Molecular Spin Sensor for In-Situ Monitoring of Crystallization Behavior and Phase Transition in Aromatic Materials

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Herein, we report a molecular spin sensor based on metallofullerene Y 2 @C 79 N for in-situ monitoring of crystallization behavior and phase transitions in aromatic materials with high precision. Two functional aromatic materials, 1-chloronaphthalene and a liquid crystal material of 4-cyano-4′-pentyl-biphenyl (5CB), were strategically selected based on their distinct crystallization behaviors and technological relevance. Temperature-dependent spin resonance signals of Y 2 @C 79 N dissolved in aromatic materials were analyzed using electron paramagnetic resonance (EPR) spectroscopy. For Y 2 @C 79 N in 1-chloronaphthalene, two distinct EPR signal changes were observed at 250 and 230 K, corresponding to its melting and crystallization points, respectively. For Y 2 @C 79 N in 5CB, three distinct EPR signal changes were observed at 290, 270, and 230 K that correspond to its crystallization-related phase transitions, significantly outperforming conventional XRD analysis which only detected the 270 K transition. Experimental results combining theoretical calculations reveal that the sensing mechanism originates from crystallization-induced alignment of fullerene molecular orientation within the host matrix. This work establishes metallofullerene-based spin probes as a powerful analytical tool for real-time monitoring of molecular ordering processes in aromatic materials, offering superior sensitivity compared to conventional characterization methods. The demonstrated quantum sensing paradigm opens new possibilities for studying fundamental phase transition phenomena. Physical sciences/Nanoscience and technology/Techniques and instrumentation/Characterization and analytical techniques Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Recently, the use of electron spins in advanced science and technology has been actively studied owing to their quantum characteristics that can result in the development of new functionalities 1 – 5 . Among these studies, molecular spins have attracted considerable attention because these systems exhibit unique features and advantages 6 – 10 , including spin preparation and modulation via chemical engineering, spin sensing, and measurements at the molecular level. Therefore, exploring the molecular spin properties and functions is essential to advance related applications. Metallofullerenes with electron spin characteristics, such as Y 2 @C 79 N 11 , Sc 3 C 2 @C 80 12 , Y@C 82 13 , and Gd 2 @C 79 N 14 , are emerging as molecular spin materials that have characteristic electron paramagnetic resonance (EPR) signals with potential applications in diverse fields including spin sensors and quantum information processing. Among them, the aza-metallofullerene Y 2 @C 79 N has an electron spin localized on the internal cluster and shows distinct EPR signals 15 , 16 . Previously, we revealed that the internal spin in Y 2 @C 79 N can sense external environmental conditions, such as cage modification 17 – 18 and orientation 11 , 20 . We investigated the spin modulation of Y 2 @C 79 N by encapsulating it within the pores of a metal–organic framework (MOF) of MOF-177 and revealed its orientation sensitivity for Y 2 @C 79 N 20 . Additionally, we studied the spin modulation of Y 2 @C 79 N when incorporated into molecular nanorings, highlighting its orientation sensitivity within supramolecular systems 19 . Our findings reveal the characteristic orientation of Y 2 @C 79 N in organic materials containing phenyl groups, demonstrating the spin capacity to sensitively detect buckyball orientation. Therefore, further exploration of the spin-sensing function of Y 2 @C 79 N corresponding to the molecular orientation is vital. Organic molecular orientation is widely observed in processes such as crystallization, phase transition, and molecular assembly 21 – 24 . For example, in organic solar cells (OSCs), the short range crystallization of donor/acceptor materials has been reported to be significant 21 . To enhance the device performance of OCSs, 1-chloronaphthalene is often used as a solvent additive because it improves the degree of order of the polymer films. This occurs because the crystallization of 1-chloronaphthalene can induce the orientation and rearrangement of polymers 25 , 26 . Additionally, crystallization-related phase transitions can also be found in liquid crystal (LC) materials 27 – 30 , which have attracted considerable interest because of their wide practical applications in displays, innovative sensors, and digital nonvolatile memory devices. These results indicate that studies on the crystallization and phase transitions corresponding to the molecular orientation of organic materials are essential. Presently, the molecular orientation in crystallization and phase transition is widely characterized by X-ray diffraction (XRD). However, the XRD technique cannot effectively identify light atoms such as C, H and O, which are the dominant components in molecular materials. Thus, exploring other methods to probe the molecular orientation more precisely is vital. Herein, we selected the aforementioned functional organic materials, including 1-chloronaphthalene and a liquid crystal material of 4-cyano-4′-pentyl-biphenyl (5CB), to explore the spin-sensing function of Y 2 @C 79 N for the crystallization behavior and phase transitions of aromatic materials. For comparison, X-ray diffraction (XRD) and polarized-light optical microscopy (POM) imaging were employed to illustrate the relationship between the spin signals and compound crystallization. These results reveal that the Y 2 @C 79 N spin can be developed as a molecular spin sensor to monitor the phase transition of aromatic materials in situ and in real time. Results Aza-metallofullerene Y 2 @C 79 N, in which one carbon atom of the cage is substituted with a nitrogen atom, has an electron spin localized on the internal Y 2 cluster (Fig. 1 a and 1 b). In solution, Y 2 @C 79 N shows three distinct groups of EPR signals owing to the hyperfine couplings between the spin and the two Y nuclei ( I Y = 1/2). Notably, the orientation of Y 2 @C 79 N in solution is disordered. However, the selective orientation of Y 2 @C 79 N can be achieved by encapsulating it within the pores that contain phenyl groups. Our previous findings reveal the characteristic orientation of Y 2 @C 79 N in aromatic materials containing phenyl groups. This orientation is facilitated by π–π interactions between the N-substituted region of the fullerene cage and the benzene unit (Fig. 1 c). The crystallization of selected organic materials (1-chloronaphthalene and 5CB) with benzene rings formed an ordered array, which could also induce the Y 2 @C 79 N orientation (Fig. 1 d). First, 1-chloronaphthalene, an aromatic compound with a melting point of 253 K, was employed to dissolve Y 2 @C 79 N, and the temperature-dependent EPR spectra were collected to study the crystallization-induced Y 2 @C 79 N orientation and spin sensing (Fig. 2 a). Subsequently, we measured the EPR signals of Y 2 @C 79 N in 1-chloronaphthalene from 290 to 90 K (Fig. 2 b). EPR spectra were recorded using an EPR spectrometer (CIQTEK EPR200-Plus) with a continuous-wave X-band frequency. From 290 to 245 K, the signal intensity gradually increased owing to weakened spin–lattice interactions; it reached its peak at 245 K (Fig. S2). From 245 to 230 K, the signal intensity began to decrease because of the restricted motion of Y 2 @C 79 N owing to the coagulation of 1-chloronaphthalene. From 290 to 230 K, the intensity of the EPR signal at a high magnetic field has a higher enhancement than that at a low magnetic field owing to paramagnetic anisotropy. This is because the two Y nuclei and spin have restricted motion at low temperatures; consequently, insufficient rotational averaging occurs in the resonance structure. Notably, a new group of EPR signals was observed at 225 K. According to our previous EPR studies on Y 2 @C 79 N, this new group of EPR signals can be ascribed to the axisymmetric EPR signals, which are caused by the orientation of Y 2 @C 79 N in 1-chloronaphthalene. The intensity of this axisymmetric group of EPR signals gradually increased when the temperature was decreased to 90 K. These results show thast at low temperatures, Y 2 @C 79 N orientates more in 1-chloronaphthalene. The linewidths of the EPR signals were analyzed to determine the motion states of Y 2 @C 79 N in 1-chloronaphthalene at varied temperatures. Generally, the linewidth of an EPR signal is related to the rotation of the spin molecule. Figure 2 c shows the linewidth changes for the EPR signals of Y 2 @C 79 N in 1-chloronaphthalene at temperatures ranging from 290 to 230 K. For Y 2 @C 79 N in 1-chloronaphthalene, the linewidths of the EPR signals have a minimum value at 250 K. From 245 to 230 K, the linewidths of the EPR signals began increasing because of the restricted motion of Y 2 @C 79 N owing to the coagulation of 1-chloronaphthalene and restricted motion of Y 2 @C 79 N. This transition point coincides with the melting point of 1-chloronaphthalene (253 K). These results indicate that