Zinc-specific fluorescent response of tris(isoquinolylmethyl)amines (isoTQAs)

Article abstract of DOI:10.1021/ic202159v

Isoquinoline-based tetradentate ligands with C₃-symmetry, tris(1- or 3-isoquinolylmethyl)amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (β = ～0.003) in DMF/H₂O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (β = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (β = 0.017, λ_em = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn-N_aromatic distances compared with isoTQENs (N,N,N′,N′-tetrakis(isoquinolylmethyl)ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium.

Full text of DOI:10.1021/ic202159v

Article
pubs.acs.org/IC
Zinc-Specific Fluorescent Response of Tris(isoquinolylmethyl)amines
(isoTQAs)
Yuji Mikata,†,* Keiko Kawata,‡ Satoshi Iwatsuki,§ and Hideo Konno⊥
‡
†KYOUSEI Science Center and Department of Chemistry, Faculty of Science, Nara Women’s University, Nara 630-8506, Japan,
§Department of Chemistry of Functional Molecules, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto,
Higashinada, Kobe 658-8501, Japan, and
⊥National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
S
* Supporting Information
ABSTRACT: Isoquinoline-based tetradentate ligands with
C3-symmetry, tris(1- or 3-isoquinolylmethyl)amine (1- or
3-isoTQA), have been prepared and their zinc-induced fluore-
scence enhancement was investigated. Upon excitation at 324 nm,
1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in
DMF/H2O (1/1) solution. In the presence of zinc ion,
1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and
470 nm. This fluorescence enhancement at 470 nm is specific
for zinc. However, 3-isoTQA exhibited a smaller fluorescence
enhancement upon zinc complexation (ϕ = 0.017, λem = 360
and 464 nm) compared with 1-isoTQA. Crystal structures of
zinc complexes of isoTQAs demonstrate the diminished steric
crowding and shorter Zn−Naromatic distances compared with isoTQENs (N,N,N′,N′-tetrakis(isoquinolylmethyl)ethylenediamines)
leads to a higher fluorescent response toward zinc relative to cadmium.
Chart 1
INTRODUCTION
■
Zinc ions are indispensable metal ions in living systems. Zinc
plays many important structural and catalytic roles in enzymes
and is one of the key components in cellular processes includ-
ing gene expression and signal transduction.1−5 Mobile zinc
pools that exist in living cells in many organisms but their exact
roles are still not fully understood. Fluorescent zinc sensor
molecules have been extensively developed in recent years,
aiming at the visualization and/or quantification of the dynamic
and transient zinc ion distribution inside the cell.4−29 Although
several well-known mechanisms altering the fluorescence pro-
perties of the probe molecules upon guest binding have been
established,30−32 there is still a need for rational design strate-
gies for specific metal ions recognition in high selectivity and
appropriate sensitivity.
by its copper complex (Chart 1).60,61 Subsequently, Hancock
and co-workers reported the zinc specific fluorescent response
of TQA and discussed its fluorescent intensity difference be-
tween zinc(II) and cadmium(II) complexes based on the dif-
ferential CHEF (chelation enhanced fluorescence) effect that is
mainly controlled by steric effects upon complexation.54 TQA
exhibits lower cadmium response (ICd/IZn) compared with
TQEN because of reduced steric hindrance that allows efficient
CHEF mechanism in TQA−Zn complex.
We have previously reported that N,N,N′,N′-tetrakis-
(2-quinolylmethyl)ethylenediamine (TQEN) can act as a fluore-
scent zinc detection molecule (Chart 1).33 The structure of
TQEN is based on well-known heavy metal chelator, N,N,
N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). The
1- and 3-isoTQEN, isoquinoline derivatives of TQEN (Chart 1),
exhibited improved properties compared to TQEN.34 In recent
years, many other quinoline-based fluorescent zinc sensors have
been developed;34−58 however, very few isoquinoline-based fluore-
scent sensor molecules have been developed.59
TQA, tris(2-quinolylmethyl)amine, is an analogue of TPA,
tris(2-pyridylmethyl)amine and was first prepared by Karlin
and co-workers to investigate the dioxygen activation chemistry
Received: October 5, 2011
Published: January 19, 2012
© 2012 American Chemical Society
1859
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
methanol, and the solution was kept at room temperature afforded
white powder. Yield, 40%.
In this article, we report the zinc-induced fluorescence
enhancement of isoquinoline derivatives of TQA, 1- and
3-isoTQA (tris(1- or 3-isoquinolylmethyl)amines) (Chart 1).
3-IsoTQA has been prepared by Canary and co-workers and its
zinc-binding property was investigated by ESI-MS;62 however, no
fluorescent response toward zinc has been reported to date. This
article reports the synthesis and evaluation of the fluorescent prop-
erties of 1- and 3-isoTQA as a new scaffold for fluorescent zinc
sensor molecules. Reduced steric hindrance of isoTQA−Zn
complexes suppresses the cadmium response (ICd/IZn) compared
with isoTQENs, in parallel with the TQA/TQEN system.
