Self-assembled he terodinuclear europium(n1)-lan t hanide( 111) chela tes of 2,6=bis [N,N=bis( car box yme t hy1)aminome t h yl I-4-benzo ylphenol and their radiative 5 D ~ -B 'Fj transitions of Eu"'

Martti Latva,* Pauliina Makinen, Sakari Kulmala and Keijo Haapakka Department of Chemistry, University of Turku, FIN-200 I4 Turku, Finland

A photochemical study has been conducted (i) to shed light on the intramolecular T,-relaxation pathways of 2,6-bis[N,N- bis(carboxymethyl)aminomethyl]-4-benzoylphenol (L) to initiate the radiative 5D0 -+ 7Fj transitions of Eu"' and (ii) to elucidate the effects of different aqua-bridged non-radiative Ln"' cations on these Eu"'-specific radiative transitions, using the self- assembled heterodinuclear LEu"'Ln"'L chelates where the Eu"' and Ln"' cations are coordinated to each other by the aqua bridge inside the chelating cage consisting of two deprotonated phenolic hydroxy groups and four partially deprotonated 2,2'- (methylenenitrilo)bis(acetic acid) moieties. Depending on its non-radiative Ln"' cation, the encapsulated aqua-bridged Eu"'Ln"' pair may be (i) as in the case of Ln"' = Er''', Ho'", Nd"' or Pr'", a double-nuclear triplet relaxation centre where the T,-relaxation occurs mainly to the excited state of Ln"', (ii) as in the case of Ln"' = Gd"' or Y'l', a single-nuclear triplet relaxation centre where the T,-relaxation is energetically capable of occurring only to the excited state 'Dj of Eu"', so that the resulting radiative 5D0 + 7Fj transitions of Eu"' are to some extent enhanced and, finally, (iii) as in the case of Ln"' = Yb"', a single-nuclear triplet relaxation centre where the T,-relaxation occurs only to the excited state of Eu"', so that the resulting excited state 5D0 of Eu"' is strongly destabilized by the aqua-bridged Yb"': however, our present experimental results do not exclude the possibility of the aqua-bridged Yb"' cation occurring also as the T,-relaxation centre despite the large energy gap A E(T, - 2F7,2) = 11 560 cm- '.

Dinuclear lanthanide(II1) complexes are currently of vital importance because of their possible utilization (i) as novel tunable photonic devices' with potential applications in bio- medical diagnostics2 and fluorescence (ii) as para- magnetic contrast enhancing agents in magnetic resonance imaging4 and ( i i i ) as atomically homogeneous precursors for oxides with well defined electronic characteristic^.^ Further- more, the dinuclear lanthanide(II1) chelate creates an inter- esting basis to study in detail lanthanide(II1)-specific energy transfer pathways inside different chelating moieties ; in these studies, the energy transfer from the radiative excited state 5D0 or 5D, of europium(m) and terbium(m), respectively, to a non-radia tive excited state of adjacent neodymium(III), holmium(Ir1) or praseodymium(II1) has been generally

Finally, the enhancement of radiative 'Do + 7Fj transitions of Eu"' and 5D4 + 7Fj transitions of Tb"' in their co-luminescent and polymer-like chelates have been reported, where the enhancement is based on an intermolecular triplet- triplet energy transfer from a coordinated non-luminescent Ln"' chelate (Ln"' = Gd"', Lu"' or YIII) to the excited states 5Dj of Eu"' and Tb"'.9*'0

In recent years, a great deal of experimental work has been conducted in our laboratory to synthesize new multidentate chelating agents, which provide the basis for the efficient sensi- tized radiative 5D0 -+ 7Fj transitions of chelated Eu"' and 5D, --* 7Fj transitions of chelated Tb"' to become utilized as time-resolved photoluminescence and electrogenerated che- miluminescence (ECL) probes for a variety of extreme trace analysis in aqueous solutions.'O*'l In connection with these studies, we have observed that depending on the pH of equi- molar sample solutions, three structurally different self- assembled luminescent chelates of europium(n1) with 2,6- bis[ N,N-bis( carboxymethyl)aminomethyl]-4-benzoylphenol (H,L) are formed. However, only a homodinuclear LEu"'Eu"'L chelate, where an aqua-bridged Eu"'Eu"'pair is located deeply inside the chelating cage consisting of two dep- rotonated phenolic hydroxy groups and four partially depro- tonated 2,2'-(methylenenitrilo)bis(acetic acid) moieties, was found to be capable of initiating the efficient ligand-sensitized

radiative 'Do + 7Fj transitions of chelated Eu"' where this property can be accounted for the strong interaction between the chelated Eu"' cation and the deprotonated phenolic hydroxy group of L with the exceptionally long-lived lifetime of z ~ ~ ~ , ~ ~ ~ ~ ~ ~ ~ ~ = 0.972 ms. l 2

