Fluorescence enhancement of Tb3+ in Tb-aromatic acid complexes: correlation of synergistic enhancement with the structure of the ligand

SPECTROCHIMICA ACTA PART A

ELSEVIER

Spectrochimica Acta Part A 51 (1995) 2289-2300

Fluorescence enhancement of T b 3+ in Tb-aromatic acid complexes: correlation of synergistic enhancement with the structure of the ligand B.S. Panigrahi, Susy Peter 1, K.S. Viswanathan *, C.K. Mathews Chemical group, 1GCAR, Kalpakkam-603 102, India

Received 18 February 1995; accepted 12 May 1995

Abstract Fluorescence enhancement of Tb 3+ has been studied, using aromatic carboxylic acids as ligands. These ligands enhance the fluorescence of Tb 3+ by about three orders of magnitude. The enhancement is observed to be due to ligand sensitized fluorescence. The fluorescence of Tb 3+ in some of these complexes is further enhanced by an order of magnitude by the addition of trioctyl phosphine oxide (TOPO), a phenomenon referred to as synergism. However, the pattern of synergism displayed by TOPO is not uniform, and is found to vary with the ligand. While with some aromatic acids, TOPO displayed maximum synergism near pH 6, in others it did so around pH 4. In a few cases, TOPO did not display any synergism at all, at any of the pH values. These observations are discussed in detail and the results are rationalized on the basis of the structure of the aromatic acids.

1. I n t r o d u c t i o n

Trace level estimation of lanthanides using spectrofluorimetry has always been a challenge, owing to their low absorptivities and p o o r fluorescent quantum yields [1,2]. Fluorescence enhancement of lanthanides in aqueous solutions has therefore been an area of active study, motivated by the wide use of the lanthanides, particularly Tb and Eu, as luminescent probes in biological systems [3,4]. The technique of ligand sensitized fluorescence has generally been employed to obtain fluorescence enhancement in the lanthanides. In this process, an organic ligand is first excited by absorption of light, followed by energy transfer from the ligand to the excited levels of a lanthanide, which then results in fluorescence of the lanthanide. It turns out that if the intramolecular energy transfer from the ligand to the lanthanide is efficient, then the upper emitting levels of the lanthanides are more efficiently pumped by this technique than by a direct excitation, resulting in an enhanced lanthanide fluorescence. Various ligands have been used for the purpose of ligand sensitization [5-10], but the more popular ones have been the diketones, such as thenoyltrifluoroacetone (TTA) [11]. In earlier reports from this laboratory, we have shown that aromatic carboxylic acids, such as benzoic acid and * Corresponding author. Present address: Spectroscopy Division, Bhabha Atomic Research Center, Bombay, India. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0584-8539(95)01464-0

2290

B.S. Panigrahi et al./Spectrochirnica Acta Part A 51 (1995) 2289-2300

trimesic acid, serve as excellent ligands for fluorescence enhancement of certain lanthanides [12,13]. Fluorescence enhancements of up to three orders of magnitude were obtained for the lanthanides Tb 3+, Dy 3+ and Eu 3+ by using these aromatic carboxylic acids as ligands. The fluorescence of the lanthanides in lanthanide complexes is further enhanced by the use of synergistic agents, such as trioctyl phosphine oxide (TOPO), organic phosphates and sulphoxides. It is believed that these compounds provide an insulating layer around the lanthanide complex, reducing the probability of radiationless energy transfer from the lanthanides to the solvent [14,15]. In our earlier work, where we had used benzoic and trimesic acids as ligands, we found that the addition of TOPO further enhanced the fluorescence of the lanthanides in the lanthanide complexes, by another order of magnitude. However, interestingly, the pH at which maximum enhancement was observed, was different for the two ligands, benzoate and trimesate. While in the l a n t h a n i d e - b e n z o a t e - T O P O system, we observed the highest enhancement near pH 6, the lanthanide-trimesate system at pH 6 showed almost no enhancement due to synergism when TOPO was added. However, when the pH was decreased to about 4, the fluorescence of the lanthanide-trimesate-TOPO system showed a maximum. In this work, we have examined this point further to see if this pH dependence is typical of all polycarboxylic acids. A variety of polycarboxylic aromatic acids were used as ligands, and some interesting aspects of fluorescence enhancement of Tb 3 +, following the addition of TOPO have emerged, which will be discussed. The aromatic acids also enhance the fluorescence of Dy 3+ and Eu 3+. However, because the general behaviour of the Dy 3+ and E u 3 + complexes was similar to that of Tb 3 +, we have studied in detail only the Tb 3 ÷ complexes.

