Article pubs.acs.org/JPCB
Interaction of Lanthanide β‑Diketonate Complexes with Polyvinylpyrrolidone: Proton-Controlled Switching of Tb3+ Luminescence M. Karbowiak,* J. Cichos, and K. Buczko Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
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S Supporting Information *
ABSTRACT: In the present study the interaction of lanthanide β-diketonate complexes with polyvinylpyrrolidone (PVP) has been systematically investigated using fluorescence spectroscopy. The emission of terbium complexes with hexafluoroacetylacetone (hfa) ligands is almost completely quenched by PVP in the absence of H+ ions, while it is enhanced by more than 30 times in the presence of H+ ions. Conversely, the H+ ions increase the quenching efficiency of terbium complex with acetylacetone ligands. The strikingly different behavior of complexes with fluorinated and nonfluorinated β-diketonates is accounted for by the interaction mechanism, in which the important role of hydrogen bonds is indicated. The proposed mechanism includes ion-dipol Coulombic interactions between negative charge localized on ligands O−C−CH2−C-O group and positive charge induced in N atom of PVP pyrrolidone group, as well as interactions through hydrogen bonds formed between ligand C−O groups and PVP, occurring directly or through solvent molecules. Additional stabilizing effect, significantly influencing the binding strength of ligands with PVP, results from hydrogen bonds formed by terminal (CF3 or CH3) substituents of ligands with C−O group of PVP. Importantly, the quenching and enhancement of luminescence of terbium complexes with hfa ligands in PVP solution is a reversible process. This enables one to obtain the emission switch-OFF−ON system triggered externally by H+ ions, which can find possible application in the development of molecule-based devices.
1. INTRODUCTION Highly luminescent rare-earth (RE) β-diketonates have been the subject of intensive studies owing to their potential applications as luminescent and laser materials, efficient organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs),1 as NMR shift reagents,2 in analytical assays and as a modern antibody catalysts in biochemistry.3 The most outstanding properties of RE3+ ions are their narrow emission bands arising from the intraconfigurational 4f−4f transitions and long emission decay times. On the other hand, the mechanical properties and processability of lanthanide complexes are usually not well-matched with thin film fabrication technologies for such applications as light sources or sensors. Therefore, for practical use the RE3+-complexes are incorporated into various hosts such as zeolites, mesoporous silicates, silica sol−gels, organically modified xerogels and polymers.4−7 Polymeric matrices are of growing importance for hosting of RE3+ ions. Complexes of lanthanides with organic ligands embedded in polymeric hosts yield nanocomposite materials, © 2013 American Chemical Society
which presents excellent properties for applications in new generation light emitting devises due to their efficient luminescence, easy color-tuning ability, thermal- and photostability.5 Importantly, the luminescent properties of RE3+complexes doped into polymeric hosts are usually retained and often increased, whereas at the same time the mechanical and processability properties of such hybride structures are greatly improved. The complex molecules can be dispersed within polymeric matrix yielding the host−guest system. Alternatively, Lncomplexes can be covalently attached to the polymer backbone. This approach enables one to obtain the materials with properties which are not just the sum of properties of the individual components.8−11 Some polymers possess reactive groups that can interact with complexes, and this allows one to Received: August 20, 2013 Revised: November 8, 2013 Published: December 13, 2013 226
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switch to the sensitizer. However, the most widely utilized external input is probably that of H+ ion concentration. Protoncontrolled switching of luminescence in lanthanide complexes has been reported, e.g., in refs 27−31. Our preliminary results suggested that emission of Ln-βdiketonate complexes in the presence of PVP was strongly pHdependent. Therefore, one of the main motivations of the present work was to investigate the possibility of achievement of reversibly switched “on−of f ” system in solution as a function of pH. Herein, we initially discuss in detail the fluorescence spectroscopy results obtained for Ln-complexes in PVP solution and show that interaction of Ln-complexes with PVP molecules is a complex process. Systematic studies in which we used complexes with different stoichiometry (i.e., NEt4Tb(hfa)4 and Tb(hfa)3(H2O)2), different types of lanthanide ion (i.e., NEt4Tb(hfa)4 and NEt4Eu(hfa)4), as well as different types of ligand (i.e., Tb(hfa)3(H2O)2 and Tb(acac)3(H2O)3) enabled us to gain insight into the occurring processes and to propose the interaction mechanism of Ln−β-diketonate complexes with PVP in the presence and absence of H+ ions. The important role of hydrogen bonds in these interactions is revealed. The most appealing finding of this study is that emission of Tb3+-complexes with hfa ligands is quenched by PVP in the absence of H+ ions, but it is strongly enhanced in the presence of H+ ions. Moreover, alternating addition of H+ and OH− ions results in reversible switch-ON and switch-OFF luminescence.
