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C: Physical Processes in Nanomaterials and Nanostructures 80

Supramolecular Complexes of C -Based Metallofullerenes with [12]Cycloparaphenylene Nanoring and Altered Property in a Confined Space Chong Zhao, haibing meng, Mingzhe Nie, Xiang Wang, ZhenFeng Cai, Ting Chen, Dong Wang, Chunru Wang, and Taishan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02451 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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The Journal of Physical Chemistry

Supramolecular Complexes of C80-based Metallofullerenes with [12]Cycloparaphenylene Nanoring and Altered Property in a Confined Space Chong Zhao,†‡ Haibing Meng,†‡ Mingzhe Nie,†‡ Xiang Wang,†‡ Zhenfeng Cai,†‡ Ting Chen,† Dong Wang,† Chunru Wang*† and Taishan Wang*† †Beijing

National Laboratory for Molecular Sciences, Laboratory of Molecular Nanostructure

and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ‡University

of Chinese Academy of Sciences, Beijing 100049, China.

ACS Paragon Plus Environment

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ABSTRACT: Metallofullerenes have cage-shaped molecular structure and special properties derived from encapsulated metal atoms. Owing to the ball-like fullerene cage, it is still a challenge to manipulate the metallofullerene molecule as well as to control its property. Herein, we

use

molecular

carbon

nanoring

of

[12]cycloparaphenylene

to

hoop

C80-based

metallofullerene Y3N@C80 and azafullerene Y2@C79N, resulting in stable supramolecular complex, induced molecule orientation, changed assembly behavior, and tunable spin state. These supramolecular complexes and their host-guest interactions were systematically characterized by DFT-calculations, spectroscopy, NMR, and electrochemical analysis. Scanning tunneling microscopy (STM) was employed to reveal that the metallofullerene guests dominate the assembly process on Au(111) surface, and [12]CPP nanoring can change the weak van der Waals forces and influence the self-assembly. Moreover, electron spin in paramagnetic Y2@C79N was employed to percept the host-guest interaction, and it showed anisotropic spinmetal couplings due to its insufficient rotational averaging in a confined space of nanoring. The special spin character of Y2@C79N⊂[12]CPP was also investigated in solid state, and it exhibits independent spin of Y2@C79N separated by nanoring. This study provides a strategy to hoop the ball-like C80-based metallofullerenes and modulate their electronic and magnetic properties using circular nanoring. Metal-induced orientation, self-assembly characters, and susceptible electron spin for these supramolecular complexes reveal a great significance in molecule science and nanotechnology.


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INTRODUCTION The curved aromatic systems have attracted significant attention from the scientific community

as

these

molecules

can

intrigue

unique

properties.

Endohedral

metallofullerenes also have curved π-system as well as special properties derived from encapsulated metal atoms, and this kind of nested structure and metal-induced physical property make metallofullerenes valuable as molecule materials. Typically, paramagnetic metallofullerenes with unpaired electron can be utilized as molecular qubit in quantum information science owing to their stable and sensitive electron spin.1-5 Owing to the balllike fullerene cage, it is still a challenge to control the metallofullerene molecules as well as to explore more of their properties. Considering the spherical form of metallofullerene, the circular carbon nanorings are suitable host to accommodate metallofullerene, and then to change its property. Molecular carbon nanorings are unique π-systems and they resemble the segment of carbon nanotubes (CNTs).6 The large π-conjugated ring structure gives the possibility to generate directional ring current,7-8 which can be utilized to design single-molecule device. Typically, the cycloparaphenylene (CPP) nanorings consisting of conjugated multi-benzene rings represent the shortest armchair carbon nanotube segment and have attracted much attention from scientists.9-12 Since the discovery that the C60 was trapped into [10]CPP by S. Yamago, et al.,13 this class of supramolecular systems have been developed, such as C70⊂[10-11]CPP,14 La@C82⊂[11]CPP,15 and Gd@C82⊂[11]CPP.16 It should be noted that C70⊂[11]CPP and La@C82⊂[11]CPP exhibit “standing” orientation in which the long axis interact with [11]CPP due to the non-uniform concave-convex π-π interactions. Recently, Rodríguez-Otero reported the [11]CPP could be capable of


