JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 66

Foundation item: Project supported by the Bulgarian Fund for Scientific Investigations (VUH 005/05 and DO 02-129/08) Corresponding author: Maria Milanova (E-mail: nhmm@wmail.chem.uni-sofia.bg; Tel.: +35928161217) DOI: 10.1016/S1002-0721(10)60372-9

Lanthanide complexes with -diketones and coumarin derivates: synthesis, thermal behaviour, optical and pharmacological properties and immobilisation

Maria Milanova1, Joana Zaharieva1, Iliya Manolov2, Miroslava Getzova1, Dimitr Todorovsky1 (1. Department of Inorganic Chemistry, University of Sofia, Bulgaria 1, J. Bourchier Blvd., 1164 Sofia, Bulgaria; 2. Department of Organic Chemistry, Medical University, 2, Dunav Str., 1000 Sofia, Bulgaria)

Received 31 August 2010; revised 1 November 2010

Abstract: The paper presents the results of the synthesis of complexes of the type Eu(DBM)3 and Eu(DBM)3·Q (DBM-dibenzoylmethane, Q-1,10-phenantroline or 4,7-diphenyl-1,10-phenantroline) and of Tb- and Nd-complexes with the newly-synthesised coumarin derivates 3,3’-[(4-chlorphenyl)methylene)bis(4-hydroxy-2H-chromen-2-one), 3,3’-[(3,5-dimethoxy-4-hydroxy)methylene)bis(4-hydroxy-2H-chromen- 2-one), etc. Elemental and thermogravimetric analysis, IR, UV, NMR and fluorescence spectroscopy and X-ray analysis were applied for characterisation of the complexes. Some peculiarities of the synthetic procedures for both types of complexes were discussed and the influ-ence of the synthetic approach, pH of the reaction medium, temperature of synthesis and drying of the complexes on the composition, stability and optical properties was reported. The immobilisation of the complexes in thin films based on sol-gel produced SiO2 and on polymethyl-methacrylate was studied. The optimal conditions for preparation of the matrices (composition of the starting system, temperature and time of sol aging, etc.) were recommended. The film morphology was evaluated by fluorescence, scanning electron and atomic force microscopy. The interaction of the lanthanide ions with the matrices and the influence of their nature, the effect of the in-situ polymerisation and other fac-tors on photoluminescent excitation and emission spectra and excited state life-times of the complexes were followed. The effect of the sec-ond ligand on the photoluminescence properties of the immobilised diketonates was further elucidated. Processes involved in the thermal de-composition of the complexes and microcomposites produced on their base were proposed. Preliminary results on the pharmacological prop-erties of the coumarin complexes reported showed unambiguously higher cytotoxicity of the Nd complex in comparison with that of the re-spective ligand.

Keywords: rare earths; complexes; thin films; -diketones; coumarin derivates; optical properties; pharmacological properties; morphology

Due to their intensive narrow-line fluorescence emission, it is relatively insensitive to the material of the immobilisa-tion matrix, the complexes of lanthanides (Ln) with -dike-tones, which are of significant interest as an active compo-nent of organic LEDs, candidates for application as high density recordable optical recording materials and in some biomedical investigations.

Coumarin derivates with various structures are known as analytical reagents, but they have been an object of investi-gation for their physiological, photodynamic, anticoagulant[1] and anticancer[2] activity. The structure and the pharmacol-ogical functions of coumarins and some of their derivates are studied[3]. Complexes of some coumarins with lanthanides like La, Pr, Sm, Eu, Gd, Tb, Dy, Y[1], Ce[4] and Nd have shown interesting properties[1,4]. Anticoagulant and antifun-gal studies indicate an enhancement of the ligand activity on complexation, particularly with lanthanides[1]. The determi-nation of coumarins can be done by fluorimetry but some of them give a low analytical signal[5] and therefore a high de-tection limit for fluorimetry is registered. The sensitivity of the fluorimetric determination can be enhanced by the for-mation of complexes, i.e. using so called sensitised fluores-cence[5].

Recently, the chemistry of lanthanide diketonates and their application have been the objects of excellent reviews[6–8]. Following the literature data it seems that the main points of interest in the complex synthesis are: initial materials (chlo-rides or nitrates of the Ln and dibenzoylmethane (HDBM) in 25%–30% excess are usually used[9,10], pH of the reaction medium (according to[11–13] it has to be adjusted between 6 and 7), temperature of synthesis (temperatures between am-bient[14] and 80 °C[11] have been recommended), recrystalli-sation (from solutions in ethanol[11] or a mixture of acetone and absolute ethanol[9]) and the drying of the final product (it has been performed in air at ambient temperature[12], 40 °C[11] or under vacuum at 125–150 °C[9]). The presence of water can influence the complex composition but the literature data are contradictory[12,15,16].

