Upconverted photosensitization of Tb visible emission by NIR Yb excitation in discrete supramolecular heteropolynuclear complexes

Article abstract of DOI:10.1021/jacs.6b12940

Addition of Tb³⁺ salts to a solution of a (YbL_D) complex in D₂O resulted in the formation of [(YbL_D)₂Tb_x] (x = 1 to 3) complexes that, upon NIR excitation at 980 nm, showed an unprecedented Yb to Tb upconversion sensitization phenomenon resulting in the observation of the typical green emission of Tb.

Full text of DOI:10.1021/jacs.6b12940

Communication
pubs.acs.org/JACS
Upconverted Photosensitization of Tb Visible Emission by NIR Yb
Excitation in Discrete Supramolecular Heteropolynuclear Complexes
†
,‡
†
§
∥
†
Nabila Souri, Pingping Tian, Carlos Platas-Iglesias, Ka-Leung Wong, Aline Nonat,
,
†
̀
and Loïc J. Charbonniere*
†
Laboratoire d’Ingen
Universite
́
ierie Molec
́
ulaire Appliquee
́
a
̀
l’Analyse, Institut Pluridisciplinaire Hubert Curien, IPHC, UMR 7178, CNRS,
́
de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex, France
Laboratoire d’electrochimie des Materiaux Moleculaires et des Complexes, LEMMC, Faculte
9000 Setif, Algeria
Centro de Investigacion
Campus da Zapateira-Rua
‡
́
́
́
́
de Technologie, Universite
́
de Set
́
if-1,
a
1
§
́
́
s Científicas Avanzadas (CICA) and Departamento de Química Fundamental, Universidade da Corun
da Fraga 10, 15008A Coruna, Spain
Department of Chemistry, Hong Kong Baptist University, Hong Kong SAR
̃
́
̃
∥
*
S Supporting Information
Scheme 1. Ligand L and Its Partially Deuterated Analogue LD.
ABSTRACT: Addition of Tb3+ salts to a solution of a
YbL ) complex in D O resulted in the formation of
(
D
2
[
(YbL ) Tb ] (x = 1 to 3) complexes that, upon NIR
D 2 x
excitation at 980 nm, showed an unprecedented Yb to Tb
upconversion sensitization phenomenon resulting in the
observation of the typical green emission of Tb.
on the luminescence properties of Yb complexes with deuterated
15
hotonic upconversion (UC) allows for the conversion of
ligands, deuteration of the methylene bridge was expected to
decrease nonradiative vibrational quenching due to the CH
oscillators and to improve the luminescence properties.
P
low energy photons, typically in the near-infrared (NIR)
region, into photons of higher energy, generally in the visible or
1
NIR regions. This unconventional anti-Stokes process has
Full synthetic details for L can be found in the Supporting
D
2
found many applications such as in lasers, and there is a growing
Information, whereas the mononuclear [LnL(D)] complexes were
14
interest to develop fluorescence free bioanalytical assays and
obtained according to reported protocols. Table 1 summarizes
the spectroscopic properties of the isolated mononuclear
(LnL(D)) complexes.
3
theranostic applications. However, UC processes are almost
entirely restricted to solid state materials and nanomaterials and
only a handle of examples have been reported for discrete
The absorption spectra of the complexes are dominated by the
strong π→π* transitions centered on the bipyridyl units,
pointing at 310 nm in the complexes (Figure 1). Upon excitation
into the maximum of absorption, the emission spectra display the
sharp and characteristic emission bands of each Ln ion, with four
main emission bands at 485, 545, 580, and 620 nm attributed to
4
−6
molecular systems
in organic solvents, or very recently in8
7
D O thanks to the formation of a supramolecular Ln dimer.
