J. Phys. Chem. 1992,96, 1112-1 120
1112
Analysis of Enantloselective Quenching of Tb( 2,6-pyridinedicarboxyiate),8Luminescence by Resolved Ru( 1,lO-phenanthroiine),'+ in Methanol and in Water Roe1 B. Rexwinkel,+ Stefan C. J. Meskers; James P. Riel&**$and Harry P. J. M. Dekkers**t Gorlaeus Laboratories, Department of Chemistry, Leiden University, Postbox 9502, 2300 RA Leiden, The Netherlands, and Department of Chemistry, University of Missouri-St. Louis, St. Louis, Missouri 63121 (Received: July 26, 1991; In Final Form: October 2, 1991)
Circularly polarized luminescence is observed from racemic solutionsof Tb(2,6-pyridinedicarbo~ylate)~~ when small amounts are added. This phenomenon has been attributed to enantioselective quenching of of resolved Ru( 1,lO-phenanthr~line)~*+ the terbium complex by the ruthenium species, resulting in a nonracemic excited-state population. In this work, a simple kinetic scheme for this quenching process is presented, and it is demonstrated that the individual diastereomeric rate constants may be determined directly through numerical fitting of the biexponential decay of the total luminescence intensity. This is shown to give essentially the same results as numerical fitting of the time decay of the luminescence dissymmetry factor when data from both of these measurement techniques are available. It is also demonstrated that the enantioselectivity in the chiral quenching is solvent dependent. In aqueous solutions, for example, addition of A-(-)-Ru(phen)?+ results in an exam of L ~ - T ~ ( D P ALe., ) ~ ~the - , A enantiomer is quenched more efficiently, whereas in methanol the opposite enantiomer is in excess.
I. htrduction
of additional experiments involving the chiral quenching of Tb(DPA)33 by resolved R ~ ( p h e n ) ~ ~A+simple . kinetic scheme for Recent developments in instrumentation have resulted in the the quenching process is developed, and it is demonstrated that ability to detect the time dependence of the usually small net circular polarization in the emission of optically active m~lecule~.~-~ either the time dependence of the total luminescence intensity or the circularly polarized luminescence intensity may be employed To date, applications of this new experimental technique have pertained to the inhomogeneous luminescence of chiral a&e.n~nes~ to determine the individual diastereomeric quenching rate constants. This don0r:quencher system is amenable for study in water and, more extensively,to studies involving the racemization kinetia and in methanol, and remarkable differences between chiral of optically active lanthanide complexess and to a small number quenching in these two solvent systems are presented and discussed. of donor:quencher systems in which there exists enantiomericselective q ~ e n c h i n g 'resulting * ~ ~ ~ in a time-dependent nonracemic population of emitting species. Most of this latter work has II. Theory involved solutions of nine-coordinate tris-terdendate complexes A simple kinetic scheme for the enantioselective quenching of of lanthanide(II1) ions with achiral ligands with approximate D3 racemic rareearth-metal complex by a chiral quencher complex a symmetry. Complexes formed with 2,6-pyridinedicarboxylate is given below. In this scheme the two enantiomeric fonns of the (dipicolinate = DPA) have been shown to be particularly table^^^ complex are denoted by A-Ln and A-Ln, and it is implied that and have been the