Basics of Lanthanide Photophysics

8/11/2019 Basics of Lanthanide Photophysics

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Basics of Lanthanide Photophysics

Jean-Claude G. Bu nzli and Svetlana V. Eliseeva

Abstract The fascination for lanthanide optical spectroscopy dates back to the 1880swhen renowned scientists such as Sir William Crookes, LeCoq de Boisbaudran,Euge ne Demarcay or, later, Georges Urbain were using luminescence as an analyticaltool to test the purity of their crystallizations and to identify potential new elements.The richness and complexity of lanthanide optical spectra are reected in an articlepublished in 1937 by J.H. van Vleck: The Puzzle of Rare Earth Spectra in Solids . After this analytical and exploratory period, lanthanide unique optical properties were takenadvantage of in optical glasses, lters, and lasers. In the mid-1970s, E. Soini and

I. Hemmila proposed lanthanide luminescent probes for time-resolved immunoassays(Soini and Hemmila in Clin Chem 25:353361, 1979) and this has been the startingpoint of the present numerous bio-applications based on optical properties of lantha-nides. In this chapter, we rst briey outline the principles underlying the simplestmodels used for describing the electronic structure and spectroscopic properties of trivalent lanthanide ions Ln III (4f n) with special emphasis on luminescence. Since thebook is intended for a broad readership within the sciences, we start from scratchdening all quantities used, but we stay at a descriptive level, leaving out detailedmathematical developments. For the latter, the reader is referred to references Liu and

Jacquier, Spectroscopic properties of rare earths in optical materials. Tsinghua Uni-versity Press & Springer, Beijing & Heidelberg, 2005 and Go rller-Walrand andBinnemans, Rationalization of crystal eld parameters. In: Gschneidner, Eyring(eds) Handbook on the physics and chemistry of rare earths, vol 23. Elsevier BV,Amsterdam, Ch 155, 1996. The second part of the chapter is devoted to practicalaspects of lanthanide luminescent probes, both from the point of view of their designand of their potential utility.

J.-C.G. Bu nzli ( * ) and S.V. EliseevaLaboratory of Lanthanide Supramolecular Chemistry, E cole Polytechnique Fe de rale de Lausanne,BCH 1402, 1015 Lausanne, Switzerlande-mail: [email protected]

P. Ha nninen and H. Ha rma (eds.), Lanthanide Luminescence: Photophysical, Analyticaland Biological Aspects , Springer Ser Fluoresc (2010), DOI 10.1007/4243_2010_3,# Springer-Verlag Berlin Heidelberg 2010


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Keywords Crystal-eld analysis Energy transfer ff Transition Intrinsicquantum yield Lanthanide bioprobe Lanthanide luminescence Lanthanidespectroscopy Lifetime Luminescence sensitization Population analysis Quantum yield Radiative lifetime Selection rule Site symmetry SternVolmer

quenching

Contents

1 Electronic Structure of Trivalent Lanthanide Ions1.1 Atomic Orbitals1.2 Electronic Conguration1.3 The Ions in a Ligand Field

2 Absorption Spectra2.1 Induced ED ff Transitions: JuddOfelt Theory [ 5, 6]2.2 4f5d and CT Transitions

3 Emission Spectra4 Sensitization of Lanthanide Luminescence

4.1 Design of Efcient Lanthanide Luminescent Bioprobes4.2 Practical Measurements of Absolute Quantum Yields

5 Information Extracted from Lanthanide Luminescent Probes5.1 Metal Ion Sites: Number, Composition, and Population Analysis5.2 Site Symmetry Through Crystal-Field Analysis5.3 Strength of MetalLigand Bonds: Vibronic Satellite Analysis5.4 Solvation State of the Metal Ion5.5 Energy Transfers: DonorAcceptor Distances and Control of the Photophysical

Properties of the Acceptor by the Donor 5.6 FRET Analysis5.7 Ligand Exchange Kinetics5.8 Analytical Probes

6 Appendices6.1 Site Symmetry Determination from Eu III Luminescence Spectra6.2 Examples of JuddOfelt Parameters6.3 Examples of Reduced Matrix Elements6.4 Emission Spectra

References

Abbreviations

AO Acridine orangeCF Crystal eldCT Charge transfer DMF DimethylformamideDNA Deoxyribonucleic aciddpa Dipicolinate (2,6-pyridine dicarboxylate)dtpa DiethylenetrinitrilopentaacetateEB Ethidium bromideED Electric dipole

J.-C.G. Bu nzli and S.V. Eliseeva


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EQ Electric quadrupoleFRET Fo rster resonant energy transfer hfa HexauoroacetylacetonateILCT Intraligand charge transfer ISC Intersystem crossingJO JuddOfeltLLB Lanthanide luminescent bioprobeLMCT Ligand-to-metal charge transfer MD Magnetic dipoleMLCT Metal-to-ligand charge transfer NIR Near-infraredPCR Polymerase chain reactionSO Spinorbit

tta ThenoyltriuoroacetylacetonateYAG Yttrium aluminum garnet

1 Electronic Structure of Trivalent Lanthanide Ions

1.1 Atomic Orbitals

In quantum mechanics, three variables depict the movement of the electrons aroundthe positively-charged nucleus, these electrons being considered as waves withwavelength l h / mv where h is Plancks constant (6.626 10 34 J s 1), m and vthe mass (9.109 10 31 kg) and velocity of the electron, respectively: The time-dependent Hamiltonian operator H describing the sum of kinetic and

potential energies in the system; it is a function of the coordinates of theelectrons and nucleus.

The wavefunction, C n , also depending on the coordinates and time, related tothe movement of the particles, and not directly observable; its square ( C n)

2

though gives the probability that the particle it describes will be found at theposition given by the coordinates; the set of all probabilities for a givenelectronic C n , is called an orbital .

The quantied energy En associated with a specic wavefunction, and indepen-dent of the coordinates.

These quantities are related by the dramatically simple Schro dinger equation,which replaces the fundamental equations of classical mechanics for atomic sys-tems:

HC n EnC n: (1)Energies En are eigenvalues of C n , themselves called eigenfunctions. In view

of the complexity brought by the multidimensional aspect of this equation



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(3 coordinates for each electron and nucleus, in addition to time) several simplica-tions are made. Firstly, the energy is assumed to be constant with time, whichremoves one coordinate. Secondly, nuclei being much heavier than electrons, theyare considered as being xed (BornOppenheimer approximation). Thirdly, since

the equation can only be solved precisely for the hydrogen atom, the resultinghydrogenoid or one-electron wavefunction is used for the other elements, with ascaling taking into account the apparent nucleus charge, i.e., including screeningeffects from the other electrons. Finally, to ease solving the equation for non-Hatoms, the various interactions occurring in the electron-nucleus system are treatedseparately, in order of decreasing importance ( perturbation method ).

For hydrogen, the Hamiltonian simply reects Coulombs attraction betweenthe nu cleus and the electron, separated by a distance r i, and the kinetic energy of thelatter: 1

H0 1r i

12

Di D @ 2

@ x2 @ 2

@ y2 @ 2

@ z2 : (2)Each wavefunction (or orbital: the two terms are very often, but wrongly, taken

as synonyms) resulting from solving (1) is dened by four quantum numbersreecting the quantied energy of the two motions of the electrons: the orbitalmotion, dened by the angular momentum ~ , and the spin, characterized by theangular momentum ~s. If polar coordinates ( r , #, ) are used, wavefunctions are

expressed as the product of a normalizing factor N , of a radial function


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1.2 Electronic Conguration

The ground state electronic conguration of Ln III ions is [Xe]4f n (n = 014).It is energetically well separated from the [Xe]4f n 15d1 conguration ( D E> 32,000 cm 1). A far reaching fact is the shielding of the 4f orbitals by thexenon core (54 electrons), particularly the larger radial expansion of the 5s 25p6

subshells, making the valence 4f orbitals inner orbitals (bottom of Fig. 1). This isthe key to the chemical and spectroscopic properties of these metal ions.

r 2

2 / a

. u .

r /a.u.

0 1 2 3

4f3

Xe core

Nd III

Fig. 1 Top: Shape of the one-electron (hydrogenoid) 4f orbitals in a Cartesian space. From top to

bottom and left to right : 4f x(x 2 3y 2), 4f y(3y 2 x 2), 4f xyz , 4f z(x 2 y 2), 4f xz 2, 4f yz 2, and 4f z3 (combina-tions of Cartesian coordinates represent the angular functions). Bottom: Radial wavefunction of thethree 4f electrons of Nd III compared with the radial wavefunction of the xenon core (a.u. = atomicunits); redrawn after [ 1]



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Each of the n electrons of the 4f n conguration is associated with one of theseven 4f wavefunctions and may have a spin of . There are a number of ways of associating the n electrons with the 4f orbitals, taking the spin into consideration;this number corresponds to the multiplicity (or degeneracy) of the conguration and

is given by the following combinatorial formula:

4 2!n!4 2 n!

14!n!14 n!

if 3: (4)

Since there are more than one electron in the conguration, the Hamiltonian in(2) has to be adjusted to take into account the number of electrons, the apparent(screened) nucleus charge Z 0, and the repulsion between electrons located at adistance r ij :

H Xn

i1 Z 0r i

12

Di Xn

i6 j 1r ij

: (5)

A given association of the electrons with the 4f wavefunctions {( m , ms)1, (m ,ms)2, . . ..(m , ms)n} is called a micro state . The latter is characterized with overallquantum numbers M L and M S derived from the projections of the sum of the angular momenta:

~ L Xn

i1~ i; ~S X

n

i1~si; M L X

n

i1mi; M S X

n

i1msi: (6)

A set of micro states such that all the M L and M S quantum numbers correspond tothe projections of one value of L and S, respectively, is called a spectroscopic term .It is written as 2S1G, where G is a capital letter (S, P, D, F, G . . .) corresponding tothe values of L (0, 1, 2, 3, 4 . . .). The multiplicity of a term, i.e., the number of microstates it regroups, is given by (2 S + 1) (2 L + 1). An electronic congurationcontains several terms and the sum of their multiplicities is equal to the degeneracyof the conguration. The procedure for nding them out is rather tedious, except for the ground term, for which Hunds rules make its determination easy. According tothese rules (to be applied in the given order), the ground term has:

Rule 1 The largest spin multiplicityRule 2 The largest orbital multiplicity

For instance, for Eu III , 4f 6, the largest multiplicity is obtained when eachelectron is associated with a unique 4f wavefunction: S 6 3; therefore,(2S 1) 7. To obtain the largest orbital multiplicity, these electrons have to berelated with wavefunctions having the largest m values, i.e., 3, 2, 1, 0, 1,and 2; the sum is 3, henceforth L 3 and G F: the ground term is a spin septet,7F with overall multiplicity 7 7 49.



