Benzothiazolium-π-thiazole-dicyanomethanides: New nonlinear optical chromophores

Article abstract of DOI:10.1016/j.tetlet.2010.10.103

A series of merocyanines with a benzothiazolylidene donor and a quinoidal thiazole moiety have been synthesized. Experimental results and theoretical calculations show that these compounds have zwitterionic ground states and negative second-order optical

Full text of DOI:10.1016/j.tetlet.2010.10.103

Tetrahedron Letters 51 (2010) 6863–6866
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Benzothiazolium-
p-thiazole-dicyanomethanides: new nonlinear
optical chromophores
Raquel Andreu a, Elena Galán a, Javier Garín a, , Jesús Orduna a, Raquel Alicante b, Belén Villacampa b
⇑
a Departamento de Química Orgánica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
b Departamento de Física de la Materia Condensada, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of merocyanines with a benzothiazolylidene donor and a quinoidal thiazole moiety have been
synthesized. Experimental results and theoretical calculations show that these compounds have zwitter-
ionic ground states and negative second-order optical nonlinearities. Unlike most merocyanines, the
degree of charge transfer does not decrease on increasing the separation between the donor and the
acceptor groups, an unexpected feature related to the presence of the proaromatic thiazole fragment.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 17 September 2010
Revised 18 October 2010
Accepted 20 October 2010
Available online 26 October 2010
Dedicated to the memory of Professor
Michel Jubault, good friend and warm-
hearted colleague
Keywords:
Benzothiazole
Thiazole
Nonlinear optics
Benzothiazole-containing cyanines and merocyanines display
interesting optical properties and have found numerous applica-
tions,1 such as sensitizing dyes in photography, photosensitizers
in medicine, fluorescent labels for bioimaging and dyes for dye-
sensitized solar cells,2 to name just a few. In the field of second-or-
der nonlinear optics (NLO),3 where push-pull compounds with an
electron donor (D) and an electron acceptor (A) linked through a
conjugated spacer have been extensively studied, benzothiazoles
have also demonstrated their versatility. Thus, they can play the
role of acceptors,4 spacers5 and even donors when 2-benzothiazol-
ylidene fragments are conjugated to acceptor groups. In that
case, the gain in aromaticity of the hetarylidene moiety (its
‘proaromatic’ character) upon charge-transfer (CT) increases the
zwitterionic (benzothiazolium-like) character of these derivatives
and lies behind their second-order NLO responses. A variety of
acceptors have been used to that end,6 but the linking of an addi-
tional proaromatic fragment to the benzothiazolylidene group as a
strategy towards efficient NLO-chromophores has been little
studied, and mainly in connection with proaromatic acceptors.7
On the other hand, there are very few reports in the literature on
merocyanines comprising of a benzothiazolylidene donor and a
proaromatic spacer,8 and in such compounds the latter fragment
is carbocyclic. To the best of our knowledge there are no examples
of related structures bearing a heterocyclic proaromatic spacer
and, given the growing interest in thiazoles as spacers (and
auxiliary acceptors) for NLO-chromophores,9 in this Letter we
report the synthesis, characterization and linear and nonlinear
optical properties of merocyanines 1, endowed with two
proaromatic fragments, a benzothiazolylidene donor and a thia-
zole-2,5-diylidene moiety (Fig. 1). According to our expectations,
and because of the increased aromaticity on charge separation,
these compounds turn out to be zwitterionic, highly efficient
second-order NLO chromophores.
Compounds 1a–c were synthesized as shown in Scheme 1.
Thus, 1a was prepared by the reaction of 2-dicyanomethylene-
2,3-dihydro-4-phenylthiazole (2)10 and 2-(2-acetanilidovinyl)-3-
ethylbenzothiazolium iodide (3)11 in the presence of triethylamine.
A complementary strategy, using a nucleophilic benzothiazole
reagent, was adopted for the synthesis of the longer derivatives
(1b,c). Thus, the new acceptors 5b,c, in turn prepared from 2
and the corresponding dianilide hydrochlorides (4b and 4c,
⇑
Corresponding author. Tel./fax: +34 976 761194.
E-mail address: jgarin@unizar.es (J. Garín).
