Chiroptical spectroscopy of lanthanide complexes

Journal of the Less-Common Metals, 149 (1989)

CHIROPTICAL

SPECTROSCOPY

161 - 177

OF LANTHANIDE

161

COMPLEXES*

F. S. RICHARDSON Department

of Chemistry,

University of Virginia, Charlottesville, VA 22901

(U.S.A.)

(Received June 29, 1988)

Summary Chiroptical properties of lanthanide complexes are extraordinarily sensitive to electronic state structure, ligand coordination geometry, ligand stereochemistry, and interactions between a complex and its environment (in solution or in crystals). The chiroptical properties most frequently measured for lanthanide systems are circular dichroism (the differential absorption of left- and right-circularly polarized light), and circularly polarized luminescence (the differential spontaneous emission of left- and rightcircularly polarized light). All systems may exhibit circular dichroism (CD) and circularly polarized luminescence (CPL) in their absorptive and luminescent transitions when subjected to an externally applied magnetic field (aligned parallel to the direction of light propagation in a CD experiment and parallel to the direction of emission detection in a CPL experiment). However, in the absence of any externally applied fields, CD and CPL are exhibited only by systems that have net chirality in their structures or are subject to chiral perturbations by their environment. In lanthanide complexes, overall (net) chirality may reflect chiral arrangements of ligands about the lanthanide ion and/or chiral conformations and atomic centers located within one or more coordinated ligands or chelate rings. Natural CD and CPL exhibited by the 4f-4f transitions of chiral lanthanide systems offer very sensitive probes of coordination and structure in solution media. Applications are limited to systems which possess some element of chirality, but in many cases this merely requires that at least one ligand of interest have a chiral atom (e.g. an asymmetric carbon atom) or carry a chiral label (such as a chiral substituent group). Recent CD and CPL studies reported for chiral lanthanide complexes in crystals demonstrate the utility of chiroptical measurements for characterizing 4fN state structure and 4f-4f transition mechanisms. 1. Introduction ments

Chiroptical spectroscopy is a generic term for spectroscopic measurethat depend on differential interactions of left- and right-circularly

*Paper presented at the 18th Rare Earth Research Conference, September 12 - 16, 1988. 0022-5088/89/$3.50

@ Elsevier Sequoia/Printed

Lake Geneva, WI,

in The Netherlands

162

polarized radiation with matter. These differential interactions are manifested in a variety of spectroscopic processes occurring in systems with chiral structures (or chiral states), and they may be measured using chiroptical absorption, emission, or scattering techniques [l - 31. Here we consider chiroptical absorption and luminescence spectra associated with f-f electronic transitions in lanthanide systems. In chiroptical absorption spectroscopy one generally measures (directly or indirectly) the differential absorbance of left- and right-circularly polarized light by a sample, and this differential absorbance is called circular dichroism (CD). In chiroptical luminescence spectroscopy one measures the differential intensity of left- and rightcircularly polarized light in the luminescence of a sample, and this technique is commonly called circularly polarized luminescence (CPL) spectroscopy [4]. Circular dichroism induced by application of a magnetic field to a sample (with the field aligned parallel to the direction of light propagation) is referred to as magnetic circular dichroism (MCD), and circularly polarized luminescence induced by a magnetic field (applied to a sample with the field aligned parallel to the direction of emission detection) is referred to as magnetic circularly polarized luminescence (MCPL) [4 - 61. All systems may exhibit MCD and MCPL in their absorptive and luminescent transitions. However, natural CD and CPL are exhibited only by systems that have inherently chiral (dissymmetric) structures or have atomic and chemicalbond constituents arranged in a chiral configuration about the direction of light propagation (which is the direction of light transmission in a CD experiment and the direction of emission detection in a CPL experiment). In this paper our main concern will be with natural CD and CPL associated with f-f electronic transitions in chiral lanthanide complexes (in solution and in crystals).

