Inorganica Chimica Acta 357 (2004) 2345–2348
www.elsevier.com/locate/ica
Phosphorescence of gadolinium(III) chelates under ambient
conditions
Andreas Strasser, Arnd Vogler
*
Institut f€ur Anorganische Chemie, Universit€at Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 2 January 2004; accepted 4 January 2004
Available online 5 March 2004
Abstract
The gadolinium(III) chelates Gd(dtpaH2 ), Gd(hfac)3 , Gd(tta)3 and Gd(qu)3 with dtpa ¼ 1,1,4,7,7-diethylenetriaminepentaacetate, hfac ¼ hexafluoroacetylacetonate, tta ¼ thenoyltrifluoroacetonate and qu ¼ 8-quinolinolate (or oxinate) show a phosphorescence under ambient conditions. While the UV emission of Gd(dtpaH2 ) at kmax ¼ 312 nm comes from a metal-centered ff state, the
bluish (kmax ¼ 462 nm), green (kmax ¼ 505 nm) and red (kmax ¼ 650 nm) luminescence of Gd(hfac)3 , Gd(tta)3 and Gd(qu)3 , respectively, originates from the lowest-energy intraligand triplets.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Gadolinium; Compounds; Phosphorescence
1. Introduction
Lanthanide compounds have attracted much attention
in the field of luminescence spectroscopy. The excited
state behavior of lanthanide ions Ln3þ have been investigated extensively [1–6]. The electronic spectra of Ln3þ
with fn (n ¼ 1–13) electron configuration are dominated
by electronic transitions between f orbitals. Transitions
between f orbitals of Ln3þ are strictly parity-forbidden. In
addition, many ff transitions are also spin-forbidden although spin–orbit coupling facilitates a mixing with spinallowed transitions. Nevertheless, ff bands have rather
small absorption coefficients and the radiative lifetime of
ff states are relatively large. Light absorption by Ln(III)
complexes can be enhanced by suitable ligands which
transfer the excitation energy to emissive Ln3þ ions.
*
Corresponding author. Tel.: +49-941-943-4485; fax: +49-941-9434488.
E-mail address: [email protected] (A. Vogler).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.01.024
However, the ff states of Gd3þ are located at exceptionally high energies owing to the extreme stability of its
half-filled f-shell (f7 ). The lowest-energy ff transition
appears as an emission line at 312 nm. Accordingly,
Gd(III) complexes are frequently characterized by
emissive intraligand (IL) states at lower energies. Owing
to its heavy-atom effect and paramagnetism, Gd3þ induces a strong singlet/triplet mixing in the ligands [7]. It
follows that the IL fluorescence is largely quenched while
the IL phosphorescence is facilitated [8–11]. The IL luminescence of Gd(III) complexes is usually studied at
low temperatures, mostly at 77 K. Quite recently, we
reported that GdCp3 with Cp ¼ cyclopentadienyl shows
an IL phosphorescence at r.t. [12]. It follows that IL
triplets of gadolinium complexes may generally emit
under ambient conditions. We explored this possibility
and selected the chelates Gd(dtpaH2 ), Gd(hfac)3 ,
Gd(tta)3 and Gd(qu)3 with dtpa ¼ 1,1,4,7,7-diethylenetriaminepentaacetate,
hfac ¼ hexafluoroacetylacetonate, tta ¼ thenoyltrifluoroacetonate and qu ¼
8-quinolinolate (or oxinate) for the present study.
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A. Strasser, A. Vogler / Inorganica Chimica Acta 357 (2004) 2345–2348
mg, 1.14 mmol), sodium acetate (1 g) in 5 ml water and
Htta (760 mg, 3.42 mmol) in 8 ml ethanol were used.
Yield: 520 mg. Anal. Calc. for C24 H15 O7:5 F9 S3 Gd: C,
34.00; H, 1.78. Found: C, 34.03; H, 1.75%.
