Synthesis and NLO properties of new trans 2-(thiophen-2-yl)vinyl heteroaromatic iodides

Article abstract of DOI:10.1039/c0ob00046a

The synthesis and characterisation of new trans 2-(thiophen-2-yl)vinyl pyridinium, imidazolium and quinoilinium iodides is reported together with their solvatochromic shifts and EFISH characterization. 2-{(E)-2-[5′- (dibutylamino)-2,2′-bithien-5-yl]vinyl}

Full text of DOI:10.1039/c0ob00046a

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PAPER
Synthesis and NLO properties of new trans 2-(thiophen-2-yl)vinyl
heteroaromatic iodides†
Cosimo Gianluca Fortuna,*a Carmela Bonaccorso,a Fadi Qamar,b Anu Anu,b Isabelle Ledouxb and
Giuseppe Musumarraa
Received 26th April 2010, Accepted 22nd November 2010
DOI: 10.1039/c0ob00046a
The synthesis and characterisation of new trans 2-(thiophen-2-yl)vinyl pyridinium, imidazolium and
quinoilinium iodides is reported together with their solvatochromic shifts and EFISH characterization.
2-{(E)-2-[5¢-(dibutylamino)-2,2¢-bithien-5-yl]vinyl}-1-methyl pyridinium and quinolinium iodides
display high m.bvec values up to 1200 ¥ 10-48 esu. The promising non-linear optical (NLO) properties of
this new family of chromophores, which can be further improved by the design of highly efficient
systems exploiting the donor and acceptor properties of both heteroaromatic rings and substituents,
make them suitable candidates for second harmonic generation imaging with interesting biological
applications.
Molecular materials for second-order NLO7 are now well-
Introduction
recognized as promising solutions to develop a new generation
of low cost optoelectronic devices.8 A wide choice of NLO
molecules, designed according to well-established molecular en-
gineering rules, have been proposed and synthesized.9–12 More
recently, NLO molecules have been used as molecular markers
of biologically active molecules, as they may help to image, by
multiphotonic interactions such as 2-photon fluorescence (TPF)13
or second harmonic generation (SHG),14 targeted structures of
cells and membranes. In particular, SHG studies provide specific,
complementary to that offered by TPF, valuable information
about symmetry properties of these structures.15 Combining
therapeutic properties with NLO responses is of particular interest
to localize and characterize the biological sites interacting with
drugs. Directly tracking these drug molecules by NLO techniques
is therefore of great interest for a better understanding of the
biological processes underlying their therapeutic activity.
The aim of this work is to provide evidence of significant
NLO response of 2-(thiophen-2-yl)vinyl heteroaromatic iodides
with potential therapeutic properties. We do not propose here an
entirely novel and comprehensive molecular engineering scheme
to maximize non linear molecular hyperpolarizabilities, but we
report on the evidence of significant quadratic NLO responses
of therapeutic molecules, offering an interesting perspective for
optical tracking of drugs and the related improved knowledge of
therapeutic mechanisms of these structures. From this perspec-
tive, we focused our work on molecular NLO properties only,
optimization at the material level not being relevant in the context
of molecular markers and drugs, where biological affinity and
compatibility are the most critical issues for a proper use of NLO
molecules in this context.
For more than a decade our research group has synthesised and
characterised new trans 1,2-diheteroaryl ethylenes, investigating
their basicity,1 their photochemical behaviour and their interaction
with DNA2,3 in view of potential applications as antitumour
agents. Indeed several trans 2-heteroaryl-vinyl-pyridinium, imi-
dazolium and quinolinium iodides exhibited interesting in vitro
antiproliferative activities4–6 which could be rationalised and
improved by molecular modelling approaches based on physico-
chemical and pharmacokinetic properties.4,5 The biological activ-
ity of the above compounds relies on an optimal balance between
hydrophilic and hydrophobic character in these molecules, due
to the peculiarities of the heteroaromatic rings linked to the
ethylene moiety, a positively charged pyridinium, imidazolium or
quinolinium cation and a five membered heterocycle. However,
the above structures belong to the class of the so called push–pull
(donor–acceptor, D–A) molecules due to the electron donating
and electron withdrawing abilities of the heteroaromatic rings
linked by a transmitting ethylene moiety. Therefore potential
non linear optical applications can be envisaged and the sol-
vatochromic shifts exhibited by new trans 1-indolyl-2-(1-methyl
pyridinium and quinolinium-2-yl) ethylene iodides6 highlighted
these new push–pull heteroaromatics as promising candidates for
NLO applications.