2. Chiroptical

intensities

and line strengths

2.1. Circular dichroism spectra Circular dichroism spectra are typically displayed as Ae us. iJ plots, where g denotes wavenumber (of the incident radiation) and Ae is the difference in molar decadic absorption coefficients of left- and right-circularly polarized light (Ae = eL - ea) propagating through the sample. The circular dichroism associated with a particular electronic transition, A + B, may be expressed as AC(~) = 4.354 x 103sz where R is a factor dependent on the refractivity of the bulk sample medium (under the specified experimental conditions), X,(T) is the fractional thermal (Boltzmann) population of level A at the sample temperature (T), gA is the electronic degeneracy of level A, PAB(p) is a unit-normalized

163

lineshape function centered at the A + B transition frequency V,, (@ is expressed in cm-‘), and RAB is the rotatory strength of the A + B transition (expressed in the CGS units, e.s.u2 cm2). Transition rotatory strengths may be expressed in terms of electric and magnetic multipolar transition moments, and they are the quantities of interest in correlating CD intensities (Ae) with the electronic and stereochemical structure of a system [l - 3, 7,8]. Explicit expressions for the rotatory strength parameter (RAB) appearing in eqn. (1) depend on how the absorbing systems are orientationally distributed in the bulk sample relative to the direction of light propagation. For a sample composed of randomly oriented chiral systems (with chromophoric units small compared to l/V), the quantum-mechanical expression for R,, is given by: R AB = Im(Al~lB)-(BlfilA)

(2)

where Im denotes the imaginary part of a quantity, I! is the electric-dipole moment operator, &I is the magneticdipole moment operator, and both the P and &I matrix elements are defined to include summations over the degenerate components of levels A and B. Expressions analogous to (1) and (2) may be written for the total absorption intensity and dipole strength of the A + B transition in an isotropic sample: e(9) = 1.089 x 1038x

where E = (eL + ~a)/2 e.s.u.2 cm2 units), D AB

=

I(AIf?B>12

(3)

DABFpABtV)

and DA, is a transition

dipole strength (expressed

+ IL@lBH2

in (4)

Now consider a sample that has rotational symmetry about the direction of light propagation and in which the molecular systems of interest have fixed and identical orientations relative to this direction. We locate an orthogonal coordinate system (x, y, z) on each molecular system, and define z to be parallel to the direction of light propagation through the sample. If we assume that the transitions of interest are localized on individual molecular systems, then the rotatory strength of the A + B transition may be expressed as [8 - lo]

Im((AI~IB)~W~iN

- (Al~zlB)(BIfi,IA))

Re((AI~xIB)W~,,IA)

-_(AI~ylBNBlf&xIA))

(5)

where Pi and &i denote the ith components of the electric- and magneticdipole moment operators respectively; Qij is the ijth component of the electric-quadrupole moment operator; PAB is the A + B transition frequency

164

(expressed in cm-‘); and Im and Re specify the imaginary and real parts of the quantities that follow. We refer to R,,(a) as an axial (a) rotatory strength, and it may be used in eqn. (1) when the sample is, for example, a uniaxial crystal aligned with its unique (optic) axis coincident with the direction of light propagation. In the derivation of expression (5), the electricdipole-magnetic-dipole (I?@) and electric-dipole-electric-quadrupole (PQ) terms occur at the same order (in the expansion of the vector potential of the radiation field). Threedimensional orientational averaging removes the PQ terms from the isotropic rotatory strength defined by expression (2). Circular dichroism (Ae) and absorption (e) measurements are often combined to produce spectra of absorption d&symmetry factors, defined by transition A --f B occurring in an isotropic g abr = AC/~. For an electronic sample and exhibiting identical CD and absorption lineshapes, eqns. (1) and (3) may be combined to obtain g abs

-

Ae(g)4Q)

4jiR.4~

(6)

XDAB

(Note that both x and x reflect bulk sample refractivity effects on the radiation field, but their values will generally differ because the E fields and H fields of the radiation do not contribute identically to the CD us. total (dipole) absorption processes.) 2.2. Circularly polarized luminescence (CPL) spectra CPL spectra are usually displayed as AI us. 0 plots, where 0 denotes the wavenumber of the emitted radiation and AI is the difference in intensities between the left- and right-circularly polarized components of the emitted radiation (AI = IL -I,). Under steady-state irradiation conditions, the CPL associated with a particular electronic transition A + B may be expressed as [49 51

u(p)

= hd&&3(@

J@B(a,

7, t) Aw,,(a,

7)) dy dt

(7)

where x’ is a factor dependent on the refractivity of the bulk sample medium, pig($) is a unit-normalized lineshape function centered at the transition frequency DAB;Ng(fi, y, t) is the number of molecules in the sample which exist in the emitting state B at time t with orientation a and structural conformation y; and AWAg(a, y) represents the difference in transition probabilities for the spontaneous emission of left- VS. right-circularly polarized photons. The integrations in eqn. (7) are over allowed molecular conformations (y) and time (for steady-state irradiation conditions), and the angled brackets denote an average over allowed molecular orientations (a). Most CPL experiments reported for lanthanide complexes have been carried out on fluid solution samples using either unpolarized or linearly polarized exciting radiation. Furthermore, most of these experiments were carried out under conditions in which the excited species could be assumed to be orientationally randomized prior to emission (thereby eliminating

