Intermolecular energy transfer between lanthanide complexes in aqueous solution—III

Z imo~.n~cl. Cium. Vol. 41, pp. 567-569 © PerlpumonPress Ltd., 1979. Printedin Great Britain

0022-190217910401..05671S02.0010

INTERMOLECULAR ENERGY TRANSFER BETWEEN LANTHANIDE COMPLEXES IN AQUEOUS SOLUTION--III STUDIES OF TERBIUM(III) AND EUROPIUM(III) COMPLEXES TRIMELLITIC, PYROMELLITIC AND TRIMESIC ACIDS

OF

HARRY O. BRI'VrAIN* Department of Chemistry, Ferrum College, Ferrum, VA 24088, U.S.A.

(Received 14 July 1978. received/or publication 20 September 1978) Abstract--lntermolecular energy transfer, emission intensity, and emission lifetime measurements were used to study the solution phase coordination chemistry of terbium(III) and europium(IIl) complexes of trimellitie and pyromellitic acids at low pH. Solid polymeric compounds were found to precipitate out of solution once the pH reached 3.0, and these compounds were characterized by analytical means. In addition, the adduct formed between Tb3+ and trimesic was also characterized and found to be polymeric. The trimellitic and pyromellitic acid complexes appear to begin metal-ligand binding as the pH is raised from 1.5 to 2.5, and polymeric association among the complexes appears to begin at a pH of 2.5. The extent of polymerization increases up to the point of precipitation.

INTRODUCTION The coordination chemistry of lanthanide ions in aqueous solution is a poorly understood area of inorganic chemistry. Formation constants[l] and solid state crystal structures[2] abound for ianthanide complexes, but information regarding the nature of metal-ligand bonding in the solution phase has proved difficult to obtain. It has been shown, however, that lanthanide complexes of amino acids are polymeric in the hydrolysis region[3]. This association among complexes makes i t difficult to accurately describe the binding that actually takes place, in spite of additional evidence from circular dichroism [4] and circularly polarized emission studies[5]. In this laboratory, intermolecular energy transfer energy transfer among lanthanide complexes has been used as a probe to investigate the bonding in metal complexes of pyridinecarboxylic acids[6], and of phthalic and hemimellitic acids[7]. The present report details further studies involving ianthanide complexes of trimellitic acid (1,2,4-benzenetricarboxylic acid, or TML), pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, or PML), and trimesic acid (1,3,5-benzenetricarboxylic acid, or TMS). EXPERIMENTAL Trimellitic, pyromellitic, and trimesic acids were obtained from Aldrich and were used as received. TbCI3.6H20 and EuCI3-tH20 were obtained from Alfa as being 99.9% pure and thus was used without subsequent purification. Solutions containing 1:5 and I : I0 metal-ligand ratios were made up from concentrated stock solutions as previously described[6,7]. Ligand solutions were standardized titrimetrically with KOH and lanthanide solutions were standardized speetrophotometrically with hydroxy anphthol blue[8]. The procedures followed in the current set of experiments is the same as previously described for complexes of phthalic and hemimellitic acids[7]. Carbon, hydrogen, and oxygen analyses of the precipitated compounds were carried out using standard methods[9]: lanthanide ion analyses were performed by ignition of sample to the rare earth oxide. All emission measurements were made on equipment con*Present address: Department of Chemistry, Seton Hall University, South Orange, NJ 07079, U.S.A.

structed in this laboratory and previously described[6], pH measurements were taken on a Fisher Acumet 144 pH meter and were made by inserting the glass microcombination electrode directly into the fluorescence cuvette. Emission lifetime measurements were obtained in the same way as before [6.7].