Previously, dinuclear lanthanide(II1) chelates using phenolates' 3*14 and pyridine-like' chelating agents have been prepared by precipitation from different organic solutions. The present contribution reports the formation of structurally analogous self-assembled heterodinuclear LEu"'Ln"'L chelates

solutions and discusses in detail the T,-relaxation pathways of photoexcited L* to the encapsulated double-nuclear Eu"'Ln"' pair finally to initiate the sensitized radiative 5D0 + 7Fj tran- sitions of Eu"' where this double-nuclear relaxation centre provides different basis for the triplet relaxation depending on the selected Ln"' cation.

(Ln"' = Er"1 7 Gd"' 7 Ho11' 3 Nd"' , p r'", Yrrl or Yb"') in aqueous

Experimental Reagents

Oxides of dysprosium(II1) (99.9%), erbium(Ir1) (99.9%), europium(Ir1) (99.95%), gadolinium(u1) (99.9%), holmium(1rr) (99.9 YO), neodymium(rI1) (99.9 YO), ytterbium(II1) (99.9 YO) and yttrium(m) (99.9999%) were purchased from Aldrich, Koch- Light Laboratories, Ventron and Johnson Matthey. Terbium(@ chloride (99.999%) and praseodymium(rI1) nitrate (99.9%) were products of Aldrich. L was synthesized as described in detail earlier.16 Boric acid and sodium hydroxide were Suprapur reagents of Merck. The 1.00 x lop2 mol 1- ' Ln"' stock solutions were prepared by dissolving a weighed amount of the appropriate solid sample in perchloric acid, excluding those of Tb"' and Pr''' which were prepared in water: the Ln"' stock solutions were standardized by the con- ventional complexometric titration with xylenol orange as the indicator. All these reagents were used without further purifi- cation. Quartz-distilled water was used for the preparation of all solutions.

J . Chem. SOC., Faraday Trans., 1996,92(18), 3321-3326 3321

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The sample solutions containing 1 : 1 EUII'L, 1 : 1 Ln'I'L, homodinuclear LEu'~'Eu'"L, homodinuclear LLn"Ln"'L and heterodinuclear LEu"'Ln"'L chelates were attained by diluting the appropriate mixture of Eu"' and Ln"' in the equimolar solution of L adjusted to pH 9.0 by the borate buffer; these sample solutions were then allowed to equilibrate overnight at 25 "C before the luminescence measurement. Table 1 lists the statistically expected amounts of these chelates under the dif- ferent mixing conditions.

Procedures

The luminescence measurements were conventional and were carried out using a Perkin Elmer LS-5 luminescence spectro- meter. The initial intensities of luminescences and excited-state lifetimes were calculated from the measured luminescence decay curves. In the present contribution, an amount of the radiative 'Do + 7Fj transitions of Eu"' of the sample solution containing homodinuclear LEu'~'Eu"'L and heterodinuclear LEU"'LU"'L chelates is expressed as a relative luminescence yield R using the radiative 'Do -+ 7F2 transitions of uncom- plexed europium(u1) as the reference luminescence on the fol- lowing basis :'

where C E ~ I ~ ~ is the concentration of uncomplexed Eu"' ion, cE,,"~L is the total concentration of homodinuclear LEu"'Eu"'L and heterodinuclear LEu"'Ln"'L chelates, kEulIl and ki are the reciprocals of the lifetimes of the excited 'Do states of uncom- plexed and chelated Eu"', respectively, and ZEurrl and I t are the initial intensities of the radiative 'Do -+ 7Fj transitions of uncomplexed and chelated Eu"', respectively; kEurll and IEurll were determined at A,, = 395 nm and A,, = 615 nm while ki and IEUIIIL were obtained at A e x = 325 nm and A,, = 615 nm and, finally, the Ln"' cations of the heterodinuclear LEu"Ln"'L chelates were selected so that the possible Lnl''- specific radiative transitions do not occur at A,, = 615 nm.

