Hydrogen Bonding Assisted Construction of Graphite-like Deep-UV

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Hydrogen Bonding Assisted Construction of Graphite-Like DeepUV Optical Materials with Two Types of Parallel #-conjugated Units Fangfang He, Qian Wang, Mengjiao Liu, Ling Huang, Daojiang Gao, Jian Bi, and Guohong Zou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00804 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Crystal Growth & Design

Hydrogen Bonding Assisted Construction of Graphite-Like Deep-UV Optical Materials with Two Types of Parallel π-conjugated Units Fangfang He, † Qian Wang, † Mengjiao Liu, † Ling Huang,* † Daojiang Gao, † Jian Bi, † and Guohong Zou*‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu,

610068, P. R. China. ‡

College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.

E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Three new formic-borates Na(COOH)[B(OH)3](H2O)2 (1), K3(COOH)3[B(OH)3]2 (2) and (HCOOH)3[B(OH)3]2·3H2O (3), containing two types planar π-conjugated BO3 and HCOO groups, have been synthesized via systematic investigations in A – HCOO –BO3 (A = alkali metal) systems using hydrothermal methods. Compounds 1 and 2 crystallize in the centrosymmetric space groups Pnma and C2/c, and compound 3 crystallizes in the non-centrosymmetric space groups Pn, respectively. All the three titled compounds exhibited 3-D frameworks with parallel arrangement of B(OH)3 and HCOO planar π-conjugated units through hydrogen bonding assisted construction. Particularly, compounds 2 and 3 showed graphite-like layer structure. In compound 1, the Na(COOH)[B(OH)3] chains are connected with water molecules by hydrogen bonding to form the 3-D framework. In 2, graphite-like {(COOH)3[B(OH)3]2}3- layers composed of HCOO and B(OH)3 groups construct the 3-D framework through the bridging of charge-balance cations K+ located at the interlayer space. Compound 3 exhibits 3-D framework through the lattice water molecules and graphite-like (HCOOH)3[B(OH)3]2 layers interconnecting with each other via hydrogen bonding. By further comparison, the size of Na+, K+ cations and water molecules brought the discrepancy and affected the framework structure and centricity of the three titled compounds. Second harmonic generation (SHG) studies indicated that compound 3 has moderate SHG responses of approximately 0.7 times that of KH2PO4 (KDP). The UV-vis diffuse reflectance spectroscopy of the powder samples indicated that the short-wavelength absorption edges of all three compounds were below 200 nm, and they are potential deep-UV optical materials.

Keywords: π-conjugated, graphite-like, formic- borate, deep-UV, nonlinear optical crystal.


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INTRODUCTION The development of deep-ultraviolet (DUV) nonlinear optical (NLO) crystals, critical materials for solid state lasers producing DUV coherent light (λ < 200 nm) through a cascaded frequency conversion, has had a profound influence on a variety of technologically important application fields such as angle resolved photoemission, high density storage, laser micromachining, and semiconductor photolithography.1-11 In the past few decades, chemists and materials scientists have made huge efforts to elucidate the inter-relationships between the structures and properties for exploring superior NLO materials. The anionic group theory12, 13 proposed by Chen et al. has achieved great success in the development of UV and DUV NLO borates, e. g. LiB3O5 (LBO),14 β-BaB2O4 (BBO),15 K2Al2B2O7(KABO),16 Li4Sr(BO3)2

17

and SrBe2BO7 (SBBO).18 Recently, the

research systems have been expanded to beryllium borates,7, phosphates,22, 23 carbonates,

24, 25

19

fluorooxoborates,20,

21

etc. and a series of new potential DUV NLO crystals have been

reported. But commercially available NLO crystals in the DUV region are still lacking. By far, KBe2BO3F2 (KBBF)26 is the sole NLO crystal to break down the “200 nm wall” for generating DUV coherent light by direct SHG response. Since KBBF possesses two fatal weaknesses: the high toxicity of beryllium and strong layer habit in crystal growth, its practical uses are limited. Thus, it is in urgent demand to explore novel beryllium-free DUV NLO materials. According to the anionic group theory, the planar π-conjugated anionic groups possessing the large microscopic second-order susceptibility and moderate birefringence are considered to be the best NLO basic structural units for designing and synthesizing UV and DUV materials, such as [B3O6]3-, [BO3]3-, [CO3]2-, [NO3]- and [C3N3O3].3,

