Circularly polarized luminescence of terbium (III) complexes in solution

I5 March 1974

CHEMICAL PHYSICS LETTERS

Volume 25, number 2

CIRCULARLY

POLARIZED

LUMINESCENCE*

OF TERBIUM (III) COMPLEXES IN SOLUTION Chun Ka LUK and F.S. RICHARDSON Department of Chemistry. University Charlottesville. Virginia 22901.

of Virginia. USA

Received 5 December 1973

Circular dichroism (CD). total luminescence, and circularly polarized luminescence (CPL) spectra of Tb(II1) : malic acid and Th(III) : aspartic acid complexes are measured in aqueous solution at room temperature. The CD is measured in the spectral region corresponding to the Tb(II1) ‘Fd - ‘D4 free-ion absorption, and the CPL spectra span the Tb(IIIj ‘Da - 7F6, 7FS. 7F4, and 7Fa free-ion emissions. All solutions included in the study were at pH = 8.5 and had ligand : Tb(III) concentration ratios of 5: l_ Experiments were conducted using the d. Q. and racemic (d. Q) optical isomers of the ligands. Whereas the total luminescence spectra associated with the individual ‘Da - 7F~ transitions are unstructured or show, at most, two components, the CPL spectra are highly structured and demonstrate considerable splitting of the Jlevels by the lowsymmetry l&and environment. Furthermore, CPL exhibits an extraordinary sensitivity to the chemical or stereochemical nature of the ligand environment as evidenced by the different sign patterns and splittings revealed in the CPL spectra of the malic acid and aspartic acid complexes.

1_ Introduction

Optical absorption and emission spectra have been of great value for eliciting information about the electronic structure of rare earthions in crystals [ 1 ] and, to a lesser extent, in solution [Z] . Circular dicliroism (CD) spectroscopy has also been used recently to study the coordination properties of rare earth ions with a variety of ligand types in solution [3,4] . We report here our initial experimental results on the circularly polarized luminescence (CPL) spectra of chiral terbium (III) complexes in solution along with the optical absorption and CD.spectra of several of these complexes. The emitting states of the Tb(II1) systems examined here derive predominately from the SD4 free ion excited state. Since the symmetries of these systems are presumably quite low, the J = 4 level of 5D is mcst certainly split and it is probable that these splittings are .* This research was supported in part dy the Petroleum.Research Fund administeredby the American Chemical Society,

-The Camille and Henry Drefus Foundation (Teacher-Scholar grant to F-R.), and by the National Science Foundation through a majo; instrumeni g&t to the University of -Virginia.

not significantly different from kT at room temperature_ For this reason, a rigorous analysis of the CPL in terms of spectral assignments of transitions, identification of split J components, and the extent of J state mixings, for example, is not straightforward and will not be attempted here. A more complete analysis of the CPL spectra of these and other chiral lanthanide ion complexes will be given in a future paper. The re- _ sults presented here, however, demonstrate the extraordinary sensitivity of CPL to the influence of coordination symmetry on the electronic states and energy levels of the metal ion--In cases where the total unpolarized luminescence band for a particular 5 DJ + 7 FJ

transition may’show, at most, two or three split components, the CPL spectrum in thesame region may ex&bit, for example, five or six barids.due to the’fact that CPL can be > 0 or < 0, whereas total ltiinescence is. always > 0. CPL involves measuring the differential intensities of Ieft: and rIght&cularly polarized radiation in the.. spontanious emission sp&+ti _of a chiral lumii++nt .I system. Oosterboff and co-wdrkers [S] have.pioneered’ the development of this spectroscopiti too!, for obtaining ‘. .-.molecular strutittire.jtif&r&tio& although several br@f : .-.

.’ -: ‘. :. .- :. _’ :

i

;::..215.:;_

Volume 25, number 2

CHEMICAL PHYSICS LETTERS

reports on CPL experiments predated their work [63 _ More recently, Gafni and Steinberg 171 have also reported the construction of CPL instrumentation and

have measured the CPL spectra of several chiral molecular systems.

2. Experimental The emission experiments were carried out with a high-sensitivity luminescence spectrometer which measures both total luminescence and the circular polarization of luminescencef. CPL spectra were ,Dbtained by analyzing the emitted radiation alternately for left- and right-circular polarization at a modulation frequency of 50 kHz. T&e excitation beam was chopped at a frequency of 13 Hz. The electrical signal at the detector consists, therefcre, of a large ac signal, f,, which is proportional to the total emission intensity (independent of state cf polarization) and is modulated at a frequency of 13 Hz, plus a small ac signal, I,, which is modulated at 50 kHz and whose magnitude and phase carry the CPL intensity and sign variables. This signal is processed by two lock-in amplifiers operating in parallel and tuned at 13 Hz and 50 kHz, respectively. The total luminescence and CPL spectra are recorded simultaneously by a two-pen recorder. The instrument is capable of measuring Iuminescence dissymmetry factors g(lum) as small as = I Oq5. g(lum) = (IL - ZR)J$(ZL +ZR) = AZ/Z.

