Multicolor fluorescence of a styrylquinoline dye tuned by metal cations

Article abstract of DOI:10.1002/chem.201101048

A styrylquinoline dye with a dipicolylamine (DPA) moiety (1) has been synthesized. The dye 1 in acetonitrile demonstrates multicolor fluorescence upon addition of different metal cations. Compound 1 shows a green fluorescence without cations. Coordination

Full text of DOI:10.1002/chem.201101048

DOI: 10.1002/chem.201101048
Multicolor Fluorescence of a Styrylquinoline Dye Tuned by Metal Cations
Yasuhiro Shiraishi,* Chizuru Ichimura, Shigehiro Sumiya, and Takayuki Hirai[a]
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FULL PAPER
Abstract: A styrylquinoline dye with a
dipicolylamine (DPA) moiety (1) has
been synthesized. The dye 1 in acetoni-
trile demonstrates multicolor fluores-
cence upon addition of different metal
cations. Compound 1 shows a green
fluorescence without cations. Coordina-
tion of 1 with Cd2+ shows a blue emis-
sion, while with Hg2+ and Pb2+ exhib-
its yellow and orange emissions, respec-
tively. The different fluorescence spec-
tra are due to the change in intramo-
different cations. The DPA and quino-
line moieties of 1 behave as the elec-
tron donor and acceptor units, respec-
tively, and both units act as the coordi-
nation site for metal cations. Cd2+ co-
ordinates with the DPA unit. This re-
duces the donor ability of the unit and
decreases the energy level of HOMO.
This results in an increase in HOMO–
LUMO gap and blue shifts the emis-
sion. Hg2+ or Pb2+ coordinate with
both DPA and quinoline units. The co-
ordination with the quinoline unit de-
creases the energy level of LUMO.
This results in a decrease in HOMO–
LUMO gap and red shifts the emission.
Addition of two different metal cations
successfully
creates
intermediate
colors; in particular, the addition of
Cd2+ and Pb2+ at once creates a bright
white fluorescence.
Keywords: charge transfer · dyes/
pigments · fluorescence · transition
metals · white emission
lecular
charge
transfer
(ICT)
properties of 1 upon coordination with
Introduction
Organic light-emitting dyes have attracted much attention in
the optical electronics industry, because of their potential
applications in displays and light sources.[1] It is, however,
known that a single dye molecule usually shows a single
fluorescence. Creation of representative fluorescence colors,
typically violet, indigo, blue, green, yellow, orange, and
red,[2] therefore requires the dyes to show respective fluores-
cence colors, and intermediate colors to be created by
mixing these dyes.[3] In particular, a white fluorescence color
is very difficult to create; three kinds of dyes that show
blue, green, and red fluorescence or two kinds of dyes that
show blue and yellow fluorescence must be mixed in appro-
priate amounts.[4] Recently, dyes that show multiple fluores-
cence colors in response to external stimuli have attracted
much attention because these can create multiple fluores-
cence colors with a single molecule.[5] Several external stim-
uli have been used for modulation of emission colors, such
as excitation wavelength,[6] temperature,[7] pH,[8] and solvent
polarity.[9]
Scheme 1. Schematic representation of the change in energy gap promot-
ed by the coordination of metal cation.
Metal cations are often used as an external stimulus for
modulation of fluorescence colors.[10] The most popular mo-
lecular systems showing metal-induced fluorescence change
are the intramolecular charge transfer (ICT) systems,[11]
which consist of an electron donor and acceptor units cou-
pled through a p-conjugated spacer (Scheme 1). These ICT
systems possess spatially separated frontier molecular orbi-
tals.[12] The interaction of the electron donor or acceptor
moiety with metal cation, therefore, affects the energetic po-
sition of either HOMO or LUMO and elicits a change in
fluorescence spectra. When the metal binding site is situated
in the donor moiety (Scheme 1a), the coordination with a
metal cation stabilizes the HOMO more strongly than
LUMO, resulting in a blue shift of fluorescence spectra.[13]
In contrast, when the binding site is situated in the acceptor
moiety (Scheme 1b), metal coordination stabilizes the
LUMO more strongly than HOMO, resulting in a red shift
of fluorescence spectra.[14] Several ICT-based fluorescent
dyes have been proposed; however, most of the dyes show
only two[14b,15] or three[5b,16] fluorescence colors among the
seven representative colors (violet, indigo, blue, green,
yellow, orange, and red).
