Study of thermal and optical properties of dibenzoylmethane Eu ( III ) organic complexes

Two Europium(III) organic complexes (ternary complex Eu(DBM)3(PHEN) and tetrakis complex [Eu(DBM)4]N(Et)4, where DBM is dibenzoylmethane and PHEN is 1,10-phenantroline) were synthesized and their structures proven with H-NMR spectroscopy, elemental analysis, FT-IR and mass spectroscopy. Their photophysical properties have been investigated in solution, solid-state and in poly-N-vinylcarbazole (PVK) films varying complex composition from 1 to 15 wt%. Both complexes exhibit intense UV light absorption and bright red-light emission with maximum wave length at 612 nm. It was shown that tetrakis complex in solution, solid-state and PVK films have enhanced photoluminescence properties compared to ternary complex due to four DBM molecules in Eu coordination sphere, which leads to more effective excited energy transfer to Eu ion. In THF solutions both complexes are characterized with low photoluminescence quantum yields (4% and 6%), in PVK films – moderate quantum yields (14–48%), but in solid state with high quantum yields (56% and 75%). Thermal properties for pure complexes and their PVK films were studied with thermogravimetric analysis. Both complexes exhibit high thermal degradation temperatures of 304 C for ternary and 267 C for tetrakis complex, but their PVK films show even higher degradation temperature (over 370 C).


INTRODUCTION
* Unique optical properties such as narrow emission bands, high photoluminescence quantum yields, large Stokes shifts and long excited state lifetimes mark out Eu 3+ organic complexes among other organic red-light emitting materials for technological application in solidstate lighting and flat panel displays [1].Luminescence of complexes arises from efficient excited energy transfer from organic ligand molecules to central metal ion, resulting in bright emission of Eu 3+ .Investigation of effective ligands for Eu 3+ organic complexes mainly focuses on -diketones due to their intense UV absorption (high extinction coefficient), relatively easy synthesis, good solubility in commonly used solvents and good thermal stability.Dibenzoylmethane (DBM) is one of the most used ligand for Eu 3+ organic complexes.Many scientific articles report DBM Eu 3+ ternary complexes, where Eu 3+ is coordinated with three DBM ligands and one secondary ligand, such as 1,10-phenantroline (PHEN) [2,3].This complex and its poly-N-vinylcarbazole films have been fully examined as emitting layers in organic light emitting diodes (OLED) [4,5].However, to our knowledge there are only 3 works devoted to investigate tetrakis Eu 3+ complexes with DBM ligands with general structure [Eu(DBM) 4 ] -(Eu 3+ complex is anionic and charge neutralization is required by quaternary ammonium ion countercation) [6][7][8].Eu 3+ tetrakis complexes usually exhibit higher luminescence efficiency and chemical stability as well as better solubility in solvents than ternary complexes.There are no data in the literature dealing with comparison of ternary and tetrakis complexes bearing the same -diketone ligands, therefore in this work DBM is synthesized, and two Eu 3+ complexesternary complex Eu(DBM) 3 (PHEN) and tetrakis complex [Eu(DBM) 4 ]N(Et) 4 are characterized and their thermal and optical properties are reported.However, pure organic complexes display low thermal-and photo-stability as well as poor mechanical properties and film formability, which limits their practical use.Therefore, another objective of this work is the investigation of optical and thermal properties of poly-N-vinylcarbazole (PVK) films containing different concentration of the Eu 3+ complexes.
1 H-NMR spectra were recorded in CDCl 3 and (CD 3 ) 2 CO solutions (5-7 mg/ml) on a Brucker Avance 300 MHz spectrometer.CHN elemental analyses were performed on Euro Vector EA 3000 analyser.The FT-IR spectra were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer using KBr pellets.Low resolution mass spectra were determined on a Waters EMD 1000MS mass detector (ESI mode, voltage 30 V). Thermal properties of solid complexes and PVK films were determined on a Perkin Elmer STA 6000 instrument.Solid complexes were heated from 30-900 C with heating rate 10 C/min, but PVK films from 30-700 C with heating rate 3 C/min in nitrogen atmosphere.The UV-Vis absorption spectra (solutions and films) were determined on Perkin-Elmer 35 UV/Vis spectrometer.Emission and excitation spectra were measured on QuantaMaster 40 steady state spectrofluorometer (Photon Technology International, Inc.).Absolute photoluminescence quantum yields were determined using 6 inch integrating sphere by LabSphere coupled to the spectrofluorometer.The values reported are the average of three independent measurements for each sample.PVK thin films were obtained with Laurell WS-400B-NPP/LITE spin coater.

