Series of Highly Luminescent Macrocyclic Sm(III) Complexes: Functional Group Modifications Together with Luminescence Performances in Solid-State, Solution, and Doped Poly(methylmethacrylate) Film

Abstract

Here, we report our trials to regulate the luminescence performance of the macrocyclic samarium(III) complex and prepare four excellent luminescent Sm(III) complex-doped poly(methylmethacrylate) (PMMA) composites. Four 23-membered [1 + 1] Schiff-base macrocyclic mononuclear Sm(III) complexes, Sm-2_a–Sm-2_d, originating from dialdehydes with different pendant arms and 1,2-bis(2-aminoethoxy)ethane, have been constructed by the template method. Crystal structures reveal that every Sm(III) ion with the coordination geometry of a distorted bicapped square antiprism is capsulated by the macrocyclic cavity environment forming the “lasso-type” protection. Relative photophysical properties of macrocyclic Sm(III) complexes are carefully investigated in solid-state, methanol solution, and doped PMMA film, and all these show characteristic emissions of the Sm(III) ion associated with satisfactory lifetimes and quantum yields in all media, which could be comparable to reported outstanding examples. Especially, the luminescence performance for this type of Sm(III) complex could be regulated in the solid state by the use of different functional groups in the pendant arm while it is not achieved in solution and the doped PMMA composite. High emitting and air-stable plastic materials could be obtained when these Sm(III) complexes are doped in PMMA with 0.1 wt % mixing ratio, and the corresponding maximum lifetime and quantum yield are 61.2 μs and 0.63% in the case of complex Sm-2_a, respectively. We believe that these highly luminescent “lasso-type” Sm(III) complexes and doped PMMA composites are valuable references in the design of luminescent lanthanide(III) hybrid materials.

Introduction

As classic optical function materials, luminescent lanthanide(III) [Ln(III)] complexes could be used in many fields such as bioimaging,¹ solar cells,^2,3 fluoroimmunoassay,⁴⁻⁶ telecommunication,⁷ light-emitting diodes,^8,9 sensors,^10,11 and so forth. Compared with other luminescent materials such as nanoparticles and quantum dots, Ln(III) complexes have characteristic and narrow emissions in the visible or near infrared region for certain Ln(III) ions because the luminescence comes from the energy transfer between two specific 4f levels of the corresponding Ln(III) ion.¹²⁻¹⁴ In fact, even if they have so many virtues, Ln(III) complexes still do not show great potential for practical applications like other candidates. Main reasons for this issue are ascribed to two aspects. One is the low luminescence efficiency for most Ln(III) ions in solid state or solution. To date, only Eu(III) and Tb(III) ions have realized the satisfactory luminescence performances no matter which environment their complexes are in.¹⁵⁻²⁵ From the “antenna effect” mechanism proposed by Weissman, we know that the luminescence emissions of Ln(III) ions are closely related to the energy levels of organic sensitizers.²⁶⁻²⁹ The high-efficient luminescence of Ln(III) ions usually could be achieved after the elaborate regulation of triplet levels (T₁) by the chromophore selection and group modification. The other is the adverse architecture of the Ln(III) complex which is hard to shield Ln(III) ions from various quenchers.³⁰⁻³⁶ In response to the abovementioned problems, some researchers propose a solution based on the hybrid materials,³⁷⁻⁴⁰ and the luminescent Ln(III) complexes are proportionally mixed into another substrate, for instance, polymer,⁴¹⁻⁴⁵ nanometer inorganic material,⁴⁶⁻⁵¹ and so forth. These works generally could ensure the reliable photophysical properties and enhance the machining performance of Ln(III) complexes to prepare four kinds of functional devices.

