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Photophysical study of the Ca2+-chelator QUIN 2 ligand: effect of divalent and trivalent cations
Volume 179, number 5,6
CHEMICAL PHYSICS LETTERS
3 May 1991
Photophysical study of the Ca2+-chelator QUIN 2 ligand: effect of divalent and trivalent cations M. Guardigli and N. Sabbatini Dipartimento Chimico “G. Ciamician” dell’liniversith Via Seltni 2, 40126 Bologna, Italy Received 14 January I99 1;in final form 14 February I99 1
The photophysical properties of complexes of the CaZ+-chelator QUIN 2 ligand with divalent and trivalent cations have been studied. The absorption of the ligand is almost independent of the nature of the complexing cations, whde the fluorescence emission strongly depends on the electric charge of the cations. Metal emission upon excitation in the l&and has been observed for the Eu3+ complex, but not for the Tb’+ complex.
1. Introduction Systems that change their absorption or emission properties upon binding ions, e.g., chrome- and fluore-ionophores, have been recently developed [ l-41. Nondestructive measurements of intracellular Ca2+ concentration have been performed using such systems [5]. However, these measurements present some drawbacks, e.g., the low selectivity of these compounds against competing cations [ 5 1. In order to improve the sensitivity, new calciumselective chelating agents have been synthesized. One of these ligands, the 2- [ (2-bis- [ carboxymethyllamino-5-methylphenoxy)methyl]d-methoxy-& bis [ carboxymethyllaminoquinoline, QUIN 2 (fig. l), is quite interesting because it shows high selectivity against competing cations like Mg’+ and its luminescence is strongly affected by complexation with the Ca2+ ion. In fact, the luminescence quantum yield of the Ca2+ complex (Q~0.15) is re-
markably higher than that of the free ligand (@x0.025). The luminescence changes have been attributed to different conformations of the I-amino substituent in going from the free ligand to the complex [6]. An important feature of the Ca2+ ion in biological systems is its presence in many proteins or enzymes, where this ion is usually bound in selective binding sites. In biological structures, Ca2+ ions can be suitably replaced by Ln’+ ions, particularly the Eu3+ ion, because they have similar chemical behavior [T-9]. The utility of such a substitution is related to the possibility to obtain important structural information on the Ca’+-binding molecule. In fact, the luminescence properties of Ln3+ ions are affected by the symmetry and nature of the binding groups and by the presence of coordinated water molecules [ 8 1. Another interesting aspect related to the luminescence properties of lanthanide complexes has been recently emphasized. These species have been proposed as luminescent labels because of their particular emission properties, e.g., line-like spectra and long lifetimes [g-lo]. It has been shown that, in order to obtain an intense luminescence, an energytransfer process from an absorbing ligand to the lanthanide ion must take place [ 1l- 131. The factors that determine the efficiency of this process are still not completely understood, so that investigation of novel compounds is of great utility.
cH30qlclLH20~ N(CH ,COOH12
Fig. I Schematic structure of the QUIN 2 ligand. Elsevier Science Publishers B.V. (North-Holland)
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CHEMICAL PHYSICS LETTERS
In this paper, we report on the excited-state properties of complexes of the QUIN 2 ligand with the Ca*+, A13+,and Ln3+ (Ln=Eu, Gd, and Tb) ions. These properties give information on the structure of these species (and the analogous Ca*+ one) and the ligand-metal interaction in the excited state.
