ESR Study of Molecular Orientation and Dynamics of Nitronyl

Article pubs.acs.org/JPCA

ESR Study of Molecular Orientation and Dynamics of Nitronyl Nitroxide Radicals in CLPOT 1D Nanochannels Hirokazu Kobayashi,* Yuka Morinaga, Etsuko Fujimori, and Tetsuo Asaji Department of Chemisty, College of Humanities and Sciences, Nihon University, 3-25-40, Sakura-jo-sui, Setagaya-ku, Tokyo 156-8550, Japan ABSTRACT: New inclusion compounds (ICs) were prepared using the organic 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) as a nanosized template and nitronyl nitroxide (NN) radicals such as phenylnitronylnitroxide (PhNN) and p-nitrophenylnitronylnitroxide (p-NPNN). ESR measurements below 255 K for the CLPOT ICs diluted with spacer molecules gave rigid limit spectra similar to that for PhNN molecules in a glassy ethanol matrix at low temperature, which suggests isolation of the radical molecules. ESR measurements for them in the range of 290−400 K gave a modulated quintet ESR signal, which suggested uniaxial rotational diffusion of NN radicals in the nanochannels approximately around the principal y-axis of the g-tensors. In the ESR measurements to 430 K for the [(CLPOT)2-(p-NPNN)0.07] IC without spacers, the broader line width than the case in dilution was observed by inter-radical dipolar interaction. In every case, the rotational diffusion activation energies of NN radicals in the CLPOT nanochannels were several times larger than those of 2,2,6,6tetramethyl-1-piperidinyloxyl (TEMPO) radical derivatives (4-X-TEMPO) in CLPOT nanochannels. This is expected due to the larger molecular size of NN radicals than 4-X-TEMPO or stronger interaction between NN radicals and the surrounding host or guest molecules.

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TEMPO in the TPP nanochannels τR, is in the range of slow motion in the time scale of ESR at temperatures range above 112 K; 10−6 s > τR > 10−9 s.15 The molecular motion of TEMPO radicals in the TPP nanochannels is expected to have some effect on the interspin exchange/dipolar interactions of the [(TPP)2-TEMPO] IC. In fact, the [(TPP)2-TEMPO] IC itself is paramagnetic. However, control of the molecular orientation and dynamics of similar ICs by the appropriate selection of host and guest compounds in [(organic 1D porous material)-(organic radical)] ICs could be useful for probing magnetic exchange between radicals, so that it offers motivation to develop new organic magnets. In the construction of molecular chains of organic radicals using organic 1D nanochannels as templates, several combinations of TPP and nitroxide radicals such as TEMPO derivatives substituted at the 4-position (4-X-TEMPO) have been used to date.16−18 However, the upper limit of pore diameter (ca. 0.9 nm) and poor solubility of TPP in polar solvents makes it difficult to use as a versatile nanosized template for the construction of organic−radical 1D molecular chains. Therefore, we recently suggested that another 1D crystalline nanochannel template, such as 2,4,6-tris(4-chlorophenoxy)1,3,5-triazine (CLPOT; Scheme 1b19,20), can accommodate TEMPO or 4-hydroxy-TEMPO (TEMPOL).21 The CLPOT crystal has similar cavities to TPP nanochannels, of which the pore diameters are larger (ca. 1.1−1.3 nm).21,22 The structure

INTRODUCTION Crystalline 1D porous materials are often used as nanosized molecular templates for the 1D alignment of functional molecules.1−6 Most of these templates have periodic and regular cavities such as 1D nanochannels. The inclusion compounds (ICs) using such 1D porous materials as templates and organic functional molecules, when introduced in a 1D manner, exhibit anisotropic physical properties. In particular, studies on the inclusion of organic radicals as guest compounds could lead to the development of new “organic” magnets. Recently, several approaches for the construction of 1D organic−radical molecular chains have been conducted using various organic and/or inorganic 1D porous materials as templates.7−10 Tris(o-phenylenedioxy)cyclotriphosphazene (TPP; Scheme 1a) is a typical organic 1D porous material with pore diameter of 0.46−0.9 nm depending on the size of the guest molecules.11 We have previously reported a TPP IC with 1D molecular chains of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radicals in the TPP nanochannels ([(TPP)2-TEMPO] IC).12,13 Isotropic electron spin resonance (ESR) spectra of [(TPP)2TEMPO] IC changed from near-Gaussian below 139 K to an intermediate between Lorentzian and Gaussian at higher temperature, which indicates 1D spin diffusion behavior (|Jintra| < 1 K) with an increase in temperature. In addition, temperature-dependent ESR measurements for the [(TPP)2(TEMPO)0.02/(TEMP)0.98] IC (TEMP = 2,2,6,6-tetramethylpiperidine; nonradical spacer) suggested the anisotropic rotational diffusion of TEMPO around the principal y-axis of the g-tensor.14 The rotational diffusion correlation time of © 2014 American Chemical Society

Received: April 15, 2014 Revised: May 27, 2014 Published: June 17, 2014 4907

dx.doi.org/10.1021/jp503681b | J. Phys. Chem. A 2014, 118, 4907−4917

The Journal of Physical Chemistry A

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Scheme 1. Chemical Structures of the Host and Guest Compounds Used in This Study

