Complexation of Eu(III) with furan monocarboxylates in aqueous medium at variable temperatures: Luminescence and computational studies

Journal of Luminescence 212 (2019) 83–91

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Complexation of Eu(III) with furan monocarboxylates in aqueous medium at variable temperatures: Luminescence and computational studies

T

Satendra Kumara,∗, S. Majia, N. Ramanathana,b, K. Sundararajana,b a b

Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

ARTICLE INFO

ABSTRACT

Keywords: Luminescence Europium Furoic acid Stability constant Temperature DFT

Luminescence of Eu(III) with furan monocarboxylates (FCA) ligands namely, 2 furoate (2FCA) and 3 furoate (3FCA) have been studied in aqueous medium. Both the ligands sensitized the luminescence of Eu(III) to a different extent. The magnitude of luminescence intensity was higher for the Eu(III)-2FCA than Eu(III)-3FCA complex due to the higher absorbance of 2FCA ligand as corroborated by UV–Vis spectroscopy. The formation of 1:1 Eu(III) complexes with both the ligands was confirmed through luminescence lifetime measurements. Stability constants of these complexes, calculated using HypSpec computation program were found to increase with temperature. The coordination mode of both the ligands was found to be monodentate. Van't Hoff equation was used to calculate enthalpy of formation of Eu(III) complexes using stability constants at different temperatures (283, 298, 313, 328, 343 K). The enthalpy of complex formation of 2FCA and 3FCA with Eu(III) was endothermic. Density functional theory (DFT) and time dependent (TD)-DFT calculations were performed to corroborate experimental observations.

1. Introduction Because of unique photo physical properties, luminescent complexes of lanthanides have been extensively studied in literature [1–5]. Spectral properties such as, narrow emission, large stokes shift, long lifetime draw the attention of researchers to use the lanthanide ions in the field of biological application [6,7], sensors [8,9], lasers [10,11], dosimeters [12,13] etc. The drawback of poor absorption coefficient of lanthanides is overcome by complexing with organic ligands and populating indirectly the higher energy states of these ions efficiently by the ‘antenna effect’ [14,15]. The luminescence intensity of lanthanide ions in their complexes mainly depends on two factors: the efficiency of energy transfer from triplet energy level of ligand to emitting level of ion and the extent of non-radiative decay of excited state of lanthanide complex. The nature of metal ion, ligand, solvent etc. can influence the factors mentioned above [16]. Therefore by controlling these factors one can design an efficient luminescent lanthanide complex for specific applications [17]. Beta diketones based ligands were the first to be explored for luminescence enhancement of lanthanides [18]. Aromatic carboxylic acids are another class of ligands, which are extensively used for luminescence enhancement of lanthanides. From our group, various benzene and naphthalene based carboxylic acids have been reported as ∗

sensitizing ligands for the luminescence enhancement of Eu(III) [19–21]. Furthermore, pyridine based carboxylic acids have also been studied for the luminescence enhancement of Eu3+ in aqueous medium [22–25]. Many of these studies were carried out with the aim for the trace level detection of lanthanides in aqueous medium. The detection of these lanthanides has been achieved at the level of part per trillion (ppt) using ligand sensitized luminescence spectroscopy [26]. Coordination chemistry of lanthanide complexes in aqueous medium is another area of ongoing research, which provides fundamental understanding between the ligand and lanthanide ion interaction. Studies on such kind of interactions are very much important to understand the migration behavior of lanthanides and provide useful information for the nuclear industry and environmental sciences. The complexation behavior of lanthanides with benzene carboxylates and nitrogen based heterocyclic ligands has been extensively studied and enormous data is available in literature [27–30]. Because of natural abundance and diverse biological applications, oxygen hetro atom based rings are another important class of heterocyclic compounds [31,32]. Furan is an example of five membered aromatic ring with four carbon and one oxygen atom, which is present in many biological molecules such as Vitamin C, cyclic form of monosaccharide, honey and so on [33]. In the field of coordination chemistry, furan based ligands are barely used among the other aromatic

Corresponding author. E-mail address: [email protected] (S. Kumar).

https://doi.org/10.1016/j.jlumin.2019.04.023 Received 20 March 2019; Received in revised form 10 April 2019; Accepted 12 April 2019 Available online 15 April 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.

