Crystal Symmetry-Driven Spin-Optical Dynamics in Cr ³⁺ Molecular Spins

Molecular rubies show promise toward applications in quantum information science (QIS), from networking to sensing and electronics. While <i>S</i> = 3/2 Cr³⁺ ions satisfy the requirements for realizing a spin-optical interface, there is a lack of information on their combined optical and spin relaxation properties. In this work, we study the effect of the matrix on both the spin and optical properties of prototypical octahedral (<i>O</i>_<i>h</i>) Cr(acac)₃ (acac = acetylacetonate) in a series of diamagnetic M³⁺ hosts: Cr_<i>x</i>M_1-<i>x</i>(acac)₃ (M = Al [<b>1</b>]; Ga [<b>2</b>]; In [<b>3</b>]; Sc [<b>4</b>]; Co [<b>5</b>]; Rh [<b>6</b>]). X-ray diffraction confirms that the metal identity of the diluent influences the crystal symmetry of the host: monoclinic <i>P</i>2₁/c (<b>1, 2, 5, 6</b>) and orthorhombic <i>Pbca</i> (<b>3</b>,<b>4</b>). Continuous-wave (cw)-EPR spectroscopy reflects that the space group of the host dictates the ground-state spin Hamiltonian parameters. Pulse EPR spectroscopy shows that spin-lattice relaxation <i>T</i>₁ depends on space group symmetry. Confocal microscopy photoluminescence confirms the low-temperature crystal symmetry of Cr(acac)₃ within the diluent matrices. These results show that Cr(acac)₃ hosts the requisite electronic structure for optical initialization and readout below 4 K, making this class of biocompatible ions primed for cryogenic nanoscale sensing.