Hetero-binuclear near-infrared (NIR) luminescent ZnLn (Ln = Nd, Yb or Er) complexes self-assembled from the benzimidazole-based ligand

Article abstract of DOI:10.1016/j.inoche.2012.05.041

With the [Zn(HL)₂(Py)] compound from the benzimidazole-based ligand H₂L (H₂L: 2-(1H-benzo[d]imidazol-2-yl)-6- methoxyohenol) as the precursor, the series of four hetero-binuclear ZnLn arrayed complexes [ZnLn(HL)₂

Full text of DOI:10.1016/j.inoche.2012.05.041

Inorganic Chemistry Communications 22 (2012) 126–130
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Hetero-binuclear near-infrared (NIR) luminescent ZnLn (Ln = Nd, Yb or Er)
complexes self-assembled from the benzimidazole-based ligand
a
a
a
a,b,
a
a
a
Guo-Xiang Shi , Wei-Xu Feng , Dan Zou , Xing-Qiang Lü
⁎, Zhao Zhang , Yao Zhang , Dai-Di Fan ,
a
c,
d
Shun-Sheng Zhao , Wai-Kwok Wong ⁎⁎, Richard A. Jones
Shaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest University, Xi'an 710069, Shaanxi, China
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, Fujian, China
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712‐0165, United States
a
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
With the [Zn(HL) (Py)] compound from the benzimidazole-based ligand H L (H L: 2-(1H-benzo[d]imidazol-
2
2
2
Received 1 February 2012
Accepted 22 May 2012
Available online 29 May 2012
2
[
-yl)-6-methoxyohenol) as the precursor, the series of four hetero-binuclear ZnLn arrayed complexes
ZnLn(HL) (Py)(NO ] (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) have been obtained by the further
reaction with Ln(NO ·6H O, respectively. The result of their photophysical properties shows that the
strong and characteristic NIR luminescence of Nd
2
3 3
)
3
)
3
2
3
+
3+
or Yb
ion with emissive lifetimes in microsecond
range, has been sensitized from the excited state ( LC and LC) of the benzimidazole-based ligand.
2012 Elsevier B.V. All rights reserved.
Keywords:
1
3
ZnLn arrayed benzimidazole-based com-
plexes
©
NIR luminescence
Sensitization and energy transfer
There is currently considerable interest in the near infrared (NIR)
the NIR luminescence of the centered Ln3+ ions. On the other hand,
based on the typical Salen-type Schiff-base ligands without the
3
+
3+
3+
3+
3+
luminescent Ln
(Ln
= Nd , Yb
or Er ) complexes because
of their potential applications in bio-analysis [1] and materials sci-
ence [2]. However, the f–f transitions for these ions are partially for-
bidden, absorption coefficients and emissive rates are very low [3].
In order to obtain efficient emissions, many organic (cyclic or acyclic)
ligands [4] and d-block metal complexes [5] have been used as anten-
nae or chromophores for the effective sensitization of NIR lumines-
outer O
2 2
O moieties from MeO groups, series of NIR luminescent
Zn Ln (Ln = Nd, Yb or Er) arrayed complexes have been obtained
2
[9], in which from the viewpoint of enhancement of NIR luminescent
properties, the use of the flexible linker while not the rigid one of the
typical Salen-type Schiff-base ligands without the outer O O moieties,
2 2
could improve the NIR luminescent quantum efficiency due to the effec-
3
+
3
cence of Ln
NIR luminescent Ln
ions. For this purpose, in the molecular design of the
tive intramolecular energy transfer from the ligand-centered LC and
3
+
1
1
3+
complexes, besides the complete avoidance
LC not just LC excited state to the Ln ions. Moreover, the occupation
of pyridine at the axial position of the Zn2 ion, is helpful for the strong
NIR luminescence, which should be due to the complete avoidance of the
luminescent quenching effect arising from OH-, CH- or NH-oscillators
around the Ln3 ion. As a matter of fact, the use of the Salen-type
+
or decrease of the luminescent quenching effect arising from OH-,
CH- or NH-oscillators around the Ln3 ion [6], another key rule to
the high NIR quantum yield should be further considered, requiring
the realization of the energy level's match of the excited state of the
chromophore to the corresponding Ln3 ion's exciting state for the
high-yield energy transfer [7], both are strongly relative to the suit-
able selection of organic chromophores.
