Substantial luminescence enhancement in ternary europium complexes by coordination of different ionic ligands

Abstract

We demonstrate in a general and comprehensive manner that a substantial enhancement of luminescence in europium complexes can be achieved by increasing ionic ligand diversity. For the complexes studied, the measured boosts in the quantum efficiency of luminescence obtained using this strategy ranged from a minimum of 100% to a maximum of 543%. We formalize this concept by means of mathematical inequalities which describe the fact that either the quantum efficiency η, or the radiative decay rate A_rad, of a mixed ligand complex will be larger than the average of the same property for the respective same-ligand complexes. We further introduce the concept of structure hardening by ligands, by linking it quantitatively for the first time with the value of the non-radiative decay rate A_nrad, and interpret it in terms of the lowest vibrational frequency of the complex. In light of all these concepts and results, we conclude that luminescence can be boosted in europium complexes by increasing ligand diversity, which, in turn, should be ideally obtained with the help of hardener ligands. This general and comprehensive strategy aims at increasing A_rad while simultaneously decreasing A_nrad.

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1. Introduction

Highly luminescent europium complexes can be used in several applications. For example: a LED was obtained from the complex [Eu₂(2,7-BTFDBC)(DBM)₄(phen)₂] which acts as a red phosphor;¹ europium carbon nanotubes were used as luminophores in an electrochemiluminescent aptasensor for thrombin;² the formation of complexes was monitored by recording luminescence information from the prepared solid compounds.³ Moreover, complexes can be used as efficient red organic light-emitting devices, OLEDs;⁴ as portable luminescent chemosensors for naked eye Cu²⁺ detection;⁵ as sensors for the determination of pH;⁶ in studies of interactions with DNA;⁷ in accurate tumor-target bioimaging through specific in vivo biosynthesis;⁸ for bioimaging;⁹ and in in situ monitoring via europium emission of the photorelease of the antitumor drug cisplatin.¹⁰ In particular, ternary β-diketonate europium complexes can be used, for example, as efficient red organic electroluminescent devices;¹¹ as luminescent pure-red-emitting complexes for full solution-processed OLEDs;¹² in mitochondria, as targetable time-gated luminescence probes for singlet oxygen;¹³ as temperature sensors, in UV light detection, and as laser devices.¹⁴ Recently, water-soluble luminescent hybrid composites consisting of oligosilsesquioxanes and lanthanide complexes were obtained.¹⁵ These composites exhibited an effective switch-off fluorescence response with high sensing selectivity and sensitivity toward Cu²⁺ over other common metal ions in aqueous media.¹⁵

Theoretical tools can predict important aspects of the europium species involved, such as: geometry optimization,¹⁶ thermodynamic properties,¹⁶ and a treatment of luminescence properties.¹⁷ The geometry optimization of europium complexes can be performed, for example, with the Sparkle/RM1 model,¹⁸ using the quantum chemical program MOPAC 2016.¹⁹ Luminescence properties such as the quantum efficiency, η, and the Judd–Ofelt parameters Ω₂, Ω₄, and Ω₆, can be calculated by the LUMPAC (LUMinescence PACkage) software.¹⁷ Through LUMPAC, it is also possible to perform a chemical partition of the global photophysical property A_rad (ref. 20) in terms of ligand effects which allows a chemical interpretation of the role of each ligand coordinated to the europium ion in the luminescence phenomenon.

Temperature affects luminescence of europium complexes significantly, with low temperatures leading to higher luminescence.²¹ Two competitive mechanisms occur when the excited state of the metal decays: radiative processes and non-radiative processes. The radiative processes are the desired ones and occur when the excited state of the trivalent europium ion, ⁵D₀, decays to the lower electronic states of the europium ion: ⁷F_J, with J ranging from 0 to 6. The sum of all seven radiative decay rates is known simply as A_rad. The other decay pathway is the non-radiative one, characterized by the non-radiative decay rate, A_nrad. The total decay rate from the ⁵D₀ excited state, A_tot, is equal to A_rad + A_nrad. Seemingly, A_nrad competes with A_rad and, therefore, whenever the radiationless processes are hindered, A_rad usually tends to increase and more luminescence ensues. For example, when water is directly coordinated to the europium ion, their low energy vibrations tend to resonate with vibrations from the solvent molecules in the first solvation layers, thus robbing energy from the complex leading to low luminescence. Raising the vibration energies of the coordinated water molecules by deuterating them usually enhances luminescence.²²

Recently, we introduced a comprehensive strategy to boost the luminescence of europium complexes by increasing the diversity of ligands around the europium trivalent ion.²³ The strategy states that a luminescence property P, where P stands for either the quantum yield Φ, the quantum efficiency η, or the radiative decay rate A_rad, of a mixed ligand complex, would be larger than the average of the same property for the respective same-ligand complexes. The strategy can be quantified by the following inequality:²³

where [Eu…] stands for a europium complex, and L and L′ are different ligands coordinated to the europium ion. We further defined the concept of a % Boost_P in the property P as the portion of P that exceeds the average, as a percentage of the average, as shown below:

