Engineering of Organic Chromophores with Large Second-Order

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Engineering of Organic Chromophores with Large SecondOrder Optical Nonlinearity and Superior Crystal Growth Ability Hong Hong Chen, Qi Ma, Yuqiao Zhou, Zhou Yang, Mojca Jazbinsek, Yongzhong Bian, Ning Ye, Dong Wang, Hui Cao, and Wanli He Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01216 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 17, 2015

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Crystal Growth & Design

Cover Page Engineering of Organic Chromophores with Large Second-Order Optical Nonlinearity and Superior Crystal Growth Ability Honghong Chena, Qi Maa, Yuqiao Zhoub, Zhou Yanga*, Mojca Jazbinsekc, Yongzhong Biana, Ning Yeb, Dong Wanga, Hui Caoa, and Wanli Hea a Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, PR China c Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8401 Winterthur, Switzerland

Abstract: We present two D-π-A organic cationic core structures leading to highly efficient nonlinear optical (NLO) salts (E)-2-(4(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo [e] indol-3-ium iodide (P-BI) and (E)-2-(2-(5(dimethylamino) thiophen -2-yl) vinyl) -1,1,3 trimethyl-1H -benzo [e] indol-3-ium iodide (S-BI ). Single crystals of the above two materials were successfully obtained by slow evaporation method. Two different polymorphic crystals of both P-BI and S-BI were obtained from different polar solvents. Kurtz powder tests revealed that the maximum second harmonic generation (SHG) efficiency of P-BI with monoclinic space group P21 is 1.14 times of the benchmark DAST( 4-N,N-dimethylamino-4,-N,-methylstilbazolium tosylate ). Bulk single crystals of P-BI were obtained with size of up to 17.0 × 6.0 × 2.0 mm3 without using seed crystals. This demonstrates that this material exhibits great crystal growth ability along with a high second-order optical nonlinearity, making it a very attractive candidate for NLO applications such as electro-optics and THz-wave generation. Corresponding Author: Prof . Zhou Yang Department of Materials Physics and Chemistry, School of Materials Science and Engineering University of Science and Technology Beijing, Beijing, 100083, P. R. China E-mail: [email protected] Tel: +86 10 62333759

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Title Page Engineering of Organic Chromophores with Large Second-Order Optical Nonlinearity and Superior Crystal Growth Ability Honghong Chena, Qi Maa, Yuqiao Zhoub, Zhou Yanga*, Mojca Jazbinsekc, Yongzhong Biana, Ning Yeb, Dong Wanga, Hui Caoa, and Wanli Hea a Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, PR China c Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8401 Winterthur, Switzerland Corresponding Author: Prof . Zhou Yang E-mail: [email protected]


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Abstract:We present two D-π-A organic cationic core structures leading to highly efficient nonlinear

optical

(NLO)

salts

(E)-2-(4-

(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo [e] indol-3-ium iodide (P-BI) and (E)-2-(2-(5- (dimethylamino) thiophen -2-yl) vinyl) -1,1,3 trimethyl-1H -benzo [e] indol-3-ium iodide (S-BI ). Single crystals of the above two materials were successfully obtained by slow evaporation method. Two different polymorphic crystals of both P-BI and S-BI were obtained from different polar solvents. Kurtz powder tests revealed that the maximum second harmonic generation (SHG) efficiency of P-BI with monoclinic space group

P21

is

1.14

times

of

the

benchmark

DAST(

4-N,N-dimethylamino-4,-N,-

methylstilbazolium tosylate ). Bulk single crystals of P-BI were obtained with size of up to 17.0 × 6.0 × 2.0 mm3 without using seed crystals. This demonstrates that this material exhibits great crystal growth ability along with a high second-order optical nonlinearity, making it a very attractive candidate for NLO applications such as electro-optics and THz-wave generation.

INTRODUCTION There has been considerable interest in organic nonlinear optical (NLO) materials, especially D-π-A chromophores, with large second-order optical nonlinearities due to their attractive potential applications in optical frequency conversion, integrated photonics, high-speed information processing, and THz wave generation and detection.1-4 A typical organic push− pull chromophore consists of an electron donor and electron acceptor connected by a πconjugated spacer.The main merits of molecular organic materials compared with inorganic ones are their large nonlinear optical properties, ultrafast response times and almost unlimited design


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possibilities.5,6 Among the various classes of organic materials investigated worldwide, ionic organic crystals are of particular interest due to their large molecular nonlinearity, long-term thermal and photochemical stability combined with a high chromophore number density, and a higher tendency to override the dipole-dipole interactions and thus form noncentrosymetric macroscopic packing.1-4,7-11 From these materials, the most well-known example is the benchmark DAST ( 4-N,N-dimethylamino-4,-N,-methylstilbazolium tosylate ) crystal with powder second harmonic generation (SHG) efficiency 3 orders of magnitude larger than that of urea at 1907 nm, a large diagonal NLO susceptibility χ(2)11 of 420 pm V-1 at 1907 nm and an electro-optic coefficient r11 of 77 pm V-1 at 800 nm.4,12-14 Single crystalline χ(2)-active organic materials have been shown particularly beneficial for THz-wave generation and detection, where they have already entered the commercial market ( www.rainbowphotonics.com). This is due to their high nonlinearity combined with unique dielectric dispersion characteristics, important for phase matching, allowing for very broadband and efficient THz-wave generation, not possible with inorganic photoconductive antennas or inorganic NLO crystals.1,3,15 However, THz-wave applications are presently still limited by the selection of commercially available materials, which comprise of DAST, its analogue DSTMS7 and a molecular crystal OH113, which considerably limits the range of possible pump laser wavelengths.15,16 This is because only few highly nonlinear organic materials can be grown into bulk crystals large enough for THz-wave applications. For example, the growth of bulk, high-quality DAST crystals from solution, requires extremely strict temperature stabilities within ±0.002 oC over several weeks.17 Therefore it is desirable to develop new organic materials with large nonlinear optical properties and different molecular packing allowing for phase matching at different pump wavelengths, combined with improved crystal growth characteristics compared with those of available crystals.