attain novel properties also in systems obtained by the simple blending of complexes with the matrix. Poly(methyl-methacrylate) (PMMA), poly(vinyl alcohol) (PVA), polyethylene (PE), polystyrene (PS), polyurethanes, and polycarbonates belong to the most popular polymer matrices used as a host for luminescent RE3+-complexes. Among other polymers, polyvinylpyrrolidone (PVP) presents interesting properties as a host matrix for Ln3+ ions. PVP is a nonionic polymer soluble in water and organic solvents and possesses ampliphilic properties. Owing to its film forming ability, surface activity, good adhesive properties, stability in acidic or basic media, good biocompatibility and low toxicity, it is one of the most important water-soluble polymers, broadly applied in medicines, foods, cosmetics and a variety of industries.12 PVP interacts with organic molecules (dyes, drugs)13,14 and exhibits a strong tendency for complex formation with metal ions and inorganic salts.15−19 Therefore, the properties of Ln-complexes embedded into PVP matrix are often different than analogous complexes in a PMMA host. Recently, we have used PVP for fabrication of double-layer PMMA-PVP films doped with lanthanide β-diketonate complexes and noticed unexpectedly that PVP significantly enhanced emission intensity and increased the emission decay time of a Tb3+ tetrakis-complex with hexafluoroacetylacetonate ligand.20 Surprisingly, no such enhancement was observed for the same complex in PMMA nor for Tb-complexes with acetylacetonate or benzoiltrufluoroacetylacetonate ligands in PVP. Verlan et al.21 noticed that the luminescence intensity of Eu(tta)2(Ph3PO)2NO3 complex increased by a factor of 2 after incorporation into a PVP host. The authors suggested that an effective energy transfer from the polymer matrix to the ligands of the complex occurred. The increase of the relative luminescent intensity of the 5D0-7F2 transition of Eu3+ was also observed in complexes with hydrogen phthalate incorporated into PVP.22 Enhanced photoluminescence from the doped ZnS nanocrystals due to efficient energy transfer form the surface adsorbed PVP molecules to luminescent centers (Cu+, Mn2+) in nanocrystals was reported in ref 23. However, a more detailed study of the interaction mechanism between complexes and PVP has not been reported until now. Particularly, the interaction of PVP with Ln-complexes in solution has not been hitherto studied, to the best of our knowledge. Therefore, it is the main objective of this work to understand the interaction between Ln-complexes and PVP molecules in solution. A possible interaction of PVP with Ln-complexes in solution would offer a good opportunity for construction of multicomponent supramolecular assemblies. Of particular interest are systems in which components are bound through weak noncovalent interactions. This enables relatively easy influence on the interaction strength by external factors and modulation of the system properties. This allows one to devise, e.g., sensing and signaling devices. One property of Ln-complexes prone to such modulation is luminescence intensity. Such luminescent switches, where the emission is modulated in response to any external factor, as, e.g., the presence of ions or molecules, are of great current interest.24,25 However, the reversible modulation of the luminescence intensity of lanthanide complexes still remains challenging, and there is difficulty in designing suitable complexes with a luminescent on/off switch. Terai et al.26 proposed that lanthanide luminescence can be modulated by photoinduced intramolecular electron transfer (PeT) from the
2. EXPERIMENTAL SECTION Polyvinylpyrrolidone K 15 (Fluka), MW ∼10 000 and polymerization number n = 90, was used in the study. The stock solutions of different concentration (1 × 10−4 M − 1 M) were obtained by dissolving appropriate amounts of PVP in spectroscopic grade ethanol. The (N(C2H5)4)[Ln(hfa)4] (Ln = Eu or Tb), denoted as NEt4Ln(hfa)4, and the (N(C2H5)4)[Eu(btfac)4], denoted as NEt4Eu(btfac)4, compounds were prepared by reaction of lanthanide trichloride hexahydrate, LnCl3·6H2O, with hexafluoroacetylacetone (hfac, 1,1,1,5,5,5-hexafluoro-2,4-pentanedione) or benzoyltrifluoroacetone (hbtfac, 4,4,4-trifluoro-1phenyl-1,3-butanedione) and N(C2H5)4Cl monohydrate in ethanol, using an analogous procedure as described elsewhere.32 Tb(acac)3(H2O)3 (acac, 2,4-pentanedione) and Tb(hfa)3(H2O)2 were synthesized using reported procedures.33,34 The details of synthesis and analysis results are presented in the Supporting Information (SI). In order to avoid the inner filter effect, the concentration of Ln-complexes for fluorescence measurements was kept at 1 × 10−5 M. Even for such diluted solutions, the green emission of Tb3+ or red emission of Eu3+ was easily visible with the naked eye. Corrected emission and excitation spectra and luminescence decay times have been recorded on an Edinburgh Instruments FLSP 920 spectrofluorimeter, equipped with a 450 W xenon lamp, microsecond flashlamp, a red-sensitive photomultiplier (Hamamatsu R-928) and an Oxford nitrogen cryostat. For lowtemperature fluorescence measurements, the complexes were dissolved in EPA (diethyl eter:isopentane:ethanol 5:5:2 by volume). 3. RESULTS AND DISCUSSION 3.1. Luminescence Quenching of Ln-Complexes by PVP. Our preliminary studies suggested that emission of 227
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lanthanide complexes with β-diketonate ligands was substantially quenched in the presence of PVP. Since the interaction of Ln-complexes with polymer macromolecule is expected to be a complex process, systematic experimental studies have been undertaken in order to obtain data enabling putting forward the quenching mechanism. Complexes of Tb3+ and Eu3+ ions with different ligands and of different stoichiometry have been chosen for these experiments. For spectroscopic properties of β-diketonate complexes in the absence of PVP, see, e.g., ref 1 and references cited therein. Quenching of Tb3+ and Eu3+ Emission in Complexes with hfa. Figure 1 presents the photoluminescence (PL) and
emission band at 614 nm are also noticeable. Similarly as for Tb-complex, emission quenching is accompanied by disappearance of the hfa band in excitation spectra (Figure S1A, SI). Influence of PVP on emission and excitation spectra of Tb(hfa)3(H2O)2 is analogous to that observed for NEt4Tb(hfa)4 complex. Decreasing of Tb3+ emission intensity is accompanied by waning of the hfa absorption band in excitation spectra (Figure S2A,B, SI). Figure 2 presents the relative emission intensity of Tb3+ and Eu3+ ions in complexes with hfa ligand as a function of PVP
Figure 2. S−V plot of F0/F against [PVP] for emission quenching of (a) NEt4Tb(hfa)4, (b) Tb(hfa)3(H2O)2, and (c) NEt4Eu(hfa)4 complexes by PVP in ethanol. The inset shows the magnified region of low PVP concentration.
Figure 1. Photoluminescence (PL) (λexc = 300 nm) and photoluminescence excitation (PLE) (λem = 543 nm) spectra of NEt4Tb(hfa)4 complex in ethanolic solution during titration with PVP. The concentration of NEt4Tb(hfa)4 was kept at 1 × 10−2 mM and concentration of PVP in solution was increased from 0 to 0.56 mM. The inset presents the PLE spectra of Tb3+ emission at 543 nm recorded in the presence (0.56 mM) and in the absence of PVP, normalized to the same intensity.