2
The development of discrete molecular UC devices would
remove the drawbacks associated with the biotoxicity of
9
nanomaterials and might open new perspectives in biological
5
7
labeling applications. NIR to NIR and NIR to red upconverters
the D
4 J
→ F electronic transitions (J = 6 to 3, respectively) for
10
would be particularly well adapted for imaging applications,
both excitation and emission being suited to the biological
Tb and a broad band with maximum at 980 nm attributed to the
2
2
18
F
5/2→ F7/2 transition for Yb. The corresponding excitation
1
1
window for a better penetration of the exciting/emitting light
through biological tissues. Additionally, the lack of spurious
fluorescent signals also generates very low background noise and
a concomitant improved sensitivity for fluoro-immunoas-
spectra (Figure S1) are in perfect agreement with the absorption
spectra confirming that the photosensitization originated from
an antenna effect with energy transfer from the ligand to the Ln
cations. It could be noticed that the deuteration had a very strong
influence on both the excited state lifetime and quantum yield of
1
2,13
says.
We have recently reported the development of the
the Yb complex, with a very long 65 μs lifetime for [YbL ] in
D
1
4
D O, among the longest reported in the literature for Yb
phosphonated bipyridyl based ligand L (Scheme 1).
2
15,19
complexes.
Considering the very good shielding of the Ln cations by the
octadentate ligand and the promising photophysical properties
For all complexes, the luminescence decays could be perfectly
fitted with monoexponential functions, pointing to the presence
of a single species in solution. Surprisingly, the luminescence
of the (YbL) complex in water and D O (Table 1), we also
2
developed the synthesis of L , the analogue of L deuterated on all
D
methylenic positions. For the purpose of clarity, the coordination
of the complexes inside the cavity of the ligand will be denoted
Received: December 16, 2016
Published: January 16, 2017
(LnL). In view of the outstanding work of Seitz and co-workers
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b12940
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Spectroscopic Properties of (TbL), (YbL), and (YbL ) Complexes
D
λmax (nm) [ε (M−1·cm−1)]
τH O (μs)
τD2O (μs)
a
ϕH2O
b
d
ϕD2O
b
q
2
c
(
(
(
TbL)
YbL)
YbLD)
310 [10700]
310 [13100]
890
2.3
3.3
899
35
0.33
0.16
0
d
e
1.3
0
e
f
d
d
310 [13100]
65
0.2
3.8
17a
0
a
b
c
d
λexc = 310 nm, estimated error ±10%. λ = 310 nm, estimated error ±15%. Using R6G in water as reference (ϕ = 0.76).
Using cardiogreen in
exc
17b
e
f
MeOH as reference (ϕ = 0.078).
Calculated according to ref 16. The absorption was assumed to be the same as that of the nondeuterated
complex.
2
2
1
τrad
8πcn ν
ul
gl
=
2303 ×
∫
ε(ν)dν
NA
gu
∫
v × ε(v)dv
ε(v)dv
with v =
ul
∫
1
0
−1
in which c is the speed of light in a vacuum (3 × 10 cm·s ), N
A
is Avogadro’s constant, n is the refractive index of the medium
−
1
(
1.33 for D O), ν is the barycenter of the transition (in cm ), g
2
u
l
l
and g are related to the degeneracies of the lower (l, J = 7/2) and
u
l
upper (u, J = 5/2) excited states (g = 2J + 1), and ε(v) is the
u
i
i
−1
−1
molar extinction coefficient (in M ·cm ) at the wavenumber v.
A value of 1.73 ms was obtained for τ . It is to be noticed that
this value is prone to a large experimental error. From this
value, the Yb centered luminescence quantum yield ϕYb can be
calculated with
Figure 1. UV−vis absorption (dotted lines), and emission (λ = 310
exc
rad
nm, full lines) spectra for (YbL) (blue) and (TbL) (red) in TRIS/HCl,
22
0
.01 M, pH 7.0.
decay of the Tb complex was rather short (890 μs). This
5
τ
τrad
particular behavior was possibly ascribed to a Tb ( D ) to ligand
obs
4
ϕYb
=
3
(
ππ*) back energy transfer process, as observed in other
20
bipyridyl-based Tb complexes. On the other hand, the
luminescence properties of the Yb complexes are quite
interesting, particularly regarding the lifetime and quantum
Where τobs is the luminescence lifetime of Yb (65 μs in D
O),
2
leading to a value 0.037 for the Yb centered quantum yield.