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In many cases, a small variation of the chemical structure of the constituent molecules is accompanied with a large change of molecular ordering in the crystalline state and may lead to a substantial change of the macroscopic physical properties.18-19 In the past decades, much effort has been spent in the design of new molecules with various degrees of achievements.7-11, 20-22 On one hand, it has already been demonstrated that varying the counter-ion to optimize the crystal packing is an effective molecular engineering strategy to develop ionic organic crystals with noncentrosymmetric structure, and some significative conclusions about this aspect have been achieved.7,

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On the other hand, it is a well-established fact that the conjugation length and

donor/acceptor strength of these D-π-A type push-pull chromophore molecules dramatically influence their second order nonlinear optical responses.1,2 However, finding the optimal combination of donor/acceptor still remains one of the most critical challenges, in particular because most of the highly nonlinear chromophores will lead to centrosymmetric crystal structures with vanishing χ(2) nonlinearity. 24 In our previous work, a series of stilbazolium derivatives were synthesized successfully, such as DSNS (4-N,N-dimethylamino-4， -N， -methyl-stilbazolium 2-naphthalenesulfonate) with 50% higher second-order powder-test efficiency compared with DAST at 1907 nm.20 Meanwhile, some heterocyclic rings were introduced into donor and/or acceptor to further improve molecular nonlinearities. Thienyl-substituted pyridinium derivatives introduced the five membered thiophene ring, providing one of the most effective electron delocalization pathway and less aromatic resonance energy, which may promote chromophores with excellent molecular hyperpolarizabilities.4, 25 In this work, to further break the molecular symmetry which should promote non-centrosymmetric packing1, the heterocyclic benzo [e] indol was introduced to substitute the generally used pyridinium as the electron acceptor in the conjugated


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chromophores. Thienyl-substituted benzo [e] indol iodide (S-BI) and phenyl-substituted benzo [e] indol iodide (P-BI) were synthesized, expecting new crystalline materials with high secondorder optical nonlinearities, different crystalline packing for expanding the range of THz-wave applications and better crystal growth properties. Both of the above mentioned salts exhibit powder second harmonic generation (SHG) activity and the maximum SHG efficiency of P-BI is 1.14 times of DAST. Single-crystal structure analysis reveals that the molecules in P-BI make an angle of about 45 degrees to the polar axis, which together with a significant red shift in the absorption spectrum (> 70 nm) promises different phase matching characteristics for THz-wave generation compared to DAST, DSTMS and OH1, where this angle is between 20 and 28 degrees. Bulk single crystals of P-BI with the dimensions of up to 17.0×6.0×2.0 mm3 have been obtained, proving that P-BI salt has excellent crystal growth ability.

EXPERIMENTAL SECTION Materials and Instrument. All reagents were purchased as high purity from J&K and used without further purification. 1H-NMR spectra were recorded on a Bruker 500 MHz spectrometer in DMSO-d6 solutions. The MS spectra were obtained on FT-ICR-MS spectrometer (ESI) in Institute of Chemistry, Chinese Academy of Sciences. Elemental analyses were performed by Element Analyzer (Vario EL III) in Tsinghua University. UV-vis spectra were recorded by a UV/VIS/NIR spectrophotometer (JASCO V-570). Thermal analyses were conducted on a PerkineElmer Pyris 6 DSC spectrometer at a heating rate of 10 oC min-1 under a dry nitrogen purge. Synthesis of P-BI and S-BI. (E)-2-(4-(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo [e] indol -3-ium- iodide (P-BI)26 and (E)-2-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1,1,3-


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trimethyl-1H-benzo [e] indol -3-ium-iodide (S-BI) were synthesized by a condensation reaction of 1,1,2,3-tetramethyl-1H-benzo [e] indol-3-ium-iodide (BI) with the respective 4-N,Ndimethyiamino-benzaldehyde and 5-(dimethylamino)thiophene-2-carbaldehyde which was obtained by previous work4 in the presence of piperidine. 1,1,2,3-tetramethyl-1H-benzo [e] indol-3-ium-iodide (BI) was synthesized by 1,1,2-trimethyl-1H- benzo [e] indol and iodomethane (Scheme 1). Purification was effected by recrystallizing at least three times from methanol. The products were dried in vacuum for 48 h to remove solvents effectively before their characterization.