The phosphorescence spectra of L to determine its T I energy level under different conditions were measured conven- tionally using appropriate Gd"' chelates in glycerol water (5 : 4) mixtures at 77 K.

Results and Discussion Self-assembled luminescent homodinuclear LEu"*Eu"'L chelate

2,6 - Bis[N,N - bis(carboxymethyl)aminomethyl] - 4 - benzoyl- phenol (H,L) is a heptadentate chelating agent furnished with one phenolic hydroxy group and two flexible 2,2'-(methylene- nitrilo)bis(acetic acid) moieties with the acidic dissociation constants of pK,, = 2.3, pK,, = 2.8, pK,, = 5.7, pK,, = 10.8 and pK,, = 12.5 where the third step depicts the dissociation of the phenolic hydroxy group of L.

We have previously pointed out that in the strictly equi- molar aqueous sample solution of L and Eu' ' ' ,~~ the disso- ciation of the phenolic hydroxy group of L at pK,, = 5.7 initiates the formation of a mixture of self-assembled 1 : 1 Eu"'L and homodinuclear LEu"'Eu"'L chelates with the fol- lowing solution structures: (i) as to the Eu'I'L chelate, Eu"' is

Q C=O I

f"l OH/N> H02C H02C C02H C02H

coordinated to six ligand atoms selected from the two par- tially deprotonated 2,2'-(methylenenitrilo)bis(acetic acid) moi- eties of L and (ii) as to the homodinuclear LEu"'Eu"'L chelate, the aqua-bridged Eu"'Eu"' pair is located in the chelating cage consisting of two deprotonated phenolic hydroxy groups and four partially deprotonated 2,2'-(methylenenitrilo)bis(acetic acid) moieties as illustrated in Fig. 1 by the small black circles, i.e., each Eu"' cation is coordinated to two partially deproton- ated 2,2'-(methylenenitrilo)bis(acetic acid) moieties by their nitrogen and two oxygen ligand atoms so that the moieties belong to the different ligands L and to the deprotonated phe- nolic hydroxy groups of both ligands L.

Unlike the monomeric Eu"L chelate where no strong inter- action between the coordinated Eu"' cation and the deproton- ated phenolic hydroxy group of L occurs, the homodinuclear LEu"'Eu"'L chelate is capable of initiating intense sensitized radiative 'Do + 7Fj transitions of Eu"'; the appropriate exci- tation and emission spectra of this homodinuclear chelate together with the phosphorescence spectrum of LEu"'Gd"'L chelate are shown in Fig. 2. By absorbing a 325 nm photon (i) the ligand moiety is raised to one of its higher excited singlet states at S, = 30770 cm-', which immediately loses some of its excitation energy by vibrational relaxation and by internal conversion and is relaxed to its lowest excited singlet state S, = 25 970 cm- ', followed by an efficient intersystem cross- ing to initiate the lowest triplet state at T, = 22830 cm-'; and (ii) this triplet state is relaxed intramolecularly to ,D, and finally results in an excited state 'Do = 17 230 cm- ' of coor- dinated Eu"' cation which, in turn, is deactivated to the vibra- tional 7F-multiplet ground state, thus inducing the well known peak emissions of Eu"' with a lifetime of Z L E ~ ~ I I E , , ~ ~ I L - 0.972 ms as displayed by the solid line in Fig. 3.

In our opinion, this exceptionally long-lived lifetime of the sensitized radiative 'Do + 7Fj transitions of Eu"' is solid evidence for the proposed structures of the homodinuclear LEu"'Eu"'L and, as well (see below) heterodinuclear

-

Fig. 1 A schematic structure of the homodinuclear LEu"'Eu"'L chelate

Table 1 Relative amounts of homodinuclear LEU"'EU'''L and LLn"'Lu"'L and heterodinuclear LEu'*'Ln'''L chelates

~ / i o + m o l l - ' LrEu"11/10-6 mol 1 - ' [Ln''']/10-6 mol 1 - ' LEU~I~EUI%( %) LLn"'Ln"'L( %) LEu"'Ln"'L( %)

1 .o 1 .o 1 .o 1 .o 1 .o 1 .o

10.0 9.0 7.0 5.0 3.0 1 .o

0 1 .o 3.0 5.0 7.0 9.0

100 81 49 25 9 1

0 1 9

25 49 81

0 18 42 50 42 18

3322 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

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- 1 I I .- I '

I] 250 300 350 400 450 500 550 600 650 700 750

wavelengthhm Fig. 2 Excitation (-) and emission spectra (...) of the homo- dinuclear LEu"'Eu"'L chelate and phosphorescence spectra (- - -) of the homodinuclear LGd"'Gd"'L chelate

LEu"'Ln"'L chelates where the coordination sphere of the chelated Eu"' cations contains only one water molecule per two Eu"' ions to form the aqua-bridged Eu"'Eu"' pair inside the aforementioned chelating cage.

