Spectral Hole-Burning Studies of a Mononuclear Eu(III) Complex Reveal Narrow Optical Linewidths of the 5 D 0 → 7 F 0 Transition and Seconds Long Nuclear Spin Lifetimes.

Coordination complexes of rare-earth ions (REI) show optical transitions with narrow linewidths enabling the creation of coherent light-matter interfaces for quantum information processing (QIP) applications. Among the REI-based complexes, Eu(III) complexes showing the 5 D 0 → 7 F 0 transition are of interest for QIP applications due to the narrow linewidths associated with the transition. Herein, we report on the synthesis, structure, and optical properties of a novel Eu(III) complex and its Gd(III) analogue composed of 2,9-bis(pyrazol-1-yl)-1,10-phenanthroline (dpphen) and three nitrate (NO 3 ) ligands. The Eu(III) complex-[Eu(dpphen)(NO 3 ) 3 ]-showed sensitized metal-centred emission ( 5 D 0 → 7 F J ; J=0,1,2,3, 4, 5, or 6) in the visible region, upon irradiation of the ligand-centred band at 369 nm, with the 5 D 0 → 7 F 0 transition centred at 580.9 nm. Spectral hole-burning (SHB) studies of the complex with stoichiometric Eu(III) concentration revealed a narrow homogeneous linewidth (Γ h ) of 1.55 MHz corresponding to a 0.205 μs long optical coherence lifetime (T 2opt ). Remarkably, long nuclear spin lifetimes (T 1spin ) of up to 41 s have been observed for the complex. The narrow optical linewidths and long T 1spin lifetimes obtained for the Eu(III) complex showcase the utility of Eu(III) complexes as tuneable, following molecular engineering principles, coherent light-matter interfaces for QIP applications.