Article

EXPERIMENTAL SECTION

2.1. Chemicals. CLPOT (solid, colorless) was synthesized according to a previously reported method.19 PhNN (solid, indigo blue), p-NPNN (solid, dark green), and N-phenyl maleimide (N-PhMI; Scheme 1e) (solid, pale green) were purchased from Tokyo Chemical Industry Co. Ltd. All chemicals were used without further purification. 2.2. Sample Preparation. Sample preparation was conducted according to the following two concepts: (1) To examine the molecular orientation and dynamics of NN radicals confined in the CLPOT nanochannels, the radical fraction in the CLPOT nanochannels is diluted by spacers (compounds 1 and 2, see below). N-PhMI with a similar molecular shape to the radicals was used as a spacer in this study. (2) For the construction of 1D molecular chains of NN radicals, the sample is prepared without spacers (compounds 4, see below). The [(CLPOT)2-(PhNN)x/(N-PhMI)y] IC (1) was prepared using the following procedure. Both N-PhMI (2.3 × 10−2 mol) and PhNN (2.1 × 10−5 mol) were mixed in 3 mL of benzene, and the solution was poured into 4 mL of boiling 5.4 × 10−2 mol L−1 CLPOT benzene solution. A pale yellow powder was obtained as the product after cooling the solution to room temperature. The [(CLPOT)2-(p-NPNN)x/(N-PhMI)y] IC (2) was prepared using the following procedure. Both N-PhMI (2.3×10−2 mol) and p-NPNN (5.4 × 10−5 mol) were mixed in 3 mL of benzene, and the solution was poured into 4 mL of boiling 5.4 × 10−2 mol L−1 CLPOT benzene solution. A pale green powder was obtained as the product after cooling the solution to room temperature. The [(CLPOT)2-(N-PhMI)x] IC (3) was prepared using the following procedure. Three milliliters of 0.77 mol L−1 N-PhMI benzene solution was poured into 4 mL of boiling 5.4 × 10−2 mol L−1 CLPOT benzene solution. A pale yellow powder was obtained as the product after cooling the solution to room temperature. The [(CLPOT)2-(p-NPNN)x] IC (4) was prepared using the following procedure. One milliliter of 6.5 × 10−2 mol L−1 pNPNN benzene solution was poured into 4 mL of boiling 5.4 × 10−2 mol L−1 CLPOT benzene solution. A light green powder was obtained as the product after cooling the solution to room temperature. Characterization of all compounds was conducted using powder X-ray diffraction (XRD), ESR spin concentration measurements, elemental analysis, (EA) and thermogravimetric analysis−differential thermal analysis (TG-DTA). After sample characterization, variable-temperature ESR measurements of the ICs were conducted. Compounds 1−4 were stable for at least 1 year in air but even longer under He gas atmosphere used for high temperature (HT) ESR measurements (up to 430 K). A compound using CLPOT and PhNN without N-PhMI was also prepared by a similar method using benzene or ethanol to that for 4. However, the ESR spectrum of each sample was changed a month after preparation, even under the He gas atmosphere. The spin concentration of each sample decreased after the HT ESR measurements due to decomposition. In addition, the inclusion of NN radicals into TPP was difficult due to the larger molecular size and polarity of the NN group. 2.3. Instrumentation. Powder XRD analyses for all samples were conducted at room temperature using a diffractometer (Rint 2100, Rigaku Corp.) with graphite

of CLPOT nanochannels is maintained even when guest-free, due to self-assembly via alternating triazine and Cl−Cl supramolecular synthons. CLPOT is soluble in either polar solvents such as ethanol or in nonpolar solvents such as benzene. However, such smaller solvent molecules as ethanol or benzene are not included in CLPOT nanochannels. The characteristics of CLPOT nanochannels are available for the construction of molecular chains of organic radicals, because many commercially available organic radicals have cross sections greater than 1 nm and contain polar substituent groups. In this study, CLPOT ICs were prepared using nitronyl nitroxide (NN) radicals such as phenylnitronylnitroxide (PhNN; Scheme 1c23−25) or p-nitrophenylnitronylnitroxide (p-NPNN; Scheme 1d26−31). This is a rare case where NN radicals with higher delocalization of an unpaired electron were used as an alternative to TEMPO derivatives for the preparation of an IC using organic 1D nanochannels. The inclusion of NN radicals used in this study or other well-known substituted phenyl-NN radicals (XPNN)32,33 into organic 1D nanochannels could realize a molecular magnetic material through the inter-radical or radical−medium interaction. The molecular orientation and dynamics of PhNN or p-NPNN molecules in the CLPOT nanochannels were clarified using computer simulations with the EasySpin program package.34 4908



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monochromated Cu Kα radiation (λ = 0.15418 nm). Data were collected in the θ−2θ scan mode using a 2θ scan rate of 2° min−1 in the 2θ range of 5−50°. TG-DTA measurements were conducted for all samples using a thermogravimetric apparatus (TG8120, Rigaku Corp.). Ten milligrams of powder specimens and Al2O3 powder as a reference were placed in Pt sample pans and set in apparatus and heated between room temperature and 400 °C with a heating rate of 10 °C/min under an Ar gas atmosphere. ESR spectra were recorded using an X-band spectrometer (JES-FA300, Jeol) with a TE011 cylindrical cavity resonator at temperatures from 4.2 to 430 K. Powdered specimen (2−3 mg) was packed in a commercial quartz glass ESR tube (270 mm long, o.d. 5 mm) and then sealed under a He atmosphere at 4 kPa. Thermal equilibrium of the sample was achieved by waiting 5−10 min after each temperature change. Signal reproduction was also confirmed for both increasing and decreasing temperature. The X-band (e.g., 9.072091 GHz) microwave power was set to 0.001−1 mW under nonsaturated conditions. When the ESR measurement for each IC was conducted below 20 K, saturation occurred above about 0.005 mW. Microwave power was set at 0.01 mW in the ESR measurements in the range of 20−300 K, and it was set at 0.1 mW above 300 K under nonsaturated conditions. The field range and field sweep rate were set at 323 ± 15 mT and 30 mT/8 min, respectively. The field modulation was 100 kHz with an amplitude of 0.1 mT. Spin concentration was measured for the samples by comparing the double integral of the cw-ESR spectra of weighted samples with that of TEMPOL standard solution via the signal intensity of a manganese standard. All measurements were conducted with the same loading of the cavity. The estimated error was 30%. Spectral simulations of compounds 1, 2, and 4 were performed using the Pepper program for the solid-state and using the Chili program software package for anisotropic slowmotion cw ESR spectra (EasySpin 4.5.5, ETH Zürich).34 Chili requires more memory to run the spectral reproduction of NN radicals; therefore, the “Allocation” function of Opt should be taken such as “Opt.Allocation = [1e7 2e5]”.35 2.4. Procedure To Obtain the Rotation Axes of the Radical in the Nanochannels. The procedure to obtain the rotation axes of the radical in the nanochannels followed that previously reported.18,21,36 NN free radicals orientated in 1D nanochannels are characterized by the spin Hamiltonian: Ĥ = βeB·g ·S ̂ + S ·̂ ∑ Ai ·Iî + βngnB· ∑ Iî i = 1,2

i = 1,2

Figure 1. Principal axes of the g and A tensors for NN radicals. The direction of the rotation axis (magenta arrow) is defined as depicted. Symbols θ and ϕ denote, respectively, the polar and azimuthal angles.

