Cu(II)-selective fluorescence of a bis-quinolylimine derivative

Article abstract of DOI:10.1016/j.jphotochem.2010.10.018

A new bis-quinolylimine ligand containing an azadiene moiety, 1,4-bis(2-quinolyl)-2,3-diaza-1,3-buthadiene (1), was synthesized by one-step facile condensation. This simple ligand, when dissolved in acetonitrile, shows a Cu²⁺-selective fluorescence enhancement. Coordination of 1 with Cu²⁺ produces two kinds of complexes with 1:1 and 1:2 stoichiometries. The 1:2 complex shows a strong fluorescence (Φ_F = 0.37), while the 1:1 complex does not (Φ_F < 0.01). Ab initio molecular orbital calculation reveals that the 1:1 complex has a distorted structure, while the 1:2 complex has a planar structure. The planar configuration of the 1:2 complex, therefore, allows an extended π-conjugation over the entire molecule and, hence, results in fluorescence enhancement.

Full text of DOI:10.1016/j.jphotochem.2010.10.018

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Cu(II)-selective fluorescence of a bis-quinolylimine derivative
Chizuru Ichimura, Yasuhiro Shiraishi∗, Takayuki Hirai
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new bis-quinolylimine ligand containing an azadiene moiety, 1,4-bis(2-quinolyl)-2,3-diaza-1,3-
buthadiene (1), was synthesized by one-step facile condensation. This simple ligand, when dissolved
in acetonitrile, shows a Cu2+-selective fluorescence enhancement. Coordination of 1 with Cu2+ produces
two kinds of complexes with 1:1 and 1:2 stoichiometries. The 1:2 complex shows a strong fluorescence
(˚F = 0.37), while the 1:1 complex does not (˚F < 0.01). Ab initio molecular orbital calculation reveals
that the 1:1 complex has a distorted structure, while the 1:2 complex has a planar structure. The planar
configuration of the 1:2 complex, therefore, allows an extended ␲-conjugation over the entire molecule
and, hence, results in fluorescence enhancement.
Received 4 September 2010
Accepted 20 October 2010
Available online 26 October 2010
Keywords:
Quinoline
Azadiene
Copper(II)
Fluorescence
Extended ␲-conjugation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Here we report a new family of quinoline-based fluorophore,
1,4-bis(2-quinolyl)-2,3-diaza-1,3-butadiene (1, Scheme 1),
The design and development of fluorescent signaling molecules
is an area of intense research activity and of tremendous signif-
icance to the field of molecular device fabrication [1]. Various
molecular systems whose emission properties can be modulated by
external stimuli (temperature, light, redox potential, and pH) have
been proposed [2]. Metal cations are often used as external stimuli,
which promote fluorescence enhancement or quenching response
associated with the coordination with ligand groups [3]. Copper(II)
usually behaves as a fluorescence quencher via an energy transfer
from the excited state fluorophore to the empty d-orbital [4]; there-
fore, many of the reported molecular systems show fluorescence
quenching response upon addition of Cu2+ [5].
showing Cu2+-selective fluorescence enhancement. This bis-
quinolylimine ligand is obtained by one-step facile condensation
with a moderate yield (60%), whereas the other quinoline-based
ligands require more than two steps with a relatively low yield
(11–57%) [11]. The ligand, 1, shows a bright blue fluorescence upon
Cu2+ coordination, while showing no fluorescence for other cations.
We describe that the Cu2+-selective fluorescence is due to the
formation of 1–Cu2+ 1:2 complex with an extended ␲-electronic
structure over the entire molecule.
2. Experimental
Recently, several molecular systems that show Cu2+-selective
fluorescence enhancement have been proposed based on several
fluorophores such as rhodamine [6], pyrene [7], naphthalimide [8],
and coumarin [9]. Quinoline-based molecules have attracted much
attention because the quinoline moieties behave as a ligand for
metal coordination as well as a fluorophore. A variety of quinoline
derivatives has been proposed for selective fluorescence enhance-
ment upon coordination of specific metal cations [10]. There are,
however, only three reports of quinoline-based fluorophores show-
ing a fluorescence enhancement response against Cu2+ [11].
2.1. Materials
All of the reagents used were purchased from Wako and Tokyo
Kasei and used without further purification. Water was purified by
a Milli Q system.
2.1.1. Synthesis of 1
2-Quinolinecarbaldehyde (0.47 g, 3.0 mmol) and hydrazine
hydrate (0.075 g, 1.5 mmol) were refluxed in MeOH (10 ml) for 17 h.
