E-ISSN:2583-9152

Research Article

Nitrobenzoic Acid

Journal of Condensed Matter

2023 Volume 1 Number 1 Jan-Jun
Publisherwww.thecmrs.in

Photo-physical properties of Pr (III) chelates of substituted nitrobenzoic acid and nitrophenols

Vyas A1*, Vyas M2, Shekhawat M3
DOI:10.61343/jcm.v1i01.7

1* Alok Vyas, Department Of Science And Humanities, GPC, Bikaner, Raj, India.

2 Mahendra Vyas, Department of Chemistry, Engineering College Bikaner, Bikaner, Raj, India.

3 Manoj S Shekhawat, Department of Physics, Engineering College Bikaner, Bikaner, Raj, India.

Electronic absorption and emission spectra were recorded for chelates of Pr (III) with 2-hydroxy-4-nirobenzoic acid, 3-hydroxy-4-nitrobenzoic acid, 4-hydroxy-3-nitrobenzoic acid, 4-methyl-2-nitrophenol, 4-chloro-2-nitrophenol and 5-fluoro-2-nitrophenol in various M: L stoichiometry and for different pH. Intensity and energy of intraconfigurational 4fn transitions have been determined from the absorption spectra. The spectroscopic parameters like Slater-Condon (Fk), Racah (Ek), Lande (ζ4f) and Judd-Oflet parameters Ωλ (λ=2, 4, 6) have been computed using statistical method like partial regression method. The Judd-Oflet intensity parameters and fluorescence spectra have been used to calculate radiative life time (τ) of two excited states 3P0 and 1D2. From the fluorescence spectra of the chelates, effective line width (Δλeff) spontaneous emission probability (A), fluorescence branching ratio (β) and stimulated emission cross section (σ) have been determined for three optical transition 3P0-3H4, 3P0-3H5 and 1D2-3H4. Spectroscopic and intensity parameters were studied with respect to the ligand field symmetry and degree of bond covalency.

Keywords: Praseodymium, Substituted nitrobenzoic acid, Substituted nitrophenols, Photo physical properties, Judd Ofelt parameter, Laser parameter

Corresponding Author How to Cite this Article To Browse
Alok Vyas, , Department Of Science And Humanities, GPC, Bikaner, Raj, India.
Email: 		Vyas A, Vyas M, Shekhawat M, Photo-physical properties of Pr (III) chelates of substituted nitrobenzoic acid and nitrophenols. J.Con.Ma. 2023;1(1):34-45.
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https://jcm.thecmrs.in/index.php/j/article/view/7

Manuscript Received Review Round 1 Review Round 2 Review Round 3 Accepted
2024-05-10 2023-05-15 2023-05-20 2023-05-25 2023-06-01
Conflict of Interest Funding Ethical Approval Plagiarism X-checker Note
None Nil Yes 20.87%

© 2023by Vyas A, Vyas M, Shekhawat Mand Published by Condensed Matter Research Society. This is an Open Access article licensed under a Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/ unported [CC BY 4.0].

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Introduction

Rare earth metal complexes in the recent days are tools for various emerging fields from ranging from laser technology to bio-medical arenas [1-9]. As a part of systematic investigation, the complexes of hydroxy nitrobenzoic acid (2-hydroxy-4-nirobenzoic acid, 3-hydroxy-4-nitrobenzoic acid and 4-hydroxy-3-nitrobenzoic acid) and substituted nitrophenols (4-methyl-2-nitrophenol, 4-chloro-2-nitrophenol, and 5-fluoro-2-nitrophenol) with rare earth metal ions were studied in detail for their photo-physical properties as literature survey indicates little work on these complexes [10-25].

In the current pursuit, Pr (III) chelates of hydroxy nitrobenzoic acid and substituted nitrophenols were selected for study and their absorption and emission spectra were recorded and analyzed for various spectroscopic parameters (energy, intensity and laser parameters). Praseodymium exhibits characteristic f-f absorption spectrum which corresponds to transitions from the ground state multiplet to the excited state multiplet. These transitions have a fixed spectral position but their intensity and fine structure may vary with embedding matrix. The intensity of these multiplet-to-multiplet transitions has been successfully described by the Judd-Oflet theory [26-27]. According to Judd-Oflet theory, mixing between 4fn configuration and another configuration having opposite parity may be occurred by the crystal field potential and cause 4f-4f transitions to be allowed by the induced electric dipole. The spectra of Praseodymium consist of four peaks spectra with narrow bands with in the visible regions, representing the transition between 3H2 (ground state) to 1D2, 3P0, 3P1, 3P2 (excited states). The primary object of this investigation is to determine the Judd-Oflet* intensity parameters from the oscillator strength of absorption peaks and to examine how intensity parameters respond to the minor changes in the ligand environment. In addition to this, spectroscopic parameters Slater-Condon [28-29] (Fk), Racah [30-33] (Ek), Lande (ζ4f) were also determined to study degree of covalency in the metal-ligand interaction. Using the Judd-Oflet intensity parameters Ωλ (λ=2, 4, 6) and fluorescence spectra of the complexes, laser parameters like radiative lifetime of excited states, spontaneous emission probability, fluorescence branching ratio and stimulated emission

