EthoxycarbonylmethylViologen Induced Metal Halide-based Hybrids: Structures, Photoluminescence and Photocurrent Response Behavior①

CHEN Jia-Yue

?

EthoxycarbonylmethylViologen Induced Metal Halide-based Hybrids: Structures, Photoluminescence and Photocurrent Response Behavior①

CHEN Jia-Yue②

(350007)

Two newethoxycarbonylmethylviologen induced metal halide-based hybrids, (AeV)2(Bi4I16)(DMF)2(1) and (AeV)(CoCl4) (2) (AeV2+= N,N?-bis(ethoxycarbonylmethyl)-4,4?- bipyridinium) have been synthesizedand structurally determined by X-ray diffraction method. Under the direction of a new template AeV2+, the (Bi4I16)4-tetramer constructed from four edge-sharing BiI6octahedra (for 1) and (CoCl4)2-mono-nuclear (for 2) were obtained. Furthermore, C–H···O andC–H···X (X = I, Cl) hydrogen bonds contribute to the extension of structures from 0-D to 1-D chains. Their energy band gaps of 2.18 and 2.41 eV indicate their semiconductor properties, and the photoluminescence was detected on 1. Interestingly, 2 exhibits good photocurrent response behavior. Electronic structure analysis was executed to correlate the structure/property.

organic-inorganic hybrid, optical adsorption spectrum, photoluminescence, photocurrent response;

1 INTRODUCTION

In the last decade, the chemistry of polynuclear halide anionic complexes based on main group metals (such as Bi(III), Sb(III), et al)[1, 2]and transi- tion metals (Cu(I), Co(II), et al)[3, 4]have attracted great attention. These interests are inspired by their versatile structures[1, 2, 5]and numerous promising physical, for example, semiconductivity[6, 7], photo/thermochroism[8, 9]and luminescence[10, 11]in organic-inorganic hybrid salts. In addition, post- transition metal or main group metal halides are very important in the advanced materials science studies due to the favorable kinetics and low energy of M-X bonding[5].Their solution configurations of (MX6)3?/(MX4)2?can act as building blocks in con- structing polynuclear motifs or other complicated inorganic systems in the solid state. So far, for the bismuth(III) halides, the discrete mono- or polynu- clear structures have been observed, for example, mononuclear BiI3, [BiI4]?, [BiI5]2?, and [BiI6]3?, dinuclear Bi2I6, [Bi2I8]2?, [Bi2I9]3?, and [Bi2I10]4?, trinuclear [Bi3I11]2?and [Bi3I12]3?, tetranuclear [Bi4I14]2?and [Bi4I16]4?, pentanuclear [Bi5I18]3?and [Bi5I19]4?, hexanuclear [Bi6I22]4?, and octanuclear [Bi8I28]4?and [Bi8I30]6?[1, 2]. And transition metal halide A2[MX4] type compounds (M = divalent Mn, Fe, Co, Cu or Zn) have been reported[12-16]. Besides, as well-known good electron acceptors, viologens have been used in solar energy storages, electrochro- mic display devices and organic electrical con- ductors[17, 18]. But in the viologen/haloplumbate- based thermochroism materials, the phase transition temperature seems high. In order to design novel, molecular-ionic crystals with phase transitions, one of the most efficient strategies is to introduce such moiety that can reorient with the change of tempera- ture. Therefore, we here introduce ethoxycarbonyl- methyl into 4,4?-bipyridine to lower the phase transi- tion barriers. In this work, we obtained two new metal halide/organic hybrids, (AeV)2(Bi4I16)(DMF)2(1), (AeV)(CoCl4) (2) (AeV2+= N,N?-bis(ethoxycar- bonylmethyl)-4,4?-bipyridinium). Their energy band gaps were discussed, and the photoluminescent pro- perty was explained by electronic structure analysis.

2 EXPERIMENTAL

2.1 Materials and methods

All chemicals except (AeV)·2Cl were of regent grade, obtained from commercial sources and used without further purification.Elemental analyses for C, H and N were performed on a Vario MICRO ele- mental analyzer. IR spectra were recorded on a Perkin-Elmer Spectrum-2000 FTIR spectrophoto- meter (4000～400 cm-1). UV-Vis spectra were measured on a Perkin-Elmer lambda 900 UV/Vis spectrophotometer equipped with an integrating sphere at 293 K, and the BaSO4plates were used as reference. Fluorescence spectrum was carried out on a PW2424 spectrometer.

