O. V. Kotova

Role of the Ancillary Ligand N,N-Dimethylaminoethanol in the Sensitization of EuIII and TbIII Luminescence in Dimeric β-Diketonates

Two types of dimeric complexes [Ln2(hfa)6(mu2-O(CH2)2NHMe2)2] and [Ln(thd)2(mu2,eta2-O(CH2)2NMe2)]2 (Ln = YIII, EuIII, GdIII, TbIII, TmIII, LuIII; hfa- = hexafluoroacetylacetonato, thd- = dipivaloylmethanato) are obtained by reacting [Ln(hfa)3(H2O)2] and [Ln(thd)3], respectively, with N,N-dimethylaminoethanol in toluene and are fully characterized. X-ray single crystal analysis performed for the TbIII compounds confirms their dimeric structure. The coordination mode of N,N-dimethylaminoethanol depends on the nature of the beta-diketonate. In [Tb2(hfa)6(mu2-O(CH2)2NHMe2)2], eight-coordinate TbIII ions adopt distorted square antiprismatic coordination environments and are O-bridged by two zwitterionic N,N-dimethylaminoethanol ligands with a Tb1...Tb2 separation of 3.684(1) A. In [Tb(thd)2(mu2,eta2-O(CH2)2NMe2)]2, the N,N-dimethylaminoethanol acts as chelating-bridging O,N-donor anion and the TbIII ions are seven-coordinate; the Tb1...Tb1A separation amounts to 3.735(2) A within centrosymmetric dimers. The dimeric complexes are thermally stable up to 180 degrees C, as shown by thermogravimetric analysis, and their volatility is sufficient for quantitative sublimation under reduced pressure. The EuIII and TbIII dimers display metal-centered luminescence, particularly [Eu2(hfa)6(O(CH2)2NHMe2)2] (quantum yield Q(L)Ln = 58%) and [Tb(thd)2(O(CH2)2NMe2)]2 (32%). Consideration of energy migration paths within the dimers, based on the study of both pure and EuIII- or TbIII-doped (0.01-0.1 mol %) LuIII analogues, leads to the conclusion that both the beta-diketone and N,N-dimethylaminoethanol ligands contribute significantly to the sensitization process of the EuIII luminescence. The ancillary ligand increases considerably the luminescence of [Eu2(hfa)6(O(CH2)2NHMe2)2], compared to [Ln(hfa)3(H2O)2], through the formation of intra-ligand states while it is detrimental to TbIII luminescence in both beta-diketonates. Thin films of the most luminescent compound [Eu2(hfa)6(O(CH2)2NHMe2)2] obtained by vacuum sublimation display photophysical properties analogous to those of the solid-state sample, thus opening perspectives for applications in electroluminescent devices.

Theoretical Study of Structure and Electronic Absorption Spectra of Some Schiff Bases and Their Zinc Complexes

Tetradentate Schiff bases (H(2)L(i)), derivatives of salicylic aldehyde (H(2)L(1), H(2)L(2)) or o-vanillin (H(2)L(3), H(2)L(4)) with ethylenediamine or o-phenylenediamine as a bridge, and their zinc complexes were studied experimentally and theoretically in view of their possible application as emitters in organic light emitting diodes (OLEDs). The composition of thin films of the complexes was analyzed using a combination of different experimental and molecular modeling techniques taking into account changes in the Gibbs free energy of dehydration and dimerization reactions. The absorption spectra of the initial Schiff bases were investigated in methanol solutions, while the absorption spectra of their zinc complexes were investigated in thin films. Experimental results of elemental analysis, IR spectroscopy, laser desorption/ionization mass spectrometry (LDI MS), and X-ray diffraction as well as theoretical analysis of electronic absorption spectra by the quantum-chemical TD DFT method demonstrate that thin films of the zinc complexes contain binuclear anhydrous molecules. This conclusion should be taken into account when considering both transport and luminescence properties of these complexes in OLED heterostructures. A comparison of the results of CIS, TD DFT/PBE, and TD DFT/PBE0 calculations reveals the crucial importance of the inclusion of the exact exchange in the E(XC) functional for the further correct description of potential energy surfaces of excited states for the systems studied.

Influence of heteroligand complexation on the thermal, photoluminescent, and film-forming properties of some aromatic terbium(III) carboxylates

With homoligand TbL3 and heteroligand complexes TbL3(Q)n (HL = HSal (salicylic acid) and HPA (2-anilinobenzoic acid); Q = TPPO (triphenylphosphine oxide) and TOPO (tri(n-octyl)phosphine oxide); n = 1 or 2) as examples, it was shown that heteroligand complexation affects not only the thermal and photoluminescent properties but also the quality of films obtained by centrifugation: the root-mean-square roughness changes in the order TbL3Q > TbL3(Q)2 ≈ TbL3. This is due to different association degrees of the complexes in solution, which was confirmed by MALDI-TOF MS data.

Photo- and electroluminescent properties of zinc(II) complexes with tetradentate Schiff bases, derivatives of salicylic aldehyde

It is studied how the introduction of various substituents into the composition of organic ligands affects the photoluminescence spectra of new zinc(II) complexes with tetradentate Schiff bases H2L (derivatives of salicylic aldehyde (H2SAL1, H2SAL2) and o-vanillin (H2MO1, H2MO2) with ethylenediamine and o-phenylenediamine) in the form of bulk solids and thin films. It is demonstrated that the emission spectra of bulk solid complexes without o-phenylenediamine bridges (ZnSAL1 and ZnMO1) contain additional long-wavelength bands compared to the spectra of corresponding thin films. In the case of films obtained from [ZnSAL1]2 dimer complexes, the long-wavelength band is dominant. At the same time, the photoluminescence spectra of ZnSAL2 and ZnMO2 complexes with o-phenylenediamine bridges are similar in the case of solid samples and thin films. The electroluminescent properties of organic light-emitting diodes (OLEDs) with the ITO/α-NPD/ZnL/Ca:Al structure are studied. The bathochromic shift of the electroluminescence peaks of OLEDs with respect to the photoluminescence spectra of bulk solid samples and thin films is probably related to the formation of exciplexes at the α-NPD/ZnL interface. The electroluminescence spectra of OLEDs based on [ZnSAL1]2 show a hypsochromic shift of the emission maximum, which can be caused by a shift of the recombination region into the α-NPD layer.

