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Article

Luminescent 1,10-Phenanthroline β-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties

by
Francesco Fagnani
1,
Alessia Colombo
1,
Graziella Malandrino
2,
Claudia Dragonetti
1 and
Anna Lucia Pellegrino
2,*
1
Dipartimento di Chimica, Università di Milano and INSTM UdR Milano, 20126 Milan, Italy
2
Dipartimento di Scienze Chimiche, Università di Catania and INSTM UdR Catania, 95125 Catania, Italy
*
Author to whom correspondence should be addressed.
Molecules 2022, 27(20), 6990; https://doi.org/10.3390/molecules27206990
Submission received: 23 September 2022 / Revised: 11 October 2022 / Accepted: 14 October 2022 / Published: 18 October 2022
(This article belongs to the Special Issue Metal Complexes for Optical and Electronics Applications)
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Abstract

:
Substitution of the diglyme ligand of [Eu(hfa)3(diglyme)] (where hfa is hexafluoroacetylacetonate) with a simple 1,10-phenanthroline leads to a six-fold increase of the product μβEFISH, as measured by the Electric-Field-Induced Second Harmonic generation (EFISH) technique. Similarly, [Eu(tta)3(1,10-phenanthroline)] (where Htta is 2-thenoyltrifluoroacetone) is characterized by a large second-order NLO response. Both 1,10-phenanthroline europium complexes have great potential as multifunctional materials for photonics.

Graphical Abstract

1. Introduction

The design and characterization of molecules with a second-order nonlinear optical (NLO) response have received considerable attention due to their application in a range of fields such as optical communications, electrooptical devices, and optical data processing [1,2]. To present good second-order NLO properties, a molecule has to be non-centrosymmetric, with a great difference between the excited state and the ground state dipole moment; in addition, it has to be characterized by charge-transfer transitions at relatively low energy. Thus, organic molecules containing electron-acceptor and electron-donor groups connected by a polarizable π-conjugated bridge can reach a good NLO response [1,2,3,4,5,6]. In the last twenty years, coordination compounds, characterized by low-energy ligand-to-metal, metal-to-ligand, ligand-to-ligand and intraligand charge-transfer (LMCT, MLCT, LLCT, and ILCT, respectively) excited states, have shown their great potential for second-order nonlinear optics [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. In the design of NLO-active complexes, a useful aspect is that the energy of the charge-transfer states is easily controlled by the nature and oxidation state of the metal and by the choice of the ligands. In particular, it appeared that the second-order NLO response of various π-delocalized nitrogen donor ligands such as pyridines, stilbazoles, phenanthrolines, bipyridines, and terpyridines can be greatly enhanced upon coordination to a metal center [8,9,10,11,12,13,14,15,20,22,23,24,25,26]. Coordination complexes with various metals have been studied. However, surprisingly, although lanthanide (Ln) complexes have been intensively investigated for their luminescent and magnetic properties [27,28], as well as for biological applications [29,30,31,32,33], there are only a few reports on their peculiar NLO properties [34,35,36,37,38,39,40,41,42,43].