Considering that the overall luminescence quantum yield
obtained by excitation through the ligand (310 nm) was
determined to be 0.038 (Table 1), the ligand to Yb energy
transfer efficiency is almost quantitative and the nonradiative loss
are essentially due to quenching of the Yb excited state.
Considering the excellent spectroscopic properties of the
(YbL(D)) complexes, the ability of ligand L to form polynuclear
complexes of [(LnL)
15
yields of (YbL ) in D O. As previously noticed, the influence of
D
2
the CH/D oscillators in the proximity of the Yb cation has a
modest contribution relative to that of second sphere OH/D
ones of the H O (D O) solvents. Luminescence lifetimes in D O
2
2
2
16
and H O allowed for the calculations of q, the number of water
2
molecules in the first coordination sphere of the cations. For the
two complexes, a value of 0 was obtained, confirming the
coordination inside the cavity of the octadentate ligand, which
perfectly shields the cation from its environment.
14
Ln ] (x = 1 to 3) composition in water,
2
x
and the high thermodynamic and kinetic stabilities of (LnL)
1
4
Noteworthy, the solubility of the complexes in D O allowed
complexes at neutral pH, we were interested in studying the
formation of [(YbL(H/D) Tb ] heteropolynuclear complexes
and to check their spectroscopic properties. Starting from a
solution of the isolated (YbL) complex in H O buffered at pH 7.0
2
for the measurement of the NIR absorption spectrum of the
)
2
x
2
2
F → F
transition of the (YbL ) complex (Figure 2).
7
/2
5/2
D
Knowing the oscillator strengths for the transition, it was possible
2
with 0.01 M TRIS/HCl, increasing aliquots of TbCl .6H O in
to calculate the radiative lifetime τ of Yb using the modified
3
2
rad
2
1
the same buffer were added. The evolution of the electronic
absorption spectra and of the emission spectra (λexc = 310 nm)
were measured during the titration (Figures S2 and S3,
respectively). Although the variations in absorption were very
weak, those observed in luminescence were very informative,
revealing the appearance of the Tb centered emission bands
Einstein relation:
5
7
attributed to the D → F (J = 6 to 3) transitions up to ca. 1.5
4
J
equiv of Tb added. Considering the high thermodynamic and
14
kinetic stability of the (LnL) complexes at neutral pD, it is
assumed that the coordination of the Yb atom inside the
coordination cavity of LD is kept during the titration.
Accordingly, the photosensitization of Tb may arise from a
direct through space ligand to metal energy transfer or possibly
by other mechanisms involving the (YbL) entities.
In a second step, the same experiment was repeated using the
Figure 2. NIR absorption spectra of the F5/2←2F7/2 transition of the
2
deuterated version of the ligand, so that (YbL ) was titrated by
D
−
3
(
YbL ) complex in D O (c = 1.07 × 10 M).
TbCl .6H O in D O (TRIS/DCl 0.01 M at pD 7.0). The
D
2
3
2
2
B
DOI: 10.1021/jacs.6b12940
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Journal of the American Chemical Society
Communication
emission spectra in the visible region (450 nm < λ < 700 nm)
em
2
2
were recorded upon excitation into the F → F absorption
7
/2
5/2
band of Yb at 980 nm with a 2.0 W laser irradiation (Figure 3). In
absence of Tb, the visible emission spectrum was devoid of any
emission signal.
Figure 4. Log−Log plots of the emitted intensity (integrated for the
5
7
D → F peak centered at 545 nm) as a function of the incident pump
4
5
power and slopes of the linear fits for mixtures of 0.3 (blue) and 1.0 (red)
equivalents of Tb added to a (YbL ) solution in D O (pD = 7.0, TRIS/
D
2
DCl).
