Luminescence modulation of a terbium complex with anions and its application as a reagent

Article abstract of DOI:10.1002/ejic.200501064

A terbium complex Tb(PMIP)₃(PhCN), namely tris(1-phenyl-3- methyl-4-isobutyl-5-pyrazolone)-terbium-(pyrazino[2,3-f][1,10]phenanthroline-2, 3-dicarbonitrile), was synthesized as a reagent for anions. Compared with Tb(PMIP)₃(H₂

Full text of DOI:10.1002/ejic.200501064

FULL PAPER
DOI: 10.1002/ejic.200501064
Luminescence Modulation of a Terbium Complex with Anions and Its
Application as a Reagent
Dengqing Zhang,[a] Mei Shi,[a] Zhiqiang Liu,[a] Fuyou Li,*[a] Tao Yi,[a] and
Chunhui Huang*[a]
Keywords: Luminescence modulations / Fluoride anions / Terbium complexes
A terbium complex Tb(PMIP)3(PhCN), namely tris(1-phen-
yl-3-methyl-4-isobutyl-5-pyrazolone)-terbium-(pyrazino[2,3-
f][1,10]phenanthroline-2,3-dicarbonitrile), was synthesized
as a reagent for anions. Compared with Tb(PMIP)3(H2O)2,
the fluorescent quantum yield of Tb(PMIP)3(PhCN) was re-
the replacement of PhCN with fluoride (or acetate) anions
took place and the above back-energy transfer was pro-
hibited, which resulted in a fluorescence enhancement of the
terbium complex. After excessive equivalents of fluoride (or
acetate) anions were added, the replacement of PMIP with
fluoride (or acetate) anions prohibited the ligand PMIP sensi-
tized energy transfer of the terbium complex, resulting in the
fluorescence quenching of the system. However, in the aque-
duced because the triplet energy level of PhCN (20920 cm–1
)
is a little higher than that of 5D4 of Tb3+ (20400 cm–1) and
lower than that of PMIP (23000 cm–1). This resulted in a back-
energy transfer from Tb3+ to PhCN. Interestingly, the photo- ous solution, the terbium complex shows a remarkable selec-
luminescent properties of Tb(PMIP)3(PhCN) drastically de-
pend on the nature of the anions added into the solution.
When apropos equivalents of fluoride (or acetate) anions
were added into the CH3CN solution of Tb(PMIP)3(PhCN),
tivity of fluoride anions over the other anions. As a reagent,
its sensitivity is about 10–8 mol·L–1 for the fluoride anions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
such as the strong affinity of a boron atom towards the
fluoride anion,[15–19] or the designed hydrogen bonding with
the fluoride anion.[20,21] These binding events have been
converted into an electrochemical[22] or fluorescent
change,[23–26] or more directly, colorimetric change seen by
the naked eye.[27,28] However, the reported fluorescent rea-
gents for fluoride anions is quite limited.[29–31]
Introduction
Anion recognition and sensing are of current interest be-
cause of their importance in biological, environmental, and
chemical systems.[1–4] The construction of anion chemosen-
sors is essentially composed of two units, the binding site
and the signaling subunit, which may be covalently linked
(binding site-signaling submit approach)[5–8] or not (dis-
placement approach).[9] The interaction with the anion and
the change in color or fluorescence are in principle revers-
ible. However, development of fluorescent reagents has
emerged as a research area of significant importance.[1,10,11]
In these systems, anion signaling using fluorescence or
color changes can also be observed using irreversible reac-
tions.[12] The underlying idea of these irreversible systems is
to take advantage of the selective reactivity that certain
anions may display. Hence, the use of an anion-induced rea-
gent system usually has high selectivity and also an ac-
cumulative effect is directly related with the anion concen-
tration. Fluoride, the smallest anion, has unique biological
and chemical properties, and its recognition and detection
are of growing interest because it is associated with dental
care[13] and the treatment of osteoporosis.[14] As a result,
there is a need to develop new sensitive methods for fluo-