cross section of optical transitions were also evaluated to examine the alteration in fluorescence properties in conjunction with that of structure of the complex.

Experimental Studies

Hydroxy nitrobenzoic acid [2-hydroxy-4-nirobenzoic acid, 3-hydroxy-4-nitrobenzoic acid and 4-hydroxy-3-nitrobenzoic acid] and substituted nitrophenols [4-methyl-2-nitrophenol, 4-chloro-2-nitrophenol, and 5-fluoro-2-nitrophenol] were obtained from Acros-Organics and used directly. Acetate salts of Praseodymium were obtained from Indian Rare Earths Ltd., India. All other chemicals were obtained from Ranbaxy, India ltd. All solutions were prepared prior to the experiments in double distilled deoxygenated water. Stock solutions of 0.01 M ligand and 0.01 M metal ions were also prepared in deoxygenated water. The absorption studies were carried out with the sample solutions of metal and ligands in stoichiometric ratio (M:L) of 1:1, 1:2, 1:3 and 1:4 to record the spectra. The absorption spectra of the sample solutions were recorded at the room temperature in the range of 400-700nm using Systronic-119 spectrophotometer with a scan speed 600 nm/min. The pH of 1:2 M:L solution has been altered in the range of 4.5-7.5 to obtained optimum pH range. The fluorescence spectra were recorded at room temperature on Hitachi Fluorescence spectrophotometer model F-3000 having a xenon lamp and spectral range of 200-900nm at the excitation wavelength of 443.5 nm.

Theory of Rare earth Spectroscopy

The effective Hamiltonian [33-35] of rare earth metal ion under the influence of a ligand field is composed of four terms

Here, first term Hc represents the undisturbed Hamiltonian (central field approximation), the second term He represents columbic interactions which were calculated by Slater [28], shortely, Codon [29] and Racah [30-32] using tensor operated methods and are expressed in terms of Slater Codon (F2, F4, F6) and Racah parameters (E1, E2, E3). The third term Hm gives spin orbit interactions, expressed as Lande parameter (ζ4f).


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The final term HL is the ligand field Hamiltonian (configuration interaction) which is expressed as interaction parameters (α).

(a) Calculation of Energy parameters:

The Taylor series expansion method used by Wong [34-38] and other workers has been utilized for the evaluation of the spectroscopic parameters from the observed spectra [figure 1.1-1.3 and 2.1-2.3] for different M:L ratios and for different pH values. The energy Ej of the jth level is given by the equation

where Eoj = Zero order energy of the jth level and ΔFk & Δζ4f are the small changes in the corresponding parameters. The values of zero order energy Eoj and partial derivatives for the observed values of Pr(III) calculated by Wong has been used in the current evaluation. The values of ΔFk & Δζ4f have been calculated by partial regression method. The values of Fk and ζ4f are then calculated using following equations [Table 1.4 and Table 2.4].

where F0k and ζ04f are the zero order values of the corresponding parameters. The Racah parameters Ek have been calculated from Fk parameters using following equations

Using the computed Fk and ζ4f values, the Ej values (Ec) have been calculated for Pr (III) complexes with three ligands [and are listed in Table 1.4 and Table 2.4]. The nephelauxetic effect and bonding parameter, important tools for determination of degree of covalency, have also been evaluated using the following equations 1.8.

where f and c refer to the free ion and complex respectively. The values of Slater-Codon (F2, F4, F6) and Racah parameters (E1, E2, E3) Lande (ζ4f) along with nephelauxetic effect and bonding parameters are listed in Table 1.1 and Table 2.1.

