一種電荷轉(zhuǎn)移晶體的磁和介電弛豫性質(zhì)

張雪梅　于姍姍　張輝　段海寶＊,(安徽科技學院化學與材料工程學院，蚌埠3300)(南京曉莊學院環(huán)境科學學院，南京7)

一種電荷轉(zhuǎn)移晶體的磁和介電弛豫性質(zhì)

張雪梅1于姍姍2張輝2段海寶＊,2
(1安徽科技學院化學與材料工程學院，蚌埠233100)
(2南京曉莊學院環(huán)境科學學院，南京211171)

在外加電場下，利用分子的旋轉(zhuǎn)和取向運動是組裝分子介電馬達以及弛豫型分子介電體的一個主要策略。在本論文中，我們制備并表征了一個新的電荷轉(zhuǎn)移晶體[C10-DMPy][Ni(mnt)2](1)(C10-DMPy+=1-decanel-N,N-dimethylpyridinium,mnt2-=馬來二氰基二硫烯)。在一定的頻率范圍內(nèi)，該化合物展現(xiàn)了介電弛豫行為，我們將此歸于平衡陽離子的動力學位置取向和陰、陽離子間的電荷轉(zhuǎn)移。該化合物的介電弛豫過程遵循Cole-Cole方程，偏離理想的Debye模型。單晶X-射線衍射表明該化合物的陰、陽離子分別獨立堆積成柱狀，柔性的有機陽離子與剛性的磁性陰離子構筑塊間存在電荷協(xié)助氫鍵作用。此外，該化合物的磁行為展現(xiàn)為弱的鐵磁耦合作用。

電荷轉(zhuǎn)移晶體；晶體結(jié)構；介電響應；磁性質(zhì)

Relaxation means a systems monotonous approach to the equilibrium state after some excitation.In the case of dielectric relaxation one considers the response of polarization to an external alternating current field.Dielectric relaxation spectroscopy can provides information about the orientation adjustment of mobile charge present in the dielectric medium[1-3]. The dielectric relaxation methods are commonly usedin studies of ionic conductivity and molecular dynamics in different dielectric materials,such as glasses,crystal and liquids[4-7].It is well known that the measurements of alternating current(ac) conductivity can provide the underling mechanisms of the dielectric relaxation[8-10].

In general,molecular rotation and orientation by applied electric field is one of the promising strategies for assembling the potential molecular ferroelectric and switchable dielectric[11-16].Approaches to creating such materials with internal reorientation or rotation include molecular crystal with rotary groups[17-18], inclusion compounds with rotating guests and rotorcoated surfaces[19-21].In our previous studies,a series of ion-pair compounds with spin-Peierls-type transition were achieved,which consist of bis(maleonitriledithio) metalate monoanion(abbr.[M(mnt)2]-and M=Ni or Pt) bearing S=1/2 spin and organic cation with tunable molecular conformation[22-24].Recently,we explored to introduce mobile organic cation into the rigid [Ni(mnt)2]-spin system and created a bifunctional compound[25].Further investigation indicated utilization of such rigid[Ni(mnt)2]2-molecular block as a stator unit can create a switchable dielectrics.However, there are a limited number of studies on the dielectric relaxation and ac conductivity of[Ni(mnt)2]-spin system.Theses compounds may show interesting dielectric features.Usually,there are weakly chargeassisted interactions between the mobile organic cation and rigid[Ni(mnt)2]-anion,and the dipole motion of the cations under an ac electrical field may give rise to an interesting dielectric response.

Encouraged by our previous study and abovementioned findings,in this paper,we present a new compound[C10-DMPy][Ni(mnt)2](1)(C10-DMPy+=1-decanel-N,N-dimethylpyridinium cation,mnt2-=maleonitriledithiolate).This compound showed novel dielectric relaxation behaviors.

1　Experimental

1.1Chem icals and reagents

All reagents and chemicals were purchased from commercial sources and used without further purification.The starting materials disodium maleonitriledithiolate(Na2mnt)and 1-decanel-N,N-dimethylpyridinium bromide were synthesized following the published procedures[26-27].

