Methyl substitution for noncentrosymmetric stacking: A promising organic single crystal for highly efficient terahertz-wave generation

Article abstract of DOI:10.1039/d0tc00408a

Regulating secondary bonds in organic compounds is one of the important routes to design practical materials with excellent performances. However, controlling the molecular arrangement and spatial configuration by changing the limited substituent groups is still very challenging. Herein, two indolium-based (C₁₉H₂₀NO)⁺ crystals, centric OHI-CBS and acentric OHI-T, were synthesized and grown using an evaporation method. An intriguing structural change from the centrosymmetric (P2₁/c) to noncentrosymmetric (Cc) space group was obtained by a simple substitution of the chloro group by the methyl group. Theoretical calculations demonstrate that hydrogen bonds and π-π interactions make the main contribution to molecular spatial alignment. Meanwhile, the OHI-T crystal exhibits prominent nonlinearity of about 0.7 times that of the benchmark OH1 crystal, a large band gap (2.47 eV), a wide transparency range (504-2100 nm), and remarkably efficient output energy in the 0.1-20 THz region, making it a potential nonlinear optical medium for THz-wave generation. This work realizes rational crystal structural regulation through simple molecular substitution and provides a feasible design strategy for other organic optoelectronic functional materials.

Full text of DOI:10.1039/d0tc00408a

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DOI: 10.1039/D0TC00408A
ARTICLE
Methyl Made Noncentrosymmetric Stacking: A Promising Organic
Single Crystal for Highly Efficient Terahertz-wave Generation†
Jingkai Shi,†a Yixin He,†bc Fei Liang,d Xinyuan Zhang,*a Degang Xu,bc Jiyong Yao,e Guochun Zhang,e
Received 00th January 20xx,
Accepted 00th January 20xx
Zhanggui Hu,a Jianquan Yao,bc and Yicheng Wua
DOI: 10.1039/x0xx00000x
Regulating secondary bonds in organic compounds is one of the important routes to design practical materials with excellent
performances. However, it is still a great challenge to control molecular arrangement and spatial configuration by changing
limited substituent groups. Herein, two indolium-based (C19H20NO)+ crystals, centric OHI-CBS and acentric OHI-T were
synthesized and grown by evaporation method. An intriguing structural change from centrosymmetric (P21/c) to
noncentrosymmetric (Cc) space group was obtained by a simple substitution from chloro- to methyl. Theoretical calculations
demonstrate hydrogen bonds and π-π interactions make a dominated contribution to molecular spatial alignment.
Meanwhile, OHI-T crystal exhibits prominent nonlinearity about 0.7 times that of the benchmark OH1 crystal, wide band
gap (2.47 eV), wide transparency range (504 - 2100 nm), and remarkably efficient output energy among 0.1 - 20 THz region,
suggesting it is a potential nonlinear optical media for THz-wave generation. This work realizes the rational crystal structural
regulation by simple molecular subsitution and provides a feasible design strategy for other organic optoelectronic
functional materials.
magnitude higher than urea) and wide transparency in working
region (0 - 20 THz).
There are countable classes of organic NLO crystals which have
Introduction
Terahertz (THz) technology is a rapid-developing field and has
various applications in optical imaging, medical diagnosis,
military security and so on.1,2 Nonlinear difference frequency
generation (DFG) and optical rectification (OR) are two
applicable methods to obtain portable THz radiation based on
highly efficient nonlinear optical (NLO) crystals,3,4 intriguing
growing attention from optics and materials communities.
Generally, the THz tunable region and output efficiency are
mainly determined by the intrinsic properties of NLO crystals.