(b) Calculation of oscillator strength and intensity parameter                           

The experimental oscillator strength (Pexp) of the observed bands have been calculated by using the relation

where ε(ν) is the molar absorption coefficient at the energy ν and the integral corresponds to the area of the absorption band for certain transition. The theoretical oscillator strength Pcal were calculated by employing Judd-Oflet theory taking into account that f-f transitions are predominately induced electric dipole transitions. According to Judd-Oflet the oscillator strength of a transition between initial J manifold and terminal J’ manifold, is given by

where ||U(λ)|| represents the square of the reduced matrix elements of the unit tensor operator connecting the initial and final states, summing over the three values λ=2,4,6. In the above equation the Ωλ quantities, called as the Judd-Oflet intensity parameters were obtained from the least square analysis of the observed oscillator strength by employing the following equation.

where ν is the energy of the band in cm-1. The values of judd-Oflet intensity parameters are listed in Table 1.2 and Table 2.2 for all the three kinds of chelates.

(c) Calculation of laser parameters:

Judd-Ofelt intensity parameters calculated above were used for the determination of radiative properties of rare earth metal complexes Thus the Ωλ values thus obtained from


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Vyas A et al: Photo-physical prop. of Pr chelates of sub nitrobenzoic acid and nitrophenols

the absorption measurements are used to calculate the spontaneous emission probability(A), branching ratios, radiative life time and stimulated emission cross section as per equations given below where the symbols have usual meanings [42-43].

(1) Spontaneous emission probability (A)- The spontaneous emission probability from an initial J manifold to terminal J’ manifold is given by

The A values for different emission transitions, are calculated by substituting the emission wave length λp reduced matrix elements for the relevant transition and the values of parameters

(2) Fluorescence branching ratio (β)- The fluorescence branching ratio for the transitions originating from a specific manifold is defined by

the values depend upon  and  parameters which in turn depend upon the coordination environment.

(3) Radiative lifetime (τ)- The radiative life time τ for a transition is reciprocal of spontaneous emission probability A. For radiative decay from the initial J’ manifold |fN(α',S',L)j'> it is given by

(4) Stimulated emission cross-section (σp)- The stimulated emission cross-section (σp) for transition from an initial J manifold  to terminal J manifold |fN(α',S',L)j'> is expressed as

where the peak fluorescence is wave length of the emission band and  is the effective fluorescence line width and  is fluorescence intensity at wavelength . The rate of energy extraction from a laser material is dependent on the stimulated emission cross-section . This parameter is most important among laser parameters and is generally used to predict laser action. The values of above mention laser parameters for Praseodymium chelates are listed in Table 1.3 and Table 2.3.




Figure 1.1: Absorption spectra of Pr(III)-2H4NBA for different molar ratios and for different pH



Figure 1.2: Absorption spectra of Pr(III)-3H4NBA for different molar ratios and for different pH


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Figure 1.3: Absorption spectra of Pr(III)-4H3NBA for different molar ratios and for different pH


Figure 1.4: Fluorescence spectra of Pr(III) -2H4NBA

 


Figure 1.5: Fluorescence spectra of Pr(III) -3H4NBA


Figure 1.6: Fluorescence spectra of Pr(III) - 4H3NBA

Figure 2.1: Absorption spectra of Pr(III)-4M2NP for different molar ratios and for different pH



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Vyas A et al: Photo-physical prop. of Pr chelates of sub nitrobenzoic acid and nitrophenols


Figure 2.2: Absorption spectra of Pr(III)-4C2NP for different molar ratios and for different pH



Figure 2.3: Absorption spectra of Pr(III)-4F2NP for different molar ratios and for different pH


Figure 2.4: Fluorescence spectra of Pr(III) -4M2NP


Figure 2.5: Fluorescence spectra of Pr(III) -4C2NP


Figure 2.6: Fluorescence spectra of Pr(III) -5F2NP

Result and Discussion

(a) Oscillator Strength:

The absorption spectra of Praseodymium chelate in the wave length range of 400-650 nm at room temperature containing substituted nitrobenzoic acid (2H4NBA, 3H4NBA, 4H3NBA) are given in figure 1.1-1.3 and for the substituted nitrophenols (4M2NP, 4C2NP, 5F2NP) in the figure 2.1-2.3. The solution spectra have been analysed by resolving each band into Gaussian curve shape to enable evaluation of oscillator strength. The bands for different transitions have been identified by comparing the values of energies with corresponding energy level in free metal-ion. The oscillator strength values which are a measure of intensities of specific electronic transitions or degree, to which a specific transition is allowed, show marked dependence on the cation environment. The four distinct bands in the visible region can be attributed to the excitation from the ground state (3H4) to various excited states (1D2, 3P0, 3P1, 3P2*). To ascertain the molar ratio for optimum molecular stacking in solution, absorbance of complexes was recorded for four different ratios (1:1, 1:2, 1:3 and 1:4) and oscillator strength was


