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An Anthracene-Based Fluorescent Sensor for Transition Metal Ions.

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The only previous examples of tris(arenejmeta1 complexes appear to be the
[Z.2.2]paracyclophane derivatives, [(p-C,H,CH,CH,j,M]+
(M = Ag, Ga),
where the metal ions are located at the edge of or within the cyclophane cavity,
respectively. See: H Schmidbaur, R. Hager, B. Huber, G. Miiller. Angew.
C/irm. 1987, 99. 354: Angen.. Chrm. Int. Ed. Engl. 1987, 26, 338.
K. M. Chi, S. R. Frerichs. S. B. Philson, J. E. Ellis. Angew. Chern. 1987. 99.
1203: Angeii. Chenr. In/. Ed. EngI. 1987, 26, 1190.
a ) R . D. Rieke. W. P. Henry, I. S. Arney, Inorg. Chrm. 1987. 26, 420; b) H.
Schiiulele. D. Hu. H. Pritzkow. U. Zenneck. Orgunome!u//ics 1989. 8, 396:
c ) R. L. Thompson, S. Lee. A. L. Rheingold. N . J. Cooper. ihid. 1991.10,1657;
d ) K . Jonas. C.-C. Hiselhoff, R. Goddard. C. Kruger. Inorg. Chini. Actu 1992,
/YX X I , -533: ej C. Brodt, S. Niu, H. Pritzkow, M. Stephan, U. Zenneck, J.
O,;qonumct. Ch~,m.1993, 455, 283.
('1-ystal data for 1 a : C,,H,,K,N,O,,Zr.
monoclinic. P2,:'ti (no. 14)
u=21.78(1): h = 12.331(6). c=26.55(1), /J= llO.ll(5)'. V=6695(l2jA3.
.Z = 4, ( I ~ , , , =
~ ~ 1.296 gcm ', p(MoK,) = 3.43 cm- I , crystal dimensions =
0.60 x 0.50 x 0.35 mm'. The intensities of 9933 reflections were measured a t
~-96 C (0 < f) < 23') on an Enraf-Nonius C A D 4 diffractometer using Mo,,
radiation. The structure was solved by direct methods and all non-hydrogen
atoms were relined anisotropically (full-matrix least-squares). For 5997 unique
observed reflections [ ( I )z 2.00(1)].R = 0.053 and R, = 0.046, GOF = 1.285.
Further details of the crystal structnre determination are available on request
from the Director of Cambridge Crystallographic Data Centre, 12 Union
Rond. GB-Cambridge CB2 1EZ (UK). on quoting the full journal citation.
P. S Skell, M. J. McGlinchey. Angew Chem. 1975.87. 215: Angew. Chem. In/.
E d Engl. 1975, 14. 195.
L. L. Guggenberger. R. R. Schrock, J. A m . Chent. Suc. 1975. 97. 6693.
Analogous dihedral angles ranging from 31 to 43' have been reported for
other rl'-naphthalene metal complexes. See refs. 15. 12, 171 and references cited
therein.
A. V. Protchenko. L. N . Zakharov. M. N . Bochkarev. Y. T. Struchkov. J.
O~,~UJIOV
Clirm.
I C / . 1993, 447. 209.
C. Kriiger. G. Muller. G. Erker. U. Dorf, K. Engel, O r g - : a n ~ n i ~ / ~
1985,
/ / i ~4.
~
215.
a) G. Erker, C. Kriiger. G. Miiller. Adr. Orgunornet. Chem. 1985.24. 1 : b) G.
Erker. J. Wicher. K. Engel, F. Rosenfeldt. W Dietrich. C. Kriiger, J. A m . Chem.
S o ( . 1980. ff).?. 6344; c) S. S. Wreford, J. E Whitney, Inorg. Ci7ern. 1981. 20,
391X: d j H. Yasuda. K. Tatsurni. A. Nakamura. Ace. Chem. RE\. 1985.18, 120:
c) J Blenkers, 9. Hessen. F. van Bolhuis. A J. Wagner, J. H . Teuben,
O r ~ . a ~ r o ~ ~ i ~ ~ 1987.
