Synthesis, stability and sensitised lanthanide luminescence of heterobimetallic d/f terpyridine complexes

Article abstract of DOI:10.1002/ejic.201403100

The synthesis, solution behavior and photophysical properties of several heteronuclear bimetallic d/f complexes that utilise a Ru^II bis-terpyridine moiety as the sensitiser for Eu^III, Nd^III and Yb^III luminescenc

Full text of DOI:10.1002/ejic.201403100

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DOI:10.1002/ejic.201403100
Synthesis, Stability and Sensitised Lanthanide
Luminescence of Heterobimetallic d/f Terpyridine
Complexes
Evan G. Moore,*[ Massimo Benaglia, Giacomo Bergamini,
a,b]
[c]
[b]
and Paola Ceroni[
b]
Keywords: Luminescence / Sensitizers / Lanthanides / Ruthenium / Terpyridine
3
The synthesis, solution behavior and photophysical proper- centred ( LC) excited state of the organic ligand or the triplet
3
ties of several heteronuclear bimetallic d/f complexes that
metal-to-ligand charge transfer ( MLCT) excited state of the
II
III
II
utilise a Ru bis-terpyridine moiety as the sensitiser for Eu ,
Ru complex. Owing to the electronic structure of this an-
tenna and the higher density of metal-centred accepting
III
III
Nd and Yb luminescence are reported and compared to
III
III
a Gd analogue. The former compounds display sensitised
states, a higher sensitization efficiency with Nd was ob-
III
III
emission in the visible and near-infrared (NIR) regions de-
pending on the choice of the LnIII cation. We illustrate that
sensitised lanthanide emission can operate by two distinctly
different pathways that involve either the triplet ligand-
served than with Yb and Eu . Lastly, we have observed
4
2
unusual sensitised emission from the upper ( F5/2, H9/2) ex-
cited state of Nd at approximately 800 nm, which we report
here for the first time from a molecular species in solution.
III
optical fibre lasers and amplifiers for telecommunications.[
7]
Introduction
II
It is well known that complexes of Ru with 2,2Ј-bipyr-
idine (bpy) can be used as visible absorbing antennae for
Indeed, the first
example of a d–f hybrid that shows this effect involved a
Ru (bpy) ] derivative covalently linked to a Ln chelat-
Since then, many groups
have investigated modified Ru –bpy compounds as poten-
Sensitised luminescence from LnIII complexes continues
to emerge as an attractive tool for sensing and imaging.
Indeed, several bioassays have been devised and are being
commercially developed for the detection and quantifica-
tion of analytes such as urate, lactate and citrate,[ which
have important implications in disease. These systems are
[
1,2]
III
III
[8,9]
sensitized Nd and Yb emission.
II
2+
III
[
3,4]
3
[
10]
II
ing m-terphenyl carboxylate.
III
III
based on changes in long-lived Eu and/or Tb lumines-
cence, which, when coupled with time-gated detection,
tial sensitisers. In 2006, Gunnlaugson et al. reported[
11]
a
[
5]
functionalised macrocyclic complex with an appended
Ru(bpy) (phen)] fragment (phen = 1,10-phenanthroline),
which sensitises NIR emission from Nd and Yb . Build-
ing on this complexity, a [Ru(bpy) ] core that links two
Ln chelating DO3A macrocycles (DO3A = 1,4,7,10-tetra-
azacyclododecane-1,4,7-triacetic acid) was recently re-
ported by Faulkner et al. In all these cases, the long-lived
triplet metal-to-ligand charge transfer ( MLCT) excited
can facilitate higher signal-to-noise ratios than organic
compounds by eliminating auto-fluorescence and scattered
excitation. Near-infrared (NIR) emitting cations such as
2+
[
2
III
III
2+
III
III
3
Nd and Yb have also received considerable attention as
III
[6]
luminescent probes. Since their longer wavelength emis-
sion bands are less susceptible to scattering in biological
tissues, and owing to the lack of endogenous NIR-emitting
fluorophores, these compounds have potential applications
in optical tomography (NIR imaging). Other applications
for these complexes also include their incorporation into
[
12]
3
II
state (nanoseconds to microseconds) of the Ru tris-biden-
tate complex acts as a sensitising chromophore.