RESULTS AND DISCUSSION The data obtained for the complexes in this study are much more limited than has been obtained with pyrio dinecarboxylic acids[6] and other benzenecarboxylic acids[7] due to the more limited solubility of the complexes investigated here. Precipitation of a Tb~*/TML complex was observed once the solution pH was raised beyond 3.0, and a Tb3*/PML precipitate formed once the pH exceeded 3.2. The Tb3÷/TMS complex was'not found to be soluble at any pH, and thus could not be studied by emission techniques. All of the precipitated compounds exhibited strong Tb ~÷ emission and all were found to be polymeric in nature. The precipitated compounds formed from solutions containing a !:5 and a l:10 metai-ligand ratio were determined to have the same composition. Analytical data indicated that the Tb3+/TML precipitate actually had the formula Tb4TML)s-3H20, the Tb3*/PML precipitate has a formula of Tbs(PML)3.3H20, and the Tb3*/TMS precipitate has a formula of Tb,o(TMS)7.6H20. All of the Tb3÷/ligand complexes showed an enhanced Tb 3÷ emission as the pH was raised above 1.5. The pH dependence of emission observed for Tb3÷/TML and Tb3*/PML is shown in Fig. I. At a pH of 1.5, very little enhancement of Tb 3÷ emission intensity is seen, which suggests that no binding takes place between the lanthanide ion and the ligand at this pH. It is well known[10] that enhanced lanthanide ion emission is observed with the metal is complexed by a ligand, and the rise in emission beyond a pH of i.5 indicates that metai-ligand binding is taking place even at very low pH values. It is also well known[10] that complexation of a rare earth ion will result in an increased emission lifetime. Table l details the variation in Tb 3÷ emission lifetimes observed for TML and PML complexes. The emission 567

568

H.G. BRITTAIN Table 2. Stern-Volmer constants for the intensity quenching of Tb3+/ligaademission by Eu3*/ligandas a functio of pH~°) 25

THL(b)

THL(c)

P}R-(b)

PHL(¢)

Ksv ~ x

Kay ~ x

Kay ~ x

Ksv 6 x

i0-3

10-3

i0"3

10-3

1.6

1.16

1.15

1.13

1.16

1.8

1.13

i.ii

1.09

I.Ii

2.0

1.14

1.14

1.17

1.15

2.2

1.19

1.18

1.18

1.17

2.4

1.31

1.35

1.20

1.21

2.6

1.40

1.56

1.29

1.26

2.8

1.53

1.78

1.31

1.30

3.0

1.87

2.21

1.39

1.47

1.52

1.71

Ligand

pH 20

£

15

3.2

I 15

I 2.0

pM

I 25

1 3D

-

a) All v a l u u of Kay ~ carry an error of

0.05.

b) Reaules observed for 1:5 metal-to-llgand

Fig. 1. Emission intensity of the ~D,-*TFs transition of TbJ+/TML and Tb3+IPMLas a function of pH. Data are shown for 1: 5 (O) and I : 10 (A) solutions of Tb3*/TML,as well as for 1: 5 (O) and i : 10 (A) solutionsof Tb3+IPML.The intensity scale is relative m the intensity of Th3+IH:O.

c) Results observed f o r 1:10 amcal-to-llgand

ratioe. ratios.

Eu3+ quencher, I is the intensity with quencher present, [Q] is the molar concentration of Eu3÷/ligand quencher. and K;o is the Stern-Volmer constant. Values found for Tablel. pH dependenceof Th3+emission lifetimes in Tb~+/ligand the intensity quenching of Th3+ complexes with TML complexes and PML ligands by corresponding Eu3÷ complexes are found in Table 2. At low pH the values of K;, are all pX Tb3+/THL Tb3+/PML approx 1.1 x 103, but as the pH is raised to 2.5 and beyond the quenching of Th3÷ by Eu3÷ becomes more 1.6 410 415 efficient. 1.8 415 420 It is known[12] that luminescence quenching may arise 2.0 425 425 from two mechanisms: coilisional deactivation of the .donor by the quencher (dynamic quenching) and 2.2 440 430 complex formation between donor and quencher (static 2.4 455 440 quenching). Dynamic quenching elects both the emis2.6 470 445 sion intensity (I) and the emission lifetime 09, while static quenching elects only the emission intensity. An 2.8 480 455 equation for lifetime quenching may be formulated 3.0 490 465 analogous to eqn (1): 3.2

480 ro - • = K;~[Q]. 1-

(2)

All values are in ~eac and are aaaoclacad wi~h an error of

10)mac.