/ r i l l i c6.
. s 459.
a) D. J. Cardin. M. F. Lappert. C . L. Raston. Chemisirj qf Orgunozircunium
und -hufniiini Cuntpurind~,Wiley, Ne& York, 1986, and references therein.
b) G . M. Diamond. M . L. H. Gi-een, N . M. Walker. J. Organunirt. Chem. 1991,
413. ('1 : c ) D. M. Rogers, S. R. Wilson, G. S. Girolami, Orgunonwrullics 1991.
1 0 . 2419: d ) J. C . Green, M. L. H. Green, N . M. Walker. J. Chem. Soc. D ~ h n
k l 7 . \ . 1991. 173.
B. M e w D. Moras. R. Weiss, Chem. Commun. 1971, 444.
P. S Skell. E. M. Van Dam, M. P. Silvon. J. Am. Chem. Soc. 1974, 96. 626.
1. E Ellis. S. R. Frerichs. B. K. Stein, Orguiiomeiullic.\ 1993. 12, 1048.
the fluorescence of the light-emitting unit. Changes of fluorescence intensity of two orders of magnitude and more are often
observed, in other words the fluorescence can be switched on
and off.[21By virtue of its strong luminescence and chemical
stability, the anthracene unit has been widely used in the design
of fluorescent sensors. In the classical approach, a tertiary
amino group is attached to the aromatic framework, and the
thermodynamically favored amine-to-anthracene electron
transfer quenches fluorescen~e.[~I
Binding of H 14] or a metal
ion (e.g., Na+,[51Zn2+)[61substantially increases the oxidation
potential of the amine, thus preventing the photoinduced electron transfer and allowing fluorescence. Incorporation of the
tertiary amino group in a crown ether type framework has
paved the way to fluorescent sensors for alkali and alkalineearth metal cations.[51
We wished to extend this concept to the recognition of transition metal cations. To this end we attached a chelating dioxotetraaza unit to the 9-position of anthracene. according to the
synthetic route outlined in Scheme 1.
BCH2
2
Scheme 1. Synthesis of fluorescent sensor 2. en
An Anthracene-Based Fluorescent Sensor
for Transition Metal Ions**
Luigi Fabbrizzi,* Maurizio Licchelli, Piersandro
Pallavicini, Angelo Perotti, and Donatella Sacchi
Fluorescence quenching and enhancement can be used effectively for the identification of ions in solution. Fluorescent sensors have been and are currently being designed for various
cations and anions.['' A fluorescent sensor is essentially a twocomponent compound, in which a light-emitting group is covalently linked to a receptor specific for a particular ion. Sensor
efficiency requires that the ion-receptor interaction modifies
["I
[**I
Prof. Dr. L. Fabbrizri, Dr. M. Licchelli. Dr. P. Pallavicini. Prof. Dr. A. Perotti,
Dr. D Sacchi
Dipartimento di Chimica Generale. Universita di Pavia
Via Taramelli 12. 1-27100 Pavia (Italy)
Telelhx: Int. code +(382) 528544
This work &as supported by the Minister0 dell'Universiti e Ricerca Scientifica
e Tecnologica, the Consiglio Nazionale delle Ricerche, and the Fondazione
Lomhardia Ambiente (fellowship for D. S.).
Atigcir. Ciriw?.I n ! . LrI. Engl. 1994, 33. No. / 9
;:J
=
ethylencdiamine
Compound 2 (H,L) displays the typical emission spectrum of
anthracene in acetonitrile/water (4: 1); the intensity does not
change over the pH range 2-12. Its pK,, and pK,, values are 8.3
and 9.6, respectively. This indicates that the following three
species are present in solution: H4LZf,
in which the two terminal amino groups are protonated. the predominant species at
p H < 8, H,Lt at p H values from 8 to 10, and H 2 L at pH > 10.
