2307
J . Phys. Chem. 1989,93, 2307-2310
Nature of Chiral- Induced Equilibrium Shifts in Racemic Labile Lanthanide Complexes Shuguang Wu, Gary L. Hilmes, and James P. Riehl* Department of Chemistry, University of Missouri-%. (Received: May 17, 1988)
Louis, St. Louis, Missouri 63121
An analysis of the chiral-induced equilibrium shift of racemic D, tris-terdendate complexes of lanthanides with 2,6pyridinedicarboxylate is presented in terms of the associated/dissociated models of Schipper. Results are presented which indicate that the so-called Pfeiffer effect in these lanthanide complexes is best described by the dissociated model, as was determined for similar labile transition-metal complexes. The nature of the chiral discriminatory interaction is shown to be largely electrostatic by measurements in mixed solvents of varying dielectric constant.
Introduction The introduction of an optically active compound to a solution containing a labile racemic metal complex may induce a shift in the equilibrium, resulting in an excess of one enantiomer over the other. This phenomenon is known as the Pfeiffer effect after the work of P. Pfeiffer, who observed increased optical rotation following the addition of tris-transition-metal complexes to solutions of optically active alkaloids.’-’ The confirmation that the effect was due to an equilibrium shift was provided by experiments that showed that for selected systems a resultant circular dichroism (CD) spectrum could be obtained that was identical with the C D of resolved enantiomer^.^ Until the past few years, all of the reports of Pfeiffer effect induced equilibrium shifts involved CD (or optical rotatory dispersion) measurements of transition-metal complexes. The development of circularly polarized luminescence spectroscopy (CPL),5 in which one measures the net circular polarization in the emission spectra of optically active molecules, has allowed for the possibility of monitoring induced equilibrium shifts through differential excited-state populations. All of the CPL reports on the Pfeiffer effects have involved 9-coordinate tris-terdendate lanthanide complexes with 2,6-pyridinedicarboxylate(dipicolinate, DPA), since these complexes are highly optically active and luminescent.+l] Because these species are not resolvable by classical methods, however, it is not possible to verify that a simple equilibrium shift has occurred by comparing CPL spectra from the perturbed equilibrium system to that of the resolved complex as was done for transition-metal systems. Furthermore, since CPL in these systems depends on the existence of a differential excited-state population, one cannot rule out the possibility that the enantiomeric excess in the excited state is not the same as that in the ground state. This would be true if there was any differential quenching of the chiral emitting states. Recently, however, we have shown that, for a number of Ln(DPA)33- complexes, it is possible to generate a differential excited-state population by using as the excitation source a circularly polarized excitation beam.129’3 This experiment can be performed on these labile racemic mixtures since the racemization rate is (1) (2) (3) (4) 201-6.
Pfeiffer, P.; Quehl, K. Chem. Ber. 1931, 64, 2667. Pfeiffer, P.; Quehl, K. Chem. Ber. 1932, 65, 560. Pfeiffer, P.; Nakasuka, Y. Chem. Ber. 1933, 66, 410. Kirschner, S.; Ahmad, N.; Magnell, K. Coord. Chem. Rev. 1968, 3,
( 5 ) Riehl, J. P.; Richardson, F. S. Chem. Reu. 1986, 86, 1-16. (6) Brittain, H. G. Inorg. Chem. 1981, 20, 3007. (7) Madras, J. S.;Brittain, H. G. Inorg. Chem. 1980, 19, 3841. (8) Madras, J. S.; Brittain, H. G. Inorg. Chim. Acta 1980, 42, 109. (9) Yan, F.; Brittain, H. G. Polyhedron 1982, 1, 195. (10) Yan, F.;Copeland, R. A,; Brittain, H. G. Inorg. Chem. 1982, 21,
1180. (1 1) Brittain, H. G.; Rispoli, L. Polyhedron 1984, 3, 1087. (12) Hilmes, G. L.; Timper, J. M.; Riehl, J. P. Inorg. Chem. 1985, 24, 172 1-23. (13) Hilmes, G. L.; Riehl, J. P. Inorg. Chem. 1986, 25, 2617.
