Aspects of lanthanide complexes for selectivity, intensity and sharpness in luminescence bands from twenty-four praseodymium, europium and gadolinium complexes with differently distorted-hexadentate ligands.

We structurally and spectroscopically investigated a series of praseodymium (Pr) complexes with eight ligands that form helicate molecular structures. The mother ligand skeleton (L) has two bipyridine moieties bridged with ethylenediamine. The bridged skeleton of PrL was changed to diamines 1-methyl-ethylenediamine, trimethylenediamine and 2,2'-dimethyl-trimethylenediamine, and the corresponding ligands were designated as Lme, Lpr and Ldmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrLpr in the visible and near infrared (NIR) regions indicate that PrL is excited by both the electronic state of the ligand and the ff absorption band, whereas PrLpr is excited through the ligand. The addition of a methyl group to PrL and PrLpr has a different effect on the Pr emission intensity with the intensity of PrLme decreasing more than that of PrL and PrLdmpr and increasing more than that of PrLpr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on Lme, Lpr and Ldmpr were reduced and formed the corresponding PrLH, PrLmeH, PrLprH and PrLdmprH complexes. The luminescence of the non-methylated series is due to transitions related to the 1D2 level and thus the methylated series luminesces due to high energy levels such as 3PJ arising from the shortened π electronic systems. We also discuss the strong red emission of a series of Eu complexes with eight ligands from the viewpoint of their molecular structures and luminescence efficiencies and evaluate the Judd-Ofelt parameters from the luminescence spectra of Eu complexes.

Multicolor upconversion luminescence of dye-coordinated Er3+ at the interface of Er2O3 and CaF2 nanoparticles.

Multicolor upconversion luminescence of Er3+ was successfully enhanced by optimizing the interface in dye-coordinated nanoparticles with a core/shell structure. Red and green upconversion emissions of Er3+ were obtained at the interface of oxide nanoparticles via the intramolecular energy transfer from the coordinating squaraine dye with high light-absorption ability, which was more efficient than emissions through the energy transfer from metal ions such as Yb3+. Additionally, CaF2 nanoparticles as a core material minimized the energy loss with nonradiative downward relaxations in Er3+, resulting in the observation of unusual blue upconversion emissions from the upper energy level of Er3+ by nonlaser excitation using a continuous-wave (CW) Xe lamp at an excitation power of 1.2 mW/cm2.

Slow Magnetic Relaxation in a Palladium-Gadolinium Complex Induced by Electron Density Donation from the Palladium Ion.

Incorporating palladium in the first coordination sphere of acetato-bridged lanthanoid complexes, [Pd2 Ln2 (H2 O)2 (AcO)10 ]⋅2 AcOH (Ln=Gd (1), Y (2), Gd0.4 Y1.6 (3), Eu (4)), led to significant bonding interactions between the palladium and the lanthanoid ions, which were demonstrated by experimental and theoretical methods. We found that electron density was donated from the d8 Pd2+ ion to Gd3+ ion in 1 and 3, leading to the observed slow magnetic relaxation by using local orbital locator (LOL) and X-ray absorption near-edge structure (XANES) analysis. Field-induced dual slow magnetic relaxation was observed for 1 up to 20 K. Complex 3 and frozen aqueous and acetonitrile solutions of 1 showed only one relaxation peak, which confirms the role of intermolecular dipolar interactions in slowing the magnetic relaxation of 1. The slow magnetic relaxation occurred through a combination of Orbach and Direct processes with the highest pre-exponential factor (τo =0.06 s) reported so far for a gadolinium complex exhibiting slow magnetic relaxation. The results revealed that transition metal-lanthanoid (TM-Ln) axial interactions indeed could lead to new physical properties by affecting both the electronic and magnetic states of the compounds.

Multiple Magnetic Relaxation Pathways and Dual-Emission Modulated by a Heterometallic Tb-Pt Bonding Environment.

A heterometallic Tb-Pt complex, [Tb2 Pt3 (SAc)12 (H2 O)2 ] (SAc=thioacetate), was synthesized. Dual emission was modulated by the presence of a heterometallic Tb-Pt bonding environment. The heterometallic Tb-Pt bond lowers the symmetry of the Tb ion and enhanced the emission efficiency. In addition, the Tb-Pt complex shows field-induced multiple magnetic relaxation pathways. Furthermore, it served as an antenna for the observed dual emission. In other words, the heterometallic Tb-Pt bond has a significant effect on the luminescence and magnetic properties of the complex.

Chiroptical Spectroscopic Studies on Lanthanide Complexes with Valinamide Derivatives in Solution.

