Amino(porphyrinato)antimony(v) complexes as a fluorosensor for selective and sensitive detection of trivalent metal cations

Article abstract of DOI:10.1246/bcsj.20090206

The additive effects of metal cations on the fluorescence quantum yield (φf) were investigated for weakemissive amino(tetraphenylporphyrinato) antimony(V) bromide (1). When the μmol dm^-3 of trivalent metal cations such as Al³⁺, Ga³⁺, In³⁺, and Lu ³⁺ were added to MeCN solution of 1, the φf was remarkably enhanced. The dependences of Soret band shift and φf upon the addition of trivalent metal cations revealed that a transition from S₂ state to the partially charge-shifted state was prevented by coordinating trivalent metal cations to the axial nitrogen atom of 1. 1 worked as a selective fluorosensor for detecting μmol dm^-3 trivalent metal cations.

Full text of DOI:10.1246/bcsj.20090206

Short Articles
Bull. Chem. Soc. Jpn. Vol. 83, No. 1, 49–51 (2010)
49
i-Pr
NH
Ph
N
ET
+
Amino(porphyrinato)antimony(V)
Complexes as a Fluorosensor for
Selective and Sensitive Detection
of Trivalent Metal Cations
Fluorophor*
(A*)
N
N
(D)
Ph
Sb
Ph
N
Mn+
Non-emissive
Br-
Ph
R
ET
1a; R = MeO- (Φfo = 0.0037)
1b; R = HO- (Φfo = 0.010)
1c; R = i-PrNH- (Φfo = 0.0042)
Fluorophor*
(A*)
Mn+
(D)
Emissive
Shin-ichiro Tsunami,1 Hirotaka Fujiwara,2
Jin Matsumoto,2 Tsutomu Shiragami,*2
and Masahide Yasuda2
Scheme 1. PET-fluorosensor and the structure of 1.
3.5
3.0
1Miyazaki Prefecture Environmental Science Association,
6258-20 Tayoshi, Miyazaki 880-0911
Φ
2Department of Applied Chemistry, Faculty of Engineering,
University of Miyazaki, Gakuen-Kibanadai,
Miyazaki 889-2192
2.0
1.0
Received August 7, 2009
E-mail: t0g109u@cc.miyazaki-u.ac.jp
0.0
none Li+ Na+ K+ Mg2+ Zn2+ Al3+ Ga3+ In3+ Lu3+ H+
The additive effects of metal cations on the fluores-
cence quantum yield (Φf) were investigated for weak-
emissive amino(tetraphenylporphyrinato)antimony(V) bro-
Figure 1. Maximum Φf of 1a in the presence of Mn+
.
¹3
mide (1). When the ¯mol dm of trivalent metal cations
594 and 645 nm in MeCN in 0.0037 of quantum yield for
fluorescence (Φf0). This value was much smaller than that of
dihydroxo(tetraphenylporphyrinato)antimony(V) bromide (2)
(Φf0 = 0.0518) without D.14 Low Φf0 can be attributed to
the transition from S2 to CS states which has the character
of paramagnetic SbIV(tpp).14,15 The Φf0 of 1b was relatively
large compared with 1a. Probably, a proton transfer from
axial hydroxo to amino ligand occurred partially to form the
such as Al3+, Ga3+, In3+, and Lu3+ were added to MeCN
solution of 1, the Φf was remarkably enhanced. The
dependences of Soret band shift and Φf upon the addition
of trivalent metal cations revealed that a transition from S2
state to the partially charge-shifted state was prevented by
coordinating trivalent metal cations to the axial nitrogen atom
of 1. 1 worked as a selective fluorosensor for detecting
¹3
¯mol dm trivalent metal cations.
emissive zwitterion ([i-PrNH2+-Sb(tpp)-O ] Br ), because a
¹ +
¹
hydroxo proton was acidic (pKa = 10.7).16
By the addition of M(ClO4)3 (M3+ = Al3+, Ga3+, In3+, and
Lu3+) and HClO414 to an MeCN solution of 1a, the fluorescence
quantum yield (Φf) increased up to >0.029 at maximum points.
The fluorescence enhancement factor (FE),11 which is defined as
Φf/Φf0, were relatively high values (FE = 7.8-8.6) for 1a
compared to a reported naphthalimide fluorosensor toward
trivalent metal cations. However, Φf remained unchanged upon
Fluorosensors have received much attention from the view-
points of chemical, biological, and environmental sciences.1-8
Most sensing mechanism of fluorescence are based on photo-
induced electron transfer (PET).9-13 PET-fluorosensors are
normally constructed from an electron-accepting fluorophore
(A) which is linked by a spacer to an electron-donating site (D).