Previously,²³ we showed eqn (1) to be true for mixed non-ionic ligand complexes only, for all combinations of the non-ionic ligands TPPO (triphenylphosphine oxide), DBSO (dibenzyl sulfoxide), and PTSO (p-tolyl sulfoxide), for all ternary complexes of the β-diketonates TTA (1-(2-thenoyl)-3,3,3-trifluoroacetone), and BTFA (4,4,4-trifluoro-1-phenyl-1,3-butanedione). For the cases studied, the observed % Boost_Φ in the quantum yield were as high as 81%.²³

In this article, we set forth a generalization of eqn (1) and prove it to be also true for ternary mixed ionic ligand europium complexes, and thus valid without exception (at least for all cases studied so far^16,23). In order to show the usefulness of eqn (1) for the design of luminescent complexes, we further replace the double TPPO ligands in the mixed ionic ligand complex, by the non-ionic bidentate ligands BIPY (2,2-bipyridyl) and PHEN (1,10-phenanthroline) in order to reduce the non-radiative decay rate due to their increased rigidities, thus enhancing luminescence even more, when compared to the double TPPO complex. We further construe the concept of structure hardening by ligands via their effects on the non-radiative decay rate A_nrad, and advance an interpretation of rigidity in terms of the lowest vibrational frequency of the complex. Finally, we use theoretical tools to obtain, for each of the complexes, the optimized geometries; values for the parameters Ω₂, Ω₄, and Ω₆; the quantum efficiency η; A_rad and A_nrad; and a chemical interpretation of the effect of each ligand coordinated to the europium ion on the global photophysical property A_rad.²⁰

2. Results and discussion

2.1. Enhancement of the luminescence properties

The complexes synthesized in this article were Eu(β-diketonate)₂(β-diketonate′)(TPPO)₂, where β-diketonate can be either DBM, BTFA, or TTA, and Eu(DBM)(BTFA)(TTA)(TPPO)₂. Finally, we also synthesized Eu(DBM)(BTFA)(TTA)(BIPY) and Eu(DBM)(BTFA)(TTA)(PHEN). The chemical structures of these ligands are shown in Fig. 1.

Fig. 1 Chemical structures of β-diketonate ionic ligands 1,3-diphenylpropane-1,3-dione (DBM), 4,4,4-trifluoro-1-phenyl-1,3-butanedione (BTFA), 1-(2-thenoyl)-3,3,3-trifluoroacetone (TTA), and non-ionic ligands triphenylphosphine oxide (TPPO), 1,10-phenanthroline (PHEN), 2,2′-bipyridyl (BIPY).

In order to quantify the effect of mixing ionic ligands on the luminescence of europium complexes, we now present the following generalization of our conjecture, eqn (1), to the case of ternary all mixed ionic ligand complexes:

where β^A, β^B and β^C stand for different ionic ligands. Eqn (4) is a special case of eqn (3) when two of the ionic ligands are identical.The % Boost_P in both cases can be defined, as before, as the portion of P that exceeds the average, as a percentage of the average, which is the right-hand side of the inequalities in eqn (3) and (4).

Table 1 shows the luminescence properties of all complexes considered in this article: the first three presented earlier^16,23 and the next nine synthesized in this article. The lifetime of complex Eu(DBM)₃(TPPO)₂ could not be observed in chloroform solution at room temperature, most likely due to the elevated symmetry of both DBM and TPPO ligands, as well as of the complex itself, where the two TPPOs lie opposite to each other as revealed by our Sparkle/RM1 calculation; a result which is in tune with X-ray crystallographic measurements^24,25 for complexes Eu(TTA)₃(TPPO)₂ and Eu(BTFA)₃(TPPO)₂, when the TPPOs always appear opposite to each other. This dual ligand and coordination symmetry of Eu(DBM)₃(TPPO)₂ is what probably reduced the luminescence lifetime of Eu(DBM)₃(TPPO)₂ to an essentially unobservable low value at room temperature. So much so, that a lifetime for this complex could only be measured by other researchers in the solid state and only at liquid nitrogen temperatures.²¹ For the purposes of this article, therefore, we consider the quantum efficiency of this complex at room temperature to be essentially zero.

Table 1 Lifetimes, τ; decay rates total, A_tot; radiative decay rates, A_rad; non-radiative decay rates, A_nrad; and quantum efficiency calculated from experimental data and LUMPAC software

Complex	τ (ms)	Atot (s^−1)	Arad (s^−1)	Anrad (s^−1)	η (%)
Eu(DBM)3(TPPO)2 (ref. 16)	—	—	335	—	—
Eu(TTA)3(TPPO)2 (ref. 23)	0.350	2857	796	2061	21
Eu(BTFA)3(TPPO)2 (ref. 23)	0.367	2725	919	1806	21
Eu(DBM)2(TTA)(TPPO)2	0.415	2410	1082	1328	45
Eu(TTA)2(DBM)(TPPO)2	0.424	2359	1052	1307	45
Eu(DBM)2(BTFA)(TPPO)2	0.423	2364	1015	1349	43
Eu(BTFA)2(DBM)(TPPO)2	0.473	2114	1131	983	53
Eu(TTA)2(BTFA)(TPPO)2	0.435	2584	1093	1491	42
Eu(BTFA)2(TTA)(TPPO)2	0.458	2184	1058	1126	48
Eu(DBM)(BTFA)(TTA)(TPPO)2	0.434	2303	1034	1269	45
Eu(DBM)(BTFA)(TTA)(PHEN)	0.645	1550	864	686	56
Eu(DBM)(BTFA)(TTA)(BIPY)	0.523	1912	894	1018	47

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Now, we are in position to verify the validity of eqn (3) and (4), a generalization of our previously proposed conjecture to mixed ionic ligand ternary complexes.

