High emission quantum yields and color-tunable properties of Ln-chelates embedded in PMMA

Abstract

This research presents a series of PMMA thin layers (labelled Ln_PMMA, where Ln = Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺) with high overall emission quantum yields of Q^L_Eu = 85%, Q^L_Tb = 66%, Q^L_Sm = 3%, and Q^L_Dy = 6% with introduced lanthanide (Ln³⁺) coordination compounds of the type [Na₂LnL₄(OTf)(DMF)] (where L = N-(diphenylphosphoryl)-pyrazine-2-carboxamide, OTf = [CF₃SO₃]⁻, DMF = N,N-dimethylformamide). This is the first analysis comparing the photophysical properties of coordination compounds encapsulated in PMMA with single crystals, which includes the influence of factors such as the inhomogeneity of the Ln³⁺ coordination polyhedra and the refractive index. A model is proposed to estimate the change in Q^L_Ln when the Ln chelate is incorporated into a PMMA medium, and it satisfactorily reproduces the experimental data with a maximum absolute error of 3% for the case of Eu³⁺ samples. At the same time, our work shows the influence of the PMMA matrix on the photophysical properties of Ln³⁺ with large (Eu, Tb) and small energy gaps (Sm, Dy) between the emitting levels and adjacent levels with lower energy. Q^L_Ln for coordination compounds introduced into PMMA decreases relative to single crystals by about 10% for Eu³⁺ and Tb³⁺ as well as by about 70% for Sm³⁺ and Dy³⁺ for which emitting levels are quenched by the electron–phonon coupling presented by the vibrational modes of the PMMA matrix. Ln_PMMA thin layers containing a mixture of Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ coordination compounds are characterized by multicolor tunable emission.

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Introduction

Lanthanides (Ln) are elements without which the development of modern technologies cannot be imagined. Their compounds are used in broadly understood materials engineering and serve in fields such as lighting, displays,¹ sensors and luminescent markers^2–7 and luminescent bar-codes.^8–12 The application possibilities of Ln³⁺ compounds stem from their spectroscopic properties, which are a consequence of their electron structure. In the series, they fill the 4fⁿ subshell, which is efficiently shielded from the influence of the environment by the closed 5s and 5p subshells. This means Ln³⁺ compounds are characterized by electronic bands of small half-width, and the multiplicity of excited electron levels results in emission in different spectral ranges.

Unfortunately, due to the small value of the molar absorption coefficient¹³ of the 4fⁿ transitions, Ln³⁺ ions suffer from low emission brightness. One way to circumvent this inconvenience is to indirectly excite Ln³⁺ through coordinated organic ligands, which transfer the excitation energy to Ln³⁺ after the absorption process (the so-called antenna effect).^13,14 In this way, sensitized emission with much higher brightness is obtained. The efficiency of achieving sensitized emission depends on many factors, such as the spectral overlap of donor (ligand) emission and acceptor (Ln³⁺) absorption, the rate of competing radiative and non-radiative processes within the coordination compound, the distance and mutual position of the donor–acceptor, and the presence of non-radiative processes quenching the Ln³⁺ emitting level.^3,15 The key issue is the appropriate selection of the ligand and the design of a rigid structure of the coordination compound without water molecules in the inner coordination sphere.^16,17 It is difficult to select a ligand that would effectively sensitize different Ln³⁺ ions, or even Eu³⁺ and Tb³⁺. This is because the emission of the former is quenched by the presence of the ligand-to-metal ion charge transfer (LMCT) state,^18–21 and the emission intensity of the latter is often influenced by the back energy transfer (BET) from the Tb³⁺ emitting state to the ligand triplet state.^22–25 Ligand selection is not the only important factor, as it has been shown that by small structural modifications of a coordination compound with the same ligand, large changes in photophysical properties can be obtained.²⁶ It is also extremely important to know the mechanisms of ligand-to-Ln energy transfer,^15,27–29 which facilitates the design of compounds with intense sensitized emission. It should be noted that the combination of experimental results and theoretical calculations is not common.³⁰

Many of the applications mentioned require materials with tunable multi-emission, which can be achieved using different lanthanide ions.³¹ An excellent platform for Ln³⁺ compounds, due to its optical and mechanical properties, is PMMA (poly(methylmethacrylate)polymer),^32,33 which has excellent transparency, UV and chemical resistance. The literature on coordination compounds blended with PMMA can be broadly divided into two groups. In the first group, one can find papers in which the introduction of a coordination compound into PMMA results in an increase in the overall emission quantum yield (Q^L_Ln) due to the displacement of water molecules from the inner coordination sphere of Ln³⁺ and the coordination of the carbonyl group of the polymer,^34–41 as well as publications on the increase in emission due to intermolecular energy transfer, including Ln–Ln.^42–45 In these cases, the PMMA matrix acts as a co-sensitizer and increases the overall emission quantum yields of the polymer films relative to the compounds in the solid state. Research on Ln³⁺ coordination compounds (mainly with β-diketones) in polymer matrices conducted before 2009 is collected in ref. 32. The second group was the PMMA matrix, in which compounds with a saturated inner coordination sphere characterized mostly by intense emission were introduced.^37,46–66 The overall emission quantum yields were comparable to the values obtained for compounds in the form of single crystals or powders. Most of the work is on Eu³⁺ compounds, where Q^L_Ln ranges from 8 to 92%.^{46,48,50,52,57,60,63,65,66} The strategy to obtain PMMA with Eu³⁺ chelate in 92% emission yield was to obtain tridentate ligands by introducing a diphenylphosphoryl oxygen-containing functional group into the bidentate ligands. The study was conducted only for Eu³⁺ compounds.⁶⁶ An exceptionally high Eu³⁺ emission intensity with Q^L_Eu = 84% was obtained by additionally introducing guest molecules into PMMA.⁵³ Most of the work deals with PMMA films with β-diketone-containing chelates for which the triplet state of the ligand does not allow sensitization of Tb³⁺ or Dy³⁺ emission. Examples of effective Tb³⁺ sensitization in the PMMA matrix can be found in ref. 51 and 50, while analogous Eu³⁺ compounds either do not show emission⁵¹ or its Q^L_Eu is low (8%).⁵⁰ Hydrotris(pyrazol-1-yl)borate ligand was used as an emission sensitizer for Eu³⁺, Tb³⁺ and Sm³⁺ (PMMA films with a single Ln³⁺ ion), the intrinsic emission quantum yield (Q^Eu_Eu) of Eu³⁺ in PMMA was 37%, and the overall emission quantum yield measured for solutions of Tb³⁺ and Eu³⁺ compounds in CH₂Cl₂ 4 and 5%, respectively.⁶¹ There are also reports of PMMA films with tunable emission color co-doped with Eu³⁺ and Tb³⁺ coordination compounds, in which metal ions are bound by two different β-diketones, making it possible to sensitize Eu³⁺ and Tb³⁺ using two different excitation wavelengths.³⁴ Multicolor tunable emission was also obtained in Eu³⁺ and Tb³⁺ double-layer polymeric films, which consisted of two polymers (PMMA and PVP) doped with Eu or Tb compounds with different β-diketones. The use of two polymers eliminated ligand exchange during film preparation.³⁷ The above literature review illustrates how difficult it is to obtain PMMA films showing emission for compounds of several Ln³⁺ luminescent in the visible range.

