Structural, spectroscopic and nonlinear optical studies on a new efficient organic donor-acceptor crystal for second harmonic generation: L-Threoninium picrate

Article abstract of DOI:10.1016/j.saa.2010.04.044

A new and efficient organic nonlinear optical (NLO) crystal from the amino acid family, viz., l-threoninium picrate (LTHP), has been grown by solvent evaporation technique from aqueous solution. The structure of LTHP was elucidated using single crystal X-ray diffraction data. The compound crystallized in the non-centrosymmetric space group P2₁ and the unit cell contains two l-threoninium cations and two picrate anions. The backbone conformation angles Ψ¹ and Ψ² are in cis and trans configurations for both the picrate and the threoninium residues. An intra-molecular hydrogen bond between the amine N atom and the nearby oxygen atom, viz., N1-H1B^...O3 is observed, with a graph set motif R ₁¹(5). The second harmonic generation (SHG) efficiency of this material was measured using Kurtz and Perry method and found to be about 43 times that of standard potassium dihydrogen phosphate (KDP) crystals. Fourier transform infrared (FTIR) and ultraviloet-visible-near infrared spectral studies were also carried out. The details are discussed.

Full text of DOI:10.1016/j.saa.2010.04.044

Spectrochimica Acta Part A 77 (2010) 160–163
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Structural, spectroscopic and nonlinear optical studies on a new efficient organic
donor–acceptor crystal for second harmonic generation: l-Threoninium picrate
S. Natarajana,∗, M. Umamaheswarana, J. Kalyana Sundara, J. Sureshb, S.A. Martin Britto Dhasa
a School of Physics, Madurai Kamaraj University, Palkalai Nagar, Madurai 625 021, India
b Department of Physics, Madura College, Madurai 625 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 January 2010
Accepted 30 April 2010
A new and efficient organic nonlinear optical (NLO) crystal from the amino acid family, viz., l-threoninium
picrate (LTHP), has been grown by solvent evaporation technique from aqueous solution. The structure
of LTHP was elucidated using single crystal X-ray diffraction data. The compound crystallized in the non-
centrosymmetric space group P21 and the unit cell contains two l-threoninium cations and two picrate
anions. The backbone conformation angles ꢀ1 and ꢀ2 are in cis and trans configurations for both the
picrate and the threoninium residues. An intra-molecular hydrogen bond between the amine N atom
and the nearby oxygen atom, viz., N1–H1B. . .O3 is observed, with a graph set motif R11(5). The second
harmonic generation (SHG) efficiency of this material was measured using Kurtz and Perry method and
found to be about 43 times that of standard potassium dihydrogen phosphate (KDP) crystals. Fourier
transform infrared (FTIR) and ultraviloet–visible–near infrared spectral studies were also carried out.
The details are discussed.
Keywords:
Growth from solution
l-Threoninium picrate
Second harmonic generation
Nonlinear optical material
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
an acceptor (A) show large NLO response and hence have been
well studied. One way of designing such type of systems is syn-
Recent advances in organic nonlinear optical (NLO) materials
have invoked a large revival of interest in this area of research
on account of their widespread industrial potential applications.
In particular, studies have concentrated on second harmonic gen-
eration (SHG), viz., the first-order NLO effect. Results have shown
that an inherent relationship exists between the structure of these
materials and their observed properties. On the molecular scale,
the extent of charge transfer is assumed to dominate the SHG
output while on the supramolecular scale, a high SHG output
requires non-centrosymmetry, strong intermolecular interactions
and good phase-matching ability [1–4]. The NLO process requires
materials that manipulate the amplitude, phase, polarization and
frequency of optical beams. Although a variety of materials includ-
ing inorganic, organometallic, organic and polymeric have been
studied for their NLO activity, it is the organic materials which
have been receiving the maximum attention. In addition to the
advantage in synthesis, organic materials have ultra fast response
time, photo-stability and large first hyperpolarizability (ˇ) val-
ues. In particular, ␲-conjugated systems linking a donor (D) and
thesizing complexes of amino acids with picric acid. Picric acid
[5] and its complexes with amino acids, viz., l-prolinium picrate,
l-valinium picrate and l-asparaginium picrate show very high sec-
ond harmonic generation efficiency [6–8]. In the present work,
another such new compound, l-threoninium picrate (LTHP) is
reported. The synthesis, elucidation of structure, details of the
hydrogen bonding, possible structural features leading to the SHG
efficiency, FTIR and UV–vis–NIR studies of the title compound are
discussed.
2. Experimental procedures
LTHP crystals were grown by solvent evaporation technique. l-
Threonine (Loba, 99% purity) and picric acid (Sd. fine, 99% purity)
were dissolved in distilled water in 1:1 stoichiometric ratio to get
a saturated solution. This was filtered into a clean glass beaker and
covered by a clean polythene sheet with few holes on it, so as to
reduce the rate of evaporation of solvent. Small yellow-coloured
crystals of size 3.0 mm × 1.0 mm × 1.0 mm were obtained within
few days. The density of the single crystal was determined by the
floatation technique using a liquid-mixture of bromoform and car-
bon tetrachloride and it is 1.64(2) g/cm3. The melting point of the
crystal was determined using a melting point apparatus and the
value is 132(2) ◦C.
∗
Corresponding author. Tel.: +91 452 2458471x352/+91 452 2459196;
fax: +91 452 2459181.
E-mail address: s natarajan50@yahoo.com (S. Natarajan).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.04.044

