Bimetallic Eu/Tb Complexes for Ratiometric Temperature Sensing with Unusual Enhancement of Eu Luminescence with Temperature

Abstract

In this paper we describe the results of the influence of temperature in the range of 280–340 K on the luminescence of bimetallic Eu/Tb complexes with N-heterocyclic ligand L based on 2,2′-bipyridyldicarboxylic acid in acetonitrile. The experiments were carried out for systems with various Eu/Tb ratios. The stability of the complexes of the ligand L with metal M (Eu or Tb) was determined using spectrophotometric titration in acetonitrile solutions. The LM complexes’ stability constants were found to be typical for these systems; however, the stability of Eu complex is slightly higher than that for Tb. Along with rising temperature, we observed a decrease in Tb emission intensity and, at the same time, an enhancement in Eu luminescence. An explanation of Eu luminescence enhancement involves the appearance of charge transfer states, bands of which can be observed in the Eu luminescence excitation spectra as difference spectra measured with two close temperatures. The unusual Eu luminescence enhancement upon heating was observed for the first time for the complex with tetradentate O,N-type heterocyclic diamide ligand L, while an inverse phenomenon was observed with the Tb luminescence. The Eu luminescence enhancement was found earlier for various carboxylate complex salts, but not for heterocyclic coordination complexes. This allows the construction of a ratiometric luminescent thermometer in the range of 280–340 K using the ratio of luminescence intensities for Eu and Tb. The stability constants for the individual Eu and Tb complexes help us to understand the equilibrium in L:Tb:Eu complex system and shed light on plausible speciation in solution.

1. Introduction

The study of the luminescent properties of Eu and Tb complexes is an urgent task since such complexes can be used as luminescent sensors in various applications in biomedical optics and targeted medicine. The creation of biosensors for imaging biological objects (in particular, luminescence nanothermometers) has attracted a wide interest due to the rapid development of methods for diagnosing and treating cancer. Luminescent materials used for such purposes provide the possibility of non-contact thermal probing with good spatial resolution (less than 1 μm) and temperature sensitivity (less than 0.5 °C) [1,2].

At present, the majority of known publications related to luminescent thermometry are devoted to the study of Eu and Tb luminescent complexes in the solid state: in polymers, films, coordination polymers, and metal–organic frameworks (MOFs) based on Eu and Tb. Such systems can change their luminescence color depending on temperature, so they are usually called “chameleons” [3,4].

For example, [5] presents heterodinuclear Eu and Tb complexes based on metal–organic frameworks (MOFs) suitable for use as self-regulating thermometers. Such complexes have good temperature sensitivity over a wide range of temperatures. Another work presents a coordination polymer with a mixed Eu and Tb complex based on metal–organic polymeric structures that contain metal cationic centers bound to organic ligands [6]. The registration of temperature changes is based on energy transfer between ligand, Eu, and Tb in the complex. The intensity ratio of Tb³⁺and Eu³⁺ emission bands is used as the parameter (ratiometric temperature sensing). The detected correlation between the luminescence intensities of Eu and Tb can be directly applied in molecular thermometers since this parameter was highly sensitive to temperature changes in the study.

The approach to increase the sensitivity of Eu and Tb-based luminescent thermometers is presented in the work [7]. In such thermometers, Eu and Tb complexes are composed of dimeric molecules in which metal ions are connected through two hydroxybenzoate anions. The presence of such a connection led to an increase in temperature sensitivity for energy transfer between Eu and Tb ions. The sensitivity of such thermometers is independent of the ratio of Eu and Tb ions, and these thermometers can be used for non-contact temperature measurement. The authors of [8] propose a theoretical description of the sensitivity of a luminescent thermometer, which was experimentally confirmed.

The synthesis of new ligands has allowed for an increase in the sensitivity of organic luminescent materials to external parameters. In particular, the synthesis of hydroxybenzoates has led to a sensitivity in thermometers that far exceeds theoretical temperature sensitivity [9]. For example, the use of organic ligands based on hydroxydimethoxyacetophenone in Eu and Tb complexes has increased the temperature sensitivity of Eu and Tb luminescence [10].

One of the most important modern developments is a thermometer for measuring intracellular temperature. Contactless thermometers for measuring temperature with high spatial–temporal resolution have been developed [2]. Many luminescent thermometers currently have certain disadvantages, such as organic fluorophores not being suitable for prolonged temperature measurements in intracellular space due to factors such as pH, viscosity, and ionic strength causing uneven distribution of fluorescence intensity. Fluorescent polymeric thermometers have a slow response due to hysteresis phase transitions, and biomolecule-based thermometers are too cumbersome and require frequent calibration. Cellular thermometry is based on the Baffou model, assuming that the temperature increases in the cell due to endogenous thermogenesis can be neglected [11]. However, there are works in which this requirement was not met [12,13]. Therefore, the development of a more comprehensive model of molecular luminescence thermometry is still relevant.

