Triethylammonium picrate: An above-room-temperature phase transition material to switch quadratic nonlinear optical properties

Article abstract of DOI:10.1016/j.cclet.2017.10.016

To switch quadratic nonlinear optical (NLO) effects has become an exciting branch of the NLO material science. However, solid-state molecular crystals showing tunable and switchable NLO behaviors remain scarce. Here, we report an organic picrate-based binary molecular crystal, triethylammonium picrate (TEAP), which undergoes an above-room-temperature phase transition at T_c = 319 K, being solidly confirmed by the thermal and dielectric measurements. A large thermal hysteresis of ～7 K discloses the first-order feature for its phase transition. More strikingly, the quadratic NLO effects of TEAP can be switched in the vicinity of T_c. That is, TEAP exhibits NLO-active response of ～1.5 times as large as that of KDP below T_c (i.e., NLO-on state), while its NLO effects totally disappear above T_c (NLO-off state). Structure analyses disclose that the order-disorder transformations of triethylammonium cations and picrate anions collectively contribute to its phase transition, as well as switchable NLO behaviors. This work opens up a new pathway to the designing and assembling of stimuli-responsive materials.

Full text of DOI:10.1016/j.cclet.2017.10.016

Accepted Manuscript
Title: Triethylammonium picrate: An above-room-temperature
phase transition material to switch quadratic nonlinear optical
properties
Authors: Muhammad Adnan Asghar, Jing Zhang, Shiguo Han,
Zhihua Sun, Chengmin Ji, Aurang Zeb, Junhua Luo
PII:
DOI:
Reference:
S1001-8417(17)30433-3
https://doi.org/10.1016/j.cclet.2017.10.016
CCLET 4289
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
3-9-2017
9-10-2017
13-10-2017
Please cite this article as: Muhammad Adnan Asghar, Jing Zhang, Shiguo Han,
Zhihua Sun, Chengmin Ji, Aurang Zeb, Junhua Luo, Triethylammonium picrate: An
above-room-temperature phase transition material to switch quadratic nonlinear optical
properties, Chinese Chemical Letters https://doi.org/10.1016/j.cclet.2017.10.016
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Communication
Triethylammonium picrate: An above-room-temperature phase transition material to
switch quadratic nonlinear optical properties
a
a
a
a,
a
a,b
Muhammad Adnan Asghar , Jing Zhang , Shiguo Han , Zhihua Sun , Chengmin Ji , Aurang Zeb ,
a,
Junhua Luo *
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
b
University of the Chinese Academy of Sciences, Beijing 100039, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
To switch quadratic nonlinear optical (NLO) effects has become an exciting branch of the NLO
material science. However, solid-state molecular crystals showing tunable and switchable NLO
behaviors remain scarce. Here, we report an organic picrate-based binary molecular crystal,
triethylammonium picrate (TEAP), which undergoes an above-room-temperature phase
Available online
transition at T
large thermal hysteresis of ~7 K discloses the first-order feature for its phase transition. More
strikingly, the quadratic NLO effects of TEAP can be switched in the vicinity of T . That is,
TEAP exhibits NLO-active response of ∼1.5 times as large as that of KDP below T (i.e., NLO-
on state), while its NLO effects totally disappear above T (NLO-off state). Structure analyses
c
= 319 K, being solidly confirmed by the thermal and dielectric measurements. A