S. Natarajan et al. / Spectrochimica Acta Part A 77 (2010) 160–163
161
Table 1
Crystal data, experimental conditions and structure refinement parameters.
Identification code
Empirical formula
Formula weight
Temperature
LTHP
C10H12N4O10
348.24
293(2) K
´
˚
Wavelength (MoK␣)
0.71069 A
Crystal system, space group
Monoclinic, P21
´
˚
Unit cell dimensions
a = 9.235(5) A
´
˚
´
˚
◦
b = 6.265(5) A, ˇ = 108.42(5) c = 12.727(5) A
´
3
˚
Volume
698.6(7) A
Z, Calculated density
Absorption coefficient
F(0 0 0)
2, 1.655 g/cm3
0.151 mm−1
360
Crystal size
Theta range for data collection
Limiting indices
0.24 mm × 0.21 mm × 0.18 mm
2.32–24.91◦
0 ≤ h ≤ 10, −1 ≤ k ≤ 7, −15 ≤ l ≤ 14
1685/1584 [R(int) = 0.0190]
99.9%
Fig. 1. Molecular structure of LTHP.
Reflections collected/unique
Completeness to theta = 24.91
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
-Scan
Full-matrix least-squares on F2
1584/1/219
3. Results and discussion
3.1. Structural features
1.053
R1 = 0.0281, wR2 = 0.0769
R1 = 0.0289, wR2 = 0.0773
0.0(14)
The loss of proton in the picrate anion is confirmed by the C–C
distances near the phenolic group (C5–C6 and C5–C10). The picrate
anion plays a vital role in forming hydrogen bonds with the threo-
nine residue. The picrate anion forms a strong asymmetric O–H. . .O
hydrogen bond and an N–H. . .O hydrogen bond with the threonine
residue. Of the three nitro groups, two (N2/O5/O6 and N4/O9/O10)
are twisted from the plane of the ring. This twisting does not depend
upon the C–N bond distances [12]. The molecular structure of LTHP
with 50% probability displacement ellipsoids is shown in Fig. 1.
Absolute structure parameter
Extinction coefficient
0.099(7)
´
−3
˚
Largest diff. peak and hole
0.298 and −0.274 e (A)
2.1. Characterization
Nonius CAD-4 MACH 3 diffractometer with MoK␣(0.71073 Å)
radiation was used to obtain the accurate unit cell parameters.
The crystal structure was determined from the single crystal X-
ray intensity data, collected at room temperature (293 K). The data
reduction was done using XCAD4 [9]. The absorption correction
was given using the method of -scan [10]. The structure solution
and refinement were performed using SHELXTL [11]. The structure
was solved by direct methods and full-matrix least-squares refine-
ments using F2 taking all the unique reflections. The crystal data,
experimental conditions and structure refinement parameters, are
presented in Table 1. All the non-hydrogen atoms were refined with
anisotropic thermal parameters. The H atoms which participated in
the H-bonds were located from the difference Fourier and refined
with isotropic thermal parameters. All the other H atoms (–CH)
were positioned geometrically and refined using a riding model
with C–H = 0.97 (–CH) or 0.98 (–CH2) Å with Uiso(H) = 1.2Ueq (par-
ent C atom). The SHG efficiency was measured using the Kurtz and
Perry method. The FTIR spectra of the sample were recorded using
a FTIR Spectrometer (SHIMADZU FTIR, model 8400S), in KBr phase
in the frequency region of 400–4000 cm−1, under a resolution of
4 cm−1 and with a scanning speed of 2 mm/s. The transmission
spectra were recorded using an AGILENT (8453) UV–vis–NIR spec-
trophotometer in the range of 190–1100 nm covering the entire
near ultraviolet, visible and NIR regions.
3.1.1. Conformational features
The amino acid exists in the cationic form with a positively
charged amino group and a protonated carboxylic acid group. The
torsion angles ꢀ1(N1–C2–C1–O1) and ꢀ2(N1–C2–C1–O1) describ-
ing the torsions of the two C–O bonds around C1–C2 are −29.8(3)◦
and 151.5(2)◦, indicating that the carboxylic acid and the amino
group are twisted with respect to each other. Such deviation
from the planarity of these groups is observed in the crystal
structures of bis(l-threoninium) sulfate monohydrate [13], O-
phospho-l-threonine and O-phospho-dl-thereonine [14]. The side
chain conformation is described by the torsion angles ꢁ11, ꢁ12 and
ꢁ13 of −54.0(2)◦, −175.3(2)◦ and −172.1(2)◦, respectively.
3.1.2. Hydrogen bonding interactions
Intermolecular N1–H1C. . .O1 interaction form a linear zigzag
head-to-tail chain of amino acid. Such
a linear chain run-
ning along the b-axis is flanked by picrate anions on either
side through O2–H2. . .O4 interaction. An intra-molecular hydro-
gen bond between the amine N atom and the nearby oxygen
atom, viz., N1–H1B. . .O3 is observed, with a graph set motif
R11(5) (Fig. 2). In addition to these marked H-bond interac-
Fig. 2. Stacking of l-threoninium cations and picrate anions.