concentration. The quenching of Eu-complex emission (Figure 2, curve c) is obviously far more efficient than of Tb-complexes. The emission quenching of Tb(hfa)3(H2O)2 (curve b in Figure 3) appears to be somewhat more efficient than for NEt4Tb(hfa)4, but significantly less efficient than for NEt4Eu(hfa)4 complex. The plots in Figure 2 correspond to the Stern− Volmer (S−V) equation F0/F = 1 + KSV[PVP] but, as it will be discussed in the next section, the quenching process is more complex and can not be described assuming a static or/and dynamic quenching. As so, the plots presented in Figure 2 will serve rather for comparison purposes of the different systems studied in this paper. The steep decrease of intensity is observed at the very beginning of plots (inset in Figure 2), but at higher concentration of PVP the intensity seems to decrease monotonically. The more detailed inspection of the plots reveal, however, that actually three or four regions of linear dependence can be distinguished. It is better seen in Figures 3A, 4A, and 5 presenting the logarithmic dependence of (F0 − F)/F on [PVP]. Such dependence should be linear for a simple equation B + nQ = BQn, describing, for example, the binding of a fluorescent molecule B and a quencher Q. If there is more than one linear region, it may be indicative of more complex binding mechanism and may suggest the existence of various forms of the complex. The time constants derived from analysis of Tb3+ (Figure S1 and S2C, SI) and Eu3+ (Figure S1C, SI) emission decay curves for different concentrations of PVP in solutions, as well as an average lifetime ⟨τ⟩ (⟨τ⟩ = ∑Aiτ2i /∑Aiτi, where τi is the ith component decay time, and Ai is a weighted amplitude), are
photoluminescence excitation (PLE) spectra recorded for NEt4Tb(hfa)4 complex in ethanolic solution during titration with PVP. The narrow emission bands at 489 and 543 nm are due to the 5D4−7F6 and 5D4−7F5 transitions of Tb3+, whereas the broad band emerging at about 385 nm for more concentrated PVP solutions is attributed to the PVP fluorescence. Although the shape of Tb3+ emission peaks does not change with increasing concentration of PVP, the emission intensity is strongly diminished, and in 0.56 mM solution of PVP, it is almost 10 times lower than in the absence of PVP. The quenching of Tb3+ emission is accompanied by decreasing intensity of the band observed at excitation spectra at about 300 nm, assigned to hfa ligand absorption. Additionally, in the presence of PVP, the red shift of the maximum of the excitation band is observed, and for higher concentration of PVP, a new band appears at about 250 nm (inset in Figure 1). Changes observed in emission spectra recorded under excitation at 320 nm for ethanolic solution of NEt4Eu(hfa)4 following incremental addition of PVP show that also the Eu3+ emission is strongly quenched (Figure S1A, SI). For [PVP] = 0.83 mM the emission intensity is more that 60 times lower than for the initial solution. Some changes in the shape of the 228
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Figure 3. Logarithmic plots of (A) emission quenching and (B) emission decay times of Tb3+ in ethanolic solution of NEt4Tb(hfa)4 (1 × 10−3 mM) treated with different concentrations (0−0.56 mM) of PVP (λex = 300 nm, λem = 543 nm).
Figure 4. Logarithmic plots of (A) emission quenching and (B) emission decay times of Eu3+ in ethanolic solution of NEt4Eu(hfa)4 (1 × 10−3 mM) treated with different concentrations (0−0.83 mM) of PVP (λex = 305 nm, λem = 614 nm).
shown in Figures 3B and 4B. Since the data cover a large range of values, the logarithmic scale is used in the figures. Based on different slopes of τ1 − τ3 and ⟨τ⟩, four regions of linear dependence, as marked in Figure 3B, can be easily discerned in curves obtained for NEt4Tb(hfa)4. Importantly, the concentration ranges identified in Figure 3B are well correlated with those distinguished on the curves presenting the log(F0 − F)/F on [PVP] dependence (Figure 3A), with the exception that the last region ([PVP] > 0.11 mM) is clearly discernible only in the former case. The curves obtained for NEt4Eu(hfa)4 (Figure 4B) are more smooth as compared to those of NEt4Tb(hfa)4 (Figure 3B), and four regions of linear dependence can be unambiguously identified. It is worth noticing that, also in this case, the PVP concentration regions derived from analysis of intensity curves in Figure 4A match exactly those obtained from the decay time curves depicted in Figure 4B. Remarkably, these distinct PVP concentration regions also match very closely those identified for NEt4Tb(hfa)4 complex (Figure 3) and Tb(hfa)3(H2O)2 complex (Figure 5). Similarly for Tb(hfa)3(H2O)2 complex, the three regions of linear dependence, which can be distinguished in the curve showing log(F0 − F)/F as a function of log[PVP] (Figure 5),
Figure 5. Logarithmic plot of emission quenching of Tb3+ in ethanolic solution of Tb(hfa)3(H2O)2 (1 × 10−3 mM) treated with different concentrations (0−0.56 mM) of PVP (λex = 320 nm, λem = 544 nm).
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Figure 6. (A) S−V plot of F0/F against [PVP] for emission quenching of Tb(acac)3(H2O)3 in the absence (a) and in the presence (b) of HCl. The inset (i) shows the magnified region of low PVP concentration. The inset (ii) compares the S−V curves for Tb(acac)3(H2O)3 (a) and Tb(hfa)3(H2O)2 (b). (B) Logarithmic plots of (a) emission quenching and (b) emission decay times of Tb3+ in ethanolic solution of Tb(acac)3(H2O)3 (1 × 10−3 mM) treated with different concentrations (0−0.22 mM) of PVP (λex = 320 nm, λem = 546 nm).