Figure 1. Merocyanines 1a–c prepared.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.103

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R. Andreu et al. / Tetrahedron Letters 51 (2010) 6863–6866
to analyse the role that the quinoidal thiazole unit plays in these
chromophores it is instructive to compare the properties of 1a
and its polyenic analogue 7 of the same conjugation length17
(Fig. 2). Thus, whereas the latter is close to the cyanine limit, as evi-
3
denced by the nearly identical JHH values along the polymethine
3
chain (12.5–12.6 Hz in DMSO-d6), the corresponding JHH value
for the dimethine spacer of 1a is considerably higher (14.2 Hz).
This fact, together with the deshielding of the NCH2 group on pass-
ing from 7 (4.17 ppm) to 1a (4.72 ppm), show that the introduction
of the quinoidal thiazole fragment in the spacer gives rise to mero-
cyanines with an enhanced zwitterionic character.
Theoretical calculations have been carried out using the polar-
ized continuum model (PCM) in DMSO, in order to correlate with
experimental measurements, and provide a useful insight into
the structure and properties of compounds 1. Thus, inspection of
Table 1 shows that these compounds have very low and negative
bond length alternation (BLA) values,18 as determined from the cal-
culated bond lengths along the polyenic spacer (Fig. 3), suggesting
a predominant contribution of the zwitterionic limiting form to the
description of their ground states, in agreement with the previ-
ously discussed 1H NMR data.
This feature is confirmed by the high Mulliken charges on the
donor and acceptor (dicyanomethylene) end groups and by the
negative values of the variation of the donor charge on excitation,
Scheme 1. Synthesis of compounds 1a–c.
D
q(D), which suggest a decreased zwitterionic character of their
respectively), were made to react with 3-ethyl-2-methylenebenzo-
thiazole, in situ generated from the commercially available
benzothiazolium salt 6 and NEt3.
first excited states.
Moreover, inspection of the bond lengths of the heterocyclic
moieties on lengthening the spacer reveals an intriguing feature.
Thus, the slight decrease in the length of benzothiazole bond a
(Fig. 3) on passing from 1c to 1a, together with the concomitant in-
crease in bond length b, reveals that the cationic character of the
benzothiazole moiety decreases in the series 1a > 1b > 1c (as it
usually happens in merocyanines, given the growing difficulty in
separating opposite charges over larger distances).19 On the other
hand, the bond lengths of the thiazole rings and of the thiazole–
C(CN)2 bonds show that the anionic character of the dicyanometh-
ylenethiazole fragment increases from 1a to 1c. Moreover, these
two opposite trends are in good agreement with the results of Mul-
liken calculations (Table 1). This seeming contradiction can be
solved taking into account the small electron donor effect of the
spacer, which increases for the longer derivatives, as revealed by
the corresponding Mulliken spacer charges (1a: +0.03; 1b: +0.07;
1c: +0.09).
IR spectroscopy provides an experimental evidence supporting
the nondecreasing amount of charge transfer on lengthening the
spacer. It is known that the m(C„N) band is very sensitive to the
electron density on the nitrile groups, downshifting upon CT in
the case of push–pull compounds bearing a dicyanomethylene
acceptor.20 The C„N stretching modes of compounds 1, measured
by attenuated total reflectance (1a: 2191, 2162 cmꢁ1; 1b: 2181,
2154 cmꢁ1; 1c: 2183, 2148 cmꢁ1), show not only the characteristic
low frequencies and splittings expected for the acceptor bearing an
excess of electron density,21 but also a noticeable downshift of the
3
Concerning the stereochemistry of compounds 1, JHH coupling
constant analysis indicates an all-trans geometry along the
polyenic chain, whereas the benzothiazole-exocyclic C@C bond is
assumed to have (Z)-geometry, in keeping with previous crystallo-