As a promising area of luminescent Ln(III) materials, the highly luminescent Sm(III) complexes as efficient emitters are of great potential for application in fields such as agricultural films, solar concentrators, light-emitting devices, anticounterfeiting materials, and so forth. Herein, we synthesize a family of luminescent macrocyclic Sm(III) complexes (Scheme 1), explore the regulation of luminescence based on pendant arm modification, and investigate relative photophysical properties in different media. By the Sm(III) ion template method,⁵² we successfully obtained four 23-membered [1 + 1] flexible Schiff-base macrocyclic⁵³⁻⁵⁵ mononuclear Sm(III) complexes (Sm-2_a–Sm-2_d), which contain different pendant arms in macrocyclic skeletons. At the same time, the group modifications are carried out by the use of different functional groups (electron-withdrawing and electron-donating groups) in pendant arms. It is found that the functional group of pendant arms does not play a critical role in changing the electron distribution of the macrocyclic ligand and photophysical properties of four Sm(III) complexes are very similar with each other in methanol solutions. However, their lifetimes and quantum yields are different in microcrystalline powders and doped poly(methylmethacrylate) (PMMA) composites, originating from the different folding patterns of macrocyclic skeletons and orientations of pendant arms. In particular, these doped PMMA plastic materials show highly luminescence performances benefitting from the “lasso-type” protection which could prevent the fluorescence quenching from the competitive coordination of polymer molecules.^13,56

Scheme 1. Chemical Structures of “lasso-type” Macrocyclic Mononuclear Sm(III) Complexes (Left) and Corresponding Pendant Groups (Right) in Macrocyclic Ligands.

Results and Discussion

Synthesis of Dialdehydes, Macrocyclic Mononuclear Sm(III) Complexes, and Doped PMMA Plastic Materials

Though pendant arms would not directly coordinate with central Ln(III) ions, they still could improve lanthanide(III) luminescence if they were efficient chromophores.¹²⁻¹⁵ Considering that possible energy transfers in pendant arms could be further influenced by different functional groups, the adjustment of the energy level for macrocyclic ligands was operated by the introduction of different groups in the benzene rings of pendant arms. In this work, we used trifluoromethyl (−CF₃) as the electron-withdrawing group and methoxyl (−OCH₃) as the electron-donating group relative to chlorine (−Cl) and explored the possible changes for Sm(III) ion luminescence. In addition, we also tried to reveal the relationship of the sensitization efficiency and medium, which was carefully investigated related to several photophysical parameters of Sm(III) ion luminescence in solid-state, methanol solution, and doped PMMA plastic materials, in order to evaluate group modifications.

The dialdehyde H₂Q_a has been reported in another published work⁵⁷ while other three extended dialdehyde precursors (H₂Q_b, H₂Q_c, and H₂Q_d) with different groups were prepared by following a one-step reaction.⁵⁸ In fact, two nucleophilic substitutions happened in this one-step synthesis of dialdehyde between the primary amine and 5-chloro-3-(chloromethyl)-2-hydroxybenzaldehyde. The yields of H₂Q_b and H₂Q_d were higher than those of H₂Q_a and H₂Q_c. It seemed that the electron-withdrawing group in the pendant arm would lead to higher yields of dialdehydes. Based on the Sm(III) ion template method, four 23-membered [1 + 1] Schiff-base macrocyclic mononuclear Sm(III) complexes (Sm-2_a [Sm(HL2_a)(NO₃)₂], Sm-2_b [Sm(HL2_b)(NO₃)₂], Sm-2_c [Sm(HL2_c)(NO₃)₂], and Sm-2_d [Sm(HL2_d)(NO₃)₂]) were finally obtained by cyclic condensation reactions between different pendant-armed dialdehydes and 1,2-bis(2-aminoethoxy)ethane. By this way, four different groups with different electron effects were successfully introduced in the macrocyclic ligands (H₂L2_a, H₂L2_b, H₂L2_c, and H₂L2_d) to regulate the macrocyclic energy levels. At the same time, methanol was used as the medium when we studied the luminescence in solution. The common polymer matrix for luminescent Ln(III) complexes was PMMA, which was a low-cost and simply prepared polymer with excellent optical and mechanical properties. Hence, here we have doped these Sm(III) complexes into the PMMA host to obtain highly luminescent transparent films by the mechanical doping method during the synthesis of PMMA and the spin-coating method before the curing treatment.

