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Dynamics of p-nitroaniline molecules in FSM-type mesoporous silicas studied by solid-state NMR
Microporous and Mesoporous Materials 68 (2004) 111–118 www.elsevier.com/locate/micromeso
Dynamics of p-nitroaniline molecules in FSM-type mesoporous silicas studied by solid-state NMR Yoshihiko Komori *, Shigenobu Hayashi
*
Institute for Materials and Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 21 August 2003; received in revised form 16 December 2003; accepted 17 December 2003
Abstract Dynamics of p-nitroaniline (pNA) in FSM-type mesoporous silicas was investigated by means of 2 H and 13 C solid-state NMR. Deuterated pNA (pNA-d) was loaded to FSM-16 (FSM) and modified FSM samples which were prepared by reactions of FSM with trimethylethoxysilane (C1FSM), n-octyltriethoxysilane (C8FSM) and 3-aminopropyltriethoxysilane (NH2 FSM). 13 C MAS NMR spectra of the samples at room temperature showed four sharp signals assigned to pNA-d, indicating a fast motion of pNA-d at room temperature. Temperature dependence of 2 H NMR spectra exhibited the presence of two modes of motions, the isotropic motion and another motion such as a wobbling one (Motion I). Analysis of spin-lattice relaxation times (T1 ) for 2 H spins demonstrated that the apparent activation energies of Motion I were 20 kJ mol1 for FSM and 31–32 kJ mol1 for C1FSM and NH2 FSM. On the other hand, temperature dependence of T1 of Motion I in C8FSM was complicated presumably because of the motion of alkyl chains of the reacted silylating agent. These results indicated that the dynamics of pNA molecules was varied with the surface properties of the mesopores. 2003 Elsevier Inc. All rights reserved. Keywords: Dynamics; FSM-16; Mesoporous silica; Nitroaniline; Solid-state NMR
1. Introduction Ordered porous materials incorporating guest molecules have potential uses as new and intelligent materials because the guest molecules are aligned and undergo unique molecular motions [1–4]. Elucidations of host– guest interactions and dynamics of guest molecules are important from the viewpoint of efficient designs of functional inorganic–organic nanohybrid materials with advanced performances. Unique behavior of small molecules in zeolites has been investigated extensively by means of solid-state NMR spectroscopy. Especially, solid-state 2 H NMR techniques have been utilized widely because both modes and rates of molecular motions can be estimated. As for molecular motions in zeolites, studies of small organic molecules such as benzene [5–20], p-xylene *
Corresponding authors. Tel./fax: +81298614515. E-mail addresses: [email protected] (Y. Komori), hayashi.s@ aist.go.jp (S. Hayashi). URL: http://unit.aist.go.jp/imcp/7G/index_e.htm. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.013
[5,8,13,14,16,21], cyclohexane [9,18,22], alcohol [5,23], acetonitrile [24] and p-nitroaniline (pNA) [25,26] have been reported. Recently, we have demonstrated that pNA molecules in siliceous ZSM-5 undergo a flip-flop motion with a rate of 50 kHz at room temperature [26]. As for molecular motions in mesopores, dynamics of benzene, cyclohexane and p-xylene has been studied and high mobility of the molecules is deduced over a wide temperature range [16,27]. Mesoporous silica has attracted increasing interest because it has pores larger than 2 nm where large molecules can be accommodated. Furthermore, silanol groups locate on the pore surface [28,29], which can be modified with various silylating agents and alcohols [29– 31]. Composite materials with functional molecules such as p-nitroaniline [32], dyes [33,34] chlorophyll [35–37] and enzymes [38] have been synthesized with the aims of stabilizing molecules and creating unique optical properties and an efficient energy transfer system. Mesoporous materials are also applied to adsorption and separation techniques [39–42]. However, dynamics of guest molecules is not shed light on in detail. Dynamics
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of guests in mesopores is supposed to be quite different from those of micropores due to the large pore size and the unique property of the pore surface. In the present study, we have investigated the molecular motion of pNA molecules in FSM type mesoporous silicas by means of solid-state NMR spectroscopy. The pNA molecule is well known as a nonlinear optical material and second harmonic generation has been observed for pNA in various inorganic host materials such as zeolites [43–50], mesoporous silica [32] and layered materials [51–53]. We have measured temperature dependence of 2 H spectra and spin-lattice relaxation times (T1 ) as well as 13 C spectra. Furthermore, we have utilized modified mesoporous silicas where silanol groups were converted to methyl, octyl and aminopropyl groups in order to reveal the effects of the pore surface environment on the dynamics of pNA molecules.
2. Experimental 2.1. Materials FSM-type mesoporous silica (FSM) was supplied by Toyota Central R&D Laboratories, Inc. (Japan) with a code LS10-550. The pore diameter was 2.75 nm and the surface area was 970 m2 g1 . Silylating agents of trimethylethoxysilane, n-octyltriethoxysilane and 3-aminopropyltriethoxysilane were purchased from Tokyo Kasei Kogyo Co., Ltd. (Japan). Deuterated pNA (ringd4 , pNA-d) was obtained from Cambridge Isotope Laboratories (USA). 2.2. Surface modification of FSM Silylation of FSM was performed as follows [31]. FSM was pretreated at 423 K for 3 h under a N2 flow to remove adsorbed water. After 3 g of FSM was dispersed in 20 ml of the silylating agent, 50 ml of toluene was added to this suspension. The suspension was refluxed for 24 h under a N2 atmosphere. After centrifugation, the product was washed with toluene and air-dried. The obtained FSM samples modified with trimethylethoxysilane, n-octyltriethoxysilane and 3-aminopropyltriethoxysilane are abbreviated as C1FSM, C8FSM and NH2 FSM, respectively. The modified FSM samples were characterized by thermogravimetry (TG), 29 Si NMR and 13 C NMR measurements. TG curves exhibited a continuous mass loss from room temperature to 1000 K due to desorption of water and decomposition of the silylating agents. The mass losses due to decomposition of the silylating agents were roughly estimated to be 3.2, 16.9 and 11.4 mass% for C1FSM, C8FSM and NH2 FSM,
