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

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.

It was reported that dipolar lanthanide complexes such as [LLn(NO 3 ) 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, β HLS , of Na 3 [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], β EFISH , initially increase rapidly with the number of f electrons, whereas the increase is much lower for the last seven f electrons; additionally, the β HLS values increase, but much less rapidly, along the Ln series [38]. Similarly, the β EFISH values of trinuclear lanthanide adducts [Ln(NO 3 ) 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 β EFISH 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.
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. Similarly, the increase in the quadratic hyperpolarizability, βHLS, of Na3[Ln(pyridyldicarboxylate)3] along the Ln series can be explained by the increased number of f e trons [35]. The unexpected fact that the quadratic hyperpolarizability depends on number of f electrons was attributed to the polarization of the 4f electrons [41]. Addit ally, it has been reported that the quadratic hyperpolarizibilities of Ln complexes bea nonadentate ligands based on triazacyclononane, functionalized with pyridyl-2-p phinate groups, reach a maximum around the center of the lanthanide series, with a b shaped trend [42]. A similar trend of the quadratic hyperpolarizibilities was observe the case of Ln complexes of trans-cinnamic acid [44]. In parallel, some of us studied second-order nonlinear optical response of [Ln(hfa)3(diglyme)] (hfa = hexafluoroac lacetonate; diglyme = bis (2-methoxyethyl) ether) by a combination of Electric-Field duced Second Harmonic generation (EFISH) and HLS techniques in solution, confirm the role of f electrons in controlling the second-order NLO properties [38]. In these tems, the molecular quadratic hyperpolarizabilities measured by the EFISH method [ βEFISH, initially increase rapidly with the number of f electrons, whereas the increas much lower for the last seven f electrons; additionally, the βHLS values increase, but m less rapidly, along the Ln series [38]. Similarly, the βEFISH values of trinuclear lanthan adducts [Ln(NO3)3(CuL)2] (Ln = La, Ce, Sm, Eu, and Er; L = N,N′-1,3-propylen-bis (sali ideniminato)) are significantly influenced by the number of f electrons: the values initi increase rapidly with the number of f electrons, starting from lanthanum to europi then the increase is less marked upon addition of the other f electrons, with the βEFISH v of the Er complex (11 f electrons) being only 1.1 times higher than that of the Eu comp (6 f electrons) [39]. This study confirmed that the surprising polarizable character of f e trons is the origin of the fascinating NLO properties. As general trend, the increase of second-order NLO response is significant up to fulfilment of half f shell, while it beco much less relevant with the addition of further f electrons up to the total fulfilment of f shell [39]. This latter observation and the fact that lighter lanthanides (Ce-Eu) are more ab dant than the heavier ones (Gd-Lu), and therefore are generally less expensive [49], ren europium complexes of particular interest for NLO studies.
The Eu complexes under investigation are schematized in Figure 1. The struct present the Eu as central metal in the most stable oxidation state 3+ and stabilized by oxygens coming from the β-diketonate ligands and by two nitrogens of the phenant line for [Eu(hfa)3(phen)] and [Eu(tta)3(phen)] (complexes 1 and 2 in Figure 1), while th coordination sphere is completed with three additional oxygens of the polyether [Eu(hfa)3(diglyme)], complex 3.

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: where µβ λ /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. B λ is the projection along the dipole moment axis of β VEC , 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 β EFISH 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 µβ EFISH that should be maximized. A compound with a µβ EFISH 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.
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(CH 3 CO 2 ) 2 " moiety produces a significant enhancement of the product µβ EFISH , which becomes 99 × 10 −48 , 254 × 10 −48 , and 616 × 10 −48 esu for X = OMe, NMe 2 , and trans-CH=CHC 6 H 4 NMe 2 , 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. Table 1. Main absorption bands in the UV-visible spectra and second-order NLO response.
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 lig- The large NLO response of 1 and 2 is thrilling also due to the simplicity of the 1,10phenanthroline 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 lig- The large NLO response of 1 and 2 is thrilling also due to the simplicity of the 1,10phenanthroline 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 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 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 5 D0 excited state to the 7 FJ multiplet of the Eu(III) ion. In particular, the peaks at 578, 590 and 612 nm are assigned to the Eu ion transitions 5 D0 → 7 F0, 5 D0 → 7 F1 and 5 D0 → 7 F2, respectively [56]. The presence of the band due to the 5 D0-7 F0 transition in the 574-582 nm spectral region due to a singlet-to-singlet transition indicates that the Eu 3+ 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 5 D0 → 7 F2 and 5 D0 → 7 F1 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]. 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)].
Molecules 2022, 27, x FOR PEER REVIEW 5 of 10 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 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 5 D0 excited state to the 7 FJ multiplet of the Eu(III) ion. In particular, the peaks at 578, 590 and 612 nm are assigned to the Eu ion transitions 5 D0 → 7 F0, 5 D0 → 7 F1 and 5 D0 → 7 F2, respectively [56]. The presence of the band due to the 5 D0-7 F0 transition in the 574-582 nm spectral region due to a singlet-to-singlet transition indicates that the Eu 3+ 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 5 D0 → 7 F2 and 5 D0 → 7 F1 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]. The spectra, recorded as CH 2 Cl 2 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 5 D 0 excited state to the 7 F J multiplet of the Eu(III) ion. In particular, the peaks at 578, 590 and 612 nm are assigned to the Eu ion transitions 5 D 0 → 7 F 0 , 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 , respectively [56]. The presence of the band due to the 5 D 0 -7 F 0 transition in the 574-582 nm spectral region due to a singlet-to-singlet transition indicates that the Eu 3+ 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 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 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].

Materials and Methods
All reagents and solvents were purchased from Sigma-Aldrich and were used without further purification.

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 µβ EFISH 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.
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.