Scheme 1. Chemical structures and synthetic routes of S-BI and P-BI chromophores. P-BI. 1H NMR (DMSO-d6, 500 MHz): δ= 8.39 (d, 1H, J=16.0 Hz, C10H6), 8.36 (d, 1H, J=8.5 Hz, C10H6), 8.22 (d, 1H, J=9.0 Hz, C10H6), 8.16 (d, 1H, J=8.0 Hz, C10H6), 8.10 (d, 1H, J=8.5 Hz, C10H6), 7.98 (d, 1H, J=8.5 Hz, C2H2), 7.75 (t, 1H, C10H6), 7.64 (t, 1H, C10H6), 7.30 (d, 1H, J=15.5 Hz, C2H2), 6.89 (d, 1H, J=9.0 Hz, C6H6), 4.10 (s, 3H, CH3), 3.17 (s, 6H, CH3), 1.99 (s, 6H, CH3) ppm. MS, m/z: 355.3 (M+). Elemental Analysis: Calcd (%) for C25H27IN2: C, 62.24; H, 5.60; N, 5.81; found: C, 62.08; H, 5.79; N, 5.75. S-BI. 1H NMR (DMSO-d6, 300 MHz): δ= 8.44 (d, 1H, J=11.1 Hz, C2H2), 8.24 (d, 1H, J=6.6 Hz, C10H6), 8.05 (t, 3H, C10H6+ C4H2S), 7.75 (d, 1H, J=6.6 Hz, C10H6), 7.64 (t, 1H, C10H6), 7.49 (t, 1H, C10H6), 6.71 (s, 1H, C4H2S), 6.11 (d, 1H, J=10.5 Hz, C2H2), 3.77 (s, 3H, CH3), 3.33 (s, 6H, C2H6N), 1.92 (s, 6H, CH3) ppm. MS, m/z: 361.2 (M+). Elemental Analysis: Calcd (%) for


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C23H27IN2OS•1/2 H2O: C, 55.53; H, 5.23; N, 5.63; S, 6.44; found: C, 55.36; H, 5.27; N, 5.52; S, 6.43. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction studies were performed using a TTR III diffractometer (Cu Kα radiation, λ=1.54056 Å) .The instrument is equipped with an Xray tube containing a copper anticathode (20-60 kV, 10-300 mA), mounted with an angular detector and a Graphite crystal monochromator. The scan speed was set to 10°/min over an angular range 5°~45° in 2θ. Single-Crystal X-ray Diffraction (SC-XRD). Data sets were collected on an Oxford Diffraction Gemini E diffractometer with Mo-Kα (λ=0.70713 Å) at 293 K. The structures were solved by the direct method (SHELXS-97) and refined by full matrix least-squares (SHELXL-97) on F2. Anisotropic thermal parameters were used for the nonhydrogen atoms and isotropic parameters for the hydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model. Crystals of S-BI-1 and S-BI-2 were obtained by growth of S-BI salt in methanol and ethanol respectively, while P-BI-1 and P-BI-2 were obtained by growth of P-BI salt in methanol and DMSO solvents respectively.The crystallographic data of all crystals have been deposited in the Cambridge Crystallographic Data centre. Powder SHG Measurements. The second harmonic generation (SHG) powder tests were carried out as described previously9. The microcrystalline powdered samples were prepared by sieving the material to a particle size of 63-90 µm and squeezing it into a 1.00 mm Hellma UV quartz sample cell to give a constant sample thickness. The SHG signals were calibrated with respect to equally prepared DAST samples. An idler wave with a wavelength 1907 nm from an optical parametrical amplifier pumped by an amplified Ti:sapphire laser was used as fundamental wave to determine the nonresonant SHG efficiency.


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RESULTS AND DISCUSSION Design and Synthesis. The chemical structure and the synthetic route of the new benzo [e] indol chromophores S-BI and P-BI are shown in Scheme 1 and details are given in Experimental Section. The synthesis of the P-BI salt has been reported previously as a part of the general research on the syntheses of organic dyes,26 while S-BI was newly synthesized in this work. The new chromophores were designed based on the high-nonlinearity stilbazolium DAST chromophore and the thienyl-substituted pyridinium chromphore4,25 by introducing the heterocyclic benzo [e] indol to substitute the generally used pyridinium as the electron acceptor. On the one hand, this is expected to increase the electron acceptor strength, and therefore increase the molecular nonlinearity. On the other hand, the bulkier benzo [e] indol increases the molecular

asymmetry,

which

should

promote

non-centrosymmetric

packing

of

the

chromophores.1 The non-centrosymmetric crystalline packing is in particular challenging for highly nonlinear chromophores and there were only a very few reports on chromophores with a higher molecular nonlinearity than that of the stilbazolium DAST chromophore actually resulting in a non-centrosymmetric packing.27 As counter ions that do not significantly contribute to molecular nonlinearity, but support the ionic molecular packing, we here chose iodine. Note that a wider range of new promising materials can be possibly obtained in future based on benzo [e] indol chromophores by using the strategy of varying the counter ion, as demonstrated for stilbazolium and other salts.7,23 Physical Properties. The melting point, the wavelength of maximum absorption λmax in methanol solution, and powder SHG efficiency of four different crystals obtained (S-BI-1, S-BI2, P-BI-1 and P-BI-2) are listed in Table 1 and compared to DAST. Crystals of S-BI-1 and S-BI2 were obtained by growth of S-BI salt in methanol and ethanol respectively, P-BI-1 and P-BI-2


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were obtained by growth of P-BI salt in methanol and DMSO solvents respectively. All of the above four crystals were obtained by slow evaporation crystal growth method. Thermal properties are important for a reliable crystal growth and stability of crystals. From Table 1 we can see that the melting points of four crystals are all above 250°C, indicating a very good thermal stability. The melting point of S-BI with the thienyl group is higher than that of P-BI with the phenyl group. UV-vis absorption spectra of all compounds were recorded in methanol solution. It can be seen from Figure 1 that the wavelength of maximal absorption λmax of P-BI and S-BI is markedly red-shifted by about 76 and 102 nm compared to DAST, respectively, which is due to the introduction of benzo [e] indol-substituted acceptor. Compared to chromophore P-BI, there is an increase in the electron-donating strength of the amino substituted fragment when the thienyl group is introduced into S-BI, thus resulting in red shift of λmax by 26 nm. According to the well-known nonlinearity-transparency tradeoff, the red-shift of λmax is usually accompanied by a higher molecular nonlinearity of such donor-acceptor substituted conjugated molecules,1 so we expect a higher optical nonlinearity of the new chromophores compared to the DAST chromophore.

