Versatile optical materials: Fluorescence, non-linear optical and mesogenic properties of selected 2-pyrazoline derivatives

Article abstract of DOI:10.1039/a802070a

A study of the structure-property relationships in a series of 3-(4-n-decyloxyphenyl)-1-(p-X-phenyl)-2-pyrazolines has been performed. By simply changing the substituent in the 1-phenyl ring we were able to tune the physical properties of the compounds. I

Full text of DOI:10.1039/a802070a

J O U R N A L O F
Versatile optical materials: fluorescence, non-linear optical and
mesogenic properties of selected 2-pyrazoline derivatives
´
´
´
Joaqu´ın Barbera,a Koen Clays,b Raquel Gimenez,a Stephan Houbrechts,b Andre Persoonsb and
´
Jose Luis Serrano*a
C H E M I S T R Y
aDepartamento de Qu´ımica Orga´nica-CSIC, Facultad de Ciencias-ICMA, Universidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain
bDepartment of Chemistry, L aboratory of Chemical and Biological Dynamics, Katholieke
Universiteit L euven, Celestijnenlaan 200D, B-3001 L euven, Belgium
A study of the structure–property relationships in a series of 3-(4-n-decyloxyphenyl)-1-(p-X-phenyl)-2-pyrazolines has been
performed. By simply changing the substituent in the 1-phenyl ring we were able to tune the physical properties of the compounds.
If this ring is non-substituted or substituted with a 4-methoxy, 4-chloro or 4-carboxy group, the pyrazoline compounds are
fluorescent. If the ring is 4-nitro- or 2,4-dinitro-substituted, the compounds have interesting second-order non-linear optical
properties. The first hyperpolarizability has been measured using the Hyper-Rayleigh Scattering technique in solution. The 4-nitro
derivative displays liquid crystalline behaviour, showing a monotropic smectic A phase with a partial bilayer structure due to an
antiparallel arrangement of molecules as confirmed by X-ray studies in the mesophase.
show the 1,3-diaryl-2-pyrazolines to be very promising mate-
rials for second-order non-linear optics.8a,b Moreover, measure-
Introduction
Materials with optical properties are very interesting for appli-
ments of the electrooptic half-wave voltage have shown
cations in many areas of technology, including fluorescent
materials which have been extensively used as optical bright-
the potential of pyrazolines as high efficiency electrooptic
materials.8c
eners, fluorescent dyes and scintillators.1 Non-linear optical
(NLO) materials have also attracted great interest for appli-
cations in optical signal processing such as frequency doubling,
amplifiers, modulators for laser technology, data storage and
telecommunications.2
Organic compounds are being extensively studied for non-
linear optical properties, as molecular materials, and their
properties are highly dependent on their structural character-
istics. For second-harmonic generation, organic materials have
to show a high first hyperpolarizability (b) and also possess a
non-centrosymmetric organization in order to achieve a macro-
scopic effect. To obtain this particular arrangement several
approaches are possible and these include the use of crystals,
poled polymers, Langmuir–Blodgett (LB) films and liquid
crystals. Moreover, organic compounds have other advantages
such as low dielectric constants, low switching times and easy
processability.3
In order to promote mesomorphism in these structures we
have introduced an n-decyloxy substituent as a terminal chain
in the 4-position of one of the aromatic rings (Scheme 1).
Moreover, this group increases the solubility of the pyrazoline
derivatives in organic solvents and favours the necessary
arrangements to obtain LB films.9
In the other aromatic ring we have introduced a variety of
groups such as H, OCH , COOH and NO , in order to study
the influence of both acceptor and donor groups on the optical
and mesogenic properties of the compounds.
3
2
In total, seven new compounds have been synthesised in
order to investigate the points outlined above.
Results and Discussion
Synthesis
Typical organic molecules with large hyperpolarizabilities
are systems with donor and acceptor groups separated by a
conjugated system (p-system) such as benzene, stilbene or
stilbazolium.2,4–6
The synthetic route to the compounds is shown in Scheme 1.
Compounds 2–6 were prepared by the reaction of 4-n-decyloxy-
phenyl vinyl ketone 1 with the appropriately substituted phe-
nylhydrazine (route c), by adapting literature methods.10
Ethanol was chosen as the solvent at a temperature of 30–40 °C
to ensure the dissolution of the reactants and the precipitation
of the cyclised products. The use of acid catalysis (acetic acid)
favours cyclisation as opposed to the formation of phenyl-
hydrazone derivatives or Michael addition products.