Special interest was paid to complexes of the type Ln(DBM)3·Q (Q is 1,10-phenantroline (phen) or 4,7-di-phenyl-1,10-phenantroline (dpp)[17]) obtained by the method described in[18], due to their higher stability and increased luminescence intensity.

The potential usage of the studied complexes requires their immobilisation in suitable matrices. Naflon mem-branes[19], organic polymers[20] as well as SiO2-based materi-

Maria Milanova et al., Lanthanide complexes with -diketones and coumarin derivates: synthesis, thermal behaviour, optical … 67

als have typically been used to provide a microporous im-mobilisation matrix which has to fulfil two contradictory re-quirements: to ensure strong enough entrapping of the dye in order to prevent its leaching under the action of the working medium (while preserving the complex’ optical properties) and at the same time to ensure the possibility of the smaller analyte species to diffuse into it and to interact with the fluorophore. The matrix properties are therefore of crucial importance for sensor sensitivity, detection limit, calibration stability, areas of application and exploitation life. A strong dependence (for the studied system) of the nature of the ma-trix material on the emission lifetime is shown[21]. The co-ligand nature may influence the same parameter due to its specific interactions with the matrix and with the main ligand.

A comparison between matrices based on pure SiO2, ZrO2, mixed oxides (SiO2+TiO2), organically modified gels with 3-glycidoxypropyl trimethoxysilane (Glymo) is made[21,22]. The quenching that takes place in sol-gel produced matrices is decreased by the association of the Eu-complexes with polyethylene glycol[23].

Wang et al.[11] report the immobilisation of Ln(DBM)3 in a SiO2-based sol-gel matrix. The sol is aged at 40 °C in a sealed vessel followed by 4–5 weeks at ambient conditions. Meng et al.[24] report fluorescence spectra, photostability and thermal stability of spin-coating deposition of films using Eu(DBM)3 or Eu(DBM)3.phen solutions in a mixture of ethanol and dimethylformamide (DMF). Despite efforts to incorporate lanthanide complexes in the silica matrix[25,26] the films produced have a short-term stability[27].

The present paper summarises results of studies on the complexes of lanthanides with -diketones and newly syn-thesised coumarin derivates emphasizing on (1) complex synthesis peculiarities, (2) the influence of the synthetic con-ditions on their properties, (3) entrapping of the diketonates in two types of immobilisation matrices and (4) properties of the prepared composites.

1 Experimental

1.1 Synthesis of complexes

1.1.1 Diketonates Methods of Wang et al.[11], Khomenko and Kuznetsova[10], Melby et al.[12] and Meng et al.[24] were applied for Eu dibenzoylmethanate synthesis. All methods are based on the interaction between an Eu salt and HDBM in ethanol solution in presence of a small amount of NaOH. The parameters varied include the pH reached, temperature and time of the process and drying conditions. The method of Meng[24] is proposed for preparation of mixed-ligand complexes of the type Ln(DBM)3·Q (Q=phen). In the ex-periments described here, dpp was also used as an additional ligand, expecting an improvement of the latter on the com-plex photoluminescence properties. Some details of the methods were specified in the course of this work. 1.1.2 Coumarin complexes The coumarin derivates used

here are poorly soluble in alcohol and acetone and practi-cally insoluble in water. On the other side the presence of hydroxyl groups in the ligands means that a deprotonation has to take place along with the eventual coordination of the metal to the ligand. These factors determined the synthetic procedure applied, which included as a first step the prepara-tion of a water solution of a sodium salt of the ligand and then mixing with the respective lanthanide salt; as a rule a lanthanide nitrate was used. In the water solution the sodium salt obtained hydrolyses easily, so very careful observation of the synthetic mixture as well as control of the time of stir-ring is required. The NaOH/ligand stoichiometry was deter-mined by the number of hydroxyl groups to be deprotonated. A similar synthetic route with a different coumarin derivate has been reported in the literature[5].

For the other procedure applied, acetonitrile solutions of both the ligand and the lanthanide salt were used. As was expected, no deprotonation occurred, which led to formation of different kinds of complexes.