The yellow solid formed was recovered by filtration, washed with
EtOH, and recrystallized from CHCl3, affording 1 as a yellow solid
(0.280 g, 0.90 mmol, yield 60%). 1H NMR (270 MHz, CDCl3, TMS):
ı (ppm) = 8.82 (s, 2H), 8.33 (d, J = 8.6 Hz, 2H), 8.26 (d, J = 8.6 Hz,
2H), 8.18 (d, J = 8.4 Hz, 2H), 7.88 (d, J = 7.9 Hz, 2H), 7.77 (t, J = 7.7 Hz,
2H), 7.61 (t, J = 7.5 Hz, 2H). 13C NMR (100 MHz, CDCl3, TMS): ı
(ppm) = 161.38, 153.38, 148.24, 136.28, 129.93, 129.73, 128.78,
127.61, 127,59, 118.92. Elemental anal.: Calcd. for C20H14N4: C
∗
Corresponding author. Research Center for Solar Energy Chemistry, Osaka Uni-
versity, 1-3 Machikaneyama-cho, Toyonaka 560-8531, Japan. Tel.: +81 6 6850 6271;
fax: +81 6 6850 6273.
E-mail address: shiraish@cheng.es.osaka-u.ac.jp (Y. Shiraishi).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.10.018

254
C. Ichimura et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
Cu2+
hydrazine
100
a
N
CHO
b
( 0.5 equiv )
N
N
N
N
100
MeOH, 338 K, 17 h
1 (yield 60 %)
Scheme 1. Synthesis of the ligand 1.
other cations
none
77.4, H 4.55, N 18.05; Found: C 77.18, H 4.34, N 18.07. 1H and 13
NMR charts are shown in Figs. S1 and S2, respectively (Supplemen-
tary Material).
C
0
400
λ (nm)
500
2.2. Analysis
0
noneNa+ Mg2+Mn2+Co2+ Ni2+Ag+Zn2+Cd2+Pb2+Fe2+Fe3+Hg2+Cu2+
Steady-state fluorescence spectra were measured using
a
10 mm path length quartz cell on a Hitachi F-4500 fluorescence
spectrophotometer at 298 1 K [12]. Absorption spectra were
measured on an UV–vis photodiode-array spectrophotometer (Shi-
madzu; Multispec-1500) at 298 1 K. Fluorescence lifetime was
measured on a PTI-3000 apparatus (Photon Technology Interna-
tional) at 298 1 K using a Xe nanoflash lamp filled with N2 [13].
Fluorescence quantum yield (˚F) was determined by comparison
of the integrated corrected emission spectrum of standard quinine,
which was excited at 366 nm in H2SO4 (0.5 M, ˚F = 0.55) [14]. Per-
chlorate salts were used as a metal source, and all measurements
were carried out in an aerated condition. 1H and 13C NMR spectra
were obtained by a JEOL JNM-GSX270 Excalibur. Elemental analysis
was performed on a Perkin-Elmer 2400II CHN Elemental Analyzer.
The program HYPERQUAD was used for determination of stability
constants for Cu2+ complexes [15].
Fig. 1. (a) Fluorescence spectra (ꢀex = 369 nm) of 1 (20 ␮M) measured in MeCN with
and without respective metal cation (3 equiv). (b) Fluorescence intensity of 1 at
430 nm. The spectra were measured after stirring the solution containing 1 and
each metal cation for 24 h at 298 K.
2.3. Ab initio calculation
The calculations were performed with the Gaussian 03 pro-
gram [16]. Geometry optimization was carried out with the density
functional theory (DFT) using the B3LYP function. Metal-free com-
pounds were calculated using the 6-31G* basis set. Cu2+ complexes
were calculated using the 6-31G* basis set for all atoms except for
Cu, for which LANL2DZ basis set with effective core potential was
used. Hg2+ complex was calculated using the 6-31G* basis set for all
atoms except for Hg, for which the Stuttgart Relativistic Small-Core
basis set with effective core potential was used.
Fig. 2. Change in fluorescence emission color of MeCN solutions containing 1 and
each metal cation.