2.2 Computational details

The band structure calculation was based on density functional theory (DFT)[19], 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 exchange-correlation functional in the CASTEP code[20]. The number of plane waves included in the basis was determined by a cutoff energyEof 550 eV.

2.3 Synthesis

Synthesis of (AeV)·2Cl (AeV)·2Cl was synthe- sized by a one-step N-alkylated reaction of 4,4?- bipyridine with ethyl chloroacetate according to literature method[21]:

Synthesis of (AeV)2(Bi4I16)(DMF)2(1) 1 was prepared by a solution method. BiI3(0.1185 g, 0.2 mmol), (AeV)·2Cl (0.0803 g, 0.2 mmol) and KI (0.0166 g, 0.1 mmol) were dissolved in 10 mL DMF and kept stirring for 3 h. Then the pH value of the suspension was adjusted to 3.0 with HI (55%) and continued reacting for 1 h till the solution became clear red. The resultant solution was filtered and the red filtrate liquor was kept at room temperature for evaporation. The red block crystals can be obtained after 10 days. The hydrolysis of ester bond of AeV2+ has not happened, because the reaction is a thermodynamic control product. Yield: 36.2% (0.5659 g, based on Bi). C42H57Bi4I16N6O10(3672.26): calcd. C, 13.72; H, 1.55; N, 2.29%. Found: C, 13.55; H, 1.35; N, 2.45%. IR(cm-1): 3453(s), 3045(m), 2975(w), 1745(s), 1660(s), 1634(s), 1551(w), 1445(m), 1370(m), 1234(m), 1213(s), 1094(w), 1013(m), 811(m), 492(w).

Synthesis of (AeV)(CoCl4)(2) The synthesis process of 2 was similar to that of 1 except that the BiI3and KI were replaced by CoCl2·6H2O (0.0724 g, 0.3 mmol) and KCl (0.0455 g, 0.6 mmol), and the pH value of the mixture was adjusted to 4.0 with HCl. The green block crystals were obtained after 10 days. Yield: 43.5% (0.0692 g, based on Co). C18H22Cl4CoN2O4(531.11): calcd. C, 40.71; H, 4.14; N, 5.27%. Found: C, 40.33; H, 4.25; N, 5.36%. IR(cm-1): 3061(m), 2919(m), 1763(s), 1754(s), 1638(s), 1558(m), 1447(m), 1366(m), 1236(s), 1205(s), 1020(m), 809(m), 475(w).

2.4 X-ray crystallography

Table 1. Important Bond Lengths (?) and Bond Angles (°) for 1 and 2

Symmetry code: a –x, y, –z+1/2

Table 2. HydrogenBonds Details in 1 and 2

2.5 Electrode preparation and photocurrent measurement

Film of 2 was prepared using the solution coating method. 0.5 mg of the new prepared compound 2 was dissolved in 1.5 mL DMF, and the solution was coated on the ITO glass (0.6 × 0.6 cm2). The coating film was obtained after the solvent was carefully removed under reduced pressure. A 150 W high- pressure xenon lamp, located 10 cm away from the surface of the ITO electrode, was employed as a full-wavelength light source. The photocurrent experiment was carried out on a CHI650E electro- chemistry workstation using a three-electrode system, in which the sample-coated ITO glass was used as the working electrode, Pt wire as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The supporting electrolyte solution was a 0.1 mol·L?1sodium sulfate aqueous solution. The applied potential was 0.5 V for all measurements. The lamp was kept on continuously, and a manual shutter was used to block exposure of the sample to the light. The sample was typically irradiated at an interval of 10 s.