It was reported that dipolar lanthanide complexes such as [LLn(NO3)3] (where L is a dibutylaminophenyl-functionalized annelated terpyridine) are characterized by a good second-order NLO response, measured by the Harmonic Light Scattering (HLS) technique in solution [44,45,46,47], which increases as the number of f electrons increases [34,36]. Similarly, the increase in the quadratic hyperpolarizability, βHLS, of Na3[Ln(pyridyl-2,6-dicarboxylate)3] along the Ln series can be explained by the increased number of f electrons [35]. The unexpected fact that the quadratic hyperpolarizability depends on the number of f electrons was attributed to the polarization of the 4f electrons [41]. Additionally, it has been reported that the quadratic hyperpolarizibilities of Ln complexes bearing nonadentate ligands based on triazacyclononane, functionalized with pyridyl-2-phosphinate groups, reach a maximum around the center of the lanthanide series, with a bell-shaped trend [42]. A similar trend of the quadratic hyperpolarizibilities was observed in the case of Ln complexes of trans-cinnamic acid [44]. In parallel, some of us studied the second-order nonlinear optical response of [Ln(hfa)3(diglyme)] (hfa = hexafluoroacetylacetonate; diglyme = bis (2-methoxyethyl) ether) by a combination of Electric-Field Induced Second Harmonic generation (EFISH) and HLS techniques in solution, confirming the role of f electrons in controlling the second-order NLO properties [38]. In these systems, the molecular quadratic hyperpolarizabilities measured by the EFISH method [48], βEFISH, initially increase rapidly with the number of f electrons, whereas the increase is much lower for the last seven f electrons; additionally, the βHLS values increase, but much less rapidly, along the Ln series [38]. Similarly, the βEFISH values of trinuclear lanthanide adducts [Ln(NO3)3(CuL)2] (Ln = La, Ce, Sm, Eu, and Er; L = N,N′-1,3-propylen-bis (salicylideniminato)) are significantly influenced by the number of f electrons: the values initially increase rapidly with the number of f electrons, starting from lanthanum to europium; then the increase is less marked upon addition of the other f electrons, with the βEFISH value of the Er complex (11 f electrons) being only 1.1 times higher than that of the Eu complex (6 f electrons) [39]. This study confirmed that the surprising polarizable character of f electrons is the origin of the fascinating NLO properties. As general trend, the increase of the second-order NLO response is significant up to fulfilment of half f shell, while it becomes much less relevant with the addition of further f electrons up to the total fulfilment of the f shell [39].
This latter observation and the fact that lighter lanthanides (Ce-Eu) are more abundant than the heavier ones (Gd-Lu), and therefore are generally less expensive [49], render europium complexes of particular interest for NLO studies.
In the present work, we found that two known luminescent β-diketonate europium complexes [Eu(hfa)3(1,10-phenanthroline)] and [Eu(tta)3(1,10-phenanthroline)], from now on [Eu(hfa)3(phen)] and [Eu(tta)3(phen)], show an unexpected large NLO response, much higher than that previously reported for the related [Eu(hfa)3(diglyme)] [38], as evidenced by the Electric-Field Induced Second Harmonic generation (EFISH) technique in solution [41], opening a promising route for easily prepared multifunctional NLO-active lanthanide complexes.
The Eu complexes under investigation are schematized in Figure 1. The structures present the Eu as central metal in the most stable oxidation state 3+ and stabilized by six oxygens coming from the β-diketonate ligands and by two nitrogens of the phenanthroline for [Eu(hfa)3(phen)] and [Eu(tta)3(phen)] (complexes 1 and 2 in Figure 1), while the Eu coordination sphere is completed with three additional oxygens of the polyether for [Eu(hfa)3(diglyme)], complex 3.

