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Thermoreversible luminescent organogels doped with Eu(TTA)3phen complex
Journal of Colloid and Interface Science 398 (2013) 95–102
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Thermoreversible luminescent organogels doped with Eu(TTA)3phen complex Maria Laura Di Lorenzo a, Mariacristina Cocca a,⇑, Gennaro Gentile a, Maurizio Avella a, David Gutierrez b, Monica Della Pirriera b, Manus Kennedy c, Hind Ahmed c, John Doran c a
Istituto di Chimica e Tecnologia dei Polimeri (CNR), Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy Leitat Technological Center, C/de la Innovació, 2, 08225 Terrassa, Barcelona, Spain c Dublin Energy Lab, Focas Institute, DIT, Kevin Street, Dublin 8, Ireland b
a r t i c l e
i n f o
Article history: Received 21 December 2012 Accepted 2 February 2013 Available online 18 February 2013 Keywords: 12-Hydroxystearic acid Europium complex Luminescent organogel Thermoreversible gel FTIR Morphology Luminescent quantum yield
a b s t r a c t This manuscript details the preparation and characterization of luminescent organogels in toluene. Gels were prepared by using 12-hydroxystearic acid (12HSA) as gelator and different amounts of thenoyltrifluoroacetonato 1,10-phenanthroline europium(III) complex (Eu(TTA)3phen). The gelation properties and the thermoreversible behavior from solid-like to liquid systems were investigated by differential scanning calorimetry. At higher concentration, an interaction of Eu complex with the polar group of the gelator was revealed by DSC and FTIR analyses. The spectroscopic behavior of the complex was investigated in toluene solution and in the gel state. TEM analysis revealed that 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The Eu(TTA)3phen in the gel state exhibits a very high emission quantum yield, U, which was found to be independent of Eu complex concentration, at least for the composition range analyzed. These results indicate that 12HSA organogels containing Eu(TTA)3phen are promising materials for optical applications. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Organogels are soft materials that are formed through immobilization of organic solvents by means of a macromolecular and/or low molecular weight gelator [1,2]. The formation of gels from macromolecular compounds can result either from chemical cross-linking or physical interactions. Gels derived from low molecular mass gelators are physical gels or supramolecular gels. As they are formed via self-assembly of the small gelator molecules into hierarchical supramolecular structures, they can exhibit diverse morphologies, such as fibers, strands, or tapes [2–5]. Among the low molar mass molecules gelators, 12-hydroxystearic acid (12HSA) is of special interest because of the exceptionally large variety of solvents, which can be gelled at low concentration (often lower than 1 wt.%), and is used in industrial applications, like in formulation of detergents and lubricating greases [6]. The solvents that can be gelified by 12HSA include carbon tetrachloride, fluorobenzene, octane, benzene, nitrobenzene, hexafluorobenzene, cyclohexane, toluene, chloroform, methanol, and soybeanoil [7–13]. Gelation of hydrocarbons in the presence of 12HSA results from formation of 12HSA fibers that involve Hbonds forming zigzag sequences along the fiber axis and ‘‘head to head’’ contacts within the orthogonal plane between carboxylic acid groups. Fibers are rigid and very long (up to micrometers), ⇑ Corresponding author. E-mail address: [email protected] (M. Cocca). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.02.012
and their cross-sectional shapes are dependent upon the liquid type. The rheological, kinetical, and thermodynamical behavior of 12HSA organogels mainly depends on the type of liquid which is gelified. Shear elasticities, yield stresses, kinetics of aggregation, deformation, and melting behavior are related to the crosssectional sizes of fibers in such rigid networks. Transparency of the 12HSA-based gels is strongly affected by the solvent. The organogel can be transparent, when benzene, toluene, or soybeanoil are used as solvents [9,11–13], or more or less turbid in other solvents, like dodecane or nitrobenzene [11]. In some cases, a change in thermal history or concentration of 12HSA can induce a variation of transparency of the gel [14]. The thermoreversibility of organogels allows these materials to be exploited for a number of potential applications, including drug delivery systems [15], templates for preparation of nano-materials [16], gel electrolytes [17] and molecular recognition [18], as well as matrices for luminescent materials [19]. Recently, new applications were proposed for gels obtained with a gelator that incorporates luminescent moieties: dissolution of a luminescent compound into supramolecular gels can result in an enhancement in the luminescence, which may allow these materials to be exploited for design of optical amplifiers, optical waveguides, OLEDs, etc. [19,20]. Luminescent organogels can be obtained not only by incorporation of luminophores in the gelator, but also by dissolution of luminescent compounds in water or organic solvent. The incorporation of lanthanide compounds in hydrogels has been
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widely investigated [21–32], whereas only very few studies of organogels containing lanthanide molecules have been reported to date [32,33]. The latter include luminescent gels doped with a hemicaged europium(III) complex using a carboxylate-based aliphatic gelator to gelate DMF [32], and chemically cross-linked gels of europium(III), terbium(III), and neodymium(III) salts of poly(methacrylic acid) swollen in methanol [33]. To our knowledge, no studies have been reported in the literature on incorporation of luminescent compounds like lanthanide complexes in 12HSA based gels. The advantages to include a rare-earth complex into a thermoreversible organogel involve a better processability than the pure molecular lanthanide complexes and may be beneficial for the thermal stability, mechanical properties, and luminescence output [22]. In this manuscript, we detail preparation and characterization of luminescent 12HSA–toluene gels containing thenoyltrifluoroacetonato 1,10-phenanthroline europium(III) complex, Eu(TTA)3phen. Rare-earth (RE) complexes of europium are attractive because of their large Stokes’ shifts, narrow emission bandwidths and long emission lifetimes, which make them suitable candidates for lighting devices [34]. The absorption spectra of Eu(TTA)3phen in toluene reveals an absorption band at 329 nm due to n ? p transition of the Eu3+ ion and b-diketonate moieties. Since toluene presents a low dipole moment and refractive index, weak association of the complex with the solvent and a blue shift of the absorption peak can be observed [35]. Incorporation of Eu(TTA)3phen into a thermoreversible organogel allows to exploit the luminescence features of the RE complex within a thermoreversible organogel, with all the advantages of the thermoreversible organogels, detailed above.