We recently published a series of Ln³⁺ coordination compounds with a ligand (N-(diphenylphosphoryl)-pyrazine-2-carboxamide), which is an excellent emission sensitizer for a wide range of lanthanides,^16,17 and now we have incorporated the chelates into PMMA to obtain bight and multicolor tunable emission materials with a single excitation wavelength.

Finding a ligand that sensitizes different Ln³⁺ ions is challenging, but it offers the advantage of using a single excitation source for multicolor emission materials, reducing costs and enabling practical applications. To our knowledge, this is the first example of PMMA thin layers containing Eu³⁺, Tb³⁺, Sm³⁺, and Dy³⁺ compounds sensitized with the same ligand. It is also the first example where the Q^L_Ln of Eu³⁺ and Tb³⁺ with the same sensitizer is exceptionally high.

Additionally, this is the first analysis comparing the photophysical properties of coordination compounds encapsulated in a PMMA matrix versus single crystals. This includes factors such as the inhomogeneity of Ln³⁺ coordination polyhedra in PMMA and the influence of the refractive index on the Q^L_Ln. Our work also involves an in-depth analysis of how PMMA affects the photophysical properties of lanthanide compounds, focusing on those with large (Eu³⁺, Tb³⁺) and small (Sm³⁺, Dy³⁺) energy gaps between the emitting level and an adjacent lower energy level. This analysis was possible due to the stability of the coordination compounds and the absence of solvent molecules in the inner coordination sphere of Ln³⁺. Combining experimental and theoretical data, including simulations of the inhomogeneity of Ln³⁺ coordination polyhedra due to slight distortions in the PMMA matrix, has provided valuable insights into how the PMMA network influences the photophysical properties of Ln³⁺ compounds. This understanding is crucial for designing new optical materials, and the materials reported here have significant potential for practical applications, such as in layers for barcodes and quick-response codes.^67,68

Experimental

Materials

LAB grade solvents were purchased from commercial sources. Acetone and methanol were dried over 3A molecular sieves. N,N-dimethylformamide (DMF) was dried over 4A molecular sieves. Chloroform was deacidified using alumina. Lanthanide trifluoromethanesulfonates (lanthanide triflates), Ln(OTf)₃, and poly(methylm methacrylate) (PMMA) were purchased from Sigma-Aldrich and used without further purification. Lanthanide coordination compounds were obtained according to the procedure described in ref. 16.

Synthesis of PMMA thin layers with [Na₂LnL₄(OTf)(DMF)]

[Na₂EuL₄(OTf)(DMF)]@PMMA (label: Eu_PMMA). PMMA 1% wt Na₂EuL₄: The set amount of PMMA (0.0990 g) was dissolved in CHCl₃ (0.5 mL). Then the weighted amount of the Na₂EuL₄ coordination compound (0.0010 g) dissolved in acetone (1 mL) was added to the clear solution of PMMA. The final mixture was placed in an ultrasonic cleaner for a few minutes and then transferred into a Petri dish (diameter 3 cm). It was dried at 50 °C and then left in a dark place for a week.

In a similar way, thin PMMA layers containing 5%, 10% and 15% wt of the europium coordination compounds were obtained. PMMA 5% wt Na₂EuL₄: 0.0380 g PMMA–0.0020 g Na₂EuL₄; PMMA 10% wt Na₂EuL₄: 0.0360 g PMMA–0.0040 g Na₂EuL₄; PMMA 15% wt Na₂EuL₄: 0.0340 g PMMA–0.0060 g Na₂EuL₄.

[Na₂LnL₄(OTf)(DMF)]@PMMA (2 wt%, label: Ln_PMMA).
Eu_PMMA . The weighted amount of PMMA (0.1470 g) was dissolved in deacidified CHCl₃ (0.7 mL) using an ultrasonic cleaner. An appropriate amount of the europium coordination compound [Na₂EuL₄(OTf)(DMF)] (0.0030 g) dissolved in acetone (1 mL) was added to the clear solution of PMMA. The mixture was placed in an ultrasonic cleaner for five minutes and then transferred into a Petri dish (diameters of 3 cm and 3.5 cm) and left in a dryer overnight to evaporate the solvents (temperature: 50 °C). During this time, the sample was not exposed to any sunlight.

Similarly, thin PMMA layers containing the coordination compounds Na₂TbL₄, Na₂DyL₄ and Na₂SmL₄ were obtained.

The thickness of the PMMA layers was 0.07–0.08 mm and 0.17–0.21 mm. Thicker layers were used to measure overall emission quantum yield.