Self-assembled heterodinuclear LEu"'Ln"'L chelates and their radiative 'Do + 'F, transitions of Eu"'

Lanthanide(Ii1) cations are essentially spherical with large and relatively similar ionic radii, changing only from 0.848 8, (Lu"') to 1.061 8, (La"') which is the basis for their ability and tendency to form thermodynamically similar chelates in aqueous solutions. This makes it possible to postulate that homodinuclear LEu"'Eu"'L and LLn"'Ln"'L and hetero- dinuclear LEu"'Eu"'L chelates are formed in the aqueous sample solutions at a higher pH than ca. 6 to deprotonate the phenolic hydroxy group of L, where the concentration of L and the total concentration of Eu"' and Ln"' are equal and, so that their relative amounts can be statistically estimated as pointed out in Table 1 under the stated conditions. An amount of the dinuclear chelate is expressed in percentage from the total amount of the dinuclear chelates in the sample solution. In addition, these sample solutions are expected to contain equal amounts of 1 : 1 Eu"'L and Ln"'L adducts which are, however, not capable of initiating the efficient sen- sitized radiative 'Do + Fj transitions of Eu"' and thus, their occurrence in the sample solutions is neglected on the sub- sequent discussion.

It is accepted that the structure of the heterodinuclear LEu"'Ln"'L chelate is similar to that of the homodinuclear LEu"'Eu"'L chelate shown in Fig. 1, i.e., the aqua-bridged

pair is strongly coordinated to the ligand moiety inside the chelating cage. Providing that a strong overlapping ~ ~ l l I ~ ~ l l l

> v) t

c

c. .- $ 0.0010 .- 8 5 0.0005 % a, c

3

.- E 0.0000 -

..... . .... : ..

'.'" . . .

" . . . , . .. .,..: . .. \ .. .. ..... : .................. ..... _,_ .. : , , . . ,,<: d . . . ....-. , :. . :.

. "..?. : :..;..'...;:.:; ..,:. _:. .

I I I I I 1 -0.0005 0.0000 0.0005 0.0010 0.0015 0.0020

time/s Fig. 3 The decay curves for the radiative 'Do + 'Fj transitions of Eu"' in the sample solutions containing 1 x lo-' mol 1-' of both L and Eu"' (-) and in the sample solution containing 1 x lo-' mol I - of L and 5 x moll-' of both Eu"' and Yb"' (. * a )

of the appropriate energy levels of L, Ln"' and Eu"' occurs, this aqua-bridged Eu"'Ln"' pair may be relaxation centre for the T, energy of 1 at least on the following basis:

L * Eu"'Ln"'L + LEu"'Ln"'* L (2a)

LEu"'Ln"'*L -+ LEu"'Ln"'L (2b)

L*Eu"'Ln"'L + LEu"'Ln"'*L ( 3 4

LEu"'Ln"'*L + LEu"'*Ln"'L (3b)

LEu"'*Ln"'L + LEu"'Ln"'L + hv (34

L* Eu"'Ln"'L + LEU"'* Ln"'L ( 4 4

LEu"'*Ln"'L + LEu"'Ln"'L + hv (4b)

L*Eu"'Ln"'L --+ LEu"'*Ln"'L ( 5 4

LEu"'*Ln"'L + LEu"'Ln"'*L (5b)

LEu"'Ln"'*L + LEu"'Ln"'L (5c)

where L* depicts the ligand moiety of the chelate in its lowest excited state T, = 22 830 cm- obtained photochemically as pointed out above, and where the intramolecular T, relax- ation of L may occur (i) to an excited state of Ln"' finally to initiate its non-radiative transitions, i.e., the pathway (2a), (2b); (ii) to an excited state of Ln"' which, in turn, is imme- diately relaxed to an excited state of Eu"' finally to initiate its radiative transitions, i.e., the pathway (3a)-(3c); (iii) to an excited-state Eu"' finally to initiate its radiative transitions, i.e., the pathway (4a), (4b) or (iv) to an excited state of Eu"' which, in turn, is immediately relaxed to an excited state of Ln"' finally to initiate its non-radiative transitions, i.e., the pathway (5a)-(5c).