The reaction follows a general mechanism that was studied
by Coispeau and Elguero.11 1,2-Addition of the primary nitro-
gen atom of the phenylhydrazine occurs at the carbonyl group
of compound 1, followed by cyclisation by means of an
electrocyclic reaction involving six electrons.
The aim of the work described in this paper is to obtain
molecular materials that combine NLO, fluorescent and meso-
genic properties. The existence of mesogenic properties helps
the macroscopic molecular orientations which are very import-
ant to both optical and technical applications.3c In this context,
we have introduced the structural characteristics of organic
NLO materials into molecules that exhibit fluorescent proper-
ties. For these reasons, we have prepared new organic com-
pounds with the 1,3-diphenyl-2-pyrazoline group as the p-
system. These compounds show fluorescent properties1a,b,7 and,
by means of relatively straightforward synthetic procedures,
allow the introduction of suitable substituents for the
promotion of second-order non-linear optical properties.
Molecular calculations (CNDO) have been carried out and
When X=COOH and NO (compounds 5 and 6), the
nucleophilic power of the secondary nitrogen atom of the
2
phenylhydrazine derivative is diminished due to the presence
of an electron withdrawing substituent, whereas the primary
nitrogen atom is affected to a lesser extent. As a consequence,
the carboxy derivative needed a longer reaction time and, in
*E-mail: joseluis@posta.unizar.es
J. Mater. Chem., 1998, 8(8), 1725–1730
1725

OH
a
OC10H21
b
O
C10H21
O
1
NO2
H2N-NH
X
H2N-NH
NO2
d
c
e
NO2
N
N
H
X
C10H21
O
NH
O
N
N
N
C10H21
O
NO2
C10H21
O
NO2
F
NO2
H
8
X =
2
OCH3
Cl
3
4
5
6
O2N
NO2
COOH
NO2
N
N
C10H21
O
7
Scheme 1 Synthetic routes to the pyrazoline and phenylhydrazone compounds
the nitro derivative, the yield is lower than in the other
compounds.
explained by the presence of the nitro groups, which cause a
non-radiative deactivation of the excited state due to a rapid
dissipation of electronic energy.14 In addition to this, the
hydrazone system present in compound 8 is less rigid and has
more rotational freedom than pyrazoline systems.
When the phenylhydrazine bears two electron withdrawing
groups, as in the 2,4-dinitrophenylhydrazine, route c proved
unsuccessful even after heating under reflux for several hours.
Indeed this method led to the formation of a phenylhydrazone
derivative resulting from the attack of a molecule of 2,4-
dinitrophenylhydrazine on the carbonyl group and of a mol-
ecule of ethanol on the double bond (route e, compound 8). It
has been reported that 2,4-dinitrophenylhydrazones can only
be cyclised under extreme conditions (boiling acetic acid
containing hydrobromic acid).12
Therefore, compound 7 was synthesised in two consecutive
steps (route d). In the first step 3-(4-decyloxyphenyl)-2-pyraz-
oline is generated in situ to react react with 1-fluoro-2,4-
dinitrobenzene by a nucleophilic aromatic substitution using
an adaptation of described methods.13
We can observe (Table 1) a relationship between the acceptor
character of the substituent X and the emission wavelength
(l ). The stronger the acceptor character of X, the lower l
.
em
em
On the other hand, the opposite tendency is observed for the
intensities of the emission maxima (I/c in Table 1): the stronger
the acceptor character of X, the higher the intensity.
Generally, the groups that produce a higher bathochromic
effect and higher molar absorptivity in the absorption spectrum
exhibit a stronger emission, except for the compound with X=
OCH , which has a slighty larger absorption l
but weaker
3
max
fluorescent emission than the compound with X=H. These
results can be accounted for by structural and electronic effects.
When the acceptor strength of X increases, the molecular
planarity increases because the contribution of resonance form
B, in which the N1 atom has an sp2 character, becomes greater.
The quantum yields of compounds 2–5 are high (see Table 1).
Absorbance and fluorescence properties
In order to compare the absorbance and fluorescence proper-
ties of these compounds we recorded the absorbance spectra
and the emission spectra in solution in THF. The pyrazoline
derivatives 2–5 exhibit fluorescent emission. The data for these
fluorescent compounds are given in Table 1, and an example
of these spectra is given in Fig. 1. The concentrations of the
solutions were of the order of 10−7 , except for the methoxy
derivative (compound 3), which needed solutions ten or
hundred times more concentrated.
The values are relative to quinine sulfate (Q =0.55)15 and
have been obtained at an excitation wavelength of 340 nm.
