Correlation between structural studies and third order NLO properties of selected new quinolinium semi-organic compounds

Article abstract of DOI:10.1016/j.chemphys.2010.07.007

New quinolinium semi-organic compounds of formula (C9H8N)2+·SO42-, H2O (I) (bis-quinolinium sulphate monohydrate) and (C9H8N)+·NO3- (II) (quinolinium nitrate) have been synthesized and characterized by UV-Vis absorption spectroscopy, nonlinear optical (NLO) measurements and by single crystal X-ray diffraction. The third order nonlinear optical properties of (I) and (II) were investigated using two methods: the degenerate four wave mixing technique (DFWM) performed in solution at λ = 532 nm and the third-harmonic generation (THG) measurements carried out on thin films at λ = 1064 nm. The NLO measurements showed that compound (I) presents better nonlinear optical properties compared to compound (II). To understand further the optical behaviour of (I) and (II), the crystal structures of both compounds were determined from accurate single crystal X-ray diffraction measurements performed at 100 K. The crystallographic studies revealed the key role of the intermolecular interactions and the molecular arrangements in the enhancement of the NLO properties.

Full text of DOI:10.1016/j.chemphys.2010.07.007

Chemical Physics 375 (2010) 1–7
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Correlation between structural studies and third order NLO properties
of selected new quinolinium semi-organic compounds
b
c
b
b
d
c
K. Bouchouit a, , E.E. Bendeif , H. EL Ouazzani , S. Dahaoui , C. Lecomte , N. Benali-cherif , B. Sahraoui
*
a Département de Chimie, Faculté des Sciences, Université de Jijel, Algeria
b Laboratoire de Cristallographie, Résonance Magnétique et Modélisation CRM2 CNRS UMR 7036, Institut Jean Barriol, Nancy Université, Faculté des Sciences,
BP 239, 54506 Vandœuvre-lès-Nancy, CEDEX, France
c Institut des Sciences et Technologies Moléculaires d’Angers UMR CNRS 6200 MOLTECH ANJOU, Université d’Angers 2, Boulevard Lavoisier, 49045 Angers, France
d Laboratoire des Structures, Propriétés et Interactions Inter Atomiques (LASPI2A), Centre Universitaire de Khenchela, 40000 Khenchela, Algeria
a r t i c l e i n f o
a b s t r a c t
New quinolinium semi-organic compounds of formula ðC9H8NÞþ ꢀ SO42ꢁ; H2O (I) (bis-quinolinium sul-
Article history:
2
phate monohydrate) and ðC9H8NÞþ ꢀ NO3ꢁ (II) (quinolinium nitrate) have been synthesized and character-
ized by UV–Vis absorption spectroscopy, nonlinear optical (NLO) measurements and by single crystal
X-ray diffraction. The third order nonlinear optical properties of (I) and (II) were investigated using
two methods: the degenerate four wave mixing technique (DFWM) performed in solution at
k = 532 nm and the third-harmonic generation (THG) measurements carried out on thin films at
k = 1064 nm. The NLO measurements showed that compound (I) presents better nonlinear optical prop-
erties compared to compound (II). To understand further the optical behaviour of (I) and (II), the crystal
structures of both compounds were determined from accurate single crystal X-ray diffraction measure-
ments performed at 100 K. The crystallographic studies revealed the key role of the intermolecular inter-
actions and the molecular arrangements in the enhancement of the NLO properties.
Received 11 May 2010
In final form 7 July 2010
Available online 13 July 2010
Keywords:
Single crystal X-ray diffraction
Structural characterization
Nonlinear optics
Semi-organic compounds
Thin films
THG
DFWM
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
third-harmonic generation is related not only to the number of
the p-conjugated systems but also to the following factors: solvent
In recent years there has been growing interest in molecular
nonlinear optical materials due to their high potential for optoelec-
tronic and photonic applications [1,2]. The molecular nature of the
materials offers considerable design possibilities which should
permit to scale up the favourable molecular nonlinear optical prop-
erties to macroscopic devices [3–8]. Organic nonlinear optical
materials with aromatic rings represent a large class of such mate-
rials with high nonlinearity, fast response and high optical damage
threshold [9–11]. However, their use for applications is limited by
their rather poor mechanical and thermal stabilities and the lack of
sufficiently large single crystals of optical quality. Organic–inor-
ganic hybrid materials are expected to perform better in this. Espe-
cially semi-organic materials with a non-centrosymmetric cell,
large polarizabilities and nonlinear optical coefficients are cur-
rently intensively investigated [12–19]. A variety of semi-organic
crystals for nonlinear optical applications has been developed,
amongst them compounds with organic cations and inorganic or
organic anions [20].
molecule effects and hydrogen bonds interactions that generate
partial charges on the hydrogen atoms which increase significantly
the dipole moment of molecules [21,22].