Spectral Characterization and Crystal Structures of Macrocyclic Sm(III) Complexes

In the Fourier transform infrared (FT-IR) spectra of four extended dialdehydes, the absorption peaks were found at 1675–1676 cm^–1 (Figures S4–S6), while a series of strong peaks of macrocyclic Sm(III) complexes could be observed at 1633–1636 cm^–1 (Figures S7–S10), indicating the successful transformation from aldehyde groups (CH=O) to imine bonds (CH=N) in the resulting products. Interestingly, their FT-IR spectra were very similar to each other in the fingerprint region. It was suggested that they had the same molecular structures, that is, they were polymorphous structures. We also observed a series of absorption peaks in the range of 1308–1316 cm^–1 derived from stretch vibrations of nitrates in four macrocyclic Sm(III) complexes, which revealed that nitrates coordinated with central Sm(III) ions. We also monitored targeted macrocyclic Sm(III) complexes by the electrospray ionization mass spectra (ESI-MS). The existence of four [1 + 1] macrocyclic mononuclear Sm(III) complexes could be clearly verified by relative positive peaks (Figures 1 and S11–S13), and these were assigned as the species of solvent cluster ions, in good agreement with the theoretic simulations. It was critical that most architectures for four macrocyclic Sm(III) complexes were still stable without solvolysis in CH₃OH because of strong coordination interactions between the Sm(III) ion and macrocyclic ligand together with two nitrates. In addition, the single crystals of three macrocyclic Sm(III) complexes Sm-2_a, Sm-2_b, and Sm-2_c were successfully obtained from the acetonitrile/ethanol mixture by slow evaporation in air at room temperature and finally determined by the X-ray diffraction experiments.

Figure 1.

ESI-MS (positive) of macrocyclic mononuclear Sm(III) complex Sm-2_a together with the inserted experimental (a,c) and simulative [(b,d), calculation for [C₃₀H₄₃Cl₃N₅O₁₆NaSm] and [C₃₂H₄₇Cl₃N₅O₁₆NaSm], respectively] peaks of isotopic distribution corresponding to the peaks at m/z = 1011.16 and 1039.15.

X-ray single-crystal diffraction results (Table S1) of three macrocyclic Sm(III) complexes Sm-2_a–Sm-2_c revealed that they belonged to the monoclinic C2/c and triclinic P1̅ space group, respectively. Their molecular structures with the atom-numbering scheme are shown in Figures 2 and S14, while the bond distances and angles for three complexes are given in Table S2. All three Sm(III) complexes had the similar architecture which was composed of one Sm(III) ion center, one 23-membered [1 + 1] Schiff-base macrocyclic monovalent anion (HL2_a^–, HL2_b^–, and HL2_c^–), and two bidentate nitrate anions. All three macrocyclic skeletons were outstretched configurations. The dihedral angles between two salicylaldehyde rings in the macrocyclic skeletons were 69.6(2)–74.7(3)°, and it was suggested that their macrocyclic skeletons were extremely similar with each other. The extended dialdehyde components had the same tripodal configuration while three benzene rings in pendant arms had different deflection angles. The dihedral angles between the pendant-armed benzene ring and two salicylaldehyde rings were different. They were 8.7(5) and 82.6(5)° in Sm-2_a, 4.4(2) and 73.9(2)° in Sm-2_b, and 84.1(2) and 89.9(2)° in Sm-2_c. The basal coordination sphere for every Sm(III) ion was composed of two phenolic oxygen atoms, two nitrogen atoms of imine bonds, two oxygen atoms of 1,2-bis(2-aminoethoxy)ethane, and four oxygen atoms of two nitrates at each side of the macrocyclic plane. The protonated tertiary nitrogen atom N1 was free of coordination in all the cases. Three 10-coordinate Sm(III) inner coordination spheres were not with the idealized geometry and could be described as a distorted bicapped square antiprism.⁵⁹ Most of all, we observed the “lasso-type” protection in the three complexes in which the central Sm(III) ions have been encapsulated by cyclic cavities of the flexible macrocyclic ligands. As a result, a beneficial microenvironment was created to shield every Sm(III) ion from further undesired deactivations. In addition, both of the two phenolic protons were removed and a proton was added in the tertiary nitrogen atom N1, which could be verified by the electroneutrality principle for the whole molecule. This result was consistent with the fact that all the macrocyclic Sm(III) complexes were synthesized and obtained in weak acidic conditions (about pH 5.5). It was worth mentioning that two strong intramolecular N–H···O hydrogen bonds (Table S3) could be determined in every complex which should be good for stabilizing the whole complex architecture. In addition, no additional intermolecular interactions between neighboring molecules were found after the analysis of the packing arrangements.

Figure 2.