27-31

Quite a few new NLO materials with

excellent NLO performance containing planar π-conjugated anionic groups have been successfully synthesized. In the beginning, the successful cases occured in borates, for instance, KABO, SBBO, and KBBF with [BO3]3- anionic groups, β-BaB2O4 with [B3O6]3-anionic groups. Then the research systems have been expanded to carbonates with [CO3]2- anionic groups which could be the alternatives for DUV application such as MNCO3F (M = K, Rb, Cs; N = Mg, Ca, Sr),32 Na8Lu2(CO3)6F2,33 and Na2Gd(CO3)2F334 etc. Recently, nitrate Sr2(OH)3NO335 with [NO3]- anionic groups also has been proved to be a promising DUV NLO material. Analogous to [B3O6]3-, [C3N3O3]3- anionic groups has been confirmed to be a potential functional units for exploring new NLO materials, e. g. Ca3(C3N3O3)2 27 and Sr3(C3N3O3)2.

28

Most of above compounds exhibit



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strong SHG response under external field irradiation because of the strong pπ−pπ interaction between adjacent parallel p orbitals and delocalized π-orbital electrons. Other anionic groups with planar π-conjugated structrue like [NO2]-, [HCOO]- may be as alternative basic structure units to construct NLO materials. An excellent DUV NLO material should be with large SHG coefficient, proper birefringence and wide UV transparency window. A proven successful method is to introduce the planar π-conjugated anionic groups and increase the density of anionic groups in the unit cell, and make them in parallel arrays at the same time. Employing two or more planar π-conjugated structural units in one compound resulting in a synergistic effect not only increase structural diversity but also induce large second harmonic generation (SHG) enhancement. compounds

exhibit

strong

SHG

responses

such

as

36-38

Several representative

Pb2(BO3)(NO3)

(9.0×KDP),39

Pb7O(OH)3(CO3)3(BO3) (4.5×KDP).40 Nevertheless, the introduction of stereochemically active lone pair cations results in the red shift of UV cutoff edge which is far above 200 nm. So alkali or alkaline earth metals should be chosen as the counter cations for exploring new NLO materials to ensure high transmission in the UV region for no d–d electron or f–f electron transitions. Similar with [CO3]2-, [HCOO]- is also an NLO-active anionic group with planar π-conjugated structrue. In the well-known crystal HCOOLi·H2O possessing large NLO coefficients (3×KDP) and broad UV transparency region, [HCOO]- makes the dominate contribution to the SHG efficient which has been proved by the subsequent experimental and theoretical calculation investigations.41 Introducing the [HCOO]- anionic group into a mature planar π-conjugated research system, that is, borates with [BO3]3-, may produce new superior performing DUV NLO materials in formic-borates which have rarely been studied. By far, only one compound containing formate and borate mixed anions, Na3(HCOO)[B5O8(OH)],42 has been reported by Yang’s group in 2017, which has been confirmed to be a promising DUV NLO materials. Nevertheless, this compound contains fundamental acentric B5O10(OH) clusters constructed of three BO3 triangles and two BO4 tetrahedra, resulting in a disorderly arrangement of planar π-conjugated anionic groups which is not benefit for producing suitable birefringence. It presents a strong chance of finding new excellent DUV NLO materials in formic-borates system. In this work, a systematically study has been done in A – HCOO –BO3 (A = alkali metal) systems to explore novel optical materials with


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mixed planar π-conjugated formate and borate anions. Three new compounds namely Na(COOH)[B(OH)3](H2O)2 (1), K3(COOH)3[B(OH)3]2 (2) and (HCOOH)3[B(OH)3]2·3H2O (3), have been successfully synthesized. All the three compounds exhibit parallel arrangement of layers composed of B(OH)3 and HCOO parallel π-conjugated units through hydrogen bonding assisted construction. In particular, K3(COOH)3[B(OH)3]2 and (HCOOH)3[B(OH)3]2·3H2O possess graphite-like layer structures. Herein the synthesis, crystal structures, thermal behaviors, spectra, and NLO properties were reported.

