A novel class of Cd(ii), Hg(ii) turn-on and Cu(ii), Zn(ii) turn-off Schiff base fluorescent probes

Article abstract of DOI:10.1039/c0dt00737d

N,N′-((5,5′-(Quinoxaline-2,3-diyl)bis(1H-pyrrole-5,2-diyl)) bis(methanylylidene))bis(4-methoxyaniline) 4 and N,N′-((5,5′- (quinoxaline-2,3-diyl)-bis(1H-pyrrole-5,2-diyl))bis(methanylylidene))dianiline 5 have been prepared and structurally characterized. The X-ray crystal structures of compounds 4 and 4a have been determined. These compounds displayed good sensitivity toward transition metal ions with Cd(ii), Zn(ii) turn-on and Cu(ii), Hg(ii) turn-off in fluorescence. It is an elegant example of on/off behavior like a lamp. When Cd(ii) or Zn(ii) is added into compounds 4 or 5, the lamp will switch on, and then when Cu(ii) or Hg(ii) is added into the mixture, the lamp will switch off. The binding properties of 4 and 5 for cations were examined by fluorescence spectroscopy. The fluorescence data and crystal structure indicate that a 1:1 stoichiometry complex is formed between compound 4 (or 5) and metal ions, and the binding affinity is very high. The recognition mechanism between compound 4 (or 5) and metal ion was discussed based on the their chemical constructions and the CHEF/CHEQ effect when they interacted with each other.

Full text of DOI:10.1039/c0dt00737d

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PAPER
www.rsc.org/dalton | Dalton Transactions
A novel class of Cd(II), Hg(II) turn-on and Cu(II), Zn(II) turn-off Schiff base
fluorescent probes†
Yuan Hu,a,b Qian-qian Li,a Hua Li,a Qian-ni Guo,a Yun-guo Lua and Zao-ying Li*a
Received 28th June 2010, Accepted 8th September 2010
DOI: 10.1039/c0dt00737d
N,N¢-((5,5¢-(Quinoxaline-2,3-diyl)bis(1H-pyrrole-5,2-diyl))bis(methanylylidene))bis(4-methoxyaniline)
4 and N,N¢-((5,5¢-(quinoxaline-2,3-diyl)-bis(1H-pyrrole-5,2-diyl))bis(methanylylidene))dianiline 5 have
been prepared and structurally characterized. The X-ray crystal structures of compounds 4 and 4a have
been determined. These compounds displayed good sensitivity toward transition metal ions with Cd(II),
Zn(II) turn-on and Cu(II), Hg(II) turn-off in fluorescence. It is an elegant example of on/off behavior
like a lamp. When Cd(II) or Zn(II) is added into compounds 4 or 5, the lamp will switch on, and then
when Cu(II) or Hg(II) is added into the mixture, the lamp will switch off. The binding properties of 4
and 5 for cations were examined by fluorescence spectroscopy. The fluorescence data and crystal
structure indicate that a 1 : 1 stoichiometry complex is formed between compound 4 (or 5) and metal
ions, and the binding affinity is very high. The recognition mechanism between compound 4 (or 5) and
metal ion was discussed based on the their chemical constructions and the CHEF/CHEQ effect when
they interacted with each other.
