum at 77 K in a 1:4
methanol/ethanol matrix. j Phosphorescence lifetimes measured at λmax
.
k Triplet energy. l Literature values.15,16,19,30 m Literature values.56
FIGURE 4. Transient absorption spectra of triplets measured in
deaerated acetonitrile 100 µs after the laser pulse at 355 nm: xanthone
(0), 3 (b), and 5 (9). The spectra were recorded under the exact
conditions using optically matched samples at the laser excitation
wavelength. Inset: Decay of xanthone triplet measured at 625 nm.
FIGURE 3. Spectroscopic properties of 6. Normalized absorption (9)
and fluorescence (0) recorded in deaerated acetonitrile, and phospho-
rescence (b) recorded at 77 K in a 1:4 methanol/ethanol glass matrix.
The emission spectra were recorded with λex ) 492 nm. Inset: The
phosphorescence lifetime measured at 700 nm with λex ) 492 nm.
flash photolysis, shown in Figure 4. The bathochromic shift of
the bisazomethine transient relative to the monoazomethine is
consistent with its higher degree of conjugation. The extent of
the week signal observed for the azomethine transients is evident
when compared to the strongly absorbing xanthone triplet
(Figure 4). The strong ground-state bleaching is also clearly
seen in the transient absorption spectra of the azomethines and
correlates well with ground-state absorption spectra. The nega-
tive bleaching signal corresponds to a strong ground-state
absorption and hence a weak transient absorption at the
wavelength studied. The observed transients are ascribed to the
low-lying triplet because they are quenched by oxygen and they
decay with first-order kinetics. Conversely, deactivation of a
radical cation reactive intermediate would follow second-order
kinetics involving bimolecular radical cation coupling. A
unimolecular decay was also observed in the phosphorescence
measurements that showed lifetimes on the order of milli-
seconds, similar to other low temperature reports.15,17,30 These
lifetimes are 2 orders of magnitude longer than conventional
thiophenes at room temperature because of the lack of diffusion-
controlled processes at 77 K. Faster phosphorescent lifetimes
are therefore expected at room temperature, and these are further
expected to be comparable to the lifetimes of other thiophenes.
Even though a triplet intermediate was detected by laser flash
photolysis, it cannot be quantified because of its extremely weak
in efficient triplet manifold population that subsequently
undergoes energy dissipation by either phosphorescence or
nonradiative intersystem crossing. The singlet to triplet manifold
shift is consistent with other thiophenes as a result of the sulfur
atom that induces spin orbit coupling by the heavy atom
effect.15,17,18 The manifold shift was further expected to be
enhanced by the azomethine nitrogen because it can potentially
contribute to the heavy atom effect. The spin orbital coupling
to populate the triplet manifold is supported by the phospho-
rescence measurements at 77 K, which show a bathochromic
emission relative to the fluorescence emission (Figure 3). The
phosphorescence quantum yields of ca. 0.1 can be measured
for all of the thiopheno azomethines at this low temperature.
Triplet energies can also be obtained by this spectroscopic
method, which are comparable to those of terthiophene.30,31
Additional evidence for the triplet nature is derived from its
direct visualization by nanosecond-laser flash photolysis. Weakly
absorbing transients are observed at 280 and 370 nm for the
monoazomethine and the bisazomethine, respectively, by laser
(30) Wasserberg, D.; Marsal, P.; Meskers, S. C. J.; Janssen, R. A. J.;
Beljonne, D. J. Phys. Chem. B 2005, 109, 4410-4415.
(31) Wasserberg, D.; Dudek, S. P.; Meskers, S. C. J.; Janssen, R. A. J.
Chem. Phys. Lett. 2005, 411, 273-277.
2636 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
increase. Because the fluorescence quantum yields at 77 K are
determined by comparing the extremely weak fluorescence
signal at room temperature relative to the intense signal at 77
K under the same experimental conditions, precise calculations
of Φfl are difficult. This notwithstanding, an estimate of the
low-temperature fluorescence quantum yield ∼0.80 for all of
the azomethines was found. Even though the predominant
deactivation mode is by IC, approximately 20% of energy
dissipation is unaccounted for and must therefore occur by ISC
to the triplet. Nonradiative energy dissipation by E/Z photoi-
somerization of the azomethine bond is an alternate deactivation
mode.35 No photoisomers were, however, found for any of the
compounds after irradiation for 12 h at 350 nm. Given the lack
of significant signal by laser flash photolysis that would
otherwise be highly visible, the imine bond must therefore be
a good triplet deactivator leading to rapid and efficient intramo-
lecular quenching of this excited state. Such is the case for
acylhydrazides, which contain an imine-type linkage.36
FIGURE 5. Fluorescence of 7 in a 1:4 methanol/ethanol mixture at
77 K (9) and room temperature (0; magnified 200 times).
ꢀazomethine ∆Absxanthone
φazomethine
)
‚
‚ φxanthone
signal. The weak signal for the triplet thiopheno azomethines
is unexpected because these compounds are highly conjugated
and should therefore exhibit strong triplet extinction coefficients
(ꢀT), such as ꢀT ) 13 000 M-1 cm-1 for bithiophene, for
example.17,32 Both the quantum yield of triplet formation and
the ꢀT contribute to the signal intensity in laser flash photolysis.
Because of their high degree of conjugation, the thiopheno
azomethines should possess a large ꢀT. Therefore, the observed
weak signal can occur only from a low yield of triplet formation.
The yield can be calculated by measuring the change in transient
signal of the azomethine (∆Abs) with laser power versus that
of xanthone, according to eq 1. Assuming a triplet extinction
coefficient similar to that of bithiophene, an upper limit of 5%
for the triplet quantum yield is calculated by this method. Even
though the low value is in agreement with the phosphorescence
quantum yields, it is surprising because thiophenes are known
to have high triplet quantum yields, ∼0.9.15 To validate our
results, we measured the ΦT for bithiophene by the widely
accepted method of relative actinometry with 1-methylnaph-
thalene.20,33 Our measured value of 0.30 is in agreement with
other studies34 and differs from the previously measured values
by photoacoustic calorimetry.18 The discrepancy between these
values can be in part due to the photoacoustic method, whose
accuracy relies heavily on triplet lifetime being shorter than the
detector’s response time. This not withstanding, the low yield
of triplet formation implies the major pathway for deactivation
of the thiopheno azomethines occurs by internal conversion
according to the following energy conversation equation: Φfl
+ ΦISC + ΦIC ≈ 1. Therefore, the contribution from non-
radiative emission by internal conversion for the thiopheno
azomethines must be higher than that for oligothiophenes.
