NMR studies of the reaction between amino-phenylene vinylene thiophene and tetracyanoethylene

Article abstract of DOI:10.1016/j.tetlet.2008.04.020

Amino tricyanovinyl thiophene chromophores (A-TCVT) are prepared by a substitution reaction of amino-phenylene vinylene thiophene (A-PVT) with tetracyanoethylene (TCNE). This reaction does not occur directly. The vinylene double bond in the PVT unit first reacts rapidly with TCNE to form a [2+2] cycloaddition product. It is then reverted to PVT unit prior to the subsequent substitution at 50 °C. This reversible cycloaddition converts the cis-isomer of PVT units into the trans-counterparts, thus the final TCNE substituted products can be expected for a better performance as non-linear optical materials. Crown Copyright

Full text of DOI:10.1016/j.tetlet.2008.04.020

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3549–3553
NMR studies of the reaction between amino-phenylene
vinylene thiophene and tetracyanoethylene q
a,
a
b
*
Jianfu Ding , Gilles P. Robertson , Jianping Lu
a Institute for Chemical Process and Environmental Technology (ICPET), National Research Council of Canada (NRC), 1200
Montreal Road, Ottawa, ON, Canada K1A 0R6
b Institute for Microstructural Sciences (IMS), National Research Council of Canada (NRC), 1200 Montreal Road, Ottawa, ON, Canada K1A 0R6
Received 26 February 2008; revised 21 March 2008; accepted 2 April 2008
Available online 6 April 2008
Abstract
Amino tricyanovinyl thiophene chromophores (A-TCVT) are prepared by a substitution reaction of amino-phenylene vinylene thio-
phene (A-PVT) with tetracyanoethylene (TCNE). This reaction does not occur directly. The vinylene double bond in the PVT unit first
reacts rapidly with TCNE to form a [2+2] cycloaddition product. It is then reverted to PVT unit prior to the subsequent substitution at
50 °C. This reversible cycloaddition converts the cis-isomer of PVT units into the trans-counterparts, thus the final TCNE substituted
products can be expected for a better performance as non-linear optical materials.
Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
Amino tricyanovinyl compounds with a phenylene
vinylene thiophene fully conjugated bridge (A-TCVT) are
an important class of non-linear optical chromophores
due to their high performance and excellent stability.1–3
They are prepared by reacting amino-phenylene vinylene
thiophene (A-PVT) with tetracyanoethylene (TCNE). This
reaction could be performed through a lithiation reaction,1
or by a nucleophilic substitution reaction.2 The latter was
intensively used for the preparation of A-TCVT containing
polymers,3 where the lithiation reaction often results in
damage to polymer structures due to the harsh reaction
conditions.
Recently we have successfully incorporated A-PVT units
into fluorinated polyethersulfone by using a mild polycon-
densation reaction.4 The structure of the obtained polymer,
P(A-PVT)SO is illustrated in Scheme 1. A post polymeriza-
tion substitution of this polymer with TCNE was tried in
DMF at <70 °C to convert it into a non-linear optical poly-
mer, P(A-TCVT)SO. However, a few initial trials of this
substitution reaction with the use of a slight excess of
TCNE (molar ratio of [TCNE]/[A-PVT] = 1.1–1.2) follow-
ing the reported procedure did not result in a quantitative
conversion of the PVT group.3 It was found that the
reaction could be easily completed at a lower reaction tem-
perature (50 °C) when higher molar ratios (e.g., [TCNE]/
[A-PVT] ꢀ 4.0) were used. Therefore, this reaction was
carefully studied by 1H NMR spectroscopy using 4.0 equiv
TCNE in DMF-d7, with the result shown in Figure 1. In
this experiment, the 1H NMR spectrum of the starting
polymer, P(A-PVT)SO, was collected first (0 min). TCNE
1
was then added to the solution and the evolution of H
NMR spectra with reaction time was recorded at 23 °C.
A clean spectrum was obtained in 240 min. No further
changes were observed after this point, indicating that the
reaction was complete at this time. However, a study of
the 240 min spectrum revealed that the product was not
consistent with the desired structure as shown in Scheme
1. An unexpected pair of doublets appeared outside of
the aromatic region at d 5.53 and 5.98 ppm, indicating that
the fully conjugated structure of the PVT unit had changed.