Figure 1. Absorption spectra of S-BI, P-BI and DAST in methanol. Table 1. Physical properties of investigated compounds


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Melting

λmaxa

Powder

point (oC)

(nm)

SHGb

S-BI-1

>300

576

0

S-BI-2

>300

576

0.23

P-BI-1

263±1

550

1.14

P-BI-2

260±1

550

0.61

DAST

256±1

474

1

Compound

a

In methanol at room temperature.

b

1907 nm fundamental radiation, relative to DAST, with an experimental error of less than 10%.

Crystal Growth. After several experimental trials, the slow evaporation technique at room temperature in methanol was found to be the best method for growing these compounds. It has already been investigated that the polarity of the solvent can affect the crystal growth characteristics for π-conjugated crystals.28 In this work, we chose single polar nonaromatic solvents methanol and DMSO to grow P-BI crystals. It was found that the crystals grown in different polar solvents exhibit different shapes. Crystals tended to grow into a block-like shape preferentially along all edges in all directions from solvent methanol. In DMSO solvent, crystals have the trend of growing into a sheet-like shape with good transmittance. These bulk crystals grown by methanol or DMSO solvents still contained defects as a cost of fast growth. However, crystals with sizes of less than 1mm exhibit perfect optical quality in most cases. The X-ray structure analysis has shown that P-BI crystals from methanol and DMSO do not only show a


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different morphology, but also have a different crystal structure. We denote the polymorph from methanol as P-BI-1 and the polymorph from DMSO as P-BI-2. The P-BI-2 polymoph incorporates DMSO solvent in the structure (P-BI·DMSO). Both crystal structures are noncentrosymmetric and therefore interesting for χ(2) nonlinear optical applications. The slow evaporation technique at room temperature in methanol was found to be the best method also for growing S-BI. However, it was found that S-BI easily forms a hydrated phase SBI·H2O in methanol (S-BI-1) with the centrosymmetric structure P21/c. Other solvents like dichloromethane, DMF, acetonitrile and ethanol were also tried. The result was that the crystalline powder of S-BI grown from ethanol (S-BI-2) exhibits SHG efficiency of 0.23 times of DAST, meaning that the crystal structure of S-BI-2 is non-centrosymmetric. Unfortunately, we could not obtain the crystallographic data from crystalline powder of S-BI-2 after several attempts due to its small particle size. Considering the superior crystal growth ability as well as the superior nonlinear optical properties as described later on, the crystal growth of P-BI-1 by the slow evaporation method has been investigated in more detail. It is essential to stir the solution for 12 h and filter the saturated solution by using 0.2 mm porosity Millipore filters at room temperature to ensure that the salt is sufficiently purified. This is because even a small amount of impurities can act as undesired additives inhibiting or accelerating selective intermolecular interactions. In order to prevent the interference of dust in air, the purified saturated solution was placed in an open beaker wrapped with a cling film having a certain number of holes for evaporating solution, and the beaker was put in a quiet, vibration-free environment and covered with a glass cover. Nevertheless, the resulting saturated solution (0.8 g solute/100 mL methanol at room temperature) was found to form small nuclei too fast to grow bulk crystals. We therefore propose using an unsaturated


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solution environment at the start of the experiment, which is expected to decrease the rate of nucleation. To 100 mL of saturated solution at room temperature, 10, 20, 30, 40, 50 or 60 mL methanol was added and bulk single crystals with dimensions of up to 17.0 ×6.0 ×2.0 mm3 (Figure 2(a)) were finally obtained starting with the undersaturated methanol solution with volume ratios (pure methanol/saturated solution) between 0.1 and 0.3 by slow evaporation method in a period of more than ten days till the whole solution was evaporated.

Figure 2. Single crystals of P-BI-1(a) and P-BI-2( b). The optical quality of the crystals P-BI-1 was investigated by placing them between crossed polarizers in a microscope (Figure 3). They turned completely dark when rotating the crystal by 45o around the direction of light propagation from the orientation with the highest transmission, which means that in this case the main axis of the optical indicatrix (or its projection to the plane of the crystal plate) was aligned parallel to the polarization of the transmitted light, and also indicates that the crystals are single crystalline. Contrary to DAST, large bulk P-BI crystals could be easily obtained without a seed given the improved growth characteristics of the latter, so larger single crystals are expected to attain in subsequent studies.


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Figure 3. Crystal of P-BI-1 between crossed polarizers in a microscope at the position of the maximum transmission (a) and rotated by 45o from this position (b).

