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Application of circularly polarized luminescence spectroscopy to the solution structure of racemic polyaminocarboxylate lanthanide (III) complexes
=N•H ~
JOURNAL OF
LUMINESCENCE
ELSEVIER
Journal of Luminescence 62 (1994) 17—23
Application of circularly polarized luminescence spectroscopy to the solution structure of racemic polyaminocarboxylate lanthanide (III) complexes Anna Mondry1, Stefan C.J. Meskers2, James P. Riehl*
Department of Chemistry, Michigan Technological University, Houghton, MI 49931-1295, USA Received 9 September 1993, revised 14 January 1994; accepted 14 January 1994
Abstract Circularly polarized luminescence from 1: 1 complexes of Eu(III) with triethylenetetra-aminehexaacetic acid (TTHA) following excitation with circularly polarized light is reported. This observation is consistent with a solution structure for this complex that is chiral, and stable on the emission time scale of Eu(III) (approximately 1.2 ms). Additional measurements show only a slight dependence on temperature, and an estimate for the activation energy for racemization (23 kcal/mol) may be obtained. Attempts at detecting enantio-selective quenching of Tb(TTHA)3 by optically active Ru(phen)~+ are also reported. Although significant quenching was observed, and the time-decay of the Tb(III) emission was not mono-exponential, no enantio-selectivity was found. This latter result may be interpreted in terms of the lack of discriminating short-range diastereomeric interactions between the chiral ion pairs. -
1. Introduction Lanthanide (III) ions are known to form a variety of solution complexes with polyaminocarboxylate ligands, and some of these complexes are becoming increasingly important due to their p0tential as contrast agents in NMR imaging [1]. The complex formed between Gd(III) and diethylenetriaminepentaacetic acid [2] (DTPA), for example, is already an FDA approved contrast agent. Various other complexes involving polyarninocarboxylic acids have been proposed and are *
Corresponding author. . . Permanent address: Institute of Chemistry, University of
Wroclaw, Wroclaw, Poland. 2 Present address: Leiden Institute of Chemistry, Leiden University, Netherlands.
being studied as potential imaging agents. Even though complexes involving triethylenetetraaminehexaacetic acid (TTHA) are probably not suitable as contrast agents, due to the fact that this ligand effectively excludes contact of the central ion with exchangeable water molecules, these cornplexes have, nevertheless, been studied fairly extensively due to their stability and related structural properties (see Scheme 1) [3—5]. In general, in order to develop a thorough understanding of the potential use of complexes of this type, it is important to learn as much as possible about the solution structure, and dynamics of these species. In thisand regard, various techniques have been proposed employed to study the structure .
of polyaminocarboxylate complexes of lanthanide (III) complexes. Although, for the specific case of NMR contrast agents, complexes containing
0022-2313/94/$07.OO © 1994 — Elsevier Science By. All rights reserved SSDIOO22-2313(94)00012-2
18
A. Mondrv eta!.
/
Journal of Luminescence 62 (1994) 17—23
HOOC ~OOH /
I COOH
/
I
~COOH
COOH
U HA Scheme
.
Gd(III) with its f7 valence configuration, and associated relaxation properties, is the target of much research effort [1], the use of other lanthanides with more suitable spectroscopic properties is a common approach. Considerable effort has been devoted, for example, to 1H and 13C NMR studies using the various diamagnetic rare earth ions, La(III), Y(III) and Lu(III) [5]. These three ions span the small range of ionic radii of the lanthanide series, and it is expected that results from these kind of studies yield a reasonably accurate description of the structure and structural integrity of the entire lanthanide series. In addition, the use of luminescent lanthanide (III) ions such as Tb(III) and Eu(III) have also been exploited [5,6]. For example, Holz and Horrocks [5] have recently characterized complexes of TTHA with Eu(III). Their studies included lifetime measurements, and site-selective 7F 5D excitation spectroscopy of the narrow 0 0 absorption. In this work it was concluded from lifetime measurements, that, below a pH of 6.0, both 2: 1 and 1: 1 metal ligand cornplexes were present, but that above a pH of 6.5 only the 1: 1 species exists. Even in solutions in which it was concluded that only 1: 1one species was lifetime, present through observation of only emission two peaks were observed in the 7F 5D 0 —f 0 region. This result was interpreted as being due to rapidly interconverting 1: 1 isomers. Brittain [6] has used the effect of added Eu(III) on the lifetime of Tb(III) to probe the existence of TTHA complexes or oligomers containing more than one lanthanide ion. From a series of lifetime measurements of 1: 1 solutions of lanthanide ion: TTHA as a function of pH, it was concluded that, between pH 4 and 6, —~
oligomeric species exist, but that above a pH of 8 only isolated 1: 1 Ln : TTHA complexes are present. Holz and Horrocks [5] also showed that in experiments in which TTHA was added to a solution of Eu(III) at a ratio of 1: 1 a small amount of the 2: 1 species was present, but that upon the addition of excess ligand only the 1: 1 species was observed. We havehigh beensymmetry involved in a numbercomplexes of studies of relatively lanthanide in which the luminescence measurement (CPL) of the hascircular been exploited polarization in order of to obtain information about the solution structure and dynamics of optically active lanthanide cornplexes [7—10].In order to measure a non-zero CPL from these systems, it is necessary to generate a net optically active excited (emitting) state. As described in more detail below, this can be obtained from a racemic mixture if a circularly polarized excitation source is used, as long as racemization in the excited state is slower than emission. The ligands used in these various studies included 2,6pyridine-dicarboxylic acid (DPA), oxydiacetic acid (ODA), irninodiacetic acid (IDA), and methyliminodiacetic acid (MIDA). Of the two possible tris-terdendate geometries based on a trigonal tncapped prism, the menidonal (D 3) and facial (C3h) isomers, only the meridonal isomer is optically active. In these studies, it was possible to measure CPL from complexes with DPA and ODA only. It was concluded, therefore, that these complexes have D3 symmetry, and lanthanide complexes with IDA and MIDA have C3h symmetry. These results were in agreement with previous measurements involving analysis of magnetic circularly polarized luminescence and total emission intensity [11]. In this paper we report CPL from solutions of 3 following circularly polarized excitaEu(TTHA) tion. This, in fact, represents the first measurement -
of CPL from a presumably racemic lanthanide complex that does not have approximate D3 symmetry. It is also demonstrated that the CPL shows very little temperature dependence from 10 to 80°C, indicating that the complex does not undergo significant rearrangements that would lead to racemization in this temperature range. Additional experiments involving quenching of 3 by resolved the Ru(phen)~ + are also Tb(TTHA) -
