Heteroleptic Cr⁴⁺ molecular color center candidates

Most quantum sensing experiments require the creation of a spin-optical interface, where optical light can polarize and read out information from spin sublevels. This combination creates a sensor capable of detection down to the single-spin limit. Heteroleptic coordination complexes are promising architectures for enabling positional control of spins in quantum sensing experiments. Towards this end, we use complementary magnetic and optical spectroscopies supported by multiconfigurational electronic structure calculations to establish the first generation of heteroleptic S = 1 Cr⁴⁺ complexes as molecular color center candidates. This family of trigonal bipyramidal tris[2-(trimethylsilylamido)ethyl]amine chromium-halide complexes, referenced here as ^TMStrenCrX (X = F, Cl, Br, and I, respectively known as 1, 2, 3, and 4), exhibits highly modular magnetic and optical properties. 1-4 display largely axial zero-field splitting (ZFS) characterized by high-field, high-frequency continuous-wave electron paramagnetic resonance (HFHF cw-EPR) spectroscopy, where the axial ZFS parameter D continually increases from +5.194(4) cm^-1 (1) to +7.34(1) cm^-1 (4) as halide donor strength weakens. 1-4 also display near-infrared spin-flip photoluminescence (PL) ranging from 959.57(1) to 1043.81(1) nm. These PL spectra exhibit traits critical for enabling optical addressability, such as optical linewidths narrow enough to rival the magnitude of ZFS. A dilute sample of 4 in its Ti⁴⁺ diamagnetic host (4') even displays optically resolvable spin sublevels under zero applied magnetic field; using variable-field PL spectroscopy, we optically determine its ZFS parameters. These results will lead toward the realization of well-defined spin-analyte interactions and subsequent spatial control of molecular color centers, wherein tunability is maintained through the donor at the axial coordination site.