ride detection. Most of the emerging detection methods
have focused on the specific Lewis acid-base interaction,
In the past decade, the synthesis, characterization, and
application of luminescent lanthanide complexes have been
the focus of much attention.[32–35] On the basis of the
unique photophysical properties of lanthanide cations (long
luminescence lifetime and very sharp emission band),[36,37]
lanthanide complexes as luminescent materials were paid
particular attention in the immunoassay[38] and organic
light-emitting diode (OLED).[39–42] Recently, some lumines-
cent lanthanide complexes have been used as a chemosensor
for anions.[43–50] For example, Ziessel et al. reported that
several lanthanide complexes were engineered from a ligand
consisting of a single P=O fragment and two methylene
linked bipyridine subunits for nitrate anion detection.[49,50]
It is well known that fluoride anions bind tightly with lan-
thanide ions, and some work has reported on the influence
of fluoride anions on the luminescence property of crown-
based lanthanide complexes by exchanging coordinated
water with fluoride anions.[51,52] In this paper, a new ter-
bium complex Tb(PMIP)3(PhCN) (see Scheme 1) was de-
signed and synthesized as a reagent for detecting anions by
another mechanism. Here, PMIP and PhCN stand for
[a] Laboratory of Advanced Materials, Fudan University,
Shanghai, 200433, P. R. China
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D. Zhang, M. Shi, Z. Liu, F. Li, T. Yi, C. Huang
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the anion of 4-isobutyl-3-methyl-1-phenyl-5-pyrazolone lateral faces O9–O10–O12–O16 and O9–O15–O13–O16,
and pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile, respectively. The average Gd–O distance is 2.42 Å [2.31(2)–
respectively. The photophysical studies of Tb(PMIP)3- 2.53(3)], which is a little smaller than the sum of the radii
(PhCN) showed that the luminescent properties of this of Gd3+ (1.05 Å, eight coordinated) and O2– (1.42 Å).
complex could be modulated upon addition of the different Furthermore, the structure of Gd(PMIP)3(C2H5OH)(H2O)
anions.
is the same as that of Gd(PMIP)3(H2O)2 except for an etha-
nol molecule occurring in place of the water molecule, the
coordination geometry is that of distorted a bicapped-trigo-
Scheme 1. Chemical structure of Tb(PMIP)3(PhCN).
Results and Discussion
Structure Characterization of Gd(PMIP)3(H2O)2·
Gd(PMIP)3(C2H5OH)(H2O) (A)
The structure of the gadolinium complex A was charac-
terized by X-ray crystallography. An ORTEP diagram for
the asymmetric unit is shown in Figure 1. Details of the
crystal data and data collection parameters for A are given
in Table 1. It can be seen form Figure 1 that a common
repeating unit exists for the two molecules of Gd(PMIP)3-
(H2O)2 and Gd(PMIP)3(C2H5OH)(H2O). In Gd(PMIP)3-
(H2O)2, the geometry around the central ions can be de-
scribed as a distorted bicapped-trigonal prism where the tri-
gonal prism is composed of six oxygen atoms (O9, O10,
O12, O13, O15, O16). Among them, O9 and O10 are from
one β-diketone, O12 and O13 are from the other two β-
diketones, and O15 and O16 are from two water molecules.
Another two oxygen atoms (O11, O14) cap the two quadri-
Table 1. Crystal data, collection and structure refinement param-
eters for complex A.
Gd(PMIP)3(H2O)2Gd(PMIP)3(C2H5OH)(H2O)
Empirical formula
Mr [gmol–1
C87H108Gd2N12O18
1924.35
]
Crystal system
Space group
triclinic
P1
¯
Crystal size [mm]
0.40×0.25×0.18
13.2931(2)
18.0432(2)
18.7608(3)
77.3215(5)
86.4151(5)
89.7183(7)
4381.29(11)
2
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
V [Å3]
Z
ρcalcd. [gcm–3
]
1.459
1.574
1972
µ [mm–1
F (000)
]
R1 [I Ͼ 2σ(I)]
wR2 [I Ͼ 2σ(I)]
R1 (all data)
wR2 (all data)
GOF
0.0422
0.0684
0.1134
0.0826
Figure 1. ORTEP diagram of A with the thermal ellipsoids drawn
at the 30% probability level and the H atoms removed for clarity.