Ligands based on 2,2'-bipyridine and valinamide moieties induce circularly polarized luminescence in their europium complexes. Both the R and S enantiomers of the complexes were successfully obtained. Single-crystal X-ray analysis of the racemic crystal confirmed that the ligand is coordinated to the europium ion in a tetradentate fashion. The π-electronic system of the ligand is co-planar with the valinamide moiety, and acts as an efficient photoantenna to sensitize europium luminescence by UV excitation. The luminescence quantum yield (QY) of europium in the valinamide-containing complex was 44 % in acetonitrile. The glum value to evaluate the circularly polarized luminescence was relatively high at |0.13| estimated from their magnetic dipole transitions around 593 nm. For comparison, we prepared hexadentate europium complexes in the S- and R-forms derived from two bipyridine moieties linked by ethylenediamines. The determined QYs were 18 % (S) and 16 % (R), and the glum value |0.12| for the hexadentate complexes. The photophysical properties of the gadolinium complexes of the ligands were also evaluated.

Lanthanide-Oligomeric Brush Films: From Luminescence Properties to Structure Resolution.

Lanthanide (Ln) based luminescent materials are experiencing an increasing interest in their applications in several fields. In this study, we report a series of new lanthanide-oligomeric brush films, supported on quartz substrates and prepared using a layer-by-layer method (LbL). Oligomeric brush films are composed of small oligomers from our previously reported coordination polymers [x-EuL] and [x-TbL] (with x = 1, 3, and 5 generations of Ln complexes), which are grown perpendicularly from a carboxylate self-assembled monolayer. Oligomers composed of our previously described helical lanthanide complex LnL (Ln: Eu and Tb) as a luminescent moiety and benzene-1,4-dicarboxylate acid (bdc) used as a linker. Mixed films having the fifth-generation Ln complexes composed of equimolar mixture of Eu and Tb ions were prepared. Oligomeric brush films are highly transparent and exhibited a colored emission under UV irradiation. Pure Ln (Eu or Tb) films showed a strong luminescence from the Ln ions. Their luminescent properties depended on the number of lanthanide layers in the films composed of the first to third generations of lanthanide complexes. Then, the increase of the complex layers induced no difference in the luminescent properties. An energy transfer from Tb to Eu ions in the mixed films indicated a short distance between lanthanide ions of a fifth layer. The structural analysis together with the observed luminescent properties and some previous studies allowed to clarify the disposition of the oligomers in the films.

Sensitive Photodetection with Photomultiplication Effect in an Interfacial Eu2+/3+ Complex on a Mesoporous TiO2 Film.

A simple device structure composed of an interfacial Eu2+/3+ complex on a mesoporous TiO2 film is developed by a solution process and acts as the high-performance photodetector with photomultiplication phenomena. The electron transfer from the photoexcited organic ligand, 2,2':6',2″-terpyridine (terpy), as a photosensitizer to TiO2 is accelerated by the reduction level of Eu3+/2+ ions chemically bonding among terpy and TiO2, resulting in the generation of a large photocurrent. It is worth noting that its external quantum efficiency is in excess of 105% under applied reverse bias. The corresponding responsivity of the device is also determined to be 464 A/W at an irradiation light intensity of 0.7 mW/cm2 (365 nm), which is more than 3 orders of magnitude larger than those of inorganic photodetectors. A dark current of the device can be reduced to 10-9 A/cm2 by introducing a Eu oxide thin-film layer as a carrier blocking layer at the interface between transparent conducting oxide (TCO) and the TiO2 layer, and the specific detectivity reaches 5.2 × 1015 jones at 365 nm with -3 V. The performance of our organic-inorganic hybrid photodetector surpasses those of existing ultraviolet photodetectors.

Alkyl chain elongation and acyl group effects in a series of Eu/Tb complexes with hexadentate π-electronic skeletons and their enhanced luminescence in solutions.

Five Eu complexes with long alkyl chain groups, abbreviated as EuLCx ("x" indicates the number of methylene groups: x = 8, 12, 14, 18, and 22), were synthesized to evaluate their structural and luminescence properties in chloroform. The mother helicate Eu complex, EuL, which has two bipyridine moieties bridged by an ethylenediamine, has been previously reported. A reduced form in which the azomethine groups of L also coordinated to the Eu ion, EuLH, was newly prepared. EuLH also adopts a helicate molecular structure based on single crystal X-ray structural analysis. The amine hydrogens of the bridging ethylenediamine of LH are active sites for substitution and were exchanged with five different alkyl chains to form EuLCx. Luminescence band positions and shapes of EuLCx in chloroform were completely identical, with a quantum yield of 37.1 ± 1.2 and a lifetime of around 1.25 ms. This indicates that the environments surrounding the Eu ion in the various complexes are all similar. Luminescence quantum yields of TbLH and TbLC18 are also strengthened, 48.7% in acetonitrile and 55% in chloroform, respectively. Potential energy surfaces were also described by using density functional theory, suggesting the possibility of a 1 : 2 complex of Eu and the ligand as a main luminescent species in solutions. This 1 : 2 complexation forms Eu-oxygen coordination using acyl groups. It indicates that the acyl group modification results in a different structure from the mother complexes.