aDipartimento di Scienze Chimiche, Universita` di Catania, Viale A. Doria,
6, 95125, Catania, Italy. E-mail: cg.fortuna@unict.it
bLaboratoire de Photonique Quantique et Moleculaire, Institut d¢Alembert -
Ecole Normale Superieure de Cachan, 1 avenue du President Wilson, 94235,
Cachan, France
The optimization of NLO dipolar dyes is related to the
molecular factor of merit m.bvec. This parameter is the scalar
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0ob00046a
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Scheme 1
Table 1 Solvatochromic shifts and m.bvec values of cations 1–3
product of the molecular dipole moment m and the vector part of
the quadratic hyperpolarizability tensor bvec. The best factors of
merits result from a sophisticated molecular design corresponding
to m.bvec = 35000 ¥ 10–48 esu16 for highly conjugated chromophores
for telecommunications.
Here we report on lower NLO performances, but with a good
potential for significant improvement, and that are compatible
with molecular biological activity
Due to their charged character, zwitterions and cations have
not been widely investigated for quadratic NLO applications.
However, there are some reports evidencing the possibility to
measure their m.bvec values in chloroform17,18 using a technique
called Electric-field induced second harmonic (EFISH) .19 It has
been shown that the application of a DC field on a zwitterion
dissolved in a non-polar solvent does not dissociate the ionic
moieties and result in large m.bvec values.17,18 In the present work
we will revisit these investigations on push–pull chromophores by
using heterocyclic conjugated moieties and various ionic nitrogen-
containing heterocycles as strong electron acceptors.
In this context we herein report the solvatochromic shifts and
EFISH characterization of trans 2-(thiophen-2-yl)vinyl heteroaro-
matic iodides 1–3 (a,b,c), including more efficient systems exploit-
ing the donor and acceptor properties of both heteroaromatic
rings and substituents designated as d (Scheme 1).
lmax CHCl3
Compounds (nm)
lmax H2O (nm) Dlmax (nm) m.bveca(10-48 esu)
1a
2a
3a
1b
2b
3b
1c
2c
3c
1d
3d
400
354
440
420
429
480
450
383
500
625
718
380
342
410
400
359
430
417
360
460
533
604
20
12
40
20
70
50
33
23
40
92
114
180
236
n.d.
213
243
n.d.
307
353
n.d.
1240
1250
a Experimental errors do not exceed 6%.
not be performed on the quinolinium cations 3 due to the low
solubility of these compounds. On the another hand, compound
2d was obtained as a mixture of cis–trans forms. The b value of
the cis-form was not being easily accessible, as it would require
low-yield separation procedures from the trans form. Moreover,
the resulting cis isomer was not expected to be not highly stable at
room temperature.
From Table 1 we can infer some information about the electron
accepting character of the ionic nitrogen heterocycles investigated
here. Comparison of m.bvec values between pyridinium salts 1
on one hand, and imidazolium salts 2 on the other hand,
clearly highlights the better electron accepting character of the
imidazolium cations as compared to the pyridinium cation. This
results in a higher m.bvec value when imidazolium is used as an
electron acceptor. Furthermore, comparison between molecules
1c and 2c on the one hand, and 1b and 2b on the other hand,
shows the higher electron donating character of the bis-thiophene
moiety as compared with the phenyl-thiophene moiety. However,
m.bvec remains modest, as these moieties do not induce a significant
intramolecular charge transfer towards the ionic heterocycles
through the conjugated path.
Results and discussion
The synthesis of trans 2-(thiophen-2-yl)vinyl heteroaromatic io-
dides 1–3 (a,b,c) is straightforward and can be easily achieved by
condensation of 1,2-dimethyl pyridinium iodide, or 1,2-dimethyl
quinolinium iodides with heteroaromatic aldehydes. As expected,
the pyridinium and quinolinium a methyls are quite reactive due
to the strong electron withdrawing effect of the positively charged
nitrogen ring.