165

excitation photoselection Under these conditions, be expressed as [4, 51

effects on the emission polarization properties). the steady-state CPL for the A + B transition may

(8) where RB denotes the number of molecules in level B under the steady-state excitation-emission conditions of the experiment, gn is the degeneracy of level B, and RaB is the isotropic rotatory strength of the A + B transition. The steady-state total luminescence (TL) of this transition may be expressed as (9) where @.+a is the transition dipole strength and I = IL + Ia. Assuming identical lineshapes for CPL and TL, eqns. (8) and (9) may be combined to yield an expression for the emission dissymmetry factor of the A + B transition: 2 AI(v) g em z-z_ I(C)

4ji’R;,

(10)

X'DLB

The rotatory and dipole strengths, RaB and DXB, are defined according to eqns. (2) and (4) respectively. Note, however, that RXB = R,B and DiB = D AB only when the molecules have identical structures in their ground (A) and emitting (B) states. Comparisons between gabs and g,, dissymmetry factors provide information about ground state us. excited state structural conformation and configuration [ 4,11,12]. CPL may be measured for rucemic mixtures of chiral systems in isotropic solution media by using circularly polarized exciting radiation [ 131, and such measurements have been reported for a number of lanthanide systems [14 - 161. In this type of experiment, one enantiomer is excited preferentially over the other and the excited-state population is no longer racemic. Expression (8) remains valid for the A * B CPL observed in this experiment if NB is replaced by (fi,, - Ran), which reflects the different numbers of L-enantiomers and R-enantiomers in the emitting state (B). If excitation is with left-circularly polarized light at a frequency pa, the emission dissymmetry factor is given by k?

(11)

where F is a factor reflecting possible racemization processes that might occur in the excited molecules prior to emission, and &bS(iia) is the absorption dissymmetry factor (*e/e) of the sample at the excitation frequency V,. polarized excitation.) If the (The sign of g,, is reversed for right-circularly

166

racemization emitting state is excited directly (A + B) and no excited-state occurs (F = l), then eqns. (6) and (11) may be combined to yield (for A + B): g em x-

8%

(12)

D2AB

where we have assumed RAB = RaB and DAB = DiB (and all sample refractivity effects have been ignored). CPL excited and detected along the optic axis of a uniaxial crystal may be analyzed in terms of eqn. (8) if the rotatory strength parameter is expressed according to eqn. (5) and multiplied by a factor of two. CPL measurements have been reported for several lanthanide ions in trigonal Nas[Ln(oxydiacetate)s] .2NaC104.6H,0 crystals [17 - 191. CPL measurements on “frozen” isotropic samples, or on solution samples in which rotational diffusion times are long compared to emission lifetimes, may be used to evaluate components of the general rotatory strength tensor not represented in either eqn. (2) or eqn. (5) [4, 5,11, 20 - 241. This is accomplished by creating particular excited-state orientational distributions in the sample using particular excitation-emission geometries and/or excitation polarizations. Excited-state populations with well-defined orientational anisotropies can be prepared (via photoselection in the excitation processes), and the observed CPL (AI) will depend on rotatory strength parameters characteristic of these anisotropies.

2.3. Rotatory strengths and dissymmetry factors The expressions given in Sections 2.1 and 2.2 may be applied to transitions between crystal-field (Stark) levels derived from electronic configuration of a lanthanide ion (in a chiral coordination ment). The rotatory strength of a crystal-field transition, A + B, determined from CD-absorption data by evaluating either RAB = 2.297 X 1O-3g

1

Ae(F) d?

directly the 4fN environmay be

(13)

B

or R AB = 2.297 x 1o-39

$

ga,,,(Wfi) l7

dp

(14)

where the integrations are over the frequency interval spanned by the A + B transition, and RAB is expressed in units of (e.s.u.2 cm2), Expressions analogous to eqns. (13) and (14) may be written for a multiplet-to-multiplet transition, J-t J’, if we assume identical populations for all crystal-field levels within the J-multiplet manifold, let X,(T) represent the fractional population of the J-multiplet relative to other multiplets (at the sample

167

temperature T), and set g, = 2J + 1. Expressed in terms of its crystal-field components, RJJl is given by