respectively, from the mass losses between 420 and 1000 K. The 29 Si magic angle spinning (MAS) NMR spectrum of FSM showed Q2 (Si*(OSi)2 (OH)2 , )92 ppm), Q3 (Si*(OSi)3 (OH), )101 ppm) and Q4 (Si*(OSi)4 , )110 ppm) signals, and the intensity ratio of Q2 :Q3 :Q4 was 4:24:72 [29]. The spectra of the modified FSM samples exhibited weaker signals for both Q2 and Q3 and a stronger signal for Q4 than those of FSM. The intensity ratios of Q2 :Q3 :Q4 were 0:17:83, 0:13:87 and 0:20:80 for C1FSM, C8FSM and NH2 FSM, respectively, and the silylated fractions of the hydroxyl groups were estimated to be 47%, 59% and 38%, respectively. Additional signals due to the reacted silylating agents were observed in the spectra of the modified FSM samples. C1FSM gave a resonance signal at 14 ppm assigned to M1 (R3 Si*(OSi), R is an organic group) [29,31]. C8FSM showed a very broad signal at around )55 ppm assigned to T n units (RSi*(OSi)n (OR03n , R0 ¼ CH3 CH2 or H, 1 6 n 6 3). NH2 FSM gave two signals at )60 and )67 ppm assigned to T 2 (RSi*(OSi)2 (OR0 )) and T 3 units (RSi*(OSi))3 , respectively. These 29 Si spectra confirmed that the silylating agents formed Si–O–Si bondings with Si–OH groups on the mesoporous wall. 13 C cross polarization (CP) MAS NMR showed signals of organic groups due to the silylating agents attached to FSM. C1FSM showed a sharp signal at )1.4 ppm assigned to Si–CH3 . C8FSM showed signals at 12.7, 16.4, 22.8, 29.6, 32.3 and 59.8 ppm due to octyl and ethoxyl groups [31]. NH2 FSM gave the broad signals at 10, 24 and 43 ppm due to the aminopropyl group and small signals at 16.3 and 57.3 ppm due to the ethoxyl group. These results of TG, 29 Si NMR and 13 C NMR demonstrated that silylating agents were grafted successfully on the surface of the mesopores in FSM. 2.3. Introduction of pNA-d The host materials were dehydrated by evacuation at 423 K for 3 h before use. A weighted amount of pNA-d was mixed with the dehydrated host materials under a N2 atmosphere. The amounts of pNA-d were about 10 mass% of the total mass, which corresponded to a surface coverage of 34% in the case of FSM when the size of pNA (1.0 · 0.7 nm) was taken into consideration. A Pyrex glass tube containing the mixture of pNA-d and the host materials were sealed in vacuo and heated at 423 K (the melting point of pNA is 419 K). The heating time was varied from 5 to 24 h and it was found that 5 h was sufficient for incorporating pNA-d into the mesoporous materials. Adsorption of pNA-d molecules was confirmed by the X-ray diffraction (XRD) analysis showing no peaks of crystalline pNA-d. The amounts of pNA-d in the samples were checked by using TG analysis. The amount of pNA-d in FSM
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was 10 mass%. As for the modified FSM samples loaded with pNA-d, the mass loss due to loss of pNA-d and silylating agents was observed above 420 K. By subtracting the mass loss of the silylating agents of the modified FSM samples from the mass loss of the pNAd-loaded samples, the amount of pNA-d was estimated as 13%, 8% and 9% for C1FSM, C8FSM and NH2 FSM, respectively.
2.4. Analyses Powder XRD patterns were measured by a Rigaku Miniflex diffractometer with Cu Ka radiation at room temperature. TG analyses were performed in an air atmosphere by Rigaku Thermoflex TG8110 controlled by Rigaku Thermal Analysis Station TAS100. The heating rate was 10 K min1 . Differential scanning calorimetry (DSC) measurements were carried out in the temperature range between 263 and 393 K by using Rigaku Thermo plus DSC8230. The heating and cooling rate was 5 K min1 . 13 C and 29 Si MAS NMR measurements were carried out by a Bruker ASX400 spectrometer with a static magnetic field strength of 9.4 T. Larmor frequencies of 29 Si and 13 C were 79.49 and 100.61 MHz, respectively. Chemical shifts were expressed with respect to neat tetramethylsilane for both nuclei. The single pulse sequence with high-power 1 H decoupling (HD) was used to measure the 29 Si MAS NMR spectra. The spinning rate was set at 3.5–4.5 kHz. The recycle time was 400 s for quantitative purposes [29]. 13 C NMR spectra were obtained by using both HD and the ordinary CP pulse sequence with the contact time of 1 or 3 ms. Spin-lattice relaxation time (T1 ) measurements were carried out using the inversion recovery pulse sequence. Time intervals between the 180 and 90 pulses were varied from 1 ms to 5 s. The spinning rate and the recycle time were set at 3.5 kHz and 15 s, respectively. 2 H NMR measurements were carried out by a Bruker MSL400 spectrometer with a static magnetic field strength of 9.4 T. 2 H spectra were recorded at 61.42 MHz for static samples using a Bruker broadband probehead. The quadrupole echo pulse sequence was used, where the latter half of the echo signal was acquired and Fourier-transformed. The 90 pulse width for D2 O was 3.2–3.6 ls and the time interval s between the two 90 pulses was set at 15 ls. The recycle time was varied between 1 and 3 s. The set temperature was varied from 140 to 400 K. Temperature of the sample was calibrated by using CD3 OD [54]. Spectra were presented with respect to the signal of D2 O. T1 measurements were carried out using the inversion recovery pulse sequence combined with the quadrupole echo pulse sequence. Time intervals between the 180 and 90 pulses were varied from 1 to 200 ms with a recycle time of 1 s.
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3. Results and discussion 3.1.
13
C NMR spectra
Fig. 1 shows 13 C HD/MAS NMR spectra of pNA-d in the mesoporous materials at room temperature. The spectra of all samples exhibit four sharp signals due to carbon atoms of pNA-d molecules. The isotropic chemical shifts for FSM/pNA-d are 113.9 (C–H), 126.8 (C–H), 137.8 (C–NO2 ) and 154.9 ppm (C–NH2 ). Chemical shifts for other samples are similar to those of FSM/pNA-d. The signals are very sharp and their spinning sidebands are not observed. In general, the 13 C line width is caused mainly by chemical shift anisotropy and 13 C–1 H dipole–dipole interactions. The HD/MAS method averages the dipole interactions, but the chemical shift anisotropy remains as spinning side bands. The results of no spinning sidebands suggest that chemical shift anisotropy is averaged by high molecular mobility. The 13 C spins have relatively short T1 values because a recycle time of 15 s is sufficient to relax them. Furthermore, the signal intensities are very weak in the CP/ MAS spectra, implying a weak dipole–dipole interaction between 13 C and 1 H spins. These results demonstrate that pNA-d molecules undergo relatively fast motions.
Fig. 1. 13 C HD/MAS NMR spectra of pNA-d in (a) FSM, (b) C1FSM, (c) C8FSM and (d) NH2 FSM, measured at room temperature. The broad hump centered at about 125 ppm is a background signal caused by the rotor cap and the stator.
