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J. Phys. Chem. C 2008, 112, 5074-5077
Dynamic Pseudo Jahn-Teller Effect and the Phase Transition Induced by Absorption of Molecules in Metal-Organic Nanotube Framework Svyatoslav P. Gabuda,* Svetlana G. Kozlova, Valeri A. Drebuschak, Danil N. Dybtsev, and Vladimir P. Fedin NikolaeV Institute of Inorganic Chemistry SB RAS, NoVosibirsk 630090, Russia ReceiVed: September 12, 2007; In Final Form: December 15, 2007
A second-order phase transition induced by absorption of acetone in nanoporous zinc terephthalate, Zn2(C8H4O4)2(C6H12N2), has been studied by 2H NMR and DSC data. The heterogeneous system [Zn2(C8H4O4)2(C6H12N2)]‚n(CH3)2CO (n ) 0-5) is tetragonal at n < ne ≈ 2.5, with the crystal symmetry being lowered at n > ne (to orthorhombic). The breaking of symmetry is related to a combination of a pseudo Jahn-Teller effect and the influence of quantum confinement on the dynamic behavior of the absorbed molecules in the metal-organic nanotube framework.
Introduction Physical properties of molecular substances located inside the internal space of nanotubes are of key interest for developing molecular machines or multicomponent systems able to undergo definite motions under the action of an external stimulus. The development of nanotube ink-jet printers1,2 presents the first example of manipulating a nanoliquid, a substance placed inside nanotubes, by virtue of piezoelectric contractions of the walls of ferroelectric nanotubes. However, molecular mobility and structural changes of such heterogeneous composite materials reaching a certain level of saturation with guest molecules remain poorly understood. Here we report a detailed 1,2H NMR study of the symmetry and order transitions occurring with acetone molecules embedded inside the channels of a recently reported3 metal-organic absorbent: zinc(II) terephthalate Zn2(C8H4O4)2(C6H12N2), comprising a tetragonal array of ∼1 nm (10 Å) nanotubes. Hereafter the absorbent is abbreviated as {A}, and the hetero composite of {A} with acetone as {A}‚n Ac, where Ac ) (CH3)2CO and n ) 0-5. It is noteworthy that the acetone is actively studied as a guest subsystem in different absorbents.4-6 Solid-state NMR certainly can be a powerful tool for examination of such dynamic processes as guest absorption and absorption-induced phase transitions in hetero structures. Samples The samples of the absorbent were prepared by heating a dimethylformamide (DMF) solution of Zn(NO3)2, terephthalic acid (1,4-benzenedicarboxylic acid, H2bdc), and 1,4-diazabicyclo[2.2.2]octane (dabco) at 110 °C for 2 days. The procedure gave a crystalline product [Zn2(bdc)2(dabco)]‚4DMF‚1/2H2O ({A}‚ 4DMF‚1/2H2O) with a yield exceeding 80%.3 According to single-crystal X-ray diffraction data, the structure of {A}‚4DMF‚ 1/2H2O is composed of dinuclear paddlewheel Zn2 units bridged by bdc dianions to form a distorted 2D square-grid [Zn2(bdc)2]. The axial sites of the Zn2 paddlewheels are occupied with dabco molecules acting as pillars and connecting the 2D layers into a 3D structure [Zn2(bdc)2(dabco)] of {A} (Scheme 1). * Corresponding author. Phone: 7(383)3307531. E-mail: gabuda@ che.nsk.su.
SCHEME 1: Extension of the 2D Square-Grid of {Zn2(1,4-bdc)2} into a 3D Structure by DABCO Linkers at the Axial Positions
The overall topology of the framework in {A}‚4DMF‚1/2H2O is best described as a slightly compressed primitive cubic (R-Po) net. The dabco pillars violate the symmetry of the crystallographic fourfold axis and are disordered.3 Interestingly, the bdc linker is unusually bent resulting in severe twisting of the Zn2 paddlewheel moieties as compared to an ideal square grid. TGA data indicate that {A}‚4DMF‚1/2H2O looses its guest molecules in the temperature range 100-200 °C, resulting in the permanently porous host framework {A}. The crystal structure of guest-free host {A} is tetragonal (space group P4/mmm) with regular nanosized channels 7.5 × 7.5 Å2 running along the C4 axis, which are large enough to include some small molecules like acetone. The host structure {A} is reported to undergo phase transitions upon inclusion of N,N′-dimethylformamide (DMF) and benzene. Space groups of the corresponding host-guest complexes (I4/mcm for {A}‚ 4DMF and Cmmm for {A}‚2C6H6) were determined by X-ray diffraction. However, there are no clues to the mechanism of this unique guest-dependent behavior. For this reason, here we present similar phase transitions occurring in the microporous host {A} upon the acetone inclusion examined by solid-state NMR methods.