Fabrication of PVK films
PVK and required amount of complexes C1 or C2 were dissolved in 1 ml THF and the resulting mixture was heated at 40 C for 0.5 h.Afterwards polymer film was spin-coated on a glass substrate using following parameters: speed 800 rpm, acceleration 800 rpm/sec 1 min.Then obtained films were dried in 45 C for 2 h.
For complex C2 mass spectra analysis show mass peaks with m/z: 130.1; 1043.8 and 1045.0 corresponding to [N(Et) 4 ] + ; [ 151 Eu(DBM) 4 ] -; [ 153 Eu(DBM) 4 ] -, which establishes, that complex is with tetrakis structure.Moreover, 1 H-NMR spectra of C2 shows two muliplets (7.77 and 7.18 ppm) and one singlet (2.85 ppm) corresponding to 44 protons of four DBM molecules, but in higher fields quartet at 4 ppm and triplet at 1.65 ppm corresponding to 20 protons of N + (Et) 4 molecule were observed, leading to conclusion, that Eu 3+ is coordinated with four DBM and one N + (Et) 4 molecule.

Thermal properties
To evaluate C1 and C2 thermal durability, thermogravimetric (TG) analysis were conducted in nitrogen atmosphere from 30-900 C and TG curves are shown in Fig. 2. Both complexes C1 and C2 exhibit high thermal decomposition temperatures T 5% (T 5% is temperature of 5% weight loss) -304 and 267 C, respectively.TG curve of complex C1 consist of two mass loss steps around 310-443, and 444-556 C.First mass loss step (45%) corresponds to the loss of two DBM molecules, second stage (22%) -to one DBM molecule.Further thermal decomposition from 557 to 900 C leads to loss of PHEN molecule and residual weight of 20%.Similar thermal degradation pattern was reported for other Eu 3+ complexes with DBM derivatives and PHEN ligands [10].Whereas TG curve of C2 exhibits three thermal degradation stages: 268-295, 296-451, 452-590 C, with mass losses of 19%, 30% and 19%, corresponding to loss of one DBM molecule, then one DBM and N + (Et) 4 molecule, and lastly one DBM molecule, respectively.Further thermal decomposition from 591 to 900 C leads to loss of last DBM molecule and residual weight of 17%.Although T 5% of complex C2 is lower, compared to C1, it is still higher for 50 C than other Eu 3+ tetrakis complexes with -diketone ligands and quaternary ammonium ions [11,12].
It is known, that thermal properties can be improved by incorporating complexes in polymer matrixes, therefore TG analysis of PVK films containing 15% C1 or C2 were also conducted.TG curves of pure PVK film and films containing complexes are depicted in inset of Fig. 2. Pure PVK film shows small mass loss till 300 C, which can be assigned to entrapped solvent removal from polymer films.Degradation of polymer occurs in a narrow temperature interval (366-453 C).
However, PVK films blended with complexes C1 or C2 show slight increase in thermal degradation temperature interval.For PVK+15% C1 major degradation occurs at 392-467 C, but for PVK+15% C2 at 379-467 C.The thermal degradation data indicate that incorporation of C1 or C2 in PVK leads to polymer films with high thermal decomposition endurance.