[a] Dr. Y. Shiraishi, Dr. C. Ichimura, S. Sumiya, Prof. T. Hirai
Research Center for Solar Energy Chemistry and
Division of Chemical Engineering
Graduate School of Engineering Science
Osaka University, Toyonaka 560-8531 (Japan)
Fax : (+81)6-6850-6271
One of the possible ways to create multiple fluorescence
colors is to construct metal binding sites on both the elec-
tron acceptor and donor moieties of the ICT systems. Coor-
dination of metal cations with either the donor or acceptor
moiety or with both moieties would promote change in the
E-mail: shiraish@cheng.es.osaka-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101048.
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Y. Shiraishi et al.
energetic positions of the HOMO and LUMO, and create
multiple fluorescence colors. On the basis of this scenario,
Bunz and co-workers synthesized a series of cruciform dyes
(1,4-distyryl-2,5-bis(arylethynyl)benzene) possessing metal
binding sites on both the donor and acceptor moieties.[17]
Addition of different metal cations to the solution contain-
ing these dyes create several emission colors, such as
indigo–green–red,[17a] blue–yellow–red,[17b]
indigo–blue–
green,[17c] blue–green–orange,[17d] and indigo–green–or-
ange.[17e] Some other ICT-based dyes with two different
metal binding sites have also been proposed.[15b,18] All of
these ICT systems,[15b,17,18] however, show only three fluores-
cence colors among the seven representative colors.
Herein, we report an ICT-based dye that exhibits four
types of fluorescence colors, namely, blue, green, yellow, and
orange, upon coordination with different metal cations. The
dye 1 consists of quinoline and dipicolylamine (DPA) moiet-
Figure 1. a) Fluorescence spectra of 1 (10 mm) measured in MeCN with
ꢀ
10 equivalents of each respective metal cation (as a ClO4 salt). The exci-
tation wavelengths are 386 nm (free 1), 350 nm (Cd2+), 413 nm (Hg2+),
and 467 nm (Pb2+), respectively. b) Fluorescence color of the respective
solutions taken under irradiation with a handheld UV lamp (365 nm). c)
Chromaticity of the fluorescence color of 1 obtained with and without
metal cations in a 1931 CIE diagram.
ies that are connected through a p-conjugated vinyl spacer.
The quinoline and DPA units behave as electron acceptor
and donor units, respectively, and both units act as the metal
binding site. The dye, when dissolved in MeCN, shows a
green fluorescence. Addition of Cd2+ shows a blue emission,
and addition of Hg2+ and Pb2+ exhibits yellow and orange
emission, respectively. These emission properties are due to
the different ICT properties of 1 in response to the coordi-
nation of the respective metal cations. Another notable fea-
ture of 1 is that the addition of two different metal cations
at once successfully creates intermediate colors in correla-
tion to the ratio of metal cations. In particular, a mixture of
Cd2+ and Pb2+ creates a near-white fluorescence. To the
best of our knowledge, this is the first report of dyes that
creates a white fluorescence by a metal cation stimulus.
Figure 1b, a bright blue emission is observed upon addition
of Cd2+. In contrast, addition of Hg2+ creates a red-shifted
fluorescence at 546 nm with a yellow emission color (FF =
0.020). Addition of Pb2+ shows a further red-shifted fluores-
cence at 571 nm with an orange emission color (FF =0.010).
