Circularly polarized luminescence of lanthanide(III) complexes with 1-ethylenediamine-N,N′-disuccinic acids

Journal of Luminescence 42 (1988) 227 234 North-Holland, Amsterdam

227

Short Communication

CIRCULARLY POLARIZED LUMINESCENCE OF LANTHANIDE(IH) COMPLEXES WITH 1-ETHYLENEDIAMINE-N,N ‘-DISUCCINIC ACIDS Kazuyuki MURATA, Makoto MORITA and Ken EGUCHI Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Musashino, Tokyo 180. Japan Received 20 January 1988 Revised 19 July 1088 Accepted 23 August 1988

Circularly polarized luminescence and total luminescence spectra are measured for the newly synthesized lanthanide complexes with 1-ethylenediamine-N,N ‘-disuccinic acids (1-edds): K[Ln(IIl)(1-edds)] . 2H 20 (Ln(III) Sm(IIl). Eu(lll), Tb(III) and Dy(III)) in aqueous solution at room temperature. Some chiroptical emission transitions of these lanthanide complexes are interpreted in terms of the total luminescence dissymetry factor, lifetimes and luminescence quantum yields.

1. Introduction Recently, spectroscopic interest has been particularly centered on optical active lanthanide complexes in the excited state [1,21.The chirality induced by lanthanide complexes can be produced by one or more of the following mechanisms: (1) chirality due to vicinal effect, induced by the proximity of the metal to a chiral ligand; that is, optical activity of lanthanide complexes in aqueous solution is induced by asymmetric atoms in the ligands coordinated to the central metal ion; (2) conformational chirality, induced by a twist within each chelate ring; and (3) configurational chirality, caused by a chiral arrangement of chelate rings about the metal ion; that is, optical activity of Na3 [Ln(III)(oxydiacetate)3] 2NaClO~6H20 (Ln(III) Eu(III) and Tb(III) in the trigonal single crystal [3]. Circularly polarized luminescence (CPL) studies for Eu(III) and Th(III) complexes have been extensively done by Bnttain and coworkers [4]. They have examined a relationship between the spectral shapes of CPL and the structures of

Present address: Canon Research Center, Atsugi, Kanagawa, Japan.

lanthanide complex ions in solution. These lanthanide complexes include ligands with a-amino acids, carboxylic acids and aminopolycarboxylic acids. A number of recent topics on CPL studies has been reviewed [5], but almost all these CPL studies have been made in basic media by mixing the lanthanide ion with corresponding organic ligands. The lanthanide complex ions in basic media easily form some aggregates or polynuclear complexes because of the dehydration effects. Hence, we could say that these CPL spectra do not reflect the intrinsic structural features of the lanthanide complex ion in solution [61. On the other hand, Richardson has proposed selection rules for CD and CPL transitions in lanthanide complexes in terms of the change of quantum numbers ~S, ~\L and ~J, between the initial and final states in the 4f 4f transitions [7]. His theory contains a concept that the magnetic dipole (Ml) transition mechanism is dominant in CD- and CPL-allowed transitions of lanthanide complexes and these transitions are principally limited to the transitions with ~ J = 0, +1. CPL studies have been mostly explored in Eu(III) and Tb(III) complexes except for some Sm(III) and Dy(III) complexes, because a bright emission of Eu(III) and Tb(III) ions in the visible region is useful to detect very weak CPL signals. Therefore,

0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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K. Murata et al.

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Circularly polarized luminescence of lanthanide(III) complexes

it is difficult to examine systematically whether the selection rules for CPL are generally available to lanthanide ions other than Eu(III) and Tb(III) ions. In this work, we prepared four Ln(III)(1-edds) complexes where Ln(III) is Sm(III), Eu(III), Th(III) and Dy(III). In these Ln(III)(l-edds) cornplexes, it is possible that either of the two geometrical isomers (~and A) becomes dominant in aqueous solution, since l-edds in these complexes is a hexadentate ligands with asymmetric atoms [X] However, the existence of the isomer has not been experimentally confirmed yet. Optical activity of Ln(III)(l-edds) complexes is considered to be due to the conformational and vicinal effects of the coordinated ligand. We measured the CPL of these lanthanide complexes which are very stable in aqueous solution. We will examine the validity of CPL selection rules in four chiral lanthanide complexes. It appears that the CPL signal for the transition with LIJ 0 was experimentally found to be very weak compared to that of the transi-

tions with complexes.

