Design, synthesis, crystal structure and photophysical studies of an emissive, terbium based sensor for zinc

Article abstract of DOI:10.1016/j.inoche.2007.05.021

The synthesis, crystal structure and photophysical studies of the probe complex Tb · L are described. Tb · L is a mononuclear complex with a 46-membered macrocyclic rings constructed by intermolecular hydrogen bond. The mononuclear complex Tb · L shows Laporte-forbidden ⁵D₄ → ⁷F_J f-f transitions and exhibits strong green luminescence emission bands in the solid state at ambient temperature which is characteristic of terbium ion. New selective luminescent lanthanide chemosensor for Zn(II) ion based on Tb · L is also described, suggesting host-guest complexation signaling transduction mechanism.

Full text of DOI:10.1016/j.inoche.2007.05.021

Inorganic Chemistry Communications 10 (2007) 1058–1062
www.elsevier.com/locate/inoche
Design, synthesis, crystal structure and photophysical studies of
an emissive, terbium based sensor for zinc
a
a,
a
a
Xue-Qin Song a,b, Wei Dou , Wei-Sheng Liu *, Jun-Na Yao , Yan-Ling Guo ,
a
Xiao-Liang Tang
a
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
b
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
Received 28 April 2007; accepted 14 May 2007
Available online 3 June 2007
Abstract
The synthesis, crystal structure and photophysical studies of the probe complex Tb Æ L are described. Tb Æ L is a mononuclear complex
with a 46-membered macrocyclic rings constructed by intermolecular hydrogen bond. The mononuclear complex Tb Æ L shows Laporte-
5
7
forbidden D4 ! FJ f–f transitions and exhibits strong green luminescence emission bands in the solid state at ambient temperature
which is characteristic of terbium ion. New selective luminescent lanthanide chemosensor for Zn(II) ion based on Tb Æ L is also described,
suggesting host–guest complexation signaling transduction mechanism.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Lanthanide complex; Crystal structure; Hydrogen bond, Supramolecular structure; Lanthanide luminescent sensor
The design of chemosensor–molecules that can selec-
tively recognize and signal the presence of a specific analyte
through the naked eye, electrochemical and optical
response is one of the main achievements of supramolecu-
lar chemistry [1]. On account of its simplicity and high sen-
sitivity, luminescence is increasingly important for trace
chemical detection [2]. Since the pioneering work on the
cation crown ether [3] using luminescence for monitoring
low concentrations of metal cations, considerable effort
has been devoted to developing luminescent chemosensors
for cations, neutral guests and anions [4].
ment of Zn2+-specific molecular probes [9,10], and many
Zn2+ fluorescent sensors have been reported, exhibiting
high selectivity and sensitivity over other biologically essen-
tial metal ions in specific ranges of concentration [11–14].
However the fluorescence lifetimes of typical organic com-
pounds, including common zinc-selective fluorescent sensor
molecules, are in the nanosecond region. On the other
hand, luminescent lanthanide complexes, in particular
Tb(III) and Eu(III) complexes, have large Stokes shifts,
sharp emission profiles and long luminescence lifetimes in
the order of milliseconds [15]. These long-lived luminescent
compounds have the advantage that short-lived back-
ground fluorescence and scattered light decay to negligible
levels when a pulse of excitation light is applied and the
emitted light is collected after an appropriate delay time.
There are only a few reports about luminescence sensors
for Zn(II) ions [16]. Moreover, design approach for lantha-
nide complexes showing a large enhancement upon Zn2+
addition is limited. From this background, we set out to
enlarge the arsenal for detecting Zn2+ by developing a
novel luminescent lanthanide complex.
Zn2+ is the second most abundant transition-metal ion
after Fe2+ or Fe3+ in humans and other mammals [5]. It
plays important roles in various biological systems such
as neurotransmission, signal transduction, and gene expres-
sion [6,7]. The emerging importance of Zn2+ in neurologi-
cal signalling and some proposed functions [8] in biological
systems have generated an urgent demand for the develop-
*
Corresponding author. Tel.: +86 931 8915151; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.05.021

X.-Q. Song et al. / Inorganic Chemistry Communications 10 (2007) 1058–1062
1059
Here we report the design, synthesis and crystal struc-
ture of the novel Zn2+-sensitive luminescent lanthanide
chemosensor Tb Æ L, upon complexation with Zn2+, it
exhibits enhanced luminescence. Picolylsalicylamide and
triethylene glycol di-p-tosylate was prepared according to
the literature methods [17,18]. The synthetic route for the
what is more, prevent the carbonyl oxygen atoms from
coordinating to the terbium ions to form a polymeric
structure.
