Download Excited state properties of lanthanide complexes: Beyond ff states

Inorganica Chimica Acta 359 (2006) 4130–4138
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Excited state properties of lanthanide complexes: Beyond ff states
Arnd Vogler *, Horst Kunkely
Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany
Received 3 May 2006; accepted 27 May 2006
Available online 7 June 2006
Abstract
Generally, metal-centered ff states dominate the discussion of the excited state properties of lanthanide complexes. In particular, the
luminescence properties of Eu(III) and Tb(III) compounds have been studied in great detail for many decades. However, other types of
excited states such as MC fd, MLCT, LMCT, MMCT and IL are also of interest. In this context, we have recently examined the excited
state behavior of selected Ce(III), Ce(IV), Eu(II) and Gd(III) complexes which are luminescent and/or photoreactive.
2006 Elsevier B.V. All rights reserved.
Keywords: Electronic spectra; Luminescence; Photochemistry; Lanthanides; Cerium; Europium; Gadolinium
1. Introduction
Lanthanide (Ln) compounds play an important role in
the field of luminescence spectroscopy. In the ground
states, the electron configuration of lanthanide cations
extends from f0 to f14. All lanthanides form stable compounds in the oxidation state III, representing the ground
state configuration f1 (Ce3+) to f14 (Lu3+). Moreover, the
empty (f0: Ce4+), half-filled (f7: Eu2+, Gd3+, Tb4+) and
the completely filled f shell (f14: Yb2+, Lu3+) are also stable
and are of special importance. The majority of spectroscopic studies deals with Ln(III) compounds, which are
characterized by electronic transitions within the 4f shell
[1–6]. Since the f electrons are largely shielded from the
environment, they behave as inner and not valence electrons. Accordingly, the absorption and emission spectra
consist very narrow bands.
Transitions between f orbitals of Ln3+ are strictly parity
forbidden. Moreover, many ff transitions are also spinforbidden although spin–orbit coupling attenuates the
forbiddenness. Nevertheless, both restrictions have important consequences. The bands have very low absorption
*
Corresponding author. Tel.: +49 941 943 4716.
E-mail address: [email protected] (A. Vogler).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.05.025
coefficients and the radiative lifetimes of ff states are rather
large (103 s). Owing to the small absorption coefficients
of Ln3+, the excitation can be facilitated by suitable ligands
which absorb the light and subsequently transfer the excitation energy to the emissive Ln3+ ion. In addition, appropriate ligands may prevent radiationless deactivations. This
behavior is illustrated by various Eu3+ and Tb3+ complexes, which emit an intense red and green luminescence,
respectively [7,8], e.g.
EuIII(TTA)3
TTA = thenoyl-trifluoro-acetonate
TbIII(acac)3
acac = acetylacetonate
kmax = 612 nm,
acetone, r.t.
/ = 0.56, s = 565 ls
kmax = 543 nm,
ethanol, r.t.
/ = 0.19, s = 820 ls
In the following sections, any further discussion of the ff
states is omitted. For more details, the reader is referred to
an extensive body of literature which is summarized in various books and reviews [1–6]. In our short report, we
emphasize some other types of excited states: MC (metalcentered) fd, MLCT (metal-to-ligand charge transfer),
LMCT (ligand-to-metal charge transfer), MMCT (metalto-metal charge transfer) and IL (intraligand) states. How-
A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
ever, these topics are not comprehensively covered. Our
account is essentially restricted to recent observations in
our laboratory.
2. Ln(III)
2.1. MC fd and MLCT states
In addition to ff transitions, MC fd transitions are principally accessible for lanthanide ions. Generally, they occur
at energies which are much higher than those of ff transitions. However, the spectroscopy of Ce(III) [6,9–15] is quite
different from that of other Ln(III) compounds. The lowestenergy transition of Ce(III) involves the promotion of an
electron from the 4f to the 5d orbitals. Since the ground
state and the excited states of this f1 ion are spin doublets,
all transitions are spin-allowed. The corresponding absorptions appear in the UV spectral region [9,10]. The emission
from this metal-centered fd state consists of essentially two
bands which are split by ca. 2000 cm1 owing to spin–orbit
coupling. Generally, this emission occurs in the UV and/or
in the blue spectral region but can be shifted to much longer
wavelength depending on the environment of the Ce3+ ion
[11–15]. Any reliable explanation for this shift is not available, but it has been emphasized that it is a consequence
of the interaction with cerium 5d orbitals since the 4f orbitals are hardly affected by the environment. The d-orbital
splitting was attributed to crystal field effects. In addition,
covalency has been mentioned as a further influence. Unfortunately, these notions have never been related to simple
MO models. However, it has been pointed out that the
metal-centered fd transition can be viewed also as a MLCT
transition since the 5d orbitals are rather diffuse and extend
to the ligands of Ce(III) [16].