Inorganic NLO crystals (e.g., LiNbO3, ZnGeP2, GaSe) with good
physical stability and mechanical property are very
advantageous for practical application despite of relatively low
NLO susceptibilities, and their high frequency lattice absorption
limits their application beyond 7 THz.5-7 Organic NLO crystals are
another kind of materials suitable for THz generation due to
remarkably high NLO susceptibilities (usually two orders of
realized THz wave generation. They can be categorized to the
following series (see Table S1): i) pridinium-based NLO crystals,
typically benchmark organic NLO crystals of DAST and DSTMS,
show very high NLO susceptibilities (> 200 pm/V at 1.9 μm);8,9
ii) isophorone-based NLO crystals, another benchmark organic
NLO crystals of OH1 (d33 = 120 pm/V at 1.9 μm) with good
physicochemical stability;10,11 iii) nitraniline-based NLO crystals,
represented by BNA (d33 = 234 pm/V at 1064 nm);12 iv)
quinolinium-based system (e.g., OHQ-TFO, HMQ-T, HMQ-
TMS);13-19 v) isoxazolone-based system (MLS);20,21 vi)
benzothiazolium-based system (e.g., HMB-TMS, PMB-T, HDB-
T);22-24 and vii) other new classes of NLO crystals such as UOH1
and PhMDA, exhibiting comparable THz output performance as
ZnTe.25,26 However, some drawbacks, such as centrosymmetric
hydrates, strong lattice vibrational absorptions, or limited
transparency region still hinder their practical application,9,27
which require more NLO crystals with excellent properties for
in-depth research.
Designing NLO crystals with desired highly efficient
performance needs to follow two simple rules, namely,
introducing strong electron donor and acceptor system to
obtain enhanced microscopic optical susceptibility, and making
the molecules crystalize in noncentrosymmetric space group to
ensure macroscopic optical nonlinearity. However, since most
organic molecules tend to crystallize in centrosymmertric space
group, it is more difficult to find out an approach to obtain
noncentrosymmertric space group than controlling large
a. Tianjin Key Laboratory of Functional Crystal Materials, Institute of Functional
Crystals, Tianjin University of Technology, Tianjin 300384, China
E-mail: xyzhang@email.tjut.edu.cn
b. The Institute of Laser & Optoelectronics, College of Precision Instruments and
Opto-electronics Engineering, Tianjin University, Tianjin 300072, China.
c. Key Laboratory of Optoelectronic Information Technology (Ministry of Education),
Tianjin University, Tianjin 300072, China.
d. State Key Laboratory of Crystal Materials and Institute of Crystal Materials,
Shandong University, Jinan 250100, China.
e. Key Laboratory of Functional Crystals and Laser Technology, Beijing Center for
Crystal Research and Development, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, China.
† These authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: Experimental details and
characterization. CCDC 1971516, 1953173. See DOI: 10.1039/x0xx00000x
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molecular first-order hyperpolarization (β) by rational 7.53 - 7.36 (m, 3H), 7.11 (d, J = 7.9 Hz, 2H), 6.97 (d, J = 8.6 Hz,
View Article Online
DOI: 10.1039/D0TC00408A
2H), 4.08 (s, 3H), 2.29 (s, 3H), 1.77 (s, 6H).
optimizing of electron donor in compound systems.
Indolium, ignored in THz crystals system for a long time, is OHI-CBS. Methyl p-chlorobenzenesulfonate was used, then
1
another strong electron acceptor. The maximum absorption followed the same synthesis procedure. H NMR (400 MHz,
wavelength (λmax) of cation chromophores with identical DMSO) δ 10.84 (s, 1H), 8.37 (d, J = 16.2 Hz, 1H), 8.13 (d, J = 8.6
electron donor (-N(CH3)2) in the same solvent (H2O), λmax, indolium Hz, 2H), 7.84 (d, J = 6.9 Hz, 2H), 7.73 – 7.52 (m, 4H), 7.47 (d, J =
(535 nm) > λmax, benzothiazolium (511 nm) > λmax, quinolium (500 nm) > 16.2 Hz, 1H), 7.38 (d, J = 8.4 Hz, 2H), 6.97 (d, J = 8.6 Hz, 2H), 4.08
λmax, pyridinium (450 nm), 28-30 suggests that indolium-based cation (s, 3H), 1.77 (s, 6H).