F
Compound 2 (X=H) displays the highest value (0.80). The
quantum yield trend is different from the I/c trend. This could
be due to the differences between the irradiation wavelength
(340 nm) and the absorption maxima (l ) of each compound.
The Stokes shift (difference between absorption and fluores-
max
cence maxima) increases as the donor character of X increases.
The fact that electron-donor substituents induce higher
Stokes shifts is in agreement with previous studies on related
pyrazolines.1a,b
On the other hand, no significant emission was found for
the nitro derivatives (pyrazolines 6 and 7, and phenylhydrazone
8). The lack of fluorescence in these compounds could be
1726
J. Mater. Chem., 1998, 8(8), 1725–1730

Table 1 Absorption and emission UV–visible spectral data of 2-pyrazolines 2–5 (in THF)
l
/nm
log ea
(l
log eb
(340 nm)
c/×10−7
(em)c
l
nm
/
Stokes shift/
cm−1e
max
em
Compound
)
I/cd
Q f
max
F
2
3
4
5
355
360
357
369, 387
4.30
4.26
4.33
4.54
4.24
4.21
4.27
4.31
5.4
15.9
4.6
426
454
424
413
1700
740
1740
2700
4695
5750
4425
3885
0.80
0.55
0.70
0.66
2.7
sh
aExtinction coefficient at the absorption maximum wavelength. bExtinction coefficient at the excitation wavelength. cConcentration of the
solution in which the emission spectrum has been performed. dIntensity maximum normalised to a concentration of 1×10−5 . eDifference
between the absorption and emission maxima in wavenumbers. fQuantum yield relative to quinine sulfate (Q =0.55). sh: shoulder.
F
were measured in chloroform solution relative to standard
solutions of p-nitroaniline (p-NA) in the same solvent
(b
=23×10−30 esu).
p-NA
The absorption spectra in chloroform and the quadratic
coefficient as a function of molar concentration in the same
solvent are shown in Fig. 2 and 3 respectively for compounds
6–8.
The absorption maxima and b values for compounds 6–8
are given in Table 2.
We would have expected that compound 7, bearing two
nitro groups in the ortho- and para-positions, would have a b
value higher than that of compound 6, bearing only one nitro
group in the para-position. This would be expected because
Fig. 1 Excitation spectrum (---) and emission spectrum (—) for com-
pound 5
X
N
N
R
R
O
O
A
X
N
N
N
N
X = acceptor group
B
C
X
Fig. 2 Normalised UV–visible spectra of the nitro derivatives in
chloroform: (—) compound 6, (---) compound
compound 8)
7 and (·······)
X = donor group
R
O
Resonance forms of the 1-(p-X-phenyl)-3-(p-alkoxyphenyl)-2-
pyrazoline
Non-linear optical properties
The first hyperpolarizability (b) of each of the nitro derivatives
(compounds 6–8) was measured using the Hyper-Rayleigh
Scattering (HRS) technique.16
This technique relies on the fact that a small part of an
intense Nd-YAG laser pulse at optical frequency v (1064 nm)
is scattered at 2v (532 nm) due to local orientational fluctu-
ations of the molecules. Because the HRS signal is directly
proportional to b2, only absolute values of b can be deduced.
A detailed description of the experimental set-up has been
described elsewhere.17
We were unable to measure the first hyperpolarizability for
the other pyrazoline derivatives (2–5) because of the inter-
ference of the HRS signal with another incoherent process:
multiphoton fluorescence in the region of 532 nm.
If we consider compounds 6–8 as asymmetric conjugated
p-electron systems with donor and acceptor groups in the
4-position, b is by far the largest component of b .18 The
Fig. 3 Quadratic coefficient (I /I 2) versus molar concentration plot
of the nitro derivatives at room temperature in chloroform (1064 nm):
($) compound 6, (+) compound 7 and (2) compound 8
zzz
HRS
2w
w
b
values (extrapolated to infinite wavelengths) have been
0
calculated according to the two-level model.19 The b
HRS
values
J. Mater. Chem., 1998, 8(8), 1725–1730
1727

Table 2 Absorption maxima (in CHCl ) and first hyperpolarizabilities
(1064 nm, CHCl ) of the nitro derivatives.