3 May 1991
30
7
0 t;
10
2. Materials and methods The complexes of the QUIN 2 ligand have been obtained by adding a stoichiometric amount of the chloride salts of the various cations to a z lop4 M aqueous solution of the tripotassium salt of QUIN 2 at its natural pH ( x 7.5). The changes in the l&and absorption and luminescence spectra upon complexation suggest a 1; 1 stoichiometry for all complexes, as previously reported for the Ca2+ and Mg*+ complexes [ 61. EuCl3.6H2O and TbC13*6Hz0 (Ventron, 99.9%), GdClJ.6H20 (Aldrich, 99.99%), CaC12.2H20 (Aldrich, >98%), AlC13.6Hz0 (Aldrich, 99%), and QUIN 2 tripotassium salt (Sigma, 97Oh)were used. D20 (Carlo Erba) was 99.95% pure and the other solvents (Merck) were spectroscopic grade. Absorption spectra were recorded with a Kontron Uvikon 860 spectrophotometer. Luminescence spectra were measured with a Perkin-Elmer LS5 spectrofluorimeter. Luminescence decays were obtained with a Perkin-Elmer LS5 spectrofluorimeter and an Edinburgh 199 single-photon-counting equipment. The emission quantum yields were obtained by the method described by Haas and Stein [ 141, using as standards Ru(bpy):+ (@=0.028 in aerated water [ 151) for the Eu3+ emission and quinine sulphate (@=0.546 in HzS04 1N [ 161) for the ligand fluorescence.
3. Results and discussion 3.1. Ligand photophysics The absorption spectra of the QUIN 2 ligand and the complexes investigated are reported in fig. 2 and some of the absorption data are collected in table 1. It can be noticed that a blue shift and an increase in intensity take place upon complexation. However, 540
0
300
.__
J
400wll
Fig. 2. Absorption spectra of the QUIN 2 ligand (a) and its corn-plexes with Ca2+ (b ) and Ln’+ (c ) ions (Lx?+= Et?+,Tb”+or Cd’+) in watersolution.
while a large effect is introduced by the presence of the complexing cation, the size and electric charge of the cation are much less relevant. The QUIN 2 ligand presents an intense fluorescence (A,,,= 492 nm, @=0.025) due to rcx* transitions in the quinoline system [ 171. A similar emission is also shown in the CaQUIN 2 complex (&,,,=500 nm, r&0.15) [5]. The fluorescence lifetimes are 1.2 and 10 ns for the free ligand and the Ca*+ complex, respectively (table 1). These results suggest that (i) the energy of the emitting level is scarcely affected by complexation with the Ca*+ ion, (ii) the radiative rate constant of the emitting state (calculated from the quantum yield and lifetime values) is the same for the free ligand and the Ca2+ complex, and (iii) the nonradiative transitions are less efftcient in going from the free ligand to the Ca” complex. We have also studied the ligand fluorescence in the Eu3+, Tb3+, Gd3”, andA13+ complexes (fig. 3). The luminescence observed in the free ligand and the Ca*+ complex is no longer observed, while a new luminescence is present at shorter wavelengths. In this region, a faint emission is also observed in the Ca2+ complex. It can be noticed that the effects produced by complexation on the absorption and fluorescence spectra of the QUIN 2 ligand are very different. In fact, no significant difference is observed in the absorption spectra of complexes with divalent and trivalent cations (fig. 2), while the corresponding emissions are strikingly different (fig. 3). The temperature dependence of the emission of the free li-
CNEMICAL PHYSICS LETTERS
Volume179,number5,6
3May 1991
Table 1 Photophysical properties of the QUIN 2 ligand Compound
CaQUIN 2 GdQUIN 2 EuQUIN 2 TbQUIN 2 ‘) ‘) ‘) t)
1 nlPX (um)
7
0.025
SIOd’
>l
10
0.15
505(4755’)
>I
f)
0.030
520(480”)
aUW (M-i cm-‘)
I nlax (nm)
5 (ns)
@
(nm)
260 355 240 333 245 333 245 333
19800 2600 29500 3000 29500 3600 29500 3600
492(435 “)
1.2
500(360”) 392(390c’) 375 d,
f)
245 333
29500 3600
390
f)
2,
QUIN 2
Phosphorescence ‘)
Fluorescence 0)
Absorption a)
(s)
0.015
In water solution at 300 K. ‘) A,.,=330 nm. In MeOH/EtOH 4 : 1 (v/v) at 77 K (&.,=330 nm). d, Very weak emission. Measured in correspondence with the first feature in the emission spectrum. Undetectable under our experimental conditions.
