ZHANG Zhi-Zhun JIN Jin-Ce GONG Lio-Kuo②DU Ke-Zho HUANG Xio-Ying②

a (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT Two new hybrid chloroantimonates, namely, [Prmim]3SbCl6 (1, Prmim = 1-propyl-3-methylimidazolium) and [Hmim]3SbCl6 (2, Hmim = 1-hexyl-3-methylimidazolium), were synthesized in ionic liquids (ILs)with the yields of 97% and 72%, respectively. Single-crystal X-ray diffraction (SCXRD) study reveals that 1 crystallizes in monoclinic, space group Pn with a = 15.2988(12), b = 13.6388(10), c = 15.6761(13) Å, β =98.677(7)°, V = 3233.5(4) Å3, Z = 4, Dc = 1.459 g·cm-3, F(000) = 1440, μ = 1.370 mm-1, R = 0.0589 and wR =0.1366 (I > 2σ(I)); 2 crystallizes in the hexagonal space group of P63 with a = 27.7471(6), b = 27.7471(6), c =8.9811(2) Å, V = 5988.2(3) Å3, Z = 6, Dc = 1.391 g·cm-3, F(000) = 2592, μ = 1.121 mm-1, R = 0.0420 and wR =0.0726 (I > 2σ(I)). The photophysical properties of the title compounds were studied by solid-state optical absorption, photoluminescent excitation/emission (PLE/PL), PL decay spectra and photoluminescent quantum yield(PLQY). 1 and 2 exhibit PL peaks at 627 and 607 nm, Stokes shifts of 257 and 242 nm, and PLQY of 32.5% and 49.2%, respectively. The distinct photo physical characteristics of 1 and 2 are highly related to the distortion extent of the [SbCl6]3- unit.

Keywords: antimony(III) chloride, ionic liquid, luminescence, distortion;

1 INTRODUCTION

Inorganic-organic hybrid metal halides (IOMHs) have received increasing attention because of their superior photophysical characteristics with potential applications in photovoltaics, solid-state lighting,etc.[1-10]. Zero dimensional (0D) IOMHs with structurally and electronically isolated halometallate species generally exhibit broadband emission mainly due to self-trapped excitons (STE) from the interaction of excitons with lattice and large structural reorganization in the excited state[11-14]. The STE emission has been observed in the hybrid compounds based on the metal ion withns2electron configuration, such as, Ge2+, Sn2+, Pb2+,Sb3+and Bi3+[15-31].

Sb3+coordinating with halogen ions X-(X = Cl, Br, I) can form haloantimonate(III) anions with rich structural moieties like [SbX4]-[32], [SbX5]2-[31,33-41], [SbX6]3-[38,42], [Sb2X7]-[43],[Sb2X8]2-[44], [Sb2X9]3-[45,46], [Sb2X10]4-[47]and [Sb2X11]5-[48].Among them, the mononuclear [SbCl5]2-unit is commonly used to construct photoluminescent (PL) 0D-IOMHs. Such IOMHs generally exhibit broadband emission over a wide range of spectrum originating from3P1→1S0transition[31,33-41],mostly with a near-unity PL quantum yield (PLQY),e.g., in(C9NH20)2SbCl5(C9NH20= 1-butyl-1-methylpyrrolidinium)[40],(TEBA)2SbCl5(TEBA = benzyltriethylammonium)[31],(Ph4P)2SbCl5(Ph4P = tetraphenylphosphonium)[41], and(PPN)2SbCl5(PPN = bis(triphenylphosphoranylidene)ammonium cation)[33]. The high PL efficiency might be attributed to the sufficient separation of the neighboring[SbCl5]2-units by cations leading to little-to-no interactions or electronic band formation[40]. Additionally, dual emission as well as white light emission in compounds (Bmim)2SbCl5(Bmim = 1-butyl-3-methlimidazolium) and (TTA)2SbCl5(TTA = tetraethylammonium) could be easily and conveniently realized by adjusting the excitation wavelength[31,39].By introducing H2O molecules to expand the distance between [SbCl5]2-species and create more local photoelectrons for the [SbCl5]2-species, the PLQY of 25.3% in(C6N2H16)2SbCl5(C6N2H16= 2,6-dimethylpiperazine) could be enhanced to 39.6% in (C6N2H16)2SbCl5·H2O[36]. These compounds have shown potential applications in emerging fields such as thermal imaging analysis[34], scintillator[33]and anti-counterfeiting luminescent paper[37,38]. 0D-IOMHs with mononuclear [SbCl6]3-unit also have been widely reported based on CCDC database. However, their PL properties have been rarely demonstrated[38,42]. Recently, it has been reported that (Bzmim)3SbCl6(Bzmim = 1-benzyl-3-methylimidazolium) could exhibit green emission with high PLQY of 87.5%[38].

