OFF-OFF-ON switching of fluorescence and electron transfer depending on stepwise complex formation of a host ligand with guest metal ions

Article abstract of DOI:10.1021/ja0748480

Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl) pyrazine (TPPZ) and a series of metal ions (Mⁿ⁺ = Sc³⁺, Y³⁺, Ho³⁺, Eu³⁺, Lu³⁺, Nd ₃₊, Zn²⁺, Mg²⁺, Ca²⁺, Ba ²⁺, Sr²⁺, Li⁺), where TPPZ forms a 2:1 complex [(TPPZ)₂-Mⁿ⁺] and a 1:1 complex [TPPZ-Mⁿ⁺] with Mⁿ⁺ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)₂-Mⁿ⁺ complex is converted to the fluorescent 1:1 TPPZ-Mⁿ⁺ complex. This is regarded as an OFF-OFF-ON fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-Mⁿ⁺ complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M²⁺) are much larger than those of the trivalent metal complexes (TPPZ-M³⁺). On the other hand, the binding constants of (TPPZ)₂-Mⁿ⁺ (K₁) and TPPZ-Mⁿ⁺ (K₂) vary depending on the Lewis acidity of metal ions (i.e., both K₁ and K₂ values increase with increasing Lewis acidity of metal ions). Sc³⁺, which acts as the strongest Lewis acid, forms the (TPPZ)₂-Sc³⁺ and TPPZ-Sc³⁺ complexes stoichiometrically with TPPZ. In such a case, OFF-OFF-ON switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O ₂ is observed in the presence of Sc³⁺ and TPPZ depending on the ratio of Sc³⁺ to TPPZ. Electron transfer from CoTPP to O ₂ occurs at Sc³⁺ concentrations above the 1:2 ratio ([Sc³⁺]/[TPPZ]₀ > 0.5), when the (TPPZ) ₂-Sc³⁺ complex is converted to the TPPZ-Sc³⁺ complex and TPPZ-(Sc³⁺)₂, which act as promoters of electron transfer (ON) by the strong binding of O₂^?- with Sc³⁺. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc³⁺ concentrations below the 1:2 ratio (OFF), when the (TPPZ)₂-Sc³⁺ complex has no binding site available for O₂^?-.

Full text of DOI:10.1021/ja0748480

Published on Web 12/20/2007
OFF-OFF-ON Switching of Fluorescence and Electron
Transfer Depending on Stepwise Complex Formation of a
Host Ligand with Guest Metal Ions
Junpei Yuasa and Shunichi Fukuzumi*
Department of Material and Life Science, DiVision of AdVanced Science and Biotechnology,
Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan
Received July 2, 2007; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and
a series of metal ions (M ) Sc3+, Y , Ho , Eu , Lu , Nd , Zn , Mg , Ca , Ba , Sr , Li ), where
n+
3+
3+
3+
3+
3+
2+
2+
2+
2+
2+
+
n+
n+
n+
TPPZ forms a 2:1 complex [(TPPZ)
2
-M ] and a 1:1 complex [TPPZ-M ] with M at low and high
concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high
concentrations of metal ions, when the 2:1 (TPPZ)
-Mn+ complex is converted to the fluorescent 1:1 TPPZ-
2
n+
M
complex. This is regarded as an “OFF-OFF-ON” fluorescence sensor for metal ions depending on
the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the
TPPZ-Mn+ complex vary depending on the metal valence state, in which the fluorescence quantum yields
2+
of the divalent metal complexes (TPPZ-M ) are much larger than those of the trivalent metal complexes
3
+
-Mn+ (K
) and TPPZ-Mn+ (K
(
TPPZ-M ). On the other hand, the binding constants of (TPPZ)
depending on the Lewis acidity of metal ions (i.e., both K and K values increase with increasing Lewis
acidity of metal ions). Sc , which acts as the strongest Lewis acid, forms the (TPPZ)
-Sc3+ and TPPZ-
Sc complexes stoichiometrically with TPPZ. In such a case, “OFF-OFF-ON” switching of electron transfer
2
1
2
) vary
1
2
3+
2
3+
3+
from cobalt(II) tetraphenylporphyrin (CoTPP) to O
on the ratio of Sc to TPPZ. Electron transfer from CoTPP to O
:2 ratio ([Sc ]/[TPPZ]
and TPPZ-(Sc
Sc . In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc
concentrations below the 1:2 ratio (OFF), when the (TPPZ)
2
is observed in the presence of Sc and TPPZ depending
3
+
3+
2
occurs at Sc concentrations above the
3+
-Sc3+ complex is converted to the TPPZ-Sc complex
3+
1
0
> 0.5), when the (TPPZ)
2
3
+
•-
)
2
, which act as promoters of electron transfer (ON) by the strong binding of O
2
with
3+
3+
-Sc3 complex has no binding site available
+
2
•
-
for O
2
.
Introduction
been a general pattern of complex formation in fluorescence
sensor, affording “OFF-ON” switchability (Scheme 1a). The
fluorescence intensity due to the 1:1 complex formation with
Fluorescence sensors for metal ions are one of the common
analytical tools for obtaining quantitative information about the
amount of those metal ions,1-5 where a probe molecule
recognizes a metal ion to emit specific fluorescence upon
binding of the metal ion.1 Simple 1:1 stoichiometric host
n+
the metal ion (M ) increases linearly with increasing metal
n+
ion concentration from 0 M ([M ] > 0 M). In such a case,
however, those fluorescence sensors bind with metal ions within
only a limited concentration range of metal ions.
-5
(probe molecules or ligands)-guest (metal ions) recognition has
In contrast to the simple 1:1 complex formation, a stepwise
complex formation (2:1 and 1:1) in host (probe molecules or
ligands)-guest (metal ions) recognition would afford nonlinear
“OFF-OFF-ON” switchability in fluorescence sensors pro-
vided that only the 1:1 complex is fluorescent (Scheme 1b). In
such a case, a probe molecule begins to fluoresce at certain
concentrations of metal ions, when the 2:1 complex starts to be
(
1) (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J.
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Soc. 2005, 127, 4124.
1
86, 297. (g) Okamoto, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126,
3922.
1
566
9
J. AM. CHEM. SOC. 2008, 130, 566-575
10.1021/ja0748480 CCC: $40.75 © 2008 American Chemical Society

OFF
−OFF
−ON of Fluorescence and Electron Transfer
A R T I C L E S
Scheme 1
Scheme 2
converted to the 1:1 complex. This is potentially quite useful
6
for routine analysis of metal ions in a broad concentration range,
because development of detection tools for metal ions in a broad
concentration range (such as Zn2+ in a cellular system, where
the rates of electron transfer increase linearly with increasing
-10
-4
concentrations of metal ions from 0 M ([Mn+] > 0 M) as the
its concentration varies over a wide range from 10
to 10
M in some vesicles) has been highly desired.7-9
case of fluorescence sensor (Scheme 1a). In contrast, a stepwise
complex formation (2:1 and 1:1) between ligands and metal
ions would afford nonlinear “OFF-OFF-ON” switchability of
electron transfer provided that only the 1:1 metal ion complex
can activate electron transfer by coordination of the products
of electron transfer to a vacant site of metal ion complexes
(Scheme 2b). It has been suggested that new frontiers of electron
The complex formation of host ligands with guest metal ions
also affects metal ion-promoted electron-transfer reactions from
electron donors to acceptors. It has been reported that the
electron-transfer processes are accelerated by binding of metal
ion complexes (Lewis acids) with the product radical anions of
electron acceptors (Lewis bases).10-19 The Lewis acidity of metal
20-22
transfer are exploited in such nonlinear dynamics.