possess the largest first-order hyperpolarization (β),31 and Single-crystal X-ray Diffraction. The diffraction data were
indolium has quite strong electron-withdrawing capacity. collected on a Rigaku AFC10 single-crystal diffractometer
Therefore, constructing the NLO crystal system using indolium equipped with graphite-monochromated Cu Kα radiation (λ =
as electron acceptor can ensure enough molecular microscopic 1.54178 Å). The face-indexed absorption correction was carried
nonlinearity. Until recently, indolium-based EHPSI-4NBS was out based on the XPREP program. Using Olex2,33 the structures
discovered by Michaelis’ group and successfully achieved were solved using Intrinsic Phasing with the ShelXT34 structure
efficient broadband generation from 0 to 3.8 THz, proving solution program and refined using Least Squares minimization
indolium-based NLO crystals as a very potential new system for with the ShelXL35 refinement package. The ADDSYM algorithm
practical application in THz generation.32 But bulk crystal from the program PLATON was used for structural verification,
36
growth is not easy because of ethyl rotation and vibration.
and no higher symmetries were found. All of the hydrogen
Therefore, there is still great exploration possibility for atoms were located on a difference Fourier map. The
indolium-based crystals for NLO practical application. crystallographic data are listed in Table 1.
In this work, we promoted an effective design strategy to obtain Single Crystal Growth. Slow evaporation method was used for
acentric packing based on rational control of hydrogen bonds crystal growth. The solid powder of OHI-T was placed in a clean
and face-to-face π-π interaction. Via a simple replacement of beaker containing methanol to form a saturated solution at
chloro- by methyl, an intriguing structural transformation from room temperature. The beaker mouth was covered with a film
centrosymmetric
C25H24ClNO4S
(OHI-CBS)
to
non- with several small holes. After ten days, high optical quality
centrosymmetric C26H27NO4S (OHI-T) was realized. Their cut-off crystals with dimensions of 7 × 4 × 1 mm3 was grown from
wavelengths, thermal stability, transmission spectra were methanol solvent (Figure S1).
measured. More importantly, OHI-T exhibited large second Powder X-ray Diffraction (PXRD). The powder X-ray diffraction
harmonic generation (SHG) response about 0.7 times of OH1. (XRD) patterns were recorded on Rigaku MiniFlex II
Based on OHI-T crystals with high quality, highly efficient THz diffractometer equipped with Cu Kα radiation in the 2θ range of
wave generation ranging from 0.1 to 20 THz was demonstrated 5-55°.
for the first time.
Rocking Curve. The high-resolution X-ray rocking curve were
examined by X-ray diffraction (XRD, Rigaku MiniFlex II),
2θ=8.0594°.
Thermal Analysis. Thermal Gravimetric Analysis (TGA) and
Differential Thermal Analysis (DTA) was performed on a
NETZSCH STA 449F5 instruments, and a heating rate of 10 °C
min-1 in nitrogen was adopted.
Experimental
Synthesis. All the reagents were purchased and used without
any further purification. The synthesis of OHI-CBS and OHI-T
followed the basically the same procedures. Herein we take
OHI-T as example.
OHI-T. 2,3,3-trimethylindolenine (31.85 g, 0.2 mol) and methyl
p-toluenesulfonate (37.25 g, 0.2 mol) were dissolved in 1,2-
dimethoxyethane (250 mL). The solution was stirred at 50 ℃ for
48 h and then cooled to room temperature. Then light purple
UV-vis Spectroscopy. The transmittance spectrum data were
collected with a UH4150 UV-visible spectrophotometer in the
range of 250 nm to 2250 nm. The UV-vis spectra in acetonitrile
solution (1×10-5 mol·L-1) were measured by a UH4150 UV-visible
spectrophotometer in the range of 300 nm to 750 nm.
Second-Harmonic Generation (SHG) Measurement. SHG test
was performed on the powder samples by means of the Kurtz–
Perry method.37 Fundamental 2.09 μm light was generated with
a Q-switched Ho:Tm:Cr:YAG laser. Different particle sizes
powders were prepared separately by mechanical grinding of
the single crystals.