1H NMR (DMSO-d6): δ 8.59 (d, J = 6.3 Hz, 3H), 8.28 (d, J =
8.4 Hz, 3H), 8.09 (d, J = 8.1 Hz, 3H), 8.03 (d, J = 6.0 Hz, 3H), 7.92
(dd, J = 7.2, 7.8 Hz, 3H), 7.83 (dd, J = 6.9, 7.5 Hz, 3H), 5.32 (s, 6H).
13C NMR (DMSO-d6): δ 155.6, 138.0, 136.0, 132.6, 128.8, 127.4,
125.3, 124.9, 122.3, 60.4.
Anal. Calcd for C30H27Cl2N4O9.5Zn ([Zn(1-isoTQA)(H2O)]-
(ClO4)2·0.5H2O): H, 3.72; C, 49.23; N, 7.66. Found: H, 3.52; C,
48.98; N, 7.59.
[Zn(3-isoTQA)(H2O)](ClO4)2. In a chloroform suspension of
3-isoTQA was added equimolar amount of Zn(ClO4)2·6H2O in
methanol, and the solution was kept at room temperature afforded
white powder. Yield, 40%.
1H NMR (CD3OD): δ 9.50 (s, 3H), 8.21 (d, J = 8.1 Hz, 3H), 7.99
(s, 3H), 7.95 (d, J = 8.1 Hz, 3H), 7.88 (dd, J = 6.6, 7.2 Hz, 3H), 7.74
(dd, J = 7.2, 7.8 Hz, 3H), 4.56 (s, 6H).
EXPERIMENTAL SECTION
■
General. All reagents and solvents used for synthesis were from
commercial sources and used as received. N,N-Dimethylformamide
(DMF, Dojin) was spectral grade (Spectrosol). All aqueous solution
13C NMR (CD3OD): δ 153.9, 147.3, 138.6, 135.1, 130.4, 130.0,
129.7, 127.8, 123.0, 59.5.
1
was prepared using Mili-Q water (Milipore). H NMR (300 Hz) and
13C NMR (75.5 Hz) spectra were recorded on a Varian GEMINI 2000
spectrometer and referenced to internal Si(CH3)4 or solvent signals.
UV−vis and fluorescence spectra were measured on a Jasco V-660
spectrophotometer and Jasco FP-6300 spectrofluorometer, respectively.
Fluorescence quantum yields were measured on a HAMAMATSU
photonics C9920−02 absolute PL quantum yield measurement system.
CAUTION: Perchlorate salts of metal complexes with organic ligands are
potentially explosive. All due precautions should be taken.
Anal. Calcd for C30H29Cl2N4O10.5Zn ([Zn(3-isoTQA)(H2O)](ClO4)2·
1.5H2O): C, 48.05; H, 3.90; N, 7.47. Found: C, 47.69; H, 3.56; N, 7.67.
X-ray Crystallography. Single crystals of 1-isoTQA were obtained
from CH3OH at 4 °C. Single crystals of [Zn(1-isoTQA)(CH3OH)]-
(ClO4)2·2CH3OH and [Zn(3-isoTQA)(H2O)](ClO4)2·2CHCl3 were
obtained by recrystallization from CHCl3−CH3OH (1:1) at 4 °C
under ether diffusion condition. These crystals were covered by Paraton-N
oil and mounted on a glass fiber. All data were collected at 123 K on a
Rigaku Mercury CCD detector, with monochromatic MoKα radiation,
operating at 50 kV/40 mA. Data were processed on a PC using
CrystalClear software (Rigaku). Structures were solved by direct
methods (SIR-92)64 and refined by full-matrix least-squares methods
on F2 (SHELXL-97).65 Crystal data are summarized in Table S1 of the
Supporting Information. CCDC-846307−846309 contain the supple-
mentary crystallographic data for this article. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/datarequest/cif.
N-(1-isoquinolylmethyl)phthalimide (2). To the DMF solution
(11 mL) of 1-chloromethylisoquinoline34,63 (1) (265 mg, 1.49 mmol)
was added potassium phthalimide (278 mg, 1.50 mmol) and stirred
overnight at room temperature. After addition of chloroform, the
organic layer was washed with water and 10% NaOHaq. The organic
layer was dried, evaporated, and washed with hot ethanol to give 2 as
white powder. Yield, 303 mg (1.05 mmol, 70%).
1H NMR (CDCl3): δ 8.31 (d, J = 5.7 Hz, 1H), 8.21 (d, J = 7.8 Hz,
1H), 7.90−7.93 (m, 2H), 7.83 (d, J = 8.4 Hz, 1H), 7.65−7.76 (m, 4H),
7.52 (d, J = 5.7 Hz, 1H), 5.54 (s, 2H).
13C NMR (CDCl3): δ 168.5, 153.0, 141.9, 136.1, 133.9, 132.5,
130.1, 127.5, 125.9, 123.7, 123.5, 120.3, 40.7.
RESULTS AND DISCUSSION
■
Ligand Synthesis. 1-IsoTQA and 3-isoTQA62 were
synthesized from corresponding chloromethylisoquinolines via
Gabriel amine synthesis followed by N-alkylation with 2 equiv
of chloromethylisoquinoline (Scheme 1). All new compounds
Anal. Calcd for C18H12N2O2 (2): H, 4.20; C, 74.99; N, 9.72. Found:
H, 4.19; C, 74.85; N, 9.65.