polluted areas has attracted considerable attention in the field of
Introduction
supramolecular chemistry.8
In recent years, the design of fluorescent probes for selective and
In addition, zinc is, after iron, the second most abundant
transition metal in mammals,9 where it plays important roles
in various biological processes such as neurotransmission, signal
transduction and gene expression.10,11 copper is not only a sig-
nificant metallic pollutant, but also an essential element for living
organisms.12–14 Deficiency of copper or zinc can lead to growth and
metabolism disorders, such as Menkes syndrome, Wilson disease,
hereditary spinal lateral sclerosis, Alzheimer’s disease, and so on.15
Due to their toxic effects and importance to organisms, it is
important to develop new chemosensors to be able to monitor the
presence of cadmium, mercury, zinc and copper quantitatively.16–18
Dipyrrolylquinoxaline (DPQ) derivatives have emerged as a new
class of ion receptors that exhibit a remarkable selectivity toward
anions,19–25 but to the best of our knowledge, few of them have
been employed as cation receptors.26–28
sensitive quantification of biologically and environmentally im-
portant ion species, especially transition metal ions, has attracted
considerable attention in the realms of chemistry, biochemistry
and environmental chemistry.1 Among the cations, developing
probes for transition metal ions is the focus, since they usually
symbolize an environmental concern when present in uncontrolled
amounts, for example, cadmium, lead, mercury, while at the same
time some of them such as iron, cobalt, copper and zinc are con-
sidered as essential elements in biological systems.2 The design and
synthesis of probes that have notable capabilities of recognizing
and sensing cations are becoming one of the most challenging
realms, not only from the viewpoint of organic and supramolecular
chemistry but also from its potential environmental and biological
applications.3
Previously, we have reported DPQ-bridged Schiff bases28 (Fig. 1)
as strong fluorescent quenching sensors. They display remarkable
selective and sensitive fluorescence quenching responses (about
90% intensity quenched) toward mercury(II) ions among ten
different metal ions.28 In order to investigate and compare the
properties of open-chain DPQ Schiff bases, benzene groups were
introduced into the DPQ derivatives to break the restriction of the
cavity. It is found that ligand 4 (or 5) shows a 5-fold (or 6-fold)
fluorescence emission increase in the presence of Cd(II) and 3-fold
Cadmium has found extensive use in the fields of Ni–Cd
batteries, phosphate fertilizers, pigments, and semiconducting
quantum dots and rods.4 The detrimental effects of cadmium
on human health are, however, being increasingly recognized:
chronic cadmium exposure can cause renal dysfunction, calcium
metabolism disorders, and an increased incidence of certain forms
of cancer,5 possibly due to direct inhibition of DNA mismatch
repair.6 Mercury contamination is widespread and arises from a
variety of natural sources, such as oceanic and volcanic emission,
as well as anthropogenic sources,7 such as gold mining and the
combustion of solid waste and fuels. The development of an easy
mercury determination system for monitoring its concentration in
aDepartment of Chemistry, Wuhan University, Wuhan, Hubei, 430072,
People’s Republic of China. E-mail: zyliwuc@whu.edu.cn; Fax: 86-27-
68754067; Tel: 86-13647213679
bDepartment of Chemistry and Biochemistry, University of Delaware,
Newark, Delaware, 19716, United States of America
† CCDC reference numbers 781632–781633. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00737d
Fig. 1 DPQ-bridged Schiff bases.
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in the presence of Zn(II). The results obtained will be discussed in
this paper.
Experimental
General information
All starting materials and solvents were purchased from
Sinopharm Chemical Reagent Co. Ltd., and used without further
purification unless otherwise stated. Pyrrole was distilled under
N2 prior to use. N,N-dimethylformamide and 1,2-dichloroethane
were distilled from CaH2 and stored over activated molecular
sieves prior to use. Pyridine was distilled from NaOH and stored
over activated molecular sieves prior to use. 4-Methoxyaniline and
aniline were recrystallized from petroleum prior to use. Methanol,
dichloromethane and tetrahydrofuran were distilled from CaH2
under N2. The infrared spectra were performed on a Nicolet 670
FT-IR spectrophotometer. 1H NMR and 13C NMR spectra were
recorded on a Varian Mercury-VX 300 spectrometer in CDCl3.
Mass spectra were obtained on a Finnigan MAT SSQ-710 in FAB
(positive) mode. Fluorescence spectra were obtained on a Perkin
Elmer LS-55 Luminescence spectrometer. Elemental analyses were
determined by Finnigan FLASH EA 1112 elemental autoanalyzer.
The synthetic procedures to obtain compound 4, 5 and 4a are
depicted in Scheme 1.