species most studied by this technique. Since the A form of the optically active quencher molecule, QA,is used. this ligand is not chiral, both of the possible D3 enantiomers (A It has also been assumed that the excited-state decay rate of the and A) are formed in equal amounts, and the solution can be complex is much faster than the racemization rate, and, therefore, described as a racemic equilibrium. Although DPA complexes this equilibrium has not been included in the following set of have been prepared for a number of the lanthanide ions, by far q u a tions: the most studied is the Tb(DPA)33- species, since it is the most luminescent of the series and, therefore, the easiest to characterize A-Ln + huab A-Ln* A-Ln + huab A-Ln* spectroscopically.loJ1 This complex has also theen prepared and characterized in methanol solutions.I2 Circularly polarized luminescence (CPL) ~pectroscopy'~ has (1) Metcalf, D. H.; Snyder, S. W.; Wu,S.; Hilmes, G. L.; Riehl, J. P.; proven to be particularly useful probe of the structure and Demas, J. N.; Richardson, F. S . J. Am. Chem. Soc. 1989, 111, 3082-3. equilibrium dynamics of several lanthanide DPA complex(2) Blok, P. M. L.; Schakel, P.;Dekkers, H. P.J. M.Meas. Sci. Technol. es.9-11J3-16Of particular interest are the experiments in which 1990, 1 , 126-30. CPL was observed following excitation of circularly polarized (3) Rexwinkel, R. B.; Schakel, P.;Makers, S. C. J.; Dekkers, H. P.J. M., to be published. light.le16 Observation of optical activity in the emission from (4) Blok, P. M. L.; Dekkers, H. P.J. M. Photochem. Photobiol. 1991,53, these complexes implies that the enantiomeric excess in the excited 421-49. state generated by the circularly polarized excitation is retained ( 5 ) Metcalf, D. H.; Snyder, S. W.; Demas, J. N.; Richardson, F. S. J. Am. throughout the emission lifetime. Thus, one concludes that, at Chem. SOC.1990, 112, 5681. ( 6 ) Metcalf, D. H.; Snyder, S. W.; Demas, J. N.; Richardson, F. S.J. room temperature, the racemization rate is slow compared to Phys. Chem. 1990,94, 7143. emission. This has been quantitatively verified by time-resolved (7) Metcalf, D. H.; Snyder, S. W.; Demas, J. N.; Richardson, F. S. Eur. CPL measurement^.^ J . Solid Stare Inorg. Chem. 1991, 28, 57. The enantioselective quenching of aqueous solutions of Tb(8) Donato, H.; Martin, R. B. J . Am. Chem. Soc. 1972, 94, 4129-31. (DPA)d- by resolved transition-metal complexes has been de(9) Yan, F.; Brittain, H. G. Polyhedron 1982, 1, 195-9. (10) Brittain, H. G. Coord. Chem. Rev. 1970, 5, 279. scribed in several recent publication~.6.~J~ Optically active Ru(11) Brittain, H. G. J. Coord. Chem. 1989, 20, 331-47. (phen)? was the first complex discovered to effect chiral (12) Murray, G. M.; Samo, R. V.;Peterson, J. R. Inorg. Chim. Acta 1990, quenchmg of this species, and it is especially suited for detailed 176, 233-40. study due to its ease of purification and stability to thermal or (13) Riehl, J. P.; Richardson, F. S . Chem. Rev. 1986, 86, 1. (14) Hilmes, G. L.; Timper, J. M.; Riehl, J. P. Inorg. Chem. 1985. 24, photoinduced racemization. In this work, we present the results
-
Leiden University. !University of Missouri-St. Louis. *To whom correspondence should be addreased.
-
1721-3. (15) Hilmes, G. L.; Riehl, J. P. Inorg. Chem. 1986, 25, 2617. (16) Hilma, G. L.; Coruh, N.; Riehl, J. P.Inorg. Chem. 1988,27,1136-9. (17) Wu, S.; Bedard, T. C.; Riehl, J. P., to be published.