less than the emission rate for the systems that have been studied. The ability to measure CPL from the racemic mixture allows one to compare the sign and relative magnitude of the CPL spectra of the Pfeiffer-perturbed system to that of the “pure” enantiomer generated in excess by the circularly polarized excitation beam. In these systems, this comparison leads one to the conclusion that the addition of L-histidine or (+)-dimethyl-L-tartrate does, indeed, result in a simple equilibrium shift; Le., the emitting species is virtually the same in the racemic mixture and the Pfeiffer-active systems. The Pfeiffer effect has been used to obtain information about chiral molecular system^.'^ In transition-metal systems, the Pfeiffer effect has been applied with some success to predict the absolute configuration of labile metal complexes and to measure chiral discriminatory interaction^.'^ Very recently, we have shown that the Pfeiffer effect can be employed in combination with CPL from racemic mixtures in LII(DPA)~~systems to yield chiroptical properties of pure enantiomers, even though these enantiomers cannot be resolved by classical methods.16 The most important contribution to an understanding of the nature of the Pfeiffer effect in terms of the discriminatory forces has been presented by Schipper,” who examined the thermodynamics of the equilibrium shift in terms of simple models for the solution structure. In this work, we modify this treatment to include Pfeiffer effect CPL and apply the equilibrium shift model as described by Schipper to solutions of Tb(DPA)33-into which L-histidine has been added.
Theory of Pfeiffer Effect CPL Consider the following equilibrium
A-A
(1)
where A and A refer to labile racemic metal complexes: In CPL spectroscopy, one determines the emission dissymmetry factor, gem,which is defined as followss gem
= AI/(I/2)
(2)
where and
I E IL(eft) + IR(ight)
(4)
in eq 2 makes the form of this equation identical The factor of with that of the absorption equivalent, gabs gab
= A€/€
(5)
(14) Kirschner, S.; Ahmad, N.; Magnell, K. Coord. Chem. Reu. 1968,3, 201-6. (15) Kirschner, S . ; Bakkar, 1. Coord. Chem. Reu. 1982, 43, 325-35. (16) Hilmes, G. L.; Coruh, N.; Riehl, J. P. Inorg. Chem. 1988, 27, 1136-39. (17) Schipper, P. E. J. Am. Chem. SOC.1978, 100, 1079-84.
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2308 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Wu et al. I
where At
E €L(eft)
-
I
(6)
€R(ight)
and (€L(eft)+ €R(ight))/2
(7)
For an equilibrium mixture of N i A-isomers and Ni A-isomers in the ground state, g, we can relate the measured CD to that which one would measure from the pure A-enantiomer through the following relation gabs
=
vgdts
(8)
where qg denotes the enantiomeric excess in the ground state.
vg = [ N t - N t ]/ [ N t + Nil
(9)
Similarly, one can relate the measured CPL from the mixture to that of the pure A-enantiomer gem
=
where in this case the enantiomeric excess refers to the emitting state, n. We assume here that there is no chiral differential quenching of the excited state, so that vg = vn = 7. SchipperI7 has considered the thermodynamics of the Pfeiffer effect in the context of two distinct simple models, namely, associated and dissociated. In the associated model, definite aggregates consisting of the metal complex and /I-molecules of the Pfeiffer agent, P, are assumed to form, Le., (A)P@nd (A)Ps. The association constants for the two aggregates are assumed to be identical and therefore do not contribute to the discrimination free energy. The discrimination free energy is assumed to be due only to short-range chiral interactions between P and the metal complex. In the dissociated model, it is assumed that P may interact with more than one metal complex, definite aggregates are not formed. Within this equilibrium shift model, there are two sources of chiral discriminations in the free energy: racemization free energy, which essentially is due to the energy of mixing and opposes any resolution, and the discrimination free energy due to the difference in interactions between the chiral agent, P, and the enantiomeric metal complexes. In the limit that the equilibrium shift is not large, Schipper derived the following expressions for the differential concentration of enantiomeric metal complexes for these two models.
ANg = -aA(P)Np(Nt
(associated)
+ Ni)/2RT
(11)
(dissociated) (12)
ANg = Nt - N i , and Np is the concentration of the Pfeiffer molecule. In these equations A(P) = E(A-P) - E(A-P) denotes the chiral discrimination energy, and a is the mean coordination number per mole of P; both of these quantities are treated as unknown constants in this work. Equations 11 and 12 can be rewritten in terms of the enantiomeric excess by dividing each equation by N , = N i N t .