RJJ'

=

F

5

(15)

RAB,

where A and B denote crystal-field levels derived from the J and J’ multiplet manifolds respectively. The quantities inside the integrals of expressions (13) and (14) will have just one sign over the frequency interval of any given crystal-field transition; however, they may vary in sign over the frequency interval spanned by a J--f J’ transition (if the component crystal-field transitions have different signs). The dissymmetry factor, gab,(p) = Ae(g)/e(p), may exhibit variations in sign and magnitude over the frequency interval of a J+ J’ transition (reflecting the relative locations, rotatory strengths, and dipole strengths of the component crystal-field transitions). The integration of gabs(Y) over this interval yields a net dissymmetry factor proportional to RJJ~IDJJ~.

To determine rotatory strengths from steady-state CPL-emission one must evaluate expressions of the form (see eqns. (8) - (10))

R

data,

(16)

(17)

where K = (3/64n3c), and ga and fia denote respectively the degeneracy and steady-state population of the emitting level B. Since NB is seldom known (nor easily determined) and because absolute values of AI and I are exceedingly difficult to measure, determinations of rotatory strengths from CPLemission measurements are not practical. On the other hand, absolute values of emission dissymmetry factors (g,,) can be measured quite accurately, and these are the most important chiroptical parameters obtained from CPL-emission experiments [ 41. Now let us turn back to the rotatory strength defined by eqn. (2) and examine how it might be expressed in terms of parameters suitable for structure analysis and spectra-structure correlations. Following our previous work [lo, 25 - 291, we express P as an “effective” electric-dipole moment operator that acts only within the 4fN electronic configuration of the lanthanide ion:

P, = -e(-l)q

2 Lt.P

A& z 01, 1 - 41 tp>U;” 1

(18)

168

where fiQ is the qth spherical component of P (Q = 0, *l), Ut is an irreducible unit-tensor operator of rank h, and the A& are parameters that contain (implicitly) structural and mechanistic details of the lanthanide-ligand-radiation field interactions responsible for f-f electricdipole transitions. The general restrictions on A, t, andp are: h = 2, 4, 6; t = h, h ? 1; andp = 0, +l, . ..) +t. Inserting this expression for p4 into eqn. (2) and assuming that &I acts only within the 4fN configuration, we obtain

A& c (Al, 1 -QI~P)(--~)~(A~IU:IB~)(A,I~;I,IB~)* (19)

c

RAB =-e Im

kt.P

l,Q

where Ai and Bf denote crystal-field state vectors constructed entirely within the 4fN configurations. The analogous expression for the axial rotatory strength, defined by eqn. (5), is

R,,(Q) = Im

c

A.t,P 7

+

0 -

5

Atp 2 W, 1 -~ltp)(Ad U;“lW 1, P

l/2

ne2(r2)&(Afl

q IBt)‘q

(20)

where (F’) is the radial expectation value (4f(r21 4f), and q = 0 corresponds to the molecular z axis (which is defined to be parallel with the direction of light propagation in the axial CD experiment) [9, lo]. The matrix elements in eqns. (19) and (20) are defined to include summations over the degenerate components of levels A and B. Equations (19) and (20) express 4f-4f rotatory strengths in terms of fi, and Up matrix elements over crystal-field state functions, and a set of parameters {A&,}, that contain structural and mechanistic information (related to 4f-4f electric-dipole transition amplitudes) [28, 291. To the first order, the crystal-field state functions depend on the even-parity components of the lanthanide-ligand field interactions, whereas the Atp parameters depend on the odd-purity components of these interactions. The rotatory strength is a pseudosccdar quantity, so the only terms that may make net contributions to expressions (19) and (20) are those that transform as pseudoscalars (under symmetry operations performed on the molecular coordinates). If IA,) and IB,) are expanded in JM bases, we note that the only terms contributing to expression (19) are those satisfying the selection rules (AJ( = 0,l (with neither J nor J’ = 0). This suggests that for the great majority of f-f transitions, rotatory strength (and chiroptical activity) will be extremely sensitive to crystal-field-induced J-level mixings. If J-level mixings are ignored, the total electricdipole strength of a J+ J’ (multipletto-multiplet) transition may be written as

(21)

169

where Q, = (2h + l)-lZt,plA&(Z. The reducible to a similarly simple form.