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The two movements of the electrons are in fact not independent, they couple andthe strength of the interaction usually increases with the atomic number. Tosimplify the treatment of this interaction, Russel and Saunders have proposed toconsider this coupling at the level of the overall angular momenta and not for each

individual electron. This model is valid for lighter elements and not quite adequatefor lanthanides for which an intermediate coupling scheme should be applied;however, it is in large use in view of its simplicity and we shall keep with it inthis chapter. The Hamiltonian becomes:

HXn

i1 Z 0r i

12

Di Xn

i6 j 1r ij l ~ L ~S with l

x2S

: (7)

The spinorbit coupling constant l is positive if the 4f subshell is less than half lled and negative if it is more than half lled. A new quantum number J , associatedwith the total angular momentum ~ J ~ L ~S, has to be introduced with valuesranging from ( L S) to ( L S). As a consequence, each term is further splitinto a number of spectroscopic levels 2S1G J each with a (2 J 1) multiplicity.Again, the sum of these multiplicities must be equal to the multiplicity of the term.For instance, the ground term of Eu III is split into 7F0,

7F1 , 7F2 ,

7F3 , 7F4,

7F5, and 7F6

with multiplicities 1 3 5 7 9 11 13 49. The ground level can befound with third Hunds rule:

Rule 3 if n < 2 1; J J min ; if n > 2 1; J J max :Note that if the sub-shell is half lled, then L = 0 and J = S. Additionally, J may

take half-integer values if S is half-integer. The set of levels is referred to as amultiplet and this multiplet is a regular one if n < (2 + 1), the energy of the levelsincreasing with increasing values of J , while it is inverted if n > (2 + 1). Thisis illustrated with Eu III (4f 6) for which the ground level is 7F0 while it is

7F6 for TbIII

(4f 8). Finally, the energy difference between two consecutive spinorbit levels withquantum numbers J and J 0 = J + 1 is directly proportional to J 0:

D E l J 0: (8)The electronic properties of the trivalent 4f free ions are summarized in Table 1.

1.3 The Ions in a Ligand Field

The above developments are valid for free ions. When a Ln III ion is inserted into achemical environment, the spherical symmetry of its electronic structure isdestroyed and the remaining (2 J 1) degeneracy of its spectroscopic levels ispartly lifted, depending on the exact symmetry of the metalion site. In view of the



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inner character of the 4f wavefunctions their mixing with the surrounding orbitalsremains small and so is the resulting level splitting (a few hundreds of cm 1), sothat this perturbation can be treated last. Nevertheless, the resulting Hamiltoniangets very complex, so that a simplifying concept has been put forward by H. Bethein 1929: the ligands are replaced by (negative) point charges generating a crystal (or ligand) electrostatic eld which, in turn, interacts with the moving 4f electrons,generating a ligand-eld (or crystal-eld, or Stark) splitting of the spectroscopiclevels.

With the parameterization introduced by B. G. Wybourne in 1965, the nalHamiltonian becomes:

H Xn

i1 Z 0r i

12

Di Xn

i6 j 1r ij l ~ L ~S Xk ;q;i Bk qCk q i; (9)

where the summation involving i is on all the 4f electrons, Bk q are ligand-eldparameters, commonly treated as phenomenological parameters, and C(k)q arecomponents of tensor operators C (k) which transform like the spherical harmonicsused for the analytical form of the 4f wavefunctions. The running number k must be

even and smaller than 2

; for 4f electrons it can, therefore, take values of 0, 2, 4,and 6. The values for q are restricted by the point group of symmetry into which theLn III ion is embedded, but in any case, qj j k .

The Bk q parameters may be complex numbers but they have to be real for anysymmetry group with a 180 rotation about the y axis or with the xy plane being amirror plane. The relationship between the 32 crystallographic symmetry groupsand the Bk q parameters is given in Table 2. In order to compare the ligand eldstrengths in different compounds, F. Auzel has proposed the following expressionfor an average total ligand-eld effect:

N v 14p Xk ;q

Bk q2

2k 1" #1=2

: (10)

Table 1 Electronic properties of Ln III free ionsf n Multiplicity No. of terms No. of levels Ground level z /cm 1,a l /cm 1,a

f 0 f 14 1 1 1 1S01S0

f 1 f 13 14 1 2 2F5/22F7/2 625 2,870 625 2,870

f 2

f 12

91 7 13 3

H43

H6 740 2,628 370 1,314f 3 f 11 364 17 41 4I9/2 4I15/2 884 2,380 295 793f 4 f 10 1,001 47 107 5I4 5I8 1,000 2,141 250 535f 5 f 9 2,002 73 198 6H5/2 6H15/2 1,157 1,932 231 386f 6 f 8 3,003 119 295 7F0 7F6 1,326 1,709 221 285f 7 3,432 119 327 8S7/2 1,450 0aFor aqua ions, except for Ce III (Ce:LaCl 3) and Yb

III (Yb 3Ga 5O12 ), from [ 2]. The rst columnrefers to f 17 and the second to f 814



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Finally, it is worth noting that electrostatic ligand-eld effects do not completelylift the degeneracy of the J levels of odd-numbered electronic congurations; in thelatter case, all ligand-eld sublevels are at least doubly degenerate (Kramersdoublets) and this degeneracy can only be removed by a magnetic eld. A partialenergy diagram, including crystal-eld splittings is given in Fig. 2. Due to their large number, energy levels may extend up to 190,000 cm 1 for n = 6, 7, 8, and arenot yet fully explored, although an extension of Carnalls diagram up to this energyhas been recently published [ 4].

The maximum numbers of ligand-eld (or Stark) sublevels depend on the pointgroup of symmetry: they are given in Table 3 versus values of J . This can beexploited for the determination of the symmetry point group from ff absorption or emission spectra, at least when J is integer.

2 Absorption Spectra

Description of the interaction between photons (massless elemental particles of light) and matter considers the former behaving as waves comprised of twoperpendicular elds, electric and magnetic, oscillating in time (henceforth thedenomination of electromagnetic wave or radiation). When a photon is absorbed,its energy is transferred to an electron which then may be pushed into an orbitalwith higher energy. The absorption is promoted by operators linked to the natureof light: the odd-parity electric dipole (ED) operator ~ P, the even-parity magneticdipole (MD) ~ M and electric quadrupole (EQ) ~Q operators:

~ P eX

n

i1 ~r i

~ M

eh

4pmc Xn

i1 ~ i 2~si

~Q

1

2 Xn

i1 ~k ~r i~r i: (11)

There are three types of electronic transitions involving lanthanide ions:sharp intracongurational 4f4f transitions, broader 4f5d transitions, and broad

Table 2 Non-zero crystal-eld parameters for f n electronic congurations and examples of corresponding crystal hosts [ 1]Symmetry Site symmetry Crystal eld parameters ExampleMonoclinic C1 , CS, C 2, C 2h , B20 ; B40 ; B60 ;


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charge-transfer transitions (metal-to-ligand, MLCT or ligand-to-metal, LMCT).Not all transitions are permitted and the allowed ones are described by selectionrules. Laportes parity selection rule implies that states with the same parity cannotbe connected by electric dipole transitions; as a consequence ff transitions areforbidden by the ED mechanism. However, when the lanthanide ion is under theinuence of a ligand-eld, non-centrosymmetric interactions allow the mixing of

Fig. 2 Energy level diagram for Ln III ions doped in a low-symmetry crystal, LaF 3 . Redrawnfrom [ 3]



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electronic states of opposite parity into the 4f wavefunctions, which somewhatrelaxes the selection rules and the transition becomes partially allowed; it is calledan induced (or forced) electric dipole transition . Magnetic dipole transitions areallowed, but their intensity is weak; in 4f4f spectra, however, they often haveintensity of the same order of magnitude as induced electric dipole transitions.Quadrupolar transitions are also parity allowed, but they are much weaker than MDtransitions so that they are usually not observed. Some induced ED transitions arehighly sensitive to minute changes in the Ln III environment and are called hyper-sensitive or sometimes pseudo-quadrupolar transitions because they apparentlyfollow the selection rules of EQ transitions. A listing of experimentally identiedsuch transitions is presented in Table 4; note, that these transitions are not neces-sarily the most intense ones in the optical spectra.

In addition to the parity selection rule, other rules are operative, for instance, onDS (spin selection rule, requiring no change of spin for all three mechanisms,DS 0), D L, and D J ; they will be detailed below. The selection rules are derivedunder several hypotheses which are not always completely fullled in reality (inparticular 4f wavefunctions are not completely pure), so that the terms forbiddenand allowed transitions are not accurate. Lets say that a forbidden transition hasa low probability and an allowed transition a high probability of occurring.