Later, luminescent ratiometric thermometers based on the molecular forms of lanthanoids encapsulated in lanthanoid MOFs were developed. Ratiometric sensors use two different peaks of luminescence to prevent the effects of optoelectronic shifts of the excitation source or detectors [14]. The incorporation of polyoxometalates containing lanthanoids into MOFs has allowed for high temperature sensitivity in the physiological temperature range [15]. Later modifications were developed, which are so-called three-bladed screw homoleptic complexes of lanthanoids based on Eu and Tb, which have high temperature sensitivity at critical low temperatures of 130–200 K [16].

High-temperature luminescent thermometry is a promising technique for non-contact temperature measurement in very hot and inaccessible environments [17]. However, the manufacture of such thermometers is strongly limited by the thermostability of lanthanoid coordination compounds [18,19]. The legitimization of Eu and Tb complexes with polymers has allowed for high sensitivity of luminescent thermometers at high temperatures and thermal stability of the compound. For example, the alloying of Eu and Tb complexes with poly (ethylene glycol)diacrylate allowed detection of temperatures up to 473 K [20].

One of the problems with luminescent thermometers is still the impact of UV radiation on the luminescent properties of lanthanoids in the complex, including at very high temperatures (>450 K) [4,20]. Therefore, the selection of UV and thermally resistant lanthanoid complexes with high temperature sensitivity over a wide temperature range remains relevant.

The other challenging task is to develop luminescent complexes working as temperature sensors in the liquid state. In this study, we have examined the luminescent properties of Eu and Tb complexes based on bipyridylcarboxamide dissolved in acetonitrile in the temperature range of 280–340 K. Lanthanides complexes with bipyridyldicarboxamide-based ligands are good candidates for the role of temperature sensors due to good stability as well as intense luminescence with high quantum yield [21].

2. Materials and Methods

2.1. Preparation of Eu/Tb Complexes with N-Heterocyclic Ligand

The spectral characteristics of solutions of diamide ligand L, Eu, and Tb in acetonitrile were studied. The structure of the diamide ligand L is shown in Figure 1; the ligand and its Eu complex LEu(NO₃)₃ synthesis are described in the article [21].

The concentration of complexes in acetonitrile was 1 × 10⁻⁵ mol/L. The solutions of diamide ligand L, Eu(NO₃)₃∙6H₂O, and Tb(NO₃)₃∙5H₂O in acetonitrile were prepared in the following order of ingredients mixing: L, Eu, and Tb. The concentration of the ligand L was always the same (1 × 10⁻⁵ mol/L), the ratios Eu:Tb in complexes varied from 0.1:0.9 to 0.9:0.1 with the step of 0.1 of mole fraction. All solutions for spectral studies were prepared just before each experiment.

2.2. Spectral Measurements

Absorption, luminescence emission and excitation spectra, and luminescence lifetimes were measured for the studied solutions in the temperature range 280–340 K; all types of spectra were recorded every 5 K. Absorption spectra of solutions of complexes were recorded using a Solar PB2201 spectrophotometer with a thermostatically controlled cuvette compartment in 10 mm path length quartz cells relative to pure acetonitrile. Luminescence spectra and luminescence kinetics were recorded using a Solar CM2203 luminescence spectrometer in 10 mm path length quartz cells. To maintain the required temperature, a thermostatically controlled cuvette compartment was used in luminescence measurements as well. To obtain luminescence emission spectra, the excitation wavelength was set to 320 nm, and registration took place in the range from 330 nm to 800 nm. When measuring the luminescence excitation spectra, registration took place at a wavelength of 615 nm (for registration of Eu luminescence) and 545 nm (for Tb luminescence), and excitation was carried out in the range from 250 nm to 500 nm.

2.3. Processing of Spectral Data

Processing of the obtained results began with correction for the effect of the internal filter of the emission and excitation spectra of luminescence:

$$I=I_0\times {10}^{\frac{D_{ex}+D_{em}}{2}}$$

(1)

where I₀—registered luminescence intensity, D_ex—absorbance at the excitation wavelength, D_em—absorbance at the registration wavelength, and I—the obtained luminescence intensity after correction for the effect of the internal filter.