3 RESULTS AND DISCUSSION

3.1 Structure description

The organic-inorganic hybrid structure of 1presents a zero-dimensional (Bi4I16)4?tetranuclear cluster templated by AeV2+cation. Intra-/intermole- cular C–H···O and C–H···I hydrogen bonds con- tribute to the structural extending from 0-D cluster to 1-D chain. In the (Bi4I16)4?tetramer, all bismuth centers are in slightly distorted BiI6octahedral geometries, and adjacent BiI6octahedra share their edges to generate this (Bi4I16)4-tetramer (Fig. 1a). Consequently, three kinds of iodide ligand bonding environments are found: terminal I (I(1), I(2), I(3), I(7), I(8), I(9), I(10), I(14), I(15), I(16), averaged distance: 2.9021 ?), μ2-I (I(4), I(5), I(12), I(13), averaged distance: 3.2022 ?) and μ3-I (I(6), I(11), averaged distance: 3.3751 ?, Table 1). The shortest I···I distance is 7.886 ?, indicating the absence of strong I···I interaction. All bond distances and bond angles of the anion are in agreement with the related iodobismuthate compounds[23-26]. The (Bi4I16)4-tetramer is one of the largest known discrete iodobis- muthate anions, which have previously been obser- ved in [CH3(CH2)2COS(CH2)2N(CH3)3]4[Bi4I16][23],[BiPc]4[Bi4I16][24], (Pc = phthalocyanine ligand), [Ru(bipy)3]2(Bi4I16)[25]and [Ru(bipy)2(bipyo]2(Bi4I16)[26].The C–C, C–N and C–O bonds of AeV2+cation are normal, and the dihedral angles 160.88 and 151.13° defined by two pyridine rings in two independent AeV2+can be observed. Two ethoxycarbonylmethyl groups locate at two sides of bipyridine plane with bending angles of 119.26 and 110.49°. Interestingly, versatile hydrogen bonds can be observed between AeV2+cations, (Bi4I16)4-tetramersand DMF solvents, which contribute to the formation of a 1-D chain along theaxis (Fig. 1b, Table 2). Most of these hydrogen bonds are all involved in the O atoms in ethoxycarbonylmethyl groups, which are led by lowering the rotation barrier of ethoxycar- bonylmethyl group. In other words, due to the rotation facility of ethoxycarbonylmethyl, various hydrogen bonds can be achieved, which contribute to the structural stabilization.

2 is composed by a CoCl42-mononuclear cluster and a AeV2+counteraction, among which C–H···Cl hydrogen bond contributes to the generation of a 1-D chain. The Cl(2) is disorder and in the structure description, only Cl(2A) is discussed. The CoCl42-mononuclear cluster adopts a slightly distorted tetrahedral geometry with the Co–Cl bond distances ranging from 2.2695(7) to 2.329(4) ? and the Cl–Co–Cl bond angles varying from 103.06(9) to 109.58(5)° (Fig. 2). The AeV2+cation adopts a chair-like configuration, in which two pyridine rings are coplanar and two ethoxycarbonylmethyl groups locate on two sides of bipyridine plane with bending angle of 112.96°. The C(6)–H(6B)···Cl(2A) hydro- gen bond between CoCl42-anion and AeV2+cation leads to the formation of a 1-D zigzag-like chain (Fig. 2, Table 2).

Fig. 1. (a) Structue of Bi4I16)4-tetramer; (b) 1-D chain based on hydrogen bonds

Fig. 2. Structure of CoCl42-mononuclear cluster and 1-D zigzag-like chain based on C–H···Cl H-bond

3.2 Adsorption spectra and linear absorption optical properties

Fig. 3a shows the diffuse reflectance UV-Vis absorption spectra of 1 and 2. Two compounds exhi- bit adsorption ranging from 250 to 500 nm, and absorption peaks at 302, 433 nm (for 1) and 294, 367 nm (for 2) can be observed. Compared the UV- Vis absorption spectra of bipyridine and relative compounds, the peaks at 302 and 294 nm can be assigned to the* and n-transfer of AeV2+cations, and peaks at 433 and 367 nm stem from metal halides[25, 26].Their optical gaps were assessed from optical diffuse reflectance data, and the Kubelka-Munk functions converted from the diffuse reflectance data were plotted in Fig. 3b[27, 28].As shown in Fig. 3b, optical gaps of 2.18 eV (for 1) and 2.41 eV (for 2) were calculated, illustrating their potential semiconductor properties.These gaps can be compared with that of [Ru(tpy)2]2[(Bi2I7Cl2)·I] (2.12 eV) and [Ru(bipy)2(bipyo]2(Bi4I16) (2.34 eV)[26]. Their relative narrower gaps can be ascribed to their weak interactions in the lattices.