EXPERIMENTAL SECTION Synthesis All the starting reagents Na2B4O7 (≥ 99%), K2B4O7 (≥ 99%), NaCOOH (99%), KCOOH (99%), and HCOOH (60%), were analytical grade from commercial sources and used without further treatment. Synthesis of Na(COOH)[B(OH)3](H2O)2 (1). Compound 1 was hydrothermally synthesized with the reaction of Na2B4O7 (0.201 g, 1mmol), and NaCOOH (1.360g, 20mmol) and 5 mL deionized water. The mixture was stirred for 20 minutes at room temperature (pH=9.0), and then sealed into a 23 mL teflon autoclave and heated at 150 °C for 5 days. The teflon autoclave was slow cooled to room temperature and the products were washed with alcohol and dried in air. Colorless rodlike crystals of Na(COOH)[B(OH)3](H2O)2 were obtained in yields of about 65% (on the basis of B). Synthesis of K3(COOH)3[B(OH)3]2 (2). A mixture of K2B4O7 (0.152 g, 0.5mmol), and KCOOH (2.52g, 30mmol) were added to 5 mL deionized water, and stirred for 20 minutes at room temperature, and then sealed into a 23 mL teflon autoclave and heated at 150 °C for 5 days. The teflon autoclave was slow cooled to room temperature and the products were washed with alcohol and dried in air. Colorless block crystals of K3(COOH)3[B(OH)3]2 were obtained in yields of about 65% (on the basis of B). Synthesis of (HCOOH)3[B(OH)3]2·3H2O (3). Hydrothermal synthesis of K2B4O7 (0.152 g, 0.5mmol), HCOOH (3 mL) and H2O (5 mL) at 150 °C for 5 days results in compound 3, and the pH value of the starting mixture is about 9.0. The products were washed with alcohol, and dried in air. Colorless rodlike crystals of (HCOOH)3[B(OH)3]2·3H2O were obtained in yields of about 65% (on the basis of B).



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Instruments X-ray diffraction patterns of the polycrystalline materials were obtained on a Smart lab powder X-ray diffractometer using Cu Kα radiation (λ=1.540598 Å) in the angular range of 2θ from 5 to 55° with a scan step width of 0.02° and a fixed time of 0.2 s at room temperature. Thermogravimetric analysis (TGA) was conducted on a Netzsch STA 409 PC, and 10mg of crystal samples were enclosed in platinum crucibles and heated from room temperature to 800℃ at a rate of 10 ℃/min under a constant flow of N2 gas. The infrared spectra were recorded as KBr pellets in the range of 4000-400 cm-1 on a Vertex 70 Fourier transform infrared (FT-IR) spectrometer, 1 mg of sample were mixed thoroughly with 100 mg of oven-dried KBr. The UV-vis diffuse reflectance spectroscopy of the three compounds were recorded using a PerkinElmer Lamda-950 UV/VIS/NIR spectrophotometer at room temperature and scanned at 200-2500 nm, BaSO4 was used as the standard for 100% reflectance and Kubelka-Munk function was used to calculate the absorption spectra.43 The powder second-harmonic generation (SHG) signals were measured using Kurtz and Perry method at room temperature.44 We know that SHG efficiency is mainly depending on particle size, samples of compound 3 were ground and divided into the particle size in the ranges of 25-45, 45-58, 58-75, 75-106, 106-150 and 150-212 µm. The UV SHG measurements were performed with a Q-switched Nd:YAG laser at 1064 nm and a frequency doubling at 532 nm, for visible and UV SHG, respectively. Microcrystalline KDP with the same particle size were used as reference for visible SHG and LBO for UV SHG. Single

crystal

X-ray

diffraction

data

of

Na(COOH)[B(OH)3](H2O)2

(1),

(HCOOH)3[B(OH)3]2·3H2O (2), K3(COOH)3[B(OH)3]2 (3) were collected at 150(2) K on a Bruker D8 Venture diffractometer equipped with graphite monochromatic Mo Kα radiation. All the structures were solved by the direct methods and refined by full-matrix least-squares fitting on F2 using SHELX-2014.45 Anisotropic displacement parameters were refined for all atomic sites except the hydrogen atoms in 1-3. All the hydrogen atoms were located geometrically or in a difference Fourier map.46 All the three structures were verified using the ADDSYM algorithm from the program PLATON, no higher symmetries were found.47 Relevant crystallographic data and experimental details are summarized in Table 1. Atomic coordinates and isotropic displacement coefficients, and selected bond lengths for 1-3 are listed in Table S1-S6.


