Evaluation of Intermolecular Weak Binding Through the Zero-Field Splitting of an Endofullerene High-Spin Probe

Weak binding between molecules is becoming a renowned aspect of molecular engineering, offering an alternative to the tuning of conventional chemical bonds. However, the subtle nature of these interactions poses challenges for characterization using standard techniques. Herein, an efficient method of weak binding characterization based on the zero-field splitting of a high-spin probe by electron paramagnetic resonance is presented. Using endofullerene as the high spin center, we achieved direct comparison of the weak binding strengths in F-MTPP (M = H₂, Zn, Ni, and Pd) and 2F-3MTPP (M = H₂, Ni, Pd, and Pt) through the zero-field splitting parameters. The high-spin resonance spectrum is sensitive to the small variation in the supramolecular structure. By examining fullerene-porphyrin co-crystals with various metal centers and packing arrangements, the relation between intermolecular weak binding and microstructural strain on the fullerene cage is elucidated.