Nonlinear
ion complexes can be regulated by host ligands acting as Lewis
bases.18,19
kinetics has thus far been found only in chemical reactions that
In the case of 1:1 complex formation (Scheme 2a),
2
3,24
consist of many elementary reactions.
3
+
(
6) A part of the preliminary results on the fluorescence sensor for Sc and
Such “OFF-OFF-ON” behaviors would expand the scope
of not only the availability of fluorescence techniques for routine
analysis of metal ions but also unique control of metal ion-
promoted electron-transfer reactions. However, stepwise host
2+
Zn has appeared. See: Yuasa, J.; Fukuzumi, S. J. Am. Chem. Soc. 2006,
28, 15976.
7) Frederickson, C. J.; Klitenick, M. A.; Manton, W. I.; Kirkpatrick, J. B.
Brain Res. 1983, 273, 335.
1
(
(
8) For “ON-OFF” fluorescence sensors using sequential binding to metal
ions, see: (a) Baxter, P. N. W. Chem.-Eur. J. 2002, 8, 5250. (b) Baxter,
P. N. W. Chem.-Eur. J. 2003, 9, 2531. (c) Baxter, P. N. W. Chem.-Eur.
J. 2003, 9, 5011.
(probe molecules or ligands)-guest (metal ions) complex
formation has yet to be examined in fluorescence sensors for
metal ions or metal ion-promoted electron-transfer reactions.
We report herein an “OFF-OFF-ON” switchable fluores-
cence sensor for metal ions and the metal ion-promoted electron-
transfer reduction of O2 in stepwise complex formation equilibria
of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) with metal ions.
TPPZ used as a probe molecule and a host ligand in this work
has recently been utilized in a variety of molecular wire
2
+
(
9) A series of fluorescence sensor molecules with distinct affinities for Zn
2+
have been applied to fluoresce assays for Zn . See: Komatsu, K.; Kikuchi,
K.; Kojima, H.; Urano, Y.; Nagano, T. J. Am. Chem. Soc. 2005, 127, 10197.
(
10) For the complex between organic radical anions and transition metals,
see: (a) Wanner, M.; Sixt, T.; Klinkhammer, K.-W.; Kaim, W. Inorg. Chem.
1
999, 38, 2753. (b) Schwederski, B.; Kasack, V.; Kaim, W.; Roth, E.;
Jordanov, J. Angew. Chem., Int. Ed. Engl. 1990, 29, 78. (c) Ernst, S.; H a¨ nel,
P.; Jordanov, J.; Kaim, W.; Kasack, V.; Roth, E. J. Am. Chem. Soc. 1989,
1
11, 1733. (d) Ghumaan, S.; Sarkar, B.; Patra, S.; van Slageren, J.; Fiedler,
J.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2005, 44, 3210.
(
11) For X-ray crystallography of contact and separated ion pairs of alkali metal
salts of radical anions, see: (a) L u¨ , J.-M.; Rosokha, S. V.; Lindeman, S.
V.; Neretin, I. S.; Kochi, J. K. J. Am. Chem. Soc. 2005, 127, 1797. (b)
Davlieva, M. G.; L u¨ , J.-M.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem.
Soc. 2004, 126, 4557. (c) L u¨ , J.-M.; Rosokha, S. V.; Neretin, I. S.; Kochi,
J. K. J. Am. Chem. Soc. 2006, 128, 16708.
25
complexes as a bridging ligand. TPPZ forms a 2:1 complex
n+
[(TPPZ)2-M ] with metal ions at high concentrations of metal
n+
ions; this complex is converted to a 1:1 complex [TPPZ-M ]
(
12) Wu, H.; Zhang, D.; Su, L.; Ohkubo, K.; Zhang, C.; Yin, S.; Mao, L.; Shuai,
Z.; Fukuzumi, S.; Zhu, D. J. Am. Chem. Soc. 2007, 129, 6839.
at high concentrations of metal ions. Such stepwise complex
formation of TPPZ with metal ions enables “OFF-OFF-ON”
switchability in metal ion fluorescence sensors and metal ion-
promoted electron-transfer reactions.
(
13) (a) Fukuzumi, S. Bull. Chem. Soc. Jpn. 1997, 70, 1. (b) Fukuzumi, S. In
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
Germany, 2001; Vol. 4, pp 3-67. (c) Fukuzumi, S. Org. Biomol. Chem.
2
003, 1, 609.
(14) (a) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Chem. Soc., Perkin Trans.
2
1
1985, 371. (b) Fukuzumi, S.; Okamoto, T. J. Am. Chem. Soc. 1993, 115,
1600.
(20) Tributsch, H.; Pohlmann, L. Science 1998, 279, 1891.
(21) Okamoto, K.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 12416.
(22) We have previously reported unusually high kinetic order in the scandium
(15) (a) Itoh, S.; Kawakami, H.; Fukuzumi, S. J. Am. Chem. Soc. 1998, 120,
3+
7
1
3
271. (b) Itoh, S.; Kawakami, H.; Fukuzumi, S. J. Am. Chem. Soc. 1997,
19, 439. (c) Itoh, S.; Kawakami, H.; Fukuzumi, S. Biochemistry 1998,
7, 6562.
ion (Sc )-promoted electron-transfer reduction of p-benzoquinone (Q) due
to the formation of π-dimer radical anion complexes of Q bridged by two
3
+
•-
3+
n
and three Sc [Q -(Sc
) -Q, n ) 2, 3]. See: Yuasa, J.; Suenobu, T.;
(
16) (a) Fukuzumi, S.; Tanaka, T. In Photoinduced Electron Transfer; Fox, M.
Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 12090.
A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chapter 11, pp
(23) (a) Fokin, A. A.; Schreiner, P. R.; Gunchenko, P. A.; Peleshanko, S. A.;
Shubina, T. E.; Isaev, S. D.; Tarasenko, P. V.; Kulik, N. I.; Schiebel, H.-
M.; Yurchenko, A. G. J. Am. Chem. Soc. 2000, 122, 7317. (b) Fokin, A.
A.; Shubina, T. E.; Gunchenko, P. A.; Isaev, S. D.; Yurchenko, A. G.;
Schreiner, P. R. J. Am. Chem. Soc. 2002, 124, 10718.
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(b) Bonner, T. G.; Hancock, R. A.; Rolle, F. R.; Yousif, G. J. Chem. Soc.
B 1970, 314.
(25) (a) Carlson, C. N.; Kuehl, C. J.; Da Re, R. E.; Veauthier, J. M.; Schelter,
E. J.; Milligan, A. E.; Scott, B. L.; Bauer, E. D.; Thompson, J. D.; Morris,
D. E.; John, K. D. J. Am. Chem. Soc. 2006, 128, 7230. (b) Fantacci, S.; De
Angelis, F.; Wang, J.; Bernhard, S.; Selloni, A. J. Am. Chem. Soc. 2004,
126, 9715. (c) Flores-Torres, S.; Hutchison, G. R.; Soltzberg, L. J.; Abru n˜ a,
H. D. J. Am. Chem. Soc. 2006, 128, 1513.
6
36-687. (b) Fukuzumi, S.; Okamoto, T.; Otera, J. J. Am. Chem. Soc.