Fig. 3. Fluorescence spectra of the QIJIN 2 ligand (a) and its complexes with Ca’+ (b), Gd’+ (c) and Al’* (d) ions at (A) 300 Kin water solution and (B) 77 K in MeOH/EtOH 4 : 1 (v/ v) (1,=330nm).
gand and Ca*+ complex is also remarkable. At 77 K, this emission shifts to the spectral region where the emission of the complexes with trivalent cations is detected. No temperature effect is observed either on the emission of the complexes with trivalent cations or on the absorption spectrum of the free ligand. These observations suggest that the effect produced on the ligand by the electric charge of the complexing cations is more important in the excited state than in the ground state. Our interpretation of these
phenomena is as follows: two conformations may be possible for the ligand in the excited state, depending on the electric charge of the complexing cation. These two conformers should exhibit different emissions. In accordance with this viewpoint, at room temperature in the excited state one conformer would be the dominant one in complexes with trivalent cations, while the other would dominate in the free ligand and complexes with divalent cations. The temperature dependence of the fluorescence emission (fig. 3) suggests that at 77 K, the first conformer dominates also in the ligand and the Ca2+ complex. At 77 Kin MeOH/EtOH 4 : 1 solution, the QUIN 2 ligand shows a long-lived emission (I,.,,,, z 5 10 nm, 7> 1 s) which is most likely the phosphorescence of the first conformer #I. An analogous emission is observed in the Gd3+ and Ca*+ complexes (fig. 4 and table 1). From the wavelength of the first feature in the phosphorescence spectrum of the Gd3+ complex, we can obtain an energy of ~20800 cm-’ for the triplet state of the ligand in the complex. No effect of the complexing cation on the energy of the ligand phosphorescence is observed, in agreement with our model. In fact, at 77 K the same conformer should
” The possibility that such an emission might be an E-type delayed fluorescence of the other conformer cannot be excluded.
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Volume 179, number 5,6
CHEMICAL PHYSICS LETTERS
I
3 May 1991
hJu 600
700 a.nm
Fig. 5. Emission spectrum ofthe Eu3+ ion in the EuQUIN 2 complex (water solution, I,,,= 330 nm). L
-I
500
6oo x,nm
Fig. 4. Phosphorescence emission of the QUIN 2 ligand in the GdQUlN 2 complex at 77 K (MeOH/EtOH 4 : I (v/v), A.,,=330 nm).
dominate in all complexes in the excited state. 3.2. Metal emission On the basis of the energies of the singlet and triplet levels of the QUIN 2 ligand, an energy-transfer process from the ligand to the 5D0 Eu3+ (17260 cm-‘) and 5D4 Tb’+ (20400 cm-‘) [ 181 emitting levels may be expected to take place. As a matter of fact, for the EuQUIN 2 complex, the emission of the Eu3+ ion upon ligand excitation is observed. Most likely, the ligand excited state involved in the energy-transfer process is the triplet state which lies x 3500 cm-’ above the ‘Do emitting level of the Eu3+ ion. The short-lived singlet excited state is not expected to be involved in the energy transfer process. The emission spectrum of the Eu3+ ion in the complex is reported in fig. 5. The presence of one peak in the O-+0 transition region suggests that only one Eu3+-containing species is formed upon complexation (of course, the presence of non-emitting Eu3+ species cannot be excluded). The pattern of the emission spectrum suggests that the Eu3+ coordination sphere has a slightly distorted CzVsymmetry, as expected from the CPK model. Finally, the relatively high intensity of the 5D0-‘7F2 (hypersensitive 542