1-Aminomethylisoquinoline (3). To the methanol solution
(11 mL) of N-(1-isoquinolylmethyl)phthalimide (2) (132 mg, 0.46
mmol) was added hydrazine monohydrate (0.38 mL, 7.8 mmol) and
refluxed for 1.5 h. After addition of water, the insoluble materials were
filtered off. The filtrate was acidified with hydrochloric acid and
filtered. The filtrate was neutralized with NaOHaq and extracted with
ethyl acetate. The organic layer was dried and evaporated to give 3 as
yellow oil. Yield, 46 mg (0.29 mmol, 63%).
1
were characterized by H and 13C NMR, and the purity of the
final compounds was ensured by elemental analysis. X-ray cry-
stallography further confirms the structure of 1-isoTQA in the
crystalline state (Figure 1).
UV−vis and Fluorescence Spectral Changes of
1-isoTQA Induced by Zinc. A 34 μM solution in aqueous
DMF (DMF/H2O = 1:1) at 25 °C was used for spectral
measurements for 1-isoTQA. Upon addition of zinc ion, the
UV−vis ligand absorption at 324 nm decreased and new peaks
at 315 and 325 nm appeared (part a of Figure 2). Distinct
isosbestic points were seen at 313, 318, and 327 nm during the
titration and spectral changes stopped at the point where 1 eq
of zinc ion was added, indicating the exclusive formation of 1:1
complex for 1-isoTQA and the zinc ion (part a of Figure S1 of
the Supporting Information).
Part b of Figure 2 shows the fluorescence spectral change of
1-isoTQA with increasing amount of zinc ion added. Although
1-isoTQA emits negligible fluorescence in the absence of zinc
ion upon excitation at 324 nm, the fluorescence increased at
359 (49-fold) and 470 nm (21-fold) respectively in the
presence of 1 equiv of zinc ion. The fluorescence quantum yield
of zinc complex of 1-isoTQA (ϕ = 0.041) is higher than that of
1-isoTQEN−Zn complex (ϕ = 0.034). On the bases of the
number of aromatic rings in 1-isoTQA (trisisoquinoline) and
1H NMR (CDCl3): δ 8.45 (d, J = 5.4 Hz, 1H), 8.08 (d, J = 8.1 Hz,
1H), 7.81 (d, J = 7.8 Hz, 1H), 7.50−7.70 (m, 3H), 4.49 (s, 2H).
13C NMR (CDCl3): δ 159.9, 141.2, 135.7, 129.7, 127.1, 127.0,
125.7, 123.8, 119.6, 44.6.
Tris(1-isoquinolylmethyl)amine (1-isoTQA). To the acetonitrile
solution (90 mL) of 1-chloromethylisoquinoline (1) (501 mg, 2.82
mmol) and 1-aminomethylisoquinoline (3) (223 mg, 1.41 mmol) was
added potassium carbonate (1.05 g, 7.60 mmol) and stirred for 4 days
under reflux. After removal of the solvent, the residue was extracted
with chloroform/water. The organic layer was dried, evaporated,
and washed with acetonitrile to give 1-isoTQA as white powder. Yield,
170 mg (0.39 mmol, 27%).
1H NMR (CDCl3): δ 8.49 (d, J = 5.7 Hz, 3H), 7.75 (d, J = 8.4 Hz,
3H), 7.58 (d, J = 6.0 Hz, 3H), 7.47 (dd, J = 6.9, 8.1 Hz, 3H), 6.98
(d, J = 8.7 Hz, 3H), 6.58 (dd, J = 6.9, 8.4 Hz, 3H), 4.35 (s, 6H).
13C NMR (CDCl3): δ 158.0, 141.4, 136.0, 129.5, 127.3, 126.5,
125.9, 120.6, 60.0.
Anal. Calcd for C30H24.4N4O0.2 (1-isoTQA·0.2H2O): H, 5.54; C,
81.13; N, 12.61. Found: H, 5.33; C, 80.98; N, 12.53.
[Zn(1-isoTQA)(H2O)](ClO4)2. In a chloroform suspension of
1-isoTQA was added equimolar amount of Zn(ClO4)2·6H2O in
1860
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
a
Scheme 1. Synthesis of 1- and 3-isoTQA
a
Reagents: (i) potassium phthalimidate; (ii) hydrazine; (iii) 1 (2 equiv), K2CO3; (iv) 4 (2 equiv), NaHCO3.
quantum yield (ϕ) of the 3-isoTQA−Zn complex (0.017) is
less than half of the 1-isoTQA−Zn complex (0.041) (Figure S3
of the Supporting Information). This is in good agreement with
our previous results in which 1-isoTQEN−Zn complex
exhibited a higher quantum yield in comparison to the
3-isoTQEN−Zn complex.34 This point is discussed further
based on the crystal structure analysis (below).
X-ray Crystallography of the Zinc Complexes of
1- and 3-isoTQA. Single crystals of the zinc complexes of
both 1- and 3-isoTQA were obtained from methanol/chloroform
(1:1) as a perchlorate salt and analyzed by X-ray crystallography.