Computational Methods. Hirshfeld analysis was performed
using the Crystal Explorer software.38 The analyses were
performed calculating a surface around a cation separately, and
then the interactions with the surrounding molecules were
studied. Using this approach, dnorm surfaces and di–de plots were
obtained, where di represents the distance between an atom
inside the surface and the closest point in the surface, and de is
defined similarly but with an atom outside of the surface.
powder
1,2,3,3-tetramethylindolenium
4-
methylbenzenesulfonate powder was obtained by filtration and
washed with a large amount of 1,2-dimethoxyethane solvent to
remove remaining reagents. 1,2,3,3-tetramethylindolenium 4-
methylbenzenesulfonate powder (10.06 g, 30 mmol) and 4-
hydroxybenzaldehyde (3.66 g, 30 mmol) were dissolved in
methanol (250 mL) and then piperidine (1.18 ml, 12.0 mmol)
was added in solution as a catalyst. The solution was stirred at
60 ℃ for 48 h and then cooled to room temperature. Target
products of orange crystalline powder were obtained by
filtration. The new compounds were purified by
recrystallization process in methanol. 1H NMR (400 MHz,
DMSO) δ 10.85 (s, 1H), 8.37 (d, J = 16.2 Hz, 1H), 8.13 (d, J = 8.6
Hz, 2H), 7.84 (d, J = 6.7 Hz, 2H), 7.60 (tt, J = 13.1, 6.7 Hz, 2H),
2 | J. Name., 2012, 00, 1-3
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Terahertz-wave Generation. The schematic diagram of DFG reverse in the adjacent anion layers. Owing to not very strong
View Article Online
DOI: 10.1039/D0TC00408A
system with OHI-T crystal is illustrated in Figure S2. A double- hydrogen bonds between O atom of sulfonate and H atoms of
pass optical parametric oscillator, consisting of cavity mirrors hydroxyl on OHI cations (1.958 Å), OHI cations remain opposite
(M2&M3), KTP1 and KTP2 crystals (7×10×15 mm3, θ=65°, φ=0°) packing pattern following antiparallel CBS anion layers.
rotated by galvano-optical beam scanners (Cambridge Moreover, by introducing -Cl with strong electronegativity to
Technology, 6230H), was pumped by the 532 nm green light, benzene on anion group, face-to-face π-π interactions (3.496 Å,
generated by second harmonic generation (SHG) in KTP (7 × 7 × 3.481 Å) are effectively strengthened (Figure 2b), leading to
10 mm3, θ = 90 °, φ = 23.5 °) crystal. M1 and dichroic mirror almost parallel stacking of OHI cation planes, which would be
(DM) were utilized to reflect and block the 532 nm pump wave, perfect if they were in same orientation.
respectively. One of the tunable dual-wavelength pump λ2 was
tuned in the range of 1.36 - 1.5 μm by rotating KTP2 and the
Table1. Crystal Data and Structure Refinements for OHI-CBS and OHI-T
OHI-CBS
C25H24ClNO4S
469.96
monoclinic
P21/c
6.98170(10)
20.5573(3)
15.9811(2)
90
91.1430(10)
90
OHI-T
C26H27NO4S
449.54
monoclinic
Cc
8.22440(10)
21.8327(3)
13.0554(2)
90
103.570(2)
90
other was fixed (λ1 = 1.364 μm). The OHI-T crystal was pumped
with the dual-wavelength pump wave focused by a convex lens
(M4, f = 200 mm) in order to achieve the high intensity.
Monochromatic THz wave was generated from the OHI-T crystal
and collected by two off-axis parabolic mirrors (OAP, f = 101.6
mm) into a helium-cooled Si bolometer (IR Labs Inc.). The
calibration of the Bolometer was 2.89 × 105 V/W. Black
polyethylene sheet with the thickness of 1 mm was employed
to block the residual dual-wavelength DFG pump.