1
994, 116, 5503. (c) Fukuzumi, S.; Itoh, S. In AdVances in Photochemistry;
Neckers, D. C., Volman, D. H., von B u¨ nau, G., Eds.; Wiley: New York,
1
998; Vol. 25, pp 107-172.
(
(
17) Yuasa, J.; Suenobu, T.; Fukuzumi, S. ChemPhysChem 2006, 7, 942.
18) Ohkubo, K.; Menon, S. C.; Orita, A.; Otera, J.; Fukuzumi, S. J. Org. Chem.
2
003, 68, 4720.
(
19) We have previously reported that the Lewis acidity of scandium ion (Sc3+)
increases with decreasing the nucleophilicity of the counteranion, leading
3+
to increase the rate constant of the Sc -promoted electron-transfer reaction.
See: (a) Yuasa, J.; Suenobu, T.; Ohkubo, K.; Fukuzumi, S. Chem. Commun.
2
2
003, 1070. (b) Fukuzumi, S.; Yuasa, J.; Suenobu, T. J. Am. Chem. Soc.
002, 124, 12566.
J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008 567

A R T I C L E S
Yuasa and Fukuzumi
3
+
Experimental Section
of 0-0.5 equiv of Sc are ascribed to a 2:1 complex formation
3
+
3+
of TPPZ with Sc [(TPPZ)2-Sc ] (the first step of eq 1).
Materials. Scandium triflate [Sc(OTf)
3
] (OTf ) OSO
], neodymium triflate [Nd-
], lutetium triflate [Lu(OTf) ], zinc
], strontium perchlorate [Sr(ClO ], and TPPZ were
obtained from Aldrich. Magnesium perchlorate [Mg(ClO ], barium
perchlorate [Ba(ClO ], lithium perchlorate [Li(ClO )], and calcium
perchlorate [Ca(ClO ] were obtained from Nacalai Tesque. Cobalt-
II) tetraphenylporphyrin (CoTPP) was prepared according to the
2 3
CF ), yttrium
triflate [Y(OTf)
OTf) ], europium triflate [Eu(OTf)
triflate [Zn(OTf)
3 3
], holmium triflate [Ho(OTf)
(
3
3
3
2
4 2
)
4 2
)
)
4 2
4
4 2
)
(
2
6
literature. Acetonitrile (MeCN) used as a solvent was purified and
2
7
2
dried by the standard procedure. [ H
obtained from EURI SO-TOP, CEA, France.
Spectral Measurements. Formation of metal ion complexes of
3 3
]Acetonitrile (CD CN) was
n+
n+
TPPZ, (TPPZ) -M and TPPZ-M , was examined from the UV-
2
vis spectral change of TPPZ in the presence of various concentrations
n+
of M by using a Hewlett Packard 8453 diode array spectrophotometer.
n+
Formation of the TPPZ-M complexes was also examined from the
fluorescence spectral change of TPPZ in the presence of various
concentrations of M by a Shimadzu spectrofluorophotometer (RF-
3
+
n+
The plot of ∆Abs/∆Abs∞ at λ ) 358 nm vs [Sc ]/[TPPZ]0
inset of Figure 1a) confirms the stoichiometry of (TPPZ)2-
(
5
000). The monitoring wavelengths were those corresponding to the
3
+
n+
Sc in the first step of eq 1. The absorption spectrum due to
(TPPZ)2-Sc (Figures 1a and 2a, blue lines) is further changed
by the presence of 0.5-1.0 equiv of Sc with isosbestic points
at 237 and 278 nm (Figure 2a). Such stepwise UV-vis spectral
maxima of the emission bands of the TPPZ-M complexes. The
formation of (TPPZ)
2+
2+
3+
2
-Zn and TPPZ-Zn complexes was examined
3+
by ESI-MS measurements. ESI-MS data were collected on an API 365
triple quadrupole mass spectrometer (PE-Sciex) in positive detection
mode, equipped with an ion spray interface. The sprayer was held at a
6
changes are ascribed to the conversion of the 2:1 complex
potential of +5.0 kV, and compressed N
2
was employed to assist liquid
3+
3+
[
(TPPZ)2-Sc ] to a 1:1 complex of TPPZ with Sc (TPPZ-
nebulization.
3+
Sc ) (the second step of eq 1). The stoichiometry of the TPPZ-
Sc complex in the second step of eq 1 is confirmed by the
plot of ∆Abs/∆Abs∞ at λ ) 303 nm vs [Sc ]/[TPPZ]0 (inset of
Figure 2a). TPPZ also shows stepwise UV-vis spectral changes,
when the other metal ions (M : Y , Ho , Eu , Lu , Nd ,
Zn , Mg , Ca , Ba , Sr , and Li ) are employed instead
of Sc (Figures 1b-d and 2b-d; Figures S1 and S2 in
Kinetic Measurements. Kinetic measurements were performed by
using a Shimadzu UV-3100 PC UV-vis-NIR scanning spectropho-
3+
3
+
-
6
tometer. Rates of electron transfer from CoTPP (1.4 × 10 M) to O
2
-3
-
3
3+
(
air saturated: 2.6 × 10 M) in the presence of Sc [(0-1.2) × 10
n+
3+
3+
3+
3+
3+
-4 -4
M] and TPPZ (1.6 × 10 to 4.8 × 10 M) in MeCN at 298 K were
2
+
2+
2+
2+
2+
+
monitored by rise and decay of the absorption bands at 434 and 412
+
3+
nm due to CoTPP and CoTPP, respectively. All kinetic measurements
were carried out under pseudo-first-order conditions where the con-
Supporting Information). This indicates that TPPZ forms the
3
+
n+
2
centrations of O , Sc , and TPPZ were maintained at more than 10-
2:1 complexes [(TPPZ) -M ] with a series of metal ions,
2
fold excess of the concentrations of CoTPP at 298 K. Pseudo-first-
order rate constants were determined by least-square curve fits using a
personal computer.
n+
which are converted to the 1:1 complexes (TPPZ-M ) at high
30
3+
concentrations of metal ions (eq 1). In the case of Sc , which
acts as the strongest Lewis acid among a series of metal ions,
Fluorescence Quantum Yield Determination. The fluorescence
3+
3+
TPPZ-Sc is further converted to TPPZ-(Sc )2 at high
n+
quantum yields (φ ) of the TPPZ-M complexes were determined in
f
3
+
-4
-2
concentration range of Sc (8.0 × 10 to 1.6 × 10 M), and
reference to a fluorescence quantum yield of quinine sulfate (φ
in 0.5 M sulfuric acid. The excitation wavelength was λ ) 349 nm.
f
) 0.55)
3+
3+
the TPPZ-(Sc )2 complex fluoresces as strong as TPPZ-Sc
2
8
3
1
(
see Supporting Information S3).
Formation of the (TPPZ) -M and TPPZ-Mn+ complexes
f
All φ values were determined under conditions such that TPPZ certainly
forms the TPPZ-Mn+ complexes in the presence of sufficient
concentrations of metal ions.
n+
2
was also examined by ESI-MS measurements in stepwise
2
+
6,32
Theoretical Calculations. Density functional theory (DFT) calcula-
tions were performed on an 8CPU workstation (PQS, Quantum Cube
QS8-2400C-064). Geometry optimizations were carried out using the
complex formation of TPPZ with Zn in MeCN.