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Table 1. Crystal Data and Structure Refinement for Na(COOH)[B(OH)3](H2O)2 (1), K3(COOH)3[B(OH)3]2(2) and (HCOOH)3[B(OH)3]2·3H2O (3). formula

formula

Na(COOH)[B(OH)3](H

K3(COOH)3[B(O H)3]2

2O)2

(HCOOH)3[B(OH)3]2·3 H2O

165.87

376.02

315.79

crystal system

orthorhombic

monoclinic

monoclinic

space group

Pnma

C2/c

Pn

a (Å)

7.0271(5)

15.222(4)

10.6446(8)

b (Å)

6.9305(6)

10.563

3.6614(3)

c (Å)

14.0299(10)

9.264(2)

18.5768(13)

α(°)

90

90

90

β(°)

90

115.21

90.475(2)

γ(°)

90

90

90

V(Å3)

683.27(9)

1347.7(5)

723.99(10)

Z

4

4

2

ρ(calcd) (g/cm3)

1.612

1.853

1.449

150(2) K

150(2) K

150(2) K

λ(Å)

0.71073

0.71073

0.71073

F(000)

344

760

332

µ (mm-1)

0.215

1.067

0.151

0.0295/ 0.0719

0.0394/ 0.0879

0.0750/ 0.2054

mass(amu)

temperature (K)

R1, wR2 (I>2σ(I))a R1, wR2 (all data) GOF on F2

0.0454/ 0.0804

0.0697/0.1057

1.020

1.026

R1(F) = Σ||Fo| – |Fc||/Σ|Fo|. wR2(Fo2) = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1/2


0.0784/ 0.2076 1.016


RESULTS AND DISCUSSION Crystal Structure Description

Figure 1. a) Ball and stick representation of 1-D chain of {Na[B(OH)3](H2O)2}+; b) the polyhedron coordination mode of NaO6; c) the Na(COOH)[B(OH)3](H2O)2 chains of 1 linked by Na+ and hydrogen bonding along the b-axis; d) view of the arrangement of 1 connected by hydrogen bonding along the a-axis; e) view of the 3-D framework of 1 along the a-axis.

Crystal Structure of Na(COOH)[B(OH)3](H2O)2 (1) crystallizes into an orthorhombic crystal system with a centrosymmetric space group of Pnma (no. 62). The asymmetric unit of 1 contains one Na atom, one COOH−, one B(OH)3 and two water molecules. As shown in Figure 1a and 1b, Na+ ion is six coordinated with two hydroxyl from two B(OH)3 triangles and four water molecules to form octahedron polyhedron with Na-O bond lengths ranging from 2.3299(15) to 2.4226(11) Å, and connected with water molecules and hydroxyls to form the 1-D chain {Na[B(OH)3](H2O)2}+. The isolated B(OH)3 and COOH- units arrange along the b axis and linked with each other by O-H…O hydrogen bonding to another 1-D (COOH)[B(OH)3] tunnels (O3-H3…O5, 2.6731(2) Å; O2-H2…O5, 2.6869(2) Å; O1-H1…O4, 2.6883(2) Å), and all the B-O bond lengths fall into the range between 1.358(3) - 1.377(3) Å and C-O range from 1.234(3) - 1.262(3) Å. Na+ ions locate in tunnels between 1D {(COOH)[B(OH)3]}- chains and they further interconnect to form complex Na(COOH)[B(OH)3] multi-chains (Figure 1c). Meanwhile, water molecules reside in the


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interlayer along the c axis and also connected with COOH- together by hydrogen bonding with O6-H6B…O4 distance of 2.7903(2) Å, and play the role of bridging the Na(COOH)[B(OH)3] chains to the 3-D framework (Figure 1d, 1e). In compound 1, although all the Na(COOH)[B(OH)3] chains exhibit parallel alignment in the same direction, internal symmetrical arrangement in the chains results in a centrosymmetric structure of 1.

Figure 2. a) The hydrogen bonding bridges between HCOOH and B(OH)3 in 2; b) view of the A layer along the a-axis; c) view of the {KxOy} chains along the b-axis; d) the polyhedron coordination mode of K1 and K2 cations; e) view of the A' layer along the a-axis; f) view of the 3-D framework of 2 along the b-axis.

K3(COOH)3[B(OH)3]2 (2) crystallizes into a monoclinic crystal system with a centrosymmetric space group of C2/c (no. 15). The asymmetric unit of 2 contains two K+ ions, two COOH- anions and one B(OH)3 molecule. Similar with 1, COOH- anions in 2 are bonded with B(OH)3 molecules through hydrogen bonding with B-O bond length in the range of 1.367(4) -1.380(4) Å and C-O bond in the range of 1.212(5) - 1.236(3) Å. Because of no metal cations in the {(COOH)3[B(OH)3]2}3- layers, they themselves tend to form a “tighter” and “stable” arrangement and result in the 6-membered ring (6-MR) (O3-H3…O1, 2.6761(1) Å; O4-H4…O2, 2.6674(3) Å; O5-H5…O6, 2.6609(0) Å) (Figure 2a), and the 6-MR further bridged with the other six ones by sharing edges through hydrogen bonding result in the 2-D layer A along the a axis (Figure 2b) which is isomorphic to graphitic layer. The similar arrangement of COOH- and B(OH)3 molecules