Syntheses
Compounds 1, 2 and 3 were prepared according to the published
procedure and confirmed.27,29
N,N¢-((5,5¢-(Quinoxaline-2,3-diyl)bis(1H-pyrrole-5,2-diyl))bis-
(methanylylidene))bis(4-methoxyaniline)
(4). 2,3-Bis(5¢-
Scheme 1 Synthesis of Schiff bases 4 and 5. (i) 1H-pyrrole, CH2Cl2,
pyridine, -80 ◦C, 24%; (ii) 1,2-diaminobenzene, AcOH, reflux in the dark,
94%; (iii) POCl3, DMF, 0 ◦C for 10 min/1,2-dichloroethene, reflux for 2 h,
79%; (iv) CH3OH, 4-methoxyaniline, 97%, (or aniline, 75%); (v) Cu(AcO)2,
CH3OH, CH2Cl2, reflux, 1 h, 97%.
formylpyrrol-2¢yl)-quinoxaline (3) (32 mg, 0.1 mmol) and
4-methoxyaniline (271 mg, 0.22 mmol) were dissolved in
methanol (10 mL). The mixture was refluxed for 2 h; no solid
appeared. After cooling to room temperature, the mixture
was evaporated to dryness. The residue was subjected to
chromatography over silica gel (eluent, CH2Cl2 : CH3OH = 100 :
1271 (s), 1193 (s), 1142 (s), 1043 (s), 911 (s), 766 (s), 693 (s); FAB-
MS (m/z): 467, (M + H)+; Anal. calcd for C29H22N6, C, 77.23; H,
4.75; N, 18.01. Found: C, 77.02; H, 4.68; N, 18.23.
1
1, v/v) to afford a yellow solid (51 mg, 97%). H NMR (300
MHz, CDCl3, TMS): d 8.31 (s, 2H, -CH N), 7.98–8.02 (dd, J =
3.3 Hz, J = 3.3 Hz, 2H, quinoxaline), 7.66–7.69 (dd, J = 3.5 Hz,
J = 3.3 Hz, 2H, quinoxaline), 7.23 (d, J = 8.4 Hz, 4H, benzene),
6.90 (t, J = 6.5 Hz, 4H, benzene), 6.64–6.73 (m, 4H, pyrrole), 3.83
(s, 6H, -OCH3), 3.74 (s, 2H, -NH);13C NMR (75 MHz, CDCl3,
TMS): d 158.24, 147.70, 144.40, 143.44, 140.44, 133.10, 129.98,
128.73, 122.22, 117.03, 114.61, 114.20, 77.24, 58.53, 55.67; IR
(KBr, cm-1): n 3477–3414 (m), 1616 (s), 1503 (s), 1244 (s), 1185
(s), 1034 (s), 912 (s), 743 (s); 616 (s); FAB-MS (m/z): 527, (M +
H)+; Anal. calcd for C32H26N6O2, C, 72.99; H, 4.98; N, 15.96.
Found: C, 72.86; H, 5.08; N, 15.79.
Complex 4-Cu (4a). Compound 4 (53 mg, 0.1 mmol) was dis-
solved in CH2Cl2 (10 mL), then the solution of (CH3CO2)2Cu·H2O
(20 mg, 0.1 mmol) in dry methanol (5 mL) was added. The color
of the solution turned from yellow to dark red. The mixture
was refluxed for 1 h. After cooling to room temperature, the
mixture was evaporated to dryness. The residue was subjected to
chromatography over silica gel (eluent, CH2Cl2), and recrystallized
from CH2Cl2–CH3OH, (57 mg, 97%). IR (KBr, cm-1): n 3437 (br),
2924 (s), 1609 (s), 1583 (s), 1505 (s), 1487 (s), 1413 (s), 1330 (s), 1243
(s), 1205 (s), 1174 (s), 1030 (s), 828 (s), 783 (s), 555 (s). FAB-MS
(m/z): 590, (M + H)+; Anal. calcd for C32H24N6O2Cu, C, 65.35;
H, 4.11; N, 14.29. Found: C, 65.32; H, 3.99; N, 14.30.
