Luminescence Schiff-base ligands

Complexation of macrocyclic ligands to lanthanide cations has been studied extensively [207,208], One main reason for the current interest in those macro-cyclic complexes are their intrinsic paramagnetic and luminescent properties. There is also the steadily increasing number of tailor-made macrocyclic ligands [209], This section will focus on complexes which contain macrocycles as discrete counterions and in particular on the coordination chemistry of phthalocyanine (Pc) and porphyrin (Por) ligands. Schiff base ligands which display another source of amine functionalities are usually not deprotonated under the prevailing reaction conditions [210]. 

Novel clusters [Ln8Cd24Li2(OAc)48] and [Ln6Cdi8L9Cl8(io)(OAe)28(26)] (Ln = Nd, Gd, Er, and Yb) with novel drum-like structures were prepared using a flexible Schiff base ligand. In the case of the Nd, upon exeitation of the ligand-centered absorption bands, the clusters show quite intense NIR luminescence due to F3/2 - transitions, (/ = 9,11 and 13). ... 

The synthesis, structures, and luminescence properties of polynuclear lanthanide complexes and lanthanide coordination polymers, which are formed from multidentate oxygen and nitrogen-containing Schiff-base ligands, are described in this chapter. 

Recently, the use of pthalocyanines and porphyrins by Ishikawa and coworkers enabled tiie synthesis of several so-called double-decker or triple-decker lanthanide complexes, described as stacked Ji-conjugate molecules. The photophysical properties of lantiianide ious depeud markedly on their coordination environments. Since a multidecker framework can effectively keep solvent molecules away from Ln centers, it is assumed that enhanced luminescent properties can be achieved witii these types of architectures. We discovered that the use of two conjugated Schiff-base ligands H2Lh can stabihze novel multidecker framewoiks (Scheme 1). 

In complexes (1-6), the outer two Schiff-base hgands display a curved bowl-like configuration, while the inner Schiff-base ligands are virtually planar. Intramolecular n-n stacking interactions between phenylene units are found in all these multidecker structures, which may further add to the stability of the complexes (Figures 1-6). The photophysical properties of complexes (16) have been studied in solution. Upon excitation of the hgand-centered absorption band, (1) and (3) show visible emission bands typical of the Tb + ion ( D4 - F transitions n = 6, 5, 4, and 3), (2) shows near infra-red (NIR) luminescence of Yb + ion ( Fs/2 F7/2... 

In these lanthanide complexes, the multidentate Schiff-base ligands can stabilize Ln centers and act as antenna sensors for lanthanide luminescence. With the Ln + centers protected by the Schiff-base hgands from solvent and water molecules, those lanthanide complexes with multidecker and cage-like structures show improved luminescence properties. 

Bai Y, Gao H, Dang D-B et al (2010) A series of metal-organic frameworks based on polydentate schiff-base ligands derived from benzil dihydrazone synthesis, crystal structures and luminescent properties. CrystEngComm 12 1422... 

In light of the rapid development in this field and the literature on other ligands for the sensitization of NIR luminescence being explosively growing, a selection of the work most representative has been made. Simpler ligands such as the Schiff base salophen, tropolonate, azulene dicarboxylate, or pyridine carboxylate molecules have been the subject of a recent review. Petoud et al. successfrilly... 

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