Ionic liquids (ILs), as a kind of “green” reagents and templates, exhibit various excellent properties, such as low volatility, large liquid ranges, nonflammability, and high stability[49-52]. Additionally, a wide variety of ILs and various substitution ways on the parent rings provide favorable conditions for obtaining diverse structures[53-55]. Herein, by using imidazolium based ILs with different-length alkyl chain as the solvent and template, two new hybrid chloroantimonates, namely, [Prmim]3SbCl6(1, Prmim = 1-propyl-3-methylimidazolium) and [Hmim]3SbCl6(2, Hmim =1-hexyl-3-methylimidazolium), were synthesized. Both feature a 0Dstructure with isolated [SbCl6]3-octahedron as confirmed by single-crystal X-ray diffraction (SCXRD).Under the excitation wavelength of 370 nm, 1 exhibits orange emission peak at 627 nm with a large Stokes shift of 257 nm,while 2 exhibits orange-yellow emission peak at 607 nm with a large Stokes shift of 242 nm excited at 365 nm. The PLQY for 1 and 2 are 32.5% and 49.2%, respectively. The distinct photophysical properties (emission peak, Stokes shift, and PLQY) of two compounds are revealed to be related to the distortion of isolated [SbCl6]3-octahedron by comparing the title [SbCl6]3-unit with those reported in literature[38,42].

2 EXPERIMENTAL

Antimony(III) chloride (SbCl3, 99%) was purchased from Adamas Reagent Co., Ltd. [Prmim]Cl (99%) and[Hmim]Cl (99%) were purchased from Lanzhou GreenChem ILs, LICP, CAS (Lanzhou, China). All the reagents were utilized without further purification.

Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Miniflex II diffractometer with CuKαradiation (λ= 1.54178 Å) at room temperature. Elemental analyses (EA)for C, H and N were conducted on a German Elementary Vario MICRO instrument. Thermogravimetric (TG) analysis was performed on a NETZSCH STA 449F3 instrument at a heating rate of 10 K·min-1under a N2atmosphere from 20 to 800 ℃. Solid-state optical diffuse reflectance spectra were performed at room temperature on a Shimadzu 2600 UV/Vis spectrometer in a range of 200～800 nm. BaSO4with 100%reflectance was used as a standard. The absorption data were obtained from reflectance spectra by using the Kubelka-Munk functionα/S= (1 -R)2/2R[56], whereαis the absorption coefficient,Sthe scattering coefficient, andRthe reflectance.Photoluminescent excitation (PLE), PL spectra and PL decay spectra were recorded on an Edinburgh FLS1000 UV/V/NIR fluorescence spectrometer. PLQY of the title compounds were measured by the FLSP920(EI) fluorescence spectrometer.