2. Experimental 2. I. Apparatus

All fluorescence measurements were made using a Shimadzu RF 5000 spectrofluorimeter. The excitation source was a 150 W continuous wave xenon lamp. The band passes for the excitation and emission monochromators were set at 5 nm. A long-wavelength-pass filter, (UV-35, Shimadzu) with a maximum and uniform transmittance (more than 85%) above 400nm, was placed in front of the emission monochromator in order to reduce the scatter of the incident beam into the emission monochromator. Solutions were taken in a 1 cm path-length quartz cell for fluorescence measurements. All spectra were blank subtracted: a blank spectrum was recorded using identical experimental conditions but without the terbium in the solution. In the RF 5000 spectrofluorimeter, the gain of the photomultiplier tube is dynamically adjusted to correct for changes in the Xe lamp intensity. This apart, we have not made any explicit correction of our spectra for instrument response, because in our experiments only relative changes in the fluorescence intensities of Tb 3 ÷ as a function of certain experimental conditions were required. 2.2. Reagents

Stock solutions of rare earth nitrates were prepared from the corresponding oxides (99.9% pure, Indian Rare Earths) as described previously [12]. Stock solutions of aromatic acids (Merck) were prepared by dissolving the acid in distilled water and adding the required amount of sodium hydroxide, wherever necessary. A 0.01 M stock solution of TOPO (Merck) in 10% Triton X-100 (Loba Chemie) was prepared by dissolving the reagents in distilled water. All compounds were used as purchased from the supplier.

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

2291

3. Results and discussion

The aromatic acids used in this study were phthalic acid (1,2-benzene dicarboxylic acid), isophthalic acid (1,3-benzene dicarboxylic acid), terephthalic acid (1,4-benzene dicarboxylic acid), hemimellitic acid (1,2,3-benzene tricarboxylic acid), trimesic acid (1,3,5-benzene tricarboxylic acid), pyromellitic acid (1,2,4,5-benzene tetracarboxlic acid), mellitic acid (1,2,3,4,5,6-benzene hexacarboxylic acid), o-toluic acid, monomethyl phthalate and monomethyl terephthalate. We will first discuss fluorescence enhancement in the terbium-aromatic acid complexes. The effect of TOPO will be discussed later. 3. I. Tb 3 + - a r o m a t i c acid complexes

At the outset, for each of the acids mentioned above, Tb 3 + fluorescence was measured as a function of the aromatic acid concentration and the pH of the solution. The acid concentration was varied from 1 x 10-SM to 1 × 10 -3 M. It was found that for the Tb 3+ concentration used in this study (10 -6 M), the optimum acid concentration was 10-4M for all the acids, except benzoic, o-toluic and monomethyl phthalic acid, for which 10 -3 M was found to be optimal. For all further experiments, these optimal acid concentrations were maintained. The dependence of the fluorescence intensity of Tb 3 + on pH is shown in Fig. 1 for the different Tb 3 + -acid complexes. It can be seen that the fluorescence intensity shows a maximum near pH 6 for all the Tb 3 + -acid complexes, except in the case of the mellitic acid complex, for which a pH of 9 was the best. In Fig. 1 the different aromatic acids have been classified into three different groups; the reasons for such a classification is discussed later.

o Benzoic acid • Monomethyl terephthalic acid o o-Toluic acid • Monomethyl phthalate

Trimesic acid o Terephthalic acid a Isophthalic acid

o Phthalic acid • Mellitic acid a Pyromellitic acid o Hemimellitic acid

I

2

I

4

i

I

6

I

1

8

r

10

pH

Fig. 1. The dependence of the fluorescence intensity on pH for the different Tb 3+ -aromatic acid complexes. Concentrations of the reagents used are given in the text. Emission wavelength monitored, 544 nm.

2292

B.S. Panigrahi et aL/Spectrochimica Acta Part A 51 (1995) 2289-2300

200

400 (nm)

Fig. 2. Excitation spectra of (a) Tb 3 + (1 × 10 -3 M); (b) Tb 3 + (0.5 × 10 - 6 M ) - i s o p h t h a l i c acid (l × 10 - 4 M); (c) Tb 3+ (0.5 × 10 - 6 M ) - t e r e p h t h a l i c acid (1 x 10 - 4 M). Emission wavelength monitored, 544 nm. Intensity scales are the same for all figures.