The first six lines of Table 2 show the values of the average quantum efficiencies,

the measured quantum efficiencies of the mixed ionic ligand complexes Euβ^A₂β^AL₂ and the % Boost_η in the quantum efficiency due to the mixing of the ionic ligands, where β^A and β^B stand for DBM, BTFA, or TTA.

Table 2 Values of average quantum efficiency, η_avg; measured quantum efficiencies of mixed ionic ligand complexes η; and percent boost in the quantum efficiencies, % Boost_η calculated via eqn (3) for all cases except for the last one when eqn (4) was used

Complex	ηavg (%)	η (%)	% Boostη
Eu(DBM)2(TTA)(TPPO)2	7	45	543%
Eu(TTA)2(DBM)(TPPO)2	14	45	221%
Eu(DBM)2(BTFA)(TPPO)2	7	43	514%
Eu(BTFA)2(DBM)(TPPO)2	14	53	279%
Eu(BTFA)2(TTA)(TPPO)2	21	48	129%
Eu(TTA)2(BTFA)(TPPO)2	21	42	100%
Eu(DBM)(BTFA)(TTA)(TPPO)2	14	45	221%

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The last line on Table 2 shows the value of the average

the measured quantum efficiency of the mixed ionic ligand complex Eu(DBM)(BTFA)(TTA)(TPPO)₂ and the % Boost_η in the quantum efficiency due to the full mixing of the ionic ligands. Results on Table 2 indicate that eqn (3) and (4) are indeed true for all cases considered in this article. Not surprisingly, the highest % Boost_η values were observed for the DBM complexes because these are the complexes that display the lowest η_avg values. Moreover, the quantum efficiency values obtained for the mixed ionic complexes Eu(BTFA)₂(TTA)(TPPO)₂ and Eu(TTA)₂(BTFA)(TPPO), are 48% and 42%, respectively, essentially twice the 21% measured for both corresponding same ionic complexes Eu(BTFA)₃(TPPO)₂ and Eu(TTA)₃(TPPO)₂.

As a result, the % Boost_ηs obtained for the mixed ionic ligand complexes were very large, ranging from 100% to 543%; values that are much larger than the % Boost_ηs obtained in our previous article on mixed non-ionic ligand complexes, in which the maximum % Boost_η obtained was 77% for the Eu(TTA)₃(PTSO,TPPO) complex.²³ Indeed, mixing ionic ligands seems to be an even more effective strategy to enhance luminescence in ternary europium complexes.

Table 3 presents equivalent data proving that eqn (3) and (4) are also valid for A_rad, another luminescent property. The % Boost_Arad values range from 34% to 121%.

Table 3 Values of average radiative decay rates, A_rad,avg, measured radiative decay rates of mixed ionic ligand complexes A_rad, and percent boost in A_rad, % Boost_Arad, calculated via eqn (3) for all cases except for the last one when eqn (4) was used

Complex	Arad,avg (s^−1)	Arad (s^−1)	% BoostArad
Eu(DBM)2(TTA)(TPPO)2	489	1082	121%
Eu(TTA)2(DBM)(TPPO)2	642	1052	64%
Eu(DBM)2(BTFA)(TPPO)2	530	1015	92%
Eu(BTFA)2(DBM)(TPPO)2	724	1131	56%
Eu(BTFA)2(TTA)(TPPO)2	878	1093	24%
Eu(TTA)2(BTFA)(TPPO)2	789	1058	34%
Eu(DBM)(BTFA)(TTA)(TPPO)2	683	1034	51%

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Both TTA and BTFA are already asymmetric ionic ligands, as well as good antennae, and, besides that, somewhat similar to one another. Hence, the ensuing observed % Boosts, both in η and in A_rad, obtained by mixing these ionic ligands tend to be smaller than by mixing either of them with DBM. So much so that the largest η of 53% was observed for complex Eu(BTFA)₂(DBM)(TPPO)₂ (Tables 1 and 2). A final verification of eqn (3) was carried out by synthesizing and measuring the luminescent properties of complex Eu(DBM)(BTFA)(TTA)(TPPO)₂. The % Boosts in η and in A_rad for this complex, computed as the portion of the property that exceeds the average, as a percentage of the average – the right-hand side of eqn (3) – are 221% and 51%, as shown in the last lines of Tables 2 and 3, respectively.

All these results point in the direction that mixing ligands to boost the luminescent properties of europium complexes is indeed a seemingly comprehensive strategy of general validity.