0022-3654/92/2096-1112$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1113
Tb(2,6-pyridinedi~arboxylate)~'- Luminescence A-Ln*
ko
+ hvlum A-Ln* + QA A-Ln* + QA A-Ln
A-Ln*
kqM
ko
A-Ln
A-Ln'
+ QA'
A-Ln'
+ QA'
kq"
-
The steady-state dissymmetry factor, glm(X),may be obtained by integrating over the entire decay:
+ hvlm
The final state of the quencher species in indicated by QA' and that of Ln* by Ln', to cover situations in which the quenching mechanism is due to vibrational or electronic energy transfer, or electron transfer. ko denotes the decay rate in the absence of quencher, and kqMand kqM are, respectively, the rate constants for quenching of the A and A enantiomer of the excited lanthanide complex. Enantioselective quenching corresponds to kqM being unequal to k,". The degree of enantioselectivity in the quenching process is designated E, and is defined as follows:6 kqAA- kqM E, = (1) kqAA k9AA
+
Following this defintion, if homochiral quenching dominates over heterochiral quenching a positive value of E, is expected, and, obviously, a negative value will be obtained if the reverse is true. The presence of enantioselective quenching in systems of this type has the following consequences: (1) Since the excited states of the two Ln enantiomers are quenched at different rates, the decay of the total concentration of excited lanthanide species must be biexponential. From the scheme outlined above, one obtains the following equation: d[A-Ln*] /dt = -(ko + kqAA[QA])t = -k-t (2) which has the simple solution [A-Ln*](t) = [A-Ln*l0 exp(-k-r) (3) where [A-Ln*lodenotes the concentration of excited A enantiomer at time 0, and k- is defined by eq 2. Similarly, one obtains the following result for the homochiral quenching process: d[A-Ln*] /dt = -(ko
+ kqAA[QA])t= -k+t
(4)
[A-Ln*](t) = [A-Ln*Ioexp(-k+t) (5) Thus, the decay of the total luminescence is given by the following expression: [Ln*](r) = [A-Ln*],, exp(-k-r) + [A-Ln*Ioexp(-k+t)) (6) (2)Due to the different decay rates for the two enantiomers, following an initial excitation pulse, the circular polarization of the emitted light will be time dependent. This is most often observed by measurement of the luminescence dissymmetry factor, gl,,(X,t), which is defined as
where the subscripts L and R denote, respectively, left and right circular polarization, and X is the emission wavelength. h l ( A , t ) and I(x,r) are directly related to the emissive rotatory strength and dipole strength, respectively, in the usual manner." Substitution of the explicit time dependence of the excited state populations into eq 7 yields the following expression for glm(A,t):5 gIum(ht) = 8 1 u d X ) tanh (fkd[Qnlt)
Following ref 5, this can be related to the rate constants given above as follows:
As will be discussed, each of the three consequences mentioned above, corresponds to an experimentally observable quantity. Thus, values for all parameters in the above kinetic scheme can be obtained by appropriate analysis of the following experimental data. Analysis of Biexponential Decay of Total Emiasion Intensity. As illustrated in eq 6, following an initial excitation pulse, the total excited-state population decays according to a biexponential function. This results in a biexponential decay of the total lanthanide luminescence intensity. For a racemic solution of complexes, if the excitation is performed with an unpolarized light pulse, we can set [A-Ln*lo = [A-Ln*],,. Therefore, the intensity of emission, I(X,t) is given by I ( x , f ) = t/zZ(X,t=O)[exp(-k+t)
exp(-k-t)]
+ Idc
(12)
where signifies experimental dark current. By fitting the observed Z(X,r) signal to this function, the decay constants &+ and k- may be determined, although it is not possible, from this fitting p r d u r e alone, to identify which of the derived decay constants corresponds to k+ and which to k-. To uniquely determine these rate constants, one must know, from other experiments, the absolute sign of and the sign of the measured gl,(X,t) from either time-resolved or steady-state measurements. If these quantities are known, then the homochiral and heterochiral quenching constants (and the sign of E,) may be assigned. Measurement of Time-Resolved Circularly Polarized Luminescence (TRCPL). Recently, time-resolved CPL measurements have become possible.'-' In this technique one either directly measures the time dependence of hl(h,t) or determines the time dependence of the dissymmetry factor from the time dependence of Z(x,r) and AZ(x,t). From these experiments, vaiues for the magnitudes of kd and &&) can be obtained by fitting to the hyperbolic tangent function given the measured glum(A,r) by eq 8. The absolute sign of kd may be determined if the sign of $,,(A) is known or vice versa. Measurement of Steady-State Circularly Polarized hmhescence. The quantity glum(X) can be measured directly by using a continuous excitation source. From this value, one can also by using eq 11 together with determine the magnitude of g'J,&) the values found for k+ and k- through the following relationship:
Again, the ambiguity in sign of kd, Le., the assignment of k+ and k- to the two diastereomeric quenching constants, may be overcome if additional experimental information is available. In addition, for measurements of g,,,,,,(X) at various quencher concentrations, the following relation must hold:
(8)
where kd kqM - kqAA (9) and $,,(A) denotes the dissymmetry factor of the enantiomer A at the wavelength of observation. (3) In measurements of circularly polarized luminescence o b tained under steady-state conditions, Le., continuous excitation and polarization detection, the dissymmetry factor need not be equal to zero, because the time-averaged population of excited enantiomers may be different.