UV-Vis absorption properties
The UV-Vis absorption bands of pure DBM and complexes C1 and C2 were measured in THF solutions (c1.510-5 M), and are depicted in Fig. 3. Absorption spectra of DBM shows broad band with maximum at 343 nm, which is assigned to enolic -* transitions of the -diketone moiety.Furthermore, DBM exhibit high molar absorbance coefficient  -26287.5 M -1 cm -1 .The absorption spectra of complexes C1 and C2 in THF solutions are similar to the one of free DBM molecule, confirming that coordination with Eu 3+ ions do not affect ligands singlet excited states.Similar observations were reported for other -diketones and their Ln 3+ complexes [7,11].The small red-shift (7-11 nm) of absorption band maximum for C1 and C2 compared to DBM can be explained with the more effective conjugation of electrons in Eu 3+ complexes.Furthermore, absorption intensity is enhanced in the complexes C1 and C2 due to expanded -conjugated system, which forms, when Eu 3+ coordinates with DBM and PHEN molecules.The  of C1 and C2 reaches 68553 and 92967 M -1 cm -1 , which are 2.61 and 3.54 times higher than DBM molar absorptivity, suggesting, that these complexes exhibit strong ability to absorb light in the 320-380 nm region.
Figure 3 also shows absorption spectra of pure PVK and PVK thin films containing 15% of complexes C1 or C2.Pure PVK film shows complicated band shape with absorption maxima at 271, 297, 332 and 346 nm, which can be assigned to -* and n-* transitions.Absorption spectra of PVK films with C1 and C2 exhibit almost the same absorption profile as pure PVK film, only absorption intensity is increased at 332 and 346 nm due to overlap of PVK and complexes absorption spectra.Furthermore, new non-pronounced absorption band arises from 356 to 410 nm, which can be assigned to ligands absorption of complexes C1 and C2.

Photoluminescence properties in solutions and solid state
Photoluminescence properties of Eu 3+ complexes were investigated in THF solutions (1.510 -5 M) and in solid-state.The excitation spectra of Eu 3+ complexes monitoring Eu 3+ transition at 612 nm are displayed in Fig. 4. The excitation spectra of C1 and C2 exhibit broad band between 300-425 nm in THF (Fig. 4(a)) and 300-480 nm in solid-state (Fig. 4(b)), which can be assigned to absorption of DBM ligands.As absorption spectra of DBM and excitation spectra of complexes overlap, one can conclude that in synthesized complexes energy transfer occurs from DBM ligands to Eu 3+ ions.The maximums of excitation spectra for C1 and C2 in THF are at 373 and 384 nm, but in solid state a red shift to 402 and 403 nm occurs.This red shift of excitation wavelength maxima could be explained with the presence of strong - interactions between ligands in the solidstate.The absorption bands of f-f* transitions of Eu 3+ ion (at 296, 362, 381, 394, 415, 466 and 527 nm) are not present in the excitation spectra of THF solutions, however in solid-state spectra transition at 466 nm ( 7 F 0 → 5 D 2 transition) is clearly seen.This transition is usually observed for Eu 3+ complexes in solid state [6,7].Emission spectra of C1 and C2 in THF solutions as well as in solid-state shows characteristic Eu 3+ 5 D 0  7 F J (J=0-4) transitions at 580, 593, 612, 651 and 703 nm.The highest intensity is observed for 5 D 0  7 F 2 transition at 612 nm, which is induced electric dipole transition and its intensity is dependent on coordination environment of Eu 3+ ion.As it was expected tetrakis Eu 3+ complex C2 exhibits higher intensity for 5 D 0  7 F 2 transition than ternary complex C1 in THF and in solid-state due to fact, that coordination sphere of Eu 3+ in C2 is filled with four DBM ligands, which efficiently transfer the excitation energy to central metal ion and also provides better shielding of metal ion from surrounding environment.Furthermore, it is known that in ternary complexes PHEN is coordinated weakly to Eu 3+ ion, which could lead to dissociation of PHEN from some complex molecules and formation of tris complex Eu(DBM) 3 .Formation of some tris complexes could also decrease luminescence intensity for C1 complex.Furthermore, emission band at 612 for complex C1 in both solution and solid-state is split and has secondary maximums at 618 and 622 nm.However, for complex C2 in solution the emission band is broadened with one maximum at 612 nm.Similar observation was reported for other tetrakis Eu 3+ complex with DBM ligands and trioctylmethylphosphionium as counteraction [7] and was explained by interactions between [Eu(DBM) 4 ] -and counteraction ions in solution.
The absolute photoluminescence quantum yields were 4% for ternary C1 complex and 6% for tetrakis C2 complex in 1.510 -5 M THF solutions.Furthermore, in a solid-state photoluminescence quantum yield reached 56% for C1 and 75% for C2.Such luminescence quantum yield enhancement in solid-state compared to solutions can be explained with deactivation of ligand molecules in solvent medium by collisions.For Eu 3+ complexes with -diketone ligands it is common to have higher luminescence quantum yields in solid-state rather than in solutions [7,10,12,14].In conclusion, comparison of emission properties of ternary and tetrakis Eu 3+ complexes shows that tetrakis complex [Eu(DBM) 4 ]N(Et) 4 has enhanced emission properties in solid-state and has considerable potential for light emitting materials.