I

J

=

±1 in the present lanthanide

2. Experimental The ligand, l-edds, was synthesized by the usual procedures [8]. K[Ln(III)(l-edds)] 2H~O cornplexes were prepared by reaction of equimolar amounts of l-edds with the Ln(III) hydroxide, freshly prepared from Ln(III) nitrites. The obtamed lanthanide complexes were identified by the elemental analysis and JR spectra. CPL and total luminescence (TL) spectra were measured by using a new CPL detection system based on a microcomputer control developed in our laboratory [9]. CPL and TL spectra were recorded simultaneously by measuring the difference and sum of the left and right circularly polarized emission intensities, respectively. As an exciting source, 365 nm radiation light of a Hg lamp was used. Lifetimes T were measured by using a PAR model .

=

I

~

I I

I

0

A I

~

15.6 V

(

3cm 1

15.2

x lO

o

16.9 ~ ( x iü~cm~ ) I

C

16.7

16.5

I

I

I

I

~

I

--

~-:

-

17.9

i~(

x lO3cm 1

17.7

.

I

17.5

Fig. 1. CPL (z~ I, solid line) and TL (I, broken line) spectra of K[Sm(III)(1-edds)] . 2H 6H 4G 6H.~2; (C) 4G 2) in 0.068 M aqueous solution at 300 K. (A) 2 ~ 9 2; (B) 5 —. 5 2 H5 2 transitions.

K Murata et al.

/ Circularly polarized luminescence of lanthanide(III) complexes

160 boxcar integrator under an AVCO C950 nitrogen laser (337 nm) excitation. Luminescence

emission bands near 20900, 17420 and 15150 6H, cm are assigned to the 4F9 2 2’ 3 2 6H and 4F 1, 2 transitions of Dy(III), respectively. The 9 2 6 H1, 2 transition (L%J 1) only revealed a weak but detectable CPL spectrum. These CPL spectra are the first example of CPL transitions due to the vicinal effect in Sm(III) and

quantum yields I~for some emission transitions were also measured and we used standard values of quinine sulfate and rhodamine B as references [10].

=

3 R es ~ts —

Dy(III) TL Eu(III) complexes. bands near Figure 17260, 3 shows 16900characteristic and 16200 7F 7F, cm 7F They are assigned to the ‘D0 0, and 2 transitions of Eu(III), 7F respectively. CPL was observed in the ‘D0 2 transitions. Figure 4 presents strong emission bands of Th(III) complexes near 20400, 18360, 5D 170007F and 16090 7F,, 7F cm which are assigned to the 4— 6, 4 and 7F 3 transitions of Th(III), respectively. 7F 7F, Strong (z.~J CPL + 1) was observed in the ‘D4 3, transitions CPL was scarcely detected in the 5D 7F while 7F 4 4, 6 (~J 0 and 2) transitions. T and

CPL andDy(III), TL spectra of Ln(III)(1-edds), Sm(III), Eu(III) and Th(III), are Ln(III) shown

~.

in figs. 1—4. As shown in fig. 1, the Sm(III) emission bands are found at around 17800, 16800 and 15500 They6H are assigned 4G cm ‘in 6H TL spectra. 6H to the 5 2—* 5/2, 7 2 and 9 2 transitions, respectively. A weak but unambiguous amount 2 6of H CPL spectrum was observed in the 7/2 transition (~1J 1), but no CPL in the other two emission transitions. In fig. 2, TL

—*

—~

=

—~

=

—*

I

=

I

I

~

I

I

)

3cm~ I

15.2

P (

x 10

B

229

I

15.0

14.8

I

I

I

-..---~

o

I 17.5

_-...--.------.-

I

I

I

(

~

x 103cm’

I

I

)

17.3 I

I

I

C

0

I

I 21 .1

20.9

P(

x

lO3crr

I

1)20.7

20.5

Fig. 2. CPL (z~I, solid line) and TL (I, broken line) spectra of K[Dy(IIIX1-edds)] 2H 6H 4F 6H 4F 6H 20 in 0.05 M aqueous solution at 300 K. (A) 2 —~ 11 2; (B) 9 2 ~. 1, 2; (C) 9/, —~ 1512 transitions.

230

K. Murata et al.

Circularly polarized luminescence of lanthanide(JII) complexes I

16.3

17.0

3cn

~ ( x lO

~ ( x lO3cr 1

1

I

I

I

I

16.1

16.8

I

____

P (

17.3

)

x 103cm~

_

17.1

Fig. 3. CPL (~I, solid line) and TL (I, broken line) spectra of K[Eu(IlI)(1-edds)].2H 5D~-. 7F 5D,, —~ 7F 5D~—~ ~F, transitions. 20 in 0.19 M aqueous solution at 300 K. (A) 2 (B) 1 (C)

~ values in these emission transitions are summarized for four lanthanide complexes in table 1.