Formula
Fw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (deg)
β(deg)
γ(deg)
V (Å3)
Z
2293.23(5)
4
2278.80(6)
4
Results and Discussion
GOF on F2
R1 (>2σ(I))
wR2 (all data)
CCDC No.
1.085
0.0590
0.1811
1971516
1.095
0.0351
0.0958
1953173
Crystal Structure. In the two new structures, as shown in Figure
1, the indolium-based (C19H20NO)+ cation (OHI) was selected for
its maximum wavelength λmax of molecule absorption indicating
large first-order hyperpolarizability (β), which guarantees
microscopic optical susceptibility. Different anions of 4-
chlorophenylsulfonate (CBS) and p-toluene sulfonate (T) were
introduced separately to regulate the molecular spatial
arrangement by slight different -R group.
퐹2표 휎 퐹2표
퐹2표 {Σ[푤(퐹표2 퐹2푐)]2
Σ
퐹표4
a
Σ
Σ
R(F) = ||Fo| - |Fc|| / |Fo| for > 2 ( ). Rw( ) =
/ w }½
-
for all data
Differently, in the OHI-T crystal structure, methyl (-CH3)
replaced chloro (-Cl) with similar space hindrance but much
weaker electronegativity. As displayed in Figure 2c, T anions
arrange in the same direction along a-axis forming anion
columns, and the neighboring columns are in the same direction
which are in contrast with OHI-CBS. Based on this advantageous
alignment of T anions, OHI cations easily form arrangement in
the same direction by stronger hydrogen bonds (1.787 Å),
turning to a noncentrosymmertric space group. Besides, as
shown in Figure 2d, OHI cations possess not very strong face-to-
face π-π interactions (3.621 Å). Therefore, slightly changes in
electronegativity effectively cause space group transformation.
Figure 1. Chemical structure of OHI-CBS and OHI-T.
Table 1 provides the refinement details and the unit cell
parameters of the obtained crystal structures. OHI-CBS belongs
to centrosymmertric P21/c space group, while OHI-T crystallized
in the noncentrosymmertric Cc space group.
Changes caused by replacement of chloro-(-Cl) by methyl (-CH3)
are unusual, because CBS and T anions generally do not induce
different stacking pattern structures in NLO crystals, such as in
pyridium-based DASC (Cc) and DAST (Cc),8,39,40 quinolinium-
based HM7ClQ-CBS (Pna21) and HM7ClQ-T(Pna21),16 as well as
benzothiazolium-based PMB-CBS (Cc) and PMB-T (Cc).23
Chloro- (-Cl) was commonly introduced to the benzene ring in
order to enhance the molecular interactions for its strong
electronegativity, forcing spatial arrangement closer to desired
alignment. Unfortunately, OHI-CBS crystallized in a centric
space group P21/c. As shown in Figure 2a, CBS anions are in the
same orientation in each anion layer, but the orientations
Table 2. Contacts distance and contribution changes of hydrogen bonds (O-H) and
π-π interactions (C-C) in the two crytstals.
-Cl (OHI-CBS)
3.07
17.5 %
1.958 Å
8.4 %
-CH3 (OHI-T)
2.32
18.8 %
1.787 Å
5.7 %
Electronegativitya
O-H contribution
O-H distance
C-C contribution
C-C distance
3.481 Å
3.621 Å
a Mullay’s data[41, 42]
The comparison between electronegativity and contacts
distance are listed in the Table 2. With the electronegativity
changes from 3.07 to 2.32, hydrogen bonds strength increases
from 1.958 Å to 1.787 Å, and face-to-face π-π interactions
strength decreases from 3.481 Å to 3.621 Å, accompanied by
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space group transforming from centric to acentric pattern. To changing from -Cl to -CH3 in anion, while the coVnietwriAbrutictlieoOnnlionef
DOI: 10.1039/D0TC00408A
confirm our analysis about contacts changing, Hirsfield surfaces carbon and carbon (C-C) to all contacts decreased by the
were respectively generated for OHI-CBS and OHI-T based on replacement of -Cl (8.4 %, OHI-CBS) to -CH3 (5.7 %, OHI-T) in
OHI cation (Figure S3). As displayed in filtered fingerprint plots anions, which clearly shows that electronegativity changes of
(Figure 3), contribution of oxygen and hydrogen (O-H) to all atoms on the same site of anion leads to obvious changes of
contacts increased from 17.5 % (OHI-CBS) to (18.8 %) by contacts distribution in the crystal structure.