Evidence for the IC deactivation mode is derived from the
relative fluorescence at 77 K that is 260 times greater than that
at room temperature (Figure 5). All modes of excited-state
deactivation involving bond rotation are suppressed at this low
temperature, leading to a dramatic fluorescence quantum yield
ꢀxanthone ∆Absazomethine
Equation 1 was used to determine the azomethine triplet
quantum yield (φazomethine) relative to the triplet quantum yield
of xanthone (φxanthone), the triplet extinction coefficients (ꢀ), and
the change in maximum signal intensity with laser power
(∆Abs).
The energy levels, and hence the band-gap and the absorption
properties, can be further modified by acid doping to give
conductive-like properties to the azomethines. Methanesulfonic
acid is a suitable p-dopant that protonates the azomethine and
extends the degree of conjugation. A ca. 100 nm bathochromic
shift for 5-7 occurs upon acid protonation. The increased
stabilization from the nitrogen protonation results in a localized
charge on the nitrogen. The remarkable visible color change
denotes an increased delocalization and an additional band-gap
stabilization of approximately 0.4 eV. Successive acid addition
results uniquely in the protonated species given the presence
of one isobestic point as seen in Figure 6. The addition of
triethylamine neutralizes the cationic species and regenerates
the neutral bisazomethine. Cycling between the protonated and
the neutral forms via the successive addition of acid and base
results in reversible color change with little degradation of the
sample. Resistance to repetitive acid/base protonation/deproto-
nation further supports the persistence nature of the conjugated
azomethine bond.
Cyclic Voltammetry. Thiophenes are readily oxidized by two
stepwise one-electron oxidative processes, which can be ob-
served in cyclic voltammetry. The first oxidation generates a
radical cation that can cross-couple, provided unsubstituted
R-positions are available. Continuous oxidization subsequently
affords higher ordered oligomers by a radical-like coupling
mechanism and ultimately the formation of polymers by
successive oxidation. At higher potentials, a second oxidation
affording the dication is possible. The thiopheno azomethines
(35) (a) Amati, M.; Bonini, C.; D’Auria, M.; Funicello, M.; Lelj, F.;
Racioppi, R. J. Org. Chem. 2006, 71, 7165-7179. (b) Lehn, J.-M. Chem.-
Eur. J. 2006, 12, 5910-5915.
(36) (a) Lygaitis, R.; Grazulevicius, J.; Van, F. T.; Chevrot, C.;
Jankauskas, V.; Jankunaite, D. J. Photochem. Photobiol., A 2006, 181, 67-
72. (b) Getautisa, V.; Grazulevicius, J. V.; Daskeviciene, M.; Malinauskas,
T.; Gaidelis, V.; Jankauskas, V.; Tokarski, Z. J. Photochem. Photobiol., A
2006, 180, 23-27.
(32) Wintgens, V.; Valat, P.; Garnier, F. J. Chem. Phys. 1994, 98, 228-
232.
(33) Scaiano, J. C.; Lissi, E. A.; Stewart, L. C. J. Am. Chem. Soc. 1984,
106, 1539-1542.
(34) (a) Scaiano, J. C.; Redmond, R. W.; Mehta, B.; Arnason, J. T.
Photochem. Photobiol. 1990, 52, 655-659. (b) Reyftmann, J. P.; Kagan,
J.; Santus, R.; Morliere, P. Photochem. Photobiol. 1985, 41, 1-7.
J. Org. Chem, Vol. 72, No. 7, 2007 2637

Guar`ın et al.
FIGURE 6. Absorption spectra change of 6 upon protonation with
methanesulfonic acid in anhydrous acetonitrile. Mole percentage of acid
added: 0 (9), 20 (b), 40 (2), 60 (f), 80 (0), 100 (O), 120 (4), 140
(g), 160 (]).
TABLE 3. Electrochemical Data of Thiopheno Azomethines, Their
Polymers, and Nitrogen-Free Derivativesa
compound
Epa1 (V)
Epa2 (V)
∆Epa (V)
1
2
3
4
5
6
7
8
1.02
0.04
0.91
0.83
1.24
0.4
0.22
0.36
0.46
0.24
0.37
0.35
0.26
0.34
0.95
0.30
0.26
0.28
1.37
1.07
1.53
1.37
1.25
1.51
1.9
1.12
1.14
1.88
1.16 (0.91)b
1.02 (1.06)b
0.99 (1.17)b
1.17 (1.05)b
0.95
9e
10f
11c
12c
0.82
0.88 (0.95)b,c
1.60 (2.09)b,d
FIGURE 7. (A) Typical cyclic voltammogram of 6 recorded in
anhydrous dichloromethane with Bu4NPF6 supporting electrolyte against
Ag/AgCl electrode at 100 mV/s and referenced against SCE. Inset:
Repeated cyclic oxidation of 6 between 0 and 1400 mV with the product
deposited on the Pt working electrode. (B) Cyclic voltammogram of
P5 (b) and P8 (9) deposited on ITO electrode after continued oxidative
cross-coupling of 5 and 8, respectively, recorded in deaerated dichlo-
romethane under the same conditions as in (A). Inset: Normalized
spectra of 8 (open symbols) in deaerated acetonitrile and P8 (closed
symbols) on ITO electrode; absorption (squares) and emission (circles).