Le., l/glum(A) should vary linearly with the inverse concentration of the quencher.
III. Experimental Section Material and Sample Preparation. Tb(II1) solutions were prepared from TbC13 (Aldrich). 2,6-Pyridinedicarboxylicacid (DPA) was obtained from Aldrich and used without further purification. Solutions of Tb(DPA)33-were typically prepared by mixing stock solutions of Tb(II1) (pH = 3) and 2,6-pyridinedi-
1114 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Rexwinkel et al.
TABLE I: Results for Numerical Fitting of Synthetic Biexpowathl Decay Data input data data set amplitude k, k2 amplitude 1 100 000 640 660 99952 f 57 660 499758 f 186 2 500 000 640 700 99980 66 3 100000 670 4 100000 700 750 100021 f 68 850 100032 f 61 5 100000 780 1600 150018 f 103 6 150 000 1100 1500 2400 120028 f 79 7 120000 2100 3300 120058 f 67 8 120 000
+
carboxylate (pH = 8) in 1:3 stoichiometric ratios. A 0.74 mM Tb(DPA)33-solution was made by mixing 15 pL of an aqueous 0.15 M Tb(II1) solution with 20.4 pL of a 0.33 M solution of DPA in water, yielding a solution of Tb(DPA)33- at pH 10. Subsequently 3.0 mL of distilled water (or methanol) was added. The pH value of the aqueous sample was 7. Samples with higher Tb(DPA)33-concentrationwere prepared similarly. The aqueous solutions had a pH value in the range 9-10. Tris(1,lOphenanthroline)ruthenium(II) chloride was purchased from AESAR and resolved into A and A enantiomers by successive recrystallizations with antimony Dtartrate.'* The diastereomers were isolated as perchlorate salts after recrystallizationsproduced no increase in optical rotation. The optical purity of these enantiomers were also checked by CD measurements, for A-(-)Ru(phen):+ Ae(267 nm) - Ae(258 nm) = -907 M-' m-' (H20).19 Methanol (Janssen Chimica) was of spectrophotometric grade and contained 0.5 f 0.5 vol % of water as determined from densitometric measurements. In the quenching experiments (-)-Ru(phen)t+ was added in solid form to the cuvette containing the Tb(DPA)33-solution. The quencher concentration was determined spectrophotometrically at the wavelength of maximal absorbance in the visible absorption band (447 nm in water, 445 nm in methanol). In samples with low (-)-R~(phen)~~+ concentration, the absorbance of a concentrated solution was measured, which was then diluted using a micropipet. All samples were freshly prepared and were checked to ensure that they had not deteriorated during the course of the measurements. The samples with methanol as the solvent contained a small amount of water (1.5 f 0.5 vol %), but the amount was approximately the same in all samples. Sample temperature in all experiments was 20 f 1 OC. Measurements of the luminescence decay of gl,(X) and gI,(A,t) were performed using a homebuilt photon-counting CPL instrument at the University of Leiden capable of both time-resolved and steady-state measurement^.^ The time-resolved experiments described in this work were accomplished using a flash-lamp (Optitron NR-1B-Xe) with excitation pulses having a width of 1 ps. Wavelength selection of the exciting flash was done with a UG-11 filter (325 50 nm). Experiments were done at emission intensities such that photopulse pileup effects were negligible. Steady-state measurements of glm using Ar-ion laser (Coherent INNOVA-70) excitation were performed at the University of Missouri-St. Louis on an instrument described previously.16 Total emission spectra were obtained at 0.8-nm resolution; decay experiments were performed at 4-nmresolution; steady-state measurements of gl,,(X) were measured at 2- or 0.4-nm resolution as indicated. Absorbance measurements were performed on a Perkin-Elmer Lambda 16 UV-vis spectrophotometer. CD measurements were performed using a Jobin-Yvon Mark I11 dichrograph calibrated just before measurement using epi-androsterone. Optical rotations were measured on a Rudolph automatic spectropolarimeter. Data Manipulation. The curve-fitting programs used in the are based on the analysis of the intensity decay and glum(X,t) (18) Oillard. R. D.; Hill, R. E. E. J. Chem. SOC.,Dalron Tram. 1974, 1217. (19) Bosnich, B. Fundamenral Aspecrs and Recenr Developmenrs in Optical Rotarory Dispersion and Circular Dichroism; Ciardelli, F., Salvadori, P.,Eds.; Heyden & Son Ltd.: New York, 1973; p 246.