Crystal data are summarized in Table S1 of the Supporting
Information. Figures 4 and 5 show the crystal structures of the
cations of [Zn(1-isoTQA)(CH3OH)](ClO4)2·2CH3OH and
[Zn(3-isoTQA)(H2O)](ClO4)2·2CHCl3. Table S2 of the Sup-
porting Information lists the bond distances around zinc ion.
The zinc complexes of 1- and 3-isoTQA adopt trigonal
bipyramidal geometry (τ = 0.76 for 1-isoTQA−Zn complex and
0.95 for 3-isoTQA−Zn complex) and no significant steric
hindrance is found in the interatomic distances and angles around
the zinc center. Between 1- and 3-isoTQA−Zn complexes,
there are considerable differences in the coordination distances
between the zinc ion and isoquinoline nitrogen atoms. The
average Zn−Naromatic bond length is 2.045 Å for the 1-isoTQA−
Zn complex and 2.080 Å for the 3-isoTQA−Zn complex. The
former value is comparable with that of [Zn(TPA)(CH3CN)]-
(ClO4)2, 2.0475 Å.66 This may account for the difference in
the fluorescence intensity of the zinc complexes with 1- and
3-isoTQA. In addition, both isoTQA complexes have shorter
Zn−Naromatic distances than 1-isoTQEN (2.1506 Å) and
3-isoTQEN (2.1590 Å).34 These differences are well correlated
with the fluorescence intensity of the zinc complexes, affording
efficient CHEF in the more tightly bound complexes em-
poloying the isoTQA ligand framework (Table 1).
Figure 1. ORTEP plot for 1-isoTQA in 50% probability. Asterisks
indicate the atoms generated by the symmetric operation. Hydrogen
atoms were omitted for clarity.
1-isoTQEN (tetrakisisoquinoline), 1-isoTQA is a more efficient
fluorophore (1.6-fold based on each isoquinoline ring) in com-
parison to 1-isoTQEN. The fluorescence increase stopped at
the point where 1 eq of zinc ion was added, supporting the
formation of fluorescent 1:1 ZnL complex (part b of Figure S1
of the Supporting Information).
UV−vis and Fluorescence Spectral Changes of
3-isoTQA Induced by Zinc. Addition of zinc ion to the solu-
tion of 3-isoTQA in DMF/H2O (1:1) changes UV−vis and fluore-
scence spectra (Figure 3). In the absorption spectra, λmax was
changed from 314 and 324 nm to 318 and 328 nm and
isosbestic points were observed at 313, 323, and 326 nm. Upon
excitation at 324 nm, free ligand exhibited negligible fluore-
scence, and addition of zinc enhances the intensity at 360 and
464 nm, 9- and 6-fold respectively. The zinc-induced spectral
changes in both spectra stopped after 1 eq of zinc was added
(Figure S2 of the Supporting Information). The fluorescent
Fluorescence Spectral Change of 1- and 3-isoTQA
Induced by Other Metal Ions. Figure 6 shows the metal ion
specificity of the fluorescent response of 1- and 3-isoTQA
1861
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
Figure 2. (a) UV−vis absorption and (b) fluorescence (λex = 324 nm) spectra of 34 μM 1-isoTQA in DMF/H2O (1:1) at 25 °C in the presence of
various concentration of Zn2+ ranging from 0 to 68 μM.
Figure 3. (a) UV−vis absorption and (b) fluorescence (λex = 324 nm) spectra of 34 μM 3-isoTQA in DMF/H2O (1:1) at 25 °C in the presence of
various concentration of Zn2+ ranging from 0 to 68 μM.
Figure 4. ORTEP plot for [Zn(1-isoTQA)(CH3OH)](ClO4)2·2CH3OH in 50% probability. Atoms of counteranions, hydrogen, and solvents were
omitted for clarity.
monitored at 470 and 464 nm, respectively. The fluorescence
enhancement was specific for the zinc ion. Cadmium exhibited
moderate fluorescence enhancement with 1-isoTQA at short
wavelength (∼360 nm), however, negligible response at lower
energy emission (∼470 nm) was observed (Figure S4 of the
Supporting Information). The difference in radius between
Cd2+ (0.96 Å) and Zn2+ (0.74 Å), and thus the overall structure,
alters the CHEF effect on the isoTQA chromophores especially
the isoquinoline−isoquinoline interaction that generates the
emission at ca. 470 nm. Interestingly, the emission spectrum of
the 1-isoTQA−Cd complex is similar to that of the 3-isoTQA−
Zn complex except for the long-wavelength emission in the
3-isoTQA−Zn complex (Figures S3 and S4 of the Supporting
Information). For the Zn and Cd complexes, the CHEF effect
on the isoquinoline chromophore is similar, but the inter-
isoquinoline interaction to generate the ∼470 nm fluorescence
is absent for cadmium complex.
In the presence of an equimolar amount of Fe2+, Co2+, Ni2+,
Cu2+, and Cd2+, the afterward addition of zinc did not enhance the
fluorescence of 1- and 3-isoTQA. On the other hand, the fluores-
cence of 1-isoTQA−Zn complex was scarcely affected by following
addition of 1eq of any metals listed in Figure 6 in several minutes.