2.2. The chemical partition of A_rad

The boost observed for the all-different ionic ligand complex Eu(β-diketonate)(β-diketonate′)(β-diketonate′′)(TPPO)₂, albeit significant, was equivalent to the one obtained for complexes of the type Eu(β-diketonate)₂(β-diketonate′)(TPPO)₂, seemingly indicating that the presence of at least one different ionic ligand in the complex structure is already enough to obtain a highly luminescent complex. Luminescence properties in Table 1 indicate that the lifetime values of the mixed ionic ligand ternary europium complexes of general formula Eu(β-diketonate)₂(β-diketonate′)(TPPO)₂ were all similar, averaging 0.438 ms, larger than those of the repeating ionic ligand complexes, which averaged 0.359 ms. Similarly, the A_rad values for the mixed ionic ligand ternary europium complexes were much higher than those of the repeating ionic ligand complexes: 1072 s⁻¹ and 683 s⁻¹, respectively, as expected by the inequality in eqn (1). As such, the radiative decay rates, A_rad, of mixed ionic ligand complexes involving TPPO are higher than those of the corresponding same ionic ligand complexes. This confirms the positive effect of the diversity of ionic ligands coordinated to the europium ion²³ on europium luminescence.

The radiative decay rate, A_rad, is a global photophysical property of luminescent europium complexes. So, by examining A_rad only, it is difficult to grasp the role of each of the ligands on the luminescence phenomenon, and, thus, interpret A_rad from a chemical perspective. In order to better understand how each ligand facilitates the decay of the trivalent europium ion from its ⁵D₀ excited state to the ⁷F_J ones, we recently introduced a partition of a subset of A_rad, A_rad′,²⁰ which corresponds to the sum of the electric dipole electronic transitions ⁵D₀ → ⁷F_J, J = 2, 4 and 6, and which constitutes approximately 91% of A_rad. Hence, an analysis of A_rad′ is essentially an analysis of A_rad itself. Fig. 2 shows all three ternary europium complexes with their A_rad values, as well as the partitioned values of A_rad′ per ligand. Since the effects of the TPPO ligands in A_rad′ for all three cases is somewhat equivalent, 102 s⁻¹, 122 s⁻¹ and 160 s⁻¹, the difference in A_rad occurs due to the ionic ligands: 190 s⁻¹ for the DBMs, 636 s⁻¹ for the TTAs, and 635 s⁻¹ for the BTFAs. From these data alone, one would be led to the conclusion that the DBM ligand is never a very good ligand for luminescence purposes, whereas TTA and BTFA are.

Fig. 2 Total A_rad values, partitioned A_rad′ values per ligand for three europium complexes with repeating both ionic and nonionic ligands. The red circle stands for the Eu³⁺ ion.

However, according to the inequality in eqn (1), adding a different ligand to make a mixed ligand complex, should intensify the A_rad value of a more symmetrical complex, regardless of which ligand is chosen. Fig. 3 shows in a pictorial format the effect of removing a DBM ligand, one at a time, from complex Eu(DBM)₃(TPPO)₂, and replacing it with either a TTA or a BTFA.

Fig. 3 Total A_rad values, partitioned A_rad′ values per ligand for europium complexes with one, two, and three DBM ligands in their structures.

Clearly, the contribution of DBM to A_rad′ increases from an average of 63 s⁻¹ for the complex with three DBMs, to an average of 80 s⁻¹ for both complexes with two DBMs, and to the much larger average value of 338 s⁻¹ for both complexes with one DBM. This trend strongly suggests that, if on one hand DBM does not seem to be a good antenna (and that is why three DBMs make a poorly luminescent complex), a single DBM seems to be important as a coordination polyhedron symmetry breaker – the value of 338 s⁻¹ being a measure of how slightly more allowed the single DBM makes the transition, from the ⁵D₀ excited state of the trivalent europium ion to the ⁷F_J states, to be. Fig. 3 also shows very clearly that the A_rad values of the four mixed ligand complexes are all similar, in average 1051 s⁻¹, indicating that a single good antenna is already enough to guarantee a high quantum efficiency, which can be further boosted by breaking the symmetry of the coordination polyhedron by different ligands – ligands that do not even need to be good antennas.

The first complex with two TPPOs is probably the most asymmetric and distorted of the three complexes in Fig. 4, because it is the one most stuffed with ligands. When the coordination polyhedron is uncongested by replacing both of them with a single ligand, either BIPY and PHEN, A_rad diminishes. Since PHEN is a more planar and thus more symmetric ligand than BIPY, not surprisingly its complex displays the lowest value of A_rad.

Fig. 4 Total A_rad values, partitioned A_rad′ values per ligand for the fully mixed ionic ligand complexes.

2.3. A_nrad and the rigidity of the non-ionic ligands

Since the radiative and non-radiative decay processes, A_rad and A_nrad, are competitive kinetic processes, it is plausible that, all other things being equal, the values of A_nrad will tend to decrease as the values of A_rad increase. Indeed, A_nrad competes with A_rad for the depopulation of the ⁵D₀ excited state of the trivalent europium ion. Accordingly, consider the data in Table 1. While the average A_nrad for the repeating ionic ligand complexes in Table 1 is 1934 s⁻¹, it goes down when symmetry is broken for the more luminescent mixed ligand complexes to the average of 1247 s⁻¹.