O
O
-O
H2C
C
H2
N
C
H2
C
H2
N
H2C
O
-
O
2.2. Methods
OH
2
Absorption spectra were measured with a Kontron
Uvikon 932 double beam spectrophotometer. Emission
and excitation spectra were recorded on a Hitachi 850
spectrofluorometer equipped with a Hamamatsu 928
photomultiplier. A band pass of 5 nm was employed.
The luminescence spectra were corrected for monochromator and photomultiplier efficiency.
dtpaH2
F3C
-
O
F3C
-
O
N
O
O
O-
S
3. Results
F3C
hfac
tta
qu
The ligands were chosen according to different emission colours (UV, blue, green and red) of the gadolinium
complexes. Various applications are conceivable. For
example, triplet emissions under ambient conditions
could be utilized in organic light emitting diodes
(OLED) and sensor technology.
2. Experimental
2.1. Materials
All solvents used for spectroscopic measurements
were of spectrograde quality. For polymer films a
polyester resin was used as a commercial product
(Paraloid B72 from Roth, Germany). The compounds
Gd(dtpaH2 ) xH2 O (Aldrich), Hhfac (ABCR), Htta
(Aldrich), Hqu (Merck) were commercially available
and used without further purification. The complex
Gd(qu)3 was prepared according to literature methods
[13]. The synthesis of the diketonates Gd(hfac)3 and
Gd(tta)3 was modified.
The electronic spectrum of Gd(dptaH2 ) (Fig. 1)
shows a weak structured absorption at kmax ¼ 273 nm
(e ¼ 2 l mol1 cm1 ) and a continuous absorption at
shorter wavelength. The emission spectrum (Fig. 1)
displays a narrow band at kmax ¼ 312 nm. The absorption spectra of Gd(hfac)3 and Gd(tta)3 (Figs. 2 and 3)
contain bands at kmax ¼ 302 nm (e ¼ 30 000 l mol1
cm1 ) and kmax ¼ 268 (20 000), 337 (58 000) nm, respectively. Both complexes emit only very weakly in
fluid solution (e.g., acetonitrile) but exhibit a strong
luminescence in a rigid polymer matrix with kmax ¼ 462
nm for Gd(hfac)3 and kmax ¼ 505 nm for Gd(tta)3 . The
complex Gd(qu)3 shows a long-wavelength absorption
at kmax ¼ 369 nm and an emission at kmax ¼ 510 nm
with a shoulder at 650 nm (Fig. 4). In the presence of
air this shoulder disappears. Gd(qu)3 undergoes a decomposition in solution as indicated by a concomitant
change of the absorption spectrum. The excitation band
2.1.1. Synthesis of Gd(hfac)3 3H2 O
To a solution of GdCl3 2H2 O (300 mg, 1.14 mmol)
and sodium acetate (1 g) in 5 ml water was added
dropwise under stirring a solution of 0.5 ml (3.52 mmol)
Hhfac in 15 ml ethanol. Upon slow addition of water a
colourless material precipitates. It was collected by filtration, washed with water and dried over P2 O5 . Yield:
100 mg. Anal. Calc. for C15 H9 O9 F18 Gd: C, 21.64; H,
1.09. Found: C, 21.73; H, 1.04%.
2.1.2. Synthesis of Gd(tta)3 1.5H2 O
The same procedure as that for the synthesis of
Gd(hfac)3 was applied. Solutions of GdCl3 2H2 O (300
Fig. 1. Electronic absorption (a) and emission (b) spectrum of
Gd(dtpaH2 ). Absorption: c ¼ 2:6 102 mol l1 ; d ¼ 1 cm; water.
Emission: water; room temperature; kexc ¼ 275 nm.
A. Strasser, A. Vogler / Inorganica Chimica Acta 357 (2004) 2345–2348
2347
of Gd(qu)3 (Fig. 4) at kmax ¼ 370 nm matches nicely to
the absorption of the complex.
4. Discussion
Fig. 2. Electronic absorption (a) and emission (b) spectrum of Gd(hfac)3 .
Absorption: c ¼ 4 105 mol l1 ; d ¼ 1 cm; ethanol. Emission: polymer
matrix of polyester resin; room temperature; kexc ¼ 300 nm.