Figure 4. Powder X-ray diffraction patterns in the 2θ range from 5° to 45° for crystals P-BI-1, P-BI-2 and S-BI-2. Table 2. Crystallographic data of S-BI-1, P-BI-1 and P-BI-2 crystals. Compound

S-BI-1

P-BI-1

P-BI-2

Formula

C23H25IN2S•H2O

C25H27IN2

C25H27IN2•C2H6OS

Formula weight

506.43

482.39

596.63

Crystal system

Monoclinic

Monoclinic

Monoclinic

Space group

P21/c

P21

P21

a/Å

11.9049(8)

7.3145(12)

12.3912（ 15）

b/Å

12.9122(9)

12.6040(14)

7.2160（ 8）

c/Å

14.2096(16)

12.3506(19)

16.781 （ 2）


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α/Å

90.00

90.00

90.00

β/Å

92.885(10)

106.747(16)

92.145（ 11）

γ/Å

90.00

90.00

90.00

V/Å3

2181.51

1090.33

1499.4

Z

4

2

2

T/K

293

293

293

WR2

0.1527

0.0568

0.1813

R1

0.0632

0.0260

0.0591

Polymorphism Study of the Grown Crystals. Powder X-ray diffraction analyses were performed on acentric S-BI-2, P-BI-1 and P-BI-2 crystals in the 2θ range from 5°to 45°at room temperature (Figure 4). As shown in Figure 4, the X-ray diffraction patterns for crystals P-BI-1 and P-BI-2 exhibited a certain difference. The crystallographic structure of S-BI-1, P-BI-1 and PBI-2 was measured by single crystal X-ray analysis. The data obtained is listed in Table 2. Crystallographic data for S-BI-1, P-BI-1 and P-BI-2 (CCDC 1063290, 1063291, 1400173) have been deposited with the Cambridge Crystallographic Data Centre. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Tel: +44 (0)1223 336408; Fax: (internat.)+44 (0)1223/336 033. The structure of one ion pair and the packing diagram of these three crystals are shown in Figure 5 and Figure 6, respectively. Note that the structures of S-BI-1 and P-BI-2 contain one solvent molecule per ion pair, water and DMSO respectively. Since the arrangement of molecular dipoles in the crystal is responsible for the magnitude of the second-order susceptibility tensor χ(2), the crystal packing analysis for the three structures is important for understanding their second-order NLO properties in the crystalline state.


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In all cases, the P-BI and S-BI cations virtually agree with normal bond lengths and bond angles as expected for aromatic compounds. We note that in the five member heterocycle including nitrogen atom, in the case of P-BI-1 for example (Figure 5(b), Table 3), the C(10)C(11) and C(13)-N(2) distances are equal to 1.38(1) Å and 1.335(8) Å, respectively, whereas C(9)-C(10) ,C(9)-C(13) and C(11)-N(2) are equal to 1.499(9) Å, 1.53(1) Å and 1.40(1) Å, respectively. This indicates that the double bonds are localized in the C(10)-C(11) and C(13)N(2) bonds, which may be due to analogous quinoidal resonance in the five member ring. The same case also applies to P-BI-2 and S-BI-1. In P-BI-1 and S-BI-1 crystals, the range of torsional angles between N, N-dimethylamino group and benzene ring, benzene ring and benzo [e] indol ring, benzo [e] indol itself are from 1.27o to 5.50o. Therefore, the cations in above crystals are nearly perfectly planar. By contrast, particular large twists between each group are observed for crystal P-BI-2 from 4.11o to 12.59o. P-BI-1 and P-BI-2 are polymorphs which were grown by PBI salt from methanol and DMSO solution respectively by the slow evaporation method. The above analysis indicates that the polarity characteristics of solvents affect the arrangement of molecules in a certain extent, which leads to polymorphism of single crystals, and the different molecular ordering in the crystalline state further influences the morphology of grown crystals. Table 3. Selected Structural Parameters (Å,°) of the three crystals (numbering as in Figure 5) S-BI-1

P-BI-1

P-BI-2

C10-C9 1.36(1)

C10-C11 1.38(1)

C8-C9 1.40(2)

C10-C11 1.51(1)

C10-C9 1.499(9)

C9-C11 1.51(2)

C9-N2 1.393(9)

C11-N2 1.40(1)

C8-N1 1.53(2)

N2-C14 1.34(1)

N2-C13 1.335(8)

C14-N1 1.25(2)


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N2-C15 1.46(1)

N2-C015 1.45(1)

C25-N1 1.43(2)

C11-C14 1.53(1)

C9-C13 1.53(1)

C11-C14 1.56(2)

C21-N1 1.321(9)

C3-N1 1.358(8)

C20-N2 1.35(2)

C9-N2-C14 111.7(6)

C11-N2-C13 112.1(6)

C8-N1-C14 110(2)

C21-N1-C23 120.1(7)

C3-N1-C14 120.1(6)

C20-N2-C24 123(2)

C21-N1-C22 121.0(7)

C3-N1-C15 120.7(6)

C20-N2-C23 122(2)

C22-N1-C23 118.8(7)

C14-N1-C15 119.2(6)

C23-N2-C24 110(2)

C22-N1-C23/C21-C19-S1 1.27

C14-N1-C15/C1-C3-C5 4.77

C23-N2-C24/C22-C20-C18 12.59

C21-C19-S1/N2-C14-C10 4.35

C1-C3-C5/N2-C13-C10 5.50

C22-C20-C18/N1-C14-C9 4.11

N2-C14-C10/C2-C4-C6 4.74

N2-C13-C10/C18-C20-C22 4.10

N1-C14-C9/C5-C1-C3


5.49

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Figure 5. Molecular geometries of S-BI-1 (a), P-BI-1 (b), and P-BI-2 (c). Hydrogen atoms are omitted for clarity.