3
3
l
nm
/
b
b
max
HRS
0HRS
Compound
log e
(×10−30 esu)
(×10−30 esu)
114
6
7
8
430
424
396
4.5
4.4
4.4
393
215
130
66
51
the intramolecular charge transfer should be higher in the first
case, and therefore, the lowest transition energy should be
lower (i.e. l
should be higher). However, the absorbance
max
spectra show that compound 7 absorbs at a wavelength lower
than compound 6. This effect has already been seen in other
related compounds such as 1-(4-nitrophenyl)-2-pyrazoline and
1-(2,4-dinitrophenyl)-2-pyrazoline.13b This phenomenon indi-
cates that there is more efficient charge transfer in compound
6 than in compound 7, which can be explained by a higher
degree of planarity and higher resonance effect in the mono-
nitro derivative. Indeed, the presence of a second nitro group
in the ortho-position of the phenyl ring (compound 7) must
cause a decrease in the charge transfer due to the deviation
from planarity of the phenyl ring at the N1 position due to
steric effects. The effect of this is a decrease in b with respect
to compound 6. To verify this, we tried to grow crystals of
these pyrazolines but all efforts proved unsuccesful. The crystal
structure analysis of similar molecules, such as 1-(2∞,4∞-dinitro-
phenyl)pyrazole20 and 4-bromo-1-(2∞,4∞-dinitrophenyl)pyra-
zole,21 taken from the literature reveal that the nitro group in
the ortho-position is not conjugated with its phenyl ring (the
dihedral angles are 69 and 65° respectively). Moreover, the
dihedral angles between the pyrazole ring and the phenyl ring
are 27 and 22° respectively, higher than the 19° of a similar
compound with only one nitro group in the para-position,
4-bromo-3-methyl-1-(4∞-nitrophenyl)pyrazole.22
Fig. 4 DSC thermogram for compound 6; solid line: first heating;
dashed line: first cooling
complete (−20 °C at 10 °C min−1). In the second heating cycle
a cold crystallization takes place and the sample melts again
at a similar temperature to the first scan (Fig. 4).
The poor mesogenic behaviour of the compounds in this
series could be due to the fact that their molecular structure
deviates significantly from linearity. Indeed, in the crystal
structure of 1,3-diphenyl-2-pyrazoline the angle between the
axis of the phenyl rings is 35°.23
The fact that compound 6 is the only derivative with liquid
crystalline properties is probably due to the aforementioned
effect that the nitro group induces a planar structure with
conjugation throughout the whole molecule. Furthermore, the
presence of terminal groups possessing a large dipole moment
is known to make intermolecular interactions stronger through
dimer associations leading to an antiparallel arrangement.24
This is supported by the X-ray diffraction study of the super-
cooled mesophase of compound 6 at room temperature, which
The b value of the hydrazone derivative (compound 8) is
lower than those measured in the pyrazoline derivatives. This
indicates that, in terms on NLO properties, the pyrazoline
derivatives are better systems than the hydrazone derivative,
due to the presence of the heterocyclic ring which prevents the
conformational freedom of the CNN–N atoms of the hydra-
zone, thus enhancing the conjugation and, consequently, b.
˚
confirms its smectic structure with a layer thickness d=37 A.
If we compare this value with the molecular length estimated
from Dreiding stereomodels, assuming an all-trans confor-
˚
mation of the hydrocarbon chain, L =29.5 A, we can conclude
that the mesophase has a partial-bilayer structure (S
mesophase).
Ad
Although compound 5 also has an acceptor group in the 4-
position, it is not mesogenic. The existence of dimers in benzoic
acid derivatives due to hydrogen bonding is a well known
phenomenon that leads to liquid crystalline behaviour in
systems such as 4-alkoxybenzoic acids.25 This type of associ-
ation is present in compound 5 in the solid state (an IR band
is present at 1676 cm−1, which is typical for a hydrogen
bonded COOH). However, this association leads to a much
higher melting point in compound 5 than in all the other
pyrazoline derivatives (see Table 3). These strong interactions
in the solid could be responsible for the lack of liquid
crystallinity.
Mesogenic properties
The thermal behaviour of all the compounds has been studied
by polarizing optical microscopy and differential scanning
calorimetry (DSC). All the compounds melt to give an isotropic
liquid (see Table 3). However, compound 6 shows mesomorphic
behaviour on cooling, displaying a monotropic smectic A
phase. The texture of this mesophase is typically fan-shaped.