The structure of the product, which was prepared from a
scaled-up reaction under the same condition,5 was there-
fore carefully studied by 1D and 2D NMR techniques in
acetone-d6. The pair of doublets now appeared at d 5.33
q
NRCC Publication Number: 49129.
Corresponding author. Tel.: +1 613 993 4456; fax: +1 613 991 2384.
*
E-mail address: jianfu.ding@nrc-cnrc.gc.ca (J. Ding).
0040-4039/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.020

3550
J. Ding et al. / Tetrahedron Letters 49 (2008) 3549–3553
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
O
S
O
S
CF
CF
3
3
O
N
O
O
O
y
x
O
O
polymer
R
reaction
before
P(A-PVT)SO
H
S
CN
after
P(A-TCVT)SO
NC
R
CN
Scheme 1. The structures of the polymers before and after reacting with TCNE. Reaction condition: 4.0 equiv TCNE in DMF at 50 °C for 24 h. x/y = 2/1.
tons was 3-bond spin coupled with a large coupling
constant (3JH–H = 12.9 Hz). Their chemical shift displace-
ments in the olefinic region suggest that they might be on a
double bond but different from the one in the highly conju-
n",o"
6"
DMF
g"
f"
5"
8"
B
2"
3"
9"
20 h
1
gated PVT group. On the other hand, the one bond H–
13C correlation spectrum (2D-HSQC) indicated that these
two hydrogen atoms were bound to two carbon atoms at
low frequencies (d 54.0 and 48.0 ppm), indicating the pres-
ence of two saturated tertiary carbon atoms rather than
olefinic ones. The 2D-HMBC spectrum (Fig. 2) displays a
60 min
large number of aromatic and aliphatic carbon atoms
20 min
3
showing long-range couplings (2JC–C–H and JC–C–C–H
)
1
with these 2 protons. Totally seven H–13C correlations
per proton were found, including five in the aromatic
region and two in the aliphatic region. Only the structure
in the [2+2] cycloaddition product, P(A-TCNE)SO as
shown in Scheme 2 can account for those specific aromatic
and aliphatic 2- and 3-bond 1H– 13C couplings. These
results clearly indicate that the PVT’s vinylene moiety
had undergone a [2+2] cycloaddition reaction with TCNE.
0 min
6.5
8.0
DMF
7.5
7.0
6.0
5.5
5'
n' o'
9'
g'
A
f'
3' 8'
2'
10'
240 min
22 min
6 min
H6'
H5'
40
C11',12'
C5'
C6'
N
1'
50
2'
3'
60
70
4'
5'
CN
11'
CN
CN
CN
80
6'
7'
12'
10,n,o
90
8'
9'
g
f
S
6
2,5
100
3
8
9
10'
0 min
CN
110
120
8.0
7.5
7.0
6.5
6.0
5.5
C4',7'
C3',8'
130
chemical shift (ppm)
140
Fig. 1. 1H NMR study of the reaction between P(A-PVT)SO and TCNE
(4.0 equiv) in DMF-d7 (A) at 23 °C and then (B) at 50 °C (see Scheme 2 for
peak assignment).
150
7.5
7.0
6.5
6.0
5.5
1H chemical shift (ppm)
and 5.74 ppm in the 1H– C heteronuclear correlation (2D-
13
HMBC) NMR spectra due to the use of a different solvent.
1H homonuclear decoupling showed that this pair of pro-
Fig. 2. 1H–13C HMBC spectrum (acetone-d6) of P(A-TCNE)SO showing
the multiple carbon correlations originated from the cyclobutane protons.