overlap exactly with regions identified considering the dependence of emission decay time constants on PVP concentration (Figure S2D, SI). This indicates that observed changes in relative emission intensity and emission decay times as a function of PVP concentration reflect the real processes occurring in solution. It also suggests that the interaction of Ln-complexes with PVP is a multistep process. This will be discussed in more detail below. Quenching of Tb3+ Emission in Tb(acac)3(H2O)3 Complex. The emission spectra recorded during titration of Tb(acac)3(H2O)3 with PVP show that Tb3+ emission is strongly quenched (Figure S4A, SI). Analogously to other systems, this is accompanied by disappearing of the acac absorption band in excitation spectra (Figure S4B, SI). Figure 6A shows the dependence of relative Tb3+ emission intensity on PVP concentration. The quenching of Tb(acac)3(H2O)3 is far more efficient than of its counterpart with hfa ligand, and already for PVP concentration as low as 5.6 × 10−4 mM the intensity drops by 37% of its initial value (insets in Figure 6A). Three linear regions, encompassing the same PVP concentration ranges as for Tb(hfa)3(H2O)2 (Figure 5), can be distinguished on the emission intensity curve shown in Figure 6B (curve a). The emission decay curves are single exponential in the whole range of PVP concentration, and the dependence of τ on log[PVP] reveals three ranges matching exactly those identified on the intensity curve (Figure 6B). The emission quenching of Tb(acac)3(H2O)3 by PVP is significantly enhanced in the presence of HCl. Changes in the relative intensity of Tb3+ emission in the solution containing Tb(acac)3(H2O)3 and HCl (0.55 μmol) titrated with PVP are compared in Figure 6A with the curve obtained in the absence of HCl. The approximated stoichiometry between Tb(acac)3(H2O)3, PVP and HCl was determined using fluorescence titration curves. The results shown in Figure 7A were obtained during titration with PVP of the ethanolic solution containing Tb(acac)3(H2O)3 and 0.55 μmol of HCl. The curves of F0/F versus PVP content show the inflection points after addition of 0.0033, 0.0067, and 0.011 μmol of PVP to the solution containing 0.01, 0.02, or 0.03 μmol of Tb(acac)3(H2O)3,
Figure 7. (A) Changes in relative emission intensity during titration with PVP of ethanolic solutions (V = 2 mL) containing 0.55 μmol of HCl and (a) 0.01 μmol, (b) 0.02 μmol, or (c) 0.03 μmol of Tb(acac)3(H2O)3. (B) Changes in relative emission intensity during titration with HCl of ethanolic solutions (V = 2 mL) containing (a) 0.02 μmol of Tb(acac)3(H2O)3 and 5.6 × 10−4 μmol of PVP, (b) 0.02 μmol of Tb(acac)3(H2O)3 and 1.1 × 10−3 μmol of PVP, (c) 0.02 μmol of Tb(acac)3(H2O)3 and 5.6 × 10−3 μmol of PVP, and (d) 0.03 μmol of Tb(acac)3(H2O)3 and 0.0011 μmol of PVP.
respectively. This corresponds, on average, to about 2.9 mol of Tb-complex bounded to 1 mol of PVP. The F0/F curves obtained during titration of 0.02 μmol of Tb(acac) 3 (H 2 O) 3 with HCl in the ethanolic solution containing 5.6 × 10−4, 1.1 × 10−3, or 5.6 × 10−3 μmol of PVP are presented in Figure 7B. In all cases, the inflection points are observed after addition of 0.22 μmol of HCl. It may seem, therefore, that there is no correlation between concentration of PVP in solution and amount of HCl required to maximizing the quenching effect. On the other hand, the amount of HCl needed clearly depends on the content of Tbcomplex. To achieve minimum intensity 0.220 or 0.286 μmol of HCl are required for 0.02 or 0.03 μmol of complex, respectively, which corresponds to 6.6 μmol of HCl per 1 230
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μmol of Tb(acac)3(H2O)3. Since there are three acac ligands binding with Tb3+ ion through six CO groups, it seems that a relationship exists between the number of coordinating atoms and moles of H+ required for emission quenching. These findings provide some insight into mechanism of interaction, which will be discussed in detail in the following sections. 3.2. Enhancement of Luminescence of Tb-Complexes with hfa in the Presence of PVP and H+. It is shown in previous sections that PVP is an efficient quencher of emission of NEt4Tb(hfa)4, NEt4Eu(hfa)4, Tb(hfa)3(H2O)2, and Tb(acac)3(H2O)3 complexes. In the case of Tb(acac)3(H2O)3 the PVP quenching effect is even more pronounced in the presence than in the absence of H+ ions. Surprisingly, the effect of H+ ions is quite the opposite for Tb-complexes with hfa ligand: not only is the emission not decreased, but it is even significantly enhanced by PVP in the presence of H+. Figure 8 shows that emission of NEt4Tb(hfa)4 in solution containing PVP (0.22 μmol) is, in the presence of HCl (0.22
PVP. However, also in other experiments it is observed that an excess of PVP added to acidified NEt4Tb(hfa)4 solution diminishes emission intensity, and a similar effect is also observed if an excess of HCl is added to solution (inset (ii) in Figure 8). The important outcome is that to achieve intensity enhancement, the sufficient amount of HCl relative to PVP must be present in solution. If the amount of H+ is insufficient, the quenching instead of enhancing effect of PVP is observed. The binding stoichiometry between Tb-complexes and PVP was obtained using a fluorescence titration (a modified Job’s method of continuous variations35). Ethanolic solutions containing various amounts of NEt 4 Tb(hfa) 4 or Tb(hfa)3(H2O)2 were acidified with the same amount of HCl (0.55 μmol) and titrated with PVP solution. The changes in relative intensity as a function of PVP molar fraction (xPVP) in solution are presented in Figure 9a,d. In all cases, the maximum intensity was achieved for xPVP = 0.9, which corresponds to the 1:9 stoichiometry between the Tb-complex and PVP polymer. In the next experiment, solutions containing various amounts of NEt4Tb(hfa)4 or Tb(hfa)3(H2O)2 and the same amount of PVP (0.222 μmol) were titrated with HCl (1.1 × 10−3 M). The fluorescence intensity was maximized after addition of 0.396, 0.484, and 0.572 μmol of HCl to the solution containing 0.01, 0.02, and 0.03 μmol of NEt4Tb(hfa)4, respectively (Figure 9b), which corresponds to 8.8 mol of H+ ions required per 1 mol of NEt4Tb(hfa)4. This experiment shows also that to bind entirely the complex to PVP the concentration of HCl of about 1.5 × 10−4 M must be achieved in solution. At the same time, this concentration is not dependent on the PVP/Tb ratio, but it is related only with the content of Tb-complex in solution. In an analogous experiment (Figure 9e), it is determined that between 7.7 and 8.8 mol of HCl is required per 1 mol of the Tb(hfa)3(H2O)2 complex. In order to determine the binding stoichiometry between H+ and PVP, various amounts of PVP were added to the solution of NEt4Tb(hfa)4 (0.02 μmol) and titrated with HCl (Figure 9c). To achieve the maximum intensity, 0.310, 0.396, and 0.484 μmol of HCl must be added to the solution containing 0.044, 0.133, and 0.220 μmol of PVP, respectively, which yields a H+/ PVP stoichiometry of 1:1. The same stoichiometry between PVP and H+ was evaluated for Tb(hfa)3(H2O)2 complex from analysis of curves shown in Figure 9f. To conclude, the fluorescence titration indicates that about 9 mol of PVP is interacting with 1 mol of NEt4Tb(hfa)4 complex in the presence of 8.8 mol of H+ ions. In the case of Tb(hfa)3(H2O)2, the determined overall binding stoichiometry corresponds to 9 mol of PVP and 7.7−8.8 mol of H+ ions per 1 mol of the complex. Regardless of whether a solution containing Tb-complex and HCl is titrated with PVP or a solution containing Tb-complex and PVP is titrated with HCl, the same emission intensity is observed for the same composition. In both cases, the average lifetime increases from 13 to 150 μs for NEt4Tb(hfa)4 complex or from 11.2 to 160 μs for Tb(hfa)3(H2O)2 complex (Figure S9, SI). The presence of PVP is a crucial factor since addition of HCl to solution of Tb-complex in the absence of PVP affects neither the emission decay time nor emission intensity. 3.3. Mechanism. It is well-known that PVP may form complexes with various organic and inorganic compounds, such as phenols and other aromatic compounds,36,37 organic acids,38 dyes,39,13,14 inorganic anions,40 iodine I2 and KI3.41 Due to the presence of CO group PVP can coordinate metal ions.42 Delocalization of the nonboding 2p2 and 2s2 electrons present
Figure 8. Changes in emission spectra (λexc = 310 nm) recorded for NEt4Tb(hfa)4 (0.01 mM) in ethanolic solution (V = 2 mL) containing 0.22 μmol of PVP in the presence of increasing amounts of HCl (0− 0.22 mM). Inset (i) presents changes in intensity of Tb3+ emission monitored for NEt4Tb(hfa)4 (0.01 mM) solution containing initially 0.33 μmol of HCl following incremental addition of PVP. Arrows indicate points where additional amounts of HCl were added. Inset (i) shows the relative intensity of Tb3+ emission as a function of HCl concentration.