+
+
+
2+
3+
Schiff-base ligands should not be necessary to bind both Zn and Ln
ions. Especially, the recent report of one magnetic [Cu(HL) Tb(NO
from the benzimidazole-based ligand H L (H L = 2-(1H-benzo[d]
2
3 3
) ]
2
2
Past studies by our groups have shown that in the formation of the
series of ZnLn heterometallic complexes [8] from the compartmental
imidazol-2-yl)-6-methoxyohenol) [10] stimulates us for further explora-
tion on the possibility of its Zn2 and Ln luminescent complexes, de-
+
3+
spite much research on the luminescent Ln3 complexes from the
+
Salen-type Schiff-base ligands with the outer O
2
O
2
moieties from
complexes based on the Salen-type Schiff-
base ligands, as the suitable chromophores, could effectively sensitize
2
+
MeO groups, the Zn
benzimidazole-based ligands [11]. Herein, with the [Zn(HL)
the benzimidazole-based ligand H L as the precursor, series of hetero-
binuclear ZnLn complexes [ZnLn(HL) (Py)(NO ] (Ln = Nd, 1; Ln =
2
(Py)] from
2
2
3 3
)
Yb, 2; Ln = Er, 3; Ln = Gd, 4) are obtained, respectively. The sensitization
and energy transfer for the NIR luminescence of the Ln ions in the ZnLn
complexes are discussed.
⁎
Correspondence to: X.-Q. Lü, Shaanxi Key Laboratory of Degradable Medical Mate-
3+
rial, Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest University,
Xi'an 710069, Shaanxi, China. Tel./fax: +86 29 88302312.
As shown in Scheme 1, treatment of o-phenylenediamine with o-
vanillin in 1:1 molar ratio in absolute EtOH at ambient temperature
⁎
⁎ Corresponding author. Tel./fax: +86 29 88302312.
E-mail addresses: lvxq@nwu.edu.cn (X-Q. Lü), wkwong@hkbu.edu.hk (W-K. Wong).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.05.041

G-X. Shi et al. / Inorganic Chemistry Communications 22 (2012) 126–130
127
0
produced the orange Schiff-base HL with the yield of ca. 75%, and the
sole presence of pure mono-imine compound was easily recognized in
the 1H NMR spectrum as a result of the presence of single peaks for
2
the \OH, imine and NH groups in a 1:1:2 signal integral ratio. Further-
more, in refluxing absolute EtOH, the mono-imine Schiff-base precursor
0
HL is converted into the off-white benzimidazole-based ligand H
good yield. Reaction of the benzimidazole-based ligand H L and
Zn(OAc) ⋅2H O in 2:1 molar ratio in the presence of excess absolute
pyridine (Py) afforded the precursor [Zn(HL) (Py)] in good yield of
ca. 74%. Further reaction of the precursor [Zn(HL) (Py)] with the
Ln(NO ⋅6H O (Ln = Nd, Yb, Er or Gd) resulted in the formation of
the series of the hetero-binuclear ZnLn complexes [Zn(HL) (Py)Ln
NO ] (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3 or Ln = Gd, 4), respectively.
It is interesting to note that although the synthesis of the
benzimidazole-based ligand H L is stepwise, not from the straightfor-
2
L in
2
2
2
2
2
3
)
3
2
2
(
3 3
)
Fig. 1. Perspective drawing of H L containing intra-molecular hydrogen bonding, H
2
atoms are omitted for clarity.
2
ward condensation with o-phenylenediamine and o-vanillin used in
the literature [10,11], the higher yield is due to the effective avoidance
of possible by-products of the Salen-type Schiff-base ligand and the N-
substituted benzimidazole ligand. Apart from the 1H NMR spectrum
spread in relative to those of the free H L ligand (δ from 13.42 to
2
3.83 ppm) and the precursor [Zn(HL) (Py)] (δ from 13.33 to 3.68 ppm).
2
2 3
identification of H L in CD CN with a sharp singlet at δ=13.27 ppm
The ESI-MS spectra of the four ZnLn complexes (1–4) exhibit the stron-
for the \OH proton, exhibiting the typical resonance-assisted hydrogen
bonded (RAHB) proton of the type O -HÁÁÁN=C [12], colorless single
gest peak at m/z 954.25 (1), 983.05 (2), 977.27 (3) or 967.26 (4), re-
spectively, corresponding to the major species [ZnLn(HL) (Py)(NO ) ]
2
3 3
crystals of H
2
L for X-ray crystallography were obtained from an EtOH
(Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4), further indicating
that the discrete ZnLn (Ln = Nd, Yb, Er or Gd) molecule exists in the re-
spective dilute MeCN solution. Thermogravimetric analysis (TGA) of the
four polycrystalline complexes shows the similar weight loss patterns, as
shown in Fig. 1S, where a gradual weight loss (about 10%) occurred be-
tween 25 and 210 °C, indicative of the loss of solvated molecules, and the
frameworks decomposed at ca. 308 °C with an observed abrupt weight
loss.