leads to the adjacent A' layer (Figure 2e) with a rotate of about 180° compared with A layer. Two types of K+ (K1 and K2) located in the interlayer between A and A' layers are connected with each other by two µ2-OH and two µ3-O to form the 1-D chain along the b axis, among which K1 is eight coordinated with four µ3-OH from two B(OH)3 triangles and four µ3-O atoms from COOH− anions to form tetragonal prism polyhedron, and K2 is bonded with four µ3-OH and four µ3-O to form square antiprism polyhedron (Figure 2c, 2d), with the K1-O bond lengths range from 2.760(2) 3.059(2) Å and K2-O from 2.773(2) - 2.981(2) Å. Graphite-like A and A' layers stack alternatively along the b-axis in the form of “AA'AA'AA'…” and linked by K+ to form the 3-D framework (Figure 2f).

Figure 3. a) Ball and stick representation of the bridges between HCOOH and B(OH)3 by hydrogen bonding in 3; b) view of the graphite-like 2-D layer connected by hydrogen bonding along the b-axis; c) view of the 3-D framework of 3 bridged by H2O and hydrogen bonding along the a-axis.

(HCOOH)3[B(OH)3]2·3H2O (3) crystallizes into a monoclinic crystal system with a noncentrosymmetric space group of Pn (no. 7). Three HCOOH, two B(OH)3 and three water molecules are contained in the asymmetric unit of 3. Similar with 2, every three HCOOH molecules are bonded with three B(OH)3 molecules through hydrogen bonding to form the 6-MR with B-O bond length in the range of 1.357(12) -1.374(11) Å, C-O bond in the range of 1.237(11) - 1.245(10) Å and O-H…O range from 2.6533(1) Å to 2.6905(2) Å (Figure 3a). Every 6-MR


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connecting with the other six ones leads to the graphite-like (HCOOH)3[B(OH)3]2 layer, which is marked as layer A in Figure 3b and 3c. The lattice water molecules reside in the interlayer and are different with the coordination water in 1, which is also reside in the interlayer. There is no covalent bond coordinated with water in 3 and the hydrogen bond is weaker and longer in it, and the O-H…O bond range from 2.8749(1) Å to 2.9307(2) Å. The hydrogen bonding plays the role of bridging and the ordered stacking of graphite-like layers in the form of “AAA…” along the a axis result in the 3-D structure of 3, and further stabilize the framework (Figure 3c).

Templating Cations Size and Hydrogen Bonding Interactions Effecting on the Framework Structure and Centricity

Figure 4. a) The diameter of the cavity and the distance between the two chains formed through hydrogen bonding in the ac- plane in 1; b) the location of the Na+ viewed along the a-axis in 1; c) the O-O distance in the cavity in 3.

Templates usually play an important role in the formation of the compounds. Alkali and alkaline-earth metal cations and various organic ions usually are employed as templates.48 In our above reported compounds, all the structural motifs are B(OH)3 and COOH- groups, and the only difference is templates, which cause the difference of framework structure and centricity of the three titled compounds. Compared Na(COOH)[B(OH)3](H2O)2 (1) with K3(COOH)3[B(OH)3]2 (2), the structure motif of both compounds are COOH−, B(OH)3 and alkali ions Na+/K+, with the ionic radii for Na+ and K+ are 1.02 and 1.51Å, respectively. COOH− and B(OH)3 arrange in a plane and the hydrogen bonding interactions between them result in the parallel arrangement in compound 1, and antiparallel arrangement in compound 2. In 1, firstly, the hydrogen bonding link COOH−, H2O and B(OH)3 to form a 1-D chains with the diameter of the cavity is about 3.66 Å, and the distance