A_nrad is dependent on the ability of the complex to dissipate the ⁵D₀ excited state energy to the ground state via molecular vibrations and/or transient geometric distortions and their interactions with the phonons or solvent molecules in the first solvation layers that surround the complex. As such, by removing the lower frequency molecular vibrations, the probabilities of non-radiative transitions are reduced, thus reducing A_nrad and concurrently increasing A_rad. Thus, in order to further enhance the luminescence of the completely mixed ionic ligand complex Eu(DBM)(BTFA)(TTA)(TPPO)₂, we replaced both TPPO non-ionic ligands by either a 1,10-phenantroline, PHEN or by a 2,2-bipyridine, BIPY. Our expectation was that the quantum efficiencies of these complexes would be larger than those of the double TPPO complex with an A_rad of 1034 s⁻¹. From Table 1, however, as we had already discussed in the previous section, the values of A_rad decreased for both BIPY and PHEN complexes, with values of 894 s⁻¹ and 864 s⁻¹, respectively. But, based on our previous reasoning, the effect of the rigidity of the non-ionic ligands, on the other hand, should emerge essentially on A_nrad. And that was precisely the case, as revealed by the values of A_nrad for the BIPY and PHEN complexes of 1018 s⁻¹ and 686 s⁻¹, much lower than the value for the double TPPO complex of 1269 s⁻¹. The impact of the rigidity on A_rad going from the double TPPO complex to the PHEN complex is therefore a reduction of 170 s⁻¹ on its value, while the same impact on A_nrad is a much larger reduction of 583 s⁻¹. Thus, the effect of the rigidity of the ligands on A_nrad overshadows the reduction of A_rad, rendering the quantum efficiencies of the BIPY and PHEN complexes, 47% and 56%, respectively; larger than the quantum efficiency of the double TPPO complex of 45%, as originally intended.

From these results, it is possible to qualitatively order the effect of the non-ionic ligands on the rigidity of the complex as a whole, which we propose to call the hardening effect of the ligand, in terms of A_nrad, from the more to the less hardener ligand as: PHEN > BIPY > TPPO. This is corroborated by the lowest vibrational frequencies calculated by Sparkle/RM1 for all three mixed ionic ligand complexes, of 2.48 cm⁻¹ for TPPO2; 4.28 cm⁻¹ for BIPY; and 4.48 cm⁻¹ for PHEN. In this sense, we can further say that BTFA is a more hardener ligand than TTA in the complexes of Table 1. In general, whenever TTA is replaced by a BTFA on Table 1, A_nrad usually tends to decrease its value. Indeed, while complex Eu(TTA)₃(TPPO)₂ displays an A_nrad of 2061 s⁻¹, Eu(BTFA)₃(TPPO)₂ displays a smaller A_nrad value of 1806 s⁻¹. Likewise, by replacing one TTA in Eu(TTA)₂(BTFA)(TPPO)₂ with the larger A_nrad of 1491 s⁻¹ by a BTFA, we arrive at Eu(BTFA)₂(TTA)(TPPO)₂ which has a smaller A_nrad of 1126 s⁻¹. Finally, replacing TTA in Eu(TTA)₂(DBM)(TPPO)₂ with a larger A_nrad of 1307 s⁻¹, we arrive at Eu(BTFA)₂(DBM)(TPPO)₂, with the smaller value of A_nrad of 983 s⁻¹.

3. Experimental

3.1. Characterization

The mixed ionic ligand complexes synthesized in this article were characterized by MALDI-TOF mass spectrometry (Autoflex 3 Smart Beam Vertical spectrometer); elemental analysis (Perkin-Elmer CHN2400); infrared spectroscopy (samples were prepared as KBr disks, and the spectra were measured in a Bruker model IFS 66 spectrophotometer, 4000–400 cm⁻¹); ¹H NMR, ³¹P NMR and ¹⁹F NMR spectroscopy (NMR spectra of all complexes were obtained in CDCl₃ solutions by a Varian Unity Plus 400 MHz). The ESI† contains details of the experimental characterizations, data, spectra and all relevant spectral attributions for all complexes synthesized.

3.2. Luminescence measurements

Luminescence spectra and quantum efficiency measurements were obtained with a Fluorolog-3 Horiba Jobin Yvon equipped with a Hamamatsu R928P photomultiplier with a SPEX 1934 D phosphorimeter and a 150 W pulsed xenon lamp. The measurements were performed at room temperature, from 10⁻⁴ M chloroform solutions at an excitation wavelength, λ_max, between 374 nm and 397 nm with a slit width of 1.0 nm for the excitation, and 1.0 nm for the emission. Data were processed by software supplied by Horiba-Jobin-Yvon. The quantum efficiency, η, is defined as η = A_rad/(A_rad + A_nrad), where A_rad is the radiative decay rate and A_nrad is the nonradiative decay rate. The non-radiative decay rates of the complexes in solution were calculated from the lifetime decay, in which: 1/τ_obs = A_rad + A_nrad.