162
S. Natarajan et al. / Spectrochimica Acta Part A 77 (2010) 160–163
Table 2
´
˚
◦
Hydrogen-bond geometry (A, ).
´
˚
´
˚
´
˚
D–H···A
d(D–H) (A)
d(H···A) (A)
d(D···A) (A)
∠(DHA) (◦)
O(2)–H(2)···O(4)i
O(2)–H(2)···O(5)i
O(3)–H(3)···O(2)ii
N(1)–H(1A)···O(4)iii
N(1)–H(1A)···O(10)
N(1)–H(1B)···O(3)iii
N(1)–H(1B)···O(7)iv
N(1)–H(1B)···O(9)
N(1)–H(1C)···O(1)v
C(2)–H(2A)···O(3)vi
C(3)–H(3A)···O(4)
0.82
0.82
0.82
0.89
0.89
0.89
0.89
0.89
0.89
0.98
0.98
1.85
2.41
2.28
2.30
2.06
2.37
2.24
2.52
1.98
2.47
2.53
2.660(3)
2.714(3)
3.015(3)
2.904(3)
2.884(3)
2.680(3)
2.063(3)
2.898(3)
2.868(3)
3.412(3)
3.416(3)
169
103
150
125
153
101
153
106
173
162
151
Symmetry transformations used to generate the equivalent atoms: (i) x, −1 + y, z;
(ii) x, 1 + y, z; (iii) 1 − x, −1/2 + y, 1 − z; (iv) x, y, 1 + z; (v) 1 − x, 1/2 + y, 1 − z; (vi) −x,
1/2 + y, 1 − z.
Fig. 3. The ␲–␲ stacking of picrate anions.
tions, a variety of other zigzag interactions are also found in the
structure (Table 2). [CCDC 752510 contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data request/cif, by e-mailing
data request@ccdc.cam.ac.uk or by contacting The Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033].
between two picrate planes is 3.275 Å, which falls in the category of
␲–␲ stacking. It will promote the charge transfer between the anion
and the cation in the presence of electromagnetic fields. (iii) The
presence of strong electron acceptors and strong electron donors;
in this case, the picrate anion is acting as the acceptor (nitro group)
and the threonine cation is acting as donor (amino group). These
donors and acceptors are bridged by the intermolecular hydrogen
bonding(N–H···O). Themagnitude ofoptical nonlinearities depends
on the strength of the donor–acceptor groups. With nitro accep-
tor group, the relative efficiency of various donor groups in an
increasing order is: OCH3 < OH < OC6H5 < OCH3 < N2H3 < NH2 [18].
Hence in the present case, the donor–acceptor strength would
be considerably high. (iv) The linear zigzag head-to-tail chain;
intermolecular N–H···O interaction form a linear zigzag head-to-
tail chain of amino acids (see Table 3). All the above, are in part,
the reasons for the large SHG efficiency possessed by this mate-
rial.
3.2. Second harmonic generation
Since LTHP has crystallized in a non-centrosymmetric space
group, a preliminary study of the powder SHG was made using a
beam of wavelength 1064 nm, using Kurtz and Perry method [15].
A Q-switched Nd:YAG laser beam of wavelength 1064 nm was used
with an input power of 3.5 mJ, pulse width of 10 ns and the repeti-
tion rate being 10 Hz. The crystals of LTHP were ground to a uniform
particle size of about 125–150 ␮m and then packed in capillaries
of uniform bore and exposed to the laser radiation. A powder of
KDP, with the same particle size was used as a reference. The out-
put from the sample was monochromated to collect only the second
harmonic (ꢂ = 532 nm), eliminating the fundamental and the inten-
sity was measured using a photomultiplier tube. It is seen that the
SHG efficiency of LTHP is about 43 times higher than that of KDP.
Second harmonic signal of 690 mV was obtained for LTHP while the
standard KDP samples gave a SHG signal of 16 mV per pulse, for the
same input energy.
3.3. Fourier transform infrared (FTIR) and UV–vis–NIR
spectroscopic studies
The recorded FTIR spectra (Fig. 4) were compared with the
standard spectra of the functional groups [19] and tentative
assignments made for the absorption peaks. All the absorption
frequencies and their tentative assignments are listed in Table 3.
For optical device applications, the transparency in the near-
IR region is significant rather than that in the visible region
3.2.1. Structural features responsible for the high SHG efficiency
of LTHP
The level of SHG response of a given material is inherently
dependent upon its structural attributes. On a molecular scale, the
extent of charge transfer (CT) across the NLO chromophore deter-
mines the level of SHG output, the greater the CT, the larger the
SHG output. The presence of intermolecular interactions, such as
hydrogen bonds, can extend this level of CT into the supramolecu-
lar realm, owing to their electrostatic and directed nature, thereby
enhancing the SHG response [16,17]. In the present material, the
large SHG efficiency possibly arises due to the following reasons:
(i) large number of hydrogen bonds; the threoninium cation and
the picrate anion are linked through O–H···O hydrogen bonding.
The amino N atom of threoninium cation forms N–H···O hydrogen
bonds with the oxygen atoms of picrate anion. Also, O–H···O intra-
molecular hydrogen bonds are available. Hence, the intermolecular
hydrogen bonding between phenolate and nitro group of picric
acid with l-threoninium residue enhances the SHG value, which
is the required property for a system to exhibit nonlinear opti-
cal processes. (ii) The ␲–␲ stacking of picrates anion; the picrates
are arranged as parallel arrays (Fig. 3) and the nearest separation
Table 3
FTIR spectral data for LTHP.
Wave number (cm−1
)
Tentative assignments
3400 s
3238 m
2914 b
1745 s
1612 b
1570 m
1541 m
1493 b
1425 m
1273 b
1163 m
1098 s
918 m
787 m
743 m
706 s
O–H stretching
N–H symmetric stretching
C–H stretching
C
O Asymmetric stretching
+
NH3 asymmetric deformation
NO2 asymmetric stretching
NH3 symmetric stretching
CH3 asymmetric deformation
NO2 stretching
CH3 deformation
C
C
O stretching
C stretching
C–C stretching
C–C skeletal stretching
NO2 scissoring
C–H bending
C–C–N deformation
542 m
s—strong, m—medium, b—broad.