the spin of Y 2 @C 79 N can effectively sense the coagulation of 1-chloronaphthalene. In addition, the linewidths for the EPR signals of Y 2 @C 79 N in 1-chloronaphthalene show a dramatic rise at 230 K, below which Y 2 @C 79 N in 1-chloronaphthalene shows axisymmetric EPR signals. As reported previously, the axisymmetric EPR signals revealed Y 2 @C 79 N orientation in 1-chloronaphthalene. Moreover, as noted earlier, 1-chloronaphthalene is often used as a solvent additive for solar cells because it induces a short-range regular arrangement of the polymers owing to crystallization. Therefore, we propose that the Y 2 @C 79 N in 1-chloronaphthalene below 230 K also has a crystallization-induced orientation. Table 1 Melting and crystallization transition points of 1-chloronaphthalene obtained by several methods Melting point Crystallization point EPR 245 K 230 K XRD —— 230 K POM —— 233 K We performed in-situ temperature-dependent XRD characterizations of 1-chloronaphthalene (Fig. 2 d) to further investigate the relationship between the EPR signals and compound crystallization. In-situ XRD is an experimental method useful for detecting material crystallization. We found that 1-chloronaphthalene shows distinct diffraction peaks at 230 K, indicating the beginning of compound crystallization. Notably, the crystallization temperature of 1-chloronaphthalene obtained by XRD is close to the transition point of the EPR signals of Y 2 @C 79 N. These results further demonstrate the relationship between the EPR signals of Y 2 @C 79 N and compound crystallization. Moreover, polarized light optical microscopy (POM) was used to characterize the crystallization of 1-chloronaphthalene at various temperatures (Fig. 2 e). The crystal morphology was first observed at 233 K for 1-chloronaphthalene, indicating the beginning of compound crystallization. This transition temperature at the beginning of the compound crystallization is also close to the transition point for the EPR signals of Y 2 @C 79 N at 230 K. In addition, when the temperature was further decreased, 1-chloronaphthalene showed a more obvious crystal morphology, indicating that it crystallizes better at low temperatures. These POM imaging results are consistent with EPR data, where Y 2 @C 79 N shows more prominent axisymmetric signals at low temperatures. The results further confirm the relationship between the EPR signals of Y 2 @C 79 N and compound crystallization. Additionally, these results show that the spin of Y 2 @C 79 N is sensitive to compound crystallization, demonstrating its potential application as a molecular spin sensor. Based on these results, the Y 2 @C 79 N spin was employed to sense the crystallization-related phase transitions of a liquid crystal material 5CB (Fig. 3 a). Liquid-crystal (LC) materials have attracted considerable research interest owing to their promising practical applications in displays, flat panels, innovative sensors, and digital nonvolatile memory devices. 5CB belongs to a class of thermotropic liquid crystals, and is one of the most widely studied liquid crystal materials 27 , 31 – 43 . It has been established that 5CB has an isotropic-nematic transition temperature of approximately 308 K and a nematic-crystalline transition temperature of approximately 297 K. Thus, we measured the temperature-dependent EPR signals of Y 2 @C 79 N dissolved in 5CB at temperatures ranging from 320 to 130 K (Fig. 3 b and S3). For EPR studies, Y 2 @C 79 N in 5CB shows three groups of EPR signals with higher signal intensity at high field than at low field between 320 and 290 K. This reveals the anisotropic EPR spectra resulting from the restricted motion of Y 2 @C 79 N in the nematic phase of 5CB. As shown in Fig. S2, from 320 to 290 K, the EPR signal intensity of Y 2 @C 79 N in 5CB reached its peak at 290 K owing to the weakened spin–lattice interaction at low temperatures. Notably, the EPR signal intensity of Y 2 @C 79 N in 5CB decreased below 290 K. According to the literature, the nematic-crystalline phase transition of 5CB occurs at approximately 297 K, below which the 5CB begins to crystallize. Therefore, it can be deduced that below 290 K, Y 2 @C 79 N in crystalline 5CB has strong spin–lattice interactions. The temperature-dependent linewidths of the EPR signals of Y 2 @C 79 N in 5CB were then analyzed. Figure 3 c shows the temperature dependence of linewidths for the EPR signals at varied temperatures. For Y 2 @C 79 N in 5CB, the linewidths of the EPR signals have a minimum value at 295 K. The linewidths of all EPR signals gradually decrease when the temperature decreases from 320 to 295 K owing to the weakened spin–lattice interactions of Y 2 @C 79 N in nematic 5CB at low temperatures. From 290 K, the linewidths of the EPR signals began to increase when the temperature decreased. It has been established that 5CB has a nematic-crystalline transition temperature of approximately 297 K. Therefore, these results demonstrate the relationship between the EPR signals of Y 2 @C 79 N and 5CB crystallization. When the temperature was decreased to 270 K, the linewidths for the EPR signals of Y 2 @C 79 N in 5CB show a dramatic rise. Under 270 K, Y 2 @C 79 N in 5CB shows relatively axisymmetric EPR signals, indicating the orientation of Y 2 @C 79 N, as discussed earlier. From 270 to 230 K, Y 2 @C 79 N in 5CB exhibited similar EPR signals. When the temperature was decreased to 225 K, Y 2 @C 79 N in 5CB clearly showed a group of axisymmetric EPR signals, which revealed a crystalline phase change of 5CB. Moreover, from 225 K to 130 K, the intensity of the axisymmetric EPR signals gradually increased, revealing the significant effect of crystalline 5CB on the spin resonance of Y 2 @C 79 N. The EPR spectra of Y 2 @C 79 N in 5CB were simulated to obtain the hyperfine coupling constants (a) and g factors. The simulated EPR spectra were obtained using the Easyspin package ( http://www.easyspin.org ), encoded on MATLAB. Figure 3 d shows the experimental EPR spectrum of Y 2 @C 79 N in 5CB measured at 130 K, and the correspondingly simulated EPR spectrum with parameters of a ⊥ = 78.21 G, a ∥ = 92.14 G, g ⊥ = 1.952, and g ∥ = 1.993. The simulated EPR spectrum exhibited axisymmetric parameters, further demonstrating the orientation of Y 2 @C 79 N in crystalline 5CB. To further investigate the relationship between the EPR signals and 5CB crystallization, we performed in-situ temperature-dependent XRD characterization of 5CB from 300 to 240 K (Fig. 3 e). 5CB exhibits distinct XRD peaks at 270 K, revealing that 5CB crystallizes at this point. Notably, the axisymmetric EPR signals for Y 2 @C 79 N in 5CB was also clearly observed at 270 K. These results further demonstrated the crystallization-induced Y 2 @C 79 N orientation in the crystalline phase of 5CB. Additionally, from 270 to 240 K, the XRD patterns of 5CB showed similar peaks without variations in the EPR spectra. POM was used to characterize the crystallization of 5CB at various temperatures (Fig. 3 f). A crystal morphology was discovered at approximately 308 K for 5CB, revealing its nematic phase. From 308 to 260 K, the POM images of 5CB were similar. From 255 K, the POM images of 5CB show a changed crystal morphology, indicating a change in the crystalline phase. Table 2 Temperature-dependent crystalline phase transition points of 5CB obtained by several methods. Nematic→Crystalline C 2 [A] →C 1 b[B] C 1 b →C 1 a[C] Ref. EPR 290 K 270 K 230 K This work XRD —— 270 K —— This work POM —— 255 K —— This work DSC 297 K ~ 260 K ~ 230 K 35–36, 44 Temperature-dependent phase transitions of 5CB have been reported using several methods, such as differential scanning calorimetry (DSC), fluorescence spectroscopy, X-ray diffraction, Raman spectroscopy, and proton nuclear magnetic resonance (NMR). Table 2 lists the temperature transition points of 5CB in different crystalline states obtained by DSC, EPR, XRD, and POM. The DSC method was employed to systematically study the phase transitions of 5CB, which showed repeated transition points. In the DSC results, the exothermic peak at approximately 260 K can be attributed to the transition from the C 2 crystalline phase to the metastable C 1 b phase. Additionally, the exothermic peak at approximately 230 K can be attributed to the transition from the metastable C 1 b crystalline phase to the C 1 a phase. The XRD and POM results for 5CB show distinct XRD peaks at 270 K and a change in crystal morphology at 255 K, respectively, which can be attributed to the transition from the C 2 crystalline phase to the metastable C 1 b phase. However, the transition from the metastable C 1 b crystalline phase to the C 1 a phase was not observed in the XRD or POM results. Notably, two marked changes in the EPR results were observed at 270 and 230 K, which are consistent with the two corresponding phase transition points in the DSC results. These results show that the spin of Y 2 @C 79 N is sensitive to 5CB crystallization, demonstrating its potential for application as a molecular spin sensor. Density functional theory (DFT) calculations were performed to further analyze the orientation of Y 2 @C 79 N in crystalline 5CB. As illustrated in Fig. 4 a– 4 d, we assessed the thermodynamic stability of the four typical