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separating the fullerenes larger than C76.17 Therefore, the size- and orientation-selectivity is very crucial for supramolecular system design of fullerene and CPP. In this manuscript, we use nanoring of [12]CPP to hoop the C80-based metallofullerene Y3N@C80 and paramagnetic azafullerene Y2@C79N, and assembly series of stable supramolecular complexes. These supramolecular complexes were analyzed by mass spectrometry, NMR, and electrochemical analysis. Density functional theory (DFT) calculation was also executed to disclose the supramolecular structures and the molecular orientation of metallofullerene induced by [12]CPP nanoring. Scanning tunneling microscopy (STM) was employed to illustrate the molecular form and assembled behaviors of these supramolecular complexes on Au(111) surface. Typically, endohedral azafullerene Y2@C79N which has a defective carbon cage originating from the Nsubstitution on the C80 cage with susceptible electron spin18-21, will percept the confinement effect from [12]CPP nanoring. The N-defect of Y2@C79N changes the molecule symmetry and polarizes the electron distributions, and it could greatly influence the host-guest structure. Therefore, the electron paramagnetic resonance (EPR) technique was employed to recognize the spin states of Y2@C79N in its supramolecular complex. EXPERIMENTAL SECTION Synthesis of Y3N@C80 and Y2@C79N: Metallofullerene Y3N@C80 and Y2@C79N were prepared by the Krätschmer-Huffman arc discharge method22 and isolated by highperformance liquid chromatography (HPLC). The pure sample was confirmed by HPLC analysis and MALDI-TOF (Figure S1 and S2). [12]CPP and [6]CPP were purchased from J&K.


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Synthesis of Y3N@C80/Y2@C79N⊂[12]CPP: Y3N@C80/Y2@C79N and [12]CPP were mildly mixed in CS2/CDCl3 solution (for NMR and EPR test) and 1,2-Dichlorobenzene or toluene (for Raman, CV, STM and UV-Vis tests) with 1:1 mole ratio at room temperature. STM experiment: STM experiments were performed using a NanoScope E scanning tunneling microscope (Bruker Inc.) with a W tip, which was electrochemically etched and sealed with transparent nail polish to minimize Faradaic currents. The measurements are under room temperature and atmospheric pressure in the 0.1 M perchloric acid. Electrochemistry: Experiments were carried out in o-DCB solvent with glassy carbon as the working electrodes, Pt flake and a SCE as the counter and reference electrodes, respectively. The electrolyte is 0.05 M TBAPF6. The potentials were referred to the E value of the Fc/Fc+ redox couple measured in the sample solution. The scan rate is 50 mV/s. EPR measurement: All EPR spectra were measured on a Bruker E500 with continuous-wave X band. The frequency was 9.4∼9.5 GHz. All of the solution samples were dissolved in CS2 solution at the same concentration. The solid samples also had the same concentration of Y2@C79N. Theoretical Section: Y3N@C80, Y2@C79N and Y3N@C80⊂[12]CPP were firstly optimized using original PM6 and B3LYP/3-21g* to speed up the computational process,23-24 the final optimizations, the frontier MOs, and the energy profiles corresponding to the movement of Y3N@C80 were carried out by B3LYP-D3 methods within lanl2de basis25 for Y and 6-31g*26 for C, H and N, here the Grimme’s DFT-D3 method provides an empirical dispersion correction.27 The above calculations were using


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the Gaussian 09 quantum chemical program package.28 The BOMD (Born-Oppenheimer molecular dynamics) calculations were performed by CP2K code29-30 and employed Velocity Verlet algorithm with the time step of 1 fs at the temperature of 298 K (total: 12000 fs), detailed basis sets are DZVP-MOLOPT-SR-GTH.31 The structures and isosurfaces were visualized with Gauss View, the trajectories were visualized with VMD.32 The simulations of the EPR spectra were carried out by the EasySpin package (http://www.easyspin.org) on the MATLAB platform.