1.2 Sol and film/membrane preparation

1.2.1 SiO2 based matrices The complexes were immobi-lised in SiO2 matrices prepared by tetraethoxysilane (TEOS, >99%, Fluka) hydrolysis, applying the method used in[28]. The preparation of SiO2 matrices is described in more details in[29]. The Eu complex was introduced as an etha-nol-dimethylformamide (DMF) solution (2.5 g/dm3). A mole ratio of ethanol:DMF:TEOS:water=16:1.1:1:4 was used; the pH was adjusted to 8 by addition of ammonia solution. The reagents were mixed in the order complex solution-TEOS- water. After 2 h of stirring, the obtained sol was aged at temperatures from ambient to 70 °C for 3 h to 4 weeks. In some experiments, sonication of the fresh sol was carried out for 30 min in an ice-water cooled ultrasound bath.

From the prepared gels, films (typical thickness ~300 nm) were produced by dip-coating at one immersion with a with-drawal speed of 0.2 mm/s. Membranes (1–2 mm in thickness) were prepared by casting of the gel in a Teflon® mould. 1.2.2 Polymethylmethacrylate matrices Two preparation methods for the matrices were used:

(1) Polymethylmethacrylate matrix was prepared by a catalytically induced polymerisation of the monomer. To 5.3 cm3 of this monomer, 3 mg of benzoyl peroxide catalyst was added under stirring, followed by a DMF solution of the complex in such an amount that the concentration of com-plexing agent in the final solution reached 1 %. The polym-erisation proceeded at 60 ° for 24 h.

(2) To a chloroform solution of polymethylmethacrylate (2 g of the polymer in 30 cm3 solvent), a solution of the com-plex (1 %) was added and magnetically stirred for 30 min.

Films from both matrices were deposited by dip-coating at a withdrawal speed of 0.4 mm/s

1.3 Analysis and characterisation

The complexes and composites were analysed for their elemental composition. Their molecular structure was stud-

68 JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010

ied using FTIR spectroscopy (Bruker spectrometer in KBr pellets) as well as NMR spectroscopy (Bruker Avance DRX250 instrument in CD3Cl solution). Their crystal struc-ture was determined using a Siemens D500 powder diffrac-tometer (Cu K radiation) and a Bruker X8 Apex, Nonius Kappa CCD device by - and -scans at MoK radiation). The thermochemical behaviour was studied using a Pau-lic-Paulic-Erdey derivatograph at a heating rate of 5 °C/min, while optical properties (excitation and emission photolumi-nescence spectra and lifetime of the excited states) were studied with a Cary Eclipse fluorescence spectrometer.

2 Results

2.1 Complex synthesis, structure and properties

2.1.1 Synthesis Diketonates: The main results[30] con-cerning the quality of the products obtained applying the different synthetic methods can be summarised as follows:

– The elemental composition of the complex produced by the method described in[11] differs significantly from the ex-pected one for Eu(DBM)3 or Eu(DBM)3·H2O. The carbon content in the residue remaining after dissolving of the initial product in ethanol as well as the IR spectra suggests forma-tion of Eu(DBM)2·OH in the course of the synthetic proce-dure or during storage. The low-crystalline initial complex is not thermally stable and as a result of its recrystallisation, an X-ray amorphous product with a composition close to Eu(Benz)3 (Benz=deprotonated benzoic acid) is formed.

– The specimens produced by the method described in[10] contain small amounts of OH (band at 3420 cm–1) and traces of H2O (band at 1630 cm–1).

– The complex produced by Melby’s method[12] is rela-tively stable under storage in a dessicator and is easily dis-solved in acetone. The content of C and H confirms the composition Eu(DBM)3 with traces of water; its amount strongly depends on the conditions (temperature and pres-sure) of drying.

– A significant amount of Eu(OH)3 is formed when Eu(DBM)3·Q (Q=phen or dpp) is prepared by the method described in[24] at pH=6. However, the complexes are stable enough and their recrystallisation leads to rather satisfactory results.

Coumarin complexes: The composition of the complexes depends on the synthetic route applied. The elemental analy-sis suggests formation of Ln2L3.nH2O, where L is 3,3’-[(4- chlorphenyl)methylene)bis(4-hydroxy-2H-chromen-2-one) (L15), when procedure goes via sodium salt of the ligand as an initial step. The presence of the water molecules as ligands in the inner coordination sphere was observed via IR spectroscopy data. Procedure of excluding of the absorbed water molecules and its distinguishing from the crystal water molecules was applied.

