Magnetically Driven Quantum Phase Transition in a Low-Dimensional Pyrazine-Bridged Cu²⁺ Chain Magnet

The magnetic properties and phase diagrams of S = 1/2 quasi-one-dimensional Heisenberg antiferromagnets are well-established with copper-containing coordination polymers as the platform of choice due to their low energy scales and ease of chemical substitution. The inability to uncover orbitally resolved components of the magnetization has, however, been a long-standing barrier to greater understanding of high field spin state transitions. In this work, we combine pulsed field magnetization, optical spectroscopy, and magnetic circular dichroism with complementary electronic structure calculations to unravel orbital-specific contributions to the magnetism in the linear chain quantum magnet [CuL₂(H₂O)₂(pyz)](ClO₄)₂ [L = 5-methyl-2-pyridone; pyz = pyrazine]. In addition to revealing a spin flop and field-driven transition to the fully saturated spin state, we untangle the green → teal color change across the 185 K structural phase transition and employ what we learn about the different Cu²⁺ → pyrazine charge transfer excitations to decompose the magnetic circular dichroism. Analysis reveals that both e_g-derived Cu²⁺ 3d orbitals play a role in the field-driven transition to the fully saturated state, not just those formally hosting unpaired electrons. We attribute the surprisingly strong dichroic signature at room temperature to the presence of uncorrelated spin.