Observation of cascade f → d → f energy transfer in sensitizing near-infrared (NIR) lanthanide complexes containing the Ru(ii) polypyridine metalloligand

Article abstract of DOI:10.1039/c6nj00089d

Distinguishable d → f or f → d energy transfer processes depending on lanthanide ions are observed in isomorphous d-f heterometallic complexes containing the Ru(ii) metalloligand (L_Ru), which lead to sensitized NIR emission (for Nd³⁺ and Yb³⁺) or enhanced red emission of L_Ru (for Eu³⁺ and Tb³⁺), and represent the first eye-detectable evidence of f → d energy transfer processes in Ln-Ru bimetallic complexes. Based on the systematic luminescence and decay lifetime study, cascade f → d → f energy transfer has been proposed in Ln1-Ru-Ln2 trimetallic systems for improved NIR sensitization.

Full text of DOI:10.1039/c6nj00089d

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Observation of cascade fdf energy transfer in
sensitizing near-infrared (NIR) lanthanide complexes
containing Ru(Ⅱ) polypyridine metalloligand
Cite this: DOI: 10.1039/x0xx00000x
Lu-Yin Zhanga, Kang Lia, Mei Pana,b*, Ya-Nan Fana, Hai-Ping Wanga, and
Cheng-Yong Sua,c*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Distinguishable df or fd energy transfers depending on lanthanide ions are observed in
isomorphous d-f heterometallic complexes containing Ru(II) metalloligand (LRu), which lead
to sensitized NIR emission (for Nd3+ and Yb3+) or enhanced red emission of LRu (for Eu3+
and Tb3+), and represent the first eye-detectable evidence of fd energy transfer progress in
Ln-Ru bimetallic complexes. Based on systematic luminescence and decay lifetime study,
cascade fdf energy transfer has been proposed in Ln1-Ru-Ln2 trimetallic systems for
improved NIR sensitization.
www.rsc.org/
been assembled by metalloligand method, and either visible11-14 or
NIR15-18 emissions of the incorporated Ln3+-centers have been
Introduction
During the past decades, sensitization of Ln(III) luminescence by
using organic ligands as light antenna has received wide attention
due to the potential applications in bioassays, sensors, lighting or
displaying devices, and so on.1-3 As an alternative approach, the use
of d-block complexes (e.g. Ir3+, Ru2+, Os2+, Pt2+, Re+, Au+) as
chromophores to sensitize the luminescence of Ln(III) centers has
also been proved successful. Many types of transition metal
chromophores have been employed to fabricate d-f heterometallic
complexes, in which the d-block chromophores were found to
display some advantages as energy donors to Ln-centers in
comparison with organic ligands, including: (a) strong and wide
absorbance of light in UV to visible regions, (b) intense emitting
states helpful for d → f energy-transfer or white-light modulation,
and especially, (c) a long-lived and relatively low 3MLCT (metal-to-
ligand charge transfer) excited state suitable for sensitizing NIR-
Ln(III) luminescence (e.g. Pr3+, Nd3+, Er3+ and Yb3+).4-7
Metalloligand method provides an efficient and reproducible way
to assemble d-f heterometallic complexes,8-10 in which the pre-
constructed metal complexes (especially polypyridyl complexes of
d6 and d8 metal ions) possess “free” donor groups to coordinate with
Ln3+ ions instead of simple organic ligands. Such metalloligands
incorporate the luminescent merits of d-block metal centers as well
as coordination capabilities of organic ligands, thus providing
versatile strategies to design novel d-f heterometallic complexes for
sensitization of different Ln3+ ions with desirable luminescence
properties. Up to now, a variety of d-f heteronuclear complexes have
achieved via d → f energy transfer from the metalloligand MLCT
3
states to Ln3+-centers. On the other hand, since the MLCT energy
state of the metalloligand is usually much lower than the * state of
organic chromophore, in principle, it is possible to observe f → d
back energy transfer from Ln3+-center to 3MLCT state of the