A. Mondry et al.
/ Journal of Luminescence 62
(1994) 17—23
19
reported. Unlike previous results [12—14] on the quenching of Tb(DPA)~ no enantio-selective quenching is found for Tb(TTHA)3 indicating a lack of diastereomeric discriminating interactions between the oppositely charged ions in this system.
systems, and the reader may consult Ref. [18] for a more complete discussion of recent theoretical concepts. The results are most often reported in terms of the luminescence dissymmetry ratio, gium, which is defined as the ratio of the differential emission intensity to the average total emission intensity:
2. Experimental
gium
Stock solutions of Eu(III) chlorides were prepared from 99.9% Eu 203 (Aldrich) and Tb(III) solutions were prepared from TbC13 (Aldrich). The final concentrations were determined by standardization with EDTA using xylenol orange as an indicator. Stock solutions of triethylenetetraaminehexaacetic acid (TTHA) (98% Aldrich) were prepared at3 pHand = 7 Eu(TTHA)3 by neutralization with NaOH. complexes were Tb(TTHA) prepared in 1: 1.2 metal: ligand ratios at a pH of 6.5. Since this pH value is higher than the sixth ligand PKa [15] value, it is assumed that the dominant TTHA species in these solutions has a 6 charge. D 2O solutions were prepared by slowly evaporating 5 ml of the aqueous solution to dryness, and then dissolving the residue in 5 ml of D20. Ru(phen)~+ was prepared and resolved according to published procedures [16]. CPL and total luminescence spectra were recorded on an instrument constructed in our laboratory operating in a photon-counting mode. The time-decay of Tb(III) luminescence was performed in the laboratory of Dr. H.P.J.M. Dekkers at the University of Leiden, Netherlands. Excitation of Eu(III) was accomplished at 476.5 nm and Tb(III) at 488 nm with a coherent INNOVA-70 argon-ion laser. For the case of Eu(III), the laser beam was made circularly polarized ( > 95%) by placing a high-quality quar ter-wave plate in the excitation path.
Unlike circular dichroism (CD) measurements, CPL
—,
-
-
—
=
(1)
1(j±J)~
is not limited to molecular systems that have been at least partially resolved into enantiomers. Even if the solution is composed of a racemic mixtures of enantiomers, it may be possible to measure CPL if a non-racemic excited state can be prepared from the racemic ground state. Two ways of preparing chiral excited ground distributions that are not states chiral,from are the use state of circularly polarized excitation, and the use of enantio-selective quenchers. These processes are pictured schematically in Fig. 1 for a simple racemic mixture. If circularly polarized excitation is used then (R) kabs (5), and the excited state population will not be racemic as long as the racemization rate is slower than the emission rate. Hilmes and Riehl [19] have presented a theoretical description of the measurement of CPL from racemic mixtures in which the effects of racemization and radiationless energy transfer on gium were determined. Neglecting excited state energy transfer between enantiomers, one obtains the following expression for the measurement of ~ / k \ glum (A) = ~ gL(A) gabs (A) ° ). (2) \2k~0~~ + k01 kabs
(
-_____
_____
1< (R)
______
I kb(R)
In CPL spectroscopy, one measures the differ ence in intensity between left (IL) and right (IR) circularly polarized emitted radiation [17]. This technique has been used for more than 25 years in various studies aimed at elucidation of stereochemical structure and dynamics of chiral luminescent
(s)
rac
I
3. Theory
k
kb(S) -______
______ ______
R
S
Fig. 1. Schematic energy level diagram for a racemic mixture (see text).