2.1 Induced ED ff Transitions: JuddOfelt Theory [ 5 , 6 ]

JuddOfelt (JO) theory has been established within the frame of the crystal-eldconcept and it provides a simple model for reproducing the intensities of ff transitions both in solids and solutions. It only takes into account the 4f n electronicconguration, that is inter-congurational 4f n 4f n 15d1 interactions are neglected.On the other hand, spinorbit coupling is treated within the frame of the intermediate

Table 3 Number of Stark levels versus the value of quantum number J Symmetry Site symmetry Integer J

0 1 2 3 4 5 6 7 8Cubic T, T d , T h , O, Oh 1 1 2 3 4 4 6 6 7

Hexagonal C 3h , D 3h , C6 , C6h , C6v , D6 , D6h 1 2 3 5 6 7 9 10 11Trigonal C 3, S6 C 3v , D 3, D 3d Tetragonal C 4, S 4, C 4h , 1 2 4 5 7 8 10 11 13

C 4v , D 4, D 2d , D 4hLow C1 , CS, C 2, C 2h , C 2v , D 2, D 2h 1 3 5 7 9 11 13 15 17

Symmetry Site symmetry Half-integer J 1/2 3/2 5/2 7/2 9/2 11/2 13/2 15/2 17/2

Cubic T, T d , T h , O, Oh 1 1 2 3 3 4 5 6 6All others a See above 1 2 3 4 5 6 7 8 9aAll Stark sublevels are doubly degenerate (Kramers doublets)



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coupling scheme. The dipole strength in esu 2cm 2 (1036 debye 2) of an induced EDff transition between states C and C 0 is given by:

DED e2 Xl 2;4;6 Ol C U l C 0

2; (12)

in which e is the electric charge of the electron, wavefunctions C and C 0 are fullintermediate-coupled functions f n[SL]J, U l are the irreducible tensor forms of theED operator, and Ol are the phenomenological JO parameters, expressed in cm 2.The bracketed expressions in (12) are dimensionless doubly-reduced matrix ele-ments which are tabulated (and insensitive to the metalion environment). Mathe-matical treatment of the parity mixing by the crystal-eld perturbation leads to theselection rules for ff transitions reproduced in Table 5.

JO parameters are adjustable parameters and they are calculated from theabsorption spectrum e (~n). For an isotropic crystal or a solution, the experimentaldipole strength is dened as:

Dexp 1036

108 :9 ~nmean X A 2 J 1

9n

n2 2 2 !Z e ~n d~n; (13a)

Table 4 Experimentally observed hypersensitive transitions for Ln III ions in optical spectra [ 5].Energies/wavelengths are approximateLn Transition ~n /cm 1 l /nmPr 3F2

3H4 5,200 1,920

Nd 4

G5/24

I9/2a

17,300 5782H9/2 ,4F5/2

4I9/2 12,400 8064G7/2 ,

3K13/24I9/2 19,200 521

Sm 4F1/2 ,4F3/2

6H5/2 6,400 1,560Eu 5D2

7F0 21,500 4655D1

7F1 18,700 5355D0 ! 7F2 16,300 613Gd 6P5/2, 6P7/2 8S7/2 32,500 308

Tb b Dy 6F11/2

6H15/2 7,700 1,3004G11/2 ,

4I15/26H15/2 23,400 427

Ho 3

H6 5

I8 27,700 3615G6 5I8 22,100 452Er 4G11/2 4I15/2 26,400 3792H11/2

4I15/2 19,200 521Tm 1G4 3H6 21,300 4693H4 3H6 12,700 7873F4 3H6 5,900 1,695aThe transition 4G5/2 4I9/2 overlaps with 2G7/2 4I9/2bNone identied positively, but the 5D4 ! 7F5 transition shows sometimes ligand-inducedpseudo-hypersensitivity



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with X A being the fractional population of the initial state while ~nmean is given by:

~nmean R ~n e ~n d~nR e ~n d~n : (13b)

The above equations assume that the absorption bands are symmetrical, i.e.,either Gaussian or Lorentzian. If not, (13a) has to be replaced with:

Dexp 1036

108 :9 X A 2 J 1 9n

n2 2 2 !Z e ~n ~n d~n: (14)

Finally, (2 J 1) is the degeneracy of the initial state and the expressioninvolving the refractive index n is known as Lorentzs local-eld correction.Calculations of transition probabilities within the frame of JO theory are usually

made assuming that all Stark sublevels within the ground level are equally popu-lated and that the material under investigation is optically isotropic. The former hypothesis is only reasonable in some cases, e.g., when transitions initiate fromnon-degenerate states such as Eu( 7F0). Otherwise, there is a Boltzmann distributionof the population among the crystal-eld sublevels. The second assumption is notvalid for uniaxial or biaxial crystals, but, of course, holds for solutions.

The phenomenological JO parameters are determined from a t of (12) to theexperimental values dened by (13a), using adequate matrix elements. The exactprocedure is described in details in reference [ 6]. In the case of Eu III the procedureis quite simple since O

2, O

4, and O

6 can be directly extracted from the dipole

strength of the 5D27F0,

5D4 7F0, and 5L6 7F0 transitions, respectively. Anexample is shown on Fig. 3 for europium tris(dipicolinate). Extensive tabulations of JO parameters can be found in reference [ 5] while spectra for all Ln III ions arepresented in reference [ 8]; note that molar absorption coefcients are, with a fewexceptions, smaller than 10 M 1 cm 1 and very often smaller than 1 or even0.1 M 1 cm 1.

2.2 4f5d and CT Transitions

The promotion of a 4f electron into the 5d sub-shell is parity allowed; thecorresponding transitions are broader than ff transitions and their energy depends

Table 5 Selection rules for intra-congurational ff transitionsOperator Parity DS D L D J a

ED Opposite 0 6 6 (2,4,6 if J or J 0 0)MD Same 0 0 0, 1EQ Same 0 0, 1, 2 0, 1, 2a J = 0 to J 0 = 0 transitions are always forbidden



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largely on the metal environment since the 5d orbitals are external and interactdirectly with the ligand orbitals. The 4f5d transitions have high energies (Fig. 4)and only those of Ce III , Pr III , and Tb III are commonly observed. Figure 5 shows thecrystal-eld splitting of both the 4f 1(2F5/2 ,

2F7/2 ) and 5d1(2D3/2 ,

2D5/2 ) electroniccongurations of Ce III in D3h symmetry. In the spectrum displayed, the thirdtransition to 2D5/2 is not observed because it lies at too high energy. Conversely,the Ce III luminescence can be tuned from about 290 to 450 nm, depending on thematrix into which the metal ion is inserted, because of large crystal-eld effects onthe 5d 1 excited conguration.

Charge-transfer transitions, both LMCT and MLCT, are allowed and have alsohigh energies (Fig. 4), so that only the LMCT of Eu III and Yb III (possibly Sm III andTm III ) are commonly observed in ordinary solvents, contrary to d-transition metal

400 500 6000

1

2

3

4

5

E /103cm 1

/ nm

30 26 20

/ M

1 c m

1

D exp / 1042 esu 2cm 2

/ 10 20 cm 2

2 = 10.3 4 = 5.6 6 = 9.2

5L6

5D4

5D2

7F0

22.65D4

7F0 16.95L6

7F0 321

5D3

5D2 5D1

(2S+1) J 7F0,1

24 18 1622

Fig. 3 Absorption spectrum of Na 3[Eu(dpa) 3] 1.8 10 2 M in TrisHCl 0.1 M and associateddipole strengths and JO parameters. Redrawn from [ 7]

30

40

50

60

70

80

300

250

200

150

120E /103cm 1 /nm

Ce Nd Sm Gd Dy Er YbPr Pm Eu Tb Ho Tm

Fig. 4 Energy of the 4f5dtransitions in Ln III :CaF 2(squares , [9]) and of the 2p(O)-4f LMCT transitions(triangles , [10])



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ions for which this type of transition is widespread. This is sometimes not wellunderstood and the literature features many wrong assignments to MLCT transi-tions made by analogy to d-metal complexes.

3 Emission Spectra

With the exception of La III and Lu III , all Ln III ions are luminescent and their ff emission lines cover the entire spectrum, from UV (Gd III ) to visible (e.g., Pr III ,Sm III , Eu III , Tb III , Dy III , Tm III ) and near-infrared (NIR, e.g., Pr III , Nd III , Ho III , Er III ,

YbIII

[11 ]) ranges. Some ions are uorescent ( DS = 0), others are phosphorescent(DS 6 0), and some are both. The ff emission lines are sharp because the rear-rangement consecutive to the promotion of an electron into a 4f orbital of higher energy does not perturb much the binding pattern in the molecules since 4f orbitalsdo not participate much in this binding (the covalency of a Ln III ligand bond isat most 57%). Therefore, the internuclear distances remain almost the same inthe excited state, which generates narrow bands and very small Stokes shifts.A different situation is met in organic molecules for which excitation leads fre-quently to a lengthening of the chemical bonds, resulting in large Stokes shifts andsince the coupling with vibrations is strong, in broad emission bands (Fig. 6). Themain emission lines observed in Ln III luminescence spectra are listed in Table 6,together with other key photophysical parameters.

As for absorption, emission of light through ff transitions is achieved by either electric dipole or magnetic dipole mechanisms, and the selection rules detailed in

40

0

2

44

48

2F7/2

2F5/2

2

D5/2

2D3/2

n.obs.

225

E /103cm

1

E /10 3cm 1

8

6

4

2

0

[Ce(H 2O)9]3+

250

48 44 40 36 32

300 nm

1 0 2 / M

1 c m

1

Fig. 5 Left : Absorption spectrum of [Ce(H 2O) 9]3+ and right : its assignment ( D3h symmetry)



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Table 5 apply. Important parameters characterizing the emission of light from aLn III ion are the lifetime of the excited state t obs = 1/ k obs and the quantum yield Q.A general expression for the latter is simply

Q number of emitted photonsnumber of absorbed photons

: (15)

The quantum yield is related to the rate at which the excited level is depopulatedk obs and to the radiative rate constant k

rad :

QLnLn k rad

k obs t obst rad

: (16)

Subscript and superscript Ln have been added to avoid confusion with theother denition of quantum yield discussed below. The quantity dened in (16) iscalled the intrinsic quantum yield , that is, the quantum yield of the metal-centeredluminescence upon direct excitation into the 4f levels. Its value reects the extentof nonradiative deactivation processes occurring both in the inner- and outer-coordination spheres of the metal ion. The rate constant k obs is the sum of therates of the various deactivation processes:

k obs k rad Xn k nr n k rad Xi k vibr i T X j k pet j T Xk k 0k nr ; (17)

where k rad and k nr are the radiative and nonradiative rate constants, respectively; thesuperscript vibr points to vibration-induced processes while pet refers to photo-induced electron transfer processes such as those generated by LMCT states, for instance; the rate constants k 0 are associated with the remaining deactivation paths.