+
7 = -A(P)Np/2RTNg
(associated)
(13)
= -aA(P)Np/2RT (dissociated) (14) These expressions can be substituted into eq 8 and 10 to relate the model to experimental measurements 17
gabs= -&,,A(P)Np/2RTN
(associated)
(15)
= -&,,aA(P)Np/2RT
(dissociated)
(16)
gem= -&A(P)Np/2RTN
(associated)
(17)
gem= -&aA(P)Np/2RT
(dissociated)
(18)
gabs
and
Since we see no evidence for chiral quenching in these systems, the equilibrium shift is the same in the ground and excited state.
550
Wavelength (nm)
(10)
vndm
ANg = -A(P)Np/2RT
540
(w
Figure 1. Circularly polarized luminescence and total luminescence (I) of 0.014 M Tb(DPA)33-after addition of 0.14 M L-histidine. Excitation wavelength = 488 nm. The spectral region displayed corresponds
-
to the 7F5 ,D4 transition.
540 550 Wavelength (nm)
Figure 2. Circularly polarized luminescence ( A I ) and total luminescence
-
(I)of Tb(DPA)33-using circularly polarized 488-nm excitation. The spectral region displayed corresponds to the 'F, 5D4transition. We, therefore, have dropped the subscript g in the last four equations.
Results Luminescence from Tb(II1) can be induced either by direct excitation of the lanthanide or by excitation of the aromatic ligand and subsequent energy transfer to the terbium ion.'* Luminescence is observed from the excited jD4state to a number of J sublevels of the 'F, ground-state manifold. The strongest emission occurs around 540 nm and corresponds to the 'F5 5D4 transition. No CPL is observed from solutions of Tb(DPA)33under unpolarized or linearly polarized excitation. In Figure 1 we show total emission and circularly polarized emission for a solution 0.014 M in Tb(DPA)33- into which L-histidine has been added to a concentration of 0.14 M. The pH of this solution and all other solutions used here were adjusted to a pH of 3.0. The spectral region shown corresponds to the 'F5 jD, emission. In Figure 2 we show similar spectra for Tb(DPA)33- using circularly polarized 488-nm excitation with no added L-histidine. These spectra show a number of different crystal field transitions, and the two spectra are virtually identical, indicating, as mentioned above, that the emitting species in the two solutions is the same. We can, therefore, interpret the effect of adding the L-histidine +-
+-
(18) Richardson, F. S . Chem. Reu. 1982, 82, 541
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2309
Equilibrium Shifts in Lanthanide Complexes
Tb(DPA) Tb(DPA)$-:L-Histidine
2.0
3- :L-Histidine
ge,(542.8nm) x 102
1.5
1.0
ge,(542.8nm) x 102
1.0
3.2
3.0
3.4
3.6
0.5
IITemperature ( x103)
Figure 3. Emission dissymmetry factor, gemat 542.8 nm, versus 1/T for Tb(DPA)33-:~-histidinesolution. [Tb(DPA),”-] = 2.74 X lo-’ M; [Lhistidine] = 6.12 X M.
12
[L-Histidine]/[Tb(DPA);-]
g,,(542 8”
Figure 5. Emission dissymmetry factor, gemat 542.8 nm, versus the ratio hi histidine] / [Tb(DPA)33-.A linear dependence would correspond to
x 102
08
the associated model (see text). 0.4 A
DM50
gC,(542.8nm) 3 x 102 10
20
30
A
Tb(DPA)g-:L-Histidine
A
40
[L-Histidine] ( x l O z )
Figure 4. Emission dissymmetry factor, gemat 542.8 nm, versus the concentration of added L-histidine. A linear dependence corresponds to
2
A A
the dissociated model (see text). as a simple equilibrium shift as described above. In this work we monitor the emission dissymmetry factor at 542.8 nm. This corresponds to the maximum intensity in the spectra shown in Figures 1 and 2. In Figure 3 we plot the emission dissymmetry factor, gemat 542.8 nm, versus l / T f o r a solution M Tb(DPA)33- and 6.12 X lo-* M Lcontaining 2.74 X histidine. The linear inverse temperature dependence displayed in this figure is predicted by both models (eq 17 and 18). The determination of which thermodynamic model best fits this system can be obtained by performing careful experiments in which one measures gemversus the ratio N P / N (eq 17) and gemversus N p (eq 18). The results of these experiments are shown in Figures 4 and 5. It is apparent from these figures that eq 18 yields a linear dependence, whereas eq 17 does not. Clearly, this system exhibits the behavior predicted by the dissociated model of Schipper and not the associated model as suggested by other workers. I o Brittain has demonstrated that the magnitude of the Pfeiffer effect induced by L-histidine is extremely sensitive to pH and concludes that L-histidine must be protonated and, therefore, charged in order for an equilibrium shift to occur! This has been confirmed in our laboratory. We thus conclude that the interaction between the lanthanide complex and the L-histidine has a large electrostatic component. This is further demonstrated by the results shown in Figure 6, where the dielectric constant of the solution is varied by using varying ratios of formamide:water and methanokwater. The relative concentrations of the lanthanide complex and L-histidine were kept constant during the course of these experiments. As is seen in this figure, as the dielectric constant of the solution is increased by the addition of formamide (e = 11 1) [e(H20) = 781, the magnitude of gemdecreases, indicating a smaller equilibrium shift due to a mediation of the L-histidine-lanthanide interaction. The opposite effect is observed when the dielectric constant of the solvent is decreased by the addition of methanol ( e = 33). In this case the magnitude of gem increases due to an increase in the equilibrium shift.