20
A. Mondrv
ci
a!. / Journal of Luminescence 62 (1994) 17—23
In this equation, g~m(A),and g~b5(A’)denote, respectively, the luminescence and absorption dissymmetry factor for the pure R enantiomer; k0 denotes the emission rate constant, and ~ is the racemization rate constant. Note that in the limit that racemization is much larger than emission, i.e. k~0~5>>k0, no CPL is detected. If racemization can be neglected, k~0~~<
(A)
=
R
R
~ glum (A) g abs (,t) 5/RT) +k ~2Aexp( E~°~ 0)~ 115 denotes the activation energy for con(______________________ —
where E~° version of enantiomers. It should also be noted that measurements of the type described above are limited to transitions possessing fairly large values for the pure dissymmetry ratios. In our experimental setup, for these moderately luminescent compounds, values of ~ium may be obtained in a reasonable time with an accuracy of approximately x io~. product 2)~must1 be greaterThus, than 2the x 10~.This Ig~m(A) gabs( limitation leads to the requirement instrumental that the individual dissymmetry ratios should be approximately equal to ±1 x 10-2 in order to be detectable. This magnitude effectively limits this kind of experiment to f.~—~f absorption and emission
ched more rapidly than the other, and the differential population of excited state species will be time dependent. This differential population will also result in the emission being circularly polarized. In a time-resolved experiment, it can be shown that the time-dependent CPL signal can be described by the following function [12] R ‘A~ hrk(k~R k~’~’ 4 gium~ t) giumk tan l2~ q q )L\~ t where the square brackets indicate the concentration of the optically active quencher, Q~,and the chiral identity of the optically active quencher has been explicitly indicated in the superscript of the quenching rate constants. Note that we have neglected racemization effects in the derivation of this equation. In this case the total emission intensity should decay according to the following biexponential function: 1(t) = A[exp( k~t)+ exp( k-t)] + ‘DC~ (5) where k~= k~[Q~]+ k 0 and k = k~[Q~]+ k0. k0 is the decay constant in the absence of the quencher, and ‘DC denotes the dark current. — —
—
—
—
4. Results and discussion In Fig. 2 we show CPL and3total data in emission D for a solution of Eu(TTHA) 2O at pH = 6.5. These solutions were prepared with a slight excess of TTHA (1.2: 1) in order to drive the equilibrium toward the stable 1: 1 complex. These emission spectra were obtained by use of a left circularly polarized 476.5 nm excitation beam. The spectral region displayed corresponds to the 5D 7F 0 1 transition, and the incident frequency corresponds 7F 5D to the absorptive transition 0 1. The fact that strong CPL is observed from this system, mdicates that the complex is, in fact, chiral, and that the integrity of the emitting species is at least partially intact during the excited state lifetime (approximately 1.2 ms). It should also be noted that no evidence was seen for the existence of a 2: 1 species under the conditions used to collect these spectra. Furthermore, in solutions containing twice the relative concentration of Eu(III), under conditions where the 2: 1 species does exist, no CPL was detected. —+
transitions that obey magnetic-dipole selection rules, i.e. LxJ = 0, ±1, since, in general, only these transitions are expected to possess large enough dissymmetry values [20]. Recently, in a number of related studies, it has been demonstrated that it is also possible to generate non-racemic excited state populations from racemic ground states through the use of optically active quencher species [12—14].This phenomena is also pictured schematically in Fig. 1. In this case, if the quenching rate constants, k~and k~are unequal, then, following an initially unpolarized excitation pulse, one of the enantiomers will be quen-
—+
A. Mondry ci al.
/ Journal of Luminescence
2.0
0.008
1.8
0.006
62 (1994) 17—23
21
0.000
—0.002
—0.004
I
___
—0.002
H
—0.014
—
300 —0.004
—
IV II I
0.6
2
-0.008
/
0.
0~6~O
—0.006
I \\
0.4
5800
0
~2o~0~010
Wavelength (A) Fig. 2. Circularly polarized luminescence (upper curve, Al) and total luminescence (lower curve, I) for a 0.05 M solution of Eu(TTHA)3 in D 20 at pH = 6.5. Excitation was via a circularly polarized 476.5 nm laser line.
Fig. 3. Temperature dependence of ~ (592nm) for a 0.05 3. Solid line is a fit to the function M solution of Eu(TTHA) displayed in Eq. (3).
E~°~ and the product g~jm(A) g~bS(A’)were allowed to vary in the fitting procedure. Values of the preexponential factor, A, ranging from 108 to 1012 were used, with only small effects on the “best” values for the two variables. Nevertheless, due to . the nature of this function, and the limited temperature range available for these measurements, the results should only be interpreted as approximate. .
The solid line shown in this figure corresponds to It
R
‘
= 0.0115, and Ea°~= 23 kcal/mol. Both of these values are quite reasonable considering the nature of the electronic transitions involved, and the measured stability of this complex. For purposes of comparison, Metcalf et al. [21] obtamed values of 11.7 kcal/mol for the activation energy for enantiomeric conversion for Eu(DPA)~, and, thus, in agreement with the measurements reported here, we obtain result 3 is significantly more the stable to that Eu(TTHA) racemization than Eu(DPA)~ In Fig. 4 we present results for excited state quenching of Tb(TTHA)3 by Ru(phen)~.In these experiments the decay of the total luminescence at 544 nm corresponding to the 5D 7F 4 5 transition of Tb(III) was monitored as a function of time, following repeated short broad band (100 nm) excitation pulses centered at 320 nm. In Fig. 4(A) we show results for the unquenched luminescence decay of this species, Fig. 4(B) (which is plotted gium(A)gab,(A)
In Fig. 3 we 5D plot the 7F measured ~ium at the peak maxima of the 0 1 transition versus temperature following right circularly polarized excitation at 476.5 nm. Note that only a gradual decrease in ~ium is measured as temperature is increased to 80°C. This is in marked contrast to previously reported results for tris-terdendate complexes of Eu(DPA)~in H20ratio and decreased D20 in which measured dissymmetry by a the factor of 4 or 5 when the temperature was increased from room temperature to 80°C [21]. We conclude, therefore, that this species is relatively much more stable to racemization than the previously studied —*
350
Temperature (K)
—
-.
—+
tris-terdendate complexes. An estimate of the racemization rate constant may be obtained by fitting a function of the form of Eq. (3) to the data given in this figure. The result of such a non-linear leastsquares fit is given by the solid line in Fig. 3. Only
A. Mondry eta!.
22
6
I
I
II
ill—ill
11111
/ Journal of Luminescence 62
111111
I
I
liii
II
(1994) 17—23
II
II
~~1~20.::1::c:.
(A) 100
(B)
.
.
I
0
~
.~:.