To shed light on these proposed relaxation pathways of L* inside the chelating cage (see Fig. l), the log R DS. [LEu"'Ln"'L]/[LEu'llEu'llL] plots, where R denotes the rela- tive intensity of the radiative 'Do + 'Fj transitions of Eu"' (see Experimental section), shown in Fig. 4, were determined

bridged non-radiative Ln"' cation in the sample solutions adjusted to pH 9.0 under the conditions listed in Table 1. These results can be divided into two categories as follows: (i) in the case of Ln"' = Gd"' or YII1, the relative intensity of the radiative 'Do + 7Fj transitions of Eu"' is enhanced to some extent and (ii) in the case of Ln"' = Er"', Ho"', Nd"' , P r 'I' or YblI1, this intensity is quenched drastically. However, these enhanced and quenched radiative 'Do + 'Fj transitions of Eu"' were found to be strictly single-exponential decay pro- cesses with the lifetime of ~LE~l l lL, ,r t lL = 0.96 ms, excluding the heterodinuclear LEu"'Yb"'L chelate where the aqua-bridged Yb"' cation destabilizes strongly the excited state 'Do of Eu"'

using ErIII, Gdlll, Ho"1 NdIlI p 111 yI I1 , , r , or Yb"' as the aqua-

I I 1 I I

0 5 10 15

[LEu"'L~'~'L]/[ LEU"~EU"'L] Fig. 4 The relative luminescence intensity (log R) of Eu"' as a func- tion of ratio between concentrations of luminescent LEu"'Ln"'L and LEu"'Ln"'L species in sample solutions adjusted to pH 9.0. Ln"' is either Gd"' (H), Y"' (a), Yb"' (A), Ho"' (V), Er"' (+), Pr"' (+) or Nd"' ( x )

J . Chem. Soc., Faraday Trans., 1996, Vol. 92 3323

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to make the radiative 5D0 + 'Fj transitions of Eu'" exception- ally short lived with ~LEulllybIllL = 0.045 ms.

Luminescent heterodinuclear LEu"'Ln"'L (Ln = Er, Ho, Nd or Pr) chelates

A generalized energy diagram shown in Fig. 5, where iXj and 'Yj denote the higher and lower excited-state energy levels of Ln"' as compared to the lowest excited state 5D0 of Eu"', respectively, and i Z j the vibrational ground-state levels of Ln"' (see Table 2), was constructed to elucidate the aforementioned Ln"'-induced quenching of the radiative 5D0 -, 7F2 transition of Eu"' in these heterodinuclear LEu"'Ln"'L chelates (Ln"' = Er"', Ho"', Nd"' or Pr'"). All these Ln"' cations possess an excited state energy level i X j which is very close to that of the triplet state T, = 22 830 cm-' of 1, i.e., the energy gaps are AE(T, - 4F3/2) = 320 cm-' for Er"', AE(T, - 5F,) = 530 cm-' for Ho"', AE(T, - 2K15/2) = 1180 cm-'

for Nd"' and, finally, AE(T, - 3P2) = 610 cm-l for Pr"' which are slightly smaller than that [i.e., AE(T, - 5D2) = 1330 cm- '1 of Eu"'. This suggests that the T, = 22 830 cm- ' relax- ation of 1 could compete energetically with the 5D2 state of Eu"' finally to initiate its radiative 5D0 -+ 7Fj transitions and with the highest excited i X j state of Ln"' finally to initiate its non-radiative iYj -+ iZj transitions. However, the radiative

ligand Eu"' - 1 Fig. 5 A schematic diagram of the energy flow in the heterodinuclear LEul"Lnl"L chelate, where Ln"' = Er"', Ho"', Nd'" or Pr"'

Table 2 Excited-state energy levels of Ln"' cations of the hetero- dinuclear LEu'"Ln"'L chelates between the T, = 22830 cm-' of L and the 'Do = 17230 cm-' of Eu"' denoted as i X j and below the 'Do = 17230 cm-' of Eu"' denoted as iYj (see Fig. 5); the energy levels are from optical spectra of Ln"' in lanthanum trichlorideI8