The protonation state of the dioxotetraaza unit does not alter
the fluorescence of the adjacent anthrdcene unit. However, if
one equivalent of Cu" is added to an acidic solution of 2, the
fluorescence intensity IF steadily decreases on titration with
NaOH. Complete fluorescence quenching is observed when the
excess strong acid has been neutralized and two further equivalents of OH- have been added. The plot of I , vs. pH (Fig. 1)
displays a sigmoidal curve typical of a switching effect. Insights
on the quenching mechanism are provided by an analogous
titration experiment performed inside a spectrophotometric cuvette. On addition of base, the solution becomes pink-violet,
and a n absorption band at 520 nm develops. The absorbance A
reaches its limiting value after neutralization of the excess strong
acid and addition of two further equivalents of base. The plot of
A vs. p H (Fig. 1 ) shows a sigmoidal curve, which is symmetrical
VCH VeriugsgeseNschufr mhH, 0.69451 Weinheim, 1594
$ 10.0Uf 2 5 . 0
0570-0833~94~1919-1U75
1975
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T
TA
V
A
t
1:
V
F
'
T
a
T
9
'F
X
-
A
v
V V
V ' h
II
2
4
6
A A A
-
8
PH
1
MA
A M &
I
0
4
6
3~
8
PH
0.02
10
0.00
12
Fig. 1. Dependence of the fluorescence intensity (IF,A) on I
ind the absorbance
of the hand at 520 nm ( A , V) for a solution of equimolar amounts of 2 and of Cu"
in MeCN;H,O ( 4 : l ) .
Flp. 2. Dependence of the fluorescence intensity ( I , . T) on pH and the absorbance
of the band at 450 nm (.4. A) for a solution of equimolar amounts of 2 and of Nil'
in MeCN/H,O ( 4 : l ) .
to the Z,/pH profile and centered at the same pH ( = 6). The
pink-violet color arises from a d-d band of the square-planar
complex 3 (M = Cu). Deprotonation of the two amide groups
yields the tetradentate ligand L2-, which chelates the metal
atom. Copper(r1)-promoted deprotonation of amide"] and peptideI8' groups is a well-documented process in coordination
chemistry and bioinorganic chemistry. Our findings demonstrate that the quenching of the anthracene fluorescence is associated with the coordination of the Cu" ion by the dioxotetraaza
unit of 2. The radiationless deactivation of the excited state of
the fluorophore can take place by means of either an energy
transfer mechanism['] (which should occur by electron exchange
with an empty d orbital of the metal center"']) or a Cu"-toanthracene electron transfer process (the oxidation of Cu" to
Cu"' is typically Favored by the coordination of the deprotonated amide group)." 'I
The absence of fluorescence quenching by Mn" and Co" ions
is explained by the fact that these ions are not coordinated by
the dioxotetraaza unit of 2. The deprotonation of the amide
group is very endothermic and can take place only if it is compensated energetically by the formation of a strong bond between the metal and the deprotonated amide group.['*' This is
the case for the Cu" and Ni" ions, which profit greatly from
ligand field stabilization effects. But it is not the case for divalent
metal ions earlier in the first transition series such as Mn" and
Co". Spectrophotometric titration experiments confirm that
Mn" and Co" ions are not coordinated by the dioxotetraaza unit
of 2: d-d bands are not observed even in strongly basic solutions. The shift of the I,/pH sigmoidal curve to higher pH values
upon transition from Cu" to Ni" reflects the stronger metal-ligand interactions and the greater stability of the Cu" complex in
solution in comparison to the Ni" complex. The Zn" ion (d")
does not profit from ligand field effects and can thus neither
promote deprotonation of amide groups nor be chelated by the
dioxotetraaza unit in 2. Zn" is not redox active and does not
possess any empty low-energy orbitals, which are essential for
energy transfer and the radiationless decay of the anthracene
excited state.
We have found that compound 2 is an efficient fluorescent
sensor able to discriminate Cu" and Ni" ions from other divalent
3d metal ions. But can Cu" and Ni" be distinguished? It can be
seen in Figures 1 and 2 that at p H ~ 7 Cu"
, promotes fluorescence quenching, while Ni" does not. Thus, when a solution of
I
7%
2
3
When one equivalent of Ni" is added to a solution of 2 in
aqueous acetonitrile, I , decreases with increasing p H (Fig. 2).