The compound melts at 93 °C on heating and shows hysteresis
in the cooling scan, with the smectic A phase appearing at
71 °C and remaining metastable until the cooling cycle is
Table 3 Optical, thermal and thermodynamical properties of 2-
pyrazolines 2–8
Experimental
General methods
Compound
Transitiona
T / °C (DH/kJ mol−1)
108
All compounds have been characterised satisfactorily by
elemental analysis, 1H NMR and IR spectroscopy and mass
spectrometry. Microanalyses were performed with a Perkin-
Elmer 240C microanalyser. 1H NMR spectra were recorded
on a Varian Unity 300 spectrometer; coupling constants J are
given in Hz. IR spectra were obtained on a Perkin-Elmer 1600
(FTIR series) spectrometer. Mass spectra were obtained on a
VG Autospec EBE (FAB+, 3-NBA matrix). Absorption spectra
were obtained on a Kontron Uvikon 940 Spectrophotometer.
2
3
4
5
6
C–I
C–I
C–I
C–I
C–I
161
143
239
93.1 (31.0)
71.4 (−1.3)
162
I–S
C–I
A
7
8
C–I
106
aC–I: crystal–isotropic liquid transition; I–S : isotropic liquid–smectic
A mesophase transition
A
Fluorescence spectra were obtained on a Perkin-Elmer
1728
J. Mater. Chem., 1998, 8(8), 1725–1730

LS-50 Luminometer using quartz cells with a 1 cm optical
path-length. The quantum yields were measured using quinine
J 8.5, 2H), 6.88 (d, J 8.3, 2H), 7.07 (d, J 8.3, 2H), 7.63 (d, J 8.5,
2H). MS m/z (%): 408 (100, [M+]).
sulfate as the standard (Q =0.5) at an excitation wavelength
of 340 nm. The melting points and the optical textures of the
F
1-(4∞-Chlorophenyl)-3-(4◊-n-decyloxyphenyl-2-pyrazoline 4
mesophase were studied with an Olympus polarizing micro-
scope equipped with a Linkam THMS 600 heating-cooling
stage and a TMS 91 central processor. The transition tempera-
tures were measured by differential scanning calorimetry with
a TA Instruments 2000 calorimeter operated at a scanning
rate of 10 °C min−1. The apparatus was calibrated with indium
(156.6 °C, 28.4 J g−1) as the standard. X-ray diffraction patterns
were obtained using a Pinhole camera (Anton-Paar) operating
with a point-focused Ni-filtered Cu-Ka beam. The sample was
held in Lindemann glass capillaries (1 mm diameter) and
heated with a variable-temperature attachment. The diffraction
pattern was collected on flat photographic film.
This compound was synthesized from 4-chlorophenylhydrazine
chlorhydrate and 1 and purified using the same procedure as
described for compound 3. Yield: 45%. Mp 143 °C. Anal. calc.
for C H ON Cl: C, 72.73; H, 8.00; N, 6.79. Found: C, 72.51;
25 33
2
H, 7.94; N, 6.79%. IR (Nujol, NaCl): n(CNN) 1604, n(CNC
arom.) 1510, n(CMO) 1261 cm−1. 1H NMR (CDCl , 293 K):
3
d 0.88 (t, J 7.3, 3H), 1.27–1.46 (m, 14H), 1.76–1.79 (m, 2H),
3.23 (t, J 10.1, 2H), 3.81 (t, J 10.1, 2H), 3.98 (t, J 6.5, 2H),
6.90 (d, J 8.1, 2H), 7.02 (d, J 8.1, 2H), 7.22 (t, J 8.1, 2H), 7.65
(d, J 8.1, 2H). MS m/z (%): 412 (100, [M+]).
1-(4∞-Carboxyphenyl)-3-(4◊-n-decyloxyphenyl)-2-pyrazoline 5
4-n-Decyloxyphenyl vinyl ketone 1
A solution of 2 mmol (0.30 g) of 4-hydrazinobenzoic acid in
15 ml of hot absolute ethanol was added to a solution of
2 mmol (0.58 g) of 1 in 10 ml of absolute ethanol. A drop of
acetic acid was added and the mixture was stirred at 40 °C for
4 h. The yellow precipitate was filtered off and washed several
times with hot ethanol. Yield. 50%. Mp 239 °C. Anal. calc. for
C H O N : C, 73.93; H, 8.06; N, 6.63. Found: C, 73.63; H,
67 mmol (8.94 g) of aluminum chloride and 65 mmol (5.88 g)
of acryloyl chloride were suspended in 150 ml of dry carbon
tetrachloride and cooled in an ice bath. 65 mmol (15.21 g) of
decyl phenyl ether was added dropwise and the mixture was
stirred for 2 h. The red mixture was poured into a suspension
of calcium chloride in water. The aqueous layer was discarded
and the organic layer washed with saturated aqueous sodium
hydrogen carbonate, dried over calcium chloride, and the
solvent removed under reduced pressure. The residue was
purified by column chromatography on silica gel using hex-
ane–ethyl acetate (2051) as the eluent. Yield: 50%. Mp 36 °C.