Fig. 2(b,c) shows the excitation spectra obtained by monitoring the 544 nm emission of Tb 3+ in Tb 3 ÷ -isophthalate and Tb 3÷ -terephthalate complexes in aqueous medium at pH 6. Also shown for comparison is the excitation spectrum for bare Tb 3 ÷ (without the aromatic acid) in aqueous medium (Fig. 2(a)). While the excitation spectrum shown in Fig. 2(a) agrees well with the absorption spectrum reported for Tb 3 + [2], those shown in Fig. 2(b,c) are completely different and resemble closely the absorption spectra typical of aromatic carboxylic acids, such as benzoic acid [16,17]. Fig. 3(b,c) shows the emission spectra for the Tb 3+ -isophthalate and Tb 3÷ -terephthalate complexes, both of which look identical and agree well with the emission spectrum reported for Tb 3 ÷ [3]. For comparison, the emission spectra of bare Tb 3 + (without the aromatic acid) is also shown in Fig. 3(a). It should be noted that though the emission spectra are identical in all cases, the excitation wavelengths used to record these spectra were all different. For Tb 3 + (Fig. 3(a)), the excitation wavelength used was 350 nm, which corresponded to an absorption maximum for the bare Tb 3 +. However, for the Tb 3+ -isophthalate and Tb 3+ -terephthalate complexes, the excitation wavelengths used were 242 nm and 265 nm, respectively, which were the corresponding excitation maxima for the two complexes. At these excitation wavelengths, uncomplexed Tb 3+ showed no perceptible fluorescence at concentrations of 10 -6 M, employed for recording emission spectra of the Tb 3 + -aromatic acid complexes. This clearly indicates that all of the Tb 3+ emission that we observed with the complexes was due to ligand sensitized fluorescence and there was no contribution to fluorescence from direct excitation of Tb 3 +. Although the emission intensities shown in Fig. 3(a-c) are comparable, the concentrations of Tb 3 ÷ used to record these spectra were different. The concentrations of Tb 3 + used to record the emission spectra of Tb 3 + -isophthalate and Tb 3 ÷ -terephthalate (Fig.

2293

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

ol kexc = 2 6 5 n m

b kexc = 2 4 2 n m

a

~xc = 350 nm

450

600

(nm)

Fig. 3. Emission spectra of (a) Tb 3+ (1 × 10 -3 M); (b) Tb 3+ (0.5 × 10 -6 M)-isophthalic acid 10 --4 M); (C) Tb 3+ (0.5 × 10 - 6 M)-terephthalic acid (1 × 10 -4 M). Intensity scales are the same for all figures. (l ×

3(b,c)) w e r e a b o u t 1000 t i m e s l o w e r t h a n t h a t u s e d to r e c o r d t h e e m i s s i o n s p e c t r u m f o r b a r e T b 3 + (Fig. 3(a)). T h e e n h a n c e m e n t in f l u o r e s c e n c e is t h e r e f o r e o b v i o u s . It is this f a c t o r o f 1000 t h a t we refer to as the f l u o r e s c e n c e e n h a n c e m e n t f a c t o r . T h e results d i s c u s s e d a b o v e f o r i s o p h t h a l a t e a n d t e r e p h t h a l a t e w e r e t y p i c a l o f all t h e a r o m a t i c a c i d s u s e d in this study. H o w e v e r , to a v o i d a r e p e t i t i v e d i s p l a y o f s p e c t r a , w e h a v e s h o w n o n l y t h o s e o f the T b 3 + - i s o p h t h a l a t e a n d T b 3 ÷ - t e r e p h t h a l a t e c o m p l e x e s . All o f the a r o m a t i c acids e n h a n c e d t h e f l u o r e s c e n c e o f T b 3 + r e l a t i v e to t h a t o f b a r e T b 3 + ( a q u o ) , by a b o u t t h r e e ~rders o f m a g n i t u d e . A s c a n be e x p e c t e d , t h e m a x i m a in t h e e x c i t a t i o n s p e c t r a f o r t h e v a r i o u s T b 3 + - a r o m a t i c a c i d c o m p l e x e s w e r e different. T h e 2ma x in the e x c i t a t i o n s p e c t r a for t h e differe" t T b 3 + - a r o m a t i c a c i d c o m p l e x e s a r e listed in Table 1 Optimum pH, excitation wavelengths and relative fluorescence intensity a for the different T b 3 ÷ aromatic acid complexes with and without TOPO-Triton X-100 Aromatic acid