Ln"' 'Y j/cm - '

Ho"'

Nd"'

21 500 19 030 17 230 22 510 22 170 20 490 19 140 18 390 22 300 22 100 21 350 20 610 21 070 18 430 21 650 21 460 21 050 19 540 19 OOO 17 170 22 220 21 300 21 080 20 470

15 280 12 470 10210

15 480 13 260 11 210

15 940 14710 13 500 12 650 12 480 11 430 16 700

5D0 -+ 7Fj transitions of Eu"' were strongly quenched with increasing amounts of Ln"' as shown in Fig. 4, so that the remaining luminescence response can be quantitatively assign- ed for the homodinuclear LEu"'Eu"'L chelate still present in the sample mixtures. These radiative 'Do -+ 7Fj transitions of Eu"' remain strictly single-exponential decay processes with a lifetime of z L E ~ ~ ~ I L ~ ~ ~ ~ L z 0.96 ms which, in our opinion, demon- strates (i) that the intercalation of Ln"' cation inside the che- lating cage to form the strongly coordinated aqua-bridged ~ ~ 1 1 1 ~ ~ 1 1 1 pair does not essentially change the chelating environment of Eu"' and (ii) that the aqua-bridged Eu'1'Ln"' pair (Ln"' = Er"', Ho"', Nd"' or Pr"') provides no basis for an efficient intramolecular Eu"'*Ln"' -+ Eu"'Ln"'* energy transfer despite the feasible donor and acceptor energy levels (see Table 2) and short distance of the aqua-bridged Eu"' and Ln"' cations.

As a conclusion, it is postulated that the TI-relaxation in heterodinuclear LEu"'Ln"'L chelate (Ln"' = Er"', Ho"', Nd"' or Pr"') does not occur to the 'D, state of Eu"' but rather to the highest excited iXj. state of Ln"' finally to initiate the Ln"'- specific non-radiative 'Yj -+ iZj transitions.

Luminescent heterodinuclear LEu"'Gd"'L and LEul"Y1"L chela tes

The relative luminescence yield R of the radiative 'Do -+ 7Fj transitions of Eu"' is somewhat enhanced in the cases of het- erodinuclear LEu"'Ln"'L chelates (Ln"' = Gd"' or Y'") (see Fig. 4); however, these transitions were also found to be well defined single-exponential decay processes with a lifetime of z L E ~ ~ ~ ~ L ~ I ~ ~ L z 0.96 ms, ix., no essential Ln"'-initiated changes in the coordination sphere of Eu"' inside the chelating cage is assumed to occur.

Fig. 6 displays a schematic diagram of the energy flow to initiate the sensitized radiative 'Do -+ 7Fj transitions of Eu"' in the heterodinuclear LEu"'Gd"'L chelate where the lowest excited state 6P712 of Gd"' is located at E = 31 950 cm-'. It is well known that the Y"' cation has no excitation transitions in the range E = 10000-5OOOO cm-', as may be expected from its closed-shell electronic configuration and the absence of unpaired electrons, which allows to postulate that the energy diagram of the heterodinuclear LEu"'Y"'L chelate is analo- gous with that shown in Fig. 6, i.e., no interstitial energy levels of Y'" is located between the So and S , energy states of L. Consequently, the intramolecular T, = 22 830 cm-' relax- ation of L in the heterodinuclear LEu"'Gd"'L and LEu"'Y"'L chelates can occur only to the 5D2 state of Eu"' finally to initiate its radiative 5D0 -+ 7Fj transitions which, in our opinion, can be regarded as an obvious reason for these enhanced radiative 'Do + 7F. transitions of Eu'", jointly with the fact that no kind of E u " ' * h ' -+ Eu"'Ln"'* energy transfer inside the chelating cage is energetically possible. The enhancement of the relative luminescence yield is based on the formation of dinuclear complex where the ratio between

24 - I

G iTi 00 1 6 - F

a .