[13]
By contrast, we have found only a single literature ex-
ample of a Ru complex with 2,2Ј:6Ј,2ЈЈ-terpyridine (tpy)
that acts as a sensitiser for Ln emission. Ostensibly, this
may be due to the notion that the shorter MLCT lifetimes
of Ru complexes with tpy can lead to less efficient energy
cation. However, as reported by
De Cola et al., room-temperature-sensitised luminescence
from Eu has been demonstrated using a bis(ortho)cyclo-
metallated PCP-pincer Ru –terpyridine complex. Further-
more, it was shown that metal-based Ln emission can be
sensitised by means of excitation of either the Ru -centred
II
[
a] School of Chemistry & Molecular Biosciences,
III
University of Queensland,
St Lucia, Brisbane, QLD, 4072, Australia
E-mail: egmoore@uq.edu.au
3
II
http://staff.scmb.uq.edu.au/staff/evan-moore
b] Dipartimento di Chimica “Giacomo Ciamician”,
Università di Bologna,
III
[
[
transfer to the Ln
Via Selmi, 2, 40126 Bologna, Italy
III
c] Istituto per la Sintesi Organica e la Fotoreattivitá (ISOF),
Consiglio Nazionale delle Ricerche,
II
Area della Ricerca, 40129 Bologna, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403100.
III
II
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1
MLCT state or the higher-energy singlet ligand-centred
LC) state.
On the basis of previous work, which has shown very
(1
[14]
1
0 –1
high rate constants (k ≈ 10 s ) for deactivation of an or-
ganic triplet in the presence of a NIR-emitting Ln cation,
III
we were interested to see if low-energy emission from cat-
III
III
ions such as Yb and Nd can be sensitised by using the
II
2+
short-lived emission of a simple [Ru (tpy) ] -type chromo-
2
phore and whether this sensitisation would occur by means
1
3
1
3
of the LC, LC, MLCT or MLCT states. To this end, we
have prepared a series of d–f hybrid materials that utilise a
1,4-aryl-linked tpy derivative {BisTpy = 1,4-di[(2,2Ј:6Ј,2ЈЈ-
terpyridin)-4Ј-yl]benzene}, as shown in Scheme 1, and our
initial results are reported herein.
1
II
2+
Figure 1. Observed H NMR spectra of [(TolTpy)Ru (BisTpy)]
III
3 3 3 2
in CD CN upon titration with Nd (NO ) ·6H O.
In the methyl region, we assign the sharp peak at δ ≈
2
.57 ppm to the TolTpy methyl group. Upon addition of
III
Nd , the intensity of this signal decreases with concomi-
tant formation of two new signals at δ ≈ 2.64 and 2.60 ppm.
The former initially increases in intensity until after the ad-
dition of approximately 0.75 equiv., at which point the latter
becomes predominant. In the aromatic region, the spectra
are more complicated owing to the presence of several over-
lapping aromatic resonances. Nonetheless, several peaks
1
were identified in the 1D H NMR spectra, including three
Scheme 1. Chemical structures and relevant solution equilibria of
II
III 5+
the investigated heterobimetallic [(TolTpy)Ru (BisTpy)Ln
]
well-separated 2H singlet resonances for the differing 3Ј/5Ј-
II
III
II
and heterotrimetallic [(TolTpy)Ru (BisTpy)Ln (BisTpy)Ru - terpyridyl protons at δ ≈ 8.98, 9.03 and 9.12 ppm. Upon
(
TolTpy)]7+ complexes.
III
titration with Nd , the formation of two products can also
be identified from changes in the aromatic proton reso-
nances. Initially, the formation of a transient complex with
a strongly deshielded aromatic peak at δ ≈ 15.1 ppm, ad-
ditional peaks at δ ≈ 11.3, 9.9, 6.9 and a highly shielded
resonance at δ = –14.2 ppm is noted, which disappears after
the addition of approximately 0.75 molar equivalents of
Results and Discussion
III
Nd . Lastly, the appearance of two resonances that appear
during the titration at δ ≈ 11.85 and 11.99 ppm show iden-
tical behaviour to that of the methyl peak region. On the
basis of the expected tridentate coordination of the free ter-
Synthesis
II
The synthesis of [(TolTpy)Ru (BisTpy)](PF ) {TolTpy
6
2
III
pyridine with Ln cations and the titration stoichiometry,
=
4Ј-(p-tolyl)-2,2Ј;6Ј,2ЈЈ-terpyridine, BisTpy
=
1,4-
these results are consistent with the initial formation
di[(2,2Ј:6Ј,2ЈЈ-terpyridin)-4Ј-yl]benzene} has been pre-
II
III
II
[
15]
of a trimetallic [(TolTpy)Ru (BisTpy)Ln (BisTpy)Ru -
viously reported.
