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Spin-Paramagnet Communication between Nitroxide Radical and Metallofullerene Bo Wu, Yongjian Li, Li Jiang, Chunru Wang, and Taishan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12447 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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The Journal of Physical Chemistry

ABSTRACT: The paramagnetic metallofullerenes wrapped lanthanide metals have many special properties and potential applications, especially as single-molecule magnet. Herein, we report a spin probe of nitroxide radical for the magnetic dysprosium metallofullerenes. The nitroxide radical was connected to dysprosium metallofullerene though a cycloaddition reaction. Two kind of dysprosium metallofullerene, DySc2N@C80 and Dy2ScN@C80 with different characters of molecule magnet, were selected. By means of analyzing electronic spin resonance (ESR) spectra of nitroxide radical, the spin-paramagnet interactions between nitroxide and dysprosium metallofullerenes were investigated. KEYWORDS: metallofullerene • nitroxide radical • spin-paramagnet interaction • dipole−dipole interaction • molecule magnet

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INTRODUCTION The paramagnetic lanthanide metallofullerenes have attracted many interests because of their special physical and chemical properties.1-4 The paramagnetic properties of these lanthanide metallofullerenes mainly origin from the unpaired f electrons in lanthanide ions.5-7 It has been reported that some dysprosium metallofullerene, i.e. DySc2N@C80 and Dy2ScN@C80 can exhibit magnetic hysteresis under 5 K temperature and show character of single-molecule magnet. As a new type of single-molecule magnet, these dysprosium metallofullerenes have many advantages, such as their self-assembly property and stability.8-13 However, there is a challenge that is how to detect their magnetic property of these metallofullerenes at the molecular level. The paramagnetic nitroxide radical such as 2, 2, 6, 6-tetramethylpiperidine-N-oxyl (TEMPO) has been widely used as spin probe and label in biological systems.14,15 The ESR technique can be taken as a powerful detection method to characterize the dynamics or electronic structures of target molecules labeling TEMPO.16,17 During the last few years, Turro et al. observed an indirect but strong magnetic communication between the electron spin of nitroxide paramagnet and the nuclear spin of the encaged H2 species in [email protected] Moreover, a linked nitroxide radical has been confirmed to act as a remote molecule magnetic switch to manipulate the magnetic property of paramagnetic metallofullerene Sc3C2@C80 through the spin-spin interaction.21 Herein, we report a spin-paramagnet communication between dysprosium metallofullerene molecule magnets and nitroxide radical. Through analyzing the varied ESR signals of nitroxide, we try to illustrate the magnetic properties of dysprosium metallofullerenes at the molecular level.


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MATERIALS AND METHODS DySc2N@C80, Dy2ScN@C80 and Sc3N@C80 (see Supporting Information Figure S1-S4) were heated with N-ethylglycine and 2,2,6,6 tetramethylpiperidine-1-oxyl 4-formylbenzoate (1a) which was synthesized as literature methods19 at 120 ℃

to give corresponding

fulleropyrrolidines with yields of nearly 70 % in toluene solution for 50 min, respectively (Figure S5). Pure 1-oxy-2,2,6,6-tetramethylpiperidin-4-yl 4-(1-ethyl DySc2N@C80-2-yl)benzoate (I), 1-oxy-2,2,6,6-tetramethylpiperidin-4-yl 4-(1-ethyl Dy2ScN@C80-2-yl)benzoate (II), and 1oxy-2,2,6,6-tetramethylpiperidin-4-yl 4-(1-ethyl Sc3N@C80-2-yl)benzoate (III) were isolated by High Performance Liquid Chromatography (HPLC) using Buckyprep column (Figure S6). UV/Vis-NIR spectra of purified metallofullerene derivatives (Figure S7) were collected on Lambda 950 UV/Vis/NIR Spectrometer (PerkinElmer Instruments). ESR spectra were measured on a JEOL JEF FA200 X-band spectrometer (Figure S8). The samples were degassed and the oxygen was removed from the solutions. All of the samples are dissolved in toluene solution at the same concentration. RESULTS AND DISCUSSION The ESR analysis of metallofullerene with nitroxide radical. Three derivatives I, II, and III were synthesized as shown in Figure 1.