1.2Physical measurements

Elemental analyses(C,H and N)were performed with an Elementar Vario ELⅢanalytical instrument. IR spectra were recorded on a Bruker Vector 22 Fourier Transform Infrared Spectrometer(170SX)(KBr disc).Differential scanning calorimetry(DSC) experiments for 1 were carried out on a Pyris 1 powercompensation differential scanning calorimeter and the heating-cooling treatments were performed up to two cycles with the heating/cooling rate of 10 K·min-1. Magnetic susceptibility data on polycrystalline-sample were collected over the temperature range of 1.8～400 K for 1 using a Quantum Design MPMS-5 superconducting quantum interference device(SQUID) magnetometer.Temperature and frequency dependent dielectric constant,ε,dielectric loss,tanδ,the impedance,Z,and ac conductivity,σ(ac)measurements were carried out employing Concept 80 system (Novocontrol,Germany);the powdered pellet,which was prepared under 10 MPa pressure,and was coated by gold films on the opposite surfaces and was sandwiched by the copper electrodes.The pellet had a thickness of ca.0.80 mm and 78.5 mm2in the area. The rang of the ac frequencies was 1～107Hz.

1.3Preparations for 1

Na2mnt(2.0 mmol)and NiCl2·6H2O(1.0mmol) were mixed under stirring in MeOH.Subsequently,a CH3CN solution of 1-decanel-N,N-dimethylpyridinium bromide(2.0 mmol)was added,and the red precipitate was washed with MeOH.A MeOH solution with I2(0.40 mmol)was added to the mixture;the mixture was allowed standing overnight after stirred for 25 minutes.The black precipitate formed were filtered off,washed with MeOH and dried at 65℃in vacuum to give compound 1.Yield:～42%.

The single crystals suitable for X-ray analysis were obtained by evaporation of the corresponding compound in MeOH to give black-needle crystals for 1.

1.4X-ray crystallography

The diffraction data for single crystals were collected with graphite monochromated Mo Kα(λ= 0.071 073 nm)on a CCD area detector(Bruker-SMART).Data reductions and absorption corrections were performed with the SAINT and SADABS software packages[28],respectively.Structures were solved by the direct method and refined by the fullmatrix least-squares procedure on F2using SHELXL-97 program[29].The non-hydrogen atoms were refined anisotropically,and the hydrogen atoms were introduced at calculated positions(C-H 0.093 0 nm for benzene and 0.097 0 nm for methylene)and refined riding on the parent atoms with U(H)=1.2U eq (bonded C or N atoms).The crystallographic details about data collection and structural refinement are summarized in Table 1.Selected bond lengths and angles together with their estimated standard deviations are listed in Table 2.

CCDC:1415204,1.

Table 1 Crystal data and structural refinements at 293 K for 1

Table 2 Selected bond lengths(nm)and bond angles(°)for 1

2　Results and discussion

2.1Crystal structure

Compound 1 crystallizes in triclinic space group P1 at room temperature,as shown in Fig.1a,and an asymmetry unit is comprised of a pair of[Ni(mnt)2]-anion and C10-DMPy+cation.The[Ni(mnt)2]-anions possess an approximated planar geometry,and the mean-molecule-plane of[Ni(mnt)2]-anion,defined through four coordinated S atoms,makes a dihedral angle of 2.9°with the pyridine ring in the cation.The bond lengths and angles are in good agreement with the reported[Ni(mnt)2]-compounds[22-23].The cations exhibit different alkyl chain conformation,namely, from C11 to C19 atoms show completely trans-planar conformation in the hydrocarbon chain,whereas the C9 and C10 atoms in the pyridyl ring tail display a cis conformation.The direction of alkyl chain isalmost parallel to the long molecular axis of[Ni(mnt)2]-anions.All the alkyl of the cations are nor disordered at room temperature.

Fig.1(a)ORTEP view labelling atom and thermal ellipsoids drawn at the 20%probability level for 1;(b)Packing structure of 1 viewed along the a axis;(c)The layer arrangement parallel to the(111)p lane,showing the alternating stacks of anions and cations;(d)Anionic dimer with the longitudinal offset mode(Symmetry code:iii1-x,0.5+y,z;iv-x,1-y,-1+z).