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Figure 6. Crystal packing diagram of S-BI-1 (a), P-BI -1 (b), and P-BI-2 (c) projected along the crystallographic axis c, a and a, respectively. The illustrations of the lower left corner show the orientation of the main charge-transfer direction β0 of the chromophores in the unit cell. Angle θ denotes the angle between the polar axis b and the molecular charge-transfer axis for noncentrosymmetric P-BI-1 and P-BI-2 respectively. Second-Order Nonlinear Optical Properties. To achieve macroscopic second-order NLO functionality in crystals, the molecules should be packed in a non-centrosymmetric fashion. The crystal structures of P-BI-1 and P-BI-2 (Figure 6) both belong to the monoclinic space group P21 (point group 2) with the polar symmetry axis along the crystalline b-axis with the monoclinic angle β of 106.747° and 92.145° respectively. This particular structure (P21) has been for example described in great detail for the well-known methyl 2-((2,4-dinitrophenyl)amino) propanoate (MAP)29 and N-(4-nitrophenyl)-L-prolinol (NPP)30 by Zyss et al. In their unit cells, there are two ion pairs which are interchanged by a 2-fold rotation around the unique polar b axis. The adjacent two ion pairs are aligned almost perpendicular in P-BI-1 (Figure 6(b)) and nearly antiparallel in P-BI-2 (Figure 6(c)), which is influenced by the interaction between the different polar solvents and the solute molecules during the crystallization process. The macroscopic second-order optical nonlinearities described by the susceptibility tensor χ(2)IJK can be related to the microscopic first hyperpolarizabilities βijk using the oriented-gas model as29 (2) eff χ IJK = NFIJK β IJK

(1)

Where N is the number of chromophores in a unit volume, FIJK the correction factors due to intermolecular interactions, most often approximated by the local-field corrections29


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eff and β IJK the effective hyperpolarizability of the unit cell in the crystal, which depends on the

molecular packing as eff β IJK = ∑ cos θ (I , i )cos θ ( J , j )cos θ (K , k )β ijk

(2)

where the summation is performed over all molecules in the unit cell and the direction cosines cosθ (I, i ) represent the angle between the crystal axis I and the molecular axis i. In the case of “push-pull” D-π-A molecules as in this work, 1D chromophore approximation can be employed, in which only the component of the hyperpolarizability tensor

β ijk

along the main charge-transfer

axis of the molecule (β0) is large and all other components can be neglected. Like this, the only eff non-zero components of the effective hyperpolarizability tensor β IJK for the point-group crystal

symmetry 2 are eff 3 the diagonal component β 222 = β 0 cos θ

(3)

eff eff eff 2 the off-diagonal components β122 = β 212 = β 221 = β 0 cosθ sin θ

(4)

which is valid for dispersion-free coefficients (away from electronic resonances). θ is the angle between the polar axis of the crystal (b) and the main charge transfer axis of the molecule. The projection factor cos3θ of β0 to determine the diagonal coefficient is known as the order parameter of the crystal. For the well-known DAST, the long axis of the cations is tilted from the polar axis by 20°12, which leads to a large order parameter of the crystal, cos3θ=0.83. For P-BI chromophores, we assume that the main charge transfer direction is approximately the direction connecting the two nitrogen atoms N1-N2. The angle between the main charge-transfer direction of P-BI-1 cation and the crystallographic b axis is about 45°, while it is close to 90° in crystal P-


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BI-2 (see Figure 6). Angle θ for P-BI-1 is illustrated in Figure 6(b) (order parameter of the crystal is cos3θ=0.35). Using this oriented-gas model, if we only consider the number density of the molecules N and the effective order parameter of the crystal, which both are the most important factors in the above equations, the diagonal coefficient of P-BI-1 is expected to be about 41% of that of DAST and the off-diagonal coefficient 310 % of that of DAST. As already mentioned, the non-resonant hyperpolarizability β0 is additionally expected to be larger for the PBI chromophore compared to DAST, because of the significant red-shift of the absorption maximum.

(a)

(b)

Figure 7. (a) SHG intensity of S-BI -2 compared to DAST. (b) SHG intensity of P-BI-1 and PBI-2 compared to DAST. A simple way to confirm the validity of the above estimations is by measuring the secondharmonic generation efficiency of the crystalline powder under nonresonant conditions. The SHG signals were calibrated with respect to similarly prepared powdered DAST samples. By this method we obtained the SHG efficiency of P-BI-1 is around 1.14 times of the DAST and of PBI-2 of around 0.61 of DAST (Fig 7(b)). The powder SHG efficiency is proportional to


21


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( )

N 2 β eff

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2

, the squared number density of chromophores and the squared susceptibility

components spatially averaged considering the corresponding crystal symmetry.31,32 The number density of chromophores N is 1.83 nm-3, 1.33 nm-3 and 1.91 nm-3 for P-BI-1, P-BI-2 and DAST respectively. Like this, assuming the same β0, the expected powder-test efficiency30 is for P-BI-1 60 % that of DAST and for P-BI-2 less than 5 % that of DAST (considering θ > 80° for P-BI-2). The experimentally measured powder test efficiencies are considerably higher compared to the theoretically estimated values, in particular for the P-BI-2 compound. This can be on the one hand due to higher hyperpolarizability values of the new chromophores β0, as already expected from the absorption properties. On the other hand, different intermolecular interactions and possible phase-matching SHG enhancements can also influence the measured powder SHG result31. In any case, the SHG powder test measurements confirm that the new compounds are very attractive for NLO applications. From the X-ray crystal structure it follows (see Equation 3 and 4 with θ = 45°) that the offdiagonal components of χ(2) of the new crystal P-BI-1 are of the same order of magnitude as the diagonal component, while e.g. for DAST, DSTMS and OH1 the diagonal components are dominant. This is very interesting for applications, where both light polarization components should be modulated or included in the NLO processes. Additionally, for THz-wave generation the range of possible pump wavelengths in the optical range or generated THz frequencies can be extended if both polarization components can be efficiently utilized.15,16 For the crystal structure of S-BI-1 (Fig 6(a)), the space group is monoclinic P21/c with four ion pairs per unit cell, which are glided antiparallel in planes and thus form centrosymmetric alignment with a hydrated phase as S-BI·H2O. State-of-the-art crystal DAST in presence of small