Figure 2. Crystal packing diagram of OHI-CBS and OHI-T. (a) alignment of cations and anions viewed along [100] direction in OHI-CBS; (b) layer-by-layer stacking in OHI-CBS with face-
to-face π-π interactions; (c) alignment of cations and anions viewed along [101] direction in OHI-T; (d) layer-by-layer stacking in OHI-T with face-to-face π-π interactions. The light
blue dot lines represent hydrogen bonds.
method. The phase purity was confirmed by powder X-ray
diffraction (XRD) (Figure 4a and Figure S4). The observed XRD
diffraction pattern of OHI-T crystallized from methanol is
consistent with with the simulated one, suggesting the absence
of impurity. XRD pattern of OHI-T grown from methanol/H2O
(1:1, v/v) solvent is totally the same, evidencing no
centrosymmetric hydrate, which is different from benchmark
pyridinium-based derivatives DAST and DSTMS crystals
suffering from centrosymmetric hydrate.
Thermal stability of the two crystals characterized by
differential thermal analysis (DTA) and thermal gravimetric
analysis (TGA) measurements (Figure 4b and S5). As shown in
Figure 4b, the heating curve clearly exhibits sharp endothermic
peaks at 245 ℃ (Tm), which is higher melting temperature than
that of benchmark crystals of OH1 (212 ℃) and comparable to
DAST (256 ℃ ).9,10 OHI-T also exhibits a high weight-loss
temperature Ti of 278 ℃, higher than that of DAST (260 ℃).
Thus, OHI-T exhibits high physicochemical stability and
advantages for stable high-power terahertz output.
Figure 3. Fingerprint plots for O-H and C-C intermolecular interactions of OHI-CBS and
OHI-T.
Optical Measurements. Transmittance spectra (Figure 4c)
demonstrates that OHI-T crystal possesses wider transparent
Phase and Thermal Stability. Single crystals of the indolium-
based crystals were obtained through slow evaporation
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region (504 - 2100 nm). Therefore, OHI-T holds large bandgap nm,44 which is comparable to that of quinolinium-VbieawseAdrticOleHOQnli-nTe
10,17
DOI: 10.1039/D0TC00408A
of 2.47 eV and higher anti-laser damage threshold (λmax = 413 nm) and benchmark OH1 (λmax = 417 nm),
theoretically.43 On the other hand, the microscopic nonlinearity evincing sufficient first-order hyperpolarizability (β) in OHI-T for
is positively dependent on the maximum wavelength λmax of microscopic nonlinearity, due to the presence of strong
molecule absorption. The λmax of OHI cation in acetonitrile indolium electron acceptor and strong hydroxyl electron donor
solution (Figure S6) is consistent with previously reported 424 as expected.
Figure 4. (a) Simulated (black), experimental (red) and methanol/H2O mixed solvents (blue) PXRD patterns. (b) TG and DTA curves of OHI-T crystals. (c) transmittance spectra of OHI-T
(0.2 mm thickness). (d) phase-matching curve of OHI-T crystal, insert: powder (224-250 µm) second harmonic generation for OHI-T and OH1 at 2090 nm.