In absence of nonradiative deactivation processes, k obs = k rad and the quantum

yield would be equal to 1, which is very rare. Examples are, in solid state and under

E

distance

broad

4f

4f*1 *

1

large Stokes shift

sharp

small Stokes shiftFig. 6 Congurationalcoordinate diagram for emission from ( left ) anorganic chromophore and(right ) a lanthanide ion



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excitation at 254 nm, Y 2O3:Eu (5%) with Q 0.99 and terbium benzoate withQ = 1 [12]; in solution, a terbium complex with a dipyrazoylpyridine bearingaminocarboxylate coordinating groups was reported having Q = 0.95 [ 13].

Temperature-dependent vibrational deactivation processes can often be tted toan Arrhenius-type of equation [ 14]:

ln

k obs

k 0

ln A

EA

RT ; (18)

where k 0 is the rate constant at 0 K (practically: at 4 K, or even at 77 K) whichallows one to decipher which vibration is responsible for it; examples are presentedin references [ 15] and [16].

Table 6 Ground (G), main emissive (I) and nal (F) states for the most important ff emissionbands, approximate corresponding wavelengths ( l ), energy gap between the emissive state and thehighest SO level of the receiving state, and radiative lifetime of Ln III ions. More NIR lines arelisted in [ 11 ]Ln G I F l / mm or nm a Gap/cm 1,a t rad /ms a

Ce 2F5/2 5d 2F5/2 Tunable, 300450

Pr 3H41D2

3F4 , 1G4 ,

3H4 , 3H5 1.0, 1.44, 600, 690 6,940 (0.05

b 0.35)3P03P0

3H J ( J = 46)3F J ( J = 24)

490, 545, 615, 640,700, 725

3,910 (0.003 b 0.02)

Nd 4I9/24F3/2

4I J ( J = 9/213/2) 900, 1.06, 1.35 5,400 0.42 (0.20.5)Sm 6H5/2

4G5/26H J ( J = 5/213/2) 560, 595, 640, 700, 775 7,400 6.26 (4.36.3)

4G5/26F J ( J = 1/29/2) 870, 887, 926, 1.01, 1.15

4G5/26H13/2 877

Eu c 7 F05D0

7F J ( J = 06) 580, 590, 615, 650, 720,750, 820

12,300 9.7 (111)

Gd 8

S7/26

P7/28

S7/2 315 32,100 10.9Tb 7F65D4

7F J ( J = 60) 490, 540, 580, 620, 650,660, 675

14,800 9.0 (19)

Dy 6H15/24F9/2

6H J ( J = 15/29/2) 475, 570, 660, 750 7,850 1.85 (0.151.9)4I15/2

6H J ( J = 15/29/2) 455, 540, 615, 695 1,000 3.22b

Ho d 5 I85S2

5I J ( J = 8,7) 545, 750 3,000 0.37 (0.51b)

5F55I8 650 2,200 0.8

b

5F55I7 965

Er e 4 I15/24S3/2

4I J ( J = 15/2, 13/2) 545, 850 3,100 0.7b

4F9/24I15/2 660 2,850 0.6

b

4I9/24I15/2 810 2,150 4.5

b

4

I13/24

I15/2 1.54 6,500 0.66 (0.712)Tm 3H61D2

3F4 , 3H4 ,

3F3, 3F2 450, 650, 740, 775 6,650 0.09

1G43H6 ,

3F4 , 3H5 470, 650, 770 6,250 1.29

3H43H6 800 4,300 3.6

b

Yb 2F7/22F5/2

2F7/2 980 10,250 1.3 or 2.0f

aValues for the aqua ions [ 8], otherwise stated, and ranges of observed lifetimes in all media, if available, between parenthesesbDoped in Y 2O3 or in YLiF 4 (Ho), or in YAl 3(BO 3)4 (Dy)cLuminescence from 5D1 ,

5D2 , and 5D3 is sometimes observed as well

dThe laser transition 5I7! 5I8 (2.12.2 mm) is used in medical surgery of the eyeseLuminescence from four other states has also been observed: 4D5/2 , 2P3/2 , 4G11/2 , 2H9/2f Complexes in solution: 1.21.3 ms; solid-state inorganic compounds: 2 ms



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The intrinsic quantum yield essentially depends on the energy gap D E betweenthe emissive state of the metal ion and the highest sublevel of its ground, or receiving, multiplet. The smaller this gap, the easier is its closing by nonradiativedeactivation processes, for instance, through vibrations of bound ligands, particu-

larly those with high energy such as OH, NH, or CH. With the assumption thatthe deactivating phonons involved have all the same energy ho , the temperature-dependent rate constant k vibr (T ) for the quenching of a single excited level isdescribed by the following expression [ 2]:

k vibr T k vibr 0 1 e ho =k BT i

with i D Eho

; (19)

where k B is Boltzmanns constant (1.38 10 23 J K 1 0.695 cm 1), i thenumber of phonons required to bridge the gap, and k

vibr

(0) the spontaneous rate at0 K. The latter heavily depends on the order n of the process. In practice, the excitedlevel possesses several crystal-eld sublevels, the population of which is in thermalequilibrium. This equilibrium is reached in times short compared to the multi-phonon decay time, but since phonon-induced decay rates are signicantly slower for the upper levels in view of the larger energy gaps, depopulation of the lower crystal eld sublevel is the major contribution to the deactivation process. A rule of thumb is that radiative de-excitation will compete efciently with multi-phononprocesses if the energy gap is more than 6 quanta of the most energetic vibration

present in the molecule. This type of nonradiative deactivation is especially detri-mental to NIR luminescence [ 11 ]: for Er III , for instance, a CH vibrator locatedoutside the inner coordination sphere at a distance between 20 and 30 A from theemitting center induces a radiationless rate equal to the radiative one.

Determination of the intrinsic quantum yield with (16) requires evaluation of theradiative lifetime which is related to Einsteins rates of spontaneous emission Afrom an initial state C J j i, characterized by a quantum number J , to a nal stateC 0 J 0j i:

AC J ; C 0 J 0 k rad 1t rad 64p4

~n3

3h 2 J 1 nn

2

22

9 DED n3 DMD" #; (20)

where ~n is the mean energy of the transition dened in (13b), h is Plancks constant,n is the refractive index; DED is given by (12) and DMD by (21):

DMD eh4pmec

2

Ch j L 2Sj j C 0j ij j2: (21)

The bracketed matrix elements are tabulated and the radiative lifetime can,therefore, be extracted from the spectral intensity, that is from (12), (20), and(21). Except in few cases, this calculation is not trivial and large errors mayoccur, including those pertaining to the hypotheses made within JO theory.



19/45


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them will have a characteristic radiative lifetime. Moreover, the radiative lifetime isnot a constant for a given ion and a given electronic level. Indeed, there is adependence on the refractive index, as clearly shown in (20), so that transpositionof a literature value to a specic compound cannot be made directly, which explains

the wide range of t rad values reported for an individual Ln III ion (Table 6).

4 Sensitization of Lanthanide Luminescence

Since the dipole strengths of ff transitions are very small, direct excitation into the4f excited levels rarely yields highly luminescent materials, even if the intrinsicquantum yield is large, unless considerable excitation power is used (laser excita-

tion, given the sharpness of the absorption bands). Therefore, an alternative pathhas been worked out which is called luminescence sensitization or antenna effect .The luminescent ion is imbedded into a matrix or an organic environment such thatthe latter is a good light harvester. Energy is then transferred from the excitedsurroundings onto the metal ion which eventually gives off its characteristic light.Note, that several of the Ln III excited states may be implied in this process. The useof charge transfer or 4f5d states for collecting and transferring energy has long beenwell established in inorganic phosphors for lighting applications. On the other hand,the tuning of the electronic properties of organic ligands to achieve the same goal

starts only to be understood since the process is more involved in view of thenumerous electronic levels and mechanisms which may be implied.

Here, we focus on the latter case for which efcient light-harvesting is mainlyperformed by the aromatic ( p ! p*) and/or (n ! p*) transitions of unsaturatedligands displaying large cross sections for one-photon absorption. Alternatively,singlet states, intra-ligand charge transfer states (ILCT), ligand-to-metal chargetransfer states (LMCT), or 3MLCT states localized on a transition-metal containingligand may also play this role [ 18]. As a result of the poor expansion of the 4f orbitals, the Lnligand bonds are mainly electrostatic and only some minute mixing

of metal and ligand electronic wavefunctions contributes to covalency. It, therefore,appears justied to consider separately ligand-centered and metal-centered excitedstates in lanthanide complexes, and a Jablonsky diagram is adequate for represent-ing energy migration paths (Fig. 7). In this diagram, grey arrows representingenergy transfer to the metal ion do not point to a specic excited state since severalof them may intervene.

One of the main energy migration path implies Laporte - and spin-allowedligand-centered absorptions followed by intersystem crossing ( 1S* ! 3T*, k ISC )reaching the long-lived ligand-centered triplet state, from which 3T* ! Ln*(k et )energy transfer occurs. Spontaneous metal-centered radiative emission completesthe light-conversion process. It is to be stressed that although important, this energytransfer path is by far not the only operative one. Kleinerman who studied over 600lanthanide chelates pointed out as early as 1969 that excited singlet states may



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22/45

Depending on the electromagnetic nature of H0, a double-electron exchange(Dexter) mechanism or an electrostatic multipolar (Fo rster) mechanism have beenproposed and theoretically modeled. They are sketched on Fig. 8 for the simple1S*- 3T*-Ln* path. Their specic dependences on the distance d separating thedonor D from the acceptor A, i.e., e b d for double-electron exchange and d 6 for dipoledipolar processes, respectively, often limit Dexter mechanism to operate atshort distance (typically 3050 pm) at which orbital overlap is signicant, whileFo rster mechanism may extend over much longer distances (up to 1,000 pm).