sites of Y 2 @C 79 N within the 5CB crystal lattice. The most favorable structure was obtained when the nitrogen atom of Y 2 @C 79 N was positioned near the benzene ring of 5CB, and the two Y atoms were in proximity to the 5CB molecule. Concurrently, molecular dynamics (MD) simulations were employed to statistically analyze the changed distances (d) between the nitrogen atom on the carbon cage and the center of a benzene ring in 5CB. For initial conformation, the nitrogen atom was placed at a distance ( d ) far away from the center of the benzene ring of 5CB (approximately 9.2 Å). Following MD simulations, the distance ( d ) between the nitrogen atom and the center of the benzene rings stabilizes at an average of 3.28 Å after 35 ps (Fig. 4 e). These results suggest that Y 2 @C 79 N exhibits distinct orientation characteristics in crystalline 5CB. Discussion This study reported a molecular spin senor of metallofullerene Y 2 @C 79 N for in-situ monitoring of the crystallization behavior and phase transition in aromatic materials. Two functional aromatic materials, including 1-chloronaphthalene and a liquid crystal material of 5CB, were selected to illustrate this spin-sensing function of Y 2 @C 79 N. Temperature-dependent EPR results of Y 2 @C 79 N dissolved in these two aromatic materials were analyzed. The EPR results indicated that Y 2 @C 79 N in 1-chloronaphthalene exhibits two distinct changes for EPR signals at 250 and 230 K, which correspond to the melting and crystallization points of 1-chloronaphthalene, respectively. For Y 2 @C 79 N in a liquid crystal of 5CB, it exhibits three distinct changes for EPR signals at 290, 270, and 230 K, corresponding to the nematic-crystalline transition, C 2 crystalline to metastable C 1 b crystalline phase transition, and metastable C 1 b crystalline to the C 1 a crystalline phase transition. These results show that the spin of Y 2 @C 79 N can sense the crystallization-related phase transitions of these aromatic materials. In addition, Y 2 @C 79 N dissolved in these aromatic materials exhibited EPR signals that changed from isotropic to axisymmetric patterns below their crystallization points owing to the orientation of Y 2 @C 79 N in these crystalline materials. Moreover, Y 2 @C 79 N was found to be capable of sensing most phase changes of a liquid crystal material of 5CB at 290, 270, and 230 K. Conversely, the XRD method can only detect one phase transition at 270 K. XRD, a common method for characterizing orientation, struggles to detect light atoms like C, H, and O, underscoring the need for more precise probing techniques. This susceptible spin character of azametallofullerene Y 2 @C 79 N has potential applications for monitoring the crystallization behavior and phase transitions of certain aromatic materials with high precision. In particular, this molecular spin probe can penetrate into the crystal lattice of aromatic materials and reflect the phase transitions in situ. The utilization of molecular spin to detect the crystallization and phase transitions can be developed into a transformative technology in the future. Methods Synthesis, isolation and characterizations of Y 2 @C 79 N Metallofullerene Y 2 @C 79 N was synthesized by the arc-discharge method. Briefly, Y/Ni 2 alloy and graphite powder were mixed in a mass ratio of 3:1 and filled into the hollow graphite rod. The furnace was vacuumed and filled with 190 Torr He and 10 Torr N 2 . Then the graphite rods were evaporated in a DC arc discharge furnace with a current of 130 A to get the carbon soot containing various fullerenes and metallofullerenes. The carbon soot was subjected to Soxhlet extraction in toluene for 12 h. Y 2 @C 79 N was isolated and purified by multi-step high performance liquid chromatography (HPLC) on Buckyprep and Buckyprep-M column (20 mm × 250 mm) with toluene as the mobile phase. The purity of Y 2 @C 79 N was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS, AXIMA Assurance, Shimadzu). XRD measurements The PANalytical Empyrean X-ray diffractometer was employed for the characterization of samples. A volume of 100 µL of the solvent was placed in a custom-designed sample cell and positioned on the sample stage for analysis. To achieve temperature control, a low-temperature nitrogen gas was used to establish an equilibrium atmosphere. A temperature probe was positioned in close proximity to the sample surface to monitor the environmental temperature. Spectra were acquired for each sample at different temperatures, with a stabilization period of ten minutes at each temperature. POM measurements An Olympus BX51 polarized-light optical microscope (POM) equipped with a charge-coupled device (CCD) digital camera and a Linkam THMS-600 heating stage was used to detect the crystallization of 1-chloronaphthalene and 5CB. The sample (3 µL) was putted in the middle of two slides. The sample was cooled at a rate of 5 K/min. EPR measurements EPR spectra were recorded on an EPR spectrometer (CIQTEK EPR200-Plus) with a continuous-wave X-band frequency (~ 9.7 GHz). Y₂@C₇₉N was completely dissolved in 1-chloronaphthalene and 5CB at a concentration of 3.5×10⁻⁴ M, ensuring thorough dissolution and uniform dispersion of the metallofullerene molecules. Then, transfer 80 µL of the solution to the bottom of a quartz tube. To probe the phase transition of aromatic materials using the molecular spin of Y 2 @C 79 N, the Variable-temperature EPR signals were recorded. Variable-temperature EPR experiments were conducted using the EPR200-Plus electron paramagnetic resonance spectrometer equipped with a liquid nitrogen cooling system to achieve low temperatures. Each temperature was maintained for 10–15 minutes to ensure that the actual sample temperature equilibrated with the temperature detected by the sensor, Subsequently, the EPR signals were recorded. The detailed test procedure is described in Supplementary Information. Statistical Information Statistics were performed either with OriginPro 2021 software. Calculation methods The calculations were performed using the Vienna ab initio simulation package (VASP) 45 – 47 and the Perdew-Burke-Ernzerhof form of the generalized gradient approximation (GGA-PBE) 48 functional was employed to obtain the exchange and correlation terms. The optimization was performed with an energy cutoff of 520 eV, using a k-mesh of 1x1x1 and achieving energy and force convergence of 1x10 − 5 eV and 0.02 eV/Å, respectively. Molecular dynamics simulations were conducted utilizing the GROMACS simulation package, version 2022.4. 49 The simulations were executed for a duration of 100 ps, with an integration time step precisely set to 0.001 ps. Each simulation was carried out at an isothermal condition, with the temperature rigorously maintained at a constant value of 300 K. To achieve temperature coupling, the v-rescale algorithm was employed. 50 Throughout all simulations, periodic boundary conditions were consistently applied in the three-dimensional space. The cutoff distance for nonbonded interactions was established at 1.0 nm. For the treatment of long-range electrostatic interactions, the particle mesh Ewald (PME) method was implemented. 51 The simulations were facilitated by the Universal Force Field (UFF), 52 which provided the force field parameters. Declarations Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgements This work was supported by the National Natural Science Foundation of China (52472041, 52022098), and the Beijing National Laboratory for Condensed Matter Physics (2023BNLCMPKF004). We also thanks the Analysis & Testing Center, BIT. Author contributions T.W. conceived the research and supervised the project. L.L. and Y.Z. performed the material preparations, magnetic measurements. C.Z. and Z.Z. performed the DFT calculations. C.W. co-supervised the project. All authors discussed the results and commented on the manuscript. Conflict of Interest The authors declare no competing financial interest. References Bayer M. All for one and one for all. Science 364, 30-31 (2019). Gaita-Ariño A, Luis F, Hill S, Coronado E. Molecular spins for quantum computation. Nature Chemistry 11, 301-309 (2019). Laorenza DW, Freedman DE. Could the Quantum Internet Be Comprised of Molecular Spins with Tunable Optical Interfaces? 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Effects of surface morphology on the anchoring and electrooptical dynamics of confined nanoscale liquid crystalline films. Journal of the American Chemical Society 124, 15020-15029 (2002). Noble-Luginbuhl AR, Blanchard RM, Nuzzo RG. Surface Effects on the Dynamics of Liquid Crystalline Thin Films Confined in Nanoscale Cavities. Journal of the American Chemical Society 122, 3917-3926 (2000). Oladepo SA. Temperature-dependent fluorescence emission of 4-cyano-4′-pentylbiphenyl and 4-cyano-4′-hexylbiphenyl liquid crystals and their bulk phase transitions. Journal of Molecular Liquids 323, (2021). Porter D, Savage JR, Cohen I, Spicer P, Caggioni M. Temperature dependence of droplet breakup in 8CB and 5CB liquid crystals. Physical Review E 85, (2012). Sandström D, Levitt MH. Structure and Molecular Ordering of a Nematic Liquid Crystal Studied by Natural-Abundance Double-Quantum 13 C NMR. Journal of the American Chemical Society 118, 6966-6974 (1996). Smart CL, Cortese AJ, Ramshaw BJ, McEuen PL. Nanocalorimetry using microscopic optical wireless integrated circuits. Proceedings of the National Academy of Sciences 119, (2022). Zhang J, Su J, Guo H. An Atomistic Simulation for 4-Cyano-4′-pentylbiphenyl and Its Homologue with a Reoptimized Force Field. The Journal of Physical Chemistry B 115, 2214-2227 (2011). Mansaré T, Decressain R, Gors C, Dolganov VKJMC, Crystals L. Phase Transformations And Dynamics Of 4-Cyano-4′-Pentylbiphenyl (5CB) By Nuclear Magnetic Resonance, Analysis Differential Scanning Calorimetry, And Wideangle X-Ray Diffraction Analysis. 