output data
kl
k2
649.521 641 f 4 675 f 9 699 f 4 182 f 4 1101 f 1 1501 f 1 2099 f 2
649.519 659 f 5 695 f 10 750 f 5 849 f 5 1599 f 3 2399 f 5 3305 f 7
reduced x2 1.043 1.08 1.07 1.11 1.004 0.965 0.945 0.899
Levenberg-Marquardt algorithm and were modified for the specific functions required from the listings given in ref 20. All fitting of timeresolved data incorporated appropriate weighting factors to account for the noise content of the experimental signals?' For the fit of Z(t), where noise is entirely due to photopulse noise, the magnitude of the weighting factor is given by 1/Z, where Zi is the number of photopulses in the ith time channel in the decay measurement.21 For the fit of gl,,(X,t), weighting factors were taken from the theory on noise characteristics of our a~paratus.~ The goodness of fit was judged from the standard deviation, the plot of the weighted residuals, the value of the reduced xz, and the autocorrelation function of the weighted residuals. For a good - Y,,, multiplied by the square fit, the weighted residuals, Y root of the weighting factor, should be randomly distributed around zero. The reduced xz of the fit is defmed as the sum of the s q d weighted residuals divided by the number of points, minus the number of degrees of freedom. For a good fit, the reduced x2 should be approximately equal to 1. This implies that Ym-,i Yfit,i, on average, equals the predicted standard error. An additional test of goodness of fit is given by examination of the autocorrelation function of the weighted residuals. The autocorrelation function for points randomly distributed around zero should have a &function shape, being unity at the origin and behg small at other points. IV. Results A. Test of Numerical Fitting F'mmhue. The performance of the intensity fitting program was studied by applying it to a number of synthetic data sets. These data sets were generated by using a computer program in which Poisson noise was added exp(-k2t)]. The Poisson to the functionflt) = A[exp(-k,t) noise generator is described by Press et a1.20 Some relevant results, obtained on data representative of the decays actually observed in this study, are shown in Table I. The data shown are for four-parameter fits in which the amplitude, the dark current, and both decay constants were treated as unknowns. As can be seen from the results presented, the computer program used to fit this synthetic data works quite satisfactorily. For an amplitude corresponding to 2 X lo5 photon counts, biexponential decay constants differing by less than 4% may be determined. This limit is reduced when a larger number of photon counts are available. In the actual experimental measurements, the number of counts depends directly on the acquisition time. In Figure lB, we show the result of application of the fitting procedure to the total luminescence decay of a 0.7 mM solution of Tb(DPA),> in water. The small circles in this fgure represent the measured photon counts, and the solid line is the result of the biexponential fit. The first five points in the luminescencedecay were not included in the set of data to be fit, due to leakage of the excitation pulse and Ru* emission into these initial emission channels. Also given in this figure is a graph of the weighted residuals and the autocorrelation function of these residuals. For purpose of comparison, in Figure 1A we plot the results of a single-exponential fit to these same data. Although it is virtually impossible to ascertain which one of these two functions is the
+
(20) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling. W. T. Numerical Recipes; Cambridge University Prw: New York, 1986. (21) Demas, J. N. Excited Slate Uferime Measurements; Academic Press: New York, 1983.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1115
Tb(2,6-pyridinedi~arboxylate)~~Luminescence 10
2.0
5
0
-5
1.6 n
-10
2 a
g
6
1 .2
v
c2
0.8
a +
-
n v)
z1 s
0.4
0 0
C
0
z .E
0.0 2.0
4
n
1 .o
B
0.5
v
0
1.6 n
0)
s 3
c
AI ( ~ 1 0 ' )
-0.5
a
1.2
: '
0.0
-1.0
v
2
1
1
Time (msec.)
1
0
Time (msec.)