A mixture of Eu³⁺, Tb³⁺, Sm³⁺, and Dy³⁺ coordination compounds embedded in PMMA thin layers with a total concentration of 2 wt%. The weight of PMMA was 0.3920 g, and the total content of the mixture of Na₂LnL₄ (Ln = Eu, Tb, Sm, Dy) compounds was 2 wt% (0.0080 g) for each PMMA sample.
Sample I. The relative mass contents of each compound were: 10 wt% Na₂EuL₄ (0.0008 g), 10 wt% Na₂TbL₄ (0.0008 g), 35% Na₂SmL₄ (0.0028 g), and 45 wt% Na₂DyL₄ (0.0036 g). Analogous to the preparation of PMMA films with a single Ln ion compound, all the weighted amounts of coordination compounds were dissolved in 4 mL of acetone and added to the PMMA solution dissolved in 2 mL of CHCl₃. Then the solution was poured into a Petri dish and heated at 50 °C overnight. An analytical scale from Mettler Toledo was used for weighing.
Sample II. The relative mass contents of each compound were: 7.5 wt% Na₂EuL₄ (0.0006 g), 12.5 wt% Na₂TbL₄ (0.0010 g), 35% Na₂SmL₄ (0.0028 g), and 45 wt% Na₂DyL₄ (0.0036 g).
Sample III. The relative mass contents of each compound were: 12.5 wt% Na₂EuL₄ (0.0010 g), 7.5 wt% Na₂TbL₄ (0.0006 g), 35% Na₂SmL₄ (0.0028 g), and 45 wt% Na₂DyL₄ (0.0036 g).
Sample IV. The relative mass contents of each compound were: 10 wt% Na₂EuL₄ (0.0008 g), 10 wt% Na₂TbL₄ (0.0008 g), 25% Na₂SmL₄ (0.0020 g), and 55 wt% Na₂DyL₄ (0.0044 g).
Sample V. The relative mass contents of each compound were: 7.5 wt% Na₂EuL₄ (0.0006 g), 2.5 wt% Na₂TbL₄ (0.0002 g), 60% Na₂SmL₄ (0.0048 g), and 30 wt% Na₂DyL₄ (0.0024 g).

Methods

The infrared spectra of PMMA and Ln_PMMA thin layers in the range 4000–400 cm⁻¹ were obtained using an ALPHA II FTIR spectrometer, from Bruker, equipped with the QuickSnap™ sampling modules, temperature-controlled DLaTGS-detector and RockSolid™ cube corner interferometer. In the far infrared range (550–50 cm⁻¹) the spectra were recorded using a Bruker VERTEX 70 FTIR spectrometer.

Absorption spectra of PMMA thin layers and diffuse reflectance spectrum of Na₂EuL₄ single crystals were recorded at 295 K using an Agilent Technologies Cary 5000 UV-Vis-NIR Spectrophotometer.

Excitation and emission spectra were recorded using an Edinburgh Instruments FLSP 920 spectrofluorometer equipped with a 450 W continuous xenon arc lamp, a single grating excitation monochromator (mechanical wavelength coverage: 200–1000 nm; groove density: 1800 grooves per mm; optimization wavelength: 250 nm (holographic); dispersion: 1.8 nm mm⁻¹) and a single grating emission monochromator (mechanical wavelength coverage: 200–1000 nm; groove density: 1800 grooves per mm; optimization wavelength: 500 nm (ruled); dispersion: 1.8 nm mm⁻¹), and a Single Photon Counting photomultiplier (R13456, Hamamatsu). The spectra were corrected for the instrument's response. Emission decay times were measured with a μF920H 60 W xenon lamp as the excitation source, triggered by the spectrometer controller (Edinburgh Instruments FLSP 920 spectrofluorometer). Measurements of the excitation and emission spectra, as well as the luminescence decay times at 77 K were carried out in a home-made quartz dewar cooled by liquid nitrogen.

The overall emission quantum yield (Q^L_Ln) measurements were carried out in an Integrating Sphere Assembly, F-M01, with an inner diameter of 120 mm. Each measurement was taken 5 times, and the average value is given. Thicker PMMA layers that lay flat on the holder were used to measure the overall emission quantum yield. Adequate thickness of the PMMA layer is important because even slight bulges or concavities of the thin layer changed the value of the quantum yield. The empty holder was measured with a PMMA layer without Ln compound. Great care must be taken when measuring absolute emission quantum yield using an integrating sphere, as various factors can affect the values obtained.⁶⁹ In the ESI,† we discuss in detail what factors affected the emission quantum yield values for compounds in the solid state and in polymer thin layers.

The refractive index values of Na₂LnL₄ measured using a Cary 6000i spectrophotometer with the Agilent Cary Universal Measurement Accessory (UMA)¹⁶ were used for the calculations in this paper. For details, please refer to the ESI.†

Thermogravimetric analysis with differential scanning calorimetry (TGA-DSC) was performed using a Mettler Toledo TGA/DSC 3+ thermogravimeter (for samples Dy_PMMA, Eu_PMMA, and Tb_PMMA). Samples in corundum crucibles (Al₂O₃) with a capacity of 70 μL were heated in the temperature range from 30 to 800 °C in a nitrogen atmosphere with a flow rate of 50 mL min⁻¹. The heating rate was 5 K min⁻¹. For Sm_PMMA and PMMA samples, thermogravimetric analysis was performed on the TG-DTA Setaram SETSYS 16/18 Thermogravimeter in 100 μL crucibles under identical conditions.

For the TG, DSC or TG-DTA measurements were used: m_{Dy_PMMA} = 4.6340 mg; m_{Eu_PMMA} = 4.4280 mg; m_{Tb_PMMA} = 7.0240 mg, m_{Sm_PMMA} = 12.202; m_PMMA = 11.434 mg.

The DCS curves were recorded on the Mettler-Toledo DSC 3 scanning calorimeter in the temperature range from160 to 230 °C.