c
Keywords:
Phase transition
Nonlinear optical properties
Dielectric properties
Order-disorder
c
c
disclose that the order-disorder transformations of triethylammonium cations and picrate anions
collectively contribute to its phase transition, as well as switchable NLO behaviors. This work
opens up a new pathway to the designing and assembling of stimuli-responsive materials.
Multifunctional materials have recently attracted substantial interests, because of their wide applications in the field of photo-
switching, signal processing, biosensors and environment monitors, etc. [1-6]. Among them, the reversible phase transition materials
with desired switchable properties have long drawn renowned importance, due to their rational molecular design and controllable
synthesis. For example, the quadratic nonlinear optical (NLO) activities of phase transition materials can be switched between different
states under external stimulus, behaving as NLO switch. In general, an essential requirement for the NLO-active material is that it
should be non-centrosymmetric (NCS), or undergo the phase transition from a centrosymmetric (CS) phase to NCS state [7-10]. For
NCS-to-CS phase transition, quadratic NLO effects will be switched from NLO-on state to NLO-off state. However, it still remains a
challenge to control the alignment of NLO moieties into a rational manner in the solid-state materials. Hence, solid-state molecular
crystals with tunable and switchable NLO behaviors remain scarce. Recently, molecular phase transition compounds are designated as
the potential candidates for high-performance NLO switches [10-12]. The advantage of structural flexibility allows for the precise
molecule design, as well as the studies on structure-property relationship [13]. Particularly, thermal-activated molecular motions or
order-disorder changes of molecules are considered one of the most important strategies for designing new NLO-switching materials
[14-16]. For example, diisopropylammonium bromide was reported as a ferroelectric showing variable NLO properties, induced by
order-disorder transformation of organic cation [8]. Moreover, order-disorder changes of anionic moieties in the binary crystal of
+
−
[
Hdabco ][CF
effects [17].
From the viewpoint of device application, the most practical and effective temperature range of T
3
COO ] (where dabco = 1,4-diazabicyclo[2.2.2]octane) also result in a reversible phase transition and switchable NLO
c
for ferroelectric, biosensors,
electronic and optoelectronic materials is 290-365 K [18-20]. To design above-room-temperature phase transition materials as NLO
switches, we chose picrate group as the primary building unit, since it has the intrinsic push-pull electronic structure [21]. Besides, the
phenolic group might favor the formation hydrogen-bonding interactions to enhance molecular hyperpolarizability and NLO effects
[
22-25]. For instance, L-leucine L-leucinium picrate was reported as NLO material with efficiency of 1.5 times as large as that of
KH PO [26]. Although numerous picrate-based NLO compounds have been reported, to our best knowledge, the quadratic NLO
switches containing picrate group is still unexplored. This may probably due to the insufficient driving force of the disordered NO
groups to induce structural phase transition [27-30]. Here, we propose to combine the highly-flexible and branched-like amine with
——
2
4
2
—

