Selective activation of lanthanide luminescence with triarylboron- functionalized ligands and visual fluoride indicators

Article abstract of DOI:10.1039/c2cc36172h

Two triarylboron-functionalized carboxylate ligands have been found to be highly effective in selective activation of Tb(iii) and Eu(iii) emission and enable the use of the Tb(iii) and Eu(iii) complexes as highly effective luminescent indicators for F

Full text of DOI:10.1039/c2cc36172h

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Showcasing research from Professor Suning Wang’s
research group, Queen’s University, Canada
As featured in:
Selective activation of lanthanide luminescence with
triarylboron-functionalized ligands and visual fluoride indicators
Carboxylate-functionalized triarylboranes have been found to be
highly effective in selective activation of Tb(III) or Eu(III)
luminescence. Such systems are highly effective for visual and
selective fluoride sensing. The triarylboron centers in these systems
can be used for switching off green or red lanthanide emission
and turning on ligand-based blue fluorescence by the addition of
fluoride ions in solution or on a paper substrate.
See Suning Wang et al.,
Chem. Commun., 2012, 48, 12059.
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COMMUNICATION
Selective activation of lanthanide luminescence with
triarylboron-functionalized ligands and visual fluoride indicatorsw
Maria Varlan, Barry A. Blight and Suning Wang*
Received 24th August 2012, Accepted 2nd October 2012
DOI: 10.1039/c2cc36172h
Two triarylboron-functionalized carboxylate ligands have been
found to be highly effective in selective activation of Tb(III) and
Eu(III) emission and enable the use of the Tb(III) and Eu(III)
complexes as highly effective luminescent indicators for FÀ and
CNÀ in solution and on a solid substrate.
activation of Tb(III) and Eu(III) ions by two BMes2-functionalized
carboxylate ligands 1 and 2 and the use of the lanthanide
compounds in sensing applications.
The synthetic procedure of ligand 2 was reported in a recent
paper from our group.12 Ligand 1 is a new molecule and was
synthesized using procedure similar to that of ligand 2
(see ESIw). These two ligands were chosen because of their
distinct triplet energy for selective activation of Tb(III) or
Eu(III) emission. The corresponding Tb(III) and Eu(III) com-
pounds of 1 and 2 were obtained as colorless solids by the
reaction of the potassium salt of 1 and 2 with Tb(NO3)3Á6H2O
and Eu(NO3)3Á6H2O, respectively, in methanol in high yield
(Chart 1).
Lanthanide-based luminescent compounds are a very attractive
class of materials owing to the distinct and exceptionally narrow
f - f emission bands, the long decay lifetimes and the large
Stokes shifts.1–7 However, because of the forbidden nature and
the low extinction coefficients of f - f transitions, lanthanide
emission is usually very weak.1 Lanthanide emission can, how-
ever, be enhanced through the introduction of an appropriate
‘‘sensitizing’’ chelating ligand.2 Sensitization of the rare earth
ion is achieved through the absorption-energy transfer-emission
(AETE) mechanism whereas the chelating ligand acts as an
‘antenna’ to harvest light and subsequently transfer the energy
to a lanthanide ion, thus enhancing lanthanide emission.3
Unlike phosphorescence of transition metal compounds that
are usually very sensitive to oxygen quenching, thus limiting
their use as sensors under ambient conditions,5 lanthanide
emission is much less prone to oxygen.6 Consequently, lumine-
scent lanthanide compounds have found use in a wide array of
fields such as electroluminescent devices,7 sensors,8 and cellular
imaging.9 Triarylboron moieties such as BMes2Ar (Mes =
mesityl) are known to be highly effective in enhancing
fluorescence or phosphorescence of organic molecules and
transition metal complexes.10 The low-lying pp orbital on the
boron center enables the use of triarylboron compounds as
highly selective sensors for CNÀ and FÀ anions monitored
readily by either the absorption or emission spectral change of
the molecule.11 Thus, combining the triarylboron functionality
with lanthanide ions may allow us to access new functional
lanthanide-based materials.
Elemental and MS analyses established that for ligand 1, the
complex has the general formula of Ln(1)3(H2O)x(MeOH)y
(x = 0, y = 2 for 1Tb, x = 2, y = 1 for 1Eu), while for ligand 2
the complex has the general formula of Ln(2)2(OMe)(MeOH)
(2Tb and 2Eu). Based on MS data, the lanthanide compounds
are most likely oligomeric in solution and the solid state via
either bridging carboxylate or methoxy ligands, which are
commonly observed for lanthanide compounds.13
The impact of ligands 1 and 2 on luminescence of Tb(III) and
Eu(III) is quite dramatic, as shown by the spectra and photo-
graphs of ligands and complexes in Fig. 1.