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The signals due to the organic parts of the reacted silylating agents are observed in the spectra of modified FSM/pNA-d (Fig. 1b–d). The profiles of these regions are identical to the samples before the introduction of pNA-d, indicating that the organic parts of the reacted silylating agents are not decomposed by the treatment in the process of the pNA-d introduction. 3.2. 2 H NMR spectra Fig. 2 shows 2 H NMR spectra of pNA-d in the mesopore of FSM at several temperatures. Resolved double peaks in Pake doublet pattern are barely observed on a very broad signal at 149 K. Spectra consist of an unresolved broad signal in the temperature range from 188 to 229 K. The signal becomes sharper with increase in temperature above 267 K, indicating that the molecules start to undergo an isotropic motion. Fig. 3 shows temperature dependence of the line width for the sharp signal. The line width is decreasing even at 398 K. Fig. 4 shows 2 H NMR spectra of pNA-d in the mesopore of C1FSM. The Pake doublet pattern is dominant in the spectrum at 149 K. A quadrupole coupling constant (QCC ¼ e2 Qq=h) and an asymmetry factor (g) are 180 kHz and 0, respectively, indicating that most of the pNA-d molecules are in a rigid state [55]. The sharp signal in the central region is ascribed to molecules undergoing an isotropic motion although its
Fig. 3. Temperature dependence of line widths of the sharp signals for FSM/pNA-d ( ), C1FSM/pNA-d ( ), C8FSM/pNA-d (N) and NH2 FSM/pNA-d (). FWHM is the full width at half maximum of the signals.
Fig. 4. Temperature dependence of 2 H NMR spectra of pNA-d in C1FSM.
Fig. 2. Temperature dependence of 2 H NMR spectra of pNA-d in FSM.
intensity ratio is less than 3%. With increase in temperature, the intensity of another signal with a broad line shape increases in the central region and that of the doublet pattern decreases. At 305 K, the doublet pattern disappears completely and a sharp signal is observed, indicating that all pNA-d molecules undergo fast iso-
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Fig. 5. Temperature dependence of 2 H NMR spectra of pNA-d in C8FSM.
Fig. 6. Temperature dependence of 2 H NMR spectra of pNA-d in NH2 FSM.
tropic motions. The signal hardly changes any more above 305 K (Fig. 3). Fig. 5 shows 2 H NMR spectra of pNA-d in the mesopore of C8FSM. Spectra are similar to those of C1FSM/pNA-d, but a narrower doublet signal with a split of 28 kHz is clearly distinguishable in the central region at 229 K. It is suggested that some portion of pNA-d molecules undergo the 180 flip-flop motion around the C2 axis of the molecule with a rate faster than 100 kHz at this temperature [26]. The Pake doublet and the narrower doublet disappear with increase in temperature, and a sharp signal is observed at 305 K. The sharp signal does not change above 305 K (Fig. 3), similarly to that of C1FSM/pNA-d. Fig. 6 shows 2 H NMR spectra of pNA-d in the mesopore of NH2 FSM. Below 229 K, the Pake doublet is observed, indicating that all pNA-d molecules are in a rigid state. At 267 K, the doublet pattern becomes small and a broad signal appears in the central region. At 305 K, the doublet pattern disappears and a sharp signal due to a fast isotropic motion is observed. The line width of the sharp signal decreases gradually with increase in temperature (Fig. 3).
motion of pNA-d around room temperature is studied by 2 H T1 measurements. Fig. 7 shows temperature dependence of T1 as a function of inverse temperature. The minimum values of T1 are observed around room temperature, and the relaxation mechanism is fluctuation of 2 H quadrupole interaction caused by molecular motions. The temperature dependence of T1 is analyzed by the Bloembergen–Purcell–Pound (BPP) type equation for the quadrupolar relaxation as [56] 2 3 De2 Qq g2 1þ T11 ¼ 40 h 3 " # sD 4sD þ ; ð1Þ 2 2 1 þ ðxD sD Þ 1 þ ð2xD sD Þ
3.3. 2 H spin-lattice relaxation times Spin-lattice relaxation times are good parameters to study motions of the order of Larmor frequency. The
where De2 Qq=h is a change of apparent QCC upon the motion considered in a unit of radian per sec, xD is the angular resonance frequency and sD is the correlation time of the motion of 2 H. The sD is described as follows: Ea sD ¼ s0 exp ; ð2Þ RT where s0 is a correlation time of 2 H at the infinite temperature, Ea is the apparent activation energy and R is the gas constant. The temperature dependence of T1 can be simulated successfully by one component for the samples of FSM, C1FSM and NH2 FSM. The parameters are listed in
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pNA-d above room temperature. Apparent activation energies are roughly estimated as 31 and 17 kJ mol1 below and above room temperature, respectively. Ordinarily, motions with larger activation energies are observed at higher temperatures. A motion with the smaller activation energy is observed at higher temperatures if a phase transition takes place. However, no DSC peaks due to phase transitions are detected in the temperature range from 260 to 380 K for C8FSM/ pNA-d. Thus, the above change in the activation energy seems to be curious. 3.4. Dynamics of the alkyl chain in C8FSM/pNA-d
Fig. 7. 2 H spin-lattice relaxation times of pNA-d in the mesopores of (a) FSM, (b) C1FSM, (c) C8FSM and (d) NH2 FSM. Simulated results are indicated by solid lines.
Table 1 and the simulated results are indicated in Fig. 7 as solid lines. The sD values at 300 K are calculated using Eq. (2), as listed also in Table 1. On the other hand, the results for C8FSM/pNA-d cannot be simulated by one component (Fig. 7c). The temperature dependence is similar to that of C1FSM/ pNA-d below room temperature and to that of FSM/ Table 1 Parameters resulting from the BPP fit to temperature dependence of 2 H T1 of pNA-d in FSM and modified FSM
FSM C1FSM C8FSMb Above RT Below RTd NH2 FSM
Ea / kJ mol1
QCC/kHz
s0 /s
a s300 D /s
20 31
85 118
1 · 1012 1 · 1014
3 · 109 3 · 109
17 31 32
118c 118 110
1 · 1012 1 · 1014 1 · 1014
1 · 109 3 · 109 4 · 109
C8FSM has the longest alkyl chains among the three modified FSM samples. Dynamics of the alkyl chain might cause the curious behavior of pNA-d in C8FSM. In this work, fast motions of the octyl chain are detected by 13 C T1 measurements as follows. Table 2 lists the obtained 13 C T1 values of four signals at 12.7 (C8), 29.6 (C4 + C5), 32.3 (C6) and 33.1 ppm (C3) (Fig. 1c), where the numbers in the parentheses express the carbon position counted from the Si atom in the silylating agent. The 13 C T1 values of the signals at 16.4 (C1) and 22.8 ppm (C2 + C7) are not available because of the low signal-to-noise ratio and the overlapping of the two signals, respectively. The values of 1=T1 are approximately proportional to sC =ð1 þ x2C s2C Þ when 13 C spins in the alkyl chain relax through fluctuation of dipolar interaction with nearby 1 H spins [56]. The xC value is the 13 C Larmor frequency in an angular frequency unit and sC is the correlation time of the motion. The results in Table 2 imply that the T1 values of C4–C8 increase with increase in temperature from 298 to 340 K, and that the 13 C correlation times are much shorter than 1 ns above room temperature. Furthermore, the T1 value increases monotonically from C3 to C8 carbons at each temperature, indicating that the rate of the motion of C8 carbon is faster than that of C3 carbon. These motions of the octyl chain might affect the dynamics of pNA-d in C8FSM, which will be discussed in a later section.