4. Discussion We will briefly summarize electronic selection rules for CPL and TL transitions [2,7]. The electric dipole (El) transition in Ln(III) is induced by a crystal field mixed with the states of opposite parity, and only transitions with ~ S 0, L 6 and L ~ 6 are allowed; the first two being valid in the limit of Russell Saunders’ coupling and the ~

~

~

last is valid in the absence of J J mixing. On the other hand, electronic selection rules for the magnetic dipole (Ml) transition in Ln(III) are /.~S 0, L~L 0 and ~\J ~ 1 in the limit of Russell Saunders’ coupling. It both El and Ml transitions are operative simultaneously, there arises non-zero rotational strength R which is defined as an imaginary part of the scalar product of El and Ml transition moments. Therefore, we could expect to detect strong CPL spectra in 1.~S 0, L 0 and ~ L +1 transitions. As we have seen in figs. 1 and 2, the chiroptical transitions of Sm(III) and Dy(III) are in the fol=

~

=

K. Murata et aL

/

Circularly polarized luminescence of lanthanide(1II) complexes

I

231

1111

A

~

16.2

~1

16.0

x

---

~—

I

—

I

--S.

I

17.1

I —

x

I

10 3cm —1

S

-

I

16.9

C

18.4

3cm~) 18.2

~ ( x 10

~---1I:;:::I::3

--~

S-S

~

Fig. 4. CPL (z~I,solid line) and TL (I, broken line) spectra of KlTb(III)(1-edds)].2H 7F,; (B) 5D 7F 5D 5D 20 in 0.12 M aqueous solution at 300 K. (A) —. 4 -. 4 (C) 4 —. ~F~ (D) 4 —. ~F,,transitions.

4G 6H lowing: the 5 2 7/2 transition with4FL’iS 1, 1 and Z~J 1 of Sm(III) and the 9 2 ~ 6 H1, 2 transition with z.~S 1, /.~L 2 and ~J 1 of Dy(III). In both complexes, El and Ml mechanisms are simultaneously dominant by considering electronic selection rules. Recently, CPL and TL spectra from Sm(III) in Na[Sm(ODA)3] 2NaClO~ 4H20 (ODA: oxydiacetic 4G, acid)2 in 6H single crystals 6H, have6H been reported in the 5 2’ 1 2 and 9 2 transitions [11]. spectra have 4F CPL 6H,, 2 and 6H,also been observed in the 9/2 3 2 3 (DPA: dipicotransitions of means racemicof Dy(DPA) linic acid) by circularly polarized excitation [12].Namely, because of configurational opti=

=

—

=

—

=

.

—*

cal activity, both LIJ 0 and z~J ± 1 transitions are detected in CPL spectra. On the other hand, in the present work, CPL due to the vicinal effect is solely detected under the J-selection rules of L~J +1. The ‘D0 ‘F0 transition of Eu(III) is inherently both El and Ml forbidden and hence it exhibits no CPL. However, CPL has been ob5D served F, (z~J 1, Ml allowed) and 5Din the 7F 0 the 0 2 (~J 2, El allowed) transitions without almost previous reports. The strong CPLany inexceptions fig. 3 are in interpreted by the following mechanism. The El transition moment in the 5D 7F 0 —s 1 transition is produced by a cou=

=

=

—*

=

~

—~

=

232

K. Murata et a!.

Circularly polarized luminescence of lanthanide(JII) complexes

Table 1 Luminescence dissymmetry factors, lifetimes and quantum yields for prominent peaks of CPL and TL spectra in K[Ln(Illl(1-edds)]. 2H 2); [Ln(IIl) Sm(IIl), Eu(I1I), Th(III) and Dy(III)] Ion

Transition

Transition position (cm 1)

Total luminescence dissymetry factors G ~ x102 G ~ b) x 102 I

Sm(III)

Th(III)

2

—s

5 2 6H 72

17800 116800 \16700

~

155011

5D, —~ 7F 7F0 -s 1 7F —~ 2 5D 7F 4 —+ 6 7F -. 5

4F Dy(III)

Lifetime (6tS)

Quantum yields ~ >< 10

6H

4G 5

Eu(III)

Luminescence dissymetry factors 2 g1~Xl0 (position cm

9

2

H55 6H —s 15

36 .

36 .

+6.6 (16800)

17200 16900

4.6

4.6

16200

2.2

2.2

20400

+0.2

18360

1.6

2.0

17000

+0.43

0.22

16090

+0.35

3.0

2

J 21000

2

\20900 17420

2

15150

‘~ b)

Calculated according to eq. (5). Calculated according to eq. (6).