(I)

where ZL and ZR are, respectively, the intensities of the left- and right circularly polarized components of the luminescence. The luminescence dissymmetry factor was obtained as a function of wavelength by determining (CPL/total luminescence) at various wavelengths_ The circular dichroism spectra reported here were obtained with a Durrum-Jasco ORD/UVS instrument. Two ligands were used in the research reported here, malic_ acid (HOOCCH2CH(OH)COOH) and aspartic acid (HOOCCH2CH(NH2)COOH). Each possesses one asymmetric carbon-atom and exists, therefore, in one qf two opticallj isomeric (enantiomeric) forms, d or l?. Cotiplexes firmed between :Re.d (or a) isomers and :

t A detailed description of our CPL. instrumentation

given in a later, more complete, report. 216

.-

will be .

15 March 1974

and Tb(iII) in solution will be optically active by virtue of the presence of the asymmetric centers in the ligands. it is also possible, of course, that stereoselective interaction mechanisms will lead to the formation of compIexes which have additional sources of dissymmetry (e.g.. configurational or conformational [8]).The structures of Tb(IlI) : d(or Q)-malic acid and Tb(Il1) : d(or .P)-aspartic acid in solution are not known. AI1 Tb(lII)-Iigand complexes were prepared in aqueous solutions which were 0.1 M in TbC13 and 0.5 M in ligand at pH = 8.50. Total luminescence and CPL spectra were obtained for the following complexes: Tb(IIl): d-malic acid, Tb(JIl):P-malic acid, Tb(III):d, Q-malic acid, Tb(IIl):d-aspartic acid, Tb(Ill):Q-aspartic acid, Tb(lII):d, Q-aspartic acid. The stoichiometries for these systems at pH 8.50 and for 5: 1 excess ligand concentrations are not known. CPL spectra were measured for the d, the 9, and the racemic d, 2 forms in order to test our instrumentation and measurement procedures for possible artifacts. In each case the CPL for the d and

J?isomers are opposite in sign but are equal in magnitude, and we obtain a null CPL for the d, Q complexes. The exciting wavelength for each emission experlment was 365 nm, the excitation band width was 100 A, and the emission band width was 10 A.

3. Results and discussion The experimental results obtained in this study are displayed in figs. l-5. The total luminescence (Z) and CPL (w observables are given in relative quanta per unit of frequency interval. Luminescence dissymmetry factors [see eq. (1 )J for wavelengths corresponding, approximately, to positive and negative extrema in the CPL spectra are given in table I_ In this preliminary report no attempt is made to quantitatively or semiquantitatively analyze the CPL spectra associated with individual Jlevels of the-ground state in terms of ligand field split components. Qualitatively, the approximate

number and relative intensities of these components can be ascertained by simple inspection of the CPL displayed in figs. I-5. The number ofg(lum) values listed in table 1 for a given 7 FJ level does not imply that we

have-assigned this nurtiber of ligand field components to that level_ We make no such assignmeritS here. For-&ml systems in isptrppic media the sign and magnitude of the CPLassociated &th an electronic

i

Voiume 2.5, number 2

15 March 1974

CHEMICAL PHYSICS LETTERS

1

250

AI

I

100

505

t

,

500

I

495

490 X

r 485

t 480

75

I 475

AI

-

,/

2s-

Fig. 1. CD (upper solid curve). CPL (lower solid curve), and total luminescence (lower dashed curve) spectra for 1 r5, Tb3+:Qmalic acid in sotution at pH 8.54 in the 480-505 nm spectral

500

:

so

(nm)

I

:

250

,, 0

o----@ -25-5os 560

RgiO,.

1 555

t 545

‘ 550 X

, 340

t 535

530

(nm)

Fig. 3. CPL and total luminescence in the 535-560 2.0

tr~i

nm specregion for 1 :S, Tb3C:P-mJic acid in solution at pH 8.54

1.5

(lower curves)and 1 :S, i’bJi*.Qasparticacid in solutionat pH

I .o

8.50

(upper

curves).