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ligands for complexation with lanthanide ions due to its poor coordination with metal ion. Bismondo et al. reported the complextaion studies of Th(IV) with 2 furoic and thenoic acids in aqueous medium [34]. Nevertheless, there are a few reports available in literature where furoic acid based ligands were used to synthesize europium complexes [35–37], to the best of our knowledge, a thorough luminescence study is lacking in the literature on the complexation of europium furoate system in aqueous medium. The aim of this report is to study the luminescence and complexation study of Eu(III) with, 2FCA and 3FCA ligands in aqueous medium. The luminescence data obtained from this study is used to calculate thermodynamic parameters in order to predict the feasibility of complexation process occurring in aqueous medium. DFT computations were carried out to optimize the structure of Eu(III)-FCA complexes. TD-DFT computations were also performed for the ligands in order to understand electronic transitions in ligands and to correlate with the experimental results.

samples was adjusted by the addition of sodium hydroxide (99.9%, Sigma make) and perchloric acid (70%, Sigma make). Ionic strength of the aqueous solutions was adjusted to 0.1 M by sodium perchlorate (99.99% Sigma make). 2.3. Computational details Computations were carried out using a Gaussian09 suite of program [38]. For geometry optimization, B3LYP/6–311++G (d,p) level of theory was used for lighter atoms (C,H,O) of different species. The Stuttgart RSC 1997 28 electron (small core) ECP basis set was used for Eu [39,40]. The geometry of Eu(III) complexes was optimized using the septet spin multiplicity (7F0) in the theoretical calculations without imposing any symmetry constraints [27]. Same level of theory was also applied for the calculation of vibrational frequency for each of the optimized geometry. The absence of imaginary frequencies ensured that the optimized geometries were minima on potential energy surface. The calculations in aqueous medium were performed with an implicit solvation model applied in water medium (dielectric constant-80). These computations involved the polarizable continuum model (PCM) solvation method [41] as implemented in Gaussian09 suite of program. Electronic excitations of the ligands were calculated using the Time Dependent Density Functional Theory (TD-DFT) [42,43] in water using PCM model. NBO analysis was performed using NBO 6 version invoked through Gaussian 09 [44].

2. Experimental details 2.1. Instrumentation Edinburgh spectrofluorimeter (model FLS920), having a 450 W xenon lamp as an excitation source was used to record all the steady state luminescence spectra. The samples were taken in fused silica cuvette with a path length of 10 mm. The excitation as well as emission monochromator were set for a spectral band pass of 3 nm. A long-wavelength pass filter, (UV - 350, Shimadzu) with a maximum and uniform transmittance (> 85%) above 350 nm, was placed in front of the emission monochromator, in order to reduce the scattering of the incident beam into the emission monochromator. The temperature of the solution was adjusted to different values (283–343 K) using a temperature controlled cuvette holder (TC-125, Quantum Northwest USA). Luminescence decay spectra of the excited states were recorded using the same instrument with a μs-Xe flash lamp. Luminescence lifetimes were determined by fitting the decay spectra to an exponential decay function. A single exponential fit was found to be adequate for the decay processes observed in this study. The χ2 values of all the fits ranged between 0.9 and 1.1. The relative standard deviation of the lifetime values was less than 5%. Triplet energies of the ligands (L) were calculated from the phosphorescence spectra recorded at liquid nitrogen temperature (77 K). Liquid nitrogen quartz Dewar (Edinburgh) was used to record the phosphorescence spectra. Gd3+-complexes with [Gd3+] = 1 × 10−5 M, −5 [L] = 3 × 10 M were prepared in 1:1 v/v mixture of water and ethanol. Quartz EPR tube containing 0.5 ml of sample was immersed into the nitrogen Dewar containing liquid nitrogen and the spectra were recorded. UV–Vis absorption spectra were recorded at room temperature using Avantes fiber optic spectrophotometer (AvaSpec-3648-USB2). Fused silica cuvette of path length 10 mm was used as a sample cell for recording the spectra. All the spectra were blank subtracted.