Oak Ridge thermal ellipsoid plot (ORTEP) of the structure of [1 + 1] Schiff-base macrocyclic mononuclear Sm(III) complex Sm-2_a with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level, and the hydroxyl protons are shown as small spheres of arbitrary radii.

Photophysical Properties of Four Macrocyclic Sm(III) Complexes in Solid-State, Solution, and Doped PMMA Film

During photophysical property studies, all the solutions were prepared by dissolving the corresponding Sm(III) complex in methanol. The solutions were diluted to a final concentration of 5 μM to ensure the linear relationship between the intensity of emitted light and the concentration of the emitting species (A ≤ 0.05). Here, the excitation wavelengths of all the samples were chosen at 358 nm. UV–vis spectra of four Schiff-base macrocyclic Sm(III) complexes were first measured to test regulation results and they showed the same absorption band centered at 358 nm (Figure S15), which could be assigned as the characteristic π–π* transition of the azomethine chromophore in the macrocyclic ligand. It must be suggested that pendant arms containing diverse functional groups with different electron effects had no considerable influence on the energy levels of macrocyclic ligands. This result showed that electron intensities in azomethine components were not obviously changed because of extremely weak inductive effects and no conjugative effects of pendant arms for macrocyclic skeletons. Hence, the dramatical regulation of macrocyclic energy levels might be successfully achieved only when the modification of the functional group was operated in the salicylaldehyde part against the pendant arm.

In this article, only emissions of the Sm(III) ion in the visible region have been taken into consideration. After careful luminescence measurements, all the Sm(III) complexes exhibited four characteristic bands at 563, 596, 643, and 709 nm (Figure 3a,c,e) in the solid-state, methanol solution, and doped PMMA film, which were responsible for the transitions from the emitting ⁴G_5/2 to the corresponding ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) states. It was suggested that all four [1 + 1] Schiff-base macrocyclic ligands (H₂L2_a–H₂L2_d) could be served as efficient sensitizers to induce the characteristic emissions of the Sm(III) ion. The luminescence intensity of electric dipole transition ⁴G_5/2 → ⁶H_9/2 at 643 nm was a little higher than that of magnetic dipole transition ⁴G_5/2 → ⁶H_7/2 at 596 nm in all three states, and resulting magenta luminescence could be observed by naked eyes. In four complexes, the intensity ratios between two transitions, I(⁴G_5/2 → ⁶H_9/2)/I(⁴G_5/2 → ⁶H_7/2), were roughly identical (about 1.30) in methanol solution. This indicated that the local environment around the Sm(III) ion in methanol remained the same for them. At the same time, it was complicated in solid-state and doped PMMA film. Overall, compared with the amorphous complex Sm-2_d, all three complexes Sm-2_a–Sm-2_c in the microcrystalline form had higher emission intensities related to the crystal field effect, which could split the energy levels of the Sm(III) ion resulting in the increasing numbers of energy levels, probabilities of nonradiative energy transfer, and energy transfer efficiencies.

Figure 3.

Emission spectra of four macrocyclic Sm(III) complexes Sm-2_a–Sm-2_d in (a) solid-state, (c) methanol solution ([M] = 5 μM), and (e) doped PMMA films (0.1 wt %) at room temperature upon λ_ex = 358 nm. Lifetimes for Sm(III) complexes in (b) solid-state, (d) methanol solution ([M] = 5 μM), and (f) doped PMMA films (0.1 wt %) at room temperature upon λ_em = 643 nm.

Quantum yields (Φ) and lifetimes (τ) of complexes Sm-2_a–Sm-2_d were also carefully measured and calculated in order to further evaluate sensitization results and local environments of central Sm(III) ions. Quantum yields for complexes Sm-2_a–Sm-2_d have been determined at 298 K and are listed in Table 1. The lifetimes have been investigated by detecting the visible transition ⁴G_5/2 → ⁶H_9/2 at 643 nm for all the samples at room temperature. Calculated values of lifetimes are given in Table 1. Overall, all four macrocyclic Sm(III) complexes showed high quantum yields and lifetimes in the solid state which were comparable to other reported Sm(III) complexes.^16,60−64 Especially, the corresponding quantum yield and lifetime of complex Sm-2_a were 4.66% and 96.3 μs, respectively, which were the biggest values among the four complexes. Though significant changes of macrocyclic sensitizer levels were not successful by modifications of different functional groups in pendant arms, the different folding patterns of macrocyclic skeleton and orientations of pendant arms were realized in their single crystals which could affect relative energy transfer efficiencies by the crystal field effect. In addition, the architecture of complex Sm-2_a in microcrystalline powder should have more efficient energy transfers.