of NN group (i.e., the principal z-axis direction) and be near 2.0023.37 In molecular orbital calculations of gxx, spin−orbit interactions between the unpaired electron on the NN group and the π orbital make a large positive contribution to gxx. The contribution of the π orbital on the NN group to gyy is positive and smaller than gxx. Thus, it is suggested that gxx >gyy >gzz in NN radicals (i.e., the intensity of the gxx component is observed at the lowest magnetic field side). In addition, with respect to the A tensor, the unpaired electron occupies the π orbital composed of the 2pz orbitals of nitrogen atoms with considerable spin density, so that Azz is the greatest component,38 whereas Axx and Ayy are expected to be less than Azz. Therefore, the A tensor of NN radicals is quite anisotropic. The local axes of A tensors of two nitrogen nuclei of NN radicals are expected to be different, although they may be related by molecular symmetry. Nevertheless, in our analysis, it was assumed that the unpaired electron interacts with the two nitrogen nuclei having an averaged A tensor. This is solely due to making the analysis simple and easy. However, we believe this is a resonably good approximation. In this approximation, the principal axes of the g and A tensors for NN radicals will be coincident. A typical rigid-limit ESR spectrum for NN radicals simulated using the Pepper program software package (EasySpin 4.5.5; ETH Zürich34) for calculation of the solid-state cw-ESR spectra is depicted in Figure 2a. The five major hyperfine interaction lines associated with the two equivalent nitrogen atoms with a hyperfine coupling constant that corresponds to Azz are shown on the horizontal axis in Figure 2a for convenience.37 Although the lines 1, 4, and 5 are well-resolved, the spectrum between 2 and 3 is more complicated. These are caused by gxx and gyy components that are larger than gzz, and by Axx and Ayy components that are much smaller than Azz. Therefore, the g and A tensors are experimentally estimated as follows: gzz and Azz are set as default from the major five hyperfine coupling lines of the measured ESR spectrum for isolated NN radicals at low temperature, in which the molecular motion of the NN radical is thought to be frozen; gxx and gyy, and Axx and Ayy are difficult to determine directly from the ESR spectra. Therefore, at first they were estimated from the spectral reproduction of

(1)

where βe, B, g, Ai, βn, gn, S,̂ and Iî are the Bohr magneton, the laboratory magnetic flux density vector, the electron spin g tensor, the hyperfine tensor by ith 14N nucleus in the NN radical (i = 1 or 2), the nuclear magneton, the nuclear spin gfactor, the electron spin operator, and the nuclear spin operator of ith 14N nucleus in the NN radical, respectively. In this study, the principal axes of NN radicals are taken as follows (see ref 37 and Figure 1); the z-axis for PhNN and pNPNN radicals is perpendicular to the molecular plane of NN group; the y-axis is parallel to the bond between the 2-position carbon atom of the NN group and the phenyl ring, and the xaxis is perpendicular to the yz plane. The g tensor of NN radicals is determined as follows. The unpaired electron of an NN group is associated with a π orbital on the NN group; therefore, the lowest component of the g tensor should be observed perpendicular to the molecular plane 4909



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line-width components were taken as 0.20 and 0.40 mT, respectively. The g and A tensors used for the reproduction in Figure 2 are shown in Table 1,37 and they are comparable with those for nitroxide radicals such as TEMPO (see ref 21 and Table 1). For reproduction of ESR spectra at higher temperature, in which molecular motion of the radicals is excited in the 1D nanochannels, it was assumed that the rotation axis of PhNN or p-NPNN is defined by polar and azimuthal angles, θ and ϕ, respectively, in the principal axis system of the g-tensor for NN radicals (see Figure 1). In the domains of 0 ≤ θ ≤ π and 0 ≤ ϕ ≤ 2π, four possible rotation axes defined by (θ0, ϕ0), (θ0, 180° − ϕ0), (θ0, 180° + ϕ0), and (θ0, 360° − ϕ0) were derived.18,21 In the following discussion, only the (θ0, ϕ0) values are given; however, it should be noted that this ambiguity remains. For a molecule that undergoes many collisions that cause small random angular reorientations, the resultant anisotropic rotational motion is a Markov process.15,40 When Ω is defined as the Euler angles for a tumbling molecular axis with respect to a fixed laboratory axis system, the probability of finding the molecule in the direction Ω at time t is defined by P(Ω, t). In a Markov process, the probability of being at state Ω1 at time t, if in state Ω2 at time t − Δt, is (a) independent of the Ω value at any time earlier than t − Δt and is (b) dependent only on Δt and not on t. P(Ω,t) can then be described by the rotational diffusion equation: ∂P(Ω, t ) 2 = R ▽Ω P(Ω, t ) ∂t

Figure 2. Simulated ESR spectra of (a) typical powder rigid NN radicals using the Pepper program in EasySpin and (b) NN radicals modulated on the basis of anisotropic rotational diffusion around the principal z (top), x (middle), and y (bottom) axes of the g-tensor using the Chili program within the slow motion regime with respect to the ESR time scale. Bars on the horizontal axis in (a) indicate the five major hyperfine coupling interactions by two magnetically equivalent nitrogen atoms (Azz ≈ 1.8 mT, see Table 1). The Gaussian and Lorentzian line-width components were respectively taken as 0.20 and 0.40 mT in (a) and 0.18 and 0.28 mT in (b). In (b), the rotational diffusion correlation time τR was taken as 3 × 10−8 s.