N,N¢-((5,5¢-(Quinoxaline-2,3-diyl)bis(1H-pyrrole-5,2-diyl))bis-
(methanylylidene))dianiline (5). As for 4 except that aniline
(21 mg, 0.22 mmol) was used in place of 4-methoxyaniline. Yellow
powder (35 mg, 75%). 1H NMR (300 MHz, CDCl3, TMS): d 8.31
(s, 2H, -CH N), 7.99–8.02 (t, J = 4.0 Hz, 2H, quinoxaline), 7.67–
7.70 (dd, J = 3.3 Hz, J = 3.3 Hz, 2H, quinoxaline), 7.39 (t, J = 7.7
Hz, 4H, benzene), 7.13–7.25 (m, 4H, benzene), 6.91 (d, J = 2.7 Hz,
2H, benzene), 6.68–6.78 (m, 4H, pyrrole), 3.66 (s, 2H, -NH); IR
(KBr, cm-1): n 3429–3063 (m), 1655 (s), 1491 (s), 1423 (s), 1342 (s),
X-ray crystallography
The crystallographic data for 4 and 4a are summarized in Table 1.
Intensities were both collected at 293 K, on a Bruker AXS SMART
APEX II CCD diffractometer with graphite-monochromated Mo-
˚
Ka radiation (l = 0.71073 A). The collected frames were processed
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Table 1 Crystallographic data for compound 4 and 4a
Compound
4
4a
Empirical formula
Formula weight
Colour and habit
Crystal size/mm
Crystal system
Space group
C32H26N6O2·CH3OH
558.63
yellow needle
0.34 ¥ 0.32 ¥ 0.28
Monoclinic
P21/c
293(2)
22.061(5)
7.7536(18)
17.062(4)
90
C32H24CuN6O2
588.11
red block
0.32 ¥ 0.31 ¥ 0.24
Triclinic
¯
P1
T/K
293(2)
˚
a/A
8.6290(18)
16.338(4)
20.422(4)
67.414(3)
80.191(4)
85.300(4)
2619.1(10)
1212.0
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
100.265(4)
90
2871.9(12)
1176
3
˚
V/A
F000
Z
4
4
Dcalc/g cm-1
1.292
0.085
15950
1.491
0.877
15909
Absorption coefficient/mm-1
Reflections collected
Independent reflections
q range/#9702;
5649 [R(int) = 0.0269]
2.43 to 26.00
-27< = h< = 27,
-6< = k< = 9,
-21< = l< = 21
R1 = 0.0450, wR2 = 0.1138
R1 = 0.0728, wR2 = 0.1302
1.030
10636 [R(int) = 0.0226]
2.03 to 26.50
-10< = h< = 10,
-20< = k< = 20,
-25< = l< = 21
R1 = 0.0439, wR2 = 0.1013
R1 = 0.0821, wR2 = 0.1151
1.023
Limiting indices
Final R indices (I > 2s(I))
R indices (all data)
Goodness-of-fit on F2
with the software SAINT30 and an absorption correction was
applied (SADABS)31 to the collected reflections.
The space groups of each crystal were determined from the
systematic absences and Laue symmetry checks and confirmed by
successful refinement of the structures.
The structures of both compounds were solved by direct
methods (SHELXTL-97)32 and refined against F2 by full-matrix
least-squares analyses. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were generated in their idealized
positions and allowed to ride on their respective parent carbon
atoms.33 Full details of the crystallographic analyses are given in
Supporting Information as CIF files.†
configuration in which the benzene ring and the pyrrole parts are
in a trans relationship. In Fig. 3, the binding mode of 4 (or 5)
towards the metal ion was revealed by X-ray structures of the
Cu(II) complex 4a. The structure indicates that this compound
forms a 1 : 1 metal–ligand complex with four N atoms in the two
arms twisting together. Selected bond lengths and bond angles of
4a are given in Table 2. There are two independent molecules
of 4a in the asymmetric unit. The distances of 1.941(2) and
˚
˚
1.970(2) A [or 1.951(2) and 1.969(2) A] for Cu(1)–Npyr [or Cu(2)–
Npyr] (pyrrole nitrogen) are slightly shorter than that of 2.034(2)
˚
˚
and 2.062(2) A [or 2.029(2) and 2.061(2) A] for Cu(1)–Nim [or
Cu(2)–Nim]. These slight deviations are as a result of the formation
of ionic bonds between the copper and the pyrrole nitrogen atoms.