mM), about 14 times more intense than for solution without HCl. Similarly, addition of HCl to solution of Tb(hfa)3(H2O)2 in the presence of PVP significantly increases the emission intensity. Importantly, the intensity of emission is also several times higher than for initial NEt4Tb(hfa)4 or Tb(hfa)3(H2O)2 solutions (without PVP and HCl). The increase in intensity is accompanied by the recurrence of hfa absorption band in excitation spectra (Figure S5, SI). Inset (i) in Figure 8 presents changes in Tb3+ emission intensity upon incremental addition of PVP to NEt4Tb(hfa)4 solution containing initially 0.33 μmol of HCl. Intensity becomes higher with increasing concentration of PVP, but a sudden drop is observed after addition of 0.33 μmol of PVP. Additional portion of HCl (0.33 μmol) recurrences intensity, but subsequent addition of PVP diminishes it. Emission can be restored again by addition of HCl (0.11 μmol and 0.22 μmol), but its intensity is lower than that achieved for 0.2−0.3 μmol of 231
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Figure 9. Fluorescence titration curves obtained for (a) solution of NEt4Tb(hfa)4 (0.01, 0.02, or 0.03 μmol) containing 0.55 μmol of HCl titrated with PVP; (b) solution of NEt4Tb(hfa)4 (0.01, 0.02, or 0.03 μmol) containing 0.22 μmol of PVP titrated with HCl; (c) solution of NEt4Tb(hfa)4 (0.02 μmol) containing 0.044, 0.133, or 0.222 μmol of PVP titrated with HCl; (d−f) the same as a-c) but using solution of Tb(hfa)3(H2O)2. The solution volume was 2 mL, emission was excited at 300 nm and observed at 543 nm. In figures a and d, x is a molar fraction of PVP in solution.
enable one to propose the mechanism of interaction of lanthanide β-diketonate complexes with PVP. Interaction of Lanthanide β-Diketonate Complexes with PVP in the Absence of H+ Ions. First of all, it should be realized that quenching of emission of β-diketonate complexes in the presence of PVP is neither the typical static nor dynamic quenching process. In the first case, the lifetime of fluorescence molecule should not change in the presence of the quencher. If dynamic quenching occurs, both the lifetimes and intensities should decrease proportionally. In our case, the intensity is suppressed in the presence of PVP, but in contrast, the emission lifetime is increased (Figures 3B, 4B, and 6B). Hence, a drop in intensity is not due to the appearance of any additional nonradiative relaxation process within the 4fN configuration. Reversely, the intrinsic quantum efficiency, QLnLn, increases in the presence of PVP. Therefore, the quenching must be due to suppression of the energy transfer process from light harvesting ligands to Ln3+ ions, which decreases the overall sensitization efficiency, ηsens, and results in a drop of overall quantum yield accordingly to the relation: QLnL = ηsens*QLnLn.45 The observed waning of ligand absorption bands in excitation spectra of complexes in the presence of PVP provides a hard proof for such interpretation. The increase of Tb3+ emission decay times suggests that efficiency of nonradiative relaxation processes is in fact reduced. Most probably it results from increased rigidity of the surrounding Tb3+ ions due to interactions with PVP molecule, which may also act as an chelating ligand, saturating the metal coordination sites and separating Tb3+ ions from quenching interactions with solvent molecules. It is also possible, especially for more concentrated PVP solutions, that Tb3+ ion is enwrapped by more than one PVP molecule. Finally, the
on nitrogen and oxygen atoms yields resonance structures. The shift of electronic density toward oxygen atom of CO group results in a tautomeric structure with partial negative charge on oxygen atom and partial positive charge on nitrogen atom.43 Owing to this, PVP possesses the ability to coordinate both cations and anions. Depending on the arrangement of side pyrrolidone groups, PVP may exist in various conformations. It is believed that syndiotactic conformations (s-PVP) are more stable than isotactic (i-PVP).44 The latter may be stabilized through hydrogen bond interactions or coordination interactions with, e.g., metal ions. The experimental data presented in previous sections show that emission of Tb- and Eu-complexes with hfa ligand, i.e., NEt4Tb(hfa)4, Tb(hfa)3(H2O)2, and NEt4Eu(hfa)4, is strongly quenched in the presence of PVP. At the same time, the significant differences are observed in quenching efficiency: the quenching is the most efficient for NEt4Eu(hfa)4 and the least efficient for NEt4Tb(hfa)4. Moreover, the quenching efficiency is much higher for Tb(acac)3(H2O)3 than for Tb-complexes with hfa ligand. Addition of HCl to solution containing Lncomplex and PVP strongly enhances emission of NEt4Tb(hfa)4 and Tb(hfa)3(H2O)2, whereas the opposite effect is observed for Tb(acac)3(H2O)3: its emission is quenched even much more efficiently than in the absence of HCl. In the case of NEt4Eu(hfa)4, the addition of HCl restores emission, but enhancement effect is not observed, and the emission intensity is, in the presence of PVP and HCl, roughly the same as for the initial solution of the complex. Finally, emission intensity of NEt4Eu(btfac)4 remains practically unaffected by PVP neither in the presence nor absence of H+ ions. These experimental findings completed with determined binding stoichiometries 232
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increase of Tb3+ emission decay time may result from alteration of rates of energy back transfer from the Tb3+ ion onto the ligand. Taking into account the above experimental facts, two probable paths of quenching of Tb-complex emission by PVP should be considered. One possible assumption is that ligands remain coordinated to the Tb3+ ion, but owing to interaction of Tb-complex with PVP, the energy absorbed by ligand is not transferred to Tb3+ levels. The additional relaxation pathway appears, and excited levels of hfa ligand are deactivated nonradiatively, possibly with participation of PVP levels. Alternatively, one may consider that Tb-complex undergoes decomposition in the presence of PVP in solution. The distance between ligands and Tb3+ ions becomes too large to allow for a transfer of excitation energy. In emission spectra recorded for solution of NEt4Tb(hfa)4 in EPA at 120 K, the f−f transitions of Tb3+ are observed exclusively (Figure 10). At 77 K the phosphorescence of hfa