‘~

Calculated according to eq. (1).

6.4 (16 870) +3.8 (16210) +0.6 (20420) 9.0 (18400) +1.3 (17080) +13.6 (16170)

0.42

+0.019

7.0 (15100) +8.5 (15 060)

2.0

pling with odd-parity charge transfer (CT) state [2], the Ml transition moment in the ‘D0 ~ transition by mixing of ‘D, with ‘D0 state. 5D As we see in CPL spectra of Th(III) in fig. 4, the 4 —s 7F 7F 3 and 5 transitions + 1) are CPL-al7F (L~J 7F lowed but the ‘D4 —s 4 6 transitions (z~J7F0 and 2) are CPL-forbidden. Although the’ D4 —.s 4 transition has Ml allowed transition moment, the corresponding CPL is scarcely seen. The observed L~J=±1CPL transition is strongly enhanced by the El transition through of 75d statemoment with odd paritythe [2].coupling From the to 41 above-mentioned CPL spectral examples, one might conclude that CPL is easily observable in —.*

=

=

18 22

0.67 0.98

24

0.33

200 230

0.05 1.5

245

3.0

270

4.3

730

1.3

400

4.1

515

2.1

22

055

27

7.3

20

0.61

the Ml allowed transitions with selection rules of ~J + 1. We will discuss quantitatively the nature of the chiroptical transitions in the present lanthanide complexes. The CPL intensity is experimentally estimated in terms of a luminescence dissymetry factor g, [2], which is defined in eq. (1): =

2~~

g1

—

—i-—

(‘L =

—

‘P.)

I ‘2’I + I /

~

L

~‘

RI

where ~ I is CPL intensity which is the difference between left (IL) and right (IR) circularly

K Murata et aL

/

Circularly polarized luminescence of lanthanide(III) complexes

polarized light and I is the total luminescence intensity. The g1 factor is related to rotational strength (Reg) and to dipole strength (Deg). In an isotropic medium, g1 is defined as follows: 4Reg/Deg, (2) g1 Deg I (e I Ill g) 2 (3)

233

define a absolute total dissymetry factor G,~for a term-to-term transition, which is given by

f

G7~ I gah(v) Id~, (6) where the band ~ab sign is set to always positive for a —

=

=

Reg

Im(e I 12 g)
=

.

(4)

where (e I and (g I and initial and final electronic states, respectively. The electric dipole and magnetic dipole operators are denoted, respectively, by 2 and i~z.The g 1 values estimated for some emission transitions of four lanthanide complexes are summanzed in table 1. The g1 value for the Sm(III) complex is determined only at the 16 800 cm 1 emission band, which corresponds to the 2 —s 6 H7 2 transition, while the g1 values for the Dy(III) complex are observed at the 15 100 and 15060 cm 1 4Femission bands, which are re6H,, 2 transition, sponsible fortothediscuss 9 2 ~ the degree of the optical In order activity in the chiroptical term-to-term transitions of the present Ln(III)-edds complexes, we used a total luminescence dissymmetry factor G,~ [7], which is defined in eq. 5: G,f=

J

g~~(v) dv,

(5)

band

where

i

and

f

are J quantum numbers in a

term-to-term transition. The G,~ factors for the chiroptical transitions of four lanthanide cornplexes were calculated by using eq. (6) and the results correspond correctly to the degree of optical activity in a term-to-term transition. The G,~ and G,~values for prominant emission transitions in the present lanthanide complexes are summarized in table 1. We shall next discuss the CPL selection rules in terms of the G,7 factor. From table 1, we find that G1~values for the chiroptical transitions of four lanthanide complexes are of the same order of magnitude. From values it appears that in Eu(III) and Tb(III) complexes only the transitions with i.~J ±1 are CPL-allowed, are strictly forbidden. This is also thewhile case others for Sm(III) and Dy(III) complexes. Consequently, we ascertained that the J + 1 transitions are the most useful selection rules in the light of G7f magnitudes in the emission transitions of chiral lanthanide complexes. The T values in the present lanthanide complexes are tabulated for important emission transitions. In order to discuss the stability of com=