The low molecular weight gelator and the Eu complex employed in this work are reported in Fig. 1. 2.2. Gel preparation 12HSA gels were prepared by dissolving 12HSA in toluene (0.02 g/ml) at 80 °C in a oil bath, until a clear solution was obtained, and then, the solution was quenched into an ice-water bath for 10 min. Transparency of the gel was maintained at room temperature. The same procedure was used to prepare 12HSA gels containing Eu(TTA)3phen at various concentrations. Solutions were prepared by dissolving at room temperature different amounts of europium complex in toluene such as 0.01 wt.%, 0.1 wt.%, and 1 wt.%, and then, 0.02 g/ml of 12HSA was added. The Eu(TTA)3phen–toluene– 12HSA mixture was heated at 80 °C in a oil bath until 12HSA was completely dissolved, and then, the solution was quenched into an ice-water bath for 10 min. Sample codes and material compositions are reported in Table 1. 2.3. Thermal analysis
2. Experimental
Differential scanning calorimetric analysis of the gels was performed by using TA-Q2000 differential scanning calorimeter (DSC) equipped with a RCS-90 cooling unit (TA Instruments). The instrument was calibrated in temperature and energy with pure indium. For all measurements, TzeroÒ hermetic aluminum pans were used at a constant nitrogen flow rate of 20 ml/min. Each measurement was repeated three times to improve accuracy. The enthalpy of transition was calculated by integrating the transition peaks using the software TA Universal Analysis 2000, Version 4.7A.
2.1. Materials
2.4. Fourier transform infrared spectroscopy (FTIR)
12-Hydroxystearic acid (12HSA), 99%, and anhydrous toluene, 99.8%, were supplied by Sigma–Aldrich Co. The materials were used as received, and without further purification. Eu(TTA)3phen complex was synthesized from the chloride, EuCl36H2O, following the procedure reported by Binnemans et al. [36]. EuCl36H2O was dissolved in methanol at concentration of 0.025 g/ml, and then, a stoichiometric quantity of thenoyltrifluoroacetate ligand in methanol was added. The solution was stirred for 30 min, and then, the pH was adjusted to pH = 8 by adding NaOH. After that, 0.22 g of phenanthroline ligand was added, and Eu(TTA)3phen precipitated. A continuous stirring for an additional half an hour was carried out. Finally, the complex was recovered by using a Büchner funnel and cleaned with methanol and then dried in vacuum. An 80% yield was obtained. The synthesized Eu(TTA)3phen was characterized by elemental analysis by means of a EA-1108 C.E Instruments, Thermo Fisher Scientific. The elemental analysis was done by combustion, using VnO5 as additive, previous calibration with sulfanilamide. The elemental analysis yielded the following composition of the precipitated: C 43.23%; H 2.04%; N 2.92%; S <9.56%. These values indicate that the composition of the ligand corresponds to C36H20N2S3, in agreement with the Eu(TTA)3phen stoichiometric composition. Further characterization was performed by 1H NMR analysis. 1H NMR spectra were recorded with a Varian Mercury Plus 400 MHz spectrometer. The Eu complex was dissolved in CDCl3, deuterized grade 99.8, provided by Sigma Aldrich Co. Results of 1H NMR analysis are in agreement with data reported by Stanimirov et al. [37] and by Freund et al. [38]. 1H NMR (CD2Cl2, 400 MHz, d ppm): 10.24 (s, 2H, Phen), 10.16 (d, 2H, Phen), 9.55 (s, 2H, Phen), 8.49 (d, 2H, Phen), 6.96 (d, 3H, TTA), 6.51 (dd, 3H, TTA), 6.20 (s, 3H, TTA), 3.15 (s, 3H, CH b-diketonate).
FTIR spectra of the organogels were recorded on a Perkin Elmer Spectrum 100 spectrometer equipped with a single-bounce diamond attenuated total reflectance (ATR) sampling accessory. Spectra were acquired at 4 cm1 resolution and 32 scans. 2.5. Morphological analysis Morphology and structure of the gels were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM analysis was performed with a FEI Quanta 200 FEG environmental scanning electron microscope (ESEM) (Eindhoven, The Netherlands) in low vacuum mode (PH2O = 0.35–0.70 torr), using an accelerating voltage ranging between 10 and 20 kV and a large field detector (LFD). SEM analysis was performed on xerogels prepared by evaporating the solvent from the organogels placed on SEM stubs (25 °C, 72 h). No metal coating was applied. TEM analysis was performed with a FEI Tecnai G12 (LAB6 source) equipped with a FEI Eagle 4 K CCD camera (Eindhoven, The Netherlands) operating with an acceleration voltage of 120 kV. Before the analysis, cryo-ultramicrotomed sections of the samples were realized from xerogels obtained as above reported and placed on 300 mesh copper grids. 2.6. Quantum yield measurements – integrating sphere method The luminescent quantum yield, U, of Eu(TTA)3phen gel samples was determined using an integrating sphere (IS) method where the number of photons emitted is quantified relative to the total excitation photons absorbed by the sample [39–41]. The sample is placed on an 8 mm diameter quartz disk and inserted inside the sample port of the integrating sphere. The sample
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Fig. 1. Chemical structure of the gelator and of the Europium complex.
Table 1 Sample codes and compositions of the 12HSA gels containing Eu(TTA)3phen. Code
Eu(TTA)3phen (wt.%)
12HSA-0 12HSA-0.01 12HSA-0.1 12HSA-1
0 0.01 0.1 1
is excited with monochromated excitation light (350 nm, 0.25 mW cm2) from a xenon discharge lamp, coupled into the integrating sphere via optical fiber. A CCD spectrometer (AVASPEC 2048USB2) is used to monitor the photon count rate exiting at the measurement port of the integrating sphere. All measured emission spectra are corrected for the spectral response of the detector system in totality, which was determined from comparison of a measured deuterium–halogen light source with that of the known calibrated deuterium–halogen spectrum. The photon count rate is measured for three cases; (a) integrating sphere empty, (b) sphere containing ‘‘test’’ sample, (c) sphere containing a ‘‘blank’’ gel sample with no active luminescent molecules, and measured using two excitation wavelengths; (1) 350 nm – within absorption range of Eu(TTA)3phen, and (2) 613 nm within Eu(TTA)3phen emission spectral range. U is then given by the following equation:
/¼
photons emitted ð ¼ photons absorbed
P
P P Es1 Þð Ln2 = Ls2 Þ P P Lb1 Ls1
ð1Þ
where E and L are the corrected emission and incident excitation light spectra, respectively. The subscripts ‘‘s’’, ‘‘b’’, and ‘‘n’’ refer to whether the integrating sphere contains the luminescent gel sample, the blank gel, or is empty, respectively, and subscripts ‘‘1’’ and ‘‘2’’ refer to the two excitation wavelengths described above. The resultant measured spectra are illustrated in Fig. 2.