Under appropriate conditions, outlined in the experimental
section, pure trans isomers 1–3 (a,b,c) were obtained, as evidenced
by the ethylenic protons J coupling constants in the NMR spectra.
Compounds 1–3 (a,b,c) possessing electron donating moieties
connected by a vinyl linker to a strong electron withdrawing
moiety are expected to exhibit significant solvatochromic shifts.
The solvatochromic shifts of cations 1–3 (a,b,c), reported in
Table 1, encouraged us to perform Electric-field induced second
harmonic generation (EFISH)19 characterization at 1.9 mm on
the above molecules. Unfortunately, EFISH measurements could
The above results point out that the bisthiophene derivatives
designated as c display valuable NLO properties and show that
both solvatochromic shifts and m.bvec values can be modulated
varying the electronic parameters in both the donor and acceptor
heteroaromatic moieties. In fact, as reported in literature, in
general b increases with increasing donor and acceptor strength
and with increasing separation as long as there is strong
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Scheme 2
electronic coupling through the conjugated bridge,7–12 although
some exceptions might occur.20 There is still active interest in
the design of more efficient donors, acceptors and conjugating
units. In this context, we decided to introduce a dialkylamino
substituent in the alpha position of the terminal thiophene ring
increasing the electron donating character of the donor group, with
the aim of improving the solvatochromic shifts and the EFISH
performances.21
value for 1d is four times higher than that of 1c, in agreement with
the trend already indicated by theoretical calculations.
It is well known that observation of second harmonic generation
(SHG) phenomena27 is essential to envisage applications in the
field of non linear optical biomedical imaging. Recent advances
in diode-pumped solid state (DPSS) lasers have made near-
IR lasers extremely compact and affordable, with green laser
pointers (532 nm) the most common product utilizing DPPS
laser (1064 nm) through an SHG process. Both 1d and 3d
exhibit a significant absorbance at 532 nm, making them suitable
for SHG excited DPPS lasers. This preliminary investigation of
a new family of chromophores for NLO can be extended by
designing structures bearing increased conjugation lengths and
more efficient donor and acceptor substituents and undergo a
more complete characterization of their tensor properties by using
Harmonic Light Scattering28,29 experiments to infer the octupolar
component of their b tensor.
In order to support this hypothesis b0 for compounds 1c and 1d
were calculated at the Hartree–Fock level of theory (See ESI† for
details). When adding the N,N-dimethylamino substituent into
the 5¢ position of the thiophene moiety the b0 (au) value is more
than three times higher, increasing from 15768 to 49772 au.
The (n-butyl)amino derivatives 1d and 3d were obtained accord-
ing to the synthetic strategy outlined in Scheme 2, similar to that
adopted for the synthesis of other dialkylamino aldehydes.22,23
Direct amidation of the acid 4 with the amine was obtained
through a DCC-BtOH catalysed reaction (i);22 subsequent treat-
ment of the amide (5) with Lawesson’s reagent (ii) afforded the
corresponding aminobithiophene 6. The synthesis of the formyl
derivative 7 was achieved by metallation, using n-BuLi, followed
by quenching with DMF23 (iii). Actually the synthesis of the
aldehyde 7 in 6.5% yield adopting a different synthetic pathway has
recently been reported.24 However, the present synthetic procedure
involving formylation of N,N-dibutyl-2,2¢-bithiophen-5-amine 6
provides a significantly better yield. Compounds 1d and 3d were
obtained by condensation of the heteroaromatic aldehyde 7 with
the corresponding iodide salt in the presence of piperidine as the
base (iv). Condensation of 7 with imidazolium iodide provided a
mixture of E and Z isomers 2d in low yields.
Conclusion
We have synthesized and characterized trans 2-(thiophen-2-
yl)vinyl heteroaromatic derivatives and evidenced their interest
for quadratic nonlinear optics. Choosing organic cationic species
does not prevent an accurate and reliable determination of their
molecular factor of merit m.bvec. The straightforward synthesis of
the above compact heterocyclic structures, their strong non linear
optical properties and their absorption peaks around 532 nm,
make them promising probes for second harmonic generation
imaging with interesting biological applications.