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calculated (table 1.4 for series A and table 2.4 for series B). For different molar ratios, value of absorbance was found to be in order 1:1 < 1:4 < 1:3 < 1:2 showing the bidentate nature of ligand and metal complex can be abbreviated as [Pr(nba)2](CH3COO)-3 and [Pr(np)2](CH3COO)3 [where nba=2H4NBA, 3H4NBA, 4H3NBA and np=4M2NP, 4C2NP, 5F2NP]. To understand the effect of pH on complexation, the absorption spectra of metal chelate were recorded in the wave length 430-460 nm for different pH range [4.5-5.5, 5.5-6.5, 6.5-7.5, and 7.5-8.5]. The absorbance value for a particular pH range represents the weighted average of three values measured at three different points (for example three pH points 4.5, 5.0, 5.5 were selected for pH range 4.5-5.5). The absorbance of metal chelates was found to be in the order 4.5-5.5 < 5.5-6.5 < 6.5-7.5 > 7.5-8.5 (see figure 1.2 and 2.2). The decrease in value at higher pH can be attributed to the precipitation of metal chelate as turbidity appeared in solution after pH 7.5

(b) Energy Parameters:

The Ej values using Wong Equation 1.2 have been calculated for all the observed transitions of Pr(III) complexes with 2 series of ligands for different pH range. The values in Table 1.1 (for ligands of series A) and in Table 2.1 (for ligands of series B) is the average of three measurements done at various pH points for a given pH range and their value has been listed in Table 1.1 and 2.2. The low value of r. m. s. between the experimental and theoretical values of energy for the observed transitions suggests the suitability of above relation. It is the non-centro-symmetrical interactions of metal ion with surrounding ligands that would cause the mixing of electronic states with even parity, thus f-f transitions become allowed ones as induced electric dipole transitions [26-27, 36-41]. During current investigation it has been observed that ligand 2H4NBA (for ligands of series A) and ligand 4M2NP (for ligands of series B) produces greater amount of vibronic coupling with central metal ion. The nature of metal–ligand bond can be analyzed in terms of Slater-Condon parameters (Fk and Ek) which represent the magnitude of interelectronic repulsions in metal ions. The decrease in value of these parameters in metal complexes (Table 1.1-series A and Table 2.1-series B) as compared to free aqua ion indicates expansion of f electron cloud which can be attributed in terms of degree of complexation.

The value of these parameters (FK, EK and ζ4f) have been computed for different pH range and it was found that value decreases with increase in pH and was found to be lowest for the range 6.5-7.5. Thus, this range appears to be optimum pH range for the molecular stacking in solution. Among two series of chelates, the decrease in value of Slater and Codon parameters (Fk and Ek) is more for series A containing substituted nitro benzoic acids than series B.

For three ligands of nitrobenzoic series (series A), the value of Fk and EK [see Table 1.1] were chosen to determine the order of complexation tendency. The values were found to be lowest for the complex [Pr(nba)2](CH3COO)3 {where nba=2H4NBA} as compared to other two chelates. This is probably due to adjacent attachment of hydroxyl to carboxylic group and meta position of nitro group as compared with hydroxy group in 2H4NBA. In case of series B (containing hydroxy nitro-phenols), the value of Fk and EK is lowest for [Pr(np)2](CH3COO)3 {where np=4M2NP} [see Table 2.1], this probably due to fact the electron withdrawing effect is more pronounced in halogens (F and Cl) than methyl group. The decrease in the value of ζ4f (as compared with free ion) clearly suggests the decrease in spin-orbit interactions indicating a general red shift in case of metal chelates. It has been found that the nephelauxetic ratio (β) for all the systems is less than one indicating that the metal-ligand interaction is not ionic but there is a mixing of metal and ligand orbitals i.e., the metal-ligand bonding in these chelates is not mere ionic but there is covalency in them.

(c) Intensity Parameters:

The effect of host matrix on local environment for a given rare earth ion can be elucidated using the Judd-Oflet theory by studying changes in the experimentally fitted Judd-Oflet intensity parameters [Ωλ (λ=2, 4, 6)]. It has been shown that among the three intensity parameters the Ω2 is very sensitive to the structural details and chemical environment of the ligand environment. During current investigation, it is found that value of Ω2, decrease with upon complexation and it is more pronounced for 1:2 M:L ratio for the given ligand. The decrease is more pronounced for series A [substituted nitrobenzoic acid] than for series B [substituted nitrophenols]. The value is lowest for 2H4NBA among nitrobenzoic acid and for 4M2NP among nitrophenols.