[ 19 ] ) transition reflects a strong ligand-metal interaction. The Eu3+ luminescence lifetimes measured at room temperature upon ligand excitation are 0.045 ms and 0.055 ms in Hz0 and DzO, respectively, and 2.2 ms in D20 at 77 K. The emission quantum yield is 1.5 X 10m3in water solution at room temperature. It can be noticed that the lifetime in Hz0 at 300 K is shorter than that of the aquo ion (~0.11 ms), in which the lifetime is essentially determined by nonradiative deactivation via coordinated HI0 molecules [ 14,201. Since HI0 molecules are partially removed by complexation, longer lifetimes are expected in the complex than in the aquo ion. The short lifetime must be due to the presence of other efficient nonradiative deactivations. The strong temperature dependence of the lifetimes indicates that a thermally activated nonradiative process is present. For Eu3+ complexes, such a process is usualiy attributed to radiationless deactivation via low-lying, ligand-tometal charge-transfer states [ 2 1,22 1. From the lifetimes, rate constant values of 15000 s-’ and 5000 s-’ are obtained for the thermally activated and vibronic deactivation mechanisms, respectively. The thermally activated process is even more efficient than the vibronic deactivation in the aquo ion ( !c,,~(OH) = 8800 s-’ ). The lifetime data in Hz0 and D20 solutions can be used to estimate the number of Hz0 molecules coordinated to the Eu3+ ion by the empirical Horrocks equation [ 8 1. In this case, this number ( a 5) does not seem to be realistic if the
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CHEMICAL PHYSICS LETTERS
steric hindrance of the ligand is considered. Efficient deactivations via the abovementioned, thermally activated process seem to prevent a correct use [23] of the Horrocks relationship. As far as the TbQUIN 2 complex is concerned, no emission of the Tb3+ ion upon ligand excitation is observed at room temperature, contrary to what is expected. The lack of the sensitized emission of the Tb3+ ion may be ascribed to an efficient back-energy-transfer process from the 5D, lb’+ level to the QUIN 2 triplet level, followed by nonradiative losses involving the ligand moiety. This hypothesis is supported by the low value ( x 400 cm-’ ) of the energy gap between the triplet state of the ligand and the emitting level of the metal ion. A weak Tb3+ luminescence is observed at 77 K. The low intensity of this emission may be explained considering that the equilibrium is still effective at this temperature. The role played by the equilibrium between the 5D, level of the Tb3+ ion and the %x* level of the bpy ligand on the intensity of the Tb3+ emission in the ]Tb c bv+xv~bm13+ cryptate has been investigated [ 12,241. Complexes where such an equilibrium cannot be present are expected to exhibit higher emission intensities, Results recently obtained in our laboratory on the [Tb c calix [4] arene tetraamide] 3+ complex appear to agree with this expectation [ 251. Acknowledgement We wish to thank Dr.A. Mecati for performing some measurements. Financial support from the Minister0 della Universith della Ricerca Scientifica is gratefully acknowledged.
3 May 1991
[2] S. Fery-Forgues, M.-T. Le Ihis, J.-P. Guette and B. Valeur, J. Phys. Chem. 92 (1988) 6233. [ 3 ] Chem. Eng. News 45 ( 1987) 26. [4]Chem.Eng.New.s31 (1988)21. [ 51R.Y. Tsien, T. Pozzan and T.J. Rink, J. Cell Biol. 94 (1982) 325. [6] R.Y. Tsien, Biochemistry 19 (1984) 2396. [7] R.J. Williams, Struct. Bonding 50 (1982) 81. [8] W. Dew. Horrocks and M. Albin, in: Progress in inorganic chemistry, Vol. 31, ed. S.J. Lippard (Wiley, New York, 1984) p. 1. [9] J.-C. BiInzli, in: Lanthanide probes in life, medical, and environmental sciences, eds. G.R. Choppin and J.-C. Bunzli (Elsevier, Amsterdam, 1989) ch. 7. [lo] E. Soini and I. Hemmila, Clin. Chem. 25 ( 1979) 353. [ II] B. Alpha, J.-M. Lehn and G. Mathis, Angew. Chem. Intern. Ed. Engl. 26 (1987) 266. 1121N. Sabbatini, S. Perathoner, V. Balzani, B. Alpha and J.-M. Lehn, in: Supramolecular photochemistry, ed. V. Balzani (Reidel, Dordrecht, 1987) p, 187. 1’31B. Alpha, V. Balzani, J.-M. Lehn, S. Perathoner and N. Sabbatini, Angew. Chem. Intern. Ed. Engl. 26 ( 1987) 1266. [l4 Y. Haas and G. Stein, J. Phys. Chem. 75 ( 1971) 3668. ]l5 K. Nakamaru, Bull. Chem. Sot. Japan 55 (1982) 2697. [ 161S.R. Meech and D. Phillips, J. Photochem. 