As expected, N,N-di-n-butylamino bisthiophene derivatives 1d
and 3d exhibit significantly higher solvatochromic shifts with
respect to the corresponding unsubstituted compounds 1c and
3c. This is paralleled by high m.bvec values up to 1200 ¥ 10-48 esu
for compounds 1d and 3d confirming the necessity to introduce
strong dialkylamino groups to induce a high quadratic molecular
NLO response. These values are in agreement with those reported
in the literature for similar structures.25,26 The experimental m.bvec
Experimental section
The values of m.bvec for the chromophores were measured using
the electric-field induced second harmonic generation (EFISH)
technique. A Nd3+:YAG laser at 1.06 mm pumps a hydrogen Raman
cell so as to obtain a larger wavelength (1.907 mm) for which both
the fundamental and harmonic frequencies are far away from the
resonance of the investigated molecule. A Schott RG 1000 filter
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was used to filter out any remaining visible light from the laser
flash lamp. Suitable neutral density filters were used to control the
power of the incident beam and a half wave plate and polariser
were used to set the incident polarisation along the direction of the
applied electric field. In addition, a band pass filter was mounted
on the front of the detection PMT along with a filter to remove
any remaining radiation at the fundamental wavelength. A high
voltage was applied across the EFISH cell containing the solution.
The EFISH cell consisted of a stainless steel container with two
quartz optical windows, which were glued to form a wedge shaped
cavity within the cell. The electrode was connected to the high
voltage supply via an isolated stainless steel rod situated above the
wedge shaped cavity. The interelectrode distance was 2 mm, giving
a static electric field of around 40 kVcm. The cell was mounted on
an electrically isolated translation stage. The whole cell was then
translated horizontally relative to the incident beam to produce
fringes at the second harmonic wavelength. Every measurement
was referenced separately to the fringes of the pure solvent that
was used to dissolve the chromophore to take into account
fluctuations in laser power. An in-house computer program was
used to calculate the interfringe distance together with the fringe
amplitude. These data were then used to calculate the m.bvec of
the chromophore, taking the pure solvent (chloroform) as the
reference solution.
Electron Spray Ionization mass spectra were recorded on a
Thermo Finnigan LCQ Deca mass spectrometer equipped with
an ESI source.
General procedure for the synthesis of compounds 1–3 (a,b,c)
Heteroaromatic carboxaldehydes were Aldrich commercial prod-
ucts.
2-Heteroaryl thiophene derivatives, all iodide salts, were ob-
tained by refluxing equimolar amounts of 1,2,3-trimethylimida-
zolium iodide in ethanol, or 1,2-dimethylpicolinium iodide or 1,2-
dimethylquinolinium iodide and the appropriate heteroaromatic
aldehyde in the presence of few drops 20% NaOH or piperidine.
The resulting precipitate was recrystallized from ethanol.
Details on the synthetic conditions and products characteriza-
tion have been reported.31
Synthesis of compounds 5–7; 1d and 3d
In principle, EFISH is precluded in the case of ionic species,
due to ion migration towards poling electrodes and a related
cancellation of the poling field. Here, we have adapted the EFISH
technique to m.bvec measurements of ionic chromophores by i)
using a short (1 ms) DC poling field, provided that ion mobility in
chloroform is not high enough to permit ion migration towards
electrodes within the short time duration of the DC pulse, ii)
operating in a solvent of low polarity stabilizing undissociated
ion pairs. Consequently, EFISH experiments were carried out in
chloroform (er = 4.7) and at 1.907 nm (provided by a hydrogen
Raman cell pumped by a Nd3+:YAG laser in order to avoid
absorption of the second harmonic. Reproducible b values (with a
variation of 6%) were obtained. Concentrations did not exceed
1 ¥ 10-3 mol L-1, and no significant influence of the chromophore
concentration on b values could be noticed in the 1 ¥ 10-4–1 ¥ 10-3
mol L-1 concentration range.