Figure 1. A (a) Total luminescence decay of 0.7 mM solution of Tb(DPA)?- in water. The solid line is the result of a numerical fit to the functionflt) = Io exp(-kt) + Idc.Results from fit: Io = 17 1 845 94, k = 3913 2, Idc= 39 1, and reduced x2 = 2.557; (b) weighted residuals from the monoexponential fit; (c) autocorrelation function of the weighted residuals. B: (a) Total luminescence decay of 0.7 mM solution of Tb(DPA)35 in water. The solid line is the result of a numerical fit to the functionflt) = Io[exp(-k+t) + exp(-k-t)] + Idc. Results from fit: Io = 87 122 66, k-/k+ = 3542 5 and 4492 13, Idc= 29 1, and reduced x2 = 1.023; (b) weighted residuals from the biexponential fit; (c) autocorrelation function of the weighted residuals.
*
*
*
*
*
better fit by visual inspection of the mono- and biexponential fits of the decay data, the plots of residuals and the autocorrelation function of the residuals clearly show that the biexponential function represents a better fit to these data. The fitting of decays of this general type to a sum of exponentials is a well-known problem, and ordinarily one can be successful only if the exponents differ by a considerable amount. The success of our fitting procedure for a biexponential curve involving exponents which differ by such a small amount undoubtedly is related to the fact that we can impose exact equality of the preexponential factors, since, at time t = 0, the excited-state concentration of the two enantiomers must be equal. It should also be noted that, due to the relative narrowness of the excitation pulses used in the decays presented below, time t = 0 is well defined. We thus avoid complications inherent in decays in which the excitation pulse must be deconvoluted from the timedependent luminescence signal. B.spectroscoW of 'I~I(DPA),~. A total emission spectrum of a 0.7 mM solution of Tb(DPA)3g in water is displayed in Figure 2A. This spectral region displayed corresponds to emission from the excited $D4level to three of the J levels of the 'F ground-state term as indicated. In this complex, the excited state may be populated either directly, by using Ar-ion laser excitation, or indirectly, through excitation of the aromatic ligand, followed by efficient energy transfer to Tb(II1). The spectra displayed in Figure 2 were excited by the 488-nm line of an Ar ion laser; however, the total emission spectra are identical when the excitation is via ligand absorption. In Figure 2B is displayed the identical spectral region under identical experimental cond i t i o ~for a 0.7 mM solution of this complex in 98.5% methanol. Also given in this figure are CPL results following the addition
Wavelength (nm) Figure 2. Total emission spectra for 0.7 mM solution of Tb(DPA)?from 520 to 640 nm. Spectrum A is for the complex in water, B in 98.5% methanol. Circularly polarized emission in the SD4 'F5 transition following the addition of 4 pM (-)-Ru(phen)l+ is also presented. Excitation wavelength is 488 nm.
-
of 4 pM (-)-Ru(phen):+ to the two solutions. The total emission spectra were measured with a 0.8-nm bandpass and, as can be seen, are very similar. The very slight differences in intensity and splitting pattern observed for Tb(DPA)33-in these two solvents are most likely due to minor structural perturbations due to solvent cage effects. A remarkable difference between the two systems is seen in comparison of the CPL spectra. Although the line shapes are similar, AI is opposite in sign in the two solvents. In addition, the magnitude of AZ is considerably larger in methanol than in water. As discussed below, the change in sign reflects a difference in the dominant enantiomer present in the excited state. The similarity in line shapes for the total emission and CPL spectra lead us to the conclusion that the Tb(DPA)33-species present in water and methanol solutions are very similar. This conclusion is in agreement with other recent experiments involving DPA complexes in anhydrous methanol.12 The racemic equilibrium involving complexes of DPA with various lanthanide(II1) ions has been of interest for several years. Although the racemization process is too fast for any chemical resolution to be possible, it is slow compared to the emission lifetime in methanol and water (or DzO), allowing one the op portunity to measure optical activity in the excited state through preferential absorption of circularly polarized laser excitation as described in several publications.SJ4-16 It is also possible to disturb the racemic equilibrium through the addition of a noncoordinating optically active environment compound, e.g., L-histidine, resulting in a measurable unequal concentration of enantiomers in the ground ~ t a t e . ~ l Circular -~~ dichroism and circularly polarized emission spectra can be obtained from these solutions. Intraconfigurational f-f transitions that obey magnetic dipole selection rulcp may possess large values for am. The magnitudes are known to depend predominantly on the coordination geometry and to a (22) Wu, S.;Hilmes, G. L.; Riehl, J. P. J. Phys. Chem. 1989,93,2307-10. (23) Richardson, F. S. Inorg. Chcm. 1980, 19, 2806.