Figure 1. (a) DFT optimized structures of Y3N@C80/Y2@C79N⊂[12]CPP at B3LYP/631G* level. The interfacial distances between the centroid of paraphenylene unit of [12]CPP and the nearest centroid of hexagon of C80 are labeled. Corresponding MALDITOF mass spectra of (b) Y3N@C80⊂[12]CPP and (c) Y2@C79N⊂[12]CPP. RESULTS AND DISCUSSION Structural Characterizations Firstly, DFT-calculations were executed to demonstrate the size selectivity of Y2@C79N encapsulation by n[CPP] (n = 10, 11, 12) (Figure S3), and the results revealed that the [12]CPP is the only suitable nanoring in them for C80-based metallofullerenes to form stable host-guest complexes. As shown in Figure 1a, for optimized structures of


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Y3N@C80/Y2@C79N⊂[12]CPP, the interfacial distances between host and guest (3.8~4.0 Å) are coincidence with the van der Waals distance between graphite layers and other convex–concave π-π systems, revealing the stability of these supramolecular complexes. In addition, the special properties of ring current and aromatic wall which will benefit the regulation of metallofullerene property through constructing supramolecular complex. Experimentally, the complexes of Y3N@C80/Y2@C79N⊂[12]CPP were prepared by a blending method with ratio 1:1 at room temperature and clearly confirmed by matrixassisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, which shows the molecular weight of whole complex, as shown in Figure 1b and 1c. Moreover, the equatorial plane of C80-Ih cage has a segment of [6]CPP as shown in Figure 2a. Experimentally, MALDI-TOF in Figure 2b disclosed that the pristine [6]CPP is matching with [12]CPP to form stable [6]CPP⊂[12]CPP supramolecular complex, which further indicates the suitable size of [12]CPP to accommodate C80-Ih cage.


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Figure 2. (a) The supramolecular structures of Y3N@C80⊂[12]CPP and [6]CPP⊂[12]CPP. (b) MALDI-TOF mass spectrum of [6]CPP⊂[12]CPP complex.

The vibration spectrum of Y3N@C80⊂[12]CPP in solid state was also characterized by Raman experiment. There are three typical Raman peaks at 1201 cm-1, 1270 cm-1, and 1592

cm-1

corresponding

to

symmetrical

hydrocarbon

vibration,

asymmetrical

hydrocarbon vibration, and C-C breathing vibration for [12]CPP according to the theoretical

calculation

(Figure

S4),

respectively.

For

Y3N@C80⊂[12]CPP,

the

characteristic peak of 1201 cm-1 shifted to 1220 cm-1 (Figure S5), which reveals that Y3N@C80 influences the symmetric vibration of hydrocarbon bonds of [12]CPP.


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Figure 3. Shielding/deshielding effects characterized by 13C NMR. (a) Diagram for the shielding effect inner the ring and the outside deshielding effect caused by ring current of [12]CPP. (b) 13C NMR for Y3N@C80, Y3N@C80⊂[12]CPP, and [12]CPP in CS2 solution at room temperature (CDCl3 lock). We further employed

13C

NMR spectroscopy to characterize the supramolecular

structure of Y3N@C80⊂[12]CPP. Figure 3 show the

13C

NMR spectra for Y3N@C80,

[12]CPP, and Y3N@C80⊂[12]CPP, respectively. Generally, the (4n+2) π electrons for aromatic molecule always generate delocalized circuit, which would exhibit a significantly raised diamagnetic susceptibility to oppose the external field (as shown in Figure 3a).33-34 In 2007, Peeks et al. reported a six-porphyrin nanoring template complex with a diameter of 2.4 nanometres, which is antiaromatic or aromatic in different oxidation states characterized by NMR technology.8 For CPP, Taubert et al. proved that the dianions or dications of [6-11]CPPs are globally aromatic whereas the neutral ones are not aromatic characters.35 For Y3N@C80⊂[12]CPP, the phenomenon is different. In detail, the