2.7. Luminescence lifetime (s0) measurements The luminescence lifetime (s0) of the Eu(TTA)3phen gel samples was measured using a Perkin Elmer LS55 spectrometer fitted with Hamamatsu R6925 gated photomultiplier tube (PMT). Samples are excited with a short (<10 ls) monochromated excitation pulse from a Xenon discharge lamp. The gate time on the PMT detector is fixed at 0.05 ms. The delay time on the PMT detector is varied from 0.1 to 2.0 ms, in increments of 0.1 ms and the luminescence detected within the time window is integrated. The decay in integrated luminescence is found to be monoexponential. A single decay lifetime is found by fitting a monoexponential decay curve to the experimental data.
Fig. 2. Measured emission (E) and excitation light (L) spectra. The subscripts ‘‘s’’, ‘‘b’’, and ‘‘n’’ refer to whether the integrating sphere contains the luminescent gel sample, the blank gel sample, or is empty, respectively. The subscripts ‘‘1’’ and ‘‘2’’ refer to excitation wavelengths of 350 nm and 613 nm, respectively.
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3. Results and discussion Fig. 3 shows a picture of 12HSA gels, plain and containing increasing amounts of Eu(TTA)3phen. A highly transparent gel was obtained when a solution of 2 wt.% of 12HSA in toluene was quickly cooled from 80 to 0 °C, in agreement with literature data [42]. The addition of the europium complex to the gelator solution does not affect the gel formation, which occurs under the same conditions (gelator concentration and temperature) as in the case of the absence of the complex. The inclusion of low concentration of Eu(TTA)3phen does not affect the gel transparency, as illustrated in Fig. 3. Only in the presence of high amounts of the Eu complex, the gel loses in part its transparency, with the appearance of a pale yellow color. An important characteristic of 12HSA organogel is its thermoreversibility, that is, the gel can be transformed into a low viscous sol and then back into gel repeatedly by heating the sample above and below the sol M gel transition temperature [1]. In order to determine whether Eu(TTA)3phen affects the sol M gel transition, DSC analyses were conducted. DSC heating traces of 12HSA gel samples are reported in Fig. 4 and compared with the DSC curve of neat 12HSA. Neat HSA is crystalline at room temperature and upon heating displays a large melting endotherm, centered at 80.5 °C, with an enthalpy of fusion of 183 J/g, in agreement with literature data [7]. As expected, the temperature and enthalpy change of the endothermic event of the 12HSA gel are considerably lower than those of pure 12HSA, due to the fibrous structure of the organogel [43]. The 12HSA fibers in the organogel have a high surface energy, which results in lower melting enthalpy and melting point than bulk 12HSA, in agreement with literature data [42,44]. The melting temperatures (Tg?s) of 12HSA gels containing 0.01%, 0.1%, and 1% of europium complex are comparable to Tg?s of the 12HSA gel without metal complex, whereas the endothermic peak widens and the melting enthalpy (DHendo) of 12HSA gel decreases with the addition of europium complex. This suggests that the introduction of Eu(TTA)3phen affects the fibrous aggregation of the gelator, as discussed below. The DSC cooling curves of the gels, reported in Fig. 5, present a sharper transition compared to the heating scan, as typical for 12HSA organogel [45]. The temperature of the sol to gel transition (Ts?g) in neat 12HSA gel was found to be 28.1 °C, but with increasing percentage of Eu(TTA)3phen a delay in Ts?g was observed. Considering the role of molecular arrangements in the sol phase during gel formation, the composition of the material influences the temperature of sol to gel transition more than that of gel to sol transition. 12HSA gel containing 1% of Eu complex gelifies at temperatures which are 2 °C lower than Ts?g of neat 12HSA gel. A decrease in the heat evolved during gel formation (DHs?g) is also observed. The sol M gel transition temperatures and enthalpies are reported in Table 2.
Fig. 4. DSC traces of 12HSA gel samples, gained upon heating at 5 °C/min. For comparison, the DSC plot of neat 12HSA is also reported.
Fig. 5. DSC curves of 12HSA gel samples, measured upon cooling at 5 °C/min.
Table 2 Sol M gel transition temperatures and their enthalpies. Sample
Tg?s (°C)
DHendo (J/g)
Ts?g (°C)
DHexo (J/g)
12HSA-0 12HSA-0.01 12HSA-0.1 12HSA-1
38.5 39.0 38.1 38.1
5.7 5.8 4.7 4.7
28.1 27.2 26.3 26.0
7.7 6.4 5.9 5.9
Fig. 6. FTIR spectra of 12HSA based gels.
Fig. 3. Gels of 12HSA in toluene containing different amounts of Eu(TTA)3phen. From left to right: 0 wt.%, 0.01 wt.%, 0.1 wt.% and 1 wt.%.
In order to evaluate the molecular arrangement of the gelator in presence of Eu(TTA)3phen, FTIR spectra of gel samples were acquired. It is well established that 12HSA fibers are due to the
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Fig. 7. Scanning electron micrographs of 12HSA based gels: (A) neat 12HSA gel and (B) 12HSA gel containing 1 wt.% of Eu(TTA)3phen.
dimerization of two monomers between carboxylic acid groups and longitudinal growth occurs via hydrogen bonding between hydroxyl groups [46,47]. Depending on the interactions between carboxylic acid groups, they may exist as free carboxylic acid monomers, acyclic and/or cyclic carboxylic acid dimer whose absorption bands are expected at 1730 cm1, 1720 cm1, and 1700 cm1 respectively [48]. In Fig. 6, the FTIR spectra of 12HSA based gel in the hydroxyl and carboxyl regions are reported. FTIR spectrum of neat 12HSA gel is characterized by a broad band centered at around 1700 cm1, which corresponds to dipole–dipole
interactions between the carboxylic groups ascribed to the formation of cyclic dimer. The addition of 1 wt.% of Eu(TTA)3phen seems to affect the intermolecular interaction in 12HSA gel samples. An increase in the signal intensity at 1730 cm1, associated with the stretching of the free carboxyl acid monomer, and a decrease in the absorption band at 1700 cm1 are observable from Fig. 6. Consistent results were obtained by analyzing the hydroxyl region, presented in the left side of Fig. 6, where changes in the HSA–OH group absorption band in the presence of the europium complex are detectable. In particular, the absorption band at
Fig. 8. Bright field TEM images of (A) neat 12HSA gel; (B–D) 12HSA gel containing 1 wt.% of Eu(TTA)3phen.
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Fig. 9. Photoluminescence (PL) spectrum of Eu(TTA)3phen in toluene solution (left) and Eu(TTA)3phen 12HSA-1 gel (right).
Fig. 10. Picture of (A) Eu(TTA)3phen in toluene and (B) Eu(TTA)3phen in 12HSA-1 gel under UV light (k = 360 nm).
3200 cm1, due to the hydrogen bonding between the hydroxyl groups, decreases when 1 wt.% of Eu(TTA)3phen is present in 12HSA gels.