0.938
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Luminescence Modulation of a Tb Complex with Anions
FULL PAPER
nal prism, as illustrated in Figure 1. Moreover, an indepen- (PhCN) in the different solvents, such as CHCl3, DMF, and
dent C2H5OH solvent molecule exists in the unit cell.
DMSO, the similarity of the emission patterns in these sol-
vents suggests that there are no solvent-dependent struc-
tural changes.
UV/Vis Spectra
UV/Vis absorption spectra of the ligands (PMIP and
PhCN) and Tb(PMIP)3(PhCN) in CH3CN solution are
shown in Figure 2. The maximum absorption band of
PMIP at 263 nm is attributed to a singlet-singlet π-π* enol
absorption of the β-diketonate. The ligand PhCN has two
absorption bands at 264 and 306 nm, which are attributed
to n-π* and singlet-singlet π-π* absorption, respectively.
Compared with the spectrum of PMIP, the absorption
maximum of Tb(PMIP)3(PhCN) is red-shifted by 5 nm,
which is agreement with the enlargement of the conjugated
structure of ligands after coordinating to the terbium ion.
It can be seen from Figure 2 that the spectral shape of
Tb(PMIP)3(PhCN) in CH3CN is similar to that of PMIP,
indicating that the coordination of the terbium ion does not
significantly influence the singlet-state energy of PMIP. The
molar absorption coefficients (ε) of PMIP, PhCN, and
Tb(PMIP)3(PhCN) are calculated as 2.55×104, 6.11×104,
and 9.61×104 L·mol–1·cm–1, respectively, revealing that the
two ligands and the terbium complex have a strong ability
to absorb light.
Figure 3. The luminescence spectrum of Tb(PMIP)3(PhCN) at
298 K in CH3CN.
Phosphorescence Spectra
Since the first excited energy level of the Gd3+ ion
(6P7/2) is high, the energy absorbed by the ligand in the
Gd3+ complex usually cannot be transferred to the Gd3+
ion. Therefore the phosphorescence emission from the Gd3+
complex can be considered as ligand-localized phosphor-
3
escence emission, and then the energy level of π-π* of the
ligand can be deduced. At room temperature, the phos-
phorescent emission spectrum of the Gd3+ complex is very
weak, however at a lower temperature (77 K), the ligand-
localized phosphorescent spectrum can be observed.
Herein, the gadolinium complexes Gd(NO3)3(PhCN) and A
were synthesized for triplet-energy level measurements. The
phosphorescent spectra of A and Gd(NO3)3(PhCN) at 77 K
are shown in Figure 4. The emission bands of A and
Gd(NO3)3(PhCN) peak at 464 nm (21552 cm–1) and 502 nm
(19920 cm–1), respectively. By referring to the lower wave-
length emission edges of the corresponding phosphorescent
Figure 2. UV/Vis spectra of PMIP, PhCN, and Tb(PMIP)3(PhCN)
in CH3CN (1.0×10–5 mol·L–1).
Fluorescence Characteristics
The fluorescence spectrum of Tb(PMIP)3(PhCN) in
CH3CN solution (see Figure 3) shows characteristic emis-
sion bands of Tb3+ (λex = 280 nm) centered at 489, 544,
5
584, and 612 nm resulting from the deactivation of the D4
7
excited state to the corresponding ground state FJ (J = 6,
5, 4, 3) of the Tb3+ ion. The strongest emission is centered
around 544 nm and corresponds to the hypersensitive tran-
5
sition of D4Ǟ7F5. The excitation spectrum of Tb(PMIP)3-
(PhCN) overlays the absorption wavelength range of PMIP
well, indicating that the emission of Tb(PMIP)3(PhCN)
originates mainly from the energy absorbed by PMIP. We
Figure 4. Phosphorescent spectra of A and Gd(NO3)3(PhCN) at
also investigated the emission spectrum of Tb(PMIP)3- 77 K.