0 0 0
Formamide
0
0
a 0
75
I
50
Mole % H20
Figure 6. Emission dissymmetry factor, gemat 542.8 nm, versus mole fraction H 2 0 for formamide:water ( 0 )and methanokwater (A)solutions.
1
4.0
Tb(DPA)i-:L-Histidine
1
A 0
A 0
A
0
a
A 0
A 0
a
0
75
50
Mole % HzO
Figure 7. Emission dissymmetry factor, gem at 542.8 nm, versus mole fraction H20for DMS0:water (A)and methanokwater ( 0 )solutions.
It has been speculated in some transition-metal studies that hydrogen bonding is an important consideration in the Pfeiffer effect. In order to examine this possibility for the L-histidineTb(DPA)?- system, varying ratios of DMS0:water solutions were studied. In Figure 7 we plot gemat 542.8 nm versus mole percent water for methanol and DMSO. The observed increase in optical
2310
J . Phys. Chem. 1989, 93, 2310-2313
activity seen for addition of DMSO parallels that observed for methanol, strongly indicating that the changes observed are again due primarily to the variation in solvent dielectric constant. DMSO has a dielectric constant (e = 47) similar to that of methanol but is devoid of hydrogen bonding.
Discussion The conclusion drawn above, that the “dissociated” model of Schipper best fits the data for ~-histidine-Tb(DPA),~-,differs from that of Brittain et al., who have described the interaction of various chiral substrates including L-histidine with Ln(DPA),,as forming 1:l complexes.I0 Our results do, however, agree with those of Schipper, who examined similar concentration data for Pfeiffer-active transition-metal systems and also concluded that the dissociated model appears to give a better fit to the experimental data than the associated model.” Although the addition of L-histidine to racemic solutions of lanthanide-DPA complexes clearly results in an equilibrium shift, exactly which enantiomer is favored has not been known until very recently. By comparison of CPL and CD spectra from an optically to that of a active crystal of Na3Eu(DPA)3.2NaC104.6H20 Pfeiffer-perturbed Eu(DPA),,- solution, we have concluded that
addition of L-histidine results in an excess of the A-enanti~mer.’~ This conclusion is in agreement with that of Brittain.” This result implies that A(P) is negative (see eq 14, 16, and 18) and that the L-histidine-A interaction, E(A-P), is of lower energy. A multipole-multipole calculation of this interaction is in progress. The results presented above for mixed solvent systems indicate that the chiral discriminatory interaction has a large electrostatic component. In addition, it is concluded that hydrogen-bonding effects are less important. No attempt has been made to take into account selective solvation or other complications of these mixed solvent systems,20but such work is under way.
Acknowledgment. Acknowledgment is made to the Weldon Spring Fund of the University of Missouri-St. Louis for partial support of this work and to the Monsanto Co. and the Missouri Research Assistance Act for the award of a postdoctoral fellowship (G.L.H.). Registry No. L-Histidine, 71-00-1. (19) Hilmes, G. L.; Coruh, N.; Riehl, J. P., submitted for publication. (20) Reicharrdt, C. In Soluent Ejfects in Organic Chemistry; Verlag Chemie: New York, 1979; Chapter 2.