0.5 As

xlo4

o

/ I

-0.5L

where $ and r2 are the electric dipole and magnetic dipole operators, respectively. Since all the transitions 4500 observed in the study reported here are primarily I f +f f, these transitions should be only weakly allowed 1 250 in electric dipole radiation but should have substantial magnetic-dipole allowed character. Assuming that the 0 Al emitting states of all the systems studied have SD;l freeion parentage, the approximate selection rule governing , I Ia magnetic dipole allowed transitions between these states. 500 495 490 485 480 475 510 505 and &e 7F,, manifold of states,is J 7 3,4, and 5. From XMnf the spectra shown in figs. i--S., it is apparent that the Fig. 2. CD (upper solid curve), CPL (lower solid curve), and toSD4 + 7 FS. transition exhibits by far the strongest CPL tal luminescence (lower dashed curve) spectra for 15, Tb”+:P: and from table I we see that the SD4 + 7 F3., 7 FS tranaspartic acid in solution at pH 8.50 in the 475-510 nm spey s&ions yield -the h&h& g(lum) values. The @urn) v&,--tral region. ‘ues arc &all &ross t&k e&ire CPL spectrum fdr the: 51)4.+ 7F6 transition, and the SD4 + 7F2. ‘Fi, andk&sition, j + k, are de&mined by the fotatory str>$, 7F 0. transitions give %PL tdo &a11 to be me_+suredaci,, ;.’ : ‘. :., E$x, of the transition,_ curately tiih’ our .i+rkme$tion. . ._:_ ‘., .. : .; -_ :.. _ : :. ._ ‘_ .-, : . . (. ‘, :. _-&r;;,:: - 750

I

1

3

I

1

CHEMICAL PHYSICS LETI’ERS

IS March-1974

J.

43-

.

AI 0

0

I 2 3

X

lmn)

Fig. 4.CPL (solid curve) and total luminescen& (dashed curve) for

Tile total dissymmetry factor associated with an electronic transition j* k may be expressed as

where D,-k =dipole strength = I$il$k$k)i~,

This quantity ma? also be expressed in te&s of the experimental obseivabIes AI and I in the case of spontaneous emission and AE and E in the case of absorption. That is,

G(j+k)=

I 5,

Tb3+:Q-maiic acid in solution at PH 8.54.

j- AI(v)v-3dv/ $ i(v)v-3 dv j--k i-k

for luminescence, Bnd

Ad+-’

Glj+-k)=

j-k

E(V)P$J

dv/ 1 j-k

+

I

(4)

t

I

(51

Volume 25, number 2

CHEMICALPHYSICS Table 1

Luminescence dissymmetry factors, g(hn)

‘F6

‘Fs

'F4

Tb3+:Q-malic acid

Tb3+:4!-aspartic acid

Mnm)

h(m)

gflum) X i03

484

9.3

487.5

12 .

489

-6.3

492.5

16

492 496

4.5 -6.7

496

-3.3

498

500

-7.9

503

7.3 -5.6 48 -4.0 22 -81 -63 IO -50 -38 29 -8.1 -34 -59 35 -64 390

7F5

539

540

33

542

-6.7

541

544

84

542.5

548

-30

551 581 582.5 587

3.6 11

593

-8.2

616 619 622

547 550 5.55 7F4

-14 20 -53 41

624

-II

627 630

8.3 140

581 584.5

-15

589 596

544

18 9.3 -7.3

585

‘F3

%

g(lum) x 10’

588 592 ‘F3

620 623.5 627 629.5

for absorption_ The integrations in eqs. (4) and (5) are

overall frequencies spanned by the vibronic manifold of thej + k ekctronic transition. If molecular geometry in the emit&g state is precisefy the same as in the ground state, then G(ltim) and @$s) have the same sign arid magnitude. Any difference in magnitude or in sign between G(rum) and G(abs) indicates a change in molecular geometry upon excitation, The values of @urn) = N/1 and of dabs) = Ae/t- determined at fre-

-15 hltich 1974

LETTERS.

The large numb& Gf components observed in thk CPL spectra of the Q-aspartic acid.complex for SD4 -$ 7F, and of the 2-m$ic acid complex for 5D4 e7Fi and SD4 + 7F3 can be.accounted, for by extensive splitting within the 5Di and ?Fi levels by vkd low syinmetry l&and fields_ However, an ~t~rRativ~ interpretation could be offered based on a heterogeneity of the complex species present in soIution. it has been suggested that at the pH values used in this study various amino- and hydroxy carboxyIic acids tend to form polymeric species in aqueous solution with lanthanide ions [4]. The dependence of the CPL and CD spectra on pH and on metakligand concentration ratios was not examined in the study reported here. Experiments in which pH and metal:ligand concentration ratios are

varied are in progress and the results will be repdrted in a subsequent, more complete, publication_ Additionally, the presence of another metal ion such as Eu(III) to which an excited Tb(III) ion may transfer energy if the two ions are in sufficiently close contact, may be useful in ascertaining the presence of dimeric or-polymeric species in solutions_ That is, the efficiency of the energy transfer T~J(III),~D~ + Eu(III),~F,