1862
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
Figure 5. ORTEP plot for [Zn(3-isoTQA)(H2O)](ClO4)2·2CHCl3 in 50% probability. Atoms of counteranions, hydrogen, and solvents were
omitted for clarity.
Table 1. Structural and Fluorescent Properties for Zinc
Complex of TPEN, 1-isoTQEN, 3-isoTQEN, TQEN, TPA,
1-isoTQA, 3-isoTQA, and TQA
correlated with the efficiency of CHEF (Table 1). The TQEN−
Zn complex exhibits significant steric crowding arising from
proximate quinoline rings to yield extremely long Zn−Naromatic
distances (Zn−N = 2.1543, 2.4007, 2.1271, and 2.3711 Å).33 In
such a distorted coordination environment, CHEF induced by
zinc coordination is far from maximal and the larger cadmium
ion fits to the coordination cavity, resulting in poor fluorescent
quantum yield of zinc complex (ϕ = 0.007) and zinc/cadmium
selectivity (ICd/IZn = 64%). Therefore, two structural modifications
to remove this steric crowding from TQEN have been investi-
gated: (1) reduction in the number of quinoline rings to afford bi-
squinoline derivatives, N,N′-bis(2-quinolylmethyl)-N,N′-dimethyl-
ethylenediamine (BQDMEN)67 and (2) replacement of quinoline
ring with the isoquinoline variant to afford 1- and 3-isoTQEN.34
Both strategies worked well and the ICd/IZn value was improved to
25% and 14% for BQDMEN and 1-isoTQEN, respectively.
In the present isoTQA system, decreasing the steric hindrance
around the central zinc and also reducing the number of coordina-
tion atoms, strengthens the zinc-isoquinoline nitrogen interaction,
enabling a more compact coordination geometry. As a result, the
1-isoTQA exhibited improved ICd/IZn value up to 6.9% (free
ligand is 5%). This is in good agreement with the steric control
theory for ICd/IZn selectivity that recently proposed by Hancock.54
pH Effect on Fluorescence Intensity of 1- and 3-isoTQA.
Figure 7 demonstrates that the pH-dependent change in
mean Zn−Naromatic fluorescent quantum yield for ICd/IZn
ligand
distance (Å)
Zn complex
(%)
a
TPEN
2.15
2.15
2.16
2.26
2.05
2.05
2.08
NA
NA
14
b
b
1-isoTQEN
3-isoTQEN
0.034
15
a
TQEN
0.007
NA
64
c
TPA
NA
6.9
18
1-isoTQA
3-isoTQA
0.041
0.017
d
TQA
2.13
22
a
b
c
d
Ref 33. Ref 34 Ref 66. Ref 54.
Figure 6. Relative fluorescence intensity of 1-isoTQA at 470 nm (filled
bars) and 3-isoTQA at 464 nm (open bars) in the presence of 1
equivalent of metal ions in DMF/H2O (1:1) at 25 °C (λex = 324 nm).
I0 is the emission intensity of free ligand.
These observations indicate that the metal exchange process for
isoTQA complexes is very slow, similarly to isoTQENs.34,59 In the
presence of Cu2+, the fluorescence of 1-isoTQA−Zn complex was
completely quenched after 1 day incubation at room temperature,
due to higher affinity of Cu2+with 1-isoTQA than Zn2+.
Relationship between Metal Selectivity, Fluorescent
Response, and Complex Structure. In these types of
fluorescence probes, the steric hindrance and/or strength of
metal binding reflected by the interatomic distances between
zinc and coordinated aromatic nitrogen atoms were well
Figure 7. Effect of pH on fluorescence intensity of 34 μM 1-isoTQA at
470 nm (circles) and 3-isoTQA at 464 nm (squares) in the absence
(open marks) and presence (filled marks) of 1 equivalent of zinc ion in
DMF/H2O (1:1) at 25 °C (λex = 324 nm).
fluorescence intensity of 1- and 3-isoTQA is negligible. In the
presence of 1 eq of zinc ion, fluorescence enhancement was
observed in the range of pH = 3−9. The zinc-isoTQAs
interaction is prevented by protonation of the nitrogen atoms
1863
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
of the compound at low pH and by competing metal hydrolysis
(formation of Zn(OH)2) at high pH. From the pH-dependent
spectral change in UV−vis spectra (0.1 M NaClO4 in 1:1
DMF/H2O, 25 °C), protonation constants of 1- and 3-isoTQA
defined as the corresponding acid dissociation constants Kai =
[isoTQAHi‑1][H+]/[isoTQAHi] were determined to be pKa1 =
5.35(2), pKa2 = 3.38(3), and pKa3 < 2 for 1-isoTQA and pKa1 =
5.42(3), pKa2 = 3.54(3), and pKa3 = 2.14(4) for 3-isoTQA,
respectively (Figures S5−S8). These values are parallel with
those for TPA in different experimental conditions.68,69
Zinc Binding Affinity of 1- and 3-isoTQA. Since the zinc
binding affinity of 1- and 3-isoTQA is significantly high as
indicated by the sharp-edged titration curves shown in Figures
S1 and S2 of the Supporting Information, it is impossible to
determine the accurate binding constant of isoTQAs with metal
ions by conventional titration. So, competition experiments
using TPEN were conducted. Many fluorescent zinc sensor
molecules release zinc upon addition of TPEN leading to fluo-
rescence quenching because of the difference in zinc binding
affinity. However, in the case of hexacoordinate zinc complexes
of 1- and 3-isoTQEN, the zinc binding affinity is too high to
observe transmetalation even in excess TPEN for a long period
(more than 1 month).34
AUTHOR INFORMATION
■
Corresponding Author
*Phone/fax: +81-742-20-3095, e-mail: mikata@cc.nara-wu.ac.jp.