13C

NMR signals of Y3N@C80 shift upshield by approximately 0.14/0.13 ppm


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for carbon a (red) and carbon b (gray) in C80 compared to that in pristine Y3N@C80 (Figure 3b). This is due to the shielding effect generated by magnetic ring current from [12]CPP. Whereas the

13C

NMR signals of [12]CPP shift downfield by approximately

0.57/0.41 ppm for carbon c (green) and carbon d (blue). The aromatic ring current was found to shield the inner space and deshield the outer [12]CPP nanoring, which may be caused by charge transfer between Y3N@C80 and [12]CPP. In addition, the distinguish 13C

NMR shift also can prove the formation of stable complex Y3N@C80⊂[12]CPP with

1:1 mole ratio.

Figure 4. Cyclic voltammetry measurements and frontier molecular orbitals. (a) Cyclic voltammetry of [12]CPP, Y3N@C80, and Y3N@C80⊂[12]CPP (1:1 mole ratio) at the same concentration. The scan rate is 50 mV/s. Dotted lines mean the first and the second quasi-reversible reduction potentials of Y3N@C80. (b) Energy levels of frontier orbitals of Y3N@C80 and Y3N@C80⊂[12]CPP. Electrochemical analyses of Y3N@C80 and Y3N@C80⊂[12]CPP conducted by cyclic voltammetry (CV) to clarify the electronic interaction between metallofullerene guest and CPP host (Figure 4a). Figure 4a shows that the first reduction potential of Y3N@C80 inside [12]CPP cavity shifts from −1.45 to −1.42 V, and the second potential shifts from


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−1.90 to −1.82 V. These results reveal that the host [12]CPP can enhance the electron accepting ability of guest Y3N@C80 due to the π-extended structure. Furthermore, the changed redox properties can be illustrated by the frontier molecular orbitals

of

HOMO

and

LUMO

for

Y3N@C80

and

its

host-guest

complex

Y3N@C80⊂[12]CPP, see Figure 4b. For the HOMO of Y3N@C80⊂[12]CPP, the electrons mainly distribute on the CPP, whereas the LUMO of it is delocalized on the central metallofullerene cage. Moreover, the LUMO of the complex is decreased to −3.13, which agrees well with the electrochemical result that metallofullerene⊂CPP complex becomes easier to get electron.

Figure 5. DFT-calculated energy process of Y3N@C80 and Y2@C79N inside [12]CPP nanoring. (a) Diagram of three typical paths ( Ⅰ , Ⅱ , and Ⅲ ) for Y3N@C80 inside [12]CPP nanoring. Path Ⅰ means that Y3N@C80 rotates along the ring horizontally. Path Ⅱ and Ⅲ mean that Y3N@C80 rotates along the ring vertically in different conformers as shown. (b) Calculated energy profiles for three paths of Y3N@C80 rotation within [12]CPP. (c) Diagram of four typical paths for Y2@C79N inside [12]CPP nanoring. Path Ⅰ and Ⅲ show that the N-guided Y2@C79N rotates horizontally. Path Ⅱ and Ⅳ show that the N-guided Y2@C79N rotates vertically. (d) Calculated energy profiles for four paths of Y2@C79N rotation within [12]CPP. The above energy profiles are all with rigid structures at the level of B3LYP/6-31G*~ lanl2dz for Y.


