Introduction

Endohedral fullerenes are constructed by putting atoms or clusters inside fullerene cages, which isolates the internal species with environments, so even those high-reactive species can be well stabilized inside the fullerene cages1,2,3,4,5,6. For example, paramagnetic endohedral fullerenes such as N@C60 (refs 7, 8, 9, 10), Sc@C82 (refs 11, 12), Y@C82 (refs 13, 14), Sc3C2@Ih-C80 (refs 15, 16, 17) and so on, encapsulating radicals inside the fullerene cages, show also remarkable high stability, and they can be kept in air under room temperature for a long time, especially for endohedral metallofullerenes (EMFs). Considering the paramagnetic endohedral fullerenes usually show long electron spin relaxation and coherence times, they are expected to have potential applications in many fields such as spin labelling, spintronics, quantum computing and so on18.

For paramagnetic EMFs, the electronic spin resonance (ESR) technique is a powerful tool to detect the spin distributions and spin–nucleus couplings on internal species19,20,21. By means of ESR, it was revealed that the magnetic property of the internal species can be roughly manipulated by changing the dynamic movement of internal species. For example, under room temperature the internal Y2 cluster in Y2@C79N has a free rotation that leads to a symmetric ESR pattern, but along with the temperature decreasing, the free motion of Y2 is hindered, leading to spin anisotropy and an asymmetric ESR pattern22. In addition, the spin characters and couplings in Sc3C2@C80 was observed to change largely upon chemical modification of the fullerene cage due to the restricted Sc3C2 cluster17.

For better applying the paramagnetic EMFs in quantum information process and molecular devices, however, it is still a challenge to finely manipulate their magnetic property. Recently, Turro et al. chemically modified the H2@C60 with a nitroxide radical, and observed an indirect but strong magnetic communication between the electron spin of nitroxide paramagnet and the nuclear spin of encaged H2 (refs 23, 24, 25, 26, 27, 28). This finding provides a valuable clue for us on manipulating the magnetic property of paramagnetic EMFs via a foreign paramagnet29. Since a strong spin–spin interaction between the paramagnetic fullerene molecule and the paramagnet is expected, thus the magnetic property of paramagnetic EMFs may be controlled by the attached paramagnet.

Herein, we report detailed studies on the fine manipulation for paramagnetic Sc3C2@C80 by connecting it with a paramagnet of nitroxide radical. The target system FSc3C2@C80PNO· contains two kinds of spins localizing on Sc3C2@C80 and nitroxide radical, respectively. The remote nitroxide group serves as a magnetic switch for the ESR signals of Sc3C2@C80 through spin–spin interactions. The paramagnetic properties of the metallofullerene Sc3C2@C80 can be delicately adjusted by changing the temperature, varying the distance between the two spin centres, or simply quenching the nitroxide radical.

Results

Preparation of metallofullerene and its derivatives

Metallofullerene Sc3C2@C80 was synthesized by the Kräschmer–Huffman arc-discharging method30 and isolated by multi-stage high-performance liquid chromatography. Two Sc3C2@C80 derivatives, FSc3C2@C80PNOH and FSc3C2@C80PNO·, were first synthesized through a Prato reaction31, respectively, as shown in Fig. 1a,b. The structures and spin density distributions of FSc3C2@C80PNOH and FSc3C2@C80PNO· were calculated as well, as shown in Fig. 1c,d. The FSc3C2@C80PNOH has one spin centre that is localized on Sc3C2@C80, whereas the FSc3C2@C80PNO· has two unpaired spins localizing on the Sc3C2@C80 moiety and nitroxide radical, respectively.

Figure 1: Magnetic switch for the ESR signals of Sc3C2@C80.
figure 1

(a) The structure of FSc3C2@C80PNOH. (b) The structure of FSc3C2@C80PNO·. (c) The calculated structure and spin density distributions of FSc3C2@C80PNOH. (d) The calculated structure and spin density distributions of FSc3C2@C80PNO·. (e) The ESR spectrum of FSc3C2@C80PNOH at 293 K in toluene. (f) The ESR spectrum of FSc3C2@C80PNO· at 293 K in toluene. The lamps in e and f show the ‘on’ and ‘off’ states of Sc3C2@C80 ESR signals, respectively.