Higher amounts of Eu(TTA)3phen affect the intermolecular interaction in 12HSA gel, probably at this concentration the Eu complex interact with the polar groups of 12HSA monomer. This
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interaction could explain the delay of sol ? gel transition temperature observed in DSC analysis. It is likely that during cooling, the interaction between europium complex and 12HSA monomer retards the free monomer diffusion to the growing crystalline surface. This results not only in a shift of the sol–gel transition to lower temperatures, but also in the decreased enthalpy of transition, as reported in Table 2. The morphology of 12HSA xerogel, obtained after toluene evaporation, was evaluated by means of SEM analysis. 12HSA forms a network structure that is made up of thin fibrils entangled with each other, as shown in Fig. 7A. Morphology of the gels is preserved after the addition of the Eu complex, as it is possible to observe in Fig. 7B, where the SEM micrograph of 12HSA gel containing 1 wt.% of Eu(TTA)3phen is reported. Results on the dispersion of the Eu complex in 12HSA gel were obtained by means of TEM analysis. In Fig. 8, bright field TEM images of 12HSA gel (Fig. 8A) and 12HSA gel containing 1 wt.% of Eu(TTA)3phen (Fig. 8B–D), obtained after toluene evaporation, are reported. As expected, due to the preparation methodology adopted, 12HSA xerogel does not show the evidence of any fibrillar structure. In fact, xerogels were obtained by evaporation of toluene from the gel, inducing its progressive shrinking and the formation of a collapsed and solid compact residue [49]. This compact structure can be observed by TEM after the obtainment of the ultramicrotomed slices, whereas the fibrillar structure of the xerogel is preserved only in the external part of the sample, as already evidenced by SEM analysis. As concerning the xerogel containing 1 wt.% of Eu(TTA)3phen (Fig. 8B–D), the Eu complex is well distributed within the 12HSA gel and forms very small agglomerates. Some of these small agglomerates show an isodimensional shape with a dimension ranging between 5 and 25 nm, but most of them are organized in elongated structures. The average distance between the Eu complexes within these agglomerates has been estimated as 3.1 ± 0.5 nm (see Fig. 8D), larger than that expected from crystallographic data of neat Eu(TTA)3phen and from the value of 0.976 nm reported for the shortest Eu–Eu distance [50] in the crystal. A comparable reduction of the packing factor in Eu diketonate complex has been already reported for Eu(BrC8TTA)3phen, that is, for the diketonate complex in which –C8H16Br groups are grafted on thiofene rings. In this case, the shortest Eu–Eu distance has been calculated as 1.7 nm [38]. Therefore, the increased Eu–Eu distance measured in our system confirms the presence of strong interactions between 12HSA and the Eu complex: 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The photoluminescence (PL) spectra of Eu(TTA)3phen in 12HSA1 gel and in toluene solution are shown in Fig. 9. In both gel and solution, PL is dominated by the red peak at 614 nm, which results from 5D0 to 7F2 energy level transition [51] of the Eu ion. The Eu(TTA)3phen/12HSA gels appear highly luminescent under UV irradiance, as observable from Fig. 10B in which for comparison is also reported the appearance of Eu(TTA)3phen in toluene (Fig. 10A). Some differences in PL spectra of Fig. 9 can be observed in the two states, as the 5D0–7F4 energy level transition is present in the gel sample PL at 694 nm, but it is not significant in solution. In both media, however, PL is dominated by the red emission peak at 614 nm, which results from 5D0 to 7F2 energy level transition in the Eu ion. The absolute luminescence quantum yield (U) is quantified in Table 3. The optical density of the Eu(TTA)3phen gel layer in practical applications may be tuned by varying the concentration of Eu complex. It is important, therefore, to determine U for varying concentration as quenching effects may arise at higher concentration. The
Table 3 Luminescent quantum yield, U, and luminescent lifetime, s0, of Eu(TTA)3phen 12HSA gels of varying concentration. Sample
Integrated Excitation wavelength emission range (nm) (nm)
Photoluminescent Luminescent quantum yield, U lifetime, s0 (%) (ms)
12HSA-1 12HSA-0.1 12HSA-0.01
350 350 350
67 ± 7 65 ± 7 68 ± 7
550–720 550–720 550–720
0.62 ± 0.02 0.60 ± 0.02 0.65 ± 0.02
complex exhibits very high U up to 68 ± 7%. No decrease in U is observed at higher concentrations up to 1% w/w. This is supported by the luminescent lifetime, s0, measurements also shown in Table 3. A proportional decrease in observed luminescent lifetime would be expected in samples exhibiting relatively lower U, as discussed by Ahn et al. [40]. No significant change in s0 is observed here for varying concentration, indicating that U is constant over the range of concentrations considered. 4. Conclusions Luminescent organogels were prepared by dissolving 12hydroxystearic acid (12HSA), in a toluene solution containing different amount of (Eu(TTA)3phen). The incorporation of the Eu complex induces a delay in the sol to gel transition temperature (Ts?g) and a decrease in the heat evolved during melting (DHendo) and gel formation (DHs?g). These results suggest that during cooling, the interaction between europium complex and 12HSA monomer retards the free monomer diffusion to the growing crystalline surface, hindering the formation of hydrogen bonds. The FTIR results confirm that the addition of 1 wt.% of Eu(TTA)3phen into 12HSA affects the intermolecular interaction in 12HSA gel. The supramolecular structure of 12HSA xerogel obtained after toluene evaporation is preserved after addition of the Eu complex. The latter appears, in TEM micrographs, homogeneously dispersed within the gel and forming very small agglomerates. 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The gels are strongly red-light emitting, indicating that no quenching processes are present also at higher complex concentration. The Eu(TTA)3phen in gel state exhibits a very high luminescent quantum yield, U, up to 68 ± 7%. U is not affected by Eu(TTA)3phen concentrations up to 1% w/w. In addition, no significant variation in measured luminescent lifetime, s0, was observed at higher concentration. The luminescent properties of the thermoreversible 12HSA organogels indicate that they are promising materials for optical applications where high luminescent quantum yield is required. In particular, the materials appear promising for the development of an efficient organolanthanide downshifter soft layer. In fact, the realization of a luminescent organogel containing Eu(TTA)3phen, showing down-shifting properties of the absorbed light, can be considered a promising preliminary step for a new generation of luminescent solar concentrators: flexible, compact, quasi-solid-state devices free of leakage and adaptable to several geometries. Acknowledgment This work was supported by the European Commission as part of the Framework 7 integrated Project EPHOCELL (FP7/2009-2013 under Grant Agreement n° 227127). References [1] N.M. Sangeetha, U. Maitra, Chem. Soc. Rev. 34 (2005) 821. [2] L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Thermoreversible luminescent organogels doped with Eu(TTA)3phen complex Maria Laura Di Lorenzo a, Mariacristina Cocca a,⇑, Gennaro Gentile a, Maurizio Avella a, David Gutierrez b, Monica Della Pirriera b, Manus Kennedy c, Hind Ahmed c, John Doran c a