Benzoic o-Toluic lsophthalic Terephthalic Trimesic Phthalic Hemimellitic Pyromellitic Mellitic Monomethyl phthalate Monomethyl terephthalate a

Tb 3 + -aromatic acid

Tb 3 + -aromatic acidTOPO-Triton X-100

pH

2exJnm

RF

pH

2¢xc/nm

RF

6.0 6.0 6.0 6.0 6.0 6.0 7.0 7.0 9.0 6.0 6.0

273 273 242 265 249 240 245 262 266 256 262

100 51 241 273 366 164 82 108 139 19 72

6.0 6.0 4.5 4.5 4.0 6.0 6.0 7.0 9.0 6.0 5.5

288 288 288 288 288 288 288 291 294 288 288

6790 7690 1737 3588 3967 74 31 78 117 66 3167

Relative fluorescence (RF) intensity is the fluorescence intensity of the complex relative to that of Tb 3 + complex (without TOPO-Triton X-100), which is taken as 100.

zoate

SA(A) 51:13-F

-ben-

2294

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

Table 1. Table I also gives the fluorescence enhancement in the different Tb 3 + aromatic acid complexes relative to that of the Tb 3 + - b e n z o a t e complex. 3.2. Tb 3 .

aromatic a c i d - T O P O

Triton X-IO0

The use of T O P O as synergistic agent to enhance the fluorescence of Tb 3+ by minimizing the collisions between the lanthanide ions and water molecules is well known [6-8,12,13]. However, as mentioned earlier, when aromatic acids were used as ligands, the synergism displayed by T O P O showed interesting differences for the various aromatic acids. We will discuss these aspects under two sections. We will first consider those aromatic carboxylic acids in which no two carboxyl groups are ortho to each other. Later, we will discuss our results for those acids, where at least two carboxyl groups are ortho to each other. The reason for such a classification will become apparent as the discussion proceeds. 3.3. Aromatic acids in which no two carboxyl groups are ortho to each other

Of the aromatic carboxylic acids that we had chosen for this study, those that fell into this category were isophthalic acid, terephthalic acid and trimesic acid. Because the results and spectra for the case of trimesic acid have already been published [13], we will present here only the results for the Tb 3 + isophthalic and Tb 3 + terephthalic systems. In our earlier work, where benzoic acid and trimesic acid were used as ligands, we found that the optimum concentration of T O P O was 1 × 1 0 - 4 M [12,13]. The same concentration of T O P O was also used in the present work. Because T O P O is insoluble in aqueous medium, it was solubilized using a non-ionic surfactant Triton X-100. The concentration of Triton X-100 was maintained at 0.1%, which again was found to be optimal in our previous work [12]. It should be noted that this concentration of Triton X-100 is above its critical micetle concentration. We observed that the fluorescence enhancement of Tb 3+ in the Tb 3 + - i s o p h t h a l a t e and Tb 3 + -terephthalate complexes following the addition of T O P O was singificant near p H 4.5, whereas at p H 6 addition of T O P O had a negligible effect on the fluorescence of Tb 3 + in these complexes (Fig. 4). Similar results were also obtained for the Tb 3 + trimesate complexes, where a p H of 4 was found to be optimal [13]. Fig. 5(a,b) shows the excitation spectra obtained by monitoring the 544 nm emission of Tb 3+ in the Tb 3+ i s o p h t h a l a t e - T O P O - T r i t o n X-100 and T b 3 + - t e r e p h t h a l a t e T O P O Triton X-100 system at different p H values. For comparison, the excitation spectra of the two T b 3 + - a c i d complexes (without T O P O Triton X-100) at their optimum pH of 6, are also shown. It can be seen that at pH 6, the addition: of T O P O - T r i t o n X-100 alters the excitation spectra somewhat. However, as the p H of the solution is lowered, the excitation spectra change significantly and increase in intensity, reaching a maximum near pH 4.5. This behaviour is identical to what was observed with trimesic acid. We therefore conclude that this sort of pH dependence is typical of polycarboxylic aromatic acids, where the carboxylic groups are not ortho to each other. These result indicate that near pH 6, when the polycarboxylic aromatic acids mentioned above are used as ligands, T O P O does not significantly participate in the complexation process. This is shown by the fact that the excitation spectra of the Tb 3 + - a r o m a t i c acid T O P O - T r i t o n X-100 system is only slightly different from the Tb 3+ aromatic acid complexes near pH 6. However, as the pH is lowered T O P O appears to complex with Tb 3 +, as shown by the change in the excitation spectra. This phenomenon could be due to a subtle participation of Triton X-100, which is basically used to solubilize T O P O in the aqueous phase. TOPO, being hydrophobic, is otherwise insoluble in water. The enhanced solubility of T O P O in the aqueous phase in the presence of Triton X-100, results from T O P O being trapped, possibly in the palisade layer of the micelle formed by Triton X-100, with the polar P-~) group oriented towards the polar head groups of the surfactant and the nonpolar octyl groups directed towards the interior of the micelle [18].

