A New Inorganic-organic Hybrid Based on Biisoquinoline and Hexachloridostannate: Structure, Photoluminescence,Electrochemical Behavior and Theoretical Study

XIAO Guang-Can

(Testing Center of Fuzhou University, Fuzhou 350002, China)

1 INTRODUCTION

Organic-inorganic hybrid compounds based on anionic main group halides and functional organic cations have attracted much attention due to not only their rich structural motifs but also their highly tunable functional properties, including opt-electronic properties, such as nonlinear optical behavior,thermochroism, semiconductivity and ferroelectricity[1-6]. Among the main group metals, as a heavy p-block metal in IVA group, tin halides such as SnXn(X = l, Br, and I) are very special for their flexible coordination environments, variable stereochemical activities and superior carrier mobility, upon which they demonstrate potential applications in display and storage technologies[7-10]. In the reported tin halide/organic hybrid system, most hybrid compounds contain only simple organic amines, in which the organic components only act as weakly interacting individual molecules, so the physical properties generally stem from the inorganic tin halides[11-16]. To our knowledge, the introduction of functional organic molecules into tin halides is still in infancy[16-19]. The incorporation of functional organic molecules into hybrids seems significant and can help to obtain multi-functional materials[20,21].Kept this in mind, we here attempt to synthesize novel hybrid tin halides with functional organic cations, which could not only modify the inorganic skeleton by weak interactions such as hydrogen bonds for dimension extending, but also improve its functions like the charge carrier mobility. Isoquino-line and its derivatives (1,ω-bis(isoquinoline)alkane dications) are good candidates for conjugated functional unit carriers or organic templates, which have received close attention because of their broad range of biological activities[22]. In this work, by introducing biisoquinoline dications into the tin halide, a new inorganic/organic hybrid (BIQBT)(SnCl6) 1 has been synthesized, and it exhibits strong fluorescence,which was further explained by theoretical calculation.

2 EXPERIMENTAL

2.1 Materials and methods

BIQBT·2Br (1,4-bis(isoquinoline) butane bromide)was synthesized according to the literature method,using isoquinoline and 1,4-dibromobutane as starting materials. Other chemicals of regent grade quality were obtained from commercial sources and used without further purification. Elemental analyses for C, H and N were performed on a Vario MICRO elemental analyzer. IR spectra were recorded on a Perkin-Elmer Spectrum-2000 FTIR spectrophotometer (4000～400 cm-1). UV-Vis spectrum was measured on a Perkin-Elmer lambda 900 UV/Vis spectrophotometer equipped with an integrating sphere at 293 K, and BaSO4plates were used as reference. Fluorescence spectrum was carried out on a PW2424 spectrometer. Cyclic voltammetry was recorded on BAS100A electrochemical analysis instrument using Pt–C as the working electrode and Ag–AgCl as the reference electrode.

2.2 Computational details

In the energy calculations of E(organicmoiety,BIQBT2+),E(inorganic moiety, SnCl62-)and E(hybrid system) constructed from their cif files, only single-point calculations were conducted. But in the calculation of free BIQBT2+dications, full optimizations were carried out with standard 6-31g basis set. In these calculations, full electron 6-31g basis set was used for C, H, N and Cl atoms, which is high enough for light atoms. And pseudo-potential basis set cep-4g was applied in Sn atoms, which has been proved to be reliable in heavy atoms. All the calculations were performed using DFT/B3LYP method with the Gaussian03 program[23]. The band structure calculation was based on density functional theory(DFT)[24], in which wave functions were explained in a plane wave basis set and the spin polarized version of PW-91 GGA was employed for the exchangecorrelation functional in the CASTEP code[25]. The number of plane waves included in the basis was determined by a cutoff energy Ecof 550 eV (All the calculated input and output files can be found in the supplemental materials).

2.3 Synthesis of BIQBT·2Br

BIQBT·2Br (1,4-bis(isoquinoline) butane bromide)was prepared with isoquinoline and 1,4-dibromobutane as starting materials according to the reported method[26].

2.4 Synthesis of (BIQBT)(SnCl6) (1)

BIQBT·2Br (0.0711 g, 0.15 mmol) and SnI2(0.0372 g, 0.1 mmol) were dissolved in 10 mL methanol, and then 2 mL condensed HCl was added.The resultant solution was stirred for 2 h in air, then transferred and sealed in a 25 mL Teflon-lined reactor. The reactor was heated in an oven to 160 ℃for 4 days and cooled to room temperature at a rate of 2.2 ℃/h. Yellow block crystals in ca. 56.3%(0.0362 g based on Sn) were obtained and washed by methanol and ether. In this work, tin is +4 although the original material was SnI2, similar to the published results[27]. Here the Sn(II) may be oxidized by oxygen in the air. Anal. Calcd. for C22H22Cl6N2Sn (645.83): C, 40.91; H, 3.41; N,4.34%. Found: C, 41.26; H, 3.25; N, 4.42%. IR (KBr,cm-1): IR(KBr): 3170(s), 1699(s), 1591(s), 1531(m),1487(s), 1188(m), 792(s), 771(m).