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The ESR analysis of metallofullerene with a nitroxide radical

Sc3C2@C80 is a typical paramagnetic endohedral fullerene. As reported previously, the ESR spectrum of the pristine Sc3C2@C80 shows a symmetric pattern with 21 resonant lines, however, those of Sc3C2@C80 derivatives showed a distorted pattern with a greatly increased amount of resonant lines17. Therefore, ESR spectroscopy was first employed to reveal the electron spin characters of FSc3C2@C80PNOH and FSc3C2@C80PNO·.

As shown in Fig. 1e, the ESR spectrum of FSc3C2@C80PNOH was measured and analysed. Because the Ih symmetry of Sc3C2@C80 is broken down after chemical modification, the original three equivalent scandium nuclei (ISc=7/2) are classified into two groups in FSc3C2@C80PNOH, in which one group contains a single Sc nucleus (g=1.9948, hyperfine coupling constants (hfcc)=8.5 G), and the other group contains two equivalent Sc nuclei (hfcc=5.0 G). In comparison, the previously studied Sc3C2@C80 fulleropyrrolidine shows a similar ESR pattern with hfcc of 8.6 G (one Sc nucleus) and 4.8 G (two Sc nuclei), respectively17. These ESR results reveal that the FSc3C2@C80PNOH has a same reaction site with that of Sc3C2@C80 fulleropyrrolidine17.

However, the ESR study of FSc3C2@C80PNO· showed only three resonant lines (g=2.0026, a=15.5 G) that are derived from nitroxide radical (IN=1), and the ESR signals of Sc3C2@C80 moiety were not observed, as illustrated in Fig. 1f. The current results reveal that the ESR signals of Sc3C2@C80 can be switched off by paramagnetic nitroxide radical through spin–spin interaction. Thus it is interesting that the nitroxide radical group can serve as a remote controller for the ESR signals of Sc3C2@C80 moiety. Vividly, if the ESR signals of Sc3C2@C80 moiety are regarded as an indicating lamp, the nitroxide radical group can switch it off.

To reveal the mechanism of how the ESR signals of Sc3C2@C80 moiety are switched off by the nitroxide radical, we synthesized FSc3N@C80PNO· in a same way for comparison. Sc3N@C80 is a diamagnetic molecule, and no spin–spin interaction is expected for FSc3N@C80PNO·. The signal intensity of FSc3N@C80PNO· was observed to be stronger than that of FSc3C2@C80PNO· at the same concentration (0.151 p.p.m.). That is to say, the spin–spin interactions weaken both of the ESR signals of Sc3C2@C80 and nitroxide radical (Supplementary Fig. 1).

For FSc3C2@C80PNO·, the spin–spin interactions can be expressed as below:

Where Edip is the energy of dipolar coupling, and r is the dipole–dipole distance.

In fact, the spin–spin interactions between Sc3C2@C80 and nitroxide radical broaden the resonance lines and lower the line intensity in the meantime, so the line width (ΔH) is adopted to represent the ESR line intensity and interpret the spin–spin interactions32,33.

In general, for paramagnetic molecules the ESR line width is inversely proportional to the relaxation time (T), including the spin–lattice relaxation time (T1) and the spin–spin relaxation time (T2):

Therefore, the strong dipole–dipole interactions between nitroxide radical and Sc3C2@C80 in FSc3C2@C80PNO· reduced the spin–spin relaxation time (T2), resulting in decreased ESR signals of both Sc3C2@C80 and nitroxide radical moieties.