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D. Zhang, M. Shi, Z. Liu, F. Li, T. Yi, C. Huang
FULL PAPER
spectra, the energies of zero-phonon transition of the triplet Tb(PMIP)3(PhCN) to Rhodamine 6G. From these mea-
states (3ππ*) at about 435 nm (23000 cm–1) and 478 nm surements and Equation (3), the probability of Tb3+ emis-
(20920 cm–1) were obtained for PMIP and PhCN, respec- sion (φLn) in Tb(PMIP)3(PhCN) was determined to be
tively.
1.1% (Table 2). In combination with the overall quantum
yield (φoverall) of Tb(PMIP)3(PhCN), the efficiency of en-
ergy transfer (φtransfer) from the ligand to Tb3+ was calcu-
lated to be 14.2% (see Table 2) according to Equation (1).
Similarly, the efficiency of energy transfer (φtransfer) of
56.3% and the probability of Tb3+ emission (φLn) of 5.3%
were obtained for Tb(PMIP)3(H2O)2.
With φLn and τd determined in acetonitrile, radiative and
nonradiative decay rates of the TbIII complex were calcu-
lated from Equation (4).
Photophysical Properties of the Terbium Complexes
By using Rhodamine 6G in ethanol (1×10–6 mol·L–1,
φref = 0.92) as a reference, the overall luminescence quan-
tum yields (φoverall) of Tb(PMIP)3(PhCN) and Tb(PMIP)3-
(H2O)2 in CH3CN were measured to be 0.15% and 2.97%
according to the well-known method.[53] The luminescence
lifetimes (τ) were also investigated for Tb(PMIP)3(PhCN)
and Tb(PMIP)3(H2O)2 in CH3CN. The measured lumines-
cent decay can be described by monoexponential kinetics,
and the luminescence lifetimes of Tb(PMIP)3(PhCN) and
Tb(PMIP)3(H2O)2 are 95.5 and 341.1 µs, respectively.
In order to understand the influence of the ligand modi-
fication on the photophysical property of Tb(PMIP)3-
(PhCN), the efficiency of energy transfer (φtransfer) from the
ligand to Tb3+ and the probability of the terbium emission
krad = φLn/τdknr = (1 – φLn)/τd
(4)
Taking 1.1% of φLn and 95.5 µs of τd into account, the
radiative and norradiative decay rates of Tb(PMIP)3-
(PhCN) were calculated as 111 and 1.04×104 s–1, respec-
tively (see Table 2). Similarly, the radiative and nonradiative
decay rates of Tb(PMIP)3(H2O)2 were determined as 151
and 2.71×103 s–1 (see Table 2), respectively.
To elucidate the energy transfer process of the terbium
complex, the energy levels of the relevant electronic states
should be estimated. By referring to their wavelengths at
the UV/Vis absorbance edges, the singlet-energy levels of
PMIP and PhCN were estimated to be 32260 cm–1 (310 nm)
and 29410 cm–1 (340 nm), respectively. In combination with
the triplet-energy levels of PMIP and PhCN, the schematic
energy level diagram and the energy transfer process are
shown in Figure 5.
(φLn
)
were investigated for Tb(PMIP)3(PhCN) and
Tb(PMIP)3(H2O)2 on the basis of the method developed by
Selvin et al.[54] The overall quantum yield (φoverall) can be
defined as Equation (1).
φoverall = φtransferφLn
(1)
When a lanthanide complex is mixed with an acceptor
of known quantum yield and very short fluorescence life-
time (ns), the efficiency of energy transfer between them
(φET) and the probability of lanthanide emission (φLn) are
calculated from both the fluorescence decay lifetime and
intensity measurements, see Equation(2) and (3).