20
0
11111
~
II
II
-1
I
5
I
(C)
I
10
I I—I
100
5
I,
10
Time (ms.) Fig. 4. (A) Luminescence decay (a) given in photon Counts for the 5D
3 in H ofTb(III) from 1.0 mM Tb(TT}IA) 20, (B) the decay following the addition of 14 ~tM A-( — )-Ru(phen) and (C) the decay following the addition of 14 .tM racemic-Ru(phen)~.Solid lines in (a) are the result of mono-exponential fits to the time-decay, (c) is a plot of the residuals from the mono-exponential fit, and (b) is the time-correlation function of the residuals. 4
-s ~F5 transition
~,
using a slightly different time axis) illustrates the time-decay following the addition of A-( Ru(phen)~ and Fig. 4(C) shows the decay results following the addition of racemic-Ru(phen)~ Also given in these figures is the result of a weighted least-squares fit to a simple mono-exponential decay (indicated by the solid line) in the lower plots (a), a plot of the residuals from the leastsquares fit (c), and the autocorrelation fit of the residuals (b). This quencher complex has been shown to be an extremely effective enantio-selective quencher of Tb(DPA)~ and considerable effort has been devoted toward developing an understanding of the fundamental pairwise interactions leading to the observed enantio-selectivity between these two D3 complexes. As can be seen, quenching of Tb(III) luminescence is observed; however, as can be seen in Figs. 4(B) and 4(C), the time-decay observed following the addition of resolved and racemic Ru(phen)~ are virtually indistinguishable. The mono-exponential fit of the decays in Figs. 4(B) and 4(C) yield identical values of 1401 s 1 for the quenching rate constant. —
~,
~.
-,
+
—
)-
As indicated in Eq. (5), if the quenching of the excited state is enantio-selective, then the decay should be a sum of two exponentials. Examination of the residuals and the time-correlation functions of the residuals in Fig. 4, do show periodic oscillations which are similar to that observed in previous enantio-selective quenching studies in which biexponential behavior was observed. However, this effect is seen both when resolved and racemic quencher has been added, and, therefore, cannot be attributed to any chiral process. These oscillations do suggest the presence of more than one sspecies in solution. Ifthese two decays are subject to a biexponential fit to the form of Eq. (5), values of 1542 ± 9 and 1303 ±5 are obtained for the optically active quencher (Fig. 4(B)) and 1528 ±8 and 1307 ±6 for the racemic quencher (Fig. 4(C)). These pairs of values are the same within the accuracy of the fitting procedure; however, the form of Eq. (5) with identical pre-exponential factors for the two decay processes, is not correct, if the source of the biexponential decay is two emitting species with unknown relative concentrations. Furthermore, it is not possible to obtain a reliable fit if an additional
A. Mondry ci a!.
/ Journal of Luminescence
pre-exponential factor is included in the numerical fitting procedure for decay processes, such as these, that differ by less than 20%.
62 (1994) 17—23
23
to the National Institutes of Health (GM42194-01) for partial support of this work.
References
5. Summary The results presented here for circularly polarized excitation of Eu(TTHA)3 represent the first use of this technique to probe the chiral structure of non-D 3 species, and show the potential of this technique as a structural probe. The lack of measurable CPL from the quenching experiments leads us to conclude that, although the electronic 3 with aspects of the Tb(TTHA) Ru(phen)~ areinteraction such that of energy transfer from Tb(III) to the Ru-complex does occur, no strong diastereomeric orientational preference is present. This is not too surprising considering the spherical nature of the TTHA complex, but this result does have important implications concerning the relative contributions of chiral electronic effects to the energy transfer between diastereomeric ion pairs in studies of this type. The fact that two species are observed, that apparently have identical unquenched lifetimes, is consistent with the observation of Holz and Horrocks [5] mentioned previously, in which two 5D 7F~peaks were detected. 0 The measurement of circularly polarized luminescence from racemic mixtures and the generation of chiral excited states through the use of optically active quenchers have tremendous potential as probes of the chiral structure and solution dynamics of lanthanide complexes. Extensions of the work presented here to other chiral complexes and chiral quenchers are underway. +
—+
Acknowledgements Acknowledgement is made to the American Chemical Society Petroleum Research Fund, and —
[I] R.B. Lauffer, Chem. Rev. 87 (1987) 901. [2] G.R. Choppin, P.A. Baisden and S.A. Khan, Inorg. Chem. 18 (1979) 1330. [3] A. Yingst and A.E. Martell, J. Am. Chem. Soc. 91(1969) 6927. [4] T.J. Wenzel, ME. Ashley and RE. Sievers, Inorg. Chem. 27 (1988) 4730. [5] R.C. Holz and W. DeW. Horrocks Jr., Inorg. Chim. Acta 171 (1990) 193.J. Coord. Chem. 21(1991)295. [6] H.G. Brittain, [7] G.L. Hilmes and J.P. Riehi, Inorg. Chem. 25 (1986) 2617. [8] G.L. Hilmes and J.P. Riehl, Inorg. Chem. 24 (1985) 1721. [9] G.L. Hilmes, N. Coruh and J.P. Riehl, Inorg. Chem. 27 (1988) 1136. [10] G.L. Hilmes, N. Coruh and J.P. Riehl, Inorg. Chem. 27 (1988) 3647. [11] D.R. Foster and F.S. Richardson, Inorg. Chem. 22 (1983) 3996. [12] D.H. Metcalf, S.W. Snyder, S. Wu, G.L. Hilmes, J.P. Riehi, J.N. Demas and F.S. Richardson, J. Am. Chem. Soc. Ill (1989) 3082. [13] RB. Rexwinkel, S.C.J. Meskers, J.P. Riehl and H.P.J.M. Dekkers, J. Phys. Chem. 96 (1992) 1112. [14] R.B. Rexwinkel, S.C.J. Meskers, H.P.J.M. Dekkers and J.P. Riehl, J. Phys. Chem. 97 (1993) 13519. [15] P. Letkeman and A.E. Martell, Inorg. Chem. 18 (1979) 1284. [16] RD. Gillard and R.E.E. Hill, J. Chem. Soc. Dalton Trans. (1974) 1217. [17] J.P Riehl and F.S. Richardson, Chem. Rev. 86 (1986) 1. [18] F.S. Richardson, in: Circular Dichroism: Principles and Applications, eds. A. Berova, K. Nakanishi and R.W. Woody (VCH), to be published. [19] S. Wu, G.L. Hilmes and J.P. Riehl, J. Phys. Chem. 93 (1989) 2307. [20] F.S. Richardson, Inorg. Chem. 19 (1980) 2906. [21] D.H. Metcalf, SW. Snyder, iN. Demas and F.S. Richardson, J. Phys. Chem. 94 (1990) 7143.
