Cu(II)-selective green fluorescence of a rhodamine-diacetic acid conjugate

Article abstract of DOI:10.1021/ol7022714

A new rhodamine derivative (1) containing an ethylenediamine-N,N-diacetic acid moiety exhibits Cu(II)-selective strong green fluorescence, while showing very weak orange fluorescence with other metal cations.

Full text of DOI:10.1021/ol7022714

ORGANIC
LETTERS
2
007
Vol. 9, No. 24
039-5042
Cu(II)-Selective Green Fluorescence of a
Rhodamine Diacetic Acid Conjugate
5
−
Xuan Zhang, Yasuhiro Shiraishi,* and Takayuki Hirai
Research Center for Solar Energy Chemistry and DiVision of Chemical Engineering,
Graduate School of Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan
shiraish@cheng.es.osaka-u.ac.jp
Received September 17, 2007
ABSTRACT
A new rhodamine derivative (1) containing an ethylenediamine-N,N-diacetic acid moiety exhibits Cu(II)-selective strong green fluorescence,
while showing very weak orange fluorescence with other metal cations.
5
Design of fluorescent chemosensors for heavy and transition
proposed; however, most of these are photoexcited by UV
1
2+
metal (HTM) cations is an area of intense research activity.
light. There are only five reports of the turn-on type Cu -
selective sensors driven by visible light excitation.
Rhodamine is a molecule used extensively as a fluorescent
labeling reagent and a dye laser source because of its
excellent photophysical properties, such as long absorption
and emission wavelengths elongated to visible region, high
2
+
5c-e,i,j
Much effort has been devoted to the development of Cu -
selective fluorescent chemosensors because of biological and
2
+ 2
2+
environmental importance of Cu . So far, various Cu
sensors have been proposed; however, most of these show a
3
2+
turn-off (fluorescence quenching) response, because Cu
usually acts as a quencher via an energy or electron-transfer
process. “Turn-on” type Cu sensors have also been
6
fluorescence quantum yield, and large absorption coefficient.
Recently, rhodamine-based fluorescent chemosensors or
4
2+
5i,7
5j,8
chemodosimeters, which are driven by visible light excita-
tion and show turn-on response to the targeted HTM cation,
(
1) (a) Amendola, V.; Fabbrizzi, L.; Licchelli, M.; Mangano, C.;
Pallavicini, P.; Parodi, L.; Poggi, A. Coord. Chem. ReV. 1999, 190-192,
49-669. (b) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord.
Chem. ReV. 2000, 205, 59-83. (c) Rurak, K. Spectrochim. Acta A 2001,
2+ 7a
2+ 5i,j,8a
2+ 7b-e,8b-d
3+ 7f-i
6+ 8e
such as Pb , Cu ,
Hg ,
Fe , and Cr ,
6
have been proposed. The cation-sensing mechanism of these
5
7, 2161-2195.
(
2) Kr a¨ mer, R. Angew. Chem., Int. Ed. 1998, 37, 772-773.
(4) (a) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. Soc.
1972, 94, 946-950. (b) Kemlo, J. A.; Shepherd, T. M. Chem. Phys. Lett.
1977, 47, 158-162.
(3) (a) Fabrizzi, L.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Sacchi, D.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1975-1977. (b) Torrado, A.;
Walkup, G. K.; Imperiali, B. J. Am. Chem. Soc. 1998, 120, 609-610. (c)
Zheng, Y.; Gatt a´ s-Asfura, K. M.; Konka, V.; Leblanc, R. M. Chem.
Commun. 2002, 2350-2351. (d) Zheng, Y.; Cao, X.; Orbulescu, J.; Konka,
V.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. Anal. Chem. 2003,
(5) (a) Mitchell, K. A.; Brown, R. G.; Yuan, D.; Chang, S.-C.; Utecht,
R. E.; Lewis, D. E. J. Photochem. Photobiol. A: Chem. 1998, 115, 157-
161. (b) Kaur, S.; Kumar, S. Chem. Commun. 2002, 2840-2841. (c) Royzen,
M.; Dai, Z.; Canary, J. W. J. Am. Chem. Soc. 2005, 127, 1612-1613. (d)
Xu, Z.; Qian, X.; Cui, J. Org. Lett. 2005, 7, 3029-3032. (e) Qi, X.; Jun, E.
J.; Xu, L.; Kim, S.-J.; Hong, J. S. J.; Yoon, Y. J.; Yoon, J. J. Org. Chem.