Corresponding authors.
E-mail addresses: sunzhihua@fjirsm.ac.cn (Z. Sun), jhluo@fjirsm.ac.cn (J. Luo).

picric acid into one system, and thus tri-ethylamine (TEA) has been used as cation [31,32]. As a result, a new above-room-temperature
phase transition material, triethylammonium picrate (TEAP), has been synthesized. It is found that TEAP shows switchable quadratic
NLO effects in the vicinity of T
c
= 319 K. That is, TEAP exhibits NLO response of ∼1.5 times larger than that of KDP below T
(“NLO-off” state). Structure analyses disclose that order-disorder
c
(“NLO-on” state), while its NLO effect fully disappears above T
c
changes of triethylammonium cation and picrate anion account for its phase transition, as well as the switchable NLO behaviors. This
work opens up a new way to design the stimuli-responsive materials in the class of binary compounds.
Yellow crystals were obtained via slow evaporation from solution containing tri-ethylamine and picric acid in ethanol with
stoichiometric molar ratio. Crystal purity was verified by elemental analyses and powder X-ray diffraction (Fig. S1 in Supporting
information). Calcd. for C12
Differential scanning calorimetry (DSC) and specific heat capacity (C
H
18
N
4
O
7
: C 144.12, H 18.18, N 56.04; Found: C 144.10, H 18.17, N 56.05.
) experiments were performed on a NETZSCH DSC 200 F3
p
under nitrogen atmosphere in aluminum crucibles. The dielectric constants () was recorded on TH2828 A impedance analyzer at
different frequencies in the temperature range of 310-330 K with the AC voltage of 1 V.
X-ray single-crystal diffraction data were collected at room temperature (280 K) and high temperature (340 K) using the Super Nova
CCD diffractometer, equipped with the graphite monochromated Cu-Kα radiation (λ = 1.54184 Å). The crystal with an approximate
3
dimension of 0.31 × 0.30 × 0.26 mm was selected. CrystalClear software package (Rigaku) was used for data collection, data
reduction and cell refinement, while crystal structures were solved by the direct methods and refined by the full-matrix method based
2
on F using the SHELXLTL software package [33]. All the non-hydrogen atoms were refined anisotropically, while the positions of
hydrogen atoms were generated geometrically. Data collection details, crystallographic data and refinement for TEAP are given in
Table S1 (Supporting information).
DSC was carried out on the polycrystalline samples of TEAP to detect its thermal-induced phase transition. As shown in Fig. 1a, the
DSC traces show an exothermic peak at 319 K (heating) and an endothermic peak at 312 K (cooling) with a large thermal hysteresis of
~
7 K, which confirm its reversible first-order phase transition. From the C -T trace (Fig. 1b), an entropy change (S) is calculated with
p
-1
-1
the value of ~7.631 J·mol ·K . Using the Boltzmann equation ΔS = RlnN, where R is the gas constant and N is the ratio of the numbers
of respective geometrically distinguishable orientations, N is obtained as 2.49. This value of N suggests that phase transition of TEAP
should be an order-disorder type [34-36]. Besides, thermogravimetric analysis also shows a small exothermic peak at ~319 K, further
confirming the phase transitions of TEAP (Fig. S2 in Supporting information).
(
a)1.0
(b)
0.6
0.5
Heating
0.4
0.0
Cooling
0
.2
-
-
0.5
1.0
0.0
2
90 300 310 320 330 340
Temperature (K)
300
310
320
330
340
Temperature (K)
p
Fig. 1. (a) DSC and (b) C -T traces of TEAP.
To further confirm phase transition of TEAP, variable-temperature powder X-ray diffraction (VT-PXRD) patterns were collected at
00 K, 340 K and back to 300 K, respectively. the patterns recorded above and below the T show obvious changes, as shown in Fig.
3
c
º
º
º
º
S3 (Supporting information). For instance, at 340 K, four new diffraction peaks at 14.45 , 20.52 , 24.21 and 28.10 were observed as
º
º
º
compared to that of 300 K. Besides, some obvious shifts or displacements of diffraction peaks also occurred at 14.44 , 17.44 , 24.72
º
º
º
º
º
º
and 26.11 , along with the disappearance of some peaks at 15.74 , 18.84 , 19.53 , 25.41 and 36.57 at 340 K. Moreover, the patterns
recorded at room temperature, before T and upon cooling back to 300 K from high temperature, appear to be the same. This confirms
the reversible nature of phase transition in TEAP, coinciding with DSC and C -T results.
c
p
A comprehensive structural analysis is quite essential for the understanding on the microscopic mechanism of reversible phase
transitions. At 280 K (room-temperature phase, RTP), TEAP crystallizes in the orthorhombic crystal system with a NCS space group
of Pca2 . Its asymmetric unit is composed of two protonated TEA cations and two picrtae anions, as shown in Fig. 2a. Strong N-H∙∙∙O
1
hydrogen-bonding interactions can be found between the N atoms of the TEA cations and O atoms in the picrate anions, resulting in
the formation of two distinct H-bonded dimers (Table S2 in Supporting information).
It is interesting that the picrate species lie on the (020) planes almost parallel to each other, but adopt the opposite orientations (Fig.
ͦ
2
b). The dihedral angle between these two picrtae species is about 8.2. Further analysis of picrate species explore that the ortho-NO
2
groups twisted away from benzene plane compared to para-NO group that locates on the benzene plane (Fig. 2b). This may arise due
2
to the steric interactions caused by the bonded phenol group with nearby cation. Further, both the anions are stacked alternately
yielding an ABABAB∙∙∙ sequence together with the possible π-π stacking interactions (Cg-Cg = 3.436 Å at RTP) offered by the adjacent
aromatic rings (Fig. S4 in Supporting information).
At 340 K (above T
group of P2 /n. The asymmetric unit becomes half as compared to that at RTP, i.e., one TEA cation and one picrtae anion, as shown in
Fig. 2c. Hence, its crystallographic symmetry changes from the CS monoclinic space group of P2 /n to NCS orthorhombic space group
c
, high-temperature phase, HTP), it is found that TEAP changes to the monoclinic crystal system with a CS space
1
1