3.3. Materials

The reagents and solvents used were 1,3-diphenylpropane-1,3-dione (DBM, Alfa Aesar, 99%); 4,4,4-trifluoro-1-phenyl-1,3-butanedione (BTFA, Alfa Aesar, 99%); 1-(2-thenoyl)-3,3,3-trifluoroacetone (TTA, Alfa Aesar, 99%); triphenylphosphine oxide (TPPO, Sigma Aldrich, 99%); 1,10-phenanthroline monohydrated (PHEN, Panreac, 99%); 2,2′-bipyridyl (BIPY, Aldrich, 99%); ethanol (J.T. Baker); hexane (Sigma Aldrich); acetone (Sigma Aldrich) and chloroform (J.T. Baker).

3.4. Syntheses

3.4.1 Synthesis of complex [EuCl₃(TPPO)₄]·3H₂O. A solution of [EuCl₂·(H₂O)₆]Cl (1 mmol) was prepared in 30 mL of pure ethanol, and was left under stirring conditions. Subsequently, 4 mmols of TPPO free ligand was dissolved in 20 mL of pure ethanol, and then added to the previous solution. The resulting reagent mixture was left overnight with stirring under reflux. The solvent was evaporated under reduced pressure until dryness. A white solid, [EuCl₃(TPPO)₄]·(H₂O)₃, was obtained, and purified by recrystallization with warm ethanol to a complete dissolution of the solid in a 250 mL beaker. It was left to evaporate until pure crystals were formed. Yield: 99%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1425.20, found (m/z) 1425.23. Elemental analysis calculated: C 60.66%, H 4.67%, found: C 60.60%, H 4.61%, IR (KBr): νO–H 3461 cm⁻¹; ν=C–H 3090–3015 cm⁻¹; νP=O 1087 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 7.60–7.31 (m, Ar.), 2.61 (s, OH), and ³¹P NMR (162 MHz, CDCl₃): δ 27 ppm.

3.4.2 Synthesis of complex Eu(DBM)₂(TTA)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of DBMK (0.8 mmol) was obtained by deprotonation of the corresponding DBMH free ligand, using KOH base (0.8 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of TTAK (0.4 mmol) obtained from TTAH by deprotonation using KOH base (0.4 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash the KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 94%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1380.26, found (m/z) 1380.39. Elemental analysis calculated: C 64.44%, H 4.31%, found: C 64.64%, H 4.37%, IR (KBr): νC–H 3075–3054 cm⁻¹; νC–H 2990 cm⁻¹; νC=O 1685 cm⁻¹; νC=O 1600 cm⁻¹; νP=O 1113 cm⁻¹; νC=F 1179 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.39 (s, CH), 7.85–5.75 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 25 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −83.90 ppm and −84.41 ppm (CF₃).

3.4.3 Synthesis of complex Eu(TTA)₂(DBM)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of DBMK (0.4 mmol) was obtained by deprotonation of the corresponding DBMH free ligand, using KOH base (0.4 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of TTAK (0.8 mmol) obtained from TTAH by deprotonation using KOH base (0.8 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash the KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 98%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1378.17, found (m/z) 1378.28. Elemental analysis calculated: C 58.43%, H 3.81%, found: C 58.45%, H 3.77%, IR (KBr): νC–H 3076–3056 cm⁻¹; νC–H 2989 cm⁻¹; νC=O 1680 cm⁻¹; νC=O 1608 cm⁻¹; νP=O 1118 cm⁻¹; νC=F 1182 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.78 (s, CH),7.82–6.07 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 25 ppm and −73 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −84.06 ppm and −84.64 ppm (CF₃).

3.4.4 Synthesis of complex Eu(DBM)₂(BTFA)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of DBMK (0.8 mmol) was obtained by deprotonation of the corresponding DBMH free ligand, using KOH base (0.8 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of BTFAK (0.4 mmol) obtained from BTFAH by deprotonation using KOH base (0.4 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 82%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1374.30, found (m/z) 1374.44. Elemental analysis calculated: C 66.47%, H 4.48%, found: C 66.67%, H 4.35%, IR (KBr): νC–H 3078–3055 cm⁻¹; νC=O 1684 cm⁻¹; νC=O 1621 cm⁻¹; νP=O 1118 cm⁻¹; νC=F 1184 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.63 (s, CH), 7.90–7.54 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 24 ppm and −71 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −83.84 ppm and −84.40 ppm (CF₃).

3.4.5 Synthesis of complex Eu(BTFA)₂(DBM)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of DBMK (0.4 mmol) was obtained by deprotonation of the corresponding DBMH free ligand, using KOH base (0.4 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of BTFAK (0.8 mmol) obtained from BTFAH by deprotonation using KOH base (0.8 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 94%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1366.26, found (m/z) 1366.43. Elemental analysis calculated: C 62.47%, H 4.13%, found: C 62.64%, H 3.93%, IR (KBr): νC–H 3076–3054 cm⁻¹; νC=O 1681 cm⁻¹; νC=O 1625 cm⁻¹; νP=O 1117 cm⁻¹; νC=F 1181 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.44 (s, CH),7.91–7.55 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 28 ppm and −72 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −84.44 ppm (CF₃).