The plot of the absorbance at 450 nm (d-d transition) versus pH
yields a symmetrical sigmoidal curve centered at p H z 8. The
absorption band at 450 nm corresponds to the yellow, low-spin,
square-planar complex 3 (M = Ni).
It should be noted that if a n acidic solution of 2 containing
equimolar amounts of the 3d ions Mn", Co", and Zn" (solution A) is titrated with base, I, remains constant over the p H
range 2- 12. If one equivalent of Ni" is added to solution A (to
give solution B) titration with NaOH is accompanied by fluorescence quenching at p H = 8 (sigmoidal curve similar to that in
Fig. 2). Finally, if one equivalent of Cu" is added to solution B
and titration with base is performed, fluorescence is quenched at
pH 6. as expected based on the results shown in Figure 1.
1976
c V C H Veu/uKsXi,.si,llsc.haftmbH. 0-69451 Wimheim, 1994
0.0
0.5
1.0
1.5
2.0
nFig. 3. Discrimination of Cu" and Ni" ions by the fluorescent sensor 2. Ni" is added
to solution of 2 in MeCN/H,O, which was adjusted to pH 7.1 with the 2.6-lutidine
(T). Then Cu" is odded ( A ) . n = number of equivalents of the added metal ion.
0570-0833~Y4,'1Y19-IY76$ iO.OO+ .25;0
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2 was adjusted to pH 7.1 with 2,6-lutidine and Ni" is added, no
substantial decrease in IFwas observed, even after the addition
of one or more equivalents of metal ion (Fig. 3). When Cu" was
added to the same solution, IF decreased linearly and reached
zero on addition of one equivalent of the metal ion (Fig. 3 ) .
Until now anthracene-based fluorescent sensors for metal
ions have been known primarily for alkali and alkaline-earth
metal ions. Typically the uncomplexed sensor does not fluorescence and the binding of the metal to the receptor results in
fluoresce; this switching mechanism is triggered by photoinduced electron transfer."] Compound 2 is the prototype of a
new family of pH-sensitive anthracene-based fluorescent sensors for transition metal ions. Here the uncomplexed sensor
fluoresces, and the interaction with the 3d metal ion leads to
fluorescence quenching; this switching mechanism is triggered
by energy transfer. Sensor selectivity towards transition metal
ions can be modified by changing the structural features of the
chelating unit: this unit can be cyclic or noncyclic, the number
and arrangement of amide groups can be changed, and other
heteroatoms can be included. We are currently designing and
testing these types of sensors.
Experinwntul Procedure
Benzobis(thiadiazo1e)s Containing Hypervalent
Sulfur Atoms: Novel Heterocycles with High
Electron Affinity and Short Intermolecular
Contacts between Heteroatoms**
Katsuhiko Ono, Shoji Tanaka, and Yoshiro
Yamashita"
Heterocycles containing hypervalent sulfur atoms have attracted considerable attention owing to their unique electronic
structures and reactivities.['I 1,2,5-Thiadiazole rings containing
a tetravalent sulfur atom are more stable than the related thiophene rings found in the stable pyrazine derivative 1[*] and the
stable bicyclic compound 2.[31Compound 1 is a strong electron
and forms a ribbonlike structure by short intermolecular contacts between the hypervalent S atoms and the N
atoms. In this context heterocycle 3a is expected to have a high
electron affinity and form a unique molecular network. This
compound resembles the framework of bis([l,2,5]thiadiazolo)tetracyanoquinodimethane (BTDA-TCNQ 4). which is an electron acceptor in organic metals and forms a sheetlike network
through short S . . . N=C contacts.[*] However, 5, which was
prepared from 4, is its only reported derivative.16] We have now
synthesized the i.2,d4-benzobis(thiadiazole)derivatives 3 b and
I : Diethyl malonate (0.75 g, 4.72 mmol) and 9-chloromethylanthracene (0.91 g.
