Elucidating Ultranarrow 2F7/2 to 2F5/2 Absorption in Ytterbium(III) Complexes

Achieving ultranarrow absorption linewidths in the condensed phase could enable optical state preparation of specific non-thermal states, a prerequisite for quantum-enabled technologies. The 4f orbitals of lanthanide(III) complexes are often referred to as “atom-like”, reflecting their isolated nature, and are promising substrates for the optical preparation of specific quantum states. To better understand the photophysical properties of 4f states and assess their promise for quantum applications, theoretical building blocks are required for rapid screening. In this study, an atomic-level perturbative calculation (spin-orbit crystal field, SOCF) is applied to various Yb(III) complexes to investigate their linear absorption and emission through a fitting mechanism of their experimentally determined transition energies and oscillator strengths. In particular, the optical properties of (thiolfan)YbCl(THF) (thiolfan = 1,1′-bis(2,4-di-tert-butyl-6-thiomethylenephenoxy)ferrocene), a recently reported complex with a ultranarrow optical linewidth, are computed and compared to those of other Yb(III) compounds. Through a symmetry descent procedure and a transition energy sampling study, major contributors to the optical linewidth are identified. We find that low-symmetry crystal fields, combined with the ultra-high similarity of states resulting from an anisotropic crystal field, create particularly isolated f-f transitions and narrow linewidths. Simultaneously, we find that these atom-like transitions have highly correlated excited-ground energy fluctuations that serve to greatly suppress inhomogeneous line-broadening. This article illustrates how SOCF can be used as a low-cost method to probe the influence of symmetry and ligands on the optical properties of Yb(III) complexes to assist the development of novel lanthanide series quantum materials.