solution in air at room temperature, and the crystallographic data and
the selected bond lengths and angles are given in Tables 1–2S, respec-
tively. As shown in Fig. 1, the ligand H L crystallizes in the monoclinic
2
space group P2(1)/n. In the molecule, the fused benzene and imidazole
rings are nearly planar, of which the dihedral angle (2.2(2)°) is smaller
than that of 6.6(2)° between the benzimidazole moiety and the
substituted aryl ring. The molecule is slightly twisted about the C6–C8
bond, with the C5C6C8N2 and C7C6C8N1 torsion angles of 6.9(3)° and
X-ray quality crystals of 1⋅1.5MeOH⋅1.5H O⋅0.5Py as the represen-
2
6
.2(3)°, respectively. The intra-molecular strong hydrogen bonding be-
tative of the four ZnLn complexes were obtained, and the tables of
tween the \OH group and the imine N of the ligand with the N1ÁÁÁO2
selected crystal properties are also given in Tables 1–2S. Complex
distance of 2.586(2) Å, may be responsible for the small twist angles.
1⋅1.5MeOH⋅1.5H O⋅0.5Py crystallizes with two independent hetero-
2
The precursor [Zn(HL)
2
(Py)] and the four ZnLn complexes 1–4 were
binuclear molecules and solvates of MeOH, H O and Py in the asymmet-
2
1
well characterized by EA, FT-IR, H NMR and ESI-MS. As to the room
rical unit shown in Fig. 1S. As shown in Fig. 2, in both ZnNd molecules,
1
−
2+
temperature H NMR spectrum in CD
3
CN of complex 1, one set while
two deprotonated (HL) ligands sandwich one Zn
ion through their
N,O sites and one Nd3 ion through their O site, endowing a relative
+
large shift (δ from 14.59 to −2.65 ppm) of the photon resonances of
2
2+
−
3+
the (HL) ligands is observed due to the Nd -induced shift, significantly
arrangement close to head-to-head. Each Zn
(Zn1 or Zn2) ion has a
five-coordinate environment and adopts a distorted square pyramidal
geometry, composed of the inner N
benzimidazole-based HL ligands as the base plane, and one N atom
2
O
2
core from two deprotonated
−
Fig. 2. Perspective drawing of one of the hetero-binuclear part in complex
Scheme 1. Syntheses of the H
binuclear ZnLn complexes 1–4.
2
L, the precursor [Zn(HL)
2
(Py)] and the series of hetero-
1⋅1.5MeOH⋅1.5H
2
O⋅0.5Py, H atoms, the other hetero-binuclear part and solvates are
omitted for clarity.

128
G-X. Shi et al. / Inorganic Chemistry Communications 22 (2012) 126–130
3
+
from the coordinated Py at the apical position. The Nd
ion in both
2
+
ZnNd molecules is bridged the respective Zn ion by two phenoxo ox-
ygen atoms of two (HL) ligands with the similar Zn⋅⋅⋅Nd separation of
.636(3) Å. Each Nd ion is ten coordinate. In addition to the four ox-
8
6
4
2
0
Ex for 1
−
NIR Em for 1
Ex for 2
3
+
3
−
ygen atoms from two deprotonated benzimidazole-based (HL) ligands,
NIR Em for 2
Ex for 3
they complete their coordination environments with six oxygen atoms
−
NIR Em for 3
from three bidentate NO
3
anions. The lengths (2.026(14)–2.053(14) or
−
2.019(12)–2.060(10) Å) of Zn\N (the benzimidazole (HL) ligands,
N1 and N3 or N11 and N13) bonds are smaller than that (2.083(12) or
.110(12) Å) of Zn\N (Py, N5 or N15) bond while between those
2.042(8)–2.117(11) or 2.041(9)–2.101(8) Å) of Zn\O (phenoxo, O2
2
(
and O3 or O15 and O16) bonds. The Nd\O bond lengths depend on
the nature of the oxygen atoms: they vary from 2.354(9)–2.667(9) Å,
−
and the bond lengths from oxygen atoms of NO
3
anions are longer
than those from phenoxo oxygen atoms, while slightly shorter than
those from \OMe groups. Moreover, all the dihedral angles (14.8(3)°
and 23.9(3)° or 20.7(3)° and 23.2(3)°) in both ZnNd molecules between
the benzimidazole moieties and the substituted aryl rings, considerably
2
00
300
400
500
800 1000 1200 1400 1600
Wavelength (nm)
Fig. 4. NIR emission and excitation spectra of complexes 1–3 in MeCN solution at
2
larger than that (6.6(2)°) in the free H L ligand, should be due to the fur-
−
5
2
×10
M at room temperature.