graphic studies on benzothiazolylidene-derived merocyanines.8b,12
Moreover, 3JHH values along the polyenic spacer afford valuable
information concerning the ground state polarization of these
3
compounds, given the existing correlation between JHH values
3
and C–C bond lengths.13 Thus, the high JHH value (14.2 Hz in
DMSO-d6) for the protons of the dimethine spacer of 1a points to
a
noticeable double bond character for the formally single
@HC–CH@ bond and, therefore, to an important zwitterionic con-
tribution to the ground state of this compound. (As expected, such
contribution slightly decreases in the less polar solvent acetone-d6,
3
as judged from the corresponding JHH value of 13.9 Hz). For the
longer derivatives, the
DJ values (defined as the difference between
the averaged 3JHH values of the formal double and single C–C bonds
along the polymethine chain)14 provide a clear indication of the
ground state polarization. Taking into account that for all-trans
polyenes
D
J ꢀ 6 Hz and for cyanines
D
J ꢀ 0 Hz,15 the
DJ values
measured for 1b (ꢁ3.1 Hz) and 1c (ꢁ2.8 Hz) in DMSO-d6 turn out
to be relatively small and negative, indicating that the ground
states of these compounds also lie between the ideal polymethine
state (cyanine limit) and the fully zwitterionic limiting form. In
agreement with the strong polarization of these merocyanines,
the chemical shifts of their polymethine protons show an oscilla-
tory behaviour16 that reflects the alternation in the electron densi-
ties of the carbon atoms to which they are bonded (e.g., for 1b in
DMSO-d6 such dH values, starting from the benzothiazole end
group, are: 7.28, 8.08, 6.63 and 7.64 ppm, respectively). In order
m(C„N) vibrations on passing from 1a to 1b,c. This reveals a larger
Table 1
Selected calculateda bond lengths, BLA and Mulliken charges
Compound
ab
bb
BLAb
q(D)c
q(A)d
D
q(D)e
1a
1b
1c
1.354
1.355
1.356
1.402
1.401
1.399
ꢁ0.016
ꢁ0.014
ꢁ0.014
+0.54
+0.52
+0.51
ꢁ0.57
ꢁ0.59
ꢁ0.60
ꢁ0.06
ꢁ0.05
ꢁ0.05
a
b
c
Calculated at the PCM-B3P86/6-31G*//B3P86/6-31G* level in DMSO.
In Å, see Figure 3.
Charge on the donor (ethylbenzothiazole fragment).
Charge on the acceptor (dicyanomethylene fragment).
Variation of the charge on the donor upon excitation.
d
e
Figure 2. Structure of model compound 7.

R. Andreu et al. / Tetrahedron Letters 51 (2010) 6863–6866
6865
Table 3
Experimental and calculated optical nonlinearitiesa and TDDFT-calculated parameters
Compound
Exp (DMSO)
CPHFb
b0
TDDFTc
e
l
bd
lb0
lf
l
Eg
f
Dl
ge(z)f
1a
1b
1c
ꢁ820
ꢁ4300
ꢁ5300
ꢁ440
ꢁ1900
ꢁ2300
30.85
37.94
45.45
ꢁ5533
2.09
1.83
1.63
1.47
1.96
2.45
ꢁ3.19
ꢁ3.58
ꢁ4.56
ꢁ16304
ꢁ45921
a
b
c
d
e
f
l
b and
l
b0 values in 10ꢁ48 esu.
Calculated at the PCM-CPHF/6-31G*//6-31G* level in DMSO.
Calculated at the PCM-B3P86/6-31G*//B3P86/6-31G* level in DMSO.
Measured at 1907 nm. Experimental accuracy: 10%.
Determined using the two-level model.
In Debyes.
In eV.
Figure 3. Selected bond lengths (from top to bottom: 1a to 1c) and structural
parameters.
g
Table 2
Electrochemical and optical properties and calculated EHOMO/ELUMO values
a
a
b
b
c
d
Compound Eox
Ered
EHOMO
ELUMO
kmax (log
e)
kmax
DMSO (Table 3), and the
lb0 values (extrapolated at zero fre-
quency) were calculated by the two-level model,22 using the lowest
energy absorption band of each compound. EFISH measurements in
dichloromethane were undertaken as well and the experimental
results pointed to negative nonlinearities, although the poor solu-
1a
1b
1c
+0.85 ꢁ0.92 ꢁ5.98
+0.64 ꢁ0.78 ꢁ5.79
+0.60 ꢁ0.67 ꢁ5.65
ꢁ3.51
ꢁ3.61
ꢁ3.70
592 (sh), 636
(5.08)
608
674 (sh), 739
(5.09)
669
681
850e
bility in this solvent precluded us to obtain reliable
lb values.
a
E in V versus Ag/AgCl, glassy carbon working electrode, TBAPF6 0.1 M in CH2Cl2,
0.1 V/s. Ferrocene internal reference Eox = +0.51 V.
(For the sake of comparison, Disperse Red 1 in dichloromethane
b0 value of 480 ꢂ 10ꢁ48 esu under the same experimental
b
gives a
conditions).
l
In eV. Calculated at the PCM-B3P86/6-31G*//B3P86/6-31G* level in DMSO.