Both free ligands display weak blue fluorescence in the solid
state and in solution. Complexes 1Tb, 2Tb, 1Eu and 2Eu
display green, blue, red and pink luminescence, respectively,
in solution and the solid state (Fig. 1, Table 1). Complex 1Tb
exhibits exceptionally bright luminescence with characteristic
Tb(III) emission bands. The ligand’s fluorescence band is
also evident in all compounds. In the solid state, ligand 1’s
Based on these considerations, we recently initiated the develop-
ment of triarylboron-functionalized luminescent lanthanide
compounds. Herein we report the effective and selective
Department of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada K7L 3N6.
E-mail: suning.wang@chem.queensu.ca
w Electronic supplementary information (ESI) available: Synthetic
details, characterization data, completed titration data. See DOI:
10.1039/c2cc36172h
Chart 1
c
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Fig. 2 Luminescence titration spectra of Tb(Bz)3 (lex = 300 nm, left)
and Eu(Bz)3 (lex = 330 nm, right) with 1 in THF (1.0 Â 10À5 M)
at 298 K.
Because of the large contribution of ligand 2’s fluorescence,
compound 2Eu has pink luminescence while 1Eu, with much
less ligand’s emission contribution, has the characteristic red
emission color of Eu(III) (Fig. 1).
Fig. 1 Absorption (solid line) and luminescence (dashed line) spectra
of complexes in THF (top, left), luminescence spectra in PMMA
(10 wt%) at r.t. (top, right), photographs of 1Tb, 2Tb, 1Eu and 2Eu
in PMMA (10 wt%) (bottom), in the solid state and in THF solutions
under irradiation at 365 nm (middle).
Since the key difference between ligands 1 and 2 and
benzoate is the BMes2 group, to examine the role of the
BMes2 group in ligands 1 and 2, we performed the titration
experiments of Tb(Bz)3 and Eu(Bz)3 by ligands 1 and 2,
respectively (Bz = benzoate). As shown in Fig. 2, the
addition of ligand 1 to the solution of Tb(Bz)3 or Eu(Bz)3
led to a drastic intensity increase of the lanthanide emission
bands. The exchange of ligand 1 with benzoate clearly enhances
the emission of the lanthanide ions. Similar phenomenon was
also observed for ligand 2 with Eu(Bz)3. These experiments
established that the BMes2 group in ligands 1 and 2 is critical
in activating Tb(III) or Eu(III) emission. The fact that ligand 1
can activate both Tb(III) and Eu(III) emission while ligand 2
can only activate Eu(III) emission may be explained by the
triplet energy difference of the two ligands. It has been well
established before that in order to activate lanthanide ion
emission, the ligand’s triplet energy should be close to and
above that of the emissive state of the lanthanide ion. To
determine the triplet excited energy level of the ligands, the
phosphorescence spectra for 1 and 2 at 77 K were recorded in
THF (see ESIw) and the T1 triplet energy level was calculated14
to be 24 398 cmÀ1 and 21 563 cmÀ1 respectively, which are
above the lowest excited resonance levels (or the emissive states)
5D4 of Tb(III) (20 500 cmÀ1) and 5D0 of Eu(III) (17 250 cmÀ1).15
The ineffective sensitization of the Tb(III) ion with ligand 2
may be attributed to the T1 level of the ligand being too close
Table 1 Photophysical properties of ligands 1 and 2 and their
respective Tb(III) and Eu(III) complexes
Fssc, Total
emission/
Lanthanide
l
ex, nm
UV-Vis, nm THF, lema, nm/t,
ms, rt
Compound (e, MÀ1 cmÀ1) rt
Fsol emission
b
1
2
1Tb
330 (15 997)
328 (25 031)
330 (43 011)
384
406
384, 489,
545, 585,
615/1.22(2)
406
0.05
0.14
—
—
—
0.70/0.56
330
2Tb
1Eu
328 (60 086)
330 (46 512)
335
345
—
0.41/0.03
0.14/0.04
384, 579, 590, —
617/0.63(1)
2Eu
328 (65 813)
350
406, 579, 590, —
617/0.54(1)
0.48/0.10
a
b
In THF at 1 Â 10À5 M. Relative to 9,10-diphenylanthracene in
c
CH2Cl2. Measured in the solid state in 10 wt% doped PMMA
polymer films using an integration sphere.
contribution to the total emission of 1Tb becomes very small,
indicating very effective energy transfer from the ligand to the
Tb(III) center, as shown in Fig. 1. Indeed, the quantum
efficiency for 1Tb in the solid state was found to be very
impressive, 0.70 for total emission and 0.56 for Tb(III) emission
bands (80% of the total emission). In contrast, 2Tb does
not display any Tb(III) emission bands in solution and
shows very weak Tb(III) bands in the solid state (o10% of
the total emission). These observations support that ligand 1 is
highly effective in activating Tb(III) emission while ligand 2
is not.