(2)

where ▽Ω is the Laplacian operator on the surface of a unit sphere and R is the rotational diffusion coefficient. If the molecule is approximated by a rigid sphere of radius a rotating in a medium of viscosity η, the rotational Stokes−Einstein relationship yields 2

R=

kT 8πa3η

(3)

In an anisotropic liquid, the average rotational diffusion correlation time τR is defined by

the experimental ESR spectra using EasySpin by eye, and then, the final value of each component of g and A tensors was estimated by varying these components (gii and Aii, i = x, y, and z), line-width parameters, and signal amplitude step by step so as to obtain the smallest value of the sum of the squares of the difference between the experimental and simulated spectra. In EasySpin, the line shape is expressed by a Voigt function, which is defined by Gaussian broadening (convolution) of the Lorentzian peaks.39 Therefore, the line-width parameters in the following consist of two parameters that correspond to Gaussian and Lorentzian functions. In the present analysis, the full width at half-maximum (fwhm) was taken as the linewidth parameter. In Figure 2a, the Gaussian and Lorentzian

τR = (6 R R⊥ )−1

(4)

where R∥ and R⊥ are defined as the parallel and perpendicular components of the rotational diffusion tensor, R. It was assumed that eqs 2−4 are applicable to the anisotropic molecular rotational diffusion in the organic 1D nanochannels.18,21 Thus, in the simulation, highly anisotropic R∥ and R⊥ components, such as R∥/R⊥ ≈ 1000 were assumed. Figure 2b shows the ESR spectrum simulated according to the uniaxial rotational diffusion around the principal z (top), x (middle), or y (bottom) axis using the Chili program package

Table 1. Tensor Components of g and A for PhNN in Rigid Duco Cement, and PhNN (1), p-NPNN (2 and 4), and TEMPO in CLPOT Nanochannels state of radicals a

PhNN in rigid Duco Cement PhNN in CLPOT diluted by spacers (this study, 1) p-NPNN in CLPOT diluted by spacers (this study, 2) p-NPNN in CLPOT without spacers (this study, 4) TEMPO in CLPOTb a

gxx

gyy

gzz

Axx/mT

Ayy/mT

Azz/mT

2.0127 2.0118 2.0116 2.0102 2.0102

2.0068 2.0075 2.0076 2.0084 2.0062

2.0028 2.0031 2.0030 2.0032 2.0023

0.52 0.34 0.32 0.64 0.73

0.52 0.12 0.16 0.08 0.60

1.80 1.74 1.77 1.64 3.35

ref 37. bref 21. 4910



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The results in Figure 3b,c are consistent with the assumption. Therefore, CLPOT nanochannnels are expected to exist in 1 and 3. Figure 4 shows XRD patterns for (a) bulk N-PhMI, (b) 2 ([(CLPOT)2-(p-NPNN)0.04/(N-PhMI)0.50], see below regard-

in EasySpin: the Gaussian and Lorentzian line-width components, and τR were, respectively, taken as 0.18 and 0.28 mT, and 3 × 10−8 s. The values of θ and ϕ were estimated by varying τR, linewidth parameters, and signal amplitude so as to obtain the best fit for an ESR spectrum at the highest temperature, at which molecular motion of guest radicals in the 1D nanochannels is in the range of slow-motion region (10−6 s > τR > 10−9 s) in the time scale of ESR. By using these θ and ϕ, τR at each temperature was estimated similarly by varying τR, line-width parameters, and signal amplitude.

3. RESULTS AND DISCUSSION 3.1. Characterization of [CLPOT-(NN Radical)] ICs. Figure 3 shows XRD patterns for (a) bulk N-PhMI, (b) 3

Figure 4. Powder XRD patterns for (a) bulk N-PhMI, (b) 2 ([(CLPOT)2-(p-NPNN)0.04/(N-PhMI)0.50]), (c) 4 ([(CLPOT)2-(pNPNN)0.07]), and (d) bulk p-NPNN at room temperature. The vertical bars under (b) and (c) are estimations with the assumption that they belong to the same space group as guest-free CLPOT (P63/ m),22 and that they have different cell parameters.

ing the compositions), (c) 4 ([(CLPOT)2-(p-NPNN)0.07]), and (d) bulk p-NPNN. As with Figure 3, the red bars under Figure 4b,c are the estimations in which it is assumed that 2 and 4 belong to the same space group as guest-free CLPOT. The results in Figures 4b,c are consistent with the assumption. Therefore, CLPOT nanochannnels are expected to exist also in 2 and 4. Experimental results for the EA, ESR spin concentration, and desorption amounts from TG measurements are given in Table 3. The bold italic entries under radicals and spacers show the estimated fraction of radical and/or spacer molecules to the CLPOT unit cell formed by 2 CLPOT molecules, that is consistent with the experimental results. The number of guest molecules per CLPOT unit cell was estimated according to the EA results, and the radical fraction of guest molecules was estimated on the basis of ESR spin concentration as listed in

Figure 3. Powder XRD patterns for (a) bulk N-PhMI, (b) 3 ([(CLPOT)2-(N-PhMI)]), (c) 1 ([(CLPOT)2-(PhNN)0.01/(NPhMI)0.50]), and (d) bulk PhNN at room temperature. The vertical bars under (b) and (c) are estimations with the assumption that 1 and 3 belong to the same space group as guest-free CLPOT (P63/m)22 and that they have different cell parameters.

([(CLPOT)2-(N-PhMI)]), (c) 1 ([(CLPOT)2-(PhNN)0.01/ (N-PhMI)0.50]), and (d) bulk PhNN at room temperature (see below regarding the compositions). The vertical bars under Figure 3b,c are the estimations assuming that 1 and 3 belong to the same space group as guest free CLPOT (P63/ m)22 and that they have different cell parameters (Table 2).

Table 2. Cell Parameters for Crystals of Guest-Free CLPOT and Those for 1−4a compounds

a/nm

c/nm

guest-free CLPOTb 1 2 3 4 [(CLPOT)2-(TEMPOL)0.01/(TEMP−OH)0.39] ICc [(CLPOT)2-(TEMPO)0.02/(TEMP)0.98] ICc

1.5364(3) 1.520 1.535 1.530 1.535 1.520 1.510

0.6855(2) 0.682 0.682 0.683 0.682 0.686 0.687

a

Values for 1−4 were estimated on the assumption that they belong to the same space group as guest-free CLPOT (P63/m), [(CLPOT)2(TEMPOL)0.01/(TEMP−OH)0.39] (TEMP−OH = 4-hydroxy-2,2,6,6-tetramethylpiperidine; non-radical spacer), and [(CLPOT)2-(TEMPO)0.02/ (TEMP)0.98] ICs. bref 22. cref 21. 4911



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Table 3. Experimental Results for the EA, ESR Spin Concentration, and Desorption Amounts from TG Measurementsa compounds radicals 1 2

PhNN 0.01 p-NPNN 0.04

3 4

p-NPNN 0.07

spacers N-PhMI 0.50 N-PhMI 0.50 N-PhMI 1.0

EA H/% 2.76 2.76 2.65 2.78 2.75 2.85 2.73 2.64

C/% 56.15 56.04 56.09 56.10 57.05 57.06 54.90 54.94

N/%

ESR spin concentration/g−1

8.89 9.04 8.86 9.09 9.08 8.95 9.10 9.10

8 8 2 2

× 10 × 1018 × 1019 × 1019 18

5 × 1019 5 × 1019

desorption amount in TG measurements/% 8.9 8.3 8.6 8.0 15.8 15.8 0.42 2.1

a

Bold italic entries under radicals and spacers show the estimated fraction of radical and/or spacer molecules to the CLPOT unit cell formed by two CLPOT molecules, which is consistent with the experimental results. The calculated values for this estimation are shown by italic.