The emission spectra were recorded with λex ) 400 nm.
a First and second oxidation potentials corresponding to the radical cation
and the dication, respectively. In anhydrous and deaerated dichloromethane
with Bu4NPF6 as the supporting electrolyte. Values are reported against
SCE with ferrocene (0.59 V) as an internal standard.23 b Values in
parentheses refer to the oxidation potentials of oxidatively coupled materials
deposited on the ITO electrode. c Literature values.43 d Literature values.37
e Literature values.57 f Literature values.58
studied are of no exception. Two consecutive one-electron
oxidations corresponding to the radical cation followed by the
dication were observed for 5-8 (Table 3). Additionally, 2-4
undergo two one-electron oxidations of the amine group. For
all of the azomethines studied, it is noteworthy that the first
oxidation forms a quasi-reversible radical cation as seen in
Figure 7A. This is in stark contrast to other azomethines such
as 12 that undergo irreversible oxidation regardless of the
experimental conditions because of their extreme reactivity. This
behavior ultimately limits their usefulness as functional
materials.8,37-39 In fact, previous azomethine examples are so
reactive they undergo both monomer and polymer oxidative
decomposition. The persistent thiopheno azomethine radical
cations imply a high degree of stability of the reactive
intermediate, unlike other azomethine radical cations.
Because previous azomethine electrochemical studies in-
volved only homologous aryl units they not only exhibited
irreversible oxidation,37,39,40 they also exhibited extremely high
oxidation potentials. The thiophene-containing azomethines are
advantageous because they possess lower oxidation potentials
and they are comparable to other thiophenes. This is due in
part to the two ester withdrawing groups, while the strong
donating capacity of the amine group influences the oxidation
potentials. This is obvious for 3 and 4 whose oxidation potentials
are similar to those of the bisazomethines even though the latter
have a higher degree of conjugation. The low oxidation
(37) Diaz, F. R.; del Valle, M. A.; Brovelli, F.; Tagle, L. H.; Bernede,
J. C. J. Appl. Polym. Sci. 2003, 89, 1614-1621.
(38) (a) Yang, C.-J.; Jenekhe, S. A. Macromolecules 1995, 28, 1180-
1196. (b) Lund, H.; Hammerich, O. Organic Electrochemistry, 4th ed.;
Marcel Dekker: New York, 2001. (c) Caballero, A.; Ta´rraga, A.; Velasco,
M. D.; Molina, P. Dalton Trans. 2006, 1390-1398. (d) Catanescu, O.;
Grigoras, M.; Colotin, G.; Dobreanu, A.; Hurduc, N.; Simionescu, C. I.
Eur. Polym. J. 2001, 37, 2213-2216. (e) Grigoras, M.; Catanescu, C. O.;
Colotin, G. Macromol. Chem. Phys. 2001, 202, 2262-2266.
(39) Higuchi, M.; Yamamoto, K. Polym. AdV. Technol. 2002, 13, 765-
770.
(40) (a) Zotti, G.; Randi, A.; Destri, S.; Porzio, W.; Schiavon, G. Chem.
Mater. 2002, 14, 4550-4557. (b) Brovelli, F.; Rivas, B. L.; Bernede, J. C.
J. Chilean Chem. Soc. 2005, 50, 597-602.
2638 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
FIGURE 8. Schematic representation of the relative band-gaps and energy levels influenced by the number of thiophene groups.
potentials observed for the monoazomethines are a result of the
stabilizing influence of the amino donating group, which
decreases the HOMO energy level. The combined influence of
the amino donating ability concomitant with the high degree of
conjugation of the azomethines cause a significant lowering of
the oxidation potentials. The monoazomethine oxidation po-
tentials are highly perturbed by the amino group such that they
are lower than their thiophene and bithiophene analogues, whose
oxidation potentials are 2.07 and 1.05 V, respectively.41
The ionization potential (IP), corresponding to the HOMO
working electrode.42,43 The oxidative coupling results in an
increase in the oxidation peak of the deposited material on the
electrode concomitant with a decrease of the first oxidation peak
of the monomer (inset Figure 7A). The increase in anodic peak
potential for the deposited film relative to the monomer is
attributed to the nonconductiveness of the deposited layer as a
result of the ion diffusion rate determining step. This is well
established for the electropolymerization of semiconductors and
is consistent with other thiophenes.44 The amount of product
deposited is the same for 5-8 and corresponds to a charge of
ca. 10-3 coulombs obtained by washing the resulting layer with
copious amounts of acetonitrile and dichloromethane followed
by measuring the redox potential of the insoluble product in
monomer-free electrolyte.45 The resulting redox properties of
the cross-coupled products are reported in Table 3.
Because of previous studies that showed the radical cation
azomethine intermediates are particularly reactive, their elec-
trochemically induced oxidative coupling was expected to result
in limited extension of their conjugation because of R-â and
â-â coupling defects.46 The EIC of 8 was investigated because
its â positions do not allow for such defects and it would
promote exclusively conjugated products via R-R coupling. The
bathochromic shifts found for both the fluorescence and the
absorption spectra (inset Figure 7B) suggest a product with a
greater degree of conjugation relative to the monomer is formed.
This can only occur by R-R coupling. Product coupling was
further confirmed by large-scale electrochemical oxidative cross-
coupling of 5 in which sufficient material was obtained for
conventional molecular weight characterization by GPC. The
energy level, can be calculated from the oxidation onset
ox
onset
according to IP ) E (SCE) + 4.4. The Eonset (SCE) is the
oxidation potential onset in volts versus the SCE electrode to
which a 0.39 V correction factor is applied. Similarly, the
electron affinity (EA), corresponding to the LUMO energy level,
red
onset
can normally be calculated from the reduction onset (E
)
red
according to EA ) E (SCE) + 4.4. Because of the lack of
onset
observed reduction under our experimental conditions, which
further supports the azomethine robustness, the electron affinity
can be calculated alternatively from the spectroscopic band-
gap and from the ionization potential according to EA ) IP -
Eg. These data provide an accurate representation of the
HOMO-LUMO energy levels and the band-gaps, which are
schematically depicted in Figure 8. The stabilizing effect of the
amino donating group on the HOMO is apparent in this figure.
Furthermore, the effect of the number of thiophene groups and
the conjugation degree upon reducing the band-gap is also
apparent. The spectroscopic band-gaps demonstrate that values
as low as 1.9 eV are possible with the thiopheno azomethines.
This is different from their analogues whose band-gap lower
limit is considerably higher at 2.4 eV.