10.1021/jp077321i CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
Absorption of Molecules in a Nanotube Framework
J. Phys. Chem. C, Vol. 112, No. 13, 2008 5075
Figure 1. 1. 2H NMR spectrum of {A}‚nAcD at n < nc (ν0 ) 13 MHz).
Experimental Section Acetone was absorbed by the step-by-step method from saturated vapor at normal conditions. The sorption was monitored by weight. During the experiments, we obtained 35 samples with varying saturation degrees: from the lowest one to the maximum saturation (0.5% and 48% increment of the sample mass, respectively). Under complete saturation, each formula unit of the sorbent framework {A} contains ∼5 molecules of (CH3)2CO. Thermocalorimethric studies of the saturated samples {A}‚5Ac and {A}‚5AcD [where AcD is 99% deuterated acetone, (CD3)CO] showed thermal anomalies at 170 K for {A}‚5Ac and 180 K for {A}‚5AcD, what is close to the freezing points of pure acetone and deuteroacetone, respectively. Raman spectra of the acetone-saturated samples were recorded under normal conditions, the vibration of the CO group (∼1740 cm-1 7) characteristic of the CO forms of acetone being clearly observed. 1,2H NMR spectroscopy was used to study molecular dynamics and ordering in the systems {A}‚nAc and {A}‚nAcD. The 1,2H NMR spectra were recorded with a pulse Bruker CXP-300 spectrometer at Larmor frequencies νo ) 300 and 46 MHz. To gain precise lineshapes of the 2H and 1H NMR spectra, we additionally used a continuous-wave autodyne differential scanning spin detector. Results 1. 1,2H NMR Spectroscopy Data. In all cases the 1H NMR spectra were a complex superposition of broad bands (δB ≈ 7 G or 100 ppm) originating from the hydrogens of the nanoporous host {A} and narrow featureless lines (half-width δB ≈ 0.3 G or ∼4 ppm) from the hydrogens in the nanoliquid subsystem (acetone). At the same time, the 2H NMR spectra of {A}‚nAcD (Figures 1 and 2) contained only the deuteron lines in the nanoliquid subsystem; therefore, no exchange was observed between the hydrogen atoms of acetone and those of the porous framework. When the saturation degree of the porous framework {A}‚ nAcD was in the range n ) 0-2.5 (or n < nc ≈ 2.5, where nc is the critical saturation degree value), the shapes of the experimental 2H NMR spectra can be represented by the Gaussian function f(ν - νo) ∼ exp[-(ν - νo)2/2β2], where β is the experimental parameter approximately equal to the halfwidth of the NMR line (Figure 1). However, when the saturation exceeds critical value (n > nc ≈ 2.5), the shape of the 2H NMR spectra of the {A}‚nAcD system drastically changes. The recorded fine structure of the spectra (Figure 2) is typical of 2H
Figure 2. 2H NMR spectrum of {A}‚nAcD at n > nc. Two sets of components (νxx; νyy; νzz) and (ν/xx; ν/yy; ν/zz) correspond to the -1 T 0 and 0 T 1 spin transitions. Dots show the experiment; solid line shows the calculated spectrum with Cq ) 3 kHz, η ) 0.2, β ) 0.45 kHz, ν0 ) 13 MHz.