J. Ding et al. / Tetrahedron Letters 49 (2008) 3549–3553
3551
F
F
F
F
F
F
F
F
F
F
j
F
F
k
r
f
g
o
b
n
O
S
c
O
S
CF3
q
m
a
e
d
h
p
l
i
O
N
1
O
O
O
CF3
O
O
y
x
2
F
F
F
F
3
4
x/y = 1/2
5
6
TCNE/DMF
23 oC
7
8
P(A-PVT)SO
S
9
10
F
F
F
F
F
F
F
F
F
F
F
r'
f'
g'
h'
j'
o'
n'
b'
O
S
c'
d'
O
S
k'
CF3
a'
p'
e'
l'
i'
m'
q'
O
N
O
O
O
CF3
O
1'
4'
O
y
x
F
F
F
F
F
2'
3'
CN
CN
5'
11'
12' CN
CN
6'
TCNE/DMF
50 oC
P(A-TCNE)SO
7'
8'
9'
S
10'
F
F
F
F
F
F
F
F
F
F
F
F
r"
f"
g"
h"
j"
o"
n'
b"
O
S
c"
d" e"
O
S
k"
CF3
q"
m"
a"
p"
i"
l"
O
N
O
x
O
O
CF3
O
O
y
1"
4"
F
F
F
F
2"
3"
5"
6"
7"
8"
9"
S
P(A-TCVT)SO
10"
CN
12"
11"
NC
CN
Scheme 2. Reaction scheme of P(A-PVT)SO with TCNE in DMF at 23 °C and then at 50 °C.
The reaction solution for the NMR test at 23 °C
depicted in Figure 1A was then heated to 50 °C and further
monitored by NMR. Figure 1B shows that most signals
shifted to higher frequencies, indicating that a new reaction
was occurring with a strong electron withdrawing group
attaching to the aromatic structure. At the same time, the
pair of doublets at 5.53 and 5.98 ppm from the cyclobutane
unit gradually decreased in intensity, indicating that this
unit had dissociated. A clean 1H NMR spectrum was
obtained in 20 h. It matches perfectly the expected substitu-
tion product, P(A-TCVT)SO. Therefore it can be con-
cluded that the nucleophilic substitution of the A-PVT
group with TCNE does not directly occur as a one-step
reaction. The double bond of the A-PVT group reacted
first with TCNE at room temperature to form a [2+2]
cycloaddition product, which further converted to the sub-
stitution product at 50 °C.6
with TCNE. These two reactions might occur simulta-
neously or one after another. The double bond in the
PVT unit will allow the rich amine electrons to delocalize
all the way to the thiophene ring to promote the nucleo-
philic substitution. Therefore, the PVT units should have
higher reactivity for this reaction than the cyclobutane
derivatives, and it is more likely that the substitution
occurred after the cyclobutane unit was reverted to the
PVT unit. A UV spectroscopy study of the purified [2+2]
cycloaddition polymer showed that it was reverted to the
original PVT polymer at 22 °C in two days. It is believed
that this cycloreversion should be faster in the reaction at
50 °C. However, the 1H NMR spectra in Figure 1B showed
no signals associated with the cycloreversion product, PVT
polymer, formed during the reaction. It may indicate that
the recovered PVT unit can react with TCNE immediately
to form the substitution product. This assumption was
1
The process at 50 °C for the formation of the substitu-
tion product actually involved two reactions, that is, the
dissociation of the cyclobutane unit and the substitution
checked using H NMR to monitor the behaviors of the
purified cycloaddition product in dried and in untreated
DMF-d7 at 50 °C. The results showed a nearly quantitative

3552
J. Ding et al. / Tetrahedron Letters 49 (2008) 3549–3553
conversion to the final substitution product in the dried sol-
vent in 24 h. However, about 90% of cycloaddition product
was reverted to PVT polymer in the wet solvent, with the
formation of a new peak at ꢀ6.23 ppm, attributing to
HCN,7 a hydrolysis product of TCNE. This result indicates
that the cycloreversion occurred first to recover PVT units
and TCNE, then the recovered TCNE reacted with the
formed PVT unit for substitution in dry solvent, or TCNE
was scavenged by the trace of water in the wet solvent due
to hydrolysis,8 so that its substitution on the PVT unit was
prevented. This result confirms that the cycloreversion
occurred prior to the substitution.