~

plexes in aqueous solution, we used T values for the ‘D 0 F0 transition. The T values for Eu(III) aquo complexes and Eu(III) 1-edds complexes are about 110 and 200 p.s, respectively. Since the exchange rate between the lanthanide ion and the coordinated H20 ligands is very fast in the aquo complexes, the T value in aquo complexes becomes shorter than in the complexes fixed with the ligands. The smaller ‘r is explained by a release of ligands through a coupling of the OH vibrations with the 41 4f transitions [13]. The difference between T value in Eu(III) aquo complex and the present Eu(III) complex indicates that the complex exists in a form with a bonding ratio of Eu(III)/!igand (1 : 1) in aqueous solution. A relationship between the experimental CPL detection limits and the 1, values has not yet been studied enough [14]. All the ~i, values in table 1 are in the similar order of magnitude ...+~

term-to-term transition, and a and b mean any stark components in a term-to-term transition. We calculated the parameters ~ in some chiroptical transitions of four lanthanide complexes by using eq. (5). According to the arguments on CPL selection rules [7], it happens occasionally that a!though the ~ab components are very large, the G,f value becomes very small. This is because in4Fthe ._~7F3 transition of Tb(III) and in the 9 2 —s 2 transition of Dy(III) both positive and negative signs of the g~8values cancel out totally. On the contrary, when the sign of all the ~ I signals observed in a term-to-term transition is positive or negative in the CPL spectral region, the G,~factor calculated by using eq. (5) reflects the real CPL intensity. This is the case with Sm(III) and Eu(III) complexes. Therefore, we want to

=

=

234

K. Murata eta!.

Circularly polarized luminescence of lanthanide(III) complexes

between 10 ~ and 10 2 If the cP, values are very small for the CPL-forbidden transitions we might easily judge whether the corresponding transitions are allowable in CPL or not. But this correlation cannot be seen in this table. However, if the ~I~1 values are smaller, the signal-to-noise ratio becomes poorer and this makes it very difficult to detect a good CPL spectrum of lanthanide complexes. Practically to say, to detect the CPL spectrum more clearly, it seems necessary to consider the transitions with larger 1i1 values and these obeying the /~J 0, + 1 CPL selection rules in emission. Concluding, we obtained the following results: (1) We have observed CPL of four Ln(III)l-edds complexes with Ln(III) Sm(III), Eu(III), Tb(III) and Dy(III) in our computer controlled CPL/TL spectrometer systems. CPL of Sm(III) and Dy(III) complexes due to vincinal and conformational effects is observed for the first time in the present work; (2) CPL-allowed transitions in these cornplexes are found to obey simple J-selection rules of z~J 0, +1 as is suggested by Richardson’s selection rules. Unfortunately, the theoretically presumed CPL transitions with z.~J 0 are not detected in our experiments; (3) A threshold for CPL detection is discussed by examining both CPL-selection rules and observed cP1 values. However, there is no apparent correlation between the Ji1 values and the g1 values of CPL. In order to discuss the CPL selection rules in the absorption processes, investigation of circular dichroism and absorption spectra is now in progress. —

—

=

Acknowledgements We gratefully acknowledge Mr. T. Osada for synthesizing the chiral lanthanide complexes. We are also indebted to Professor M. Miwa for allowing us to use the absorption and CD spectrophotometers.

References [1] S.F. Mason, ed., Optical Activity and Chiral Discrimination (Reidel, Boston, 1979) ch. 6; M. Morita, Bunko Kenkyu. 29 (1980) 357 [2) ES. Richardson and J.P. Riehl, Chem. Rev. 77 (1977) 773 J.P. Rid and F.S. Richardson, ibid., 86 (1986) 1. [3] J.P. Morley, J D. Saxe and F.S. Richardson, Mol. Phys. 47 (1982) 379; J.D. Saxe, J.P. Morley and F.S. Richardson, ibid.. 47 (1982) 407. [4] L. Spaulding, HG. Brittain, L.H. O’Connor and K.H. Pearson, Inorg. Chem. 25 (1986) 188. [5] HG. Brittain, Coord. Chem. Rev. 48 (1983) 243. [6] K. Murata and M. Morita, J. Lumin. 26(1981)207. [7] F.S. Richardson, Inorg. Chem. 19 (1980) 2806. [8] J.A. Neal and N.J. Rose, Inorg. Chem. 7 (1968) 2405. [91M. Morita and K. Eguchi, Riken Reports 78 (1984) 157. [10] W.H. Melhuish, J. Phys. Chem. 64 (1960) 762. [11] R.C. Carter. CE. Miller, R.A. Palmer, P.S. May, D.H. Metcalf and F.S. Richardson, Chem. Phys. Lett. 131 (1986) 37. [12] G.L Hilmes and J.P. Riehl, lnorg. Chem. 25 (1986) 2617. [13] S.P. Sinha, ed., Systematics and the Properties of the Lanthanides, edited by NATO ASI Series (Reidel, Dordrecht, 1982) Ch. 10, p. 451. [14] P.H. Schippers, J.P.M. van der Ploeg and P.H J.M. Dekkers, J. Am. Chem. Soc lOS (1983) 84.