Toward liquid cell quantum sensing: Ytterbium complexes with ultra-narrow absorption

Chemical synthesis and design primarily occur in condensed phase environments, where concentration, solubility, temperature, and polarizability can be tuned to access specific reactivity and functionality. However, in quantum technologies (such as atomic vapor cells used in precision magnetometry), the energetic disorder induced by a fluctuating liquid environment acts in direct opposition to the precise control required for coherence-based sensing. Overcoming fluctuations requires a protected quantum subspace that only weakly interacts with the local environment. We report aferrocene-supported ytterbium complex ((thiolfan)YbCl(THF), thiolfan = 1,1′-bis(2,4-di-tert-butyl-6-thiomethylenephenoxy)ferrocene) that exhibits an extraordinarily narrow absorption linewidth in solution at room temperature with a full-width at half-maximum of 0.625 ± 0.006 meV. Detailed spectroscopic analysis allows us to assign all near infrared transitions to strongly atom-centered f-f transitions, protected from the solvent environment. A combination of density functional theory and multireference methods match experimental transition energies and oscillator strengths, illustrating the role of spin-orbit and asymmetric ligand field in enhancing absorption and pointing toward molecular design principles that create well-protected yet observable electronic transitions in lanthanide complexes. Narrow linewidths allow for a demonstration of extremely low-field magnetic circular dichroism at room temperature, employed to sense and image magnetic fields, down to Earth scale.