metalloligand. In this way, the luminescence of the metalloligand
itself can be intensified, benefitting from the additional * → f → d
1
3
energy immigration besides the normal MLCT → MLCT energy
transfer. However, although Bünzli and others have suggested such
kinds of energy transfer, seldom evidence has been observed yet.19,20
Herein, we use a Ru(II) polypyridine metalloligand to
construct a series of isostructural heterometallic complexes
with different Ln3+ ions. Interestingly, we found that the red-
emission of Ru(II)-metalloligand is significantly affected by the
incorporated Ln3+-centers, showing remarkable enhancement
with Eu3+ and Tb3+ while quenched by Nd3+ and Yb3+, which
can be directly detected by naked eye. Further
photoluminescence studies clearly indicate diversified energy
transfer pathways between Ru(II)-metalloligand and different
Ln3+ ions upon UV-vis excitations, including normal d → f
transitions for Nd3+ and Yb3+ and unprecedented f → d
transitions for Eu3+ and Tb3+. This is in consistent with the fact
3
that the MLCT energy level of Ru(II)-metalloligand (~15 700
cm-1) lies between the accepting levels of visible-emitting Tb3+
(5D4, ~20,400 cm-1), Eu3+ (5D0, ~17,500 cm-1), and NIR-
emitting Nd3+ (4F3/2, ~11,000 cm-1), and Yb3+ (2F5/2, ~10,000
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cm-1). Based on these observations, a cascade f → d → f energy (d, J = 5.4 Hz, 2H), 7.69 (d, J =5.1 Hz, 2H), 7.55 (t, J = 6.6 Hz, 2H),
transfer process has been proposed and utilized to achieve 7.50 (t, J = 6.6 Hz, 2H). EA anal. Calcd for C, 40.56; H, 2.55; N,
better NIR sensitization in trimetallic systems combining both 8.87 %; Found: C, 40.52; H, 2.27; N, 8.98 %.
Eu/Tb and Nd/Yb centers with Ru(II)-metalloligand.
Syntheses of Ln-Ru and Ln1-Ru-Ln2 Complexes. Bis-(2,2’-
bipyridine)-2,2’-bipyridine-4,4'-dicarboxylic acid-ruthenium (II)
Experimental
(LRu·(PF6)2, 19 mg, 0.03 mmol) was dissolved in water (3 ml) by the
addition of LiOH (1.4 mg, 0.06 mmol) and followed by the addition
of Tb(NO3)3·6H2O (4.5 mg, 0.01 mmol). A large quantity of
precipitates appeared, which were separated by centrifugation. The
mother liquid was left for natural evaporation and after two weeks,
pink lamellar crystals ([Tb(LRu)3(H2O)3](PF6)2.5(NO3)0.5(H2O)n, Tb-
Ru) suitable for single crystal diffraction were afforded. On the other
hand, the precipitates separated by centrifugation in the above
process were washed with water (1 ml × 2) to get the precipitate
samples for luminescence study. All the other heterometallic Ln-Ru
complexes were prepared in a similar way, and the trimetallic Ln1-
Ru-Ln2 complexes were prepared by mixing two kinds of lanthanide
nitrates with appropriate ratios in the reaction system.
Materials and methods
Solvents were purchased from Sigma Aldrich and distilled from
appropriate drying agents prior to use. Commercially available
reagents were used without further purification unless otherwise
stated. 1H NMR spectra were recorded on a Varian/Mercury-Plus
300 NMR spectrometer. The elemental analyses were performed
with Perkin-Elmer 240 elemental analyzer. The powder X-ray
diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE
X-ray powder diffractometer (Cu Kα, 1.5418 Å). UV-Vis absorption
spectra were recorded using
a
Shimadzu/ UV-250PC
spectrophotometer. The photoluminescence spectra were measured
on EDINBURGH FLS980 fluorescence spectrophotometer, which is
equipped with an R5509-72 PMT detector. The measurements were
performed in the solid state, and the emission/excitation spectra were
corrected according to the instrumental sets. Luminescence lifetimes
were measured by Time-Correlated Single Photon Counting
(TCMPC) method using 405 nm picosecond pulsed diode laser as
the excitation. Energy dispersive X-ray analysis (EDX) was
measured on a Quanta 400 thermal FE environment scanning
electron microscope with an INCA energy dispersive X-ray
spectrometer.