Briefly, reaction of TolTpy with
(TolTpy)]7 heteronuclear complex. The addition of more
than approximately 0.75 molar equivalents of Ln ion
+
III
III
Ru Cl ·xH O in EtOH yields the [Ru (TolTpy)Cl ] com-
3
2
3
III
plex, which is treated with BisTpy. Purification by means of
chromatography and precipitation with NH PF yields the
desired antenna chromophore. The corresponding Ln
complexes (Ln = Nd, Eu, Gd, Yb) were readily prepared in
leads to a shift in the equilibrium to favour formation of
4
6
II
III 5+
III
the desired heterobimetallic [(TolTpy)Ru (BisTpy)Ln ]
complex, as shown in Scheme 1. As observed crystallo-
[
16,17]
graphically for similar complexes,
the remaining coor-
dination sites of the Ln cation will be occupied by solvent
CH CN, and a summary of the observed speciation is
3
III
shown in Scheme 1.
–
II
and coordinated NO3 anions.
To monitor complexation of the [(TolTpy)Ru -
2
+
III
1
(BisTpy)] complex with Ln cations, we used H NMR
spectroscopy. Although often broad and difficult to inter-
pret owing to the presence of unpaired 4f electrons on the
Solution Stability
II
2+
metal cation, we were able to successfully monitor complex-
Corresponding titrations of [(TolTpy)Ru (BisTpy)] in
III
ation with Nd , which showed only weak paramagnetic CH CN with Ln(NO ) ·xH O (Ln = Nd, Gd, Eu, Yb)
3
3 3
2
effects. Shown in Figure 1 are the resulting spectra of the using absorption and emission spectroscopy were under-
II
2+
III
[
(TolTpy)Ru (BisTpy)] complex in CD CN and upon ad- taken to quantify the stability constants of Ln complex
3
III
dition of aliquots of Nd (NO ) ·6H O.
formation, reported as log10βMLH values in which M, L and
3
3
2
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H denote the complex stoichiometry in terms of metal, li- vidual values for log β and log10β120 should be consid-
10 110
gand and proton equivalents. The resulting absorption and ered highly correlated owing to the spectral similarity of the
III
[19]
emission spectrum are similar for each of the Ln cations absorbing and emitting LnL and LnL species.
Hence,
2
(
see Figures S1–S3 in the Supporting Information) and the we also report average values, which we note are similar to
III
III
spectra with Gd are shown as an example in Figure 2.
those reported for terpyridine complexes with Ln (ClO )
4 3
[
20]
in CH CN. An increase in stability is observed in moving
3
III
from left to right across the Ln series owing to the in-
creasing Lewis acidic character of the metal.
Computational Studies
To gain further insight into the structural features of the
II
2+
mononuclear [(TolTpy)Ru (BisTpy)] and heterobimetal-
II
III
2+
lic [(TolTpy)Ru (BisTpy)Ln (NO ) (H O) ] complexes,
3
3
2
x
we undertook static density functional theory (DFT) calcu-
lations using the B3LYP/LANLD2Z basis set. However, to
avoid computationally expensive calculations when involv-
III
ing open-shell 4f metals, we have substituted the Ln cat-
Figure 2. Observed change in absorption (left axis) and emission
III
II
2+
ion with a diamagnetic Y , which nonetheless has an iden-
spectra (right axis, λex = 490 nm) for [(TolTpy)Ru (BisTpy)] in
III
III
CH
abs (left axis) and ΔIemis (right axis) for selected wavelengths (indi-
3 3 3 2
CN upon titration with Gd (NO ) ·xH
O. Inset: Values of tical charge and similar ionic radius to the middle Ln
[
21]
Δ
ions.
We have found this approach to be useful in the
III
cated by coloured arrows) versus equivalents of Gd added.
past for estimating the geometry and excited-state energies
[
22]
of metal complexes.
are shown in Figure 3.
