Emission "off-On" effect from europium complexes triggered by AcO anion: Synthesis, characterization and sensing performance

Article abstract of DOI:10.1016/j.saa.2014.02.042

In this paper, a series of Eu(III) complexes based on four diamine ligands and two diketone ligands were synthesized. Their single crystal structures were studied, where intermolecular π-π stacking was found. The photophysical parameters of these Eu(III)

Full text of DOI:10.1016/j.saa.2014.02.042

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Emission ‘‘Off–On’’ effect from europium complexes triggered by AcO
anion: Synthesis, characterization and sensing performance
b
c
Xiaogang Li a, , Dong Zhang , Jing Li
⇑
a Industrial & Commercial College, Hebei University, Baoding 071000, China
b Institute of Penology, The Central Institute for Correctional Police, Baoding 071000, China
c Agricultural University of Hebei, Baoding 071000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
The correlation between emissive
performance and ligand structure was
studied.
ꢀ
Conjugation chain might trigger
energy transfer roll-back and
compromise emission.
ꢀ
ꢀ
The presence of AcO anion could stop
the energy transfer roll-back.
A sensing system was developed with
high sensitivity and selectivity.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a series of Eu(III) complexes based on four diamine ligands and two diketone ligands were
synthesized. Their single crystal structures were studied, where intermolecular p–p stacking was found.
Received 20 December 2013
Received in revised form 24 January 2014
Accepted 9 February 2014
The photophysical parameters of these Eu(III) complexes were measured, along with their ligand triplet
levels. The energy transfer mechanism between ligand and metal center was discussed in detail. Energy
transfer roll-back was found in Eu(III) complexes owing large-conjugated diamine ligands, compromising
emissive performance. This energy transfer roll-back, however, could be stopped by the presence of AcO
anion, leading to Eu(III) complex emission enhancement. The sensing performance of such Eu(III) com-
plexes was thus investigated in detail. High sensitivity and selectivity were observed.
Ó 2014 Elsevier B.V. All rights reserved.
Available online 24 February 2014
Keywords:
Eu(III)
Sensor
Emission turn-on
AcO
Anion
Introduction
activities such as cell division, DNA genetics and cell senescence
[2]. The switch missing or malfunction leads to a series of diseases,
Owing to their participation in biological reaction of many ani-
mal species, including human beings, more and more research
interests have been focused on the detection of anions [1]. Partic-
ularly, AcO anion has been recently reported as an important
molecular switcher for proteins and enzymes in vital biological
including Alzheimer disease, Parkinson’s disease and even cancer.
Thus, the exploration for AcO anion detection and sensing has been
sparked.
Traditionally, AcO anion can be precisely detected using high
performance liquid chromatography (HPLC) [3]. Then optical
sensors have shown their promising aspects with simple, fast
and non-destructive characters, along with the fact that the detec-
tion operation has a low need for instrumentation and sample
⇑
Corresponding author.
E-mail address: xiaogangli389@163.com (X. Li).
http://dx.doi.org/10.1016/j.saa.2014.02.042
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

2
X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
pretreatment [4]. Literatures have reported some optical sensing
systems for AcO anion which are, however, generally emission
‘‘On–Off’’ ones [5–7]. In other words, the detection is based on
probe emission quenching triggered by AcO anion. Considering
that some competing species and energy acceptors may quench
probe emission as well, those emission ‘‘On–Off’’ sensors suffer
from limited selectivity. In addition, the interaction between AcO
anion and probe usually causes probe emission spectral shift, lead-
ing to emission intensity variations and thus compromising
accuracy.
Due to the unique f–f radiative transitions, rare earth based
emitters can give sharp and narrow emissions which are immune
to surrounding environment or reagents, offering high stability to-
wards environmental variations [8]. The antenna ligand excitation
mechanism makes the modification on ligands feasible to be car-
ried out, without compromising mental-centered f–f emissions.
By modulating ligand energy level, the energy transfer between
ligand, metal center and analyte can be finely controlled, which
means that emission ‘‘Off–On’’ effect towards analyte may be real-
ized by choosing ligands with suitable energy level [9]. In addition,
rare earth based emitters own good solubility in common solvents,
allowing themselves to be easily loaded into various supporting
matrixes. All these excellent characters make rare earth based
emitters a promising candidate for anion detection.
CCD-1000 detector using graphite-monochromated Mo Ka radia-
tion at 298 K. Elemental analysis data were collected on a Carlo
Erba 1106 elemental analyzer. UV–Visible absorption spectra were
recorded using a HP 8453 UV–Vis–NIR diode array spectrophotom-
eter. Photoluminescence (PL) spectra were measured with a F-4500
fluorescence spectrophotometer. Excited state lifetimes were mea-
sured using 355 nm light excited by a pulsed Nd:yttrium alumi-
num garnet (YAG) laser, with line width of 1.0 cmꢁ1, pulse
duration of 10 ns and repetition frequency of 10 Hz, respectively.
PL quantum yields were determined using a literature method
[10]. All operations were finished at room temperature and ambi-
ent condition without being specified.
Synthesis of diamine ligands
The starting reagents of 4-(diphenylamino)benzaldehyde
(TPA-CHO), 9-ethyl-9H-carbazole-2-carbaldehyde (Cab-CHO), 1,
10-phenanthroline-5,6-dione (Phen-O), 1,10-phenanthroline-5,6-
dione-dioxime (Phen-NOH) and 1,10-phenanthroline-5,6-diamine
(Phen-NH2) were prepared following literature procedures [11–15].
10H-indolo[20,30:5,6]pyrazino[2,3-f][1,10]phenanthroline (IPP).
5 mmol of Phen-NH2, 5.5 mmol of indoline-2,3-dione and
0.1 mmol of 4-methylbenzenesulfonic acid were added into
25 mL of ethanol and heated to reflux overnight. After cooling,
the solution was poured into 300 mL of cold water. The crude prod-
uct was collected and recrystalized in ethanol to yield IPP as gray
Enlightened by above considerations, in this paper, we design a
series of Eu(III) complexes with similar diamine ligands. Their
emission parameters, as well as the energy transfer between li-
gand, metal center and AcO anion, are carefully measured and dis-
cussed. The correlation between ligand structure and emission
‘‘Off–On’’ is tentatively explained.
powder. 1H NMR (300 Hz, CDCl3, 25 °C):
d 10.13 (d, 1H,
J = 8.5 Hz), 9.24 (d,1H, J = 2.5 Hz), 9.17 (d, 1H, J = 2.0 Hz), 9.09 (d,
1H, J = 5.5 Hz), 8.55 (d, 1H, J = 5.5 Hz), 7.60–7.64 (m, 2H), 7.55 (d,
1H, J = 8.5 Hz), 7.34 (m, 2H). Anal. Calcd for C20H11N5: C, 74.76;
H, 3.45; N, 21.79. Found: C, 74.67, H, 3.60; N, 21.68.
Experimental details
4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-dipheny-
laniline (IPD). 5 mmol of Phen-O, 5 mmol of TPA-CHO and 20 g of
NH4Ac were added into 30 mL of HAc and heated to reflux over-
night. After cooling, the solution was poured into 500 mL of cold
water and extracted with CH2Cl2. After the evaporation of solvent,
the crude product was recrystallized in ethanol to give IPD as pale
yellow powder. 1H NMR (300 Hz, CDCl3): d 9.18 (t, 2 H), 7.74 (m, 2
H), 7.67 (m, 2 H), 7.31–7.17 (m, 14 H). Anal. Calcd for C31H21N5: C,
80.32; H, 4.57; N, 15.11. Found: C, 80.27, H, 4.67; N, 15.02.
2-(9H-carbazol-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline
(CIP). The synthetic route for CIP was similar to that for IPD, except
that Cab-CHO was replaced by TPA-CHO. 1H (300 MHz, CDCl3), d:
9.14 (2H), 8.99 (1H), 8.93 (1H), 8.67 (1H), 8.35 (1H), 7.92 (3H),
7.86 (1H), 7.85 (1H), 7.58 (1H), 7.31 (1H), 4.70 (2H), 1.48 (3H).
Anal. Calcd for C27H19N5: C, 78.43; H, 4.63; N, 16.94. Found: C,
78.25, H, 4.67; N, 17.02.
General chemicals and methods
Scheme 1 shows the molecular structures of those Eu(III)
complexes studied in this paper. The starting chemicals, including
1,10-phenanline (Phen), 2-thenoyltrifluoroacetonate (TTA), 1,3-di-
phenyl-propane-1,3-dione (DBM), indoline-2,3-dione, 9H-carba-
zole and triphenylamine were bought from Shanghai Chemical
Company (Shanghai, China) and used for synthesis directly. The
inorganic salts and reagents were bought from Tianjin Chemical
Company (Tianjin, China) and used as received. The organic sol-
vents were redistilled prior to use.
The equipments and measuring methods used in this paper are
listed as follows. 1H NMR spectra were taken from a Varian INOVA
300 spectrometer. Single crystal data were detected by a Siemens
P4 single-crystal X-ray diffractometer equipped with a Smart
Scheme 1. The molecular structures of Eu(III) complexes studied in this paper.

X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
3
Synthesis of Eu(III) complexes
Eu(TTA)3Phen. Eu(TTA)3Phen was synthesized following a liter-
ature procedure [10]. Anal. Calcd. for C36H20F9O6S3N2Eu: C, 43.43,
H, 2.02, N, 2.81. Found: C, 43.33, H, 2.14, N, 2.74. Its single crystal
structure (CCDC 891935) will be discussed below.
Eu(TTA)3IPP. The synthetic route for Eu(TTA)3IPP was similar to
that for Eu(TTA)3Phen except that Phen was replaced by IPP. Anal.
Calcd. for C44H23F9O6S3N5Eu: C, 46.49, H, 2.04, N, 6.16. Found: C,
46.35, H, 2.12, N, 6.24.
Eu(TTA)3IPD. The synthetic route for Eu(TTA)3IPD was similar to
that for Eu(TTA)3Phen except that Phen was replaced by IPD. Anal.
Calcd. for C55H33F9O6S3N5Eu: C, 51.65, H, 2.60, N, 5.48. Found: C,
51.54, H, 2.72, N, 5.44.
Eu(TTA)3CIP. The synthetic route for Eu(TTA)3CIP was similar to
that for Eu(TTA)3Phen except that Phen was replaced by CIP. Anal.
Calcd. for C51H31F9O6S3N5Eu: C, 49.84, H, 2.54, N, 5.70. Found: C,
49.76, H, 2.65, N, 5.59.
Eu(DBM)3Phen. The synthetic route for Eu(DBM)3Phen was sim-
ilar to that for Eu(TTA)3Phen except that TTA was replaced by DBM.
Anal. Calcd. for C57H41O6N2Eu: C, 68.33, H, 4.12, N, 2.80. Found: C,
68.43, H, 4.01, N, 2.63.
Eu(DBM)3IPP. The synthetic route for Eu(DBM)3IPP was similar
to that for Eu(DBM)3Phen except that Phen was replaced by IPP.
Anal. Calcd. for C65H44O6N5Eu: C, 68.30, H, 3.88, N, 6.13. Found:
C, 68.21, H, 3.97, N, 6.04.
Eu(DBM)3IPD. The synthetic route for Eu(DBM)3IPD was similar
to that for Eu(DBM)3Phen except that Phen was replaced by IPD.
Anal. Calcd. for C76H54O6N5Eu: C, 71.02, H, 4.23, N, 5.45. Found:
C, 71.13, H, 4.34, N, 5.54. Its single crystal structure (CCDC
891937) will be discussed below.
Fig. 1B. The intermolecular
(down).
p
–
p
stacking in Eu(TTA)3Phen (up) and Eu(DBM)3IPD
Table 1
Selected geometric parameters of Eu(TTA)3Phen.
Eu(DBM)3CIP. The synthetic route for Eu(DBM)3CIP was similar
to that for Eu(DBM)3Phen except that Phen was replaced by CIP.
Anal. Calcd. for C72H52O6N5Eu: C, 70.01, H, 4.24, N, 5.67. Found:
C, 70.12, H, 4.39, N, 5.52.
Bond length
(Å)
Bond angle
(°)
Eu–O1
Eu–O2
Eu–O3
Eu–O4
Eu–O5
Eu–O6
Eu–N1
Eu–N2
2.322(9)
2.292(8)
2.312(8)
2.311(9)
2.304(9)
2.309(9)
2.554(10)
2.536(10)
O1–Eu–O2
O3–Eu–O5
O4–Eu–O6
N1–Eu–N2
73.24(3)
73.35(3)
72.54(3)
64.62(3)
Results and discussion
Single crystal structures
The single crystal structures of Eu(TTA)3Phen and Eu(DBM)3IPD
are obtained and shown in Fig. 1. The selected geometric parame-
ters are summarized in Tables 1 and 2. It is observed that each
Eu(III) ion is coordinated with six O atoms from three diketone li-
gands (TTA or DBM) and two N atoms from a diamine ligand, form-
ing a square antiprism coordination environment. The Eu–O bonds
in Eu(TTA)3Phen are similar to each other with mean value of
ꢂ2.31 Å. While, the Eu–O bonds in Eu(DBM)3IPD are found to be
obviously longer than those in Eu(TTA)3Phen with mean value of
ꢂ2.36 Å. Similar case is also found from the Eu–N bonds in
Eu(TTA)3Phen and Eu(DBM)3IPD. On the other hand, the O–Eu–O
and N–Eu–N bite angles in Eu(TTA)3Phen are larger than the corre-
sponding values in Eu(DBM)3IPD. The less crowded coordination
Fig. 1A. Single crystal structure of Eu(TTA)3Phen (left) and Eu(DBM)3IPD (right). All H atoms were omitted for clarity.

4
X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
Table 2
TTA based complexes generally own higher absorption intensity
compared with the corresponding values of DBM based ones.
Fig. 3 shows the PL spectra of the eight Eu(III) complexes in CH3-
Selected geometric parameters of Eu(DBM)3IPD.
Bond length
(Å)
Bond angle
(°)
CN solution (5 lM) upon excitation wavelength of 350 nm. Not
Eu–O1
Eu–O2
Eu–O3
Eu–O4
Eu–O5
Eu–O6
Eu–N1
Eu–N2
2.342(10)
2.354(8)
2.366(10)
2.345(8)
2.357(8)
2.362(8)
2.575(10)
2.581(10)
O1–Eu–O5
O2–Eu–O6
O3–Eu–O4
N1–Eu–N2
71.6(3)
70.9(3)
71.1(3)
63.6(3)
surprisingly, characteristic lines from Eu(III) ion are observed,
peaking at 578 nm from 5D0 ? 7F0, 591 nm from 5D0 ? 7F1 and
610 nm from 5D0 ? 7F2, respectively. Although the ligands of those
Eu(III) complexes are different, their peaks are exactly the same.
Their f–f radiative transitions are perfectly shielded from environ-
mental influence by filled 5d and 5p orbitals, leading to the immu-
nity of emissions to various ligands. Despite of their identical
emission peaks, it seems that TTA based emissions own sharper
emission bands (FWHM610nm = 8 nm) compared with DBM based
ones do (FWHM610nm = 12 nm), where FWHM means full width
at half maximum. According to J–O theory [10], these widened
emission bands in DBM based complexes may be explained by
their highly distorted coordination sphere, as suggested by above
discussion on their single crystals. Never the less, this emission sta-
bility is a promising factor as potential sensing probe.
environment in Eu(TTA)3Phen, owing to the smaller diketone li-
gand of TTA compared with DBM, should be responsible for above
fact. It seems that all ligands in Eu(DBM)3IPD try to minimize the
steric hindrance by extending coordination bonds and distorting
bite angles. This observation can be further strengthened when
compared with corresponding values of tetrahedral coordination
complexes, where metal-N bond length values and N-metal-N bite
angles are ꢂ2.1 Å and ꢂ80°, respectively [16,17]. Considering that
the square antiprism coordination environment is much more
crowded than tetrahedral one, it is reasonable to see the longer
bonds and smaller bite angles in Eu(TTA)3Phen and Eu(DBM)3IPD.
In addition, there is intermolecular
tals owing to the large conjugation planes in Eu(TTA)3Phen and
Eu(DBM)3IPD. It can be seen that the face-to-face attraction
between conjugation planes makes them align nearly parallel to
each other. The face-to-face distance and intersection angle are
3.331 Å and 0.18° for Eu(TTA)3Phen and 2.910 Å and 18.04° for
PL quantum yields and excited state lifetimes
PL quantum yield (U) and excited state lifetime (s) are two key
factors for emissive materials. The former one is defined as the ra-
tio of emitted photon number to absorbed photon number. Here PL
quantum yields of the eight Eu(III) complexes are determined by
Formula 1 and listed in Table 3, where subscripts s and r refer to
sample and reference solutions respectively, n is solvent refractive
index, D is integrated intensity [10]. Quantity B is calculated by
B = 1–10AL, where A is the absorption coefficient at excitation
wavelength and L is optical length.
p–p stacking in both crys-
p–p
Eu(DBM)3IPD, respectively, which confirms the existence of
p–p
attraction [18,19].
2
Us
¼
UrðBr=BsÞðns=nrÞ ðDs=DrÞ
ðFormula 1Þ
It can be seen that Eu(TTA)3Phen and Eu(DBM)3Phen are supe-
Photophysical parameters
rior emitters with high quantum yields. As for the other six com-
plexes, their quantum yields generally decrease with the
increasing conjugation chain. Thus, although diketone ligands are
the dominant energy donors for Eu(III) center, the molecular struc-
ture of diamine ligands also plays an important role for emissive
performance.
To get a further understanding on the causation for the de-
creased quantum yields, the excited state lifetimes of 5D0 ? 7F2
transition are measured. Here, the lifetimes of other transitions
are ignored since 5D0 ? 7F1 transition lifetime is independent on
ligand variation and other emission peaks are too weak to be de-
tected [10]. All emissions follow single exponential decay pattern
with decay time of hundreds of microseconds, as shown in Fig. 4,
suggesting that the emissive Eu(III) center is uniformly distributed
UV–Vis absorption and PL emission spectra
Fig. 2 shows the UV–Vis absorption spectra of the eight Eu(III)
complexes in CH3CN solution (5 M). It is observed that all spectra
l
own multiple intense absorption bands in UV-region, extending to
blue region of ꢂ420 nm. Despite of their various absorption peaks
in high energy region, there exists an universal absorption band
peaking at 350 nm for all complexes. After being compared with
literature results [10], the high energy bands can be attributed to
the absorption from diamine ligand, admixed with some contribu-
tions from TTA and DBM ligands. While, the universal band is as-
signed to the dominant absorption from diketone ligands (TTA or
DBM), which explains their similar wavelength values. In addition,
Fig. 2. The UV–Vis absorption spectra of the eight Eu(III) complexes in CH3CN
solution (5 lM).
Fig. 3. The PL spectra of the eight Eu(III) complexes in CH3CN solution (5
kex = 350 nm).
lM,

X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
5
Table 3
PL quantum yields, excited state lifetimes and T1 levels of the eight Eu(III) complexes.
Complexes
U
s
(
ls)
Kr (ꢃ103 Sꢁ1
)
Knr (ꢃ103 Sꢁ1
)
T1 (diketone/diamine, nm)
Eu(TTA)3Phen
Eu(TTA)3IPP
Eu(TTA)3IPD
Eu(TTA)3CIP
Eu(DBM)3Phen
Eu(DBM)3IPP
Eu(DBM)3IPD
Eu(DBM)3CIP
0.92
0.46
0.33
0.15
0.88
0.53
0.34
0.11
588
308
269
190
592
403
301
190
1.565
1.495
1.227
0.789
1.486
1.315
1.130
0.579
0.136
1.753
2.491
4.474
0.203
1.166
2.193
4.684
400,561/402,427,458,485
400,561/558
400,561/570
400,561/604
447/402,427,458,485
447/558
447/570
447/604
To confirm this hypothesis, T1 levels of the four diamine ligands
and the two diketone ligands are determined by their low temper-
ature phosphorescence spectra shown in Fig. 5, with their values
summarized in Table 3. As for the two diketone ligands, their T1
levels are obviously higher than (5D0 + 1500 cmꢁ1), which means
that the energy transfer from their T1 states to Eu(III) center is irre-
versible. With the increasing conjugation chain, the T1 level de-
creases largely from 485 nm for Phen to 604 nm for CIP, which
means that the energy transfer roll-back from 5D0 to T1 is possible.
Particularly, CIP T1 level is even lower than 5D0 level (17,200 cmꢁ1
or 581.4 nm), which means that the long-lived excited state Eu(III)
center can easily transfer its energy to CIP T1 state, leading to the
two lowest quantum yields in those eight Eu(III) complexes. Thus,
it is hereby confirmed that energy transfer roll-back is activated by
the increasing conjugation chain, compromising emissive perfor-
mance. On the positive side, such energy transfer roll-back may
be affected by interactions with other energy acceptors or anions,
causing emission intensity variation and thus showing potential
as sensing probe.
Fig. 4. The emission decay dynamics of the eight Eu(III) complexes in CH3CN
solution (5 M).
l
As for the case of diketone ligands owing long conjugation
chain, Zhang and coworkers have reported a series of Eu(III) com-
plexes owing triphenylamine derived diketone ligands [10]. The
long conjugation chain in those diketone ligands can also decrease
their triplet levels. However, no obvious energy roll-back was
found in corresponding Eu(III) complexes. Zhang and coworkers
suggested that those triphenylamine derived diketone ligands
transfer their singlet energy directly to Eu(III) center, avoiding trip-
let states and the potential energy roll-back.
in each sample. The emissive ratio (Kr) and non-emissive ratio (Knr)
are calculated using the following formulas and listed in Table 3.
Kr
Kr þ Knr
U
¼
ðFormula 2Þ
ðFormula 3Þ
1
s
¼ Kr þ Knr
It is observed that the non-emissive ratio greatly increases with
the increasing conjugation chain. While the emissive ratio slightly
decreases with the increasing conjugation chain, which may be
caused by the competing effect from non-emissive decay path. In
other words, the decreased quantum yields are mainly caused by
the increased non-emissive ratio originated from diamine ligands,
which will be discussed in detail below.
Sensing performance
Towards AcO anion
In order to testify the potential as sensing probe, the emission
spectra of the eight Eu(III) complexes in CH3CN solution (5 lM)
Diamine ligand energy levels
According to literature report, a general antenna energy transfer
mechanism from excited state ligand to Eu(III) center in Eu(III)
complexes can be described as follows [20]. After absorbing excita-
tion light, the exited state ligand experiences an intersystem cross-
ing (ISC) between the lowest singlet state (S1) and the lowest
triplet state (T1). Upon proper energy difference between T1 and
Eu(III) center (5D0 = 17,200 cmꢁ1), T1 energy can be transferred to
Eu(III) center, leading to metal-centered f–f radiation. If the energy
difference between T1 and 5D0 is smaller than 1500 cmꢁ1, energy
transfer roll-back from 5D0 to T1 can be activated, compromising
metal-centered f–f radiation [21,22]. Above result has suggested
that the growing conjugation chain in the Eu(III) complexes is
responsible for the increasing non-emissive ratio. Considering that
conjugation chain is effective on lowering ligand T1 level [18], it is
thus estimated that the decreasing diamine ligand T1 should be
claimed to be responsible for the decreased quantum yields.
Fig. 5. The low temperature (77 K, kex = 350 nm) phosphorescence spectra of
ligands.

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X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
anion concentration. It is thus concluded that both complexes own
emission ‘‘Off–On’’ effect and linear response towards AcO anion,
making sensing operation feasible.
upon presence of AcO anion (10 lM) are shown as Fig. 6. It is ob-
served that the emissions from two Phen based Eu(III) complexes
increase very slightly. With the increasing conjugation chain, the
emission ‘‘Off–On’’ effect becomes obvious. As for TTA series, the
emission intensity ratios of 5D0 ? 7F2 (free)/5D0 ? 7F2 (AcO anion)
are 1.10 for Eu(TTA)3Phen, 1.41 for Eu(TTA)3IPP, 2.16 for Eu(TTA)3-
IPD and 6.69 for Eu(TTA)3CIP, respectively. As for DBM series, those
values are 1.07 for Eu(DBM)3Phen, 1.94 for Eu(DBM)3IPP, 2.95 for
Eu(DBM)3IPD and 7.18 for Eu(DBM)3CIP, respectively. It seems that
CIP based Eu(III) complexes are more sensitive towards AcO anion
than the others do.
Sensing mechanism
To explain above emission ‘‘Off–On’’ effect, we are proposing a
possible sensing mechanism shown as Fig. 9 which can be de-
scribed as follows. We have above mentioned that there exits po-
tential energy transfer roll-back from excited state Eu(III) to
ligand T1, compromising metal-centered f–f radiation, which ex-
plains why Eu(TTA)3CIP and Eu(DBM)3CIP own lower PL quantum
yields than the others. On the other hand, if this energy transfer
roll-back can be stopped or decreased, the efficient metal-centered
f–f radiation can be restored, showing emission ‘‘Off–On’’ effect.
There are multiple reacting sites for AcO aion in CIP ligands such
as N–H and N lone pairs. The presence of AcO anion may increase
CIP T1 level and thus minimize the energy transfer roll-back, lead-
ing to the emission enhancement. As for the others, cases are sim-
ilar. Their emission enhancement is limited, however, since the
energy transfer roll-back is intrinsically low.
The detailed AcO anion sensing performance of Eu(TTA)3CIP and
Eu(DBM)3CIP in CH3CN solution (5 lM) is investigated by their
emission spectra upon various AcO anion concentrations. As shown
in Fig. 7, the emission intensity increases gradually with increasing
AcO anion concentration, showing emission ‘‘Off–On’’ effect. Both
emission band shape and peaks are found to be similar to those
of free complexes, which means that the metal-centered f–f radia-
tion is highly stable, favoring sensing accuracy.
Generally, as for a probe localized in homogeneous medium
with single exponential decay dynamics, the intensity form of
Stern–Volmer equation can be described by Formula 4 [11], where
F means luminescent intensity at 610 nm. The subscript 0 denotes
a value in the absence of analyte, KSV is the Stern–Volmer constant,
and [AcOꢁ] is AcO anion concentration.
To confirm this hypothesis, the excited state lifetimes and quan-
tum yields of Eu(TTA)3CIP and Eu(DBM)3CIP in CH3CN solution
(5
which are obtained as 439
417 s and 0.71 for Eu(DBM)3CIP, respectively. Both excited state
lM) upon the presence of AcO anion (10
lM) are measured,
ls and 0.83 for Eu(TTA)3CIP and
l
F0=F ¼ 1 þ KSV ½AcOꢁꢄ
The plot of F0/F against [AcOꢁ] should be a linear one with identical
slope of KSV. This hypothesis can be confirmed by the working plots
of Eu(TTA)3CIP and Eu(DBM)3CIP in CH3CN solution (5 lM) upon
various AcO anion concentrations shown as Fig. 8. All plots fit For-
ðFormula 4Þ
lifetimes and quantum yields are greatly increased. Using Formula
2 and Formula 3, Kr and Knr values are obtained as 1.891 ꢃ 103 Sꢁ1
and 0.387 ꢃ 103 Sꢁ1 for Eu(TTA)3CIP and 1.703 ꢃ 103 Sꢁ1 and
0.695 ꢃ 103 Sꢁ1 for Eu(DBM)3CIP, respectively. Compared with
those of free Eu(TTA)3CIP and Eu(DBM)3CIP listed in Table 3, it
can be seen that the increased excited state lifetimes and quantum
yields are mainly caused by the decreased Knr values. In other
mula 4 well, showing good linear response towards increasing AcO
Fig. 6A. The emission spectra of TTA based Eu(III) complexes in CH3CN solution (5 lM, kex = 350 nm) upon presence of AcO anion (10 lM).

X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
7
Fig. 6B. The emission spectra of DBM based Eu(III) complexes in CH3CN solution (5 lM, kex = 350 nm) upon presence of AcO anion (10 lM).
Fig. 7B. The emission spectra of Eu(DBM)3CIP (kex = 350 nm) upon various AcO
anion concentrations from 0 to 10 M.
Fig. 7A. The emission spectra of Eu(TTA)3CIP (kex = 350 nm) upon various AcO
anion concentrations from 0 to 10 M.
l
l
words, the presence of AcO anion stops the energy transfer roll-
back pathway, confirming the correctness of our proposed sensing
mechanism.
only triggered by the presence of AcO anion. The emission from
Eu(TTA)3CIPꢁand Eu(DBM)3CIP is immune to the presence of Clꢁ,
Brꢁ and ClO4 anions, while Fꢁ and H2POꢁ4 tend to slightly quench
the emission, which can be explained as follows. According to
above analysis, the emission ‘‘Off–On’’ effect is achieved by the
limitation of energy transfer roll-back from excited Eu(III) center
to CIP T1 state. As for Clꢁ, Brꢁ and ClOꢁ4 anions, they have little
interaction between CIP ligand and thus cannot limit the energy
transfer roll-back, leading to the missing of emission ‘‘Off–On’’ ef-
fect. As for Fꢁ and H2POꢁ4 , they own strong interaction with CIP li-
gand through hydrogen bond. This interaction, however, not only
Towards other competing anions and oxygen
Aiming at precise and reliable sensing readings, good recogniz-
ing ability towards specific analyte in complicated background full
of competing ions and matters is desired for all sensors. Fig. 10
shows the emission intensity contrast bars of Eu(TTA)3CIP and
Eu(DBM)3CIP in CH3CN solution (5
lM) upon some common an-
ions (10 M). It can be seen that the emission ‘‘Off–On’’ effect is
l

8
X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
Fig. 10A. The emission intensity contrast bars of Eu(TTA)3CIP in CH3CN solution
(5 M, kex = 350 nm) upon some common anions (10 M). Inset: photo of
corresponding anions.
l
l
a
Fig. 8A. The working plot for Eu(TTA)3CIP (610 nm, kex = 350 nm) upon various AcO
anion concentrations from 0 to 10 lM.
Fig. 10B. The emission intensity contrast bars of Eu(DBM)3CIP in CH3CN solution
(5 M, kex = 350 nm) upon some common anions (10 M). Inset: photo of
corresponding anions.
l
l
a
Fig. 8B. The working plot for Eu(DBM)3CIP (610 nm, kex = 350 nm) upon various
AcO anion concentrations from 0 to 10 M.
l
Fig. 9. A potential sensing mechanism in Eu(TTA)3CIP and Eu(DBM)3CIP.
can increase CIP T1 level but can also directly quench excited Eu(III)
center, leading to the emission quenching [23].
Considering that both CIP T1 and excited Eu(III) center are vul-
nerable to oxygen molecule owing to their long-lived excited
states, the emission response measurement towards oxygen mole-
cule should be carried out [24,25]. Fig. 11 shows the emission
intensity contrast bars of Eu(TTA)3CIP and Eu(DBM)3CIP in O2 bub-
Fig. 11. The emission intensity contrast bars of Eu(TTA)3CIP and Eu(DBM)3CIP in O2
bubbled CH3CN solution (5 M, kex = 350 nm) and N2 bubbled CH3CN solution
(5 M, kex = 350 nm).
l
l
bled CH3CN solution (5
(5 lM). No surprise, both emissions are found to be quenchable
lM) and N2 bubbled CH3CN solution
Conclusion
by oxygen molecules. We have attributed the causation to the oxy-
gen quenching on the long-lived excited CIP T1 and excited Eu(III)
center. The decrease amount, however, is not so obvious, which
may be explained by the fast energy transfer between CIP T1 and
Eu(III) center, as well as the well-protected f–f radiations. This
sensing system shows an improved oxygen immunity towards
oxygen compared with literature reports [8,26].
In conclusion, this paper reported eight Eu(III) complexes,
including their single crystal structures and photophysical param-
eters. The correlation between emissive performance and diamine
ligand structure was carefully investigated, which found that the
large conjugation chain in diamine ligands could decrease ligand
triplet level and thus trigger energy transfer roll-back from excited
Eu(III) center to ligand T1 state, leading to decreased excited state

X. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 1–9
9
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AcO anion could increase T1 level and thus stop the energy transfer
roll-back, showing emission ‘‘Off–On’’ effect. This emission
enhancement effect was obvious and immune to other common
anions. As a consequence, AcO sensing system was developed, with
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