[Tb(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)11(Tb-Ru):
(based on LRu). EA anal. Calcd for C, 41.60; H, 3.42; N, 9.35 %;
Found: C, 41.52; H, 3.27; N, 9.08%.
[Eu(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)12 (Eu-Ru): Yield: 65%. EA
anal. Calcd for C, 41.44; H, 3.48; N, 9.31 %; Found: C, 41.63; H,
3.27; N, 9.10%.
[Gd(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)7(Gd-Ru): Yield: 60%. EA
anal. Calcd for C, 42.74; H, 3.21; N, 9.61 %; Found: C, 42.99; H,
3.58; N, 9.17%.
Yield:
60%
[Nd(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)10(Nd-Ru): Yield: 50%. EA
anal. Calcd for C, 42.10; H, 3.39; N, 9.46 %; Found: C, 42.38; H,
3.75; N, 9.25%.
[Yb(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)9 (Yb-Ru): Yield: 55%. EA
anal. Calcd for C, 41.94; H, 3.31; N, 9.42 %; Found: C, 42.34; H,
3.42; N, 9.15 %.
[EuxNdy(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)15 (Eu-Ru-Nd, x + y = 1,
x : y = 1 : 3, 1 : 1 and 3 : 1, respectively). Yield: 45 ~ 50%. EDX
results: feeding ratio, Ru : Eu : Nd = 1 : 0.08 : 0.25 (x : y = 1 : 3);
Found, Ru : Eu : Nd = 1 : 0.09 : 0.25; feeding ratio, Ru : Eu : Nd = 1 :
0.17 : 0.17 (x : y = 1 : 1); Found, Ru : Eu : Nd = 1 : 0.16 : 0.17;
feeding ratio, Ru : Eu : Nd = 1 : 0.25 : 0.08 (x : y = 3 : 1); Found, Ru :
Eu : Nd = 1 : 0.22 : 0.08.
[EuxYby(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)15 (Eu-Ru-Yb, x + y = 1,
x : y = 1 : 3, 1 : 1 and 3 : 1, respectively). Yield: 50 ~ 55%. EDX
results: feeding ratio, Ru : Eu : Yb = 1 : 0.08 : 0.25 (x : y = 1 : 3);
Found, Ru : Eu : Yb = 1 : 0.08 : 0.25; feeding ratio, Ru : Eu : Yb = 1 :
0.17 : 0.17 (x : y = 1 : 1); Found, Ru : Eu : Yb = 1 : 0.15 : 0.17;
feeding ratio, Ru : Eu : Yb = 1 : 0.25 : 0.08 ( x : y = 3 : 1); Found,
Ru : Eu : Yb= 1 : 0.22 : 0.10.
[TbxNdy(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)15 (Tb-Ru-Nd, x + y = 1,
x : y = 1 : 3, 1 : 1 and 3 : 1, respectively). Yield: 45 ~ 50%. EDX
results: feeding ratio, Ru : Tb : Nd = 1 : 0.08 : 0.25 (x : y = 1 : 3);
Found, Ru : Tb : Nd = 1 : 0.07 : 0.25; feeding ratio, Ru : Tb : Nd = 1 :
0.17 : 0.17 (x : y = 1 : 1); Found, Ru : Tb : Nd = 1 : 0.16 : 0.17;
feeding ratio, Ru : Tb : Nd = 1 : 0.25 : 0.08 ( x : y = 3 : 1); Found,
Ru : Tb : Nd= 1 : 0.23 : 0.09.
Scheme 1. Synthesis route for the Ru(II) metalloligand and
lanthanide complexes.