3. Results and discussion
3.1. Synthesis and properties of the ligand
The bis-quinolylimine ligand, 1, was synthesized by condensa-
tion of 2-quinolinecarboxyaldehyde and hydrazine with 60% yield
(Scheme 1). Fig. 1a shows the fluorescence spectra (ꢀex = 369 nm)
of 1 (20 ␮M) measured in MeCN with and without metal cation (3
equiv). Without cation, 1 shows almost no fluorescence with the
fluorescence quantum yield (˚F) <0.004. Addition of Cu2+ to the
solution, however, creates a strong fluorescence at 380–520 nm. As
shown in Fig. 2, a bright blue fluorescence is observed upon Cu2+
Hg2+
300
200
other cations
none
Cu2+
addition. In contrast, addition of other metal cations (Na+, Mg2+
,
100
Mn2+, Co2+, Ni2+, Ag+, Zn2+, Cd2+, Pb2+, Fe2+, Fe3+, and Hg2+) to the
solution shows almost no fluorescence (Fig. 1b). This suggests that 1
shows selective fluorescence enhancement against Cu2+. It is noted
that, as shown in Fig. S3 (Supplementary Material), the fluorescence
enhancement upon Cu2+ addition occurs very slowly; 15 h stirring
is required for saturation of the fluorescence intensity.
Fe3+
0
250
300
350
λ (nm)
400
450
Fig. 3 shows the fluorescence excitation spectra of 1 monitored
at 430 nm with and without metal cation. With Cu2+, a vibra-
tional excitation band appears at 350–410 nm. Addition of Hg2+ or
Fe3+ also exhibits excitation band, but their wavelengths are much
Fig. 3. Fluorescence excitation spectra (ꢀem = 430 nm) of 1 (20 ␮M) measured in
MeCN with and without respective metal cation (3 equiv). The spectra were mea-
sured after stirring the solution containing 1 and each metal cation for 24 h at 298 K.

C. Ichimura et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
100
255
50
0.67
100
0
0
1
2
3
4
5
[Cu2+]/[1]
0.5
0
350
0
400
450
500
550
600
0
0.2
0.4
0.6
0.8
1
X
λ ( nm )
Fig. 5. Fluorescence-based Job’s plot of 1 with Cu2+ (X = [Cu2+]/([Cu2+] + [1]) mea-
sured at ꢀex = 369 nm and ꢀem = 430 nm. The total concentration of 1 and Cu2+ is set
20 ␮M.
Fig. 4. Fluorescence spectra (ꢀex = 369 nm) of 1 (20 ␮M) measured in MeCN with
different amount of Cu2+. (Inset) Fluorescence intensity monitored at 430 nm. The
spectra were measured after stirring the solution containing 1 and different amount
of Cu2+ for 24 h at 298 K.
shorter (<350 nm). This suggests that the interaction between 1 and
Cu2+ produces an emitting species that is photoexcited at a longer
wavelength. The photoexcitation at ꢀ > 350 nm (Fig. 1) therefore
allows selective fluorescence enhancement against Cu2+. It must
be noted that the Cu2+ sample, when left for 30 days (exposed
to interior lighting at room temperature under aerated condition)
does not show fluorescence decrease (Fig. S3, Supplementary Mate-
rial). This indicates that the formed emitting species is chemically
stable. It is also noted that the fluorescence response of 1 to Cu2+
is unaffected by many cations. As shown in Fig. S4 (Supplemen-
0.63
0.05
0.45
0-0.36
1.0
0.72
1
1.3
0
1.5
0.18
0.36
1.7-5.0
369 nm
0.45
389 nm
0.63
0
0.72
350
400
tary Material), addition of other metal cation (Na+, Mg2+, Mn2+
,
1.0
1.3
λ (nm)
Co2+, Ni2+, Ag+, Zn2+, Cd2+, Pb2+, Fe2+, or Fe3+) to the solution
containing 1 and Cu2+ scarcely affects the fluorescence intensity,
although addition of Hg2+ leads to an intensity decrease. These indi-
cate that 1 detects Cu2+ selectively even in the presence of many
other cations.
1.5
1.7-5.0
0
250
300
350
λ (nm)
400
450
500
Fig. 6. Absorption spectra of 1 (20 ␮M) measured in MeCN at different Cu2+ concen-
tration. The numbers in the Fig. denote the equivalents of Cu2+ added (0, 0.18, 0.36,
0.45, 0.63, 0.72, 1.0, 1.3, 1.5, 1.7, 2.0, 2.1, 2.3, 3, 3.75, and 5 equiv). The grey lines are
the spectra obtained with 1.7–5.0 equiv of Cu2+. The spectra were measured after
stirring the solution containing 1 and different amount of Cu2+ for 24 h at 298 K.
3.2. Stoichiometry for Cu2+ coordination
Fig. 4 shows the fluorescence spectra of 1 measured at different
Cu2+ concentration. With <1.5 equiv of Cu2+, only a weak fluores-
cence enhancement is observed (˚F < 0.01), indicating that, at this
concentration range, a nonfluorescent species is produced via the
interaction with Cu2+. Addition of >1.7 equiv of Cu2+, however,
leads to a strong fluorescence enhancement, where the inten-
sity is saturated upon addition of >2.5 equiv of Cu2+ (˚F = 0.37).