In the present work, we report on a detailed study performed on
two new quinolinium semi-organic compounds (bis-quinolinium
sulphate monohydrate, ðC9H8NÞþ ꢀ SO42ꢁ, H2O and quinolinium
2
nitrate, ðC9H8NþÞþ ꢀ NO3ꢁÞ, displaying interesting NLO properties.
The results of the degenerate four wave mixing (DFWM) experi-
ments performed at 532 nm and the third-harmonic generation
(THG) measurements carried out at 1064 nm of the studied com-
pounds are presented and discussed. On the basis of the structural
analysis, we show the important role of the intermolecular interac-
tions and the molecular arrangement for the modification of the
NLO properties.
2. Experimental details
2.1. Crystallization and sample preparation
Recently, we have reported on the origin of the nonlinear
optical response showing that the nonlinear susceptibility of
The studied compounds were prepared by slow evaporation of
quinoline (99%, Aldrich) solutions in nitric and sulfuric acid at
room temperature. The obtained colourless crystals were washed
with water and dried in air.
* Corresponding author.
E-mail address: karim.bouchouit@laposte.net (K. Bouchouit).
0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2010.07.007

2
K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
For the DFWM experiments solutions of very low concentration
scaling using the intensity of equivalent reflections was checked
and did not show any crystal decay during data collection, this scal-
ing procedure was nevertheless applied.
The collected reflections were integrated with the DENZO pro-
gram implemented in the HKL2000 package [24]. The unit-cell
parameters were refined by the program SCALEPACK, which is also
part of the HKL2000 package. Data reduction was performed with
the program SCALEPACK. The absorption was small, but the data
were nevertheless corrected by means of Gaussian numerical inte-
gration using the ABSORB program [25].
(c = 10ꢁ2 mol/l) were obtained by dissolving crystals in a gently
heated aqueous solution. We also prepared thin films for the
TGH measurements: a semi-organic layer was deposited on the
top of glass (Chevallier S.A glasses) first, then the samples were
dried in oven for 20 min at 120 °C. The resulting thin layers were
then moved into the glove box (under inert atmosphere). Finally,
semi-organic thin films were formed by slow evaporation (Joule ef-
fect). The current used for the evaporation process was 3 A at
4 ꢀ 10ꢁ7 mbar.
2.3.2. Crystal structures refinements
2.2. UV spectra
The structures were solved by direct methods and refined by
full-matrix least-squares on F2 using SHELXL97 [26] with no con-
straints applied. All calculations were carried out using the WinGX
software package [27]. Atomic scattering factors were taken from
the International Tables for Crystallography (1992, Vol. C, Tables
4.2.6.8 and 6.1.1.4). The displacement parameters of the non-H
atoms were refined anisotropically using all reflections with
The UV–Vis spectra of I and II are shown in Fig. 1. The measure-
ments were performed in 1 cm (quartz) cell using a solution of low
concentration (c = 10ꢁ5 mol/l). The spectra clearly show that both
compounds absorb in a small zone ranging from 250 nm to
350 nm. This absorption area can be attributed to the quinolinium
aromatic moieties absorption bands.
I > 2r(I). The electron density of the H atoms was clearly identified
in the Fourier difference maps, and their atomic coordinates and
isotropic displacements parameters were refined. The full experi-
mental details and refinements results for compound (I) and (II)
are summarized in Table 1.