The
-
5
positive-ion ESI mass spectrum of TPPZ (3.9 × 10 M) in
-
5
MeCN in the presence of low (3.0 × 10 M; Figure 3a) and
2
9
Becke3LYP functional and 6-31G** basis set with the restricted
Hartree-Fock formalism and as implemented in the Gaussian 03
program revision C.02.
-5
2+
high (6.8 × 10 M; Figure 3c) concentrations of Zn shows
signals at m/z +989.3 and at +601.1 m/z, which correspond to
+
+
{
[Zn(TPPZ)2](OSO2CF3)} and {[Zn(TPPZ)](OSO2CF3)} , re-
Results and Discussion
(
30) We have also examined complex formation of TPPZ with the potentially
interfering metal ion such as Fe(II). TPPZ also forms the complex with
Fe(II) in MeCN. However, the Fe(II) complex of TPPZ exhibits no
fluorescence. For the Fe(II) complex of TPPZ, see: Campos-Fern a´ ndez,
C. S.; Smucker, B. W.; Cl e´ rac, R.; Dunbar, K. R. Isr. J. Chem. 2001, 41,
Stepwise Complex Formation of TPPZ with Metal Ions.
Upon addition of 0-0.5 equiv of scandium triflate [Sc(OTf)3]
-
5
(
OTf ) OSO2CF3) [(0-2.8) × 10 M] to an MeCN solution
207.
-5
(31) The Lewis acidities of the other metal ions are not strong enough to form
of TPPZ (5.7 × 10 M), UV-vis spectral changes of TPPZ
n+
the 1:2 complexes [TPPZ-(M
)
2
].
2
32) Formation of (TPPZ) -Sc and TPPZ-Sc is confirmed by H NMR.
1
were observed with isosbestic points at 262, 276, 302, and 311
3+
3+
1
(
6
The H NMR signals of TPPZ exhibit a downfield shift in the presence of
nm (Figure 1a). Such spectral changes of TPPZ in the presence
3
+
0
.5 equiv of Sc , demonstrating development of positive charge by
3 1
+
3+
formation of (TPPZ)
2
-Sc . The H NMR signals of (TPPZ)
2
-Sc exhibit
3
+
(
26) Shirazi, A.; Goff, H. M. Inorg. Chem. 1982, 21, 3420.
a further downfield shift in the presence of 1 equiv of Sc due to the
3
+
3+
(
27) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals,
additional binding of Sc to yield the 1:1 complex, TPPZ-Sc . However,
3+ 3+
5
th ed.; Butterworth-Heinemann: Amsterdam, 2003.
2
(TPPZ) -Sc and TPPZ-Sc could not be detected by positive-ion ESI
3+
(28) Crosby, G. A.; Demas, J. N. J. Phys. Chem. 1971, 75, 991.
29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
mass spectrum, which may be due to multivalency of the Sc complex of
TPPZ.
(
568 J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008

OFF
−OFF
−ON of Fluorescence and Electron Transfer
A R T I C L E S
Figure 1. UV-vis absorption spectra of (a) TPPZ (5.7 × 10 M) in the presence of Sc3+ [0 M (red line) to 2.8 × 10 M (blue line)], (b) TPPZ (4.9 ×
-
5
-5
-
-
5
5
3+
-5
-5
2+
1
0
0
M) in the presence of Lu [0 M (red line) to 4.0 × 10 M (blue line)], (c) TPPZ (5.7 × 10 M) in the presence of Zn [0 M (red line) to 3.0 ×
-5 2+ -4
1
M (blue line)], and (d) TPPZ (5.8 × 10 M) in the presence of Ca [0 M (red line) to 2.0 × 10 M (blue line)] in MeCN at 298 K. Insets: Plots
3
+
3+
2+
of ∆Abs/∆Abs∞ (a) at λ ) 358 nm vs [Sc ]/[TPPZ]0, (b) at 360 nm vs [Lu ]/[TPPZ]0, (c) at λ ) 355 nm vs [Zn ]/[TPPZ]0, and (d) at λ ) 355 nm vs
2
+
-2
3+
-2
3+
-2
2+
[
Ca ]/[TPPZ]0. Photographs of MeCN solutions of TPPZ (2.0 × 10 M) in the presence of (a) Sc (1.0 × 10 M), (b) Lu (1.0 × 10 M), (c) Zn
-2 2+ -2
(
1.0 × 10 M), and (d) Ca (1.0 × 10 M) under UV light irradiation.
spectively.33 Those signals have a characteristic distribution of
isotopomers (Figure 3a,c) that matches well with the calculated
in the 2:1 complex [(TPPZ)2-M ], in which the lowest excited
n+
3
7
state may still be the n,π* triplet.
+
n+
isotopic distribution for {[Zn(TPPZ)2](OSO2CF3)} and {[Zn-
The fluorescence spectral titration of TPPZ by M was
examined under the same experimental conditions as employed
for the UV-vis spectral titration (Figures 1 and 2). The results
are shown in Figure 4. Typically, TPPZ shows stepwise
fluorescence spectral changes in response to [Sc ]/[TPPZ] at
0-0.5 (red and blue lines) and 0.5-1.0 (blue and green lines)
Figure 4a) as in the case of the UV-vis spectral titration of
TPPZ by Sc (Figures 1a and 2a). Such stepwise fluorescence
spectral changes are also ascribed to stepwise complex formation
between TPPZ and Sc (eq 1). The fluorescence intensity (F.
L. intensity) at λ ) 453 nm (inset of Figure 4a) due to the
TPPZ-Sc complex is plotted against [Sc ]/[TPPZ] , where
fluorescence intensity starts to increase at Sc concentrations
above the 1:2 ratio ([Sc ]/[TPPZ] > 0.5). TPPZ also shows
stepwise fluorescence changes in the fluorescence spectral
titration of TPPZ by the other metal ions (Figure 4b-d; Figure
S5 in Supporting Information). There is very weak fluorescence
due to the (TPPZ)2-M complexes at low concentrations of
(
TPPZ)](OSO2CF3)}+ as shown in Figure 3b,d, respectively.
The visible fluorescence photographs of TPPZ in the presence
3
+
3+
2+
of 0.5 and more than 1 equiv of metal ions (Sc , Lu , Zn ,
3
+
2
+
and Ca ) are shown in the insets of Figures 1 and 2,
respectively. The 1:1 complexes (TPPZ-M ) exhibit strong
0
n+
(
fluorescence (insets of Figure 2). Since TPPZ itself hardly
3+
6
n+
fluoresces, the fluorescence of the TPPZ-M complexes may
result from the change in the lowest excited state from the n,π*
triplet to the π,π* singlet, which becomes lower in energy than
3
+
n+
the n,π* triplet due to the complexation with M acting as a
3
+
3+
strong Lewis acid.3
4-36
In contrast, the (TPPZ)2-M complexes
n+
0
3
+
n+
hardly fluoresce (insets of Figure 1). The strong binding of M
in the TPPZ-Mn+ complexes may be significantly weakened
3
+
0
2
+
(
33) The crystal structure of the TPPZ-Zn complex has been previously
reported. See: Graf, M.; Greaves, B.; Stoeckli-Evans, H. S. Inorg. Chim.