Electrochemical Oxidative Coupling. Even though the
thiopheno azomethine radical cation formation is quasi-revers-
ible, it is, however, sufficiently reactive to undergo electro-
chemically induced oxidative cross-coupling (EIC). EIC of 5-8
was possible either by repetitive oxidative cycling between 0
and 1400 mV or by applying a potential greater than the
corresponding radical cation for a period of 10 min. The
resulting products were deposited as dark colored films on the
(42) (a) Hansford, K. A.; Guarin, S. A. P.; Skene, W. G.; Lubell, W. D.
J. Org. Chem. 2005, 70, 7996-8000. (b) Haare, J. A. E. H. v.; Havinga, E.
E.; Dongen, J. L. J. v.; Janssen, R. A. J.; Cornil, J.; Bre´das, J.-L. Chem.-
Eur. J. 1998, 4, 1509-1522.
(43) (a) Je´roˆme, C.; Maertens, C.; Mertens, M.; Je´roˆme, R.; Quattrocchi,
C.; Lazzaroni, R.; Bre´das, J. L. Synth. Met. 1996, 83, 103-109. (b) Je´roˆme,
C.; Maertens, C.; Mertens, M.; Je´roˆme, R.; Quattrocchi, C.; Lazzaroni, R.;
Bre´das, J. L. Synth. Met. 1997, 84, 163-164.
(44) (a) Sun, X. B.; Liu, Y. Q.; Chen, S. Y.; Qiu, W. F.; Yu, G.; Ma, Y.
Q.; Qi, T.; Zhang, H. J.; Xu, X. J.; Zhu, D. B. AdV. Funct. Mater. 2006,
16, 917-925. (b) Yassar, A.; Roncali, J.; Garnier, F. Macromolecules 1989,
22, 804-809. (c) Henderson, P. T.; Collard, D. M. Chem. Mater. 1996, 7,
1879-1889.
(45) Tourillon, G. In Handbook of Conducting Polymers; Skotheim, T.
A., Ed.; Marcel Dekker, Inc.: New York, 1986; Vol. 1, pp 293-350.
(46) Roncali, J. Chem. ReV. 1992, 92, 711-738.
(41) Diaz, A. F.; Martinez, A.; Kanazawa, K. K.; Salmon, M. J.
Electroanal. Chem. Interfacial Electrochem. 1981, 130, 181-187.
J. Org. Chem, Vol. 72, No. 7, 2007 2639

Guar`ın et al.
TABLE 4. Representative Bond Distances and Mean Plane Angle
for 7 and Its Nitrogen-Free Analogue
7
11a
side Bd
monothiopheneb bithiophenec side Ad
plane angleg
-ÌdX-e
180°
170.1°
177.9° 180°
1.343 Å
1.387 Å
1.423 Å
antiparallel
1.306 Å
1.372 Å
1.413 Å
1.300 Å 1.326 Å
1.460 Å 1.440 Å
1.452 Å 1.453 Å
-X-aryle
dCH-aryl
sulfur orientationg
antiparallel parallel antiparallel
a From Zobel.51 b Refers to distances and angles from S1 through S2 in
Figure 9A. c Refers to distances and angles from S2 through S4 in Figure
9A. d Refers to each side of the unsymmetric molecule. e X ) N for 7 and
C for 11. f Refers to angles between the central and the terminal thiophene
mean planes. f Sulfur orientation relative to the central thiophene.
analyses were subsequently dissolved in deuterated acetone. The
resulting NMR spectra showed an imine chemical shift identical
to that of the original reaction mixture, confirming that the E
isomer is uniquely formed.
FIGURE 9. (A) Schematic representation of the crystal structure 7
showing the atomic labels. (B) Schematic representation of crystal
structure 7 seen along the axis parallel to the thiophene units.
In addition to absolute assignment of the geometric isomer,
the isolated crystal structure illustrated in Figure 9 shows the
heteroatom units are orientated in an antiparallel arrangement.
This is predominantly a result of the intramolecular hydrogen
bonding of the donor-acceptor pairs between the central sulfur
(S2) and the imine hydrogens (H5 and H10). The antiparallel
orientation is similar to that of bithiophene and other higher
ordered oligothiophenes. Moreover, the planar configuration
between the thiophene units and the azomethine is visible, in
which the mean planes of the terminal thiophenes are twisted
2.6° and 8.0° from the azomethine plane (Table 4). This
represents among the first few examples of reduced twisting of
the azomethine mean planes.25 This is in contrast to homoaryl
aromatic rings that are normally twisted between 45° and 65°
from the mean planes of azomethines to which they are directly
bound.14,47,48 This arrangement prevents any steric hindrance
between the imine hydrogen and that of the aryl ortho
hydrogen,14,47 which would lead to an orthogonal biphenyl-type
conformation.49 The observed planarity disruption for such
compounds limits their conjugation50 and subsequently limits
their usefulness as functional materials. The crystallographic
data show the azomethine bond distances are shorter than those
of its nitrogen-free derivative,51,52 while the aryl-aryl distance
remains unchanged.53 The enhanced planarity observed for the
studied azomethines is further responsible for the increased
conjugation and ensuing narrow band-gaps and potentially
results in ideal conductive when doped.14,54 The high degree of
planarity and linearity further results in an extremely tight
average molecular weight calculated corresponds to a product
with 9 thiophene units. This represents a lower limit for the
molecular weight of the coupled products and implies that the
molecular weight of the bulk insoluble material is likely much
higher. Further evidence for the increased conjugation of the
coupled product is derived from the spectroscopic band-gap of
the electrochemically produced product that is 0.5 eV lower
than the corresponding monomer. Furthermore, both the absorp-
tion and the fluorescence spectra undergo pronounced batho-
chromic shifts relative to the monomer (inset Figure 7B) because
of increased conjugation. The similarity between the spectro-
scopic and the redox properties of the products derived from 5
and 8 confirms the coupling occurs exclusively via the R
position and results in an increased degree of conjugation. This
is further supported by the broad reversible oxidation potentials
that are indicative of a distribution of oxidation states associated
with oligomers and polymers. The resulting coupled products
exhibit oxidations similar to those of their nitrogen-free deriva-
tives, and they can be reversibly oxidized at low potentials such
that the azomethines can be p-doped with standard chemical
dopants. This is in stark contrast to other conjugated azomethines
that undergo complete irreversible radical cation formation
regardless of the experimental conditions and scan rates. This
irreversible behavior has limited the usefulness of other azo-
methines as functional materials.37 The low oxidation potential
concomitant with the reversible radical cation formation illustrate
the suitability of thiophene azomethines as functional p-dopable
materials.