Radiative rates and sensitization efficiency

The radiative (A_rad) and non-radiative (A_nrad) rates, intrinsic ⁵D₀ emission quantum yield and sensitization efficiency (η_sens) for the PMMA doped with Na₂EuL₄ and for the single crystals were determined based on the Eu_PMMA and Na₂EuL₄ emission spectra according to the following expression:⁷⁰where A_0J is the Einstein spontaneous emission coefficients of ⁵D₀ → ⁷F_J transitions (J = 0–6). The magnetic dipole transition ⁵D₀ → ⁷F₁ is taken into calculations as the reference, as this transition is practically insensitive to the Eu³⁺ ion environment.⁷¹S_0J and S₀₁ are the areas under the curves of the ⁵D₀ → ⁷F_J and ⁵D₀ → ⁷F₁ transitions.

In eqn (1), the value of A₀₁ can be obtained by the expression:³⁴

A₀₁ = 0.31 × 10⁻¹¹(n)³(ν₀₁)³ (2)

where n is the refractive index, and this value is 1.61 for Na₂EuL₄¹⁶ and 1.49 for PMMA, respectively.

The radiative rate (A_rad) is given by the expression:

where each A_0→J (J = 0–6) component can be estimated from the emission spectra for the Eu³⁺ compound, as detailed in the ESI.† Based on the experimental lifetime of the ⁵D₀ emitting level (τ) and (A_rad) rate, it is possible to determine the nonradiative rates (A_nrad) by the relationship:The intrinsic ⁵D₀ emission quantum yield is described by the equation:

The sensitization efficiency is determined by:²²

Theoretical calculations

To simulate the structural distortions that might occur in the coordination sphere when the Na₂EuL₄ compound is embedded in a PMMA medium (Eu_PMMA), we calculated the intensity parameters by analyzing distortions from the crystal structure across a million slightly varied configurations. These configurations were randomly generated using the JOYSpectra (https://joyspectra.website) platform.⁷² The maximum displacement (L) of each configuration was determined by a model based on a Bose–Einstein distribution:⁷³where ℏ is the reduced Planck's constant, M_r is the reduced mass lanthanide–ligating atom, and [small omega, Greek, macron] is an average phonon frequency associated with this effective reduced mass, usually known as the promoting mode. Thus, with this model, thermal effects and distortions of the coordination sphere along the PMMA matrix are taken into account, making vibronic interactions particularly relevant for the so-called sideband transitions.⁷⁴ Given the general trend that high phonon energies are accessible with increasing temperature, it is plausible that the mean phonon energy at 77 K is lower than at 300 K.

The theoretical intensity parameters [capital Omega, Greek, macron]_λ (λ = 2, 4, and 6) are considered as the average of all N-random configurations:

where Ω_λ(0) are the intensity parameters of the crystallographic structure, with non-distorted geometry at the (x₀; y₀; z₀) coordinates. For distorted geometries around the equilibrium (n ≥ 1), the maximum displacement in each x, y and z coordinate of the ligating atoms is limited by the edges of a cube (x₀ ± L/2; y₀ ± L/2; z₀ ± L/2). See the ESI† (eqn (S3)–(S5) and the related discussions) for further details on the calculation of the theoretical intensity parameters for a single configuration.

Results and discussion

Characteristic

Ln_PMMA samples obtained according to the procedure described above have the form of transparent thin layers with a thickness of about 0.07 mm (see Fig. 1), and in the case of thicker samples of about 0.20 mm.

Fig. 1 Photograph of the PMMA thin layers doped with 2 wt% of Na₂LnL₄: (a) without a UV lamp and (b) under a UV lamp.

The analysis of Ln_PMMA thermograms indicates slight changes in mass loss in the thermal decomposition process compared to the undoped polymer (Fig. S1, ESI†). PMMA samples are thermally stable up to a temperature of about 100 °C. For all Ln_PMMA samples, the first weight loss occurs in the temperature range around 100–230 °C, and this value is lower than that observed for single crystals.¹⁶ This is not a typical phenomenon, as in many papers the authors indicate that the thermal stability of the compound increases when introduced into the PMMA matrix.^{31–33,37,47} However, the range of highest weight loss, which falls in the 300–440 °C temperature range, is analogous for Ln_PMMA layers and single crystals. Based on the analysis of the DCS curve (Fig. S2, ESI†), it was found that during cooling there are no phase transitions and thermal anomalies in the range of 30 °C to 160 °C. During subsequent cycles (cooling and heating, see Fig. S2, ESI†), only a glass transition occurs at temperatures of 105–125 °C.

The IR spectra of Ln_PMMA samples and pure PMMA (Fig. S3, ESI†) are almost identical due to the very low concentration (2 wt%) of the coordination compound. Slight differences can be seen in the range of 1650–1513 and 710–550 cm⁻¹. There are bands corresponding to vibrations of atoms of the Na₂LnL₄ molecule, characterized by very low intensity, with frequencies of 1600 (ν_C=O amide), 1567, 1538 and 696, 638, 553 cm⁻¹. The most intense bands in the IR spectrum with frequencies of 1726 cm⁻¹ and 1191, 1146 cm⁻¹ are attributed to the vibrations ν(C=O) and ν(C–O–C) of the PMMA matrix, respectively.