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

/

2295

\

/

\ o o-Tolule acid o b e n z o i c acid • M o n o m e t h y l t e r e p h t h a l i c acid • M o n o m e t h y l p h t h a l i c acid

Trimesic acid o Terephthalic acid o Isophthalic acid

o • a o

i 2

I 4

'

I 6

l

I 8

Phthalic acid Mellitic acid P y r o m e l l i t i c acid H e m i m e l l i t i c acid

i

10

pH

Fig. 4. The dependence of the fluorescence intensity on pH, for the different Tb 3 + -aromatic a c i d - T O P O - T r i t o n X-100 complexes. Concentrations of the reagents used are given in the text. Emission wavelength monitored, 544 nm.

For TOPO to complex with Tb 3+ in the Tb 3 + - a r o m a t i c acid complexes, it is necessary for TOPO, embedded in the micelle, to approach the Tb 3+ - a r o m a t i c acid complex closely, so that the P--O group in TOPO can be in close proximity to the Tb 3 + ion. However, such a close approach of T O P O - T r i t o n X-100 to Tb 3 + may be impeded if the ligand has charged groups on it, such as a C O 0 - . This is so, because ion-dipolar interactions between these charged groups and the polar head groups of the micelle can localize the micelle (and hence the TOPO inside it) near the charged groups and prevent the approach of TOPO to the central metal ion Tb 3 +. In such an event TOPO would not complex with the Tb 3+ in the Tb 3 + - a c i d complex to display any synergism. Such a situation exists near pH 6, where the aromatic polycarboxylic acids have more than one carboxyl group ionized, thereby carrying a multiple charge on the ligand. For example, at pH 6 terephthalic acid exists largely in the doubly ionized form (96%) as calcualted from its pKa values (pK~ = 3.54 and pK2 = 4.46) [19]. Assuming that one of the ionized carboxyl groups were to complex with Tb 3 +, there would still be an other ionized carboxyl group in the terephthalate ligand (not complexed with the metal ion) which would interfere with TOPO complexing with Tb 3 +. However, near pH 4 is is the singly ionized form of this acid that dominates. When this singly ionized carboxyl group enters into bonding with Tb 3+, the Tb 3+-terephthalate complex is left with only an unionized carboxyl group, which would not interact as strongly with the polar head groups of Triton X-100 as the carboxylate anion, and therefore does not interfere with TOPO complexing with Tb 3 +. Hence, in the case of iigands with no charged groups on them (other than the one that has already bonded to Tb 3 +) a close approach of TOPO to Tb 3 + becomes possible leading to synergism. Clearly for maximum synergism to be observed with T O P O - T r i t o n X-100 the lower pH (about 4) in the case of

2296

B.S. Panigrahi et al./Spectrochimica Acta Part ,4 51 (1995) 2289-2300

polycarboxylic acids is required to keep these acids in the singly ionized form. In the case of monocarboxylic acids such as benzoic acid the singly ionized form is the only one possible and therefore synergism with T O P O - T r i t o n X-100 was observed even at p H 6. The above rationalization implies that if terephthalic acid were to be converted to a monocarboxylic ligand by esterifying one of its carboxyl groups, then it should behave in a manner similar to benzoic acid. It was indeed found that when monomethyl terephthalate was used as the ligand, the optimum pH for fluorescence enhancement due to synergism was now near p H 6.0 (Fig. 4). Fig. 6 shows the excitation spectra of the Tb 3 + - m o n o m e t h y l terephthalate T O P O Triton X-100 complex as a function of pH, clearly revealing the greater similarity of the monomethyl terephthalate ligand to benzoate than to terephthalate. It should be noted that for synergism to be observed both T O P O and Triton X-100 are required. We have seen that the addition of only Triton X-100 to the T b 3 + - a c i d complex does not enhance the fluorescence of the complexes at any of the p H values. 3.4. Aromatic acids in which carboxylic groups are ortho to each other