(47) Skene, W. G.; Dufresne, S. Acta Crystallogr. 2006, E62, o1116-
o1117.
(48) (a) Kuder, J. E.; Gibson, H. W.; Wychick, D. J. Org. Chem. 1975,
40, 875-879. (b) Manecke, G.; Wille, W. E.; Kossmehl, G. Makromol.
Chem. 1972, 160, 111-126. (c) Bu¨rgi, H. B.; Dunitz, J. D. Chem. Commun.
1969, 472-473.
X-ray Crystallography. Like their carbon analogues, two
geometry isomers (E and Z) are possible with azomethines.
Because the azomethine has only one proton that does not
couple, both E and Z isomers are expected to provide only a
singlet in 1H NMR in the region of 8.5 ppm (Figure 1).
Differentiation between these two is not readily possible by 1D
and 2D NMR unlike their carbon analogues. Absolute assign-
ment of the azomethine to the E isomer can, however, be derived
from the crystallographic data (Figure 9). Formation of the
thermodynamically stable E isomer is assumed for all of the
azomethines given the presence of only one imine peak in all
of the 1H NMR spectra. This was confirmed by repeated
crystallizations of 4, 5, and 7, which consistently gave the same
crystal structure and the E isomer. The crystals used for X-ray
(49) Narasegowda, R. S.; Yathirajana, H. S.; Bolteb, M. Acta Crystallogr.,
Sect. E 2005, E61, o939-o940.
(50) (a) Wan, C.-W.; Burghart, A.; Chen, J.; Bergstro¨m, F.; Johansson,
L. B.-Å.; Wolford, M. F.; Kim, T. G.; Topp, M. R.; Hochstrasser, R. M.;
Burgess, K. Chem.-Eur. J. 2003, 9, 4430-4441. (b) Jiao, G.-S.; Thoresen,
L. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 14668-14669.
(51) (a) Ruban, G.; Zobel, D. Acta Crystallogr., Sect. B 1975, B31, 2632-
2634. (b) Zobel, D.; Ruban, G. Acta Crystallogr., Sect. B 1978, B34, 1652-
1657.
(52) Blanchard, P.; Brisset, H.; Illien, B.; Riou, A.; Roncali, J. J. Org.
Chem. 1997, 62, 2401-2408.
(53) Pelletier, M.; Brisse, F.; Cloutier, R.; Leclerc, M. Acta Crystallogr.,
Sect. C 1995, C51, 1394-1397.
(54) Roncali, J.; Thobie-Gautier, C. AdV. Mater. 1994, 6, 846-848.
2640 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
FIGURE 10. (A) Crystal packing arrangement of 7 illustrating the intermolecular hydrogen bonding between donor and acceptor pairs O1‚‚‚H10
1
1
3
1
1
1
and O3‚‚‚H20 (i (x, /2 - y, /2 + z); ii (x, 1 + y, z); iii (x, /2 - y, /2 + z); iv (x, -1/2 - y, /2 + z); v (1 - x, 1 - y, 1 - z); vi (1 - x, /2 + y,
1/2 - z); vii (1 - x, -y, 1 - z); viii (1 - x, -1/2 + y, /2 - z); ix (1 - x, /2 + y, /2 - y)). (B) Crystal packing arrangement of 7 illustrating the
π-stacking interactions between adjacent bithiophenes.
1
3
3
packing arrangement in the crystal structure that is further
Conclusion
enhanced by intermolecular hydrogen bonding between O1‚‚‚
The first example of a simple modular route for conjugated
thiophene analogues was presented. This self-assembled ap-
proach leads to quantitative and selective addition of the
thiophenes units that can be controlled by either stoichiometry
or solvents and allows for fine-tuneable physical properties of
the end products. Through selective unsymmetric and symmetric
conjugated compounds, materials with dial-in properties are
accessible. Reversible-type oxidation and moderately low
H
10 (Figure 10A) and O3‚‚‚H20, π-stacking (Figure 10B) between
S3 and S4 with their corresponding sulfurs in adjacent molecules,
and C-H-π interactions between H16 and H17 with the
π-system of S3 and S4. The resulting combined strong interac-
tions result in a three-dimensional herringbone-like network.24,55
(55) Skene, W. G.; Trefz, T. Polym. Prepr. 2004, 45, 563-564.
J. Org. Chem, Vol. 72, No. 7, 2007 2641

Guar`ın et al.
10.47; S, 12.01. HR-MS calculated for C10H14O4N2SNa (MNa+),
281.05665; found, 281.05649.
potentials make these azomethines suitable for p-type doping
applications, while their linear and planar conformations impart
a high degree of conjugation. Such attributes open a new realm
of stable thiophene-containing materials with a property of
tailoring, and of importance the band-gaps and emissions, which
can easily be had unlike their nitrogen-free derivatives. Access
to these synthetically simple compounds with varible properties
will ultimately yield new materials suitable for functional
devices with new operating properties. The inherently weak
emission of the thiophene azomethines, low oxidation potential,
and reversible radical cation formation potentially make these
compounds suitable as PEDOT hole injection replacements in
organic light emitting diodes.
(E)-Diethyl
2-((Thiophen-2-yl)methyleneamino)-5-ami-
nothiophene-3,4-dicarboxylate (3). To 2 (100 mg, 0.39 mmol)
was added 2-thiophene carboxaldehyde (43 mg, 0.38 mmol) in
ethanol followed by refluxing for 2 h after the addition of a catalytic
amount of trifluoroacetic acid (TFA). The solvent was removed,
and the product was isolated (126 mg, 94%) as a yellow solid after
purification by flash chromatography (SiO2) with 40% ethyl acetate
and 60% hexanes. Mp: 114-116 °C. 1H NMR (400 MHz, acetone-
d6): δ ) 8.26 (s, 1H), 7.64 (d, J ) 5.2 Hz, 1H), 7.52 (d, J ) 4.0
Hz, 1H), 7.48 (bs, 2H), 7.14 (dd, J ) 5.2, 4.0 Hz, 1H), 4.33 (q, J
) 7.1 Hz, 2H), 4.20 (q, J ) 7.1 Hz, 2H), 1.38 (t, J ) 7.1 Hz, 3H),
1.26 (t, J ) 7.1 Hz, 3H). 13C NMR (100 MHz, acetone-d6): δ )
165.0, 164.3, 161.1, 161.0, 146.1, 143.2, 132.9, 132.0, 130.5, 128.4,
101.9, 61.0, 60.0, 14.3, 14.1. HR-MS calculated for C15H17O4N2S2
(MH+), 353.06242; found, 353.06251.