All the distances between Cu and the four N atoms are all shorter
CCDC reference numbers 781633 (4) and 781632 (4a).
˚
than 2.5 A. Driven by metal-coordination, rotation of the pyrrol
fragment by nearly 180◦ around C–C bond orients the nitrogen
atoms N(3) and N(4) [or N(8) and N(10)] of the pyrrol and imino
nitrogen atoms N(1) and N(2) [or N(7) and N(9)] in the same
direction, thus enabling the molecule to function as a planar
quadridentate N4 chelator. Two benzene planes of the arms in
each molecule are almost parallel to each other with the dihedral
angles about 4.1◦ and 3.9◦, respectively. In the structures of 4a,
the copper atom is bound to four nitrogen atoms, two from the
Results and discussion
Synthesis and characterization of compounds
The new fluorescent sensor 4 (or 5) was synthesized by refluxing
the methanol solution of 2,3-bis(5-formylpyrrol-2-yl)quinoxaline
(3) and 4-methoxyaniline (or aniline) for 2 h. The products
were obtained as a yellow powder in high yield and charac-
terized by IR, 1H NMR, 13C NMR, FAB-MS and elemental
analysis.
In order to determine the complex model, single crystals of
4 and 4a suitable for X-ray analysis were obtained from their
dichloromethane–methanol–petroleum solution by slow evapora-
tion at room temperature. Perspective drawings of compounds 4
and 4a are shown in Fig. 2 and 3, respectively.
deprotonated pyrroles and two from the adjacent imines, and is
ꢀ
almost coplanar to the respective N4 plane ( angles at Cu(1) =
ꢀ
359.90,
angles at Cu(2) = 359.74).
Fluorescence titration of 4 and 5 with metal ions
In Fig. 2, compound 4 has two identical arms on the opposite
side. Two arms twist small angles with the quinoxaline plane, one
angle 33.2◦, the other angle 11.6◦. Each arm adopts a unique
To obtain an insight into the binding properties of 4 and 5
toward metal ions, the changes of the fluorescence properties
of 1 ¥ 10-5 mol L-1 of 4 and 5 were investigated upon addition
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Fig. 2 ORTEP view of the X-Ray structure of compound 4 with all heavy atoms labeled.
Fig. 3 ORTEP view of the X-Ray structure of compound 4a with all heavy atoms labeled.
of a wide range of 4 equiv. cations including Pb2+, Mg2+, Ca2+,
Ni2+, Ba2+, Co2+, Cd2+, Hg2+, Zn2+ and Cu2+ in THF solution at
293 K. The chemosensory behavior of 4 and 5 is similar, which
is depicted in Fig. 4: the THF solution of 4 or 5 displayed a
remarkable emission increase when Zn2+ or Cd2+ ion was added
in; when Hg2+ or Cu2+ was added into the solution of 4 or
5, a significant fluorescent emission quenching was observed;
by contrast, relatively insignificant spectral changes were beheld
upon addition of most of the other metal ions. So it can be
concluded that 4 and 5 have a high selectivity for recognition
of Zn2+, Cd2+, Hg2+ and Cu2+. The sensitivity of the fluorescence
emission response of 4 and 5 toward Zn2+, Cd2+, Hg2+ and Cu2+ was
subsequently examined under the same conditions with various
metal ion concentrations (Fig. 5 to 8).
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Fig. 4 Fluorescent emission changes of (a) compound 4 (1.0 ¥ 10-5 mol L-1), lex = 406 nm or (b) compound 5 (1.0 ¥ 10-5 mol L-1), lex = 398 nm in THF
solution in the presence of 4 equiv. various metal ions.
when the concentration of Zn2+ is increased up to 1.2 equiv, the
Table 2 Selected bond lengths and bond angles of compound 4a
fluorescence intensity of 5 is increased by 3-fold at 530 nm.
Compound 4a Bond length/A Compound 4a
Bond angle (◦)
˚
As shown in Fig. 7 and Fig. 8, a strong decrease of the emission
intensity was observed upon addition of Cu2+ or Hg2+. After
addition of 1.5 equiv. of Cu2+ or Hg2+, the fluorescence intensity at
516 nm of 4 were reduced to 51% and 24% of the initial one,
respectively. After addition of 2.5 equiv. of Cu2+ or Hg2+, the
fluorescence intensity at 516 nm of 4 were both reduced by 7%.