Under this category of acids, we have studied phthalic acid, hemimellitic acid, pyromellitic acid and mellitic acid. The emission spectra of Tb 3 + - a r o m a t i c a c i d - T O P O - T r i t o n X-100 complexes over the p H range 3 to 7 for the different acids mentioned above, were recorded and compared with the emission spectra of the Tb 3 + - a r o m a t i c acid (without T O P O - T r i t o n X-100). It was found that with these acids, the addition of T O P O did not enhance the fluorescence of the Tb 3 + - a r o m a t i c acid complex at any of the pH values. If anything, the fluorescence intensity was a little lower than that obtained before the addition of T O P O - T r i t o n X-100. Furthermore, near pH 4, the fluroescence intensity was slightly lower than that observed near p H 6. Fig. 7 shows the excitation spectra of the Tb 3 + - a r o m a t i c a c i d - T O P O - T r i t o n X-100 complexes over the same p H range. Again to avoid a repetitive display of spectra, we

pH 4.5

pH 4.5

~em = 544 n m

~'m = 544 nm

pH 4.0 pH 4.0

pH 5.0

pH 5.0

Lv]ll 2~

400 Inm)

200

400

(nm)

Fig. 5. Excitation spectra of Tb3+ -aromatic acid TOPO Triton X-100 complexesas a function of pH for the two ligands, (a) isophthalic acid and (b) terephthalic acid. Excitation spectrum of the Tb3+-acid complex without TOPO-Triton X-100 is also shown (dotted line). Concentrations used are Tb3+ (1 x 10 -6 M), TOPO (1 x 10-4 M), aromatic acid (1 × l0 -4 M).

B.S• Panigrahi et al,/Spectrochimica Acta Part A 51 (1995) 2289-2300

2297

pH 5.0 pH 6.0 ~'em = 5 4 4 n m

pH 4.5

pH 4.0 pH 7.0

....

,

f

,,,

200

400 (nm)

Fig. 6. Excitation spectra of Tb 3+ - m o n o m e t h y l t e r e p h t h a l a t e - T O P O - T r i t o n X-100 complex as a function of pH. Excitation spectrum of the Tb 3 + - a c i d complex without T O P O - T r i t o n X-100 is also shown (dotted line). Concentration of monomethyl terephthalate (1 x 10 - 4 M); T O P O and Tb 3 ÷ concentrations are as given in Fig. 5.

show only those of phthalic acid and hemimellitic acid complexes which are typical. It is clear from the figure that the excitation spectra change very little at any of the pH values. Though all these aromatic acids, which had at least two carboxyl groups ortho to each other, enhanced the fluorescence of T b 3 + itself by about three orders of magnitude, the

:

!;i.

Aem= 544 nm

:

Z,,m = 5 4 4 n m

".

:

',

k./ pH 6.0 pH 5.0 pH 4.0

• ..,.,...,

200

(nm)

pH 6.0 pH 5.0 p H 4.0

:/

...............

400 : 2 0 0

(nm)

400

Fig. 7. Excitation spectra of Tb 3 + - a r o m a t i c a c i d - T O P O - T r i t o n X-100 complex as a function of pH for the two ligands (a) phthalic acid and (b) hemimellitic acid. Excitation spectrum of the Tb 3 ÷ - a c i d complex without T O P O - T r i t o n X-100 is also shown (dotted line)• Concentration of aromatic acid (1 × 10 -4 M); T O P O and Tb 3 + concentrations are as given in Fig. 5.

