The Correlation Between f–f Absorption and Sensitized Visible Light Emission of Luminescent Pr(III) Complexes: Role of Solvents and Ancillary Ligands on Sensitivity

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 L^me, L^pr and L^dmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrL^pr 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 PrL^pr is excited through the ligand. The addition of a methyl group to PrL and PrL^pr has a different effect on the Pr emission intensity with the intensity of PrL^me decreasing more than that of PrL and PrL^dmpr and increasing more than that of PrL^pr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on L^me, L^pr and L^dmpr were reduced and formed the corresponding PrLH, PrL^meH, PrL^prH and PrL^dmprH 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 ³P_J 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.

White-Light Emitting Di-Ureasil Hybrids

White-light emitting materials have emerged as important components for solid state lighting devices with high potential for the replacement of conventional light sources. Herein, amine-functionalized organic-inorganic di-ureasil hybrids consisting of a siliceous skeleton and oligopolyether chains codoped with lanthanide-based complexes, with Eu3+ and Tb3+ ions and 4,4'-oxybis(benzoic acid) and 1,10-phenanthroline ligands, and the coumarin 1 dye were synthesized by in situ sol⁻gel method. The resulting luminescent di-ureasils show red, green, and blue colors originated from the Eu3+, Tb3+, and C1 emissions, respectively. The emission colors can be modulated either by variation of the relative concentration between the emitting centers or by changing the excitation wavelength. White light emission is achieved under UV excitation with absolute quantum yields of 0.148 ± 0.015, 0.167 ± 0.017, and 0.202 ± 0.020 at 350, 332, and 305 nm excitation, respectively. The emission mechanism was investigated by photoluminescence and UV⁻visible absorption spectroscopy, revealing an efficient energy transfer from the organic ligands to the Ln3+ ions and the organic dye, whereas negligible interaction between the dopants is discerned. The obtained luminescent di-ureasils have potential for optoelectronic applications, such as in white-light emitting diodes.

Highly Efficient Luminescent Polycarboxylate Lanthanide Complexes Incorporated into Di-Ureasils by an In-Situ Sol-Gel Process

In order to prepare efficient luminescent organic–inorganic hybrid materials embedded with a lanthanide (Ln³⁺) complex with polycarboxylate ligands, Ln³⁺-doped di-ureasils with 4,4-oxybis(benzoic acid) and 1,10-phenanthroline ligands were synthesized via an in-situ sol–gel route. The resulting hybrids were structurally, thermally, and optically characterized. The energy levels of the ligands and the host-to-ion and ligand-to-ion energy transfer mechanisms were investigated (including DFT/TD–DFT calculations). The results show that these Ln³⁺-based di-ureasil hybrids exhibit promising luminescent features, e.g., Eu³⁺-based materials are bright red emitters displaying quantum yields up to 0.50 ± 0.05. The luminescent color can be fine-tuned either by selection of adequate Ln³⁺ ions or by variation of the excitation wavelength. Accordingly, white light emission with CIE coordinates of (0.33, 0.35) under 310 nm irradiation was obtained.

Pr(III) luminescence enhancement by chelation in solution and in sol-gel glass

Due to the weak emission of lanthanide ions in solution, it is common practice to form complexes of the lanthanide ions with organic ligands that strongly absorbs light and transfers the energy to the lanthanide ion center via the antenna effect. The organic ligands 2-6-pyridinedicarboxylate (L1) and the polytonic diazine (N-N) ligand L2 (C22H16N12O2) were used to synthesize two Pr(III) complexes, namely: Pr-L1 (Na3[Pr(C7H3NO4)3]) and Pr-L2. The prepared complexes were further encapsulated in an optically transparent sol-gel glass. The synthesized ligands and complexes were characterized by FTIR and (1)H NMR. Room temperature luminescence of Pr-L1 and Pr-L2 complexes in solution and in sol-gel glass were investigated using a spectrofluorometer. Excitation at the maximum absorption wavelength of the ligands (280nm) resulted in the typical visible luminescence (centered at around 600nm) resulting from the (1)D2→(3)H4 transition of the Pr(III) ion, which contributes to the efficient energy transfer from the absorbing ligand L1 to the chelated Pr(III) ion (an antenna effect) while the Pr(III) luminescence is not efficiently sensitized by ligand L2. The obtained emission spectra indicated that the excitation energy level for the central Pr(III) is in a slightly lower location than ligand L1 excitation triplet (T1) level and can accept the energy transfer from T1 efficiently.

A new and efficient luminescence enhancement system of Eu-N-(3,5-dibromosalicylidene)-2-aminopyridine-1,10-phenanthroline and its application in the determination of trace amounts of europium

A sensitive luminescence enhancement system has been developed for the determination of trace amounts of trivalent europium. The luminescence intensity of europium complex with N-(3,5-dibromosalicylidene)-2-aminopyridine (HL) was greatly enhanced after the addition of 1,10-phenanthroline (Phen) in acetonitrile solution. The excitation and emission wavelengths were 274 and 617 nm respectively. Under optimal conditions, the luminescence intensities varied linearly with Eu(3+) concentration in the range of 4.0 × 10(-6)∼2.4 × 10(-5) mol L(-1) with a detection limit of 4.3 × 10(-10) mol L(-1). This method was successfully applied to determine the trace amounts of Eu(3+) in a high purity Gd(2)O(3) matrix and in a mixed lanthanide sample. Energy transfer mechanism and luminescence enhancement of HL-Phen-Eu ternary system were studied. The results indicate that both HL and Phen are good sensitizers for the luminescence of Eu(3+) ions, as energy gap between the lowest triplet level of HL/Phen and the resonant level of Eu(3+) ((5)D(1)) exists around the optimal value (3000 ± 500 cm(-1)). The interference by some other lanthanide ions and common ions were also studied.