2. Results and Discussion

The europium fluorinated β-diketonate complexes have been obtained through a facile synthesis starting from the europium acetate and the ligands. The present approach finds counterparts in the route previously reported for the analogous [Eu(hfa)3(diglyme)] complex [28,38] and offers several advantages, such as high yield, synthesis in a single step, low-cost route from commercially available chemicals. Additionally, all the complexes can be handled in air, are non-hygroscopic, soluble in common organic solvents and present high thermal and chemical stability.
Furthermore, the second-order nonlinear optical properties in chloroform solution of complexes 1 and 2 have been deeply studied, working with an incident radiation of low energy (λ = 1.907 µm), by the EFISH method in solution [48].
This technique, suitable for dipolar molecules, provides information on the molecular NLO properties through the following equation:
γEFISH = (μβλ/5kT) + γ (−2ω; ω, ω, 0)
where μβλ/5kT is the dipolar orientational contribution and γ (−2ω; ω, ω, 0) is the electronic cubic contribution, which can usually be neglected when studying the second-order NLO properties of dipolar molecules. Βλ is the projection along the dipole moment axis of βVEC, which is the vectorial component of the tensor of the quadratic hyperpolarizability, working with an incident wavelength λ of a pulsed laser. To avoid overestimation of the quadratic hyperpolarizability value, due to resonance enhancements, it is necessary to work with an incident wavelength λ whose second harmonic λ/2 is far from any absorption band of the compound investigated. For this reason, a wavelength of 1.907 µm was chosen to study complexes 1 and 2. To obtain the value of βEFISH it would be necessary to know the ground state dipole moment μ of the molecule. However, from an applicative point of view, it is the product μβEFISH that should be maximized. A compound with a μβEFISH value higher than that of Disperse Red One (500 × 10−48 esu), proposed for electrooptic polymeric poled films [50,51], can be considered of interest for photonic applications.
It turned out that [Eu(hfa)3(phen)] (1; Figure 1) is characterized, in solution, by a μβEFISH of 1016 × 10−48 esu (Table 1), 6.3 times higher than that previously reported [38] for the related complex [Eu(hfa)3(diglyme)] (3). Therefore, remarkably, replacement of diglyme with a simple 1,10-phenanthroline leads to a huge increase of the second-order NLO properties. Such a large NLO response is also observed for [Eu(tta)3(phen)] (2; μβEFISH = 920 × 10−48 esu) in which the hexafluoroacetylacetonate ligand has been replaced by the 2-thenoyltrifluoroacetonate. It is worth pointing out that these values are higher than that observed for the europium adduct [Eu(NO3)3(CuL)2] in solution (μβEFISH = 720 × 10−48 esu), which is much more difficult to prepare [39].
The large NLO response of 1 and 2 is thrilling also due to the simplicity of the 1,10-phenanthroline ligand. In fact, it is known that coordination of 5-X-1,10-phenanthrolines to a “Zn(CH3CO2)2” moiety produces a significant enhancement of the product μβEFISH, which becomes 99 × 10−48, 254 × 10−48, and 616 × 10−48 esu for X = OMe, NMe2, and trans-CH=CHC6H4NMe2, respectively) [52,53], but the best NLO response of these Zn(II) complexes is lower than that obtained for complexes 1 and 2, although the 1,10-phenanthroline ligand is functionalized in the Zn systems.
The absorption spectra of the complexes, reported in Figure 2, display various features as a function of the different ligands which compose the structures.
In particular, the [Eu(hfa)3(diglyme)] adduct shows a strong band around 306 nm, whereas the [Eu(hfa)3(phen)] presents bands centered at 233, 272 and 293 nm arising from the phen and hfa contributions. Notably, in both complexes, a shoulder around 325 nm can be assigned to the lowest spin-allowed π-π* transition of the β-diketonate hfa ligand [54,55]. Finally, the absorption spectrum of the [Eu(tta)3(phen)] displays, together with the bands at 230 and 272 nm due to the 1,10-phenanthroline, a broad and intense signal at 341 nm arising from the tta ligand. For an easier comparison of the different contribution arising from each ligand, overlays of the UV-vis spectra of the complexes and the associated ligands are reported in Figure 3. Thus, the UV-vis spectra of 1 and 2 are due to the contribution of the relative β-diketonate, hfa and tta for 1 and 2, respectively, and antenna ligand phen, while the UV spectrum of 3 is only due to the hfa contribution, the diglyme beeing inactive in the UV-vis region.
In addition, the luminescence spectra of the adducts, registered at room temperature, are reported in Figure 4. The spectra were obtained using an excitation wavelength of 348 nm for the [Eu(hfa)3(phen)], [Eu(tta)3(phen)] and [Eu(hfa)3(diglyme)].
The spectra, recorded as CH2Cl2 solutions, are reported normalized in intensity, but similar intensity values have been obtained with concentrations of 10−3 M, 10−5 M and 10−3 M for the complexes 1, 2 and 3, respectively. Therefore, the [Eu(tta)3(phen)] has a much higher luminescence intensity. The emission peaks observed in Figure 4 consist of f—f emission transitions from the 5D0 excited state to the 7FJ multiplet of the Eu(III) ion. In particular, the peaks at 578, 590 and 612 nm are assigned to the Eu ion transitions 5D0 → 7F0, 5D0 → 7F1 and 5D0 → 7F2, respectively [56]. The presence of the band due to the 5D0-7F0 transition in the 574–582 nm spectral region due to a singlet-to-singlet transition indicates that the Eu3+ ion occupies a low symmetry environment in all the three compounds [57]. This feature is also supported by the asymmetry ratio, i.e. the ratio between the 5D0 → 7F2 and 5D0 →7F1 electronic transitions, which, with a value ranging from 9.05, to 11.89 and 17.6 for 3, 1 and 2, respectively, indicates a highly asymmetric environment [57].