2298

B.S. Panigrahi et al./Spectrochimk:a Acta Part A 51 (1995) 2289-2300

I

pH6.0

Z~,m= 544nm pH 5.0

pH7.0

200

(nm)

400

Fig. 8. Excitation spectra of Tb 3 + - m o n o m e t h y l p h t h a l a t e - T O P O - T r i t o n X-100 complex as a function of pH. Excitation spectrum of the Tb 3 + - a c i d complex without TOPO-Triton X-100 is also shown (dotted line). Concentration of monomethyl phthalate (1 x 10 -3 M); TOPO and Tb 3 + concentrations are as given in Fig. 5.

fluorescence of Tb 3+ in these acid complexes was not enhanced any further by the addition of T O P O at any of the p H values. In this regard, these acids behaved differently from benzoate or any of the acids discussed in the earlier section. At p H values near 6, these acids are again multiply charged and hence it is not surprising that they do not show any synergism with TOPO, for the reasons discussed in the previous section. However, near p H 4 these acids exist predominantly in the singly ionized form and hence a close approach of T O P O with the Tb 3 + now becomes possible. Complexation of Tb 3 + with T O P O should now be possible and probably does take place. However, the presence of an unionized carboxyl group in the ortho position leaves an O - H group (carboxylic) in close proximity to Tb 3 + which could quench the Tb 3 + fluorescence in a manner similar to the O - - H oscillator in H20. While T O P O can successfully shield the Tb 3 + from collisions with the solvent H 2 0 molecules, it can do very little to shield the Tb 3 ÷ from this "inbuilt" quencher. Hence no enhancement is observed at any of the p H values following the addition of T O P O in cases where there are carboxyl groups ortho to each other. If, however, one of the carboxyl groups were to be esterified, then this "inbuilt" O - H quencher would essentially be removed, and fluorescence enhancement following the addition of T O P O - T r i t o n X-100 should be observed. This was indeed found to be the case when monomethyl phthalate was used as the ligand. Though the fluorescence enhancement of Tb 3 ÷ - m o n o m e t h y l phthalate complex itself was about five times lower than that observed with benzoate, synergism was observed when T O P O - T r i t o n X-100 was added to the Tb 3 ÷ - m o n o m e t h y l phthalate complex. A factor of three enhancement in fluorescence due to synergism was observed in this case, unlike with the phthalate complex where no enhancement was seen following the addition of T O P O - T r i t o n X-100. Though the enhancement due to synergism in the case of the monomethyl phthalate ligand was not as dramatic as was observed with ligands such as benzoate, the fact that enhancement was observed at all, supports our explanation. Fig. 8 shows the p H dependence of the excitation spectra of the Tb 3 + - m o n o m e t h y l p h t h a l a t e - T O P O - T r i t o n X- 100 system.

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

2299

The reason for the rather moderate enhancement due to synergism in the case of the monomethyl phthalate ligand is probably due to the presence of C - H oscillators in close proximity to Tb 3+ in the Tb 3 + - m o n o m e t h y l phthalate complex. Similar to the O - H vibrations, the C - H bonds can also quench the Tb 3+ fluorescence, though less efficiently, because the vibrational frequency of C - H (about 2950 c m - 1) is less than that of O - H (about 3450 c m - l ) [20]. In the Tb 3 + -monomethyl phthalate complex, the Tb 3 + fluorescence is quenched by both the O - H of the solvent water molecules and the C - H of the methyl group in the ester. When T O P O - T r i t o n X-100 is added to the Tb 3+-monomethyl phthalate complex, the stronger quencher, i.e. O - H oscillator of the solvent molecules, is kept out leading to a corresponding fluorescence enhancement. However, the C - H oscillators in close proximity to the Tb 3+ continue to quench the Tb 3÷ fluorescence. T O P O - T r i t o n X-100 therefore reduce the quenching processes only partially and hence only a moderate enhancement due to synergism was observed in this case. It is also interesting to note that the monomethyl phthalate displayed maximum synergism with T O P O - T r i t o n X-100 near pH 6 (Fig. 4); behaviour in keeping with its monocarboxylic nature. If the carboxymethyl group (COOMe) in the monomethyl phthalate ligand is replaced by a methyl group (o-toluic acid), the distance of the C - H oscillators from the Tb 3+ would increase. The increased distance would reduce the quenching efficiency of the C - H oscillators, resulting in a greater fluorescence enhancement in the case of the o-toluic acid ligand. To confirm this point, we studied the fluorescence enhancement of Tb 3 + using o-toluic acid as the ligand. The fluorescence enhancement of the Tb ~+-toluic acid complex itself was about a factor of two lower than that observed using benzoate. However, when TOPO Triton X-100 was added, the toluic acid ligand showed an enhancement due to synergism by almost two orders of magnitude and near pH 6. It must be noted that the pH dependence of synergism in the case of toluic acid also is identical to that of benzoic acid, and not surprisingly so because both are monocarboxylic acids. Table 1 summarizes the data on the optimum pH, excitation maxima and the fluorescence enhancement in the different Tb 3 + - a r o m a t i c a c i d - T O P O - T r i t o n X-100 complexes relative to that of the Tb 3 + - b e n z o i c a c i d - T O P O Triton X-100 complex.