Table 1. Lifetimes (τ) and Quantum Yields (Φ) for Four Macrocyclic Sm(III) Complexes in Solid-State, Methanol Solution ([M] = 5 μM), and PMMA Film (0.1 wt %)^a.

	solid stateb		methanol solutionb		PMMA filmb
complex	τ (μs)	Φ (%)c	τ (μs)	Φ (%)c	τ (μs)	Φ (%)c
Sm-2a	96.3 ± 0.4	4.66 ± 0.11	37.7 ± 0.1	0.42 ± 0.02	61.2 ± 0.3	0.63 ± 0.04
Sm-2b	62.1 ± 0.3	2.40 ± 0.05	38.2 ± 0.1	0.43 ± 0.03	59.7 ± 0.3	0.58 ± 0.04
Sm-2c	59.1 ± 0.3	3.05 ± 0.08	38.6 ± 0.1	0.44 ± 0.03	58.5 ± 0.3	0.61 ± 0.04
Sm-2d	70.2 ± 0.3	2.32 ± 0.06	39.2 ± 0.1	0.46 ± 0.03	53.4 ± 0.2	0.52 ± 0.03

^aλ_ex = 358 nm was used for lifetime and quantum yield determinations.

^bThe samples of complexes Sm-2_a–Sm-2_c were microcrystalline powders and complex Sm-2_d was amorphous powder.

^cQuantum yields were determined by the use of an integrating sphere at room temperature.

As shown in Figure 3c, in the four complexes, we observed the symmetrical shapes and absence of splitting for the peak at 598 nm, derived from the ⁴G_5/2 → ⁶H_7/2 transition of the Sm(III) ion, which should be because of solvent temperature fluctuations. All the luminescence decay curves (Figure 3d) for four complexes in methanol have been fitted with monoexponential functions which also suggested the presence of one major luminescent species in every solution. Interestingly, for the four complexes, their lifetimes (37.7–39.2 μs) were very similar with each other in methanol. It was suggested that they had the similar energy-transfer process, and the modification of the functional group in the pendant arm did not obviously change the degree of the solvation effect for each Sm(III) complex. For the four complexes, quenching effects, originating from methanol molecules, were remarkably strengthened compared with the solid state which could be proven by the obvious reduction of lifetimes and quantum yields. Their similar quantum yields, determined to be 0.42–0.46%, proved these points again.

In order to improve the machinability of the luminescent Sm(III) complex for practical applications, we also selected PMMA as the substrate to prepare excellent composites, which possessed the luminescence properties of the Sm(III) ion together with the properties of PMMA matrices, for instance, low-weight, transparency in the visible range, and good mechanical properties. The abovementioned four complexes have been doped into PMMA matrices, and films (about 10 μm thick) of each Sm(III) complex doped (0.1 wt %) in PMMA were deposited on glass substrates. As expected, four characteristic emission bonds could also be observed for each film doped with the corresponding Sm(III) complex (Figure 3e), revealing the structural stability for every complex in PMMA. The decay curves (Figure 3f) for four complexes in PMMA were also fitted with monoexponential functions indicating one luminescent species in each composite. As listed in Table 1, their lifetimes were also similar with each other that suggested the similar local environments of Sm(III) complexes surrounded by PMMA polymer molecules just like methanol molecules in solution. Compared to the solid state, the insertion of four Sm(III) complexes into PMMA matrices led to two results: the lifetimes reached at the same values shortened by about 29, 4, 1, and 24%, respectively, and their quantum yields tended to be the same as well. The abovementioned results revealed that the crystal field effect had little impact on the energy transfer of sensitization when macrocyclic Sm(III) complexes were mixed into methanol or PMMA under the low concentration. Simultaneously, it was indicated that the coordination sphere of the central Sm(III) ion could not be fully shielded by the macrocyclic cavity microenvironment in this “lasso-type” architecture. For all four complexes, the existence of additional interactions was probable between the methanol molecules and Sm(III) complexes in solution compared with the solid state. Finally, the same photophysical results were obtained after half a year (Figure S16) in air without phase separation suggesting the satisfactory stability of these doped PMMA films of the four Sm(III) complexes. The doped PMMA films would split, and obvious phase separation phenomenon could be observed at the same time if the doped content of the Sm(III) complex was more than about 2 wt %.