JOURNAL OF
LUMINESCENCE
ELSEVIER
Journal of Luminescence 62 (1994) 17—23
Application of circularly polarized luminescence spectroscopy to the solution structure of racemic polyaminocarboxylate lanthanide (III) complexes Anna Mondry1, Stefan C.J. Meskers2, James P. Riehl*
Department of Chemistry, Michigan Technological University, Houghton, MI 49931-1295, USA Received 9 September 1993, revised 14 January 1994; accepted 14 January 1994
Abstract Circularly polarized luminescence from 1: 1 complexes of Eu(III) with triethylenetetra-aminehexaacetic acid (TTHA) following excitation with circularly polarized light is reported. This observation is consistent with a solution structure for this complex that is chiral, and stable on the emission time scale of Eu(III) (approximately 1.2 ms). Additional measurements show only a slight dependence on temperature, and an estimate for the activation energy for racemization (23 kcal/mol) may be obtained. Attempts at detecting enantio-selective quenching of Tb(TTHA)3 by optically active Ru(phen)~+ are also reported. Although significant quenching was observed, and the time-decay of the Tb(III) emission was not mono-exponential, no enantio-selectivity was found. This latter result may be interpreted in terms of the lack of discriminating short-range diastereomeric interactions between the chiral ion pairs. -
1. Introduction Lanthanide (III) ions are known to form a variety of solution complexes with polyaminocarboxylate ligands, and some of these complexes are becoming increasingly important due to their p0tential as contrast agents in NMR imaging [1]. The complex formed between Gd(III) and diethylenetriaminepentaacetic acid [2] (DTPA), for example, is already an FDA approved contrast agent. Various other complexes involving polyarninocarboxylic acids have been proposed and are *
Corresponding author. . . Permanent address: Institute of Chemistry, University of
Wroclaw, Wroclaw, Poland. 2 Present address: Leiden Institute of Chemistry, Leiden University, Netherlands.
being studied as potential imaging agents. Even though complexes involving triethylenetetraaminehexaacetic acid (TTHA) are probably not suitable as contrast agents, due to the fact that this ligand effectively excludes contact of the central ion with exchangeable water molecules, these cornplexes have, nevertheless, been studied fairly extensively due to their stability and related structural properties (see Scheme 1) [3—5]. In general, in order to develop a thorough understanding of the potential use of complexes of this type, it is important to learn as much as possible about the solution structure, and dynamics of these species. In thisand regard, various techniques have been proposed employed to study the structure .
of polyaminocarboxylate complexes of lanthanide (III) complexes. Although, for the specific case of NMR contrast agents, complexes containing
0022-2313/94/$07.OO © 1994 — Elsevier Science By. All rights reserved SSDIOO22-2313(94)00012-2
18
A. Mondrv eta!.
/
Journal of Luminescence 62 (1994) 17—23
HOOC ~OOH /
I COOH
/
I
~COOH
COOH
U HA Scheme
.
Gd(III) with its f7 valence configuration, and associated relaxation properties, is the target of much research effort [1], the use of other lanthanides with more suitable spectroscopic properties is a common approach. Considerable effort has been devoted, for example, to 1H and 13C NMR studies using the various diamagnetic rare earth ions, La(III), Y(III) and Lu(III) [5]. These three ions span the small range of ionic radii of the lanthanide series, and it is expected that results from these kind of studies yield a reasonably accurate description of the structure and structural integrity of the entire lanthanide series. In addition, the use of luminescent lanthanide (III) ions such as Tb(III) and Eu(III) have also been exploited [5,6]. For example, Holz and Horrocks [5] have recently characterized complexes of TTHA with Eu(III). Their studies included lifetime measurements, and site-selective 7F 5D excitation spectroscopy of the narrow 0 0 absorption. In this work it was concluded from lifetime measurements, that, below a pH of 6.0, both 2: 1 and 1: 1 metal ligand cornplexes were present, but that above a pH of 6.5 only the 1: 1 species exists. Even in solutions in which it was concluded that only 1: 1one species was lifetime, present through observation of only emission two peaks were observed in the 7F 5D 0 —f 0 region. This result was interpreted as being due to rapidly interconverting 1: 1 isomers. Brittain [6] has used the effect of added Eu(III) on the lifetime of Tb(III) to probe the existence of TTHA complexes or oligomers containing more than one lanthanide ion. From a series of lifetime measurements of 1: 1 solutions of lanthanide ion: TTHA as a function of pH, it was concluded that, between pH 4 and 6, —~
oligomeric species exist, but that above a pH of 8 only isolated 1: 1 Ln : TTHA complexes are present. Holz and Horrocks [5] also showed that in experiments in which TTHA was added to a solution of Eu(III) at a ratio of 1: 1 a small amount of the 2: 1 species was present, but that upon the addition of excess ligand only the 1: 1 species was observed. We havehigh beensymmetry involved in a numbercomplexes of studies of relatively lanthanide in which the luminescence measurement (CPL) of the hascircular been exploited polarization in order of to obtain information about the solution structure and dynamics of optically active lanthanide cornplexes [7—10].In order to measure a non-zero CPL from these systems, it is necessary to generate a net optically active excited (emitting) state. As described in more detail below, this can be obtained from a racemic mixture if a circularly polarized excitation source is used, as long as racemization in the excited state is slower than emission. The ligands used in these various studies included 2,6pyridine-dicarboxylic acid (DPA), oxydiacetic acid (ODA), irninodiacetic acid (IDA), and methyliminodiacetic acid (MIDA). Of the two possible tris-terdendate geometries based on a trigonal tncapped prism, the menidonal (D 3) and facial (C3h) isomers, only the meridonal isomer is optically active. In these studies, it was possible to measure CPL from complexes with DPA and ODA only. It was concluded, therefore, that these complexes have D3 symmetry, and lanthanide complexes with IDA and MIDA have C3h symmetry. These results were in agreement with previous measurements involving analysis of magnetic circularly polarized luminescence and total emission intensity [11]. In this paper we report CPL from solutions of 3 following circularly polarized excitaEu(TTHA) tion. This, in fact, represents the first measurement -
of CPL from a presumably racemic lanthanide complex that does not have approximate D3 symmetry. It is also demonstrated that the CPL shows very little temperature dependence from 10 to 80°C, indicating that the complex does not undergo significant rearrangements that would lead to racemization in this temperature range. Additional experiments involving quenching of 3 by resolved the Ru(phen)~ + are also Tb(TTHA) -