2006, 71, 2881-2884. (f) Mart ´ı nez, R.; Zapata, F.; Caballero, A.; Espinosa,
A.; T a´ rraga, A.; Molina, P. Org. Lett. 2006, 8, 3235-3238. (g) Wen, Z.-
C.; Yang, R.; He, H.; Jiang, Y.-B. Chem. Commun. 2006, 106-108. (h)
Jun, E. J.; Won, H. N.; Kim, J. S.; Lee, K.-H.; Yoon, J. Tetrahedron Lett.
2006, 47, 4577-4580. (i) Xiang, Y.; Tong, A.-J.; Jin, P.-Y.; Ju, Y. Org.
Lett. 2006, 8, 2863-2866. (j) Mei, L.; Xiang, Y.; Li, N.; Tong, A.-J. Talanta
2007, 72, 1717-1722.
7
2
2
5, 1706-1712. (e) Li, Y.; Yang, C. M. Chem. Commun. 2003, 2884-
885. (f) Mokhir, A.; Kiel, A.; Herten, D.-P.; Kraemer, R. Inorg. Chem.,
005, 44, 5661-5666. (g) Mei, Y.-J.; Bentley, P. A.; Wang, W. Tetrahedron
Lett. 2006, 47, 2447-2449. (h) Comba, P.; Kr a¨ mer, R.; Mokhir, A.; Naing,
K.; Schatz, E. Eur. J. Inorg. Chem. 2006, 4442-4448. (i) Choi, J. K.; Kim,
S. H.; Yoon, J.; Lee, K.-H.; Bartsch, R. A.; Kim, J. S. J. Org. Chem. 2006,
7
1, 8011-8015. (j) Park, S. M.; Kim, M. H.; Choe, J.-I.; No, K. T.; Chang,
S.-K. J. Org. Chem. 2007, 72, 3550-3553. (k) Xie, J.; M e´ nand, M.;
Maisonneuve, S.; M e´ tivier, R. J. Org. Chem. 2007, 72, 5980-5985. (l)
Wen, Y.-Q.; Yue, F.; Zhong, Y.-R.; Ye, B.-H. Inorg. Chem. 2007, 46,
(6) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; p 67.
7
749-7755.
1
0.1021/ol7022714 CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/02/2007

probes is based on the change in structure between spiro-
cyclic and open-cycle forms. Without cations, these probes
exist in a spirocyclic form, which is colorless and nonfluo-
rescent. Addition of metal cation leads to a spirocycle
opening via coordination5i,7 or irreversible chemical reaction,5j,8
resulting in an appearance of pink color and orange
fluorescence.
Here we report that a new rhodamine derivative (1),
containing an ethylenediamine-N,N-diacetic acid moiety, an
analogue of a classical chelator EDTA (Scheme 1; Synthe-
Scheme 1. Synthesis of 1
Figure 1. Fluorescence spectra (λex ) 480 nm) of 1 (25 µM)
measured in CH CN with respective metal cations (6 equiv). For
3
9
detailed intensity and spectra, see Figures S1 and S2.
ment, but the enhancement is very small (2.8-fold) and the
emission appears at 570-580 nm (orange fluorescence),
which is similar to that obtained with the already-reported
5
i,j,7,8
rhodamine-based probes.
Figure 2 shows the results of fluorescence titration of 1
a
b
N,N-Diisopropylethylamine. Trifluoroacetic acid.
9
2+
sis ), acts as a Cu -selective fluorescent sensor. This
demonstrates a strong “green” fluorescence upon binding
2
+
with Cu , while showing a very weak ordinary orange
fluorescence with other metal cations. To the best of our
knowledge, this is the first rhodamine-based fluorescent
chemosensor showing green fluorescence.
Figure 1 shows fluorescence spectra (λex ) 480 nm) of 1
(
3
25 µM) measured in CH CN with respective metal cations
(
6 equiv). Without cations, 1 shows a very weak fluorescence
2+
3
Figure 2. Change in fluorescence spectra of 1 (25 µM) in CH -
at 575 nm; however, Cu addition creates a remarkably
enhanced (49-fold) and blue-shifted (45 nm) green fluores-
cence at 530 nm. Hg2 also induces an emission enhance-
CN with the Cu2 amount added (λex ) 480 nm). Inset: Change
+
in the emission intensity monitored at 530 nm. For detailed change
+
in spectra, see Figure S3.9
(7) (a) Kwon, J. Y.; Jang, Y. J.; Lee, Y. J.; Kim, K. M.; Seo, M. S.;
Nam, W.; Yoon, J. J. Am. Chem. Soc. 2005, 127, 10107-10111. (b) Zheng,
2
+
2+
with Cu . Upon addition of <2 equiv of Cu , 575 nm
emission increases. However, addition of >2 equiv of Cu2
leads to a drastic increase in the shorter wavelength emission,
along with a continuous blue-shift of the emission band. As
shown in Figure 3 (bottom), the emission color continuously
changes from orange to green with increasing Cu2 amount.