Conclusions

In summary, we have demonstrated that a series of [1 + 1] pendant-armed Schiff-base macrocyclic mononuclear Sm(III) complexes (Sm-2_a–Sm-2_d) are synthesized by the Sm(III) ion template. X-ray structures confirm that each ten-coordinate Sm(III) center with the coordination geometry of a distorted bicapped square antiprism is fully capsulated by the cavity environment of the corresponding macrocyclic ligand to form “lasso-type” architecture. Moreover, all four macrocyclic ligands (H₂L2_a–H₂L2_d) could serve as effective sensitizers for the Sm(III) ion, exhibiting characteristic emissions. It simultaneously proves that the luminescence performance of the macrocyclic Sm(III) complex in the solid state could be efficaciously regulated by different functional groups in pendant arms without the same results in methanol solution. These macrocyclic Sm(III) complexes are also used as molecular emitter materials to prepare four highly luminescent doped PMMA films with 0.1 wt % contents. The average quantum yield of four polymeric materials is 0.58%, and the maximal value is as high as 0.63%. It is also found that these doped PMMA composites are air-stable for at least half a year. These properties are at the top rank of reported emissive Sm(III) polymeric materials with a Sm(III) complex doped in the PMMA matrixes. Further work is being undertaken on this luminescent “lasso-type” Sm(III) complex to continuously improve its luminescence performances and application potentials.

Experimental Section

Materials and Methods

Unless otherwise specified, reagents of analytical grade were purchased directly from commercial sources and used without any further purification.

¹H NMR spectroscopic measurements were performed on a Bruker DPX 300 MHz spectrometer in CDCl₃, using tetramethylsilane (SiMe₄) as an internal reference at room temperature. Elemental analyses were measured using a PerkinElmer 1400C analyzer. ESI-MS were recorded using a Thermo Fisher Scientific LCQ-Fleet mass spectrometer in a scan range of 100–2000 amu. Infrared spectra (4000–400 cm^–1) were collected using a Nicolet FT-IR 170X spectrophotometer at 25 °C using KBr plates. UV–vis spectra were recorded using a Shimadzu UV-3150 double-beam spectrophotometer using a quartz glass cell with a path length of 10 mm. The lifetimes and visible emission measurements were determined using the Edinberge FLS920 fluorometer equipped with a PMT R 928-P detector in the visible region. The quantum yield data for Sm(III) complexes were acquired at room temperature on the HORIBA Jobin Yvon FluoroMax-4 fluorometer using a integrating sphere.

Synthesis of H₂Q_b

The extended dialdehyde H₂Q_b was prepared according to the literature method.⁵⁸ 2-(4-trifluoromethylphenyl)ethylamine (0.477 g, 2.52 mmol) was dissolved in 60 mL of acetone, and then K₂CO₃ solid (7.010 g, 50.72 mmol) was added directly into the solution under magnetic stirring. The stirring procedure was kept for 1 h and acetone solution (20 mL) of 5-chloro-3-(chloromethyl)-2-hydroxybenzaldehyde (1.256 g, 6.13 mmol) was added dropwise to the mixture. After stirring for another 4 h at room temperature, the solid was filtered off and washed with acetone. The filtrate was concentrated to dry, and the orange crude product was obtained. H₂Q_b was finally purified by silica gel column chromatography using ethyl acetate/petroleum ether (v/v = 1:6) as the eluant. Yield, 1.048 g, 79%. mp: 191–192 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.99 (s, 2H), 7.54 (d, J = 2.4 Hz, 4H), 7.28 (s, 2H), 7.23 (d, J = 7.8 Hz, 2H), 3.89 (d, J = 3.3 Hz, 4H), 3.01 (d, J = 5.3 Hz, 2H), 2.92 (d, J = 6.0 Hz, 2H). Anal. Calcd for C₂₅H₂₀Cl₂F₃NO₄: C, 57.05; H, 3.83; N, 2.66%. Found: C, 56.95; H, 3.80; N, 2.63%. Main FT-IR absorptions (KBr pellets, cm^–1): 3415 (m), 2922 (m), 2853 (m), 1676 (s, −CH=O), 1325 (s), 1111 (s), 714 (w).