The effect of the [2+2] cycloaddition on the subsequent
substitution was further examined using a model com-
pound (PVT-BPE) as shown in Scheme 3. In this com-
pound, two ethylene linkages instead of two phenylene
moieties were attached to the amino group in order to sim-
plify the spectrum in the aromatic region. Figure 3 illus-
trates the changes in the NMR spectra during the
reaction. As soon as TCNE (4.0 equiv) was added into
the PVT-BPE solution in DMF-d7 at 23 °C, a clean, stable
but completely different spectrum was observed in 2 min
with two new doublets at d 5.50 and 6.07 ppm, which is
attributed to the formation of the cyclobutane unit by
the [2+2] cycloaddition reaction of PVT-BPE with TCNE
as discussed above, indicating that the cycloaddition of this
model compound is very fast. A [2+2] cycloaddition reac-
tion can be initiated photochemically or thermally. The
combination of an electron-rich alkene and an electron-
deficient one (nitro- or polycyano-alkene) as the reactants
will facilitate the thermal process.9 It might explain that
the cycloaddition of A-PVT with TCNE could be con-
ducted at such unusually low temperature in dark.
5",8"
6"
e"
f"
3"
2"
9"
24 h
16 h
2.5 h
e'
e
f'
2'
3',10'
8'
6'
5'
9'
6
2 min
3
f
2,5
7
9
10
8
PVT-BPE
8
6
5
4
chemical shift (ppm)
Fig. 3. 1H NMR spectra of PVT-BPE and its reaction solution in DMF-d7
in the presence of TCNE (4.0 equiv) at 23 °C for 2 min, and then at 50 °C
for 2.5, 16, and 24 h (see Scheme 3 for peak assignment).
good agreement with the formation of a highly conjugated
double bond, further confirming that the cyclobutane unit
was dissociated. On the other hand, the spectrum of the
cycloaddition product (2 min) showed three thiophene pro-
ton peaks (two d-lets and one dd-let), which were easily
identified due to their smaller coupling constants. During
the reaction, the dd-let gradually disappeared, while the
two d-lets shifted to much higher frequencies, indicating a
strong electron-withdrawing group attached to C-10 of
the PVT unit. The IR spectra of the TCNE substituted
products displayed two new peaks at 1588 and 1422 cmꢁ1
corresponding to the formation of disubstituted thiophene
ring.10
Figure 3 also reveals a significant impact of the revers-
ible [2+2] cycloaddition on the steric conformation of the
PVT vinylene moiety of the final product. Several weak
peaks can be seen in the aromatic region of the starting
molecule, PVT-BPE. These peaks cannot be removed by
a careful purification, meaning they are not attributed to
The reaction temperature was then raised to 50 °C and
NMR spectra recorded at 2.5, 16, and 24 h are displayed
in Figure 3. The reaction resulted in a gradual change in
1
the H NMR spectra within 24 h. During this period, the
two doublets at d 5.44 and 6.00 ppm from the cyclobutane
unit decreased in intensity while accompanied by the for-
mation of two new coupled doublets at d 7.60 and
7.70 ppm with a much larger coupling constant. This is in
Ar
Ar
Ar
Ar
Ar
Ar
O
O
O
O
f"
O
O
f'
f
e"
e'
e
N
N
N
1"
1'
1
2"
3"
TCNE
50 oC
2'
3'
TCNE
23 oC
2
3
CN
CN
4"
4'
4
5"
8"
6"
7"
5'
5
11'
6
7
CN
6'
12'
7'
CN
S
F
F
8
9
8'
9'
c b
S
9"
S
10"
d
a
F
Ar =
10
CN
12"
CN
10'
NC
11"
PVT-BPE
F
F
TCNE-BPE
TCVT-BPE
Scheme 3. Reaction scheme of PVT-BPE with TCNE in DMF-d7 at 23 °C for 1 h and then at 50 °C for 24 h.

J. Ding et al. / Tetrahedron Letters 49 (2008) 3549–3553
3553
M.; Jiang, J.; Callender, C. L.; Stupak, J. Macromolecules 2007, 40,
3145; (c) Ding, J.; Day, M. Macromolecules 2006, 39, 6054; (d) Ding,
J.; Qi, Y.; Day, M.; Jiang, J.; Callender, C. L. Macromol. Chem. Phys.