Table 3. The results of TG measurements also support these results. In addition, the ESR signal of 3 was silent. According to these results, compounds 1-4 are respectively assigned as follows: (1) [(CLPOT)2-(PhNN)0.01/(N-PhMI)0.50], (2) [(CLPOT)2-(p-NPNN)0.04/(N-PhMI)0.50], (3) [(CLPOT)2(N-PhMI)], and (4) [(CLPOT)2-(p-NPNN)0.07]. The total numbers of guest molecules (i.e., radical and spacer) of 1 and 2 per CLPOT unit cell were estimated to be below 0.6, which are as much as the [(CLPOT) 2 (TEMPOL)0.01/(TEMP−OH)0.39] IC (TEMP−OH = 4hydroxy-2,2,6,6-tetramethylpiperidine; nonradical spacer), and are smaller than [(CLPOT)2-(TEMPO)0.02/(TEMP)0.98] IC in our previous study.21 These results imply that there are many vacancies in the CLPOT nanochannels in 1, 2, and 4. The small amounts of NN radical inclusion in 1, 2, and 4 are assumed to be caused by a lower concentration of radicals in solution during sample preparation due to the lower solubility of NN radicals in benzene (cf. 7.4 mol L−1 of TEMPO mesitylene solution in the preparation of [(TPP)2-TEMPO] IC, 10−100 times as much as this study). Thus, the choice of an appropriate solvent in which CLPOT and NN radicals are well dissolved is necessary. Figure 5 shows the TG-DTA curves for 1 ([(CLPOT)2(PhNN) 0.01 /(N-PhMI) 0.50 ]) and DTA curves for 3 ([(CLPOT)2-(N-PhMI)]), guest-free CLPOT, and bulk NPhMI. The TG-DTA results for 2 ([(CLPOT)2-(p-NPNN)0.04/ (N-PhMI)0.50]) were similar to those for 1. Each DTA curve for 1 and 3 was similar to that for bulk CLPOT. The large endothermic peak of 1, 3, and bulk CLPOT at 220 °C indicates melting of the CLPOT crystal,20 at which the weight loss was observed in the TG curve. However, two additional small endothermic peaks were also observed at 71 and 200 °C both for 1 (including radicals) and 3 (without radicals). These are distinguishable from the endothermic peaks of bulk N-PhMI at 90 °C for melting and at 234 °C for boiling, which indicates that these endothermic peaks originated from [(CLPOT)2-(NPhMI)] itself and implies the inclusion of PhNN or N-PhMI molecules in the CLPOT nanochannels in 1−3 (see below). Figure 6 shows the TG-DTA curves for 4 ([(CLPOT)2-(pNPNN)0.07]) and DTA curves for guest-free CLPOT and bulk p-NPNN. The DTA curve for 4 was also similar to that for bulk CLPOT, whereas the two additional small endothermic peaks as observed in Figure 5 were also observed at 110 and 199 °C in Figure 6. Each endothermic peak in 4 was also distinguishable from that of bulk p-NPNN observed at 135 °C,29 as well as the anomaly that indicates decomposition at 170 °C. Therefore, the inclusion of p-NPNN in the CLPOT nanochannels in 4 is also expected (see below).

Figure 5. TG-DTA curves for 1 ([(CLPOT)2-(PhNN)0.01/(NPhMI)0.50]) and DTA curves for 3 ([(CLPOT)2-(N-PhMI)]), guestfree CLPOT, and bulk N-PhMI. All measurements were conducted with a heating rate of 10 °C/min in an Ar atmosphere. The TG-DTA curves for 2 ([(CLPOT)2-(p-NPNN)0.04/(N-PhMI)0.50]) were similar to those for 1.

3.2. ESR Spectra for PhNN or p-NPNN Molecules Diluted by Spacers in the CLPOT Nanochannels. D’Anna et al. reported temperature-dependent ESR measurements of PhNN radicals dispersed in rigid glassy ethanol matrices.37 In their study, a rigid-limit ESR spectrum of PhNN was observed in a rigid ethanol matrix at 128 K (note: ethanol forms a glass near 148 K) or in Duco Cement at 77 K, and the principal components of the g and A tensors of PhNN radicals were determined as shown in Table 1. The g and A tensors of PhNN and p-NPNN in the CLPOT nanochannels estimated in our study using EasySpin are thus comparable with their results. In addition, D’Anna et al. reported that the rigid-limit ESR spectrum of PhNN observed at low temperature changed with an increase in temperature to 168 K into two narrower peaks between lines 2 and 3 depicted in Figure 2a. Such a change is easily expected to be due to molecular motion in viscous ethanol solution; however, the details of molecular motion were not reported in their study. Figure 7 shows a temperature-dependent experimental (black) and simulated (red) ESR spectra for 1 ([(CLPOT)24912