The kinetics of luminescence were measured at the excitation wavelength of 320 nm and the registration wavelength of 615 (Eu emission) or 545 nm (Tb emission). In each experiment the kinetics of luminescence was recorded 5 times and then averaged. The lifetime (τ) of the excited state of a rare earth element ion was calculated using the formula:

$$\tau =\frac{t}{\ln I\left(0\right)-\ln I\left(t\right)}$$

(2)

where I(0)—luminescence intensity at the initial time and I(t)—luminescence intensity at the time t.

The luminescence quantum yield was determined by the method of a reference dye, where the reference luminescence quantum yield (Φ_et) was taken as the quantum yield of luminescence of a solution of ligand L and Eu in acetonitrile at a concentration of 1 × 10⁻⁵ mol/L [21]:

$$\mathsf{\Phi }=\frac{I_{\mathrm{x}}}{D_{\mathrm{x}}}·\frac{D_{\mathrm{e}\mathrm{t}}}{I_{\mathrm{e}\mathrm{t}}}·{\left(\frac{n}{n_{\mathrm{e}\mathrm{t}}}\right)}^2·{\mathsf{\Phi }}_{\mathrm{e}\mathrm{t}}$$

(3)

where Φ—luminescence quantum yield, I_x and I_et—wavelength-integrated luminescence intensities of the studied sample and the reference, D_x and D_et—absorbances at the excitation wavelength of studied solution and the reference, n, n_et—refractive indices of studied solution and the solution with reference complex, and Φ_et—luminescence quantum yield of the reference complex.

2.4. Measurements of Constants of Complex Formation

Ultraviolet–visible (UV–vis) absorption spectra were recorded at temperature (293 ± 1 K) in the wavelength region 200–500 nm using a Hitachi U-1900 spectrophotometer with 10 mm path length quartz cells. A ligand L solution was prepared for spectrophotometric titration with concentration ca. 0.1–0.4 mmol∙L⁻¹. A titrant solution M(NO₃)₃·nH₂O (M = Eu, Tb) with concentration ca. 1–4 mmol∙L⁻¹ was prepared by dissolution of a nitrate hydrate salt in the ligand L solution. A 2 mL ligand L solution was titrated with the required aliquot of the M(NO₃)₃·nH₂O solution. The stability constants of the lanthanide complexes were calculated using nonlinear least-squares regression analysis using the HypSpec2014 program [22].

2.5. 1H NMR Spectrum of Eu Complex with the Ligand L

1H NMR (600 MHz, ACETONITRILE-d3) Hppm 1.23 (t, J = 7.06 Hz, 6H) 3.21–3.39 (m, 4H) 5.59 (br.s., 1H) 6.23 (br.s., 1H) 6.74 (d, J = 6.97 Hz, 1H) 6.77 (d, J = 6.60 Hz, 1H) 7.16 (br. s., 1H) 7.84 (d, J = 5.59 Hz, 1H) 7.99 (d, J = 5.14 Hz, 1H) 11.79 (br. s., 1H). %).

3. Results

3.1. Absorption Spectra

Figure 2 shows the absorption spectrum of a mixed complex of Eu and Tb at different temperatures for the ratio of L:Eu:Tb = 1:0.2:0.8 (the order of substances in the record coincides with the order of mixing during the preparation of the solution). The spectra for other studied solutions are shown in the Supplementary Material (Figure S1).

The absorption spectra of the studied solutions show a wide band in the 325–330 nm, which corresponds to the absorption of light by the metal-linked ligand L in a complex with lanthanide ions [23]. We could compare those spectra with absorption spectra for a ligand L solution titrated with the aliquot of the M(NO₃)₃·nH₂O solution (M = Eu, Tb), where the ligand L absorption at 290 nm is transferred into the band with the maximum at 320–330 nm due to complexation of M with the ligand L. Thus, it can be argued that while mixing solutions of ligand L, Eu(NO₃)₃∙6H₂O, and Tb(NO₃)₃∙5H₂O, complex formation occurred; that is, the studied solutions contain Eu and/or Tb complexes in appropriate ratios.

3.2. Constants of Complexation

Stability of the complexes of the ligand L with Eu and Tb was determined by spectrophotometric titration technique in acetonitrile solutions. The absorbance spectra of the system are changed upon addition of aliquots of the metal salts: the peak near 280 nm corresponding to the π→π* transition of pyridine rings drops, while the peak at 320 nm rises (Figure 3). A good isosbestic point and monotonous behavior of absorbances during titration were observed for both systems, with Eu and Tb (Figure 3 and Figures S2 and S3 in the Supplementary Materials).