Laser-Induced Fluorescence of Jet-Cooled IBr: B3&+
-
X‘Z’ Excitation Spectra
Thomas A. Stephenson,* William R. Simpson, Julie R. Wright,+ Henry P. Schneider, Joshua W. Miller,$ and Karen E. Schultzs Department of Chemistry, Swarthmore College, Swarthmore, Pennsylvania 19081 (Received: June 28, 1988; In Final Form: September 21, 1988)
-
The laser-induced fluorescenceexcitation spectrum of the B X transition in IBr seeded in a He supersonic free jet expansion has been recorded. The (l,O), (2,0), and (3,O) transitions are observed with sufficient signal-to-noise to extract the relative fluorescence quantum yields for the v ’ = 1, 2, and 3 vibrational levels. All three levels are found to undergo predissociation to some degree, in contrast to the case of IC1, in which the v’= 1 level in the B state appears stable with respect to dissociation. These differences are discussed in light of the available curve-crossing parameters. No evidence is found in the laser-induced fluorescence excitation spectra for transitions associated with the NeIBr van der Waals complex, possibly due to rapid relaxation mechanisms.
Introduction Since the report by Brown’ in 1932 of the rich visible and near-infrared electronic spectroscopy of IBr, this interhalogen molecule has served as an important model system for tests of the photofragmentation dynamics of electronically excited and the importance of adiabatic and diabatic descriptions of molecular potential energy curves.Islz The low-lying excited electronic states of IBr (A3111,B3110+,B’(0’)) were extensively documented by Selin and co-workers using high-resolution absorption spectroscopy.”-15 The B3110+ X’Z+ system involves excitation to an excited state that, in the diabatic limit, correlates with ground-state (2P3/2)iodine and excited-state (2Pl 2) bromine atoms. A repulsive Q = 0 state (often labeled Y(O+$)that correlates with ground-state atoms intersects the B state at internuclear distances greater than the equilibrium internuclear separation. The result is an avoided crossing and an adiabatic B state potential curve that supports only a limited number of vibrational levels. Energetics dictate that all of these levels are metastable
-
* Author to whom correspondence
should be. addressed. Current address: Department of Chemistry, Columbia University, New York. NY 10027. *Current address: School of Nutrition, Tufts University, Medford, MA 02155. 5 Current address: Department of Chemistry, University of California, Berkeley, CA 94720.
with respect to dissociation to ground-state atoms by tunneling through the potential barrier generated by the avoided crossing. The predissociation dynamics of the metastable B state vibrational levels in IBr have been examined by using both absorption and fluorescence spectroscopy. Selin and Soderborg describe the B X absorption spectrum and note that bands
-
(1) Brown, W. G. Phy. Rev. 1932, 42, 355.
(2) Petersen, A. B.; Smith, I. W. M. Chem. Phys. 1978, 30, 407. (3) Baunhcum. S. L.; Hofmann, H.; Leone, S. R.; Nesbitt. D. J. Discuss. Faraday S&. 1979, 67, 306. (4) De Vries, M. S.; Van Veen, N. J. A.; De Vries, A. E. Chem. Phys. Letr. 1978, 56, 15. (5) Haugen, H. K.; Weitz, E.; Leone, S. R. J . Chem. Phys. 1985,83,3402. ( 6 ) Siese. M.: Tiemann. E. Z . Phvs. D 1987. 7. 147. (7j De Vries,’M. S.; Van Veen, N, J. A,; Hutchinson, M.; De Vries, A. E. Chem. Phys. 1980, 51, 159. (8) Knijckel, H.; Tiemann, E.; Zoglowek, D. J . Mol. Specfrosc.1981, 85, 22s. __.
(9) Bandrauk, A. D.; Turcotte, G.; Lefebvre, R. J. Chem. Phys. 1982, 76, 225. (IO) Child, M. S.; Bernstein, R. B. J . Chem. Phys. 1973, 59, 5916. (1 1) Couillaud, B.; Ducasse, A.; Garrido, L.;Joly, F. J . Phys. B 1976, 9, 2091. (12) Child, M. S. Mol. Phys. 1976, 32, 1495. (13) Selin, L. E. Ark. Fys. 1962, 21, 479. (14) Selin, L. E.; SMerborg, B. Ark. Fys. 1962, 21, 515. (15) Selin, L. E. Ark. Fys. 1962, 21, 529.
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