In addition, a d 8-dependent dipolequadrupolar mechanism may also be quiteeffective at short to medium-range distances; in fact, depending on ODA , it may beas efcient as the dipoledipole mechanism up to distances as long as 300 pm.

For lanthanides possessing low-lying charge-transfer excited states (e.g., Eu III ,Sm III ) or for complexes having low-lying ILCT states, the energy transfer process isfurther affected by additional nonradiative quenching arising from back energytransfer onto the ligand (not shown on Fig. 7). Since in this case the accepting statesare quite broad, minute differences in their energy may lead to large differences inthe spectral overlap, and therefore, in the overall quantum yield. Yb III represents aspecial case since it has only one, low-lying, excited state ( 2F5/2 ) and severalexcitation mechanisms have been proposed [ 11 ].

For the molecular edices discussed here, another denition of quantumyield ought to be made: the overall quantum yield , that is the quantum yield of

Ligand * Ligand

4f*

4f

4f*

4f

3T*

3T*

1S 1*

1

S 1*

1S 1*

1S 1*

3 T1*

3 T1*

1S 01S 0

1S 01S 0

LnIII LnIII*

LnIII*

E

Ligand * Ligand

4f*

4f

4f*

4f

LnIIIE

Fig. 8 Dexter ( top ) and Fo rster ( bottom ) energy transfer mechanisms



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the metal-centered luminescence upon ligand excitation. It is related to the intrinsicquantum yield by the following equation:

QLnL

D

pop et Q

LnLn

sens Q

LnLn : (25)

The three components in the middle term represent (1) the efciency Dpop withwhich the feeding level ( 3T, ILCT, LMCT, 3MLCT, possibly a 4f5d state) ispopulated by the initially excited state (the corresponding rate constant is k etISC if 1S* is excited and 3T* is the donor level, see Fig. 7), (2) the efciency of the energytransfer ( et ) from the donor state to the accepting Ln III ion, and (3) the intrinsicquantum yield. The overall sensitization efciency, sens can be accessed experi-mentally if both the overall and intrinsic quantum yields are known or, alternatively,the overall quantum yield and the observed and radiative lifetimes:

sens QLnLQLnLn Q

LnL

t rad

t obs: (26)

The lifetime method is especially easy to implement for Eu III compounds sincethe radiative lifetime is readily determined from the emission spectrum via (23).Some data are reported in Table 8.

An important remark at this stage is that the intrinsic quantum yield is directlyproportional to t obs , but not necessarily the overall quantum yield since a change in

Table 8 Quantum yields, observed and radiative lifetimes, as well as sensitization efciency for Eu III tris(dipicolinate) and bimetallic Eu III helicates; all data are at room temperature, for solutionsin TrisHCl 0.1 M (pH = 7.4); 2 s are given between parentheses [ 7, 21]. See Fig. 9 for formulaeSample Q Eu L Q

Eu Eu t obs /ms t

rad /ms sens[Eu(dpa) 3]

3 0.29(2) 0.41(2) 1.7(1) 4.1(3) 0.76(6)[Eu 2(L

C1 )3] 0.24(2) 0.37(4) 2.4(1) 6.8(3) 0.67(10)[Eu 2(L

C2 )3] 0.21(2) 0.37(4) 2.4(1) 6.9(3) 0.58(8)[Eu 2(L

C3 )3] 0.11(2) 0.36(4) 2.2(1) 6.2(3) 0.30(5)

Fig. 9 Chemical structures of the ligands mentioned in Table 8



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the inner coordination sphere inducing, for instance, a smaller nonradiative deacti-vation rate may also inuence sens (in one way or the other) through the resultingelectronic changes in the molecular edice. Another point is that the distinctionbetween intrinsic and overall quantum yields is often unclear in the literature,

particularly for NIR-emitting ions for which direct experimental determination of the quantum yield is rarely performed. Authors commonly rely on (16) to evaluatethe intrinsic quantum yield from lifetime determination and from a literature valueof t rad , so that extreme caution must be exercised in discussing these estimates [ 11 ].

4.1 Design of Efcient Lanthanide Luminescent Bioprobes

The ligand design for building efcient lanthanide luminescent bioprobes (LLBs)must meet several requirements, both chemical, photophysical, and biochemical:(1) efcient sensitization of the metal luminescence, (2) embedding of the emittingion into a rigid and protective cavity minimizing nonradiative deactivation, (3) longexcited state lifetime, (4) water solubility, (5) large thermodynamic stability,(6) kinetic inertness, (7) intense absorption above 330 nm, and (8) whenever relevant, ability to couple to bioactive molecules while retaining their photophysi-cal properties and not altering the bio-afnity of the host.

From the chemical point of view, it is best when the coordination sphere is

saturated, i.e., when 810 donor atoms are bound to the metal ion, and when thecoordinating groups are strong since in vivo experiments require large pLns(dened as log[Ln III ]free in water, at pH 7.4, [Ln III ]t = 1 mM, and [Ligand] t 10 mM; ideally pLn should be > 20). Carboxylates, aminocarboxylates, phospho-nates, hydroxyquinolinates, and hydroxypyridinones are good candidates, while b-diketonates which have excellent photophysical properties have the tendency to beless stable. In aqueous solutions, the enthalpy and entropy changes upon complexformation between Ln III cations and many ionic ligands are predominantly inu-enced by changes in hydration of both the cation and the ligand(s). Complexation

results in a decrease in hydration, yielding positive entropy changes favorable to thecomplexation process. On the other hand, dehydration is endothermic and contri-bution from bond formation between the cation and the ligand(s) often does notcompensate this unfavorable energy contribution to the variation in Gibbs freeenergy, so that the overall complexation process is generally entropy driven.Therefore, it is advantageous to resort to polydentate ligands for building a coordi-nation environment around Ln III ions. Macrocyclic complexes based on the cyclenframework [ 22] or on cryptands [ 23] are also proved to be quite adequate, as well asself-assembled mono- and bi-nuclear triple helical edices [ 21].

The photophysical requirements are related to the two aspects described in (25):energy transfer ( sens ) and minimization of nonradiative processes ( QLnLn ). The rstone is difcult to master in view of the intricate processes going on (Fig. 7, [20]).Some authors have nevertheless tried to establish phenomenological rules. One has,however, to be cautious in applying them since these rules rely on a rather simple



25/45

and naive picture: the 1S* 3T*Ln* energy transfer path on one hand, and theconsideration that the only parameter of importance is the energy gap between 3T*and the emitting Ln III level. Some of the relationships are exemplied on Fig. 10and examination of these data clearly point to the difculty in establishing a

dependable relationship. The following lessons can be drawn from these data:

The maximum values of the quantum yields usually occur when the triplet stateenergy is close to the energy of one of the higher excited states of the metal ion,consistent with the fact that the emissive level is usually not directly fed by theligand excited states (except maybe in the case of the Schiff base complexesdepicted at the bottom of Fig. 10). When the energy of the feeding state becomescloser to the energy of the emitting state, back-energy transfer operates and thequantum yield goes down: this is true for both Eu III and Tb III and a safe energydifference minimizing this process is around 2,5003,000 cm 1.

Inspection of the Eu III quantum yields clearly demonstrates that the energy of the triplet state corresponding to the larger values depends on the type of ligand:it is close to the 5D0 level for Schiff base complexes, to the

5D1 level for b-diketonates, and to the 5D2 level for polyaminocarboxylates.

For the two series of Eu III and Tb III complexes with the same polyaminocarbox-ylate ligands, the maximum values reached by the quantum yield of Tb III arelarger than those of Eu III : this reects the smaller Eu( 5D0

7F6) energy gapcompared to the Tb( 5D4

7F0) gap (Table 6).

It has been shown for calixarenes that more efcient ISC transfers take placewhen the energy difference between the singlet and triplet states is around5,000 cm 1; therefore, ligand designers try to keep to the following phenomeno-logical rules: D E(1S* 3T*) 5,000 cm 1 and D E(3T* Ln* emissive level) in therange 2,5003,500 cm 1 . These are, however, golden rules only and sometimesminute energy differences in the ligand states lead to large differences in overlapbetween the emission spectrum of the donor and the absorption spectrum of theacceptor, resulting in large differences in quantum yield [ 27].

The second aspect, namely minimization of nonradiative deactivation has two

facets: avoiding low-lying LMCT states, essentially for SmIII

, EuIII

, and YbIII

, andavoiding high energy vibrations in the rst and second coordination spheres; thelatter aspect is dealt with in Sect. 5.4.

4.2 Practical Measurements of Absolute Quantum Yields

Quantum yield measurements are simple in their principle, but very difcult tocarry out experimentally, particularly when it comes to the luminescence of lantha-nide ions and to intrinsic quantum yield, ff absorptions being faint. There are twomain methods: the comparative method in which the sample under examination iscompared to a standard with known quantum yield, and the absolute method which



26/45

E ( 3 T) / 10 3 cm 1

E ( 3 T) / 10 3 cm 1

E (3

T) / 103

cm1

Eu III

5D0

5D4

5D05D1

5D2

5D3

5D15D2

Q E u

L

Q E u

L

Q T b

L

18 20 22 24 26 28

0.0

0.1

0.2

0.3

0.4

0.5

0.6

18 20 22 24 26 28

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Tb III

0.0

0.1

0.2

0.3

0.4

0.5

0.6

12 14 16 18 20 22

O

H

FF

Ar

O

OH

N

N

OH

R

R

X

Y

F

Fig. 10 Relationships between the triplet state energy of the feeding ligand and the quantum yieldsof 39 Eu III (top ) and Tb III (middle ) polyaminocarboxylates [ 24] and of Eu III Schiff bases ( triangles ,[25]) and b-diketonates ( squares , [26])



27/45

determines the amount of absorbed and emitted light with an integrating spheresince luminescence is emitted in all directions.