Figure 1c shows the CIE (International Commission on Illu-
mination) chromaticity diagram[19] for the fluorescence spec-
tra. The free 1 shows a green fluorescence color with a chro-
maticity coordinate (0.17, 0.46), while the addition of re-
spective cations shows a broad color range; Cd2+ (blue;
0.15, 0.13), Hg2+ (yellow; 0.33, 0.61), and Pb2+ (orange;
0.46, 0.52). These indicate that 1 creates four types of fluo-
rescence colors among the seven representative colors upon
addition of different metal cations. The fluorescence life-
times of these emissions are similar to each other (0.6–
1.2 ns, Figure S4, Supporting Information). It is also noted
that, as shown in Figure S5 in the Supporting Information,
addition of Zn2+ to the solution containing 1 shows a blue
emission (441 nm; FF =0.031) similar to that of Cd2+. Addi-
tion of Fe2+, Fe3+, or Cr3+ gives an orange emission (at
571 nm) similar to that of Pb2+, although with decreased
quantum yield (FF <0.008). Addition of other cations, such
as Mn2+, Ni2+, Co2+, and Cu2+ leads to almost no fluores-
cence.
Results and Discussion
Fluorescence properties: The dye 1 was synthesized by the
condensation of 2-methylquinoline with 4-(di-2-picolylami-
no)-benzaldehyde[13a] in 47% yield (see Experimental Sec-
tion). The purity of 1 was fully confirmed by 1H and
13C NMR spectroscopy, and FAB-MS analysis (Figures S1–
S3 in the Supporting Information).
Figure 1a shows the fluorescence spectra of 1 (10 mm)
measured in MeCN with and without each respective metal
cation (10 equiv). Without a cation, 1 shows a green fluores-
cence at 508 nm (FF =0.061). Upon addition of Cd2+, the
spectrum blue shifts to 454 nm (FF =0.033). As shown in
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FULL PAPER
Coordination properties: Fluorescence titration of 1 was car-
ried out to clarify the coordination properties with metal
cations. Figure 2a shows the result of fluorescence titration
with Cd2+. The Cd2+ addition leads to a decrease in the
508 nm fluorescence for free 1 and an increase in a blue-
shifted fluorescence at 454 nm, with an isoemissive point at
461 nm. The spectral change almost terminates by the addi-
tion of one equivalent of Cd2+. Figure 3a shows the result
of absorption titration of 1 with Cd2+. The absorption band
at 386 nm associated with free 1 decreases with the addition
of Cd2+, with an isosbestic point at 350 nm. The spectral
change is virtually finished with the addition of one equiva-
lent of Cd2+, which is consistent with the fluorescence titra-
tion data (Figure 2a). Nonlinear fitting of the absorption ti-
tration data by the Hyperquad program[20] reveals that 1 co-
ordinates with Cd2+ in a 1:1 stoichiometry with an associa-
tion constant logK=6.5 (Table 1). The dotted lines in the
Table 1. The apparent stability constants (logK) of 1 with metal cations
measured in MeCN at 298 K.[a]
Reaction
Cd2+
6.5
Hg2+
Pb2+
1+M2+![M(1)]2+
7.0
12.5
5.1
10.2
1+2M2+![M2(1)]4+
[a] Determined by Hyperquad program with the absorption titration data
(Figure 3).
inset of Figure 2a show the mole fraction distribution of the
species determined by the Hyss program[21] with the associa-
tion constant. The decrease in the 508 nm fluorescence
agrees reasonably well with the formation of 1:1 1/Cd2+
complex, suggesting that 1:1 coordination produces the
blue-shifted fluorescence. The DPA ligand strongly coordi-
[22]
nates with Cd2+
,
indicating that the coordination of DPA
moiety of 1 with Cd2+ produces the 1:1 1/Cd2+ complex.