Spectroscopic and photophysical properties of PMMA thin layers with a single coordination compound

Absorption and luminescence spectroscopy. The absorption spectra (Fig. 2) of Ln_PMMA show a broad band with a maximum at 273.5 nm, corresponding to the π* ← π transition of the ligand of the coordination compound introduced into the PMMA layer. In the spectral range from about 300 to 350 nm there is a band corresponding to the π* ← n transition. The absorption bands of Ln_PMMA are characterized by a smaller half-width compared to the absorption spectrum of the coordination compound in the solid state. The observed differences are typical and often occur between the absorption bands of ligands of coordination compounds in the solid state and their dilute solutions¹⁶ or dilute solid solutions (compounds introduced into polymers). The network of non-covalent interactions present in the crystal structure is responsible for broadening the absorption bands of compounds in the solid state. From a comparison of the absorption spectra of [Na₂Eu(L)₄(OTf)(DMF)] (labeled as Na₂EuL₄) and Ln_PMMA, it can be seen that the introduction of coordination compounds into PMMA has practically no effect on the energy of the ligand singlet state (S₁), and the energy of the 0-phonon line (ZPL) has changed little from 28 800 cm⁻¹ for single crystals to 28 200 cm⁻¹ for Ln_PMMA. These values were determined from the band edge (inset in Fig. S4, ESI†). The ZPL energies determined as the maximum of the band with the lowest energy obtained from the Gaussian distribution (see Fig. S4, ESI†) are 31 150 cm⁻¹ (single crystals) and 30 870 cm⁻¹ (Ln_PMMA). The values of ZPL energy, determined by the two methods, are given in Table 1. We give two values because ZPL as the band edge is often found in the literature, so it will allow comparison with other systems. Comparing the ZPL energy for different systems is only possible if the method of estimating the ZPL value was the same. Fitting of the bands (Fig. S4, ESI†), was performed with a multiple Gaussian peak fitting procedure, yielding R² equal to 0.9990 (Eu_PMMA) and 0.9998 (Na₂EuL₄).

Fig. 2 Absorption spectra of PMMA thin layers with a single coordination compound and diffuse reflectance spectrum of [Na₂Eu(L)₄(OTf)(DMF)] (labeled as Na₂EuL₄) single crystals at 295 K.

Table 1 Spectroscopic properties of the ligand in Gd_PMMA and in Na₂GdL₄ single crystals (ZPL – 0-phonon line)

	S1 energy (cm^−1)	T1 energy (cm^−1)	T1 lifetime (ms)
PMMA	28 200 (ZPL as a band edge)	21 685 (barycenter)	τ 1 = 0.53 ± 0.04
	30 870 (ZPL as a band maximum from the Gaussian distribution)	25 640 (ZPL as a band edge)	τ 2 = 3.3 ± 0.1
			τ 3 = 11.6 ± 0.2
Single crystals^16	28 800 (ZPL as a band edge)	21 700 (barycenter)	τ 1 = 1.41 ± 0.06
	31 150 (ZPL as a band maximum from the Gaussian distribution)	25 470 (ZPL as a band edge)	τ 2 = 3.9 ± 0.2
			τ 3 = 19 ± 1

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The introduction of coordination compounds into PMMA caused a small but noticeable hypsochromic shift in the phosphorescence spectrum recorded at 77 K for Gd_PMMA relative to Na₂GdL₄ single crystals. This results in a slight change in the ZPL energy, defined as the band edge and the barycenter, as presented in Table 1 and Fig. S5 (ESI†). For the Gd_PMMA, the emission decay time of the ligand triplet state (T₁) is three-exponential (Table 1 and Fig. S5, ESI†), as in the case of the single crystal, but with shorter lifetimes for each component.

In order to select PMMA layers with very good mechanical parameters, thin layers were obtained for different Na₂EuL₄ contents in the range of 15–1 wt% of the coordination compound, for which excitation spectra can be seen in Fig. S6 (ESI†). Taking into account that from the content of 5 wt% of Na₂EuL₄ the layers were characterized by very good quality (transparency and homogeneity) further studies were carried out for PMMA layers with 2 wt% of Na₂LnL₄ content.

When studying PMMA thin films with the introduction of Ln³⁺ coordination compounds, the question arises to what extent the structure of the compounds has been preserved in the polymer and how this, together with the polymer matrix lattice, affects the radiative and non-radiative properties of the chelate. Understanding the processes that take place during the incorporation of compounds into polymers is crucial for material design. The above comparisons of the absorption and phosphorescence spectra of Ln_PMMAversusNa₂LnL₄ single crystals may suggest that the chelate structure is reasonably preserved. However, this conclusion can only be made by comparing the emission spectra of Eu_PMMA and Na₂EuL₄, where the Eu³⁺ ion is used as a structural probe. This is possible because of the emitting level of Eu³⁺ (⁵D₀), which, due to J = 0, is not split by the crystal field.⁷⁵ The band profile and energy of the components ⁵D₀ → ⁷F_J transitions (J = 0–4) for Na₂EuL₄ and Eu_PMMA are identical at 295 and 77 K, as shown in Fig. 3.

Fig. 3 (a) Emission spectra of Eu_PMMA and Na₂EuL₄ at 295 and 77 K. (b) Enlargement of the spectral region of the ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₁ transitions.

The broadening of emission lines for Eu_PMMA and the lack of formation of separated electronic components at 77 K can be observed. This is due to inhomogeneous broadening of the emission lines resulting from the presence of Ln³⁺ coordination polyhedra in the polymer matrix, which differ slightly. It is important to note that the packing in the single crystal structure and the network of intermolecular non-covalent interactions may affect the position of the ligands and the symmetry of the Ln³⁺ coordination polyhedra. However, comparing the emission spectra within the series of Ln_PMMA and Ln³⁺ chelates in the solid state, there is no change in either the shape of the bands or their energy range regardless of the lanthanide ion (see Fig. 4).

Fig. 4 Emission spectra of Ln_PMMA (2 wt% of Na₂LnL₄) and Na₂LnL₄ at 295 K.

Photophysical properties of PMMA thin layers with a single coordination compound

The values of the emission band areas corresponding to the ⁵D₀ → ⁷F_J (J = 0–4) transitions and selected photophysical parameters for Eu_PMMA and Na₂EuL₄ are collected in Table 2. The relative integral intensity ratios ⁵D₀ → ⁷F_J/⁵D₀ → ⁷F₁ (equivalent to S_0→J/S_0→1 in eqn (S2), ESI†) have a higher value for Eu_PMMA than for single crystals, which is caused by the broadening of emission bands due to the inhomogeneity of Eu³⁺ coordination polyhedra in PMMA, i.e. slight distortions of atoms surroundings the Eu³⁺ ion. This will lead to slightly higher intensity parameters, as will be modeled and discussed later in the theoretical section.