3. Results and discussion 3.1. Luminescence enhancement and energy transfer At the outset, luminescence intensity of Eu(III)-FCA was monitored with respect to ligand concentration and pH of the aqueous solution. Maximum luminescence intensity was found at 5 × 10−4 M of ligand concentration at pH 5.0 for 2 × 10−5 M of Eu(III). Therefore a concentration of FCA and pH of the solution was maintained at 5 × 10−4 M and 5.0, respectively, for luminescence studies. It should be noted that the pKa values for 2FCA and 3FCA are 3.16 and 3.9, respectively [45]. Hence, at pH 5.0, both the ligands exist completely in de-protonated form (furoate ions). Fig. 1a shows the excitation spectra of 5 × 10−3 M of Eu(III) in aqueous medium. The different peaks are due to 4f intratransitions of Eu(III) with the most intense peak appearing at 394 nm corresponds to 7F0-5L6 transition [46]. The excitation spectra of Eu(III)2FCA and Eu(III)-3FCA complexes are shown in Fig. 1b and c, respectively. The concentration of Eu(III) in these complexes is 2 × 10−5 M. The excitation spectra were recorded by monitoring the emission of Eu (III) at 592 nm. The excitation spectra did show a broad band at 260 and 252 nm for Eu(III)-2FCA and Eu(III)-3FCA, respectively, which can be attributed to π-π* absorption bands of ligands (which are discussed later in detail in Fig. 3). The absence of 4f intra transitions peak in the excitation spectra of Eu(III)-2FCA and Eu(III)-3FCA indicates the energy transfer from ligand to Eu(III) via ‘antenna effect’. Fig. 2a and b shows the emission spectra of Eu(III)-2FCA and Eu(III)-3FCA, respectively. The five emission peaks centered at 555, 591, 615, 650 and 698 nm are due to the transitions between 5D1-7F2, 5D0-7F1, 5D0-7F2, 5D0-7F3, 5 D0-7F4, respectively. The absence of emission peaks from the ligands ensured efficient energy transfer. About two order enhancement in luminescence intensity was observed in the Eu(III)-FCA complexes compared to bare Eu(III) luminescence in aqueous medium. Furthermore, the luminescence intensity of Eu(III)-2FCA was nearly six times more as compared to Eu(III)-3FCA. Fig. 3 shows the absorption spectra of both the FCA ligands. The absorption maxima (239 nm for 2FCA and 234 nm for 3FCA) are due to the π-π* absorption bands of the ligands. It can be seen from the absorption spectra that the absorbance of 2FCA at 262 nm (∼0.24) is more than that of 3FCA at 252 nm (∼0.04), which results in the enhanced luminescence intensity of the Eu(III)-2FCA compared to Eu(III)-3FCA.

2.2. Chemicals and sample preparation Eu2O3 (Indian Rare Earth Limited, India) was used to prepare the stock solution of Eu(III) in perchlorate form. Europium perchlorate was prepared by dissolving Eu2O3 powder in concentrated perchloric acid (Sigma make) and then slowly evaporated to dryness under IR lamp. The residue was then dissolved in water to get the stock solution of Eu (III) in water. This solution was acidified with a few drops of perchloric acid in order to avoid any hydroxide formation. Stock solutions of ligands, 2FCA and 3FCA (Fluka make, A.R grade) were prepared by dissolving the required amount in water. De-ionized water (18 MΩ) obtained with a Milli-Q (Millipore) system was used in preparing all solutions. Eu(III)-FCA complexes were prepared by mixing the required amount of ligand and Eu(III) from the stock solution. The pH of the 84

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Fig. 3. Absorption spectra of (a) 2FCA(5 × 10−5 M); (b) 3FCA(5 × 10−5 M) in aqueous medium, pH = 5.0. The excitation wavelength at which maximum emission occurs is shown by star.