In this context, we have recently studied the electronic
spectra of cerium(III) halides [17]. The emission spectra
of solid anhydrous CeCl3, CeBr3 (Fig. 1) and CeI3 display
a rather simple pattern. The emission is relatively intense
also at r.t., but the bands are better resolved at 77 K. They
appear at kmax = 340 and 362 nm for CeCl3, 362 and
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390 nm for CeBr3, 464 and 514 nm for CeI3. For all three
compounds, the separation of both emission maxima
amounts to approximately 2000 cm1 corresponding to
the energy difference of both f states (2F7/2 and 2F5/2). In
the excitation spectra, these transitions appear as longest
wavelength bands at kmax = 294 and 312 nm for CeCl3,
295 and 325 nm for CeBr3 (Fig. 1), 384 and 417 nm for
CeI3.
What is the reason for the red shift of the excitation and
emission in the series CeCl3, CeBr3 and CeI3? The cerium
5d orbitals must be modified by overlap with the valence
orbitals of the halide ligands. Generally, these are the nsnp
orbitals which are filled. They are certainly located at much
lower energies than the empty Ce 5d orbital. It follows that
this interaction should shift the fd transitions to higher
energies from CeCl3 to CeI3 since the s and p orbital energies increase from Cl to I. On the contrary, the emission
shows the opposite behavior. We suggest that the valence
orbitals of X are so stable and contracted that their overlap with the diffuse, high energy 5d orbitals of Ce3+ is negligible. However, the empty 3d, 4d and 5d orbitals of Cl,
Br and I, respectively, are also located at quite high energies and are well-suited for the overlap with the 5d metal
orbitals (Scheme 1).
The energy of the empty halide d-orbital should increase
from Cl to Br to I. In the case of Cl, the 3d-orbital
energy is apparently much higher than that of the Ce3+
5d orbital. Accordingly, the overlap is also moderate. For
CeI3, the 5d orbital energy of I may come close to that
Ce
−
III
3+
Ce −X
X
nd
5d
E
A
4f
Qualitative MO scheme for CeIII -halide complexes including the lowest-energy transition
Fig. 1. Electronic excitation (kem = 350 nm) and emission (kexc = 290 nm)
spectrum of solid CeBr3 under argon at 77 K, intensity in arbitrary units.
in absorption (A) and emission (E)
Scheme 1.
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of the 5d Ce orbital. The overlap now becomes much larger. As a result, the lowest-energy empty MO of CeX3 must
shift to lower energies from X = Cl to Br and to I in agreement with our observation. Simultaneously, this MO contains an increasing nd halide contribution in this order. It
follows that in the case of CeCl3 the lowest-energy transition may still be considered to be largely metal-based, while
for CeI3 a considerable 4f(Ce) ! 5d(I) MLCT contribution must be taken into account. On the basis of this model,
it is also concluded that in the ground state metal–halide
interaction is ionic while in the fd/MLCT excited state
metal–ligand bonding exists, but it should be rather weak
since it is caused by just one single electron. In this context,
it is of interest to if low-energy MLCT states of Ce(III)
complexes with conventional CT accepting ligands can also
be observed.
Metal-to-ligand charge transfer (MLCT) excited states
play a very important role in the photophysics and photochemistry of metal complexes. MLCT states occur at low
energies if a ligand with empty low-energy orbitals is coordinated to an electron-rich metal center. The overwhelming
number of observations have been made on d ! p* MLCT
states of polypyridyl (or 1,2-diimine) complexes with electron-donating transition metals such as Ru(II) [18,19],
Re(I) [19–22] and Cu(I) [19,23,24]. However, the occurence
of MLCT states is not restricted to transition metals.