Figure 5 displays the DFT models calculated for the
[
(YbL) Tb ] species (x = 1 to 3). They revealed two important
2
x
Figure 3. Evolution of the emission spectra (450 nm < λ < 700 nm)
em
−4
upon titration of a 7.1 × 10 M solution of (YbL ) (D O, pD 7.0,
D
2
TRIS/DCl) by TbCl ·6H O, with excitation at 980 nm (P = 2.0 W).
3
2
5
7
Inset: Emission intensity of the D → F emission band of Tb at 485 nm
4
5
as a function of the Tb/(YbL ) ratio.
D
As soon as Tb was added, emission bands appeared at 485,
45, 580, and 620 nm, which could be clearly associated with the
5
characteristic Tb centered emission bands. The intensity of the
signal increased up to one equivalent of added Tb and then
decreased to reach a plateau after 1.5 equiv of added Tb cations.
Although weak with Ln cations, the possibility of photo-
23
sensitization of the Tb atom by non linear processes was
discarded by performing a similar experiment with the (LuL )
H
1
4
complex in place of the (YbL ) one, which revealed a total
D
absence of Tb emission upon 980 nm excitation. From these
experiments, it appears that the observed emission is the result of
an up conversion mechanism in which the Tb emission is arising
from a Yb to Tb energy transfer, a phenomenon previously
24
observed in the solid state, but reported here for the first time at
the molecular level in solution. To ascertain the UC mechanism,
the intensity of the visible emission was integrated as a function
of the pump power of the laser. The corresponding Log−Log
plot is reported in Figure 4, for mixtures containing 0.3 and 1.0
equiv of Tb in the D O solution of (YbL ). In both cases, the
Figure 5. DFT models of the [(YbL) Tb] (top), [(YbL) Tb ] (middle),
2
2
2
and [(YbL) Tb ] heteropolynuclear complexes.
2
D
2
3
slope of the linear fit is close to two, in perfect agreement with a
two photon UC mechanism.
points. Whatever the structure, the two Yb atoms are very close
to the incoming Tb atoms (dYb-Tb ranged from 3.85 to 4.56 Å in
the different structures). This close proximity seems to be a
major parameter to allow for the possibility of intermetallic
Analysis of the data by nonlinear least-squares methods using
25
the Specfit software revealed, as previously observed for pure
14
Tb and Yb titrations, the presence of three new emitting
species and the variations could be accurately fitted to the
following model:
energy transfer in upconversion processes as recently observed in
7
a homodinuclear Er complex in D O (d
= 4.48 Å). A second
2
Er−Er
3
+
3+
lesson from DFT is that the income of the second and third Tb
+
Tb
+Tb
2
(YbL ) Xoooo oo Y [(YbL ) Tb] Xoooo oo Y [(YbL ) Tb ]
D
+
D 2
D 2
2
atoms in the [(YbL) Tb] complex did not perturb much the
2
3
+
Tb
structure of the complex, explaining the possibility to form these
intricate heteropolynuclear structures.
Xoooo oo Y [(YbL ) Tb ]
D 2
3
The calculated distributions of the species formed during the
titration and the relative UC emission spectra of the
heteropolynuclear species are presented in Figures S4 and S5,
respectively (Supporting Information).
The observation of this unique photosensitization upconver-
sion process is the result of an accumulation of advantageous
properties of the system studied here. First, summation of two
2
2
excited states level of the F → F transition of Yb at 980 nm
7/2
5/2
C
DOI: 10.1021/jacs.6b12940
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Journal of the American Chemical Society
is almost perfectly matching the energy level of the D → F
Communication
5
7
(4) (a) Aboshyan-Sorgho, L.; Besnard, C.; Pattison, P.; Kittilstved, K.