J+ J’

rotatory

strength

is not

3. CD W. CPL measurements In general, CD and CPL measurements provide similar information about molecular structure and about interactions between a chromophore (or lumiphore) and its chiral environment. However, whereas CD reflects structure in the ground state of a system, CPL reflects structure characteristic of the luminescent state. Comparisons of the CD and CPL exhibited by absorptive and emissive transitions between a specific pair of states in a system (e.g. A + B and A + B) may be used to assess ground state us. emitting state structural differences [4,11,12, 30 - 321. For lanthanide complexes in which A + B and A + B correspond to intraconfigurational 4f-4f transitions, the A f, B CD and CPL should be nearly identical unless the CPL excitation process has photoselected or photochemically produced distributions of structures in the excited state which are different from the equilibrium distribution in the ground state. CD and CPL are generally used as complementary chiroptical techniques in investigations of lanthanide systems. Many transitions in these systems are accessible to study by either CD or CPL, but not by both. CDabsorption measurements offer the opportunity for determining the rotatory strengths and absorption dissymmetry factors of individual transitions, whereas CPL-emission measurements can yield absolute values of emission dissymmetry factors but not rotatory strengths (except under very unusual circumstances). On the other hand, CPL-emission experiments offer special advantages for examining particular populations of species in a sample through the use of excitation photoselection and/or time-resolved measurements. It was noted in Section 2.2, for example, that CPL may be obtained from racemic mixtures by using circularly polarized excitation to effect chiral photoselection (i.e. the preferential excitation of one enantiomer over the other). Most optically active (chiral) lanthanide systems examined to date have relatively large and structurally complex ligands (or coordination environments) with many degrees of freedom available for non-radiative excited state energy disposal. Among these systems, only those with Sm3+, Eu3+, Gd3+, Tb3+ or Dy 3+ have shown luminescence quantum efficiencies sufficient for supporting CPL measurements. Most lanthanide CPL studies have been carried out on europium and terbium complexes in solution [ 4, 11, 32 - 341. The 7F, + 5D, transition of Tb3+ exhibits particularly strong CPL in a wide variety of systems. CD-absorption experiments afford access to many more transitions (in ions spanning the entire lanthanide series) than do CPL-emission experiments. However, most 4f-4f transitions have very low molar absorptivities, so one generally must use optically thick samples (high concentrations of

170

complexes and/or long optical path lengths) in making CD-absorption measurements. This does not pose a problem in many studies, but it generally precludes the use of CD-absorption measurements for investigating very dilute solutions or sample systems available in very small quantities. However, laser-excited CPL-emission measurements can be carried out on small samples and dilute solutions, with the greatest sensitivities being observed for terbium ( 7FJ +- 5D,), europium ( 7FJ + 5D0), samarium ( 6HJ + 4G5,2), and dysprosium (6HJ + 4F 9/z) systems. Generally, it is either necessary or highly desirable to study lanthanide biomolecular complexes in solution at low concentrations (typically 10 ,uM or less in the bound lanthanide ions). For these systems, CPL must be the chiroptical technique of choice for probing the lanthanide coordination environment.

4. Crystal studies Chiroptical studies of lanthanide ions in enantiomorphic crystals have been reported for only three types of systems: cubic K,Ln,(NO&, [35, 361; tetragonal Ln,?Ge,O, [ 37,381; and trigonal Nas[Ln(ODA),] -2NaC104- 6Hz0, where ODA denotes an oxydiacetate ligand (see ref. 10 and references cited therein). The most detailed studies have been carried out on the latter systems, abbreviated hereafter as LnODA. These systems have been proposed as highly suitable models for developing, calibrating, and testing various theories of lanthanide 4f-4f chiroptical activity [S, 9,39 - 411. Single crystals of Nas[Ln(ODA),] -2NaC104-6H,O grow separately (and spontaneously) in two enantiomorphic forms. The crystals have the space group R32, with three Ln(ODA)s3complexes per unit cell. The Ln3+ ions are located at sites with D, symmetry, the tris-terdentate Ln(ODA)33- complexes have D3 point-group symmetry, and the LnO, coordination polyhedra each have slightly distorted tri-capped trigonal prism structures (also with II3 symmetry). The two crystallographic enantiomorphs differ with respect to the absolute configuration (A or A) of their constituent Ln(ODA)s3complexes and the chiral (left-handed or right-handed) arrangement of these complexes about the trigonal axis of the crystal. The structure of these crystals is sufficiently simple (or at least well-defined) to permit measurement and detailed analysis of their chiroptical properties, yet there is sufficient complexity in the lanthanide-ligand chelate systems to make them “relevant” as model systems (in the general study of lanthanide chiroptical activity, where most of the interest is in structurally complex coordination environments). The chiroptical properties of LnODA crystals have been studied extensively over the past 15 years in a number of laboratories. Until recently, however, progress in measurement, analysis, and interpretation was confined to just a few transition regions in any given system, and the prospects for rationalizing CD and CPL data in terms of rotatory strengths of individual crystal-field transitions seemed remote. That picture was changed