φET = 1 – (τda/τd)
(2)
φLn = φa(Ida/Iad)/(1/φET – 1) = φaIda(τd – τad)/(Iadτad
)
(3)
Here τad and τd are the excited state lifetime of the lan-
thanide complex in the presence and absence of the ac-
ceptor, respectively; φa is the fluorescence quantum yield
of the acceptor; Ida is the integral area under the residual
lanthanide emission in the presence of the acceptor; and Iad
is the integral area of the fluorescent emission part of the
acceptor.
Here, Rhodamine 6G was used as the acceptor in ethanol
(φa = 0.92 and τ Ͻ 10 ns). For example, in a mixture solu-
tion consisting of Tb(PMIP)3(PhCN) (10 µ) and Rhoda-
mine 6G (1.25 µ), the emission lifetime of the TbIII com-
Figure 5. Schematic energy level diagram and the energy transfer
process in the system of Tb(PMIP)3(PhCN). S1: the first excited
singlet state, T1: the first excited triplet state.
Generally, the sensitization pathway in luminescent ter-
plex (λem = 544 nm) decreased from 95.5 to 80.0 µs, indicat- bium complexes consists of the excitation of the ligands
ing that 16.2% of the energy was transferred from from the ground state to their excited singlet states, and
Table 2. Photophyscical properties of Tb(PMIP)3(PhCN) and Tb(PMIP)3(H2O)2 in CH3CN.
PL
System
λmax
τ
[µs]
φoverall
φLn
[%]
φtransfer
[%]
krad
knr
[nm]
[×103]
[s–1]
[s–1]
Tb(PMIP)3(H2O)2
Tb(PMIP)3(PhCN)
544
544
341.1
95.5
114.9
29.7
1.5
10.5
5.3
1.1
3.7
56.3
14.2
28.4
151
111
321
2.71×103
1.04×104
8.4×103
Tb(PMIP)3(PhCN) + 3F– 544
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Luminescence Modulation of a Tb Complex with Anions
FULL PAPER
subsequently through the intersystem crossing of the li- the terbium ion, Cl–, Br–, and I– had difficultly in displacing
gands to their triplet states, following the energy transfer the ligands PhCN and PMIP, which was confirmed by the
from the triplet state of the ligand to the central ion. In this fact that there was no obvious change in the luminescent
process the 4f electrons of the Tb3+ ion are excited to the emission of Tb(PMIP)3(PhCN) with further addition of
5DJ manifold ion from the ground state, finally the Tb3+ Cl–, Br–, or I– (see Figure 6).
ion emits when the 4f electrons undergo a transition from
the excited state of 5D4 to the ground state.[55,56] We noticed
1
that the energy gaps ∆E (1ππ*-3ππ*) between the ππ* and
3ππ* levels are 9260 and 8490 cm–1 for PMIP and PhCN,
respectively (see Figure 5). According to Reinhoudt’s em-
pirical rule that the intersystem crossing process will be ef-
fective when ∆E (1ππ*-3ππ*) is at least 5000 cm–1,[57] the
intersystem crossing processes are effective for PMIP and
PhCN. On the other hand, the triplet-energy level of PMIP
5
(Ϸ23000 cm–1) is higher than that of D4 (20400 cm–1) for
Tb3+, and their energy gap ∆E (3ππ*-5D4) between the li-
gand- and metal-centered level is 2600 cm–1. According to
Latva’s empirical rule,[58] an optimal ligand-to-metal trans-
fer process for TbIII is when ∆E (3ππ*-5D0) Ͼ 2000 cm–1. It
can be concluded that the transfer process is also effective
from PMIP to Tb3+ and that PMIP is suitable as a sensi-
tizer for TbIII. On the contrary, the 3ππ* state (ca.
Figure 6.