ther coordination of metal ions. Besides the inter-molecular N10–
H10⋅⋅⋅N17 hydrogen bonding with N⋅⋅⋅N distance of 2.850(2) Å be-
tween the solvate Py and one of the ZnNd molecules, as shown in
Fig. 2S, the other solvate molecules (MeOH or H
the framework and they exhibit no observable interactions with the
host structure. It is worth noting that in the formation of ZnNd arrayed
2
O) are not bound to
temperature are observed, but photo excitation of the antennae at
the range of 275–500 nm (λex =377 nm for 1, 384 nm for 2 or
383 nm for 3), as shown in Fig. 4, gives rise to the characteristic emissions
3
+
4
4
3+
2
2
complex 1⋅1.5MeOH⋅1.5H
2
O⋅0.5Py, incomparable of that of magnetic
] [10], the occupation of Py at the axial position of
ions is considered to completely avoid the further coordination of
of the Nd
ion ( F → I , J=9, 11, 13), the Yb ion ( F → F7/2)
3
/2
J/2
4
5/2
and the Er3 ion ( I1 → I ) in the NIR region. For complex 1, the
+
4
[Cu(HL)
2
Tb(NO
3
)
3
3/2
15/2
2
+
4
4
Zn
emissions at 907, 1085 and 1354 nm can be assigned to F3/2 → I9/2,
3
+
4
4
and 4F3/2 → I
4
3+
solvents around the Nd ions. As to the bulk purity of the four polycrys-
talline complexes, it is convincingly established by X-ray powder diffrac-
tion measurements. As shown in Fig. 3S, for each of the four complexes,
the peak positions of the measured pattern closely match those of the
F3/2 → I
13/2
transitions of the Nd
ion, respectively.
11/2
For complexes 2 and 3, the strong emission at 1003 and the weak emis-
−
1
2
2
5/2
sion at 1548 cm can be attributed to the F → F transition of the
7/2
3+
3
+
ion and the 4I13/2 → I
4
Yb
15/2
transition of the Er
ion, respectively.
simulated 1⋅1.5MeOH⋅1.5H
formed for each complex.
The photophysical properties of the precursor [Zn(HL)
complexes 1–4 have been examined in dilute MeCN solution at room
temperature or 77 K, and summarized in Table 3S and Figs. 3–5. As
shown in Fig. 3, the similar ligand-centered solution absorption spectra
2
O⋅0.5Py, confirming that a single phase is
The precursor [Zn(HL) (Py)] or the complex 4 does not exhibit the NIR
2
luminescence under the same condition, just has the typically strong lu-
−
3
2
(Py)] and
minescence (λem =528 nm, τ=1.37 ns and ϕ=14.3×10 for the pre-
−
3
cursor) or weakened (λem =503 nm, τ=0.52 ns and ϕ=0.11×10 for
4) benzimidazole-based ligand H L in the visible region, as shown in
2
Fig. 5. It is worth noting that for complexes 1–3, the similar excitation
(
3
225–231, 298–301 and 339–340 nm) of complexes 1–4 to that (226,
01 and 358 nm) of the precursor [Zn(HL) (Py)] in the UV–visible
spectrum (λex =377 nm for 1, λex =384 nm for 2 or λex =383 nm for
3) monitored at the respective NIR emission peak (λem=1085 nm for
1, λem =1003 nm for 2 or λem =1548 nm for 3) to that (λex=377 nm
for 4) at the weakened visible emission peak (λem=503 nm), clearly
showing that both the visible and NIR emissions are originated from
2
region are observed, while the lowest energy absorptions of com-
plexes 1–4 are blue-shifted by 18–19 nm upon the further coordina-
tion of Ln
3
+
ions. For complexes 1–3, the similar residual visible
emission bands (ca. 503 nm and τb1 ns, almost undetectable) and low
quantum yields (Фem b10 ) in dilute absolute MeCN solution at room
the same π–π* transitions of the benzimidazole-based ligand H L, sug-
2
−
5
3+
gests that the energy transfer from the antenna to the Ln
ions takes
0
0
0
0
0
.8
.6
.4
.2
.0
6
2
.5x10
Ex for [Zn(HL) (Py)]
Zn(HL)2
(Py)]
2
[
Em for [Zn(HL) (Py)]
2
1
2
3
4
6
6
6
5
Ex for 4
Em for 4
2.0x10
1.5x10
1.0x10
5.0x10
0
.0
3
00
400
500
600
700
800
2
00
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 3. UV–vis spectra of precursor [Zn(HL)
2
(Py)] and complexes 1–4 in MeCN solution
2
Fig. 5. Visible emission and excitation spectra of precursor [Zn(HL) (Py)] and complex
at 2×10− M at room temperature.