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Figure 1. The structures of a, I, b, II, c, III and d, 1a. The ESR spectroscopy was employed to characterize the paramagnetic property of these three derivatives. III has diamagnetic Sc3N@C80 moiety, was studied for comparison. These derivatives all exhibit three ESR peaks. It was revealed that the hyperfine coupling constant (a) of 15.4 G, 15.56 G, and 15.66 G were observed for I, II, and III respectively at room temperature. The g-values for all these derivatives were same with 2.0021. In the system of I, the relative intensity and line width of its nitroxide radical ESR signals are changing due to the strong spin-paramagnet interaction compared to those of III. The signal intensity of nitroxide radical in I was observed to be weaker than that of III at the same concentration in Figure S8. Generally, the ESR signal line width is inversely proportional to the relaxation time (T) for paramagnetic molecules, including the spin-lattice relaxation time (T1) and the spin-spin relaxation time (T2).22,23 Considering the same concentration and similar structure of DySc2N@C80 and Sc3N@C80, the weaker nitroxide ESR signal in I can be ascribed to the shortened relaxation time of this radical system, which originates from the spinparamagnet interaction between DySc2N@C80 and nitroxide radical group. It is known that when a fast-relaxing paramagnet is present, the relaxation of a slow-relaxing paramagent will be enhanced. The DySc2N@C80 has very short relaxation time due to the fast spin-orbital coupling of its f-electrons. Consequently, the linked nitroxide radical exhibits increased relaxation and reduced signal intensity for its ESR peaks. The spin-paramagnet interactions of metallofullerene derivatives. We can use the change of ESR signals to reflect the spin-paramagnet interactions between Dy-metallofullerenes and nitroxide radical under low temperature. As in frozen solutions at low temperature, the ESR spectra are sensitive to the dipole−dipole interaction between neighboring nitroxide radicals.


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Usually, the peaks in the ESR spectra of nitroxide radicals become broader in Figure 2 at low temperatures due to the incomplete averaging the anisotropy of g and a tensor. This effect could be estimated by the empirical ratio of peak heights d1/d of frozen ESR spectra (Figure 2). At frozen states, this parameter was a convenient measure of the strength of the dipole−dipole interactions. A larger d1/d value means a stronger dipolar interaction.24

Figure 2. The ESR spectra of a, I, b, II and c, 1a at 153 K, 133 K and 113 K in toluene. As shown in Table 1, I has the highest d1/d value (0.55-0.64) than those of II and individual nitroxide radical 1a group, which have similar d1/d values. These results reveal that in I the neighboring nitroxide radicals have greater dipole−dipole interactions.


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Table 1. The parameter d1/d of the different paramagnetic system at various temperatures. Compound/Temperature

153 K

133 K

113 K

I

0.55

0.64

0.57

II

0.41

0.43

0.37

1a

0.42

0.40

0.39

The enhanced dipole−dipole interactions between neighboring nitroxide radicals in I disclose an obvious spin-paramagnet interaction between DySc2N@C80 and nitroxide radical moieties. As a result, the linked nitroxide radical has an increased relaxation and changed paramagnetic property. For I, however, the unchanged d1/d value revealed that a weak spin-paramagnet interaction between Dy2ScN@C80 and nitroxide radical moiety. The progressive microwave power saturation of the ESR is a convenient method to determine the spin relaxation rates and spin-spin interactions. So we choose I, II and individual nitroxide radical 1a to do the microwave power saturation experiment to show the spin-paramagnet interactions as shown in Figure 3.

Figure 3. Samples simulated power-saturation curves for a, I, b, II and c, 1a at 113 K in toluene. The individual symbols are the output of the simulation program, and the solid lines are fits to


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the simulated data. P1/2 values obtained from fits using equation (1) are as follows: a, 1.61 mw, b, 0.44 mw and c, 0.42 mw respectively. In this experiment, the intensity of the spectrum is recorded at each observing power P. Because the signal intensity S in the non-saturating spectrum is proportional to P1/2, the power at half saturation, and the log-log intensity versus P will have non-saturating and saturating regimes.25-27 Generally, the intersection of these two linear regimes when extrapolated has been used to determine P1/2. More recently, this plot is fit using least-squares fitting to an equation of the form

√ =

)) √

( (

(1)

Where k is an instrumental scaling factor, S is the intensity of the signal.