When the synthetic medium is acetonitrile without of de-protonation a type of complex NdL(NO3)3.2CH3CN where L is 3,3’-[(3,5-dimethoxy-4-hydroxy)methylene)bis(4-hydroxy-

2H-chromen-2-one) (L11) was formed. The synthetic procedure and characterization of the com-

plexes prepared will be presented in more details in a com-ing report. 2.1.2 Molecular and crystal structure of the complexes diketonates: The differences in IR spectra of the complexes prepared by the investigated synthetic methods are due mainly to the differences in OH and H2O content and gener-ally follow the findings reported in[11]. A detailed description and interpretation of the spectra of all the prepared com-plexes is given in[30].

The NMR study[31] reveals: – Good stoichiometry of the complexes. Table 1 shows, as

an example, the shifts in the 1H spectrum and the rather good agreement between the theoretically expected number of H-atoms in Eu(DBM)3·dpp (taking the intensity of the me-thyne CH shift at 2.3 ppm as a base) and the number found experimentally.

– The content of water (derived from the intensity of the peak at 3.96 ppm). The product prepared by Melby’s proce-dure contains up to 1 mol H2O/mol complex if the drying is done at pressure higher than 0.7 kPa.

– The completeness of the complexation process and the eventual presence of unreacted free ligands in the final product (indicated by the peaks in the 7.0–7.9 ppm region in the 1H spectrum and around 128 ppm in the 13C spectrum).

Table 1 Shifts and signals values in 1H NMR spectrum of Eu(DBM)3·dpp

Number of atoms Atom position

(see Fig. 1)

Shift/

ppm

Signal

intensity Theoretically Experimentally found

1 3.26 1.84 3

2 6.22 6.95 12 11.3

3,4 6.86 11.26 18 18.4

Total from DBM 33 32.6

2,9 11.34 1.00 2 1.6

3,8; 3(Ph) 8.90 3.82 6 6.2

5,6 10.13 1.05 2 1.7

2(Ph) 8.36 2.33 4 3.8

4(Ph) 8.09 1.70 2 2.8

Total from dpp 16 16.1

Overall 49 48.7

Fig. 1 Schematic formula of Eu(DBM)3·dpp

Maria Milanova et al., Lanthanide complexes with -diketones and coumarin derivates: synthesis, thermal behaviour, optical … 69

X-ray analysis revealed that the interplanar distances (in nm and, in brackets, relative intensity in %) of Eu(DBM)3 are 1.523 (51), 1.341(57),1.202 (100), 1.148 (60), 0.988 (54), 0.847 (29), 0.690 (15), 0.601 (25), 0.521 (25), 0.493 (23), 0.452 (29), 0.420 (20), 0.338 (30). The complex Eu(DBM)3·dpp is X-ray amorphous.

Single crystals were prepared by dissolving the powdered material in a solution of tetrabutylammonium hexafluoro-phosphate (TBA) and dimethyl sulfoxide (DMSO). After a few weeks, needle-like crystals of TBA+[Eu(DBM)4]– were obtained at ambient temperature. The symmetry unit of the crystal structure consists of two crystallographically inde-pendent [Eu(DBM)4]– complexes, two TBA+ ions as counter ions and two DMSO solvent molecules. Each of the Eu at-oms is coordinated by four bidentate DBM ligands, resulting in the coordination number of eight (Fig. 2). The Eu–O dis-tances vary between 0.235 and 0.242 nm with average values for the two Eu coordinations of 0.2388 nm and 0.2398 nm, respectively. All the crystallographic data are given in[32].

Coumarin complexes: The main problem when studying the lanthanide complexes with coumarin derivates was to eluci-date the coordination of the carbonyl oxygen to the metal ion. An additional obstacle was the space hindrance and interfer-ence caused by the position of the carbonyl oxygen in the bi-cycle coumarin ring, namely at the 2nd carbon atom as well as the hydroxyl groups at the 4th carbon atom. The formation of single crystals is the only way to solve the problem.

So far, infrared spectroscopy data were used to find out the possible change in the C=O band position in the infrared spectra as well as the presence of water in the molecule. No significant differences in the IR spectra of the complexes were observed. The valence band of the C=O group in the ligands IR spectra was at rather low wavenumbers (down to 1655 cm–1), which is probably due to its participation in very stable hydrogen bridges (Fig. 3).