Istituto di Chimica e Tecnologia dei Polimeri (CNR), Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy Leitat Technological Center, C/de la Innovació, 2, 08225 Terrassa, Barcelona, Spain c Dublin Energy Lab, Focas Institute, DIT, Kevin Street, Dublin 8, Ireland b
a r t i c l e
i n f o
Article history: Received 21 December 2012 Accepted 2 February 2013 Available online 18 February 2013 Keywords: 12-Hydroxystearic acid Europium complex Luminescent organogel Thermoreversible gel FTIR Morphology Luminescent quantum yield
a b s t r a c t This manuscript details the preparation and characterization of luminescent organogels in toluene. Gels were prepared by using 12-hydroxystearic acid (12HSA) as gelator and different amounts of thenoyltrifluoroacetonato 1,10-phenanthroline europium(III) complex (Eu(TTA)3phen). The gelation properties and the thermoreversible behavior from solid-like to liquid systems were investigated by differential scanning calorimetry. At higher concentration, an interaction of Eu complex with the polar group of the gelator was revealed by DSC and FTIR analyses. The spectroscopic behavior of the complex was investigated in toluene solution and in the gel state. TEM analysis revealed that 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The Eu(TTA)3phen in the gel state exhibits a very high emission quantum yield, U, which was found to be independent of Eu complex concentration, at least for the composition range analyzed. These results indicate that 12HSA organogels containing Eu(TTA)3phen are promising materials for optical applications. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Organogels are soft materials that are formed through immobilization of organic solvents by means of a macromolecular and/or low molecular weight gelator [1,2]. The formation of gels from macromolecular compounds can result either from chemical cross-linking or physical interactions. Gels derived from low molecular mass gelators are physical gels or supramolecular gels. As they are formed via self-assembly of the small gelator molecules into hierarchical supramolecular structures, they can exhibit diverse morphologies, such as fibers, strands, or tapes [2–5]. Among the low molar mass molecules gelators, 12-hydroxystearic acid (12HSA) is of special interest because of the exceptionally large variety of solvents, which can be gelled at low concentration (often lower than 1 wt.%), and is used in industrial applications, like in formulation of detergents and lubricating greases [6]. The solvents that can be gelified by 12HSA include carbon tetrachloride, fluorobenzene, octane, benzene, nitrobenzene, hexafluorobenzene, cyclohexane, toluene, chloroform, methanol, and soybeanoil [7–13]. Gelation of hydrocarbons in the presence of 12HSA results from formation of 12HSA fibers that involve Hbonds forming zigzag sequences along the fiber axis and ‘‘head to head’’ contacts within the orthogonal plane between carboxylic acid groups. Fibers are rigid and very long (up to micrometers), ⇑ Corresponding author. E-mail address: [email protected] (M. Cocca). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.02.012
and their cross-sectional shapes are dependent upon the liquid type. The rheological, kinetical, and thermodynamical behavior of 12HSA organogels mainly depends on the type of liquid which is gelified. Shear elasticities, yield stresses, kinetics of aggregation, deformation, and melting behavior are related to the crosssectional sizes of fibers in such rigid networks. Transparency of the 12HSA-based gels is strongly affected by the solvent. The organogel can be transparent, when benzene, toluene, or soybeanoil are used as solvents [9,11–13], or more or less turbid in other solvents, like dodecane or nitrobenzene [11]. In some cases, a change in thermal history or concentration of 12HSA can induce a variation of transparency of the gel [14]. The thermoreversibility of organogels allows these materials to be exploited for a number of potential applications, including drug delivery systems [15], templates for preparation of nano-materials [16], gel electrolytes [17] and molecular recognition [18], as well as matrices for luminescent materials [19]. Recently, new applications were proposed for gels obtained with a gelator that incorporates luminescent moieties: dissolution of a luminescent compound into supramolecular gels can result in an enhancement in the luminescence, which may allow these materials to be exploited for design of optical amplifiers, optical waveguides, OLEDs, etc. [19,20]. Luminescent organogels can be obtained not only by incorporation of luminophores in the gelator, but also by dissolution of luminescent compounds in water or organic solvent. The incorporation of lanthanide compounds in hydrogels has been
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widely investigated [21–32], whereas only very few studies of organogels containing lanthanide molecules have been reported to date [32,33]. The latter include luminescent gels doped with a hemicaged europium(III) complex using a carboxylate-based aliphatic gelator to gelate DMF [32], and chemically cross-linked gels of europium(III), terbium(III), and neodymium(III) salts of poly(methacrylic acid) swollen in methanol [33]. To our knowledge, no studies have been reported in the literature on incorporation of luminescent compounds like lanthanide complexes in 12HSA based gels. The advantages to include a rare-earth complex into a thermoreversible organogel involve a better processability than the pure molecular lanthanide complexes and may be beneficial for the thermal stability, mechanical properties, and luminescence output [22]. In this manuscript, we detail preparation and characterization of luminescent 12HSA–toluene gels containing thenoyltrifluoroacetonato 1,10-phenanthroline europium(III) complex, Eu(TTA)3phen. Rare-earth (RE) complexes of europium are attractive because of their large Stokes’ shifts, narrow emission bandwidths and long emission lifetimes, which make them suitable candidates for lighting devices [34]. The absorption spectra of Eu(TTA)3phen in toluene reveals an absorption band at 329 nm due to n ? p transition of the Eu3+ ion and b-diketonate moieties. Since toluene presents a low dipole moment and refractive index, weak association of the complex with the solvent and a blue shift of the absorption peak can be observed [35]. Incorporation of Eu(TTA)3phen into a thermoreversible organogel allows to exploit the luminescence features of the RE complex within a thermoreversible organogel, with all the advantages of the thermoreversible organogels, detailed above.