Fluorescent
titrations
of
Tb(PMIP)3(PhCN)
20920 cm–1) of PhCN is so close to 5D4 (20400 cm–1) of
Tb3+, giving ∆E (3ππ*-5D0) = 520 cm–1, which is too low to
prevent the back-energy transfer from the Tb3+ excited state
to the triplet state of PhCN. Moreover, the triplet energy
(1.0×10–5 mol·L–1) in CH3CN upon addition of F–, Cl–, Br–, I–,
–
–
–
ClO4 , NO3 , NO2 , and AcO–.
It is well known that the lanthanide complex receptor
level of PhCN is lower than that of PMIP, and the energy prefers smaller (harder) F– (or AcO–) to larger (softer) Cl–,
transfer from PMIP to PhCN may occur. Therefore, it is Br–, or I–. In fact, a special fluorescent modulation of
not difficult to understand the fact that Tb(PMIP)3(PhCN) Tb(PMIP)3(PhCN) was observed upon addition of fluoride
exhibits a lower fluorescence quantum yield (φoverall
(0.15%) compared to that (2.97%) of Tb(PMIP)3(H2O).
)
and acetate anions. For example, Figure 7 shows fluorescent
spectral changes of Tb(PMIP)3(PhCN) in CH3CN solution
(10 µ) upon addition of tetra-n-butylammonium fluoride
(Bu4NF). Upon addition of the fluoride anion, the charac-
teristic terbium emission (λex = 280 nm) greatly increased
along with an increase of the emission intensity of PhCN
Fluorescence Modulation of the Complex with Anions
The fluorescence modulation of Tb(PMIP)3(PhCN) with centred at 400 nm (see Figure 7, a). When Bu4NF reached
anions was investigated by the fluorescence titration experi- about three equivalents of Tb(PMIP)3(PhCN) (see Figure 7,
ments in CH3CN. Figure 6 shows the titration curves of a), this change became saturated with a fluorescent quan-
the spectral parameters of Tb(PMIP)3(PhCN) against the tum yield of 1.05%. Under these conditions, the efficiency
[anion]/[Tb(PMIP)3(PhCN)] ratio in acetonitrile. Interest- of energy transfer (φtransfer) and the probability of Tb3+
ingly, a different fluorescent modulation of Tb(PMIP)3- emission (φLn) for this mixture were measured to be 28.4%
(PhCN) was observed upon addition of the different anions. and 3.7%, respectively (see Table 2). It can be seen from
For example, a slight variation of the luminescent proper- Table 2 that the mixture exhibited a higher energy transfer
ties of Tb(PMIP)3(PhCN) was observed upon addition of efficiency (φtransfer) and larger probability of Tb3+ emission
–
–
–
other anions such as ClO4 , NO3 , or NO2 . When one compared with Tb(PMIP)3(PhCN). Because of its strong
equivalent of Cl–, Br–, or I– was added, the fluorescent in- bonding ability, the fluoride anions were able to substitute
tensity of Tb(PMIP)3(PhCN) reached the maximum (see one solvent molecule and one PhCN molecule that were
Figure 6), and then it was almost unchanged with further weakly coordinated to TbIII in the complex, and so TbIII
addition of Cl–, Br–, or I–. Similarly, Mahajan et al. re- reached a coordination number of nine, revealing that the
5
ported that the luminescence intensity of tris(β-diketonate) back-energy transfer from D4 of Tb3+ and the triplet state
europium (λem = 611 nm) was enhanced twofold when three of PMIP to the triplet state of PhCN was restrained (see
equivalents of Cl– were added.[47] Generally, the coordina- Figure 5); consequently, both the probability of Tb3+ emis-
tion number of the TbIII complexes in solution is nine.[59] sion (φLn) and the fluorescent quantum yield of the mixture
The coordination number of Tb3+ in Tb(PMIP)3(PhCN) is were improved. However, further addition of a large excess
eight, implying that the complex can bond one anion to of Bu4NF caused a decrease of the characteristic fluores-
form a coordination of nine, hence, the increasing fluores- cent emission of Tb3+, and the emission was quenched com-
cent emission was observed upon addition of one equiva- pletely when nine equivalents of Bu4NF were added (see
lent of the anions. Because of their weak bonding ability to Figure 7b). These changes can be interpreted as the follow-
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D. Zhang, M. Shi, Z. Liu, F. Li, T. Yi, C. Huang
FULL PAPER
ing: upon addition of more fluoride anions, PMIP ligands other anions, which may be explained by the small size, the
were completely replaced by fluoride anions, resulting in high surface charge density of the fluoride anion and its
the breakage of the ligand-sensitized energy transfer from strong affinity to Tb3+ [60]
Even in this situation, the solu-
.