2. 1 Synthesis of [Prmim]3SbCl6 (1)

A mixture of SbCl3(0.2335 g, 1 mmol) and [Prmim]Cl(0.4882 g, 3 mmol) was sealed into a 28 mL Teflon-lined stainless-steel autoclave. In this reaction system, the ionic liquid [Prmim]Cl acts as both solvent and template. The container was closed, heated at 120 ℃ for 3 hours, and then cooled to room temperature naturally. Colourless and transparent block-like crystals were formed after certain time(around two weeks) at room temperature. Long crystallization time possibly relates to the viscosity of the ILs. Once the crystal nuclei were formed, many crystals would be precipitated, and finally 1 could be obtained in a high yield(yield: 0.7042 g, 97% based on Sb). Elemental analysis: calcd.(%) for C21H39N6SbCl6: C, 35.52; H, 5.53; N, 11.83. Found: C,35.70; H, 6.11; N, 11.99.

2. 2 Synthesis of [Hmim]3SbCl6 (2)

A similar synthesis procedure as that for 1 was adopted except that a mixture of SbCl3(1.1641 g, 5 mmol) and[Hmim]Cl (2.1408 g, 10 mmol) was used. Colourless and transparent block-like crystals were formed with the yield of 3.1045 g (72% based on Sb). Elemental analysis: calcd. (%)for C30H57N6SbCl6: C, 43.08; H, 6.86; N, 10.04. Found: C,40.94; H, 6.83; N, 9.70.

2. 3 Structure refinements

The colorless-transparent block-like crystals 1 and 2 were selected for SCXRD experiment with dimensions of 0.39mm × 0.39mm × 0.20mm and 0.50mm × 0.25mm ×0.20mm, respectively. For 1, a total of 31441 reflections were collected in the range of 1.99°≤θ≤30.31° withRint=0.0371, 15049 of which are independent. Crystal 1 crystallizes in monoclinic, space groupPnwitha=15.2988(12),b= 13.6388(10),c= 15.6761(13) Å,β=98.677(7)°,V= 3233.5(4) Å3,Z= 4,Dc= 1.459 g·cm-3,F(000) = 1440,μ= 1.370 mm-1,R= 0.0589 andwR=0.1366 (I> 2σ(I)). For 2, 37899 total reflections were collected in 2.24°≤θ≤29.24° region withRint= 0.0946, of which 9758 were independent. Crystal 2 is of hexagonal system, space groupP63witha= 27.7471(6),b=27.7471(6),c= 8.9811(2) Å,V= 5988.2(3) Å3,Z= 6,Dc=1.391 g·cm-3,F(000) = 2592,μ= 1.121 mm-1,R= 0.0420 andwR= 0.0726 (I> 2σ(I)).SHELX2018 package was used to solve and refine the structure onF2by full-matrix least-square methods[57]. Selected bond lengths and bond angles of 1 and 2 are shown in Table 1, and selected hydrogen bond parameters in Table 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (o) for 1 and 2

Table 2. Selected Hydrogen Bond Lengths (Å) and Bond Angles (°) for 1 and 2

Compound 1: Symmetry transformations: #1: x + 1/2, -y + 2, z - 1/2; #2: x + 1/2, -y + 1, z + 1/2; #3: x + 1/2, -y + 1, z - 1/2;#4: x, y - 1, z-1; #5: x - 1/2, -y + 2, z - 1/2; #6: x - 1/2, -y + 1, z - 1/2; #7: x, y, z - 1; #8: x, y - 1, z; #9: x - 1/2, -y + 1, z + 1/2 Compound 2: Symmetry transformations: #1: x, y, z + 1; #2: y, -x + y, z + 1/2; #3: -x + 1, -y + 1, z + 1/2