4
6
R.; Aebischer, A.; Bunzli, J.-C. G.; Hauser, A.; Piguet, C. Angew. Chem.,
́
Int. Ed. 2011, 50, 4108. (b) Suffren, Y.; Golesorkhi, B.; Zare, D.; Guenee,
̈
transition of Tb at 485 nm. Then, the excellent luminescence
́
properties of the (YbL ) complex place it as an excellent
D
L.; Nozary, H.; Eliseeva, S. V.; Petoud, S.; Hauser, A.; Piguet, C. Inorg.
Chem. 2016, 55, 9964.
photosensitizer for upconversion purpose. Of great importance,
the Tb3+ cations are the less sensitive of the Ln series toward
(
5) Blackburn, O. A.; Tropiano, M.; Sørensen, T. J.; Thom, J.; Beeby,
A.; Bushby, L. M.; Parker, D.; Natrajan, L. S.; Faulkner, S. Phys. Chem.
Chem. Phys. 2012, 14, 13378.
nonradiative deactivation from solvents and its emission could be
observed eventhough some water molecules are coordinated to it
in the heteronuclear complexes. Finally, the phosphonate
functions of the L(D) ligands not only allow a strong coordination
of one Ln cation in the cavity, but the formed mononuclear LnL
(
6) Hyppanen, I.; Lahtinen, S.; Aaritalo, T.; Makela, J.; Kankare, J.;
Soukka, T. ACS Photonics 2014, 1, 394.
(7) Nonat, A.; Chan, C. F.; Liu, T.; Platas-Iglesias, C.; Wong, K.-L.;
3+
complexes are prone to bind strongly with incoming Ln cations
through bridging phosphonate interactions. A still unanswered
question of this upconversion process is that of its mechanism.
Charbonnier
8) Liu, T.; Nonat, A.; Beyler, M.; Regueiro-Figueroa, M.; Nchimi
Nono, K.; Jeannin, O.; Camerel, F.; Debaene, F.; Cianferani-Sanglier, S.;
Tripier, R.; Platas-Iglesias, C.; Charbonniere, L. J. Angew. Chem., Int. Ed.
014, 53, 7259.
9) Gnach, A.; Lipinski, T.; Bednarkiewicz, A.; Rybka, J.; Capobianco,
J. A. Chem. Soc. Rev. 2015, 44, 1561.
10) Sy, M.; Nonat, A.; Hildebrandt, N.; Charbonnier
Commun. 2016, 52, 5080.
̀
e, L. J. Nat. Commun. 2016, 7, 11978.
(
́
24a
̀
̈
Following Gudel’s work, the two main mechanisms are (i)
2
(
excited state absorption (ESA), in which an Yb atom absorb a
first photon and the excited state formed absorb a second one to
reach a higher lying excited state which transfers energy to the Tb
atom; or (ii) energy transfer upconversion (ETU), in which the
excitation of the first Yb atom is followed by absorption of a
photon by a second Yb atom, and an energy transfer occurs
cooperatively to excite the Tb atom.
(
̀
e, L. J. Chem.
(11) Weissleder, R.; Ntziachristos, V. Nat. Med. 2003, 9, 123.
(12) Mattsson, L.; Wegner, K. D.; Hildebrandt, N.; Soukka, T. RSC
Adv. 2015, 5, 13270.
(
13) Harvey, P.; Oakland, C.; Driscoll, M. D.; Hay, S.; Natrajan, L. S.
Dalton Trans. 2014, 43, 5265.
14) Souri, N.; Tian, P.; Lecointre, A.; Lemaire, Z.; Chafaa, S.; Strub, J.-
M.; Cianferani, S.; Elhabiri, M.; Platas-Iglesias, C.; Charbonniere, L. J.
Inorg. Chem. 2016, 55, 12962.