Acta 1993, 204, 239.
n+
(
(
(
34) Lewis, F. D.; Reddy, G. D.; Elbert, J. E.; Tillberg, B. E.; Meltzer, J. A.;
Kojima, M. J. Org. Chem. 1991, 56, 5311.
metal ions (Figures 4 and S5, blue lines). However, strong
35) Fukuzumi, S.; Satoh, N.; Okamoto, T.; Yasui, K.; Suenobu, T.; Seko, Y.;
Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 7756.
n+
2+
2+
36) The partial structure fixation of TPPZ by complex formation with M
(37) The optimized structures of TPPZ-Mg and (TPPZ)
2
-Mg obtained by
(
TPPZ-M ) may also be ascribed to strong fluorescence of TPPZ-Mn+,
n+
using DFT at the B3LYP/6-31G basis support the stronger binding of
**
2
+
2+
where the rotation of coordinated pyridine rings is restricted. This rotation
Mg in the 1:1 complex (TPPZ-Mg ) than that in the 2:1 complex
2+ 2+
n+
may be less restricted in the 2:1 complexes [TPPZ-(M
)
2
] because of
[(TPPZ)
2
nitrogen in the (TPPZ) -Mg complex (2.17 Å) becomes shorter (2.01
2+
2
-Mg ], in which the bond length between Mg and the pyrazine
n+
n+
2+
the weaker binding of M in the TPPZ-(M
)
2
complexes than that in
n+
the TPPZ-M complexes.
Å) in the TPPZ-Mg complex (see Supporting Information S4).
J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008 569

A R T I C L E S
Yuasa and Fukuzumi
-
5
3+
-5
-5
Figure 2. UV-vis absorption spectra of (a) TPPZ (5.7 × 10 M) in the presence of Sc [2.8 × 10 M (blue line) to 5.9 × 10 M (green line)], (b)
-
5
3+
-4
-3
-5
2+
-1
TPPZ (4.9 × 10 M) in the presence of Lu [1.0 × 10 M (blue line) to 1.0 × 10 M (green line)], (c) TPPZ (5.7 × 10 M) in the presence of Zn
-
5
-3
-5
2+
-3
[
3.0 × 10 M (blue line) to 3.1 × 10 M (green line)], and (d) TPPZ (5.8 × 10 M) in the presence of Ca [5.1 × 10 M (blue line) to 7.9 × 10
3
+
3
+
M (green line)] in MeCN at 298 K. Insets: Plots of (a) ∆Abs at λ ) 303 nm vs [Sc ]/[TPPZ]0, (b) absorbance at 330 nm vs [Lu ], (c) absorbance at 381
2
+
2+
-2
3+
-2
nm vs [Zn ], and (d) absorbance at 380 nm vs [Ca ]. Photographs of MeCN solutions of TPPZ (2.0 × 10 M) in the presence of (a) Sc (2.0 × 10
3
+
-2
2+
-2
2+
-2
M), (b) Lu (7.0 × 10 M), (c) Zn (2.0 × 10 M), and (d) Ca (2.0 × 10 M) under UV light irradiation.
-
5
2+
-5
-5
Figure 3. Positive-ion ESI mass spectra of an MeCN solution of TPPZ (3.9 × 10 M) in the presence of Zn [(a) 3.0 × 10 M and (c) 6.8 × 10 M].
+
+
The signals at (a) m/z 989.3 and (c) m/z 601.1 correspond to {[Zn(TPPZ)2](OSO2CF3)} and {[Zn(TPPZ)](OSO2CF3)} , respectively. Calculated isotopic
distributions are shown for (b) {[Zn(TPPZ)2](OSO2CF3)} and (d) {[Zn(TPPZ)](OSO2CF3)} .
+
+
3
+
n+
fluorescence starts to appear at λ ) 453 nm (Lu ), λ ) 432
Normalized absorbance for the titration of TPPZ by M at
2+
2+
n+
n+
nm (Zn ), and λ ) 429 nm (Ca ) (Figure 4, green lines) at
high concentrations of metal ions, which are ascribed to
low concentrations of M is plotted against logarithm of M
concentrations (log[M ]) (Figure 5a), which corresponds to the
n+
n+
n+
formation of the TPPZ-M
complexes (see Supporting
2:1 complex formation of TPPZ with M . In the case of
3
+
3+
3+
3+
3+
3+
Information S5).
trivalent metal ions (Sc , Y , Ho , Eu , Lu , and Nd )
570 J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008

OFF
−OFF
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A R T I C L E S
expressed by eq 3 (for derivation of eq 3, see Supporting
Information S8).
(
A - A ) ) (A - A ) ×
0
∞
0
n+
n+
4
K [M ][TPPZ] + 1 - 8K [M ][TPPZ] + 1
1
0
x
1
0
(3)
n+
4
K [M ][TPPZ]
1
0
n+
The K1 values of TPPZ-M are determined as (1.2 ( 0.1) ×
7
-2
2+
7
-2
2+
1
0 M (Ba ), (7.5 ( 0.4) × 10 M (Sr ), and (3.1 ( 0.1)
6
-2
+
×
10 M (Li ) from the best-fit lines for the dependence of
Abs on [M ] (Figure S1).
n+
∆
Plots of normalized absorbance and fluorescence intensity
n+
n+
due to TPPZ-M vs log [M ] for the titration of TPPZ by
M
n+
at high concentrations of Mn+ are shown in Figure 5b,c,
respectively. The dependence of normalized fluorescence in-
n+
n+
tensity due to TPPZ-M on log[M ] (Figure 5c) agrees with
n+
that of normalized absorbance on log [M ] (Figure 5b). Such
absorbance and fluorescence intensity changes due to formation
n+
of the TPPZ-M complex are expressed by eq 4, where A is
n+
n+
absorbance of TPPZ in the presence of M (2[M ] >
n+
[
TPPZ]0), and A0 and A∞ are absorbance due to (TPPZ)2-M
and TPPZ-M , respectively (for derivation of eq 4, see
n+
Supporting Information S9).
-
5
Figure 4. Fluorescence spectra of (a) TPPZ (5.7 × 10 M) in the presence
3
+
-5
-5
of Sc [0 M (red line) to 2.8 × 10 M (blue line) to 5.9 × 10 M (green
(
A - A0)
(A - A )
(or I∞)
-
5
3+
-5
)
∞
0
×
line)], (b) TPPZ (4.9 × 10 M) in the presence of Lu [4.6 × 10
M
-
4
-5
(or I)
(
blue line) to 9.0 × 10 M (green line)], (c) TPPZ (5.7 × 10 M) in the
2+ -5 -3
presence of Zn [3.0 × 10 M (blue line) to 3.1 × 10 M (green line)],
2
2
n+ 2
-K
[Mn+] +
x
K
[M ] + (4 - K )K [TPPZ] )
(2[Mn+] - [TPPZ]
-
5
2+
-4
and (d) TPPZ (5.8 × 10 M) in the presence of Ca [6.7 × 10 M (blue
2
2 2 0 0
-
1
line) to 3.5 × 10 M (green line)] in MeCN at 298 K. The excitation
(4 - K )[TPPZ]
wavelengths were (a) λ ) 315 nm, (b) λ ) 355 nm, (c) λ ) 343 nm, and
2 0
(4)
(
d) λ ) 324 nm. Insets: Plots of fluorescence intensity (a) at λ ) 453 nm
3
+
3+
vs [Sc ]/[TPPZ]0, (b) at λ ) 453 nm vs [Lu ], (c) at λ ) 432 nm vs
2
+
2+
The formation constants (K2) of the TPPZ-Mn+ complex are
[
Zn ], and (d) at λ ) 429 nm vs [Ca ].