Fig. 3. Room temperature solid UV-Vis absorption spectra with thickness of 2 mm (a) and diffuse re?ectance spectra in Kubelka-Munk units (b) of 1 and 2

3.3 Fluorescence properties

The luminescence properties of 1 and 2 were studied in the solid state at room temperature. 1 exhibits emission peaks at 449, 506 and 610 nm upon irradiation at 320 nm, but 2 is non-luminescent (Fig. 4). For 1, compared with the emissions of 2,2?-bipyridine and its derivants (365～442 nm)[29], the emission band at 449 nm can be attributed to the intra-ligand charge transfer of AeV2+dication, and these shifts may be led by the perturbation of substi- tuted groups on pyridine ring[30]. And the emission at 506 nm could be attributed to metal-assisted ligand- to-ligand charge transfer due to the presence of strong hydrogen bonds[31].Finally, the emission at 610 nm can be assigned to I→Bi electronic transi- tions within the (Bi4I16)4-iodobismuthate inorganic part, but Bi–Bi transfer can be ruled out because the Bi···Bi interaction is absent[32].

So as to clarify the nature of photoluminescent emissions, the density of states (DOS) of 1 was calculated using the CASTEP program[20].The calculated DOS of 1 (Fig. 5) shows that the top of the valence band derives from thecooperative contribution of I-5andorbitals of AeV2+dication, while the bottom of the conduction band is almost the contribution from the* antibonding orbitals of the AeV2+dication, and Bi-6orbitals also appear in the conductive band. Therefore, intra-ligand charge transfer of the AeV2+dication and I-5to Bi-6s coexist in the absorption transition of 1. Thus, the origin of the low wavelength emis- sion at 449 nm may be assigned to the intra-ligand* transition of the AeV2+ligand, and the longer wavelength emission at 506 nm may be led by metal-assisted ligand-to-ligand charge transfer, and that at 610 nm should be caused by I-5to Bi-6transfer in the iodobismuthate cluster.

Fig. 4. Room temperature solid-state luminescence spectrum of 1 (ex= 320 nm)

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

3.4 Photocurrent response behavior

In order to investigate the photoelectric con- version behavior of 2, a three-electrode system was used in the photocurrent response experiment, and the result can be seen in Fig. 6. Upon repetitive irradiation with xenon light on and off with an interval of 10 s, repeatable and steadyphotocurrents with rapid responses can be achieved, and there is not any decay after ten on/off cycles of illumination. The photocurrent was about 1.6 μA, which is much larger than that of Zn4L2(bpca)4·4DMF·9H2O (0.014 μA)[33]. The photocurrent response mechanism can be explained as the electron-transfer among the (AeV)(CoCl4)/ITO electrodes in solution: upon irradiation, electron transfers can occur from the (CoCl4)2-donors to the (AeV)2+cations to give (AeV)?+species, then the (AeV)?+radicals transfer their electrons to the ITO electrodes to produce the effective electron flow. The relative higher photo- current can also be led by the presence of electron- withdrawing ethoxycarbonylmethyl groups, which can stabilize the (AeV)?+species.

Fig. 6. Photocurrent response behavior of 2

4 CONCLUSION

By using the template of a viologen (AeV2+), two metal halide clusters, (Bi4I16)4-tetramer and (CoCl4)2-mononuclear cluster, were present. C–H···O andC–H···X (X = I, Cl) hydrogen bonds contribute to the structural extending from 0-D to 1-D chain. The energy band gaps of 2.18 and 2.41 eV indicate their semiconductor nature. Besides, photoluminescence was detected on 1 and 2, and 2 also exhibits good photocurrent response behavior.

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28 March 2018;

4 June 2018 (CCDC 1505780 and 1505781)

① This work was supported by the Science and Technology Funding Project of Fujian Provincial Department of Transportation (No. 201337)

. E-mail: fjfzcjy@126.com

10.14102/j.cnki.0254-5861.2011-2017