23 ( 1983) 193. [ 171 I.B. Berlman, Handbook of fluorescence spectra of aromatic molecules (Academic Press, New York, 1975) [IS] R. Reisfeld and C.K. Jorgensen, in: Inorganic chemistry concepts, Vol. 1, eds. M. Becke, M.F. Lappert, J.L. Margrave, R.W. Parry, C.K. Jorgensen, S.J. Lippard, K. Niedeuzu and H. Yamatera (Springer, Berlin, 1977) p. 1. [ 191C.K. Jorgensen and B.R. Judd, Mol. Phys. 8 (1965) 281. [ 201 J.-C. Bilnzli and J.-R. Yenin, Inorg. Chem. 18 (1979) 605. [ 211 G. Blasse, Struct. Bonding 26 ( 1976) 43. [ 221 G. Blasse, in: Handbook on the physics and chemistry of rare earths, Vol. 3, eds. K.A. Gschneidner and L. Eyring (North-Holland, Amsterdam, 1979) p. 171. [ 231 W.DeW. Horrocks and D.R. Sudnick, Accounts Chem. Res. 14 (1981) 384. [ 241 B. Alpha, R. Ballardini, V. Balzani, J.-M. Lehn, S. Perathoner and N. Sabbatini, Photocbem. Photobiol. 52 ( 1990) 299. [25] N. Sabbatini, M. Guard@, A. Mecati, V. Balzani, R. Ungaro, E? Ghidini, A. Casnati and A. Pochini, J. Chem. Sot. Chem. Commun. ( 1990) 878.
References [ I] H.G. Liihr and F. Viigtle, Accounts Chem. Res. 18 ( 1985) 65.
543
CHEMICAL PHYSICS LETTERS
3 May 1991
Photophysical study of the Ca2+-chelator QUIN 2 ligand: effect of divalent and trivalent cations M. Guardigli and N. Sabbatini Dipartimento Chimico “G. Ciamician” dell’liniversith Via Seltni 2, 40126 Bologna, Italy Received 14 January I99 1;in final form 14 February I99 1
The photophysical properties of complexes of the CaZ+-chelator QUIN 2 ligand with divalent and trivalent cations have been studied. The absorption of the ligand is almost independent of the nature of the complexing cations, whde the fluorescence emission strongly depends on the electric charge of the cations. Metal emission upon excitation in the l&and has been observed for the Eu3+ complex, but not for the Tb’+ complex.
1. Introduction Systems that change their absorption or emission properties upon binding ions, e.g., chrome- and fluore-ionophores, have been recently developed [ l-41. Nondestructive measurements of intracellular Ca2+ concentration have been performed using such systems [5]. However, these measurements present some drawbacks, e.g., the low selectivity of these compounds against competing cations [ 5 1. In order to improve the sensitivity, new calciumselective chelating agents have been synthesized. One of these ligands, the 2- [ (2-bis- [ carboxymethyllamino-5-methylphenoxy)methyl]d-methoxy-& bis [ carboxymethyllaminoquinoline, QUIN 2 (fig. l), is quite interesting because it shows high selectivity against competing cations like Mg’+ and its luminescence is strongly affected by complexation with the Ca2+ ion. In fact, the luminescence quantum yield of the Ca2+ complex (Q~0.15) is re-
markably higher than that of the free ligand (@x0.025). The luminescence changes have been attributed to different conformations of the I-amino substituent in going from the free ligand to the complex [6]. An important feature of the Ca2+ ion in biological systems is its presence in many proteins or enzymes, where this ion is usually bound in selective binding sites. In biological structures, Ca2+ ions can be suitably replaced by Ln’+ ions, particularly the Eu3+ ion, because they have similar chemical behavior [T-9]. The utility of such a substitution is related to the possibility to obtain important structural information on the Ca’+-binding molecule. In fact, the luminescence properties of Ln3+ ions are affected by the symmetry and nature of the binding groups and by the presence of coordinated water molecules [ 8 1. Another interesting aspect related to the luminescence properties of lanthanide complexes has been recently emphasized. These species have been proposed as luminescent labels because of their particular emission properties, e.g., line-like spectra and long lifetimes [g-lo]. It has been shown that, in order to obtain an intense luminescence, an energytransfer process from an absorbing ligand to the lanthanide ion must take place [ 1l- 131. The factors that determine the efficiency of this process are still not completely understood, so that investigation of novel compounds is of great utility.