Table 2 13 C T1 results of the octyl group in C8FSM/pNA-d Temperature/K
C3a
C4 + C5b
C6a
C8c
a
298 320 340
0.74 0.64 1.0
0.78 0.92 1.4
1.2 1.7 2.2
1.6 2.3 2.7
b
a
sD at 300 K. Assuming different components above and below room temperature. c Assuming the same value as that of C8FSM below RT. d Assuming the same values as those of C1FSM.
T1 /s
The signals of C3 and C6 are deconvoluted. Assuming that C4 and C5 have the same T1 value. c The value might have been affected by partially overlapped signal of the methyl group in the ethoxyl group. b
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3.5. Dynamics of pNA-d in the mesopore of FSM The 2 H correlation time of FSM/pNA-d at 300 K estimated from the T1 analysis is quite short (3 · 109 s, Table 1). On the other hand, the 2 H NMR spectrum at 305 K shows a broad signal in the central region (Figs. 2 and 3), indicating that the isotropic motion is relatively slow at room temperature. Namely, the isotropic motion is not the dominant mechanism to relax 2 H spins, but other motions (named Motion IFSM ) such as a wobbling one relax 2 H spins in the temperature range from 267 to 398 K. The relaxation mechanism is supported by the QCC value effective for the relaxation (85 kHz, Table 1) which is much smaller than that in a rigid state (180 kHz). The featureless broad signal in the 2 H spectra around 200 K is considered to originate from Motion IFSM . In summary, temperature dependence of the dynamics of pNA-d is described as follows. At 149 K, pNA-d molecules undergo Motion IFSM with the apparent activation energy of 20 kJ mol1 . The correlation time of Motion IFSM is of the order of 100 kHz at 149 K, being predicted using the parameters in Table 1. The central broad signal in the 2 H NMR spectrum at 149 K is consistent with the above motion. With increase in temperature, the correlation time of Motion IFSM decreases, and it becomes 3 · 109 s at room temperature. On the other hand, the isotropic motion is recognized around room temperature in the 2 H NMR spectra (Fig. 2). The rate of the isotropic motion is of the order of 100 kHz at room temperature, considering the relatively broad 2 H signal. Dynamics of pNA-d in the mesopores is different from that in the micropores of zeolites. The pNA-d molecules undergo a 180 flip-flop motion with an activation energy of 57 kJ mol1 in siliceous ZSM-5 zeolite with a pore diameter of about 0.54 nm [26]. The large pore space (2.75 nm) in FSM allows the isotropic motion of pNA-d molecules as well as Motion IFSM with the smaller activation energy (20 kJ mol1 ).
3.6. Dynamics of pNA-d in modified FSM In a similar way to FSM/pNA-d, dynamics of pNA-d in modified FSM can be described by assuming two modes of motions; the isotropic motion and the other motion (named Motion IC1 , IC8 and INH2 for C1FSM/ pNA-d, C8FSM/pNA-d and NH2 FSM/pNA-d, respectively). The isotropic motion for the modified samples is recognized as the sharp signals in the 2 H NMR spectra around room temperature. However, the narrower line widths and the smaller magnitudes of the temperature dependence than that of FSM/pNA-d (Fig. 3) indicate faster rates of the isotropic motion for the modified samples. On the other hand, the spectral changes from
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267 to 305 K (Figs. 4–6) are more evident than that of FSM/pNA-d (Fig. 2), indicating that the activation energy of the isotropic motion for the modified samples is larger than that of FSM/pNA-d. The 2 H broad signals in the central region at 229 K are caused by Motion IC1 , IC8 and INH2 of pNA-d (Figs. 4–6). Furthermore, it is reasonable to assume that the T1 values (Fig. 7) are related with Motion IC1 , IC8 and INH2 . The larger QCC values (110–118 kHz) than that of FSM (85 kHz) may imply that modes of Motion IC1 , IC8 and INH2 are different from that of Motion IFSM (Table 1). Temperature dependence of the dynamics of pNA-d in C1FSM is described as follows. At 149 K, Motion IC1 is slow and most of the molecules are in a rigid state. With increase in temperature, the correlation time of Motion IC1 decreases, and it becomes 3 · 109 s at room temperature. The apparent activation energy of Motion IC1 is much larger (31 kJ mol1 ) than that of FSM (Table 1). On the other hand, the isotropic motion is recognized around room temperature in the 2 H NMR spectra (Fig. 4). The activation energy for the isotropic motion is also larger than that of FSM, and the rate of the isotropic motion is faster than that of FSM at room temperature. The larger activation energies of both Motion IC1 and the isotropic motion indicate stronger interaction between pNA-d and C1FSM than that of FSM. The most plausible interaction is the interaction between the aromatic ring of pNA-d and methyl groups of the reacted silylating agent on the pore surface. As for pNA-d in C8FSM, the 180 flip-flop motion is detected clearly in the spectra at 229 K (Fig. 5). Motion IC8 might be ascribed to the 180 flip-flop motion with a wobbling motion. In this case, the C2 axis of pNA-d must be fixed considerably during the flip-flop motion, possibly on octyl chains. The curious behavior of Motion IC8 is observed at about room temperature. The interaction between pNA-d and octyl chains might be changed at about room temperature. The fast motion of octyl chains deduced by the 13 C T1 measurements might affect Motion IC8 in the higher temperature range. Dynamics of pNA-d in NH2 FSM is also described by assuming Motion INH2 and the isotropic motion. At 149 K, Pake doublet pattern is clearly observed (Fig. 6), indicating that all the molecules are in a rigid state and that Motion INH2 is the slowest in all the samples. Furthermore, the line width of the 2 H signal decreases with increase in temperature and broader than those of C1FSM/pNA-d and C8FSM/pNA-d at room temperature (Fig. 3), indicating that the rate of the isotropic motion is slower at room temperature. These motional properties might be caused by a hydrogen bonding between an NO2 group of pNA-d and an NH2 group of the reacted silylating agent on the pore surface. Conclusively, the rates of the isotropic motion and Motion IFSM , IC1 , IC8 and INH2 are varied with the surface environments of mesopores.
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4. Conclusion Molecular motions of pNA-d in the FSM-type mesoporous silicas have been studied successfully by means of 2 H and 13 C NMR. The isotropic motion is observed above room temperature, and the rates of the isotropic motion for modified FSM/pNA-d are faster than that of FSM/pNA-d at about room temperature. The 2 H spectra below room temperature indicate another motion such as a wobbling motion (Motion I). Analysis of spin-lattice relaxation times (T1 ) for 2 H spins demonstrates that the apparent activation energies of Motion I in C1FSM and NH2 FSM (31–32 kJ mol1 ) are larger than that of Motion I in FSM (20 kJ mol1 ). On the other hand, Motion I in C8FSM is complicated probably because of the motion of octyl chains of the reacted silylating agent. It is concluded that pNA molecules undergo the isotropic motion and Motion I, and that their motions are varied with the surface properties of the mesopores.