After addition of 2.4 equiv. of Cu2+ or Hg2+, the fluorescence
intensity of 5 at 505 nm were reduced to 2%, 3% of the initial one,
respectively.
Hence, 4 and 5 can be used as “turn-on” molecular probes
toward Cd2+ and Zn2+, while “turn-off” molecular probes toward
Hg2+ and Cu2+.
Fig. 9 shows the Job’s plots of receptor 4 and 5 with Zn2+, Cd2+,
Hg2+ and Cu2+. The total concentration of the host and guest
was constant (3.0 ¥ 10-5 mol L-1) in THF, with a continuously
variable molar fraction of the guest ([G]/([H]+[G])). When the
molar fraction of the guest was 0.50, the difference of fluorescence
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(2)–N(7)
Cu(2)–N(8)
Cu(2)–N(9)
Cu(2)–N(10)
2.034(2)
2.062(2)
1.941(2)
1.970(2)
2.061(2)
1.951(2)
2.029(2)
1.969(2)
N(1)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
N(3)–Cu(1)–N(4)
N(4)–Cu(1)–N(1)
N(8)–Cu(2)–N(7)
N(8)–Cu(2)–N(10) 92.76(9)
N(9)–Cu(2)–N(7) 99.10(9)
N(10)–Cu(2)–N(9) 84.11(9)
99.09(10)
83.98(9)
92.81(9)
84.02(9)
83.77(9)
Fig. 5 shows the titration profiles of 4 on Cd2+ and Zn2+ addition.
When the concentration of Cd2+ is increased up to 1.2 equiv,
the fluorescence intensity of 4 is increased by 5-fold at 573 nm;
when the concentration of Zn2+ is increased up to 0.9 equiv, the
fluorescence intensity of 4 is increased by 3-fold at 553 nm.
Fig. 6 shows the titration profiles of 5 on Cd2+ and Zn2+ addition.
When the concentration of Cd2+ is increased up to 1.2 equiv,
the fluorescence intensity of 5 is increased by 6-fold at 573 nm;
Fig. 5 Fluorescence spectra of 4 (1.0 ¥ 10-5 mol L-1) upon the addition of various amounts of (a) Cd2+ or (b) Zn2+ in THF solution, lex = 406 nm. The
inset shows the fluorescence intensity as a function of (a) [Cd2+] at lmax (em) = 573 nm and (b) [Zn2+] at lmax (em) = 553 nm.
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Fig. 6 Fluorescence spectra of 5 (1.0 ¥ 10-5 mol L-1) upon the addition of various amounts of (a) Cd2+ or (b) Zn2+ in THF solution, lex = 398 nm. The
inset shows the fluorescence intensity as a function of (a) [Cd2+] at lmax (em) = 573 nm and (b) [Zn2+] at lmax (em) = 530 nm.
Fig. 7 Fluorescence spectra of 4 (1.0 ¥ 10-5 mol L-1) upon the addition of various amounts of (a) Cu2+ or (b) Hg2+ in THF solution, lex = 406 nm. The
inset shows the fluorescence intensity as a function of (a) [Cu2+] or (b) [Hg2+] at lmax (em) = 516 nm.
Fig. 8 Fluorescence spectra of 5 (1.0 ¥ 10-5 mol L-1) upon the addition of various amounts of (a) Cu2+ or (b) Hg2+ in THF solution, lex = 398 nm. The
inset shows the fluorescence intensity as a function of (a) [Cu2+] or (b) [Hg2+] at lmax (em) = 505 nm.