determined from the best-fit lines in insets of Figures 2 and
2
+
2+
2+
and Zn , Mg , and Ca , acting as strong Lewis acids, those
2
3+
S2, where the K2 values vary from (1.4 ( 0.1) × 10 (Sc ) to
n+
metal ions form the 2:1 complex [(TPPZ)2-M ] almost
stoichiometrically with TPPZ (Figure 5a, closed circles). In such
a case, the formation constants (K1) of the (TPPZ)2-M
complexes can be determined by using eq 2, where A is
-5
2+
(
4.4 ( 1.3) × 10 (Ba ). The K1 and K2 values are listed in
n+
Table 1 together with the absorption maxima of (TPPZ)2-M
λ1) and TPPZ-M (λ2), emission maxima (λe), excitation
n+
n+
(
energies (∆E0,0), and the fluorescence quantum yields (φf) of
n+
absorbance of TPPZ in the presence of M , R ) (A - A0)(A∞
n+
40
the TPPZ-M complexes. We have previously derived the
-
1
-
(
A0) , and A0 and A∞ are absorbance due to TPPZ and
binding energies (∆E) of superoxide-metal ion complexes
n+
TPPZ)2-M , respectively (for derivation of eq 2, see Sup-
porting Information S6).
•-
n+
•-
n+
(
O2 -M ) from the gzz values of ESR spectra of O2 -M
38
complexes, which are highly sensitive to the Lewis acidity of a
series of metal ions and provide a quantitative measure of Lewis
acidity of metal ions. Linear correlations are observed in plots
-2
n+ -1
(
1 - R) ) K [TPPZ] (2[M ]R - [TPPZ]0) (2)
42
1
0
of log K1 and log K2 vs ∆E (Figure 6, open and closed circles,
The linear plots of (1 - R)-2 vs [M ]R are obtained. The
n+ -1
respectively). This indicates that the binding strength of metal
10
-2
3+
K1 values are (1.4 ( 0.1) × 10
M
(Ho ), (2.4 ( 0.3) ×
n+
n+
ion with TPPZ in (TPPZ)2-M and TPPZ-M is mainly
determined by the Lewis acidity of metal ions. Some deviations
in the linear plots of log K1 and log K2 vs ∆E indicate that the
counteranions of metal ions may also affect the binding constants
10
-2
3+
10
-2
3+
1
0
M
(Eu ), (2.3 ( 0.2) × 10
M
(Lu ), (5.9 ( 1.3)
×
1010
M
-2
(Nd ), (2.3 ( 0.1) × 10 M (Mg ), and (5.2
9 -2 2+ 39
3+
9
-2
2+
(
0.4) × 10 M (Ca ) (see Supporting Information S7).
2+ 2+ +
In contrast, the other metal ions (Ba , Sr , and Li ) acting as
n+
weak Lewis acids form the (TPPZ)2-M complexes in the
(40) Absorption maxima, fluorescence maxima, and fluorescence quantum yields
n+
of TPPZ-M were determined in the presence of sufficient concentrations
presence of much more than 0.5 equiv of metal ions (Figure
a, closed triangles). In such a case, the absorbance change (A
A0 ) ∆Abs) due to formation of the TPPZ-M complex is
of metal ions such that TPPZ certainly forms the 1:1 complexes (TPPZ-
n+
+
5
-
M ). In the case of TPPZ-Li , however, only 53% of TPPZ forms the
+ -1
TPPZ-Li complex even in the presence of 8.1 × 10 M of Li+ due to
n+
+
the extremely small binding constant of the TPPZ-Li complex. On the
n+
2
other hand, absorption maxima of the (TPPZ) -M complexes were
n+
(
38) Absorbance of TPPZ in the presence of Mn+ (A) is derived from the sum
2
determined under conditions such that the 2:1 complexes [(TPPZ) -M ]
n+
n+
of absorbance due to TPPZ and (TPPZ)
2
-M at any wavelength, which
are not converted to the 1:1 complexes (TPPZ-M ) as shown in Figures
1 and S1.
can be expressed by eq 2 (see derivation of eq 2 in Supporting Information
S6).
(41) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(42) Fukuzumi, S.; Ohkubo, K. Chem.-Eur. J. 2000, 6, 4532.
(43) Fukuzumi, S.; Ohkubo, K. J. Am. Chem. Soc. 2002, 124, 10270.
(
39) In the case of Sc +, Y3+, and Zn2+, the K
3
1
values are too large to be
determined actually.
J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008 571

A R T I C L E S
Yuasa and Fukuzumi
Figure 5. (a) Plots of normalized absorbance vs log [Mn+] for the titration of TPPZ by M at low concentrations of Mn+. Plots of normalized (b) absorbance
n+
n+
n+
n+
and (c) fluorescence intensity vs log[M ] for the titration of TPPZ by M at high concentrations of M
.
Table 1. Ion Radii (r) and Binding Energies (∆E) of O2•--Metal Ion Complexes, Formation Constants of (TPPZ)2-M (K1) and TPPZ-Mn+
n+
n+
n+
n+
(
K2), Absorption Maxima of (TPPZ)2-M (λ1) and TPPZ-M (λ2), Emission Maxima of TPPZ-M (λe), Excitation Energies (∆E0,0) of
n
+
n
+
TPPZ-M , and the Fluorescence Quantum Yields (φf) of TPPZ-M in MeCN at 298 K
ra
Eb
g
∆
(eV)
K
1
λ
(nm)
1
λ
(nm)
2
λ
(nm)
e
∆E
0,0
(eV)
metal salt
(Å)
(M-2)
K
2
φh
absence of
metal salt
Sc(OTf)3
Y(OTf)3
Ho(OTf)3
Eu(OTf)3
Lu(OTf)3
Nd(OTf)3
Zn(OTf)2
Mg(ClO4)2
Ca(ClO4)2
Ba(ClO4)2
Sr(ClO4)2
Li(ClO4)
308f
2
0.81
1.02
1.02
1.25
0.98
1.11
0.90
0.89
1.12
1.42
1.26
0.92
1.00
0.85
d
d
(1.4 ( 0.1) × 10
(8.2 ( 1.4) × 10
(6.0 ( 0.3) × 10
(6.0 ( 0.3) × 10
(2.7 ( 0.1) × 10
(2.9 ( 0.3) × 10
(2.1 ( 0.3) × 10
(9.5 ( 5.3) × 10
(3.7 ( 0.6) × 10
(4.4 ( 1.3) × 10
(3.7 ( 0.7) × 10
e
325
331
345
353
332
354
352
344
330
328
330
330
331
330
345
365
330
380
379
362
350
330
341
453
450
455
451
453
439
432
432
429
403
412
404
3.16
3.18
3.10
3.04
3.17
3.03
3.06
3.12
3.18
3.38
3.29
0.022
0.017
0.011
0.006
0.029
0.005
0.19
0.26
0.19
0.069
0.14
0.018
-
1
1
1
1
1
0
0
0
0
-2
-2
-1
-2
-1
-5
-4
-5
-4
(1.4 ( 0.1) × 10
(2.4 ( 0.3) × 10
(2.3 ( 0.2) × 10
(5.9 ( 1.3) × 10
d
(2.3 ( 0.1) × 10
(5.2 ( 0.4) × 10
(1.2 ( 0.1) × 10
(7.5 ( 0.4) × 10
(3.1 ( 0.1) × 10
0.82
0.83
0.71c
0.65
0.58
0.49
0.52
0.53
9
9
7
7
6
a
41
b
Taken from ref 42. c Taken from ref 43. d Too large to be determined accurately. e Too small to be
Effective ion radius (coordination number ) 8).
f
g
determined accurately. Absorption maxima of TPPZ in the absence of metal salt. Determined from the absorption maxima and emission maxima in
MeCN. Quinine sulfate (φf ) 0.55) in 0.5 M sulfuric acid was used as a reference.
h
of (TPPZ)2-Mn+ and TPPZ-M . In the case of Zn , the
n+ 44
2+
than those of the trivalent metal complexes (TPPZ-M ). The
3+ 46
2+
n+
binding constant (K2) of the 1:1 complex (TPPZ-Zn ) is much
formation constants (K2) of the TPPZ-M complexes may not
affect the fluorescence quantum yields, because TPPZ-Zn ,
which has virtually the same formation constant as that of
TPPZ-M , has a much larger fluorescence quantum yield
45
2+
2+
larger than the other divalent metal ions. In such a case, Zn
forms the fluorescent TPPZ-Mn+ complex with TPPZ in water
see Supporting Information S10).