Nonlinear Optics Test. The SHG signals of OHI-T crystals were approximately 0.7 times that of OH1 in the size range from 224
collected using the Kurtz-Perry method under 2.09 µm - 250 µm. Such good performance benefited from near-perfect
fundamental laser with crushed OH1 crystals as reference. As spatial alignment of indolium cations and demonstrated
shown in Figure 4d, the SHG response of OHI-T gradually indolium cation is indeed a good building block for organic NLO
improves with the increasing particle size, indicating phase crystal.
matching behavior of OHI-T. Moreover, the SHG intensity is
Figure 5 (a) OHI-T crystal THz prototype device; (b) rocking curves of OHI-T prototype device; (c) tuning THz generation in OHI-T crystal (0.8 mm thickness) and OH1 crystal (1.32 mm
thickness); (d)input-output relationship of DFG THz process in OHI-T crystal under different pumping power.
Terahertz-wave Generation. For further evaluation of the compact THz generator device with 3mm aperture (Figure 5a).
practical THz coherent generation capacity, we fabricated a A b-plate of OHI-T crystal with thickness of 0.8 mm was utilized.
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1
2
3
M. Tonouchi, Nat. Photonics, 2007, 1, 97-105.
The full width at half maximum (FWHM) of rocking curve is only
144 arc sec, indicating enough high optical quality of OHI-T
crystal (Figure 5b). Besides, because thicker OH1 crystal
corresponds to higher output energy and wider tuning range,
not same thickness but 1.32 mm OH1 prototype device was
prepared as reference.45 Pumped by a dual-wavelength of 1.36
- 1.5 μm laser (9.0 mJ/pulse, 10 ns) (Figure S2), efficient
ultrawideband tuning range of 0.1 - 20 THz was successfully
achieved via type-0 phase-matched DFG technique, and the
maximum THz pulse energy reached 25.3 nJ/pulse at 11 THz
(Figure 5c). The conversion efficiency reached 2.8×10-6, and
there was no obvious damage after highest 9 mJ pumping. Such
widely tunable monochromic THz output performance can be
comparable with commercial organic NLO crystals of DAST (0.3
- 19.6 THz), DSTMS (0.88 - 19.27 THz), OH1 (0.02 - 20 THz).45-47
The input-output energy of OHI-T measured at 4.8 THz, 6.15 THz
and 11 THz generation are well linear fitted (Figure 5d),
corresponding to the characteristic of a second-order nonlinear
process. Therefore, OHI-T can be seen a promising THz NLO
crystal candidate for practical terahertz generator. In-depth
study of higher THz output power and conversion efficiency
application can be expected in future experiments via
optimizing the crystal quality and the experimental parameters
(e.g. pump wavelength, phase matching condition).
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Conclusions
In summary, a new acentric indolium-based crystal OHI-T for
terahertz region application has been successfully developed by
rational control between hydrogen bonds and face-to-face π-π
interactions. OHI-T crystal exhibited a large SHG response of 0.7
times of benchmark OH1 crystal benefitting from synergistic
effect of enhanced first-order hyperpolarizability and
advantageous spatial arrangements. In addition, it also
possessed excellent thermal stability and formed no
centrosymmetric hydrate. For the first time, highly-efficient
tunable monochromatic terahertz wave was obtained in
indolium-based crystal system. The discovery of OHI-T not only
provides a new promising core NLO crystal candidate for
practical THz generation, but also confirms that slightly
regulating in molecular interactions is of great significance to
designing promising optoelectronic functional crystal materials
with desired performances.
Conflicts of interest
There are no conflicts to declare.
24 J. Shi, F. Liang, Y. He, X. Zhang, Z. Lin, D. Xu, Z. Hu, J. Yao, Y.
Wu, Chem. Commun., 2019, 55, 7950-7953.
25 U. Venkataramudu, C. Sahoo, S. Leelashree, M. Venkatesh, D.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 51702232, 51890864,
61771332).
27 J. Kim, O-P. Kwon, F. D. J. Brunner, M. Jazbinsek, S-H. Lee,
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Table of Contents
Acentric indolium-based OHI-T (C26H27NO4S) was obtained by rational design, and highly
efficient terahertz-wave generation was realized through OHI-T single crystal.