3.4.6 Synthesis of complex Eu(BTFA)₂(TTA)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of DBMK (0.8 mmol) was obtained by deprotonation of the corresponding DBMH free ligand, using KOH base (0.8 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of TTAK (0.4 mmol) obtained from TTAH by deprotonation using KOH base (0.4 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash the KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 89%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1364.17, found (m/z) 1364.27. Elemental analysis calculated: C 56.40%, H 3.62%, found: C 56.86%, H 3.74%, IR (KBr): νC–H 3061–3026 cm⁻¹; νC=O 1595 cm⁻¹; νC=O 1545 cm⁻¹; νP=O 1068 cm⁻¹; νC=F 1177 cm⁻¹; νC=F 1118 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.97 (s, CH),7.84–6.06 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 25 ppm and −71 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −84.48 ppm and −84.59 ppm (CF₃).

3.4.7 Synthesis of complex Eu(TTA)₂(BTFA)(TPPO)₂. A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.4 mmol) was prepared in 30 mL of pure ethanol. At the same time, a solution of BTFAK (0.4 mmol) was obtained by deprotonation of the corresponding BTFAH free ligand, using KOH base (0.4 mmol) in 20 mL of pure ethanol. Subsequently, this solution was slowly added to the initial solution of Eu(TPPO)₄Cl₃(H₂O)₃, and then left under reflux with magnetic stirring during 15 hours. Further, a solution of TTAK (0.8 mmol) obtained from TTAH by deprotonation using KOH base (0.8 mmol) was added to the reaction system. In both steps of the ionic ligand addition, the solution pH was adjusted to 6.5 by means of an ethanolic solution of KOH base, 0.1 mol L⁻¹. Furthermore, the solution was left stirring and under reflux for 24 hours. Finally, the solvent was slowly evaporated at room temperature for a few days. A yellow solid was obtained and washed with water to wash the KCl salt formed, and then hot hexane was added to remove the displaced TPPO ligand. The solid was recrystallized using ethanol. Yield: 84%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 1370.13 found (m/z) 1370.18. Elemental analysis calculated: C 54.39%, H 3.46%, found: C 54.53%, H 3.48%, IR (KBr): νC–H 3074–3056 cm⁻¹; νC=O 1684 cm⁻¹; νC=O 1610 cm⁻¹; νP=O 1120 cm⁻¹; νC=F 1188 cm⁻¹; νC==F 1168 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.17 (s, CH), 7.85–7.34 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 30 ppm and −76 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ −84.65 ppm and −84.73 ppm (CF₃).

3.4.8 Synthesis of complex Eu(DBM)(BTFA)(TTA)(TPPO)₂. A solution of europium salt [EuCl₂·(H₂O)₆]Cl (0.4 mmol) was prepared in 30 mL of pure ethanol. The TPPO (1.2 mmol) ligand was dissolved in 20 mL of pure ethanol and was slowly added to the europium salt solution. The resulting solution was left under stirring and reflux at 75 °C for 24 h. The system was then cooled and the 20 mL of an ethanolic solution of DBMK (0.4 mmol) was added. The temperature of the resulting solution was slowly increased during 2 h, until the temperature reached 75 °C. Next, it was stirred under reflux for 18 h then the system was cooled down. This same procedure was repeated twice: first by adding BTFAK (0.4 mmol), followed by the addition of TTAK (0.4 mmol), in this order. In all steps of ionic ligand addition, if necessary, the solution pH was adjusted to 6.5 by use of an ethanolic solution of NaOH 0.1 mol L⁻¹. Finally, the ethanol of the resulting reagent solution slowly evaporated at room temperature for a few days. The yellow solid obtained was washed with water to remove the KCl formed, and then washed with hot hexane to remove the TPPO ligand displaced. The solid was recrystallized using a solution ethanol. Yield: 85%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z), found (m/z) 1372.25. Elemental analysis calculated: C 60.44%, H 3.97%, found: C 60.30%, H 3.88%, IR (KBr): νO–H 3452 cm⁻¹, νC–H 3060–3039 cm⁻¹, νC=O 1619 cm⁻¹, νP=O 1115 cm⁻¹; νC=F 1182 cm⁻¹; νC=F 1174 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.21 (s, CH), 7.79–6.43 (m, Ar.); ³¹P NMR (162 MHz, CDCl₃): δ 28 ppm and 72.43 ppm; ¹⁹F NMR (376 MHz, CDCl₃): δ –79.38 ppm and −80.00 ppm (CF₃).

3.4.9 Synthesis of complex Eu(DBM) (BTFA) (TTA) (PHEN). A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.2 mmol) was prepared in 20 mL of pure ethanol and slowly added to an ethanolic solution of the PHEN ligand (0.2 mmol). The mixture was left stirring under reflux at 75 °C for 12 h. Next, the system was cooled down to room temperature, and then DBMK (0.2 mmol) was slowly added to the solution. The mixture was left under stirring and reflux at 78 °C for 12 h. The same procedure was performed twice, first to add BTFAK (0.2 mmol), and then to add TTAK (0.2 mmol). After the TTAK ligand was added to the system, it was left under stirring and reflux for 24 h. Then, the ethanol was slowly evaporated at room temperature for a few days. An orange solid was obtained and washed with water to remove the KCl formed, and then washed with hot hexane to remove the excess of TPPO ligand. Yield: 94%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 992.75, found (m/z) 992.76. Elemental analysis calculated: C 54.50%, H 2.95%, found: C 54.71%, H 3.14%; IR (KBr): ν=C–H 3079–3049 cm⁻¹, νC=N 1615 cm⁻¹, νC–F 1179 cm⁻¹, νC=O 1694 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 8.68 (s, CH), 8.27–6.12 (m, Ar.); ¹⁹F NMR (376 MHz, CDCl₃): δ −79.26 to 74.82 ppm (CF₃).