All calculations in this work were carried out using the
GAUSSIAN 03 package. The geometrical structures for the model
compounds were obtained at the density functional theory (DFT)
B3LYP/6-311+G(d,p) level; after the minimization process, the
vibrational spectra have been evaluated to check that no imaginary
frequencies are present. The static first hyperpolarizabilities (b0)
were evaluated by the coupled perturbed Hartree Fock (CPHF)
approach at the Hartree–Fock level with 6-311+G(d,p) basis set
for all the atoms. Solvation effects were taken into account using
the Polarizable Continuum Model.30
N,N-dibutyl-4-oxo-4-(2-thienyl)butanamide
(5). 1,3-Dicy-
clohexylcarbodiimide (DCC) (3350 mg, 16.3 mmol) and 1-
hydroxybenzotriazole (BtOH) (2.50 g, 16.3 mmol) were added
to a stirred solution of the acid (4) (2500 mg, 13.6 mmol) in
dichloromethane (100 mL) at 0 ◦C. The mixture was stirred for
30 min at rt after which the amine (2.29 mL, 13.6 mmol) was added
and the mixture was stirred overnight. The by-products formed
were separated by filtration to give a pale brown solution. This
organic solution was extracted with a solution of citric acid 5%
(4 ¥ 100 mL), a solution of sodium bicarbonate (5%) (2 ¥ 100 mL),
dried over MgSO4 and evaporated to give an oily brown residue.
The product was purified by chromatography on silica gel (eluent:
hexane\ethylacetate) and gave pure (5) as an orange oil (2010 mg,
6,78 mmol, 51%)
1H-NMR (500 MHz, CDCl3) d = 7.81 (dd, 3J(H,H)1 = 3.8 Hz,
3
3J(H,H)2 = 1.1 Hz, 1H; 4-ArH), 7.62 (dd, J(H,H)1 = 5.0, Hz,
3
3J(H,H)2 = 1.1 Hz, 1H; 2-ArH), 7.13 (dd, J(H,H)1 = 5.0 Hz,
3J(H,H)2 = 3.8 Hz, 1H; 3-ArH), 3.32 (m, 3J(H,H) = 6.9 Hz,
1H NMR spectra were recorded at 27 ◦C using a Varian Inova
500 spectrometer. Chemical shifts (d) are expressed in ppm and
referenced to residual undeuterated solvent. NMR data were
processed using MestReC software (http://www.mestrec.com).
Two-dimensional (2D) NMR experiments (gCOSY; gHSQCAD;
gHMBCAD) were carried out on all new compounds using the
pulse sequences from the Varian user library. On the basis of 2D-
NMR analyses, complete assignments of 1H and 13C signals were
obtained (see ESI†).
4H; NCH2), 3.30 (t, J(H,H) = 6.7 Hz, 2H; ArCOCH2), 2.77 (t,
3
3J(H,H) = 6.7 Hz, 2H; NCOCH2), 1.63–1.47 (m, 4H; NCH2CH2),
1.39–1.25 (m, 4H; NCH2CH2CH2), 0.98–0.89 (t, 3J(H,H) = 6.9 Hz,
6H; CH3). 13C-NMR (125 MHz, CDCl3) d = 192.29 (ArCO),
170.88 (CON), 144.08 (5 CAr), 133.34 (2 CAr), 132.01 (4 CAr),
128.07 (3 CAr), 47.71 (13 CH2), 45.98 (17 CH2), 34.48 (8 CH2),
31.06 (14 CH2), 29.90 (18 CH2), 27.17 (9 CH2), 20.26 (15 CH2),
20.16 (19 CH2), 13.86 (16 CH3), 13.83 (20 CH2). ESI-MS: m/z
(%): 612.8 (95) [2M+Na+], 296.2 (100) [M+H+].
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into water (10 mL) and extracted with (3 ¥ 25 mL) of ethyl acetate.
The whole organic phase was washed with water (100 mL), dried
over Na2SO4, filtered and the solvent was removed under reduced
pressure to give the crude 5-formyl-bithiophene (7) as a dark oil.
Chromatography on silica gel (eluent: exane\dichloromethane)
gave pure (7) as a red solid (183.7 mg, 5.7 ¥ 10-1 mmol, 57%).