2005, 206, 2396.
impurities. NMR analyses indicated that they were related
to cis-isomer of PVT-BPE. The PVT unit was synthesized
from the Wittig reaction. Accompanying the predominant
trans-isomer, a small amount of the cis-isomer was also
formed.11 The content of the cis-isomer was estimated to
5. Synthesis of P(A-TCNE)SO: P(A-PVT)SO (0.109 g, 0.1 mmol of
PVT unit) and TCNE (0.051 g, 0.4 mmol) were dissolved in 0.5 mL of
anhydrous DMF at 0 °C. The solution was stirred at 0 °C for 3 h and
then dropped into 10 mL of methanol to precipitate the polymer at 0
°C; the pale green powder was collected using centrifuge and washed
with cold methanol 3 times and dried at room temperature under
vacuum for 12 h (0.095 g, 78% yield). 1H NMR (400 MHz, acetone-
1
be ꢀ8% from the H NMR analysis. If this cis-structure
remained in the final TCNE substituted product, it would
act as a bending linkage between the electron-donating and
electron-accepting units, resulting in lower electro-optic
efficiency than the linear trans-counterpart. The reversible
process of the [2+2] cycloaddition has favorably converted
all the cis-isomer into the trans-isomer, showing a similar
effect as the iodine treatment.12 This effect is evidenced by
the absence of peaks from the cis-isomer in the NMR spec-
trum of the final product (24 h) in Figure 3.
d6):
d (ppm) 7.710–7.620 m, 7.46 (d, J = 8.2 Hz), 7.330 (d,
J = 8.58 Hz), 7.220–7.090 (m), 7.040 (d, J = 8.58 Hz), 7.010–6.930
(m), 5.737 (d, J = 12.8 Hz), 5.337 (d, J = 12.8 Hz); 13C NMR
(100 MHz, acetone-d6): d (ppm) 157.7, 154.0, 153.4, 150.5, 148.1,
147.9, 145.5, 145.3, 145.1, 144.5, 144.0, 143.9, 141.4, 134.5 132.9,
132.5, 130.4, 130.2, 129.9, 129.8, 128.9, 128.5, 128.3, 127.8, 126.9,
126.7, 125.1, 124.8, 123.7, 122.2, 121.4, 118.4, 118.3, 116.9, 111.9,
111.6, 110.62, 110.57, 53.9, 47.8, 43.9, 43.1. 19F NMR (376 MHz,
acetone-d6): d (ppm) ꢁ63.80 (6F, m), ꢁ137.44 (4F, m), ꢁ137.65 (8F,
m), ꢁ151.97 (4F, m), 152.46 (8F, m). IR (cmꢁ1): 2976, 2872, (CH
stretching), 1637, 1604, 1501, 1491 (sh) (phenyl ring), 1389, 1319,
1295, 1260, 1197, 1177, 1097, 1065, 997, 829, 599. Mn: 19,000 Da.
Mw/Mn: 1.8.
It is worth to note that a new singlet appeared at d
6.23 ppm in all of the spectra recorded from reaction solu-
tions in DMF-d7 except in carefully dried solvent. Its inten-
sity increased gradually with reaction time. Some additional
features of this peak are following: (1) It appeared immedi-
ately after the addition of TCNE and was accompanied
with the shift or disappearance of the water signal at
3.61 ppm; (2) its maximum intensity was similar to the
intensity of water signal in the PVT-BPE spectrum; (3) it
6. Synthesis of P(A-TCVT)SO: P(A-PVT)SO (0.1091 g, 0.1 mmol of
PVT unit) and TCNE (0.051 g, 0.4 mmol) were dissolved in 0.5 mL of
anhydrous DMF. The solution was stirred at 50 °C for 16 h and then
dropped into 10 mL of methanol to precipitate the polymer; the deep
blue powder was collected by centrifuge and was washed with
methanol 3 times and dried at room temperature under vacuum for
1
was not found in the purified final product. H and 2D
1
12 h (0.090 g, 76% yield). H NMR (400 MHz, acetone- d6): d (ppm)
NMR studies of TCNE in DMF-d7 indicated that this
peak is attributed to HCN formed from the hydrolysis of
TCNE.7 This side reaction could compete with the substitu-
tion reaction by consuming extra TCNE, and it could be
prevented when an anhydrous solvent was used.