Data were taken f o r 1:5 m e c a l - t o - l i g a n d r a t i o s .

lifetime of an uncomplexed Th3÷ ion is known to be 400/~sec[ll]; at low pH values the observed Tb 3÷ lifetime is very close to this value. The emission lifetime increases as the pH is raised, and this increase provides additional support for extensive metai-ligand binding at these very low pH values. Addition of Eu3+/ligand to a solution of ThJ+/ligand results in a quenching of the emission intensity and lifetime of the Tb 3+ ion. the intensity quenching was analyzed by the standard Stern-Volmer equation:

Io~ I K*,.[Q], =

where

Io is the Tb3÷ emission

(1)

intensity in the absence of

If KY~ and K:~ are the same, then one may conclude that no static quenching is taking place and that complex formation between donor and quencher is absent. On the other hand, if K~, is greater than K:~, then association of the two must be taking place. In all of the Th3÷/ligand solutions studied in the present work, the value of K~o was found to be l.l-+ 0.1 x 103. This value is identical to the values of KY~ at low pH, but not in the more basic solutions, The quenching data then indicate that once the pH exceeds about 2.5, association of Th~÷/ligand and Eu3÷/iigand complexes begins to take place. This association is seen to increase up to the point of precipitation, where polymeric complexes may be isolated. Consideration of all the data enables one to describe the bonding situations in these lanthanide complexes of TML and PML ligands. At a very low pH of 1.5 no complexation of any sort is taking place; no enhance-

lntermolecular energy transfer ment of Tb ~÷ emission intensity is seen and the Tb 3~" emission lifetime is the same as for an uncoordinated Tb 3. ion. As the pH is raised from 1.5 to 2.5, some metal-ligand binding takes place (as evidenced in emission intensity enhancements and in the increase in emission lifetime), but no association among the complexes takes place. As the pH is raised from 2.5 to 3.0, metaltigand binding becomes more extensive (greater enhancements in emission intensity and lifetime) and association among the lanthanide complexes begins to take place. After the pH of 3.0 is passed, precipitation of polymeric complexes is observed, and these may be isolated and characterized. The Tb3+/TMS complex could not be studied by any of the emission spectroscopy techniques since the ligand was found to be insoluble below a pH of 5.25 and since the lanthanide complex was insoluble above this pH. The isolation of a polymeric complex indicates similar behavior to the TML and PML complexes, in that extensive association of lanthanide complexes takes place even at medium pH values.

Acknowledgement--This work was supported by a Cotrell grant

569

from the Research Corporation which made possible the purchase of the luminescence apparatus. REFERENCES 1. T. Moeller, D. F. Martin, L. C. Thompson, R. Ferrus, G R Feistel and W. J. Randall, Chem. Rev. 65, I (1965). 2. K. W~ Bagnall, Lanthanide and actinide carboxylates~ In Int. Rev. Science lnorg. Chem. (Series II) (Edited by K. W Bagnall), Vol. 7, ppo 42-63 (1975). 3. R. Prados, L. G. Stadtherr, H. Donato and R. B~ Martin. J lnorg. Nucl. Chem. 16, 689 (1974). 4. (a) L. I. Katzin and E. Gulyas, Inorg. Chem. 7. 2442/1%8): (b) L. I. Katzin, lnorg. Chem. 8, 1649 (1969). 4. H. G. Bfittain and F. S. richardson. Bioinorg Chem. 7 233 (1977). 6. H. G. BfittaJn, lno~+ Chem. 17, 2762 (1978). 7. H. G. Brittain, J. Inorg. Nucl. Chem. in press. 8. H. G. Brittain, Anal. Chem. Acta 96, 165 (1978). 9. J. B. Niederl and V. Nierderl, Micromethods of Quantitative Organic Analysis. Wiley, New York (1942). 10. L. N Lugina, N. K. Davidenko and K. B. YatsimirskiL Zh. Neorg. Khim. 18, 1453 (1973). I1. G. Stein and E. Wurzberg. J. Chem. Phys. 62, 208 (1975). 12. C. A. Parker, Photoluminescence of Solutions. Elsevier. New York (1968)