ACKNOWLEDGMENTS
■
This work was supported by the Research for Promoting
Technological Seeds, JST, Adaptable and Seamless Technology
Transfer Program through Target-driven R&D, JST, Grant-in
Aid for Scientific Research from the MEXT, Japan, and the
Nara Women’s University Intramural Grant for Project
Research.
REFERENCES
■
(1) Vallee, B. L.; Falchuk, K. H. Physiol. Rev. 1993, 73, 79−118.
(2) Vallee, B. L.; Auld, D. S. Acc. Chem. Res. 1993, 26, 543−551.
(3) Frederickson, C. J.; Suh, S. W.; Silva, D.; Frederickson, C. J.;
Thompson, R. B. J. Nutr. 2000, 130, 1471S−1483S.
(4) Que, E. L.; Domaille, D. W.; Chang, C. J. Chem. Rev. 2008, 108,
1517−1549.
(5) Dai, Z.; Canary, J. W. New J. Chem. 2007, 31, 1708−1718.
(6) Burdette, S. C.; Walkup, G. K.; Spingler, B.; Tsien, R. Y.; Lippard,
S. J. J. Am. Chem. Soc. 2001, 123, 7831−7841.
Upon addition of 1 equiv of TPEN to the zinc complex of
1-isoTQA in DMF/H2O (1:1), gradual (∼10 h to completion)
decrease of fluorescence due to the slow zinc transfer from the
1-isoTQA−Zn complex to TPEN was observed. Thus, fairly
strong zinc binding affinity of isoTQAs resist to immediate
fluorescence quenching by TPEN.
(7) Burdette, S. C.; Frederickson, C. J.; Bu, W.; Lippard, S. J. J. Am.
Chem. Soc. 2003, 125, 1778−1787.
(8) Zhang, X.-A.; Hayes, D.; Smith, S. J.; Friedle, S.; Lippard, S. J.
J. Am. Chem. Soc. 2008, 130, 15788−15789.
(9) Hirano, T.; Kikuchi, K.; Urano, Y.; Nagano, T. J. Am. Chem. Soc.
2002, 124, 6555−6562.
(10) Hanaoka, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T.
J. Am. Chem. Soc. 2004, 126, 12470−12476.
CONCLUSIONS
■
(11) Komatsu, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T.
J. Am. Chem. Soc. 2005, 127, 10197−10204.
1- and 3-isoTQA exhibit zinc-specific fluorescence enhance-
ment. No other metals studied, including Cd2+, afford an emis-
sion response at ∼470 nm upon binding to the isoTQA ligand
scaffold. The fluorescence intensity of 1-isoTQA is improved in
comparison to the tetrakisquinoline derivatives (1-isoTQEN)
due to a more efficient CHEF mechanism.
(12) Komatsu, K.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem.
Soc. 2007, 129, 13447−13454.
(13) Mizukami, S.; Okada, S.; Kimura, S.; Kikuchi, K. Inorg. Chem.
2009, 48, 7630−7638.
(14) Koike, T.; Watanabe, T.; Aoki, S.; Kimura, E.; Shiro, M. J. Am.
Chem. Soc. 1996, 118, 12696−12703.
Quinoline ring interactions in the TQEN−Zn complex result
in significant fluorescence quenching; however, the isoquino-
line−isoquinoline interaction generates long wavelength
emission that is specific for zinc binding for isoTQEN deriva-
tives. Such excitonic interactions are well documented in phenyl-
substituted tripodal systems based on TPA.70 Similarly, the
∼470 nm emission of isoTQAs contain isoquinoline−isoquinoline
interaction or energy transfer that is valid only for the asso-
ciated zinc complexes.
Although the intensity of the low energy fluorescence for
isoTQA−Zn complexes is weaker in comparison to the isoTQEN−
Zn complexes because of reduced interisoquinoline interaction,
it should be mentioned that such a strict structure-based
photophysical requirement is still available for the tetradentate,
trisisoquinoline derivatives studied in this work. Elucidation
of the detailed fluorescent mechanism in these complexes is
ongoing. In addition, preparation of water-soluble 1-isoTQA
derivatives could serve as an important molecular design for
high affinity intracellular zinc probes.
(15) Kimura, E.; Aoki, S.; Kikuta, E.; Koike, T. Proc. Natl. Acad. Sci.,
USA 2003, 100, 3731−3736.