Contrary to the previous titration experiments [21], we found two complexes in systems with 10 times higher ligand L concentration due to formation of complexes with LM and LM2 stoichiometry (

Table 1. Log β values for the stability of trivalent Eu and Tb with the ligand L in acetonitrile.

Metal	Ratio L:M	Log βi
Eu	1:1	6.64 ± 0.02
	1:2	6.81 ± 0.02
Tb	1:1	6.17 ± 0.04
	1:2	8.07 ± 0.04

). The latter complex has low stability and its formation observed only in concentrated solutions. As we test the conditions of the system with concentrations possessing a high temperature response, we find the additional complex species in solution. The LM complexes stability constants fall within the typical range for such systems [21,23,24,25]. The stability of Eu complex is slightly higher than stability of Tb complex. The same relation was found for a series of substituted anilides of 2,2′-bipytidyl-6,6′-dicarboxamide [23]. The structure of the complexes was estimated for Eu complex by the 1H NMR investigation of the synthesized compound [21].

Chemical synthesis of the Eu complex (Scheme 1, [21]) confirms monometallic complex formation with the composition of LEu(NO₃)₃. The chemical synthesis of the Tb complex is identical.

The paramagnetic chemical shifts in ¹H NMR spectrum of the complexes allows the estimation of the metal ion coordination with two nitrogen atoms of pyridine rings of bipyridine moiety. The chemical shift of α-protons of pyridine substitutes are extremely low because of unusual position of the rings near the paramagnetic Eu atom, at the same time, the chemical shifts of N-ethyl group protons are close to the shifts in the free ligand L.

The formation constant of the LM2 complexes is low compared with their monometallic precursor that shows low affinity of the complex species toward the addition of the second metal. From these data we assume that the LM2 complexes form by coordination of M(NO₃)₃ specie with pyridine nitrogen (Scheme 2).

The affinity of the LM complex compared to the second metal ion was lower for Eu (lgK₂ = 0.17 ± 0.04) than for Tb (lgK₂ = 1.9 ± 0.08). We expected a higher concentration of LTb₂ complexes in the system.

3.3. Ligand Triplet State

The energy of the ligand triplet state in the europium and terbium complexes was determined from the phosphorescence spectrum of the gadolinium complex measured at liquid nitrogen temperature (Figure 4). The shortest wavelength component in the spectrum is usually used to determine the ligand triplet state energy in complexes [26]. It was found that the energy of the triplet level of the ligand L was E(Tr) = 21,690 ± 80 cm⁻¹. Thus, the ligand has a higher energy triplet state compared to the resonance levels of Eu (E(Eu) = 17,240 cm⁻¹) and Tb (E(Tb) = 20,410 cm⁻¹) ions.

The energy gap between the triplet level of the ligand and the resonance level of terbium is E(Tr) − E(Tb) = 1280 cm⁻¹, which is noticeably smaller than the energy gap between the energy levels of the ligand and europium E(Tr) − E(Eu) = 4450 cm⁻¹. The energy gap between the resonance levels of Eu and Tb ions is 3170 cm⁻¹. These energy gap values make back transfers of energy from Eu ion to the resonance level of Tb almost improbable.

3.4. Luminescence Emission Spectra

The luminescence spectra of a mixed complex L:Eu:Tb = 1:0.2:0.8 at various temperatures are shown in Figure 5. The spectra for other studied ratios L:Eu:Tb are given in the Supplementary Material, Figure S4. It can be seen from the graph that along with rising temperature, the luminescence intensity of the Eu ion with a maximum at 615 nm corresponding to the energy transition ⁵D₀ → ⁷F₂ increases, and the luminescence intensity of the Tb ion at 545 nm corresponding to the energy transition ⁵D₄ → ⁷F₅ decreases.

3.5. Luminescence Excitation Spectra

Figure 6 shows the luminescence excitation spectra of a mixed complex L:Eu:Tb = 1:0.2:0.8 at a detection wavelength of 545 nm corresponding to the maximum intensity of Tb luminescence measured at various temperatures. The luminescence excitation spectra for other studied solutions are shown in the Supplementary Material, Figure S5. With increasing temperature, the intensity of Tb luminescence decreases.

3.6. Luminescence Lifetime

Temperature variation has almost no effect on the luminescence lifetimes of the Eu or Tb emission. Figure 7 demonstrates the luminescence lifetimes of Eu and Tb emission as dependence of temperature for the solution L:Eu:Tb = 1:0.2:0.8; the dependences for other solutions are given in the Supplementary Material, Figures S6 and S7.