Synthesis of the Metalloligand LRu·(PF6)2. 1.16 g (2.39 mmol)
Ru(Bpy)2Cl2 (Bpy = 2,2’-bipyridine) and 1.04 g (3.46 mmol) diethyl
2,2'-bipyridine-4,4'-dicarboxylate were added into 250 ml round
bottom flask, and then the mixture of 160 ml ethanol/water (v : v =
1 : 1) was added. The solution was refluxed under the protection of
N2 atmosphere for 8 hours. After that, the solution was cooled and
distilled at low pressure to remove ethanol. 5 ml saturated water
solution of KPF6 was added to result in large quantity of red
precipitates, which were filtered and washed with diethyl ether. The
crude product was then added into 100 ml HCl solution (4 mol L-1)
and refluxed under the protection of N2 atmosphere for 12 hours. 5
ml saturated water solution of KPF6 was further added into the
resulting solution to result in red solids, which were filtered and
washed with water and diethyl ether to get pure product of
1
LRu·(PF6)2. Yield: 1.79 g (80%). H NMR (300 MHz, DMSO-d6):
9.24 (s, 2H), 8.86 (dd, J1 = 2.4 Hz, J2 = 8.4 Hz, 4H), 8.17 (m, 4H),
7.95 (d, J = 5.7 Hz, 2H), 7.87 (dd, J1 = 1.5 Hz, J2 = 6 Hz, 2H), 7.73
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[TbxYby(LRu)3(H2O)3]·(PF6)2.5(NO3)0.5(H2O)15 (Tb-Ru-Yb, x + y = 1, Interdigitation of two Tb(LRu 3
)
structural units through hydrogen
x : y = 1 : 3, 1 : 1 and 3 : 1, respectively). Yield: 45 ~ 50%. EDX bonding and π-π stacking.
results: feeding ratio, Ru : Tb : Yb = 1 : 0.08 : 0.25 (x : y = 1 : 3);
Construction of d-f Heteronuclear Complexes from Ru(II)
Metalloligand. The series of d-f heteronuclear complexes were
prepared by using the pre-constructed mononuclear Ru(II) complex
Found, Ru : Tb : Yb = 1 : 0.07 : 0.24; feeding ratio, Ru : Tb : Yb = 1 :
0.17 : 0.17 (x : y = 1 : 1); Found, Ru : Tb : Yb = 1 : 0.17 : 0.17;
feeding ratio, Ru : Tb : Yb = 1 : 0.25 : 0.08 ( x : y = 3 : 1); Found,
Ru : Tb : Yb= 1 : 0.24 : 0.08.
bis-(2,2'-bipyridine)-2,2'-bipyridine-4,4'-dicarboxylic
acid-
ruthenium(II) (LRu) as the metalloligand. In LRu, each Ru(II) ion is
coordinated by three bipyridine organic ligands with one of them
possessing two dicarboxylic groups for further coordination to Ln3+
ions (Fig. 1). The bimetallic Ln-Ru (Ln = Eu, Tb, Gd, Nd and Yb)
complexes were obtained by dissolving the deprotonated Ru(II)
metalloligand in water with the aid of LiOH and followed by the
addition of hydrated Ln(NO3)3 salts, offering both polycrystalline
and single-crystal samples. The crystalline products are apt to
effloresce in the air, therefore, the powder X-ray diffraction (PXRD)
analysis only afforded a poor scattering pattern (Fig. S1) with
informative small peaks compared with the simulation. However, the
elemental analyses and the PXRD patterns of the freshly precipitated
samples of Ln-Ru series proved that all as-prepared bimetallic
complexes are isostructural (Fig. S2). So we only analyzed the
single-crystal structure of Tb-Ru complex as the representative. The
trimetallic Eu(Tb)-Ru-Nd(Yb) systems were generated in a similar
way by mixing two Ln(NO3)3 in varied ratios together with the
metalloligand LRu (Table 1). Formation of isomorphous
polycrystalline phases was confirmed by PXRD analyses, which
present major diffraction peaks as the single-lanthanide samples,
although displaying obvious broadening in the peak profiles (Fig.
S3).