PMIP to Tb3+, consequently reducing the fluorescent quan- tion is very clear and transparent, the observable sensitivity
tum yield of the complex. Compared with the fluoride is about 10–8 mol·L–1 of fluoride anions.
anion, the acetate anion exhibited a similar modulation be-
havior for the fluorescent emission of the terbium complex
(see Figure 6). From the above results, we can see that the
addition of fluoride and acetate anions affects the quantum
yield of Tb3+ emission and modulates the energy transfer
path, which finally results in the modulation of the overall
emission quantum yield.
Figure 8.
Fluorescent
titrations
of
Tb(PMIP)3(PhCN)
(1.0×10–5 mol·L–1) in CH3CN/H2O (9:1 v/v) upon addition of F–,
–
–
–
Cl–, Br–, I–, ClO4 , NO3 , NO2 , and AcO–.
Conclusions
In summary, we have designed and synthesized a novel
lanthanide-based luminescent reagent Tb(PMIP)3(PhCN)
for anions. By using Tb3+ as the fluorophore, Tb(PMIP)3-
(PhCN) displays high selective fluorescent modulation on
the fluoride and acetate anions in acetonitrile. The large
fluorescent enhancement at first and then fluorescence
quenching were observed for the terbium complex upon ad-
dition of fluoride and acetate anions. However, in aqueous
solution, Tb(PMIP)3(PhCN) shows a unique fluorescent
quenching upon addition of fluoride, indicating its remark-
able selectivity for fluoride anions over the other anions. At
the same time, the reason for the fluorescence modulation
with the anions was described. This system is particularly
useful relative to other molecules because it allows detec-
tion of traces of fluoride and acetate anions by lumines-
cence monitoring at a terbium center.
Figure 7.
Fluorescent
titrations
of
Tb(PMIP)3(PhCN)
(1.0×10–5 mol·L–1) upon addition of Bu4NF.
However, when Tb(PMIP)3(PhCN) was dissolved in the
mixture of CH3CN/H2O (9:1 v/v), only a decrease in fluo-
rescent emission of the complex was observed upon ad-
dition of fluoride and acetate anions. Under these condi-
tions, the coordinating water molecules can compete with
PhCN fragments and gradually remove them from the first
coordination sphere of Tb3+, then back-energy transfer
from the Tb3+ excited state to the triplet state of PhCN
was broken. In fact, the complex exhibited more intense
fluorescent emission (φ = 0.62%) in CH3CN/H2O (9:1 v/v).
When further fluoride anions were added, PMIP was re-
placed by fluoride anions, resulting in the breakage of the
ligand-sensitized energy transfer from PMIP to Tb3+ (see
Figure 5), and then the fluorescence of Tb3+ was quenched.