Photoluminescence properties in PVK thin films
In this work PVK was used as polymer matrix for Eu 3+ complexes due to its excellent hole-transporting properties.Furthermore, only few polymers have been established as good hosts for organic lanthanide complexes and PVK is one of them due to good match between triplet energy level of the polymer host and complex and also suitable overlap between emission spectra of polymer and absorption spectra of Eu 3+ complex.PVK thin films doped with 1, 4, 8 and 15 wt% of complex C1 or C2 were prepared by spin-coating method from THF. Excitation spectra of polymer films with complex C1 and pure PVK thin film are shown in Fig. 5, inset shows emission spectra of pure PVK film.Excitation spectra of doped polymer films overlaps with pure PVK emission spectra, indicating possibility of absorbed excitation energy transfer from polymer host to Eu 3+ complex.Such energy transfer would give an advantage for thin PVK films, because polymer host would provide additional excitation energy for complex, which would lead to enhancement for film emission intensity.
The excitation spectra of PVK films shows broad bands in region from 300 to 410 nm with maximum at 348 nm, which corresponds to absorption of PVK matrix.However excitation bands have shoulder at 370 nm, which corresponds to absorption of C1.
The emission spectra of polymer films (depicted in Fig. 6) exhibit the same emission bands as those for complexes in THF solutions or solid-state.PVK films with 1% C1 or C2 also shows a weak polymer emission band from 360 till 560 nm indicating, that energy transfer from PVK to complexes is incomplete.This is due to great distance between polymer chain and complex in these films.However, for PVK films with higher complex concentration, the energy is completely transferred and no emission band of PVK is observed in spectra.Similar observations were reported for PVK films with tetrakis and ternary Eu(III) complexes with 2-thenoyltrifluoroacetonato ligands [15,16].Intensity of emission band at 612 nm and absolute photoluminescence quantum yields increases with increasing complex concentration in PVK matrix.The quantum yields for series of ternary complex C1 PVK films (1, 4, 8, 15 wt%) increase in following order 14%, 21%, 26% and 30%, respectively, but for tetrakis complex C2 films (1, 4, 8, 15 wt%) -17%, 30%, 46% and 48%.For tetrakis complex C2 increasing complex mass in the PVK from 8% to 15% has an insignificant effect on 5 D 0  7 F 2 (612 nm) transition intensity and quantum yield.This can be explained with the fact that in film with high Eu 3+ ion concentration an energy transfer between Eu 3+ ions occurs as a non-radiative process.Therefore intensity of complex luminescence does not increase and sometimes even decreases in films with high complex concentration [13,17].Registered quantum yield values for investigated PVK films are at the same level as the ones for the known films of Eu 3+ β-diketone complexes in other polymers [18].It can be concluded, that tetrakis complex [Eu(DBM) 4 ]N(Et) 4 exhibits more intense emission not only in solution and solid-state, but also in PVK films.

CONCLUSIONS
In summary ternary and tetrakis Eu 3+ complexes with general structures Eu(DBM) 3 (PHEN) and [Eu(DBM) 4 ]N(Et) 4 were prepared and their structures were confirmed by elemental analysis, FT-IR, 1 H-NMR and mass spectroscopy.Investigation of photoluminescence properties of complexes in THF, solid-state and PVK films shows that emission spectra contains characteristic Eu 3+ ion emission lines with the most intense transition at 612 nm.Excitation window for both complexes lies in the near-UV region (300-400 nm), providing large Stokes shifts.Enhanced emission intensity and photoluminescence quantum yields were found for both complexes in solid-state (quantum yields 56% and 75%).PVK polymer films containing Eu 3+ DBM complexes also showed moderate quantum yields (14-48%).The luminescence intensity and quantum yields increased with increasing complex concentration in PVK films and reached maximum at doping concentration of 15 wt%.Tetrakis complex [Eu(DBM) 4 ]N(Et) 4 in solution, solid-state and PVK films have enhanced photoluminescence properties compared to ternary complex due to four DBM molecules in Eu 3+ coordination sphere, which leads to more effective excited energy transfer to Eu 3+ ion.The results of thermogravimetric analysis of complexes revealed that they exhibit high thermal degradation temperatures.