(16) Qian, F.; Zhang, C.; Zhang, Y.; He, W.; Gao, X.; Hu, P.; Guo, Z.
J. Am. Chem. Soc. 2009, 131, 1460−1468.
(17) Lim, N. C.; Schuster, J. V.; Porto, M. C.; Tanudra, M. A.; Yao,
L.; Freake, H. C.; Bruckner, C. Inorg. Chem. 2005, 44, 2018−2030.
̈
(18) Taki, M.; Wolford, J. L.; O’Halloran, T. V. J. Am. Chem. Soc.
2004, 126, 712−713.
(19) Lu, C.; Xu, Z.; Cui, J.; Zhang, R.; Qian, X. J. Org. Chem. 2007,
72, 3554−3557.
(20) Tamanini, E.; Flavin, K.; Motevalli, M.; Piperno, S.; Gheber, L.
A.; Todd, M. H.; Watkinson, M. Inorg. Chem. 2010, 49, 3789−3800.
(21) Hanaoka, K.; Muramatsu, Y.; Urano, Y.; Terai, T.; Nagano, T.
Chem.Eur. J. 2010, 16, 568−572.
(22) Xu, Z.; Baek, K.-H.; Kim, H. N.; Cui, J.; Qian, X.; Spring, D. R.;
Shin, I.; Yoon, J. J. Am. Chem. Soc. 2010, 132, 601−610.
(23) Tomat, E.; Lippard, S. J. Curr. Opi. Chem. Biol. 2010, 14, 225−
230.
(24) Ahmed, N.; Geronimo, I.; Hwang, I.-C.; Singh, N. J.; Kim, K. S.
Chem.Eur. J. 2011, 17, 8542−8548.
(25) Yang, X.-B.; Yang, B.-X.; Ge, J.-F.; Xu, Y.-J.; Xu, Q.-F.; Liang, J.;
Lu, J.-M. Org. Lett. 2011, 13, 2710−2713.
ASSOCIATED CONTENT
* Supporting Information
■
(26) Buccella, D.; Horowitz, J. A.; Lippard, S. J. J. Am. Chem. Soc.
2011, 133, 4101−4114.
S
Tables and figures of experimental procedure for synthesis of
the compounds, and crystallographic data in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
́
(27) Majzoub, A. E.; Cadiou, C.; Dechamps-Olivier, I.; Tinant, B.;
Chuburu, F. Inorg. Chem. 2011, 50, 4029−4038.
(28) Jia, J.; Gu, Z.-Y.; Li, R.-C.; Huang, M.-H.; Xu, C.-S.; Wang, Y.-F.;
Xing, G.-W.; Huang, Y.-S. Eur. J. Org. Chem. 2011, 4609−4615.
1864
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865

Inorganic Chemistry
Article
(29) Xie, G.; Xi, P.; Wang, X.; Zhao, X.; Huang, L.; Chen, F.; Wu, Y.;
Yao, X.; Zeng, Z. Eur. J. Inorg. Chem. 2011, 2927−2931.
(30) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997,
97, 1515−1566.
(31) Urano, Y.; Kamiya, M.; Kanda, K.; Ueno, T.; Hirose, K.;
Nagano, T. J. Am. Chem. Soc. 2005, 127, 4888−4894.
(32) Egawa, T.; Koide, Y.; Hanaoka, K.; Komatsu, T.; Terai, T.;
Nagano, T. Chem. Commun. 2011, 47, 4162−4164.
(33) Mikata, Y.; Wakamatsu, M.; Yano, S. Dalton Trans. 2005, 545−
550.
(61) Wei, N.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1994, 33,
6093−6100.
(62) Xu, J.; Chuang, C.-L.; Canary, J. W. Inorg. Chim. Acta 1997, 256,
125−128.
(63) Newkome, G. R.; Kiefer, G. E.; Xia, Y.-J.; Gupta, V. K. Synthesis
1984, 676−679.
(64) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(65) Sheldrick, G. M. SHELXL-97, Program for Refinement of Crystal
Structures; University of Gottingen, Germany, 1997.
̈
(66) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.;
Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. Inorg. Chem. 2003, 42,
7472−7488.
(34) Mikata, Y.; Yamanaka, A.; Yamashita, A.; Yano, S. Inorg. Chem.
2008, 47, 7295−7301.
(35) Mikata, Y.; Wakamatsu, M.; Kawamura, A.; Yamanaka, N.; Yano,
S.; Odani, A.; Morihiro, K.; Tamotsu, S. Inorg. Chem. 2006, 45, 9262−
9268.
(67) Mikata, Y.; Yamashita, A.; Kawamura, A.; Konno, H.; Miyamoto,
Y.; Tamotsu, S. Dalton Trans. 2009, 3800−3806.
(68) Bravard, F.; Rosset, C.; Delangle, P. Dalton Trans. 2004, 2012−
(36) Ichimura, C.; Shiraishi, Y.; Hirai, T. Tetrahedron 2010, 66,
2018.
5594−5601.
(69) Ambundo, E. A.; Deydier, M.-V.; Grall, A. J.; Aguera-Vega, N.;
Dressel, L. T.; Cooper, T. H.; Heeg, M. J.; Ochrymowycz, L. A.;
Rorabacher, D. B. Inorg. Chem. 1999, 38, 4233−4242.