2.5 X-ray crystallography

X-ray data on suitable single crystal of 1 with dimensions of 0.30mm × 0.25mm × 0.20mm were collected at 293(2) K with a Rigaku Weissenbery IP diffractometer using graphite-monochromated MoKα radiation (λ(MoKα) = 0.71073 Å) by using an ω-2θ scan mode. In the range of 3.35≤θ≤27.48°,out of the 12010 total reflections, 5018 were independent with Rint= 0.0190, of which 4614 were considered to be observed (I > 2σ(I)) and used in the succeeding refinement. The multi-scan absorption correction was applied. The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-squares techniques on F2using SHELXL-97 program[28]. Hydrogen atoms of C–H were generated geometrically. All non-hydrogen atoms were refined by full-matrix least-squares techniques for 4614 observed reflections with I >2σ(I) to the final R = 0.0197, wR = 0.0493 (w =1/[σ2(Fo2) + (0.0295P)2+ 0.6768P], where P = (Fo2+ 2Fc2)/3), S = 1.021, (Δ/σ)max= 0.001, (Δρ)min=–0.656 and (Δ/σ)max= 0.388 e/Å3. Important bond lengths and bond angles are listed in Table 1. Hydrogen bond details are given in Table 2.

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

Table 2. Hydrogen Bond Details in 1 (Length in Å and Angle in °)

3 RESULTS AND DISCUSSION

3.1 Structure description

According to structural analysis, the asymmetric unit of 1 consists of (SnCl6)2-anion and (BIQBT)2+dication. C–H···Cl hydrogen bonds among them contribute to the formation of 1-D chain. Besides,π···π interactions and electrostatic interactions stabilize the structure. The structure of (SnCl6)2-anion and (BIQBT)2+dication are given in Fig. 1(a)and (b), respectively. The 1-D chain based on C–H···Cl hydrogen bonds can be seen in Fig. 2, and its packing diagram along the b axis showing relative positions of organic and inorganic moieties is revealed in Fig. 3. In (SnCl6)2-mononuclear anion,the tin(IV) center is surrounded by six Cl-donors to give an octahedral geometry (Fig. 1(a)). In this SnCl6octahedron, Cl(2), Cl(3), Cl(4) and Cl(6)locate at the equator plane, while Cl(1) and Cl(5)occupy the axial positions. This SnCl6octahedron is ideal with Sn–Cl ranging among 2.4267(18)～2.4507(18) Å (Table 1), and Cl–Sn–Cl bond angles are also normal (88.63(3)～90.78(7)°, 179.37(8)～179.60(9)°), which are very close to 90° and 180°for an ideal octahedron. These geometric parameters are similar to the previously published results[27,29].

The C–C and C–N bonds of BIQBT2+dications are in normal ranges (Fig. 2(a)). And the bond angles on butuan spacer are 112.86(53) and 116.86(57)º,larger than that of 1,1΄-(nutane-1,4-diyl)dipyridi-nium dibromide dihydrate (110.16º)[30], n-butuan(109.48º) and free BIQBT2+(111.0239º and 110.0820º, optimized result by DFT calculation,supplemental materials). So, a slightly unfolding of BIQBT2+dications has occurred. In the solid situation of 1, two quinoline rings are generally parallel(dihedral angle 2.58º). But in free BIQBT2+, two quinoline rings are perpendicular with dihedral angle of 96.68º. This structural inversion should be driven by the formation of hydrogen bonds between C–H···Cl hydrogen bonds and π···π interactions,which will be discussed in section 3.2.

Fig. 1. Structure of (SnCl6)2- anion (a) and (BIQBT)2+ dication with atomic labeling scheme

Fig. 2. 1-D chain arrangement of 1 based on C–H···Cl hydrogen bonds

Fig. 3. Packing diagram of 1 along the c axis

In the crystal, three kinds of weak interactions can be observed for the structural stabilization. The first is the C–H···Cl hydrogen bonds among neighboring BIQBT2+dications and (SnCl6)2-mononuclear anions (Table 2), upon which an infinite one-dimensional strand extending along the a axis is given(Fig. 2). Secondly, intermolecular strong π···π stacking interactions with centroid-centroid distances of 3.692(4) and 3.711(4) Å among quinoline conjugated rings (dihedral angles: 7.4(3) and 6.0(3)°,perpendicular distances: 3.455(3)/3.572(2) and 3.508(3)/3.375(3) Å, respectively) also help to stabilize the structure. Based on the hydrogen bonds and π···π stacking interactions, a 3-D framework is given (Fig. 3). Finally, the electrostatic interactions between cations and anions contribute to the structural stabilization. A particularly obvious feature of this hybrid structure is the (SnCl6)2-mononuclear anion trapped within the cavity of chargebalanced catioins BIQBT2+cavities (Fig. 3), and its formation may be achieved by the synergistic interaction between the (SnCl6)2-and BIQBT2+dications.