3
+
(
3
+
n+
(φ ) 0.19) than that of TPPZ-M (φ ) 0.005-0.022)
The fluorescence quantum yields of the TPPZ-M com-
f
f
plexes were determined in a series of metal ions.6 The results
,40
(Table 1).
are shown in Figure 7, where the fluorescence quantum yields
of the divalent metal complexes (TPPZ-M ) are much larger
Fluorescence Sensor for Metal Ions in a Wide Concentra-
tion Range. The availability of such an “OFF-OFF-ON”
fluorescence sensor for detection of metal ions in a wide
2+
(
44) A series of metal ions with different counteranions are chosen in this work,
n+
since the ∆E values of those metal ions have been given in ref 42.
(46) The fluorescence quantum yields of TPPZ-M may be related to the
n+
(45) With regard to metal ion selectivity, TPPZ has a much larger K
2
value for
fluorescence maxima of the TPPZ-M complexes, because fluorescence
2
+
3+
3+
Zn as compared with other biologically important metal ions such as
lifetimes of the trivalent metal complexes (TPPZ-Sc and TPPZ-Y )
2
+
2+
Mg and Ca (Table 1). This means that TPPZ forms the fluorescent 1:1
complex (TPPZ-M ) selectively with Zn among biologically relevant
that fluoresce at longer wavelength are much shorter than those of divalent
n+
2+
2+
2+
metal complexes (TPPZ-Mg and TPPZ-Ca ) (see Supporting Informa-
metal ions.
tion S11).
572 J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008

OFF
−OFF
−ON of Fluorescence and Electron Transfer
A R T I C L E S
Figure 6. Plots of log K1 (open circles) and log K2 (closed circles) vs ∆E.
Figure 8. (a) Change in visible fluorescence of MeCN solutions of TPPZ
-
5
-5
3+
-5
(
1.0 × 10 to 5.0 × 10 M) in the presence of Sc [(0-3.6) × 10
M]. Plots of (b) I/I∞ at 453 nm and (c) ∆Abs/∆Abs∞ [) (A - A0)/(A∞ -
A0)] at 303 nm vs [Sc3 ] for the titration of TPPZ [3.4 × 10 M (black
+
-5
-
5
-5
closed circles), 4.7 × 10 M (red closed circles), and 5.7 × 10 M (blue
closed circles)] by Sc3 in MeCN at 298 K.
+
Figure 7. Visible fluorescence and fluorescence quantum yields (φf) of
-
7
2+
-2
2+
TPPZ (6.0 × 10 M) in the presence of Zn (1.1 × 10 M); Ba (9.9
-
1
2+
-1
2+
-1
2+
-1
×
10 M); Sr (7.4 × 10 M); Ca (8.8 × 10 M); Mg (8.2 × 10
3+ -2 3+ -2 3+ -3
M); Sc (1.2 × 10 M); Eu (1.1 × 10 M); Y (6.2 × 10 M);
3
+
-3
3+
-3
3+
-2
Lu (4.0 × 10 M); Ho (3.4 × 10 M); Nd (2.8 × 10 M); and
+
-1
Li (8.1 × 10 M) in MeCN at 298 K.
concentration range is confirmed by fluorescence sensing of
3
+
47,48
Sc (vide infra).
The fluorescence in the presence of various
-5
-5
3+
concentrations of TPPZ (1.0 × 10 to 5.0 × 10 M) and Sc
-
5
6
[
(0-3.6) × 10 M] was visualized as shown in Figure 8a.
3+
The strong fluorescence due to the TPPZ-Sc complex was
3+
observed in the presence of more than 0.5 equiv of Sc at each
concentration of TPPZ.49 Dependence of the ratio of the
fluorescence intensity to the final fluorescence intensity (I/I∞)
3
+
on [Sc ] is shown in Figure 8b, where I/I∞ starts to increase at
(
47) TPPZ also acts as the “OFF-OFF-ON” fluorescence sensor for other metal
ions (see insets of Figures 1 and 2).
3+
-6
Figure 9. Dependence of ket on [Sc ] for electron transfer from CoTPP
(
48) The detection limit was determined to be 2.0 × 10 M in fluorescence
-
6
-3
3+
sensing of Sc3+, that is, detectable fluorescence was observed in an MeCN
(1.4 × 10
M) to O2 (air saturated, 2.6 × 10 M) in the presence of Sc
-
6
-6
-4
-4
solution of TPPZ (1.0 × 10 M) in the presence of 2.0 × 10 M of
and TPPZ [1.6 × 10 M (black closed circles), 3.2 × 10 M (red closed
3+
-4
Sc . Although the detection limit of this system is not completely
satisfactory, our concept should be general enough to apply other
fluorescence sensors that show stepwise complexation with metal ions. Thus,
the detection limit of this system would be improved by using other probe
molecules (including derivation of TPPZ) that have high affinities for metal
ions.
circles), and 4.8 × 10 M (blue closed circles)] in MeCN at 298 K.
Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5),
which is shifted to the larger value with increasing the initial
-5
concentration of TPPZ [3.4 × 10 M (black closed circles),
n+
n+
(
2
49) Formation of TPPZ-M and (TPPZ) -M is also dependent on the
-
5
-5
concentrations of TPPZ and metal ions. Typically, no sufficient 1:1 complex
4.7 × 10 M (red closed circles), and 5.7 × 10 M (blue
6 3+
n+
(
(
TPPZ-M ) is formed in the presence of low concentrations of TPPZ
closed circles)]. The dependence of I/I∞ on [Sc ] (Figure 8b)
agrees with that of ∆Abs/∆Abs∞ on [Sc ] (Figure 8c).
-6
n+
-6
[TPPZ] < 1.0 × 10 M) and metal ions ([M ] < 1.0 × 10 M). This
48
3+
is the detection limit of this system.
J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008 573

A R T I C L E S
Scheme 3
Yuasa and Fukuzumi
of 0-0.5 equiv of Sc3 ([Sc ]/[TPPZ]0 < 0.5) in a deaerated
+
3+
OFF-OFF-ON Switching in Metal Ion-Promoted Elec-
tron-Transfer Reduction of O2. We have also examined the
effects of stepwise complex formation of TPPZ with metal ions
in metal ion-promoted electron transfer from CoTPP to O2 (vide
infra).50 Since Sc acting as strongest Lewis acid forms the
-
4
MeCN solution of TPPZ (4.8 × 10 M) at 298 K. In the
3
+
3+
presence of more than 0.5 equiv of Sc ([Sc ]/[TPPZ] >
0
0.5), however, an efficient electron transfer from CoTPP to O2
occurs to yield CoTPP . The rate of electron transfer was
3+
+
2
:1 and 1:1 complexes stoichiometrically with TPPZ, the
determined by monitoring a decrease in absorbance at 412 nm
due to CoTPP and an increase in absorbance at 434 nm due to
electron-transfer reduction of O2 with CoTPP was examined in
the presence of different ratios of Sc3+ to TPPZ.