Fig. 2 ORTEP plot of the TBA+[Eu1(DBM)4]– complex with num-

bering scheme and displacement ellipsoids at the 50 % prob-ability level (hydrogen atoms omitted) (the second europium complex and the tetrabutylammonium counter ions of the asymmetric unit are omitted for clarity)

Fig. 3 IR spectra in the 1750–1350 cm–1 of L15 (top) and NdL15

(Nd2L3·4H2O) (bottom) The complex formation is causing changes in the 1700–

1350 cm–1 region (Fig. 3). No band occured at 1655 cm-1, which is in agreement with literature data[4,5] and is an indi-cation for coordination by carbonyl groups.

Based on the IR data, a structure formula (Fig. 4) is pro-posed that requires further proof.

XRD revealed that the complexes in powder form were amorphous. Cubic like crystals of Nd2L3·4H2O (NdL15) with size about 250 nm were observed by scanning electron microscopy in a thin layer obtained from a DMSO solution (Fig. 5) after evaporation.

Fig. 4 Proposed structure formula of Nd2L3·4H2O (NdL15)

Fig. 5 Film of NdL15/DMSO on glass substrate

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2.1.3 Thermochemical behaviour of the complexes The DTA data show that the melting of the complex prepared by the method[12] is completed at ~210 °C and its destruction starts (with weak mass-loss) in the interval 275–305 °C. Burning of organic ligand decomposition products proceeds around 500 °C and traces of remaining carbon above 540 °C, causing a mass loss of 6%–7%. The overall mass loss of 80.0% is rather close to the expected 78.7% calculated if Eu2O3 is assumed as the final product of the thermal destruc-tion. The decomposition of Eu(DBM)3·dpp occurs in a simi-lar way. The destruction of the complex proceeds above 260 °C with the separation of the dpp-ligand (mass loss in the inter-val 260–390 °C: found 27.8 %, calculated 28.8 %). 2.1.4 Optical properties of the complexes Bands in the range of 200–400 nm in the excitation spectra of all of the synthesised complexes belong to absorption of the organic ligands ( - * electron transition). The bands at 300–400 nm (maximum at ~350 nm) and at 260–270 nm are assigned to the DBM ligands and that at 210–300 nm with maximum around 250 nm is related to phenanthroline ligands. The ad-dition of dpp to Eu(DBM)3 causes a small change of the peak positions.

The emission spectra contain all the typical bands of Eu3+. The substitution of phen with dpp does not cause any change in the solid spectra of the complexes. The data for the ob-served luminescence life-time are perfectly fitted with a sin-gle exponent. The life-times of the excited states (in solid) are 82 s for Eu(DBM)3, 326 s for Eu(DBM)3·phen and 363 s for Eu(DBM)3·dpp.

The partial substitution of DBM ligands with OH– does not influence the band positions in the excitation and emis-sion spectra but the intensities significantly depend on the synthesis conditions of the complexes. The presence of OH decreases the fluorescence emission intensity with ~80% compared with the OH-free product. The recrystallisation of the initial products leads to better expressed emission spectra.

No significant differences in the emission spectra are ob-served as a result of the substitution of phen with dpp.

Excitation and emission spectra of the coumarin derivates and the complexes were obtained. Not all of the coumarins have the same emission spectra, which deserves further elu-cidation. The spectra of L15 and Tb2L3·2H2O are shown in Fig. 6. In the spectrum of Tb2L3·2H2O, the typical energy transfers are visible, including the most intensive 5D4–7F5 transfer at 548 nm. An emission spectrum for the corre-sponding Nd2L3·4H2O (NdL15) was not obtained, possibly because of the stronger non-radiative relaxation processes taking place that influence the Nd energy transfers.

2.2 Immobilisation of the complexes

2.2.1 Composite synthesis The procedure described in Ref. [28] proposes the following conditions for the SiO2 ma-trix preparation: (1) Sol preparation: mole ratio TEOS:C2H5OH: H2O=1:16:4, adjusted adding H2O with pH=8, thus leading to a pH of the solution 6.5. A decrease of the pH (especially below 6) leads to partial destruction of the

Fig. 6 Emission spectra of L15 (top) and TbL15 (bottom)

complex. The decrease of the ethanol relative content (at constant ratio TEOS:H2O=1:4) renders difficulties in the control of hydrolysis. The complex is introduced in the sol as ethanol-DMF solution (volume ratio 14). (2) Films (typical thickness of ~300 nm) are deposited after aging of the sol at 50 °C for 5 h or 15 h if membranes will be produced. The deposition is done by dip-coating at a withdrawal speed of 0.2 mm/s. Membranes (1–2 mm in thickness) were prepared by casting the gel in Teflon® moulds. Specimens were dried at 40 °C for 72 h (films) or 90 h (membranes) and needed 2 weeks of storage at ambient temperature in a vacuum dessi-cator for complete solidification. 2.2.2 Properties of the composites SiO2-based compos-ites: The composites’ IR spectra contain mainly the bands characteristic for the pure matrix and are practically inde-pendent of the preparation method of the complexes.