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255 to 398 K. All spectra were normalized by each spectral maximum peak height. The spin concentration of each sample remained unchanged after the temperature-dependent ESR measurements. The ESR spectra below 255 K for 1 and 2 were respectively unchanged within the experimental error and were well-reproduced on the basis of a rigid-limit powder pattern of an isolated PhNN radical shown in Figure 2a. Therefore, the PhNN and p-NPNN radicals included in the CLPOT nanochannels of 1 and 2, respectively, are expected to be adequately isolated by spacers, and the molecular motion is frozen with respect to the ESR time scale (i.e., τR > 10−6 s). The spectra for PhNN and p-NPNN at 255 K, respectively, were simulated using the g and A tensor components shown in Table 1, and the respective Gaussian and Lorentzian line-width components were 0.25 and 0.23 mT for 1, and 0.06 and 0.88 mT for 2. The values of the g and A tensors for 1 and 2 estimated using EasySpin were consistent with the results reported by D’Anna et al.37 The ESR line widths for isolated p-NPNN molecules in the CLPOT nanochannels, as shown in Figure 7b, are broader than those for PhNN in Figure 7a. In 1 or 2, the guest inclusion amount is similar. Moreover, in ESR measurements of PhNN or p-NPNN benzene solution of 1 × 10−5 mol L−1, each spectrum was similar, and was an isotropic quintet with the peak-to-peak line width of 0.26 mT and aN = 0.74 mT. Therefore, the broader line width in 2 implies the larger interaction between radicals and host/spacers. For TEMPO or TEMPOL molecules in CLPOT nanochannels diluted by spacers, a change of the ESR spectra from a rigid limit was observed above 100 K. However, the temperature dependence of the ESR spectra for the NN radicals in CLPOT was observed above 255 K. These results are ascribable to the fact that the NN radicals are larger and heavier than TEMPO derivatives. Figure 7a,b show a continuous change in the ESR spectra with the increase in temperature. In the range of 312−339 K for 1 and around 351 K for 2, two narrower peaks were observed between lines 2 and 3, which are similar to those observed by D’Anna et al.37 In addition, the modulated quintet from the two 14 N atoms with I = 1 of the NN group were observed above 349 K for PhNN in the CLPOT nanochannels and above 369 K for p-NPNN, respectively. The temperature-dependent ESR spectra of 1 and 2 were reproducible in the model for the anisotropic rotational diffusion of isolated PhNN and p-NPNN molecules in CLPOT nanochannels within the slow motion regime with respect to the ESR time scale (10−9 s < τR < 10−6 s) over the entire temperature range in Figure 7. The ESR spectrum of 1 at 368 K was reproducible on the assumption of rotational diffusion around the axis tilted by (θ, ϕ) = (68°, 78°) to the principal system of the g-tensor, with Gaussian and Lorentzian line-width components and τR of 0.09 and 0.24 mT, and 1.1 × 10−8 s, respectively. The ESR spectra were also wellreproduced at other temperatures by the anisotropic rotational diffusion model around the same rotation axis. In the case of 2, the ESR spectrum at 398 K was reproducible on the assumption of rotational diffusion around the axis tilted by (θ, ϕ) = (74°, 90°) to the principal system of the g-tensor, with the Gaussian and Lorentzian line-width components and τR of 0.11 and 0.30 mT, and 1.0 × 10−8 s, respectively (see Figure 7). According to these results, PhNN and p-NPNN radicals are assumed to be included in the CLPOT nanochannels. Therefore, 1 and 2, and probably also 3 (due to the analogy

Figure 6. TG-DTA curves for 4 ([(CLPOT)2-(p-NPNN)0.07]) and DTA curves for guest-free CLPOT and bulk p-NPNN. All measurements were conducted with a heating rate of 10 °C/min in an Ar atmosphere.

Figure 7. Temperature dependence of the ESR spectra for 1 ([(CLPOT)2-(PhNN)0.01/(N-PhMI)0.50]); left) and 2 ([(CLPOT)2(p-NPNN)0.04/(N-PhMI)0.50]); right). All spectra were normalized by each spectral maximum peak height. Each red line indicates the spectrum simulated using the EasySpin program package.34 The spectra for 1 and 2 below 255 K were well-reproduced on the basis of a rigid-limit powder pattern of isolated PhNN or p-NPNN radicals. The spectra for 1 and 2 above 294 K were reproduced using an anisotropic rotational diffusion model for PhNN or p-NPNN in CLPOT nanochannels, respectively.

(PhNN) 0.01 /(N-PhMI) 0.50 ] IC) and 2 ([(CLPOT)2 -(pNPNN)0.04/(N-PhMI)0.50] IC) in a temperature range from 4913



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radicals, as opposed to TEMPO, of which the spin density is wrapped inside. 3.3. ESR Spectra for [(CLPOT)2-(p-NPNN)0.07] ICs (without Spacers). Because the ESR spectra for 4 ([(CLPOT)2-(p-NPNN)0.07]) below room temperature were unchanged within the experimental error, the measurement was mainly conducted above room temperature. After the first HT ESR measurements of 4 in the range of 293−424 K, the ESR spectral line-width at room temperature became narrower than before under the same experimental condition, for example, in microwave power, signal amplitude, modulation field, and so on (Figure 9a). The spin concentrations estimated from the signal intensities were coincident within the experimental error. These results may be caused by the reduction of the defects of CLPOT nanochannels or other sources of line broadening other than electron spin−spin dipolar interaction originated from the CLPOT nanochannels (i.e., plenty of atoms composing CLPOT molecules) by annealing. As a result, the molecular rotation of p-NPNN in the CLPOT nanochannels is assumed to be easier, so that dipolar interaction are expected to be suppressed. Figure 9b shows the temperature dependence of the ESR spectra for 4 in the second HT ESR measurements in the range of 293−424 K. All spectra were normalized by each spectral maximum peak height. The ESR spectra and their spin concentrations at each temperature before and after second measurements were coincident within the experimental error. Including the case of 2, it should be noted that the CLPOT IC with p-NPNN is stable even at temperatures above 400 K. These results support the inclusion of guest molecules in the CLPOT nanochannels of 4 as well as 1−3. The spectrum for p-NPNN molecules in the CLPOT nanochannels in 4 at 293 K was simulated using the g and A tensor components shown in Table 1. The xx and yy components of the g and A tensor of p-NPNN molecules in 4 are slightly different from those in 2. In addition, the Gaussian and Lorentzian line-width components were 0.22 and 0.96 mT, respectively, and are larger than the results for 2, in which p-NPNN molecules are adequately isolated by spacers (see section 3.2). These results suggest that the electron spin− spin dipolar interaction between neighboring p-NPNN molecules still contributes significantly to the ESR line-width also after annealing. The temperature-dependent ESR spectra for 4 in Figure 9b were reproducible above 342 K, as with 2, by the model of anisotropic rotational diffusion of isolated p-NPNN molecules in the CLPOT nanochannels within the slow motion regime with respect to the ESR time scale (10−9 s < τR < 10−6 s). The ESR spectrum for 4 at 424 K shown in Figure 9b was reproducible on the assumption that the axis of the rotational diffusion was tilted by (θ, ϕ) = (61°, 90°) to the principal system of the g-tensor for p-NPNN in the CLPOT nanochannels. The Gaussian and Lorentzian line-width components and τR were estimated to be 0.04 and 0.58 mT, and 8.8 × 10−9 s, respectively. These (θ, ϕ) indicate that the longest molecular axis is approximately parallel to the rotation axis. In addition, τR for each spectrum in Figure 9b in the range of 342−424 K was longer than that for 2 in the comparison at the same temperature (see Figure 8). These may be caused by the difference of neighboring molecules of each radical included in the CLPOT nanochannels (N-PhMI in 2 but p-NPNN in 4). Ea for p-NPNN molecules in the CLPOT nanochannels of 4 was estimated from Figure 8 to be 45 kJ mol−1 and was smaller than