Note that the transformation between FSc3C2@C80PNO· and FSc3C2@C80PNOH is reversible, that is, the nitroxide radical in FSc3C2@C80PNO· turns into the corresponding hydroxylamine derivative (FSc3C2@C80PNOH) using p-toluenesulfonohydrazide, and the FSc3C2@C80NOH can be back to FSc3C2@C80PNO· by means of oxidation with copper acetate (Supplementary Fig. 2). Therefore, the magnetic property of Sc3C2@C80 can be easily manipulated by a chemical method, and the remote nitroxide group serves as a switch in this process.

The distance-dependent ESR signals

On the basis of equation (1), the spin–spin interactions between Sc3C2@C80 and nitroxide radical moieties can be efficiently reduced by elongating their distance, thus two other Sc3C2@C80 and nitroxide radical derivatives, that is, FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3, were synthesized with longer distances between these two spin centres. The pulsed ESR measurements on FSc3C2@C80PNO· and FSc3C2@C80PNO·-2 revealed that the T2 of nitroxide radical becomes longer when the distance of these two spins increases (Supplementary Fig. 3). As shown in Fig. 2, it is obvious that the ESR signals of Sc3C2@C80 also gradually boost up along with the distance increasing, and this process is like lighting a lamp and making it brighter.

Figure 2: Distance-dependent ESR signals of Sc3C2@C80 derivatives.
figure 2

(a–c) Structures of FSc3C2@C80PNO·, FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3. (d–f) ESR signals of FSc3C2@C80PNO·, FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3 at 293 K. The lamps in d–f show the strengthened ESR signals of Sc3C2@C80 moiety along with the enlarged distance from nitroxide radical.

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However, the increased chain length between Sc3C2@C80 and nitroxide radical would result in a strengthened spin–lattice interaction, which will bring another line-broadening effect for both nitroxide radical and Sc3C2@C80 moiety. The spin–lattice relaxation time (T1) can be expressed as below:

Where and are mean square amplitudes of the fluctuating fields along the x- and y-directions, and τc is the correlation time of the motion that causes the fluctuation. For molecules with a spherical shape, τc in liquid solution corresponds to the rotational correlation time τr, which can be approximated by the Stokes–Einstein relation:

Where a is the rotationally effective radius of the molecule, and η is the viscosity of the solvent. In liquids with a low viscosity (τ2cω2s≈1), the T1 is dependent of a3, and T1 decreases with increasing the rotationally effective radius of the molecule. Therefore, increasement of the molecular size would shorten the T1, leading to weaker ESR signals.

Since FSc3C2@C80PNO·, FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3 are all rigid structural molecules, the intramolecular dipolar coupling strength (D) can be estimated, in which the coupling strength of FSc3C2@C80PNO· with r=1.38 nm was estimated to be about 24.3 MHz following the classical point dipole approximation, and those of FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3 were estimated to be about 11.2 and 5.48 MHz, respectively.

The temperature-dependent ESR signals

It is known that the spin–lattice relaxation time (T1) of unpaired spin is tightly related to the temperature. As expressed in equation (3), the decreasement of temperature would reduce the Bx and By, and then increase the T1 and lead to higher ESR line intensity. Therefore, the temperature-dependent ESR studies of FSc3C2@C80PNO· were performed, as shown in Fig. 3. It can be observed that no ESR signal of Sc3C2@C80 was observed at 293 K, but since 253 K, the ESR signals of Sc3C2@C80 appeared together with three strong resonant lines of nitroxide radical, and continuously increased along with the temperature further decreasing. Finally at 213 K, the ESR signals of Sc3C2@C80 moiety can be clearly observed. The electrostatic spin–phonon interaction was also analysed for these temperature-dependent ESR spectra34. As the lowest temperature in our system is 213 K, under this condition the FSc3C2@C80PNO· toluene solution is still in liquid state, so the spin–phonon interaction is rather small and negligible. From the temperature-dependent ESR spectra, it can be seen that the temperature also can light the signals of Sc3C2@C80 moiety in FSc3C2@C80PNO· and make it brighter.