S. Natarajan et al. / Spectrochimica Acta Part A 77 (2010) 160–163
163
4. Conclusions
The crystals of a new NLO material from the amino acid family,
viz., l-threoninium picrate (LTHP) were grown using slow evap-
oration technique. The crystal structure of LTHP was elucidated
using single crystal X-ray diffraction data. The proton in the picric
acid migrated to the L-threonine forming l-threoninium picrate.
The SHG efficiency of this material was measured using Kurtz and
Perry method and found to be about 43 times that of standard KDP
crystals. Large number of hydrogen bonds, the ␲–␲ stacking of
picrate anions, the presence of strong electron acceptors and elec-
tron donors and the linear zigzag head-to-tail chains are, in part, the
reasons for the large SHG efficiency possessed by this material. The
FTIR spectrum shows the presence of the functional groups and the
mode of vibrations. The absorption increases rapidly around 200
and 357 nm, which are due to ␲–␲* and n–␲* transitions, respec-
tively. The absence of absorption in the region between 450 and
1100 nm shows that these crystals are useful for the second har-
monic generation of Nd:YAG laser (ꢂ = 1064 nm).
Fig. 4. FTIR spectrum of LTHP.
Acknowledgement
The authors thank the DST, Government of India, for establishing
the single crystal diffractometer facility at the School of Physics,
Madurai Kamaraj University, through the FIST Programme.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.04.044.
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