useful to provide insights for energy transfer. The triplet state energy was calculated from the 0-0 transition of phosphorescence bands of the ligands and found to be 24870 and 24940 cm−1 for 2FCA and 3FCA, respectively. Hence, it can be concluded that the triplet level of both the ligands lies above the emitting level of Eu(III) and therefore effectively sensitizes the Eu(III) luminescence. The schematic for proposed mechanism of energy transfer is shown in Fig. S1. The luminescence decay spectra of the complexes were found to fit with a single exponential decay, which eventually indicates the presence of single specie. The luminescence lifetime (τ) was found to be nearly the same (118 μs) for both Eu(III)-2FCA and Eu(III)-3FCA complexes. The luminescence lifetime of Eu(III) is very sensitive to number of water molecules present in first coordination sphere. Therefore an empirical relationship, NH2O = 1.05/τ - 0.70, is derived in the literature between the number of average water molecules (NH2O) present in the inner coordination sphere and lifetime of Eu(III) [47]. The value of NH2O can provide direct information on stoichiometry of the Eu(III) complexes. The lifetime of bare Eu(III) in aqueous medium is 108 μs which corresponds to 9 water molecules in the inner coordination sphere (Eu(H2O)93+). The number of water molecules in both the complexes studied here was calculated to be ∼8 using the above formalism. It is therefore clear that one water molecule is exchanged with one molecule of ligand implying the formation of 1:1 complexes of Eu(III)-2FCA and Eu(III)-3FCA in aqueous medium.

Fig. 1. Excitation spectra of (a) Eu(III) (5 × 10−3 M); (b) Eu(III) (2 × 10−5 M)2FCA(5 × 10−4 M); (c) Eu(III) (2 × 10−5 M)-3FCA(5 × 10−4 M) in aqueous medium. Emission wavelength is 592 nm for all the cases.

3.2. Complexation studies The electric dipole transition of Eu(III) 5D0→7F2 is hypersensitive transition, whose intensity is sensitive to the nature of the ligands coordinated to Eu(III) than the intensities of the other transitions [46]. In previous work, the hypersensitivity of the 5D0 -7F2 transition and lifetime were utilized for studying complexation behavior of Eu(III)-benzoic acid system in acetonitrile [21]. For the present Eu(III)-FCA system, lifetime values do vary marginally from Eu(III)-aqua (108 μs) to Eu(III)-FCA (118 μs). Therefore, the sensitivity of the 5D0 - 7F2 transition, generally represented in terms of asymmetric ratio (R), which is the ratio of the intensities of the transitions I (5D0→7F2)/I (5D0→7F1) is used to study the complexation of Eu(III)-FCA system. For this purpose, the luminescence titration was performed by recording spectra of different solutions having Eu(III) concentration of 1 × 10−4 M with different metal to ligand ratio. Fig. 4 shows the variation of R for both the complexes as a function of ligand concentration. It can be seen from this figure that asymmetry ratio gradually increases as a function of ligand to metal ratio. It attributes to the fact that complexation with ligand

Fig. 2. Emission spectra of (a) Eu(III) (2 × 10−5 M)-2FCA(5 × 10−4 M); (b) Eu (III) (2 × 10−5 M)- 3FCA(5 × 10−4 M) in aqueous medium, pH = 5.0. Excitation wavelength is 260 nm for (a) and 252 nm for (b).

In order to have ‘antenna effect’, the triplet energy of the ligand must be higher than the emitting level of luminescent ion (17227 cm−1 for Eu(III)) [46]. Therefore, triplet state energy calculation of ligands is 85

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Fig. 6. Correlation between log β and pKa for complexation of Eu(III) with different mono carboxylic acids, Δ from this work (I = 0.1 M NaClO4, T = 298 K). Other values were taken from literature.

Fig. 4. Variation of asymmetry ratio (I615/I592) of Eu(III) (1 × 10−4 M) with 2FCA & 3FCA.

changes the geometry of Eu(III) ion, causing 615 nm peak to be more intense. The value of R increases from 0.54 to 0.96 for Eu(III)-2FCA and 0.64 to 1.32 for Eu(III)-3FCA as ligand to metal ratio was varied from 10 to 100. These values of asymmetry ratio were used in the HypSpec 2009 computation program as an input parameter [48] and stability constants (log β) were obtained for both 1:1 complexes. The values of log β calculated for Eu(III)-2FCA and Eu(III)-3FCA are 1.24 ± 0.02 and 1.54 ± 0.02, respectively.

Hence, it can be concluded that the interaction nature of furoates is much similar to that of other carboxylates, where coordination mode is monodentate in nature. Furthermore, lifetime data also indicated the removal of one water molecule, which strengthens our argument that the coordination mode of FCA ligands with Eu(III) is monodentate. Based on these experimental observations, the possibility of the bidentate coordination of the ligand to the metal (Fig. 5a and b) is ruled out and confirms that the coordination takes place monodentately via carboxylate oxygen (Fig. 5c) exclusively and other carboxylate oxygen of 2FCA/3FCA ligand does not participate in complexation with Eu(III).