Crystallography. Single-crystal reflection data were collected on an
Oxford Gemini S Ultra diffractometer with the Enhance X-ray
Source of Cu-Kα radiation (λ = 1.54178 Å) using the ω-φ scan
technique.21 Empirical absorption correction was applied using
spherical harmonics implemented in SCALE3 ABSPACK scaling
algorithm.22 Structural solution and refinement against F2 were
carried out using the SHELXL programs.23 All the non-hydrogen
atoms were refined with anisotropic parameters, while H atoms were
placed in calculated positions and refined using a riding model,
except for the H atoms of water molecules, which were found by
electron cloud density (Q peaks). Crystal data for complex Tb-Ru:
C96H104F15N18.5O32.5P2.5Ru3Tb, Mr = 2861.53, monoclinic, space
group C2/c, a = 23.9537(5), b = 41.5149(9), c = 23.8634(4) Å,  =
o
108.316(2) , V = 22528.4(8) Å3, Z = 8, Dc = 1.687 g cm-3, T = 150
K. Refinement of 17406 parameters converged at final R1 [for data
with I > 2σ (I)] = 0.0826, wR2 (all data) = 0.2516. CCDC number:
1047975. The selected bond lengths and bond angles for compounds
are listed in Table S1.
Results and discussion
As shown in Fig. 1a, in complex Tb-Ru, the Ru(II)-center in LRu is
coordinated with six N atoms from three bipyridine ligands in an
octahedral geometry, and the Tb(III)-center is 9-coordinated with six
O atoms from three metalloligands (each using only one bidentate
carboxylic group, and the other remaining uncoordinated) together
with three water molecules, having an approximately three-capped
trigonal prismatic geometry. Every two Tb(LRu)3 structural units are
interdigitated and further extended in the crystal lattice with the aid
of hydrogen bonding and π-π stacking (Figs. 1b and S4). Bond
distances and angles around individual metal centers are in
accordance with usual Ru(II) and Tb(III) complexes as listed in
Table S1 (Ru-N: 2.02(1)~2.08(1) Å; Tb-O: 2.37(1)~2.61(1) Å), and
the closest RuTb, RuRu and TbTb separations are about 8.4,
8.1 and 6.1 Å. It is well known that either - or - stereo-
configuration can be formed around the 6-coordinated Ru(II) center,
and herein, in each Tb(LRu)3 structural unit, the same handedness is
found for the three LRu metalloligands. However, different
handedness is found for the LRu metalloligands in neighboring
Tb(LRu)3 structural units. Therefore, as a whole, the crystal structure
is racemic.
a)
Photoluminescence and d f / f d Energy Transfer in Ln-Ru
Complexes. The UV-vis absorption spectra (Fig. 2a) of LRu
metalloligand and Ln-Ru complexes in water solution cover a wide
range from 220-500 nm. In the UV region (<400 nm), the
absorptions are mainly associated with the π-π* electronic transitions
from the bpy moieties and d-d* transitions of Ru(II) in the
b)
Fig. 1 (a) Molecular structure of complex Tb-Ru (green for Tb, pink
for Ru, blue for N, gray for C, red for O, percentage of probability
30%. Solvated water, anions and H atoms are omitted for clarity). (b)
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metalloligand.24 From the major peak centered at 287 nm, we emission is significantly blue-shifted to ~630 nm, owing to the
estimate the lowest excited ππ* state of the organic chromophore at changes in metalloligand structure brought by the Eu/Tb3+
about 34 843 cm-1. Whereas the salient peak centered at 460 nm in coordination. Noticeably, the decay lifetimes of this Ru-based
the visible region indicates the 1MLCT (metal-ligand charge transfer) MLCT emission are increased to 820 and 760 ns at r.t. in Eu-Ru and
of LRu metalloligand at about 21 739 cm-1.25 In the solid state, the Tb-Ru complexes, together with the distinct increase of absolute
reflectance spectra (Fig. S5) of metalloligand and complexes extend quantum yields (12.7% and 11.7%) in comparison with the pure
to ~700 nm due to packing effect, but still bearing the characteristic metalloligand (5.1%). As seen in Fig. 2d, complexes Eu-Ru and Tb-
peaks around 460 and 300 nm. Excitation of LRu in the solid state Ru look much brighter than LRu under UV radiation, indicative of
produces a red emission centered at 693 nm (Fig. 2b, Table 1), obviously enhanced Ru-based emission upon coordination with Eu3+
originated from the 3MLCT state of the metalloligand with the and Tb3+. Furthermore, the Tb-Ru crystals show similar emitting
lifetime and absolute quantum yield tested to be 469 ns and 4.0% at profile (Fig. S6), but brighter emission and higher quantum yield
room temperature (r.t.), respectively, which are comparable with (21.8%) than the corresponding precipitated powders, probably due
other [Ru(bipy)3]2+-based chromophores.26-28
to better crystalline packing and less quenching effect of the solvated
water molecules.