Ligand 1,9-bis[2-(2’-picolyaminoformyl)-1,4,7,9-tetra-
oxadecane] luminescent in solution and in the solid state.
The emission band of the ligand is in the UV region and
with the emission in the solid state at 347 and 441 nm
which may correspond to the singlet and triplet states. In
addition to being fluorescent in the UV and near-UV
regions, the ligands can be excited from 270 to 320 nm,
making them possible candidates as activators for lantha-
nide luminescence. Intramolecular energy transfer from
the triplet state of the ligand to the resonance level of the
lanthanide ion is one of the most important processes influ-
encing the fluorescence quantum yields of lanthanide com-
plexes [22]. In order to acquire the triplet excited state T1 of
the ligand L, the phosphorescence spectra of the Gd(III)
nitrate complexes were measured at 77 K in a methanol
solution. The triplet state energy levels T1 of the ligand L
in the nitrate complexes, which were calculated from the
shortest wavelength phosphorescence bands [22] of the cor-
responding Gd(III) complexes, is 22,727 cmꢁ1. The energy
level is above the lowest excited resonance level 5D4
(20,400 cmꢁ1) of Tb(III). Looking at the energy of the
ligand triplet states in 42 terbium complexes, Latva et al.
[23] conclude that the energy back transfer is observed
when the energy difference between the 5D4 level of Tb(III)
and the lowest triplet state energy level of the ligand is less
than about 1850 cmꢁ1. Thus the absorbed energy could be
transferred from ligand to the Tb ions without the energy
back transfer process. When the compound Tb Æ L is
excited at 323 nm, the luminescence spectrum collected is
impressive as the transitions to the ground-state manifold
from the emitting 5D4 level are observed in moderate detail.
The dominant peak is the hypersensitive transition,
ligand
1,9-bis[2-(2’-picolylaminoformyl)-1,4,7,9-tetraox-
adecane] (L) is shown in Scheme 1 [19]. Tb Æ L was
obtained by the reaction of terbium nitrate, L in the molar
ratio of 1:1 in methanol-ethyl acetate solution [20] and was
revealed by X-ray diffraction [21].
Single-crystal X-ray diffraction studies shows Tb Æ L is a
mononuclear complex and the terbium(III) ion is nine
coordinated by two aqua molecules, six O atoms of biden-
tate nitrate anions and one carbonyl atoms of L. It is
important to note that the molecular geometry of the
ligand displays cis–cis conformation as shown in Fig. 1a.
The bond angles around the terbium centre range from
51.34(1) to 154.67(1)ꢁ indicating that the terbium ion is in
a somewhat distorted monocapped anti-square prism
Fig. 1b. Interestingly, one of the strong coordinated car-
bonyl oxygen atoms in L are not coordinated in Tb Æ L
which is very important for the construction of supramo-
lecular structures as follows. As shown in Fig. 1c, the coor-
dinated water molecules with nitrogen atoms of the
pyridine rings, oxygen atoms of nitrate and carbonyl oxy-
gen atoms of uncoordinated arms, generate strong inter-
molecular hydrogen bonds. The O(O16)–Hꢀ ꢀ ꢀNpy(N4)
˚
distance was found to be 1.854 A, with an Oꢀ ꢀ ꢀN distance
˚
of 2.698 A and the O–Hꢀ ꢀ ꢀN angle of 172.7ꢁ, thus the 46-
membered macrocyclic rings were formed between two
adjacent units. The O(O16)–Hꢀ ꢀ ꢀOcarbonyl(O6) distance
˚
was found to be 1.894 A, with an Oꢀ ꢀ ꢀO distance of
˚
2.729 A and O–Hꢀ ꢀ ꢀO angle of 167.1ꢁ which is crucial
important to stabilize the mentioned macrocyclic rings.