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Molecule Orientation and Thermodynamic Process The optimized structures of Y3N@C80⊂[12]CPP showed that Y3N cluster is tend to in the quasi-C12 axis plane. We then investigated the orientation of Y3N@C80 and Y2@C79N in the supramolecular host. In detail, we simulated three typical rotating processes by calculating the energy profiles of Y3N@C80 within [12]CPP along rotation paths, as shown in Figure 5a. Path Ⅰ denotes the most advantageous path, suggesting that three Y atoms keep adhering to the wall of nanoring during rotation process. Path Ⅰ has a very small barrier of 3 kcal/mol when rotating 360 degree horizontally (Figure 5b). The axial rotation of Y3N@C80 in [12]CPP nanoring described in path Ⅰ is more advantageous compared to other paths. For path Ⅱ , Y3-guided Y3N@C80 moves along the axis of N-Y bond with energy barrier of 15.5 kcal/mol when rotating 360 degrees in [12]CPP. And when two Y atoms are on the vertical sites of C12 plane (θ = 90° or 270°), the conformers are at the maximum value of energy. For path Ⅲ , three Y atoms move perpendicularly along the quasi-C12 axis plane with energy barrier about 7 kcal/mol at a higher energy trajectory. These results show that although [12]CPP has high symmetry with C12 point-group and the guest C80-cage has a Ih-symmetry, the endohedral Y3N cluster could induce special supramolecular geometry. Moreover, the optimized structure of Y2@C79N⊂[12]CPP shows that the Y2 cluster of Y2@C79N is also in the quasi-C12 axis plane of [12]CPP and tend to adhere to the wall of nanoring. The calculation results also show that the Y2 atoms of Y2@C79N are inclined to retain in the hoop plane. They show similar thermodynamic process by calculating the energy profiles of Y2@C79N within [12]CPP along rotation paths of Ⅰ to Ⅳ (Figure 5c).


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Among them, path Ⅰ denotes the most advantageous rotation type, for which the N and two Y atoms of Y2@C79N keep adhering to the wall of [12]CPP nanoring. Path Ⅰ has a small barrier of 4 kcal/mol when rotating Y2@C79N horizontally (Figure 5d). For path Ⅱ, the N-guided Y2@C79N rotates vertically with two Y atoms adhering to the wall of nanorings with an energy barrier of 15 kcal/mol when rotating 360 degrees in [12]CPP. For path Ⅲ, only the N atom keeps adhering to the wall of nanoring and two Y atoms are perpendicular to quasi-C12 axis plane with low energy barrier (about 6 kcal/mol) but at a higher energy trajectory of 28 kcal/mol than path Ⅰ

when rotating 360 degrees

horizontally for Y2@C79N in [12]CPP. For path Ⅳ , the N-guided Y2@C79N rotates vertically with an energy barrier of 20 kcal/mol when rotating 360 degrees in [12]CPP. We also performed a DFT-based BOMD (Born Openheimer Molecular Dynamics) trajectory for Y3N@C80⊂[12]CPP by CP2K. The Y3N cluster is placed in two different initial positions that one is in the coplanar position with [12]CPP and another one is in the vertical plane with [12]CPP. As the supporting trajectories shown (two animations, 12 ps, 298 K, PBE/DZ(P)), the coplanar Y3N keeps small amplitude rotating in the plane (Animation A), whereas the vertical Y3N cluster is distorted into the [12]CPP plane (Animation B). That is, the revolving Y3N@C80 tends to with one degree of freedom in the host (maintained in the quasi-C12 plane in the [12]CPP) and such unique supramolecular dynamics make the Y3N@C80⊂[12]CPP be potential application for the future molecular machines.


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Figure 6. STM images of [12]CPP, Y3N@C80, Y2@C79N, and Y3N@C80/Y2@C79N⊂[12]CPP. (a-b) Large-scale and high-resolution STM images of the [12]CPP layer on Au(111) surface. (c) High-resolution STM image of Y2@C79N monolayer on Au(111) surface. (d) High-resolution STM image of Y2@C79N⊂[12]CPP monolayer on Au(111) surface. (e) Large-scale and (f) high-resolution STM images of the Y3N@C80 monolayer on Au(111) surface; (g) Cross-section profile among the white dotted line in (f). (h) Large-scale and (i) high-resolution STM images of the Y3N@C80⊂[12]CPP monolayer on Au(111) surface; (j) Cross-section profile along the white dotted line in (i). Image conditions: (a-b) E = −400 mV, Ebias = −390 mV, It = 0.997 nA. (c) E = -400 mV, Ebias = -229 mV, It = 1.000 nA. (d) E = −400 mV, Ebias = −378 mV, It = 1.000 nA. (e–f) E = −400 mV, Ebias = −360 mV, It = 1.000 nA; (h) E = −427 mV, Ebias = −521 mV, It = 2.395 nA; (i) E = −400 mV, Ebias = −335 mV, It = 0.736 nA. Self-Assembly on Surface STM was employed to provide insights into self-assembly property and structure illustration for Y3N@C80/Y2@C79N⊂[12]CPP. In detail, [12]CPP, Y3N@C80, Y2@C79N, and Y3N@C80/Y2@C79N⊂[12]CPP layers on Au(111) were prepared by dip-coating method. Figure 6a and Figure 6b display the highly disordered distribution (the yellow rings represent the [12]CPP) of hollow [12]CPP molecule on Au(111) surface. Figure 6c