Usually fluorescence of A disappears through ET-quenching by
D. In the presence of analytes such as metal ions or cationic
molecules, however, they disturb the PET, resulting in
fluorescence from A (Scheme 1). Recently, we have success-
fully synthesized amino(methoxo)(tetraphenylporphyrinato)-
antimony(V) (1a) which has an amino group as an axial
ligand.14 Weak fluorescence appears from 1a under S2-excita-
tion because of the contribution of non-emissive charge-shifted
(CS) state.14 Therefore, 1a is the simplest directly connected
D-A system (Scheme 1). However, the question of whether the
CS state of 1a is controlled by a guest molecule has arisen.
Here, we will investigate the ability of 1a and its analogous 1b
and 1c as PET-fluorosensors for metal cations.
addition of mono- and divalent ions such as Li+, Na+, K+, Zn2+
,
and Mg2+ (Figure 1). Moreover, a dependence of Φf on the
concentration of M3+ was observed. A typical example is the
dependence of the Φf of 1a on ¯M concentration of Al3+
(Figure 2). The sensitivity toward M3+ was evaluated by the
concentrations (C1/2F) required to enhance up to the half of the
maximum fluorescence intensity. The C1/2F of 1a were 3.6 ¯M
for Al3+, 3.9 ¯M for Ga3+, 3.6 ¯M for In3+, 3.6 ¯M for Lu3+
,
and 6.1 ¯M for H+, respectively (Table 1). Also the Φf of 1b
gave smaller C1/2F values than that of 1a, as shown in Table 1.
In the case of 1c, double changes were observed at 0.80 and
F
F
13 ¯M of C1/2 (Figure 3). The C1/2 values of 1a-1c are
summarized in Table 1.
¹6
Under Soret band excitation of 1a (0.5 ¯M, ¯M = 10
mol dm¹3) at 420 nm, a weak S1 fluorescence was observed at
Since the S2 excited state is too short-lived to interact with
¯M of M3+, it should be taken into account that M3+ interact
Published on the web December 18, 2009; doi:10.1246/bcsj.20090206

50
Bull. Chem. Soc. Jpn. Vol. 83, No. 1 (2010)
© 2010 The Chemical Society of Japan
3.0
2.0
3.0
(B)
(A)
4.0
(A)
(B)
Φ
3.0
2.0
Φ
Φ
2.0
1.0
0.0
1.0
0.0
1.0
500
600
700
0
5
10
15
20
[Al(ClO4)3] / 10-6 mol dm-3
Wavelength / nm
Figure 2. (A) Spectral change of fluorescence spectra of 1a
when 0-20 ¯mol of Al(ClO4)3 was added to MeCN
solution of 1a. (B) Plots of Φf vs. [Al(ClO4)3] in 1a ( )
and 1b ( ).
0.0
0
1
2
3
4
5
0
10
[Al(ClO4)3] / 10-6 mol dm-3
20
30
40
50
[Al(ClO4)3] / 10-6 mol dm-3
Figure 3. Dependence of Φf of 1c on [Al3+].
Table 1. C1/2 Values of 1 toward M3+ and H+ Ions
C1/2F/10¹6 M (C1/2A/10¹6 M)d)
1
Φf0 a)
Eox/Vb)
Ered/Vc)
Al3+
Ga3+
In3+
Lu3+
H+
1a
1b
1c
0.0037
0.0100
0.0042
1.59
®
¹0.79
¹0.81
¹0.83
3.6 (3.2)
2.6 (3.2)
0.8
3.9 (3.9)
3.0 (9.8)
0.6
3.6 (3.5)
2.3 (5.9)
1.5
3.6 (3.2)
2.3 (2.3)
2.2
6.1 (10.2)
4.4 (10.0)
3.0
®
a) The fluorescence quantum yields in the absence of additive. b) The oxidation potentials vs. Ag/Ag+ (See Ref. 14).
F
c) The reduction potentials vs. Ag/Ag+. d) C1/2 is the concentration required to enhance up to half maximum
determined by dependence on fluorescence quantum yields. The values in parenthesis were C1/2A determined from on
absorption spectra.
metal cations, might only lead to the selective interaction with
an axial amino ligand on 1. The concentrations (C1/2A) required
to show half the maximum difference absorbance (¦A) were
4.5
4.0
0.3
0.2
0.1
0
3.0
A
determined, as shown in Table 1. These C1/2 values are in
2.0
1.0
F
good agreement with C1/2 values. Therefore, it suggested that
the dependence of Φf on [M3+] could be attributed to the
complexation of 1 with M3+ in the ground state.