of Pca2 (at RTP). As far as we are aware, this symmetry transformation differs from the most of common phase transitions obeying
1
the Curie principle. Actually, some examples showing the similar symmetry breaking have been reported to exhibit interesting
properties [37-39]. For example, the piezoelectric crystal of guanidinium iodide exhibits an unusual phase transition from hexagonal
polar space group of P6
reported an unusual phase transition occurres from P4/mnc to Pa3in (NH
one of the fourfold TiF octahedra in the cubic space group of Pa3 [38].
3
mc (below T
c
) to a monoclinic non-polar P2
1
/m (above T
c
) at T
c
= 349 K [37]. In addition, Molokeev et al.
)
4 3
TiF upon cooling. This might be caused by the ordering of
7
6
Fig. 2. Crystal structures of TEAP. (a, b) The asymmetric unit and packing viewed along c-axis at RTP; (c, d) RTP and at (b) The asymmetric unit and packing
viewed along c-axis at HTP.
At HTP, the thermal-induced atomic vibrations become much severer, which lead to highly-disordered state of both cationic and
anionic moieties (Fig. 2c and Fig. S5 in Supporting information). Take the triethylammonium cation as example, all the flexible ethyl
groups acquire higher thermal ellipsoidal states, which will be more suitable with two of three ethyl groups being splitted to two
different occupied positions. In addition, the picrate anion also contributes to phase transition of TEAP, because the NO groups
2
become highly-disordered at HTP. As a result, all the components adopt the CS crystallographic symmetry (Fig. 2d), leading to the
disappearance of quadratic NLO activities. Hence, it is proposed that the order-disorder changes of both cations and anions lead to its
phase transition and switchable NLO properties.
Second harmonic generation (SHG) effect is a well-known and sensitive technique for detecting the symmetry-breaking phase
transition from CS phase to a NCS phase. Exploration of above-room-temperature molecular NLO-switches with the remarkable NLO
responses would be applied in the optoelectronic field, such as high-power laser system [18]. At RTP, TEAP exhibits notable SHG
properties with the efficiency of ~1.5 times as large as that of KH
2
PO
as ~0.58 pm/V (χ KDP ≈ 0.39 pm/V). With the temperature approaching to T
even fully disappear above the T (Fig. 3a). Such a variation of temperature-dependent NLO effects coincides well with its
crystallographic symmetry transformation, which changes from the NCS space group (Pca2 , at RTP) to CS space group (P2 /n, at
4
(i.e., KDP, Fig. 3b). Thus, its quadratic coefficient is estimated
(2)
c
upon heating, the NLO responses decrease sharply and
c
1
1
HTP). The presence of a large thermal hysteresis (~6.5 K) also agrees with the DSC result, revealing the first-order feature for its phase
transition. Besides, its NLO switching contrast, defined as the ratio of NLO signals at high- and low-NLO states, is calculated to be
~
40 (NLO contrast was estimated as the ratio of the NLO signals at high- and low-NLO states. At the centrosymmetric space group
P2 /n, the deviation of noise level was used for the approximate calculation.). This value is slightly higher than those of other solid-
state materials (~38 for metal-organic hybrid [39]; ~20 for ferroelectric liquid-crystalline polymer [11]; ~35 for organic ionic salts
17]). Moreover, the switching reversibility of TEAP was estimated by repeating the “on/off” cycles. The result in Fig. 3c shows that
1
[
the NLO signals at high-NLO state can recover after several cycles without any obvious fatigue. This finding discloses TEAP might be
a potential candidate as NLO-switching material.
(
a)2.5
(b)
(c)2
.0
.5
.0
.5
.0
0
.0
2.0
1.5
1.0
0.5
0.0
KDP
1
1
0
0
-
0.6
Heating
Cooling
TEAP
-1.2
-1.8
-
2.4
3
05
310
315
320
325
330
-0.003-0.002-0.0010.000 0.001 0.002 0.003
Time (s)
1
2
3
4
5
6
Temperature (K)
Cycle number
Fig. 3. (a) Temperature-dependent variation of SHG effects of TEAP in the heating mode. (b) Comparison of NLO intensities for TEAP and KDP. (c)
Reversible switching of SHG effects.
Dielectric properties usually display distinct anomalies in the vicinity of T during phase transition. Fig. 4 shows the temperature
c
dependence of dielectric constants () at 300, 500, 800 and 1000 kHz, respectively. Step-like dielectric anomalies were clearly
observed at ~318.5 K in the heating mode, which match well with the thermal and NLO studies. In addition, the powder-sample of
TEAP exhibits an initial dielectric constant value of approximately 10.42 at 318 K. With temperature approaching T , the dielectric
c

constants jump to 11 above 319 K (f = 1000 Hz); such step-like dielectric anomalies reveal that structural phase transition of TEAP is
neither ferroelectric nor anti-ferroelectric type.
11.4
11.2
11.0
10.8
10.6
10.4
1000 kHz
800 kHz
500 kHz
300 kHz
314 316 318 320 322 324 326 328
Temperature (K)
Fig. 4. Temperature-dependent dielectric constants of TEAP.
In conclusion, we have reported a new organic quadratic NLO-switching material, which shows an above-room-temperature phase
transition. The combination of flexible cationic moiety and conjugated picrate anion constructs the targeted phase-transition materials
with switchable NLO properties. Structural analyses reveal that the order-disorder changes of flexible cations and anionic moieties
account for its NLO-switching behaviors. It is believed that this finding affords an opportunity for the design of stimuli-responsive
materials in the class of binary compounds.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 21622108, 21525104, 21601188, 91422301,
2
1373220, 51402296 and 51502290), the Natural Science Foundation of Fujian Province (No. 2015J05040), the Strategic Priority
Research Program of the Chinese Academy of Sciences (CAS) (No. XDB20000000), the Youth Innovation Promotion of CAS (No.
014262) and the State Key Laboratory of Luminescence and Applications (No. SKLA-2016-09).
2
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