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and Figure 6d show the dispersed distributions of Y2@C79N and Y2@C79N⊂[12]CPP on Au(111) surface, respectively. Figure 6e shows the monolayer formed by Y3N@C80 molecule, well-ordered structure can be observed on Au(111) surface. High-resolution STM image in Figure 6f reveals that Y3N@C80 molecules form a typical six-fold symmetric close-packed structure. The intermolecular distance of Y3N@C80 is measured to be a2 = 1.1 ± 0.1 nm (Figure 6g). Figure 6h and Figure 6i displays the distribution images

of

the

Y3N@C80⊂[12]CPP

molecules

on

Au(111)

surface.

The

Y3N@C80⊂[12]CPP molecules randomly absorb on the surface. The intermolecular distance of Y3N@C80⊂[12]CPP is measured to be a1 = 1.6 ± 0.1 nm according to the cross-section analysis in Figure 6j, which is in consistent with the modified layer measured as vdW diameter (1.64 nm). XRD study of the membranes for Y3N@C80, [12]CPP, and their complex on quartz also discussed in Figure S6. Thus, it can be concluded that the supramolecular structure of Y3N@C80⊂[12]CPP is formed and the [12]CPP nanoring can change the weak van der Waals forces and influence the selfassembly character of Y3N@C80⊂[12]CPP. In addition, STM illustrated showed that metallofullerene guest molecule can improve the self-assembly behavior of [12]CPP and dominate the self-assembled process of these supramolecular complexes.


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Figure 7. Spin density distributions and temperature-dependent EPR spectra in solution. (a-b) Spin density distribution of Y2@C79N and Y2@C79N⊂[12]CPP; the isovalue is 0.003. (c-d) Temperature-dependent EPR spectra of Y2@C79N and Y2@C79N⊂[12]CPP. Samples were dissolved in CS2. (e) EPR signal comparison for Y2@C79N and Y2@C79N⊂[12]CPP at 298 K. The EPR measurement frequency is 9.4457 GHz, the continuous-wave power is 10.02 mW, the intensity is 13 dB. Spin Probe and Manipulation For paramagnetic Y2@C79N, its unpaired spin is distributed on the internal Y2 cluster as shown in Figure 7a. The spin density distributions for the host-guest complex is shown in Figure 7b. We then performed the temperature-dependent EPR measurements of