Experimental Section
(E)-Diethyl 2-((5-(Thiophen-2-yl)thiophen-2-yl)methylene-
amino)-5-aminothiophene-3,4-dicarboxylate (4). To 2 (30 mg,
0.12 mmol) was added 1 (40 mg, 0.21 mmol) in isopropanol. The
solution was then refluxed for 5 h after the addition of a catalytic
amount of trifluoroacetic acid (TFA). The solvent was removed,
and the product was isolated as a yellow solid (42 mg, 81%) after
purification by flash chromatography (SiO2) with 40% ethyl acetate
and 60% hexanes. Mp: 125 °C. 1H NMR (400 MHz, acetone-d6):
δ ) 8.20 (s, 1H), 7.50 (d, J ) 5.1, 1H), 7.46 (d, J ) 3.9 Hz, 1H),
7.40 (d, J ) 3.9, 1H), 7.30 (d, J ) 3.9 Hz, 1H), 7.12 (dd, J ) 5.1,
3.9 Hz, 1H), 4.35 (q, J ) 7.1 Hz, 2H), 4.21 (q, J ) 7.1 Hz, 2H),
1.40 (t, J ) 7.1 Hz, 3H), 1.27 (t, J ) 7.1 Hz, 3H). 13C NMR (100
MHz, acetone-d6): δ ) 165.5, 164.1, 161.8, 146.6, 141.6, 141.1,
137.0, 134.3, 132.6, 130.2, 129.6, 127.7, 126.2, 125.8, 101.0, 61.6,
60.4, 15.1, 15.0. EI-MS: m/z 434.9 ([M]+, 96%). HR-MS calculated
for C19H19N2O4S3 (MH+), 435.05015; found, 435.05022.
Synthesis. 5-(Thiophen-2-yl)thiophene-2-carbaldehyde (1). In
a round-bottom flask was added phosphorus oxychloride (1.83 g,
12 mmol) at 0 °C to DMF (15 mL). After 30 min, 2,2′-bithiophene
(500 mg, 3.0 mmol) was added, and the solution was stirred at
room temperature for 30 min. The temperature was raised to 50
°C and then held at this temperature until the reaction was
completed. Diluted hydrochloric acid was added to the reaction
mixture at 0 °C. The crude product was extracted with ethyl acetate
after the mixture was warmed to room temperature. Purification
by flash chromatography (SiO2) with 20% ethyl acetate and 80%
hexanes yielded the title product as a light brown powder (81%).
1H NMR (400 MHz, acetone-d6): δ ) 9.93 (s, 1H), 7.90 (d, 1H,
J ) 3.9 Hz), 7.60 (d, 1H, J ) 5.2 Hz), 7.52 (d, 1H, J ) 4.0 Hz)
7.43 (d, 1H, J ) 3.9 Hz), 7.11 (t, 1H, J ) 5.2 Hz). 13C NMR (100
MHz, acetone-d6): δ ) 183.2, 146.5, 142.5, 138.5, 136.2, 129.0,
127.9, 126.8, 125.1. HR-MS calculated for C9H7OS2 (MH+),
194.99328; found, 194.99274.
(2E,5E)-Diethyl
2,5-Bis((thiophen-2-yl)methyleneamino)-
thiophene-3,4-dicarboxylate (5). 2 (100 mg, 0.4 mmol) and
2-thiophene carboxaldehyde (198.8 mg, 1.8 mmol) were refluxed
in anhydrous isopropanol (10 mL) in a 25 mL flask, during which
the solution turned orange and then red in color within 5 h of
refluxing under nitrogen with a catalytic amount of TFA. The
solution was then concentrated under vacuum to near dryness. The
crude product was loaded onto a silica column and eluted with
hexanes/ethyl acetate (85%/15% v/v) up to hexanes/ethyl acetate
(75%/25% v/v) to give the products as a red solid (65 mg, 36%).
Mp: 125-126 °C. 1H NMR (400 MHz, acetone-d6): δ ) 8.75 (s,
2H), 7.85 (d, 2H, J ) 5.2), 7.76 (d, 2H, J ) 3.7), 7.26 (dd, 2H, J
) 5.2, 3.7), 4.31 (q, 4H, J ) 7.2), 1.36 (t, 6H, J ) 7.2). 13C NMR
(100 MHz, acetone-d6): δ ) 163.0, 153.60, 149.2, 142.4, 135.1,
133.2, 128.9, 127.5, 61.2, 14.2. HRMS calculated for C20H19O4N2S3
(MH+), 447.05015; found, 447.04921.