Anal. calc. for C H O : C, 79.17; H, 9.72. Found: C, 78.94;
26 34
3 2
7.94; N, 6.47%. IR (Nujol, NaCl): n(CNO) 1676, n(CNN)
1606, n(CNC arom.) 1511, n(CMO) 1258 cm−1. 1H NMR
1.75–1.77 (m, 2H), 3.29 (t, J 10.4, 2H), 3.90 (t, J 10.4, 2H),
(CDCl , 293K): d 0.86 (t, J 7.3, 3H), 1.25–1.50 (m, 14H),
3
3.93 (t, J 7.1, 2H), 6.91 (d, J 9.0, 2H), 7.06 (d, J 8.7, 2H), 7.68
(d, J 9.0, 2H), 7.99 (d, J 8.7, 2H). MS m/z (%): 422 (100,
[M+]), 261 (63).
19 28
2
H, 10.31%. IR (Nujol, NaCl): n(CNO) 1664, n(CNC) 1602,
n(CNC arom.) 1509 cm−1. 1H NMR (CDCl , 293 K): d 0.86
3
3-(4∞-n-Decyloxyphenyl)-1-(4◊-nitrophenyl)-2-pyrazoline 6
(t, J 6.8, 3H), 1.25–1.44 (m, 14H), 1.75–1.80 (m, 2H), 4.00
(t, J 6.6, 2H), 5.85 (d, J 10.5, 1H), 6.41 (d, J 17.0, 1H), 6.92
(d, J 8.8, 2H), 7.16 (dd, J 16.8, J 10.1, 1H), 7.94 (d, J 8.6,
2H). MS m/z (%): 289 (43, [M+]), 261 (100).
This compound was synthesized from 4-nitrophenylhydrazine
and 1 using the same procedure as described for compound 5.
The reaction mixture was stirred for 6 h at 40 °C. The solvent
was evaporated and the crude product purified by column
chromatography on silica gel using hexane–ethyl acetate (151)
as the eluent. Yield: 30%. Mp (DSC) 93 °C (I 71 °C SmA on
cooling). Anal. calc. for C H O N : C, 70.92; H, 7.80; N,
3-(4∞-n-Decyloxyphenyl)-1-phenyl-2-pyrazoline 2
2 mmol (0.58 g) of 1 was dissolved in 20 ml of absolute ethanol
and 2 mmol (0.22 g) of phenylhydrazine. A drop of acetic acid
was added and the mixture was stirred at room temperature
for two hours. The yellow precipitate was filtered off under
vacuum and recrystallized from ethanol. Yield: 55%. Mp
108 °C. Anal. calc. for C H ON : C, 79.36; H, 8.99; N, 7.41.
25 33
3 3
9.93. Found: C, 70.44; H, 7.63; N, 10.06%. IR (Nujol, NaCl):
n(CNN) 1593, n(CNC arom.) 1507, n(NO ) 1299, n(CMO)
2
1246 cm−1. 1H NMR (CDCl , 293 K): d 0.88 (t, J 7.1, 3H),
3
1.20–1.34 (m, 14H), 1.78–1.80 (m, 2H), 3.36 (t, J 10.2, 2H),
3.98 (t, J 10.2, 2H), 4.00 (t, J 6.6, 2H), 6.94 (d, J 9.0, 2H), 7.03
(d, J 9.2, 2H), 7.69 (d, J 9.5, 2H), 8.18 (d, J 9.5, 2H). MS m/z
(%): 423 (100, [M+]).
25 34
2
Found: C, 79.04; H, 8.58; N, 7.45%. IR (Nujol, NaCl): n(CNN)
1600, n(CNC arom.) 1501, n(CMO) 1258 cm−1. 1H NMR
1.76–1.83 (m, 2H), 3.23 (t, J 10.2, 2H), 3.86 (t, J 10.2, 2H),
(CDCl , 293 K): d 0.89 (t, J 6.8, 3H), 1.28–1.50 (m, 14H),
3
3-(4∞-n-Decyloxyphenyl)-1-(2◊, 4◊-dinitrophenyl)-2-pyrazoline 7
3.98 (t, J 6.5, 2H), 6.87 (t, J 7.8, 1H), 6.91 (d, J 8.7, 2H), 7.12
(d, J 7.8, 2H), 7.26 (t, 2H), 7.67 (d, J 8.8, 2H). MS m/z (%):
378 (100, [M+]), 237 (27).