These imply that two kinds of 1–Cu2+ complexes are produced
in response to Cu2+ concentration. Fluorescence-based Job’s plot
analysis was performed to clarify the stoichiometry for associa-
tion of 1 with Cu2+. As shown in Fig. 5, two inflection points are
observed at X (=[Cu2+]/([Cu2+] + [1])) = 0.5 and 0.67, suggesting that
two kinds of 1–Cu2+ complexes with 1:1 and 1:2 stoichiometries
are produced. The apparent stability constants for the 1:1 and
1:2 complexes were determined by the Hyperquad program [15]
to be log K(Cu1/Cu·1) = 8.9 and log K(Cu21/2Cu·1) = 14.5, respec-
tively.
Fig. 6 shows the absorption spectra of 1 obtained at different
Cu2+ concentration. Without cation, 1 exhibits a strong absorption
band centered at 325 nm (ε = 37 940 M−1 cm−1), which is assigned
to the imine moiety (Ar–CH N) [17]. Addition of <1.5 equiv of
Cu2+ leads to a decrease in the 325 nm band, suggesting that the
imine nitrogens are involved in Cu2+ coordination. Fig. 7 shows
the fluorescence excitation spectra of 1 obtained at different Cu2+
concentration. Addition of <1.5 equiv of Cu2+ leads to a formation
of excitation band at 270–350 nm, but the long wavelength band
(350–410 nm) does not appear. The result is consistent with the flu-
100
0
100
0
1
2
3
4
5
[Cu2+]/[1]
2.3-5.0
2.1
1.5
1.3
2.0
1.7
1.0
0.74
0-0.63
350
0
250
300
400
450
λ (nm)
Fig. 7. Fluorescence excitation spectra (ꢀem = 430 nm) of 1 (20 ␮M) measured in
MeCN at differentCu2+ concentration. Thenumbers in theFig. denote the equivalents
of Cu2+ added (0, 0.18, 0.36, 0.45, 0.63, 0.72, 1.0, 1.3, 1.5, 1.7, 2.0, 2.1, 2.3, 3, 3.75,
and 5 equiv). The grey lines are the spectra obtained with 1.7–5.0 equiv of Cu2+
.
(Inset) Intensity monitored at 369 nm. The spectra were measured after stirring the
solution containing 1 and different amount of Cu2+ for 24 h at 298 K.

256
C. Ichimura et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
quinoline
PET
a
quinoline
1
1
1
H+
N
N
N
N
H+
PET
PET
b
H+
H+
N
0
250
N
N
N
300
350
400
λ (nm)
450
500
550
H+
H+
Fig. 8. Fluorescence (ꢀex = 316 nm) and excitation (ꢀem = 410 nm) spectra of (dotted
line) quinoline (40 ␮M) and (solid line) 1 (20 ␮M) measured in MeCN with 1 mM
HClO4.
PET
Scheme 2. Effect of protonation of 1 on the intramolecular PET mechanism.
orescence data (Fig. 4). These indicate that the addition of <1.5 equiv
of Cu2+ produces the 1:1 complex, which has no excitation band at
350–410 nm and shows almost no fluorescence upon excitation at
>350 nm.
necessary for the formation of a longer wavelength excitation
band.
As shown in Fig. 6 (gray lines), addition of >1.7 equiv of Cu2+
leads to a minor absorption change, where weak absorption bands
appear at 369 and 389 nm (inset). As shown in Fig. 7 (gray lines),
addition of >1.7 equiv of Cu2+ leads to a formation of long wave-
length excitation band at 350–410 nm. The band maxima at 350,
369, and 389 nm agree with the absorption band maxima (Fig. 6,
inset). The inset of Fig. 7 shows the change in intensity of 369 nm
band with the Cu2+ amount. The profile is similar to the fluores-
cence intensity profile (Fig. 4, inset). These clearly suggest that the
addition of >1.7 equiv of Cu2+ produces the 1:2 complex, which has
an excitation band at 350–410 nm and shows fluorescence upon
excitation at >350 nm.
The properties of fluorescence obtained upon excitation of the
1:2 complex at longer wavelength must be clarified. As shown in
Fig. 9 (solid line), the excitation spectrum consists of two major
bands at 270–350 nm and 350–410 nm, respectively. The former
is similar to the band for the protonated quinoline and 1 (Fig. 8).