However, fluorescent quenching to different degrees was
observed for Tb(PMIP)3(PhCN) upon addition of the dif-
ferent anions (see Figure 8). Consequently, the complex
Experimental Section
General: Diaminomaleonitrile and tetrabutylammonium fluoride
trihydrate were obtained from Acros. TbCl3·6H2O, Gd(NO3)3·
6H2O, and 1-phenyl-3-methyl-4-isobutyl-5-pyrazolone were pur-
chased from Shanghai Sinopharm Chemical Reagent Co. Ltd.,
Phenanthroline-5,6-dione was synthesized according to the litera-
ture.[61] The complexes Tb(PMIP)3(H2O)2 and Gd(PMIP)3(H2O)2·
Gd(PMIP)3(C2H5OH)(H2O) (A) were synthesized according to a
similar method reported previously[62] and characterized by ele-
mental analyses. Moreover, the structure of the complex A was
showed a remarkable selectivity of fluoride anions over the characterized by X-ray crystallography. UV/Visible absorption
2282 Eur. J. Inorg. Chem. 2006, 2277–2284
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Luminescence Modulation of a Tb Complex with Anions
FULL PAPER
were used. The crystal data and details of the structure
4 1/2
spectra were recorded with a UV 2550 spectrophotometer of Shim-
adzu. The element analyses were performed with Vario EL III O-
ΣwFo
]
determinations are summarized in Table 1.
1
Element analyzer. HNMR spectra were recorded with a Mercury
CCDC-288368 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Plus 400NB NMR spectrometer. The luminescence and phospho-
rescence spectra were measured with an Edinburgh LFS920 fluo-
rescence spectrophotometer and LP-920 laser flash photolysis spec-
trophotometer, respectively. For the phosphorescence measure-
ments, the fourth harmonic 266 nm of the pulsed GCR-4 Nd:YAG
Laser (Spectra-Physics, USA) with 30 Hz repetition rate and 6 ns
pulse width was used as the excitation source, and the experiments
were completed at 77 K in ethanol/methanol (1:1 v/v). Fluorescence
lifetimes were recorded with a single photon counting spectrometer
from Edinburgh LFS920 fluorescence spectrophotometer with
microsecond pulse lamp as the excitation source. The data were
analyzed by an iterative convolution of the luminescence decay pro-
file with the instrument response function using a software package
provided by Edinburgh Instruments.
Fluorescence Modulation: Fluorescence modulations were carried
out in the following manner: the solution of Tb(PMIP)3(PhCN) in
a 1.0 cm quartz cuvette was titrated with the concentrated solutions
of tetrabutylammonium salts of different anions (X = F–, Cl–, Br–,
I–, ClO4 , NO3 , NO2 , and AcO–) by a micro sample injector. In
order to account for the dilution effect, the volume of these concen-
trated solutions was negligible. The concentration of Tb(PMIP)3-
(PhCN) in all experiments was 1.0×10–5 mol·L–1.
–
–
–
Acknowledgments
Synthesis of Pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile
(PhCN): PhCN was synthesized according to a similar method re-
ported previously.[63] 2,3-Diaminomaleonitrile (6.0 mmol) was
added to a solution consisting of 1,10-phenanthroline-5,6-dione
(1.0 g, 4.8 mmol) and absolute ethanol (30 mL) in a 50 mL flask.
The reaction mixture was refluxed for 3 h and then cooled to room
temperature. The solution was concentrated to ca. 10 mL with a
rotary evaporator and the solid product was filtered. After
recrystallization from ethanol twice, the desired pale yellow powder
(1.0 g) was obtained with a yield of 74%. 1HNMR (400 MHz,
CDCl3): δ = 7.93 (q, 2 H), 9.4–9.46 (d, 2 H), 9.46–9.52 (d, 2
H) ppm. C16H6N6(282.26): calcd. C 68.08, H 2.14, N 29.77; found
C 68.00, H 2.10, N 30.02.
The authors thank the National Science Foundation of China
(Grant 20490210 and 20501006), National Basic Research 973 Pro-
gram (2006CB601103), Shanghai Sci. Tech. Comm. (05DJ14004),
and China Postdoctoral Science Foundation for financial support.
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www.eurjic.org
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Received: November 29, 2005
Published Online: March 30, 2006
2284
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Eur. J. Inorg. Chem. 2006, 2277–2284