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Dynamics of p-nitroaniline molecules in FSM-type mesoporous silicas studied by solid-state NMR Yoshihiko Komori *, Shigenobu Hayashi
*
Institute for Materials and Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 21 August 2003; received in revised form 16 December 2003; accepted 17 December 2003
Abstract Dynamics of p-nitroaniline (pNA) in FSM-type mesoporous silicas was investigated by means of 2 H and 13 C solid-state NMR. Deuterated pNA (pNA-d) was loaded to FSM-16 (FSM) and modified FSM samples which were prepared by reactions of FSM with trimethylethoxysilane (C1FSM), n-octyltriethoxysilane (C8FSM) and 3-aminopropyltriethoxysilane (NH2 FSM). 13 C MAS NMR spectra of the samples at room temperature showed four sharp signals assigned to pNA-d, indicating a fast motion of pNA-d at room temperature. Temperature dependence of 2 H NMR spectra exhibited the presence of two modes of motions, the isotropic motion and another motion such as a wobbling one (Motion I). Analysis of spin-lattice relaxation times (T1 ) for 2 H spins demonstrated that the apparent activation energies of Motion I were 20 kJ mol1 for FSM and 31–32 kJ mol1 for C1FSM and NH2 FSM. On the other hand, temperature dependence of T1 of Motion I in C8FSM was complicated presumably because of the motion of alkyl chains of the reacted silylating agent. These results indicated that the dynamics of pNA molecules was varied with the surface properties of the mesopores. 2003 Elsevier Inc. All rights reserved. Keywords: Dynamics; FSM-16; Mesoporous silica; Nitroaniline; Solid-state NMR
1. Introduction Ordered porous materials incorporating guest molecules have potential uses as new and intelligent materials because the guest molecules are aligned and undergo unique molecular motions [1–4]. Elucidations of host– guest interactions and dynamics of guest molecules are important from the viewpoint of efficient designs of functional inorganic–organic nanohybrid materials with advanced performances. Unique behavior of small molecules in zeolites has been investigated extensively by means of solid-state NMR spectroscopy. Especially, solid-state 2 H NMR techniques have been utilized widely because both modes and rates of molecular motions can be estimated. As for molecular motions in zeolites, studies of small organic molecules such as benzene [5–20], p-xylene *
Corresponding authors. Tel./fax: +81298614515. E-mail addresses: [email protected] (Y. Komori), hayashi.s@ aist.go.jp (S. Hayashi). URL: http://unit.aist.go.jp/imcp/7G/index_e.htm. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.013
[5,8,13,14,16,21], cyclohexane [9,18,22], alcohol [5,23], acetonitrile [24] and p-nitroaniline (pNA) [25,26] have been reported. Recently, we have demonstrated that pNA molecules in siliceous ZSM-5 undergo a flip-flop motion with a rate of 50 kHz at room temperature [26]. As for molecular motions in mesopores, dynamics of benzene, cyclohexane and p-xylene has been studied and high mobility of the molecules is deduced over a wide temperature range [16,27]. Mesoporous silica has attracted increasing interest because it has pores larger than 2 nm where large molecules can be accommodated. Furthermore, silanol groups locate on the pore surface [28,29], which can be modified with various silylating agents and alcohols [29– 31]. Composite materials with functional molecules such as p-nitroaniline [32], dyes [33,34] chlorophyll [35–37] and enzymes [38] have been synthesized with the aims of stabilizing molecules and creating unique optical properties and an efficient energy transfer system. Mesoporous materials are also applied to adsorption and separation techniques [39–42]. However, dynamics of guest molecules is not shed light on in detail. Dynamics
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of guests in mesopores is supposed to be quite different from those of micropores due to the large pore size and the unique property of the pore surface. In the present study, we have investigated the molecular motion of pNA molecules in FSM type mesoporous silicas by means of solid-state NMR spectroscopy. The pNA molecule is well known as a nonlinear optical material and second harmonic generation has been observed for pNA in various inorganic host materials such as zeolites [43–50], mesoporous silica [32] and layered materials [51–53]. We have measured temperature dependence of 2 H spectra and spin-lattice relaxation times (T1 ) as well as 13 C spectra. Furthermore, we have utilized modified mesoporous silicas where silanol groups were converted to methyl, octyl and aminopropyl groups in order to reveal the effects of the pore surface environment on the dynamics of pNA molecules.
2. Experimental 2.1. Materials FSM-type mesoporous silica (FSM) was supplied by Toyota Central R&D Laboratories, Inc. (Japan) with a code LS10-550. The pore diameter was 2.75 nm and the surface area was 970 m2 g1 . Silylating agents of trimethylethoxysilane, n-octyltriethoxysilane and 3-aminopropyltriethoxysilane were purchased from Tokyo Kasei Kogyo Co., Ltd. (Japan). Deuterated pNA (ringd4 , pNA-d) was obtained from Cambridge Isotope Laboratories (USA). 2.2. Surface modification of FSM Silylation of FSM was performed as follows [31]. FSM was pretreated at 423 K for 3 h under a N2 flow to remove adsorbed water. After 3 g of FSM was dispersed in 20 ml of the silylating agent, 50 ml of toluene was added to this suspension. The suspension was refluxed for 24 h under a N2 atmosphere. After centrifugation, the product was washed with toluene and air-dried. The obtained FSM samples modified with trimethylethoxysilane, n-octyltriethoxysilane and 3-aminopropyltriethoxysilane are abbreviated as C1FSM, C8FSM and NH2 FSM, respectively. The modified FSM samples were characterized by thermogravimetry (TG), 29 Si NMR and 13 C NMR measurements. TG curves exhibited a continuous mass loss from room temperature to 1000 K due to desorption of water and decomposition of the silylating agents. The mass losses due to decomposition of the silylating agents were roughly estimated to be 3.2, 16.9 and 11.4 mass% for C1FSM, C8FSM and NH2 FSM,