-+ Tb(111),7F -I-Eu(III),!D

should be greatly enhanced by the formation of poly- _ merit species in which Tb(II1) and Eu(III) ions may exist in close and rigid contact. CPL experiments on malic acid and aspartic acid solutions containing both Tb(II1) and Eu(III) are in progress. The primary purpose of this comm&kation is td report the first CPL of dissymmetric Ianthanide iori systems. The spectra displayed in figs. l-5 and the da-._ ta given in table ‘1 demonstrate that this technique l&s considerable potential as a probe of the electronic s&u& t&I featurei of lanthanide ions.& complex em&n’ ments_~Furthermore, it is possible that CPi’wilI p&tide ’

a sensitive.probe of the stereoch+stry

of~sticlj systems

:in solution. Whereas the totailuminescenc~ spectrti:a+ ‘.I sokiated with the sD4 .+ 7Fi transiiions are%m&+tu~~d-quencies corresponding to the %+ous ektrema of the or.&ow; ai riiost, two componentti, the CPL spectra & i’ CPL-and CD spectra of figs. 1 and 2 are niarly eq&l,. highly str@zttij$d And’~&+I -consideiable:Sqtitiing of’ : I- .’ indicating similar structtk-es foi the cdmplex species .the Jievels by:$e low symm?try ligand’eri+oriment;:. I.-. in the ‘F6 and 5D4st&~+. : _., : . Furthkmore, CPL exhibits &i exkaiirdinary se&&~~:, ’ ste~eochkriicaf ,tiatuie’& &lit&d ~7. * An example o
Volume 25. number

CHEhlICALPHYSlCSLE'f'l'ERS

2

m&c acid and aspartic acid complexes. It however, that detailed theoretical analyses tionai systematic experimental studies are befdre these results can be translated into in1 structure infcrrnation.

is apparent, and addinecessary precise chem-

References [ 11 G.H. Dieke, in: Spectra and energy levels of rare earth iorrs in crystals, eds. H.M. Crosswhite and H. Crosswhite science,

(Inter-

New York, 1968);

B.G. Wybournc. Spectroscopic properties (interscience, New York, 1965);

of rare earths

B.R. Judd, F’hys.Rev. 127 (1962) 750; C.S. Of&, J. Chem. Phys. 37 (1962) 511; K.S. Thomas. S. Sin& and GM. Dieke, J. Chem. Phys. 38 (1963) 2180: G. Gnshurov end O.J. Sovers. J. Chem. Phys. SO (1969) 429;

S-P. Sinha and E. Butter, Mol. Phys. 16 (1969) 285. 121 W.T. Carnell, P.R. Fields and B.C. Wybourne, J. Chem. Phys. 42 (I965) 2797; J.L. Ryan and C-K. J$rBensen, J. Phys. Chem. 70 (1968) 2845; 1V.T. Carnell, P.R. Fields and K. Rajnak, J. Chem. Phys.

49(1968,4i24,4443,4450;

15 n;iarch1974

W-T. Crrrnell, P.R. Fields and B.G. Wyboume, J. aem.

Phys.49(1968)4412. 131 L-1. Katzin, Inorg. Chem. 7 (1968) 1183; L-1. Katzin and E. Gulyas, Inorg. Chem. 7 (1968) 2442; L.I. Katzin, Inorg. Chem. 8 (1969) 1649; 5. Misumi, S. Kida and T. Robe, Spectrochim. Acta 24A (1967) 271. R. &ados. LG. Stadtherr. H. Donato Jr. and R.B. Martin, J. Inorg. Nucl. Chem., to be pubhshed. CA. Emeis and L.J. Oosterhoff, Chem. Phys. Letters 1 (1967) 129; H.P.J.M. Dekkers, CA. Emeis and L-3. Oosterhoff. J. Am. Chem. Sot. 91 (1969) 4.589; C.A. Emeis and L.J. Oosterhoff, J. Chem. Phys. 54 (197 1) 4809. [61 B.N. Samoflov. J. Espt. Theor. Phys. (Soviet Phys.) 18

(1948) 1030; MS. Brodin and V_Ja. Reznichenko,

Ukmin. Phys. I. l0 (1965) 178: 0. Neunhoeffer and H. Ulrich, Z. Elektrochem- 59 (1955) 122. 171 A. Gafni and 1.2. Steinberg, Photochem. Photobioi. I5 I.Z. Steinberg and A. Gsfni, Rev. $5. Instr. 43 (l972)409, 181 C. 3. fiawkins, Absolute configuration of metal complexes (Wiley-Interscience, New York, 1971) ch. 5.