Fig. 3. Electronic absorption (a) and emission (b) spectrum of
Gd(tta)3 . Absorption: c ¼ 2 105 mol l1 ; d ¼ 1 cm; ethanol.
Emission: polymer matrix of polyester resin; room temperature;
kexc ¼ 370 nm.
Fig. 4. Electronic excitation spectrum (a) and emission spectra (b, c) of
Gd(qu)3 . Excitation: ethanol; room temperature; kem ¼ 650 nm. Emission: air (b) and argon (c) saturated ethanol; room temperature;
kexc ¼ 370 nm.
The complex Gd(dtpaH2 ) is widely used and commercially available as a MRI agent for medical diagnosis. The dtpaH2 ligand does not provide IL excited
states at low energies owing to the absence of a conjugated p-electron system. Accordingly, the IL absorption
of Gd(dtpaH2 ) (Fig. 1) appears only at rather shortwavelength (k < 265 nm). The metal-centered ff absorptions occur at longer wavelength. In particular, the
structured absorption at kmax ¼ 273 nm is rather characteristic [2]. It is thus not surprising that Gd(dtpaH2 )
does not exhibit an IL luminescence, but the typical
narrow UV emission at kmax ¼ 312 nm which belongs to
the spin-forbidden 6 P7=2 ! 8 S7=2 ff transition of the Gd3þ
ion [6]. Since it is associated with a multiplicity change
of two it is equivalent with a phosphorescence (or triplet
emission) of diamagnetic compounds.
The other chelates used in this work have their IL
states available at energies well below the 6 P7=2 ff state.
The longest-wavelength absorption of Gd(hfac)3 ,
Gd(tta)3 and Gd(qu)3 at kmax ¼ 302, 337 and 368 nm,
respectively, are assigned to the lowest-energy spinallowed IL transition of the hfac [8], tta [8] and qu [14]
ligand. Spin forbidden IL absorptions are not observed
since they are apparently too weak.
As previously shown, Gd(hfac)3 [8], Gd(tta)3 [8] and
Gd(qu)3 [15] show an IL phosphorescence at low temperatures. Gd(qu)3 displays an additional fluorescence
at shorter wavelength [15]. However, singlet/triplet
mixing in Gd(III) chelates is so strong that the IL
phosphorescence of these chelates does not only appear
at low temperatures but also under ambient conditions
in analogy to the phosphorescence of many other complexes of heavy metals [16].
The greenish blue phosphorescence of Gd(hfac)3 at
kmax ¼ 470 nm and the green phosphorescence of
Gd(tta)3 at kmax ¼ 510 nm in fluid acetonitrile solution
are very weak. In contrast, these emissions are quite
strong if the compounds are incorporated in a rigid
matrix (Figs. 2 and 3). Generally, Gd(III) complexes
have variable coordination numbers, mainly six and
nine. Accordingly, the chelate structures are certainly
rather flexible. This flexibility may provide a channel for
radiationless deactivation which is restricted in a rigid
matrix. Besides, flexibility of the diketonate ligand itself
might play a role.
In analogy to various other oxinates of heavy metals
[17], Gd(qu)3 shows a fluorescence and a phosphorescence at r.t. (Fig. 4). The red phosphorescence which
appears as a shoulder at 650 nm is quenched by
oxygen. The appearance of a r.t. phosphorescence in
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A. Strasser, A. Vogler / Inorganica Chimica Acta 357 (2004) 2345–2348
solution which can be quenched by O2 is a typical
feature of heavy metal oxinates including the Th(IV)
complex [17].
In summary, several Gd(III) chelates are characterized by a r.t. phosphorescence which extends from the
UV to the red spectral region depending on the ligands.
While the UV luminescence of Gd(dtpaH2 ) originates
from a metal-centered ff state, the bluish, green and red
emissions of Gd(hfac)3 , Gd(tta)3 and Gd(qu)3 , respectively, are of the IL type. Since these triplet emissions
occur under ambient conditions, various applications
are conceivable.
Acknowledgements
Financial support by BASF is gratefully acknowledged.
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