Fig. 9 (dotted line) shows the fluorescence spectrum of 1:2 complex
obtained upon excitation at 316 nm. A broad fluorescence centered
at 410 nm appears, which is similar to the spectra for the proto-
nated quinoline and 1 (Fig. 8). In contrast, as shown in Fig. 9 (dashed
line), the excitation at 369 nm shows a red-shifted fluorescence
centered at 430 nm, indicating that the longer wavelength excita-
tion produces a different emitting species. Time-resolved emission
decay analysis of the 1:2 complex (Fig. S6, Supplementary Mate-
rial) reveals that the fluorescence obtained upon longer wavelength
excitation has 3.3 ns lifetime, whereas that obtained upon shorter
wavelength excitation has a much longer lifetime (30.1 ns). The
fluorescence of the protonated quinoline (35.6 ns) and 1 (31.4 ns)
also has a longer lifetime (Fig. S7, Supplementary material). This
indicates that the fluorescence obtained upon shorter wavelength
excitation of the 1:2 complex is due to the ␲, ␲* transition of the
quinoline moieties [21], and that obtained upon longer wavelength
excitation is due to the different emitting species.
3.3. Coordination and fluorescence properties of the ligand
It is well known that a quinoline molecule is nonfluorescent
because of its n, ␲* transition [18]. Protonation or metal coor-
dination of quinoline nitrogen, however, converts the transition
to ␲, ␲* and results in fluorescence enhancement. To clarify the
fluorescence enhancement mechanism for the 1–Cu2+ 1:2 com-
plex, effects of proton on the fluorescence properties of quinoline
and 1 were studied. Fluorescence titration of quinoline was per-
formed in MeCN with HClO4 (Fig. S5, Supplementary Material). The
absence of H+ shows no fluorescence, but H+ addition (<0.05 mM)
shows a strong fluorescence centered at ca. 410 nm, along with
a formation of excitation band at 270–350 nm [19], as shown in
Fig. 8 (dotted line). In contrast, 1 shows different fluorescence
behaviors upon H+ titration (Fig. S5, Supplementary Material).
Addition of H+ (<0.05 mM) does not show fluorescence, although
the quinoline nitrogens of 1 are fully protonated. This is because,
as shown in Scheme 2a, the photoinduced electron transfer (PET)
from the imine nitrogens to the excited state quinoline moi-
eties quenches the fluorescence [20]. However, as shown in Fig. 8
(solid line), further H+ addition (>0.05 mM) promotes a fluores-
cence enhancement along with a formation of excitation band
at 270–350 nm. This is because, as shown in Scheme 2b, pro-
tonation of imine nitrogens suppresses PET [20]. These indicate
that the protonation (or metal coordination) of both quino-
λex = 316 nm
λem = 430 nm
1
λex = 369 nm
0
250
300
350
400
450
500
550
λ (nm)
line and imine nitrogens of
1 is necessary for fluorescence
enhancement. However, as shown in Fig. 8, the excitation spec-
trum of 1 measured with H+ does not show longer wavelength
excitation band at 350–410 nm, which is observed in the pres-
ence of Cu2+ (Fig. 7). This indicates that Cu2+ coordination is
Fig. 9. Fluorescence spectra of 1 (20 ␮M) measured in MeCN with 3 equiv of Cu2+
when excited at (dotted line) 316 nm and (dashed line) 369 nm. (Solid line) Fluores-
cence excitation spectrum monitored at 430 nm. The spectra were measured after
stirring the solution containing 1 and Cu2+ for 24 h at 298 K.
,

C. Ichimura et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
257
Table 1
The above ␲-conjugation mechanism is further confirmed by
the configuration of a 1–Hg2+ 1:2 complex. As shown in Fig. 3,
addition of Hg2+ to a solution containing 1 leads to a formation
of ␲, ␲* transition band at 270–350 nm, but does not show longer
wavelength band. As shown in Fig. S8 (Supplementary Material),
Job’s plot analysis indicates that 1 coordinates with Hg2+ in a
1:2 stoichiometry, as is the case for Cu2+. However, as shown in
Table 1e, the 1–Hg2+ 1:2 complex has a distorted structure with
dihedral angle 47.5◦. The average distance of Hg–quinoline nitro-
Geometry optimized structures of (a) 1, (b) fully protonated form of 1, (c) 1–Cu2+
1:1 complex, (d) 1–Cu2+ 1:2 complex, and (e) 1–Hg2+ 1:2 complex determined by
ab initio calculation.a
Top view
Side view
(a) 1
˚
gen is 2.16 A, which is shorter than that of Hg–imine nitrogen
(b) fully protonated form of 1
˚
˚
(2.61 A). In contrast, the distance of Cu–quinoline nitrogen (2.23 A)
of the 1–Cu2+ 1:2 complex is longer than that of Cu–imine nitrogen
(2.09 A). The different distances for coordination of Hg and Cu2+
2+
˚
are probably due to the stronger affinity of quinoline nitrogen with
Hg2+ than Cu2+ [23]. The different coordination distance around
Hg2+ probably leads to a distortion of the 1–Hg2+ 1:2 complex
(Table 1e). The above results again suggest that the ␲-conjugational
extension of the 1–Cu2+ 1:2 complex is due to the planar configu-
ration of the complex.