only f−f transitions of Eu3+ are observed in the emission spectra recorded at 77 K in the absence as well as in the presence of PVP. This implies that, unlike others, the NEt4Eu(btfac)4 complex is stable in the presence of PVP. Binding of Ln3+ ions to the PVP polymer is not an unexpected process. PVP possesses the CO group with induced partial negative charge on it, which facilitates coordination of metal cations. Then one may assume that PVP coordinates the Tb3+ ions, which, in consequence, shifts the Tb(hfa)4− ↔ Tb3+ + 4hfa− equilibrium, existing in solution of NEt4Tb(hfa)4, toward the dissociated form of the complex. Keeping in mind this supposition, it is of interest to follow to what extent this process is mirrored in excitation spectra. Two main bands, with maxima at 251 and 328 nm can be distinguished in excitation spectra of PVP fluorescence monitored at 400 nm (Figure 11, curves e and f). Analogously,
Figure 11. Excitation spectra recorded for Tb3+ emission at 543 nm and PVP emission at 400 nm in solutions containing NEt4Tb(hfa)4 complex (2 mL, 0.02 mM) and different amounts of PVP.
Figure 10. Emission spectra recorded under excitation at 305 nm for EPA solutions of (a) NEt4Tb(hfa)4 at 120 K, (b) NEt4Tb(hfa)4 at 77 K, (c) NEt4Tb(hfa)4 in the presence of PVP at 77 K, and (d) NEt4Tb(hfa)4 in the presence of both PVP and HCl at 77 K.
two bands are observed in excitation spectra recorded for pure PVP solution (Figure S11, SI). The relative intensity of the band at a shorter wavelength decreases for higher concentration of PVP. The emission spectrum of PVP recorded under excitation at 245 nm is shifted toward UV, as compared to that recorded under 325 nm excitation (Figure S10, SI), which implies the existence of at least two different forms of PVP. It has been suggested23 that the excitation band at ∼250 nm arises from the enol-tautomer of PVP, whereas that at a longer wavelength from the keto form of pyrrolidone moiety. Excitation and emission spectra measured at 77 K for PVP solution suggests the existence of one more form, labeled as PVP-A (Figure S11, SI). The excitation band characteristic for this form is observed at ∼275 nm, and its distinguishing feature is a weak phosphorescence detected at 77 K. Addition of PVP to NEt4Tb(hfa)4 solution initially increases the intensity of the Tb3+ excitation band at about 325 nm (Figure 11, curve b). This band is likely to correspond to that attributed to the keto form and observed in the excitation spectrum of PVP at 328 nm. The occurrence of hfa absorption bands at 299 and 310 nm shows that energy transfer from the hfa ligand is still present, which suggests that PVP only partially substituted for hfa in the Tb-complex. At higher concentration of PVP, the intensity of 325 nm band gradually decreases, and a new broad band with maximum at about 266 nm appears,
ligand additionally appears, but its intensity is weak. By contrast, if PVP is present in solution, the 77 K spectrum is dominated by a very strong phosphorescence of hfa, whereas the f−f transitions of Tb3+ are barely visible, only as very faint lines. Hence, the excitation energy absorbed by hfa ligand is neither transferred to Tb3+ levels nor relaxed nonradiatively. If no additional relaxation process appears the energy transfer from hfa ligand to Tb3+ ion must be switched-off. This means that, in the presence of PVP, the Tb-complex undergoes decomposition. In the absence of PVP in solution, the equilibrium, Tb(hfa)4− ↔ Tb3+ + 4hfa−, is shifted almost completely to the left, toward complex formation. The observed weak phosphorescence is due to the presence of only small amounts of free ligand being in the equilibrium with the complex. To shift this equilibrium toward dissociated forms, PVP must bind Tb3+ ions or hfa ligands or both of them. Similarly, for solution of NEt4Eu(hfa)4, Tb(hfa)3(H2O)2, and Tb(acac)3(H2O)3 complexes, the ligand phosphorescene observed at 77 K is very weak in the absence and strongly enhanced in the presence of PVP. This suggests that the quenching mechanism is analogous to that for NEt4Tb(hfa)4 complex. On the other hand, in the case of NEt4Eu(btfac)4, 233
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curves presenting log(F0 − F)/F as a function of log[PVP], extending in both cases in the same ranges of PVP concentration: (i) [PVP] < 5 × 10−4 mM, (ii) 5 × 10−4 mM < [PVP] < 0.017 mM and (iii) [PVP] > 0.017 mM (Figure 5 and Figure 6). Exactly the same characteristic points are observed for tetrakis complexes (Figure 3 and Figure 4), with the only difference that the last region is split into two ones: 0.017 mM < [PVP] < 0.11 mM and [PVP] > 0.11 mM for NEt4Tb(hfa)4, and 0.017 mM < [PVP] < 0.17 mM and [PVP] > 0.17 mM for NEt4Eu(hfa)4. In all cases, the regions distinguished on the intensity curves match very well those identified from analysis of the decay time constants (Figures 3, 4, 5, 6, and S2D). In the first region, i.e., for [PVP] < 5 × 10−4 mM, the initial most sudden drop of emission intensity is observed in all cases (inset in Figure 2). It would be tempting to correlate the number of distinguished linear regions with the stoichiometry of complexesthree and four regions were found for tris and tetrakis complexes, respectivelyand to consider that they mirror the successive substitution of ligands by PVP. On the other hand, taking into account the different values of formation constants, substitution of successive ligands is expected to occur at different PVP concentrations, whereas the identified linear ranges are exactly the same for Tb(hfa)3(H2O)2 and Tb(acac)3(H2O)3 (Figure 5 and Figure 6) and only a minor difference is observed between NEt4Tb(hfa)4 and NEt4Eu(hfa)4 (Figure 3 and Figure 4). Hence, more likely is that the PVP concentration ranges distinguished on log(F0 − F)/F versus log[PVP] curves do not reflect successive substitution of β-diketonate ligands, but rather reveal formation of PVP−Tb complexes of different conformations. In spite of similarities between log(F0 − F)/F versus log[PVP] curves, there are significant differences in quenching efficiency, particularly between Tb(acac)3(H2O)3 complex and those with hfa ligand. The observed effect is too large to result solely from differences in formation