No electron transfer from CoTPP (Eox ) 0.35 V vs SCE)
to O2 (Ered ) -0.86 V vs SCE)52 has occurred in the presence
+
CoTPP . The rates obeyed pseudo-first-order kinetics in the
5
1
3+
presence of large excess of O2, TPPZ, and Sc relative to the
concentration of CoTPP (see the first-order plot in Supporting
Information S12). The pseudo-first-order rate constant (kobs)
increases proportionally with increasing O2 concentration. The
second-order rate constant of electron transfer (ket) starts to
(50) Metal ions acting as Lewis acids also accelerate the electron-transfer
•-
2 2
reduction of O , when metal ions bind with the product O ; see ref 42.
(
51) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459.
52) Sawyer, D. T.; Calderwood, T. S.; Yamaguchi, K.; Angelis, C. T. Inorg.
Chem. 1983, 22, 2577.
(
3
+
3+
increase at Sc concentrations above the 1:2 ratio ([Sc ]/
574 J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008

OFF
−OFF
−ON of Fluorescence and Electron Transfer
A R T I C L E S
n+
[
TPPZ]0 > 0.5) irrespective of the initial concentration of TPPZ
nor the 2:1 complex [(TPPZ)2-M ] exhibits fluorescence
-4
-4
[
1.6 × 10 M (black closed circles), 3.2 × 10 M (red closed
(OFF). With increasing concentrations of metal ions, the
complex is converted to the 1:1 complex
(TPPZ-M ) that exhibits strong fluorescence (ON). In the case
of Sc , which acts as the strongest Lewis acid among a series
of examined metal ions, the TPPZ-Sc complex is further
converted to the 1:2 complex [TPPZ-(Sc )2] at high concen-
tration range of Sc , which is also strongly fluorescent.
-
4
n+
circles), and 4.8 × 10 M (blue closed circles)] as shown in
Figure 9. Since Sc complexes of TPPZ [(TPPZ)2-Sc and
(TPPZ)2-M
3
+
3+
n+
3+
3+
TPPZ-Sc ] have no interaction with O2, the promoting effects
of Sc on electron transfer at Sc concentrations above the
1
formation of TPPZ-Sc with O2
of O2 and TPPZ-Sc should result in the positive shift of
3
+
3+
3+
3+
3+
:2 ratio ([Sc ]/[TPPZ]0 > 0.5) are ascribed to the complex
3
+
•- 53
3+
.
The complex formation
•
-
3+
3+
Similarly, neither TPPZ nor the (TPPZ)2-Sc complex has
the one-electron reduction potential of O2, leading to accelerate
the electron-transfer reduction of O2 (Scheme 3c). In contrast
ability to promote electron transfer from CoTPP to O2, which
50
is thermodynamically infeasible (OFF). However, the electron
3
+
3+
3+
to the case of TPPZ-Sc , (TPPZ)2-Sc (and TPPZ itself)
transfer starts to occur (ON) at Sc concentrations above the
•-
3+
3+
cannot bind with O2 , since (TPPZ)2-Sc has no binding site
for O2 (Scheme 3a,b). In such a case, no electron transfer
from CoTPP to O2 occurs at Sc concentrations below the 1:2
ratio ([Sc ]/[TPPZ]0 < 0.5). The 1:1 complex (TPPZ-Sc )
is further converted to the 1:2 complex [TPPZ-(Sc )2] in the
presence of a large excess of Sc (vide supra). In such a case,
TPPZ-(Sc )2 as well as excess free Sc accelerate the
1:2 ratio ([Sc ]/[TPPZ]0 > 0.5), when the 2:1 complex
•
-
3+
3+
(TPPZ)2-Sc is converted to the 1:1 complex (TPPZ-Sc )
3+
3+
and the 1:2 complex is converted to TPPZ-(Sc )2, which can
bind strongly with O2 . Thus, TPPZ acts as unique ligands for
3+
3+
•-
3+
metal ions, enabling “OFF-OFF-ON” switchability for metal
ion fluorescence sensors and metal ion-promoted electron-
transfer reactions.
3+
3+
3+
electron-transfer reduction of O2 by CoTPP (Scheme 3d), when
3+
Acknowledgment. This work was partially supported by
Grants-in-Aid (No. 19205019) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
the ket values further increase with increasing Sc concentra-
3+
tions above the 1:1 ratio ([Sc ]/[TPPZ]0 > 1). The amount of
excess free Sc3 should vary depending on the initial concentra-
+
3+
tion of TPPZ ([TPPZ]0), because formation of the TPPZ-(Sc )2
Supporting Information Available: UV-vis spectra of TPPZ
complex is not stoichiometric judging from the small formation
n+
in the presence of low and high concentrations of M (S1 and
2
-1
constant [K3 ) (2.2 ( 0.1) × 10 M ; see Supporting
S2, respectively), UV-vis spectra of TPPZ in the presence of
Information S3]. This causes some deviations with regard to
3
+
-4
-2
Sc (8.0 × 10 to 1.6 × 10 M) and the fluorescence
spectrum of TPPZ-(Sc )2 (S3), the optimized structures of
3+
54
slopes above the 2:1 ratio ([Sc ]/[TPPZ]0 > 2) (Figure 9).
3
+
2
+
2+
TPPZ-Mg and (TPPZ)2-Mg calculated by DFT at the
Summary and Conclusions
*
*
B3LYP/6-31G basis (S4), fluorescence titration of TPPZ by
n+
-1
We have demonstrated OFF-OFF-ON switching of fluo-
M
(S5), the derivation of eq 2 (S6), plots of (1 - R) vs
n+
rescence and electron transfer depending on stepwise complex
[M ]/R (S7), the derivation of eq 3 (S8) and eq 4 (S9), visible
n+
2+
formation of TPPZ with metal ions (M ). Neither TPPZ itself
fluorescence of TPPZ-Zn in water (S10), time profiles of
the fluorescence decay for TPPZ-Sc , TPPZ-Y , TPPZ-
Mg , and TPPZ-Ca (S11), and first-order plot for Sc -
promoted electron transfer from CoTPP to O2 in the presence
of TPPZ (S12). This material is available free of charge via the
Internet at http://pubs.acs.org.
3
+
3+
•
-
3+
2+
2+
3+
(
53) We have previously reported ESR spectra of O
2
-Sc complexes in fluid
solution. See: Fukuzumi, S.; Patz, M.; Suenobu, T.; Kuwahara, Y.; Itoh,
S. J. Am. Chem. Soc. 1999, 121, 1605.
(
54) The slopes would be the same irrespective of the initial concentration of
3
+
TPPZ at extremely high concentrations of Sc , in which electron transfer
3
+
is mainly promoted by excess free Sc . However, the rate of electron
transfer was too fast to be determined accurately at extremely high
3
+
concentrations of Sc
.
JA0748480
J. AM. CHEM. SOC.
9
VOL. 130, NO. 2, 2008 575