between adjacent chains is about 4.69 Å (Figure 4a). The distance between two chains is long enough to accommodate the Na+ through covalent bonding O-Na-O and made it reside in the same plane constructed by COOH− and B(OH)3 units. Meanwhile 3.66 Å is too short to accommodate the bridge Na+ in the same plane so as to extrude it to the adjacent interlayer and play the role of bridge (Figure 4b). In compound 2 with the larger alkali K+, COOH− and B(OH)3 are connected with each other through hydrogen bonding resulting in a layered compound. Although the closest interlayer O-O distance with proper interspaces is 4.59 Å, and the nearest K-O bond is about 2.76 Å, thus the K+ should reside in the interlayer space and no K+ could accommodate in the layer (Figure 4c). Na+ ions in 1 and K+ ions in 2 are lined up alternatively for more effective packing resulting in CS structures with two different arrangements. So we can see that the templating cations size is very important in influencing the structure of the compounds by changing the bonding nature of the constituent polyhedra. The hydrogen bonding interaction is also an important factor to regulate the crystallographic centricity of solid-state materials. The availability of hydrogen bonding interactions between anionic ligands and organic templating cations are critical for the effect. Compared K3(COOH)3[B(OH)3]2 (2) with (HCOOH)3[B(OH)3]2·3H2O (3), the hydrogen bonding induce to form similar graphite-like layers composed of COOH− and B(OH)3 groups. Compound 2 and 3 exhibit different crystallographic centricity because of the different stacking of graphite-like layers, with the form of “AA'AA'AA'…” in 2 with rotation of A layer 180° resulting in the other A' layer and “AAA…” in 3. Meanwhile alkali cations K+ are contained in 2 and no metal cation exists in 3. Because of impulsion between the limit space in the hydrogen bonding interaction induced layers and relative bigger K+ caions in 2, the unfavourable repulsion was minimized by adopting an inversion at A layer and alternatively arranged K1+ and K2+ cations, which render the CS nonpolar compound of 2. Abundant O-K-O covalent bonds further stabilize the structure. In 3 no conflicts from metal cations made the configuration of the A layer maintain and render the NCS polar compound of it, while the configuration is further stabilized by H2O molecules with hydrogen bonding. From the above discussion, we can see that templating cations size and hydrogen bonding interactions play an important role in effecting the framework structure and the centricity of the compounds.

Powder X-ray Diffraction


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The powder XRD patterns of Na(COOH)[B(OH)3](H2O)2 (1), K3(COOH)3[B(OH)3]2 (2) and (HCOOH)3[B(OH)3]2·3H2O (3) show that experimental patterns are in good agreement with the calculated patterns based on the results fitted from single-crystal X-ray diffraction (Figure 5), which confirms that the three titled compounds are pure.

Figure 5. Experimental and calculated XRD patterns for a) Na(COOH)[B(OH)3](H2O)2 (1), b) K3(COOH)3[B(OH)3]2 (2), and c) (HCOOH)3[B(OH)3]2·3H2O (3).

Thermal Properties The

thermogravimetric

analysis

(TGA)

curves

of

Na(COOH)[B(OH)3](H2O)2

(1),

K3(COOH)3[B(OH)3]2 (2) and (HCOOH)3[B(OH)3]2·3H2O (3) are shown in Figure 6. The TGA curve shows that compound 1 exhibits two major weight loss stages in the regions about 50-150°C and 150-430 °C, The first weight loss is 38.90% assigned to the departure of two coordination water molecules and the decomposition of B(OH)3 (calcd 38.00%), followed by the loss of 19.92% corresponding to the decomposition of formate groups (calcd 22.31%). Be similar with compound 1, TGA curve of compound 2 shows two major weight loss stages in the regions about 90-160°C and 160-753 °C, The first weight loss is about 15.02% assigned to the decomposition of B(OH)3 (calcd14.36%), followed by the loss of 26.10% corresponding to the decomposition of KCOOH units (calcd 29.50%). The weight loss stage of 3 can not be clearly parted and displays only one weight loss stage of about 55.4% from 60-403 °C corresponding to the release of three water molecules and the decomposition of B(OH)3 and HCOOH groups (calculated value 51.30%).



Figure 6. TGA curves for a) Na(COOH)[B(OH)3](H2O)2 (1), b) K3(COOH)3[B(OH)3]2 (2), and c) (HCOOH)3[B(OH)3]2·3H2O (3).