constants of Tb-hfa and Tb-acac complexes. Moreover, the Tb-complexes with βdiketonate ligands are relatively stable, e.g., for Tb(acac)3(H2O)3 the formation constants are log β1 = 6.0, log β2 =10.6, and log β3 =14.46 Therefore, it is to some extent surprising, that more stable complexes are formed between Tb3+ and PVP, even assuming a possible stronger chelate effect of PVP. Therefore, besides binding of Tb3+ ions to PVP, the interaction of ligands with PVP polymer should be considered as an additional important factor causing decomposition of Tb−β-diketonate complexes. The expected differences in interaction strength between PVP and hfa or acac ligands would account for a different quenching efficiency, even if formation constants of Tb-complexes with both ligands are similar. In accord with these expectations, analysis of phosphorescence and phosphorescence excitation spectra (Figure S9 and S10, SI) recorded for hfa ligand and Ln-hfa complexes in the absence as well as in the presence of PVP, provides direct evidence that not only the Ln-hfa complexes undergo decomposition in the presence of PVP, but also that the hfa ligand is bounded to the polymer. The mechanism of interaction of PVP with anions is more complicated than with cations. Although the anion binding ability of PVP is well established now, at the same time, depending on the type of anion, solvent, etc., a variety of possible mechanisms of interactions are proposed, e.g., binding
which is most closely related to the band assigned as PVP-A in the excitation spectrum of PVP polymer (Figure S11, SI). A shoulder observed at the shorter wavelength side of this band seems to correspond to the band observed in the excitation spectrum of PVP at 251 nm, attributed to the enol form. It can be assumed that the band at 266 nm, the position of which does not change with increasing concentration of PVP, corresponds to Tb-PVP complex forming when all hfa ligands are substituted by PVP. The proposed assignment is further supported by changes observed in Tb3+ emission decay times. For pure solution of NEt4Tb(hfa)4 complex, the emission at 543 nm decays with τ = 13 μs. In the presence of PVP the second, and for [PVP] > 0.028 mM also the third component appear on the decay curve recorded under excitation at 310 nm, with time constants increasing from 40 to 90 μs and from 160 to 600 μs, respectively, as the concentration of PVP increases (Figure S1a, SI). Such continuous changes in emission decay times accompanying the alteration of excitation bands (Figure 11) indicate that substitution of hfa by PVP is a step-by-step process. By contrast, the behavior of the emission decay recorded under excitation at 250 nm is completely different (Figure S1B, SI). The decay is single-exponential in the whole range of PVP concentration, with time constants of 1300−1400 μs. It shows that contrary to 310 nm band, the excitation band at 250 nm is related with a single well-defined form resulting from binding of Tb3+ to PVP. It is not excluded that conformation of PVP binding Tb3+ ions differs from conformation of free PVP in solution. The excitation bands of PVP interacting with Tb3+ are located at higher energy than those of free PVP. In fact, they appear at energy close to that observed for the form exhibiting phosphorescence (PVP-A, Figure S11, SI). Depending on the arrangement of side pyrrolidone groups, PVP may exist in various conformations. It is believed that in solution, the syndiotactic s-PVP is the most stable conformation. Less stable isotactic i-PVP form can be stabilized through hydrogen bond interactions or interactions with metal ions, if, e.g., one metal ion is coordinated trough CO groups from two pyrrolidone moieties adjacent in the PVP chain.44 Hence, the band appearing at 260 nm, which intensity increases upon coordination of Tb3+ ions, may be characteristic for the i-PVP. Actually, excitation bands of PVP may be composed of a number of overlapping bands corresponding to different PVP conformations with similar structure of energy levels. It is observed that if excitation wavelength is varied within the 315− 335 nm range, the emission band of PVP is also shifted accordingly. On the other hand, emission observed under excitation at higher energy band (250 − 285 nm) is almost independent of the excitation wavelength. Importantly, the appearance of a new band (at 266 nm) in the excitation spectra of Tb3+, located outside the range of hfa absorption, presents a direct proof for occurrence of excitation energy transfer from PVP to Tb3+ ions. This process requires the short distance between Tb3+ and pyrrolidone moieties, and thus provides evidence that Tb3+ ions are bounded to PVP. At the same time, disappearance of hfa bands in excitation spectra of Tb3+ emission shows that hfa ligands are completely abstracted from Tb3+ ions. The influence of PVP on excitation spectra of NEt4Eu(hfa)4 and Tb(hfa)3(H2O)2 is analogous to that described above (Figures S1 and S2, SI). In the case of tris Tb(hfa)3(H2O)2 and Tb(acac)3(H2O)3 complexes, three linear regions can be distinguished on the 234
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ligand complex. Formation of hydrogen bonds is necessary for shifting the appropriate equilibrium toward binding of ligand with PVP, as is observed for hfa and acac. If stabilization through hydrogen bonds is not sufficient, the ligand remains coordinated to the metal ion, which is the case for btfac ligand. Considering formation of hydrogen bonds between PVP and ligands, three possible mechanisms should be evoked: (i) the hydrogen bonds can be formed between ligand C−O groups (acceptors) and CH2 or CH groups of PVP; (ii) assuming solvatation of PVP, the hydrogen bonds can be formed between ligand C−O groups (acceptors) and protons of solvent molecules (C2H5OH or H2O) bound to PVP; (iii) assuming protonation of ligand, the hydrogen bond can be formed between H+ ions coordinated to ligand C−O groups and acceptor CO groups of PVP. Taking into account the above-mentioned experimental findings, the less plausible is mechanism (iii). It does not explain why interaction of acac with PVP is stronger than that of hfa ligand. Perfluorinated alkyl groups increase the Lewis acidity, so hfaH as a stronger acid possesses larger charge on hydrogen, and should be a better hydrogen bond