171

dramatically by a series of recent studies carried out on the NdODA, SmODA, and EuODA systems at temperatures between 4.2 and 295 K [lo, 19,27,42]. May and co-workers were able to assign over 100 transitions contributing to the CD spectra of SmODA within the 17 600 - 30 500 cm-’ spectral range, and at low temperatures the CD spectra were sufficiently resolved to permit quantitative rotatory strength determinations for over 50 of these transitions (between crystal-field levels) 126, 27, 42, 431. High-resolution CPL spectra obtained on SmODA crystals elucidated yet additional transitions contributing to chiroptical emissions from the 4Gs,2 levels of Sm3+ [19, 421. May obtained similarly detailed and quantitative chiroptical results for the NdODA system [42], and M. T. Berry et al. [lo, 251 have reported quantitative CD-absorption results over the 16 000 37 700 cm-’ spectral range for the EuODA system. The results obtained on the SmODA, NdODA, and EuODA systems (cited above) presented the first opportunity for carrying out truly quantitative analyses of 4f-4f chiroptical spectra based on transition rotatory strengths defined by expressions (13) and (20). Equation (20) expresses rotatory strengths in terms of &, and Ut matrix elements over crystal-field state vectors (specific to the transitions of interest), and a set of parameters {A&} that are characteristic of the system (and are, in principle, applicable to all the 4f-4f transitions in the system). The JM compositions of the 4fN state vectors are determined by the even-parity components of the crystalfield hamiltonian (to the first order), and the Ah, parameters depend on lanthanide-ligand interactions that will couple 4f-4f transitions to the electricdipolar components of a resonant radiation field (in the oneelectron-one-photon approximation) [28, 291. For a system with D, symmetry, there are 12 independent, symmetry-allowed A&, parameters. Analyses carried out on the SmODA, NdODA, and EuODA systems showed that rotatory strengths are extraordinarily sensitive to the JM compositions of the crystal-field state functions and to the signs and magnitudes of the A& parameters [lo, 27, 421. They reflect aspects of ligand-field interactions and structure that have only minimal influence on transition energies and dipole strengths, and they can provide much more precise information about state structure and lanthanide-ligand-radiation field interaction mechanisms than is obtainable from energy-level analyses and dipole strength analyses. The chiroptical properties of LnODA systems are finally yielding to detailed interpretation and analysis, and these systems have renewed promise as models for investigating the influence of structurally complex ligands on 4fN state structure and 4f-4f transition processes. However, it is important that other chiroptically active crystals containing lanthanide complexes be identified for study. 5. Solution studies The main objectives of chiroptical studies on crystals are to obtain detailed information about electronic state structure, ligand-field inter-