22

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amounts of water also forms a hydrate polymorph with centrosymmetric structure thus without second-order NLO properties. This leads us to infer that if controlling conditions properly, noncentrosymmetric structure crystals of S-BI may also be obtained. Crystalline powder of S-BI2 obtained from ethanol as described above leads to a SHG efficiency 0.23 times of DAST, which confirmed our deduction (Figure 7(a)).

CONCLUSIONS In summary, two organic NLO chromophores for highly efficient organic NLO salts have been synthesized and systematically characterized. The noncentrosymmetric crystal structure of P-BI-1 with monoclinic space group P21 exhibits the maximum second harmonic generation (SHG) efficiency, which is higher than for the state-of-the-art DAST crystal. However, crystal SBI-1 obtained from methanol solution shows centrosymmetric structure with monoclinic space group P21/c. P-BI shows a much higher solubility in methanol than S-BI and much better crystal growth characteristics. Bulk single crystals of P-BI with dimensions of up to 17.0×6.0×2.0 mm3 were easily obtained without using seed crystals and crystals with sizes of less than 1 mm exhibit good optical quality. Furthermore, two different polymorphic crystals P-BI-1 and P-BI-2 were obtained by P-BI salt growing in different solvents. The grown P-BI-1 crystals exhibit a blocklike morphology in methanol, P-BI-2 crystals exhibit flake-like appearance in DMSO solvent. The formation of crystals S-BI-1, S-BI-2, P-BI-1 and P-BI-2 demonstrates that this kind of materials have the trend of crystallizing into polymorphic crystals with a variety of physical characteristics. The studies about the influence of the polarity of solvents on the polymorphism and morphology of ionic organic crystals are beneficial to fundamental research especially for organic π-conjugated materials. There is only a little difference in π-conjunction between the molecular structure of P-BI and S-BI but is accompanied with a large change of molecular


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ordering in the crystalline state and a substantial change of the macroscopic physical properties. These results demonstrate that the crystal packing of this type of salts is very sensitive to the nature of the molecule structure. In conclusion, P-BI-1 crystals are very promising materials for NLO applications, having not only a large macroscopic nonlinearity but also good crystal characteristics including the bulk crystal growth ability, solubility and high thermal stability. ASSOCIATED CONTENT ACKNOWLEDGMENT The authors thank Professor Zeda Xu for the technical support. This work was supported by the Major Project of International Cooperation of the Ministry of Science and Technology (Grant No. 2013DFB50340), the National Natural Science Foundation of China (Grant No. 51173017, 51373024, 51473020, 61370048), the Beijing Higher Education Young Elite Teacher Project (Grant No. YETP0356), Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-14-001A2， FRF-TP-14-013A2).

Supporting Information The X-ray crystallographic information files of P-BI-1, P-BI-2 and S-BI-1 as well as Figure S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.


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REFERENCES (1) Dalton, L. R.; Günter, P; Jazbinsek, M.; Kwon, O. P.; Sullivan, P. A. Organic Electro-Optics and Photonics: Molecules, Polymers and Crystals; Cambridge University Press, Cambridge UK,

2015. (2) Dalton, L. R.; Sullivan, P. A.; Bale, D.H. Chem. Rev. 2010,110, 25-55. (3) Jazbinsek, M.; .Günter, P. in Handbook of Organic Materials for Optical and (Opto)electronic Devices, ed. Handbook of Organic Materials for Optical and (Opto)electronic Devices, ed. Ostroverkhova, O. Woodhead Publishing Series in Electronic and Optical Materials, Cambridge UK, 2013, 39, 190-216. (4) Li, L.; Cui, H.; Yang, Z.; Tao, X.; Lin, X.; Ye, N.; Yang, H. CrystEngComm. 2012, 14,10311037. (5) Ahlheim, M. et al. Science. 1996, 271, 335-337. (6) Bosshard, C.; Spreiter, R.; Degiorgi, L.; Günter, P. Phys.Rev. B. 2002, 66, 205107. (7) Yang, Z.; Mutter, L.; Stillhart, M.; Ruiz, B.; Aravazhi, S.; Jazbinsek, M.; Schneider, A.; Gramlich, V.; Günter, P. Adv. Funct. Mater. 2007, 17, 2018-2023. (8) Yang, Z.; Wörle, M.; Mutter, L.; Jazbinsek, M.; Günter, P. Cryst. Growth Des. 2006, 7, 8386. (9) Sun S.; Liu, X.; Wang, X.; Li, L.; Shi, X.; Li, S.; Ji, C.; Luo, J.; Hong, M. Cryst. Growth Des.

2012, 12, 6181-6187. (10) Okada, S.; Oikawa, H.; Nakanishi, H. Chem. Mater. 2000, 12, 1162-1170.