3 RESULTS AND DISCUSSION

SCXRD analysis reveals that 1 crystallizes in the monoclinic space group ofPn. As shown in Fig. 1a, the crystallographic asymmetric unit of 1 consists of six [Prmim]+cations and two isolated [SbCl6]3-anions. Each Sb3+atom in the crystal is coordinated with six Cl-atoms, forming a mononuclear [SbCl6]3-octahedron. The [SbCl6]3-units are completely separated from each other by [Prmim]+cations(Fig. 1b). The bond lengths of Sb-Cl fall in the range of 2.529(3)～2.838(4) Å (Table 1), close to those in previously reported [Bzmim]3SbCl6with a range from 2.4983(19) to 2.8679(19) Å[38]. 1 exhibits a three-dimensional supramolecular network considering the hydrogen bonds among[SbCl6]3-anions and [Prmim]+cations (Fig. 1c, Table 2). As there are two unique [SbCl6]3-octahedra in the crystal with Sb(1)…Sb(2) distances of 10.1335(11) Å, the hydrogen bonding environments for them are dissimilar (Fig. 1c).Moreover, PLATON calculations indicate different patterns ofπ…πaccumulation between two imidazole rings (Fig. 1d,Table 3).

Fig. 1. (a) Asymmetric unit of compound 1. (b) A diagram showing packing of anions and cations in one unit cell of 1.(c) Three-dimensional supramolecular network for 1 considering hydrogen bonds (left) and the hydrogen bonding environment around the [Sb(1)Cl6]3- and [Sb(2)Cl6]3- anions (right). (d) Selected π…π interactions in 1

Table 3. π…π Interactions in Compound 1

Single crystal of 2 belongs to the hexagonal space group ofP63and the crystallographic asymmetric unit contains three[Hmim]+cations and one [SbCl6]3-anion, as shown in Fig. 2a.The mononuclear six-coordination [SbCl6]3-units are surrounded by imidazolium cations with a relatively long hexyl chain, resulting in a 0Dstructure (Fig. 2b). The Sb-Cl bond distances range from 2.5480(15) to 2.7880(15) Å (Table 1), also comparable to those of 2.4983(19)～2.8679(19) Å in[Bzmim]3SbCl6[38]. Hydrogen bonds are also observed in 2(Table 2). The detailed hydrogen bonding environment for[SbCl6]3-anions are presented in Fig. 2c. The isolated[SbCl6]3-anions are connected to the neighbouring [Hmim]+cations via C-H···Cl hydrogen bonds, some of which, such as C(13)-H(13A)···Cl(2), C(15)-H(13B)···Cl(6), C(25)-H(25B)···Cl(3) and C(25)-H(25A)···Cl(4), result in a 2Dlayer along theabplane (Fig. 2d). Further connected by hydrogen bonds along thecaxis (e.g., C(24)-H(24A)···Cl(4)and C(24)-H(24B)···Cl(6), Fig. 2c), a 3Dsupramolecular structure finally could be obtained. Additionally,π···πinteractions between two imidazole rings in 2 were observed(Fig. 2e, Table 4).

Fig. 2. (a) Asymmetric unit of compound 2. (b) A diagram showing packing of anions and cations in one unit cell of 2. (c) Three-dimensional supramolecular network for 2 considering hydrogen bonds (left) and the hydrogen bonding environment around the [Sb(1)Cl6]3- anion as well as a one-dimensional supramolecular chain along the c axis for 2 considering partial hydrogen bonds (right).(d) Two-dimensional supramolecular network for 2, considering various hydrogen bonds along he ab plane.t Partial [Hmim]+ cations and hydrogen bonds are omitted for clarity. (e) Selected π…π interactions in 2

Table 4. π···π Interactions in Compound 2

The phase purity of the title compounds was confirmed by PXRD (Fig. 3) and EA (Experimental section). TG analyses of 1 and 2 were performed under a N2atmosphere from 20 to 800 ℃. As shown in Fig. 4, both compounds display a one-step weight loss from 250 to 370 ℃. Compared with the thermal decomposition procedure of SbCl3and the corresponding ILs, clearly the thermal properties of the title compounds are highly related to the high thermal stability of ILs.