The thermal destruction of the composites proceeds mainly at 260–355 °C and is completed at ~650 °C. The general pattern of DTA, DTG and TG curves of the compos-ite containing the ternary complex Eu(DBM)3·dpp is practi-cally the same as that of the Eu(DBM)3-containing compos-ite.

The main excitation band in the spectrum of the latter is shifted to longer wave lengths compared with the pure com-plex ( max=350 nm) and appears in the interval 390–450 nm with a maximum at 417 nm. No significant shift in the main excitation band is observed in the composite with Eu(DBM)3·dpp compared with the pure complex.

Maria Milanova et al., Lanthanide complexes with -diketones and coumarin derivates: synthesis, thermal behaviour, optical … 71

The emission spectrum exhibits typical ligand-sensitised emission of Eu3+ ions. The bands of the 5D0

7F2 transition are not broadened compared to the pure complex and the number of Stark components is preserved. In the emission spectrum of the composite with Eu(DBM)3·dpp, bands be-low 560 nm (some of which are hinted only in the pure com-plexes’ spectra and in the spectrum of the Eu(DBM)3-con-taining material) are well expressed.

Luminescence lifetime decay curves of the composites were satisfactory fitted with first order exponential functions. The excited states decay half lives (ms) are increased (com-pared with the complexes in the solid state) to 527 and 506 for the composites with Eu(DBM)3 and Eu(DBM)3·dpp, re-spectively.

Composites preserved their photoluminescence for at least two years storage at ambient conditions.

Polymethylmethacrylate based composites: Polymethyl-methacrylate films produced from monomer are rather flat. The colour darkening suggests some partial destruction of the complexes. No such changes are observed in films from polymethylmethacrylate solution. They are dense, uniform (Fig. 7) with regularly distributed complex.

In both types of matrices, excitation and emission spectra of the respective complexes are preserved (Fig. 8, compare with the data in[30]) and the difference in peak positions be-tween the two matrices is less than 0.8 nm. Soaking of the composites in water for two weeks does not change the spectra but leads to a decrease of the photoluminescence in-tensity with approximately 15%.

The response to O2 of the microcomposite produced from

Fig. 7 SEM (top) and AFM (middle, bottom) images of film pro-

duced from polymethyl-methacrylate solution (top, middle) and polymerized monomer (bottom)

Fig. 8 Emission spectrum of the composite with Eu(DBM)3·phen

the polymer is shown in Fig. 9 through the well known Stern-Volmer,s law I0/I=1+K[O2], where I0 and I are the emission intensities in the absence and presence of oxygen, respectively, [O2] is the concentration of O2 and K is the Stern-Volmer quenching constant. The value of the latter is 5.1×10–3%–1 for gas phase oxygen and 13.5×10–3 ppm–1 for oxygen dissolved in water; the correlation coefficients for the linear fitting are 0.992 and 0.990, respectively. The line-arity of the film produced by monomer polymerisation is also very good but the value of the constant is ~3.5 times less.

Fig. 9 Stern-Volmer dependence for the polymethylmethacrylate

based composite (gas phase)

3 Discussion

3.1 Peculiarities of complex synthesis

3.1.1 Diketonates The main problem in the synthesis of the dibenzoylmethanates is the chemical and thermal insta-bility of the complexes. Three crucial parameters can be out-lined in their preparation: the pH of the solution and the temperature and pressure of heating. The pH-value has to be high enough (above 6) to ensure stability of the DBM-com-plexes, which are easily destroyed by acids. At the same time the pH has to be low enough to avoid or minimise the precipitation of Ln(OH)3. Increasing the pH value to 7 at 80 °C leads to the formation of significant amounts of Ln(DBM)2OH. The experimental results suggest that a pH of

72 JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010

6.5 is optimal. The Eu(DBM)3 produced by the method of Melby[12] as well as Eu(DBM)3·dpp are thermally more sta-ble and the commonly applied isothermal recrystallisation from ethanol is of use for removal of any hydroxides present. The most important stage in Melby’s process is the drying of the complex. Practically H2O-free complex can be obtained if the drying is done at a temperature not higher than 110 °C if the pressure is kept below 700 Pa.