As shown in Fig.1b and 1c,the anions and cations are aligned into segregated stacks, respectively.The two neighboring Ni[(mnt)2]-anions are formed intoπ-type dimer along the crystallographic a axis direction with the shorter interatomic separations is 0.3995 nm of Ni(1)…Ni(1)i(Symmetry code:ix,-1+y,z).The adjacent anions dimers with a slippage arrangement along both short and long molecular axes of the anions form a zigzag chain.Each anions chain was surrounded by four nonmagnetic cations stacks.There existed charge assisted C-H…N and N-H…N interactions between the adjacent anion and cation stacks,as demonstrated in Fig.1d,the shorter inter-atomic contacts are found (C(19)…N(1)0.331 8 nm,H(19)…N(1)0.256 8 nm, C(19)-H(19)…N(1)146.75°,C(23)ii…N(2)0.351 4 nm, H(23)…N(2)0.265 2 nm,C(19)-H(19)…N(1)161.19°, N(25)iii…N(3)0.352 3 nm,H(25A)…N(3)0.255 7 nm, C(19)-H(19)…N(1)175.14°,C(17)iv…N(4)0.349 6 nm, H(17B)…N(4)0.272 8 nm,C(19)-H(19)…N(1)138.57° (Symmetry code:iix,y,-1+z;iii1-x,0.5+y,z;iv-x,1-y, -1+z).Along the c axis direction,two C10-DMPy+cations are arranged in an anti-parallel arrangement, a longer centroid-to-centroid separation(0.680 750 nm)of pyridine owing to the steric hindrance between the N,N-dimethyl groups and the alkyl chain.The cations are further arranged into the individual cation layers,which are also parallel to the ab-plane.

2.2M agnetic property of 1

The plots of χmas a function of temperature of 1 in the temperature range of 1.8～400 K is displayed in Fig.2,whereχmrepresents the molar magnetic susceptibility with one[Ni(mnt)2]-per formula unit andthe diamagnetism contributed from the atomic cores was not removed.The overall magnetic behavior of 1 corresponds to a paramagnetic system with ferromagnetic coupling interaction.

Fig.2 Plots of χm-T for 1

The χmT value at 300 K is 0.390 emu·K·mol-1which is slightly higher than the spin-only value expected for system with S=1/2(0.375 emu·K·mol-1). With the temperature decrease,the value of χmslightly increases.Below 25 K,the χmvalues decrease steeply, and such the typical Curie paramagnetic behavior of 1 in low temperature region arises from the magnetic impurity caused by the lattice defects.In order to estimate the magnetic exchange nature of 1,the simple Curie-Weiss law was used to analyze the magnetic susceptibility data over the range of 1.8～400 K:

where the symbols of χ0is contributed by the core diamagnetism and the possible temperatureindependent van Vleck-type paramagnetic susceptibility originated from the coupling of the ground and excited states through a magnetic field, and then C/T term represents the paramagnetism from the magnetic impurity.However,the obtained C values from fits are too large and seem unreasonable. The inability of Curie-Weiss models to describe the magnetic behavior indicates this 1-D spin system probably posses strong magnetic anisotropy,which leads to the temperature dependent magnetic susceptibility deviating from the isotropic magnetic coupling model.

2.3Dielectric p roperties

The relative permittivity ε*(ω)of the dielectric material as function of frequency is given by:

Frequency dependent of the dielectric permittivity ε′and dielectric loss tanδ=ε″/ε′for 1 are shown in Fig.3 in the temperature range of 50～130℃. From the Fig.3,it can be seen that the values ε′rapidly drops from about 40 to 7 with the increase of the field frequency from 1 to 104Hz at 130℃.The nature of the dielectric permittivity related to oscillating free dipoles(like long alkyl chain of the organic cation)in an alternating field.At very low frequencies,the dipoles motion can follow the applied electric field.As the frequency increases and reaches a characteristic value(ω=1/τ),the dielectric constant slightly decreases and exhibit relaxation process.The different model of mechanisms leads to the resonance dielectric relaxation spectra in the case of electronic polarization or molecular vibrations which occur at frequency beyond 1012Hz.Below this frequency,the dielectric relaxation spectra prevail relating to the behavior of dipole motion or ionic polarization.For 1, [Ni(mnt)2]-anions have rigid planar configuration and the motion at small electric field is very difficult. Therefore,the relaxation that appears in 1～104Hz could be attributed to dipole motion of the organic cations in 1.Dielectric relaxation process were also observed in the tanδ-f plot(Fig.3b).The dielectric relaxation spectra of 1,transformed into electric modulus spectra in Fig.4a by using Eq.(3),the dielectric modulus representation minimizes the unwanted effects of the extrinsic relaxation and is often used in the analysis of the dynamic conductivity of solids[30].It can essentially eliminate the problem ofthe electrode polarization and space charge injection phenomena.