As shown in Figure 2 (inset), the emission intensity increases
H.; Qian, Z.-H.; Xu, L.; Yuan, F.-F.; Lan, L.-D.; Xu, J.-G. Org. Lett. 2006,
+
8
, 859-861. (c) Lee, M. H.; Wu, J.-S.; Lee, J. W.; Jung, J. H.; Kim, J. S.
Org. Lett. 2007, 9, 2501-2504. (d) Wu, D.; Huang, W.; Duan, C.-Y.; Lin,
Z.-H.; Meng, Q.-J. Inorg. Chem. 2007, 46, 1538-1540. (e) Soh, J. H.;
Swamy, K. M. K.; Kim, S. K.; Kim, S.; Lee, S.-H.; Yoon, J. Tetrahedron
Lett. 2007, 48, 5966-5969. (f) Xiang, Y.; Tong, A.-J. Org. Lett. 2006, 8,
1
549-1552. (g) Zhang, M.; Gao, Y.-H.; Li, M.-Y.; Yu, M.-X.; Li, F.-Y.;
+
Li, L.; Zhu, M.-W.; Zhang, J.-P.; Yi, T.; Huang, C.-H. Tetrahedron Lett.
2
5
4
007, 48, 3709-3712. (h) Bae, S.; Tae, J. Tetrahedron Lett. 2007, 48,
389-5392. (i) Zhang, X.; Shiraishi, Y.; Hirai, T. Tetrahedron Lett. 2007,
8, 5455-5459.
2
+
with increasing Cu amount up to 6 equiv, but decreases at
>6 equiv. The blue-shift of the emission band also stops
with >6 equiv of Cu (Figure S3 ). In contrast, Hg
1
0
(
8) (a) Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997,
1
19, 7386-7387. (b) Yang, Y.-K.; Yook, K.-J.; Tae, J. J. Am. Chem. Soc.
2+
9
2+
2
005, 127, 16760-16761. (c) Ko, S.-K.; Yang, Y.-K.; Tae, J.; Shin, I. J.
Am. Chem. Soc. 2006, 128, 14150-14155. (d) Wu, J.-S.; Hwang, I.-C.;
Kim, K. S.; Kim, J. S. Org. Lett. 2007, 9, 907-910. (e) Xiang, Y.; Mei,
L.; Li, N.; Tong, A.-J. Anal. Chim. Acta 2007, 581, 132-136.
(10) Fluorescence quantum yield of 1 obtained with 6 equiv of Cu2 is
0.32, where fluorescein (ΦF ) 0.85 in 0.1 M NaOH) is used as a standard:
Parker, C. A.; Rees, W. T. Analyst 1960, 85, 587-600.
+
(9) See the Supporting Information.
5040
Org. Lett., Vol. 9, No. 24, 2007

Figure 3. Change in (top) color and (bottom) fluorescence of 1
25 µM) in CH
3
CN with different Cu2 amounts.
+
(
addition leads to a monotonous enhancement of the orange
Figure 5. Change in absorption spectra of 1 (25 µM) in CH
3
CN
with Cu2 addition. Inset: Change in absorbance at (open circle)
56 and (closed circle) 506 nm. For detailed absorption spectra
emission at 570-580 nm, where the emission blue-shift does
+
2
+
9
not occur at any Hg amount (Figure S4 ), as is usually
5
observed for the rhodamine-based probes.5
i,j,7,8
with >6 equiv of Cu , see Figure S5.9
2+
Figure 4 shows absorption spectra of 1 (25 µM). Without
9
IR analysis of 1 in CH
3
CN (Figure S6 ) reveals that both
carboxylic and amide carbonyl absorption at 1749.1 and
-
1
2+
1
(
674.4 cm shifts to lower frequency upon addition of Cu
-
1
2+
-1
1634.9 cm ) and Hg (1661.4 cm ). This indicates that
both carbonyl groups are involved in the coordination with
7
a,i 1
9
these cations.