3.7. Luminescence Intensity of Tb and Eu as a Function of Temperature and the Eu/Tb Ratio

To understand how Eu and Tb luminescence intensities correlate with each other, the intensity ratios of their emission were plotted versus temperature (Figure 8). As the temperature increases, the luminescence of Tb decreases, while that of Eu increases, and those dependences of temperature are close to linear. Therefore, one can calculate the coefficient of luminescence sensitivity to temperature as the slope of luminescence intensity plotted versus temperature. Increasing the proportion of Eu in solution leads to a higher slope in temperature–dependence of luminescence.

The coefficients of luminescence sensitivity to temperature for Tb emission at 545 nm and Eu emission at 615 nm are provided in Figure 9.

From the figure, it follows that an increase in the proportion of Eu in solution leads to a decrease in the coefficient of linear regression of the temperature dependence of luminescence for Tb and to an increase for Eu, and these dependences are monotonous and close to linear. We conclude that if one wants to use single luminescence bands (Tb emission or Eu emission) for temperature evaluation, better sensitivity is achieved at higher concentration of ions.

Discussion of mechanisms of luminescence quenching for Tb and Eu enhancement with temperature will be given later in the following Section.

4. Discussion of Results: Enhancement of Eu Luminescence with Temperature

For Tb complexes, the quenching of luminescence along with temperature can be explained simply by the mechanism of reverse transfer of excitation energy [27]. This mechanism leads to a decrease in Tb luminescence intensity with rising the temperature of the complex framed in the solid form (powder and film) or dissolved in the solution. The quenching of Tb luminescence is used in thermometric sensors [28,29].

In contrast to Tb luminescence, the luminescence intensity of the Eu in studied complexes is increasing with temperature. This unusual enhancement of the Eu luminescence upon heating the samples containing Eu ions can be explained by the process of deactivation of its high-energy levels (⁵D_J, ⁵L_J, ⁵G_J) through the charge transfer state (Figure 10) [30].

As a result of thermal activation of higher energy levels of the Eu ion, the transfer is possible to the close in energy state of complex with charge transfer (CTS), which is followed by transfer to the resonance level of the Eu ion. Thus, there is an additional “pumping” of the resonance level of the Eu ion, and, consequently, an increase in the luminescence intensity upon heating.

Such an explanation of luminescence enhancement involves the appearance of a CTS in the complex. The appearance of a CTS band was apparently observed in the luminescence excitation spectra recorded at 615 nm for Eu emission (Figure 11). More clearly it can be illustrated by difference spectra which we received by subtracting from the spectrum measured at certain temperature the spectrum for a solution with temperature lower in 5 K.

The energy transfer mechanism described for Eu complexes does not work for the complexes with Tb ion due to the too high energy of the ⁵D₃ level. Therefore, when the solution of Tb complex is heated, a decrease in the luminescence intensity is observed with increasing temperature.

Resulted from enhancement of Eu luminescence with temperature and the constancy of absorption spectra, the luminescence quantum yield (measured in Eu emission band) for mixed solutions increases with rising temperature. Figure 12 shows the dependences of the absolute luminescence quantum yield of temperature at different Eu/Tb ratios in the mixed complex. One can notice that with an increase in the Eu fraction in the solution, the luminescence quantum yield becomes higher while absorption at the excitation wavelength 320 nm keeps constant for the complex in solution.

5. Ratiometric Temperature Sensing Using Eu and Tb Luminescence Bands

If temperature dependence for Eu and Tb luminescence intensity goes in the opposite directions, that makes sense to use the intensities ratio of Eu and Tb emissions for ratiometric temperature sensing. Figure 13 shows the dependences of the Eu/Tb luminescence intensities ratio of temperature at different Eu/Tb ratios in the mixed systems. From the figure it follows that with an increase in temperature, the ratio of the Eu/Tb luminescence intensities increases, since the luminescence intensity of Eu increases, while that of Tb decreases.

6. Conclusions

The unusual Eu luminescence enhancement upon heating was observed for the complex with tetradentate O,N-type heterocyclic diamide ligand L for the first time, while an inverse phenomenon was observed with the Tb luminescence. The Eu luminescence enhancement was found earlier for various carboxylate complex salts but not for heterocyclic coordination complexes. This allows constructing a ratiometric luminescent thermometer in the 280–340 K range. The stability constants for the individual Eu and Tb complexes help us in understanding the equilibrium in L:Tb:Eu complex system and shed light on plausible speciation in studied solutions.