with the XRD and TG-DTA results) are expected to be inclusion compounds. These rotation axes in 1 and 2 are approximately parallel to the principal y-axis of the g-tensor (see Figure 1); therefore, both PhNN and p-NPNN are expected to be included in the CLPOT nanochannels keeping the longest molecular axis parallel to the channel axis, if it is assumed that the rotation axis is parallel to the channel axis. The (θ, ϕ) parameters of the rotation axis to the principal axis for p-NPNN are larger than those for PhNN. The difference is expected to be dependent on the ratio of the molecular length of each NN radical (0.9 nm for PhNN, and 1.1 nm for p-NPNN) to the CLPOT pore diameter for the CLPOT nanochannels (1.1−1.3 nm). Figure 8 shows the temperature dependence of the rotational diffusion correlation time τR for PhNN (1; diagonal crosses)

Figure 8. Temperature dependence of the rotational diffusion correlation time τR for PhNN (1; diagonal crosses) and p-NPNN (2; open circles) diluted with N-PhMI in the CLPOT nanochannels, and [(CLPOT)2-(p-NPNN)0.07] (4: green circles). Each rotational activation energy for PhNN or p-NPNN in the CLPOT nanochannels of 1, 2, and 4 was estimated to be 37, 54, and 45 kJ mol−1, respectively, using the τR values plotted in this figure.

and p-NPNN (2; open circles), diluted by N-PhMI in the CLPOT nanochannels. Each τR for the PhNN or p-NPNN radical in the CLPOT nanochannels diluted by spacers is much longer than that for TEMPO or TEMPOL radicals in the nanochannels; for example, τR for TEMPO or TEMPOL is ca. 10−9 s at room temperature,21 whereas τR for the NN radical is ca. 10−7 s. In addition, each rotational activation energy for PhNN and p-NPNN in the CLPOT nanochannels of 1 and 2 was estimated to be 37 and 54 kJ mol−1, respectively, using the τR values in the range of 293−400 K, the magnitude of which is several times larger than that for TEMPO or TEMPOL in the CLPOT nanochannels (8 or 7 kJ mol−1, respectively). This may be due to (1) the molecular size and weight of the NN radicals, which are larger and heavier than TEMPO or TEMPOL, and (2) stronger interaction between NN radicals and the surrounding host or guest molecules caused by the delocalization of spin density on the nitronylnitroxide group of NN 4914



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new molecular magnet with 1D molecular chains of organic radicals after the fact that the β phase of p-NPNN is a quasi-1D Heisenberg ferromagnet with Tc = 0.65 K.28−30 Instead, the ESR study of isolated NN radicals suggests the availability of NN groups as an ESR spin probe like many of TEMPO derivatives for porous material, polymer, membrane, and so forth.15 Further investigations, including the development of an appropriate methodology for sample preparation, are now underway.

4. CONCLUSION The molecular orientation and dynamics of PhNN and pNPNN included in CLPOT nanochannels were examined using ESR measurements. With N-PhMI as spacers, the ESR spectra of PhNN and p-NPNN in CLPOT nanochannels were simulated using a model of the anisotropic rotational diffusion around the rotation axis tilted to the principal axis system of the g-tensor by (θ, ϕ) = (68°, 78°) for PhNN and (74°, 90°) for pNPNN. Each τR and Ea for the anisotropic rotational diffusion of the NN radical in the CLPOT nanochannels were relatively longer and much larger than those for TEMPO or TEMPOL in the CLPOT nanochannels. These results are considered to be due to the larger molecular size and weight of the NN radicals than TEMPO derivatives and/or stronger interaction between NN radicals and the surrounding host or guest molecules. In the case without spacers, the reduction by annealing of the defects of CLPOT nanochannels or other sources of line broadening originated from the CLPOT nanochannels was suggested from the line narrowing due to the first HT ESR measurements. The broader ESR line-width in the case without spacers than the line-width in dilution imply the contribution of remaining dipolar interaction between neighboring p-NPNN in the CLPOT nanochannels. These nearby p-NPNN molecules are expected to affect the g and A tensor components and the rotation axis direction to the principal axes of the g-tensor of guest radicals in 4. Ea for the p-NPNN in the CLPOT nanochannels diluted with spacers was larger than that without spacers presumably due to the larger total inclusion amount in the case of the former. This is only the beginning of studies on the construction of 1D chains of NN radicals in a CLPOT template, and further investigation is now underway.