(c) 1–Cu2+ 1:1 complex [1·Cu(MeCN)]2+b
(d) 1–Cu2+ 1:2 complex [1·Cu2(MeCN)2]4+b
3.5. Effect of other factors on fluorescence properties of 1:2
complex
The above results suggest that the coordination geometry of
the 1–Cu2+ 1:2 complex is crucial for fluorescence enhancement.
It is well known that coordination of imine ligands with metal
cations depends on several factors such as solvents and counter
anions [24]. The fluorescence properties of the 1:2 complex are
also affected by these factors: (i) the fluorescence intensity depends
strongly on solvents. As shown in Fig. S9 (Supplementary Material),
cyanide solvents such as MeCN and butylonitrile show strong fluo-
rescence, while other common solvents such as alcohols show very
weak fluorescence. This is probably because the cyanide molecules
behave as a ligand for Cu2+ center (Table 1) [25]. (ii) Addition of
water leads to a decrease in fluorescence intensity. As shown in
Fig. S10 (Supplementary Material), the fluorescence intensity of
the 1:2 complex in MeCN is not affected by the addition of 0.05%
water, but 0.2% and 1% additions lead to 54% and 93% intensity
decrease. (iii) Counter anion of Cu2+ also strongly affects the inten-
sity. As shown in Fig. S11 (Supplementary Material), addition of
Cu(ClO4)2 or Cu(OTf)2 to 1 shows strong fluorescence, while CuCl2
or Cu(NO3)2 does not. These findings suggest that application of
this ligand for fluorescence sensing of Cu2+ in real environmental
(e) 1–Hg2+ 1:2 complex [1·Hg2(MeCN)2]4+
a
The gray, blue, orange, black, and white atoms denote C, N, Cu, Hg, and H atoms,
respectively.
b
Counter anions of the complexes were omitted for clarity.
3.4. Emission mechanism for 1:2 complex
The longer wavelength excitation band for 1:2 complex is
formed by extended ␲-conjugation over the entire molecule due
to the planar configuration of the complex, as observed for related
alkyne- or imine-containing molecules [22]. This is confirmed by
coordination geometry of the complex, calculated with the Gaus-
sian 03 program at the DFT level [16]. Table 1 shows the optimized
geometry of the molecules. As shown in Table 1a, 1 has a planar
structure, where the dihedral angle between two quinoline moi-
eties is 0.0◦. As shown in Fig. 8, the protonated form of 1 does
not show longer wavelength excitation band, while showing ␲, ␲*
band. As shown in Table 1b, the protonated 1 has a distorted struc-
ture with dihedral angle 72.2◦. This may be due to the electrostatic
repulsion of the positive charges. This probably leads to a discon-
nection of the ␲-conjugation over the entire molecule and results
in the absence of long wavelength excitation band. In the case of
1–Cu2+ 1:1 complex (Table 1c), all four nitrogens of 1 coordinate
with Cu2+, forming a (E,Z)-configuration. The complex also has a dis-
torted structure with dihedral angle 59.5◦. This may also suppress
the ␲-conjugational extension, resulting in no formation of longer
wavelength band. In contrast, for 1–Cu2+ 1:2 complex (Table 1d),
two Cu2+ are coordinated with each set of adjacent quinoline and
imine nitrogens, forming a (E,E)-configuration. The dihedral angle
of the complex is 5.2◦, suggesting that the complex has a planar
structure with a sp2 hybridized bridging unit [22]. The 1:2 com-
plex therefore allows an extended ␲-conjugation over the entire
molecule and, hence, shows longer wavelength excitation band at
350–410 nm.
samples is difficult at present. However, the mechanism for Cu2+
-
selective fluorescence enhancement based on the structure change
upon Cu2+ coordination may contribute to the development of new
Cu2+-selective fluorescent ligands.