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(d) Radiative Properties and Fluorescence Spectra:

The fluorescence spectra of three chelates recorded at room temperature using a xenon laser with excitation wavelength of 443.5 nm is shown in figures 1.4-1.6. The spectra consist of three transitions corresponding to 3Po-3H4, 3Po - 3H5 and 1D2-3H4 arising from two excited states 1D2 and 3Po. Using the Ωλ values thus obtained from the absorption measurements are used to calculate spontaneous emission probability, branching ratios, radiative life time and stimulated emission cross section for the metal chelates as per the equations given above. Transitions having spontaneous emission probability (A) greater than 500 sec-1 and fluorescence branching ratio (β) ~ 0.5 are considered to be good radiation transition. It has been observed that among three transitions, value of spontaneous emission probability is maximum for the transition 3Po-3H4 for both series of ligands. The value is 722.633 for Pr(III)-2H4NBA, 620.089 for Pr(III)-3H4NBA and 562.093 cm-1 for Pr(III)-4H3NBA. This trend clearly indicates that spontaneous emission probability is related to structural dynamics of chelates. In the case of Pr (III) chelates of nitrophenols, the stimulated emission probability (A) for the transition 3Po-3H4 was found to be 606.287 with ligand 4M2NP, 597.944 with ligand 4C2NP and 589.613 with ligand 5F2NP. These values clearly indicate that among two ligand series, ligands of first category have higher value of stimulated emission probability, probably because they have higher degree of covalency in the metal -ligand linkage.

The luminescence branching ratio (β) is a critical parameter, to the laser designer, because it characterizes the possibility of attaining stimulated emission from any specific transition and among three transitions observed in two series of ligands, only transition 3Po-3H4 have the adequate value of branching ratio. For the Pr (III) chelates of nitrobenzoic acid and nitrophenols, the observed values are very close (~ 0.76 for the nitrophenols and ~ 0.75 for the nitrobenzoic acid). These values indicate that two kinds of ligands have similar branching ratio for the transition 3Po-3H4.

Stimulated emission cross section (σ) is most important laser parameter. Its value signifies the rate of energy extraction from the laser material. Value of σ for the Pr (III) chelates with nitrobenzoic

series ranges from 1.62-1.39 for the transition 3P0-3H4, 0.81-0.84 for the transition 3P0-3H5 and 0.21-0.22 for the 1D2-3H4. For the Pr (III) chelates with nitrophenols, the observed value ranges from 1.42-1.61 for the transition 3Po-3H4, 0.78-0.85 for the transition 3Po-3H5 and 0.24-0.26 for the 1D2-3H4. Thus, among three transitions observed with both series of ligand 3P0-3H4 has the maximum value of stimulates emission cross section.

The fluorescence life time (τ) for a transition is reciprocal of A. The minimum value of τ has been obtained for the transition 3P0-3H4 for the two series of Pr (III) chelates. This fact coupled with other laser parameters clearly indicate that the transition 3P0-3H4 to be most probable laser transition and 3P0 level to be efficient fluorescence level respectively for two series of metal chelates.

Table 1.1: Computed values of (Fk), (ζ4f), (Ek) (in cm-1), (β) and (b1/2) parameters of Pr (III) chelate containing hydroxy nitrobenzoic acid in various metal-ligand stoichiometries and at different pH