382, 111 - 197 (2002). Kresse G, Furthmüller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 6, 15-50 (1996). Kresse G, Furthmüller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B 54, 11169-11186 (1996). 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Supplementary Files NCSI20250220.docx Supplementary Information Molecular Spin Sensor for In-Situ Monitoring of Crystallization Behavior and Phase Transition in Aromatic Materials Cite Share Download PDF Status: Published Journal Publication published 04 Aug, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6145981","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":430106388,"identity":"b7a8c2ec-d4da-4b58-9111-4ff9c912675c","order_by":0,"name":"Taishan Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3ElEQVRIie3PMQuCQBTA8TuCWoTW59B3EA5EcPCreEsu4uwQcRLolqvQl2hqvhBsUVqFGtxruKkxOpfG69qC7j+9g/eDdwiZTL8boDlgJoeJJuCS2BXO2DcEIadHWI84p66+iZUXkMumGFDqUzbruJq0ydLjDdDD9SgPayPKrCRUEpfHrsOnELo9zRjOa8rActTkfJfkCQGpRvLUIX1MhmMOeA8jYRok6O8u6rZAK/mXKmwikluxmthlTET6WAfzXTEIsfIX5axVE9kU3mM4Pj/tyyZCY8lkMpn+uRe6UEvqj7sJ+gAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-1834-3610","institution":"Beijing Institute of Technology","correspondingAuthor":true,"prefix":"","firstName":"Taishan","middleName":"","lastName":"Wang","suffix":""},{"id":430106389,"identity":"12c77182-62be-4af9-82fb-c26f0f155faf","order_by":1,"name":"Linshan Liu","email":"","orcid":"","institution":"Institute of Chemistry, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Linshan","middleName":"","lastName":"Liu","suffix":""},{"id":430106390,"identity":"2f1002d6-08dc-424a-a28d-e9e1728f49b9","order_by":2,"name":"Chong Zhao","email":"","orcid":"","institution":"East China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Chong","middleName":"","lastName":"Zhao","suffix":""},{"id":430106391,"identity":"0814cbe9-d48c-4c9c-acc5-1b73b9938c28","order_by":3,"name":"Yingjian Zhang","email":"","orcid":"","institution":"Beijing Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Yingjian","middleName":"","lastName":"Zhang","suffix":""},{"id":430106392,"identity":"e1a8ba28-e07b-48a6-bab9-e6599fd2ef5c","order_by":4,"name":"Zhuxia Zhang","email":"","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhuxia","middleName":"","lastName":"Zhang","suffix":""},{"id":430106393,"identity":"a53ae1f0-099b-4c1f-a788-3845f8053ff9","order_by":5,"name":"Chunru Wang","email":"","orcid":"https://orcid.org/0000-0001-7984-6639","institution":"Institute of Chemistry, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Chunru","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-03-03 11:56:37","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6145981/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6145981/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-62649-2","type":"published","date":"2025-08-04T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":78802623,"identity":"e376ca29-e6a2-430e-9401-af9a936af908","added_by":"auto","created_at":"2025-03-19 07:05:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":329591,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure and schematic illustration. a\u003c/strong\u003e DFT-optimized molecular structure of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN. \u003cstrong\u003eb\u003c/strong\u003e Calculated spin density distributions of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN. \u003cstrong\u003ec\u003c/strong\u003e Schematic of the interaction between the N-substituted region in C\u003csub\u003e79\u003c/sub\u003eN cage and benzene ring. \u003cstrong\u003ed \u003c/strong\u003eSchematic of the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in \u0026nbsp;crystalline aromatic materials.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/084d57471cb961ff2e41becf.png"},{"id":78802497,"identity":"b2ec4fd8-6c48-4fcb-be82-8ae5c5d95a2e","added_by":"auto","created_at":"2025-03-19 06:57:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":356516,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMolecular spin sensor for 1-chloronaphthalene. a\u003c/strong\u003e Schematic diagram of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in the disordered and ordered matrix of 1-chloronaphthalene. \u003cstrong\u003eb\u003c/strong\u003e Temperature-dependent EPR spectra of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene ranging from 290 to 90 K. The EPR measurement frequency is 9.7033 GHz, the continuous-wave power is 0.63 mW, and the intensity is 25 dB. \u003cstrong\u003ec\u003c/strong\u003e Linewidth changes for the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene under varied temperatures from 290 to 230 K. T\u003csub\u003e1\u003c/sub\u003e and T\u003csub\u003e2\u003c/sub\u003e denote the transition temperatures of 245 and 230 K, respectively. \u003cstrong\u003ed\u003c/strong\u003e In-situ temperature-dependent XRD characterizations on 1-chloronaphthalene under varied temperatures. \u003cstrong\u003ee\u003c/strong\u003e In-situ polarized-light optical microscope (POM) images of 1-chloronaphthalene under varied temperatures. \u003cstrong\u003ef\u003c/strong\u003e Experimental EPR spectrum of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene measured at 130 K, and correspondingly simulated EPR spectrum.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/cf700574f19f00005a291458.png"},{"id":78802624,"identity":"b945b21b-1b8e-4edf-bca9-8b8bab61e323","added_by":"auto","created_at":"2025-03-19 07:05:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":410428,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMolecular spin sensor for liquid crystal 5CB. a\u003c/strong\u003e Schematic diagram of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB from nematic to crystalline states. 5CB molecules are colored green. \u003cstrong\u003eb\u003c/strong\u003e Temperature-dependent EPR spectra of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB ranging from 320 to 130 K. The EPR measurement frequency is 9.5399 GHz, continuous-wave power is 0.63 mW, and intensity is 25 dB. \u003cstrong\u003ec\u003c/strong\u003e Temperature dependence of the linewidth of EPR signals for Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB. T\u003csub\u003e1\u003c/sub\u003e and T\u003csub\u003e2\u003c/sub\u003e denote the transition temperatures of 290 and 270 K, respectively. \u003cstrong\u003ed\u003c/strong\u003e Temperature-dependent XRD characterizations on 5CB. \u003cstrong\u003ee\u003c/strong\u003e Temperature-dependent polarized-light optical microscopy images of 5CB. \u003cstrong\u003ef\u003c/strong\u003e Experimental EPR spectrum of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB measured at 130 K, and correspondingly simulated EPR spectrum.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/0aea48e3e707d96b386e9b6c.png"},{"id":78802625,"identity":"3626e6a9-f275-446e-89f2-e7f5425f11ac","added_by":"auto","created_at":"2025-03-19 07:05:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":298821,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMolecular dynamics simulations. a–d\u003c/strong\u003e Four typical conformations of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in crystalline 5CB and their relative energies. \u003cstrong\u003ee\u003c/strong\u003e Molecular dynamics simulation (100 ps) of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in crystalline 5CB and statistical analyses of the distance changes between the nitrogen atom and the center of a benzene ring in 5CB.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/04ceb558dcd4028d14b0c451.png"},{"id":88311883,"identity":"1c90670e-62ff-4132-b6eb-4ee87592cb54","added_by":"auto","created_at":"2025-08-05 07:07:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2053901,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/7fc8e142-d994-4a2c-8325-a152a812b760.pdf"},{"id":78802499,"identity":"9c74d071-89e6-404d-9cf0-0b48e65a3f47","added_by":"auto","created_at":"2025-03-19 06:57:07","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":824119,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information\u003c/p\u003e\n\u003cp\u003eMolecular Spin Sensor for In-Situ Monitoring of Crystallization Behavior and Phase Transition in Aromatic Materials\u003c/p\u003e","description":"","filename":"NCSI20250220.docx","url":"https://assets-eu.researchsquare.com/files/rs-6145981/v1/9e74f9bd67bf1d4eda8ed068.