In nm, measured in CH2Cl2.
In nm, measured in DMSO.
log
c
d
It can be seen that compounds 1 show increasingly negative lb
e
e not determined due to its very low solubility.
values on lengthening the spacer. This trend is reproduced by
Coupled-perturbed Hartree-Fock (CPHF) calculations, although
the inclusion of the solvent gives rise to largely overestimated
NLO responses. In order to get a better understanding of the non-
linear optical properties of 1a–c, TDDFT calculations have also been
carried out, since they provide the parameters involved in the two-
zwitterionic contribution for the longer derivatives, in line with the
results of theoretical calculations.
The redox properties of the target compounds were measured
by cyclic voltammetry (Table 2). Compounds 1a–c show two irre-
versible waves, corresponding to the oxidation of the donor and
the reduction of the acceptor, respectively, and it can be seen that
the oxidation and the reduction processes become easier on chain
lengthening, in agreement with the trends shown by the calculated
HOMO and LUMO energies. This behaviour can be related to the
stability of the electrochemically generated ion-radicals, which
are more effectively delocalized in the longer derivatives due to
level model: in this approach b0 /
ference between the excited- and ground-state dipole moments
e and g, respectively), f is the oscillator strength and E is the first
Dl Dlge is the dif-
gef/E3, where
(l
l
excitation energy. These parameters are also gathered in Table 3
although, to provide a clearer picture of the CT direction on excita-
tion,
of lge onto the ground state dipole moment direction. It can be
seen that the negative ge(z) values lie behind the negative NLO
responses and that the decrease in E, together with the calculated
increases in f and in | ge| are responsible for the observed
increase in b0 values on passing from 1a to 1c. Therefore, com-
Dlge has been replaced by Dlge(z), defined as the projection
D
the increased extension of the
p
system.
Dl
The UV–vis absorption data of chromophores 1, studied in
dichloromethane and DMSO, are collected in Table 2. All com-
pounds show strong absorption bands in the visible region, extend-
ing into the near-infrared for the longest derivative 1c. It can be
seen that in dichloromethane, lengthening the spacer gives rise
to large vinylene shifts, even exceeding the typical value for cya-
nines (ca. 100 nm), whereas in DMSO such shifts are much lower
(1a to 1b: 61 nm, 1b to 1c: 12 nm). These trends reveal that com-
pounds 1 are close to the ideal polymethine state in dichlorometh-
ane and that the increase in solvent polarity gives rise to more
localized structures, in agreement with 1H NMR data. Moreover,
the negative solvatochromism shown by 1a–c points to a decrease
in their dipole moments upon excitation and, therefore, to zwitter-
ionic ground states. The observed band broadening in DMSO is
indicative of stronger polar interactions in this solvent, as con-
firmed by the increase in the negative solvatochromism on length-
ening the spacer (28 nm (724 cmꢁ1) for 1a; 70 nm (1416 cmꢁ1) for
1b; 169 nm (2920 cmꢁ1) for 1c). Besides, a comparison of the UV–
vis data of 7 (slightly positive solvatochromism and decreased
bandwidth on increasing solvent polarity)17 and 1a confirms that
the replacement of one C@C bond in the spacer by a quinoidal thi-
azole results in merocyanines with an increased charge separation.
D
l
l
pounds 1 can be classed as ‘right-hand side’ NLO-chromophores.23
Unfortunately, their limited solubility has precluded a comparative
study of their second-order NLO responses in different solvents
and, therefore, a more precise placement of such compounds in
either region D or E of Marder’s plot.