The resulting optimised geometries
Generally, we note only slight changes to the absorption
II 1
spectra, particularly for the Ru MLCT peak at 492 nm,
although closer inspection reveals an approximately 5% in-
crease in the intensity of this peak. In the UV region, ob-
served absorption changes are larger, with a decrease in ab-
sorption at 244 nm, an increase in absorption at approxi-
mately 370 nm and two pseudo-isosbestic points at approxi-
mately 234 and 287 nm. These changes can be attributed
primarily to a change from the “trans–trans” to “cis–cis”
configuration of non-coordinated N-donor atoms on the
III
BisTpy ligand upon Ln complex formation. The observed
emission spectra are also quite similar upon titration with
III
III
each of the Ln cations (data for Gd shown in Figure 2),
thus demonstrating an approximately 30–50% enhancement
II 3
in the Ru MLCT emission intensity (see Table 2) together
with a small redshift in the peak maximum from approxi-
mately 660 to 670 nm upon complex formation. Analysis
of these titration results using pHAb software[ yielded
stability constants for the LnL (log β110) and LnL2
18]
1
0
(
log β120) complexes, as summarised in Table 1.
10
2
Table 1. Stability constants of LnL and LnL complexes by absorp-
II
tion and emission titration of [(TolTpy)Ru (BisTpy)](PF
6
)
2
with a
II
Figure 3. Optimised DFT geometries of (a) [(TolTpy)Ru (Bis-
III
solution of Ln (NO
3
)
3
·xH
2
O (Ln = Nd, Eu, Gd, Yb; x = 5, 6) in
2+
II
III
2+
Tpy)] and (b) [(TolTpy)Ru (BisTpy)Y (NO
3
)
3
(H
2
O)] com-
CH
3
CN.
plexes showing LUMO (upper) and HOMO (lower).
log10
β
110 log10β110 log10β110 log10β120 log10β120 log10β
120
Ln
II
2+
The calculated structure of [(BisTpy)Ru (TolTpy)] is
as expected. It shows an essentially planar orientation of
the BisTpy and TolTpy ligands and average Ru–N bond
lengths of approximately 2.08 Å, which are similar to those
(
abs)
(emis)
(avg.)
(abs)
(emis)
(avg.)
Nd
Eu
Gd
Yb
7.0(1)
7.9(6)
8.7(4)
9.7(1)
6.3(1)
7.2(7)
8.0(9)
9.8(3)
6.7(5)
7.6(5)
8.4(5)
9.8(1)
10.4(6) 11.2(1) 10.8(6)
12.2(3) 11.4(6) 11.8(6)
12.8(9) 11.7(1) 12.3(8)
12.7(4) 13.1(9) 12.9(3)
[
23,24]
found in X-ray structural reports.
N bond lengths to the central Tpy N atom are apparent
from absorption measure- owing to the strained chelate ring, which is also manifest in
Slightly shorter Ru–
Values obtained for log β
1
0
110
ments are slightly higher than those from emission spectra. the N–Ru–N bond angles for each of the chelated Tpy
We expect the former to be more accurate owing to the groups and deviates significantly from the ideal value of
larger observable changes during titration. However, indi- 90° to approximately 79°. Notably, the non-coordinated N
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atoms of the BisTpy ligand adopt the preferred “trans– whereas for the YbIII complex, a single NIR emission peak
2
2
trans” arrangement of N donors to minimise steric interac- is observed at 985 nm that corresponds to the F Ǟ F
5/2
7/2
1
tions between adjacent protons, as observed in the X-ray transition. Upon direct excitation of the MLCT absorption
[25]
III
III
structure of this ligand.
band (λex = 490 nm), the same Nd and Yb peaks are
III
For the Y complex, chelation of the metal cation leads also observed in the NIR with similar intensities (see Figure
to a rearrangement of these N donors into the necessary S4 in the Supporting Information), thereby unequivocally
II
II
“
cis–cis” configuration, whereas the remainder of the Ru
demonstrating that the Ru –tpy core can act as a sensitiser
III
complex remains essentially unchanged. The trivalent metal for Ln -centred NIR emission.
ion binds three bidentate NO anions, with an average
Y –O bond length of 2.49 Å, and bite angles of approxi-
–
3
III
mately 53°, similar to the previously reported structure of
III
3+
[26]
the [Y (tpy)(NO )(H O)] complex. We have modelled
3
2
the remainder of the coordination sphere of the metal with
one solvent water molecule, in accordance with this struc-
tural report. Whereas this is most certainly the case for the
III
III
III
III
larger Ln cations (i.e., Nd , Eu and Gd ), the much
III
smaller ionic radius of the Yb cation will likely preclude
[
16,21]
solvent binding.