3.2 Properties of complexes

3.2.1 Diketonates The synthetic method has no signifi-cant impact on the IR spectra of the complexes. To our best knowledge, the only published X-ray powder diffraction data (without exact values of interplanar distances and relative intensities) for complexes of the type Ln(DBM)3 (Ln-lantha-nides) are those for La- and Tb-dibenzoylmethanates[33]. Comparison with data obtained here reveals similarity in the diffractograms of the complexes of the different lanthanides, which are rather different from the one of the pure ligand. The interplanar distances are large, as can be expected from the large ligand size. The crystallisation is not complete and an X-ray amorphous phase exists in the studied sample. In-troducing an additional large ligand in Eu(DBM)3·dpp inhib-its the crystallisation and the complex is X-ray amorphous. The X-ray single crystal analysis show that after dissolving of Eu(DBM)3 in DMSO in the presence of tetrabutyl- am-monium, a reconstruction of the complex takes place leading to formation of TBA+[Eu(DBM)4]– [32].

The analysis of the excitation and emission spectra of the complexes leads to the following conclusions: (1) The DBM electronic system is rather strong and the addition of dpp to Eu(DBM)3 causes an insignificant perturbation in it. The in-ternal coordination sphere of the Eu3+-ions contains oxygen atoms of the main ligands while dpp is located in the exter-nal sphere and does not have a significant effect on the symmetry properties of the Eu3+ polyhedron. (2) The in-crease of the intensities as a result of the decrease of the OH–

content in the complex coordination sphere can be explained with the decreased possibility for efficient non-radiative re-laxation processes taking place via vibronic coupling with the vibrational states of the OH oscillators[34]. (3) The perfect fitting of the data for the observed luminescence life-time with a single exponent as well as the splitting of the band at 612 nm shows the existence of only one emission centre in all of the studied specimens, suggesting a relatively ordered Eu3+ environment. (4) The observed life-times of the excited states of the solids are consistent with other literature data but are lower than the value of 1993 s for Eu(DBM)3 re-ported in Ref. [35]. 3.1.2 Coumarin complexes The complexes obtained are stable in air at room temperature (they were usually kept in a dessicator). At ignition of a powder sample in air, no changes were observed up to about 200 °C. The compounds were observed to be very soluble in DMSO and DMF, poorly soluble in ethanol and acetone and practically insolu-ble in water.

The pharmacological properties of the ligands and com-plexes were tested towards three different tumor cell lines (SKW-3, BV-173 and K-562). The complex Nd2L3·4H2O (NdL15) showed higher toxicity in comparison with the ligand L15 (Fig. 10).

Fig. 10 Cytotoxicity of Nd2L3·4H2O (NdL15, dark grey) and L15

(light grey) towards three cell lines

3.2 Composite properties

3.2.1 SiO2-based composites In the FTIR spectrum of SiO2-based composite, the bands for Si-O-Si bonds (1088, 793 and 460 cm–1) are dominating along with the one spe-cific for Si-OH (965-970 cm–1). Very weak bands of the complexes are observed for the first time in the spectra of this type of systems. Apart from the low content of the com-plexes in the composite samples, another reason for the ab-sence of their bands could be the hindered vibration of the complex ligands by the surrounding matrix structure[34]. The composite spectrum is a superposition of the matrix and of immobilised complex spectra without significant shifts of the bands.

Due to the low relative content, the embedded complex has an insignificant influence on the thermally induced changes in the microcomposite. Combining the results re-ported in[36] and findings in this work, the thermal destruc-tion of the composites includes evolution of traces of ethanol (40–140 °C) and DMF (140–260 °C) left from the synthetic procedure, followed by condensation between Si-OH groups (>205 °C). The results confirm the formula Si4O5OHO5/2 proposed in Ref. [37] as a constructive unit of the matrix Si-O-Si network, leading to a theoretically expected mass-loss of 26.8% for the complex-free matrix, which is very close to what was found experimentally. It seems that the neutral ligand of the composite containing ternary com-plex is lost at temperatures up to 280 °C.

The optical properties of the composite are practically in-dependent of the complex synthesis method. However, they are significantly influenced by the synthesis conditions of the composite, mainly due to the thermal instability of the com-plexes and their limited solubility in ethanol. These facts impose some limitations on the synthetic procedure:

– The temperatures for aging of the sol and drying of the films or membranes have to be limited to 40 °C for thin films and 50 °C for the ticker membranes. Above these temperatures the fluorescence strongly decreases (ternary complex) or disappears (binary complex). At the same time,

Maria Milanova et al., Lanthanide complexes with -diketones and coumarin derivates: synthesis, thermal behaviour, optical … 73

drying at ambient temperature does not allow complete so-lidification of the specimens even for a prolonged period.