cH30qlclLH20~ N(CH ,COOH12
Fig. I Schematic structure of the QUIN 2 ligand. Elsevier Science Publishers B.V. (North-Holland)
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CHEMICAL PHYSICS LETTERS
In this paper, we report on the excited-state properties of complexes of the QUIN 2 ligand with the Ca*+, A13+,and Ln3+ (Ln=Eu, Gd, and Tb) ions. These properties give information on the structure of these species (and the analogous Ca*+ one) and the ligand-metal interaction in the excited state.
3 May 1991
30
7
0 t;
10
2. Materials and methods The complexes of the QUIN 2 ligand have been obtained by adding a stoichiometric amount of the chloride salts of the various cations to a z lop4 M aqueous solution of the tripotassium salt of QUIN 2 at its natural pH ( x 7.5). The changes in the l&and absorption and luminescence spectra upon complexation suggest a 1; 1 stoichiometry for all complexes, as previously reported for the Ca2+ and Mg*+ complexes [ 61. EuCl3.6H2O and TbC13*6Hz0 (Ventron, 99.9%), GdClJ.6H20 (Aldrich, 99.99%), CaC12.2H20 (Aldrich, >98%), AlC13.6Hz0 (Aldrich, 99%), and QUIN 2 tripotassium salt (Sigma, 97Oh)were used. D20 (Carlo Erba) was 99.95% pure and the other solvents (Merck) were spectroscopic grade. Absorption spectra were recorded with a Kontron Uvikon 860 spectrophotometer. Luminescence spectra were measured with a Perkin-Elmer LS5 spectrofluorimeter. Luminescence decays were obtained with a Perkin-Elmer LS5 spectrofluorimeter and an Edinburgh 199 single-photon-counting equipment. The emission quantum yields were obtained by the method described by Haas and Stein [ 141, using as standards Ru(bpy):+ (@=0.028 in aerated water [ 151) for the Eu3+ emission and quinine sulphate (@=0.546 in HzS04 1N [ 161) for the ligand fluorescence.
3. Results and discussion 3.1. Ligand photophysics The absorption spectra of the QUIN 2 ligand and the complexes investigated are reported in fig. 2 and some of the absorption data are collected in table 1. It can be noticed that a blue shift and an increase in intensity take place upon complexation. However, 540
0
300
.__
J
400wll
Fig. 2. Absorption spectra of the QUIN 2 ligand (a) and its corn-plexes with Ca2+ (b ) and Ln’+ (c ) ions (Lx?+= Et?+,Tb”+or Cd’+) in watersolution.