In summary, benzothiazolium-thiazole-dicyanomethanides 1,
comprising of a formal benzothiazolylidene donor and a quinoidal
thiazole fragment, presumably acting as spacer and auxiliary
acceptor, have been prepared for the first time and shown to dis-
play negative second-order optical nonlinearities. The gain in aro-
maticity experienced by the two heterocyclic moieties upon charge
transfer results in merocyanines with a strong zwitterionic charac-
ter that, surprisingly enough, does not decrease on lengthening the
spacer. The increased charge separation due to the presence of the
quinoidal thiazole fragment, ascertained by comparison with a
polyenic analogue, can be relevant in the design of new zwitter-
ionic NLO-chromophores.
Acknowledgements
This observation suggests that the thiazole ring is not only a
jugated spacer, but also an auxiliary acceptor group.
The second-order NLO figures of merit, lb, (where b is the first
molecular hyperpolarizability) of 1a–c were measured by electric
p-con-
We thank Dr. J. Casado (Universidad de Málaga) for the ATR-IR
spectra. Financial support from MICINN-FEDER (CTQ2008-02942
and MAT2008-06522C02-02) and Gobierno de Aragón-Fondo
Social Europeo (E39) is gratefully acknowledged. Predoctoral
field-induced second harmonic generation (EFISH) at 1907 nm in

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R. Andreu et al. / Tetrahedron Letters 51 (2010) 6863–6866
7. (a) Ikeda, H.; Sakai, T.; Kawasaki, K. Chem. Phys. Lett. 1991, 179, 551–554; (b)
Cho, B. R.; Je, J. T.; Lee, S. J.; Lee, S. H.; Kim, H. S.; Jeon, S. J.; Song, O.-K.; Wang, C.
H. J. Chem. Soc., Perkin Trans. 2 1996, 2141–2144; (c) Kay, A. J.; Woolhouse, A.
D.; Gainsford, G. J.; Haskell, T. G.; Barnes, T. H.; McKinnie, I. T.; Wyss, C. P. J.
Mater. Chem. 2001, 11, 996–1002.
8. (a) Ashwell, G. J.; Malhotra, M.; Bryce, M. R.; Grainger, A. M. Synth. Met. 1991,
43, 3173–3176; (b) Bell, N. A.; Crouch, D. J.; Simmonds, D. J.; Goeta, A. E.;
Gelbrich, T.; Hursthouse, M. B. J. Mater. Chem. 2002, 12, 1274–1279.
9. (a) Breitung, E. M.; Shu, C.-F.; McMahon, R. J. J. Am. Chem. Soc. 2000, 122, 1154–
1160; (b) Schmidt, K.; Leclercq, A.; Zojer, E.; Lawson, P. V.; Jang, S.-H.; Barlow,
S.; Jen, A. K.-Y.; Marder, S. R.; Brédas, J.-L. Adv. Funct. Mater. 2008, 18, 794–801;
(c) Ma, X.; Ma, F.; Zhao, Z.; Song, N.; Zhang, J. J. Mater. Chem. 2010, 20, 2369–
2380.
fellowships to E.G. (CSIC, JAE 2008) and R. Alicante (FPI BES 2006-
12104) are also acknowledged.
Supplementary data
Supplementary data (general experimental methods, synthesis
and characterization of new compounds, UV–vis spectra, NLO mea-
surements and computational procedures) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.10.103.
10. Seybold, G. U.S. Patent 4,371,734, 1983; Chem. Abstr. 1983, 98, 198202.
11. Brooker, L. G. S.; White, F. L.; Keyes, G. H.; Smyth, C. P.; Oesper, P. F. J. Am. Chem.
Soc. 1941, 63, 3192–3203.
References and notes
12. Nüesch, F.; Grätzel, M.; Nesper, R.; Shklover, V. Acta Crystallogr., Sect. B 1996,
52, 277–286.
13. Radeglia, R.; Dähne, S. J. Mol. Struct. 1970, 5, 399–411.
14. Marder, S. R.; Perry, J. W.; Bourhill, G.; Gorman, C. B.; Tiemann, B. G.; Mansour,
K. Science 1993, 261, 186–189.
1. (a) Hamer, F. M. The Cyanine Dyes and Related Compounds; Wiley-Interscience:
New York, 1964; (b) Larivé, H.; Dennilauler, R. In Thiazole and its Derivatives;
Metzger, J. V., Ed.; Wiley-Interscience: New York, 1979; pp 23–216. Part 3;
(c)Functional Dyes; Kim, S.-H., Ed.; Elsevier: Amsterdam, 2006; (d)Heterocyclic
Polymethine Dyes: Synthesis, Properties and Applications; Strekowski, L., Ed.;
Springer: Berlin, 2008.