4. Conclusion
We found that a bis-quinolylimine ligand, 1, shows a selective
fluorescence enhancement against Cu2+. Coordination of 1 with
two Cu2+ (1:2 complex formation) creates a new emitting species
that is photoexcited at 350–410 nm, while the addition of other
metal cations does not create such species. The 1–Cu2+ 1:2 com-
plex has a planar structure and, hence, promotes an extension
of ␲-electrons over the entire molecule. This thus allows fluo-
rescence enhancement upon photoexcitation at 350–410 nm. The
fluorescence properties of the 1:2 complex depend strongly on sev-
eral factors such as solvents and counter anions. Although further
investigation on the extended ␲-conjugation mechanism for flu-
orescence enhancement is still required, the simple mechanism
driven by Cu2+-induced structure change would be effective for
creation of new Cu2+-selective fluorescent ligands.

258
C. Ichimura et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 253–258
Acknowledgments
[9] (a) S.Y. Lee, H.J. Kim, J.-S. Wu, K. No, J.S. Kim, Tetrahedron Lett. 49 (2008)
6141–6144;
(b) M.H. Kim, H.H. Jang, S. Yi, S.-K. Chang, M.S. Han, Chem. Commun. (2009)
4838–4840.
This work was supported by the Grant-in-Aid for Scientific
Research (No. 21760619) from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan (MEXT). C.I. thanks the
Global COE Program “Global Education and Research Center for
Bio-Environmental Chemistry” of Osaka University.
[10] (a) Y. Shiraishi, C. Ichimura, T. Hirai, Tetrahedron Lett. 48 (2007) 7769–7773;
(b) S. Ou, Z. Lin, C. Duan, H. Zhang, Z. Bai, Chem. Commun. (2006) 4392–4394;
(c) Y. Zhang, X. Guo, W. Si, L. Jia, X. Qian, Org. Lett. 10 (2008) 473–476;
(d) M. Royzen, A.V.C. Durandin Jr., N.E. Young, J.W. Geacintov, Canary, J. Am.
Chem. Soc. 128 (2006) 3854–3855;
(e) Y. Mikata, M. Wakamatsu, S. Yano, Dalton Trans. (2005) 545–550.
[11] (a) G.-K. Li, Z.-X. Xu, C.-F. Chen, Z.-T. Huang, Chem. Commun. (2008) 1774–1776;
(b) T. Moriuchi-Kawakami, J. Sato, Y. Shibutani, Anal. Sci. 25 (2009) 449–452;
(c) E. Ballesteros, D. Moreno, T. Gómez, T. Rodríguez, J. Rojo, M. García-Valverde,
T. Torroba, Org. Lett. 11 (2009) 1269–1272.
[12] (a) Y. Shiraishi, Y. Tokitoh, G. Nishimura, T. Hirai, Org. Lett. 7 (2005) 2611–2614;
(b) Y. Shiraishi, Y. Tokitoh, T. Hirai, Chem. Commun. (2005) 5316–5318.
[13] (a) Y. Shiraishi, Y. Tokitoh, T. Hirai, Org. Lett. 8 (2006) 3841–3844;
(b) Y. Shiraishi, Y. Tokitoh, G. Nishimura, T. Hirai, J. Phys. Chem. B 111 (2007)
5090–5100.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.10.018.
References
[14] J.E. Sabol, M.G. Rockley, J. Photochem. Photobiol. A 40 (1987) 245–257.
[15] A. Sabatini, A. Vacca, P. Gans, Coord. Chem. Rev. 120 (1992) 389–405.
[16] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, J.A. Montgomery, T. Vreven Jr., K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitano, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.
Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-
mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,
J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Ste-
fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B.G. John-
son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B.05,
Gaussian, Inc., Pittsburgh, PA, 2003.
[17] P. Bose, P. Ghosh, Chem. Commun. 46 (2010) 2962–2964.
[18] N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 29 (1956) 373–379.
[19] C. Ichimura, Y. Shiraishi, T. Hirai, Tetrahedron 66 (2010) 5594–5601.
[20] (a) Y. Shiraishi, H. Maehara, K. Ishizumi, T. Hirai, Org. Lett. 9 (2007) 3125–
3128;
(b) A. Caballrero, R. Martínez, V. Lloveras, I. Retera, J. Vidal-Gancedo, K. Wurst,
A. Tárraga, P. Molina, J. Veciana, J. Am. Chem. Soc. 127 (2005) 15666–15667.
[21] M.F. Anton, M. Nicol, J. Lumin. 18–19 (1979) 131–134.
[1] (a) V. Balzani, Molecular Devices and Machines: A Journey into the Nano World,
Wiley-VCH, Weinheim, 2003;
(b) A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy,
J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515–1566;
(c) A.P. de Silva, N.D. McClenaghan, C.P. McCoy, Molecular Switches, Wiley-VCH,
New York, 2000.