Figure 3: Temperature-dependent ESR signals of Sc3C2@C80 derivatives.
figure 3

The ESR spectra of FSc3C2@C80PNO· at variable temperatures in toluene solution. The lamps represent the strengthened ESR signals of Sc3C2@C80 moiety along with the decreased temperatures.

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Moreover, it should be noted that the ESR signals of the nitroxide were also enhanced. Therefore, along with the temperature decreasing, the prolonged T1 enhances not only the ESR signals of Sc3C2@C80 moiety, but also those of the nitroxide radical (Supplementary Fig. 4). The line widths of Sc3C2@C80 moiety and nitroxide radical are listed in Table 1.

Table 1 ESR data.
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Discussion

Through connecting the paramagnetic metallofullerene Sc3C2@C80 with a nitroxide radical, we have realized the manipulation of ESR signals of Sc3C2@C80. The remote nitroxide group serves as a magnetic switch for ESR signals of Sc3C2@C80, that is, the paramagnetic nitroxide group can ‘switch off’ the ESR signals of Sc3C2@C80 moiety. It was revealed that the spin–spin interactions between Sc3C2@C80 and nitroxide radical play a key role in realizing this kind of magnetic switch. Moreover, through increasing the distance between Sc3C2@C80 and nitroxide radical, or decreasing the temperature, we can finely adjust the paramagnetic property of Sc3C2@C80.

Such controllable paramagnetism and switchable ESR signals have potential applications in quantum information processing and molecular devices. For example, these magnetic molecules can be fabricated to a single molecule membrane for data storage considering their transferrable two electron spin states (0/1), which can be written and read out by means of scanning tunnelling microscope. In addition, these magnetic molecules can be utilized as a probe for the reaction transition state considering the susceptible magnetic switch of the nitroxide group.

Methods

Synthesis of Sc3C2@C80 and Sc3N@C80 derivatives

Sc3C2@C80 and Sc3N@C80 were heated with N-ethylglycine and 2,2,6,6 tetramethylpiperidine-1-oxyl 4-formylbenzoate (1a) (Supplementary Fig. 5), which were synthesized as literature methods24 at 120 °C to give corresponding fulleropyrrolidines with yields of nearly 50% in toluene solution for 15 and 50 min, respectively. Pure FSc3C2@C80PNO· and FSc3N@C80PNO· were isolated by HPLC using Buckyprep column (Supplementary Fig. 6). FSc3C2@C80PNO·-2 and FSc3C2@C80PNO·-3 were synthesized according to same procedure used for compound FSc3C2@C80PNO·, except 2,2,6,6 tetramethylpiperidine-1-oxyl 4'-formylbiphenyl-4-carboxylate (2a) and 2,2,6,6 tetramethylpiperidine-1-oxyl 4'-p-terphenyl-4-carboxylate (3a) (Supplementary Fig. 7) were used.

Synthesis of FSc3C2@C80PNOH

To a solution of ~0.5 mg of the nitroxide derivative FSc3C2@C80PNO· in ~2 ml toluene was added ~1 mg p-toluenesulfonohydrazide, and stirred under air for about 15 min.

Characterization of metallofullerene derivatives

Ultraviolet/visible–near-infrared spectra of purified metallofullerene derivatives (Supplementary Fig. 8) were collected on Lambda 950 UV/Vis/NIR Spectrometer (PerkinElmer Instruments). 1H NMR spectra of FSc3N@C80PNOH was measured in chloroform-d on a Bruker 600 MHz spectrometer (Supplementary Fig. 9).

ESR measurements of metallofullerene derivatives

ESR spectra were measured on a JEOL JEF FA200 X-band spectrometer (Supplementary Fig. 10). 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.

Calculations on Sc3C2@C80 derivatives

Density functional theory calculations were investigated by Perdew, Burke and Enzerhof/double numerical plus polarization using the DMol3 code in Accelrys Materials Studio35,36.

Additional information

How to cite this article: Wu, B. et al. Molecular magnetic switch for a metallofullerene. Nat. Commun. 6:6468 doi: 10.1038/ncomms7468 (2015).