Syntheses And Photoluminescent Properties Of Rare Earth Complexes Based On Bi-aromatic-bridged Bis-β-diketone

The unique optical properties of lanthanide ions have made their complexes ofintense interest for a wide range of photonic applications such as tunable lasers, lightemitting diodes, optical amplifier, luminescence probes for bio-analysis and medicalimaging. Bis-diketones are derived from the diketones through the bridging spacers,resulting in a series of tetra-ketones, which promise their rich structural chemistry withthe metal ions in various modes. Typical structures are featured by the double-strandedhelical, triple-stranded helical, tetra-stranded helical, triangular, and chainlike motifs.Reviewing the reported literatures, it is found that the fluorescent intensity andefficiency of the multi-nuclear lanthanide complexes are much better than thecorresponding mono-nuclear lanthanide complexes, unfortunately, we do not know toomuch about why. We have designed and synthesized a series of lanthanide complexesassembled about the bis-diketones and their luminescent properties have beeninvestigated, which would pave the way for further study the theoretical work on therelationship of the structures and luminescent behaviors.In this work, we have prepared three bis-diketones BTB, BPB and BTT throughthe coupling of two mono-ketones (BTFA, DBM and TTA) via spacers, respectively.Interestingly, the energy levels of the excited state are controllable through the couplingat the proper position. The rigidity of the aryl groups prevents the two binding sitesfrom chelating a single metal ion, and the flexibility originating from the free rotation ofC-C single bond, allows the formation of structures (Ln: ligand=2:3) in a twistconformation, which has been confirmed by the single crystal structural analysis andMS-TOF. The control on the energy level of triple state of the ligand is achievedthrough introduction of the spacers between the two mono-ketone moieties, whichmatches well with the energy level of the excited state of the lanthanide ions, leading tothe significant sensitization to the lanthanide ions. The fluorescent investigation reveals that dinuclear complexes possess better behavior than the mononuclear complex. Thedesigns, syntheses, and fluorescent studies of the three series of triple-strandedlanthanide complexes have supplied valuable information for the theoretical workconcerning the relationship of the structure and the luminescence.