Table 1. Photophysical data for the Ru-based MLCT emission and
Ln-based NIR emission in LRu, Ln-Ru and Ln1-Ru-Ln2.
Em-MLCT
Em-Ln(III)
Complex
QY*
(%)
max


max


(nm) (ns, r.t.)
(ns, 77K)
(nm) (ns, r.t.) (ns, 77 K)
LRu
693
469
1567
5.1
-
-
-
Eu-Ru
Tb-Ru
Gd-Ru
Nd-Ru
820
760
908
14
2276
1658
2588
247
12.7
11.7
11.0
0.4
-
-
-
-
-
-
630
106
144
424
Eu-Ru-Nd
(Eu:Nd=1:3)
27
602
776
0.5
0.8
1.9
0.8
0.9
1090
Eu-Ru-Nd
(Eu:Nd=1:1)
112
164
56
191
262
153
181
1342
1519
695
Eu-Ru-Nd
(Eu:Nd=3:1)
1593
321
650
1059
Tb-Ru-Nd
(Tb:Nd=1:3)
Tb-Ru-Nd
(Tb:Nd=1:1)
102
1263
743
Tb-Ru-Nd
(Tb:Nd=3:1)
186
71
1670
1135
621
1.8
1.2
1.6
292
595
758
1070
1662
1265
Yb-Ru
Eu-Ru-Yb
(Eu:Yb=1:3)
125
Eu-Ru-Yb
(Eu:Yb=1:1)
200
312
132
170
334
1301
1705
1207
1547
2005
2.7
4.3
1.5
2.7
4.4
816
935
778
814
991
1614
2040
1740
2276
2508
Eu-Ru-Yb
(Eu:Yb=3:1)
630
998
Tb-Ru-Yb
(Tb:Yb=1:3)
Tb-Ru-Yb
(Tb:Yb=1:1)
Tb-Ru-Yb
(Tb:Yb=3:1)
*The quantum yield (QY) for the complexes in visible region
(400-800 nm) were measured at the excitation of 335 nm.
After coordination with Eu/Tb3+ ions, the resulting d-f
heterometallic Eu-Ru and Tb-Ru complexes do not emit the
characteristic f → f luminescence of Ln3+ ions, but still give out a
broad red emission similar to that of pure LRu. However, the
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Fig. 2 (a) UV-vis absorption spectra of the metalloligand and Ln-Ru
complexes at room temperature (5  10-5 mol L-1, water); (b) Solid
state excitation and emission spectra of LRu and Eu-Ru, Tb-Ru
complexes; (c) Solid state excitation and NIR emission spectra of
Nd-Ru, Yb-Ru complexes (the broad emission in visible region is
from LRu metalloligand); (d) Photographs for LRu and different Ln-
Ru complexes under visible light (upper) and 365 nm UV radiation
(lower).
The absence of f → f emissions of Eu/Tb3+ in two Ln-Ru
3
complexes indicates inefficient MLCT  f (d  f) energy transfer
from LRu metalloligand to the excited f-levels, which is expectable
since the triplet state (~15 698 cm-1) of the metalloligand estimated
from phosphorescent data of Gd-Ru complex at 77 K (Fig. S7) is
5
5
lower than D0 of Eu3+ (17 500 cm-1) and D4 of Tb3+ (20 400 cm-1).