M:L Ratio pH chelates  Slater-Condon parameters (Fk) F2 F4 F6Lande parameterz4fRacah parameters (Ek)E1 E2 E3NephelauxeticbBonding parameterb1/2
 1:16.5-7.52H4NBA310.40642.8514.690705.0234557.32023.839460.7660.96370.1346
3H4NBA310.54242.8704.692705.4734559.32623.849460.9690.96410.1338
4H3NBA311.10942.9484.700704.4164567.64923.893461.8110.96590.1305
1:24.5-5.52H4NBA310.15542.8164.686707.5794553.63823.819460.3940.96290.1361
3H4NBA310.26042.8314.688708.3644555.1723.827460.5500.96320.1355
4H3NBA310.99642.9334.699704.5174565.98823.884461.6430.96550.1312
5.5-6.52H4NBA310.06542.8044.685708.6494552.32623.813460.2610.96260.1366
3H4NBA310.18042.8204.686708.5154554.00223.821460.4310.96300.1359
4H3NBA310.92942.9234.698704.4054565.00023.879461.5430.96530.1316
6.5-7.52H4NBA309.97642.7924.683709.7184551.01523.806460.1290.96230.1371
3H4NBA310.03142.7994.684713.9484551.82123.810460.2100.96250.1368
4H3NBA310.88242.9174.697704.3204564.31523.875461.4740.96520.1319
7.5-8.52H4NBA310.31842.8394.688704.8694556.03923.832460.6370.96340.1351
3H4NBA310.36042.8454.689702.8784556.64923.835460.6980.96350.1349
4H3NBA310.95242.9274.698704.5754565.34423.881461.5780.96540.1314
 1:36.5-7.52H4NBA310.24642.8294.687706.4524554.97223.826460.5290.96320.1355
3H4NBA310.29542.8364.688708.2354555.69223.830460.6020.96330.1353
4H3NBA311.02242.9364.699704.6724566.37623.886461.6820.96560.1310
 1:46.5-7.52H4NBA310.30342.8374.688706.1634555.80923.831460.6140.96340.1352
3H4NBA310.34442.8434.689708.1464556.41723.834460.6750.96350.1350
4H3NBA311.08742.9454.700704.0824567.33023.891461.7780.96580.1306
Free aqua ion322.09044.4604.867741.0004729.00024.740748.140 

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Table 1.2: Computed values of Judd-Ofelt (tλ) intensity parameters for Pr (III) chelates containing hydroxy nitrobezoic acid for different metal-ligand stoichiometries and for different pH

M:L RatiopH Parameters(Ωλ x 109) Pr (III) - 2H4NBA Pr (III) – 3H4NBA Pr (III) – 4H3NBA
  1:1 6.5-7.5Ω2 -14.3015-17.2159-19.9995
Ω4 1.31191.29211.21562
Ω6 4.12264.09483.91060
Ω4 / Ω60.31820.31550.31085
1:2 4.5-5.5Ω2 -11.8053-13.4723-14.0019
Ω4 1.42301.36691.2900
Ω6 4.36734.22694.0435
Ω4 / Ω60.32580.32330.3190
5.5-6.5Ω2 -11.3025-12.2095-12.7652
Ω4 1.43321.39201.3149
Ω6 4.38544.27104.0887
Ω4 / Ω60.32680.32590.3215
6.5-7.5Ω2 -10.3445-10.9524-11.5282
Ω4 1.42921.41481.3398
Ω6 4.33984.31544.1333
Ω4 / Ω60.32930.32780.3241
7.5-8.5Ω2 -11.5555-12.6991-13.3771
Ω4 1.42791.37951.3025
Ω6 4.37654.24894.0659
Ω4 / Ω60.32620.32460.3203
  1:36.5-7.5Ω2 -12.2089-14.7203-17.5204
Ω4 1.39161.34211.2652
Ω6 4.27324.18293.9991
Ω4 / Ω60.32560.32080.3163
  1:46.5-7.5Ω2 -13.0713-15.9532-18.7639
Ω4 1.33631.31721.2404
Ω6 4.16634.13803.9550
Ω4 / Ω60.32070.31830.3136

Table 1.3: The Spontaneous emission probability (A), fluorescence branching ratio (β), total spontaneous probability (AT,) measured [τ] and radiative fluorescence life time [τR ] for Pr (III) chelates containing hydroxy nitrobenzoic acid with fluorescence peak value.

Metal chelatesParameters3P0 →3H43P0 → 3H51D2 → 3H4
Pr(III)-2H4NBAλp(nm)486525.8602.6
Δλeff (nm)22.0515.4620.22
A (sec-1)722.633184.39736.378
Β0.7658550.1955470.038598
AT (sec-1)942.981
τ (μs)138.5542.32747.5
τR (μs)106.1
σ (pm2)1.6220.8100.211
Pr(III)-3H4NBAλp(nm)488526602.9
Δλeff (nm)21.2213.3218.45
A (sec-1)620.0869163.757433.23951
Β0.7589020.2004170.040681
AT (sec-1)817.083
τ (μs)161.3610.73008.5
τR (μs)122.4
σ (pm2)1.4610.8420.212
Pr(III)-4H3NBAλp(nm)487.1527.2602
Δλeff (nm)20.0711.0418.45
A (sec-1)562.093152.59131.491
β0.7532990.2044970.042204
AT (sec-1)746.175
τ (μs)177.9655.33175.5
τR (μs)134.0
σ (pm2)1.3900.8120.211