3.3. pKa-log β relationship and mode of coordination There are three possible modes of coordination that should exist in the 1:1 complexes of Eu(III)-FCA systems as shown in Fig. 5. The coordination mode shown in Fig. 5b is precluded in Eu(III)-3FCA as ether oxygen is far away to coordinate to Eu(III). The interaction between the Eu(III) and carboxylate ligand is believed to be electrostatic in nature. Therefore a correlation can be found between the logβ of the Eu(III)ligand complex and pKa value of the ligand [49]. Fig. 6 shows the plot between the pKa and logβ of different complexes of Eu(III)-monodentate carboxylates taken from literature [28,50–52]. The data of both the Eu(III)- furoate system from this study fit nicely in this correlation.

3.4. Effect of temperature on complexation In order to calculate thermodynamic parameters, luminescence studies were performed at different temperatures. Fig. 7a and b shows the luminescence spectra of Eu(III)-2FCA and Eu(III)-3FCA, respectively at different temperatures (283, 298, 313, 328 and 343 K). The asymmetric ratio increases from 0.68 at 283 K to 1.02 at 343 K for Eu(III)2FCA whereas it increases from 0.94 at 283 K to 1.33 at 343 K for Eu (III)-3FCA complex. The log β values were calculated for Eu(III)-2FCA

Fig. 5. Possible coordination modes of FCA with Eu(III) in its 1:1 complexes. (a) bidentate with carboxylate (b) bidentate which is possible only with 2FCA ligand (caboxylate and ether oxygen) (c) monodentate. 86

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Fig. 8. Van't Hoff plots of the (a) Eu(III)-2FCA and (b) Eu(III)-3FCA complexes. Table 2 Thermodynamic parameters for the complexation of Eu(III) with FCA at T = 298 K, I = 0.1 M NaClO4.

−4

−3

Fig. 7. Luminescence spectra of Eu(III) (1 × 10 M)eFCA(5 × 10 M) as a function of temperature, I = 0.1 M NaClO4, pH-5.0. The spectra have been normalized to the luminescence intensity at 592 nm peak.

283 298 313 328 343

Eu(III)-3FCA

1.17 1.24 1.34 1.46 1.64

1.48 1.54 1.68 1.77 1.81

± ± ± ± ±

0.0.2 0.02 0.02 0.03 0.02

± ± ± ± ±

0.02 0.02 0.02 0.01 0.01

and Eu(III)-3FCA complexes by performing luminescence titration at all temperatures mentioned above and are given in Table 1. It can be discerned that the stability constants increase as a function of temperature. These values of stability constants were utilized to calculate thermodynamic parameters using the Van't Hoff equation [53]:

ln

=

H/RT +

ΔH (kJ/mol)

ΔG (kJ/mol)

TΔS (kJ/mol)

Eu-2FCA Eu-3FCA

1.24 ± 0.02 1.54 ± 0.02

14.2 ± 1.1 11.3 ± 1.0

−8.1 −9.0

21.3 20.3

Fig. 9 shows the structures of both the 1:1 complexes optimized in water medium using PCM model. At the outset, the optimization was achieved in the gas phase and subsequently, re-optimization was carried out in water medium. The re-optimization process contributed slightly different minima (with no imaginary frequencies) with a very marginal change in the geometry of the complex. The starting geometry for both the complexes was provided by considering the carboxylate moiety both as monodentate and as bidentate. In addition, for Eu(III)2FCA complex the bidentate mode with the oxygen of the ring and carboxylate oxygen was also considered. However, only carboxylate moiety coordinated in the final optimized geometries. The reason for nonparticipation of ring oxygen in bonding is due to the non-availability of lone pair which is delocalized inside the aromatic furan ring. The structural parameters of both the optimized geometries are shown in Table 3. The binding energy (ΔE) of the 1:1 complexes in aqueous medium were calculated for the complextaion reaction-

Stability constants (log β) of Eu(III)-2FCA

log β

3.5. Structural optimization and stabilization energy

Table 1 Stability constants of Eu(III) complexes at different temperatures (I = 0.1 M NaClO4). Temperature (K)