This agrees with our observation (see
below) that NIR emission from the YbIII complex upon
III
4
90 nm excitation has a similar magnitude to that of Nd
emission, despite the latter being typically more intense ow-
III
ing to a difference in the hydration state of the Ln cation.
Figure 4. Observed emission spectra (λex = 308 nm) in the visible
II
(
left axis) and NIR region (right axis) for [(TolTpy)Ru (Bis-
2
+
II
III
2+
Tpy)]
(RuL) and [(TolTpy)Ru (BisTpy)Ln (NO
3 3 2 x
) (H O) ]
(
RuL–Ln; Ln = Nd, Eu, Gd, Yb) in CH CN. Inset: Expansion of
3
Photophysics
II 3
Ru MLCT emission band (λex = 490 nm).
II
2+
The absorption spectra of the [(TolTpy)Ru (BisTpy)]
Also shown in Figure 4 (inset) is an expansion of the
and [(TolTpy)Ru (BisTpy)Ln (NO ) (H O) ] complexes RuII 3MLCT emission upon MLCT excitation (λex
II
III
2+
1
=
3
3
2
x
(
Ln = Nd, Eu, Gd, Yb) in CH CN show a high degree of 490 nm) for each of the complexes. Consistent with the de-
3
1
similarity, as discussed and shown in Figure 2. On the basis crease in LC emission owing to strong intersystem cross-
of previously reported work,[ we can assign the absorp- ing, we see an increase in the intensity of the MLCT emis-
27]
3
II
tion band with a peak at approximately 492 nm to the Ru
d–π* ( MLCT) transition, whereas the UV absorption band approximately 10 nm upon Ln complex formation. The
with a peak at approximately 308 nm can be attributed to relative increase in intensity is most significant for the Gd
a ligand-centred ( LC) π–π* transition that involves the aro- complex, whereas the smallest increase was apparent with
sion for each of the 4f complexes and a small redshift of
1
III
III
1
III
3
matic terpyridine moieties.
Nd . In addition to the broad MLCT emission, we also
Figure 4 shows the observed emission spectra for observe a weak but characteristic emission peak at 616 nm
II
2+
II
III
2+
[
(TolTpy)Ru (BisTpy)] and the corresponding [(TolTpy)- for the [(TolTpy)Ru (BisTpy)Eu (NO ) (H O)]
com-
3
3
2
II
III
2+
III
5
7
Ru (BisTpy)Ln (NO ) (H O)] complexes (Ln = Nd, Eu, plex, which corresponds to the Eu -centred D Ǟ F
Gd, Yb) in CH CN upon LC excitation at λ = 308 nm. transition.
For the [(TolTpy)Ru (BisTpy)] complex, emission peaks
at approximately 355 nm ( LC) and approximately 660 nm measured for each of the complexes to further characterise
MLCT) are observed, which are redshifted to approxi- the Ru -based MLCT emission, with results summarised
mately 373 nm and approximately 667 nm upon complex- in Table 2. The lifetime of the MLCT state for the
ation with Gd . Notably, the intensity of the LC emission [(BisTpy)Ru (TolTpy)] complex is similar to the reported
peak is strongly diminished in the presence of the lanthan- value of 1.0 ns for the structurally related [Ru (PhTpy) ]
3
3
2
0
2
1
3
ex
II
2+
The luminescence lifetimes at approximately 670 nm were
1
3
II
3
(
3
III
1
II
2+
II
2+
2
ide cations. When λex = 308 nm is used, emission peaks complex (PhTpy = 4Ј-phenyl-2,2Ј:6Ј,2ЈЈ-terpyridine) in
characteristic to each of the emissive LnIII metal ions are CH CN.