while a large effect is introduced by the presence of the complexing cation, the size and electric charge of the cation are much less relevant. The QUIN 2 ligand presents an intense fluorescence (A,,,= 492 nm, @=0.025) due to rcx* transitions in the quinoline system [ 171. A similar emission is also shown in the CaQUIN 2 complex (&,,,=500 nm, r&0.15) [5]. The fluorescence lifetimes are 1.2 and 10 ns for the free ligand and the Ca*+ complex, respectively (table 1). These results suggest that (i) the energy of the emitting level is scarcely affected by complexation with the Ca*+ ion, (ii) the radiative rate constant of the emitting state (calculated from the quantum yield and lifetime values) is the same for the free ligand and the Ca2+ complex, and (iii) the nonradiative transitions are less efftcient in going from the free ligand to the Ca” complex. We have also studied the ligand fluorescence in the Eu3+, Tb3+, Gd3”, andA13+ complexes (fig. 3). The luminescence observed in the free ligand and the Ca*+ complex is no longer observed, while a new luminescence is present at shorter wavelengths. In this region, a faint emission is also observed in the Ca2+ complex. It can be noticed that the effects produced by complexation on the absorption and fluorescence spectra of the QUIN 2 ligand are very different. In fact, no significant difference is observed in the absorption spectra of complexes with divalent and trivalent cations (fig. 2), while the corresponding emissions are strikingly different (fig. 3). The temperature dependence of the emission of the free li-
CNEMICAL PHYSICS LETTERS
Volume179,number5,6
3May 1991
Table 1 Photophysical properties of the QUIN 2 ligand Compound
CaQUIN 2 GdQUIN 2 EuQUIN 2 TbQUIN 2 ‘) ‘) ‘) t)
1 nlPX (um)
7
0.025
SIOd’
>l
10
0.15
505(4755’)
>I
f)
0.030
520(480”)
aUW (M-i cm-‘)
I nlax (nm)
5 (ns)
@
(nm)
260 355 240 333 245 333 245 333
19800 2600 29500 3000 29500 3600 29500 3600
492(435 “)
1.2
500(360”) 392(390c’) 375 d,
f)
245 333
29500 3600
390
f)
2,
QUIN 2
Phosphorescence ‘)
Fluorescence 0)
Absorption a)
(s)
0.015
In water solution at 300 K. ‘) A,.,=330 nm. In MeOH/EtOH 4 : 1 (v/v) at 77 K (&.,=330 nm). d, Very weak emission. Measured in correspondence with the first feature in the emission spectrum. Undetectable under our experimental conditions.
Fig. 3. Fluorescence spectra of the QIJIN 2 ligand (a) and its complexes with Ca’+ (b), Gd’+ (c) and Al’* (d) ions at (A) 300 Kin water solution and (B) 77 K in MeOH/EtOH 4 : 1 (v/ v) (1,=330nm).
gand and Ca*+ complex is also remarkable. At 77 K, this emission shifts to the spectral region where the emission of the complexes with trivalent cations is detected. No temperature effect is observed either on the emission of the complexes with trivalent cations or on the absorption spectrum of the free ligand. These observations suggest that the effect produced on the ligand by the electric charge of the complexing cations is more important in the excited state than in the ground state. Our interpretation of these
phenomena is as follows: two conformations may be possible for the ligand in the excited state, depending on the electric charge of the complexing cation. These two conformers should exhibit different emissions. In accordance with this viewpoint, at room temperature in the excited state one conformer would be the dominant one in complexes with trivalent cations, while the other would dominate in the free ligand and complexes with divalent cations. The temperature dependence of the fluorescence emission (fig. 3) suggests that at 77 K, the first conformer dominates also in the ligand and the Ca2+ complex. At 77 Kin MeOH/EtOH 4 : 1 solution, the QUIN 2 ligand shows a long-lived emission (I,.,,,, z 5 10 nm, 7> 1 s) which is most likely the phosphorescence of the first conformer #I. An analogous emission is observed in the Gd3+ and Ca*+ complexes (fig. 4 and table 1). From the wavelength of the first feature in the phosphorescence spectrum of the Gd3+ complex, we can obtain an energy of ~20800 cm-’ for the triplet state of the ligand in the complex. No effect of the complexing cation on the energy of the ligand phosphorescence is observed, in agreement with our model. In fact, at 77 K the same conformer should
” The possibility that such an emission might be an E-type delayed fluorescence of the other conformer cannot be excluded.
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Volume 179, number 5,6
CHEMICAL PHYSICS LETTERS
I
3 May 1991
hJu 600
700 a.nm
Fig. 5. Emission spectrum ofthe Eu3+ ion in the EuQUIN 2 complex (water solution, I,,,= 330 nm). L
-I
500
6oo x,nm
Fig. 4. Phosphorescence emission of the QUIN 2 ligand in the GdQUlN 2 complex at 77 K (MeOH/EtOH 4 : I (v/v), A.,,=330 nm).