P1/2 ∝ = +

= ki +kd

(2)

kd = 3(r-6)γ µ T1f sin2θ cos2θ

(3)

The form of kd depends on the properties of the two interacting spins. Where r is the distance between the paramagnets, and γs is the magnetogyric ratio of the slow-relaxing electron. Additionally, μ and T1f are the magnetic moment and spin-lattice relaxation time of the fastrelaxing paramagnet, respectively. The P1/2 values for these molecules are similar at room temperature because of the fast-motion of these paramagnetic molecules in toluene. The interaction of the two spin centers was observed at 113 K in frozen solution. As a result of the spin-paramagnet interaction of DySc2N@C80 with nitroxide group, the P1/2 of I (1.61 mw) is larger than those of II (0.44 mw) and 1a (0.42 mw) at 113 K, as shown in Figure 3. The results can also be revealed in equation (2) and (3). There are two Dy inside II, and the different directions of the magnetic moments will reduce the final


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magnetic properties. So the spin-paramagnet interactions are not obviously at low temperature. The enhanced P1/2 of I can be ascribed to the great magnetic moment of linked DySc2N@C80. In addition, at frozen state, the nitroxide radical also can be influenced by intermolecular DySc2N@C80 moiety, which can equally change its paramagnetic property. CONCLUSIONS In conclusion, we report a spin-paramagnet communication between dysprosium metallofullerenes and nitroxide radical. At frozen states, a larger d1/d value means a stronger dipolar interaction between the neighboring nitroxide radicals in I. The progressive microwave power saturation of the ESR was also executed to determine this kind of spin-paramagnet interaction. To summarize, we revealed a spin probe of nitroxide radical for the magnetic dysprosium metallofullerenes and this result is helpful to disclose the single molecule magnet at molecular level. ACKNOWLEDGMENT This work was supported by the National Basic Research Program (2012CB932901), National Natural Science Foundation of China (21203205, 61227902, 51472248), Beijing Natural Science Foundation (2162050), and the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-T02). T. Wang particularly thanks the Youth Innovation Promotion Association of CAS. ASSOCIATED CONTENT Supporting Information. Experimental details, the HPLC data, mass spectra, UV/Vis-NIR spectra and ESR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *[email protected] Notes The authors declare no competing financial interest. REFERENCES (1) Huang, H.; Yang, S. Relative Yields of Endohedral Lanthanide Metallofullerenes by Arc Synthesis and Their Correlation with the Elution Behavior. J. Phys. Chem. B. 1998, 102, 1019610200. (2) Bondino, F.; Cepek, C.; Tagmatarchis, N.; Prato, M.; Shinohara, H.; Goldoni, A. ElementSpecific Probe of the Magnetic and Electronic Properties of Dy incar-Fullerenes. J. Phys. Chem. B. 2006, 110, 7289-7295. (3) Popov, A. A.; Yang, S.; Dunsch, L. Endohedral Fullerenes. Chem. Rev. 2013, 113, 59896113. (4) Wang, T.; Wang, C. Endohedral Metallofullerenes Based on Spherical Ih-C80 Cage: Molecular Structures and Paramagnetic Properties. Acc. Chem. Res. 2014, 47, 450-458. (5) Furukawa, K.; Okubo, S.; Kato, H.; Shinohara, H.; Kato, T. High-Field/High-Frequency ESR Study of Gd@C82-I. J. Phys. Chem. A. 2003, 107, 10933-10937. (6) Matsuoka, H.; Ozawa, N.; Kodama, T.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K.; Furukawa, K.;

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Figure 2. The ESR spectra of a, I, b, II and c, 1a at 153 K, 133 K and 113 K in toluene. 78x170mm (300 x 300 DPI)


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