Addition of Hg2+ or Pb2+ to 1 shows different fluores-
cence properties. In the case of Hg2+ (Figure 2b), addition
of <1.5 equivalents of Hg2+ leads to a quenching of 508 nm
fluorescence for the free 1, but further Hg2+ addition cre-
ates a red-shifted fluorescence at 546 nm. This implies that
two kinds of species are produced by the coordination of 1
with Hg2+. Absorption titration data show a similar trend
(Figure 3b). Addition of <1.5 equivalents of Hg2+ leads to
a decrease in 386 nm absorption for the free 1 along with an
increase in 356 nm band; however, further Hg2+ addition
leads to a decrease in this band with an appearance of a
new band at 402 nm. Nonlinear fitting analysis of the titra-
tion data indicates that 1:1 1/Hg2+ (logK=7.0) and 1:2 1/
Hg2+ complexes (logK=12.5) are produced via a coordina-
tion of 1 with Hg2+ (Table 1). As shown in Figure 2b (inset),
the fluorescence intensity decreases with the addition of ap-
proximately 1.5 equivalents of Hg2+, which is in accordance
with the 1:1 1/Hg2+ complex formation. Further Hg2+ addi-
tion creates a red-shifted emission along with the 1:2 1/Hg2+
complex formation. This suggests that the 1:2 1/Hg2+ com-
plex is the yellow emitting species.
Figure 2. Fluorescence spectra of 1 (10 mm) measured in MeCN with dif-
ferent amount of a) Cd2+ (lex =350 nm), b) Hg2+ (lex =413 nm), and c)
Pb2+ (lex =427 nm). Inset: Change in fluorescence intensity, in which the
dotted lines are the mole fraction distribution of species determined by
the stability constants (Table 1). The photographs of the solutions were
taken under irradiation with a handheld UV lamp (365 nm).
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Y. Shiraishi et al.
Change in electronic properties by metal cation binding: To
clarify the change in electronic properties of 1 upon coordi-
nation with different metal cations, ab initio calculations
were performed within the Gaussian 03 program by using
the time-dependent density functional theory (TDDFT).[24]
As shown in Table S1 (in the Supporting Information), the
singlet electronic transitions of free 1 and its metal com-
plexes are mainly contributed by HOMO!LUMO or
HOMO!LUMO+1 transition (S0!S1 or S0!S2), and the
calculated transition energies are close to the observed ab-
sorption maxima of the spectra (Figure 3). This suggests that
the calculated results represent the electronic properties of
the compounds precisely.
Fluorescent dyes exhibiting ratiometric response to metal
cations have spatially separated frontier molecular orbitals.
The change in fluorescence properties upon metal cation co-
ordination are therefore explained by the HOMO–LUMO
gap and their electronic distributions.[12] Figure 4 summarizes
the energy levels of HOMO and LUMO for respective mol-
ecules and their electronic distributions. In the case of free 1
(Figure 4a), p electrons on the HOMO are mainly located
on the aminostyrene moiety, whereas those on the LUMO
are mainly located on the quinoline moiety. This indicates
that the ICT from the donor (DPA) to the acceptor (quino-
line) indeed occurs in the compound 1.
Upon coordination of Cd2+ with the DPA unit (Fig-
ure 4b), the energy levels of both HOMO and LUMO de-
crease relative to those of free 1. The HOMO level decreas-
es more significantly, indicating that the HOMO is more sta-
bilized than LUMO. Upon coordination of Cd2+, p electrons
on HOMO are distributed on the styrylquinoline moiety, in
which the electrons are not located on the aniline nitrogen.
This suggests that coordination of Cd2+ with the DPA
moiety indeed decreases the electron-donating ability of the
nitrogen atom of DPA moiety, as schematically shown in
Scheme 1a.[13a] The HOMO–LUMO gap therefore becomes
larger relative to free 1. This thus results in a blue shift of
Figure 3. Absorption spectra of 1 (10 mm) measured in MeCN with differ-
ent amount of a) Cd2+, b) Hg2+, and c) Pb2+. The gray lines are the spec-
tra obtained with 1.5–10.0 equivalents of metal cations.