A. Mondry et al.
/ Journal of Luminescence 62
(1994) 17—23
19
reported. Unlike previous results [12—14] on the quenching of Tb(DPA)~ no enantio-selective quenching is found for Tb(TTHA)3 indicating a lack of diastereomeric discriminating interactions between the oppositely charged ions in this system.
systems, and the reader may consult Ref. [18] for a more complete discussion of recent theoretical concepts. The results are most often reported in terms of the luminescence dissymmetry ratio, gium, which is defined as the ratio of the differential emission intensity to the average total emission intensity:
2. Experimental
gium
Stock solutions of Eu(III) chlorides were prepared from 99.9% Eu 203 (Aldrich) and Tb(III) solutions were prepared from TbC13 (Aldrich). The final concentrations were determined by standardization with EDTA using xylenol orange as an indicator. Stock solutions of triethylenetetraaminehexaacetic acid (TTHA) (98% Aldrich) were prepared at3 pHand = 7 Eu(TTHA)3 by neutralization with NaOH. complexes were Tb(TTHA) prepared in 1: 1.2 metal: ligand ratios at a pH of 6.5. Since this pH value is higher than the sixth ligand PKa [15] value, it is assumed that the dominant TTHA species in these solutions has a 6 charge. D 2O solutions were prepared by slowly evaporating 5 ml of the aqueous solution to dryness, and then dissolving the residue in 5 ml of D20. Ru(phen)~+ was prepared and resolved according to published procedures [16]. CPL and total luminescence spectra were recorded on an instrument constructed in our laboratory operating in a photon-counting mode. The time-decay of Tb(III) luminescence was performed in the laboratory of Dr. H.P.J.M. Dekkers at the University of Leiden, Netherlands. Excitation of Eu(III) was accomplished at 476.5 nm and Tb(III) at 488 nm with a coherent INNOVA-70 argon-ion laser. For the case of Eu(III), the laser beam was made circularly polarized ( > 95%) by placing a high-quality quar ter-wave plate in the excitation path.
Unlike circular dichroism (CD) measurements, CPL
—,
-
-
—
=
(1)
1(j±J)~
is not limited to molecular systems that have been at least partially resolved into enantiomers. Even if the solution is composed of a racemic mixtures of enantiomers, it may be possible to measure CPL if a non-racemic excited state can be prepared from the racemic ground state. Two ways of preparing chiral excited ground distributions that are not states chiral,from are the use state of circularly polarized excitation, and the use of enantio-selective quenchers. These processes are pictured schematically in Fig. 1 for a simple racemic mixture. If circularly polarized excitation is used then (R) kabs (5), and the excited state population will not be racemic as long as the racemization rate is slower than the emission rate. Hilmes and Riehl [19] have presented a theoretical description of the measurement of CPL from racemic mixtures in which the effects of racemization and radiationless energy transfer on gium were determined. Neglecting excited state energy transfer between enantiomers, one obtains the following expression for the measurement of ~ / k \ glum (A) = ~ gL(A) gabs (A) ° ). (2) \2k~0~~ + k01 kabs
(
-_____
_____
1< (R)
______
I kb(R)
In CPL spectroscopy, one measures the differ ence in intensity between left (IL) and right (IR) circularly polarized emitted radiation [17]. This technique has been used for more than 25 years in various studies aimed at elucidation of stereochemical structure and dynamics of chiral luminescent
(s)
rac
I
3. Theory
k
kb(S) -______
______ ______
R
S
Fig. 1. Schematic energy level diagram for a racemic mixture (see text).