(70) Liang, J.; Zhang, J.; Zhu, L.; Duarandin, A.; Young, J., V. G.;
Geacintov, N.; Canary, J. W. Inorg. Chem. 2009, 48, 11196−11208.
(37) Zhou, X.; Yu, B.; Guo, Y.; Tang, X.; Zhang, H.; Liu, W. Inorg.
Chem. 2010, 49, 4002−4007.
(38) Zhu, J.-F.; Yuan, H.; Chan, W.-H.; Lee, A. W. M. Tetrahedron
Lett. 2010, 51, 3550−3554.
(39) Liu, Z.-C.; Wang, B.-D.; Yang, Z.-Y.; Li, T.-R.; Li, Y. Inorg. Chem.
Commun. 2010, 13, 606−608.
(40) Xue, L.; Liu, C.; Jiang, H. Chem. Commun. 2009, 1061−1063.
(41) Xue, L.; Wang, H.-H.; Wang, X.-J.; Jiang, H. Inorg. Chem. 2008,
47, 4310−4318.
(42) Xue, L.; Liu, C.; Jiang, H. Org. Lett. 2009, 11, 1655−1658.
(43) Weng, Y.; Chen, Z.; Wang, F.; Xue, L.; Jiang, H. Anal. Chim.
Acta 2009, 647, 215−218.
(44) Wang, H.-H.; Gan, Q.; Wang, X.-J.; Xue, L.; Liu, S.-H.; Jiang, H.
Org. Lett. 2007, 9, 4995−4998.
(45) Zhang, Y.; Guo, X.; Si, W.; Jia, L.; Qian, X. Org. Lett. 2008, 10,
473−476.
(46) Shiraishi, Y.; Ichimura, C.; Hirai, T. Tetrahedron Lett. 2007, 48,
7769−7773.
(47) Royzen, M.; Durandin, A.; Young, J., V. G.; Geacintov, N. E.;
Canary, J. W. J. Am. Chem. Soc. 2006, 128, 3854−3855.
(48) Chen, H.; Wu, Y.; Cheng, Y.; Yang, H.; Li, F.; Yang, P.; Huang,
C. Inorg. Chem. Commun. 2007, 10, 1413−1415.
(49) Liu, Y.; Zhang, N.; Chen, Y.; Wang, L.-H. Org. Lett. 2007, 9,
315−318.
(50) Chen, Y.; Han, K.-Y.; Liu, Y. BIoorg. Med. Chem. 2007, 15,
4537−4542.
(51) Qiu, L.; Jiang, P.; He, W.; Tu, C.; Lin, J.; Li, Y.; Gao, X.; Guo, Z.
Inorg. Chim. Acta 2007, 360, 431−438.
(52) Wu, D.-Y.; Xie, L.-X.; Zhang, C.-L.; Duan, C.-Y.; Zhao, Y.-G.;
Guo, Z.-J. Dalton Trans. 2006, 3528−3533.
(53) Aragoni, M. C.; Arca, M.; Bencini, A.; Blake, A. J.; Caltagirone,
C.; De Filippo, G.; Devillanova, F. A.; Garau, A.; Gelbrich, T.;
Hursthouse, M. B.; Isaia, F.; Lippolis, V.; Mameli, M.; Mariani, P.;
Valtanconi, B.; Wilson, C. Inorg. Chem. 2007, 46, 4548−4559.
(54) Williams, N. J.; Gan, W.; Reibenspies, J. H.; Hancock, R. D.
Inorg. Chem. 2009, 48, 1407−1415.
(55) Mameli, M.; Aragoni, M. C.; Arca, M.; Atzori, M.; Bencini, A.;
Bazzicalupi, C.; Blake, A. J.; Caltagirone, C.; Devillanova, F. A.; Garau,
A.; Hursthouse, M. B.; Isaia, F.; Lippolis, V.; Valtancoli, B. Inorg. Chem.
2009, 48, 9236−9249.
(56) Zhou, X.; Lu, Y.; Zhu, J.-F.; Chan, W.-H.; Lee, A. W. M.; Chan,
P.-S.; Wong, R. N. S.; Mak, N. K. Tetrahedron 2011, 67, 3412−3419.
(57) Zhang, C.; Zhang, Y.; Chen, Y.; Xie, Z.; Liu, Z.; Dong, X.; He,
W.; Shen, C.; Guo, Z. Inorg. Chem. Commun. 2011, 14, 304−307.
(58) Mikata, Y.; Yamashita, A.; Kawata, K.; Konno, H.; Itami, S.;
Yasuda, K.; Tamotsu, S. Dalton Trans. 2011, 40, 4976−4981.
(59) Mikata, Y.; Yamashita, A.; Kawata, K.; Konno, H.; Itami, S.;
Yasuda, K.; Tamotsu, S. Dalton Trans. 2011, 40, 4059−4066.
(60) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D.
Inorg. Chem. 1994, 33, 1953−1965.
1865
dx.doi.org/10.1021/ic202159v | Inorg. Chem. 2012, 51, 1859−1865