Addition of Pb2+ shows spectral behaviors similar to that
of Hg2+. Nonlinear fitting of the absorption titration data
(Figure 3c) also confirms the formation of 1:1 1/Pb2+
(logK=5.1) and 1:2 1/Pb2+ (logK=10.2) complexes
(Table 1). As shown in Figure 2c (inset), addition of
<1.5 equivalents of Pb2+ leads to a quenching of 508 nm
fluorescence for the free 1 along with the 1:1 1/Pb2+ com-
plex formation. Further Pb2+ addition creates a red-shifted
emission at 571 nm, which is in accordance with the 1:2 1/
Pb2+ complex formation. These suggest that the 1:2 1/Pb2+
complex is the orange emitting species. As shown in Fig-
ure 3b and c, addition of <1.5 equivalents of Hg2+ or Pb2+
promotes a blue-shift of the absorption spectra, as is the
case for Cd2+. This indicates that the DPA unit of 1 firstly
coordinates with Hg2+ or Pb2+, producing nonemissive 1:1
1/Hg2+ and 1/Pb2+ complexes. As quinoline nitrogen atoms
fluorescence spectra upon coordination with Cd2+
.
Coordination of Hg2+ or Pb2+ with the DPA moiety of 1
shows different electronic properties. As shown in Figure 4c
and d, the HOMO and LUMO levels of the 1:1 1/Hg2+ and
1/Pb2+ complexes are similar to those of 1:1 1/Cd2+ complex
(Figure 4b). The p electrons on LUMO of the 1:1 1/Cd2+
complex are distributed on the styrylquinoline moiety, but
those of the 1:1 1/Hg2+ and 1/Pb2+ complexes are localized
on the pyridine moieties. As shown in Table S2 (in the Sup-
porting Information), p electron distribution on the styryl-
quinoline moiety exists at higher energy levels than
LUMO+1. This is probably because the pyridine nitrogen
atoms strongly coordinate with Hg2+ and Pb2+, and the p
electrons on the pyridine moieties are stabilized strongly.[23]
These results indicate that photoexcitation of these 1:1 1/
Hg2+ and 1/Pb2+ complexes promotes an electronic popula-
tion of pyridine moieties by means of an electron-transfer
process[25] and, hence, shows almost no fluorescence.
can also coordinate with heavy metal cations such as Hg2+
[23]
and Pb2+
,
further addition of these ions leads to a coordi-
nation with the quinoline nitrogen, resulting in the forma-
tion of emissive 1:2 1/Hg2+ and 1/Pb2+ complexes.
Coordination of Hg2+ or Pb2+ with a quinoline moiety
(formation of the 1:2 1/Hg2+ and 1/Pb2+ complexes) alters
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Figure 4. The HOMO–LUMO energy gaps for respective compounds and interfacial plots of the orbitals. a) free 1, b) 1:1 1/Cd2+ complex, c) 1:1 1/Hg2+
complex, d) 1:1 1/Pb2+ complex, e) 1:2 1/Hg2+ complex, and f) 1:2 1/Pb2+ complex. Gray, blue, and black atoms of the molecular frameworks indicate
the C, N, and metal atoms. Green and deep red parts on the interfacial plots refer to the different phase of the molecular wave functions, for which the
isovalue is 0.02 a.u.
the electronic properties. As shown in Figure 4e and f, p
electron distributions on HOMO of the 1:2 1/Hg2+ and 1/
Pb2+ complexes are similar to those of the corresponding
1:1 complexes. However, p electrons on LUMO of the 1:2
complexes are distributed on the quinoline moiety, whereas
the 1:1 complexes have a p electron distribution on the pyri-
dine moieties. This is probably because the coordination of
the quinoline nitrogen with Hg2+ or Pb2+ stabilizes p elec-
trons on the quinoline moiety. The 1:2 complexes have
lower HOMO and LUMO energy levels than those of free
1. The decrease in the LUMO level is more significant than
that of the HOMO, indicating that the LUMO is more stabi-
lized. The p electrons on the LUMO of the 1:2 1/Hg2+ and
1/Pb2+ complexes are localized on the quinoline moiety,
whereas the free 1 and 1:1 1/Cd2+ complex have a p elec-
tron distribution on the aminostyrene moieties. This suggests
that the 1:2 1/Hg2+ and 1/Pb2+ complexes strongly promote
the ICT from the donor DPA to the acceptor quinoline moi-
eties, through the stabilization of LUMO by the coordina-
tion of Hg2+ or Pb2+ with the quinoline nitrogen, as sche-
matically shown in Scheme 1b. This therefore leads to a de-
crease in the HOMO–LUMO gap and results in a red-shift
of the fluorescence spectra.[14a,26]
complexes, and the intensity of respective bands changes
with the Cd2+/Hg2+ ratio. At high Cd2+ concentration, the
1:1 1/Cd2+ complex exists and shows 454 nm fluorescence.