Fig.3 Frequency dependences of ε′(a)and tanδ(b)for 1 in the range of 50～130℃

Fig.4 Frequency dependences of dielectric modulus M″(a)and p lots of τ versus T for 1(b)at selected temperature

It is observed that the maximum in M″peak shift to higher frequency with the temperature increase.In the frequency region below peak maximum,the charge carrier drifts to long distance.For the frequency above peak maximum M″,the carrier seem to be confined to potential well,thus drifts to short distances or spatially localized.In order to get the deep insight into the dielectric relaxation process,the frequencydependence of peak for the dielectric loss at different temperature is plotted(Fig.4b)and the following relation is:

Where τ=1/fmaxand fmaxis the frequency at maximum in the plot of tanδ-f under a selected temperature;τ0represents the characteristic macroscopic relation time,Eais the activation energy or potential barrier required for the dielectric relaxation,kBis Boltzmanns constant.The best fits giving the following results using Eq.(4):τ0=8.928(3)× 10-13s and Ea=0.63(1)eV.According to our previous work,this dielectric relaxation is attributed to dynamic orientation motion segmental motions of the organic cation.Similar relaxation have been reported in polymer.The Eaand τ0values in this compound are slightly larger than our reported compound 1,10-bis(1-methylimidazolium)decane bis(maleonitriodithiolato) nickelate[31].For this compound,the segregated stacks of anions and cations and diverse charge-assisted H…N,C…N and H…S interactions between the adjacent anion and cation stacks(as shown in Fig.1d)stabilize the cations in its lattice,furthermore,the rigid pyridine ring compared to imidazolyl ring in 1 give seldom freedom for the motion of the cations,which may be the reason for its relatively larger activation energy and τ0.

Fig.5 Temperature dependences of ε″(a)and tanδ(b)for 1 at the selected frequency

Carefully check the temperature dependence dielectric constant(ε″)and dielectric loss(tanδ)at selected frequency for 1(Fig.5a and 5b),the second step dielectric relaxation was observed in the range of 50～110℃and at the frequency from 105to 106Hz. The dielectric constant and loss are almost constants at low temperature.With the temperature increasing,the wide dielectric loss and constant peak become visible in the range of 50～110℃and maxima of all peaks shift toward high frequencies.This dielectric behavior is typical thermal assisted dielectric relaxation,and may be related to charge transfer of the cations and anions.At high frequencies(105to 106Hz),the dipoles motion(organic cation segments motion,the first step dielectric relaxation)cannot follow the applied electric field,and second relaxation becomes apparent.

The relaxation process for 1 at selected temperatures was fitted using Cole-Cole model function:

Fig.6 Plots of ε″versus ε′at selected temperatures for 1 (Open:experimental data;lines:theoretically reproduced using Eq.(5)).

Where the symbols ε0and ε∞are respectively the static and high frequencies limits of dielectric permittivity,is relaxation time related to the frequency of maximum dielectric loss and is parameter which describe the shape of the relaxation spectra with the range of 0～1.For an ideal Debye relaxation,α=0.Fig.6 shows the Cole-Cole plots of the relaxation process for 1 at selected temperatures,and the best fits using Eq.(5)for the plots of ε”-ε yielded the corresponding parameters ε0,ε∞and α for 1, which are summarized in Table 3.The fitted ε∞parameter is closed to the dielectric constant at higher frequency(f＞105Hz)for 1,and the fitted α parameters deviate from zero,indicating that the relaxation process depart from the ideal Debye dielectric response.