Complex

[Eu(H2O)9]3+ + 2FCA = [Eu(H2O)82FCA]2+ + H2O

S/R

[Eu(H2O)9]3+ + 3FCA = [Eu(H2O)83FCA]2+ + H2O

The Van't Hoff plots for both the Eu(III) complexes are shown in Fig. 8. All the thermodynamic parameters (ΔH, ΔG and TΔS) calculated are shown in Table 2. The enthalpy of formation (ΔH) of 1:1 Eu(III)2FCA and Eu(III)-3FCA complexes was 14.2 ± 1.1 kJ/mol and 11.3 ± 1.0 kJ/mol, respectively. It suggests that the complex formation of Eu(III)-2FCA is more endothermic than Eu(III)-3FCA. It is evident that although complexation of Eu(III) with FCA ligands is endothermic, positive entropy favors the complexation. For the interactions between Eu(III) and FCA ligands, the positive enthalpy predominates due to the fact that large energy is required for dehydration of both the Eu(III) and the ligands in aqueous medium. On the other hand, the favorable entropy is due to the enhancement in the randomness of both the coordination sphere and the bulk solvent.

The thermal corrections to the electronic energy (E) and enthalpy (H) of the optimized complexes have been performed. The calculated binding energy for both the complexes in aqueous phase is shown in Table 4. The binding energy of Eu(III)-3FCA being more than that of Eu (III)-2FCA, is in good agreement with log β value measured experimentally. A clear divergence was noted between the computations and experiments in predicting the enthalpy of the reaction. Experimentally, the complexation reaction was observed to be endothermic in contrast to the computational prediction, which is exothermic. Interestingly, with respect to gas phase, when solvent effect of water was introduced into the computations, the enthalpy decreased by ∼10 times in comparison to the gas phase value. Similar observations have been reported in literature for various metal complexes in gas and water phase [27]. 87

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Fig. 9. Optimized geometries of the 1:1 complexes of Eu(III) with (a) 2FCA, (b) 3FCA in water using PCM Model.

3.6. TD-DFT absorption spectra

In order to provide insights into large variation of experimental and PCM predicted energies, the extensive computation was performed in different solvents (dielectric constant of ∼20–∼180) and is shown in Fig. 10. It is clear from figure that as the dielectric constant increases, PCM predicted enthalpy of the reaction decreases. It is to be noted that the enthalpy of Eu(III)-3FCA is more than that of Eu(III)-2FCA in all the solvents, which is in line with the experimental observation. The above exercise demonstrates that the solvent with large dielectric constant directs the reaction more towards endothermicity; however, a comprehensible match between the computations and experiments could not be observed. It is likely due to the continuum model used in the calculations, whose empirical nature of introducing the solvent does not allow the complete reproducibility of experimental results.

To investigate the electronic transitions of ligands, TD-DFT calculations were performed on the optimized geometries using B3LYP/6-311G (d,p) level of theory. Commutations were performed in water at PCM model. Computed spectra and parameters are summarized in Fig. 11 and Table 5, respectively. It is to be noted that although 20 transitions were calculated to generate spectra for both the ligands but only one transition falls in our experimental range of spectrophotometer (> 220 nm). Here, theoretical spectra are plotted beyond 200 nm for both the ligands. The theoretical peaks were calculated at 249.5 and 247.8 nm, which were in good agreement with experimentally observed intense bands of 2FCA and 3FCA ligands at 239 and 234 nm, respectively. Furthermore, a higher oscillator strength for 2FCA (0.40) compared to 3FCA (0.07) as predicted by computations is also in good agreement with the experimentally observed absorbance values of the ligands.

Table 3 Structural parameters for the optimized complexes of Eu(III) using PCM model. Complex

dEu-O(COO) (Angstrom)

dC-O (Angstrom)

dEu-O(H2O) (Angstrom)

Bond angle Eu-O-C

Dihedral angle (Eu-O-C-O)

Eu-2FCA Eu-3FCA

2.342 2.314

1.284, 1.249 1.289, 1.250

2.536, 2.493, 2.495, 2.519, 2.544, 2.556, 2.528, 2.556 2.521, 2.458, 2.484, 2.630, 2.561, 2.556, 2.550, 2.511

142.67 143.14

7.16 −5.62

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Table 4 Calculated energies for all the species and binding energies of both the complexes in gaseous phase and water using PCM model. Complex In gaseous phase Eu-2FCA Eu-3FCA In aqueous phase Eu-2FCA Eu-3FCA

E

comp

(a.u.)