MLCT bands have also been observed in the electronic
spectra of complexes that contain reducing main group
metals, including Sn(II), Sb(III) and Bi(III) [25]. In contrast to these transition and main group metal compounds,
MLCT states of f-group metal complexes have apparently
not yet been identified. The reason for this lack is not quite
clear, but may be related to the fact that complexes of lanthanides or actinides are only of limited stability. This
applies in particular to complexes with neutral ligands such
as 2,2 0 -bipyridine (bipy) or 9,10-phenanthroline. While
such lanthanide complexes are known [26,27], the affinity
of Ln3+ for these ligands seems to be rather small. The electronic spectra provide evidence for this notion. Generally,
the longest-wavelength band of the bipy ligand undergoes
a distinct red shift upon complex formation [28,29]. However, in the case of Ce(III) bipy complexes such a shift
has not been observed [30], indicating a rather weak elec-
tronic interaction between cerium and bipy. On the other
hand, anionic ligands form relatively stable complexes with
Ln3+, owing to the electrostatic attraction between metal
cations and ligand anions. Accordingly, a complex consisting of a reducing f-group metal cation and an electronaccepting anionic ligand should be a promising candidate
for the observation of an optical MLCT transition. We
explored this possibility and selected the compound CeIII
(pyz-COO)3 (Structure 1) with pyz-COO = pyrazine-2carboxylate for a recent study [31].
This choice was based on the following considerations.
Ce(III) is a one-electron donor of moderate reducing
strength. Pyrazine has been shown to be a rather strong
acceptor for MLCT transitions [32]. As an electron-withdrawing substituent, the carboxylate group of pyz-COO
should even enhance the acceptor strength of pyrazine.
Generally, simple Ce(III) compounds are colourless,
since the metal-centered f ! d transition gives rise to an
absorption in the near UV region [9,10]. The free acid
pyz-COOH or its deprotonated anion pyz-COO is also
colourless, because its absorption appears below 400 nm
[31]. Upon coordination these intraligand bands remain
in the UV region, as indicated by the observation that
ZnII(pyz-COO)2 is a white compound which does not show
any absorption in the visible region [33]. In distinction to
this zinc complex, CeIII(pyz-COO)3 is a yellow substance.
This colour is caused by the longest-wavelength absorption
of the complex at kmax = 388 nm, which extends into the
visible spectral region (Fig. 2). We suggest that this band
belongs to a MLCT transition from the Ce(III) 4f orbitals
to the p* orbitals of the pyz-COO ligand. This assignment
is consistent with the reducing character of Ce(III) and
electron-accepting nature of the pyz-COO ligand. CeIII
(pyz-COO)3 shows a weak blue-green luminescence
(Fig. 2) at kmax = 470 nm [31]. It is assumed to originate
from its MLCT state. Owing to the f1 electron configuration, absorption and emission are both spin-allowed processes. The small Stokes shift of DE = 4497 cm1 reflects
the fact that the f-electron that takes part in the MLCT
transition is hardly involved in any bonding interaction.
N
N
Ce III
O
O
3
Structure 1.
Fig. 2. Electronic absorption (a) and emission (e) spectrum of 1.31 · 103
CeIII(pyz-COO)3 Æ 1.5H2O in CH3CN/DMF = 100/1 at room temperature. Absorption: 1 cm cell (- - -) and 0.01 cm cell (––). Emission:
kexc = 360 nm, intensity in arbitrary units.
A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
The excited state behavior of lanthanide(III) compounds
is not only determined by photophysical processes. Some
Ln3+ ions which can be oxidized to Ln4+ have been shown
to undergo this oxidation also by photolysis [34]. For
example, Ce3+ in an acidic aqueous solution is photooxidized according to the equation [35]
IV
þ
Ce3þ
þ 12H2
aq þ H hm ! Ce
ð1Þ
The nature of the reactive excited state is not quite clear.
Generally, photooxidations of metal complexes which proceed with a concomitant reduction of the solvent are initiated by CTTS (charge transfer to solvent) excitation but a
CTTS absorption of Ce3+ in H2O has apparently not been
identified. However, as mentioned above the MC f ! d
transition of Ce3+ can also be viewed as a special type of
(CeIII ! H2O) MLCT transition, since the 5d orbital of
Ce(III) is very large and diffuse and extends thus to the
water molecules in the first and possibly second coordination sphere of the cerium ion [16]. Nevertheless, the molecular process of the photolysis of Ce3+ in water remains
obscure.
The photooxidation of Ce3+ also takes place in the presence of oxidants such as Cu2+, S2 O8 2 and Eu2+ [34]. In
this case, an excited state electron transfer involving the
fd state of Ce(III) seems to be in operation.
Photooxidations of Ln(III) to Ln(IV) have also been
observed for Tb3+ and Pr3+ [34]. In contrast to Ce(III),
the lowest excited states of Tb(III) and Pr(III) are ff states
which are apparently also able to participate in electron
transfer processes.