The 1H- and 13C-NMR spectra (500 MHz, CDCl3) and the mass
spectrum were coincident with those reported in the literature24
2-{(E)-2-[5¢-(dibutylamino)-2,2¢-bithien-5-yl]vinyl}-1-methyl-
pyridinium iodide (1d). 1,2-Dimethyl-pyridinium iodide (47 mg,
0.2 mmol) was added to a solution of aldehyde 7 (67 mg, 0.2 mmol)
in 2 mL ethanol and in the presence of few drops of piperidine;
the mixture was stirred for 6 h at 50 ◦C. The solvent was removed
under reduced pressure to give (1d) as a dark oil. Chromatography
purification on silica gel (eluent: dichloromethane\methanol) gave
pure (1d) as dark solid (46.06 mg, 1.12 ¥ 10-1 mmol, 56%).
N,N-dibutyl-2,2¢-bithiophen-5-amine (6). A mixture of Lawes-
son’s reagent (1250 mg, 3.1 mmol) and the amide (5) (890 mg,
3 mmol) was heated in toluene (15 mL) for 60 min. The mixture
was cooled and the solvent was evaporated under reduced pressure
to give the crude bithiophene (6) as an oil. Chromatography
purification on silica gel (eluent: hexane\dichloromethane) gave
pure (6) as yellow oil (757,2 mg, 2.57 mmol, 83%).
1H-NMR (500 MHz, CDCl3) d = 7.05 (dd, 3J(H,H)1 = 3.8 Hz,
3
1H-NMR (500 MHz, CDCl3) d = 9.11 (d, J(H,H) = 6.3 Hz,
3
3J(H,H)2 = 1.1 Hz, 1H; 13-ArH), 6.95 (dt, J(H,H)1 = 3.8 Hz,
3
1H; 26-ArH), 8.26 (d, J(H,H) = 8.0 Hz, 1H; 23-ArH), 8.19 (t,
3
3J(H,H)2 = 1.1 Hz, 1H; 14-ArH), 6.93 (dd, J(H,H)1 = 3.8 Hz,
3J(H,H) = 7.9 Hz, 1H; 24-ArH), 7.94 (d, 3J(H,H) = 15.2 Hz, 1H;
21-ArH), 7.57 (dt, 3J(H,H)1 = 6.4 Hz, 3J(H,H)2 = 1.2 Hz, 1H; 25-
ArH), 7.42 (d, 3J(H,H) = 3.9 Hz, 1H; 14-ArH), 7.09 (d, 3J(H,H) =
4.2 Hz, 1H; 4-ArH), 6.85 (d, 3J(H,H) = 3.9 Hz, 1H; 15-ArH), 6.65
3J(H,H)2 = 1.1 Hz, 1H; 15-ArH), 6.86 (d, 3J(H,H) = 3.8 Hz, 1H;
3
4-ArH), 5.77 (bs, 1H; 3-ArH)), 3.21 (t, J(H,H) = 6.9 Hz, 4H;
NCH2), 1.61 (m, 4H; NCH2CH2), 1.36 (m, 4H; NCH2CH2CH2),
0.95 (t, 3J(H,H) = 7.2 Hz, 4H; CH). 13C-NMR (125 MHz, CDCl3)
d = 156.93 (2 CAr), 139.01 (11 CAr), 127.47 (14 CAr), 123.69 (4
CAr), 121.68 (13 CAr), 120.62 (15 CAr), 120.13 (5 CAr), 101.78
(3 CAr), 53.55 (16–7 CH2), 29.17 (17–8 CH2), 20.27 (18–9 CH2),
13.90 (19–10 CH3). ESI-MS: m/z (%): 585.3 (95) [2M+H]+, 294.3
(100) [M+H] +.
3
3
(d, J(H,H) = 15.2 Hz, 1H; 20-ArH), 5.79 (d, J(H,H) = 4.2 Hz,
1H; 3-ArH), 4.45 (s, 3H; 28-CH3), 3.29 (t, 3J(H,H) = 7.6 Hz, 4H;
NCH2), 1.65 (m, 4H; NCH2CH2), 1.38 (m, 4H; NCH2CH2CH2),
0.99 (t, 3J(H,H) = 7.6 Hz, 4H; CH3). 13C-NMR (125 MHz, CDCl3)
d = 159.83 (2 CAr), 152.56 (22 CAr), 146.94 (11 CAr), 145.66 (26 CAr),
142.93 (24 CAr), 137.45 (21 CAr), 136.67 (14 CAr), 134.28 (13 CAr),
127.88 (4 CAr), 123.78 (23 CAr), 122.81 (25 CAr), 120.85 (15 CAr),
117.63 (5 CAr), 109.52 (20 CAr), 101.91 (3 CAr), 53.47 (7,16 CH2),
46.63 (28 NCH3), 29.25 (8,17 CH2), 20.08 (9,18 CH2), 13.82 (9,18
CH3). ESI-MS: m/z (%): 411.3 (100) [M+].