2.3. X-ray diffraction
2.3.1. Data collection and reduction
The crystal structures of two new organic–inorganic salts, i.e.
bis-quinolinium sulphate monohydrate, ðC9H8NÞþ ꢀ SO42ꢁ, H2O (I)
2
2.4. Nonlinear optical properties
and quinolinium nitrate, ðC9H8NÞþ ꢀ NO3ꢁ (II) have been determined
by single-crystal X-ray diffraction analysis. Single crystals of
dimensions 0.20 ꢂ 0.12 ꢂ 0.10 mm for (I) and 0.18 ꢂ 0.15 ꢂ
0.08 mm for (II) were fixed on a glass fibre with silicon grease,
mounted on the goniometer head of the Nonius Kappa CCD diffrac-
tometer equipped with a two-dimensional CCD detector and con-
2.4.1. DFWM measurements
Third order nonlinear optical susceptibilities ðvh3iÞ of I and II
solutions were measured by the standard backward degenerate
four wave mixing (DFWM) method using a Nd:YAG laser at
k = 532 nm with 30 ps pulses and 1 Hz repetition rate [28,29]. In
ventional X-ray tube (Mo K
a radiation, k = 0.71073 Å). To
decrease thermal smearing effects, the samples were cooled at
100(2) K with a liquid-nitrogen Oxford Cryostream cooling device
using the cooling rate 120 K h ꢁ1. The temperature was beforehand
calibrated using a K-type Chromel–Alumel thermocouple posi-
tioned at the same place on the crystal. The crystal temperature
was stable to within 2 K. Data collection was controlled by the pro-
gram COLLECT [23]. The cell parameters were determined from an
analysis of the Bragg peak positions collected on the same sets of
15 images. X-ray diffraction data were collected at a fixed detector
Table 1
Crystal data and structure refinement for I and II.
I
II
Chemical formula
Formula weight
C
18H18N2O5S
C9H8N2O3
192.17
374.40
(g molꢁ1
)
Temperature (K)
Wavelength (Å)
Cell setting, space group
Unit cell dimensions
100(2)
0.71073
Triclinic, P1
a = 7.2660(10) Å,
100(2)
0.71073
Monoclinic, C2/c
a = 24.006(5) Å
ꢀ
position using
x step scans repeated at different values of the u an-
gle. Each frame covered a 1° omega rotation step. Coverage of reci-
procal space was more than 99% complete to (sin hmax/k) of 0.7 Åꢁ1
for both crystals. Final cell parameters were deduced from a least-
squares post-refinement on all measured reflections. Interframe
a
= 88.382(9)°
b = 7.2830(10) Å,
b = 77.015(8)°
c = 15.6680(10) Å,
c
806.43(17)
2
1.542
b = 3.6758(7) Å,
b = 99.96(3)°
c = 19.154(4) Å
= 86.637(8)°
Volume (Å3)
Z
1664.7(6)
8
1.534
1.1
Calculated density
(Mg mꢁ3
)
Compound (I)
Compound (II)
1.0
0.9
l
(mmꢁ1
)
0.236
0.118
Absorption correction
Tmin, Tmax
F(0 0 0)
Crystal size (mm)
Theta range for data
collection
Integration
0.911, 0.946
392
Integration
0.938, 0.963
800
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.15 ꢂ 0.12 ꢂ 0.10
0.15 ꢂ 0.12 ꢂ 0.10
2.67–30°
2.74–30°
Limiting indices
h = ꢁ10 ? 10
k = ꢁ10 ? 0
l = ꢁ21 ? 22
4678
h = 0 ? 33
k = 0 ? 5
l = ꢁ26 ? 26
Reflections collected
Parameters
Independent reflections
(I > 2r(I))
Refinement method
2403
235
159
3725 [R(int) = 0.068]
2153 [R(int) = 0.067]
Full-matrix least-
Full-matrix least-
300
400
500
600
700
800
900 1000 1100
squares on F2
squares on F2
Wavlength (nm)
Largest diffraction peak
and hole
0.546 and ꢁ0.502 e Åꢁ3 0.463 and
ꢁ0.362 e Åꢁ3
Fig. 1. The linear absorption spectra for compound I and II.