Table 2 Relative integrated ⁵D₀ →⁷F_J (J = 0–4) transition intensities for Na₂EuL₄ in the PMMA thin layer and in the solid state at 295 K

		PMMA	Single crystals
Integrated intensity (dimensionless)	^5D0 → ^7F0	0.034	0.011
	^5D0 → ^7F1	1	1
	^5D0 → ^7F2	6.16	5.31
	^5D0 → ^7F3	0.23	0.23
	^5D0 → ^7F4	1.83	1.33
Photophysical property	A rad (s^−1)	501	582
	A nrad (s^−1)	68	6
	τ exp (μs) (λexc = 464 nm)	1758 ± 4	1700 ± 3
	Q ^EuEu (%)	88	99
	Q ^LEu (%)	85 ± 8	95 ± 10
	η sens (%)	97	96

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In contrast to the similarity of the luminescence and excitation spectra of Eu_PMMA and Na₂EuL₄, significant differences are observed in the values of their photophysical parameters, except for the sensitization efficiency, which is similar in both cases (η_sens ≥ 95%). This indicates that the sensitization process (i.e., the intramolecular energy transfer rates) is quite similar. There was an increase in the radiative lifetime from 1.718 ms (Na₂EuL₄) to 1.996 ms (Eu_PMMA), resulting in a tuning of the intrinsic (Q^Eu_Eu) and overall (Q^L_Eu) emission quantum yield from 99% to 88% and from 95% to 85%, respectively. The change in the refractive index (n), which equals 1.61 for single crystals and 1.49 for PMMA thin layers, is mainly responsible for the change in the value of A_rad (A_rad = 1/τ_rad) and results in tuning the radiative lifetime (τ_rad) from a smaller to a larger value according to eqn (1)–(4).

There are few reports on the influence of τ_rad on Q^Eu_Eu and Q^L_Eu and they concern the comparison between the chelate in the solid state and in solution.^13,68,76 The refractive index is, of course, not the only factor determining the Q^L_Eu value for specific non-radiative processes. As shown in ref. 16 regarding the analysis of Na₂LnL₄ (Ln = Eu³⁺, Gd³⁺, Tb³⁺) in the form of single crystals, Q^L_Eu is also a function of the lifetime of the ligand triplet state (τ_T). The longer the lifetime of the T₁ state, the greater the Q^L_Eu observed. In the case of Eu_PMMA, Q^Eu_Eu equals 88%, while the contribution of non-radiative processes for Eu_PMMA (A_nrad = 68 s⁻¹) is higher compared to single crystals (A_nrad = 6 s⁻¹), as presented in Table 2. This is a different situation than that reported by Biju et al.⁶⁵ and Buczko et al.³⁷ for Eu³⁺ compounds with β-diketones enclosed in PMMA, who observed the reduction of non-radiative processes in polymers and an increase in the ⁵D₀ emission intensity. Andreiadis et al.,⁶³ on the other hand, showed a slight reduction in Q^L_Eu upon incorporation of Eu³⁺ chelate into PMMA relative to the solid state. The aforementioned studies concern Eu³⁺ coordination compounds with a saturated coordination sphere and ligands with fluorinated CF₃ groups. It should be emphasized that in the case of our compounds using a non-fluorinated ligand, we are able to obtain PMMA thin layers with intense multicolor sensitized emission of various Ln³⁺, despite the increase in non-radiative emission quenching processes relative to the single crystals (see Fig. 1, Table 2 and Table S1, ESI†). The increase in A_nrad for Eu_PMMA is most likely due to a slight loss of stiffness of the compound as a result of the absence of a network of intermolecular non-covalent interactions present in the crystal structure, as well as the presence of a network of high-energy C–H vibrations (2995 and 2950 cm⁻¹) of PMMA visible in the IR spectra (Fig. S3, ESI†). Thus, the Q^L_Eu value of 85% is the resultant refractive index change, which correlates well with the dependence of Q^L_Eu on n and τ_T¹⁶ and the presence of additional non-radiative processes. The slight temperature dependence of the ⁵D₀ emission decay time for Eu_PMMA (see Table S1, ESI†) indicates the presence of temperature-dependent non-radiative processes.

A similar temperature dependence of the ⁵D₄ emission decay time is not observed for Tb_PMMA. It should be noted that the ⁵D₀ and ⁵D₄ emission decay curves for Eu_PMMA and Tb_PMMA are monoexponential (see Fig. S7, ESI†), confirming the presence of a single form of the coordination compound Eu³⁺ and Tb³⁺ in the polymer matrix. In addition, the curves do not depend on the excitation wavelength, as shown by the data collected in Table S1 (ESI†). The coordination compound Na₂TbL₄, which had been stored in a desiccator for two years, was used to obtain thin films. The compound was used because its IR, absorption, and emission spectra, as well as emission decay times, were unchanged from those of “fresh” crystals,¹⁶ while the Q^L_Ln decreased. We introduced this Tb³⁺ compound into PMMA to determine if it would behave similarly to Na₂EuL₄. Analogous to the Eu³⁺ compound, we observed a slight reduction in Q^L_Ln of about 10% in Tb_PMMA.