20
A. Mondrv
ci
a!. / Journal of Luminescence 62 (1994) 17—23
In this equation, g~m(A),and g~b5(A’)denote, respectively, the luminescence and absorption dissymmetry factor for the pure R enantiomer; k0 denotes the emission rate constant, and ~ is the racemization rate constant. Note that in the limit that racemization is much larger than emission, i.e. k~0~5>>k0, no CPL is detected. If racemization can be neglected, k~0~~<
(A)
=
R
R
~ glum (A) g abs (,t) 5/RT) +k ~2Aexp( E~°~ 0)~ 115 denotes the activation energy for con(______________________ —
where E~° version of enantiomers. It should also be noted that measurements of the type described above are limited to transitions possessing fairly large values for the pure dissymmetry ratios. In our experimental setup, for these moderately luminescent compounds, values of ~ium may be obtained in a reasonable time with an accuracy of approximately x io~. product 2)~must1 be greaterThus, than 2the x 10~.This Ig~m(A) gabs( limitation leads to the requirement instrumental that the individual dissymmetry ratios should be approximately equal to ±1 x 10-2 in order to be detectable. This magnitude effectively limits this kind of experiment to f.~—~f absorption and emission
ched more rapidly than the other, and the differential population of excited state species will be time dependent. This differential population will also result in the emission being circularly polarized. In a time-resolved experiment, it can be shown that the time-dependent CPL signal can be described by the following function [12] R ‘A~ hrk(k~R k~’~’ 4 gium~ t) giumk tan l2~ q q )L\~ t where the square brackets indicate the concentration of the optically active quencher, Q~,and the chiral identity of the optically active quencher has been explicitly indicated in the superscript of the quenching rate constants. Note that we have neglected racemization effects in the derivation of this equation. In this case the total emission intensity should decay according to the following biexponential function: 1(t) = A[exp( k~t)+ exp( k-t)] + ‘DC~ (5) where k~= k~[Q~]+ k 0 and k = k~[Q~]+ k0. k0 is the decay constant in the absence of the quencher, and ‘DC denotes the dark current. — —
—
—
—
4. Results and discussion In Fig. 2 we show CPL and3total data in emission D for a solution of Eu(TTHA) 2O at pH = 6.5. These solutions were prepared with a slight excess of TTHA (1.2: 1) in order to drive the equilibrium toward the stable 1: 1 complex. These emission spectra were obtained by use of a left circularly polarized 476.5 nm excitation beam. The spectral region displayed corresponds to the 5D 7F 0 1 transition, and the incident frequency corresponds 7F 5D to the absorptive transition 0 1. The fact that strong CPL is observed from this system, mdicates that the complex is, in fact, chiral, and that the integrity of the emitting species is at least partially intact during the excited state lifetime (approximately 1.2 ms). It should also be noted that no evidence was seen for the existence of a 2: 1 species under the conditions used to collect these spectra. Furthermore, in solutions containing twice the relative concentration of Eu(III), under conditions where the 2: 1 species does exist, no CPL was detected. —+
transitions that obey magnetic-dipole selection rules, i.e. LxJ = 0, ±1, since, in general, only these transitions are expected to possess large enough dissymmetry values [20]. Recently, in a number of related studies, it has been demonstrated that it is also possible to generate non-racemic excited state populations from racemic ground states through the use of optically active quencher species [12—14].This phenomena is also pictured schematically in Fig. 1. In this case, if the quenching rate constants, k~and k~are unequal, then, following an initially unpolarized excitation pulse, one of the enantiomers will be quen-
—+
A. Mondry ci al.
/ Journal of Luminescence
2.0
0.008
1.8
0.006
62 (1994) 17—23
21
0.000
—0.002
—0.004
I
___
—0.002
H
—0.014
—
300 —0.004
—
IV II I
0.6
2
-0.008
/
0.
0~6~O
—0.006
I \\
0.4
5800
0
~2o~0~010
Wavelength (A) Fig. 2. Circularly polarized luminescence (upper curve, Al) and total luminescence (lower curve, I) for a 0.05 M solution of Eu(TTHA)3 in D 20 at pH = 6.5. Excitation was via a circularly polarized 476.5 nm laser line.
Fig. 3. Temperature dependence of ~ (592nm) for a 0.05 3. Solid line is a fit to the function M solution of Eu(TTHA) displayed in Eq. (3).
E~°~ and the product g~jm(A) g~bS(A’)were allowed to vary in the fitting procedure. Values of the preexponential factor, A, ranging from 108 to 1012 were used, with only small effects on the “best” values for the two variables. Nevertheless, due to . the nature of this function, and the limited temperature range available for these measurements, the results should only be interpreted as approximate. .
The solid line shown in this figure corresponds to It
R
‘
= 0.0115, and Ea°~= 23 kcal/mol. Both of these values are quite reasonable considering the nature of the electronic transitions involved, and the measured stability of this complex. For purposes of comparison, Metcalf et al. [21] obtamed values of 11.7 kcal/mol for the activation energy for enantiomeric conversion for Eu(DPA)~, and, thus, in agreement with the measurements reported here, we obtain result 3 is significantly more the stable to that Eu(TTHA) racemization than Eu(DPA)~ In Fig. 4 we present results for excited state quenching of Tb(TTHA)3 by Ru(phen)~.In these experiments the decay of the total luminescence at 544 nm corresponding to the 5D 7F 4 5 transition of Tb(III) was monitored as a function of time, following repeated short broad band (100 nm) excitation pulses centered at 320 nm. In Fig. 4(A) we show results for the unquenched luminescence decay of this species, Fig. 4(B) (which is plotted gium(A)gab,(A)
In Fig. 3 we 5D plot the 7F measured ~ium at the peak maxima of the 0 1 transition versus temperature following right circularly polarized excitation at 476.5 nm. Note that only a gradual decrease in ~ium is measured as temperature is increased to 80°C. This is in marked contrast to previously reported results for tris-terdendate complexes of Eu(DPA)~in H20ratio and decreased D20 in which measured dissymmetry by a the factor of 4 or 5 when the temperature was increased from room temperature to 80°C [21]. We conclude, therefore, that this species is relatively much more stable to racemization than the previously studied —*
350
Temperature (K)
—
-.
—+
tris-terdendate complexes. An estimate of the racemization rate constant may be obtained by fitting a function of the form of Eq. (3) to the data given in this figure. The result of such a non-linear leastsquares fit is given by the solid line in Fig. 3. Only
A. Mondry eta!.
22
6
I
I
II
ill—ill
11111
/ Journal of Luminescence 62
111111
I
I
liii
II
(1994) 17—23
II
II
~~1~20.::1::c:.
(A) 100
(B)
.
.
I
0
~
.~:.
20
0
11111
~
II
II
-1
I
5
I
(C)
I
10
I I—I
100
5
I,
10
Time (ms.) Fig. 4. (A) Luminescence decay (a) given in photon Counts for the 5D
3 in H ofTb(III) from 1.0 mM Tb(TT}IA) 20, (B) the decay following the addition of 14 ~tM A-( — )-Ru(phen) and (C) the decay following the addition of 14 .tM racemic-Ru(phen)~.Solid lines in (a) are the result of mono-exponential fits to the time-decay, (c) is a plot of the residuals from the mono-exponential fit, and (b) is the time-correlation function of the residuals. 4
-s ~F5 transition
~,
using a slightly different time axis) illustrates the time-decay following the addition of A-( Ru(phen)~ and Fig. 4(C) shows the decay results following the addition of racemic-Ru(phen)~ Also given in these figures is the result of a weighted least-squares fit to a simple mono-exponential decay (indicated by the solid line) in the lower plots (a), a plot of the residuals from the leastsquares fit (c), and the autocorrelation fit of the residuals (b). This quencher complex has been shown to be an extremely effective enantio-selective quencher of Tb(DPA)~ and considerable effort has been devoted toward developing an understanding of the fundamental pairwise interactions leading to the observed enantio-selectivity between these two D3 complexes. As can be seen, quenching of Tb(III) luminescence is observed; however, as can be seen in Figs. 4(B) and 4(C), the time-decay observed following the addition of resolved and racemic Ru(phen)~ are virtually indistinguishable. The mono-exponential fit of the decays in Figs. 4(B) and 4(C) yield identical values of 1401 s 1 for the quenching rate constant. —
~,
~.