3.4.10 Synthesis of complex Eu(DBM) (BTFA) (TTA) (BIPY). A solution of [EuCl₃(TPPO)₄]·(H₂O)₃ (0.2 mmol) was prepared in 20 mL of pure ethanol and slowly added to a ethanolic solution of the BIPY ligand (0.2 mmol). The mixture was left stirring under reflux at 75 °C for 12 h. Next, the system was cooled down to room temperature, and then DBMK (0.2 mmol) was slowly added to the solution. The mixture was left under stirring and reflux at 78 °C for 12 h. The same procedure was performed twice, first to add BTFAK (0.2 mmol), and then to add TTAK (0.2 mmol). After the TTAK ligand was added to the system, it was left under stirring and reflux for 24 h. Then, the ethanol was slowly evaporated at room temperature for a few days. An orange solid was obtained and washed with water to remove the KCl formed, and then washed with hot hexane to remove the excess of TPPO ligand. Yield: 96%. Characterization: calculated MALDI-TOF/MS [M + H]⁺ (m/z) 968.73, found (m/z) 968.84. Elemental analysis calculated: C 53.37.44%, H 3.02%, found: C 53.88%, H 3.51%; IR (KBr): ν=C–H 3062–3048 cm⁻¹, νC=N 1625 cm⁻¹, νC–F ⁻¹, νC=O 1684 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ 8.62 (s, CH), 7.86–6.03 (m, Ar.); ¹⁹F NMR (376 MHz, CDCl₃): δ −79.22 to 79.87 ppm (CF₃).

3.5. Computational procedure

All geometries of europium complexes were fully optimized using Sparkle/RM1 model¹⁸ implemented in MOPAC 2016 software.¹⁹ The keywords used in the MOPAC calculations were SPARKLE RM1 GNORM = 0.25 BFGS XYZ T = 10D ALLVEC. The vibrational modes were calculated from the optimized geometries, when we made sure the geometries were true minima with no imaginary frequencies. In this step we used the keywords: SPARKLE RM1 FREQ THERMO XYZ T = 10D ALLVEC. The luminescence parameters and properties were calculated from experimental data of emission spectra and lifetime curves using the LUMPAC software.¹⁹ The refractive index used in the calculations was 1.45 because the photophysical experiments were all carried out in chloroform solution.

4. Conclusions

We demonstrate that mixing ionic ligands in europium complexes does enhance their luminescence over and above what can be observed for the luminescence of related compounds with repeating ionic ligands. Indeed, this proposition of mixing ionic ligands for the purpose of enhancing luminescence, pairs with the results we obtained previously by mixing non-ionic ligands.²³

Therefore, our strategy of increasing the diversity of ligands in the coordination polyhedron to enhance the luminescence properties of europium complexes is strengthened as a general and comprehensive one, which can be easily understood and applied to a variety of situations. The strategy is quantified by means of inequalities, as further generalized in eqn (3) and (4).

By means of the chemical partition of A_rad′, we discovered that coordination symmetry breaking by a single ionic ligand which can, in itself, even be a poor antenna, does indeed enhance luminescence. This suggests that there is no need for a multitude of antennas around a europium ion – coordination diversity being more important; a result which is in tune with the inequalities present in eqn (3) and (4).

We also introduce the concept of structure hardening by ligands, by linking it for the first time with the values of A_nrad. Accordingly, we call ligand L a hardener in comparison with ligand L′, a softener, if the value of A_nrad for a complex increases when L′ is replaced by L. Such a concept allows us to order ligands in terms of their hardening effect in a given class of compounds, showing that – for the complexes studied – PHEN is the best non-ionic hardener, followed by BIPY and then by TPPO. Likewise, the ionic ligands can also be ordered, with DBM being the most ionic hardener, followed by both BTFA and TTA, for the class of compounds studied in this article.

In light of these concepts, we conclude that luminescence enhancement can be obtained in europium complexes by increasing ligand diversity, ideally with the help of hardening ligands defined as those capable of lowering A_nrad. This general and comprehensive strategy thus boosts the quantum efficiency by increasing A_rad while simultaneously decreasing A_nrad.

Extension of this rationale of luminescence enhancement to other lanthanide trications and different ligands is being presently carried out in our laboratories.

Acknowledgements

The authors appreciate the financial support of the following Brazilian Agencies: FACEPE (Pronex), and CNPq. The authors also thank Centro de Tecnologias Estratégicas do Nordeste, CETENE, and Dr Júlia F. Campos for the MALDI-TOF mass spectrometry measurements.

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Footnote

† Electronic supplementary information (ESI) available: excitation spectra; emission spectra; lifetime curves; infrared spectra; and ¹H, ¹⁹F, and ³¹P NMR spectra. See DOI: 10.1039/c6ra20609c

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