Gas-Phase Synthesis of Lanthanide(III) Benzoates Ln(Bz) 3 (Ln = La, Tb, Lu)

A new method for the synthesis and film deposition of nonvolatile aromatic lanthanide(III) carboxylates by ligand exchange reaction between the starting volatile components in the gas phase was proposed. The complexes Ln(Bz)3 (Ln = La3+, Tb3+, Lu3+, HBz = benzoic acid) were synthesized by gas-phase ligand exchange reaction between the volatile Ln(Thd)3 and HBz (HThd = 2,2,6,6-tetramethylheptane-3,5-dione). The composition of the complexes was confirmed by elemental, thermal, IR-spectroscopic, and photoluminescence analyses and, in the case of lanthanum and lutetium complexes, by 1H NMR.

Gas-phase synthesis of terbium and lutetium carboxylates

Gas-phase ligand exchange between volatile lanthanide dipivaloylmethanates (Ln(dpm)3; Hdpm is dipivaloylmethane, Ln = Tb, Lu) and o-substituted aromatic carboxylic acids (HCarb = Hsal is o-hydroxybenzoic acid, Habz is o-aminobenzoic acid, Hpobz is o-phenoxybenzoic acid, Hpa is o-anilinobenzoic acid). The gas-phase reaction involves the formation of the mixed-ligand complex Ln (dpm)3−n(Carb)n, which is subsequently converted into tris-carboxylate (Ln(Carb)3) on heating of the product in vacuum.

IR Spectra of N,N′-Ethylene-Bis(salicylaldiminates) and N,N′-ethylene-Bis(acetylacetoniminates) of Ni(II), Cu(II), and Zn(II)

Vibrational spectra of N,N′-ethylene-Bis(salicylaldiminaates) and N,N′-ethylene-Bis(acetylacetoniminates) of nickel (II), copper (II), and zinc (II) are studied experimentally (IR spectroscopy, 400–4000 cm−1) and theoretically (B3LYP), band assignment is given, and the distribution of potential energy of normal vibrations in internal coordinates is studied. Differences between vibrational spectra of the complexes are discussed. Thermodynamic functions of gas-phase complexes corresponding to temperatures of 298 and 800 K are calculated.

Thin Films of Tb(pobz)3 (Hpobz = 2-phenoxybenzoic acid): Reactive CVD and Optical Properties

Thin films of aromatic terbium carboxylates are promising materials for application in different optical devices. However, thin films deposition of these compounds by common methods is almost impossible due to their low solubility in most common organic solvents and nonvolatility. In our previous work it was shown that thin films of one of the simplest aromatic terbium carboxylate Tb(bz)3 (Hbz - benzoic acid) can be obtained by the original approach based on the exchange reaction between volatile precursors - reactive CVD (RCVD). Herein we improved this approach and demonstrated its possibilities and advantages on deposition of thin films of new emissive material - Tb(pobz)3 (Hpobz - phenoxybenzoic acid). The bulk products and thin films of Tb(pobz)3, obtained by RCVD method were characterized by elemental analysis, IR-, Raman and photoluminescent spectroscopy as well as thin films were studied by AFM and SEM techniques.

Reactive Chemical Vapor Deposition Method as New Approach for Obtaining Electroluminescent Thin Film Materials

The new reactive chemical vapor deposition (RCVD) method has been proposed for thin film deposition of luminescent nonvolatile lanthanide aromatic carboxylates. This method is based on metathesis reaction between the vapors of volatile lanthanide dipivaloylmethanate (Ln(dpm)3) and carboxylic acid (HCarb orH2Carb′) and was successfully used in case of HCarb. Advantages of the method were demonstrated on example of terbium benzoate (Tb(bz)3) and o-phenoxybenzoate thin films, and Tb(bz)3 thin films were successfully examined in the OLED with the following structure glass/ITO/PEDOT:PSS/TPD/Tb(bz)3/Ca/Al. Electroluminescence spectra of Tb(bz)3 showed only typical luminescent bands, originated from transitions of the terbium ion. Method peculiarities for deposition of compounds of dibasic acids H2Carb′ are established on example of terbium and europium terephtalates and europium 2,6-naphtalenedicarboxylate.

Mass spectrometric study of the overheated vapor over Ni(II), Cu(II), and Zn(II) N,N′-ethylenebis(acetylacetoniminato) complexes

A mass spectrometric study of the overheated vapor over the complexes Ni(acacen), Cu(acacen), and Zn(acacen) (H2acacen = N,N′-ethylenebis(acetylacetonimine)) has been carried out in the temperature range of 180–760°C. Irrespective of the degree of overheating, the vapor phases over all of these compounds contain no ions heavier than the molecular ion [MO2N2C12H18]+. The existence of molecular ions in the overheated vapor in the double-chamber two-temperature effusion cell is evidence of the high thermal stability of the complexes. The onset temperature of the thermal decomposition of Ni(acacen), Cu(acacen), and Zn(acacen) is 690, 610, and 560°C, respectively. The way of fragmentation of the chelates under electron impact ionization depends on the nature of the metal.