Figure 3. Cq(n) and β(n) dependences of 2H NMR spectra in the system ZnTFt‚nDAc. Dotted line shows the expected dependence Cq(n) ) λCq(n ) 5), where λ is order parameter for the molecular field model.10
nuclei with spin I ) 1 and an electric quadrupole moment.8 Rather unexpected was the splitting of frequencies νxx and νyy corresponding to the perpendicular components of the slightly nonaxial tensor of electric quadrupole interactions between the 2H nuclei with the principal value νzz. The asymmetry parameter of the tensor η ) (νxx - νyy)/(νo - νzz) was found to be η ≈ 0.2 for all spectra of the inclusion complexes {A}‚nAcD for n > nc ≈ 2.5. Therefore, at n > nc, the nonaxial symmetry of the tensor of electric quadrupole interactions of the 2H nuclei no longer corresponds to the tetragonal symmetry of the host framework {A}. To describe the phase transition in the system {A}‚nAcD in more detail, we numerically analyzed the obtained 2H NMR spectra following the standard procedures.8 Figure 3 shows experimental dependences Cq(n) and β(n), where Cq is the constants of electric quadrupole interactions of 2H nuclei, Cq ≈ 4/3(νo - νzz),8 and β(n) is the parameter of nonuniform broadening of 2H NMR spectra specifying the mean-square fluctuations of Cq as functions of the saturation degree n. These data are presented in Figure 3 together with the β(n) plot for n < nc. The observed phenomenon suggests that the abrupt change in the shape of the 2H NMR spectra of {A}‚nDAc in the vicinity of n ) n ≈ 2.5 is related to a phase transition and the break of C4 axial symmetry of the studied system. The phase transition and the break of C4 axial symmetry of the studied binary system
5076 J. Phys. Chem. C, Vol. 112, No. 13, 2008
Figure 4. DSC results for {A} (1), {A}‚5Ac (2), and {A}‚5AcD (3). Measurements of thermal properties of three samples were carried out using differential scanning calorimeter DSC-204 (Netzsch) in a temperature range of 140-320 K at a heating rate of 6 K min-1. Samples mass was 5.3 mg for the sample {A}‚5 AcD, 6.3 mg for the sample {A}‚5Ac, and 5.5 mg for starting sample {A} (empty matrix).
{A}‚nAcD above the critical point (n > nc ≈ 2.5) may be tentatively attributed to vibronic instability of the square group [ZnO4] of {A} due to the pseudo Jahn-Teller effect9 associated with the interaction of the ground dsp2 state and the excited sp states of the central Zn2+ ion. The observed higher (tetragonal) symmetry of the system below n < nc can be explained by the dynamic pseudo Jahn-Teller effect.9 To elaborate these hypotheses, we undertook DSC and NMR studies of the guestfree framework {A}. 2. DSC and 1H NMR Data for the Guest-Free Absorbent {A}. The results of the differential scanning calorimetric measurements of the starting sample {A} are shown in the Figure 4, curve 1. The most significant feature of the thermal properties of {A} is a huge endothermic peak near 160 K presumably related to a first-order phase transition in the empty matrix. The latent heat of the transition was preliminary evaluated as approximately 140 J/g, or 80 kJ/mol. According to 1H NMR spectroscopy data, the empty absorbent {A} is characterized by rotational mobility of both [C8H4O4]2- anions (bdc) and pillar C6H12N2 molecules (dabco). It is essential, that this mobility does not undergo any appreciable changes nearby the phase transition. In full accordance with the molecular mobility data, the measurements of the temperature dependence of 1H NMR spin-lattice relaxation times T1 revealed featureless and nearly linear decrease of T1 ranging from 35 ms at 180 K to 20 ms at 120 K. These results may prompt the conclusion that the phase transition in the empty matrix {A} can be related only to some supramolecular effects, i.e., to the orthorhombic distortion of the starting tetragonal structure. Such distortion may be governed by the pseudo Jahn-Teller mechanism discussed above. However, the mechanism, which shifts the phase transition temperature from 160 K (in the guest-free matrix {A}) to ∼300 K in acetone-filled samples {A}‚nAc and {A}‚ nAcD (n > nc ≈ 2.5) remains unclear. 3. DSC Data for the Hetero Systems {A}‚5Ac and {A}‚ 5AcD. Figure 4 shows DSC data of acetone-filled {A}‚5Ac and {A}‚5AcD hetero composites (curves 2 and 3). The lowtemperature endothermic peak near 160 K is almost 10 times less than the above-mentioned large heat anomaly in the guestfree matrix{A}. This observation is in good accordance with the above data suggesting the structural distortion in acetonefilled samples at all temperatures. Moreover, it can be assumed that the decreased peak intensity at 160 K is related to a minor