3
On the contrary, the reverse energy transfer f  MLCT (f  d) is
adequate to take place if Eu/Tb3+ ions are directly sensitized by the
excited ππ* state of the organic chromophore. The above speculation
is supported by the observation that the Ru-based red emission in
two Ln-Ru complexes is more effectively excited in the UV region
(< 400 nm) while the pure metalloligand LRu shows the highest
excitation in visible region (> 400 nm). As seen from Figure 3, the
excitation-emission dependence studies evidently reveal that the
most intensive Ru-based red emission of the free metalloligand LRu
is achieved at excitation wavelength ~470 nm, indicating a dominant
3
1MLCT  MLCT energy transfer process. In contrast, for the two
Ln-Ru complexes, the Ru-based red emission is more preferably
excited at shorter wavelengths below 400 nm, suggesting that the
higher excited ππ* state of the organic chromophore is involved in
3
energy transfer. This result evidently justifies a *  f  MLCT
energy immigration process, accounting for the enhanced 3MLCT
emission in bimetallic Eu/Tb-Ru complexes. Therefore, the two Ln-
Ru complexes can emit brighter Ru-based red emission with longer
decay lifetimes and higher quantum yields than the free
metalloligand LRu. In former reports, Bünzli and co-workers have
suggested that there might be f → d energy transfer from Ln(III) to
Ru(II) in a LnIII–DOTA–Bipy–RuII system,19 yet they did not
observe any intensified luminescence of Ru-metalloligand after Ln3+
coordination. While our cases represent direct and eye-detectable
evidence for sensitizing Ru-based MLCT emission by Ln3+ ions
coordination, which provides a secondary sensitizing pathway via a
1
3
*  f  d process besides normal MLCT  MLCT energy
transfer approach (Scheme 2).
Fig. 3 (a-c) Solid state excitation-wavelength-dependent emission
spectra of LRu ligand (a), and Eu-Ru (b) and Tb-Ru (c) complexes. (d)
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Emission intensity-excitation wavelength relationship of LRu ligand
and Eu-Ru and Tb-Ru complexes.
systems to achieve a cascade f  d  f energy transfer process
by incorporating both Eu/Tb3+ and Nd/Yb3+ ions with the Ru-
metalloligand. Since the Ln-Ru structural units are densely
packed in crystal lattice with suitable RuLn separation (~8 Å),
it is possible to accomplish an energy immigration from Eu/Tb-
center to LRu and then to Nd/Yb-center with the Ru-
metalloligand acting as both a wide UV-vis absorption antenna
and energy transfer bridge. We therefore prepared a series of
trimetallic Ln1-Ru-Ln2 samples by mixing Eu/Tb3+ and
Nd/Yb3+ ions in the isomorphous crystals as listed in Table 1.
The ratios of the different Ln3+ ions integrated in the trimetallic
samples are confirmed by energy-dispersive X-ray
spectroscopy (EDX), which are basically in consistent with the
feeding ratios.
On the other hand, the 3MLCT energy state of LRu (~15 698 cm-1)
is suitable for sensitizing the NIR luminescence of Yb3+ and Nd3+
via d  f energy transfer. As demonstrated in Scheme 2, the 4F3/2 of
2
Nd3+ and F5/2 of Yb3+ energy levels are around 10 000 cm-1, which
are appropriate to happen d  f transition from LRu-metalloligand.