K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
3
DFWM geometry, two strong, equally intense, counter-propagating
pump beams and a weak probe beam were temporally and spa-
tially overlapped in the sample. The probe beam makes a small an-
gle (h = 12°) with respect to the pump beams and its intensity was
adjusted at Iprobe = 6 ꢂ 10ꢁ2Ipump. A phase-conjugated signal gener-
and 1.6 mJ pulse power. A half wave plate has been placed between
two polarizers in order to adjust the polarization and the power of
the fundamental beam. The intensity at the input face of the sam-
ple is taken for being a Gaussian distribution in space and time. The
beam diameter was 0.65 mm at the sample position and the ap-
plied power density was 2 GW/cm2. The beam was focused on
the sample passing through a lens (f = 25 cm). The films were
mounted on a rotation stage. A 10 nm narrow line interferential fil-
ter (FL355) centred at 355 nm wavelength was used to block the
pump beam before crossing the photodetector. We also used
density attenuator to reduce the intensity generated from the non-
linear medium. The third harmonic signal was detected by photo-
multiplier tube (Model: Hamamatsu) which was integrated with a
boxcar and processed by a computer. A portion of the input beam
was selected and measured by a fast photodiode to monitor the in-
put energy. Finally, we obtained the so-called Maker fringes [31]
which were generated by rotating the sample in the range 60°
to the normal. We used SiO2 as a reference material to calibrate
the THG measurements. The third order nonlinear optical suscep-
tibility of SiO2 was determined to be vh3i ¼ 2:00 ꢀ 10ꢁ22 ½m2=V2ꢃ in
agreement with the literature values [32]. The intensity of the gen-
erated TH is given by the following formula [33,34]:
ated from these three laser beams of the same frequency (x) was
collected at the reverse direction of the probe beam. This signal
was measured by a photomultiplier tube (PMT) and fed to a digital
storage oscilloscope (Tektronix TDS 3054). A part of the input laser
beam was split off and measured by a photodiode in order to mon-
itor the input laser energies.
The studied compounds were dissolved in water in a 1 mm
thick cuvette. We used carbon disulfide (CS2) as a reference mate-
rial to calibrate the DFWM measurements. The third order nonlin-
ear optical susceptibility of CS2
good agreement with the literature values.
ð
vh3i ¼ 1:94 ꢀ 10ꢁ20 ½m2=V2ꢃÞ is in
The DFWM reflectivity (R) was calculated from the propagation
equations of the interacting beams, which were deduced from the
Maxwell equations using the approximation of slow amplitude
variation and taking into account the linear and nonlinear absorp-
tion or only the linear absorption coefficients (Eq. (1)). The DFWM
reflectivity (R) is defined as the total DFWM signal intensity di-
vided by the incident probe-pulse intensity, is plotted as a function
of the pump-pulse intensity (Fig. 2). The DFWM reflectivity (R) can
be expressed as follows [28–30]:
A3I3
x
vð3Þ ¼ Cte ꢀ
ð3Þ
A1fexp½i3
x
lðNxð2Þ ꢁ Nð32xÞ Þ=cꢃ ꢁ 1g
!
!
Nð32Þ þ Nðx2Þ
Nð31xÞ ꢁ Nð32Þ
Nð33xÞ ꢁ Nð32Þ
8
2
a
x
ei6
Nð32Þ l=c
x
p2þ 4
x
x
x
>
<
;
p2 P 0
>
A1 ¼
;
A3 ¼ 1 ꢁ
2
Nð33xÞ þ Nð32Þ
Nð31xÞ þ Nð32Þ
Nð33xÞ þ Nð32Þ
I
I
h4ið0Þ
h3ið0Þ
a
pðctgðp‘ÞÞþ
½
½
2ꢃ
x
x
x
Rijkl
¼
¼
ð1Þ
2
a
p2þ 4
Njx ¼ njx x cos hjx
>
>
:
p2 < 0
;3
x
;3
;3x
;
2
a
qðctghðq‘ÞÞþ
2ꢃ
where nðxjÞ is the refractive index of the jth medium corresponding
ꢀ
ꢁ
vijkl 2Ih1ið0Þ expðꢁ
a‘Þ ꢁ 4 , q = ip and the parame-
48p3
n2ck
a2
h3i
2
;3x
where p2
¼
to the fundamental frequency
x; 3x, the frequency of the third har-
monic signal and l, the film thickness. The vh3i is the third order
ter ‘ is the cell length.
nonlinear optical susceptibility of the film; I3 , the power of the
generated third harmonic signal and h, the rotation angle.
In order to characterise the individual molecule we determined
the second order hyperpolarizability using the following relation:
x
c
¼
vh3iðF4NÞꢁ1
ð2Þ
3. Results and discussion
where F = (n2 + 2)/3 is the Lorentz field factor correction, N = NA ꢀ C/
M is number of solved molecules per unit volume and NA is Avoga-
dro number.