172

actions, and electronic transition processes (between crystal-field levels). The objectives of chiroptical studies on lanthanide systems in solution are generally (and often necessarily) quite different. Seldom is it possible to carry out unambiguous chiroptical measurements on solid solution samples at low temperatures due to strain polarization effects and interferences between linear and circular dichroic (and birefringent) phenomena. Generally, one must work with fluid solution samples under conditions that preclude the resolution of CD (or CPL) spectral features assignable to individual (and specific) transitions between crystal-field levels. (Only the 7F, * ‘D, transitions of Eu(II1) complexes represent a consistent exception to this situation.) Total (or net) rotatory strengths of multiplet-to-multiplet transition manifolds may be defined and experimentally determined (see Section 2.3), but they cannot be codified in terms of a set of structure-related parameters that are generic to all the transitions of a particular system (such as, for example, the Ahtp parameters for crystal-field transitions and the Judd-Ofelt Z& parameters for multiplet-to-multiplet transition dipole strengths). Therefore, J -+J’ rotatory strengths are seldom useful for eliciting precise structural information about a lanthanide complex. However, CD, CPL, and absorption and emission dissymmetry factors measured within a multiplet-to-multiplet transition manifold generally exhibit sign and magnitude patterns that are very sensitive to changes in the coordination environment of lanthanide ion. These CD and CPL “signatures”, characterized with respect to the relative signs, magnitudes and spectral frequencies of features appearing in a given transition region, provide the bases for spectrastructure correlations in most solution-phase studies. Solution-phase CD and CPL studies on lanthanide complexes have been reviewed in several, relatively recent papers [4, 32 - 341, and the reader is referred to these papers for surveys of applications. CPL-emission studies of Eu(II1) and Tb(II1) complexes have been overwhelmingly favored, primarily because of the superior measurement sensitivities achievable in such studies. Many Sm(III), Gd(III), and Dy(II1) complexes should also be amenable to CPL-emission studies in solution, but only a few studies on Sm(II1) and Dy(II1) complexes have been reported in the literature [15,16]. Nearly all studies have been carried out on systems belonging in one of the following categories: (i) complexes in which at least one directly coordinated ligand is inherently dissymmetric (i.e. contains one or more chiral centers); (ii) complexes in which the inner-coordination sphere is occupied entirely by nondissymmetric (achiral) ligands, but which are codissolved in solution with chiral molecules that may interact with the lanthanide ion through outersphere coordination or long-lived collisional events; (iii) achiral complexes dissolved in chiral solvents; and, (iv) racemic mixtures of chiral complexes co-dissolved in solution with chiral molecules that may shift the thermodynamic equilibrium concentration of complexes from a 50:50 (racemic) mixture of enantiomers to a mixture with one enantiomer in excess. A wide variety of ligand types are represented in these studies: from simple carboxylic acids and sugars to fl-diketonates, polyether ionophores (cyclic and

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noncyclic), nucleosides, nucleotides, nucleic acids, and protein molecules (see refs. 4, 11, 14 - 16, 32 - 34, 44 - 47 and references cited therein). Studies have been carried out on both aqueous and nonaqueous solutions, and a great deal of attention has been given to effects associated with ion pairing and adduct formation, and to structural (and coordination) changes accompanying changes in solution pH and ionic strength. Earlier (in Section 2.2) it was noted that CPL may be obtained from a racemic mixture of chiral complexes by using chiral photoselection in the excitation process. Circularly polarized exciting radiation is used to prepare an excited state population that contains different numbers of the two enantiomers in the mixture, and if this difference (or enantiomeric excess) is maintained during the time interval between excitation and emission, then CPL will be observed. Decay of the enantiomeric excess with time can occur via structural racemization processes and excited state energy-transfer between opposite enantiomers [4, 131. Consider a steady state excitationemission experiment in which left-circularly polarized light is used to excite a transition G -+ E, and CPL-emission is measured for the transition A +- B. The emission dissymmetry factor (g,, = Al/l) for the A +- B transition in this experiment may be expressed in terms of transition rotatory strengths (RAB and Ros) and transition dipole strengths (DAB and Des) as follows (ignoring sample refractivity effects) :

(22) where F is inversely proportional to the ratio of enantiomeric excess produced in the excited state E at the time of excitation us. the enantiomeric excess present in excited state B during A + B luminescence (under steady state conditions). Direct excitation of the emitting level (G + E = B) will probably enhance the F ratio (since in this case the B +- E relaxation step is avoided); however, this will not necessarily increase the magnitude of g,, if 1(RGB/DGB)) < 1(RGE/DGE)l. In the absence of fast excited-state racemization processes (i.e. when F = l), this experiment is optimized by choosing a G + E transition having a large absorption dissymmetry factor: Igabs = lAe/el a I (RGE/DGE)I. Riehl and co-workers have carried out a number of studies in which they used circularly polarized light to excite CPL from samples showing no CD [14 - 16, 481. Two systems of particular interest were Ln(ODA)s3- and Ln(DPA)33- complexes in aqueous solution under mildly alkaline (pH 8 - 9) conditions. These complexes have tris-terdentate chelate structures with trigonal dihedral (03) symmetry in the crystalline state [49, 501, and the ligand backbone of each chelate ring stretches diagonally across a rectangular face of the coordination polyhedron (which forms a slightly distorted tricapped trigonal prism). These structures have configurational chirality about their trigonal axis and a chiral center at the lanthanide ion. Neither ODA (oxydiacetate) nor DPA (dipicolinate or 2,6-pyridinedicarboxylate) contains