The low molecular weight gelator and the Eu complex employed in this work are reported in Fig. 1. 2.2. Gel preparation 12HSA gels were prepared by dissolving 12HSA in toluene (0.02 g/ml) at 80 °C in a oil bath, until a clear solution was obtained, and then, the solution was quenched into an ice-water bath for 10 min. Transparency of the gel was maintained at room temperature. The same procedure was used to prepare 12HSA gels containing Eu(TTA)3phen at various concentrations. Solutions were prepared by dissolving at room temperature different amounts of europium complex in toluene such as 0.01 wt.%, 0.1 wt.%, and 1 wt.%, and then, 0.02 g/ml of 12HSA was added. The Eu(TTA)3phen–toluene– 12HSA mixture was heated at 80 °C in a oil bath until 12HSA was completely dissolved, and then, the solution was quenched into an ice-water bath for 10 min. Sample codes and material compositions are reported in Table 1. 2.3. Thermal analysis
2. Experimental
Differential scanning calorimetric analysis of the gels was performed by using TA-Q2000 differential scanning calorimeter (DSC) equipped with a RCS-90 cooling unit (TA Instruments). The instrument was calibrated in temperature and energy with pure indium. For all measurements, TzeroÒ hermetic aluminum pans were used at a constant nitrogen flow rate of 20 ml/min. Each measurement was repeated three times to improve accuracy. The enthalpy of transition was calculated by integrating the transition peaks using the software TA Universal Analysis 2000, Version 4.7A.
2.1. Materials
2.4. Fourier transform infrared spectroscopy (FTIR)
12-Hydroxystearic acid (12HSA), 99%, and anhydrous toluene, 99.8%, were supplied by Sigma–Aldrich Co. The materials were used as received, and without further purification. Eu(TTA)3phen complex was synthesized from the chloride, EuCl36H2O, following the procedure reported by Binnemans et al. [36]. EuCl36H2O was dissolved in methanol at concentration of 0.025 g/ml, and then, a stoichiometric quantity of thenoyltrifluoroacetate ligand in methanol was added. The solution was stirred for 30 min, and then, the pH was adjusted to pH = 8 by adding NaOH. After that, 0.22 g of phenanthroline ligand was added, and Eu(TTA)3phen precipitated. A continuous stirring for an additional half an hour was carried out. Finally, the complex was recovered by using a Büchner funnel and cleaned with methanol and then dried in vacuum. An 80% yield was obtained. The synthesized Eu(TTA)3phen was characterized by elemental analysis by means of a EA-1108 C.E Instruments, Thermo Fisher Scientific. The elemental analysis was done by combustion, using VnO5 as additive, previous calibration with sulfanilamide. The elemental analysis yielded the following composition of the precipitated: C 43.23%; H 2.04%; N 2.92%; S <9.56%. These values indicate that the composition of the ligand corresponds to C36H20N2S3, in agreement with the Eu(TTA)3phen stoichiometric composition. Further characterization was performed by 1H NMR analysis. 1H NMR spectra were recorded with a Varian Mercury Plus 400 MHz spectrometer. The Eu complex was dissolved in CDCl3, deuterized grade 99.8, provided by Sigma Aldrich Co. Results of 1H NMR analysis are in agreement with data reported by Stanimirov et al. [37] and by Freund et al. [38]. 1H NMR (CD2Cl2, 400 MHz, d ppm): 10.24 (s, 2H, Phen), 10.16 (d, 2H, Phen), 9.55 (s, 2H, Phen), 8.49 (d, 2H, Phen), 6.96 (d, 3H, TTA), 6.51 (dd, 3H, TTA), 6.20 (s, 3H, TTA), 3.15 (s, 3H, CH b-diketonate).
FTIR spectra of the organogels were recorded on a Perkin Elmer Spectrum 100 spectrometer equipped with a single-bounce diamond attenuated total reflectance (ATR) sampling accessory. Spectra were acquired at 4 cm1 resolution and 32 scans. 2.5. Morphological analysis Morphology and structure of the gels were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM analysis was performed with a FEI Quanta 200 FEG environmental scanning electron microscope (ESEM) (Eindhoven, The Netherlands) in low vacuum mode (PH2O = 0.35–0.70 torr), using an accelerating voltage ranging between 10 and 20 kV and a large field detector (LFD). SEM analysis was performed on xerogels prepared by evaporating the solvent from the organogels placed on SEM stubs (25 °C, 72 h). No metal coating was applied. TEM analysis was performed with a FEI Tecnai G12 (LAB6 source) equipped with a FEI Eagle 4 K CCD camera (Eindhoven, The Netherlands) operating with an acceleration voltage of 120 kV. Before the analysis, cryo-ultramicrotomed sections of the samples were realized from xerogels obtained as above reported and placed on 300 mesh copper grids. 2.6. Quantum yield measurements – integrating sphere method The luminescent quantum yield, U, of Eu(TTA)3phen gel samples was determined using an integrating sphere (IS) method where the number of photons emitted is quantified relative to the total excitation photons absorbed by the sample [39–41]. The sample is placed on an 8 mm diameter quartz disk and inserted inside the sample port of the integrating sphere. The sample
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Fig. 1. Chemical structure of the gelator and of the Europium complex.
Table 1 Sample codes and compositions of the 12HSA gels containing Eu(TTA)3phen. Code
Eu(TTA)3phen (wt.%)
12HSA-0 12HSA-0.01 12HSA-0.1 12HSA-1
0 0.01 0.1 1
is excited with monochromated excitation light (350 nm, 0.25 mW cm2) from a xenon discharge lamp, coupled into the integrating sphere via optical fiber. A CCD spectrometer (AVASPEC 2048USB2) is used to monitor the photon count rate exiting at the measurement port of the integrating sphere. All measured emission spectra are corrected for the spectral response of the detector system in totality, which was determined from comparison of a measured deuterium–halogen light source with that of the known calibrated deuterium–halogen spectrum. The photon count rate is measured for three cases; (a) integrating sphere empty, (b) sphere containing ‘‘test’’ sample, (c) sphere containing a ‘‘blank’’ gel sample with no active luminescent molecules, and measured using two excitation wavelengths; (1) 350 nm – within absorption range of Eu(TTA)3phen, and (2) 613 nm within Eu(TTA)3phen emission spectral range. U is then given by the following equation:
/¼
photons emitted ð ¼ photons absorbed
P
P P Es1 Þð Ln2 = Ls2 Þ P P Lb1 Ls1
ð1Þ
where E and L are the corrected emission and incident excitation light spectra, respectively. The subscripts ‘‘s’’, ‘‘b’’, and ‘‘n’’ refer to whether the integrating sphere contains the luminescent gel sample, the blank gel, or is empty, respectively, and subscripts ‘‘1’’ and ‘‘2’’ refer to the two excitation wavelengths described above. The resultant measured spectra are illustrated in Fig. 2.
2.7. Luminescence lifetime (s0) measurements The luminescence lifetime (s0) of the Eu(TTA)3phen gel samples was measured using a Perkin Elmer LS55 spectrometer fitted with Hamamatsu R6925 gated photomultiplier tube (PMT). Samples are excited with a short (<10 ls) monochromated excitation pulse from a Xenon discharge lamp. The gate time on the PMT detector is fixed at 0.05 ms. The delay time on the PMT detector is varied from 0.1 to 2.0 ms, in increments of 0.1 ms and the luminescence detected within the time window is integrated. The decay in integrated luminescence is found to be monoexponential. A single decay lifetime is found by fitting a monoexponential decay curve to the experimental data.