2-{(E)-2-[5¢-(dibutylamino)-2,2¢-bithien-5-yl]vinyl}-1-methyl-
quinolinium iodide (3d). 1,2-Dimethyl-quinolinium iodide
(55 mg, 0.2 mmol) was added to a solution of aldehyde 7 (67 mg,
0.2 mmol) in 2 mL ethanol and in the presence of few drops
of piperidine; the mixture was stirred for 6 h at 50 ◦C. The
solvent was removed under reduced pressure to give (3d) as
a dark oil. Chromatography purification on silica gel (eluent:
dichloromethane\methanol) gave pure (3d) as a dark solid
(37.82 mg, 8.2 ¥ 10-2 mmol, 41%).
5¢-(dibutylamino)-2,2¢-bithiophene-5-carbaldehyde (7). This
compound, recently reported,24 was synthesized adopting a
different procedure providing a better yield with respect to the
literature one (6.5%). A 2.5 M solution of BuLi in hexane (0.8
mL, 2.0 mmol) was dropped at 10 ◦C to a stirred solution of
bithiophene (6) (0.30 g, 1.0 mmol) in anhydrous ether (2 mL).
After 30 min DMF (0.13 mL, 1.5 mmol) was slowly added, and the
mixture was allowed to react for 50 min. The mixture was poured
3
1H-NMR (500 MHz, CDCl3) d = 8.64 (d, J(H,H) = 9.3 Hz,
1H; 24-ArH), 8.59 (d, 3J(H,H) = 14.9 Hz, 1H; 21-ArH), 8.36 (d,
3
3J(H,H) = 9.3 Hz, 1H; 23-ArH), 8.05 (d, J(H,H) = 8.8 Hz, 1H;
3
3
27-ArH), 7.83 (dt, J(H,H)1 = 7.9 Hz, J(H,H)2 = 1.3 Hz, 1H;
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29-ArH), 7.78 (dd, 3J(H,H)1 = 7.9 Hz, 3J(H,H)2 = 1.1 Hz, 1H; 30-
ArH), 7.71 (d, 3J(H,H) = 4.2 Hz, 1H; 14-ArH), 7.51 (t, 3J(H,H) =
7.7 Hz, 1H; 28-ArH), 7.06 (d, 3J(H,H) = 4.2 Hz, 1H; 4-ArH), 6.88
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3
3
(d, J(H,H) = 15.2 Hz, 1H; 20-ArH), 6.57 (d, J(H,H) = 4.2 Hz,
3
1H; 15-ArH), 5.79 (d, J(H,H) = 4.2 Hz, 1H; 3-ArH), 4.49 (s,
3
3H; 32-CH3), 3.31 (t, J(H,H) = 7.7 Hz, 4H; NCH2), 1.68 (m,
4H; NCH2CH2), 1.42 (m, 4H; NCH2CH2CH2), 1.02 (t, 3J(H,H) =
7.3 Hz, 6H; CH3). 13C-NMR (125 MHz, CDCl3) d = 160.91 (2
CAr), 155.22 (22 CAr), 149.20 (11 CAr), 141.77 (24 CAr), 141.20 (23
CAr), 139.74 (14 CAr), 139.31 (25 CAr), 135.32 (13 CAr), 133.91 (29
CAr), 129.71 (30 CAr), 129.38 (4 CAr), 127.43 (26 CAr), 126.73 (28
CAr), 121.39 (21 CAr), 121.34 (15 CAr), 117.86 (27 CAr), 117.76 (5
CAr), 110.87 (20 CAr), 102.72 (3 CAr), 53.54 (7,16 CH2), 40.41 (32
NCH3), 29.23 (8,17 CH2), 20.24 (9,18 CH2), 13.92 (9,18 CH3).
ESI-MS: m/z (%): 461.3 (100) [M+].
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