respectively, from the mass losses between 420 and 1000 K. The 29 Si magic angle spinning (MAS) NMR spectrum of FSM showed Q2 (Si*(OSi)2 (OH)2 , )92 ppm), Q3 (Si*(OSi)3 (OH), )101 ppm) and Q4 (Si*(OSi)4 , )110 ppm) signals, and the intensity ratio of Q2 :Q3 :Q4 was 4:24:72 [29]. The spectra of the modified FSM samples exhibited weaker signals for both Q2 and Q3 and a stronger signal for Q4 than those of FSM. The intensity ratios of Q2 :Q3 :Q4 were 0:17:83, 0:13:87 and 0:20:80 for C1FSM, C8FSM and NH2 FSM, respectively, and the silylated fractions of the hydroxyl groups were estimated to be 47%, 59% and 38%, respectively. Additional signals due to the reacted silylating agents were observed in the spectra of the modified FSM samples. C1FSM gave a resonance signal at 14 ppm assigned to M1 (R3 Si*(OSi), R is an organic group) [29,31]. C8FSM showed a very broad signal at around )55 ppm assigned to T n units (RSi*(OSi)n (OR03n , R0 ¼ CH3 CH2 or H, 1 6 n 6 3). NH2 FSM gave two signals at )60 and )67 ppm assigned to T 2 (RSi*(OSi)2 (OR0 )) and T 3 units (RSi*(OSi))3 , respectively. These 29 Si spectra confirmed that the silylating agents formed Si–O–Si bondings with Si–OH groups on the mesoporous wall. 13 C cross polarization (CP) MAS NMR showed signals of organic groups due to the silylating agents attached to FSM. C1FSM showed a sharp signal at )1.4 ppm assigned to Si–CH3 . C8FSM showed signals at 12.7, 16.4, 22.8, 29.6, 32.3 and 59.8 ppm due to octyl and ethoxyl groups [31]. NH2 FSM gave the broad signals at 10, 24 and 43 ppm due to the aminopropyl group and small signals at 16.3 and 57.3 ppm due to the ethoxyl group. These results of TG, 29 Si NMR and 13 C NMR demonstrated that silylating agents were grafted successfully on the surface of the mesopores in FSM. 2.3. Introduction of pNA-d The host materials were dehydrated by evacuation at 423 K for 3 h before use. A weighted amount of pNA-d was mixed with the dehydrated host materials under a N2 atmosphere. The amounts of pNA-d were about 10 mass% of the total mass, which corresponded to a surface coverage of 34% in the case of FSM when the size of pNA (1.0 · 0.7 nm) was taken into consideration. A Pyrex glass tube containing the mixture of pNA-d and the host materials were sealed in vacuo and heated at 423 K (the melting point of pNA is 419 K). The heating time was varied from 5 to 24 h and it was found that 5 h was sufficient for incorporating pNA-d into the mesoporous materials. Adsorption of pNA-d molecules was confirmed by the X-ray diffraction (XRD) analysis showing no peaks of crystalline pNA-d. The amounts of pNA-d in the samples were checked by using TG analysis. The amount of pNA-d in FSM
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was 10 mass%. As for the modified FSM samples loaded with pNA-d, the mass loss due to loss of pNA-d and silylating agents was observed above 420 K. By subtracting the mass loss of the silylating agents of the modified FSM samples from the mass loss of the pNAd-loaded samples, the amount of pNA-d was estimated as 13%, 8% and 9% for C1FSM, C8FSM and NH2 FSM, respectively.
2.4. Analyses Powder XRD patterns were measured by a Rigaku Miniflex diffractometer with Cu Ka radiation at room temperature. TG analyses were performed in an air atmosphere by Rigaku Thermoflex TG8110 controlled by Rigaku Thermal Analysis Station TAS100. The heating rate was 10 K min1 . Differential scanning calorimetry (DSC) measurements were carried out in the temperature range between 263 and 393 K by using Rigaku Thermo plus DSC8230. The heating and cooling rate was 5 K min1 . 13 C and 29 Si MAS NMR measurements were carried out by a Bruker ASX400 spectrometer with a static magnetic field strength of 9.4 T. Larmor frequencies of 29 Si and 13 C were 79.49 and 100.61 MHz, respectively. Chemical shifts were expressed with respect to neat tetramethylsilane for both nuclei. The single pulse sequence with high-power 1 H decoupling (HD) was used to measure the 29 Si MAS NMR spectra. The spinning rate was set at 3.5–4.5 kHz. The recycle time was 400 s for quantitative purposes [29]. 13 C NMR spectra were obtained by using both HD and the ordinary CP pulse sequence with the contact time of 1 or 3 ms. Spin-lattice relaxation time (T1 ) measurements were carried out using the inversion recovery pulse sequence. Time intervals between the 180 and 90 pulses were varied from 1 ms to 5 s. The spinning rate and the recycle time were set at 3.5 kHz and 15 s, respectively. 2 H NMR measurements were carried out by a Bruker MSL400 spectrometer with a static magnetic field strength of 9.4 T. 2 H spectra were recorded at 61.42 MHz for static samples using a Bruker broadband probehead. The quadrupole echo pulse sequence was used, where the latter half of the echo signal was acquired and Fourier-transformed. The 90 pulse width for D2 O was 3.2–3.6 ls and the time interval s between the two 90 pulses was set at 15 ls. The recycle time was varied between 1 and 3 s. The set temperature was varied from 140 to 400 K. Temperature of the sample was calibrated by using CD3 OD [54]. Spectra were presented with respect to the signal of D2 O. T1 measurements were carried out using the inversion recovery pulse sequence combined with the quadrupole echo pulse sequence. Time intervals between the 180 and 90 pulses were varied from 1 to 200 ms with a recycle time of 1 s.
113
3. Results and discussion 3.1.
13
C NMR spectra
Fig. 1 shows 13 C HD/MAS NMR spectra of pNA-d in the mesoporous materials at room temperature. The spectra of all samples exhibit four sharp signals due to carbon atoms of pNA-d molecules. The isotropic chemical shifts for FSM/pNA-d are 113.9 (C–H), 126.8 (C–H), 137.8 (C–NO2 ) and 154.9 ppm (C–NH2 ). Chemical shifts for other samples are similar to those of FSM/pNA-d. The signals are very sharp and their spinning sidebands are not observed. In general, the 13 C line width is caused mainly by chemical shift anisotropy and 13 C–1 H dipole–dipole interactions. The HD/MAS method averages the dipole interactions, but the chemical shift anisotropy remains as spinning side bands. The results of no spinning sidebands suggest that chemical shift anisotropy is averaged by high molecular mobility. The 13 C spins have relatively short T1 values because a recycle time of 15 s is sufficient to relax them. Furthermore, the signal intensities are very weak in the CP/ MAS spectra, implying a weak dipole–dipole interaction between 13 C and 1 H spins. These results demonstrate that pNA-d molecules undergo relatively fast motions.
Fig. 1. 13 C HD/MAS NMR spectra of pNA-d in (a) FSM, (b) C1FSM, (c) C8FSM and (d) NH2 FSM, measured at room temperature. The broad hump centered at about 125 ppm is a background signal caused by the rotor cap and the stator.