0.0
400
410
420
430
440
450
0
5
10
15
20
Wavelength / nm
[Al(ClO4)3] / 10-6 mol dm-3
The energy level of the charge-separated state of 1a was
estimated to be 0.53 eV lower than the S2 state (ES2 = 2.92 eV)
by Rehm-Weller calculation assuming that Eox of i-PrNH
ligands equaled that of n-PrNH2 (Eox = 1.60 V).17 Under
excitation of the Q-band at 550 nm, however, the fluorescence
quenching occurred, although the energy level of the charge
separated state of 1a was higher than that of the S1 state
(ES1 = 2.08 eV). It is suggested that the charge transfer in the
Sb-N bond to make the partially CS state in the ground state,
judging from the -max (421.8 nm) of 1a appearing at longer
wavelength compared with that of 2 (417 nm). On the other
hand, in the presence of the proton and trivalent metal cations,
the selective formation of 1/M3+ prevents the formation of
partial CS state in the ground state (Scheme 2).
Figure 4. Spectral change of absorption spectra of the Soret
band of 1a when 0-20 ¯M of Al(ClO4)3 was added to
MeCN solution of 1a and a plot of ¦A versus [Al3+].
with 1 in the ground state.15 Figure 4 shows the dependence of
absorption spectra of 1a on [Al3+]. With an increase of [Al3+],
absorbance (A) increases along the red shift of -max from 421.8
to 422.8 nm. An isosbestic point appears at 419.1 nm. Fur-
thermore, the reduction potential of 1a shifts positively
direction (¦Ered = 0.26 V) in the presence of Al3+. These
results suggest that the complex between 1 and Al3+ (1/Al3+
)
should be formed in the ground state through the coordination
of M3+ with the nitrogen lone pair of the axial amino ligand.
From Hill plot analysis based on Figure 4, the stoichiometric
ratio of the complex (1a/Al3+) was determined to be 1:1 (see
Supporting Information). On the other hand, M+ and M2+
cations affected neither the Soret band nor reduction potential
of 1 at all (see Supporting Information). Furthermore, neither
spectral change of the Soret band nor the shift of reduction
potential was observed in the case of 2. The basicity of the
axial amino ligand of 1a was very poor since the acid
dissociation constant (pKa) for the conjugated acid of the axial
amino ligand was reported to be 4.41, a value smaller than that
of aniline (pKa = 4.63).14 Accordingly, trivalent metal cations,
which are more strongly Lewis acidic than mono- or divalent
In conclusion, we elucidated that 1 operated as a selective
fluorosensor to detect M3+ ions such as Al3+, Ga3+, In3+, and
Lu3+ on a ¯M-order.
Experimental
Instruments.
UV-vis absorption spectra and fluorescence
spectra of solutions were measured on a Hitachi U2001 spec-
trometer and on a Hitachi F4500 spectrometer, respectively.
1H NMR spectra were taken in CDCl3 using Me4Si as an inter-
nal standard on a Bruker AC 250P spectrometer at 250 MHz.
SIMS spectra were obtained on a Hitachi M-2000AM spectro-
photometer. All chemicals were of the best commercial grades
available.

S. Tsunami et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 1 (2010)
51
partially charge-shifted state
(non-emissive)
S1 and S2 state
(emissive)
Measurements of Fluorescence Quantum Yields.
The
concentration of 1 in MeCN solutions was adjusted for the
absorbance to be less than 0.1 at the excitation wavelength
(420 nm). The fluorescence quantum yields (Φf) were determined
by using an MeCN solution of 2 (Φf = 0.0518) as an actinom-
eter.14
δ+ NHPri
Mn+
NHPri
SbV(tpp)*
R
SbV(tpp)
δ-
R
Measurements of Redox Potentials.
The oxidation and
reduction potentials of a dried MeCN solution of 1 (1 © 10¹2 M) in
the presence of a supporting electrolyte (Et4NBF4; 0.1 M) were
hν`
Fluorescence
h
ν
hν
hν` (weak)
¹1
measured by cyclic voltammetry at a scan rate of 300-500 mV s
+
NHPri
Mn+
NHPri
NHPri
at 25 °C on a BAS CV-50W cyclic voltammeter using a carbon-
disk working electrode, a Pt counter electrode, and an Ag/AgNO3
reference electrode.
Mn+
SbV(tpp)
SbV(tpp)
R
SbIV(tpp)
R
R
1/Mn+
Supporting Information
1
The Hill plot in the case of 1a and Al3+ and the absorption
spectra of 1a in the presence of M+ and M2+. This material is
available free of charge on the web at http://www.csj.jp/journals/
bcsj/.
Scheme 2. Plausible mechanism on enhancement of Φf.