2,5-Diamino-thiophene-3,4-dicarboxylic Acid Diethyl Ester
(2). The optimized procedure is based on similar reports.11,24,25
Sulfur (4.53 g, 0.14 mol) and triethylamine (7.09 mL, 0.05 mol)
were stirred in DMF (15 mL) in a 250 mL three-necked flask
whereupon the solution turned red within 30 min of stirring at room
temperature. Ethyl cyanoacetate (20.4 mL, 0.19 mol) diluted in
DMF (5 mL) was subsequently added dropwise over 30 min. The
opaque solution was allowed to stir under ambient conditions for
3 days, after which the solvent was removed under vacuum, to yield
a brown solid. The solid was loaded onto a silica column and eluted
with 100% hexanes gradient up to 35% ethyl acetate. The procedure
was repeated a second time to obtain the title compound (2.15 g,
9%) as gold flaky crystals. Mp: 155-156 °C. 1H NMR (400 MHz,
acetone-d6): δ ) 6.15 (s, 4H), 4.17 (q, 4H, J ) 7.1 Hz), 1.25 (t,
6H, J ) 7.1 Hz). 13C NMR (100 MHz, acetone-d6): δ ) 166.7,
151.3, 103.7, 61.0, 15.7. EI-MS: m/z 258.1 ([M]+, 80%), 212 ([M
- C2H5O]+, 100%), Anal. Calcd for C10H14O4N2S: C, 46.50; H,
5.46; N, 10.85; O, 24.74; S, 12.41. Found: C, 45.89; H, 5.10; N,
(2E,5E)-Diethyl 2,5-Bis((5-(thiophen-2-yl)thiophen-2-yl)meth-
yleneamino)thiophene-3,4-dicarboxylate (6). To 2 (49 mg, 0.19
mmol) was added 1 (75 mg, 0.39 mmol), and the solution was then
refluxed in isopropanol for 4 h along with a catalytic amount of
TFA. The product was isolated as a red powder (58 mg, 50%) after
column chromatography (SiO2), eluted first with 50% ethyl
acetate and 50% hexanes, and then with 100% acetone. Mp: 130-
(56) (a) Fre`re, P.; Raimundo, J.-M.; Blanchard, P.; Delaunay, J.;
Richomme, P.; Sauvajol, J.-L.; Orduna, J.; Garin, J.; Roncali, J. J. Org.
Chem. 2003, 68, 7254-7265. (b) Jestin, I.; Fre`re, P.; Mercier, N.; Levillain,
E.; Stievenard, D.; Roncali, J. J. Am. Chem. Soc. 1998, 120, 8150-8158.
(c) Elandaloussi, E. H.; Fre`re, P.; Richomme, P.; Orduna, J.; Garin, J.;
Roncali, J. J. Am. Chem. Soc. 1997, 119, 10774-10784. (d) Geisler, T.;
Petersen, J. C.; Bjornholm, T.; Fischer, E.; Larsen, J.; Dehu, C.; Bredas,
J.-L.; Tormos, G. V.; Nugara, P. N. J. Phys. Chem. 1994, 98, 10102-
10111.
1
132 °C. H NMR (400 MHz, acetone-d6): δ ) 8.68 (s, 2H), 7.70
(d, 2H, J ) 4.0 Hz), 7.57 (d, 2H, J ) 5.2 Hz), 7.49 (d, 2H, J ) 3.6
Hz), 7.41 (d, 2H, J ) 4.0 Hz), 7.16 (dd, 2H, J ) 5.2, 3.6 Hz), 4.34
(q, 4H, J ) 6.9 Hz), 1.39 (t, 6H, J ) 6.9 Hz). 13C NMR (100
MHz, acetone-d6): δ ) 162.9, 153.8, 149.1, 143.7, 140.2, 137.4,
136.3, 129.4, 128.2, 126.8, 126.7, 125.8, 61.5, 14.6. EI-MS: m/z
610.9 ([M]+, 100%). HR-MS calculated for (MH+) exact mass,
611.0255; found, 611.0252.
(57) (a) Wei, Y.; Chan, C. C.; Tian, J.; Jang, G. W.; Hsueh, K. F. Chem.
Mater. 1991, 3, 888-897. (b) Meerholz, K.; Heinze, J. Electrochim. Acta
1996, 41, 1839-1854.
(58) (a) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.;
Hapiot, P. J. Phys. Chem. 1995, 99, 11923-11929. (b) Facchetti, A.; Yoon,
M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am.
Chem. Soc. 2004, 126, 13480-13501. (c) Barbarella, G.; Favaretto, L.;
Sotgiu, G.; Zambianchi, M.; Fattori, V.; Cocchi, M.; Cacialli, F.; Gigli, G.;
Cingolani, R. AdV. Mater. 1999, 11, 1375-1379.
(2E,5E)-Diethyl 2-((5-(Thiophen-2-yl)thiophen-2-yl)methyl-
eneamino)-5-((thiophen-2-yl)methyleneamino)thiophene-3,4-di-
carboxylate (7). This compound was synthesized either in a
stepwise manner or by a one-pot approach. Stepwise formation was
achieved from 2 (48 mg, 0.19 mmol) to which was added 1 (30
2642 J. Org. Chem., Vol. 72, No. 7, 2007

Self-Assembled Conjugated Thiopheno Azomethines
mg, 0.15 mmol), and the solution was refluxed with isopropanol
for 12 h along with a catalytic amount of TFA. The intermediate
product was isolated as a yellow powder (50 mg, 77%) after
purification by flash column chromatography (SiO2) with 40% ethyl
acetate and 60% hexanes as solvent. To the isolated product was
added 2-thiophene carboxaldehyde (13 mg, 0.12 mmol) in isopro-
panol in addition to a catalytic amount of TFA. The reaction was
refluxed for 12 h. A red colored powder (37.5 mg, 63%) was
isolated after flash column chromatography (SiO2) eluted with
hexanes up to hexanes/ethyl acetate (80%/20% v/v). Mp: 128 °C.
1H NMR (400 MHz, acetone-d6): δ ) 8.74 (s, 1H), 8.68 (s, 1H),
7.84 (d, J ) 5.2 Hz, 1H), 7.76 (d, J ) 3.9 Hz, 1H), 7.70 (d, J )
3.9 Hz, 1H), 7.57 (d, J ) 5.2, 1H), 7.50 (d, J ) 3.9 Hz, 1H), 7.41
(d, J ) 3.9 Hz, 1H), 7.25 (dd, J ) 5.2, 3.9 Hz, 1H), 7.16 (dd, J )
5.2, 3.7 Hz, 1H), 4.40-4.25 (m, 4H), 1.45-1.30 (m, 6H). 13C NMR
(100 MHz, acetone-d6): δ ) 162.6, 153.1, 152.5, 148.8, 144.4,
144.3, 143.7, 142.1, 140.5, 136.4, 135.8, 134.7, 134.0, 132.8, 128.6,
128.5, 126.9, 125.7, 124.8, 60.8, 60.7, 13.7, 13.6. HR-MS calculated
for C24H21N2O4S4 (MH+) exact mass, 529.03787; found, 529.03676.