[28]
Upon Ln complexation, the luminescence
III
3
1
also clearly evident in addition to the identified LC and lifetimes are extended to varying extents, an effect that is
MLCT emission. For the Eu complex, peaks at 578, 592, especially pronounced for the Gd complex. Similar results
3
III
III
[
29]
II
II
6
15, 648, 684, 749 and 816 nm are observed, and are as- have been noted
upon complexation of [(BisTpy)Ru -
5
7
2+
signed to the expected D Ǟ F transitions (J = 0–6). In (TolTpy)] to Zn , which can be attributed to a decrease
0
J
addition, two weaker yet easily resolved high-energy peaks in nonradiative deactivation pathways and rigidification of
were evident at 537 and 554 nm, which we assign to “hot” the complex upon metal binding.
transitions from the D1 excited state to F0,1 and 7F2
5
7
It is well known that Gd cannot act as an energy-trans-
III
III
ground-state levels. For the Nd complex, NIR emission fer acceptor owing to the very high energy of its 4f excited
bands at 880, 1060 and 1335 nm are evident, which corre- states. However, for the other lanthanides, the observation
spond to the F3/2 Ǟ I9/2, I11/2 and I13/2 transitions, of 4f-centred emission clearly points to an effective energy-
4
4
4
4
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Table 2. Emission intensities, lifetimes, energy-transfer rate con- excited state as a consequence of their slightly lower
II
III
stants and efficiencies for [(TolTpy)Ru (BisTpy)Ln (NO
3
)
3
-
3
–1
MLCT energies of 14925 cm relative to values of approx-
2+
(
H
2
O)
x
]
complexes (RuL–Ln, Ln = Nd, Eu, Gd, Yb) in CH
3
CN
–1
II
2+
imately 16650 cm that are typical of [Ru (bpy) ] deriva-
tives. Similarly, the values of k
and Eu as acceptors are much lower than those of Nd
owing to their poorer overlap of accepting 4f-centred states.
By using the room-temperature absorption and 77 K
3
(N/A: not applicable).
III
we obtained with Yb
EnT
–1
[a]
EnT [%] [b]
I
em [a.u.]
MLCT
τ [ns]
MLCT
k
EnT [s ]
Φ
III
III
3
3
[b]
II
III
Ru ǞLn
efficiency
(λex = 490 nm) (λex = 490 nm)
RuL
1.00Ϯ0.10
1.52Ϯ0.15
1.27Ϯ0.13
1.49Ϯ0.15
1.44Ϯ0.14
1.30Ϯ0.07
2.43Ϯ0.12
1.76Ϯ0.09
2.37Ϯ0.12
2.21Ϯ0.11
N/A
N/A
N/A
N/A
27.6
2.5
II
III
emission spectra of the [(TolTpy)Ru (BisTpy)Gd (NO ) -
(H
3
3
RuL–Gd
RuL–Nd
RuL–Yb
RuL–Eu
2+
8
O)
2
x
]
complex, we have estimated the energies and full
1.56ϫ10
8.84ϫ10
4.07ϫ10
1
1
6
width at half-maximum (FWHM) for the LC, MLCT and
7
3
3
9.1
MLCT, LC states, respectively (see Figures S5–S6 in the
Supporting Information), and have used this data to con-
struct the energy-level diagram shown in Figure 5. As noted
elsewhere,
[
q u q u
a] Calculated from 1/τ – 1/τ . [b] Calculated from 1 – (τ /τ ).
[
13,27]
an intermediate 3LC state lies above the
3
transfer pathway with the MLCT state acting as an energy
donor. Hence, by using the MLCT lifetime of the Gd
and other Ln complexes, we can calculate rates and effi-
3
MLCT level and can be populated after 1MLCT exci-
tation, thus leading to sensitised emission from the lower-
energy 4f excited states after energy transfer. However, as
discussed, the observed decreases in luminescence lifetimes
3
III
III
II
III
ciencies for the Ru ǞLn energy-transfer (EnT) process
using Equations (1) and (2) [
30,31]
in which τ and τ are the
q
u
3
herein clearly indicate that the MLCT state acts as the do-
nor for sensitised Ln emission. For Nd and Yb , we
note this energy-transfer step is thermodynamically favour-
able, whereas in the case of Eu , the very close proximity
and hence small overlap between the MLCT and Eu
lifetimes in the presence or absence of quencher. Results for
these calculations are summarised in Table 2.