1116 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Rexwinkel et al.
TABLE Ik Results for 0.7 m M Solution of [Tb(DPA),”] in Methanol“ [A-R~(phen)~*~], X106 M 0 0.27 0.538 0.799 0.924 1.33 1.98 3.26 3.90 4.16 5.23d 5.97 8.18 11.8 16.5
k-, s-I
k+, s-I
483 835 1195 1550 1645 2375 3307 5202 6230 6235 7929 928 1 12653 17990 24460
483 681 899 1099 1142 1535 2099 3098 3619 3672 458 1 5335 7093 9960 13586
ghm
P U lm
(543 nm) -0.280 -0.262 -0.268 -0.276 -0.285 -0.272 -0.287 -0,294 -0.287 -0.290 -0.289 -0.296 -0,298 -0.293 av -0.284
(543 nm)b
-0.0280 -0,0385 -0.0440
-0.276 -0.272 -0.258
-0.0468 -0.0567 -0.0590 -0.062oC -0.0627 -0.0632 -0.0653 -0.0660 -0.0668 -0.0668
-0.218 -0.254 -0.233 -0.234 -0.242 -0.236 -0.242 -0.234 -0.233 -0.241 av -0.244 & 0.006
0.004
k- and kt were obtained from the biex nential fit of I(?), and glm(543 nm) was obtained under steady-state conditions: Eq calculated according to eq 1, and g$ym calculated from eq 13. calculated from gl,(543 nm) and values for k- and kt obtained from fit of Z(t). CThisvalue changed to this sample, measurement of g,,,(h,t) was performed (cf. Table 111). -0.0883 when measured with 0.4-nm spectral resolution.
t C 0
u
16 12
-
1 /[Ru(phen),*’] 0.000
5.000
10.000
15.000
20.000
[R~(phen)~’’] (106M)
Figure 3. Stern-Volmer plot for the quenching of 0.7 mM solution of in 98.5%methanol. Solid lines repment Tb(DPA)3f by (-)-R~(phen)~~+ a linear least-squares fit to the data. The insert is an enlargement of the low-concentration region.
much lesser extent to the nature of the ligand.24 Thus, an assignment of the absolute sign of glum to a specific enantiomer of Tb(DPA)33-has been made possible through comparison of the spectra obtained for a similarly perturbed Eu(DPA)~~solution to spectra obtained from an optically active crystal of a related D3 Eu(II1) compound, namely, Na3Eu(oxydiacetate),. 2NaC104-6H20.25.26This analysis relies upon the reasonable assumption that the equilibrium perturbation resulting from the addition of the chiral environment compound would be independent of the identity of the lanthanide ion. It has, therefore, been concluded that a positive sign for glm at the peak of the emission of the 5D4 ’FStransition corresponds to an excess of the A enantiomerF2 With reference to the theoretical treatment preaented above, this,in fact, uniquely defmes the sign of gjm(543 nm) to be negative. This quantity should be independent of the solvent used. Methanol Solution. In Figure 3 we plot results of the biexponential analysis of the total luminescence decay of Tb(DPA)3* in methanol following addition of small amounts of A-(-)-Ru( ~ h e n ) ~ ~The + . insert in this figure is an expanded plot of the
-
(24) Coruh, N.;Hilmes, G. L.; Riehl, J. P. Imrg. Chem. 1988.27.3647. (25) Sen,A. C.; Chowdhury, M.;Schwartz, R. W.J. Chem. Soc.,Faraday Tram. 2 1981, 77, 1293. (26) Morley, J. P.; Saxe, J. D.; Richardson, F. S.Mol. Phys. 1982,47, 379.