The situation is different for the Dy³⁺ and Sm³⁺ compounds, which have a small energy gap between the emitting level and the adjacent lower energy level. For Dy_PMMA and Sm_PMMA, we observe a large reduction in the Q^L_Ln values by 65% and 73%, respectively, compared to the single crystals (see Table S1, ESI†). Despite this significant reduction, the Q^L_Ln value remains typical for fluorinated Sm³⁺ β-diketonates in PMMA.⁶² Reports on Sm³⁺ coordination compounds in PMMA matrices are scarce, and often do not include Q^L_Sm values or ⁴G_5/2 emission decay lifetimes, focusing instead on other material properties.^40,47,49,61 Biju et al. noted a slight increase in Q^L_Sm for Sm³⁺ β-diketonate in PMMA compared to the solid state, from 3.4% to 4%, and a slight reduction in the ⁴G_5/2 emission lifetime from 79 to 73 μs.⁶² The appearance of two decay times of Sm³⁺ and Dy³⁺ emission in thin layers compared to the monoexponential decay time of Eu³⁺ and Tb³⁺ compounds in PMMA is unexpected and difficult to explain based on the available results and applied research techniques. The use of the transient absorption technique could help in the interpretation of this phenomenon.⁷⁷

There are no literature reports, to our knowledge, of Dy³⁺ coordination compounds encapsulated in a PMMA thin layer showing an antenna effect. This may be due to the small energy gap of Dy³⁺, which leads to a large contribution of non-radiative processes in the depopulation of the ⁴F_9/2 level. Additionally, the Dy³⁺ emitting level is at a higher energy (∼21 200 cm⁻¹), which favors back energy transfer from ⁴F_9/2 to the T₁ ligand state. However, compared to the quantum yield of known Dy³⁺ compounds in solid states, our Q^L_Dy = 6% is still an interesting value.¹³ It should be mentioned that there is a paper on polymeric PMMA dispersions containing single and multiple Ln coordination compounds (including Dy³⁺ chelate) affording multicolored nanolabels.⁸ The value of emission quantum yield is given only for Eu dispersion.

Considering the excellent photophysical properties of the single crystals,¹⁶ and the rigidity and reduced multiphoton quenching provided by the well-designed ligand and coordination compound structure of Na₂LnL₄, it seems that PMMA does not offer the same structural rigidity. Instead, PMMA favors the lability of the Ln³⁺ coordination polyhedra and provides a pathway for non-radiative deactivation of Sm³⁺ and Dy³⁺ emitting levels through its high-energy C–H vibration network.

The PMMA matrix has five main vibrational modes:⁷⁸ –O–CH₂ (v̄ = 3000 cm⁻¹) and –CH₃ (v̄ = 2950 cm⁻¹); the –C=O stretch (v̄ = 1730 cm⁻¹); deformation of the –O–CH₃ (v̄ = 1380 cm⁻¹); and the symmetric stretch –C–O–C– (v̄ = 990 cm⁻¹). These modes, combined with the energy gap between the emitting and the lower adjacent level for the studied Ln³⁺ ions, indicate that the electron–phonon coupling with the emitting levels of Eu³⁺ and Tb³⁺ is less likely to occur, as more phonons are needed. Conversely, due to a lower energy gap, the emitting levels of Dy³⁺ and Sm³⁺ are more susceptible to quenching by PMMA vibrational modes. Thus, considering the combination of the above vibrational modes present in PMMA, Tb³⁺ and Eu³⁺ require more phonons to bridge their energy gaps ∼14 800 and ∼12 400 cm⁻¹, respectively, compared to Dy³⁺ and Sm³⁺, which have gaps of ∼7350 and 7400 cm⁻¹, respectively.⁷⁹

In addition, the emission decay curves of the Dy_PMMA and Sm_PMMA emitting levels exhibit a biexponential character, as shown in Table S1 and Fig. S8 (ESI†). The value of the longer lifetime components is slightly dependent on the temperature and the excitation wavelength. Even with direct excitation, the decay remains biexponential. The experiment was repeated several times, and the photophysical properties of Na₂DyL₄ and Na₂SmL₄ single crystals were checked just before incorporation into PMMA.

Theoretical modeling

Since the emission spectra of solid-state Na₂LnL₄ and the compound embedded in a PMMA medium (Ln_PMMA) show the same energy of Stark components (Fig. 3 and 4), it is evident that in the PMMA medium, Na₂LnL₄ undergoes slight deviations around the crystallographic geometry. These deviations affect the odd-rank crystal field (intensity parameters, eqn (S4)–(S6), ESI†) more than the even part of the ligand field (energies of the transitions). Therefore, the model represented by eqn (7) and (8) can be applied, as there is no evidence of other symmetries involved. Thus, the displacement model maintained the same symmetry for one million configurations (N = 10⁶ in eqn (8)), as shown in the Graphics Interchange Format (GIF) animation (see Displacement_Ln_PMMA.gjf in the ESI†).

The values of the theoretical intensity parameters (eqn (S4), ESI†) for the Na₂LnL₄ crystal and Ln_PMMA samples are shown in Table S2 (ESI†). In comparison with the experimental parameters Ω^exp_λ (λ = 2 and 4) obtained for Na₂EuL₄ and Eu_PMMA, there is a good agreement with the displacement model, with a maximum absolute deviation of Ω^exp₄ − [capital Omega, Greek, macron]^theo₄ = 0.9 × 10⁻²⁰ cm² for Eu_PMMA at 77 K, or a maximum percent deviation of 14%.

As mentioned before, the main factor for the reduction of the quantum yields when Na₂EuL₄ and Na₂TbL₄ are incorporated into PMMA (Eu_PMMA and Tb_PMMA) can be attributed to the change in the index of refraction. Thus, a model can be proposed based on the analyses of the definition of theoretical quantum yield:^80,81

where Q^L_Ln(P) and Q^L_Ln(C) are the quantum yields of the samples in the PMMA and crystal medium, respectively. n_P and n_C are the indices of refraction of the PMMA and the crystal. Λ is the changing of the electric dipole strength induced by small structural deviations, which specifically depend on the changing of the intensity parameters from the crystal to the PMMA (P), i.e., Ω_λ → [capital Omega, Greek, macron]_λ (Table S2, ESI†), induced by structural changes. Thus, Λ can be obtained by:The detailed procedure to obtain the formulation of eqn (9) and (10) is presented in the ESI.†

Just for illustration, by applying the values of n_P = 1.49, n_C = 1.61, Q^L_Eu(C) = 95%, Q^L_Tb(C) = 75%, and Λ ≈ 1.1 (eqn (S17)–(S20), ESI†) in the right-side of eqn (9), we obtain Q^L_Eu(P) = 82% and Q^L_Tb(P) = 64%, which is very close to the measured values of Ln_PMMA samples in Table S1 (ESI†) (85% and 66%, respectively). It is important to reinforce that this model cannot be applied for the specific case of Sm³⁺ and Dy³⁺ due to the quenching interaction of the PMMA with the emitting levels of these ions.