III
III
III
k
EnT = 1/τ
q
– 1/τ
/τ
u
(1)
(2)
III
3
III 5
D0
Φ
EnT = 1 – τ
q
u
excited electronic states points to a thermally populated
[
13,32]
The rate constants and efficiencies we obtain can be com- pathway.
2
+
pared to reported values in which [Ru(bpy)3] derivatives
have been used as energy donors. EnT rates on the order of
5
6 –1
approximately 10 –10 s were observed by Van Veggel et
al.[ for several Yb and Nd complexes, for which it
10]
III
III
was noted that poor orbital overlap between the LUMO of
II 3
III
the Ru MLCT donor and Ln -centred 4f orbitals re-
sulted in a weak electron-exchange interaction.
2
+
By contrast, a series of [Ru(bpy) ] derivatives with an
3
III
appended Ln chelating group were prepared in an elegant
study reported by Ward et al.[ In this case, the presence of
an extended aromatic π system can provide a superexchange
pathway between the two metal ions, with an intramolec-
ular separation of 15.6 Å giving a much higher EnT rate
31]
III
7 –1
constant with Nd as the acceptor of 1.9ϫ10 s . In this
work, Förster and Dexter energy-transfer rates were also
calculated from the appropriate overlap integrals, and the
latter mechanism was implicated as the dominant pathway
for the observed sensitisation.
II
Figure 5. Electronic energy-level diagram for [(BisTpy)Ru -
2
+
II
III
(
(
TolTpy)]
(RuL) and [(TolTpy)Ru (BisTpy)Ln (NO
3 3
) -
2+
2 x
H O) ]
(RuL–Ln; Ln = Nd, Eu, Gd, Yb) complexes (emissive
III
Ln transitions shown by solid arrows).
By using the structure obtained from DFT calculations
II
III
2+
for [(TolTpy)Ru (BisTpy)Y (NO ) (H O)] , we can esti-
Lastly, inspection of the emission spectra (λ = 490 nm)
3
3
II
2
ex
2+
III
III
III
mate the distance between the Ru donor and Ln ac- for [(TolTpy)Ru (BisTpy)Nd (NO ) (H O) ] reveals an
3
3
2
x
ceptor to be approximately 16 Å in these complexes. unusual sharp emission peak centred at approximately
[
33,34]
Furthermore, an examination of the relevant molecular or- 800 nm. On the basis of previous work,
we assign this
bitals (see Figure 3) shows that the HOMO and LUMO peak to the Nd -centred ( F , H9/2)Ǟ4I9/2 transition.
III
4
2
[35]
5/2
have significant orbital density located on the bridging To the best of our knowledge, this transition has not been
BisTpy ligand. As such, a similar superexchange pathway previously observed in solution for a molecular complex,
II
III
can be expected to be operant in the [(TolTpy)Ru (BisTpy)- although its appearance in inorganic glasses such as Nd -
III
2+
[33,34]
Ln (NO ) (H O)] complexes, which can facilitate ef- doped PbBr and KPb Br has been well documented.
3
3
2
2
2
5
ficient energy transfer by means of the Dexter mechanism. In these cases, a lack of low-energy phonon modes is ac-
Given the similar separations involved, the slightly higher cepted as the rationale for the observed radiative intensity
values we obtain for kEnT can thus be attributed to a more of this transition, despite being located only 1000–
3
–1
4
favourable spectral overlap between the MLCT state of the 1100 cm above the emitting F3 state. In the present case,
/2
II
III
current tpy-based Ru derivatives and the Ln -centred 4f this explanation is untenable, and the reason for the appear-
Eur. J. Inorg. Chem. 0000, 0–0
5
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FULL PAPER
1
ance of this peak remains unclear, although it can be clearly equipped with a 5 mm probe, and H chemical shifts are reported
in parts per million relative to tetramethylsilane (TMS).