7
The O(O17)–Hꢀ ꢀ ꢀNpy(N2) distance was found to be
5D4 ꢁ F5, which is made up of a single intense peak with
˚
˚
1.950 A, with an Nꢀ ꢀ ꢀO distance of 2.799 A and the O–
Hꢀ ꢀ ꢀN angle of 176.9ꢁ, the O(O17)–Hꢀ ꢀ ꢀOnitrate(O10) dis-
a shoulder at about one-third the intensity. The
7
5D4 ꢁ F6 is the second largest peak observed and consists
˚
tance was found to be 2.054 A, with an Oꢀ ꢀ ꢀO distance of
of an intense peak with a shoulder at approximately one-
7
2.900 A and O–Hꢀ ꢀ ꢀO angle of 173.3ꢁ, led to this series
fifth intensity at lower frequency. The 5D4 ꢁ F4 transitions
˚
of hydrogen bonds being defined as linear, which further
assemble macrocyclic rings into a 1D cage-like supramolec-
ular structure (Fig. 1d). These strong hydrogen bonds
together with those intramolecular hydrogen bonds play
an important role in the stabilization of the structure and
consist of two peaks of equal intensity. The typical emis-
5
7
sion band at 620 nm corresponding to D4 ꢁ F3 disap-
pears due to double-frequency effect [24]. The
a
fluorescence quantum yield U of the terbium nitrate com-
plex in spectroscopic grade acetonitrile (concentration:
N
H
O
OCH3
O
N
NH2
N
N
O
H
K2CO3
DMF
OH
N
O
OH
O
O
O
O
H3C
SO2Cl
O
N
OTs
TsO
O
O
OH
H
HO
O
N
Scheme 1. The synthesis route of the ligand L.

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X.-Q. Song et al. / Inorganic Chemistry Communications 10 (2007) 1058–1062
Fig. 1. (a) ORTEP diagram (30% probability ellipsoids) showing the coordination sphere of Tb Æ L; (b) the coordination polyhedron of the Tb ion (pale
˚
green: Tb ion; red: oxygen atom). The key bond distances and angles (A and ꢁ): Tb(1)–O(5) 2.288(3), Tb(1)–O(16) 2.325(3), Tb(1)–O(17) 2.370(3), Tb(1)–
O(10) 2.444(3), Tb(1)–O(13) 2.448(4), Tb(1)–O(14) 2.454(3), Tb(1)–O(7) 2.469(3), Tb(1)–O(8) 2.479(3), Tb(1)–O(11) 2.521(3); O(5)–Tb(1)–O(16) 77.81(12),
O(5)–Tb(1)–O(17) 82.31(11), O(16)–Tb(1)–O(17) 149.60(11), O(5)–Tb(1)–O(10) 148.34(13), O(16)–Tb(1)–O(10) 125.88(11), O(17)–Tb(1)–O(10) 81.94(11),
O(5)–Tb(1)–O(13) 129.77(13), O(16)–Tb(1)–O(13) 89.95(14), O(17)–Tb(1)–O(13) 85.30(14), O(10)–Tb(1)–O(13) 75.84(13), O(5)–Tb(1)–O(14) 78.21(12),
O(16)–Tb(1)–O(14) 77.03(11), O(17)–Tb(1)–O(14) 76.55(11), O(10)–Tb(1)–O(14) 123.98(12), O(13)–Tb(1)–O(14) 51.56(12), O(5)–Tb(1)–O(7) 77.97(13),
O(16)–Tb(1)–O(7) 124.60(13), O(17)–Tb(1)–O(7) 72.20(12), O(10)–Tb(1)–O(7) 71.12(13), O(13)–Tb(1)–O(7) 142.04(14), O(14)–Tb(1)–O(7) 142.74(12),
O(5)–Tb(1)–O(8) 88.64(11), O(16)–Tb(1)–O(8) 79.08(12), O(17)–Tb(1)–O(8) 123.43(12), O(10)–Tb(1)–O(8) 77.53(12), O(13)–Tb(1)–O(8) 136.97(13),
O(14)–Tb(1)–O(8) 154.67(12), O(7)–Tb(1)–O(8) 51.34(11) O(5)–Tb(1)–O(11) 146.67(12), O(16)–Tb(1)–O(11) 74.54(11), O(17)–Tb(1)–O(11) 130.23(11),
O(10)–Tb(1)–O(11) 51.51(11), O(13)–Tb(1)–O(11) 68.74(13), O(14)–Tb(1)–O(11) 112.66(12), O(7)–Tb(1)–O(11) 102.96(12), O(8)–Tb(1)–O(11) 68.23(12).