8.090–7.940 (m), 7.650–7.380 (m), 7.36–6.84 (m); 13C NMR
(100 MHz, acetone-d6): d (ppm) 159.4, 157.8, 154.2, 150.3, 148.1,
148.0, 145.5, 145.4, 145.2, 144.3, 144.0, 143.9, 142.7, 141.5, 140.6,
139.6, 137.3, 132.9, 132.3, 130.1, 129.9, 128.7, 128.1, 126.8, 126.5,
123.7, 121.7, 118.5, 117.8, 117.0, 114.0, 113.9, 81.6, 64.7 (m), 49.8; 19
F
NMR (376 MHz, acetone-d6): d (ppm) ꢁ63.80 (6F, m), ꢁ137.54 (4F,
m), ꢁ137.96 (8F, m), ꢁ152.40 (4F, m), 152.92 (8F, m); IR (cmꢁ1):
2972, 2873, (CH stretching), 2218, 1637, 1604, 1588, 1499, 1492 (sh)
(phenyl ring), 1422, 1391, 1321, 1296, 1261, 1183, 1168, 1097, 1075,
997, 824, 598. Tg: 217 °C. Mp: 20,100 Da.
In conclusion, the substitution reaction of PVT units in
polymer and in a model compound with TCNE does not
occur straightaway in DMF. TCNE first adds to the vinyl-
ene moiety in PVT to form a [2+2] cycloaddition product
rapidly at room temperature. It is then reverted to PVT
unit at 50 °C, and immediately followed by the substitution
reaction. This reversible cycloaddition converts the cis-iso-
mer of PVT units into the trans-counterparts. It is expected
to produce better performance of the final TCNE substi-
tuted product as a non-linear optical material.
7. Dombi, G.; Diehl, P.; Lounila, J.; Wasser, R. Org. Magn. Reson.
1984, 22, 573.
8. Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508.
9. (a) Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 5th
ed.; John Wiley & Son: New York, 1992; p 1077; (b) Williams, J. K.;
Wiley, D. W.; McKusick, B. C. J. Am. Chem. Soc. 1962, 84, 2210; (c)
Huisgen, R. Acc. Chem. Res. 1977, 10, 117.
10. Characterization of TCVT-BPE: 1H NMR (400 MHz, acetone-d6): d
(ppm) 8.048 (1H, d, J = 4.4 Hz, H-900), 7.622 (2H, d, J = 8.8 Hz, H-
300), 7.519 (1H, d, J = 16.0 Hz, H-500), 7.506 (1H, d, J = 4.4 Hz, H-800),
7.410 (1H, d, J = 16.0 Hz, H-600), 6.968 (2H, d, J = 8.8 Hz, H-200),
4.546 (4H, t, J = 5.6 Hz, H-e00), 4.098 (4H, t, J = 5.6 Hz, H-f00); 13C
NMR (100 MHz, acetone-d6): d (ppm) 160.8 (s, C-700), 150.1 (s, C-100),
142.9 (s, C-900), 142.7 (d, JC–F = 238 Hz, C-b00 or C-c00), 139.0 (d, JC–F
= 238 Hz, C-b00 or C-c00), 138.8 (s, c-500), 138.1 (d, JCꢁF = 235 Hz, C-
a00), 134.8 (m, C-d00), 132.5 (s, C-1000), 131.9 (s, C-1100), 130.8 (s, C-300),
128.1 (s, C-800), 125.3 (s, C-400), 116.5 (s, C-600), 114.2 (s, CNs), 113.5 (s,
C-200), 80.2 (s, C-1200), 74.2 (s, C-e00), 51.7 (s, C-f00). IR (cmꢁ1): 3062,
2972, 2875 (CH stretching), 2218, 1637, 1603 (sh), 1588, 1522 (sh),
1500, 1422, 1320, 1296, 1261, 1176, 1097, 1075, 998, 824, 673, 646,
598. Tm: 189.0 °C. MS: calcd for C33H16F10N4O2S: 722.1; found m/z:
723.1 (M+1).
References and notes
1. (a) Cai, C.; Liakatas, I.; Wong, M.-S.; Bo¨sch, M.; Bosshard, C.;
Gunter, P.; Concilio, S.; Tirelli, N.; Suter, U. W.. Org. Lett. 1999,
¨
1, 1847; (b) Shu, C.-F.; Wang, Y.-K. . J. Mater. Chem. 1998, 8,
833.
2. (a) Spraul, B. K.; Suresh, S.; Sassa, T.; Herranz, M. A.; Echegoyen, L.;
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