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Y2@C79N (Figure 7c) and Y2@C79N⊂[12]CPP (Figure 7d) in CS2 at 298 K and 233 K, respectively. The concentrations of Y2@C79N are the same for the two samples. Pristine Y2@C79N exhibited three groups of EPR signals with 1:2:1 intensity ratio at 298 K, which is the consequence from triplet couplings between unpaired spin and two equivalent 89Y nuclei (I = 1/2). The isotropic hyperfine coupling constant (hfcc) is 81.5 G, and the g value is 1.9875 (Figure S7). However, Y2@C79N⊂[12]CPP showed obvious anisotropic EPR peaks in which the peak intensity at high field increased at 298 K (Figure 7e). This kind of anisotropy was also observed for pristine Y2@C79N under low temperature due to the slowed rotation of internal Y2 cluster. When temperature reduced to 233 K, the asymmetry paramagnetic property became more obvious caused by the slowed rotation of Y2 cluster as well as the confined effect from nanoring, which limits the rotation of Y2@C79N inside [12]CPP nanoring. Considering the tight interaction between Y2@C79N and [12]CPP, the rotation of Y2@C79N is hindered, resulting in more insufficient rotational averaging of electron resonance and a much larger temperature effect on the correlation time as compared for the naked Y2@C79N. In addition, the slow rotation of the whole complex will intensify the insufficient rotational averaging of electron resonance. To further illustrate the spin character of Y2@C79N⊂[12]CPP, we investigated the paramagnetic properties for Y2@C79N⊂[12]CPP layer on the surface of quartz in solid state. For the layer of Y2@C79N⊂[12]CPP in Figure 8a, it exhibits axisymmetric parameters at 298 K with a⊥ = 85.64 G and a∥ = 74.93 G for the two equivalent Y nuclei, and g⊥ = 1.968 and g∥ = 1.9795, indicating unique states for Y2@C79N molecules within [12]CPP. The agreement of simulated and experimental EPR spectra reveals the highly


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isolated Y2@C79N molecules without serious agglomeration. Moreover, the axisymmetric parameters in Y2@C79N⊂[12]CPP solid were also observed in Y2@C79N⊂MOF-177 complex as well as Y2@C80(CH2Ph) derivative.5,20-21 Generally, the EPR tensors reflect the structural characteristics of spin molecule, and low-symmetry molecule is often accompanied by irregular coupling parameters since the low-symmetry lattice field. Here the axisymmetric hyperfine couplings of Y2@C79N⊂[12]CPP indicate that the Y2 cluster companying with unpaired spin in Y2@C79N molecule has somewhat orientation in solid state rather than random directions. It was proved the guest spin tends to be in a symmetrical position, which is consistent of above DFT calculation that the external N atom as well as two internal Y atoms in Y2@C79N are retaining in the hoop plane of nanoring, the weak interaction between Y2@C79N and [12]CPP also discussed in supporting information (Figure S8).

Figure 8. EPR properties in solid state. EPR measurements of Y2@C79N⊂[12]CPP (a) and Y2@C79N in biphenyl (b) in solid state. Samples were measured at 298 K. The frequency is 9.4471 GHz. (c) Mono molecular layer STM image of Y2@C79N⊂[12]CPP layer on Au(111) surface. Image conditions: E = −400 mV, Ebias = −378 mV, It = 1.000 nA. (d) Scanning electron microscope (SEM) of Y2@C79N in biphenyl deposited onto the surface of quartz with 1×1 cm2.


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For comparison, we introduced the biphenyl (a fragment unit of [12]CPP) and prepared its Y2@C79N complexes in membrane states. It can be seen in Figure 8b that the Y2@C79N dispersed in biphenyl solid presents an isotropic EPR signals (aiso = 80.28 G, g = 1.973) at 298 K with two equivalent Y nuclei. These results revealed that the Y2 unit of Y2@C79N is still rotating in biphenyl matrix, whereas in [12]CPP matrix the Y2 rotation is hindered. In addition, isotropic EPR signals also indicate disordered Y2@C79N molecules among biphenyl. It should be noted that the wide peak at central field is caused by the agglomeration of Y2@C79N in biphenyl (the EPR spectrum of pure Y2@C79N solid is shown in Figure S9). For Y2@C79N⊂[12]CPP, STM helps to understand the layer image and interfacial assembly property. The layer was prepared by the dip-coating method using Y2@C79N⊂[12]CPP solution. As shown in Figure 8c, Y2@C79N⊂[12]CPP molecules are dispersed on the Au(111) surface. This assembly behavior effectively reduces the spinspin interaction and benefits the design and preparation of molecular devices with magnetic function. In addition, we use scanning electron microscope (SEM) to investigate the membrane of Y2@C79N and biphenyl complex deposited on the surface of quartz with 1×1 cm2, and Figure 8d shows a characteristic membrane surface. CONCLUSIONS We took advantage of nanoring of [12]CPP to hoop C80-based metallofullerene Y3N@C80 and azafullerene Y2@C79N, resulting in controllable molecule orientation, changed selfassembly behaviors, and tunable spin state. DFT-calculations and dynamic simulations demonstrated the unique molecular orientation of the guest Y3N@C80 within [12]CPP, where the Y atoms tend to adhere the wall of the hosts caused by the regional selectivity