Table 1.4: Observed and computed values of oscillator strengths (P) & energies (E) of Pr(III) chelates containing hydroxy nitrobenzoic acid

 1:2M:L ratio pHEnergy Wavelength and Oscillator Strength Energy levels 3P2* 3P1 3P0 1D2 σr.m.s. deviation
2H4NBA6.5-7.5Eexpt (cm-1)2253821358207251701691.55
Ecal (cm-1)22450212572072517140
Pexpt x 10615.7307.8693.5813.9911.47x 10–6
Pcal x 10615.7305.7595.6523.991
3H4NBA6.5-7.5Eexpt (cm-1)2255321397207311702596.33
Ecal (cm-1)22476212762073317155
Pexpt x 10614.7846.9203.1783.8451.28x 10–6
Pcal x 10614.7845.0804.9833.845
4H3NBA6.5-7.5Eexpt (cm-1)2255421400207731705483.25
Ecal (cm-1)22485212982077417166
Pexpt x 10613.9896.3622.8003.3251.23x 10-6
Pcal x 10613.9894.6064.5263.325

Table 2.1: Computed values of (Fk), (ζ4f), (Ek) (in cm-1), (β) and (b1/2) parameters of Pr(III) chelates containing substituted nitrophenols in various metal-ligand stoichiometries and at different pH

M:L Ratio pH chelates  Slater-Condon parameters (Fk)  F2  F4  F6Lande parameterz4fRacah parameters (Ek)  E1  E2  E3NephelauxeticbBonding parameterb1/2
 1:16.5-7.54M2NP310.51042.8664.691703.8094558.85823.847460.9220.96400.1340
4C2NP310.63642.8834.693696.9064560.70223.856461.1080.96440.1333
5F2NP310.69242.8914.694697.0804561.52923.861461.1920.96460.1330
1:24.5-5.54M2NP310.45642.8584.690703.1044558.06123.843460.8410.96380.1343
4C2NP310.59642.8774.693697.0244560.12023.853461.0490.96430.1335
5F2NP310.58042.8754.692697.1834559.87523.852461.0250.96420.1336
5.5-6.54M2NP310.33442.8414.689704.5224556.26223.833460.6590.96350.1350
4C2NP310.47442.8604.691698.4424558.32123.844460.8680.96390.1342
5F2NP310.51242.8664.691697.0744558.88923.847460.9250.96400.1340
6.5-7.54M2NP310.30042.8364.688704.2774555.76523.831460.6090.96330.1352
4C2NP310.44042.8564.690698.1984557.82323.841460.8170.96380.1344
5F2NP310.46642.8594.691696.9914558.20723.843460.8560.96390.1343
7.5-8.54M2NP310.38142.8484.689703.6854556.95123.837460.7290.96360.1348
4C2NP310.52142.8674.691697.6064559.01023.848460.9370.96400.1340
5F2NP310.53642.8694.692697.2424559.23323.849460.9600.96410.1339
 1:36.5-7.54M2NP310.43642.8554.690703.3834557.77323.841460.8120.96380.1344
4C2NP310.57742.8754.692697.3014559.83223.852461.0200.96420.1336
5F2NP310.60642.8794.693697.3374560.26123.854461.0640.96430.1335
 1:46.5-7.54M2NP310.47742.8614.691703.7034558.36323.844460.8720.96390.1342
4C2NP310.65942.8864.694696.9764561.0323.858461.1420.96450.1332
5F2NP310.67142.8884.694696.7484561.21223.859461.1600.96450.1331
Free aqua ion322.09044.4604.867741.0004729.00024.740748.140322.090

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Table 2.2: Computed values of Judd-Ofelt (Tλ) intensity parameters for Pr (III) chelates containing substituted nitrophenols for different metal-ligands stoichiometries and for different pH