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2012, 94, 120-126. (12) Jazbinsek, M.; Mutter, L.; Günter, P. IEEE J. Sel. Top. Quantum Electron. 2008, 14, 12981311. (13) Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Brunner, F. D. J.; Seo, J. I.; Hunziker, C.; Schneider, A.; Yun, H.; Lee, Y. S.; Günter, P. Adv. Funct. Mater. 2008, 18, 3242-3250. (14) Pan, F.; Knopfle, G.; Bosshard, C.; Follonier, S.; Spreiter, R.; Wong, M. S.; Günter, P. Appl. Phys. Lett. 1996, 69, 13-15. (15) Vicario, C.; Jazbinsek, M.; Ovchinnikov, A. V.; Chefonov, O. V.; Ashitkov, S. I.; Agranat, M. B.; Hauri, C. P. Opt. Express. 2015, 23, 4573-4580. (16) Schneider, A.; Neis, M.; Stillhart, M.; Ruiz, B.; Khan, R. U. A.; Günter, P. J. Opt. Soc. Am. B. 2006, 23,1822-1835. (17) Ruiz, B.; Jazbinsek, M.; Günter, P. Cryst. Growth Des. 2008, 8, 4173-4184. 18) Kwon, O.; Kwon, S.; Jazbinsek, M.; Seo, J.; Kim, J.; Seo, J.; Lee, Y.; Yun, H.; Günter, P. Chem. Mater. 2011, 23, 239-246. (19) Silva, P.; Cardoso, C.; Silva, M.; Paixäo, J.; Beja, A.; Garcia, M.; Lopes, N. J. Phys. Chem. A. 2010, 114, 2607-2617. (20) Ruiz, B.; Yang, Z.; Gramlich, V.; Jazbinsek, M.; Günter, P. J. Mater. Chem. 2006, 16, 28392842. (21) Ogawa, J.; Okada, S.; Glavcheva, Z.; Nakanishi, H. J. Cryst. Growth. 2008, 310, 836-842. (22) Wu, W.; Wu, D.; Cheng, W.; Zhang, H.; Dai, J. Cryst. Growth Des. 2007, 7, 2316-2323. (23) Yang, Z.; Aravazhi, S.; Schneider, A.; Seiler, P.; Jazbinsek, M.; Günter, P. Adv. Funct. Mater. 2005, 15, 1072-1076.


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(24) Xu, H.; Zhang, M.; Zhang, A.; Deng, G.; Si, P.; Huang, H.; Peng, C.; Fu, M.; Liu, J.; Qiu, L.; Zhen, Z.; Bo, S.; Liu, X. Dyes & Pigments. 2014, 102, 142–149. (25) Dai, Y.; Tan, J.; Ye, N.; Yang, Z. CrystEngComm, 2014, 4, 636-641. (26) Pazenok, S.; Kondratenko, N.; Popov, V.; Troitskaya, V.; II'chenko, A.; Al'perovich, M.; Yagupol'skii, L. Chem. Heterocycl. Compd., 1983, 19, 1182. (27) Coe, B. J.; Harris, J. A.; Asselberghs, I.; Wostyn, K.; Clays, K.; Persoons, A.; Brunschwig, B. S.; Coles, S. J.; Gelbrich, T.; Light, M. E.; Hursthouse, M. B.; Nakatani, K. Adv. Funct. Mater.

2003, 13,347. (28) Lee, S.; Koo, M.; Jazbinsek, M.; Kwon, O. Cryst. Growth Des. 2014, 14, 6024-6032. (29) Oudar, J.; Zyss, J. Phys. Rev. A. 1982, 26, 2016-2027. (30) Zyss, J.; Nicoud, J.; Coquillay, M. J. Chem. Phys. 1984, 81,4160-416. (31) Kurtz S. K.; Perry T. T. J. Appl. Phys. 1968, 39, 3798-3813. (32) In 1D-chromophore approximation, for point group symmetry 2 or m:

(β )

eff 2

= β 02 (19 cos 6 θ + 13 cos 4 θ sin 2 θ + 44 cos 2 θ sin 4 θ ) / 105


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Table of Contents Use Only

Two different polymorphic crystals of P-BI were obtained by growing in different polar solvents. The noncentrosymmetric crystal structure of P-BI-1 with monoclinic space group P21 exhibits large second-order optical nonlinearity, and bulk single crystals with dimensions of up to 17.0×6.0×2.0 mm3 were easily obtained without using seed crystals.


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Figure 1. Absorption spectra of S-BI, P-BI and DAST in methanol



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Figure 2. Single crystals of P-BI-1(a) and P-BI-2(b).


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Figure 3. Crystal of P-BI-1 between crossed polarizers in a microscope at the position of the maximum transmission (a) and rotated by 45o from this position (b).



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Figure 4. Powder X-ray diffraction patterns in the 2θ range from 5° to 45° for crystals P-BI-1, P-BI-2 and S-BI-2.


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Figure 5. Molecular geometries of S-BI-1 (a), P-BI-1 (b), and P-BI-2 (c). Hydrogen atoms are omitted for clarity.



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Figure 6. Crystal packing diagram of S-BI-1 (a), P-BI -1 (b), and P-BI-2 (c) projected along the crystallographic axis c, a and a, respectively. The illustrations of the lower left corner show the orientation of the main charge-transfer direction β0 of the chromophores in the unit cell. Angle θ denotes the angle between the polar axis b and the molecular charge-transfer axis for non-centrosymmetric P-BI-1 and P-BI-2 respectively.


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Figure 7. (a) SHG intensity of S-BI -2 compared to DAST. (b) SHG intensity of P-BI-1 and P-BI-2 compared to DAST.



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Abstract


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Scheme 1. Chemical structure and synthetic route of S-BI and P-BI chromophores.


Engineering of Organic Chromophores with Large Second-Order

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