0 - ligand Gd"' ligand

Fig. 6 A schematic diagram of the energy flow in the heterodinuclear LEu"'Gd"'L chelate

3324 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

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energy donors and acceptors is changed from 1 (homodinuclear complex) to 2 (heterodinuclear complex, where Ln"' is either Gd"' or Y"'). Thus, the relative lumines- cence yield can increase by factor of two. In fact similar kind of enhancement of Eu"' and Tb"' ion luminescences based on the use of excess of energy donors relative to amount of accep- tors has been exploited both in co-luminescence and in polymer structure luminescence, where excess of ligand and non-luminescent Gd'", Lu"' or Y'" ions relative to amount of Eu"' ion is used in order to approach the situation where all the Eu"' ions in the system will be excited and emit lumines- cence~ .~ . O

Luminescent heterodinuclear LEu'I'Y b"'L chelate

The log R us. log ([LEu"'Yb"'L]/[LEu"'Eu'''L]) plot (see Fig. 4) shows that the radiative 'Do -+ 7Fj transitions of Eu"' are not so efficient in the heterodinuclear LEu"'Yb"'L chelate as in the homodinuclear LEu"Eu"'L chelate. Fig. 3, in turn, dis- plays the decay curves for the radiative 5D0 -+ 7Fj transitions of Eu"' in the sample solutions containing (see Table 1) (i) the homodinuclear LEu"'Eu"'L chelate (solid line) where the decay is a well defined single-exponential process with a life- time of ~ L E ~ l i l E ~ l l l L = 0.960 ms and (ii) the mixture of homo- dinuclear LEu"'Eu'''L and heterodinuclear LEu"'Yb"'L chelates (dotted line) where this double-exponential decay can be resolved into two well defined components with the 5D0 -, 7Fj transitions lifetimes of Z L E , , , ~ ~ E ~ ~ ~ ~ L = 0.960 ms and ~ L E ~ i i i y b l i i L = 0.045 ms, which can be stated for the hetero- dinuclear LEu"'Yb"'L chelate.

The lifetime of the radiative 'Do + 7Fj transitions of Eu"' is a sensitive measure of the number of non-radiative OH oscil- lators in the coordination sphere of Eu"', so (i) that the life- time of zmin = 0.110 ms corresponds to the Eu"' cation where all of its nine coordination sites are filled with the non- radiative OH oscillators (i.e., it is a kind of minimum lifetime of these Ed"-specific transitions) and (ii) that the lifetime of z,,, = 2.500 ms corresponds to the Eu"' cation in D 2 0 where all the nine non-radiative OH oscillators have been blocked out from the coordination sphere of Eu"' by D 2 0 molecules (i.e., it is a kind of maximum lifetime of these Eu"'-specific transitions). The lifetime of the radiative 'Do + 7Fj transitions of Eu"' inside the chelating cage of the heterodinuclear LEu"'Yb"'L chelate was, however, found to be even shorter than that of the totally hydrated Eu"' cation, which, in our opinion, provides a solid basis to propose that these Yb"'- induced exceptionally short-lived radiative 'Do --+ 7Fj tran- sitions of Eu"' have to be a result of an intramolecular Eu"'*Ln"' -, Eu"'Ln"'* energy transfer, rather than of an Y b"'-induced structural change of the heterodinuclear LEu"'Yb"'L chelate as compared with the heterodinuclear LEu"'Ln"'L chelates (Ln"' = Er'", Eu"', Gd"', Ho"', Nd"', Pr"' or Y'").

A schematic energy diagram displayed in Fig. 7 was con- structed to explain the Yb"'-induced quenching of the radi- ative 5Do + 7Fj transitions of Eu"' and their exceptionally short lifetime occurring inside the chelating cage of the hetero- dinuclear LEu"Yb"'L chelate: the energy states of L and Eu"' are those discussed in connection with Fig. 2, and the only excited state of Yb"' denoted as 'Fsi2 is located at E = 10420 cm-'. On the basis of this energy diagram, it is obvious that a direct intramolecular T,-relaxation of L to the excited state 2F5/2 of Yb"' is not possible in heterodinuclear LEu"Yb"'L chelate, because of much larger energy gap AE(T1 - 2F7/2) = 11 560 cm-' for Yb"' than for Eu"' [AE(T, - 'D,) = 490 cm- '1. Thus, energy transfer occurs in heterodinuclear chelate efficiently from T, of L to the closely located excited state 'D, of Eu"' finally to initiate the excited state 'Do of Eu"'. The aqua-bridged Yb"' cation is, however, capable of strongly interacting with this excited state of Eu"', thus resulting in the

Yb "' ligand ligand Eu"'