4.03 mmol) were added to a solution of sodium ethoxide in anhydrous ethanol
(0.1 1 g. 4.72 mmol of Na in 20 mL of EtOH). The solution was heated at reflux for
1X h. The NaCl precipitate formed during the reaction was filtered off and the
solvent evaporated in vacuo. The residue was redissolved in diethyl ether. and
yellow crystals were obtained by slow evaporation. Yield: 68%. Correct C.H,N
analysis for C?,H,,O,
R
S
"
N
T
)
2 : Ethylenediamine (30 mL, freshly distilled over CaO) and 1 (0.5 g, 1.43 mmol)
were stirred at room temperature under nitrogen for 7 d. Excess ethylenediamine
was disMed off under reduced pressure. Treatment of the yellow residue with
diethyl ether gave a pale yellow precipitate. which was filtered offand recrystallized
fromethanol. Yield 74%. M.P. ?05--208 C. MS (7OeV)'m!z37X(Mt. 73%). 349
( [ M CHNH2]'. 5 2 % ) . 191 (C,,H,CHi. 100%). Correct C.H.N analysis for
C,,H,,Y,OI.
I
1
R
2
3a:RzH
3b: R = Br
3C: R = Ph
~
R
Received: May 13, 1994 [Z 6932 IE]
German version: A n g i w . Chrm. 1994. 106. 2051
R. A. Bissell. A. P. de Silva. H. Q. N Gunaratne. P. L. M. Lynch, G. E. M.
Maguire. C. P. McCoy, K . R. A. S. Sandanayake. T o p . Curr. Chrm. 1993. 168,
223
V. Goulle. A. Harriman, J.-M. Lehn,J. Chm7. Soc. Chrm. Commun. 1993. 1034.
R. A . Blssell. A . P. de Silva. H. Q. N . Gunaratne, P. L. M. Lynch. G. E. M.
Maguire. K. R A. S. Sandanayake, Clieni. Sot. Rev. 1992. 187.
A. P. de Silva. R. A . D. D. Rupasinghe, J. Chem. Soc. Cliem. Commur7. 1986,
I 70').
A . P de Silva, S. A. de Silva, J. Cllmm. Sor. Cherv. Commun. 1986, 1709.
E. U. Akkaqa. M. E. Huston, A. W. Crarntk. J. Ani. Chrm. Soc. 1990, 11-7,
3590.
M. Kodama. E Kimura, J Chem. Sor. Dalton Trans. 1979, 325.
D. W. Margerum. G. R . Dukes in Metal Ions in BiologiculSj~tems,Vol. I (Ed.
H. Sigel). Dckker, New York, 1974, p. 157. an references therein.
P. Suppan. Chpmi.sfrvand Lifiht, The Royal Society of Chemistry, Cambridge,
1994. p. 66.
V. Balrani. F. Scandola, Suprun?oleculur Photochemistrj,, Horwood. London.
1991. p. 71
L. Fabbrizri. A. Perotti, A. Poggi, lnorg. Chem. 1983, 22. 1411.
1.Fabbrizri. T. A . Kaden. A. Perotti, B. Seghi, L. Siegfried. /norg. Chrm. 1986.
-75,321.
NC+CN
NC
R
I
+
5 : R = H, C(CH&CN
R
k
CN
R
?
6a:R=H
6b: R = Br
6~ : R = Ph
5:w;
N-
R
R
7a : R = Br
7b : R = Ph
[*I
8a : R = Br
8b : R = Ph
9
Prof. Dr. Y Yamashita. K . Ono. Dr. S. Tanaka
Department of Structural Molecular Science
The Graduate University for Advanced Studies
and Institute for Molecular Science
Myodaiji. Okazaki 444 (Japan)
Telefax: Int. code (564)54-2254
+
[**I
Angtw. Clwni, /nt.
Ed. EngI. 1994. 33. N o . 19
j?
This work was supported by research fellowships from the Japan Society for
the Promotion of Science for Young Scientists. We thank Prof. Dr. K. Tdnaka,
Institute for Molecular Science. for conducting the X-ray crystallographic
study, and Prof. Dr. S. Hirdyama. Kyoto Institute of Technology. for the
measurement of the fluorescence spectrum of 3c.
VCH Verlufisfie.tell.sclia~tmhH, 0-69451 Weinheim, 1994
0570-0833iY4,'1919-1Y77 d 10.00+ .25/U
1977
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