Novel Lanthanide-Based Polymeric Chains and Corresponding Ultrafast Dynamics in Solution

Two types of structurally related one-dimensional coordination polymers were prepared by reacting lanthanide trichloride hydrates [LnCl3·(H2O)m] with dibenzoylmethane (Ph2acacH) and a base. Using cesium carbonate (Cs2CO3) and praseodymium, neodymium, samarium, or dysprosium salts yielded [Cs{Ln(Ph2acac)4}]n (Ln = Pr (1), Nd (2), Sm (3), Dy (4)) in considerable yields. Reaction of potassium tert-butoxide (KOtBu) and the neodymium salt [NdCl3·(H2O)6] with Ph2acacH resulted in [K{Nd(Ph2acac)4}]n (5). All polymers exhibit a heterobimetallic backbone composed of alternating lanthanide and alkali metal atoms which are bridged by the Ph2acac ligands in a linear fashion. ESI-MS investigations on DMF solutions of 1−5 revealed a dissociation of all the five compounds upon dissolution, irrespective of the individual lanthanide and alkali metal present. Temporal profiles of changes in optical density were acquired performing pump/probe experiments with DMF solutions of 1−5 via femtosecond laser spectroscopy, highlighting a lanthanide-specific relaxation dynamic. The corresponding relaxation times ranging from seven picoseconds to a few hundred picoseconds are strongly dependent on the central lanthanide atom, indicating an intramolecular energy transfer from ligands to lanthanides. This interpretation also demands efficient intersystem crossing within one to two picoseconds from the S1 to T1 level of the ligands. Magnetic studies show that [Cs{Dy(Ph2acac)4}]n (4) has slow relaxation of the magnetization arising from the single Dy3+ ions and can be described as a single-ion single molecule magnet (SMM). Below 0.5 K, hysteresis loops of the magnetization are observed, which show weak single chain magnet (SCM) behavior.