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Figure 9. (a) ESR spectra for 4 ([(CLPOT)2-(p-NPNN)0.07]) at 293 K before and after the first high-temperature (HT) ESR measurements up to 424 K. These measurements were conducted under the same experimental condition, for example, microwave power, signal amplitude, modulation field, and so on. The spin concentrations before and after HT ESR measurements were coincident within the experimental error. (b) Temperature dependence of the ESR spectra for 4 in the range of 293−424 K during the second HT ESR measurements. All spectra were normalized by each spectral maximum peak height. Each red line indicates the simulated spectrum using the EasySpin program package.34 Each spectrum was reproducible with increasing and decreasing in temperature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +81-3-53179739. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Strategic Research Base Development program for Private Universities subsidized by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), 2009-2013. The authors appreciate Professor T. Hashimoto at Nihon University for assistance with the TG-DTA measurements, and Organic Elemental Analysis Research Center, Kyoto University for the elemental analyses. We also thank Assistant Professor S. Stoll at the University of Washington and Professor T. Takizawa at Nihon University for advice regarding EasySpin. Finally, we

that for 2 (see section 3.2). This may be due to smaller total inclusion amount than 2. The interspin interaction between neighboring p-NPNN molecules in 4 implies the possibility of NN radical chain construction in the CLPOT nanochannels by the large inclusion amount of guest radicals. This could be realized with selection of an appropriate solvent that can dissolve both CLPOT and the guest radicals. Then, if it is possible, NN radicals may be potential candidates to be building blocks for a 4915



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(17) Kobayashi, H.; Asaji, T.; Tani, A. Preparation and Characterization of New Inclusion Compounds Using Stable Nitroxide Radicals and an Organic 1-D Nanochannel as a Template. Materials 2010, 3, 3625−3641, DOI: 10.3390/ma3063625. (18) Kobayashi, H.; Takeuchi, K.; Asaji, T. Molecular orientation and dynamics of different sized radicals included in organic 1-D nanochannels. J. Phys. Chem. A 2013, 117, 2093−2101, DOI: 10.1021/jp311784a. (19) Jetti, R. K. R.; Thallapally, P. K.; Xue, F.; Mak, T. C. W.; Nangia, A. Hexagonal nanoporous host structures based on 2,4,6-tris-4-(halophenoxy)-1,3,5-triazines (halo = chloro, bromo). Tetrahedron 2000, 56, 6707−6719, http://dx.doi.org/10.1016/S0040-4020(00)00491-9. (20) Schaefer, F. C.; Thurston, J. T.; Dudley, J. R. Cyanuric Chloride Derivatives. IV. Aryloxy-s-triazines. J. Am. Chem. Soc. 1951, 73, 2990− 2992, DOI: 10.1021/ja01151a004. (21) Kobayashi, H.; Asaji, T.; Tani, A. ESR study of the molecular orientation and dynamics of stable organic radicals included in the 1-D organic nanochannels of 2,4,6-tris-4-(chlorophenoxy)-1,3,5-triazine. Magn. Reson. Chem. 2012, 50, 221−228, DOI: 10.1002/mrc.3802. (22) Thalladi, V. R.; Brasselet, S.; Weiss, H.-C.; Bläser, D.; Katz, A. K.; Carrell, H. L.; Boese, R.; Zyss, J.; Nangia, A.; Desiraju, G. R. Crystal Engineering of Some 2,4,6-Triaryloxy-1,3,5-triazines: Octupolar Nonlinear Materials. J. Am. Chem. Soc. 1998, 120, 2563−2577, DOI: 10.1021/ja972830r. (23) Osiecki, J. H.; Ullman, E. F. Studies of free radicals. I. .alpha.Nitronyl nitroxides, a new class of stable radicals. J. Am. Chem. Soc. 1968, 90, 1078−1079, DOI: 10.1021/ja01006a053. (24) Wong, W.; Watkins, S. F. X-Ray structures of a free radical nitronylnitroxide and diamagnetic succinimide. J. C. S. Chem. Commun. 1973, 888−889, DOI: 10.1039/C39730000888. (25) Zheludev, A.; Barone, V.; Bonnet, M.; Delley, B.; Grand, A.; Ressouche, E.; Rey, P.; Subra, R.; Schweizer, J. Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculations. J. Am. Chem. Soc. 1994, 116, 2019− 2027, DOI: 10.1021/ja00084a048. (26) Awaga, K.; Inabe, T.; Nagashima, U.; Maruyama, Y. Twodimensional network of the ferromagnetic organic radical, 2-(4nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-Noxide. J. C. S. Chem. Commun. 1989, 1617−1618, DOI: 10.1039/ C39890001617. (27) Turek, P.; Nozawa, K.; Shiomi, D.; Awaga, K.; Inabe, T.; Maruyama, Y.; Kinoshita, M. Ferromagnetic coupling in a new phase of the p-nitrophenyl nitronyl nitroxide radical. Chem. Phys. Lett. 1991, 180, 327−331, http://dx.doi.org/10.1016/0009-2614(91)90328-7. (28) Kinoshita, M.; Turek, P.; Tamura, M.; Nozawa, K.; Shiomi, D.; Nakazawa, Y.; Ishikawa, M.; Takahashi, M.; Awaga, K.; Inabe, T.; Maruyama, Y. An Organic Radical Ferromagnet. Chem. Lett. 1991, 1225−1228, http://dx.doi.org/10.1246/cl.1991.1225. (29) Nakazawa, Y.; Tamura, M.; Shirakawa, N.; Shiomi, D.; Takahashi, M.; Kinoshita, M.; Ishikawa, M. Low-temperature magnetic properties of the ferromagnetic organic radical, p-nitrophenyl nitronyl nitroxide. Phys. Rev. B 1992, 46, 8906−8914, DOI: 10.1103/ PhysRevB.46.8906. (30) Kinoshita, M. Bulk ferromagnetism in the crystal of the organic radical, p-NPNN. Synth. Met. 1993, 55−57, 3285−3290, http://dx.doi. org/10.1016/0379-6779(93)90116-E. (31) Zheludev, A.; Bonnet, M.; Ressouche, E.; Schweizer, J.; Wan, M.; Wang, H. Experimental spin density in the first purely organic ferromagnet: the β para-nitrophenyl nitronyl nitroxide. J. Magn. Magn. Mater. 1994, 135, 147−160, http://dx.doi.org/10.1016/03048853(94)90341-7. (32) Deumal, M.; Bearpark, M. J.; Novoa, J. J.; Robb, M. A. Magnetic Properties of Organic Molecular Crystals via an Algebraic Heisenberg Hamiltonian. Applications to WILVIW, TOLKEK, and KAXHAS Nitronyl Nitroxide Crystals. J. Phys. Chem. A 2002, 106, 1299−1315, DOI: 10.1021/jp015512u. (33) Matsushita, M. M.; Izuoka, A.; Sugawara, T.; Kobayashi, T.; Wada, N.; Takeda, N.; Ishikawa, M. Hydrogen-Bonded Organic

thank Associate Professor F. Iwahori at Nihon University for advice on the properties of NN radicals.

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ESR Study of Molecular Orientation and Dynamics of Nitronyl

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