2.2. LMCT states
Complexes of the cations Ln3+, which can be reduced
to Ln2+, are expected to display LMCT absorptions at
relatively low energies. Owing to their f7 and f14 configuration, Eu(II) and Yb(II) compounds, respectively, are
quite stable. Moreover, the oxidation state II is also
accessible for Nd, Tm, and in particular for Sm. Indeed,
LMCT absorptions have been detected for complexes of
the corresponding Ln3+ ions [36–38]. However, the
LMCT states and MC ff states have comparable energies.
With less reducing ligands, the ff states may occur below
the LMCT states.
The LMCT states of Ln(III) compounds are apparently
not luminescent, but are frequently reactive [34]. Generally,
LMCT excitation of metal complexes leads to the reduction of the metal ions and the concomitant oxidation of
ligands. In agreement with this notion, the photoreduction
of Eu(III) and Sm(III) to Eu(II) and Sm(II) has been
reported but a clear relationship to LMCT excitation has
been rarely established [34]. Moreover, oxidation products
are often unknown. In addition, an accumulation of Ln(II)
does usually not take place since it undergoes an efficient
reoxidation. With these shortcomings in mind, we have
recently studied Eu3+ in the presence of azide in aqueous
solution [39].
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The system Eu3þ =N3 in water has been selected for various reasons. Since N3 has a rather low optical electronegativity (2.8) [32], N3 to Eu3+ LMCT absorptions should
occur at relatively long wavelengths. Secondary photooxidation of Eu2+ which requires short-wavelength irradiation
could thus be diminished or avoided. Even more important, azide is well known to undergo a rapid irreversible
oxidation to nitrogen. Recombination of the radical pair
Eu2+/N3, which is generated by LMCT excitation of europium(III) azide complexes, is then less efficient. Finally, the
choice of water as solvent offers several advantages. In particular, any interference by organic radicals which are
formed in organic solvents is excluded.
The LMCT band of Eu(III) bromide complexes appears
at 320 nm [36]. Since Br and N3 have the same optical
electronegativity (2.8) [32], the N3 to Eu3+ LMCT absorption is expected to appear at nearly the same wavelength.
Indeed, upon addition of azide to an aqueous solution of
EuCl3 a new band shows up (Fig. 3) at kmax = 324 nm
(e = 43.5) [39]. Upon irradiation (kirr > 300 nm) of an aqueous solution containing 0.01 M EuCl3 Æ 6H2O and 0.05 M
NaN3, photolysis takes place as indicated by an increase
of the absorption above 260 nm. Simultaneously, N2 bubbles evolve. The photolysis of the Eu3+ azide complex proceeds according to the simple equation
2þ
Eu3þ N
þ 1:5N2
3 ! Eu
ð2Þ
2+
The photochemical formation of Eu was confirmed by
luminescence spectroscopy. While aqueous Eu2+ is not
emissive, it forms a fairly stable complex with the crown
ether 15-crown-5 which shows an intense luminescence at
kmax = 432 nm [40]. Indeed, when 15-crown-5 in methanol
is added to the photolyzed solution, this emission is nicely
reproduced (Fig. 3). At kirr = 333 nm, the quantum yield of
the Eu2+ formation is / = 7 · 104. When the photolysis of
the Eu3+ azide complex is performed in the presence of O2,
spectral changes are not observed since the Eu3+ azide
complexes are completely regenerated.
Fig. 3. Absorption spectrum (a) of an aqueous solution of 0.02 M
EuCl3 Æ 6H2O and 0.1 M NaN3. Emission spectrum (e) of this solution
after irradiation and addition of a solution of 15-crown-5 in methanol
(kexc = 320 nm, 1-cm cell, emission intensity in arbitrary units). All
solutions were saturated with argon.
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In the absence of oxygen, Eu2+ ions can be photochemically oxidized by water [34,41,42]. This process prevents
the efficient accumulation of Eu2+ as a final product of
the Eu(III) photoreduction. However, in the region of the
LMCT band of Eu(III) azide complexes the extinction
coefficient of Eu2+ is sufficiently small (e < 500) to restrict
this interference. In contrast, previous studies have been
carried out with EuCl3 or Eu(ClO4)3 in aqueous or alcoholic solution [34,43–45]. In these cases, LMCT excitation
requires much shorter-wavelength irradiation. Since in this
spectral region Eu2+ is strongly absorbing and efficiently
photooxidized (e.g. at 250 nm: e = 1778 and / = 0.2) [41],
the secondary photolysis severely interferes with the primary photoreduction of Eu3+. Moreover, while the azide
radicals as primary photooxidation products undergo a
rapid irreversible decay, chloride atoms which are generated by LMCT excitation of EuCl3 are much more stable
and accordingly favor a recombination (EuCl2 + Cl !
EuCl3). Even if a cage escape of chlorine atoms should
be successful and the formation of Cl2 takes place, a subsequent reoxidation of Eu2+ by Cl2 would certainly occur.
All these unfavorable conditions prevent the accumulation
of Eu2+ as a permanent photoproduct of EuCl3. These
observations may also be of importance for the photoreduction of other Ln3+ ions such as Sm3+ [46].
2.3. MMCT states
Since Ln(III) can serve as electron donor (see MLCT) or
electron acceptor (see LMCT), it can be anticipated that
MMCT transitions in suitable mixed metal systems will
occur. Indeed, Ce3+ and Tb3+ in combination with oxidizing d0 metals such as Ti(IV), V(V) and Ta(V) show
(Ln3+ ! d0) MMCT absorptions [6,47,48]. On the other
hand, the ion pair [EuIII(2.2.1.cryptand)]3+[MII(CN)6]4
with M = Fe, Ru and Os is characterized by MMCT transitions from the reducing cyanide complexes to the oxidizing Ln3+ cation [49,50].
Very little is known about the excited state processes
from such MMCT states. They are apparently not emissive
[6,47]. On the contrary, they quench the emission of other
states which are located at higher energies.
At this point, it should be mentioned that lanthanides
also form deeply coloured mixed-valence compounds
which contain the combination of either Ln3+/Ln4+ or
Ln2+/Ln3+ [47]. However, the extent of mixed-valence
interaction is largely unknown in these cases.
but is of considerable interest. It has been known for a long
time that lanthanides can mediate their heavy-atom effect
to ligands by inducing increased spin–orbit coupling. As
a consequence, the fluorescence of the ligand is quenched
since intersystem crossing becomes faster. For the same
reason, the radiative lifetime of the IL triplet decreases
and the phosphorescence quantum yield grows. However,
previous studies were essentially limited to measurements
at low temperatures, but we have recently shown that this
IL phosphorescence appears also at r.t. [52,53]. This observation is important for potential applications. In order to
observe an IL emission, other excited states of different origin (e.g. MC, CT) must be absent or at least located at
energies well above the emitting IL state. An excellent candidate for this purpose is Gd3+. Owing to the very high stability of its half-filled f shell (f7), ff transitions occur at very
high energies. Since neither Gd2+ nor Gd4+ is accessible,
MLCT or LMCT excited states are also not available.
Finally, the paramagnetism of Gd3+ with 7 unpaired electrons also facilitates intersystem crossing in ligands.
In this context, we have recently examined the emission
behavior of the gadolinium(III) chelates Gd(dtpaH2),
Gd(hfac)3, Gd(tta)3 and Gd(qu)3 with dtpa = 1,1,4,7,7diethylene-triaminepentaacetate, hfac = hexafluoroacetylacetonate, tta = thenoyltrifluoroacetonate and qu =
8-quinolinolate (or oxinate) (Structure 2) under ambient
conditions [52].
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) appears only at rather short-wavelength
(k < 265 nm). The metal-centered ff absorptions occur at
longer wavelength. In particular, the structured absorption
at kmax = 273 nm is rather characteristic. It is thus not surprising that Gd(dtpaH2) does not exhibit an IL luminescence, but rather exhibits the typical narrow UV emission
at kmax = 312 nm which belongs to the spin-forbidden
6
P7/2 ! 8S7/2 ff transition of the Gd3+ ion. Since it is associated with a multiplicity change of two, it is equivalent
with a phosphorescence (or triplet emission) of diamagnetic
compounds.
O
O
H2C
H2
C
O
N
H2
C
H2
C
O
N
H2C
dtpaH2
OH
2.4. IL states
O
2
F 3C
Intraligand transitions play an important role in photochemistry and photophysics of Ln(III) complexes. In particular, the antenna effect has been studied in great detail
[38,51]. This effect refers to the ability of a ligand to absorb
light and to transfer subsequently the excitation energy to
the Ln3+ ion, which finally emits from ff states. We will discuss here another aspect of IL states which is less known
O
F 3C
N
O
O
S
O
O
F 3C
hfac
tta
Structure 2.
qu
A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
The other chelates used in this work have their IL states
available at energies well below the 6P7/2 ff state. The longest-wavelength absorptions of Gd(hfac)3, Gd(tta)3 and
Gd(qu)3 at kmax = 302, 337 and 368 nm, respectively, are
assigned to the lowest-energy spin-allowed IL transition
of the hfac, tta and qu ligands. Spin forbidden IL absorptions are not observed since they are apparently too weak.