3. Materials and Methods

All reagents and solvents were purchased from Sigma-Aldrich and were used without further purification.
[Eu(hfa)3(1,10-phenanthroline)], (1). The adduct was synthetized through a one-step reaction from the Eu(III) acetate hydrate, Hhfa, 1,10-phenanthroline. Specifically, 1.5797 g (4.8 mmol) of Eu(CH3COO)3∙xH2O was first suspended in dichloromethane (50 mL). 1,10-phenanthroline 0.8650 g (4.8 mmol) was added to the suspension. H-hfa 2.04 mL (14.4 mmol; d = 1.47 g/mL) was added under vigorous stirring after 10 min and the mixture was refluxed under stirring for 1 h. After solvent evaporation, the complex appears in form of very light orange crystals. The crystals were collected, washed several times with pentane in order to ensure a high degree of purity, and filtered. The yield was about 90%. Elemental analysis (EuC27H11O6N2F18): Calc: C, 34.02; H, 1.16. Found: C, 33.69; H, 1.20.
[Eu(tta)3(1,10-phenanthroline)], (2). The adduct was synthetized following a reaction similar to that of 1 from 1.5797 g (4.8 mmol) of the Eu(III) acetate hydrate suspended in ethanol (100 mL), 0.8650 g (4.8 mmol) of 1,10-phenanthroline, and 3.1993 g (14.4 mmol) of Htta. The adduct was collected after solvent evaporation, washed several times with pentane, and a yield of about 88% was obtained. Elemental analysis (EuC36H20O6N2S3F9): Calc: C, 43.43; H, 2.02. Found: C, 43.88; H, 1.97.
[Eu(hfa)3(diglyme)], (3). The adduct was synthetized following a procedure similar to that of 1, from 1.5797 g (4.8 mmol) of the Eu(III) acetate hydrate, 2.04 mL (14.4 mmol; d = 1.47 g/mL) Hhfa and 0.69 mL (4.8 mmol; d = 0.940 g/mL) of diglyme. A similar purification process was also executed for this product. The yield was about 92%. Elemental analysis (EuC21H17O9F18): Calc: C, 27.80; H, 1.89. Found: C, 27.35; H, 1.79.
Characterization. All EFISH measurements were carried out at the Dipartimento di Chimica of the Università degli Studi di Milano, in CHCl3 solutions at a concentration of 1 × 10−3 M, working with a non-resonant incident wavelength of 1.907 µm, obtained by Raman shifting the fundamental 1.064 µm wavelength produced by a Q-switched, mode-locked Nd3+: YAG laser manufactured by Atalaser. The spectrometer for the EFISH measurements was a prototype made by SOPRA (France). The reported µβEFISH values are the mean of 16 successive measurements performed on the same sample.
The UV-Vis spectra of the three adducts were collected using a JASCO V-650 UV − vis spectrophotometer, starting from 1 × 10−4 M solutions in CH2Cl2. Photoluminescence spectra were collected at room temperature using a JASCO FP-8300 spectrofluorimeter at a λ excitation of 348 nm for the [Eu(hfa)3(diglyme)], [Eu(hfa)3(phen)] and [Eu(tta)3(phen)] complexes. Elemental microanalyses were carried out using a Carlo Erba 1106 elemental analyzer.

4. Conclusions

Lanthanide complexes have been intensively studied for their luminescent and magnetic properties, but recently, their NLO properties have also attracted great interest. Among them, the Eu non-centrosymmetric molecules, containing the 1,10-phenanthroline antenna ligand, have been the object of interest in the present study. This work has evidenced the unexpected huge second-order NLO response of luminescent β-diketonate europium complexes bearing a simple 1,10-phenanthroline, a result of particular relevance in the search of new multifunctional building blocks for photonic nanomaterials. Their μβEFISH values, higher than that of the benchmark Disperse Red One, open the way to the use of these Eu complexes in a wide range of technological fields such as photonic applications. Thus, the 1,10-phenanthroline ligand, a well-known antenna ligand for the photoluminescence of Eu(III), plays a crucial role in boosting the NLO response of these systems. In fact, the μβ value goes from 161(×10−48 esu) for the [Eu(hfa)3(diglyme)] complex to 1061 (×10−48 esu) for Eu(hfa)3(1,10-phenanthroline), due to the substitution of diglyme by 1,10-phenanthroline. Because the two compounds have the same β-diketonate, the unique difference, and thus the factor responsible for the significant increase in the μβ value, is the phenanthroline. This observation is further supported by the μβ value of 920 (×10−48 esu) found for [Eu(tta)3(1,10-phenanthroline)]. Other important advantages of the present work are, on the one hand, the facile, one-pot, low-cost synthetic approach, and on the other hand, the non-hygroscopic, high-solubility and air stability features of the complexes, which represent added values and open a promising route for easily preparing multifunctional NLO-active lanthanide complexes.
Finally, the present results open the door to other intriguing EFISH investigations such as the study of the effect of donor substituents on the 1,10-phenanthroline coordinated to lanthanides.