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Molecular Spin Sensor for In-Situ Monitoring of Crystallization Behavior and Phase Transition in Aromatic Materials","fulltext":[{"header":"Introduction","content":"\u003cp\u003eRecently, the use of electron spins in advanced science and technology has been actively studied owing to their quantum characteristics that can result in the development of new functionalities\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Among these studies, molecular spins have attracted considerable attention because these systems exhibit unique features and advantages\u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, including spin preparation and modulation via chemical engineering, spin sensing, and measurements at the molecular level. Therefore, exploring the molecular spin properties and functions is essential to advance related applications.\u003c/p\u003e \u003cp\u003eMetallofullerenes with electron spin characteristics, such as Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, Sc\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e\u003csup\u003e12\u003c/sup\u003e, Y@C\u003csub\u003e82\u003c/sub\u003e\u003csup\u003e13\u003c/sup\u003e, and Gd\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, are emerging as molecular spin materials that have characteristic electron paramagnetic resonance (EPR) signals with potential applications in diverse fields including spin sensors and quantum information processing. Among them, the aza-metallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN has an electron spin localized on the internal cluster and shows distinct EPR signals\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Previously, we revealed that the internal spin in Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN can sense external environmental conditions, such as cage modification\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e and orientation\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. We investigated the spin modulation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN by encapsulating it within the pores of a metal\u0026ndash;organic framework (MOF) of MOF-177 and revealed its orientation sensitivity for Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Additionally, we studied the spin modulation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN when incorporated into molecular nanorings, highlighting its orientation sensitivity within supramolecular systems\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Our findings reveal the characteristic orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in organic materials containing phenyl groups, demonstrating the spin capacity to sensitively detect buckyball orientation. Therefore, further exploration of the spin-sensing function of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN corresponding to the molecular orientation is vital.\u003c/p\u003e \u003cp\u003eOrganic molecular orientation is widely observed in processes such as crystallization, phase transition, and molecular assembly\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. For example, in organic solar cells (OSCs), the short range crystallization of donor/acceptor materials has been reported to be significant\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. To enhance the device performance of OCSs, 1-chloronaphthalene is often used as a solvent additive because it improves the degree of order of the polymer films. This occurs because the crystallization of 1-chloronaphthalene can induce the orientation and rearrangement of polymers\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Additionally, crystallization-related phase transitions can also be found in liquid crystal (LC) materials\u003csup\u003e\u003cspan additionalcitationids=\"CR28 CR29\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, which have attracted considerable interest because of their wide practical applications in displays, innovative sensors, and digital nonvolatile memory devices. These results indicate that studies on the crystallization and phase transitions corresponding to the molecular orientation of organic materials are essential. Presently, the molecular orientation in crystallization and phase transition is widely characterized by X-ray diffraction (XRD). However, the XRD technique cannot effectively identify light atoms such as C, H and O, which are the dominant components in molecular materials. Thus, exploring other methods to probe the molecular orientation more precisely is vital.\u003c/p\u003e \u003cp\u003eHerein, we selected the aforementioned functional organic materials, including 1-chloronaphthalene and a liquid crystal material of 4-cyano-4\u0026prime;-pentyl-biphenyl (5CB), to explore the spin-sensing function of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN for the crystallization behavior and phase transitions of aromatic materials. For comparison, X-ray diffraction (XRD) and polarized-light optical microscopy (POM) imaging were employed to illustrate the relationship between the spin signals and compound crystallization. These results reveal that the Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN spin can be developed as a molecular spin sensor to monitor the phase transition of aromatic materials in situ and in real time.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003eAza-metallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN, in which one carbon atom of the cage is substituted with a nitrogen atom, has an electron spin localized on the internal Y\u003csub\u003e2\u003c/sub\u003e cluster (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). In solution, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN shows three distinct groups of EPR signals owing to the hyperfine couplings between the spin and the two Y nuclei (\u003cem\u003eI\u003c/em\u003e\u003csub\u003eY\u003c/sub\u003e = 1/2). Notably, the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in solution is disordered. However, the selective orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN can be achieved by encapsulating it within the pores that contain phenyl groups. Our previous findings reveal the characteristic orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in aromatic materials containing phenyl groups. This orientation is facilitated by π\u0026ndash;π interactions between the N-substituted region of the fullerene cage and the benzene unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The crystallization of selected organic materials (1-chloronaphthalene and 5CB) with benzene rings formed an ordered array, which could also induce the Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN orientation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003eFirst, 1-chloronaphthalene, an aromatic compound with a melting point of 253 K, was employed to dissolve Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN, and the temperature-dependent EPR spectra were collected to study the crystallization-induced Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN orientation and spin sensing (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). Subsequently, we measured the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene from 290 to 90 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). EPR spectra were recorded using an EPR spectrometer (CIQTEK EPR200-Plus) with a continuous-wave X-band frequency. From 290 to 245 K, the signal intensity gradually increased owing to weakened spin\u0026ndash;lattice interactions; it reached its peak at 245 K (Fig. S2). From 245 to 230 K, the signal intensity began to decrease because of the restricted motion of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN owing to the coagulation of 1-chloronaphthalene. From 290 to 230 K, the intensity of the EPR signal at a high magnetic field has a higher enhancement than that at a low magnetic field owing to paramagnetic anisotropy. This is because the two Y nuclei and spin have restricted motion at low temperatures; consequently, insufficient rotational averaging occurs in the resonance structure. Notably, a new group of EPR signals was observed at 225 K.\u003c/p\u003e \u003cp\u003eAccording to our previous EPR studies on Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN, this new group of EPR signals can be ascribed to the axisymmetric EPR signals, which are caused by the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene. The intensity of this axisymmetric group of EPR signals gradually increased when the temperature was decreased to 90 K. These results show thast at low temperatures, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN orientates more in 1-chloronaphthalene.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe linewidths of the EPR signals were analyzed to determine the motion states of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene at varied temperatures. Generally, the linewidth of an EPR signal is related to the rotation of the spin molecule. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the linewidth changes for the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene at temperatures ranging from 290 to 230 K. For Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene, the linewidths of the EPR signals have a minimum value at 250 K. From 245 to 230 K, the linewidths of the EPR signals began increasing because of the restricted motion of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN owing to the coagulation of 1-chloronaphthalene and restricted motion of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN. This transition point coincides with the melting point of 1-chloronaphthalene (253 K). These results indicate that the spin of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN can effectively sense the coagulation of 1-chloronaphthalene.