E

water

(a.u.)

E

Eu-aqua

(a.u.)

EL (a.u.)

ΔE (kJ/mol)

ΔH (kJ/mol)

−1739.74555 −1739.72617

−76.437246 −76.437246

−1397.62069 −1397.62069

−418.103375 −418.102145

−1204.3 −1156.7

−1199.8 −1153.1

−1739.98339 −1739.98134

−76.445192 −76.445192

−1398.18138 −1398.18138

−418.20336 −418.199809

−115.1 −119.1

−109.4 −113.3

Table 5 TD-DFT calculated electronic excitation parameters of 2FCA and 3FCA ligands. Ligand

Excitation energy (ev)

λexc (nm)

Oscillator strength

Configuration

λexpt (nm)

2FCA

4.9700 5.9327

249.46 208.99

0.4062 0.0453

239 N.Aa

3FCA

5.0039

247.78

0.0717

6.1465

201.72

0.0900

HOMO L UMO HOMO-1 LUMO HOMO LUMO+3 HOMO-2 LUMO HOMO LUMO HOMO-2 LUMO HOMO LUMO HOMO LUMO+2

234 N.Aa

a These bands could not be observed experimentally as the spectrophotometer range is above 220 nm.

oxygen) of 2FCA was also found to be more than 3FCA. The corresponding E2 energy for delocalization of π(C9eO10) to adjacent anti bonding orbital of furan ring is more for 3FCA in comparison to 2FCA, which is a clear marker for the facile delocalization of π orbital of 3FCA more than 2FCA ligand, with the concomitant decrease in the π occupancy of the donor orbital. It is this π orbital occupancy, responsible for a higher absorbance of 2FCA in contrast to 3FCA. It can be recalled that the TD-DFT calculations also predicted a larger absorbance for 2FCA. The NBO plot showing the overlap of π orbital with furan ring is shown in Fig. 12. Interestingly, NBO analysis divulged for Eu(III)-complexes that there exists a strong hydrogen bonding between the carboxylate oxygen (not coordinated to Eu) and one of the adjoining H2O ligands. The E2 energy in excess of ∼10 kcal/mol was discerned both for 2FCA and 3FCA ligands of Eu(III) complexes, which therefore does not deserve any further comment in view of the influence of ligands to overall complex formation.

Fig. 10. Variation of ΔH in the complexation reaction of Eu(III)-2FCA and Eu (III)-3FCA in different solvents using PCM Model.

4. Conclusions A systematic luminescence study was carried out to investigate complexation of Eu(III)-2FCA and Eu(III)-3FCA in aqueous medium. Triplet energy calculation along with excitation spectra revealed the efficient energy transfer from both of the ligands to Eu(III). Lifetime data indicated the formation of 1:1 Eu(III) complexes with both the ligands. Asymmetric ratios were used to calculate stability constants using HysSpec Program. The endothermic nature of complexation of both the complexes was evidenced by Van't Hoff plot. The theoretical results arrived from DFT, TD-DFT and NBO calculations were in agreement with the experimental assignments of Eu(III)-FCA complexes. Overall, comprehensive complexation studies of Eu(III)-FCA through experimental and theoretical probes of our kind are extremely helpful to provide significant insights in the migration behavior of lanthanides.

Fig. 11. Calculated absorption spectrum of 2FCA and 3FCA by TD-DFT B3LYP/ 6-311G (d,p) using PCM model in water.

3.7. NBO analysis To provide further insights as to how the absorbance of 2FCA is much higher of than its isomeric 3FCA counterpart, NBO analysis was performed both on 2FCA and 3FCA ligands. NBO analysis of π orbital of carboxylate unit revealed that the electron occupancy of the 2FCA is more than the 3FCA ligand. Furthermore, the occupancy of the negative charge on oxygen (which is being represented as the third lone pair on

89

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Fig. 12. NBO plot of delocalization of π(C9O10) of carboxylate unit of 2FCA and 3FCA to adjacent furan ring.

Appendix A. Supplementary data

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