Fig. 2. Measured emission (E) and excitation light (L) spectra. The subscripts ‘‘s’’, ‘‘b’’, and ‘‘n’’ refer to whether the integrating sphere contains the luminescent gel sample, the blank gel sample, or is empty, respectively. The subscripts ‘‘1’’ and ‘‘2’’ refer to excitation wavelengths of 350 nm and 613 nm, respectively.
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3. Results and discussion Fig. 3 shows a picture of 12HSA gels, plain and containing increasing amounts of Eu(TTA)3phen. A highly transparent gel was obtained when a solution of 2 wt.% of 12HSA in toluene was quickly cooled from 80 to 0 °C, in agreement with literature data [42]. The addition of the europium complex to the gelator solution does not affect the gel formation, which occurs under the same conditions (gelator concentration and temperature) as in the case of the absence of the complex. The inclusion of low concentration of Eu(TTA)3phen does not affect the gel transparency, as illustrated in Fig. 3. Only in the presence of high amounts of the Eu complex, the gel loses in part its transparency, with the appearance of a pale yellow color. An important characteristic of 12HSA organogel is its thermoreversibility, that is, the gel can be transformed into a low viscous sol and then back into gel repeatedly by heating the sample above and below the sol M gel transition temperature [1]. In order to determine whether Eu(TTA)3phen affects the sol M gel transition, DSC analyses were conducted. DSC heating traces of 12HSA gel samples are reported in Fig. 4 and compared with the DSC curve of neat 12HSA. Neat HSA is crystalline at room temperature and upon heating displays a large melting endotherm, centered at 80.5 °C, with an enthalpy of fusion of 183 J/g, in agreement with literature data [7]. As expected, the temperature and enthalpy change of the endothermic event of the 12HSA gel are considerably lower than those of pure 12HSA, due to the fibrous structure of the organogel [43]. The 12HSA fibers in the organogel have a high surface energy, which results in lower melting enthalpy and melting point than bulk 12HSA, in agreement with literature data [42,44]. The melting temperatures (Tg?s) of 12HSA gels containing 0.01%, 0.1%, and 1% of europium complex are comparable to Tg?s of the 12HSA gel without metal complex, whereas the endothermic peak widens and the melting enthalpy (DHendo) of 12HSA gel decreases with the addition of europium complex. This suggests that the introduction of Eu(TTA)3phen affects the fibrous aggregation of the gelator, as discussed below. The DSC cooling curves of the gels, reported in Fig. 5, present a sharper transition compared to the heating scan, as typical for 12HSA organogel [45]. The temperature of the sol to gel transition (Ts?g) in neat 12HSA gel was found to be 28.1 °C, but with increasing percentage of Eu(TTA)3phen a delay in Ts?g was observed. Considering the role of molecular arrangements in the sol phase during gel formation, the composition of the material influences the temperature of sol to gel transition more than that of gel to sol transition. 12HSA gel containing 1% of Eu complex gelifies at temperatures which are 2 °C lower than Ts?g of neat 12HSA gel. A decrease in the heat evolved during gel formation (DHs?g) is also observed. The sol M gel transition temperatures and enthalpies are reported in Table 2.
Fig. 4. DSC traces of 12HSA gel samples, gained upon heating at 5 °C/min. For comparison, the DSC plot of neat 12HSA is also reported.
Fig. 5. DSC curves of 12HSA gel samples, measured upon cooling at 5 °C/min.
Table 2 Sol M gel transition temperatures and their enthalpies. Sample
Tg?s (°C)
DHendo (J/g)
Ts?g (°C)
DHexo (J/g)
12HSA-0 12HSA-0.01 12HSA-0.1 12HSA-1
38.5 39.0 38.1 38.1
5.7 5.8 4.7 4.7
28.1 27.2 26.3 26.0
7.7 6.4 5.9 5.9
Fig. 6. FTIR spectra of 12HSA based gels.
Fig. 3. Gels of 12HSA in toluene containing different amounts of Eu(TTA)3phen. From left to right: 0 wt.%, 0.01 wt.%, 0.1 wt.% and 1 wt.%.
In order to evaluate the molecular arrangement of the gelator in presence of Eu(TTA)3phen, FTIR spectra of gel samples were acquired. It is well established that 12HSA fibers are due to the
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Fig. 7. Scanning electron micrographs of 12HSA based gels: (A) neat 12HSA gel and (B) 12HSA gel containing 1 wt.% of Eu(TTA)3phen.
dimerization of two monomers between carboxylic acid groups and longitudinal growth occurs via hydrogen bonding between hydroxyl groups [46,47]. Depending on the interactions between carboxylic acid groups, they may exist as free carboxylic acid monomers, acyclic and/or cyclic carboxylic acid dimer whose absorption bands are expected at 1730 cm1, 1720 cm1, and 1700 cm1 respectively [48]. In Fig. 6, the FTIR spectra of 12HSA based gel in the hydroxyl and carboxyl regions are reported. FTIR spectrum of neat 12HSA gel is characterized by a broad band centered at around 1700 cm1, which corresponds to dipole–dipole
interactions between the carboxylic groups ascribed to the formation of cyclic dimer. The addition of 1 wt.% of Eu(TTA)3phen seems to affect the intermolecular interaction in 12HSA gel samples. An increase in the signal intensity at 1730 cm1, associated with the stretching of the free carboxyl acid monomer, and a decrease in the absorption band at 1700 cm1 are observable from Fig. 6. Consistent results were obtained by analyzing the hydroxyl region, presented in the left side of Fig. 6, where changes in the HSA–OH group absorption band in the presence of the europium complex are detectable. In particular, the absorption band at
Fig. 8. Bright field TEM images of (A) neat 12HSA gel; (B–D) 12HSA gel containing 1 wt.% of Eu(TTA)3phen.
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Fig. 9. Photoluminescence (PL) spectrum of Eu(TTA)3phen in toluene solution (left) and Eu(TTA)3phen 12HSA-1 gel (right).
Fig. 10. Picture of (A) Eu(TTA)3phen in toluene and (B) Eu(TTA)3phen in 12HSA-1 gel under UV light (k = 360 nm).
3200 cm1, due to the hydrogen bonding between the hydroxyl groups, decreases when 1 wt.% of Eu(TTA)3phen is present in 12HSA gels.