[2] (a) L. Fabbrizzi, A. Poggi, Chem. Soc. Rev. (1995) 197–202;
(b) A.W. Czarnik, Acc. Chem. Res. 27 (1994) 302–308;
(c) A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Coord. Chem. Rev. 205
(2000) 41–57.
[3] (a) A. Czarnik, Fluorescent Chemosensors for Ion and Molecule Recognition,
American Chemical Society, Washington, DC, 1993;
(b) B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3–40;
(c) P. Jiang, Z. Guo, Coord. Chem. Rev. 248 (2004) 205–229.
[4] (a) D. Parker, J.A.G. Williams, J. Chem. Soc. Perkin Trans. 2 (1995) 1305–1314;
(b) S. Alves, F. Pina, M.T. Albelda, E. García-Espan˜a, C. Soriano, S.V. Luis, Eur. J.
Inorg. Chem. (2001) 405–412;
(c) Y. Shiraishi, Y. Kohno, T. Hirai, J. Phys. Chem. B 109 (2005) 19139–19147.
[5] (a) G. Nishimura, K. Ishizumi, Y. Shiraishi, T. Hirai, J. Phys. Chem. B 110 (2006)
21596–21602;
(b) A. Mokhir, A. Kiel, D.-P. Herten, R. Kraemer, Inorg. Chem. 44 (2005)
5661–5666;
(c) Y.-Q. Wen, F. Yue, Y.-R. Zhong, B.-H. Ye, Inorg. Chem. 46 (2007) 7749–7755.
[6] (a) Y. Xiang, A.-J. Tong, P.-Y. Jin, Y. Ju, Org. Lett. 8 (2006) 2863–2866;
(b) W. Lin, L. Long, B. Chen, W. Tan, W. Gao, Chem. Commun. 46 (2010)
1311–1313;
[22] (a) D.-Y. Wu, J.-X. Xie, C.-L. Zhang, C.-Y. Duan, Y.-G. Zhao, Z.-J. Guo, Dalton Trans.
(2006) 3528–3533;
(b) W.-K. Dong, J.-G. Duan, Y.-H. Guan, J.-Y. Shi, C.-Y. Zhao, Inorg. Chim. Acta
362 (2009) 1129–1134;
(c) T. Terashima, T. Nakashima, T. Kawai, Org. Lett. 9 (2007) 4195–4198.
[23] A.E. Martell, R.M. Smith, Critical Stability Constants, vol. 2, Plenum Press, New
York, 1974.
[24] (a) Z. Wu, Y. Zhang, J.S. Ma, G. Yang, Inorg. Chem. 45 (2006) 3140–3142;
(b) H.S. Hur, S.H. Kim, S.Y. Yun, S.W. Lee, Polyhedron 27 (2008) 1229–1237;
(c) A.R. Kennedy, K.G. Brown, D. Graham, J.B. Kirkhouse, M. Kittner, C. Major,
C.J. McHugh, P. Murdoch, W.E. Smith, New J. Chem. 29 (2005) 826–832.
[25] (a) P. Comba, A. Fath, T.W. Hambley, A. Kühner, D.T. Richens, A. Vielfort, Inorg.
Chem. 37 (1998) 4389–4401;
(c) M.H. Lee, H.J. Kim, S. Yoon, N. Park, J.S. Kim, Org. Lett. 10 (2008) 213–216;
(d) X. Zhang, Y. Shiraishi, T. Hirai, Org. Lett. 9 (2007) 5039–5042.
[7] (a) H.S. Jung, M. Park, D.Y. Han, E. Kim, C. Lee, S. Ham, J.S. Kim, Org. Lett. 11
(2009) 3378–3381;
(b) E.J. Jun, H.N. Won, J.S. Kim, K.-H. Lee, J. Yoon, Tetrahedron Lett. 47 (2006)
4577–4580;
(c) R. Martínez, F. Zepata, A. Caballero, A. Espinosa, A. Tárraga, P. Molina, Org.
Lett. 8 (2006) 3235–3238.
[8] (a) Z. Xu, X. Qian, J. Cui, Org. Lett. 7 (2005) 3029–3032;
(b) Z. Xu, Y. Xiao, J. Cui, D. Cui, Org. Lett. 7 (2005) 889–892;
(c) Z. Xu, S.J. Han, C. Lee, J. Yoon, D.R. Spring, Chem. Commun. 46 (2010)
1679–1681.
(b) J. Reedijk, W.L. Groeneveld, Rec. Trav. Chim. 87 (1968) 1079–1088.