3.1. Structures and crystal packing
3.1.1. Bis-quinolinium sulphate monohydrate, ðC9H8NÞþ ꢀ SO42ꢁ, H2O
2
2.4.2. THG measurements
(I)
Third order nonlinear optical susceptibility of the investigated
compounds was also evaluated by THG measurements. The mea-
surements were performed on thin films using a Q-switched
Nd:YAG laser at k = 1064 nm with 16 ps pulse, 10 Hz frequency
The crystal structure of (I) (Fig. 3) can be described as being
composed of chains of SO24ꢁ groups and water molecules extending
along the a-axis and alternating with C9H8N+ quinolinium-stacked
layers (Fig. 4). The asymmetric unit contains two quinolinium cat-
ion (C9H8N)+, one sulphate anion (SO4)2ꢁ and one water molecule.
The inorganic chains and the quinolinium layers are connected
through five interactions: firstly via two strong N–Hꢀ ꢀ ꢀO hydrogen
bonds (N1–H1nꢀ ꢀ ꢀO2 and N2–H2nꢀ ꢀ ꢀO4) and secondly via three
weaker C–Hꢀ ꢀ ꢀO intermolecular interactions (C17–H17ꢀ ꢀ ꢀO2, C3–
H3ꢀ ꢀ ꢀO3 and C5–H5ꢀ ꢀ ꢀO1w). The quinolinium entities are linked
1.6x10-4
Compound (I)
Compound (II)
1.4x10-4
1.2x10-4
1.0x10-4
8.0x10-5
6.0x10-5
4.0x10-5
2.0x10-5
together by only aromatic
p–p stacking interactions to form infi-
nite perpendicular layers. The average spacing between the quino-
linium molecules in the column is 3.332(2) Å. The water molecules
play an important role in the three-dimensional network of hydro-
gen bonding. In fact, the water molecules act as bridges between
the sulphate groups through two hydrogen bonds: (Ow1–
H1wꢀ ꢀ ꢀO3 and O1w–H2wꢀ ꢀ ꢀO1). Besides these two hydrogen
bonds, the water molecules are also connected to the quinolinium
cations by a weaker (C5–H5ꢀ ꢀ ꢀO1w) interaction. The intermolecu-
lar packing appears to be controlled by a three-dimensional net-
work of N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO hydrogen bonds.
0.0
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
I1 [GW/cm2 ]
As discussed above, all the sulphate oxygen atoms are involved
in hydrogen bonds: O1 and O4 are acceptors only once, while O2
and O3 are acceptor twice. One notices, first, that the sulphate
Fig. 2. The DFWM reflectivity (R) of bis-quinolinium sulphate hydrate and
quinolinium nitrate as a function of input pump intensity.

4
K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
Fig. 3. Ortep III representation [35] of the bis-quinolinium sulphate monohydrate with thermal displacement ellipsoids plotted at 50% probability. H atoms are represented
by spheres.
Fig. 4. View along the b-axis of the crystal parking of (I) showing the alternating ðC9H8NÞþ ꢀ SO42ꢁ and H2O moieties.
2
anion has a distorted tetrahedral configuration. Inside the (SO4)2ꢁ
tetrahedron the S1–O2 [1.499(2) Å] and S1–O4 [1.485(2) Å] are
longer (by ꢄ0.027 and 0.013 Å, respectively) than S1–O1
[1.472(2) Å] and S1–O3 [1.472(2) Å]. These significant differences
in S–O bond lengths are due to the fact that O2 and O4 are involved
in strong hydrogen bonding (N1ꢀ ꢀ ꢀO2: 2.550(3) Å and N2ꢀ ꢀ ꢀO4:
2.626(3) Å) compared to that involving O1 and O3 (Ow1ꢀ ꢀ ꢀO1:
2.875(3) Å and Ow1ꢀ ꢀ ꢀO3: 2.837(3) Å).
Examining the interatomic distances and angles of the quinolin-
ium cations in (I) shows that the majority of the bond lengths and
angles found in (I) are fairly similar to those in the Quinoline
molecule [36]. However, it is worth noting the enlargement of the
C–N–C angle [121.9(2)° and 122.1(2)°] due to the capture of a pro-
ton, compared to the C–N–C angle in Quinoline [117.0(1)°] [36]. This
protonation elongates the N–C bond lengths by ꢄ0.01 Å (in aver-
age), and enlarges the C–N–C angle by about +5°. The benzene and
the pyridinium rings are nearly coplanar and their deviation from
the mean plane is only 1.53(2)°. The dihedral angle observed in
the molecular structure of quinoline is 1.52(2)° [36].