to the lanthanide 5D4 resonance level (Dn = 1063 cmÀ1
)
resulting in an inefficient energy transfer. The relatively low
Eu(III) emission quantum efficiencies of 1Eu and 2Eu may be
caused by the very large energy difference of the ligand’s triplet
5
state and the D0 that is also known to result in low emission
efficiency due to non-radiative deactivation of the lanthanide
emitting state.16
To further probe the impact of BMes2 on the lanthanide
emission and the possible use of the lanthanide compounds in
detecting anions such as fluoride and cyanide, we examined the
absorption and luminescence spectral change of complexes
1Tb, 1Eu and 2Eu in response to the addition of NBu4F
(TBAF) and NBu4CN (TBACN) in THF. The complete
titration data can be found in the supporting materials. For
1Tb, the addition of TBAF or TBACN caused a general
decrease of the Tb(III) emission bands and a red shift of the
For the 1Eu and 2Eu complexes, characteristic Eu(III) emission
bands from the 5D0 - 7Fn transitions with n = 0, 1 and 2 were
observed. These emission bands have moderate quantum
efficiencies (0.04 for 1Eu, 29% of the total emission; and
0.10 for 2Eu, 21% of the total emission), indicating that
ligands 1 and 2 are capable of activating Eu(III) emission, albeit
much less efficient, compared to 1Tb. The ligand’s emission
band contributes significantly in both Eu(III) compounds.
c
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to the replacement of carboxylate ligand by the anion. These
experiments confirmed that the boryl group has indeed a
strong impact on the emission of the lanthanide ions, which
may in turn be used for anion sensing/detection by monitoring
the lanthanide emission bands.
To test if the lanthanide compounds can be used for
practical sensing of FÀ, we loaded 1Tb and 1Eu onto filter
papers. Such prepared filter papers have weak response to
aqueous solutions of FÀ. However, they do show distinct
response to TBAF in MeOH (1Tb and 1Eu are insoluble in
MeOH), changing emission color to blue, as shown by Fig. 3
(see also ESIw), with a visual detection limit of 50–100 ppm.
Similarly prepared Tb(Bz)3 or Eu(Bz)3 filter papers did not
show any emission color change toward FÀ. The 1Tb and 1Eu
filter papers also respond to TBACN, albeit much less distinct
color change, compared to TBAF (see ESIw).
In summary, the first examples of triarylboron function-
alized Tb(III) and Eu(III) compounds have been achieved. The
BMes2 group has been found to be highly effective in activating
Tb(III) or Eu(III) emissions and the resulting new lanthanide
complexes are promising as luminescent sensors/probes for
CNÀ and FÀ.
Fig. 3 Top: Luminescence titration spectra of 1Tb (left) and 1Eu
(right) (1.0 Â 10À5 M in THF) by TBAF at 298 K (lex = 300 nm),
inset: photographs showing the solution color change. Bottom:
photographs showing the emission color change of 1Tb and 1Eu
on filter papers either after the addition of one drop of TBAF
(large excess) MeOH solution (left) or the dipping of the papers in
MeOH of 100 ppm TBAF (right). Blank MeOH solution was used
as a control.
ligand’s fluorescence band from B385 nm to 430 nm, leading
to the emission color change from green to blue, as shown in
Fig. 3. Because the ligand’s fluorescence band change and
the absorption spectral change of 1Tb with the addition of
o3.0 eq. anions resemble those of the free ligand 1 and a
Cu2 paddlewheel compound containing ligand 1 (see ESIw),
the quenching of the Tb(III) emission bands can be attri-
buted to the binding of the anions to the boron atom, that
decreases the first excited state energy of the ligand, thus
lowering of the triplet energy, leading to the diminished
emission intensity of the Tb(III) ion. The decrease of the first
excited energy of ligand 1 upon the addition of FÀ and CNÀ to
the boron center is caused by the low energy charge transfer
transition of the BMes2X group (X = FÀ or CNÀ) to the
carboxylate, in agreement with the behaviour of the free
ligand. A control experiment with 1Tb being titrated by
TBACl shows that the Tb(III) emission did not display any
appreciable quenching until more than 10 eq. of TBACl was
added (see ESIw) owing to the displacement of ligand 1 from
the Tb(III) center, thus further supporting that the color and
spectral change of 1Tb upon the addition of B3 eq. FÀ or
CNÀ is indeed caused by selective binding of the anion to the
B center.
For 1Eu, the addition of FÀ or CNÀ causes the lumines-
cence of the solution to undergo multi-stage color change as
shown in Fig. 3, a consequence of the combined change of the
ligand-based fluorescence band and the Eu(III) emission bands.
It is noteworthy that the addition of anions to the 1Eu causes
first a great increase of the Eu(III) emission peaks, and only
after the addition of more than 2 eq. of the anion the Eu(III)
peaks experience intensity decrease. Furthermore, the ligand’s
fluorescence band experiences a red shift in the same manner
as that observed in 1Tb. The initial intensity gain with the
addition of B2 eq. anions may thus be attributed to the
lowering of the triplet state of the ligand after anion binding
to the B atom, making the ligand more effective in Eu(III)
emission activation. The subsequent quenching can be attributed
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
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c
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