Two New Antimony(III) Chloride Hybrids Composed of Mononuclear [SbCl₆]^3- Unit and Ionic Liquid Cations with Different Length of Alkyl Chain

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 with ns² electron configuration, such as, Ge²⁺, Sn²⁺, Pb²⁺, Sb³⁺ and Bi^3+[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 from ³P₁→¹S₀ transition^{[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 con-veniently 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 chloroan-timonates, namely, [Prmim]₃SbCl₆ (1, Prmim = 1-propyl-3-methylimidazolium) and [Hmim]₃SbCl₆ (2, Hmim = 1-hexyl-3-methylimidazolium), were synthesized. Both feature a 0D structure 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 CuKα radiation (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^–1 under a N₂ atmosphere 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. BaSO₄ with 100% reflectance was used as a standard. The absorption data were obtained from reflectance spectra by using the Kubelka-Munk function α/S = (1 – R)²/2R^[56], where α is the absorption coefficient, S the scattering coefficient, and R the 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° with R_int = 0.0371, 15049 of which are independent. Crystal 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) ų, Z = 4, D_c = 1.459 g·cm^-3, F(000) = 1440, μ = 1.370 mm^-1, R = 0.0589 and wR = 0.1366 (I > 2σ(I)). For 2, 37899 total reflections were collected in 2.24°≤θ≤29.24° region with R_int = 0.0946, of which 9758 were independent. Crystal 2 is of hexagonal system, space group P6₃ with a = 27.7471(6), b = 27.7471(6), c = 8.9811(2) Å, V = 5988.2(3) ų, Z = 6, D_c = 1.391 g·cm^-3, F(000) = 2592, μ = 1.121 mm^-1, R = 0.0420 and wR = 0.0726 (I > 2σ(I)). SHELX 2018 package was used to solve and refine the structure on F² by 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

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

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Compound 1
Bond	Dist.		Bond	Dist.
Sb(1)−Cl(2)	2.552(3)		Sb(2)−Cl(10)	2.529(3)
Sb(1)−Cl(5)	2.568(3)		Sb(2)−Cl(8)	2.605(4)
Sb(1)−Cl(3)	2.576(4)		Sb(2)−Cl(12)	2.617(4)
Sb(1)−Cl(4)	2.752(4)		Sb(2)−Cl(9)	2.685(4)
Sb(1)−Cl(1)	2.754(4)		Sb(2)−Cl(11)	2.738(4)
Sb(1)−Cl(6)	2.789(3)		Sb(2)−Cl(7)	2.838(4)
Angle	(°)		Angle	(°)
Cl(2)−Sb(1)−Cl(5)	89.77(13)		Cl(10)−Sb(2)−Cl(8)	89.91(13)
Cl(2)−Sb(1)−Cl(3)	87.59(14)		Cl(10)−Sb(2)−Cl(12)	90.11(14)
Cl(5)−Sb(1)−Cl(3)	90.89(14)		Cl(8)−Sb(2)−Cl(12)	92.27(13)
Cl(2)−Sb(1)−Cl(4)	89.57(13)		Cl(10)−Sb(2)−Cl(9)	86.76(13)
Cl(5)−Sb(1)−Cl(4)	178.50(16)		Cl(8)−Sb(2)−Cl(9)	89.21(12)
Cl(3)−Sb(1)−Cl(4)	87.74(13)		Cl(12)−Sb(2)−Cl(9)	176.53(13)
Cl(2)−Sb(1)−Cl(1)	88.75(12)		Cl(10)−Sb(2)−Cl(11)	88.94(13)
Cl(5)−Sb(1)−Cl(1)	87.71(14)		Cl(8)−Sb(2)−Cl(11)	178.46(12)
Cl(3)−Sb(1)−Cl(1)	176.08(15)		Cl(12)−Sb(2)−Cl(11)	88.74(14)
Cl(4)−Sb(1)−Cl(1)	93.62(13)		Cl(9)−Sb(2)−Cl(11)	89.72(13)
Cl(2)−Sb(1)−Cl(6)	178.27(14)		Cl(10)−Sb(2)−Cl(7)	177.55(17)
Cl(5)−Sb(1)−Cl(6)	88.74(12)		Cl(8)−Sb(2)−Cl(7)	88.10(13)
Cl(3)−Sb(1)−Cl(6)	91.55(13)		Cl(12)−Sb(2)−Cl(7)	91.40(13)
Cl(4)−Sb(1)−Cl(6)	91.89(12)		Cl(9)−Sb(2)−Cl(7)	91.79(12)
Cl(1)−Sb(1)−Cl(6)	92.08(12)		Cl(11)−Sb(2)−Cl(7)	93.03(13)
Compound 2
Bond	Dist.		Bond	Dist.
Sb(1)−Cl(5)	2.5480(15)		Sb(1)−Cl(6)	2.6324(14)
Sb(1)−Cl(1)	2.6136(16)		Sb(1)−Cl(4)	2.7367(15)
Sb(1)−Cl(3)	2.6324(15)		Sb(1)−Cl(2)	2.7880(15)
Angle	(°)		Angle	(°)
Cl(5)−Sb(1)−Cl(1)	89.60(5)		Cl(3)−Sb(1)−Cl(4)	91.94(5)
Cl(5)−Sb(1)−Cl(3)	89.66(5)		Cl(6)−Sb(1)−Cl(4)	85.57(5)
Cl(1)−Sb(1)−Cl(3)	90.41(5)		Cl(5)−Sb(1)−Cl(2)	176.56(5)
Cl(5)−Sb(1)−Cl(6)	88.44(5)		Cl(1)−Sb(1)−Cl(2)	90.00(5)
Cl(1)−Sb(1)−Cl(6)	92.06(5)		Cl(3)−Sb(1)−Cl(2)	93.75(5)
Cl(3)−Sb(1)−Cl(6)	176.88(5)		Cl(6)−Sb(1)−Cl(2)	88.16(4)
Cl(5)−Sb(1)−Cl(4)	89.67(5)		Cl(4)−Sb(1)−Cl(2)	90.59(5)
Cl(1)−Sb(1)−Cl(4)	177.54(5)

Table 2

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

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Compound 1
D−H···A	d(D−H)	d(H···A)	d(D···A)	∠DHA
C(1)−H(1A)···Cl(7)#1	0.93	2.94	3.692(18)	138.8
C(2)−H(2A)···Cl(5)#2	0.93	2.88	3.743(17)	154.1
C(3)−H(3A)···Cl(6)	0.93	2.76	3.549(15)	142.7
C(4)−H(4A)···Cl(7)#1	0.96	2.97	3.822(18)	148.1
C(4)−H(4B)···Cl(3)	0.96	2.87	3.659(18)	139.8
C(8)−H(8A)···Cl(1)#3	0.93	2.82	3.585(15)	140.7
C(9)−H(9A)···Cl(7)#4	0.93	2.79	3.673(15)	160.0
C(10)−H(10A)···Cl(10)#3	0.93	2.94	3.710(14)	141.4
C(11)−H(11A)···Cl(4)#3	0.96	2.78	3.631(17)	148.5
C(11)−H(11C)···Cl(11)#3	0.96	2.78	3.606(17)	144.9
C(12)−H(12A)···Cl(9)#3	0.97	2.82	3.750(15)	160.9
C(12)−H(12B)···Cl(12)#4	0.97	2.75	3.617(15)	148.6
C(15)−H(15A)···Cl(11)#5	0.93	2.77	3.609(19)	151.2
C(16)−H(16A)···Cl(6)#6	0.93	2.88	3.763(18)	159.9
C(17)−H(17A)···Cl(8)#7	0.93	2.90	3.549(16)	127.7
C(17)−H(17A)···Cl(9)#7	0.93	2.89	3.618(16)	135.6
C(19)−H(19A)···Cl(10)#7	0.97	2.94	3.832(17)	153.2
C(19)−H(19B)···Cl(3)#6	0.97	2.73	3.616(16)	151.6
C(23)−H(23A)···Cl(1)	0.93	2.62	3.46(2)	150.0
C(24)−H(24A)···Cl(9)#8	0.93	2.96	3.68(2)	135.0
C(25)−H(25A)···Cl(9)#8	0.96	2.56	3.50(3)	165.4
C(25)−H(25B)···Cl(12)#6	0.96	2.64	3.53(3)	155.9
C(26)−H(26A)···Cl(6)	0.97	2.72	3.63(3)	156.9
C(27)−H(27B)···Cl(3)#9	0.97	2.95	3.74(3)	140.4
C(29)−H(29A)···Cl(1)#3	0.93	2.84	3.56(3)	134.3
C(30)−H(30A)···Cl(12)#7	0.93	2.84	3.73(3)	162.3
C(31)−H(31A)···Cl(2)	0.93	2.61	3.34(3)	135.8
C(31)−H(31A)···Cl(3)	0.93	2.87	3.66(2)	143.6
C(33)−H(33B)···Cl(8)#7	0.97	2.77	3.56(3)	139.2
C(36)−H(36A···Cl(2)	0.93	2.39	3.275(19)	158.4
C(38)−H(38A)···Cl(11)#5	0.93	2.66	3.468(19)	145.5
C(39)−H(39A)···Cl(7)#5	0.96	2.83	3.60(3)	137.7
Compound 2
D−H···A	d(D−H)	d(H···A)	d(D···A)	∠DHA
C(1)−H(1A)···Cl(5)	0.95	2.72	3.582(7)	151.1
C(2)−H(2A)···Cl(3)#1	0.95	2.64	3.470(6)	146.3
C(3)−H(3A)···Cl(4)#2	0.95	2.77	3.573(6)	143.3
C(3)−H(3A)···Cl(6)#2	0.95	2.91	3.617(6)	132.6
C(4)−H(4A)···Cl(2)#2	0.98	2.68	3.570(6)	151.8
C(4)−H(4C)···Cl(1)	0.98	2.90	3.761(7)	146.6
C(5)−H(5A)···Cl(6)#2	0.99	2.66	3.447(7)	136.9
C(5)−H(5B)···Cl(3)#1	0.99	2.91	3.808(7)	151.0
C(12)−H(12A)···Cl(2)#2	0.95	2.75	3.468(6)	132.6
C(13)−H(13A)···Cl(2)#1	0.95	2.64	3.449(6)	143.1
C(14)−H(14A)···Cl(6)	0.98	2.79	3.368(6)	118.0
C(14)−H(14C)···Cl(2)#1	0.98	2.73	3.636(6)	154.1
C(15)−H(15A)···Cl(1)#1	0.99	2.72	3.590(6)	146.3
C(15)−H(15B)···Cl(6)#2	0.99	2.78	3.568(6)	137.3
C(15)−H(15A)···Cl(1)#1	0.99	2.72	3.590(6)	146.3
C(15)−H(15B)···Cl(6)#2	0.99	2.78	3.568(6)	137.3
C(21)−H(21A)···Cl(4)#1	0.95	2.90	3.683(6)	140.8
C(22)−H(22A)···Cl(5)#3	0.95	2.86	3.649(6)	141.5
C(23)−H(23A)···Cl(4)	0.95	2.94	3.714(5)	138.9
C(23)−H(23A)···Cl(5)	0.95	2.80	3.552(6)	136.2
C(24)−H(24A)···Cl(4)#1	0.98	2.80	3.736(5)	159.2
C(24)−H(24C)···Cl(6)	0.98	2.79	3.495(6)	129.4
C(25)−H(25A)···Cl(4)	0.99	2.86	3.668(6)	139.8
C(25)−H(25B)···Cl(3)#3	0.99	2.84	3.616(6)	135.7
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 mono-clinic space group of Pn. 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]₃SbCl₆ with 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 of π…π accumulation between two imidazole rings (Fig. 1d, Table 3).

Figure 1

Figure 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

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Table 3

Table 3.  π…π Interactions in Compound 1

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Cg(I)→Cg(J)	Cg···Cg (Å)	α (°)	β (°)	γ (°)
Cg(3)→Cg(6)	4.1369	26.51	19.68	8.15
Cg(4)→Cg(3)	5.2276	57.60	20.93	78.44
Cg(6)→Cg(2)	4.7628	62.21	9.23	61.86
Cg(6)→Cg(3)	4.1369	26.51	8.15	19.68
Cg(3): N(5)→C(15)→C(16)→N(6)→C(17); Cg(4): N(7)→C(22)→C(23)→N(8)→C(24); Cg(6): N(11)→C(36)→C(37)→N(12)→C(38)

Single crystal of 2 belongs to the hexagonal space group of P6₃ and 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 0D structure (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 2D layer along the ab plane (Fig. 2d). Further connected by hydrogen bonds along the c axis (e.g., C(24)–H(24A)···Cl(4) and C(24)–H(24B)···Cl(6), Fig. 2c), a 3D supramolecular structure finally could be obtained. Additionally, π···π interactions between two imidazole rings in 2 were observed (Fig. 2e, Table 4).

Figure 2

Figure 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

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Table 4

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

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Cg(I)→Cg(J)	Cg···Cg (Å)	α (°)	β (°)	γ (°)
Cg(1)→Cg(2)	3.6355	6.54	26.46	20.59
Cg(2)→Cg(1)	3.6355	6.54	20.59	26.46
Cg(3)→Cg(3)	5.6011	22.02	43.29	65.06
Cg(1): N(3)→C(11)→C(12)→N(4)→C(13); Cg(2): N(1)→C(1)→C(2)→N(2) →C(3); Cg(3): N(5)→C(21)→C(22)→N(6)→C(23)

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 N₂ atmosphere 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 SbCl₃ and the corresponding ILs, clearly the thermal properties of the title compounds are highly related to the high thermal stability of ILs.

Figure 3

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

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Figure 4

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

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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 of ns² electron configuration ions, the two obvious absorption peaks in the absorption spectra can be assigned to ¹S₀ → ¹P₁ and ¹S₀ → ³P₁ transition, 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 to ³P₁→¹S₀ transition 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]}.

Figure 5

Figure 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

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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]:

where θ_n are the Cl−Sb−Cl bond angles, d_n are the Sb−Cl bond lengths, and d is 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 of σ² and 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

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

DownLoad: CSV

Compounds	PL (nm)	Emission colour	Stokes shift (nm)	PLQY (%)	σ2	Δd (*10-4)	Ref
[(C6H22N4)2(Sb2Cl10) (SbCl6)(Cl)2(H3O)]·3(H2O)	517	Green	165	45	5.95	0.03	[42]
(C6H22N4)4(SbCl6)3(Cl)7·4(H2O)	580	Yellow	200	43	9.57	0.18	[42]
(C6H22N4)2(SbCl6)2(Cl)2·3(H2O)	638	Red	290	6	105.14	90.79	[42]
(Bzmim)3SbCl6	560	Green	202	87.5	5.54	10.08	[38]
[Hmim]3SbCl6 (2)	607	Orange-yellow	242	49.2	6.91	9.07	Thiswork
[Prmim]3SbCl6 (1)	627	Orange	257	32.5	6.24	14.30	Thiswork

4. CONCLUSION

In summary, by combining SbCl₃ and 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.

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