donor than acacH. More importantly, it also does not account for increased efficiency of interaction of acac with PVP in the presence of H+. If acac was already protonated, additional amount of H+ should have not had a significant influence. To some extent the observed differences between affinity of hfa and acac ligands to PVP can be explained by mechanisms (i) and (ii). The acac ligand is a stronger basis then hfa, and the O atom of acac’s C−O group acting as an proton acceptor should form stronger hydrogen bond. However, this dissimilarity in proton−acceptor properties is not likely to result in such large difference in interaction strength between hfa and acac as it is observed. Moreover, mechanisms (i) and (ii) still do not account for the lack of interaction of btfac ligand with PVP. The phenyl group is weakly electron-donating, thus the btfcH is a weaker acid than hfaH, but stronger than acacH. Therefore, to account for observed differences in interaction strength, it is indispensable to include participation of terminal substituents in the formation of hydrogen bonds. The CF3 groups are acceptors of proton in hydrogen bond, whereas CH3 are donors. Consequently, hfa ligand can be only an acceptor in a hydrogen bond (CF3 and CO groups), whereas acac can be an acceptor (CO) as well as a donor (CH3). Considering mechanism (i), the terminal CF3 groups of hfa ligand can form C−F···H−C hydrogen bonds with CH or CH2 groups of PVP, whereas in the case of acac, the additional possibility exists of formation of stronger C−H···OC hydrogen bonds between acac CH3 groups and CO groups of PVP (Figure S12a, SI). Therefore, in the case of acac, there are two possible ways of formation of C−O···H−C hydrogen bonds, i.e., between PVP’s CO group and ligands protons, as well as between ligands C−O group and PVP as a proton donor. In the case of hfa, there is no proton donor for a hydrogen bond with PVP’s C O groups. Assuming that solvent molecules (C2H5OH or H2O) bounded to PVP participate in the formation of hydrogen bonds (mechanism ii)) the terminal CF3 groups of hfa can form C−F···H−O bonds, whereas CH3 groups of acac the C−H··· O−H bonds (Figure S12b, SI). Also, in this case, the interaction of acac with PVP should be stronger than that of hfa ligand. Comparing mechanisms (i) and (ii), stronger interactions are expected for the latter, due to the formation of PVP>COδ‑···
of anions to positive charge induced on N atom in pyrrolidone moiety, van der Waals interactions, dispersive interactions, or hydrogen bonds.41,37 In the case of protonated β-diketonate ligands, the most plausible interaction would be through formation of hydrogen bonds. If ligand is deprotonated and exists as an anion, the Coulombic ion−dipole interaction between a negative charge localized on the ligands −C(O)−CH−C(O)- group and a positive charge induced on the tertiary nitrogen atom of pyrrolidone ring is more likely. At the same time, it cannot be excluded that both mechanisms are operating simultaneously. The binding through electrostatic interaction may be additionally stabilized by formation of hydrogen bonds between ligand and PVP. In addition, it is well-known that PVP is strongly solvated in solution. Then, the solvent molecules may participate in the formation of hydrogen bonds between PVP and ligand. In the previous section, we have indicated the strong influence of H+ ions on the emission spectra, which is also suggestive of the important role the hydrogen bonds play in interactions between PVP and Ln-complexes. The observed differences in interaction strength of PVP with NEt4Eu(hfa)4 and NEt4Eu(btfac)4 complexes provide another hints about possible mechanism. The first complex is efficiently quenched by PVP, whereas the latter is almost not at all. The formation constants are very similar for both complexes, so they are not a decisive factor. The difference must result, then, from different affinity of hfa and btfac ligands to PVP. The fluorescence study performed for the mixed solution of NEt4Tb(hfa)4 and NEt4Eu(btfac)4 confirms these expectations showing that, in contrast to hfa, the btfac ligand interacts very weekly, or does not interact at all, with PVP. The observed differences reveal also the important role of terminal R1 and R2 substituents of R1-(CO)−CH2−(CO)−R2 β-diketonate ligands. Again, in order to account for a stronger interaction of −CF3 than −C6H5 groups with PVP, the formation of hydrogen bonds must be evoked. Even larger discrepancy in susceptibility to quenching by PVP is observed between Tb(hfa) 3 (H 2 O) 2 and Tb(acac)3(H2O)3. Although the stability constants for Tb(hfa)3(H2O)2 have yet not been determined, they are not expected to be larger than for Tb(acac)3(H2O)3, since it is well established that if ligand contains electron withdrawing groups, its ability for formation of complexes with Ln3+ ions decreases. Moreover, in both cases the interaction strength of PVP with Tb3+ cations is similar. Then the differences in quenching efficiency must be accounted for by different interaction strength of hfa and acac ligands with PVP, which must result from their disparate chemical character determining the tendency for formation of hydrogen bonds. Based on this arguments the electrostatic interactions and hydrogen bonding should be considered as two main mechanisms of interaction of ligands with PVP. The latter can occur directly between ligands and PVP atoms or be mediated through solvent molecules (C2H5OH or H2O). Only hydrogen bonds, which provide additional stabilizing effect can account for observed differences in interaction strength of various ligands with PVP. In the case of pure electrostatic interaction, there should not be large differences between complexes with btfac and hfa ligands. In solution containing lanthanide β-diketonate complex and PVP, there are competitive interactions of ligands with PVP and metal cation. The solely electrostatic interaction between ligand and PVP is not strong enough to cause dissociation of the Ln235
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Figure 12. Schematic diagram showing interaction of Tb(hfa)3(H2O)2 complex with PVP leading to quenching or enhancement of Tb3+ emission depending on the absence or presence of H+ ions in solution.
Hδ+OHδ+···Oδ‑−CCOδ‑···H−CC−H···Oδ‑C(CO)2H+...F−C(CO)2H+ and H−C