15) (a) Doffek, C.; Alzakhem, N.; Bischof, C.; Wahsner, J.; Gu
Silber, T.; Lugger, J.; Platas-Iglesias, C.; Seitz, M. J. Am. Chem. Soc. 2012,
34, 16413. (b) Doffek, C.; Seitz, M. Angew. Chem., Int. Ed. 2015, 54,
Almost 50 years after the first observations of the
1
upconversion phenomenon in the solid state, this unconven-
(
tional luminescence process is now fully integrated into the
luminescence toolbox. Nevertheless, its demonstration at the
molecular level is only 6 years old, with the pioneering work of
́
̀
(
̈
den-
4
Piguet and co-workers. We postulate that the findings reported
̈
here open very interesting perspectives within the field of
molecular upconversion, paving the way for the design of new
molecular systems, showing upconversion in water.
1
9719.
(16) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.;
Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem. Soc.,
Perkin Trans. 2 1999, 493.
ASSOCIATED CONTENT
Supporting Information
■
(
17) (a) Olmsted, J. J. Phys. Chem. 1979, 83, 2581. (b) Benson, R. C.;
*
S
Kues, H. A. Phys. Med. Biol. 1978, 23, 159.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b12940.
(18) Bu
̈
nzli, J.-C. G. Chem. Rev. 2010, 110, 2729.
(
19) (a) Zhang, T.; Zhu, X.; Cheng, C. C. W.; Kwok, W. M.; Tam, H.
L.; Hao, J.; Kwong, D. W. J.; Wong, W.-K.; Wong, K.-L. J. J. Am. Chem.
Soc. 2011, 133, 20120. (b) D’Aleo, A.; Bourdolle, A.; Brustlein, S.;
Fauquier, T.; Grichine, A.; Duperray, A.; Baldeck, P. L.; Andraud, C.;
Brasselet, S.; Maury, O. Angew. Chem., Int. Ed. 2012, 51, 6622.
1
13
31
Full synthetic details for ligand L , with H-, C-, and P
D
́
NMR spectra and mass spectra, excitation spectra of the
complexes and UV−vis absorption, luminescence spectra
and speciation diagram for titration of [YbL ] by TbCl
D
3
(
2
(
20) Bu
000, 1917.
21) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. Phys. Chem.
Chem. Phys. 2002, 4, 1542.
̈
̀
nzli, J.-C. G.; Charbonniere, L. J.; Ziessel, R. Dalton Trans.
(
PDF)
AUTHOR INFORMATION
■
(
(
(
22) Eliseeva, S. V.; Bu
23) Andraud, C.; Maury, O. Eur. J. Inorg. Chem. 2009, 2009, 4357.
24) See for example: (a) Salley, G. M.; Valiente, R.; Gudel, H. U. J.
̈
nzli, J.-C. G. Chem. Soc. Rev. 2010, 39, 189.
Corresponding Author
l.charbonn@unistra.fr
ORCID
*
̈
Phys.: Condens. Matter 2002, 14, 5461. (b) Puchalska, M.; Zych, E.;
Sobczyk, M.; Watras, A.; Deren, P. Mater. Chem. Phys. 2015, 156, 220.
̀
Loïc J. Charbonniere: 0000-0003-0328-9842
Author Contributions
N.S. and P.T. contributed equally.
(
c) Duan, Q.; Qin, F.; Wang, P.; Zhang, Z.; Cao, W. J. Opt. Soc. Am. B
013, 30, 456.
(25) (a) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbuhler, A. D.
2
̈
Talanta 1985, 32, 1133. (b) Maeder, M.; Zuberbu
990, 62, 2220.
̈
hler, A. D. Anal. Chem.
Notes
1
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The Algerian Ministry of Research is gratefully acknowledge for
the financial support of N.S.
■
REFERENCES
■
(
Gu
1) (a) Auzel, F. Chem. Rev. 2004, 104, 139−173. (b) Gamelin, D. R.;
̈
del, H. U. Acc. Chem. Res. 2000, 33, 235.
2) Lenth, W.; Macfarlane, R. M. Opt. Photonics News 1992, 3, 8.
3) Chen, G.; Qiu, H.; Prasad, P. R.; Chen, X. Chem. Rev. 2014, 114,
(
(
5
161.
D
DOI: 10.1021/jacs.6b12940
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX