A highly selective copper fluorescent indicator based on aminoquinoline substituted BODIPY

Article abstract of DOI:10.1016/j.dyepig.2013.02.013

A highly selective fluorescent indicator for copper, substituted with an aminoquinoline group at the position 3 or 5 of the BODIPY has been synthesized. The indicator has very low fluorescence quantum yield in acetonitrile with emission maxima at 580 nm. Upon binding copper it shows 50 nm bathochromic shifts with a change of solution color and a large increase in the fluorescence intensity. The fluorescence lifetime in acetonitrile was too short and could not be determined; whereas fluorescence decay profile of the copper complex was described by a mono-exponential decay with the lifetime of 4.38 ns. ESI-MS enabled detection of a Cu·1 CN complexes which was fromed by C-C bond cleavage of acetonitrile.

Full text of DOI:10.1016/j.dyepig.2013.02.013

Dyes and Pigments 98 (2013) 195e200
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A highly selective copper fluorescent indicator based on
aminoquinoline substituted BODIPY
*
Xu Jia, Xiaoting Yu, Xiaolong Yang, Jie Cui, Xiaoliang Tang, Weisheng Liu, Wenwu Qin
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly selective fluorescent indicator for copper, substituted with an aminoquinoline group at the
position 3 or 5 of the BODIPY has been synthesized. The indicator has very low fluorescence quantum
yield in acetonitrile with emission maxima at 580 nm. Upon binding copper it shows 50 nm bath-
ochromic shifts with a change of solution color and a large increase in the fluorescence intensity. The
fluorescence lifetime in acetonitrile was too short and could not be determined; whereas fluorescence
decay profile of the copper complex was described by a mono-exponential decay with the lifetime of
4.38 ns. ESI-MS enabled detection of a Cu$1 CN complexes which was fromed by CeC bond cleavage of
acetonitrile.
Received 23 December 2012
Received in revised form
16 February 2013
Accepted 19 February 2013
Available online 5 March 2013
Keywords:
Aminoquinoline
Copper probe
Ó 2013 Elsevier Ltd. All rights reserved.
Bathochromic shifts
Time-resolved fluorescence
CeC bond cleavage
BODIPY
1. Introduction
markedly higher selectivity for Cu2þ. Later, some good fluorescent
chemosensors for Cu2þ based on the BODIPY were also reported
Fluorescence spectroscopy is an attractive analytical technique
to measure intracellular concentrations of biologically important
ions and molecules. The design and development of fluorescent
chemosensors for the detection of analytically important species is
still an expanding area of research [1e3] Because of their excellent
photophysical characteristics and easy synthesis, 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene [4,5] (better known as BODIPY,
difluoroboron dipyrromethene) derivatives are favored fluo-
rophores in the design of fluorescent probes.
Hu et al. reported synthesis of three new 8-aminoquinoline-5-
azo derivatives, which were found to be good fluorescence re-
agents for the determination of Cu2þ in ultraviolet region [6]. Since
Cu2þ is a notorious fluorescence quencher, very few ratiometric
fluorescent chemosensors for Cu2þ are available in the literature
[7]. Therefore, the rapid monitoring of Cu2þ, especial the ratio-
metric and colorimetric detection, is very important. The first
fluorescent chemosensor for Cu2þ based on the BODIPY as a re-
[8b,c].
In a recent paper [9,10], we described the photophysical prop-
erties of the nonsymmetric 3-phenylamino and the symmetric 3,5-
diphenylamino-substituted difluoroboron dipyrromethene dye.
They showed some special spectroscopic properties that previous
BODIPY derivatives did not have. Unlike BODIPY analogs with a 4-
N,N-dimethylaminophenyl substituent at the meso-position [11,12],
red-shifted, solvent polarity-dependent broad fluorescence band
originating from an intramolecular charge transfer state (ICT) was
not found in the emission spectrum of these aminophenyl-
substituted BODIPY dyes. Moreover, there is a large difference in
photophysical properties between the mono and bis-substituted
aminophenyl BODIPY derivative. This means that spectroscopic
properties of these BODIPY derivatives are mainly effected by the
aminophenyl group at the 3- (and 5-) position(s). Consequently,
one can expect that replacement of the aminophenyl group by
other chelator having similar structure, would introduce a pro-
nounced change of both absorption and emission maxima due to
change of the properties upon complexation. Therefore, we syn-
thesized a new BODIPY derivative 1 (Scheme 1) with the amino-
quinoline attached via the N atom at the position 3 or 5 of the
BODIPY moiety.
porter subunit (Kd ¼ 3
mM) was published in 2006 [8a]. It had 8-
hydroxyquinoline moiety as a receptor and showed significant
fluorescence quenching in the presence of Cu2þ and Hg2þ with
In this paper, we investigate the photophysical properties of
new BODIPY derivative in a variety of solvents and its copper
* Corresponding author. Tel./fax: þ86 931 8912582.
E-mail address: qinww@lzu.edu.cn (W. Qin).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.02.013

196
X. Jia et al. / Dyes and Pigments 98 (2013) 195e200
ꢀ
monochromatized Mo K
a
radiation (
l
¼ 0.71073 A). The struc-
tures were solved by direct methods. All non-hydrogen atoms were
subjected to anisotropic refinement by full-matrix least-squares
methods on F2 by using the program package SHELXS-97 [15].
Hydrogen atoms were placed at calculated positions. Final R indices
[I > 2
s
(I)] were R1 ¼ 0.0758, wR2 ¼ 0.1939; max./min. residual
ꢂ ꢀꢂ3
electron density 0.42/ꢂ0.31 e
A
.
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC 825211 for 1. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax: þ44(0)-1223-336033 or e-mail: deposit@ccdc.
cam.ac.uk or www.ccdc.cam.ac.uk/conts/retrieving.html).
Scheme 1. Synthesis of sensor 1.
2.4. Steady-state UVevis absorption and fluorescence spectroscopy
complex in acetonitrile, using UV/Vis absorption and steady-state
and time-resolved fluorescence techniques.
UVevis absorption spectra were recorded on a Varian UV-
Cary100 spectrophotometer and for the corrected steady-state
excitation and emission spectra, a Hitachi F-4500 spectrofluorom-
eter was employed. Freshly prepared samples in 1-cm quartz cells
were used to perform all UVevis absorption and emission mea-
surements. For the determination of the fluorescence quantum
yields ff of 1, only dilute solutions with an absorbance below 0.1 at
the excitation wavelength (lex ¼ 488 or 500 nm) were used.
Rhodamine 6G in water (ff ¼ 0.76) was used as fluorescence
standard [16]. The ff values reported in this work are the averages
of multiple independent measurements. The majority of the ff
determinations were done using undegassed samples. In all cases,
correction for the solvent refractive index was applied. All spectra
were recorded at 20 ꢀC using undegassed samples. To check the
influence of dissolved O2 on ff, some samples were purged with
nitrogen for 15 min prior to measurement. The obtained ff values
were within experimental error equal for aerated and degassed
samples. The titration experiments with copper were carried out by
adding small quantities of a stock solution of metal perchlorate
salts in MeCN to a much larger volume (25 mL) of solutions of 1.
2. Experimental section
2.1. Instrument and reagent
All solvents (Aldrich, SigmaeAldrich, Acros, or Riedel-Dehaën)
were reagent grade quality and were used without further purifi-
cation. 1H and 13C NMR spectra were taken on a Bruker DRX-400
and DRX-400/4 spectrometer with TMS as an internal standard
and CDCl3 as solvent. Chemical shift multiplicities are reported as
s ¼ singlet, d ¼ doublet, and m ¼ multiplet. 13C spectra were
referenced to the CDCl3 (77.67 ppm) signal. Mass spectra were
recorded in E.I. mode. Melting points were taken on an X-4 precise
micro melting point cryoscope (Beijing Fukai Instrument Co.) and
are uncorrected.
2.2. The synthesis of sensor 1
Compound 2 was synthesized as described previously [13].
50 mg (0.15 mmol) of compound 2 was dissolved in 20 mL aceto-
nitrile, 20 mg (0.14 mmol) of 8-aminoquinoline was added to it. The
reaction mixture was stirred at room temperature for 72 h. After
solvent was remove by under reduced pressure, the crude solid was
purified by column chromatography on silica purified through
column chromatography over silica with ethyl acetate and petro-
leum ether (1:4) to obtain 37 mg of 1 in 60% of yield [13,14]. Reddish
2.5. Copper binding properties
The ground-state dissociation constants Kd of the complexes
between Cu2þ and the probes were determined in an acetonitrile
solution by fluorometric titration as a function of Cu2þ concentra-
tion using the fluorescence excitation and/or emission spectra.
Nonlinear fitting of Eq. (1) [17] to the steady-state fluorescence data
F recorded as a function of [Cu2þ] yields values of Kd, the fluores-
cence signals Fmin and Fmax at minimal and maximal [Cu2þ],
respectively (corresponding to the free and Cu2þ bound forms of
the probe, respectively), and n (the number of copper ions bound
per probe). Equation (1) assumes that the absorbance of the sample
is small (<0.1) and that Cu2þ complex formation in the excited state
is negligible.
solid, mp 62e64 ꢀC. 1H NMR (CDCl3, 400 MHz)
d 6.27 (d, 1H,
J ¼ 4.0 Hz, H-a), 6.46 (d, 1H, J ¼ 4.0 Hz, H-b), 6.83 (d, 1H, J ¼ 4.0 Hz,
H-c), 6.96 (d, 1H, J ¼ 4.0 Hz, H-d), 7.44e7.53 (m, 9H), 8.12 (d, 1H,
J ¼ 8.0 Hz, H-g), 8.92 (s, 1H, H-f), 10.52 (s, 1H, H-e); 13C NMR (CDCl3,
100 MHz): d111.83, 113.65, 114.75, 122.30, 122.65, 126.39, 128.23,
128.40, 129.45, 130.28, 132.17, 133.56, 134.27, 135.52, 135.93, 149.35,
156.73. MS (ESI) m/z calcd. For C24H16BClF2N4 444.1; found 445.1
(Mþ1); 425.3 (M-F, 100%).
2.3. Crystal structure determination
ꢀ
ꢁ
Fmax Cu2þ n þ FminKd
F ¼
(1)
ꢀ
ꢁ
Crystals of 1 were obtained by slow evaporation of the ethyl ac-
etate in air over two weeks, yielding red stick crystals with approx-
imate dimensions of 0.18 ꢁ 0.18 ꢁ 0.20 mm3 suitable for X-ray
diffraction. The crystals belonged to the monoclinic space group P-1
n
Kd þ Cu2þ
Fitting Eq. (1) to the steady-state fluorescence data F with n, Kd,
Fmin, and Fmax as freely adjustable parameters always gave values of
n close to 1 (n ¼ 1.06e1.30 in this experiment), indicating that one
copper ion is bound per fluorescent indicator. Therefore, n was kept
fixed at 1 in the final fittings of Eq. (1) to the fluorescence excitation
or emission spectral data, from which the estimated values of Kd,
Fmin, and Fmax are reported.
ꢀ
ꢀ
(number 2) with cell dimensions a ¼ 7.596(3) A, b ¼ 8.866(4) A,
ꢀ
c ¼ 15.385(7) A,
a
3
¼ 79.944(5)ꢀ,
b
¼ 84.348(5)ꢀ,
g
¼ 88.261(5)ꢀ,
V ¼ 1015.1(7) A , Z ¼ 2 rcalc ¼ 1.455 g cmꢂ3, 2qmax ¼ 50.00ꢀ,
ꢀ
m
(MoK
a
) ¼ 0.227 mmꢂ1
.
The data for 1 was measured on a Bruker Smart APEX II CCD
diffractometer at 296
ꢃ
2
K
equipped with graphite-

X. Jia et al. / Dyes and Pigments 98 (2013) 195e200
197
Since large spectral shifts are observed in the absorption spectra
of 1, the ratiometric method can be used to estimate the ground-
state dissociation constants Kd (Eq. (2)).
ꢀ
ꢁ
Rmax Cu2þ n þ Rmin
K
dx
R ¼
(2)
ꢀ
ꢁ
n
K
dx Cu2þ
R ¼ Aðl1absÞ=Aðl2absÞ is the ratio of the absorbances at the two
indicated wavelengths. Nonlinear fitting of Eq. (2) to the absorption
ratiometric data R recorded as a function of [Cu2þ] yields values of
Kd, the ratios Rmin and Rmax at minimal and maximal [Cu2þ],
respectively (corresponding to the free and complex forms of the
copper probe, respectively), and n. Since
x
¼ Amin
ð
l2absÞ=Amax
ð
l2abs
Þ
e the ratio of the absorbances at the indicated wavelength e is
experimentally accessible, a value for Kd can be extracted from
ratiometric absorption data.
Fig. 1. ORTEP representation of compound 1 with displacement ellipsoids at the 20%
probability level.
2.6. Time-resolved fluorescence spectroscopy
3.2. Spectroscopic properties
Fluorescence lifetimes were measured by a FLS920 at room
temperature. The samples were dissolved in CH3CN and the con-
centrations were adjusted to have optical densities at the excitation
wavelength <0.1. Solutions were purged with nitrogen for 15 min
prior to analysis. The monitored wavelength was 580 nm and
590 nm for 1 in solvents when excite the samples at 490 nm. For the
copper complex, the monitored wavelength was from 580 nm to
630 nm when excited at 570 nm.
The absorption and emission spectra of molecule 1 were
recorded in several solvents (Fig. 2). The absorption spectrum is of
comparable shape as those of the described BODIPY dyes [18,19],
with an intense absorption band somewhat above 560 nm and a
less pronounced shoulder at shorter wavelengths. The absorption
spectra of 1 in toluene and chloroform are analogous to that in
cyclohexane and they all show two absorption bands: the 0e0 band
of a strong S0eS1 transition with a maximum ranging at 560 nm
and a more pronounced shoulder on the high-energy side. The
absorption spectra are affected by solvent polarity; the maximum
being blue-shifted (by w40 nm) when the solvent is changed from
cyclohexane (560 nm) to acetonitrile (518 nm). Additionally, a
weaker, broad absorption band is found in the UV at around
350 nm. This broader and weaker absorption band is attributed to
the S0eS2 transition.
Compound 1 in cyclohexane shows an emission spectrum with a
maximum at w580 nm, displaying the typical emission features of
BODIPY. The maximum of the fluorescence band isn’t affected by
solvent polarity. Contrary to the near-invariance of the emission
wavelength maximum lem(max) in solvent. The fluorescence
quantum yield ff of 1 is strongly solvent dependent. The ff values
are moderately high in cyclohexane (0.17), somewhat lower in
toluene (0.04), while in all other solvents of higher polarity the
emission intensity is strongly quenched (ff ¼ 0.01 in CHCl3 and
0.0024 in MeCN). The low ff value for 1 in polar solvents can be
attributed to an efficient quenching via an excited-state ICT process
Fluorescence decay histograms were obtained on an Edinburgh
instrument FLS920 spectrometer equipped with a supercontinue
white laser (400e700 nm), using the time-correlated single photon
counting technique in 2048 channels. Histograms of the instrument
response functions (using a LUDOX scatterer) and sample decays
were recorded until they typically reached 5 ꢁ 103 counts in the
peak channel. Obtained histograms were fitted as sums of the
exponential, using Gaussian-weighted nonlinear least squares
fitting based on MarquardteLevenberg minimization implemented
in the software package of the instrument. The fitting parameters
(decay times and preexponential factors) were determined by
minimizing the reduced chi-square
c
2. An additional graphical
method was used to judge the quality of the fit that included plots
of surfaces (“carpets”) of the weighted residuals versus channel
number. All curve fittings presented here had c2 values below 1.1.
All measurements were done at 20 ꢀC.
3. Result and discussion
3.1. Crystal structure of 1
As shown in Fig. 1, in the BODIPY ring system, the two planar
pyrrole subunits and the boron atom constitute a plane for which
the deviation of all non-hydrogen atoms is within the 0.005e
ꢀ
0.070 A range. The angle between the two pyrrole moieties is
4.41ꢀ, which is near the average 6.5ꢀ (range 0ꢀe18.1ꢀ) found in the
Cambridge Structural Database (CSD, updated to October 2008).
The two fluorine atoms are equidistant above and under the plane
of the pyrrole moieties, and the FeBeF plane is 109.86ꢀ to the plane
of the BODIPY core. However, the phenyl ring at the meso-position,
the aminoquinoline ring and the pyrrole rings are not coplanar. The
phenyl ring at the meso-position makes an angle of 59.04ꢀ with the
BODIPY plane, which is in the range of most BODIPY derivatives
(40.3ꢀe90ꢀ), but is smaller than the average value of 76.4ꢀ found in
the CSD. The dihedral angles between aminoquinoline ring and
pyrroles is 33.09ꢀ.
Fig. 2. Normalized absorbance (dash curves) and fluorescence emission (solid curves)
spectra of 1 in several solvents.

198
X. Jia et al. / Dyes and Pigments 98 (2013) 195e200
from the nitrogen atom of the aminoquinoline amine NH moiety to
the strongly electron deficient BODIPY acceptor. As the solvent
polarity increases, ICT becomes more favored, which induces the
quenching of fluorescence and shifting of spectra. The solvent
dependence of ff may be interpreted as an indication of a small
contribution of a dipolar resonance state to the ground or excited
state of 1 [10]. The blue shift of the absorption spectrum from
cyclohexane to MeCN suggests that the increase of the electron
density in the conjugated BODIPY system by the aminoquinoline
moiety is less pronounced in the excited state than in the ground
state.
detected by the naked eye. The relative contributions of the 602/
518 nm signals change with varying [Cu2þ] and the vis absorption
spectra show isosbestic points at 500 and 548 nm. The fluorescence
emission spectra of 1 are shown as a function of [Cu2þ] in Fig. 3b.
The maximum of the fluorescence emission band shifts bath-
ochromically from 580 nm in ion-free acetonitrile to 630 nm in the
presence of Cu2þ and is accompanied by an increase in intensity.
The excitation spectra (see Fig. S1) show an excitation maximum at
614 nm with increasing concentration of copper, implying that the
fluorescence originates mainly from the excited state of the copper
complex. The highest measured ff value (0.04 for excitation at
570 nm) is found at the copper ion concentration of 15 mM. Upon
binding of Cu2þ, the electron-donating properties of the amine are
reduced, partially suppressing the ICT effect, causing a red shift and
an enhancement in the fluorophore’s fluorescence intensity. Com-
plex formation causes rotation of the aminoquinoline ring resulting
in more extended planarity of the chromophore. Beside the ICT
effect, it caused an enhancement in fluorophore’s fluorescence in-
tensity. In a previous paper, we have shown that the behavior of
phenylamino substituted BODIPY dye is different from that
observed in related BODIPY dyes, in which the amine donor group
is separated from the BODIPY core by a styryl spacer. The amino-
quinoline moiety plays the role of a donor in both the ground and
excited state of 1, which results in the appearance of a dipole
pointing with its negative head toward the BODIPY core. The
ground-state dipole moment in 1 is larger than the excited-state
dipole moment so that the dipole moment difference points from
the BODIPY to the aminoquinoline moiety. This is responsible for
the negative solvatochromic effect observed for this dye: the
ground-state absorption spectrum shows a large blue shift with
increasing solvent polarity (the inertial contribution to the dielec-
tric response stabilizes the ground state to a larger extent than the
excited state, hence the blue shift). When the Cu2þ coordinated
with the nitrogen atoms in the complex with 1, the charge transfer
1 was blocked, causing a red shift in the fluorescence spectrum in
acetonitrile solution (polar solvent). Consequently, the complex
absorbs and fluoresces more intensely at longer wavelengths
relative to the uncomplexed dye 1 [9] (Table 1).
3.3. Spectroscopic properties of 1 with Cu2þ
The changes in optical properties induced by addition of metal
ions perchlorates to acetonitrile solutions of 1 are shown in Fig. 3.
While the fluorescence emission spectra of 1 are hardly altered in
the presence of other metal ions, pronounced changes are observed
upon addition of Cu2þ ions. The addition of Cu2þ to a solution of 1 in
CH3CN strongly affects the absorption spectra. Upon increasing the
metal ion concentration, the lowest energy absorption band (with
maximum at 518 and 547 nm in the absence of metal ions) de-
creases in intensity whereas a new peak appears at approximately
610 nm whose intensity increases with higher metal ion concen-
tration The change in the absorption spectra causes the immediate
change of the solution color from red to violet, which can be
The ground-state dissociation constants Kd of the complexes
between Cu2þ and the probe were determined in acetonitrile so-
lution by fluorimetric titration as a function of Cu2þ concentrations
using the fluorescence excitation and/or emission spectra. The re-
sults obtained at lexc ¼ 570 nm indicated a complex of 1:1 stoi-
chiometry and yielded a value of 22 mM for the Kd. Since there is a
large shift of the absorption spectra, the ratiometric absorption
measurements could be performed. By using the ratio of decrease/
increase in the absorption bands at la1bs
Kd value of 31 mM was determined. The Kd value of 22 mM obtained
=
l2abs ¼ 602=518 nm, the
from direct fluorimetric titrations using emission spectra is in good
agreement with that from (ratiometric) absorption measurements.
This 1:1 stoichiometry binding mode was also supported by the
data of Job’s plots (Fig. S2). The detection limit was calculated based
on the fluorescence titration. The emission intensity of 1 without
Cu2þ was measured 10 times and the standard deviation of blank
measurements was determined. A good linear relationship be-
tween the fluorescence intensity and the Cu2þ concentration could
be obtained in the 0e15
Fig. S3). The detection limit was then calculated with the equation:
/m, where is the standard deviation of blank
mM concentration range (R ¼ 0.991;
detection limit ¼ 3
s
s
measurements, m is the slope between intensity versus sample
concentration. The detection limit was measured to be
9.0 ꢁ 10ꢂ9 M.
Fig. 3. Absorption spectra (a) and fluorescence emission spectra (lex ¼ 570 nm) (b) of 1
(15 m
M) in acetonitrile as a function of [Cu2þ]. The insets of (a) and (b) show the best
Lu has reported CeC bond cleavage of acetonitrile by a dinuclear
copper cryptate [20a], The dinuclear copper (II) cryptate [Cu2L](-
ClO4)4 cleaves the CeC bond of acetonitrile at room temperature to
fits to the ratiometric ðla1bs
=
l2abs ¼ 602=518 nmÞ absorption titration data (for a) and
the direct (lex ¼ 570 nm, lem ¼ 630 nm) fluorimetric emission titration data (for b) of 1
as a function of [Cu2þ].

X. Jia et al. / Dyes and Pigments 98 (2013) 195e200
199
Table 1
Photophysical data of 1 in different solvents, and in the presence of copper ion in acetonitrile. Kd is the average value determined from all direct analyses of the fluorometric and
spectrophotometric titrations.
Complex
Solvent
labs(max)/nm
lem(max)/nm
Ff
s/ns
kf/107 Sꢂ1
Kd/mM
1
Cyclohexane
Toluene
Chloroform
Acetonitrile
Acetonitrile
560
561
555
518,547
610
580
586
583
580
630
0.17
0.04
0.01
0.0024
0.04
3.21
0.82
5.3
4.9
1-Cu2þ
4.26 (610e630 nm)
22
produce
a
cyanide bridged complex of [Cu2L(CN)](-
those obtained from the common BODIPY compounds. The fluo-
rescence lifetimes of 1 in acetonitrile and other polar solvents were
too short and could not be determined. Upon complexation by Cu2þ
in acetonitrile, in addition to the change of the solution color and
the increase in fluorescence intensity, the fluorescence decay
became slower.
Fig. 5b displays the fluorescence decay traces of 1 complex by
Cu2þ in acetonitrile at lem ¼ 590, lem ¼ 610 and lem ¼ 630 nm. The
curve at 590 nm is a sum of two exponentials. At 590 nm, the
contributions of the s1 (4.37 ns) and s2 (1.84 ns) components are
ClO4)3$2CH3CN$4H2O. Cleavage of the CeCN bond of acetonitrile
with monomeric Cu2þ complex has also been observed before
[20b,c]. In our experiment, the formation of Cu$1 adduct was
further confirmed by mass spectrometry. The electrospray ioniza-
tion mass spectrum of the copper adducts with 1 showed a mo-
lecular mass of 619.4, (Fig. SI) which may be correspond to the
formula of [Cu$1(2CN)](H2O$CH3CN), that is, the MS data suggest
the presence of CN complexes. Thus, it might be possible that these
species are formed upon ESI ionization.
The expected structural change of 1 in the presence of copper
metal ions is depicted in Fig. 4. The detection system in this work
was pure acetonitrile. The electrospray ionization mass spectrum
conformed that the product of 1 and Cu2þ was formed from CH3CN.
When we change detection system to aqueous/MeOH systems, the
probe doesn’t show the high selectivity for Cu2þ anymore. More-
over, same as nonsymmetric 3-phenylamino and the symmetric
3,5-diphenylamino-substituted difluoroboron dipyrromethene
dye, pH doesn’t affect the response of the probe’s fluorescence
intensity.
76% and 24%, respectively. The contribution from the s1 component
increases, whereas that of the s2 component decreases with
increasing emission wavelength (See Table S1). Above 610 nm, the
fluorescence decays are single exponential with lifetime is
4.38 ꢃ 0.02 ns, implying that the fluorescence emission wave-
lengths above 610 nm and the fluorescence excitation wavelengths
Fluorescence decay traces of BODIPY derivative 1 and its copper
complex were recorded at several emission wavelengths by the
single-photon timing method [21]. Fluorescence decay profiles of 1
could be described by a single-exponential decay in cyclohexane
and toluene. The lifetimes
s estimated via single curve analysis
were independent of the observation wavelength. The lifetime is
about 3.21 ns in cyclohexane and 0.82 ns in toluene.
From the decay traces displayed in Fig. 5 it is clear that the
fluorescence lifetimes decrease with increasing solvent polarity
(from toluene to acetonitrile).
The rate constant of radiative (kf) and non-radiative (knr) deac-
tivation can be calculated from the measured fluorescence quan-
tum yield ff and the single-exponential fluorescence lifetime
according to Eqs. (3) and (4):
kf
¼
ff
=
s
(3)
ꢂ
ꢃ.
knr
¼
1 ꢂ ff
s
(4)
The values of kf are 5.3 ꢁ 107 sꢂ1 and 4.9 ꢁ 107 sꢂ1, respectively,
indicating that kf is nearly independent of the low-polarity solvent
used. The kf values obtained for 1 are different when compared to
Fig. 5. Representative fluorescence decay curves of 1 in toluene (a) and in cyclohexane
(b, black curve-lem ¼ 580 nm) excited at 490 nm and 1 in acetonitrile in addition of
copper ions (b, lem ¼ 590e630 nm) excited at 570 nm at different time windows (20 ns
and 50 ns, respectively).
Fig. 4. Schematic representation of the proposed structure changed after bound with
Cu2þ in acetonitrile.

200
X. Jia et al. / Dyes and Pigments 98 (2013) 195e200
‘International Cooperation Program of Gansu Province’
(1104WCGA182). This study was supported in part by the ‘Key
Program of National Natural Science Foundation of China’
(20931003).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.02.013.
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Fig. 6. Relative fluorescence intensity of the acetonitrile solution of 1 (15 mM) in the
presence of various cations in the concentrations of 300
tration is 15 M) in acetonitrile.
m
M (for Cu2þ the concen-
m
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about 610 nm are mainly from the excited state of the copper
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It is known that achieving a high selectivity for the analyte of
interest over a complex background of potentially competing spe-
cies is a challenging task in sensor development. Fig. 6 illustrates
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Since the emission intensity of compound 1 almost does not
change on addition of other metal ions, it means that the detection
of Cu2þ by 1 is hardly affected by these common coexistent metal
ꢀ
ions. The distance of two N atoms in aminoquinoline is 2.662 A. In
addition, the steric hindrance from two F atoms and the electro-
static repulsion between the chlorine and fluorine atoms made
only copper ion fit well with aminoquinoline. The shift of the ab-
sorption and fluorescence spectra of 1 upon addition of Cu2þ is
completely opposite to the intramolecular charge transfer (ICT)
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4. Conclusion
ꢁ
We have synthesized the visible light excitable, fluorescent
BODIPY-based dye 1, which shows solvent-dependent spectro-
scopic/photophysical properties. The aminoquinoline substituted
BODIPY dye forms 1:1 complexes with several transition-metal
ions Cu2þ. The new boradiazaindacene dye is an example of a
very high selectivity for fluorescent probe for copper metal ions
displaying large absorption and fluorescence changes in an
analytically interesting wavelength region.
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Acknowledgments
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W.Q. thanks the ‘Program for New Century Excellent Talents in
University’ (NCET-09-0444), the Natural Science Foundation of
China (No. 21271094) and the ‘Fundamental Research Funds for the
Central Universities’ (lzujbky-2011-22) and the ‘National Science
Foundation for Fostering Talents in Basic Research of the National
Natural Science Foundation of China’ (Grant No. J1103307) and the
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