Synthesis of H₂Q_c

The synthetic process of H₂Q_c was the same as that of H₂Q_b except that 2-(3,4-dimethoxyphenyl)ethylamine (0.453 g, 2.50 mmol) was used. H₂Q_c was finally purified by silica gel column chromatography using ethyl acetate/petroleum ether (v/v = 1:10) as the eluant. Yield, 0.892 g, 69%. mp: 178–179 °C. ¹H NMR (400 MHz, CDCl₃): δ 10.02 (s, 2H), 7.54 (s, 2H), 7.28 (s, 2H), 6.78 (d, J = 8.1 Hz, 1H), 6.66 (d, J = 8.2 Hz, 1H), 6.62 (s, 1H), 3.87 (s, 4H), 3.84 (s, 6H), 2.86 (s, 4H). Anal. Calcd for C₂₆H₂₅Cl₂NO₆: C, 60.24; H, 4.86; N, 2.70%. Found: C, 60.19; H, 4.82; N, 2.67%. Main FT-IR absorptions (KBr pellets, cm^–1): 3418 (w), 2924 (s), 2853 (s), 1660 (s, −CH=O), 1443 (s), 1265 (s), 723 (m).

Synthesis of H₂Q_d

The synthetic process of H₂Q_d was the same as that of H₂Q_b except that 2-(3,4-dichlorophenyl)ethylamine (0.452 g, 2.50 mmol) was used. H₂Q_d was finally purified by silica gel column chromatography using ethyl acetate/petroleum ether (v/v = 1:4) as the eluant. Yield, 1.117 g, 85%. mp: 195–196 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.87 (s, 2H), 7.41 (d, J = 2.3 Hz, 2H), 7.21 (d, J = 8.2 Hz, 1H), 7.16 (s, 2H), 7.06 (d, J = 1.8 Hz, 1H), 6.82 (dd, J = 8.2, 1.8 Hz, 1H), 3.71 (s, 4H), 2.74 (s, 4H). Anal. Calcd for C₂₄H₁₉Cl₄NO₄: C, 54.67; H, 3.63; N, 2.66%. Found: C, 54.57; H, 3.58; N, 2.62%. Main FT-IR absorptions (KBr pellets, cm^–1): 3415 (w), 2856 (w), 1675 (s, −CH=O), 1459 (s), 1281 (m), 1025 (w).

Syntheis of Sm-2_a ([Sm(HL2_a)(NO₃)₂])

The macrocyclic Sm(III) complex Sm-2_a was prepared following our previously reported procedure.⁵⁷ Sm(NO₃)₃·6H₂O (0.045 g, 0.10 mmol) was dissolved in ethanol (20 mL) and added to a solution of H₂Q_a (0.048 g, 0.10 mmol) in acetonitrile (10 mL). The mixture was refluxed for 10 min, and then ethanol solution (20 mL) of 1,2-bis(2-aminoethoxy)ethane (0.017 g, 0.11 mmol) was added. The yellow green solution was refluxed for additional 2 h, cooled to room temperature, and filtered. The filtrate was concentrated to give complex Sm-2_a in a yield of 76% (0.067 g). Anal. Calcd for C₃₀H₃₁Cl₃N₅O₁₀Sm: C, 41.02; H, 3.56; N, 7.97%. Found: C, 40.93; H, 3.52; N, 7.93%. ESI-MS (positive mode, m/z): 1011.16 {[Sm(HL2_a)(NO₃)₂] + 6H₂O + Na}⁺ (100%) and 1039.15 {[Sm(HL2_a)(NO₃)₂] + 2CH₃OH + 4H₂O + Na}⁺ (72%). Main FT-IR absorptions (KBr pellets, cm^–1): 3431 (w), 2935 (w), 2881 (w), 1633 (s, CH=N), 1458 (s), 1384 (s), 1313 (s), 1075 (w), 769 (w). Yellow green single crystals of complex Sm-2_a in a rectangular shape were grown from a mixture of ethanol/acetonitrile (v/v = 4:1) by slow evaporation in air at room temperature for 2 weeks.