3.1.2. Quinolinium nitrate, ðC9H8NÞþ ꢀ NO3ꢁ (II)
The molecular structure of quinolinium nitrate is given in Fig. 5.
In this structure one can observe NOꢁ3 anionic columns parallel to
Fig. 5. Ortep III representation [35] of the quinolinium nitrate with thermal
displacement ellipsoids plotted at 50% probability. H atoms are represented by
the b-axis surrounded by six quinolinium (C9H8N)+ cationic groups.
The molecular structure can be viewed as a donor–acceptor ad-
spheres.

K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
5
Fig. 6. The crystal structure parking of (II) viewed along the b-axis.
Fig. 7. Quinolinium molecules arrangement: (a) in (I) and (b) in (II).
duct, in which the nitrate anions and the quinolinium cations play
an important role in the hydrogen bonding pattern (Fig. 6). Never-
theless this hydrogen bonding network does not point out any di-
rect hydrogen-bond between the quinolinium cations.
The detailed geometry of the nitrate entities shows that the
N1–O1 [1.253(2) Å] and N1–O2 [1.245(2) Å] bond lengths are
significantly shorter than the N1–O3 bond [1.271(2) Å], which is
in accordance with the relatively strong interaction involving the

6
K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
Table 2
The third order NLO results obtained from THG experiments performed on thin films of compounds (I) and (II). (
nonlinear optical susceptibilities. (
a
v
) is the linear absorption coefficient m, (vh3i) is the third order
ꢀ
ꢁ
h3i
c
) is the second order hyperpolarizability, (vh3i
/
a
) is the factor of merit and
is the third order nonlinear optical.
THG
Compounds
M [g/mol]
a
[cmꢁ1
]
vhe3xip1020 ½m2 Vꢁ2
ꢃ
vhe3xip1020a ½m3 Vꢁ2
ꢃ
vhT3HiG1020 ½m2 Vꢁ2
ꢃ
c
exp10ꢁ43 ½m5 Vꢁ2
ꢃ
I
II
CS2
374.40
192.17
76.1
0.250
0.472
ꢄ0
0.98
0.71
1.94
4.85
1.80
–
3.92
1.68
0.56
–
1.50
4.71 ꢂ 10ꢁ2
O3 atom with the imino group N2 atom of the pyridinium ring
(N2–H22ꢀ ꢀ ꢀO3) [2.741(3) Å]. On the other hand, the nitrate group
acts as a hydrogen-bond acceptor from five neighboring quinolin-
ium cations through a weak C–Hꢀ ꢀ ꢀO interactions and ensure
therefore the anion–cation connection along the a- and c-axes.
The geometrical features of the quinolinium cations are similar
to those previously described in (I). The organic rings atoms are al-
most coplanar and the angle between the mean plane of the pyrid-
inium and the benzene rings (i.e. the dihedral angle) is 1.15(2)° and
this puckering along the C4–C9 bond is commonly found in quin-
olinium salts structures [36,37]. The presence of such puckering
has already been noticed in compounds containing similar pyridi-
nium rings [38]. The interplanar separation between quinolinium
The second order nonlinear hyperpolarizability (c) values
(4.85 ꢀ 10ꢁ43 and 1.80 ꢀ 10ꢁ43 [m5 Vꢁ2] for I and II respectively)
and the calculated merit factors
(
v
/
h3i a
)
(3.92 ꢀ 1020 and
1.50 ꢀ 1020 [m3 Vꢁ2] for I and II respectively) clearly bring out that
compound (I) has a better NLO properties.
It is of interest to note that the obtained results are in line with
those found in several recent studies [39–42]. In fact, the compar-
ison of the obtained values and those described in the literature
indicates that the third order NLO susceptibilities of the studied
compounds are of the same order of magnitude as those used in
the NLO applications.
