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

Spin-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 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.