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a chiral (atomic) center, and both form nearly planar chelate rings (which precludes significant conformational chirality in the chelate systems). In solutions containing no chiral reagents, the Ln(ODA),3and Ln(DPA)33complexes should exist as racemic mixtures of optical isomers (or enantiomers) with A and A absolute configurations (defined with respect to the arrangement of chelate rings about the lanthanide ion). Hilmes and Riehl [15] investigated Ln(ODA)33and Ln(DPA)33complexes of Pr3+, Sm3+, Eu3+, Tb3+ and Dy3+ in aqueous solution. Using circularly polarized laser radiation, they were able to excite CPL from the DPA complexes of Sm3+, Eu3+, Tb3+ and Dy3+, and from the ODA complex of Dy3+. They were unable to detect CPL from the ODA complexes of Sm3+, Eu3+ and Tb3+, and neither the DPA nor ODA complexes of Pr3+ exhibited CPL. Qualitative differences (i.e. detection us. no detection of CPL) between the results obtained for the various systems were attributed to the relative racemization kinetics of DPA us. ODA complexes and the luminescence lifetimes of the transitions examined for the different ions. The ODA complexes racemize faster than the DPA complexes and, except for DY(ODA),~-, they do not maintain an enantiomeric excess throughout the time interval between excitation and emission. The use of circularly polarized radiation to excite CPL from racemic mixtures has many interesting applications in lanthanide coordination chemistry. However, the most important of these applications will require time-resolved CPL measurements with which changes in excited-state structural distributions may be followed and characterized [13]. Time-resolved CPL measurements on lanthanide systems have not yet been reported, but their feasibility is imminent with currently available CPL-emission spectrometers based on pulsed-excitation-photon-counting emission-detection techniques.

6. Conclusion Motivating factors in lanthanide chiroptical studies generally derive from one or more of the following: (i) a general interest in chiroptical activity; (ii) a particular interest in the chemical and physical properties of chiral lanthanide complexes and relationships between these properties and specific aspects of structural chirality; and, (iii) an interest in exploiting chiroptical measurement techniques to probe coordination and structure in systems which have chirality, but for which chirality is of little interest beyond providing a “handle” for chiroptical measurements. Point (iii) accounts for the great majority of studies reported to date, and will probably motivate most future chiroptical measurements on lanthanide systems. CD and CPL spectra generally provide much more sensitive probes of coordination and structure than do their non-chiroptical counterparts, but they remain somewhat under-utilized in lanthanide coordination chemistry and spectroscopy. Their application requires intrinsic or induced chirality in the coordination

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environment, but this requirement is not as limiting (with respect to scope of applications) as it might appear. Many types of ligands of general interest either contain chiral centers (e.g. asymmetric carbon atoms) intrinsic to their structure or can easily accommodate chiral centers or substituents in their structure (without causing significant changes in their coordination chemistry). Furthermore, it is not always necessary to have chiral ligands (or ligands with chiral labels) bound directly to the lanthanide ion via innersphere coordination. In many cases, lanthanide chiroptical activity can be induced by chiral molecules that have no direct access to the inner-coordination sphere. Finally, for experiments in which CPL is excited by circularly polarized light, one may use samples in which complexes exist as racemic mixtures of optical enantiomers (in their equilibrium ground state). Chiroptical activity and other chirality-related phenomena have been investigated extensively and in great detail for transition-metal complexes, organic molecules, and biopolymer systems [ 1 - 3, 51 - 531, and chiroptical spectroscopy has played an enormously important (and sometimes crucial) role in characterizing electronic structure and stereochemistry in these classes of systems. By comparison, chiroptical activity and spectroscopy of lanthanide systems have received only meager attention. Chiroptical spectra of most lanthanide systems are complex and not easily amenable to detailed analysis, and precise spectra-structure correlations remain somewhat elusive. However, the chiroptical spectra of certain lanthanide systems have yielded to detailed analysis [lo, 19, 27, 421, and there is now considerable evidence that lanthanide chiroptical spectra can provide information regarding coordination and structure not obtainable from other types of spectroscopic data [4,10,11,14 - 19,27, 32 - 47 3. Lanthanide chiroptical spectroscopy merits additional attention and effort, with respect to theory, experiment, and new systems for study.

Acknowledgments Many discussions and collaborations with Dr. Mary Berry, Dr. Harry Brittain, Dr. Stan May, Dr. David Metcalf, Dr. Richard Palmer, Dr. Michael Reid, Dr. James Riehl and Dr. Robert Schwartz were important to this work, and financial support from the U.S. National Science Foundation (NSF Grant CHE-8215815) is gratefully acknowledged.

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