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The signals due to the organic parts of the reacted silylating agents are observed in the spectra of modified FSM/pNA-d (Fig. 1b–d). The profiles of these regions are identical to the samples before the introduction of pNA-d, indicating that the organic parts of the reacted silylating agents are not decomposed by the treatment in the process of the pNA-d introduction. 3.2. 2 H NMR spectra Fig. 2 shows 2 H NMR spectra of pNA-d in the mesopore of FSM at several temperatures. Resolved double peaks in Pake doublet pattern are barely observed on a very broad signal at 149 K. Spectra consist of an unresolved broad signal in the temperature range from 188 to 229 K. The signal becomes sharper with increase in temperature above 267 K, indicating that the molecules start to undergo an isotropic motion. Fig. 3 shows temperature dependence of the line width for the sharp signal. The line width is decreasing even at 398 K. Fig. 4 shows 2 H NMR spectra of pNA-d in the mesopore of C1FSM. The Pake doublet pattern is dominant in the spectrum at 149 K. A quadrupole coupling constant (QCC ¼ e2 Qq=h) and an asymmetry factor (g) are 180 kHz and 0, respectively, indicating that most of the pNA-d molecules are in a rigid state [55]. The sharp signal in the central region is ascribed to molecules undergoing an isotropic motion although its
Fig. 3. Temperature dependence of line widths of the sharp signals for FSM/pNA-d ( ), C1FSM/pNA-d ( ), C8FSM/pNA-d (N) and NH2 FSM/pNA-d (). FWHM is the full width at half maximum of the signals.
Fig. 4. Temperature dependence of 2 H NMR spectra of pNA-d in C1FSM.
Fig. 2. Temperature dependence of 2 H NMR spectra of pNA-d in FSM.
intensity ratio is less than 3%. With increase in temperature, the intensity of another signal with a broad line shape increases in the central region and that of the doublet pattern decreases. At 305 K, the doublet pattern disappears completely and a sharp signal is observed, indicating that all pNA-d molecules undergo fast iso-
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115
Fig. 5. Temperature dependence of 2 H NMR spectra of pNA-d in C8FSM.
Fig. 6. Temperature dependence of 2 H NMR spectra of pNA-d in NH2 FSM.
tropic motions. The signal hardly changes any more above 305 K (Fig. 3). Fig. 5 shows 2 H NMR spectra of pNA-d in the mesopore of C8FSM. Spectra are similar to those of C1FSM/pNA-d, but a narrower doublet signal with a split of 28 kHz is clearly distinguishable in the central region at 229 K. It is suggested that some portion of pNA-d molecules undergo the 180 flip-flop motion around the C2 axis of the molecule with a rate faster than 100 kHz at this temperature [26]. The Pake doublet and the narrower doublet disappear with increase in temperature, and a sharp signal is observed at 305 K. The sharp signal does not change above 305 K (Fig. 3), similarly to that of C1FSM/pNA-d. Fig. 6 shows 2 H NMR spectra of pNA-d in the mesopore of NH2 FSM. Below 229 K, the Pake doublet is observed, indicating that all pNA-d molecules are in a rigid state. At 267 K, the doublet pattern becomes small and a broad signal appears in the central region. At 305 K, the doublet pattern disappears and a sharp signal due to a fast isotropic motion is observed. The line width of the sharp signal decreases gradually with increase in temperature (Fig. 3).
motion of pNA-d around room temperature is studied by 2 H T1 measurements. Fig. 7 shows temperature dependence of T1 as a function of inverse temperature. The minimum values of T1 are observed around room temperature, and the relaxation mechanism is fluctuation of 2 H quadrupole interaction caused by molecular motions. The temperature dependence of T1 is analyzed by the Bloembergen–Purcell–Pound (BPP) type equation for the quadrupolar relaxation as [56] 2 3 De2 Qq g2 1þ T11 ¼ 40 h 3 " # sD 4sD þ ; ð1Þ 2 2 1 þ ðxD sD Þ 1 þ ð2xD sD Þ
3.3. 2 H spin-lattice relaxation times Spin-lattice relaxation times are good parameters to study motions of the order of Larmor frequency. The
where De2 Qq=h is a change of apparent QCC upon the motion considered in a unit of radian per sec, xD is the angular resonance frequency and sD is the correlation time of the motion of 2 H. The sD is described as follows: Ea sD ¼ s0 exp ; ð2Þ RT where s0 is a correlation time of 2 H at the infinite temperature, Ea is the apparent activation energy and R is the gas constant. The temperature dependence of T1 can be simulated successfully by one component for the samples of FSM, C1FSM and NH2 FSM. The parameters are listed in
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pNA-d above room temperature. Apparent activation energies are roughly estimated as 31 and 17 kJ mol1 below and above room temperature, respectively. Ordinarily, motions with larger activation energies are observed at higher temperatures. A motion with the smaller activation energy is observed at higher temperatures if a phase transition takes place. However, no DSC peaks due to phase transitions are detected in the temperature range from 260 to 380 K for C8FSM/ pNA-d. Thus, the above change in the activation energy seems to be curious. 3.4. Dynamics of the alkyl chain in C8FSM/pNA-d
Fig. 7. 2 H spin-lattice relaxation times of pNA-d in the mesopores of (a) FSM, (b) C1FSM, (c) C8FSM and (d) NH2 FSM. Simulated results are indicated by solid lines.
Table 1 and the simulated results are indicated in Fig. 7 as solid lines. The sD values at 300 K are calculated using Eq. (2), as listed also in Table 1. On the other hand, the results for C8FSM/pNA-d cannot be simulated by one component (Fig. 7c). The temperature dependence is similar to that of C1FSM/ pNA-d below room temperature and to that of FSM/ Table 1 Parameters resulting from the BPP fit to temperature dependence of 2 H T1 of pNA-d in FSM and modified FSM
FSM C1FSM C8FSMb Above RT Below RTd NH2 FSM
Ea / kJ mol1
QCC/kHz
s0 /s
a s300 D /s
20 31
85 118
1 · 1012 1 · 1014
3 · 109 3 · 109
17 31 32
118c 118 110
1 · 1012 1 · 1014 1 · 1014
1 · 109 3 · 109 4 · 109
C8FSM has the longest alkyl chains among the three modified FSM samples. Dynamics of the alkyl chain might cause the curious behavior of pNA-d in C8FSM. In this work, fast motions of the octyl chain are detected by 13 C T1 measurements as follows. Table 2 lists the obtained 13 C T1 values of four signals at 12.7 (C8), 29.6 (C4 + C5), 32.3 (C6) and 33.1 ppm (C3) (Fig. 1c), where the numbers in the parentheses express the carbon position counted from the Si atom in the silylating agent. The 13 C T1 values of the signals at 16.4 (C1) and 22.8 ppm (C2 + C7) are not available because of the low signal-to-noise ratio and the overlapping of the two signals, respectively. The values of 1=T1 are approximately proportional to sC =ð1 þ x2C s2C Þ when 13 C spins in the alkyl chain relax through fluctuation of dipolar interaction with nearby 1 H spins [56]. The xC value is the 13 C Larmor frequency in an angular frequency unit and sC is the correlation time of the motion. The results in Table 2 imply that the T1 values of C4–C8 increase with increase in temperature from 298 to 340 K, and that the 13 C correlation times are much shorter than 1 ns above room temperature. Furthermore, the T1 value increases monotonically from C3 to C8 carbons at each temperature, indicating that the rate of the motion of C8 carbon is faster than that of C3 carbon. These motions of the octyl chain might affect the dynamics of pNA-d in C8FSM, which will be discussed in a later section.