(c) The macrocyclic ring formed by hydrogen bond. (d) The cage-like super molecular structure of Tb Æ L.
1.0 · 10ꢁ5 mol Lꢁ1) was found to be 0.21 with eosin
(U = 0.23) as reference. The absence of the ligand-based
emission in the spectrum of Tb Æ L also suggests that the
energy transfer from the ligand to the Tb(III) center is
effective.
the intrinsic emission of the Tb(III). Upon addition of
K+, Na+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+
,
Cu2+, Cd2+ and Hg+ in a 20-fold excess, no apparent
changes in emission spectra were found, however, a strong
fluorescence enhancement was obtained in the presence of
a small amount of Zn2+ cation. Similar fluorescent
enhancement was also observed upon addition of Al3+ cat-
ion. However, a smaller emission enhancement was
observed even upon addition of Al3+ cation in a 20-fold
excess. The shape of the titration curve with the platform
upon addition of the Zn2+ cation suggested the formation
of a 1:1 stoichiometry host–guest complex. Meanwhile, the
complexity of the titration curve with Al3+ cation did not
The fluorescence spectra were recorded from a solution
of Tb Æ L (5.0 · 10ꢁ6 M) in the absence or presence of var-
ious metal ions found at high concentrations in cells, such
as K+, Na+, Mg2+, Ca2+ and Zn2+ as well as transition-
metal ions such as Cr3+, Al3+, Mn2+, Fe2+, Co2+, Ni2+
,
Cu2+, Cd2+ and Hg+. As shown in Fig. 2, the fluorescence
spectra of compound Tb Æ L in acetonitrile exhibited a weak
emission band at about 545 nm, which was attributed to

X.-Q. Song et al. / Inorganic Chemistry Communications 10 (2007) 1058–1062
1061
Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+ and Hg+ do not
cause any detectable changes in the UV–vis spectra, even
when more than 20 stoichiometric ratios of cation are
added.
A possible reason for the enhanced luminescence may be
as follows. Before coordination with Zn2+, nitrogen atoms
of Tb Æ L could form an intramolecular hydrogen bond
with hydrogen atoms, which resulted in a photoinduced
electron-transfer, and the de-excitation occured mainly
via a nonradiative pathway. These processes consequently
led to the weak fluorescence of Tb Æ L in acetonitrile solu-
tion. Once Tb Æ L was coordinated with Zn2+, the elec-
tron-transfer process was forbidden [25], and an extended
p-electron conjugation system was formed synchronously.
This conjugation system was involved in an intramolecular
charge transfer (ICT) process from the ligand donor to the
Zn2+ acceptor. Owing to the formation of the extended p-
electron conjugation system, the Tb Æ L/Zn2+ system exhib-
ited an enhanced Tb-centred luminescence. Therefore, the
participation of nitrogen atoms in binding with Zn(II)
complexation helps in the release of fluorescence through
efficient intramolecular energy transfer from the pyridyl
group to the Tb(III) ion. Thus Tb Æ L represents a new case
of luminescence enhancement with Zn(II) ion. This finding
may not only enlarge the arsenal for detecting Zn(II) ion,
but also add new merits to the chemistry of salicylamide
derivatives, which are usually used as the chromophore
for the lanthanide fluorescence and constituents for tunable
supramolecular chemistry.
Fig. 2. Emission spectra (excitation at 308 nm) of Tb Æ L(5lM) in the
presence of various concentrations of Zn2+: 0, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0,
3.0, 4.0, 5.0 with respect to Tb Æ L. The bands arise from 5D4–7FJ and the J
values of the bands are labelled.
suggest the formation of a simple 1:1 stoichiometry host–
guest complex.
The UV–vis absorption spectral change was also moni-
tored during Zn2+ addition. The absorption spectrum of
Tb Æ L in acetonitrile without Zn2+ had kmax = 203 nm
and broad band at 291 nm, tailing out to 320 nm (Fig. 3).