3
H NMR titration in CD CN (Figure S7 )
shows that the aromatic protons of 1 shift downfield and
2
+
become broader upon Cu addition. This is due to the
decrease in electron density of the aromatic ring, indicating
2
+ 7a,i
that 1 actually coordinates with Cu . Addition of ethyl-
2+
enediamine to the solution of 1 containing either Cu or
2+
Hg leads to disappearance of both absorption and emission
spectra, indicative of a reversible coordination between 1
5
i,7
and these cations. The above findings indicate that the
emission enhancement of 1 upon addition of Cu2 and Hg
+
2+
involves the spirocycle-opening mechanism, as is also the
case for related rhodamine-based sensors: coordination of
metal cations with carboxylic and amide carbonyl groups of
Figure 4. Absorption spectra of 1 (25 µM) upon addition of
respective metal cations (6 equiv) in CH CN.
3
5i,7
1
leads to the spirocycle opening, resulting in appearance of
9
cations, 1 scarcely shows absorption at 500-600 nm,
the absorption and emission spectra (Figures 3 and S4 ). The
5i,j,7,8
indicating that 1 exists as a spirocycle-closed form.
This
blue-shift of both absorption and emission spectra of 1 upon
addition of >2 equiv of Cu indicates that the system
involves different mechanisms.
2
+
is confirmed by a distinctive spirocycle carbon shift at 65.84
1
3
9
ppm in the C NMR spectrum of 1. Addition of metal
+
+
+
+
cations (except for Li , Na , K , and Ag ) leads to an
appearance of 556 nm absorbance, as is also the case for
Figure 6 shows the contour map of the fluorescence
intensity of 1 as a function of excitation/emission wave-
the reported rhodamine-based probes.5
i,j,7,8
In contrast, Cu
addition shows a blue-shifted absorption at 506 nm.
Figure 5 shows the results of absorption titration of 1 with
2+
lengths measured with different Cu amounts. With <2
2+
2
+
equiv of Cu (Figure 6a,b), excitation/emission maximum
appears at 560/575 nm (orange emission). In contrast, with
2
+
2+
2+
Cu . With <2 equiv of Cu , 556 nm absorption increases
>2 equiv of Cu (Figure 6c-f), a new maximum appears
with increasing Cu2 amount, along with an appearance of
+
at 510/530 nm (green emission). With >5 equiv of Cu
2+
2+
pink color (Figure 3, top). In contrast, with >2 equiv of Cu
Figure 5), the 556 nm absorbance decreases, and 506 nm
absorbance increases. As shown in Figure 3 (top), the pink
(Figure 6e,f), the latter maximum becomes predominant
(
along with disappearance of the former maximum. In
2
+
contrast, Hg shows a single set of maximum at 560/580
2+
9
color of the solution fades with >2 equiv of Cu . The 506
nm (orange emission) (Figure S8 ), which is similar to that
2
+
2+
nm absorbance, however, decreases with >6 equiv of Cu
obtained with <2 equiv of Cu . This suggests that the
(
Figure 5, inset, closed circle). In contrast, Hg2+ addition
spirocycle-opening process is actually involved in the emis-
2
+
leads to a monotonous increase in the 556 nm absorption,
where the shorter wavelength absorption does not appear at
sion enhancement of 1 with Cu . The appearance of the
2
+
510/530 nm maximun with >2 equiv of Cu (Figure 6c-
f) indicates that the green emission of 1 originates from the
2
+
9
any Hg amount (Figure S4 ).
Org. Lett., Vol. 9, No. 24, 2007
5041

9
(A556nm) shows a clear 1:1 stoichiometry (Figure S10 ). This
means that the green emitting species have a complicated
stoichiometry. In addition, as shown in Figure 5, the
absorption spectra of 1 do not show a clear isosbestic point.
This suggests that multiple species form through association
2
+
of 1 with different amounts of Cu , leading to formation
of the green emitting species. It must also be noted that the
2+
saturation of the emission increase after Cu addition
2
+
requires >60 min (300 K), whereas Hg shows rapid
9
saturation (<5 min) (Figure S11 ). These findings and the
inherent aggregation property of rhodamine suggest that the
green emitting species may be a self-assembled “aggregate”
containing multiple 1 molecules, formed by coordination
2
+
13
association with multiple Cu ions. Rhodamine aggrega-
tion is enhanced at high temperature due to a decrease in a
1
4
9
solvation interaction. As shown in Figure S11, the satura-
2+
tion of the green emission increase after Cu addition occurs
more rapidly at higher temperature. This means that the
aggregation is the crucial factor for the green emitting species
formation.