Table 3ε0,ε∞and α parameters for 1

2.4Com p lex impedance analysis

The increase of dielectric loss and dielectric permittivity at higher temperature is due to the conductivity increase of the sample.To understand this dielectric relaxation and analyze dynamics of the ionic movement in crystal,the complex impedance(Z′-Z″)plot at different temperature was made(Fig.7a). The plot shows a single semicircle for selected temperature related to bulk effects.It indicates that single conductivity process take place in the sample. These impedance plots were solved by fitting using equivalent circuit where each impedance semicircle can be represent by a resistor,R,and capacitor,C,in parallel.It is clearly seen from the Fig.7a that the radius of semicircle decreases with increasing temperature,which indicates that the decrease of the bulk resistance with an increases of the temperature. The temperature dependent conductivities σdcare plotted in the form of lgσdcversus 1 000/T,as shown in Fig.7b,the lgσdcas a function of 1 000/T shows linear relationship in the temperature range of 403～473 K, and the activation energy(Edc)was estimated as 0.57(2)eV,which is similar with the activation energy obtained from dielectric relaxation.The similar active energy implies that charge carrier has to overcome the same energy barrier while conducting as well as relaxation.The conduction of 1 due to a process of charge transfer between the cation and anion.Fromthe single crystal structure,there are diverse weak interactions between the anion and cation.As the temperature increasing,the motion of the alkyl chain of the cation induces the enhancement charge transfer and charge carrier.The motion of charge carriers in the low-mobility solids is accompanied by an electric relaxation.The charge transfer between the cation and anion is accompanied by the change in the direction of the dipole movement in the samples.

Fig.7 Complex-plane impedance plots for 1 at various temperatures(a)and temperature dependence of σdcfor 1(b) (Black dot:obtained from using an equivalent circuit;lines:theoretically reproduced)

3　Conclusions

In summary,a molecular magnetic based on [Ni(mnt)2]-spin system have been synthesized and characterized structurally via incorporating the easily mobile organic cation.Some weakly charge-assisted interactions between the mobile organic cation and rigid[Ni(mnt)2]-anion were observed in the crystal structure of 1.The dielectric results indicated that the dielectric relaxation in the investigated frequency range originates from the dynamic orientation motion of alkyl chain of the organic cation.The overall magnetic behavior of 1 corresponds to a paramagnetic system with ferromagnetic coupling interaction.This study provided a new strategy for the design of relaxlike dielectric.

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Magnetic and Relax-like Dielectric Response Behavior in a Charge-Transfer Crystal

ZHANG Xue-Mei1YU Shan-Shan2ZHANG Hui2DUAN Hai-Bao＊,2
(1School of Chem ical and Material Engineering,Anhui Science and Technology University,Bengbu,Anhui 233100,China)
(2School of Environmental Science,Nanjing Xiaozhuang University,Nanjing 211171,China)

Molecular rotation and orientation by applied electric field is one of the promising strategies for assembling the potential molecular dielectric rotors and relax-like dielectric.Here,a new charge transfer compound [C10-DMPy][Ni(mnt)2](1)(C10-DMPy+=1-decanel-N,N-dimethylpyridinium cation,mnt2-=maleonitriledithiolate), which shows interesting dielectric relaxation process,is synthesized and characterized.The anions and cations of 1 are aligned into segregated stacks.There existed weakly charge-assisted hydrogen bonding interactions between the mobile organic cation and rigid[Ni(mnt)2]-anion.Large temperature-dependent dielectric constant values and dielectric relaxation process of 1 can be ascribed to the dynamic orientation motion of alkyl chain of the organic cation and charge transfer between anions and cations.The overall magnetic behavior of 1 corresponds to a paramagnetic system with ferromagnetic coupling interaction.CCDC:1415204,1.

charge-transfer crystal;crystal structure;dielectric response;magnetic properties

O614.81+3

A

1001-4861(2016)01-0025-09

10.11862/CJIC.2016.002

2015-07-27。收修改稿日期：2015-10-16。

國家自然科學基金(No.21201103，21301093)資助項目。

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