( 1 06M(-’)
Figwe 4. Plot of blum]-l at 543 nm (spectral resolution = 2 nm) versus [(-)-Ru(phen)?+]- in 98.5%methanol. Excitation wavelength = 300 nm. The slope of the solid line yields a value of E.&u, (see Table VI). The ratio of intercept to slope yields a value of the average quenching constant, l/JkqM + kqM] = 1.23 X lo9 s-l.
low-concentration region. The amplitude of the biexponential fits, corresponding to the number of counts in the first time window was usually more than 2 X lo5. Both rate constants in the biexponential decay show a linear Stern-Volmer behavior as indicated by the solid lines in this figure. These results are also presented in Table 11. The sign of gIm(543nm) in methanol is negative, showing that the dominant emitting species is the A enantiomer, and, therefore, the larger slope corresponds to more efficient quenching of the A enantiomer (k). A simple straight line fit to this data yields values for k+ and k,and substituting the value for ko (see eqs 2 and 4) obtained from the single-exponential decay when no quencher is present, one gets the following values for the diastereomeric quenching rate constants: k,~* = (1.47 0.01) x 109 M-1 s-1
kqM = (0.797 f 0.004)
X
lo9 M-’ s-l
The errors in these rate constants were determined from simple unweighted least-squares analysis. Thus, Eq = -0.30 0.02 and from the definition given in eq 9 kd = (0.67 f 0.01) X lo9 M-l s-l
*
Table I1 also contains E values calculated from the decay constants for each addition o? R ~ @ h e n ) ~ Note ~ + . that these Eq values are independent of errors in quencher concentration. Therefore, a somewhat more reliable value of Eqis obtained by taking the average value from this table, 0.284 0.004,where the specified error is the standard error. Again, the negative sign for Eq (positive
*
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1117
Tb(2,6-pyridinedi~arboxylate)~~Luminescence 4 2
Residuals1
0 -2 -4 0.6
0.4
0.2 n
E C
M
;0.0 U
-s
I
0.000
5.000
10.000
15.000
20.000
0,
[ R ~ ( p h e n ) ~ * +( 1] O-6M)
-0.2
Figure 6. Stern-Volmer plot for the quenching of Tb(DPA),"- by (-)-Ru(phen)32+in water. Squares correspond to 0.7 mM Tb(DPA)3*, circles to 10 mM Tb(DPA),'-. Solid lines represent linear least-squares fits to the data.
-0.4
. * : . * ' .I 1.000
0.000
2.000
Time (msec.)
Figure 5. (a) Time dependence of glmat 543 nm (spectral resolution = 2 nm) for a 0.7 mM solution of Tb(DPA),> in methanol quenched by 5.2 pM (-)-Ru(phen),". The time axis displayed corresponds to 500 time channels and equals more than 12 times the excited-state lifetime. The solid line is the result of a two-parameter fit of eq 8 (see text). (b) Weighted residuals from the tanh fit. (c) Autocorrelationfunction of the weighted residuals. See Table 111 for parameter values.
for &Jimplies that, for this system, heterochiral quenching is faster than homochiral quenching. It does not appear that Eq depends on the concentration of quencher. Results for glummeasured at 543 nm under steady-state conditions are also given in Table 11. In F i i 4 we plot l/gh versus the inverse of the quencher concentration. These data follow the linear behavior expected from eq 14. The solid line in this figure represents a simple linear least-squares fit to this data. Unless otherwise stated, all glmdata reported in this work pertain to the emission wavelength of 543 nm (ie., at the wavelength where gh, of the Tb(II1) 5D4 'F5 transition has its maximum value) and to a bandwidth of 2 nm. Due to the fact that glm shows pronounced fine structure in this band, because of overlapping values increase considerably upon crystal-field transitions, glum decreasing the spectral bandwidth. At a bandwidth of 0.4 nm, for example, glum values at the peak increase by approximately 50%. It should also be noted that the glum data have not been corrected for dark current, which is negligible under the experimental conditions used in the work presented here, or for nonideal behavior of the sheet polarizer used with the photoelastic modulator. This latter effect would lead to an error of approximately -2% at this wavelength (543 nm). Furthermore, since the Ruhen)^^+ is optically active, one must be concerned with differential absorption of Tb(II1) emission. However, it was observed that, even at the highest concentration of quencher used in this study, the absorption at 543 nm was very small, and, therefore, no correction for partial circular polarization due to reabsorbed 543-nm light was necessary. Finally, we disregard that at 543 nm a very small fraction of the emission intensity is due to Ruhen)^^+ emission. The emission dissymmetry factor for (-)Ru(phen)32+is, in fact, very small,*'