Addition of Hg2+ leads to a formation of non-emissive 1:1
1/Hg2+ complex, through the replacement of Cd2+ with
Hg2+, due to the higher stability constant between the DPA
unit and Hg2+ than Cd2+ (Table 1) and results in the
quenching of 454 nm fluorescence. Further Hg2+ addition
promotes coordination of quinoline nitrogen with Hg2+ (for-
mation of 1:2 1/Hg2+ complex) and shows 546 nm fluores-
cence. As shown in Figure 6 (white circle), the chromaticity
coordinates of Cd2+/Hg2+ mixtures lie in a near straight line
connecting those for pure Cd2+ and Hg2+ solutions. This in-
dicates that, in the mixtures, the replacement and sequential
coordination of metal cations successfully creates intermedi-
ate colors.[19]
Figure 5b shows the fluorescence spectra of Hg2+/Pb2+
mixtures. The change in Hg2+/Pb2+ ratio leads to a continu-
ous shift of the spectra at between 546 and 571 nm, which
are assigned to the 1:2 1/Hg2+ and 1/Pb2+ complexes, re-
spectively. At high Pb2+ concentration, 1:2 1/Pb2+ complex
exists and shows fluorescence at 571 nm. The increase in
Hg2+ concentration probably leads to a formation of binu-
clear 1:1:1 1/Hg2+/Pb2+ complex, in which the DPA and
quinoline units coordinate with Hg2+ and Pb2+, respectively,
due to the higher stability constant between the DPA unit
and Hg2+ than Pb2+ (Table 1). This probably results in con-
tinuous blue shift of the spectra. Further Hg2+ increase
leads to a replacement of Pb2+ coordinated by quinoline ni-
trogen with Hg2+ (formation of 1:2 1/Hg2+ complex). This
results in further blue shift of the spectra. As shown in
Effect of mixing two metal cations: The effect of mixing two
metal cations on the fluorescence spectra of 1 was studied
to clarify whether the intermediate colors can be created ac-
cording to the ratio of mixed metal cations. Figure 5 show
the fluorescence spectra of 1 (20 mm) measured with two
metal cations (Cd2+/Hg2+, Hg2+/Pb2+, and Cd2+/Pb2+) as a
function of the metal cation ratio, for which the total metal
cation concentrations are set at 200 mm. As shown in Fig-
ure 5a, Cd2+/Hg2+ mixtures show two fluorescence bands at
454 and 546 nm, assigned to the 1:1 1/Cd2+ and 1:2 1/Hg2+
Figure 6 (square), the chromaticity coordinates of Hg2+
/
Pb2+ mixture lies in a straight line connecting those for pure
Hg2+ and Pb2+ solutions. This indicates that continuous blue
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Y. Shiraishi et al.
Figure 6. Chromaticity coordinates for fluorescence spectra of 1 (20 mm)
*
measured with and without metal cations in a 1931 CIE diagram. : with
pure metal cations, :Cd2+/Hg2+ mixtures, &: Hg2+/Pb2+ mixtures,
:
~
*
Cd2+/Pb2+ mixtures. The total concentrations of cations in solutions are
set at 200 mm. ꢁ denotes the ideal coordinate for white color fluorescence
(0.33, 0.33). The photographs for respective solutions are summarized in
Figure S7 (Supporting Information).
pure Cd2+ and Pb2+ solutions because of the formation of
binuclear 1:1:1 1/Cd2+/Pb2+ complex. Nevertheless, the data
indicate that the mixing of these metal cations successfully
creates intermediate colors.