In the comparative method, the quantum yield of the unknown sample (indices x)is given relative to the quantum yield of the standard (indices S) by:

QLnL QS E x ES

ASl S A xl x

I Sl S I xl x

n x2

nS2 ; (27a)

with E being the integrated and corrected emission spectrum, A the absorbance atthe excitation wavelength l , I the intensity of the excitation source at the excitationwavelength, and n the refractive index. In principle, linearity between the intensityof the emitted light and the concentration of the sample is only achieved if A < 0.05, so that samples should be diluted to reach this value of absorbance,

while being cautious not to dissociate the complex. In practice, if both the standardand the sample are excited at the same wavelength l exc (a highly desirable, althoughnot always achievable situation), measurements can be safely carried out up to A = 0.5 since the inner-lter effect would be the same for both samples. In this case,(27a) simplies to:

QLnL QS E x ES

ASl exc A xl exc

n x2

nS 2 ; (27b)

or to (27c) if the standard and the unknown sample are in the same solvent:

QLnL QS E x ES

ASl exc A xl exc

: (27c)

In case A x 6 AS and at least one of them is > 0.05, then the absorbances in (27ac)should be replaced by:

A ! 1 10 A; (28)to take into account the different inner-lter effects. The correction is small for small differences in absorbances but can become very important: e.g., if AS = 0.1and A x = 0.15, 0.2, 0.4, and 0.6, respectively, then the corrected AS / A x ratios wouldbe 0.42, 1.79, 2.93, and 3.64 instead of 0.5, 2, 4 and 6, respectively, correspondingto corrections of 5, 10, 19, 27, and 39%, respectively.

It is also essential that emission spectra are corrected for the instrumentalfunction established with a standard calibrated lamp. It is wise not to use thecalibration curve given by the manufacturer of the spectrometer and to re-measurethis instrumental function at regular intervals because many items inuence it,particularly the emission intensity of the excitation lamp and the quantum ef-ciency of the detector (which both decrease with time). In case (27a) is used, theexcitation instrumental function has to be known as well. Regarding the standard, itis best when its emission spectrum overlaps the emission spectrum of the unknown



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sample; a safe way to proceed is to use two different standards and to measure them

against each other as well. Each measurement should also be repeated severaltimes. Selected standards useful in lanthanide photophysics are listed in Table 9.

The absolute method using an integration sphere has been in use for a long time,particularly by physicists or scientists involved in the design of phosphor materials[30]. A specially designed integration sphere has been produced in our laboratorywhich fairly well fullls the needs of chemists and biochemists. It has a small 2-inch diameter to ensure maximum sensitivity and is manufactured in Zenith 1

Teon [ 7]. Samples are put in 2.4-mm I.D. quartz capillaries, themselves insertedinto a protective quartz tube. The modied de Mello et al. method [ 30] requires the

measurement of (1) La , the integrated intensity of light exiting the sphere when theempty capillary is illuminated at the excitation wavelength (Rayleigh scatteringband); (2) Lc , the same integrated intensity at the excitation wavelength when thesample is introduced into the sphere; these two measurements often necessitatethe use of attenuators (transmission 0.0110%); (3) Ec the integrated intensity of theentire emission spectrum. The absolute quantum yield is then given by:

Qabs Ec

Lal exc Lcl exc Fattl exc; (29)

whereby Fatt (l exc ) is the correction for the attenuators used . Reproducible andaccurate data can be obtained when the fraction of absorbed light a = ( La Lc) / Lais in the range 0.100.90 [ 7]; both solid state samples and solutions (minimumvolume: 60 mL) can be measured. This method also requires carefully establishedinstrumental functions; it is illustrated in Fig. 11 .

5 Information Extracted from Lanthanide Luminescent Probes

The most important applications of lanthanide luminescent stains used as structuralor analytical probes are summarized below. Any luminescent Ln III may act as aluminescent probe, but some ions either bear more information or are more

Table 9 Selected useful standards for quantum yield determinations at room temperature. Moreextensive listings can be found in [ 28, 29]Compound a conc./solvent b Range (nm) QQuinine sulfate aq. H 2SO4 (0.5 M) 400600 0.546

Cresyl violet Methanol 600650 0.54(3)Cs 3[Tb(dpa) 3] 6.5 10 5 M, TrisHCl 0.1 M 480670 0.22(2)[Ru(bpy) 3](ClO 4)3 10 5 M, aerated water 550800 0.028(2)10 5 M, de-aerated water 0.043(2)

Cs 3[Eu(dpa) 3] 7.5 10 5 M, TrisHCl 0.1 M 580690 0.24(2)[Yb(tta) 3(H 2O)2] 10 3 M, toluene 9501,080 0.0035abpy = bipyridine; tta = thenoyltriuoroacetylacetonatebWhen not given, the concentration should be such that A < 0.1 (usually c < 10 5 M)



29/45

luminescent than others, so that they are preferentially used. This is the case of Eu III , which will be extensively referred to in the following description [ 31].

5.1 Metal Ion Sites: Number, Composition, and Population Analysis

Lanthanide ions have been used as substitutes for Ca II and Zn II in proteins to obtaininformation on the number of metallic sites (by simple titration) and on their composition. This may of course be extended to any molecule or materials. Onevery useful transition in this respect is the highly forbidden and faint Eu(5D0 ! 7F0) transition which is best detected in excitation mode by analyzing theemission of the hypersensitive transition 5D0 ! 7F2; since both the emitting andend states are non-degenerate, its number of components indicates the number of different metalion sites. Moreover, the energy of this transition depends on thenephelauxetic effect di generated by coordinated atoms and ions; at 298 K:

~ncalc 17 ; 374 CCN XCN

i1ni di; (30)

no sample

La E a = 0 L c Ec

bafflessam ple

300 /nm

Intensity (a.u.)

La (without sample)

Lc (with sample)

E c (Tb emission x10)

La -Lc =lightabsorbed

300

with sample

400 500 600 700

Fig. 11 Top : Integration sphere and bottom : example of quantum yield determination on a TbIII

sample (this work)



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with CCN being a constant depending on the coordination number and ni the number of coordinating groups with nephelauxetic effect di. The latter are tabulated for themost common ligands [ 32] and some predictions are rather accurate [ 21], whichallows one to check the composition of the inner coordination sphere.

When several metal ion sites are present in a compound, population analysis canbe carried out in two main ways. For Eu III , since the intensity of the MD transition5D0 ! 7F1 is independent of the metalion environment, a spectral decompositionof the transition recorded under broad band excitation into its componentsmeasured under selective laser excitation, followed by integration yields the popu-lation P i of each site [ 31]. More generally, one can rely on lifetime measurement,since the luminescence decay will be a multi-exponential function which may beanalyzed, for instance, with Origin 1 , using the following equations:

I t A I 0 Xn

i1 Bie k i t ; (31a)

P i Bi=k i

Pn

i1 Bi=k i

: (31b)

In recording the decay, one has to make sure that (1) there is no artifact at thebeginning of the decay (remaining light from the light pulse), (2) the decay isrecorded during at least 56 lifetimes, (3) the signal at the end goes back to thebackground value, and (4) the decay is dened by a sufcient number of data points.Even if experimental data are of high quality, it is difcult to determine populationssmaller than 5 % and to decompose decays with more than 2 or 3 exponentialfunctions or when the two lifetimes are either very different or quite similar. Theexample given on Fig. 12 illustrates a bi-exponential analysis of an Eu( 5D0) decay.

0 2 4 6 810

8

6

4

2

0

A = 0.00033(4)B 1 = 0.088(3)B 2 = 0.938(3)

R 2 = 0.9997

P 1 = 0.22(2) P 2 = 0.78(2)

Eu III

l n I ( t )

t /ms

k 2k 1

1 = 0.54(1) ms 2 = 0.185(1) ms

Fig. 12 Luminescence decay for an Eu III sample with its bi-exponential analysis; straight linescorrespond to the two decay rates and the red line is the calculated t (this work)



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5.2 Site Symmetry Through Crystal-Field Analysis

This aspect is related to the Stark splitting of the levels described in Table 3. Againhere, Eu III luminescence is the easiest to analyze given the non-degeneracy of theemissive 5D0 level. When allied to high-resolution selective laser excitation of components of the 5D0 7F0 transition, symmetry of multi-site molecules andmaterials can be worked out easily, based on group-theoretical considerations [ 31,33]. Such detailed analyses are not discussed here.

On the other hand, the Eu( 5D0 ! 7F1) transition represents an interesting case.A simple examination of its splitting tells immediately at which crystal system thecompound belongs: cubic if only 1 component is seen, axial (i.e., hexagonal,tetragonal, or trigonal, labeled A and E in group-theoretical notation) if there aretwo components, and low symmetry if the maximum splitting of three appears. For truly low-symmetry species, the three components are equally spaced and tend tohave the same intensity. However, when the coordination sphere is close to anidealized higher symmetry, the splitting is unsymmetrical. In this case, threeimportant pieces of information can be extracted for symmetries close to axialsymmetry: (1) the sign of the B 02 crystal-eld parameter which depends on therelative energetic position of the A and E sublevels of 7F1 , (2) its value thanks to aphenomenological relationship between D E(AE) and this parameter [ 34], and (3)the extent of the deviation from the idealized symmetry given by the splitting of theE sublevel. In the example depicted on Fig. 13, the crystal eld parameter has avalue of ca 600 cm 1 and the coordination polyhedron EuN 6O3 appears to be onlyslightly distorted from the idealized D 3h symmetry with D E(EE) equal to 31 cm

1.

5.3 Strength of MetalLigand Bonds: Vibronic Satellite Analysis

The analysis depicted above requires high-resolution spectra. It is sometimescomplicated by the occurrence of vibronic satellites which may articially increasethe number of components of a given transition, so again care has to be exercised.Vibronic transitions have the tendency to be strongest when associated with

161 cm 1

A E

31 cm1

0 200

580 590 600

400 600E /cm 1

/nm

02B >0

02B


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hypersensitive transitions, for instance, Eu( 5D0 ! 7F2); in addition, the 7F2 levelcorresponds to an energy range (1,0001,500 cm 1) in which the density of phononstates is usually large. On the other hand, these satellites provide useful informationon the strength of the Lnligand bond: the larger this strength, the more intense the

satellites [ 35].