- Due to lower solubility of the complex in ethanol, the usage of DMF (or similar) as solvent combined with ethanol is practically unavoidable. However, a higher content of DMF in the solution (e.g. at volume ratio ethanol/DMF=6/7) results in the appearance of opalescence after 6 weeks of storage of the composite and a change in the optical proper-ties of the composite that is probably due to the disturbance of the Eu3+ coordination shell caused by the strong donor ability of DMF.

– Despite the positive effect of sonication[37] on the uni-form distribution of the complex in the immobilisation ma-trix, it causes quenching of the photoluminescence that is probably due to some destruction of the complexes and is not to be recommended.

– The presence of contaminant ions (using NaOH instead of NH4OH for adjusting of pH of the water added for alkox-ide hydrolysis) strongly decreases the excitation state life-time to 31 s for Eu(DBM)3 and to19 s for Eu(DBM)3·dpp- containing composites.

The immobilisation of the complex in the supporting ma-trix leads to the following conclusions:

– The matrix effect on the coordination shell of the Eu3+ as well as the energy transfer between ligands and Eu3+ ions is realised not only when the complex is prepared in situ during the composite preparation[38] but also when the latter is in-troduced in the matrix as finished product. The interaction is confirmed by: (1) the slight narrowing of the Eu(DBM)3 ex-citation band; (2) the appearance of a band at 443 nm in the spectrum of the composite with Eu(DBM)3.dpp (present as a shoulder in the composite with the binary complex), which can be attributed to some specific interaction of the ligands (mainly the neutral one) with the matrix; (3) the red shift of the main excitation band in the composite spectrum (417 nm) compared with the one of the pure complex solution (350 nm), which can be attributed to a saturation occurring in the crys-tal[39].

– Single average site distribution (a uniform surrounding environment) of Eu3+ ions is preserved in the composites in which only one type of emission centre exists. This result is in agreement with the results reported in[34] and confirms the relatively ordered environment of the Eu3+ in the immobili-sation environment.

– An increase of the luminescence lifetime of both com-plexes after their embedding in the matrices. The effect has been observed before[11,34] and is ascribed to a less efficient non-radiative 5D0 relaxation process due to the decrease of the number of OH groups coordinated to the lanthanide ion[11]. Considering the rather low content of H2O in the pure complex used in the present paper, the effect can more probably be explained with the restriction of ligand vibra-tions when embedded in the rigid structure of the silica gel[34]. 3.2.2 Polymethylmetacrylate-based composites The ex-perimental data (change of the colour of the complex as a

result of its entrapping in the matrix and low sensitivity of the microcomposite to oxygen) show that the complexes used in this study are unstable in presence of peroxide and therefore the preparation of the matrix by monomer polym-erisation is not a suitable method for this purpose.

The nature of the immobilised complex influences the film morphology[40] and its roughness increases in the or-der Rudpp<Eu(DBM)3<Eu(DBM)3·phen<Eu(DBM)3·dpp< Eu(TTA)3<Eu(TTA)3·phen<Eu(TTA)3·dpp, (TTA=thenoyl-trefluoracetone). Crystals of the complexes are not observed in the films (such crystallisation was reported in Ref. [37]).

The microcomposite prepared from polymer solution dis-plays a higher sensitivity to oxygen. The difference can be due to (1) the partial destruction of the complexes in the po-lymerisation process when monomer is applied as starting material and (2) the AFM observed a higher porosity of the produced polymer film.

4 Conclusions

The paper presented an overview and results from the ex-perimental testing of known methods for synthesis of some lanthanide complexes with -diketones and coumarin deri-vates as well as for preparation of matrices based on SiO2 or polymethylmethacrylate for their entrapping. On this ground the optimal synthetic methods and procedure details were recommended. The thermal destruction of the complexes was evaluated. The impact of the synthetic conditions and the influence of the immobilisation of the complexes on their fluorescence properties were partially elucidated.

The lanthanide complexes with 3,3’-[(4-chlorphenyl) me-thylene)bis(4-hydroxy-2H-chromen-2-one) showed potential for pharmaceutical application.

Acknowledgment: The pharmacological tests were performed by Prof. S. Konstantinov and his colleagues from Medical University, Sofia.

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