Especially, Nd3+ has dense and multiple energy levels between 10
3
000~15 000 cm-1, which allows significant overlap with the MLCT
state of LRu. Therefore, in Nd-Ru complex, efficient energy transfer
occurs from Ru(II) to Nd(III) and intense NIR emissions at 900 and
4
1059 nm (4F3/2 → I9/2
,
4I11/2) were observed (Fig. 2c). The room
temperature decay lifetime detected at 1059 nm is about 106 ns,
amounting to a quantum yield Nd of 0.04% as calculated from the
natural lifetime of Nd3+ ions (0.25 ms). Simultaneously, the emission
intensity of LRu itself was greatly hampered and the decay lifetime of
its red emission decreased to 14 ns (Table 1). In Yb-Ru complex, the
2
NIR luminescence of Yb3+ was detected at 998 nm (2F5/2 → F7/2
)
with the decay lifetime and calculated Yb of 595 ns and 0.03%,
respectively. And the lifetime of LRu emission was decreased to 71
ns. According to the energy transfer efficiency () equation,
-1
 = 1 - q-1/u
,
in which q refers to the “quenched” lifetime of Ru-metalloligand
with Yb/Nd coordination and u refers to its “unquenched” lifetime
in Gd-Ru complex, the Ru → Nd and Ru → Yb energy-transfer
efficiency  were calculated to be 0.98 and 0.92 at room temperature,
respectively.29
Fig. 4 Solid state emission of Ln1-Ru-Ln2 (Ln1 = Eu or Tb,
Ln2 = Nd or Yb) complexes compared with Nd-Ru and Yb-Ru
(the broad emission in visible region is from LRu metalloligand,
ex = 335 nm).
As shown in Fig. 4, the trimetallic Ln1-Ru-Ln2 complexes
show similar emission profiles as those for Nd-Ru and Yb-Ru
complexes. Both the metalloligand based broad emissions and
characteristic NIR emissions of Nd3+ and Yb3+ ions are
observed, while the absence of characteristic emissions from
Eu3+ and Tb3+ ions manifests the similar f  d energy transfer
as described in Eu/Tb-Ru systems. The decay lifetime studies
provide more evidance about the energy transfer processes in
these complexes. For Eu-Ru-Nd system with Eu:Nd metal
ratios ranging from 1:3 to 1:1 and 3:1, the lifetime of the Ru-
Scheme 2. Diagram indicating the main energy levels and potential
energy transfer processes in different Ln-Ru complexes, showing a
cascade f → d → f energy transfer pathway by designing trimetallic
systems.
Cascade
f  d  f energy transfer in Ln1-Ru-Ln2
Trimetallic Complexes. Observation of above distinguishable
d  f and f  d energy transfer in isomorphous bimetallic Ln-
Ru complexes prompts us to design trimetallic Ln1-Ru-Ln2
6 | J. Name., 2012, 00, 1-3
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ARTICLE
based emission changes steadily from 27 to 112 and 164 ns,
respectively, falling between the pure Eu-Ru (820 ns) and Nd-
Ru (14 ns) complexes. This means that the increase of Eu:Nd
ratio leads to an increase in the decay lifetime of 3MLCT
emission, showing an intensified energy transfer from Eu3+ to
LRu metalloligand. Moreover, this increase of Eu:Nd ratio also
results in a constant increase in the decay lifetime of Nd3+-NIR
emission (144, 191 and 262 ns). The calculated quantum yield
based on Nd3+-emission also increases from 0.04% of Nd-Ru to
0.10% of Eu-Ru-Nd with Eu:Nd ratio of 3:1 at room
temperature, and reaches to 0.61% under 77 K. These results
indicate that the energy transfer from Eu3+ to LRu can further
immigrate to Nd3+, leading to a steady increase of decay
lifetime and luminescence quantum efficiency. Similar results
have also been testified in Tb-Ru-Nd, Eu-Ru-Yb and Tb-Ru-Yb
trimetallic systems (Table 1).
panm@mail.sysu.edu.cn; cesscy@mail.sysu.edu.cn
b
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, China
c
State Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou 730000, China
Electronic Supplementary Information (ESI) available: [PXRD, TG, more
emission and crystallographic information]. See DOI: 10.1039/b000000x/
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Acknowledgements
This work was supported by the 973 Program (2012CB821701),
the NSFC Projects (91222201, 21373276, 21450110063), the
Fundamental Research Funds for the Central Universities
(15lgzd05), the RFDP of Higher Education of China
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Table of Contents (TOC) entry
A possible approach to achieve multiple f → d, d → f and cascade f
→ d → f energy transfer processes
in heteronuclear Ru-Ln and Ln1-Ru-Ln2 complexes.