5.4 Solvation State of the Metal Ion

Quenching of the lanthanide luminescence by high-energy vibrations is a major concern in the design of highly luminescent probes. Multi-phonon deactivation isvery sensitive to the metalligand distance and the phenomenon can be reasonablymodeled by Fo rsters dipoledipole mechanism [ 2]. The more phonons neededto bridge the energy gap, the less likely the quenching phenomenon to occur.Table 10 illustrates this phenomenon for solutions of complexes in water (~n(OH) 3,600 cm 1) and deuterated water ( ~n(OD) 2,200 cm 1) by listingthe lifetimes of the excited level in these two solvents.

Although detrimental to the emission intensity, vibrational quenching allowsone to assess the number of water molecules q interacting in the inner-coordinationsphere. Several phenomenological equations have been proposed, based on theassumptions that OD oscillators contribute little to deactivation and that all theother deactivation paths are the same in water and in deuterated water and canhenceforth be determined by measuring the lifetime in the deuterated solvent. Animportant point for their application is to make sure that quenching by solventvibrations is by far the most important deactivation process in the molecule. If other temperature-dependent phenomena (e.g., phonon-assisted back transfer) areoperating, these relationships become unreliable. This has often been observedwith Tb III [27].

Table 10 Illustration of the energy gap law with respect to quenching of the Ln III luminescenceby high-energy vibrations. Samples are dilute solutions of perchlorates or triates at roomtemperature [ 11 ]Ln D E /cm 1 No. of phonons Lifetime/ ms

OH OD H 2O D2OGd 32,100 9 15 2,300 n.a.Tb 14,800 4 7 467 3,800Eu 12,300 34 56 108 4,100Yb 10,250 3 4.5 0.17 a 3.95

Dy 7,850 23 34 2.6 42Sm 7,400 2 3 2.7 60Er 6,600 2 3 n.a. 0.37Nd 5,400 12 23 0.031 0.14aEstimated from quantum yields in water and deuterated water and from t (D2O)



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Establishing q for Nd III is really problematic and other relationships have beenput forward, which do not yield very satisfying results, owing to too limitedcalibration range, as, by the way, for (38)(40).

The best way to minimize vibration-induced deactivation processes is to design a

rigid metalion environment, devoid of high-energy vibrations and protecting theLn III ion from solvent interactions. Such an environment also contributes to reducecollision-induced deactivation in solution. Further protection may be gained byinserting the luminescent edice into micelles, a strategy used in bioanalyses [ 23].Recent reports have also demonstrated a considerable weakening of the quenchingability of OH vibrations if the coordinated water molecules are involved in strongintra- or inter-molecular H-bonding. Combining this effect with encapsulation intoa rigid receptor turns the weakly emitting aqua ions into entities with sizeableluminescence.

5.5 Energy Transfers: DonorAcceptor Distances and Control of the Photophysical Properties of the Acceptor by the Donor

Distances between a chromophore and a metalion site, as well as between metalions, may be inferred from the determination of energy transfer efciency withinthe frame of Fo rsters dipoledipole mechanism. In this case, the following sim-plied equations hold to estimate the efciency of transfer between the donor D andthe acceptor A:

et 1t obst 0

k 0k obs

1

1 RDA = R06 ; (41)

in which t obs and t 0 are the lifetimes of the donor in presence and in absence of theacceptor, respectively, RDA is the distance between the donor and the acceptor, and R0 is the critical distance for 50 % transfer, which depends on (1) an orientationfactor k having an isotropic limit of 2/3, (2) the quantum yield QD of the donor (inabsence of the acceptor), (3) the refractive index n of the medium, and (4) theoverlap integral J between the emission spectrum E~nof the donor and the absorp-tion spectrum e~n of the acceptor:

R60 8:75 10 25k2 QD n 4 J ; (42a)

J

R e~n E~n ~n

4d~n E

~n

d~n

: (42b)

Estimation of R0 is, therefore, accessible from the experimental optical andstructural properties of the system. If a crystal structure is at hand, the problem



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simplies in that RDA is known and if the lifetimes of (41) can be determined, thencalculation of R0 is straightforward. Energy transfer between lanthanide probes hasallowed determining the distance between Ca II and Zn II ions in proteins (substitutedby Ln III ions), as well as the distance between tryptophan chromophores and these

metalion sites [ 40].Another interesting application of directional energy transfer is the control

of the photophysical properties of a metal ion (nd or 4f) by another one. For instance, Cr III can be used to populate the excited state of Nd III or Yb III . If the rateconstant of the energy transfer is fast enough and if k obs (Cr) < < k obs (Ln), thenthe excited Ln III ions will decay with an apparent lifetime equal to the (long)lifetime of the 3d partner. In this case, the lifetime on Nd III and Yb III can beshifted in the millisecond range, which is an advantage for time-resolveddetection [ 41 , 42].

5.6 FRET Analysis

In fact, FRET (Fo rster resonant energy transfer) analysis has the same basis as theenergy transfer described in the above section. It is used either in simple bio-analyses or to detect protein interactions and DNA hybridization. Its principle isshown on Fig. 14 in the case of a homogeneous immunoassay [ 43].

In homogeneous immunoassays, the analyte is biochemically coupled to twospecic antibodies labeled one with a LLB and the other by an organic acceptor.Emission from the organic acceptor is detected in time-resolved mode becausethe population of its excited state by intramolecular transfer from the LLB shiftsits lifetime in the millisecond range. In this way, it is easy to discriminatebetween the luminescence emitted by uncoupled and coupled antibody moleculeslabeled with A; similarly, since the luminescence of A is spectrally differentfrom that of the LLB, interference from Ln III luminescence emitted by theuncoupled antibody labeled with the Ln III chelate is also discriminated. There

is, therefore, no need to wash out unused reactants. A method using FRET for the

DLLB

A

FRET

Labeledantibodies

Analyte

Fig. 14 Principle of ahomogeneous immunoassaybased of FRET technology(redrawn from Ref. [ 43])



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determination of quantum yields of lanthanide chelates and organic dyes has alsobeen described [ 44 ].

5.7 Ligand Exchange Kinetics

As a starting point, assume that two Ln III complexes are present simultaneously insolution at equilibrium and are related by a ligand exchange process. The time-dependence of the luminescence emission following an excitation pulse will dependon the rate of chemical exchange relative to the photophysical deactivation rates. If the exchange is slow or fast, no information can be gathered from lifetime measure-ments. However, if the chemical exchange process occurs at a rate comparable to

the de-excitation rate, the time dependence of the luminescence decay is a functionof both the excited state lifetimes and the interconversion rate. Equations have beenworked out and the exchange kinetics of several Eu III and Tb III complexes has beenelucidated. The Eu III ion lends itself more easily to such experiments becauseselective excitation of one species through the 5D0 7F0 transition can be easilyachieved [ 45 , 46].

5.8 Analytical Probes

In analytical applications, the Ln III absorption or emission properties are either simply detected or modulated by a process depending on the concentration of theanalyte, itself reversibly binding to the lanthanide tag. If absorption is used,hypersensitive transitions (Table 4) are good reporters in view of their sensitivityto minute changes in the Ln III environment. When it comes to luminescence, amuch more sensitive technique (especially if time-resolved detection is used), thereare several ways of modulating the emission (Fig. 15). One obvious way is to

modulate the solvation in the rst coordination sphere (a); alternatively, interactionof the analyte with the ligand molecules may modify the energy transfer ability of the latter (b), and the analyte itself may transfer energy onto the reporter ion (c).Note that situations may be reverse, in that either sensitization or quenching may beinduced. Cations, anions, pH, pO 2, aromatic molecule sensors have been designedalong these lines, while time-resolved immunoassays often take advantage of FRETtechnology [ 23].

Molecular interaction between the luminescent tag and other molecules presentin solution can also result in luminescence quenching and quantitative investigationof the phenomenon provides both analytical and photophysical information. Incollisional (dynamic) quenching, the quencher molecule diffuses to the luminescentprobe during the lifetime of the excited state; upon collision, the latter returns to theground state without emission of light. The average distance that a molecule having



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a diffusion coefcient D can travel in solution during the lifetime of the excitedstate is given by:

x ffiffiffiffiffiffiffiffi2 Dt obsp : (43)A typical collisional quencher is molecular oxygen which has a diffusion

coefcient of 2.5 10 5 cm 2s 1 in water at 298 K. During the lifetime of theEu( 5D0) level, typically 1 ms, it can, therefore, diffuse over 2.2 mm, that is adistance comparable to the size of a biological cell. In some instances, a non-luminescent complex may result from the collision (static quenching), similar to thecases shown on Fig. 15. Stern and Volmer have worked out the equation for

dynamic quenching:

E0 E 1 K DQ 1 k q t 0Q ; (44)

in which K D is the dynamic quenching constant, k q the bimolecular rate constant, E0and E the emission intensities in absence and in presence of quencher, respectively,and t 0 the observed lifetime in absence of quencher. When collisional quenchingoccurs, the lifetime decreases in parallel to the luminescence intensity:

E0 E

t 0t

: (45)

h

h

h

h

h h

h

h h

h

+

an

a

b

c

an

an

Fig. 15 Modulation of lanthanide luminescence byan analyte through reversiblebinding: ( a ) removal of solvent quenching, ( b )modulation of the ligandability to transfer energy ontothe Ln III ion, and ( c) bindingof a sensitizing analyte tothe ligand(s). Redrawnfrom [ 42]



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A similar equation can be derived for static quenching, with K S being the staticquenching constant:

E0 E 1 K SQ : (46)

When both dynamic and static quenching occurs, the equations combine into:

E0 E 1 K DQ 1 K SQ ; (47a)

E0 E 1 K D K S Q K D K S Q

2: (47b)

Here again, (45) holds and luminescence intensities may be substituted withlifetimes. It turns out from these equations that if the SternVolmer plot is linear, it

reects the sole presence of dynamic quenching. This is, for instance, the case for the quenching of the bimetallic [Eu 2(L

C2 )3] helicate with acridine orange (AO), asshown on Fig. 16. The corresponding constants are K D 6.7(1) 105 M 1 andk q 2.7(1) 108 M 1 s 1. On the other hand, quenching of the same chelate withethidium bromide (EB) is typical of both dynamic and static quenching with K D 3.0(1) 104 M 1, K S 2.0(1) 103 M 1and k q 1.23(4) 107 M 1s 1 . The bimolecular rate constants are relatively small compared to diffusion (10 10 M 1 s 1) because of the shielding of the Eu III ion embedded inside the helicaledice. This quenching has been taken advantage of to develop a versatile and

robust method for the detection of various types of DNA and of PCR products [ 47].

Acknowledgments This work is supported through grants from the Swiss National ScienceFoundation. The authors are grateful to Fre deric Gumy for his help in recording luminescencedata and to Dr Jonathan Dumke for pertinent comments.

0 5 10 15 20 25 30 350

5

10

15

20

25

[AO] / M0

0

3

6

9

12

15

[EB] / M

E 0

/ E o r 0

/

E 0

/ E o r 0

/

[Eu 2(LC2 )3][Eu 2(L

C2 )3]

40 80 120 160 200

Fig. 16 SternVolmer plots for the quenching of the luminescence of [Eu 2(LC2 )3] by acridine

orange ( left ) and ethidium bromide ( right ) in TrisHCl buffer (pH 7.4). Open squares : lifetimes,

solid squares : intensities. Redrawn after [ 47]



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6 Appendices

6.1 Site Symmetry Determination from Eu III Luminescence

Spectra

The exact site symmetry can be determined usually only if polarization measure-ments are made, that is, on single crystals. A light with s polarization has itselectric vector perpendicular to the crystallographic c axis and its magnetic vector parallel to it; the reverse holds for p polarization. The following scheme sketcheshow the site symmetries other than cubic ( T x , O x , see Table 3) may be found fromEu( 5D0) emission spectra. This is the simplest procedure since

5D0 is non-degenerate.However, site symmetry can be worked out with other luminescent ions as well and,

also, from absorption spectra.

5D07F1

5D07F1

5D07F1

5D07F4

5D0 7F4

5D07F6

5D07F6

3 2 AxialLow symmetry

5C 1, C 2, C S

4 C 2v 3 D 2 1 +2

S 4

2 D 3

2 +1

3 +2 C 3v

3 +3 C 3

1

2 +2C 3h

2 +21 D 6

1+1

2 +1 D 3h

3 +1 D 4

2 +2 C 6v

2 +3 C 6

Number of components in the transition

1 Cubic ( O h , O , T h , T d , T )

6.2 Examples of JuddOfelt Parameters

Table 11 JuddOfelt parameters for Ln III aqua ions in dilute acidic solution [ 8]Ln 10 20 O2 /cm

2 1020 O4 /cm2 1020 O6 /cm

2

Pr 32.6 5.7 32.0Nd 0.93 5.00 7.91Sm 0.91 4.13 2.70Eu 1.46 6.66 5.40Gd 2.56 4.70 4.73Tb 0.004 7.19 3.45Dy 1.50 3.44 3.46Ho 0.36 3.14 3.07Er 1.59 1.95 1.90Tm 0.80 2.08 1.86



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Table 12 JuddOfelt parameters for Nd III in selected environments [ 11 , 48]Sample 10 20 O2 /cm

2 1020 O4 /cm2 1020 O6 /cm

2

Nd:YAG 0.37 2.29 5.97Cs 3[Nd(dpa) 3]/H 2O 7.13 3.78 13.21

Nd(ClO 4)3 /MeCN 1.2 7.7 7.8Nd(ClO 4)3 /DMF 1.2 8.9 8.4Nd(NO 3)3 /MeCN 11.8 2.1 6.6Nd(NO 3)3 /DMF 6.7 5.0 7.6

6.3 Examples of Reduced Matrix Elements

Table 13 Doubly reduced matrix elements used in the calculations of the dipole strengths for absorption and emission of Cs 3[Eu(dpa) 3], from [ 7]

Transition Element Value Transition Element Value5D2

7F0 C U 2 C 0k 2 0.0008 5D0! 7F2 C U 2 C 0k

2 0.00325L6

7F1 C U 6 C 0k 2 0.0090 5D0! 7F4 C U 4 C 0k

2 0.00235L6

7F0 C U 6 C 0k 2 0.0155 5D0! 7F6 C U 6 C 0k

2 0.00025D4

7F0 C U 4 C 0k 2 0.0011

6.4 Emission Spectra

In the following we give typical examples of luminescence spectra of the Ln III ions,with emission from the main luminescent levels. Depending on the chemicalenvironment of the ion, the shape of the spectra may differ substantially (for instance, the relative intensity and CF splitting of the bands), but the energy of the transitions remains relatively insensitive; in addition vibronic transitions as well

600 650 700 750

17 16 15 14

E / 10 3 cm 1 E / 10 3 cm 1

Pr III(a) Pr III(b)

3 P

0

3 F

2

3 P

0

3 H

4

3 P 0

3 H

5

3 P

0

3 H

6

3 P

0

3 H

6

1 D 2

3 H

5

1 D

2

3 H

5

1 D 2

3 H

4

1 D

2

3 H

4

3 P 0

3

F 3

3 P 0

3

F 2

3 P

0

3 F

3 , 4

3 P 0

3

F 4

1 D 2 3 F 4

900 1000 1100

11

450 550 650 750

22 20 18 16 14

/ nm / nm

10

Fig. 17 (continued)



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500

20 18 16 6.8 6.4

Er III Tm III

500

22

925 975 1025 1075 1125

10.5 10 9.5 9

Yb III

E / 10 3 cm 1 E / 10 3 cm

1

E / 10 3 cm 1

4 S

3 / 2

4 I 1

5 / 2

4 F

9 / 2

4 I 1

5 / 2

4 I 1

3 / 2

4 I 1

5 / 2

1 G 4 3 H 6

2 F 5/2 2 F 7/2

1 G4

3 F4

1 D 2

3

F 2

3 H

4

3 H

6

1 G

4

3 H

5

1 D 2 3 H 4

/ nm / nm

/ nm

600 1500 1600

20 18 16 14

600 700 800

500 600 700 800

22 20 18 16 14

Dy III

J = 15/2

13/2

11/2 9/2

600 650 950 1000 1050

16 15 10.5 10.0

Ho III

J = 8 7

E / 10 3 cm 1 E / 10 3 cm

1

4 F 9/2 6 H J

5 F 5 5 IJ

/ nm / nm

Fig. 17 (continued) Typical examples of Ln III emission spectra under ligand excitation (320 340 nm): microcrystalline samples of dimeric [Ln(hfa) 3(L)] 2 (L = 4-cyanopyridine N -oxide) for Ln = Pr(a), Nd, Sm, Dy, Ho, Tm, Yb [ 49]; solutions of [Pr(b)(L 1)2(NO 3)] in CH 2Cl2 (L

1 = dihy-drobis-[3-(2-pyridyl)pyrazolyl]borate) [ 50] and Cs 3[Ln(dpa) 3] 23.7 10 2 M in TrisHCl0.1 M (pH 7.4), Ln = Eu, Tb [ 7]. Emission of Gd is measured on a microcrystalline sample of Gd 2O3 and of Er (up-conversion, l exc = 980 nm) on a doped sample of NaYF 4:Yb/Er (18/4 %) (thiswork). All spectra are recorded at room temperature (except for Tm, Ho, and Pr(a) in the visiblerange, 77 K), corrected for the instrumental function, and normalized



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as emission from other levels may also show up. In the case of Pr III , the two spectrashown are quite different because different emissive states are sensitized.

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2. Hu fner S (1978) Optical spectra of transparent rare earth compounds. Academic, New York3. Carnall WT, Goodman GL, Rajnak K, Rana RS (1989) A systematic analysis of the spectra of

lanthanides doped into single crystal LaF 3. J Chem Phys 90:344334574. Peijzel PS, Meijerink A, Wegh RT, Reid MF, Burdick GW (2005) A complete 4f n energy level

diagram for all trivalent lanthanide ions. J Solid State Chem 178(2):4484535. Go rller-Walrand C, Binnemans K (1998) Spectral intensities of f-f transitions. In:

Gschneidner KA Jr, Eyring L (eds) Handbook on the physics and chemistry of rare earths,vol 25. Elsevier BV, Amsterdam, Ch. 167

6. Walsh BM (2006) JuddOfelt theory: principles and practices. In: Di Bartolo B, Forte O (eds)Advances in spectroscopy for lasers and sensing. Springer Verlag, Berlin, pp 403433

7. Aebischer A, Gumy F, Bu nzli JCG (2009) Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates). Chem Phys Phys Chem 11:13461353

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9. Dorenbos P (2000) The f-d transitions of the trivalent lanthanides in halogenides and chalco-genides. J Lumin 91:91106

10. Shionoya S, Yen WM (1999) Principal phosphor materials and their optical properties. In:Shionoya S, Yen WM (eds) Phosphor handbook. CRC, Boca Raton, Ch. 235

11. Comby S, Bunzli JCG (2007) Lanthanide near-infrared luminescence in molecular probes anddevices. In: Gschneidner KA Jr, Bu nzli JCG, Pecharsky V (eds) Handbook on the physics andchemistry of rare earths, vol 37. Elsevier BV, Amsterdam, Ch. 24

12. Bredol M, Kynast U, Ronda C (1991) Designing luminescent materials. Adv Mater 3:361367

13. Brunet E, Juanes O, Sedano R, Rodriguez-Ubis JC (2002) Lanthanide complexes of poly-carboxylate-bearing dipyrazolylpyridine ligands with near-unity luminescence quantumyields: the effect of pyridine substitution. Photochem Photobiol Sci 1:613618

14. Kleinerman M, Choi SI (1968) Exciton-migration processes in crystalline lanthanide chelatesI. Triplet exciton migration in lanthanide chelates of 1, 10-phenantholine. J Chem Phys49:39013908

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