As previously shown, Gd(hfac)3 [54], Gd(tta)3 [54] and
Gd(qu)3 [55] show an IL phosphorescence at low temperatures. Gd(qu)3 displays an additional fluorescence at
shorter wavelength. 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 appears under ambient conditions [52] in analogy
to the phosphorescence of many other complexes of heavy
metals [56].
The greenish blue phosphorescence of Gd(hfac)3 at
kmax = 470 nm and the green phosphorescence of Gd(tta)3
at kmax = 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. 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
[57], Gd(qu)3 shows fluorescence at kmax = 510 nm and
phosphorescence at r.t. The red phosphorescence which
appears as a shoulder at 650 nm is quenched by oxygen.
The appearance of a r.t. phosphorescence in solution which
can be quenched by O2 is a typical feature of heavy metal
oxinates [57].
The organometallic compound GdCp3 (Structure 3)
with Cp = cyclopentadienyl is another interesting example
of the heavy-atom effect [53]. Generally, the lanthanides
form LnCp3 complexes which in solution exist as discrete
LnCp3 molecules or as solvates of the type LnCp3L with
L = solvent [58]. The structure of LnCp3 consists of a regular triangle with the centers of the g5-coordinated Cp
planes at the corners and the metal in the middle (D3h symmetry). The LnCp3L complexes have a pseudotetrahedral
structure with a trigonal-pyramidal LnCp3 fragment (C3v).
Since the bonding in LnCp3 is largely ionic, the Cp3 3
ligand frame can be treated separately [59,60]. The interligand interaction in LnCp3 leads to a HOMO/LUMO separation, which is smaller than that of a single Cp ligand.
4135
GdCp3 shows an emission from this type of interligand
excited state (Fig. 4) [53], which is bonding with respect
to the interaction within the Cp3 3 moiety. However,
fluorescence does not occur while a strong green phosphorescence (kmax 500 nm) appears even in solution at r.t.
(/ = 0.2 in ether).
The interligand triplet of solid GdCp3 decays with
s = 2.3 ls. Of course, other LnCp3 complexes with lanthanides which have available low-energy ff states (e.g. TbCp3
and YbCp3) do not show interligand emissions. In this
case, the luminescence originates from MC ff states [61–63].
Recently, we have observed another type of heavy-atom
effect. In this case, it is not transmitted to an organic ligand
but to another metal complex as a whole [64]. With regard
to transition metals, the heavy-atom effect is largely
restricted to the second and third transition series [56].
Accordingly, the phosphorescence of first-row transition
metal complexes has frequently rather long lifetimes. The
emission may be then quite strong at low temperatures
but is absent or only very weak at r.t. However, it is conceivable that a heavy-atom effect can be induced in firstrow transition metal complexes by the introduction of a
suitable second metal. We explored this possibility and
selected the compounds GdIII[MIII(CN)6] with M = Cr
and Co for a recent study [64]. This choice was guided by
the following considerations. The salts K3Cr(CN)6 [65]
and K3Co(CN)6 [66,67] show an intense long-lived LF
(ligand-field) phosphorescence but only at low temperatures. It follows that a strong LF phosphorescence may
appear at r.t. if gadolinium transmits its heavy-atom effect
to Cr(III) and Co(III). This expectation is based on the
observation that Gd(III) coordinates to [Co(CN)6]3 [68]
and [Cr(CN)6]3 [69–72] via the nitrogen atoms. Moreover,
it has been shown that the cyanide bridges mediate an electronic interaction between both metals as indicated by the
magnetic coupling in complexes which contain, for example, the CrIII–CN–GdIII moiety [69–72]. It follows that
L
Gd
Gd
Structure 3.
Fig. 4. Electronic emission (a) and excitation (b) spectrum of GdCp3 in
dry diethylether at room temperature, kexc = 250 nm and kem = 500 nm,
intensity in arbitrary units.
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A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
Fig. 5. Electronic excitation (kmax = 626 nm) and emission (kexc =
300 nm) spectrum of solid Gd[Co(CN)6] at r.t., intensity in arbitrary units.
the compounds Gd[M(CN)6] with M = Cr and Co are
quite promising candidates for the appearance of an
intense LF phosphorescence at r.t.
The electronic spectrum of [Co(CN)6]3 is characterized
by long-wavelength absorptions at kmax = 313 (e =
243 M1 cm1) and 260 (180) nm [73], which belong to
the spin-allowed LF transitions 1A1g ! 1T1g and 1T2g,
respectively [32,73]. The lowest-energy spin-forbidden LF
transition 1A1g ! 3T1g is only observed in absorption as
a shoulder at 385 nm for solid K3[Co(CN)6] at 15 K [67].
However, the 3T1g state is emissive but also only at low
temperatures (T < 77 K) [66,67]. The phosphorescence of
K3[Co(CN)6] appears at kmax = 694 nm with a lifetime of
s = 0.65 ms at 77 K. This lifetime is quite large, since
cobalt as a member of the first transition series does not
exert a strong heavy-atom effect. In solution or low-temperature glasses, [Co(CN)6]3 is not luminescent owing to
a facile photosubstitution which is induced by LF excitation. In contrast to the potassium salt, the solid compound
Gd[Co(CN)6] exhibits a strong LF phosphorescence
(kmax = 626 nm) at r.t. (Fig. 5) [64].
Compared to the potassium salt (kmax = 694 nm), it
undergoes a considerable blue shift. This is a clear indication that Gd3+ does not only act as a counterion, but is also
directly coordinated to cyanide via nitrogen. The formation of a cyanide bridge between Gd3+ and Co3+ is certainly also required for an electronic interaction of both
metals. As a consequence, Gd(III) exerts a strong heavyatom effect at Co(III) which in turn leads to the appearance
of an intense LF phosphorescence at r.t. In agreement with
this assumption, the LF triplet of CoIII in Gd[Co(CN)6]
undergoes a relatively fast decay (s = 7.5 · 107 s). This
type of heavy-atom effect applies also to Gd[Cr(CN)6].
The complex [Cr(CN)6]3 shows a weak phosphorescece
at kmax = 820 nm, which originates from the lowest-energy
LF doublet. For Gd[Cr(CN)6], this LF phosphorescence is
quite intense even at r.t. [64].
respectively. However, Ln4+ ions are oxidizing. Accordingly, complexes with those metals will display low-energy
LMCT absorptions. Studies of excited state properties had
been essentially restricted to Ce(IV) complexes [34]. Since
the f shell is empty, any interference by MC transitions is
excluded.
Numerous Cer(IV) compounds are coloured because
their LMCT bands appear in the visible spectral region.
Such LMCT excited states are apparently not emissive
but reactive [34]. Generally, Ce(IV) complexes undergo a
photoreduction to Ce(III) and a concomitant oxidation
of ligands or the solvent. Recently, we have studied the
excited state behavior of CeIV(tmhd)4 (Structure 4) with
tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionato and made
some surprising observations [74].
In agreement with a previous report, the broad band of
Ce(tmhd)4 at kmax = 372 nm (Fig. 6) is assigned to a tmhd
to Ce(IV) LMCT transition. The shorter-wavelength
absorption at kmax = 276 nm is attributed to a tmhd IL
transition. The photolysis of Ce(tmhd)4 proceeds according
to the equation:
hm=LMCT
CeIV ðtmhdÞ4 ƒƒƒƒ! CeIII ðtmhdÞ3 þ oxidized tmhd
ð3Þ
While the photoredox behavior of Ce(tmhd)4 is not unusual, the luminescence of this complex (Fig. 6) is quite surprising. First of all, in addition to the UO2 2þ ion [56],
(H3C)3 C
(H3C)3 C
C
H
C
C
C
O
O
O
C(CH3)3
C(CH3)3
O
C
O
C
CeIV
HC
C
O
(H3C)3C
O
O
C
(H3C)3C
CH
C(CH3 )3
C
C
H
C(CH3)3
Structure 4.
3. Ln(IV)
The oxidation state IV is relatively stable for cerium and
terbium owing to the electron configuration f0 and f7,
Fig. 6. Electronic absorption (a) and emission (e) spectrum of
3.34 · 105 M CeIV(tmhd)4 in CH3CN at r.t., 1-cm cell. Emission:
kexc = 370 nm, intensity in arbitrary units.
A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
Ce(tmhd)4 seems to be only the second example of an emitting f0 complex. Even more intriguing is the fact that the
luminescence does apparently not originate from the lowest-energy LMCT state since the emission and the longest-wavelength LMCT absorption appear at similar
energies (Fig. 6). We suggest that this emission involves a
radiative transition from a higher-energy LMCT state to
the lowest-energy LMCT state. Ce(IV) has available two
different acceptor orbitals: the 5d orbitals at higher energy
and the 4f orbitals at lower energies. Accordingly, the longest-wavelength band of Ce(tmhd)4 at kmax = 372 nm is
attributed to the (ligand ! 4f CeIV) LMCT transition.