The above findings indicate that the mixing two metal cat-
ions with 1 enables creation of various intermediate fluores-
cence colors. Another notable feature of 1 is that a near
white fluorescence color can be created. As indicated by the
arrow in Figure 6 (triangle), the chromaticity coordinate of
the mixture consisting of Cd2+ (7.5 equiv) and Pb2+
(2.5 equiv) is (0.31, 0.37), which is close to the standard
white light illuminate (0.33, 0.33)[4] indicated by a cross. As
shown by the inset photograph, a bright white fluorescence
(FF =0.014) is observed. Generally, a white fluorescence has
to be created by mixing three kinds of dyes that show blue,
green, and red fluorescence, or two kinds of dyes that show
blue and yellow fluorescence.[4] To the best of our knowl-
edge, this is the first report of a dye that shows a white fluo-
rescence induced by metal cations.
Figure 5. Fluorescence spectra of 1 (20 mm) measured in MeCN in the
presence of a) Cd2+/Hg2+ mixture (Cd2+ +Hg2+ =10 equiv, lex =365 nm),
b) Hg2+/Pb2+ (Hg2+ +Pb2+ =10 equiv, lex =454 nm), and c) Cd2+/Pb2+
(Cd2+ +Pb2+ =10 equiv, lex =390 nm), as a function of the ratio of metal
cations. The metal cation ratios are: a) Cd2+/Hg2+ =10:0, 9.5:0.5, 9.2:0.8,
9:1, 8.9:1.1, 8.7:1.3, 8:2, 0:10; b) Hg2+/Pb2+ =10:0, 2:8, 1:9, 0.8:9.2,
0.5:9.5, 0.3:9.7, 0.1:9.9, 0:10; c) Cd2+/Pb2+ =10:0, 8.5:1.5, 8:2, 7.5:2.5, 7:3,
6:4, 2:8, 0:10. The gray lines are the spectra obtained ratios with 10:0 and
0:10. The absorption spectra and photographs for respective solutions are
summarized in Figure S6 and S7, respectively (Supporting Information).
shift of the spectra by the sequential replacement of metal
cations allows successful creation of intermediate colors.
Figure 5c shows the fluorescence spectra of Cd2+/Pb2+
mixture. At high Cd2+ concentration, 1:1 1/Cd2+ complex
exists and shows 454 nm fluorescence. Addition of Pb2+
leads to a decrease in the 454 nm fluorescence along with an
appearance of 563 nm fluorescence, with an isoemissive
point at 512 nm. The ratiometric spectral change is probably
Conclusion
We clarified that a styrylquinoline dye containing a DPA
moiety (1) shows multiple fluorescence colors upon coordi-
nation with different metal cations. The dye 1 has a green
fluorescence, and shows different fluorescence colors in the
presence of Cd2+ (blue), Hg2+ (yellow), and Pb2+ (orange).
These color changes are explained by the ICT process
within the molecule. The dye 1 possesses DPA and quinoline
units that act as electron donor and acceptor, respectively,
and both units behave as coordination sites for metal cat-
ions. The energy levels of HOMO and LUMO orbitals of 1
are critically changed by the coordination of different metal
due to the coordination of quinoline nitrogen with Pb2+
,
leading to a formation of binuclear 1:1:1 1/Cd2+/Pb2+ com-
plex, in which the DPA and quinoline units coordinate with
Cd2+ and Pb2+, respectively. Further increase in Pb2+ con-
centration leads to a decrease in 563 nm fluorescence with a
red shift of the spectrum (571 nm). This is because the re-
placement of Cd2+ coordinated by DPA unit with Pb2+
leads to a formation 1:2 1/Pb2+ complex. As shown in
Figure 6 (triangle), the chromaticity coordinates of Cd2+
Pb2+ mixture draws a gentle curve connecting those for
/
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Multicolor Fluorescence
FULL PAPER
cations and results in blue or red shift of the fluorescence
spectra. Another notable feature of the dye is that the addi-
tion of two metal cations at once successfully creates inter-
mediate colors according to the ratio of metal cations. In
particular, the addition of the mixture of Cd2+ and Pb2+
with an appropriate ratio creates a near-white fluorescence.