Fig. 3. Comparison of PXRD patterns of the title compounds with the corresponding simulated ones

Fig. 4. Comparison of the thermogravimetric curves of the title compounds and SbCl3 as well as the corresponding ionic liquids

The photophysical properties of the title compounds were further characterized by solid-state optical absorption spectra,PLE, PL as well as PL decay spectra at room temperature. As shown in Figs. 5a and 5b, 1 exhibits bright orange emission under the irradiation of 370 nm light, slightly different from 2 which exhibits bright orange-yellow emission under the irradiation of 365 nm light. Both 1 and 2 represent a similarly colorless and transparent appearance under ambient light,suggesting nearly no absorption in the visible region, which is consistent with the optical absorption spectra, as shown in Figs. 5c and 5d. Based on the electron absorption transition ofns2electron configuration ions, the two obvious absorption peaks in the absorption spectra can be assigned to1S0→1P1and1S0→3P1transition, respectively[58-60]. The PLE bands of 1 and 2 were consistent with the corresponding optical absorption spectra. Under the excitation of 370 nm light, the PL band of 1 is centered at 627 nm with a large Stokes shift of 257 nm, while the PL band of 2 peaks at 607 nm with a large Stokes shift of 242 nm excited at 365 nm. Broadband emissions of 1 and 2 could be ascribed to3P1→1S0transition from the Sb-based isolated halometallate species[31,36,38,42,60].The microsecond PL lifetime for 1 and 2 is in accordance with that of the reported antimony halide hybrids (Figs. 5e and 5f)[38,42].

Fig. 5. (a) Single crystal of 1 under ambient light (up) and UV light (down). (b) Single crystal of 2 under ambient light (up) and UV light (down).(c) Optical absorption/PLE/PL spectra for 1. (d) Optical absorption/PLE/PL spectra for 2. (e) PL decay spectra for 1. (f) PL decay spectra for 2

In order to deeply understand the factor that affects the photophysical properties (PL band, Stokes shift, PLQY) of 1 and 2, the distortion degree of the isolated [SbCl6]3-unit in the title compounds and corresponding structures in literature was evaluated by using the following formulas[61-63]:

whereθnare the Cl-Sb-Cl bond angles,dnare the Sb-Cl bond lengths, anddis the average of the Sb-Cl bond distances. The results of distortion associated with photophysical properties are summarized in Table 5. It can be found that the PL wavelength and Stokes shift increase with the increasing values ofσ2and Δd, especially in the case of compounds with a similar organic ligand (C6H22N4= tris(2-aminoethyl)amine), which eliminates the influence of organic parts on luminescence.The trend is also observed for the compounds constructed from the ILs with different substituents (e.g., BzmimCl,PrmimCl, and HmimCl). Therefore, the red shift of PL wavelength and larger Stokes shift for 1 should be due to the relatively larger bond length variance (Δd) compared to that for 2. For PLQY, the trend is in contrast. It is decreasing with more distortion of the halometallate species. The higher extent of structural distortion might lead to more energy dissipation from non-radiative transition in the procedure of excited state reorganization into the symmetric structure, finally resulting in less radiative emission with weak luminescent intensity or weak PLQY in 1[42]

Table 5. Summary of the Photophysical Characteristics and Distortion Degree of [SbCl6]3- Octahedron for Selected Hybrid Chloroantimonates (III)

4 CONCLUSION

In summary, by combining SbCl3and the imidazolium based ILs with different lengths of alkyl chain, two new antimony(III) chloride hybrids featuring a mononuclear[SbCl6]3-unit were obtained. Compound 1 exhibits bright orange emission peaking at 627 nm with relatively larger Stokes shift of 257 nm. 2 exhibits bright orange-yellow emission peaking at 607 nm with Stokes shift of 242 nm. The PL peak, Stokes shift and PLQY for 1 and 2 are highly related to the extent of distortion of halometallate unit. Our studies enrich the family of [SbCl6]3-based luminescent IOMHs.