-,
+
—
)-
As indicated in Eq. (5), if the quenching of the excited state is enantio-selective, then the decay should be a sum of two exponentials. Examination of the residuals and the time-correlation functions of the residuals in Fig. 4, do show periodic oscillations which are similar to that observed in previous enantio-selective quenching studies in which biexponential behavior was observed. However, this effect is seen both when resolved and racemic quencher has been added, and, therefore, cannot be attributed to any chiral process. These oscillations do suggest the presence of more than one sspecies in solution. Ifthese two decays are subject to a biexponential fit to the form of Eq. (5), values of 1542 ± 9 and 1303 ±5 are obtained for the optically active quencher (Fig. 4(B)) and 1528 ±8 and 1307 ±6 for the racemic quencher (Fig. 4(C)). These pairs of values are the same within the accuracy of the fitting procedure; however, the form of Eq. (5) with identical pre-exponential factors for the two decay processes, is not correct, if the source of the biexponential decay is two emitting species with unknown relative concentrations. Furthermore, it is not possible to obtain a reliable fit if an additional
A. Mondry ci a!.
/ Journal of Luminescence
pre-exponential factor is included in the numerical fitting procedure for decay processes, such as these, that differ by less than 20%.
62 (1994) 17—23
23
to the National Institutes of Health (GM42194-01) for partial support of this work.
References
5. Summary The results presented here for circularly polarized excitation of Eu(TTHA)3 represent the first use of this technique to probe the chiral structure of non-D 3 species, and show the potential of this technique as a structural probe. The lack of measurable CPL from the quenching experiments leads us to conclude that, although the electronic 3 with aspects of the Tb(TTHA) Ru(phen)~ areinteraction such that of energy transfer from Tb(III) to the Ru-complex does occur, no strong diastereomeric orientational preference is present. This is not too surprising considering the spherical nature of the TTHA complex, but this result does have important implications concerning the relative contributions of chiral electronic effects to the energy transfer between diastereomeric ion pairs in studies of this type. The fact that two species are observed, that apparently have identical unquenched lifetimes, is consistent with the observation of Holz and Horrocks [5] mentioned previously, in which two 5D 7F~peaks were detected. 0 The measurement of circularly polarized luminescence from racemic mixtures and the generation of chiral excited states through the use of optically active quenchers have tremendous potential as probes of the chiral structure and solution dynamics of lanthanide complexes. Extensions of the work presented here to other chiral complexes and chiral quenchers are underway. +
—+
Acknowledgements Acknowledgement is made to the American Chemical Society Petroleum Research Fund, and —
[I] R.B. Lauffer, Chem. Rev. 87 (1987) 901. [2] G.R. Choppin, P.A. Baisden and S.A. Khan, Inorg. Chem. 18 (1979) 1330. [3] A. Yingst and A.E. Martell, J. Am. Chem. Soc. 91(1969) 6927. [4] T.J. Wenzel, ME. Ashley and RE. Sievers, Inorg. Chem. 27 (1988) 4730. [5] R.C. Holz and W. DeW. Horrocks Jr., Inorg. Chim. Acta 171 (1990) 193.J. Coord. Chem. 21(1991)295. [6] H.G. Brittain, [7] G.L. Hilmes and J.P. Riehi, Inorg. Chem. 25 (1986) 2617. [8] G.L. Hilmes and J.P. Riehl, Inorg. Chem. 24 (1985) 1721. [9] G.L. Hilmes, N. Coruh and J.P. Riehl, Inorg. Chem. 27 (1988) 1136. [10] G.L. Hilmes, N. Coruh and J.P. Riehl, Inorg. Chem. 27 (1988) 3647. [11] D.R. Foster and F.S. Richardson, Inorg. Chem. 22 (1983) 3996. [12] D.H. Metcalf, S.W. Snyder, S. Wu, G.L. Hilmes, J.P. Riehi, J.N. Demas and F.S. Richardson, J. Am. Chem. Soc. Ill (1989) 3082. [13] RB. Rexwinkel, S.C.J. Meskers, J.P. Riehl and H.P.J.M. Dekkers, J. Phys. Chem. 96 (1992) 1112. [14] R.B. Rexwinkel, S.C.J. Meskers, H.P.J.M. Dekkers and J.P. Riehl, J. Phys. Chem. 97 (1993) 13519. [15] P. Letkeman and A.E. Martell, Inorg. Chem. 18 (1979) 1284. [16] RD. Gillard and R.E.E. Hill, J. Chem. Soc. Dalton Trans. (1974) 1217. [17] J.P Riehl and F.S. Richardson, Chem. Rev. 86 (1986) 1. [18] F.S. Richardson, in: Circular Dichroism: Principles and Applications, eds. A. Berova, K. Nakanishi and R.W. Woody (VCH), to be published. [19] S. Wu, G.L. Hilmes and J.P. Riehl, J. Phys. Chem. 93 (1989) 2307. [20] F.S. Richardson, Inorg. Chem. 19 (1980) 2906. [21] D.H. Metcalf, SW. Snyder, iN. Demas and F.S. Richardson, J. Phys. Chem. 94 (1990) 7143.