Br
Br
+
+
Ph
Ph
Ph
Ph
N
N
N
N
Sb
N
Sb
ROH
N
N
N
Ph
Ph
References
Br-
MeCN
reflux
Ph
Ph
Br-
OR
Br
1
A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
4a; R = Me (90%)
4b; R = H (75%)
3
A. J. M. Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem.
Rev. 1997, 97, 1515.
i-Pr-NH2
MeCN, room temp.
i-Pr-NH2
MeCN, room temp
2
A. P. de Silva, B. McCaughan, B. O. F. McKinney, M.
Querol, Dalton Trans. 2003, 1902.
1b (1.3%) + 1c (1.2%)
1a (76%), 1b (68%)
3
4
P. Jiang, Z. Guo, Coord. Chem. Rev. 2004, 248, 205.
K. Rurack, U. Resch-Genger, Chem. Soc. Rev. 2002, 31,
Scheme 3. Synthesis of 1.
116.
5
V. Balzani, J. van de Loosdrecht, A. Mecati, N. Sabbatini,
Preparation of 1. An MeCN solution (50 cm3) containing 4b
(1.1 mmol) and i-PrNH2 (2 cm3) was stirred for 4 h at room
temperature. The solvent was evaporated and the residue was
dissolved in CH2Cl2. The CH2Cl2 solution was washed three times
with 50 cm3 portions of H2O. After evaporation, the crude product
was chromatographed on silica gel (Fuji Silysia BW-300) using
CHCl3-MeOH (10:1 v/v) as an eluent to give 1b (Scheme 3). The
synthesis of 1c was performed by stirring an MeCN solution
(50 cm3) containing 3 (1.1 mmol) and i-PrNH2 (2 cm3) and
purifying by the same procedures as with 1b. The spectral data
J.-M. Lehn, R. Ziessel, Angew. Chem., Int. Ed. Engl. 1991, 30,
190.
6
S. Aoki, H. Kawatani, T. Goto, E. Kimura, M. Shiro, J. Am.
Chem. Soc. 2001, 123, 1123.
7
13922.
8
K. Okamoto, S. Fukuzumi, J. Am. Chem. Soc. 2004, 126,
A. P. de Silva, H. Q. N. Gunaratne, C. P. McCoy, Nature
1993, 364, 42.
9
B. Ramachandram, G. Saroja, N. B. Sankaran, A. Samanta,
J. Phys. Chem. B 2000, 104, 11824.
1
of 1 are shown below. On H NMR measurements, peaks of NH
10 J. Yuasa, S. Fukuzumi, J. Am. Chem. Soc. 2008, 130, 566.
11 D. Staneva, I. Grabchev, J.-P. Soumillion, V. Bojinov,
J. Photochem. Photobiol., A 2007, 189, 192.
12 H.-Y. Luo, J.-H. Jiang, X.-B. Zhang, C.-Y. Li, G.-L. Shen,
R.-Q. Yu, Talanta 2007, 72, 575.
proton in 1 were not all observed due to extremely broad lines. The
preparation of 1a from 4a was already reported in a previous
paper.14
Hydroxo(isopropylamino)(tetraphenylporphyrinato)antimony(V)
bromide (1b). Yield 68% from 4b; SIMS: m/z 808 (M+); 1H NMR
(CDCl3): ¤ ¹3.92 (1H, m, Sb-N-CH-), ¹2.29 (6H, d, J = 6.2 Hz,
-C(CH3)2), 7.82-7.88 (12H, m, Ph), 8.27 (4H, d, J = 6.4 Hz, Ph),
8.58 (4H, d, J = 6.4 Hz, Ph), 9.33 (8H, s, pyrrole).
Di(isopropylamino)(tetraphenylporphyrinato)antimony(V) bro-
mide (1c). Yield 1.2% from 3; SIMS: m/z 847 (M+ ¹ 2); 1H NMR
(CDCl3): ¤ ¹4.01 (2H, m, Sb-N-CH-), ¹2.36 (12H, d, J =
6.2 Hz, -C(CH3)2), 7.87-7.96 (12H, m, Ph), 8.31 (8H, m, Ph), 9.40
(8H, s, pyrrole).
13 Y. Andou, K. Shima, T. Shiragami, M. Yasuda, Chem. Lett.
2001, 1198.
14 S. Tsunami, K. Tanaka, J. Matsumoto, T. Shiragami, M.
Yasuda, Bull. Chem. Soc. Jpn. 2008, 81, 583.
15 M. Fujitsuka, D. W. Cho, T. Shiragami, M. Yasuda, T.
Majima, J. Phys. Chem. B 2006, 110, 9368.
16 T. Shiragami, Y. Andou, Y. Hamasuna, F. Yamaguchi, K.
Shima, M. Yasuda, Bull. Chem. Soc. Jpn. 2002, 75, 1577.
17 C. K. Mann, Anal. Chem. 1964, 36, 2424.