Stoichio-metric ratio of M:LpH Parameters(Ωλ x 109) Pr (III) – 4M2NP Pr (III) – 4C2NP Pr (III) – 5F2NP
  1:1 6.5-7.5Ω2 -20.8025-22.1557-23.2043
Ω4 1.18481.16051.1342
Ω6 3.71673.68563.6442
Ω4 / Ω60.31870.31480.3112
1:2    4.5-5.5Ω2 -17.6318-18.6767-19.6258
Ω4 1.26321.24671.2233
Ω6 3.83963.82013.7836
Ω4 / Ω60.32900.32630.3233
5.5-6.5Ω2 -16.5676-17.6183-18.5772
Ω4 1.28941.27291.2495
Ω6 3.88053.86113.8255
Ω4 / Ω60.33220.32960.3266
6.5-7.5Ω2 -15.5121-16.5685-17.3158
Ω4 1.31561.29911.2810
Ω6 3.92193.90263.8749
Ω4 / Ω60.33540.33280.3305
7.5-8.5Ω2 -17.2091-18.3620-19.2014
Ω4 1.27381.25471.2338
Ω6 3.85633.83293.8003
Ω4 / Ω60.33030.327350.3246
  1:36.5-7.5Ω2 -18.6892-19.8343-20.8873
Ω4 1.23721.21801.1918
Ω6 3.79903.77543.7343
Ω4 / Ω60.32560.32260.3191
  1:46.5-7.5Ω2 -19.7448-20.8840-21.7324
Ω4 1.21091.19171.1709
Ω6 3.75773.73403.7016
Ω4 / Ω60.32220.31910.3163

Table 2.3: The Spontaneous emission probability (A), fluorescence branching ratio (β), total spontaneous probability (AT,) measured [τ] and radiative fluorescence life time [τR ] for Pr (III) chelates containing substituted nitrophenols with fluorescence peak value

Metal chelatesParameters3P0 3H43P0 3H5 1D2 3H4 
Pr(III)-4M2NPλp(nm)485.8525.7603
Δλeff (nm)20.113.2314.22
A (sec-1)606.287154.15130.253
Β0.7670.1940.0380
AT (sec-1)790.691
τ (μs)164.9648.73305.4
τR (μs)126.5
σ (pm2)1.4910.7890.249
Pr(III)-4C2NPλp(nm)486525.9603.2
Δλeff (nm)19.0212.6513.42
A (sec-1)597.944152.61030.051
β0.7660.1950.0384
AT (sec-1)780.606
τ (μs)167.2655.33327.7
τR (μs)128.1
σ (pm2)1.5560.8190.263
Pr(III)-5F2NPλp(nm)486525.9603.2
Δλeff (nm)18.0712.0313.11
A (sec-1)589.613150.98929.817
β0.7650.1950.0387
AT (sec-1)770.420
τ (μs)169.6662.33353.8
τR (μs)129.8
σ (pm2)1.6150.8520.267

Table 2.4: Observed and computed values of oscillator strengths (P) & energies (E) of Pr(III) chelates containing substituted nitrophenols

 1:2M:L ratio pHEnergy Wavelength and Oscillator Strength Energy levels 3P2* 3P1 3P0 1D2σr.m.s. deviation
4M2NP6.5-7.5Eexpt (cm-1)2255821404207321696978.19
Ecal (cm-1)22446212572073317139
Pexpt x 10613.7836.4003.7653.3410.82x 10–6
Pcal x 10613.7835.1205.0203.341
4C2NP6.5-7.5Eexpt (cm-1)2250721358207301699896.22
Ecal (cm-1)22425212422073217128
Pexpt x 10613.4836.1903.5813.1550.89x 10–6
Pcal x 10613.4834.9174.8323.155
5F2NP6.5-7.5Eexpt (cm-1)2250321354207301699695.97
Ecal (cm-1)22420212392073117125
Pexpt x 10613.1845.9803.3982.9700.88x 10–6
Pcal x 10613.1844.7194.6372.970

Conclusion

In the present pursuit, the equilibrium, spectroscopic and luminescence studies of Pr(III) metal-chelates have been carried out with two series of ligands. The first series consist of substituted nitrobenzoic acids (2-hydroxy-4-nitrobenzoic acid [2H4NBA], 3-hydroxy-4-nitrobenzoic acid [3H4NBA], and 4-hydroxy-3-nitrobenzoic acid [4H3NBA]) and second series consist of substituted nitrophenols (includes 4-methyl-2-nitrophenol [4M2NP], 4-chloro-2-nitrophenol [4C2NP], and 5-fluoro-2-nitrophenol [5F2NP]). It can be concluded that both series of ligands shows chelation with Praseodymium (III) metal salt and chelation is more pronounced for substituted nitrophenols than substituted nitrobenzoic acids and optimum pH range is found to be 6.5-7. From various energy and intensity parameters, it can be concluded that among substituted nitrobenzoic acids 2H4NBA is more effective in coordination and in substituted nitrophenols 4M2NP show maximum chelation. The Laser parameters also follows the same trend.

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