The absorption spectrum of Tb Æ L changed upon addition
of Zn2+ (0–1.0 equiv), and then remained at a plateau upon
further addition of Zn2+ in accordance with the lumines-
cence spectra. The absorption wavelength changes below
300 nm were supposed to be due to pyridine-Zn2+ coordi-
nation [15(a)]. Thus, 1:1 complex stoichiometry was
observed in both the absorption and the luminescence
emission spectra of Tb Æ L. Upon addition of a stoichiome-
Acknowledgement
This work was supported by the National Natural Sci-
ence Foundation of China (Grants 20431010, 20621091
and J0630962).
tric ratio cations such as K+, Na+, Mg2+, Ca2+, Cr3+, Al3+
,
Appendix A. Supplementary material
CCDC 644770 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.
2007.05.021.
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X.-Q. Song et al. / Inorganic Chemistry Communications 10 (2007) 1058–1062
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90–95 ꢁC for 48 h. After cooling down, the mixture was poured into
water (100 cm3) and extracted with CHCl3. The CHCl3 layer was
evaporated to dryness to give the crude product, which was
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cmꢁ1): 3382(m), 2920(m), 2880(m), 1649(s, C@O), 1601(m), 1537(s),
1489(m), 1452(m), 1304(s), 1251(s, Ar–O), 1133(s, C–O–C), 1035(s),
755(s), 617(m). 1H NMR (300 MHz; CDCl3,): d 3.44 (4H, s), 3.82
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45.25, 68.04, 69.11, 70.52, 99.29, 112.60, 121.49, 121.65, 122.07,
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[20] The detailed synthesis for Tb Æ L is as follows: The ligand (0.1 mmol)
was dissolved in the minimum amount of ethyl acetate at 60 ꢁC.
Tb(NO3)3 Æ 6H2O (0.1 mmol) was also dissolved in the minimum
amount of ethyl acetate and added to the ligand solution and left to
stir at room temperature for 4 h. 1 cm3 of methanol was added and
filtered. Upon standing the filtrate at room temperature for several
weeks, [TbL(NO3)3(H2O)2] (Tb Æ L) was obtained as colorless needles.
Yield 52%. Anal. Found (%): C, 39.98; H, 4.24; N, 10.40; Tb, 16.48.
Calcd for C32H38N7O17O16Tb (%): C, 40.39; H, 4.02; N, 10.30; Tb,
16.70. Major IR bands (KBr; m, cmꢁ1): 1589 (w), 1569 (w), 1446 (s),
1394 (w), 1276 (w), 1230 (s), 1215 (s), 1146 (s), 1108 (s), 1088 (s), 996
(w) 855 (s), 842 (s) 822 (w), 772 (s), 750 (w) 726 (w), 637 (s) 626 (s), 577
(s), 557 (w), 527(w). UV–vis (CH3CN): 203 nm (20,400 Mꢁ1 cmꢁ1).
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CCD area detector with graphite-monochromated Mo Ka radiation
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˚
(k = 0.71073 A). The structures were solved with direct methods and
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refined with the full-matrix least-squares technique based on F2 using
the SHELXTL program package. Anisotropic thermal parameters
were applied to all non-hydrogen atoms. The organic hydrogen atoms
were generated geometrically, the water hydrogen atoms were located
from difference maps and refined with isotropic temperature factors.
Crystal data for Tb Æ L: crystal dimensions 0.34 · 0.23 · 0.19 mm,
ꢀ
˚
˚
Triclinic, space P1, a = 12.457(2) A, b = 12.900(2) A, c =
˚
[16] (a) O. Reany, T. Gunnlaugsson, D. Parker, J. Chem. Soc., Perkin
Trans. 2 (2000) 1819;
14.545(3) A,
a = 65.437(3)ꢁ,
b = 77.343(3)ꢁ,
c = 64.425(3)ꢁ,
V = 1915.3(6) A , Z = 2, qcalcd = 1.650 g cmꢁ3, M= 951.61. Of the
10016 symmetry independent reflections (2.54ꢁ< h < 25.01ꢁ), 6640
reflections are observed [I > 2r(I)]. On the basis of all these data and
514 refined parameters, R1 = 0.0339, wR2 = 0.0750 and GOF on F2
of 1.005 were obtained.
3
˚
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