The shorter-wavelength (ligand ! 5d CeIV) LMCT absorption has not been identified but may be obscured by the intense IL band at kmax = 276 nm. However, irrespective of
the nature of the shorter wavelength bands, higher-energy
excitation can lead to the population of the (ligand ! 5d
CeIV) LMCT state. Its radiative transition to the
(ligand ! CeIV 4f) LMCT state is then nothing else but
an emission from a fd excited state of Ce(III). Our suggestion is supported by the observation that [CeIII(tmhd)4]
and CeIII(acac)3 emit at similar energies as CeIV(tmhd)4
[74]. Usually, the emission of Ce(III) compounds is relatively intense and short-lived (ns-range) since it is a spin-allowed process. According to these considerations, the
excited state behavior of Ce(tmhd)4 can now be summarized by the following qualitative energy diagram (Scheme
2).
The population of the LMCT (CeIII d1) state may be
facilitated by its energetic proximity to the IL state and
by the larger extension or the cerium 5d orbitals, which certainly provide a better electronic coupling with the ligands
than the well-shielded 4f orbitals. Since the emission is a
rather rapid process, it apparently competes successfully
with radiationless deactivation including photoredox processes, which are assumed to start from the lowest-energy
LMCT state. This conclusion is supported by the observation that the photolysis of Ce(tmhd)4 is also initiated by
long-wavelength irradiation [74].
IL state
III
1
LMCT(Ce d )
Emission
Absorption
4137
At this point, an analogy between the emission of
CeIV(tmhd)4 and the chemiluminescence of [RuIII(bipy)3]3+
should be emphasized. The luminescence of Ce(tmhd)4 can
be viewed as an emission of Ce(III) from a MC fd excited
state, which is generated by electron transfer from a tmhd
ligand to Ce(IV). This behavior corresponds to that of
[Ru(bipy)3]3+. Electron transfer from strong reductants
yields [Ru(bipy)3]2+ not only in the ground state but also
in the emissive MLCT excited state [18,19].
4. Ln(II)
In agreement with the stability of the f7 and f14 configuration, Eu(II) and Yb(II) compounds are accessible and
well characterized. In addition, Sm(II) plays an important
role in the chemistry of lanthanides. All three M2+ ions
are strongly reducing. Accordingly, the corresponding
Ln(II) complexes are not expected to display LMCT
absorptions. However, with suitable ligands MLCT transitions should occur at low energies, but have not yet been
identified. The reason for this is not quite clear but may
be related to the lack of stability of such Ln(II) complexes.
So we are left with MC transitions. Indeed, compounds of
Sm2+, Eu2+ and Yb2+ are characterized by low-energy MC
fd and ff states which are of comparable energies. It
depends on the environment of Ln2+ if the fd or ff states
are lowest [6].
Ln(II) compounds are frequently emissive and the nature of the emitting state can be recognized by the structure
of the spectrum. In particular, the line-type pattern of ff
transitions is rather characteristic while the emission from
fd states gives broad bands which, however, can show
vibrational features. The majority of Eu(II) compounds
[6] including EuI2 [75] emits from fd states. In addition,
SmI2 and YbI2 also show luminescence from fd states even
in solution at r.t. [75]. In contrast, Sm2+ photochemically
generated in a variety of glasses displays a ff emission
[76–78].
Owing to their reducing character, it is not surprising
that Ln2+ ions are easily photooxidized. In particular,
the photoreactivity of Eu(II) [34] and Sm(II) [79–81]
compounds has been studied in some detail. In analogy
to Ce(III) (see above), the photooxidations of Ln(II)
seem to originate from fd states since the fd transitions
terminate at d orbitals which are exposed to the ligands
or the solvent. Water (or H+) has been shown to be a
photooxidant for Eu2+ [41,42]. Recently, SmI2 has been
applied as a very useful photoreductant in organic synthesis [79].
III 1
LMCT (Ce f )
photoredox reaction
ground state
Scheme 2.
5. Summary
Generally, the discussion of excited state properties of
lanthanide complexes is restricted to MC ff states. In our
short account, other types of excited states are emphasized.
It is shown that the luminescence and photochemistry of
various lanthanide complexes in the oxidation states II,
4138
A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138
III and IV originate from MC fd, MLCT, LMCT and IL
states.
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