Fig. 7 LE u"'Y b"' L chelate

A schematic diagram of the energy flow in the heterodinuclear

short-lived radiative 'Do -+ 7Fj transitions of Eu"' with a life- time of ~ L E ~ l j l y b 1 l l L = 0.045 ms. It is tempting to propose that this Yb"'-initiated destabilization of the excited state 'Do of Eu"' to induce the efficient intramolecular 'Do -+ 'F5/2 energy transfer is based on a mechanism of the aqua-bridged Eu"'Yb''' pair being a kind of vibrational mediator, despite the wide energy gap AE('DO + 2F5/2) = 6840 cm-' (see Fig. 7) which does not provide the conventional conditions of strong- ly overlapping emission and excitation spectra. Support for this proposal can be obtained from the work of Crosby and Kashalg who have observed an energy transfer to the excited 'F5/2 level of Yb"' in a 1 : 3 chelate of Yb"' with dibenzoylme- thane over the energy gap AE z 100o0 cm-' (ie., in the cir- cumstances without any strong spectral overlap) and assigned this exceptional and surprisingly efficient intramolecular energy transfer for a strong vibrational coupling of Yb"' to the six carbonyl oxygens of the ligands. Furthermore, no phos- phorescence response was obtained for the homodinuclear LYb"Yb"'L chelate, as well for the other homodinuclear che- lates excluding Gd"' and Y'" used in this work, which can be regarded at least as a partial support for the intramolecular TI-relaxation of L to the excited state 'F5/2 of Yb"' in spite of the large energy gap of AE(T, - 'F,/,) = 11 560 cm-'.

The photoluminescence of chelated Yb"' in aqueous solu- tions has been a less studied subject despite its obvious advan- tages of intense, long-lived and narrow-band radiative 2F5/2 -+ ,F7.,, transitions which, in our opinion, create inter- esting possibilities for its usage as an analytical tool for a variety of purposes.

Conclusion This work reports the formation of self-assembled luminescent heterodinuclear LEu"'Ln"'L chelates (Ln"' = Er"', Gd'", Ho"', Nd'", Pr'", Y"' or Yb"') where two deprotonated phenolic hydroxy groups and four partially deprotonated 2,2'-(methy- lenenitrilo)bis(acetic acid) moieties form a chelating cage con- taining the aqua-bridged Eu"'Ln"' pair coordinated to the all fourteen ligand atoms of the chelating moiety. This Eu"'Ln"' pair can be regarded as the binuclear intramolecular relax- ation centre for the triplet energy T I of L finally to initiate the sensitized radiative 'Do -+ 'Fj transitions of Eu"' and/or the non-radiative iZj -+ iYj transitions of Ln"', depending on the energy states of selected Ln"' cation on the following basis: as to the Eu"'Ln"' relaxation centre, ( i ) where Ln"' is Er"', Ho'", Nd'" or Pr"', the T,-relaxation of L occurs to the appropriate excited levels of Ln"' without any intramolecular energy trans- fer to the excited state 'Dj of Eu"' or Ln"'-initiated destabi- lization of the excited state 'Do of Eu"', (ii) where Ln"' is Gd"' or Y'", the T,-relaxation of L is energetically possible only to the excited level of Eu"' which results in the more efficient radiative 5D0 -+ 7F,j transitions of Eu"' than in the analogous

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 3325

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homodinuclear LEu"'Eu"L chelate and, finally, (iii) where Ln"' is Yb'", the TI-relaxation of L occurs only to the excited level 'Dj of Eu"' which is, however, strongly destabilized by the aqua-bridged Yb"' cation despite the absence of the strongly overlapping spectra of the 'Do -+ 7Fj emission tran- sitions of Eu"' and 2F7/2 + 2F5,t excitation transition of Yb'", thus resulting in the exceptionally short-lived radiative 'Do --+ 7Fj transitions of Eu"' in the chelating cage of the het- erodinuclear LEu"'Yb"'L chelate.

In general, the photoluminescence of chelated Yb"' in aqueous solutions has been a less studied subject despite its analytically beneficial properties, such as its intense and long- lived radiative 2F5/2 + 2F,/2 transition with a narrow emis- sion peak at ca. 1000 nm which, in our opinion, may well provide a feasible basis for its usage as a luminescence tool for trace element analysis; work along these lines is currently in progress in our laboratory.

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PaDer 6/01424K: Received 28th Februarv. 1996

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