Author Contributions

Conceptualization, G.M. and C.D., methodology, A.L.P. and A.C. and F.F.; investigation, A.L.P. and F.F.; writing—original draft preparation, A.L.P. and F.F.; writing—review and editing, A.L.P., G.M., F.F., A.C. and C.D.; supervision, G.M. and C.D.; funding acquisition, A.C., F.F. and G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Università degli Studi di Milano (Project PSR2020_DIP_005_PI_ACOLO “Synthesis and characterization of organic and coordination compounds for application in luminescent devices or in bioimaging”).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

We deeply thank Stefania Righetto for EFISH measurements. F. Fagnani thanks MUR and Università degli Studi di Milano for a PhD fellowship. A. L. Pellegrino thanks the Ministero dell’Università e della Ricerca within the PON “Ricerca e Innovazione” 2014–2020 Azioni IV.4 program. Fondazione Cariplo and Regione Lombardia are acknowledged for the instrumentation bought during the SmartMatLab Centre project (2014).

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds are not available.

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Molecules 27 06990 g001 550
Figure 1. Molecular structures of (1) [Eu(hfa)3(phen)], (2) [Eu(tta)3(phen)] and (3) [Eu(hfa)3(diglyme)] complexes.
Figure 1. Molecular structures of (1) [Eu(hfa)3(phen)], (2) [Eu(tta)3(phen)] and (3) [Eu(hfa)3(diglyme)] complexes.
Molecules 27 06990 g001
Molecules 27 06990 g002 550
Figure 2. UV-vis spectra of [Eu(hfa)3(phen)], [Eu(tta)3(phen)] and [Eu(hfa)3(diglyme)], complexes from 1 × 10−4 M solutions in CH2Cl2.
Figure 2. UV-vis spectra of [Eu(hfa)3(phen)], [Eu(tta)3(phen)] and [Eu(hfa)3(diglyme)], complexes from 1 × 10−4 M solutions in CH2Cl2.
Molecules 27 06990 g002
Molecules 27 06990 g003 550
Figure 3. UV-vis spectra comparison of (a) [Eu(hfa)3(phen)], (b) [Eu(tta)3(phen)] and (c) [Eu(hfa)3(diglyme)] complexes with the corresponding ligands (1 × 10−4 M CH2Cl2).
Figure 3. UV-vis spectra comparison of (a) [Eu(hfa)3(phen)], (b) [Eu(tta)3(phen)] and (c) [Eu(hfa)3(diglyme)] complexes with the corresponding ligands (1 × 10−4 M CH2Cl2).
Molecules 27 06990 g003
Molecules 27 06990 g004 550
Figure 4. Luminescence spectra (λexc = 348 nm), of [Eu(hfa)3(phen)], [Eu(tta)3(phen)] and [Eu(hfa)3(diglyme)], complexes in CH2Cl2.
Figure 4. Luminescence spectra (λexc = 348 nm), of [Eu(hfa)3(phen)], [Eu(tta)3(phen)] and [Eu(hfa)3(diglyme)], complexes in CH2Cl2.
Molecules 27 06990 g004
Table
Table 1. Main absorption bands in the UV-visible spectra and second-order NLO response.
Table 1. Main absorption bands in the UV-visible spectra and second-order NLO response.
CompoundAbsorption a λmax/nm (ε/M−1 cm−1)μβ (×10−48 esu) b
1
Molecules 27 06990 i001		233, 272 (11,851), 293 1016
2
Molecules 27 06990 i002		230, 272 (9347), 341 (10,000)920
3
Molecules 27 06990 i003		306 (16,617)161
a In CH2Cl2 10−4 M; b In CHCl3 10−3 M working at 1.907 μm; the experimental error is ±10%.
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Fagnani, F.; Colombo, A.; Malandrino, G.; Dragonetti, C.; Pellegrino, A.L. Luminescent 1,10-Phenanthroline β-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties. Molecules 2022, 27, 6990. https://doi.org/10.3390/molecules27206990

AMA Style

Fagnani F, Colombo A, Malandrino G, Dragonetti C, Pellegrino AL. Luminescent 1,10-Phenanthroline β-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties. Molecules. 2022; 27(20):6990. https://doi.org/10.3390/molecules27206990

Chicago/Turabian Style

Fagnani, Francesco, Alessia Colombo, Graziella Malandrino, Claudia Dragonetti, and Anna Lucia Pellegrino. 2022. "Luminescent 1,10-Phenanthroline β-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties" Molecules 27, no. 20: 6990. https://doi.org/10.3390/molecules27206990

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