Higher amounts of Eu(TTA)3phen affect the intermolecular interaction in 12HSA gel, probably at this concentration the Eu complex interact with the polar groups of 12HSA monomer. This
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interaction could explain the delay of sol ? gel transition temperature observed in DSC analysis. It is likely that during cooling, the interaction between europium complex and 12HSA monomer retards the free monomer diffusion to the growing crystalline surface. This results not only in a shift of the sol–gel transition to lower temperatures, but also in the decreased enthalpy of transition, as reported in Table 2. The morphology of 12HSA xerogel, obtained after toluene evaporation, was evaluated by means of SEM analysis. 12HSA forms a network structure that is made up of thin fibrils entangled with each other, as shown in Fig. 7A. Morphology of the gels is preserved after the addition of the Eu complex, as it is possible to observe in Fig. 7B, where the SEM micrograph of 12HSA gel containing 1 wt.% of Eu(TTA)3phen is reported. Results on the dispersion of the Eu complex in 12HSA gel were obtained by means of TEM analysis. In Fig. 8, bright field TEM images of 12HSA gel (Fig. 8A) and 12HSA gel containing 1 wt.% of Eu(TTA)3phen (Fig. 8B–D), obtained after toluene evaporation, are reported. As expected, due to the preparation methodology adopted, 12HSA xerogel does not show the evidence of any fibrillar structure. In fact, xerogels were obtained by evaporation of toluene from the gel, inducing its progressive shrinking and the formation of a collapsed and solid compact residue [49]. This compact structure can be observed by TEM after the obtainment of the ultramicrotomed slices, whereas the fibrillar structure of the xerogel is preserved only in the external part of the sample, as already evidenced by SEM analysis. As concerning the xerogel containing 1 wt.% of Eu(TTA)3phen (Fig. 8B–D), the Eu complex is well distributed within the 12HSA gel and forms very small agglomerates. Some of these small agglomerates show an isodimensional shape with a dimension ranging between 5 and 25 nm, but most of them are organized in elongated structures. The average distance between the Eu complexes within these agglomerates has been estimated as 3.1 ± 0.5 nm (see Fig. 8D), larger than that expected from crystallographic data of neat Eu(TTA)3phen and from the value of 0.976 nm reported for the shortest Eu–Eu distance [50] in the crystal. A comparable reduction of the packing factor in Eu diketonate complex has been already reported for Eu(BrC8TTA)3phen, that is, for the diketonate complex in which –C8H16Br groups are grafted on thiofene rings. In this case, the shortest Eu–Eu distance has been calculated as 1.7 nm [38]. Therefore, the increased Eu–Eu distance measured in our system confirms the presence of strong interactions between 12HSA and the Eu complex: 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The photoluminescence (PL) spectra of Eu(TTA)3phen in 12HSA1 gel and in toluene solution are shown in Fig. 9. In both gel and solution, PL is dominated by the red peak at 614 nm, which results from 5D0 to 7F2 energy level transition [51] of the Eu ion. The Eu(TTA)3phen/12HSA gels appear highly luminescent under UV irradiance, as observable from Fig. 10B in which for comparison is also reported the appearance of Eu(TTA)3phen in toluene (Fig. 10A). Some differences in PL spectra of Fig. 9 can be observed in the two states, as the 5D0–7F4 energy level transition is present in the gel sample PL at 694 nm, but it is not significant in solution. In both media, however, PL is dominated by the red emission peak at 614 nm, which results from 5D0 to 7F2 energy level transition in the Eu ion. The absolute luminescence quantum yield (U) is quantified in Table 3. The optical density of the Eu(TTA)3phen gel layer in practical applications may be tuned by varying the concentration of Eu complex. It is important, therefore, to determine U for varying concentration as quenching effects may arise at higher concentration. The
Table 3 Luminescent quantum yield, U, and luminescent lifetime, s0, of Eu(TTA)3phen 12HSA gels of varying concentration. Sample
Integrated Excitation wavelength emission range (nm) (nm)
Photoluminescent Luminescent quantum yield, U lifetime, s0 (%) (ms)
12HSA-1 12HSA-0.1 12HSA-0.01
350 350 350
67 ± 7 65 ± 7 68 ± 7
550–720 550–720 550–720
0.62 ± 0.02 0.60 ± 0.02 0.65 ± 0.02
complex exhibits very high U up to 68 ± 7%. No decrease in U is observed at higher concentrations up to 1% w/w. This is supported by the luminescent lifetime, s0, measurements also shown in Table 3. A proportional decrease in observed luminescent lifetime would be expected in samples exhibiting relatively lower U, as discussed by Ahn et al. [40]. No significant change in s0 is observed here for varying concentration, indicating that U is constant over the range of concentrations considered. 4. Conclusions Luminescent organogels were prepared by dissolving 12hydroxystearic acid (12HSA), in a toluene solution containing different amount of (Eu(TTA)3phen). The incorporation of the Eu complex induces a delay in the sol to gel transition temperature (Ts?g) and a decrease in the heat evolved during melting (DHendo) and gel formation (DHs?g). These results suggest that during cooling, the interaction between europium complex and 12HSA monomer retards the free monomer diffusion to the growing crystalline surface, hindering the formation of hydrogen bonds. The FTIR results confirm that the addition of 1 wt.% of Eu(TTA)3phen into 12HSA affects the intermolecular interaction in 12HSA gel. The supramolecular structure of 12HSA xerogel obtained after toluene evaporation is preserved after addition of the Eu complex. The latter appears, in TEM micrographs, homogeneously dispersed within the gel and forming very small agglomerates. 12HSA is able to solvate the Eu diketonate complex inducing a remarkable increase in the Eu–Eu distance. The gels are strongly red-light emitting, indicating that no quenching processes are present also at higher complex concentration. The Eu(TTA)3phen in gel state exhibits a very high luminescent quantum yield, U, up to 68 ± 7%. U is not affected by Eu(TTA)3phen concentrations up to 1% w/w. In addition, no significant variation in measured luminescent lifetime, s0, was observed at higher concentration. The luminescent properties of the thermoreversible 12HSA organogels indicate that they are promising materials for optical applications where high luminescent quantum yield is required. In particular, the materials appear promising for the development of an efficient organolanthanide downshifter soft layer. In fact, the realization of a luminescent organogel containing Eu(TTA)3phen, showing down-shifting properties of the absorbed light, can be considered a promising preliminary step for a new generation of luminescent solar concentrators: flexible, compact, quasi-solid-state devices free of leakage and adaptable to several geometries. Acknowledgment This work was supported by the European Commission as part of the Framework 7 integrated Project EPHOCELL (FP7/2009-2013 under Grant Agreement n° 227127). References [1] N.M. Sangeetha, U. Maitra, Chem. Soc. Rev. 34 (2005) 821. [2] L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201.
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