\u003c/p\u003e \u003cp\u003eIn addition, the linewidths for the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene show a dramatic rise at 230 K, below which Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene shows axisymmetric EPR signals. As reported previously, the axisymmetric EPR signals revealed Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN orientation in 1-chloronaphthalene. Moreover, as noted earlier, 1-chloronaphthalene is often used as a solvent additive for solar cells because it induces a short-range regular arrangement of the polymers owing to crystallization. Therefore, we propose that the Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene below 230 K also has a crystallization-induced orientation.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMelting and crystallization transition points of 1-chloronaphthalene obtained by several methods\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMelting point\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCrystallization point\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEPR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e245 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e230 K\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eXRD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e230 K\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePOM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e233 K\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWe performed in-situ temperature-dependent XRD characterizations of 1-chloronaphthalene (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) to further investigate the relationship between the EPR signals and compound crystallization. In-situ XRD is an experimental method useful for detecting material crystallization. We found that 1-chloronaphthalene shows distinct diffraction peaks at 230 K, indicating the beginning of compound crystallization. Notably, the crystallization temperature of 1-chloronaphthalene obtained by XRD is close to the transition point of the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN. These results further demonstrate the relationship between the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN and compound crystallization.\u003c/p\u003e \u003cp\u003eMoreover, polarized light optical microscopy (POM) was used to characterize the crystallization of 1-chloronaphthalene at various temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). The crystal morphology was first observed at 233 K for 1-chloronaphthalene, indicating the beginning of compound crystallization. This transition temperature at the beginning of the compound crystallization is also close to the transition point for the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN at 230 K. In addition, when the temperature was further decreased, 1-chloronaphthalene showed a more obvious crystal morphology, indicating that it crystallizes better at low temperatures. These POM imaging results are consistent with EPR data, where Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN shows more prominent axisymmetric signals at low temperatures. The results further confirm the relationship between the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN and compound crystallization. Additionally, these results show that the spin of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN is sensitive to compound crystallization, demonstrating its potential application as a molecular spin sensor.\u003c/p\u003e \u003cp\u003eBased on these results, the Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN spin was employed to sense the crystallization-related phase transitions of a liquid crystal material 5CB (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Liquid-crystal (LC) materials have attracted considerable research interest owing to their promising practical applications in displays, flat panels, innovative sensors, and digital nonvolatile memory devices. 5CB belongs to a class of thermotropic liquid crystals, and is one of the most widely studied liquid crystal materials\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan additionalcitationids=\"CR32 CR33 CR34 CR35 CR36 CR37 CR38 CR39 CR40 CR41 CR42\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. It has been established that 5CB has an isotropic-nematic transition temperature of approximately 308 K and a nematic-crystalline transition temperature of approximately 297 K. Thus, we measured the temperature-dependent EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN dissolved in 5CB at temperatures ranging from 320 to 130 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and S3).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor EPR studies, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB shows three groups of EPR signals with higher signal intensity at high field than at low field between 320 and 290 K. This reveals the anisotropic EPR spectra resulting from the restricted motion of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in the nematic phase of 5CB. As shown in Fig. S2, from 320 to 290 K, the EPR signal intensity of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB reached its peak at 290 K owing to the weakened spin\u0026ndash;lattice interaction at low temperatures. Notably, the EPR signal intensity of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB decreased below 290 K. According to the literature, the nematic-crystalline phase transition of 5CB occurs at approximately 297 K, below which the 5CB begins to crystallize. Therefore, it can be deduced that below 290 K, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in crystalline 5CB has strong spin\u0026ndash;lattice interactions.\u003c/p\u003e \u003cp\u003eThe temperature-dependent linewidths of the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB were then analyzed. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec shows the temperature dependence of linewidths for the EPR signals at varied temperatures. For Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB, the linewidths of the EPR signals have a minimum value at 295 K. The linewidths of all EPR signals gradually decrease when the temperature decreases from 320 to 295 K owing to the weakened spin\u0026ndash;lattice interactions of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in nematic 5CB at low temperatures. From 290 K, the linewidths of the EPR signals began to increase when the temperature decreased. It has been established that 5CB has a nematic-crystalline transition temperature of approximately 297 K. Therefore, these results demonstrate the relationship between the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN and 5CB crystallization. When the temperature was decreased to 270 K, the linewidths for the EPR signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB show a dramatic rise. Under 270 K, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB shows relatively axisymmetric EPR signals, indicating the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN, as discussed earlier. From 270 to 230 K, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB exhibited similar EPR signals. When the temperature was decreased to 225 K, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB clearly showed a group of axisymmetric EPR signals, which revealed a crystalline phase change of 5CB. Moreover, from 225 K to 130 K, the intensity of the axisymmetric EPR signals gradually increased, revealing the significant effect of crystalline 5CB on the spin resonance of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN.\u003c/p\u003e \u003cp\u003eThe EPR spectra of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB were simulated to obtain the hyperfine coupling constants (a) and g factors. The simulated EPR spectra were obtained using the Easyspin package (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.easyspin.org\u003c/span\u003e\u003cspan address=\"http://www.easyspin.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), encoded on MATLAB. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed shows the experimental EPR spectrum of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB measured at 130 K, and the correspondingly simulated EPR spectrum with parameters of a\u003csub\u003e\u0026perp;\u003c/sub\u003e= 78.21 G, a\u003csub\u003e∥\u003c/sub\u003e= 92.14 G, g\u003csub\u003e\u0026perp;\u003c/sub\u003e= 1.952, and g\u003csub\u003e∥\u003c/sub\u003e= 1.993. The simulated EPR spectrum exhibited axisymmetric parameters, further demonstrating the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in crystalline 5CB.\u003c/p\u003e \u003cp\u003eTo further investigate the relationship between the EPR signals and 5CB crystallization, we performed in-situ temperature-dependent XRD characterization of 5CB from 300 to 240 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee). 