4. Conclusions

Aromatic carboxylic acids serve as excellent ligands in enhancing the fluorescence of the lanthanides Tb 3+, Dy 3+ and Eu 3+. The use of these ligands resulted in an enhancement of the lanthanide fluorescence by about three order of magnitude. When these complexes were treated with T O P O - T r i t o n X-100, these ligands behaved differently and could be classified into three groups. The monocarboxylic acids formed the first class; at pH 6 these acids showed synergism when TOPO was added. Polycarboxylic acids with no carboxyl groups ortho to each other form the second class. In these cases, addition of TOPO does show synergism, but near pH 4. In the third class, we have the polycarboxylic acids that have at least two carboxyl groups ortho to each other. In these cases we found that the addition of TOPO does not lead to any synergism at any of the pH values. Fig. 4 summarizes the pH dependence of fluroescence enhancement of Tb 3 + following the addition of TOPO for all aromatic acids that we have studied. The three classes have been shown separately in the figure. This study has clearly revealed the utility of aromatic carboxylic acids as ligands to sensitize the fluorescence of the trivalent lanthanides (Tb, Dy and Eu), and together with T O P O - T r i t o n X-100 they enhance the lanthanide fluorescence by four orders of magnitude. This opens up the possibility for the use of these ligands in the fluorimetric estimation of the lanthanides at trace levels.

2300

B.S. Panigrahi et al./Spectrochimica Acta Part A 51 (1995) 2289-2300

References [1] G. Stein and E. Wurzberg, J. Chem. Phys., 62 (1975) 208. [2] W.T. Carnall, Handbook of the Physics and Chemistry of Rare Earths, Vol. 3, Elsevier, Amsterdam, 1979, Chap. 24. [3] J.-C.G. Bunzli, in J.-C.G. Bunzli and G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, New York, 1989, Chap. 7. [4] F.S. Richardson, Chem. Rev., 82 (1982) 541. [5] J.-H. Yang, G.-Y. Zhu and B. Wu, Anal. Chim. Acta, 198 (1987) 287. [6] T. Taketatsu and A. Sato, Anal. Chim. Acta, 108 (1979) 429. [7] T. Taketatsu, Talanta, 29 (1982) 397. [8] G. Zhu, Z. Si, J. Yang and J. Ding, Anal. Chim. Acta, 231 (1990) 157. [9] M. Morin, R. Bador and H. Dechaud, Anal. Chim. Acta, 219 (1989) 67. [10] L.M. Perry and J.D. Winfordner, Anal. Chim. Acta, 237 (1990) 273. [ll] Y.-Y. Xu, I.A. Hemmila and T.N.E. Lovgren, Analyst, ll7 (1992) 1061. [12] S. Peter, B.S. Panigrahi, K.S. Viswanathan and C.K. Mathews, Anal. Chim. Acta, 260 (1992) 135. [13] B.S. Panigrahi, S. Peter, K.S. Viswanathan and C.K. Mathews, Anal. Chim. Acta, 282 (1993) 117. [14] F. Halverson, J. Brinen and J. Leto, J. Chem. Phys., 41 (1964) 157. [15] F. Halverson, J. Brinen and J. Leto, J. Chem. Phys., 41 (1964) 2752. [16] H. Hosoyo, J. Tanaka and S. Nagakura, J. Mol. Spectrosc., 8 (1962) 257. [17] M. lto, J. Mol. Spectrosc., 4 (1960) 144. [18] M.J. Rosen, Surfactants and Interfacial Phenomenon, John Wiley, New York, 1989. [19] J.A. Dean, Handbook of Organic Chemistry, McGraw-Hill, Singapore, 1987, Sect. 8, pp. 8-9, 8-10, 8-39, 8-47. [20] Y. Haas, G. Stein and E. Wurzburg, J. Chem. Phys., 60 (1974) 258.