Gabuda et al. acetone-free parts of the absorbent {A} structure, in which the nanotubes are blocked probably by some admixture molecules. Another thermal anomalies are observed at 170 K (in {A}‚ 5Ac) and at 180 K (in {A}‚5AcD). The temperatures of these anomalies are close to the freezing points of pure acetone and deuteroacetone, respectively. It means that the properties of the acetone absorbed in nanotubes of {A} correspond, to some extent, to those of bulk liquid. However, it is surprising that some additional peaks of heat anomalies are observed at higher temperatures T′(Ac) ) 224 K and T′′(Ac) ) 235 K for the acetone-filled sample. Similar heat anomalies are also observed in the deuterated sample {A}‚5 AcD: T′(AcD) ) 215 K, and T′′ (AcD) ) 227 K, showing a pronounced isotope effect of almost 10 K as compared to {A}‚5Ac. The striking feature of the discovered anomalies is that the ratios T′(Ac)/T′(AcD) and T′′(Ac)/T′′(AcD) are inversely proportional to the ratio of Ac and AcD molecular weights. This fact may be explained by plausible influence of quantum confinement on molecular dynamics of acetone molecules in the nanotubes of {A}. 4. Shottky Heat Anomaly in Hetero Systems {A}‚5Ac and {A}‚5AcD. The dynamic behavior of absorbed molecules may be regarded as a special case of the problem of a material particle with mass m placed in a box with dimensions a, b, and c. According to the quantum rules,11 the particle is characterized by series of allowed energy states Eijk
Eijl ) h2/8m(i2/a2+ j2/b2+ l2/c2)
(1)
where i, j, l ) 1, 2, 3, ... are the quantum numbers and h is Planck’s constant. If the box is cubic (a ) b ) c ) a0), the energy gap ∆E between the ground (i ) 1) and the first exited (i ) 2) states is ∆E ) 9h2/8ma02. For an absorbent framework built of such cubic boxes, one can expect a Shottky heat anomaly at the temperature TS ) ∆E/k, where k is the Boltzmann constant. Under this assumption, and T′ and T′′ attributed to a Shottky heat anomaly centered at the average temperature TS ) (T′ + T′′)/2, the “experimental” parameter a- is equal to 0.4 Å. The calculated value can be considered as an extra contribution in the molecular diameter of acetone (about 6 Å) related to the conditions of quantum confinement in nano pores of {A}. Significant advantage of the above model is a simple explanation of the TS splitting into two peaks at T′ and T′′. Within the context of the Shottky model, the splitting may be explained by the orthorhombic distortion of cubic cell, where the corrected box parameters are a ) 0.45 Å and b ) 0.35 Å. Such distortion was observed before for the {A}‚2C6H6 material3 and correlates well with the above conclusion on the orthorhombic structure of {A}‚5AcD based on the 2H NMR data. Discussion The obtained results make it possible to clarify the induction mechanism responsible for the concentration phase transition in the {A}‚nAcD, nearby the critical point nc ≈ 2.5. Here, the influence of quantum confinement on the state of absorbed molecules is of fundamental importance. The conditions of quantum confinement determine that the absorbed molecule can occupy only some allowed energy states defined by formula 1. In the axially symmetric system, the first excited energy level (with quantum number i ) 2) is represented by a double degenerate state of odd symmetry (“ungerade” term). The degeneration can be broken by any accidental structural fluctuations, including the distortions due to dynamic Jahn-Teller effect in the porous framework. As a result, the total energy of the system may be lowered. However, the expected lowering
Absorption of Molecules in a Nanotube Framework of total energy of the nanoporous hetero system {A}‚nAcD may be achieved only if the framework distortions are correlated. Within of the lattice gas model,10 such correlation takes place only at n g 2.5, when the guest molecules can be ordered at defined sites in the pores of the structure.
J. Phys. Chem. C, Vol. 112, No. 13, 2008 5077 Acknowledgment. The authors are indebted to A. R. Semenov for NMR spin-lattice T1 measurements. This work was supported by the Russian Foundation for Basic Research (Grant Nos. 05-03-32263 and 07-03-00436). References and Notes
Conclusions The considered hetero composites based on sorption of molecular substances into vibronically instable metal organic nanotube framework are interesting in several aspects. The vibronic instability is most often responsible for the spontaneous ferroelectric polarization, as it takes place in some metal oxide nanotube systems.1,2 Some heterogeneous or the “relaxor” ferroelectrics are known to exhibit giant electromechanical (piezoelectric) response12 that is extensively used in a broad range of fields from ultrasonic and medical applications to telecommunications. Relaxors do not have a polar ground state and frequently they have disordered polarization with small ordered “polar nanoregions”13,14 which individually get polarized15 below definite temperature.16 However, typical relaxors are heterogeneous disordered materials manifesting different mesoscopic and microscopic heterogeneities over a range of lengths and timescales. This diversity impedes systematization of experimental observations and even the determination of entire phase diagram of these materials, the jumping-off point for every study of a material. In this respect, a highly ordered hetero composite based on an impregnated metal organic nanotube frameworks represents a promising approach which can be helpful to clarify such basic physical ideas as the nature of polarization and molecular dynamics and can also have extensive applications.
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