5
4 in MeCN solution at 2×10
−5
M at room temperature.

G-X. Shi et al. / Inorganic Chemistry Communications 22 (2012) 126–130
129
place efficiently [13]. Moreover, for complexes 1–2, the respective lumi-
nescent decay curves obtained from time-resolved luminescent exper-
iments can be fitted mono-exponentially with time constant of
microseconds (1.28 μs for 1 at 1085 nm and 13.4 μs for 2 at 1003 nm),
and the intrinsic quantum yield ФLn (0.51% for 1 or 0.67% for 2) of the
characteristic NIR luminescence of Nd3+ or Yb3+ ion with emissive life-
times in the microsecond range, has been sensitized from the excited
state (both LC and LC) of the benzimidazole-based ligand due to the ef-
fective intramolecular energy transfer. While in facilitating the NIR sensi-
tization, the specific design of hetero-metallic polynuclear complexes
from the benzimidazole-based ligands is now under way.
1
3
3
+
0
Ln emission may be estimated by ФLn=τobs/τ , where τobs is the ob-
served emission lifetime and τ
0
is the “natural lifetime”, viz 0.25 ms and
ions, respectively [14], which indicates
3
+
3+
2
.0 ms for the Nd
and Yb
Acknowledgement
the presence of single emitting center for both 1 and 2 in dilute MeCN
solutions [15]. Due to the limitation of our instrument, we were unable
to determine the τobs value at 1354 nm for Nd3 ion or at 1548 nm for
This work is funded by the National Natural Science Foundation
+
(21173165, 20871098), the Program for New Century Excellent Talents
3
+
3+
the Er ion and thus could not estimate the ФLn value for the Er ion,
besides the reason to the rather weak NIR emission intensity for the Er3
ion in complex 3 with at least one order of magnitude weaker than the
in University from the Ministry of Education of China (NCET-10-0936),
the research fund for the Doctoral Program (20116101110003) of Higher
Education of China, the State Key Laboratory of Structure Chemistry
+
3
+
3+
corresponding Nd
or Yb
ion of complexes 1–2.
(20100014), the Provincial Natural Foundation (2011JQ2011) of Shaanxi,
As a reference compound, complex 4 allows the further study of
the Education Committee Foundation of Shaanxi Province (11JK0588),
Hong Kong Research Grants Council (HKBU 202407 and FRG/06-07/II-
the antennae luminescence in the absence of energy transfer, because
3
+
−1
the Gd
ion has no energy levels below 32,000 cm , and therefore
16) in P. R. of China, the Robert A. Welch Foundation (Grant F-816), the
cannot accept any energy from the antennae excited state [16]. In di-
Texas Higher Education Coordinating Board (ARP 003658-0010-2006)
and the Petroleum Research Fund, administered by the American Chem-
ical Society (47014-AC5).
lute MeCN solution at 77 K, complex 4 displays the stronger antennae
−
3
fluorescence than that (λem =503 nm, τ=0.52 ns and ϕ=0.11×10
)
at room temperature on the same condition, which shows the higher lu-
minescent intensity (λem =486 nm and 535 nm) and the distinctively
longer luminescence lifetimes (1.13 ns and 2.24 ms). This result shows
that the sensitization of the NIR luminescence for complexes 1–3 should
Appendix A. Supplementary material
1
−1
3
−1
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.05.041.
arise from both the LC (20,576 cm ) and the LC (18,692 cm ) excit-
ed state of the benzimidazole-based ligand H L at low temperature. If the
2
antennae luminescence lifetime of complex 4 is to represent the
excited-state lifetime in the absence of the energy transfer, the energy
transfer rate (kET) in the complexes 1–3 can thus be calculated from
References
2
+
k
ET=1/τ
based emission undergoing quenching by the respective Ln
and τ is the weakened while unquenched lifetime in the reference
q
−1/τ
u
[17], where τ
q
is the residual lifetime of the Zn
-
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[
2
(
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(
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(
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In conclusion, with the compound [Zn(HL)
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binuclear ZnLn complexes [ZnLn(HL) (Py)(NO ] (Ln = Nd, Yb, Er or
Gd) with two energy donors around the Ln ion are obtained. The re-
sults of their photophysical studies show that the strong and
2
(Py)] from the
2
(
2
3 3
)
3
+

130
G-X. Shi et al. / Inorganic Chemistry Communications 22 (2012) 126–130
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