5CB exhibits distinct XRD peaks at 270 K, revealing that 5CB crystallizes at this point. Notably, the axisymmetric EPR signals for Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB was also clearly observed at 270 K. These results further demonstrated the crystallization-induced Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN orientation in the crystalline phase of 5CB. Additionally, from 270 to 240 K, the XRD patterns of 5CB showed similar peaks without variations in the EPR spectra. POM was used to characterize the crystallization of 5CB at various temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). A crystal morphology was discovered at approximately 308 K for 5CB, revealing its nematic phase. From 308 to 260 K, the POM images of 5CB were similar. From 255 K, the POM images of 5CB show a changed crystal morphology, indicating a change in the crystalline phase.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTemperature-dependent crystalline phase transition points of 5CB obtained by several methods.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNematic\u0026rarr;Crystalline\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eC\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[A]\u003c/sup\u003e\u0026rarr;C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb[B]\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e\u0026rarr;C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003ea[C]\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eRef.\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEPR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e290 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e270 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e230 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eXRD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e270 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePOM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e255 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026mdash;\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e297 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e~\u0026thinsp;260 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;230 K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e35\u0026ndash;36, 44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTemperature-dependent phase transitions of 5CB have been reported using several methods, such as differential scanning calorimetry (DSC), fluorescence spectroscopy, X-ray diffraction, Raman spectroscopy, and proton nuclear magnetic resonance (NMR). Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e lists the temperature transition points of 5CB in different crystalline states obtained by DSC, EPR, XRD, and POM. The DSC method was employed to systematically study the phase transitions of 5CB, which showed repeated transition points. In the DSC results, the exothermic peak at approximately 260 K can be attributed to the transition from the C\u003csub\u003e2\u003c/sub\u003e crystalline phase to the metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e phase. Additionally, the exothermic peak at approximately 230 K can be attributed to the transition from the metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e crystalline phase to the C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e phase. The XRD and POM results for 5CB show distinct XRD peaks at 270 K and a change in crystal morphology at 255 K, respectively, which can be attributed to the transition from the C\u003csub\u003e2\u003c/sub\u003e crystalline phase to the metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e phase. However, the transition from the metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e crystalline phase to the C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e phase was not observed in the XRD or POM results. Notably, two marked changes in the EPR results were observed at 270 and 230 K, which are consistent with the two corresponding phase transition points in the DSC results. These results show that the spin of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN is sensitive to 5CB crystallization, demonstrating its potential for application as a molecular spin sensor.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDensity functional theory (DFT) calculations were performed to further analyze the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in crystalline 5CB. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea\u0026ndash;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, we assessed the thermodynamic stability of the four typical sites of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN within the 5CB crystal lattice. The most favorable structure was obtained when the nitrogen atom of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN was positioned near the benzene ring of 5CB, and the two Y atoms were in proximity to the 5CB molecule. Concurrently, molecular dynamics (MD) simulations were employed to statistically analyze the changed distances (d) between the nitrogen atom on the carbon cage and the center of a benzene ring in 5CB. For initial conformation, the nitrogen atom was placed at a distance (\u003cem\u003ed\u003c/em\u003e) far away from the center of the benzene ring of 5CB (approximately 9.2 \u0026Aring;). Following MD simulations, the distance (\u003cem\u003ed\u003c/em\u003e) between the nitrogen atom and the center of the benzene rings stabilizes at an average of 3.28 \u0026Aring; after 35 ps (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). These results suggest that Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN exhibits distinct orientation characteristics in crystalline 5CB.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study reported a molecular spin senor of metallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN for in-situ monitoring of the crystallization behavior and phase transition in aromatic materials. Two functional aromatic materials, including 1-chloronaphthalene and a liquid crystal material of 5CB, were selected to illustrate this spin-sensing function of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN.\u003c/p\u003e \u003cp\u003eTemperature-dependent EPR results of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN dissolved in these two aromatic materials were analyzed. The EPR results indicated that Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene exhibits two distinct changes for EPR signals at 250 and 230 K, which correspond to the melting and crystallization points of 1-chloronaphthalene, respectively. For Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in a liquid crystal of 5CB, it exhibits three distinct changes for EPR signals at 290, 270, and 230 K, corresponding to the nematic-crystalline transition, C\u003csub\u003e2\u003c/sub\u003e crystalline to metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e crystalline phase transition, and metastable C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e crystalline to the C\u003csub\u003e1\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e crystalline phase transition. These results show that the spin of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN can sense the crystallization-related phase transitions of these aromatic materials. In addition, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN dissolved in these aromatic materials exhibited EPR signals that changed from isotropic to axisymmetric patterns below their crystallization points owing to the orientation of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in these crystalline materials.\u003c/p\u003e \u003cp\u003eMoreover, Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN was found to be capable of sensing most phase changes of a liquid crystal material of 5CB at 290, 270, and 230 K. Conversely, the XRD method can only detect one phase transition at 270 K. XRD, a common method for characterizing orientation, struggles to detect light atoms like C, H, and O, underscoring the need for more precise probing techniques. This susceptible spin character of azametallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN has potential applications for monitoring the crystallization behavior and phase transitions of certain aromatic materials with high precision. In particular, this molecular spin probe can penetrate into the crystal lattice of aromatic materials and reflect the phase transitions in situ. The utilization of molecular spin to detect the crystallization and phase transitions can be developed into a transformative technology in the future.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003eSynthesis, isolation and characterizations of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN\u003c/h2\u003e\n \u003cp\u003eMetallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN was synthesized by the arc-discharge method. Briefly, Y/Ni\u003csub\u003e2\u003c/sub\u003e alloy and graphite powder were mixed in a mass ratio of 3:1 and filled into the hollow graphite rod. The furnace was vacuumed and filled with 190 Torr He and 10 Torr N\u003csub\u003e2\u003c/sub\u003e. Then the graphite rods were evaporated in a DC arc discharge furnace with a current of 130 A to get the carbon soot containing various fullerenes and metallofullerenes. The carbon soot was subjected to Soxhlet extraction in toluene for 12 h. Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN was isolated and purified by multi-step high performance liquid chromatography (HPLC) on Buckyprep and Buckyprep-M column (20 mm \u0026times; 250 mm) with toluene as the mobile phase. The purity of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS, AXIMA Assurance, Shimadzu).\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eXRD measurements\u003c/h3\u003e\n\u003cp\u003eThe PANalytical Empyrean X-ray diffractometer was employed for the characterization of samples. A volume of 100 \u0026micro;L of the solvent was placed in a custom-designed sample cell and positioned on the sample stage for analysis. To achieve temperature control, a low-temperature nitrogen gas was used to establish an equilibrium atmosphere. A temperature probe was positioned in close proximity to the sample surface to monitor the environmental temperature. Spectra were acquired for each sample at different temperatures, with a stabilization period of ten minutes at each temperature.\u003c/p\u003e\n\u003ch3\u003ePOM measurements\u003c/h3\u003e\n\u003cp\u003eAn Olympus BX51 polarized-light optical microscope (POM) equipped with a charge-coupled device (CCD) digital camera and a Linkam THMS-600 heating stage was used to detect the crystallization of 1-chloronaphthalene and 5CB. The sample (3 \u0026micro;L) was putted in the middle of two slides. The sample was cooled at a rate of 5 K/min.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eEPR measurements\u003c/h2\u003e\n \u003cp\u003eEPR spectra were recorded on an EPR spectrometer (CIQTEK EPR200-Plus) with a continuous-wave X-band frequency (~\u0026thinsp;9.7 GHz). Y₂@C₇₉N was completely dissolved in 1-chloronaphthalene and 5CB at a concentration of 3.5\u0026times;10⁻⁴ M, ensuring thorough dissolution and uniform dispersion of the metallofullerene molecules. Then, transfer 80 \u0026micro;L of the solution to the bottom of a quartz tube. To probe the phase transition of aromatic materials using the molecular spin of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN, the Variable-temperature EPR signals were recorded. Variable-temperature EPR experiments were conducted using the EPR200-Plus electron paramagnetic resonance spectrometer equipped with a liquid nitrogen cooling system to achieve low temperatures. Each temperature was maintained for 10\u0026ndash;15 minutes to ensure that the actual sample temperature equilibrated with the temperature detected by the sensor, Subsequently, the EPR signals were recorded. The detailed test procedure is described in Supplementary Information.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eStatistical Information\u003c/h3\u003e\n\u003cp\u003eStatistics were performed either with OriginPro 2021 software.\u003c/p\u003e\n\u003ch3\u003eCalculation methods\u003c/h3\u003e\n\u003cp\u003eThe calculations were performed using the Vienna ab initio simulation package (VASP)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e and the Perdew-Burke-Ernzerhof form of the generalized gradient approximation (GGA-PBE)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e functional was employed to obtain the exchange and correlation terms. The optimization was performed with an energy cutoff of 520 eV, using a k-mesh of 1x1x1 and achieving energy and force convergence of 1x10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV and 0.02 eV/\u0026Aring;, respectively.\u003c/p\u003e\n\u003cp\u003eMolecular dynamics simulations were conducted utilizing the GROMACS simulation package, version 2022.4.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e The simulations were executed for a duration of 100 ps, with an integration time step precisely set to 0.001 ps. Each simulation was carried out at an isothermal condition, with the temperature rigorously maintained at a constant value of 300 K. To achieve temperature coupling, the v-rescale algorithm was employed.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e Throughout all simulations, periodic boundary conditions were consistently applied in the three-dimensional space. The cutoff distance for nonbonded interactions was established at 1.0 nm. For the treatment of long-range electrostatic interactions, the particle mesh Ewald (PME) method was implemented.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e The simulations were facilitated by the Universal Force Field (UFF),\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e which provided the force field parameters.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eData availability\u003c/h2\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (52472041, 52022098), and the Beijing National Laboratory for Condensed Matter Physics (2023BNLCMPKF004). We also thanks the Analysis \u0026amp; Testing Center, BIT.\u003c/p\u003e\n\u003ch2\u003eAuthor contributions\u003c/h2\u003e\n\u003cp\u003eT.W. conceived the research and supervised the project. L.L. and Y.Z. performed the material preparations, magnetic measurements. C.Z. and Z.Z. performed the DFT calculations. C.W. co-supervised the project. All authors discussed the results and commented on the manuscript.\u003c/p\u003e\n\u003ch2\u003eConflict of Interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBayer M. All for one and one for all. Science 364, 30-31 (2019).\u003c/li\u003e\n\u003cli\u003eGaita-Ari\u0026ntilde;o A, Luis F, Hill S, Coronado E. Molecular spins for quantum computation. Nature Chemistry 11, 301-309 (2019).\u003c/li\u003e\n\u003cli\u003eLaorenza DW, Freedman DE. 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UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations. 114, 10024-10035 (1992).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6145981/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6145981/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSpin-active materials with sensitive electron spin centers have drawn significant attention in quantum sensing due to their unique quantum characteristics. Herein, we report a molecular spin sensor based on metallofullerene Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN for in-situ monitoring of crystallization behavior and phase transitions in aromatic materials with high precision. Two functional aromatic materials, 1-chloronaphthalene and a liquid crystal material of 4-cyano-4\u0026prime;-pentyl-biphenyl (5CB), were strategically selected based on their distinct crystallization behaviors and technological relevance. Temperature-dependent spin resonance signals of Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN dissolved in aromatic materials were analyzed using electron paramagnetic resonance (EPR) spectroscopy. For Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 1-chloronaphthalene, two distinct EPR signal changes were observed at 250 and 230 K, corresponding to its melting and crystallization points, respectively. For Y\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e79\u003c/sub\u003eN in 5CB, three distinct EPR signal changes were observed at 290, 270, and 230 K that correspond to its crystallization-related phase transitions, significantly outperforming conventional XRD analysis which only detected the 270 K transition. Experimental results combining theoretical calculations reveal that the sensing mechanism originates from crystallization-induced alignment of fullerene molecular orientation within the host matrix. This work establishes metallofullerene-based spin probes as a powerful analytical tool for real-time monitoring of molecular ordering processes in aromatic materials, offering superior sensitivity compared to conventional characterization methods. The demonstrated quantum sensing paradigm opens new possibilities for studying fundamental phase transition phenomena.\u003c/p\u003e","manuscriptTitle":"Molecular Spin Sensor for In-Situ Monitoring of Crystallization Behavior and Phase Transition in Aromatic Materials","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-19 06:57:03","doi":"10.21203/rs.3.rs-6145981/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0970d8e0-3f4e-4a87-98ea-7235d1fbe119","owner":[],"postedDate":"March 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":45809811,"name":"Physical sciences/Nanoscience and technology/Techniques and instrumentation/Characterization and analytical techniques"},{"id":45809812,"name":"Physical sciences/Materials science/Nanoscale materials/Magnetic properties and materials"}],"tags":[],"updatedAt":"2025-08-05T07:07:27+00:00","versionOfRecord":{"articleIdentity":"rs-6145981","link":"https://doi.org/10.1038/s41467-025-62649-2","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-08-04 04:00:00","publishedOnDateReadable":"August 4th, 2025"},"versionCreatedAt":"2025-03-19 06:57:03","video":"","vorDoi":"10.1038/s41467-025-62649-2","vorDoiUrl":"https://doi.org/10.1038/s41467-025-62649-2","workflowStages":[]},"version":"v1","identity":"rs-6145981","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6145981","identity":"rs-6145981","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}