Optical Properties As shown in Figure 7, the IR spectra of three compounds display absorption peaks at 3337-3555 cm−1, 3140, 3620-3838 cm−1 owing to the existence of H2O molecules and OH− groups. The IR absorption peaks around 1610 cm−1 are due to H−O−H bending mode for compounds 1-3, the strong peaks at around 1395, 1405 and 1421 cm−1 could be assigned to the deformation vibrations of COO−, and the peaks around 1347, 1353, and 1347 cm−1 can be assigned to the asymmetrical stretching of COO−. The asymmetrical stretching of the BO3 groups appear around 1243, 1211 and 1217 cm−1 and the intense bands at 1180, 1170, and 1175 cm-1 were attributed to the B−O stretching vibrations in the BO3 triangles. It is difficult to assign the absorption bands at 774/761/755, 672/682/682, and 572/509/510 cm–1 because of the overlap of deformation vibration of COO− and the bending modes of BO4 groups in the low frequency vibrations for all compounds. These assignments are in accordance with other previous reported compounds.49, 50

Figure 7. The IR spectra of compounds a) Na(COOH)[B(OH)3](H2O)2 (1), b) K3(COOH)3[B(OH)3]2 (2), and c) (HCOOH)3[B(OH)3]2·3H2O (3).

The UV-vis diffuse reflectance spectra were measured for compounds 1-3. As shown in Figure 8, the three titled compounds exhibit high transmission in the range of 200-800 nm, which indicate


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that these three compounds are potential deep-ultraviolet optical materials.

Figure 8. UV absorption spectra of compounds 1-3.

Nonlinear Optical (NLO) Properties Among the three compounds, compounds 3 (HCOOH)3[B(OH)3]2·3H2O is non-centrosymmetric, so nonlinear optical properties have been studied. As shown in Figure 9, the SHG responses of (HCOOH)3[B(OH)3]2·3H2O were measured by the Kurtz-Perry method44 on sieved power samples with a laser at 1064 nm and 532 nm as the fundamental waves. Two famous crystals were used as the references, KDP for visible SHG and LBO for UV SHG, respectively. The intensity of the SHG signals increases gradually with the increasing sample size at first and then it tends to be

Figure 9. (a) SHG measurements of ground (HCOOH)3[B(OH)3]2·3H2O crystals (blue solid circle) and KDP (red diamond) as the reference with the laser at 1064 nm wavelength, and (b) SHG measurements of ground (HCOOH)3[B(OH)3]2·3H2O crystals (blue solid circle) and LBO (black square) as the reference with the laser at 532 nm wavelength.



constant from 150µm, which indicates that (HCOOH)3[B(OH)3]2·3H2O is type I phase-matchable in the UV and visible region. The compound exhibits a moderate SHG response about 0.7 times of KDP.51 On the basis of the NLO coefficient of LBO (3.0 × KDP), the relative magnitude of SHG coefficients in the UV region and that in the visible region are in accordance with each other. According to the anionic group theory, the SHG coefficients of compound 3 mainly originate from planar π-conjugated groups, B(OH)3 and HCOO-, with delocalized π-orbital electrons and strong pπ−pπ interaction between adjacent p orbitals. As we know, increasing the density of the planar π-conjugated groups and making the NLO active units in a parallel arrangement can improve the NLO response of the materials. In (HCOOH)3[B(OH)3]2·3H2O, the molecule is filled with planar π-conjugated groups B(OH)3 and HCOO- with high density except slight water molecules, and all the units are parallel to each other in the graphite-like layers which orient in the same direction. Such arrangement provides the possibility to enhance the SHG response. Unfortunately, (HCOOH)3[B(OH)3]2·3H2O just exhibits a moderate SHG response. Due to lacking the metal-oxygen covalent bonding, all the π-conjugated ions have to arrange in a proper direction to construct enough hydrogen bonding to stabilize the framework, resulting in all the B(OH)3 groups antiparallel arrangement to cancel most of effective contribution (Figure 3a). How to maintain the parallel arrangement and the consistency between the functionalized layers to get enhanced SHG response is the research emphasis and difficulties. The Arrangement of Layers Composed of Planar π-conjugated Groups Affecting on Optical Properties It is generally accepted that the crystal structure of compound dominates the fundamental attributes of a phase more than the chemical composition, thus the physical and chemical properties of the compound are deeply affected by the arrangement of fundamental building blocks. Understanding the relationship between structures and properties is critical for design and synthesis of new materials. The birefringence and the nonlinear optical coefficient are two key parameters to evaluate the optic performance of NLO materials. The former determines the capability of phase matching in the specific spectral region; the latter affects the conversion


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Figure 9. Different arrangement of the layers composed of planar π-conjugated groups in (a) K2Be2BO3F2; (b) Sr2Be2B2O7; (c) Sr2(OH)3NO3; (d) K3(COOH)3[B(OH)3]2; and (e) (HCOOH)3[B(OH)3]2·3H2O.