1.39 mmol (0.4 g) of 1 was added to a solution of 4.17 mmol
(0.26 ml, 80%) of hydrazine hydrate in 10 ml of absolute ethanol.
This reaction was carried out under an argon atmosphere. The
reaction mixture was heated under reflux for 3 h. Distillation
under argon afforded ethanol, water and hydrazine. The white
residue of 3-(4∞-decyloxyphenyl)-2-pyrazoline was suspended in
10 ml of absolute ethanol and placed into an ice bath. 70 mmol
(130 mg) of 2,4-dinitrofluorobenzene was added. The dinitrophe-
nyl derivative precipitated instantaneously. The suspension was
stirred for 3 h and the orange precipitate was filtered off and
recrystallized from ethanol. Yield: 55%. Mp 162°C. Anal. calc.
for C H O N : C, 64.10; H, 6.84; N, 11.96. Found: C, 64.04;
3-(4∞-n-Decyloxyphenyl)-1-(4◊-methoxyphenyl-2-pyrazoline 3
2 mmol (0.35 g) of 4-methoxyphenylhydrazine chlorhydrate
was dissolved in a mixture of ethanol–water (551) and a
solution of 2 mmol (80 mg) of sodium hydroxide in 5 ml of
ethanol was added. The mixture was poured into a solution
of 2 mmol (0.58 g) of 1 in 15 ml of ethanol. Several drops of
acetic acid were added (until pH=5) and the reaction mixture
was stirred for two hours at 40 °C. The yellow precipitate was
filtered off under vacuum and recrystallized from acetonitrile.
Yield: 50%. Mp 161 °C. Anal. calc. for C H O N : C, 76.47;
25 32
5 4
H, 6.94; N, 11.88%. IR (Nujol, NaCl): n(CNN) 1603, n(CNC
arom.) 1510, n(NO ) 1306, n(CMO) 1252 cm−1. 1H NMR
1.75–1.80 (m, 2H), 3.35 (t, J 9.6, 2H), 3.95 (t, J 9.6, 2H), 3.97
26 36
2
2
2
H, 8.82; N, 6.86. Found: C, 76.04; H, 7.91; N, 6.92%. IR (Nujol,
(CDCl , 293 K): d 0.86 (t, J 6.6, 3H), 1.26–1.44 (m, 14H),
3
NaCl): n(CNN) 1604, n(CNC arom.) 1510, n(CMO)
1251 cm−1. 1H NMR (CDCl , 293 K): d 0.87 (t, J 7.3, 3H),
3.77 (t, J 10.0, 2H), 3.77 (s, 3H), 3.96 (t, J 6.6, 2H), 6.86 (d,
(t, J 6.3, 2H), 6.90 (d, J 8.8, 2H), 7.06 (d, J 9.3, 1H), 7.59 (d,
J 8.8, 2H), 8.20 (dd, J 9.3, J 2.5, 1H), 8.46 (d, J 2.5, 1H). MS
m/z (%): 468 (100, [M+]).
3
1.25–1.55 (m, 14H), 1.75–1.77 (m, 2H), 3.18 (t, J 10.0, 2H),
J. Mater. Chem., 1998, 8(8), 1725–1730
1729

Chemical Society, Washington DC, 1983; (b) Materials for
Nonlinear Optics: Chemical Perspectives, ed. S. R. Marder,
J. E. Sohn and G. D. Stucky, ACS Symposium Series 455, American
Chemical Society, Washington DC, 1991; (c) Molecular Nonlinear
Optics, ed. J. Zyss, Academic Press, New York, 1994.
(a) D. J. Williams, Angew. Chem., Int. Ed. Engl., 1984, 23, 690;
(b) Nonlinear Optical Properties of Organic Molecules and Crystals;
ed. D. S. Chemla, J. Zyss, Academic Press., Orlando, 1987;
(c) Introduction to Nonlinear Optical EVects in Molecules and
Polymers; ed. P. N. Prasad and D. J. Williams, John Wiley and
Sons, New York, 1991.