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of π-π interaction and van der Waals attractive interaction. The electrochemical and the frontier molecular orbitals analyses were both concluded that the Y3N@C80⊂[12]CPP complex is easier to get electrons than the original state. STM illustrated the molecular form and self-assembly behavior of Y3N@C80/Y2@C79N⊂[12]CPP on Au(111) surface, and the results showed that metallofullerene guest molecule can improve the selfassembly behavior of [12]CPP and dominate the self-assembled process. On the other side, [12]CPP nanoring can change the weak van der Waals forces and influence the selfassembly of its supramolecular complexes. Moreover, the electron spin in paramagnetic Y2@C79N was employed to percept the host-guest interaction. The strong confinement effect makes guest Y2@C79N molecule rotate slowly in the host [12]CPP, and results in anisotropic EPR signals due to insufficient rotational averaging of electron spin. We also investigated the paramagnetic properties for Y2@C79N⊂[12]CPP in solid state. Y2@C79N⊂[12]CPP layer onto the surface of quartz in solid state exhibits axisymmetric parameters due to the molecular orientation of guest Y2@C79N within [12]CPP. Moreover, for the solid state of Y2@C79N⊂[12]CPP, it exhibits independent and undisturbed spin state for Y2@C79N molecule, revealing its potential in single-molecular device. This study provides a new strategy to control the C80-based metallofullerenes and modulate their electronic and magnetic properties. This kind of supramolecular complexes has potential applications in functional molecular devices. ASSOCIATED CONTENT Supporting Information. Experimental details, HPLC chromatogram, MALDI-TOF-MS, EPR, Raman, XRD and Calculation results are included in the supporting information.


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Animation A: Molecular dynamics process for Y3N@C80 rotating in [12]CPP by CP2K, initial Y3N cluster is coplanar position with [12]CPP. Animation B: Molecular dynamics process for Y3N@C80 rotating in [12]CPP by CP2K, initial Y3N cluster is vertical position with [12]CPP. AUTHOR INFORMATION Corresponding author *E-mail: [email protected], [email protected] ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51672281, 61227902, 51832008). T. Wang particularly thanks the Youth Innovation Promotion Association of CAS (2015025). REFERENCES (1) Wang, T.; Wu, J.; Xu, W.; Xiang, J.; Lu, X.; Li, B.; Jiang, L.; Shu, C.; Wang, C. Spin divergence induced by exohedral modification: ESR study of Sc3C2@C80 fulleropyrrolidine. Angew. Chem., Int. Ed. 2010, 49, 1786-1789. (2) Liu, Z.; Dong, B. W.; Meng, H. B.; Xu, M. X.; Wang, T. S.; Wang, B. W.; Wang, C. R.; Jiang, S. D.; Gao, S. Qubit crossover in the endohedral fullerene Sc3C2@C80. Chem. Sci. 2018, 9, 457-462. (3) Hu, Z.; Dong, B. W.; Liu, Z.; Liu, J. J.; Su, J.; Yu, C.; Xiong, J.; Shi, D. E.; Wang, Y.; Wang, B. W; et al. Endohedral metallofullerene as molecular high spin qubit: diverse rabi cycles in Gd2@C79N. J. Am. Chem. Soc. 2018, 140, 1123-1130. (4) Meng, H.; Zhao, C.; Nie, M.; Wang, C.; Wang, T. Triptycene molecular rotors mounted on metallofullerene Sc3C2@C80 and their spin-rotation couplings. Nanoscale 2018, 10, 1811918123.


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Supramolecular Complexes of C80-Based ... - ACS Publications

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