The basic molecular design used here based on the ICT mol-
ecules possessing metal-binding sites on both the electron
donor and acceptor moieties may become a powerful tool
for creation of multicolor fluorescence by metal cation
inputs.
Acknowledgements
This work was partly supported by the Grant-in-Aid for Scientific Re-
search (No. 23656503) from the Ministry of Education, Culture, 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.
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Experimental Section
Compound 1: 4-(Di-2-picolylamino)benzaldehyde[13a] (0.39 g, 1.29 mmol),
2-methylquinoline (0.18 g, 1.29 mmol), and piperidine (100 mL) were dis-
solved in toluene (1 mL) and stirred at 1108C for 16 h under dry nitro-
gen. Aqueous NaOH (0.1m, 70 mL) solution was added to the resulting
solution, and solution was extracted with CH2Cl2 (3ꢁ20 mL). The com-
bined organic layer was dried over Na2SO4, and concentrated by evapora-
tion. The obtained solid was recrystallized from EtOH, affording 1 as an
orange powder (0.26 g, yield 47%). 1H NMR (270 MHz, CD3Cl, TMS):
d=4.87 (s, 4H; CH2), 6.74 (d, J=8.90 Hz, 2H), 7.14–7.28 (m, 5H), 7.41–
7.75 (m, 9H), 8.04 (t, J=8.49 Hz, 2H), 8.61 ppm (d, J=4.29 Hz, 2H);
13C NMR (67.8 MHz, CDCl3, TMS): d=158.11, 156.49, 149.56, 148.48,
148.06, 136.65, 135.80, 134.18, 130.57, 129.34, 128.75, 128.53, 127.21,
126.86, 125.60, 125.46, 124.88, 121.97, 120.61, 118.82, 112.47, 111.87,
57.17 ppm; FAB-MS: m/z calcd for C29H24N4: 428.20; found: 429.22
[M+H]+. HRMS (FAB+): m/z calcd for C29H25N4: 429.2079; found:
429.2061 [M+H]+. H, 13C NMR and FAB-MS charts are shown in Figur-
es S1–S3 (Supporting Information).
1
Analysis: Fluorescence spectra were measured on a Hitachi F-4500 fluo-
rescence spectrophotometer.[27] Absorption spectra were measured on an
UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec-
1500). The measurements were carried out at (298ꢁ1) K using a 10 mm
path length quartz cell. The spectra in the presence of metal cations were
measured after stirring the solution for 1 min. Perchlorate salts were used
as a metal source, and all measurements were carried out in an aerated
condition. Fluorescence quantum yield (FF) was determined by compari-
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which was excited at 366 nm in H2SO4 (0.5m, FF =0.55).[28] 1H and
13C NMR spectra were obtained by a JEOL JNM-GSX270 Excalibur.
FAB-MS spectra were obtained by a JEOL JMS-700 Mass Spectrometer.
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density functional theory (DFT) using the B3LYP function. The metal-
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Cd2+, for which the LANL2DZ basis set with effective core potential
was used. The Hg2+ complex was calculated by using the 6-31G* basis
set for all atoms except for Hg2+, for which the Stuttgart relativistic
small-core basis set with effective core potential was used. The Pb2+ com-
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Pb2+, for which SDD basis set with effective core potential was used.
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Received: April 6, 2011
Published online: June 27, 2011
8332
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Chem. Eur. J. 2011, 17, 8324 – 8332