Synthesis of Sm-2_b ([Sm(HL2_b)(NO₃)₂])

The synthetic process of Sm-2_b was the same as that of Sm-2_a except that H₂Q_b (0.052 g, 0.10 mmol) was used. Yield: 73%, (0.066 g). Anal. Calcd for C₃₁H₃₁Cl₂F₃N₅O₁₀Sm: C, 40.83; H, 3.43; N, 7.68%. Found: C, 40.72; H, 3.37; N, 7.65%. ESI-MS (positive mode, m/z): 1045.13 {[Sm(HL2_b)(NO₃)₂] + 6H₂O + Na}⁺ (100%) and 1071.15 {[Sm(HL2_b)(NO₃)₂] + 2CH₃OH + 4H₂O + Na}⁺ (68%). Main FT-IR absorptions (KBr pellets, cm^–1): 3422 (w), 2941 (w), 1636 (s, CH=N), 1459 (s), 1316 (s), 1114 (m), 770 (w). Yellow green single crystals of complex Sm-2_b in a rectangular shape were obtained from a mixture of ethanol/acetonitrile (v/v = 2:1) by slow evaporation in air at room temperature for 2 weeks.

Synthesis of Sm-2_c ([Sm(HL2_c)(NO₃)₂])

The synthetic process of Sm-2_c was the same as that of Sm-2_a except that H₂Q_c (0.051 g, 0.10 mmol) was used. Yield: 69%, (0.062 g). Anal. Calcd for C₃₂H₃₆Cl₂N₅O₁₂Sm: C, 42.52; H, 4.01; N, 7.75%. Found: C, 42.43; H, 3.95; N, 7.70%. ESI-MS (positive mode, m/z): 1037.21 {[Sm(HL2_c)(NO₃)₂] + 6H₂O + Na}⁺ (100%) and 1065.15 {[Sm(HL2_c)(NO₃)₂] + 2CH₃OH + 4H₂O + Na}⁺ (96%). Main FT-IR absorptions (KBr pellets, cm^–1): 3442 (w), 2939 (w), 1635 (s, CH=N), 1459 (s), 1308 (s), 1065 (s), 771 (m). Yellow green single crystals of complex Sm-2_c in a rectangular shape were obtained from a mixture of ethanol/acetonitrile (v/v = 1:1) by slow evaporation in air at room temperature for 2 weeks.

Synthesis of Sm-2_d ([Sm(HL2_d)(NO₃)₂])

The synthetic process of Sm-2_d was the same as that of Sm-2_a except that H₂Q_d (0.052 g, 0.10 mmol) was used. Yield: 80%, (0.073 g). Anal. Calcd for C₃₀H₃₀Cl₄N₅O₁₀Sm: C, 39.48; H, 3.31; N, 7.67%. Found: C, 39.38; H, 3.23; N, 7.60%. ESI-MS (positive mode, m/z): 1045.14 {[Sm(HL2_d)(NO₃)₂] + 6H₂O + Na}⁺ (29%) and 1071.24 {[Sm(HL2_d)(NO₃)₂] + 2CH₃OH + 4H₂O + Na}⁺ (30%). Main FT-IR absorptions (KBr pellets, cm^–1): 3431 (w), 2936 (w), 2883 (w), 1634 (s, CH=N), 1459 (s), 1310 (s), 1075 (s), 768 (m).

All the films (about 10 μm thick) of Sm(III) complexes doped (0.1 wt %) in PMMA were obtained by the same procedure: 9.5 mg Sm(III) complexes and 0.050 g azobisisobutyronitrile were directly added into 10 mL of methylmethacrylate under mechanical stirring, and the mixture was treated under ultrasound for 30 min. The prepolymerization reaction was carried out at 55 °C for 2 h under magnetical stirring, and then the viscous mixture was kept at 55 °C without stirring for another 2 h. The films of Sm(III) complexes doped in PMMA were deposited on glass substrates by the spin coater together with a one-night drying treatment at 45 °C.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02576.

• Selected bond distances (Å) and angles (°); hydrogen bonding parameters; and UV–vis, FT-IR, and ¹H NMR spectra (PDF)

• X-ray crystallographic data (CCDC nos. 1889347–1889349) (CIF)

The authors declare no competing financial interest.