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Fig. 9 Job plots of (a) compound 4 or (b) compound 5 with Cd2+, Cu2+, Hg2+ and Zn2+. The total concentration of the host and guest is 3.0 ¥ 10-5 mol
L-1 in THF solution. I0: fluorescence intensity of the host; I: fluorescence intensity of host in the presence of the guest.
or Zn2+ ion, the intensity increases to about 3 or 5-fold. Then
Table 3 Association constants Kass of receptors 4 and 5 with guest cations
adding 4 equiv. Hg2+ or Cu2+ to the above Cd2+ or Zn2+ solution
Cation
Receptor 4
Receptor 5
induced an evident fluorescent change from significant enhancing
to nearly quenching, which displayed a pronounced ON/OFF-
type signalling behaviour. The quenching effects of Hg2+ or Cu2+
do abolish the ratiometric effect on binding of Cd2+ and Zn2+. This
character can be employed as an ion switch. When Cd2+ or Zn2+ is
added into 4 or 5, the power switches on, and then when Hg2+ or
Cu2+ is added into the system, the power switches off.
Kass (M-1)
2.7 ¥ 106
5.2 ¥ 104
1.4 ¥ 106
5.1 ¥ 106
R
Kass (M-1)
1.7 ¥ 105
3.4 ¥ 104
2.8 ¥ 107
2.0 ¥ 106
R
Hg(II)
Cu(II)
Cd(II)
Zn(II)
0.9855
0.9550
0.9884
0.9768
0.9950
0.9873
0.9994
0.9809
A strong CHEQ effect (Chelation Enhancement of the Quench-
ing) 4 and 5 toward Cu(II) and Hg(II) can be observed in Fig. 7 and
Fig. 8. This CHEQ effect can be attributed to an energy/electron
transfer quenching of the p* emissive state though low-lying
metal-centred unfilled d-orbitals for Cu(II), and to an intersystem
crossing mechanism due to the heavy atom effect for Hg(II).35,36
Zn(II) and Cd(II) coordinated to ligands 4 or 5 are generally
emissive species, causing a CHEF effect (Chelation Enhancement
of the Fluorescence Emission).35 The stronger the involvement of
the nitrogen atoms in the complexation, the stronger the effect on
the fluorescence of the ligand. This can be explained by assuming
that the deprotonation and complexation of the pyrrole with Zn(II)
and Cd(II) block its lone pair of electrons from transferring to the
DPQ moiety via PET (photo-induced electron transfer).36
intensity between the host and the guest reached a maximum,
which demonstrated that both receptors 4 and 5 formed a
1 : 1 complex with Zn2+, Cd2+, Hg2+ or Cu2+, respectively.34 The
association constants (Kass) of 4 and 5 with guest cations are
shown in Table 3.
As shown in Fig. 10, both compounds 4 and 5 have a low
level fluorescence intensities, but after addition of 1.5 equiv. Cd2+
Determination of the association constants (Kass) of complexes
For the complex of 1 : 1 stoichiometry, according to the following
relation:18
2
X = X0 + (Xlim - X0)/2c0 {cH + cG + 1/Kass - [(cH + cG + 1/Kass
)
- 4cHcG]1/2
}
Where X represents the absorbance intensity, cH and cG are the
corresponding concentration of the host and guest. The associa-
tion constants obtained by a non-linear least-square analysis (the
inset of the Fig. 5 to 8) of X vs. cH and cG are listed in Table 3. The
data show that both receptors 4 and 5 have an excellent sensitivity
for Zn(II), Cd(II), Hg(II) and Cu(II).
Fig. 10 Fluorescence intensity changes of 4 or 5 (1.0 ¥ 10-5 mol L-1) upon
the addition of 1.5 equiv. Cd2+ or Zn2+ and 4 equiv. Hg2+ or Cu2+ in THF
solution.
11350 | Dalton Trans., 2010, 39, 11344–11352
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Conclusions
In summary, a new class of fluorescent probes, 4 and 5 derived
from DPQ, have been synthesized and characterized. The analyses
of fluorescence properties of these compounds reveal that these
species display sensitive response of fluorescence toward Zn(II),
Cd(II), Hg(II) and Cu(II) ions. The emission intensity increases
with gradual addition of Zn(II) or Cd(II) and quenches when
Hg(II) or Cu(II) is added to the system. Our findings will help
to improve the direct detection of these ions in the environment.
Further modification37 and investigation38–42 of these compounds
and analogs on DNA, protein, and tumor cells is planned in due
course.
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This work was supported by the National Natural Science
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