Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties.

Abstract

The ligand, bis-β-diketone with an azobenzene bridge (4,4′-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H2L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln2L3(DMSO)4] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln2L3(DMSO)4] complexes in ethanol and acetonitrile solutions are similar to those of the pure H2L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10–3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H2L ligand is similar to those of azobenzene derivatives, while those for the [Ln2L3(DMSO)4] complexes (kiso = 10–4 s–1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.