For PMMA with a mixture of Na₂LnL₄ (samples I–V), the result of the quantum yield can be obtained as proportional to the concentration of Ln³⁺ in the total. Thus, the emission quantum yield of PMMA layers with a total 2% wt of mixed Na₂LnL₄ (Ln = Eu, Tb, Dy, and Sm) can be estimated as the proportion of each Ln³⁺ in the composition.

For PMMA with a mixture of Na₂LnL₄, the quantum yield can be obtained as proportional to the quantum yield with the concentration of Ln³⁺. For instance, the emission quantum yield of PMMA layers with a total 2% wt of mixed Na₂LnL₄ (Ln = Eu, Tb, Dy, and Sm) can be estimated as the proportion of each Ln³⁺ in the composition:

where a_i is the composition (Table 3) of i-th Ln³⁺ in the mixed PMMA.

Table 3 Composition of five PMMA layers with the total content of all Na₂LnL₄ compounds being 2% wt, and the measured and calculated Q^L_Ln

Sample number	Composition (%)				CIE coordinates [x, y]	Q ^LLn/% measured	Q ^LLn/% calculated
	Eu^3+	Tb^3+	Dy^3+	Sm^3+
I	10.0	10.0	45.0	35.0	[0.42, 0.49]	19	19
II	7.5	12.5	45.0	35.0	[0.44, 0.49]	19	18
III	12.5	7.5	45.0	35.0	[0.47, 0.46]	19	19
IV	10.0	10.0	55.0	25.0	[0.49, 0.46]	20	18
V	7.5	2.5	30.0	60.0	[0.51, 0.45]	14	13

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Photophysical properties of PMMA thin layers with a mixture of coordination compounds

The introduction of a mixture of four coordination compounds (Na₂EuL₄, Na₂TbL₄, Na₂SmL₄, Na₂DyL₄) into the PMMA layer allowed materials with multicolor tunable emission to be obtained. The emission is tunable depending on the content of the individual chelates, due to the different relative intensities of the bands corresponding to the Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ 4f → 4f transitions. Emission spectra for five samples of PMMA layers can be seen in Fig. S9 (ESI†), while Table 3 gives the composition of the five PMMA layers with a total Na₂LnL₄ of 2 wt%. Their emission color is described by the color coordinates seen in the CIE chromaticity diagram (Fig. 5).

Fig. 5 Color coordinates of thin PMMA layers with different concentration of the coordination compounds within the total content of 2 wt% of Na₂LnL₄.

The measured Q^L_Ln values are also given in Table 3. The measured values correspond very well with those calculated based on knowledge of the Q^L_Ln values measured earlier for layers with a single Ln³⁺ ion and based on the content of individual compounds.⁸ The emission color of the layers can also be tuned by changing the time scale range, due to differences in the lifetimes of the ⁵D₀ (Eu³⁺), ⁵D₄ (Tb³⁺), ⁴G_5/2 (Sm³⁺), and ⁴F_9/2 (Dy³⁺), which range from 100 to 2000 μs. Mixtures of these four coordination compounds can be used to create multicolor barcodes, so it is theoretically possible to obtain thousands of codes by changing the composition of the mixtures, the spectral range of emission and the time scale range.

Conclusions

[Na₂LnL₄(OTf)(DMF)] coordination compounds retain their coordination properties when incorporated into PMMA thin layers (labeled as Ln_PMMA). In contrast, the photophysical properties, such as emission decay time and overall emission quantum yield of Ln_PMMA compared to single crystals, depend on the type of the Ln³⁺ ions. For Eu_PMMA and Tb_PMMA, changes in photophysical properties are mainly influenced by the refractive index, while for Sm_PMMA and Dy_PMMA by electron–phonon coupling with PMMA vibrations.

Based on a combination of experimental and theoretical data, a theoretical model was proposed that accounts for simulations of inhomogeneities of Ln³⁺ coordination polyhedra due to small distortions in the PMMA matrix. Additionally, this model enables the prediction of Q^L_Ln in PMMA, showing good agreement with the measured data. This provided valuable insight into how the PMMA network affects the photophysical properties of Ln³⁺ coordination compounds. Despite the reduction in overall emission quantum yields in PMMA relative to single crystals, layers containing a mixture of Ln³⁺ compounds have potential for use as barcodes and quick-response codes.

Author contributions

Aneta Lipa: validation, formal analysis, investigation, data curation, visualization; Yen Hoang Pham: validation, formal analysis, investigation; Albano N. Carneiro Neto: methodology, software, validation, formal analysis, visualization, writing – original draft; writing – review & editing; Viktor A. Trush: investigation, resources, validation; Huanrong Li: validation, writing – review & editing; Oscar L. Malta: methodology, software, writing – review & editing; Volodymyr M. Amirkhanov: validation, resources; Paula Gawryszewska: conceptualization, validation, writing – original draft; writing – review & editing; supervision; project administration; funding acquisition.

Data availability

The data supporting this article have been included as part of the ESI.† Source Data Files are also provided via the Figshare repository under the accession code https://figshare.com/s/096df1dc3f642e58a484. The data contains: TGA measurements; IR, absorption and emission spectra; emission decay time measurements and theoretical data.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the University of Wroclaw for supporting these investigations and Dr W. Wrzeszcz for the photos in Fig. 1. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020 and LogicALL (PTDC/CTMCTM/0340/2021) financed by Portuguese funds through the FCT/MEC (PIDDAC).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03939d

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