3.2 Theoretical calculations

In this section, we want to disclose the role of weak interactions in the formation of solid by using DFT calculations. We conduct single point calculations on compound 1, (SnCl6)2-anion and BIQBT2+cation. In order to compare the energy differences of each moiety, full optimizations on (SnCl6)2-anion and BIQBT2+cation are also carried out. Furthermore, to investigate the strength of hydrogen bonds,we elongate the C···Cl distance to 4.10 Å so as to eliminate the hydrogen bond. All the starting geometries are constructed according to crystallographic parameters and calculated with G03 program at the DFT/B3LYP/6-31g/cep-4g level. For the BIQBT2+cation, the input and output files can be found in the supplemental materials. The interaction energy (including hydrogen bond, π···π stacking interaction and electrostatic force) was defined as follows: Eb= E(organic moiety)+ E(inorganic moiety)- E(hybridsystem). As indicated by calculation results, the interaction energy of 1 is 221.6992 kcal/mol. When concerning about the conformation change of BIQBT2+from free configuration (optimized geometry) to its solid situation, a 299.5010 kcal/mol energy increase is needed. This conformation change aims to the formation of hydrogen bonds and π···π stacking interaction in the lattice. But for (SnCl6)2-anion, because its ideal geometry, only 9.41 kcal/mol energy arise is observed, which is also promoted by the formation of C–H···Cl hydrogen bonds. If setting the C···Cl distance as 4.10 Å to eliminate the hydrogen bond, it exhibits destabilization with an energy arise of 14.8092 kcal/mol.And if we set the configuration of BIQBT2+in 1 as its free configuration (the dihedral angle between two quinoline rings: 96.68º) to turn off the π···π stacking interaction, the interaction energy decreases to 173.0223 kcal/mol. Therefore, their difference of 48.6758 kcal/mol is the contribution of π···π stacking interaction. Thus in all, in the hydrothermal reaction,the formation of (BIQBT)(SnCl6) hybrid using BIQBT2+and (SnCl6)2-building units must overcome an energy barrier of 308.911 kcal/mol. And among the interaction energy of 221.6992 kcal/mol in 1, hydrogen bonds give a contribution of 14.8092 kcal/mol, the contribution from π···π stacking interactions is 48.6758 kcal/mol, and the electrostatic force is the most important with amount of 158.2141 kcal/mol. Thereby, in this hybrid system with charge transfer character, if the electrostatic force is not taken into account, the π···π stacking interaction is dominant for their structural stabilization.

3.3 UV-Vis and fluorescence spectrum

The room-temperature UV-Vis absorption spectrum of 1 in DMF solution with concentration of 10-5mol/L is given in Fig. 4a, in which excitonic absorption at 273 and 335 nm can be observed. The UV-Vis spectra show typical absorption spectra for quinoline-based complexes[31]. The bands at 234 and 335 nm are assigned as ligand-centered (LC)transitions (π-π* transfer).

Fig. 4. (a) UV-Vis spectrum and (b) Room temperature solid-state luminescence spectrum of 1 (λex = 340 nm)

The fluorescence spectrum of 1 was measured in the solid state at room temperature. 1 exhibits strong emission band at 382 nm upon irradiation at 340 nm(Fig. 4b). The emission bands relative to quinoline derivates have been found at about 370 nm[32],suggesting that the emission at 382 nm can be assigned to the contribution of BIQBT2+cations. In order to understand the nature of their photoluminescent emissions, the density of states (DOS) of 1 was calculated using the CASTEP program[24,25].The calculated DOS (Fig. 5) of 1 shows that the top of the valence band derives from the cooperative contribution of Cl-3p orbitals and p-π orbitals of BIQBT2+dication, while the bottom of the conduction band is almost the contribution from p-π*anti-bonding orbitals of the BIQBT2+dication (Fig. 6).Therefore, the origin of low wavelength emission at 382 nm may be assigned to the LLCT (ligand-toligand charge transfer) stemming from BIQBT2+.

3.4 Electrochemical properties

The electrochemical property of 1 was gauged by voltammetric techniques in CH2Cl2solvent with concentration of 10-3mol/L. The scanning rate is set as 0.1 V/s and the scanning range is –2.0～2.0 V.The result is depicted in Fig. 6. The cyclic voltammogram (CV) of 1 shows three couples of oxidation reduction peaks at 1.106/–1.023 V,0.692/–0.758 V and 0.392/–0.435 V, which can be attributed to the electron transfers among Sn(IV)/Sn(II)/Sn(0). The ipa/ipc≈ 1, indicating that the reactions are reversible[33].

Fig. 5. Total and partial DOS of 1. Its position of the Fermi level is set at 0 eV

Fig. 6. Cyclic voltammogram of 1

4 CONCLUSION

In conclusion, a new inorganic-organic hybrid[(BIQBT)(SnCl6)]n(1, BIQBT = 1,4-bis(isoquinoline) butane) has been synthesized and structurally described. 1 consists of 1,4-bis(isoquinoline)butane dications and mononuclear hexachloridostannate SnCl62-anion, and hydrogen bonds among them contribute to the formation of a 1-D chain.Theoretical calculations indicate that the π···π stacking interaction is dominated for their structural stabilization. Its electrochemical behavior and photoluminescence were also discussed.

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