15. Scheibe, G.; Seiffert, W.; Hohlneicher, G.; Jutz, Ch.; Springer, H. J. Tetrahedron
Lett. 1966, 7, 5053–5059.
16. Kulinich, A. V.; Ishchenko, A. A.; Groth, U. M. Spectrochim. Acta, Part A 2007, 68,
6–14.
2. Sayama, K.; Tsukagoshi, S.; Mori, T.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.;
Suga, S.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 80, 47–71.
17. Kulinich, A. V.; Derevyanko, N. A.; Ishchenko, A. A. Russ. Chem. Bull. 2005, 54,
2820–2830.
18. Marder, S. R.; Perry, J. W.; Tiemann, B. G.; Gorman, C. B.; Gilmour, S.; Biddle, S.
L.; Bourhill, G. J. Am. Chem. Soc. 1993, 115, 2524–2526.
3. (a) Cho, M. J.; Choi, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton, L. R. Prog.
Polym. Sci. 2008, 33, 1013–1058; (b) Luo, J.; Zhou, X.-H.; Jen, A. K.-Y. J. Mater.
Chem. 2009, 19, 7410–7424; (c) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem.
Rev. 2010, 110, 25–55.
19. (a) Alías, S.; Andreu, R.; Blesa, M. J.; Franco, S.; Garín, J.; Gragera, A.; Orduna, J.;
Romero, P.; Villacampa, B.; Allain, M. J. Org. Chem. 2007, 72, 6440–6446; (b)
Andreu, R.; Carrasquer, L.; Franco, S.; Garín, J.; Orduna, J.; Martínez de
Baroja, N.; Alicante, R.; Villacampa, B.; Allain, M. J. Org. Chem. 2009, 74, 6647–
6657.
20. Casado, J.; Moreno Oliva, M.; Ruiz Delgado, M. C.; López Navarrete, J. T.;
Sánchez, L.; Martín, N.; Andreu, R.; Carrasquer, L.; Garín, J.; Orduna, J. J. Chem.
Phys. 2007, 126, 074701-1–074701-8.
4. (a) Coe, B. J.; Harris, J. A.; Hall, J. J.; Brunschwig, B. S.; Hung, S.-T.; Libaers, W.;
Clays, K.; Coles, S. J.; Horton, P. N.; Light, M. E.; Hursthouse, M. B.; Garín, J.;
Orduna, J. Chem. Mater. 2006, 18, 5907–5918; (b) Costa, S. P. G.; Batista, R. M. F.;
Cardoso, P.; Belsley, M.; Raposo, M. M. M. Eur. J. Org. Chem. 2006, 3938–3946;
(c) Batista, R. M. F.; Costa, S. P. G.; Malheiro, E. L.; Belsley, M.; Raposo, M. M. M.
Tetrahedron 2007, 63, 4258–4265; (d) Zajac, M.; Hrobárik, P.; Magdolen, P.;
Foltínová, P.; Zahradník, P. Tetrahedron 2008, 64, 10605–10618.
5. (a) Hrobarik, P.; Zahradnik, P.; Fabian, W. M. F. Phys. Chem. Chem. Phys. 2004, 6,
ˇ
21. Binev, Y. I.; Georgieva, M. K.; Novkova, S. I. Spectrochim. Acta, Part A 2003, 59,
3041–3052.
495–502; (b) Benková, Z.; Cernušák, I.; Zahradník, P. Struct. Chem. 2006, 17,
287–300.
22. Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664–2668.
23. Bourhill, G.; Brédas, J.-L.; Cheng, L.-T.; Marder, S. R.; Meyers, F.; Perry, J. W.;
Tiemann, B. G. J. Am. Chem. Soc. 1994, 116, 2619–2620.
6. (a) Kay, A. J.; Woolhouse, A. D.; Zhao, Y.; Clays, K. J. Mater. Chem. 2004, 14,
1321–1330; (b) Latorre, S.; Moreira, I. deP. R.; Villacampa, B.; Julià, L.; Velasco,
D.; Bofill, J. M.; López-Calahorra, F. ChemPhysChem 2010, 11, 912–919.