J. L. Oudar and D. S. Chemla, J. Chem. Phys., 1977, 66, 2664.
J. L. Oudar, J. Chem. Phys., 1977, 67, 446.
(a) L.-T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith,
G. Rikken and S. R. Marder, J. Phys. Chem., 1991, 95, 10 631;
(b) L.-T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith,
G. Rikken and S. R. Marder, J. Phys. Chem., 1991, 95, 10 643.
N1-[4∞-Decyloxy-a-(2◊-ethoxyethyl)benzylidene]-N2-(2,4-
dinitrophenyl)hydrazine 8
A mixture of 8 mmol (2.30 g) of 1, 8 mmol (1.58 g) of 2,4-
dinitrophenylhydrazine, 60 ml of absolute ethanol and several
drops of acetic acid was heated under reflux for 12 h. The
mixture was evaporated to dryness and the orange solid was
purified by column chromatography on silica gel using hex-
ane–ethyl acetate (1551) as the eluent and recrystallized from
ethanol. Yield: 30%. Mp 106 °C. Anal. calc. for C H O N :
3
27 38
6 4
4
5
6
C, 63.03; H, 7.39; N, 10.89. Found: C, 62.98; H, 7.42; N, 10.95%.
IR (Nujol, NaCl): n(NH) 3278, n(CNN) 1615, n(NO ) 1615,
2
n(CNC arom.) 1508, n(NO ) 1326, 1307, n(CMO) 1250 cm−1.
3H), 1.25–1.40 (m, 12H), 1.4–1.5 (m, 12H), 1.77–1.82 (m, 2H),
2
1H NMR (CDCl , 293 K): d 0.86 (t, J 6.6, 3H), 1.1 (t, J 7.0,
3
7
8
(a) H. Strahle, W. Seitz and H. Gusten, Ber. Bunsen-Ges. Phys.
¨
¨
3.13 (t, J 5.6, 2H), 3.51 (q, J 7.0, 2H), 3.78 (t, J 5.6, 2H), 4.00
(t, J 6.6, 2H), 6.93 (d, J 8.9, 2H), 7.80 (d, J 9.1, 2H), 7.59 (d,
J 8.8, 2H), 7.99 (d, J 9.6, 1H), 8.27 (dd, J 9.6, J 2.6, 1H), 9.13
(d, J 2.6, 1H), 11.88 (s, 1H). MS m/z (%): 515 (100, [M+]),
332 (67).
Chem., 1976, 80, 288; (b) H.Gusten, G. Heinrich and H. Fruhbeis,
¨
¨
Ber. Bunsenges. Phys. Chem., 1977, 81, 810; (c) H. Gusten,
¨
P. Schuster and W. Seitz, J. Phys. Chem., 1978, 82, 459.
(a) J. O. Morley and D. Pugh, in Organic Materials for Nonlinear
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Nonlinear Optics., ed. R. A. Hann and D. Bloor, Spec. Publ. R. Soc.
Chem. 69, Kent, 1989, p. 137.
Conclusions
The pyrazoline structure has shown a special ability to strongly
interact with UV and visible radiation in both linear and non-
linear optical senses. Thus, the compounds with the terminal
9
Y. R. Peterson, in Molecular Electronics, ed. O. J. Ashwell,
Research Studies Press LTD, Exeter, 1992, ch. 3.
10 R. S. Theobald, in Rodd’s Chemistry of Carbon Compounds, 2nd
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11 G. Coispeau and J. Elguero, Bull. Soc. Chim. Fr., 1970, 2717.
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groups OCH , H, Cl and COOH behave as fluorescent mate-
rials with large Stokes shifts and high quantum yields. The
3
nitro derivatives exhibit significant b values measured by the
HRS technique. Surprisingly, the compound with only one
o
nitro group shows a higher first hyperpolarizability than that
with two nitro groups. All of these optical phenomena can be
explained on the basis of structural factors. Furthermore, these
compounds tend to be oriented in a parallel form behaving as
promising promesogenic materials. Elongation of the central
unit could provide the desirable liquid crystal properties. The
combination in the same structure of all these properties
can open new technological applications for these classical
materials.
We thank Drs J. Galba´n and S. de Marcos (Universidad de
Zaragoza) for performing fluorescence measurements. This
work was supported by the Comisio´n Interministerial de
Ciencia y Tecnologia´ (projects MAT94-0717-CO2-01 and
MAT96-1073-CO2-02), the Programa Europa CAI-DGA (col-
laborative visit of R. G. to Leuven) and the Diputacio´n General
de Arago´n (Research studentship to R. G.). S. H. is a Research
Assistant and K. C. is a Senior Research Associate of the Fund
for Scientific Research—Flanders (FWO-V).
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Paper 8/02070A; Received 16th March, 1998
1730
J. Mater. Chem., 1998, 8(8), 1725–1730