The NLO characterization has also been performed by THG mea-
surements using maker fringes technique. The third order nonlin-
h3i
layers is 3.398(2) Å leading to a
p
–p
stacking interactions between
ear optical susceptibility ð
v
Þ values of the organic–inorganic
THG
the cations as also observed in (I).
thin films of the investigated compounds were evaluated by com-
paring the THG signal with that of the reference material (SiO2). In
our calculations using formula (3), the refractive index of the
organic–inorganic thin films was assumed to be the same as that
of the fused silica glass substrate. The data were corrected taken
In summary the structural investigation shows that the inspec-
tion of hydrogen bonding network in both salts shows a direct
hydrogen-bond interaction between the quinolinium cation and
the sulphate (nitrate) anion. The (N–Hꢀ ꢀ ꢀO) hydrogen bonds that
connect the sulphate anions (SO4)2ꢁ and the imino groups N1
and N2 atoms in (I) are strong, while those between the nitrate an-
ions NOꢁ3 and the imino group N2 atom in (II) are of moderate
into account the air contribution.
h3i
The values of the ð
v
Þ at k = 1064 nm range from 0.56 ꢀ 10ꢁ20
THG
to 1.68 ꢀ 10ꢁ20 [m2 Vꢁ2] (Table 2). These values are two orders of
h3i
strength. In addition to the
p–p
stacking interactions between
magnitude larger than the
v
value of silica, which is the refer-
THG
the quinolinium layers, two categories of intermolecular interac-
tions can be distinguished in both compounds: (N–Hꢀ ꢀ ꢀO) and
(C–Hꢀ ꢀ ꢀO).
ence material for THG method. One can also notice the good agree-
ment between the results obtained using THG and DFWM
techniques. These values are comparable with the one characteris-
tic of other semi-organic materials [21,22].
The benzene and pyridinium aromatic rings are almost coplanar
in both salts with a slight difference in the dihedral angle. It is
interesting to note that the molecular arrangement of the quinolin-
ium cations in (I) is completely different to that observed in (II)
(Fig. 7). While in (I) a parallel cationic layers arrangement is found
(Fig. 7 a), the organic (C9H8N)+ quinolinium moieties build a zigzag
layers in (II) with a longer interlayer spacing (Fig. 7b).
4. Conclusion
In summary, we have reported here the syntheses, third order
nonlinear optical properties, and structural characterization of
two new semi-organic compounds containing quinolinium cations.
The degenerate four wave mixing (DFWM) measurements at
532 nm, and the third-harmonic generation (THG) results at
k = 1064 nm, clearly showed that the studied compounds possess
not only high third nonlinear optical properties but also important
optical limitation properties. The two compounds display different
optical behaviours since compound (I) has more interesting non-
linear optical properties than compound (II). The accurate struc-
tural analysis showed clearly that the differences between the
two semi-organic compounds are mostly due to the intermolecular
interactions and the molecular arrangements rather to than to any
anions effect as previously shown in similar semi-organic com-
pounds. The efficiency of the hydrogen-bond interactions is di-
rectly related to the donor–acceptor charge transfer and appears
to play, by that way, a key role in the net significant enhancement
of the third-harmonic generation NLO properties in the semi-
organic compounds.
3.2. NLO proprieties
The linear absorption coefficient (
a) values of the investigated
compounds were calculated from the relation T = exp(ꢁ
a
L). Taking
into account the absorption coefficients obtained from transmis-
sion experiment, the third order NLO properties of each compound
were deduced from the DFWM modelled data (Fig. 2). The obtained
results are summarized in Table 2. a, , c and v /
vh3i h3i a represent,
respectively, the linear absorption coefficients, the susceptibilities
of third order, the second order nonlinear hyperpolarisabilities va-
lue and the figure of merit (v /
h3i a), which is an important param-
eter for optical or optoelectronic applications.
One can notice from Table 2 that the value of the third order
nonlinear optical susceptibility (vh3i) of compound (I) is three
times larger than that observed in compound (II). Such a behaviour
may be explained by the fact that the hydrogen-bond interactions
in compound (I) are stronger than those in compound (II). More-
over, the molecular packing in compound (I) helps strengthen
the donor–acceptor interactions and provides an appropriate path-
way for intermolecular charge transfer leading to a significant
enhancement of the NLO properties.
5. Supplementary material
CCDC 670758 and -671321 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif.

K. Bouchouit et al. / Chemical Physics 375 (2010) 1–7
7
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Acknowledgements
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The authors would like to thank the Service Commun de Dif-
fraction X sur Monocristaux (Nancy Université) for providing ac-
cess to crystallographic experimental facilities. Authors are
indebted to Professor Dominik Schaniel and to Doctor Pierrick Dur-
and for many helpful discussions.
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