efficiency of fundamental laser. As we know, layered architecture in a parallel arrangement with high density of the planar π-conjugated groups are beneficial to induce excellent optic properties. In the previous researches,24, 30-32, 52 the researchers put more emphasis on structural modulation of NLO-active anionic groups by cations and ingnord the microstructural investigation of interlamellar connectivity. There are multiple connection modes in the interlamination of the compounds with planar π-conjugated groups, such as electrostatic interactions in K2Be2BO3F2 (Figure 9a), covalently connection in Sr2Be2B2O7 (Figure 9b) and hydrogen bond interaction in (HCOOH)3[B(OH)3]2·3H2O (Figure 9e), or mixed modes in Sr2(OH)3NO3(Figure 9c) and K3(COOH)3[B(OH)3]2 (Figure 9d) with hydrogen bond interaction and covalently connection. Due to relatively weak force, electrostatic interactions and hydrogen bond interaction tend to form the flexible connection; while covalently connection with strong covalent bonds easily realize the rigidity connection. It has been known that increasing the density of NLO-active planar π-conjugated anionic groups and optimizing the structural criterion could enlarge the macroscopic NLO coefficients; and increasing the degree of anisotropy in polarizability could produce large birefringence. Hence, a dense parallel layered arrangement is beneficial to produce good optical properties. Most of the existing researches focused on the intraformational configuration of planar π-conjugated units, for instance, structural modulation of anionic group architectures by cations to optimize SHG effects. When the functionalized single layers formed, how to make them interconnect is also crucial for the optical properties, coparallel to enhance the contribution of



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functional groups or antiparallel to cancel the contribution. In general, flexible connection through electrostatic interactions or hydrogen bond interaction is conducive to structural modulation for the

ordered

stacking

of

layers

in

the

form

of

“AAA…”

such

as

in

KBBF,

(HCOOH)3[B(OH)3]2·3H2O. And small linkers easily fill between layers to increase the density functionalized layers.While rigidity connection through strong covalent bonds is hard to break the natrual patterns, resulting in alternating stacking in the form of “AA'AA'…” like in SBBO, K3(COOH)3[B(OH)3]2. And big ions staying in the interlayers lead to large layer-spacing. Thus, designing new NLO materials with planar π-conjugated groups for good optical properties needs attention not only on the arrangement of functional groups in single layer but also on the interlamellar connection.

CONCLUSION By combining two types planar π-conjugated BO3 and COOH groups, three novel formic-borates Na(COOH)[B(OH)3](H2O)2, K3(COOH)3[B(OH)3]2 and (HCOOH)3[B(OH)3]2·3H2O have been synthesized via hydrothermal methods. In the three titled compounds, hydrogen bonding assisted the construction of 3-D frameworks with all the planar π-conjugated units parallel arrangement. Particularly, K3(COOH)3[B(OH)3]2 and (HCOOH)3[B(OH)3]2·3H2O exhibited graphite-like layer structure. SHG studies indicated that (HCOOH)3[B(OH)3]2·3H2O has moderate SHG responses of approximately 0.7 times that of KDP. The short-wavelength absorption edges of all three compounds were below 200 nm indicated that they are potential deep-UV optical materials. Furthermore, the discovery of new series of formic-borates enriches the family of deep-ultraviolet optical materials, and our studies on templating cation size and hydrogen bonding interactions effecting on the framework structure and centricity will be valuable for designing other new NLO materials.

ASSOCIATED CONTENT Supporting Information Atomic coordinates, select bond lengths and angles, and crystal photographs are provided in Supporting Information. Accession Codes


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CCDC 1840523-1840525 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (no. 21501161 and 21401178). DEDICATION Dedicated to Professor Xin-Tao Wu on the occasion of his 80th birthday.

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Table of Contents Use Only

Hydrogen Bonding Assisted Construction of Graphite-Like Deep-UV Optical Materials with Two Types of Parallel π-conjugated Units Fangfang He, † Qian Wang, † Mengjiao Liu, † Ling Huang,* † Daojiang Gao, † Jian Bi, † and Guohong Zou*‡

Brief synopsis: Three new compounds with two mixed planar π-conjugated BO3 and HCOO groups have been synthesized via systematic investigations in A – HCOO –BO3 (A = alkali metal) systems. The UV-vis diffuse reflectance spectroscopy study indicated that the short-wavelength absorption edge of all the three titled compounds were below 200 nm, suggesting that they are potential deep-ultraviolet optical materials.


Hydrogen Bonding Assisted Construction of Graphite-like Deep-UV

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