In conclusion, we found that a new rhodamine derivative
Figure 6. Contour map of the fluorescence intensity of 1 (25 µM)
1
5
as a function of excitation/emission wavelengths measured with
2+
(
3
a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 equiv of Cu in CH CN.
9
2+
16
For detailed excitation spectra, see Figure S9.
(
3
1) exhibits Cu -selective green fluorescence in CH CN,
while showing very weak orange fluorescence with other
metal cations. Although the detailed mechanism remains to
be explained, the results presented here may contribute to
the development of more useful fluorescent sensors for HTM
cations based on the rhodamine platform.
direct photoexcitation of the ground-state species formed
through association between 1 and Cu2+ (the green emitting
species). This is supported by the appearance of the blue-
shifted absorption at 506 nm (Figure 5).
It is well-known that rhodamine dye tends to aggregate
in solution.11 This leads to a bathochromic (J-type) or
hypsochromic (H-type) shift of the absorption band. The
Acknowledgment. This work is financially supported by
Grants-in-Aid for Scientific Research (No. 19760536) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan (MEXT). We thank H. Koizumi and M.
Morishita in the same laboratory for their experimental
assistance.
2+
blue-shifted absorption of 1 obtained with >2 equiv of Cu
Figure 5) agrees well with the hypsochromic shift of the
(
11
H-aggregate of rhodamine dye. It is, however, well-known
that the H-aggregate is nonfluorescent on the basis of the
1
1
exciton theory. A few exceptions have been reported for
the fluorescent H-aggregate; however, they show a “red-
shifted” emission.12 The detailed mechanism of the Cu -
induced green emission of 1 cannot be clarified at this stage;
however, some characteristic properties were observed in the
Supporting Information Available: Materials, synthesis,
methods, and supplementary data (Figures S1-S21). This
material is available free of charge via the Internet at
http://pubs.acs.org.
2+
OL7022714
preliminary investigation. A Job’s plot (A556nm and A506nm
)
2
+
shows unresolved maximum absorption at X ()[Cu ]/
(13) Formation of insoluble material was not detected by dynamic light
scattering analysis (detection limit: 3 nm) at any Cu2 amount, indicating
that the green emitting species is the “soluble” aggregate.
+
2+
2+
(
[Cu ] + [1])) ) 0.55-0.65, while the plot with Hg
(
14) (a) Ruiz Ojeda, P.; Katime, Amashta, I. A.; Ram o´ n Ochoa, J.; L o´ pez
(
11) (a) Rohatgi, K. K.; Singhal, G. S. J. Phys. Chem. 1966, 70, 1695-
Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1-8. (b) Kemnitz,
1
(
5
1
701. (b) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762-774.
K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 6095-6104.
+
c) L o´ pez Arbeloa, I.; Ruiz Ojeda, P. Chem. Phys. Lett. 1982, 87, 556-
(15) Without Cu2 , niether absorption nor fluorescence of 1 blue-shifts
9
60. (d) Valdes-Aguilera, O.; Neckers, D. C. Acc. Chem. Res. 1989, 22,
71-177.
even with high concentration of 1 up to 0.33 mM (Figure S12 ). This
indicates that the coordination association of 1 with Cu2 triggers the
aggregation, leading to formation of the green emitting species.
(16) The fluorescence intensity of 1 (25 µM), when measured with 6
equiv of Cu2 in CH3CN containing 0.1% water, is similar to that obtained
without water; however, 30% and 100% intensity reductions occur in the
presence of 1% and 5% water, respectively.
+
(12) (a) Chambers, R. W.; Kajiwara, T.; Kearns, D. R. J. Phys. Chem.
1
974, 74, 380-387. (b) Van der Auweraer, M.; Biesmans, G.; De Schryver,
+
F. C. Chem. Phys. 1988, 119, 355-375. (c) Mandal, A. K.; Pal, M. K.
Chem. Phys. 2000, 253, 115-124. (d) R o¨ sch, U.; Yao, S.; Wortmann, R.;
W u¨ rthner, F. Angew. Chem., Int. Ed. 2006, 45, 7026-7030.
5042
Org. Lett., Vol. 9, No. 24, 2007