dominate in all complexes in the excited state. 3.2. Metal emission On the basis of the energies of the singlet and triplet levels of the QUIN 2 ligand, an energy-transfer process from the ligand to the 5D0 Eu3+ (17260 cm-‘) and 5D4 Tb’+ (20400 cm-‘) [ 181 emitting levels may be expected to take place. As a matter of fact, for the EuQUIN 2 complex, the emission of the Eu3+ ion upon ligand excitation is observed. Most likely, the ligand excited state involved in the energy-transfer process is the triplet state which lies x 3500 cm-’ above the ‘Do emitting level of the Eu3+ ion. The short-lived singlet excited state is not expected to be involved in the energy transfer process. The emission spectrum of the Eu3+ ion in the complex is reported in fig. 5. The presence of one peak in the O-+0 transition region suggests that only one Eu3+-containing species is formed upon complexation (of course, the presence of non-emitting Eu3+ species cannot be excluded). The pattern of the emission spectrum suggests that the Eu3+ coordination sphere has a slightly distorted CzVsymmetry, as expected from the CPK model. Finally, the relatively high intensity of the 5D0-‘7F2 (hypersensitive 542
[ 19 ] ) transition reflects a strong ligand-metal interaction. The Eu3+ luminescence lifetimes measured at room temperature upon ligand excitation are 0.045 ms and 0.055 ms in Hz0 and DzO, respectively, and 2.2 ms in D20 at 77 K. The emission quantum yield is 1.5 X 10m3in water solution at room temperature. It can be noticed that the lifetime in Hz0 at 300 K is shorter than that of the aquo ion (~0.11 ms), in which the lifetime is essentially determined by nonradiative deactivation via coordinated HI0 molecules [ 14,201. Since HI0 molecules are partially removed by complexation, longer lifetimes are expected in the complex than in the aquo ion. The short lifetime must be due to the presence of other efficient nonradiative deactivations. The strong temperature dependence of the lifetimes indicates that a thermally activated nonradiative process is present. For Eu3+ complexes, such a process is usualiy attributed to radiationless deactivation via low-lying, ligand-tometal charge-transfer states [ 2 1,22 1. From the lifetimes, rate constant values of 15000 s-’ and 5000 s-’ are obtained for the thermally activated and vibronic deactivation mechanisms, respectively. The thermally activated process is even more efficient than the vibronic deactivation in the aquo ion ( !c,,~(OH) = 8800 s-’ ). The lifetime data in Hz0 and D20 solutions can be used to estimate the number of Hz0 molecules coordinated to the Eu3+ ion by the empirical Horrocks equation [ 8 1. In this case, this number ( a 5) does not seem to be realistic if the
Volume 179, number 56
CHEMICAL PHYSICS LETTERS
steric hindrance of the ligand is considered. Efficient deactivations via the abovementioned, thermally activated process seem to prevent a correct use [23] of the Horrocks relationship. As far as the TbQUIN 2 complex is concerned, no emission of the Tb3+ ion upon ligand excitation is observed at room temperature, contrary to what is expected. The lack of the sensitized emission of the Tb3+ ion may be ascribed to an efficient back-energy-transfer process from the 5D, lb’+ level to the QUIN 2 triplet level, followed by nonradiative losses involving the ligand moiety. This hypothesis is supported by the low value ( x 400 cm-’ ) of the energy gap between the triplet state of the ligand and the emitting level of the metal ion. A weak Tb3+ luminescence is observed at 77 K. The low intensity of this emission may be explained considering that the equilibrium is still effective at this temperature. The role played by the equilibrium between the 5D, level of the Tb3+ ion and the %x* level of the bpy ligand on the intensity of the Tb3+ emission in the ]Tb c bv+xv~bm13+ cryptate has been investigated [ 12,241. Complexes where such an equilibrium cannot be present are expected to exhibit higher emission intensities, Results recently obtained in our laboratory on the [Tb c calix [4] arene tetraamide] 3+ complex appear to agree with this expectation [ 251. Acknowledgement We wish to thank Dr.A. Mecati for performing some measurements. Financial support from the Minister0 della Universith della Ricerca Scientifica is gratefully acknowledged.
3 May 1991
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