Table 2 13 C T1 results of the octyl group in C8FSM/pNA-d Temperature/K
C3a
C4 + C5b
C6a
C8c
a
298 320 340
0.74 0.64 1.0
0.78 0.92 1.4
1.2 1.7 2.2
1.6 2.3 2.7
b
a
sD at 300 K. Assuming different components above and below room temperature. c Assuming the same value as that of C8FSM below RT. d Assuming the same values as those of C1FSM.
T1 /s
The signals of C3 and C6 are deconvoluted. Assuming that C4 and C5 have the same T1 value. c The value might have been affected by partially overlapped signal of the methyl group in the ethoxyl group. b
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3.5. Dynamics of pNA-d in the mesopore of FSM The 2 H correlation time of FSM/pNA-d at 300 K estimated from the T1 analysis is quite short (3 · 109 s, Table 1). On the other hand, the 2 H NMR spectrum at 305 K shows a broad signal in the central region (Figs. 2 and 3), indicating that the isotropic motion is relatively slow at room temperature. Namely, the isotropic motion is not the dominant mechanism to relax 2 H spins, but other motions (named Motion IFSM ) such as a wobbling one relax 2 H spins in the temperature range from 267 to 398 K. The relaxation mechanism is supported by the QCC value effective for the relaxation (85 kHz, Table 1) which is much smaller than that in a rigid state (180 kHz). The featureless broad signal in the 2 H spectra around 200 K is considered to originate from Motion IFSM . In summary, temperature dependence of the dynamics of pNA-d is described as follows. At 149 K, pNA-d molecules undergo Motion IFSM with the apparent activation energy of 20 kJ mol1 . The correlation time of Motion IFSM is of the order of 100 kHz at 149 K, being predicted using the parameters in Table 1. The central broad signal in the 2 H NMR spectrum at 149 K is consistent with the above motion. With increase in temperature, the correlation time of Motion IFSM decreases, and it becomes 3 · 109 s at room temperature. On the other hand, the isotropic motion is recognized around room temperature in the 2 H NMR spectra (Fig. 2). The rate of the isotropic motion is of the order of 100 kHz at room temperature, considering the relatively broad 2 H signal. Dynamics of pNA-d in the mesopores is different from that in the micropores of zeolites. The pNA-d molecules undergo a 180 flip-flop motion with an activation energy of 57 kJ mol1 in siliceous ZSM-5 zeolite with a pore diameter of about 0.54 nm [26]. The large pore space (2.75 nm) in FSM allows the isotropic motion of pNA-d molecules as well as Motion IFSM with the smaller activation energy (20 kJ mol1 ).
3.6. Dynamics of pNA-d in modified FSM In a similar way to FSM/pNA-d, dynamics of pNA-d in modified FSM can be described by assuming two modes of motions; the isotropic motion and the other motion (named Motion IC1 , IC8 and INH2 for C1FSM/ pNA-d, C8FSM/pNA-d and NH2 FSM/pNA-d, respectively). The isotropic motion for the modified samples is recognized as the sharp signals in the 2 H NMR spectra around room temperature. However, the narrower line widths and the smaller magnitudes of the temperature dependence than that of FSM/pNA-d (Fig. 3) indicate faster rates of the isotropic motion for the modified samples. On the other hand, the spectral changes from
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267 to 305 K (Figs. 4–6) are more evident than that of FSM/pNA-d (Fig. 2), indicating that the activation energy of the isotropic motion for the modified samples is larger than that of FSM/pNA-d. The 2 H broad signals in the central region at 229 K are caused by Motion IC1 , IC8 and INH2 of pNA-d (Figs. 4–6). Furthermore, it is reasonable to assume that the T1 values (Fig. 7) are related with Motion IC1 , IC8 and INH2 . The larger QCC values (110–118 kHz) than that of FSM (85 kHz) may imply that modes of Motion IC1 , IC8 and INH2 are different from that of Motion IFSM (Table 1). Temperature dependence of the dynamics of pNA-d in C1FSM is described as follows. At 149 K, Motion IC1 is slow and most of the molecules are in a rigid state. With increase in temperature, the correlation time of Motion IC1 decreases, and it becomes 3 · 109 s at room temperature. The apparent activation energy of Motion IC1 is much larger (31 kJ mol1 ) than that of FSM (Table 1). On the other hand, the isotropic motion is recognized around room temperature in the 2 H NMR spectra (Fig. 4). The activation energy for the isotropic motion is also larger than that of FSM, and the rate of the isotropic motion is faster than that of FSM at room temperature. The larger activation energies of both Motion IC1 and the isotropic motion indicate stronger interaction between pNA-d and C1FSM than that of FSM. The most plausible interaction is the interaction between the aromatic ring of pNA-d and methyl groups of the reacted silylating agent on the pore surface. As for pNA-d in C8FSM, the 180 flip-flop motion is detected clearly in the spectra at 229 K (Fig. 5). Motion IC8 might be ascribed to the 180 flip-flop motion with a wobbling motion. In this case, the C2 axis of pNA-d must be fixed considerably during the flip-flop motion, possibly on octyl chains. The curious behavior of Motion IC8 is observed at about room temperature. The interaction between pNA-d and octyl chains might be changed at about room temperature. The fast motion of octyl chains deduced by the 13 C T1 measurements might affect Motion IC8 in the higher temperature range. Dynamics of pNA-d in NH2 FSM is also described by assuming Motion INH2 and the isotropic motion. At 149 K, Pake doublet pattern is clearly observed (Fig. 6), indicating that all the molecules are in a rigid state and that Motion INH2 is the slowest in all the samples. Furthermore, the line width of the 2 H signal decreases with increase in temperature and broader than those of C1FSM/pNA-d and C8FSM/pNA-d at room temperature (Fig. 3), indicating that the rate of the isotropic motion is slower at room temperature. These motional properties might be caused by a hydrogen bonding between an NO2 group of pNA-d and an NH2 group of the reacted silylating agent on the pore surface. Conclusively, the rates of the isotropic motion and Motion IFSM , IC1 , IC8 and INH2 are varied with the surface environments of mesopores.
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4. Conclusion Molecular motions of pNA-d in the FSM-type mesoporous silicas have been studied successfully by means of 2 H and 13 C NMR. The isotropic motion is observed above room temperature, and the rates of the isotropic motion for modified FSM/pNA-d are faster than that of FSM/pNA-d at about room temperature. The 2 H spectra below room temperature indicate another motion such as a wobbling motion (Motion I). Analysis of spin-lattice relaxation times (T1 ) for 2 H spins demonstrates that the apparent activation energies of Motion I in C1FSM and NH2 FSM (31–32 kJ mol1 ) are larger than that of Motion I in FSM (20 kJ mol1 ). On the other hand, Motion I in C8FSM is complicated probably because of the motion of octyl chains of the reacted silylating agent. It is concluded that pNA molecules undergo the isotropic motion and Motion I, and that their motions are varied with the surface properties of the mesopores.
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