Alternatively, 2 (100 mg, 0.39 mmol) and 2-thiophene carbox-
aldehyde (36 mg, 0.32 mmol) were combined in ethanol along with
a catalytic amount of TFA. After the mixture was refluxed for 12
h, a yellow powder was isolated after purification by flash
chromatography (103 mg, 91%). To the isolated product was added
1 (147 mg, 0.76 mmol) in isopropanol along with a catalytic amount
of TFA. A red colored powder (61 mg, 40%) was isolated after
flash column chromatography (SiO2).
One-pot formation of the title compound was achieved by
combining 2 (15 mg, 0.06 mmol) with 2-thiophene carboxaldehyde
(6.5 mg, 0.06 mmol) followed by refluxing in ethanol for 12 h in
the presence of a catalytic amount of TFA. After removal of the
solvent, isopropanol was added along with 1 (11.3 mg, 0.06 mmol)
and a catalytic amount of TFA. The mixture was then refluxed for
12 h, and the title compound was quantitatively isolated as a red
powder after flash chromatography (SiO2).
The unsymmetric 7 was also prepared stepwise by the addition
of 3 (126 mg, 0.4 mmol) to 1 (70 mg, 0.4 mmol) or by the addition
of 4 (10 mg, 0.02 mmol) to thiophene carboxaldehyde (3 mg, 0.02
mmol). The general procedure is as follows: The aldehyde was
dissolved in anhydrous toluene under nitrogen at 0 °C to which
was subsequently added 1,4-diazabicyclo[2.2.2]octane (DABCO)
(5 equiv) followed by the slow addition of titanium(IV) chloride
(TiCl4) in toluene [1 M] (1 equiv) over 30 min. The reaction was
subsequently warmed to room temperature followed by the addition
of the corresponding amine, which was dissolved in anhydrous
toluene and then refluxed for 1 h. Upon cooling to room temper-
ature, the reaction was filtered through a plug of silica (SiO2), and
the title compound was isolated as a red color powder by C-18
reverse phase preparative HPLC with 70% acetonitrile/30% water
as the mobile phase.
TMEDA (0.1 mL, 0.66 mmol) in anhydrous hexane (5 mL) under
nitrogen was added a solution of n-BuLi (2.18 M in hexanes) (0.25
mL, 0.55 mmol) dropwise at room temperature. After the mixture
was refluxed for 1 h, THF (3 mL) was added, and the solution was
cooled at -78 °C, to which was added DMF (0.1 mL, 1.36 mmol)
dropwise. The reaction mixture was hydrolyzed with water (20 mL)
after 1.5 h at room temperature, and the mixture was extracted with
ethyl acetate. The organic layers were dried over MgSO4 and then
concentrated. The crude product was loaded onto a 12+M Biotage
column and eluted with hexanes/ethyl acetate up to (98%/2% v/v)
over 20 column volumes to afford the product as a colorless oil
1
(61 mg, 30%). H NMR (400 MHz, CDCl3): δ ) 10.01 (s, 1H),
7.34 (s, 1H), 2.87 (t, 2H, J ) 7.6 Hz), 2.53 (t, 2H, J ) 7.6 Hz),
1.60 (qt, 2H, J ) 5.4 Hz), 1.56 (qt, 2H, J ) 5.4 Hz), 1.26 (bs,
28H), 0.88 (t, 6H, J ) 5.1 Hz). 13C NMR (100 MHz, CDCl3): δ
) 182.7, 151.6, 144.4, 138.2, 130.3, 31.9, 31.9, 29.8, 29.6, 29.6,
29.5, 29.5, 29.4, 29.3, 29.3, 28.3, 27.0, 22.7, 14.1. HR-MS
calculated for C25H45OS (MH+), 393.3185; found, 393.3177.
2,5-Bis-[(thiophen-2-ylmethylene)-amino]-thiophene-3,4-di-
carboxylic Acid Diethyl Ester (8). 2 (20 mg, 0.077 mmol) and
3,4-didecyl-thiophene-2-carbaldehyde (60 mg, 0.15 mmol) were
refluxed in isopropanol with a catalytic amount of TFA. The mixture
was concentrated after refluxing for 4 h. The crude product was
loaded onto a silica column and eluted with ether/ethyl acetate
(10%/90% v/v) to afford the title compound as a red solid (49 mg,
65%). Mp: 46-48 °C. 1H NMR (400 MHz, acetone-d6): δ ) 8.69
(s, 2H), 7.45 (s, 2H), 4.29 (q, 4H, J ) 7.2 Hz), 2.91 (t, 4H, J ) 7.6
Hz), 2.60 (t, 4H, J ) 7.6 Hz), 1.67 (qt, 4H, J ) 7.6 Hz), 1.62 (qt,
4H, J ) 7.6 Hz), 1.30 (bs, 62H), 0.88 (t, 6H, J ) 7.6 Hz), 0.85 (t,
6H, J ) 7.6 Hz). 13C NMR (100 MHz, acetone-d6): δ ) 164.6,
153.6, 151.1, 150.7, 145.9, 138.2, 128.9, 128.3, 62.6, 33.7, 33.7,
33.3, 31.7, 31.4, 30.1, 28.6, 24.3, 15.4. HR-MS calculated for
C60H99O4N2S3 (MH+), 1007.6761; found, 1007.6733.
Acknowledgment. We acknowledge financial support from
the Natural Sciences and Engineering Research Council Canada,
Canada Foundation for Innovation, Fonds de Recherche sur la
Nature et les Technologies, and the Centre for Self-Assembled
Chemical Structures. Appreciation is extended to Dr. M. Simard
for assistance with the crystal structure analysis and to Prof. D.
Zargarian for helpful suggestions. M.B. thanks the Universite´
de Montre´al for a graduate scholarship.
Supporting Information Available: General experimental
1
procedure, reagents, and materials. H and 13C NMR spectra of
compounds 1-7, absorption, fluorescence, and phosphorescence
spectra of compounds 2-7, and uncorrected cyclic voltammograms
of compounds 3-7. Details of the crystal structure determination
of 7. This material is available free of charge via the Internet at
http://pubs.acs.org.
3,4-Didecyl-thiophene-2-carbaldehyde. To a solution of 3,4-
didecyl-thiophene (188.5 mg, 0.51 mmol) and freshly distilled
JO070100O
J. Org. Chem, Vol. 72, No. 7, 2007 2643