related to a competition between radiative and nonradiative
4
2
deactivation pathways available to the F and H ex-
5
/2
9/2
Photophysics: Typical solution concentrations for absorption and
–
5
cited states. To more completely understand the presence emission measurements were approximately 10 , and 1.0 cm path-
of this emission peak, we are currently undertaking more length cuvettes in quartz suprasil or equivalent were used. Experi-
3
detailed time-resolved studies using a series of structurally ments were carried out in CH CN solution at 298 K. UV/Vis ab-
III
sorption spectra were recorded with a Perkin–Elmer Lambda 40
spectrophotometer. Emission spectra in the visible region were ac-
quired with a HORIBA Jobin Yvon FL-3 spectrofluorimeter. The
spectra were reference-corrected for both the excitation light source
variation (lamp and grating) and the emission spectral response
similar rigid Nd complexes, which will be reported else-
5
7
where. As suggested by the appearance of the D Ǟ F
and D Ǟ F “hot” bands in the emission spectrum for
the Eu complex, the presence of this unusual emission
peak for Nd also establishes that sensitisation through en-
ergy transfer initially populates a higher-energy metal-
centred excited state, which undergoes subsequent internal
conversion prior to emission.
1
0,1
5
7
1
III
2
III
(
detector and grating). In the near-infrared region, emission spectra
were measured with an Edinburgh Instruments FLS-920-STM
spectrofluorimeter by using a liquid-N -cooled Ge (EI-L) detector.
Low-temperature (77 K) emission spectra were measured in a cylin-
drical tube immersed in a quartz dewar that contained liquid N
2
2
.
Global fitting of absorption and emission spectra was performed
using the pHAb software package.[ Fluorescence lifetimes were
measured with an Edinburgh Instruments FLS-920-STM spectro-
fluorimeter equipped with a TCC900 card for data acquisition in
time-correlated single-photon counting (TCSPC) mode and with a
LDH-P-C-470 pulsed diode laser as the excitation source (IRF ≈
18]
Conclusion
A series of LnIII complexes (Ln = Nd, Eu, Gd, Yb) with
II
2+
a visible-light-absorbing [(TolTpy)Ru (BisTpy)] chromo-
phore have been prepared, and their solution behaviour has
been thoroughly characterised by using a combination of
90 ps FWHM). The estimated error of these measurements is Ϯ5%.
1
Computational Studies: Ground-state density functional theory
H NMR, UV/Vis and emission spectroscopy. The obtained
III
III
2+
(DFT) calculations were performed using the B3LYP/LANLD2Z
heterobimetallic [(TolTpy)Ru (BisTpy)Ln (NO ) (H O) ]
complexes were found to be effective sensitisers of Ln -
based emission both in the visible or near-infrared (NIR)
region depending on the identity of the added Ln cation.
Similarly, depending on the excitation wavelength used, two
3
3
2
x
basis set provided in Gaussian 03[ with input structures derived
36]
III
from available X-ray crystal structure data[
23–26]
and optimised
MM2 molecular models. Calculations were performed in the gas
phase with no symmetry restraints.
III
3
3
distinct pathways that involve either the LC or MLCT
excited state as energy donor have been observed.
Acknowledgments
These compounds are the first reported examples that
II
III
The Research Executive Agency (REA) of the European Commis-
sion is gratefully acknowledged for the award of a Marie-Curie
International Incoming Fellowship to E. G. M. (FP7-PIIF-GA-
utilise a Ru –tpy complex as a sensitiser for Ln emission
in the NIR region. Moreover, they demonstrate that despite
the much shorter luminescence lifetime than more com-
2010-275606) and the Australian Research Council (ARC) is ac-
2
+
II
monly used [Ru(bpy)3] derivatives, Ru –tpy derivatives
can still act as efficient antennae. In particular, for the Nd
knowledged for funding (FT100100795).
III
cation, electronic energy-transfer efficiencies as high as ap-
proximately 28% were determined that correspond to a
[
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Pages: 8
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FULL PAPER
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FULL PAPER
Lanthanide Luminescence
E. G. Moore,* M. Benaglia, G. Bergamini,
P. Ceroni ........................................... 1–8
Synthesis, Stability and Sensitised Lan-
thanide Luminescence of Heterobimetallic
d/f Terpyridine Complexes
Keywords: Luminescence / Sensitizers /
Lanthanides / Ruthenium / Terpyridine
The synthesis and solution properties for a
near-infrared emission for the first time.
Energy transfer is shown to operate
through both the triplet ligand-centred and
triplet metal-to-ligand charge-transfer
pathways.
II
III
series of heterobimetallic Ru –Ln com-
plexes are reported. These compounds
II
demonstrate the use of a Ru bis-terpyrid-
III
ine unit as the sensitiser for Ln -based
Eur. J. Inorg. Chem. 0000, 0–0
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim