Nickel(ii) and iron(iii) selective off-on-type fluorescence probes based on perylene tetracarboxylic diimide

Article abstract of DOI:10.1039/b921342b

Two novel "turn-on" fluorescent probes with perylene tetracarboxylic diimide (PDI) as the fluorophore and two different di-(2-picolyl)-amine (DPA) groups as the metal ion receptor (PDI-1 and PDI-2) were successfully synthesized with satisfactory yields. PDI-1 exhibited high selectivity toward Ni²⁺ in the presence of various other metal cations including Zn²⁺, Cd²⁺ and Cu²⁺ which were expected to interfere significantly. A 1:2 stoichiometry was found for the complex formed by PDI-1 and Ni²⁺ by a Job's plot and by non-linear least square fitting of the fluorescence titration curves. By introducing an extra diamino ethylene group between DPA and the phenyl bridge, the receptor was modified and the high selectivity of the sensor toward Ni²⁺ shifted to Fe³⁺. The enhancement factor of the fluorescence response of PDI-2 to Fe³⁺ was as high as 138. The binding behavior of the receptors in these two compounds is affected significantly by the PDI fluorophores. Most interestingly, both Ni²⁺ and Fe³⁺ are paramagnetic metal ions, which are known as fluorescence quenchers and are rarely targeted with "turn-on" fluorescence probes. This result suggests that PDIs are favorable fluorophores for a "turn-on" fluorescence probe for paramagnetic transition metal ions because of their high oxidation potential. The Royal Society of Chemistry.

Full text of DOI:10.1039/b921342b

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Nickel(II) and iron(III) selective off-on-type fluorescence probes based on
perylene tetracarboxylic diimide†‡
Haixia Wang,a,b Delou Wang,a Qi Wang,c Xiyou Li*a and Christoph A. Schalley*c
Received 13th October 2009, Accepted 20th November 2009
First published as an Advance Article on the web 5th January 2010
DOI: 10.1039/b921342b
Two novel “turn-on” fluorescent probes with perylene tetracarboxylic diimide (PDI) as the fluorophore
and two different di-(2-picolyl)-amine (DPA) groups as the metal ion receptor (PDI-1 and PDI-2) were
successfully synthesized with satisfactory yields. PDI-1 exhibited high selectivity toward Ni2+ in the
presence of various other metal cations including Zn2+, Cd2+ and Cu2+ which were expected to interfere
significantly. A 1 : 2 stoichiometry was found for the complex formed by PDI-1 and Ni2+ by a Job’s plot
and by non-linear least square fitting of the fluorescence titration curves. By introducing an extra
diamino ethylene group between DPA and the phenyl bridge, the receptor was modified and the high
selectivity of the sensor toward Ni2+ shifted to Fe3+. The enhancement factor of the fluorescence
response of PDI-2 to Fe3+ was as high as 138. The binding behavior of the receptors in these two
compounds is affected significantly by the PDI fluorophores. Most interestingly, both Ni2+ and Fe3+ are
paramagnetic metal ions, which are known as fluorescence quenchers and are rarely targeted with
“turn-on” fluorescence probes. This result suggests that PDIs are favorable fluorophores for a “turn-on”
fluorescence probe for paramagnetic transition metal ions because of their high oxidation potential.
to its inherently higher sensitivity as compared to the normal
Introduction
fluorescence quenching motif.2a,b,3
Fluorescent probes for sensing and monitoring chemical analytes
Perylene tetracarboxylic diimides (PDIs) have recently gener-
are a topical and attractive field for chemistry, biology and
ated great interest in the field of photonic materials, because of
environmental science due to their high sensitivity and simplicity.1
their excellent thermal- and photo-stability, high luminescence
efficiency, and novel optoelectronic properties.4 PDIs are good
electron acceptors with low reduction potential.5 Therefore PDIs
should be promising candidates for application as fluorophores
in fluorescent probes based on PET. However, probes based
on PDIs have been rarely reported so far.6 Recently, Zhang
and co-workers reported a PDI compound with photochromic
spiropyranes connected at the bay positions.6a The fluorescence
of this compound is cooperatively controlled by UV light, ferric
ions and protons. Li and co-workers have developed a novel
PDI compound by connecting two tridentate ligands, i.e. 4-[3,5-
bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl] benzoic acid, to the PDI
core.6b The fluorescence of this PDI was quenched significantly
upon coordination to ferric ions. Very recently, Li and co-
workers have successfully grafted the PDIs onto the surface of
gold nanoparticles and developed a fluorometric and colorimetric
sensor for Cu2+.6d
In the present paper, we report two PDI based probes (PDI-
1 and PDI-2) with “turn-on” fluorescence output. In these two
probes, the PDI unit is connected to two di-(2-picolyl)-amine
(DPA) groups by different bridges. The DPA groups can selectively
coordinate with different metal ions. According to the literature,
Zn2+ can be expected to coordinate particularly efficiently.7 As long
as the DPA receptor units are not involved in metal ion binding,
they can act as electron donors when PDI is excited and thus
quench the fluorescence of the PDI (DG = -0.26 eV, see ESI‡ for
the calculation details). The binding of metal ions at the DPA unit
will block the PET between DPA and PDI and thus restore the
fluorescence of PDI. To the best of our knowledge, PDI-1 and
The design and synthesis of highly effective fluorescent probes
is thus a fundamental task for organic and analytical chemists.
To make a useful probe, a compound must contain a “receptor”,
which can selectively interact with the analytes, and a “signaling
site”, normally a strongly emitting fluorophore.2 Furthermore,
a communication mechanism between the binding and signaling
site must exist. Among the numerous mechanisms which induce
signal changes upon binding of analytes, photoinduced receptor-
to-fluorophore electron transfer (PET) has been widely used in
the design of new sensors. A “turn-on” motif based on PET,
which changes from a non-fluorescence state to a fluorescent
state upon the analyte binding, was developed in the 1980s
and is widely employed in many fluorophore chemosensors due
aDepartment of Chemistry, Shandong University, Jinan, 250014, China.
E-mail: xiyouli@sdu.edu.cn; Fax: +86-531-88564464; Tel: +86-531-
88369877
bDepartment of Chemistry, Henan Normal University, Xinxiang, 453007,
China
cInstitut fu¨r Chemie und Biochemie, Freie Universita¨t Berlin, Takustrasse 3,
D-14195 Berlin, Germany. E-mail: schalley@chemie.fu-berlin.de
† Dedicated to Professor Michael R. Wasielewski on the occasion of his
60th birthday.
‡ Electronic supplementary information (ESI) available: 1H and 13C NMR
spectra of PDI-1 and PDI-2; free energy calculation for PET from DPA
to PDI; competition experiments of PDI-1 and PDI-2; Stern–Volmer plot
of the fluorescence quenching of PDI-2 in the presence of excess of Fe3+
;
fluorescence lifetime measurements for PDI-2 in the presence of excess of
1
Fe3+; H NMR spectra of PDI-2 obtained during the titration with Zn2+
ions in DMSO-d6; fluorescence spectra of PDI-2 analog with excess of Fe3+
in DMF. See DOI: 10.1039/b921342b
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Scheme 1 Synthesis of PDI-1
Scheme 2 Synthesis of PDI-2
PDI-2 (Schemes 1 and 2) represent the first examples of “turn-on”
fluorescent probes based on PDI signaling units.
maintain complete fluorescence quenching. For the given electron
donor–acceptor pair, PDI–DPA, the electron transfer efficiency
is determined foremost by the linker between them. For the
present study, a phenyl group was chosen to connect the PDI
and DPA units in our sensors, because it was earlier proven to be
favorable in other systems.7a,c,e,g,i,l,m,8
Results and discussion
Molecular design and synthesis
The synthesis of PDI-1 is depicted in Scheme 1. Compound
1 was synthesized by treatment of aniline with 2-(chloromethyl)-
pyridine hydrochloride in aqueous NaOH solution with hexade-
cyltrimethylammonium chloride as a phase transfer catalyst with
a yield of 30%. Nitration of 1 with silica supported nitric acid at
room temperature in dichloromethane produces nitro derivative
To obtain high sensitivity for a fluorescent chemosensor with
“turn-on” out-put based on the PET mechanism, the background
fluorescence (fluorescence of the chemosensor without analyte)
must be as low in intensity as possible. Therefore the PET between
the donor and acceptor must be very quick and efficient in order to
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2 with 71% yield. Reduction of 2 with SnCl2·2H2O in dilute
HCl afforded 3, which was condensed with 1,6;7,12-tetra-(4-tert-
butylphenoxy)-perylene-3,4:9,10-tetracarboxylic dianhydride9 in
toluene with imidazole as the base to give rise to PDI-1 in 47%. The
preparation of PDI-2 is described in Scheme 2. Compound 4 was
synthesized by the condensation of 1-chloro-4-nitrobenzene with
ethylene diamine in CH3CN. Alkylation with 2-(chloromethyl)-
pyridine hydrochloride afforded N-(4-nitrophenyl)-N¢,N¢-di-(2-
picolyl) ethylene diamine 5. Reduction of 5 with NH2NH2·H2O
over graphite powder in ethanol affords N-(4-amino-phenyl)-
N¢,N¢-di-(2-picolyl) ethylene diamine 6 in 65% yield. The con-
densation of 6 with 1,6;7,12-tetra(4-tert-butylphenoxy)-perylene-
3,4:9,10-tetracarboxylic dianhydride follows the procedure used
for PDI-1 and provides PDI-2 in 87% yield. Both PDI-1 and PDI-
2 were fully characterized by 1H and 13C NMR and MALDI-TOF
mass spectra.
Fig. 1 Fluorescence spectra of PDI-1 in the presence of different metal
ions Na+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and
Pb2+ in DMF solutions. lex = 440 nm, [PDI-1] = 6.0 ¥ 10-6 M, [Mn+] =
4.8 ¥ 10-5 M (8 equiv.).
Absorption and fluorescence spectra of PDI-1 and PDI-2
The absorption spectra of these two PDI compounds show a strong
absorption band centered around 580 nm, which is typical for PDIs
with four substituents at the bay positions.10 These results suggest
that the connection of DPA groups at the imide nitrogens does
not affect the ground state of PDI. This is reasonable because
the frontier molecular orbital knots at the imide nitrogens block
the interactions between the DPA and PDI units.4a,5b For the
same reason, the UV–vis absorption spectra of PDI-1 and PDI-2
showed almost negligible changes when metal ions were present in
the solution.
As expected, the fluorescence from the PDI fluorophore in both
PDI-1 and PDI-2 was completely quenched, indicating that the
electron transfer from DPA to PDI in both PDI-1 and PDI-2
is quick and efficient. Usually, the fluorescence lifetime for the
PDI with four p-t-butylphenoxyl groups connected at the bay
positions is around 6 ns as reported in the literature.9,11 The
fluorescence lifetime of PDI-1 and PDI-2 cannot be determined
by an instrument with a 0.05 ns time limitation, which means that
the lifetimes in PDI-1 and PDI-2 are shorter than 50 ps and the
electron transfer rate constant from DPA to PDI is larger than
1.86 ¥ 1010 s-1.
fluorescence intensities of the chemosensor with (F) or without
metal ions (F0). Fig. 2 compares the EFs of various metal ions
(14 equiv.) towards the fluorescence of PDI-1 in DMF. In the
absence of metal ions, the fluorescence of PDI-1 was strongly
quenched by the PET process with a fluorescence quantum yield
(U) as small as ~0.0003. But in the presence of Ni2+, the fluorescent
intensity of PDI-1 increased by over 49-fold, with U increased to
0.016. The EFs of Co2+, Zn2+ and Cd2+ calculated following the
same method were 8 (U = 0.0025), 6 (U = 0.0019) and 1 (U =
~0.0004), respectively. PDI-1 thus shows “turn-on” fluorescence,
and displays a quite remarkably high selectivity for Ni2+ among
the various ions under study.
Fluorescence response of PDI-1 to different metal cations
The PDI fluorescence was restored in the presence of different
metal ions. Fig. 1 shows the corresponding fluorescence spectra
of PDI-1 in DMF (6 mM) after addition of different metal ions
(8 equiv.). The emission peak appears at about 609 nm, which is
the typical emission of PDI without any significant peak shift.
This result suggests no other interaction between the DPA and the
PDI unit exists in the excited state besides the efficient electron
transfer between both units in the absence of metal coordination.
As shown in Fig. 1, the most distinctive fluorescence intensity
enhancement resulted when Ni2+ was added. Addition of Co2+
and Zn2+ enhanced the fluorescence intensity of PDI-1 too, but
by an obviously much smaller magnitude. The presence of Cd2+
and other metal ions resulted in negligible fluorescence intensity
changes under identical experimental conditions.
Fig. 2 Fluorescence responses of PDI-1 (8 mM) to various metal cations
(14 equiv.) in DMF (lex = 440 nm).
The selective response of PDI-1 towards Ni2+ is also reflected
by the fluorescence lifetimes of the different fluorescent PDI-
1–metal complexes: Ni2+: 0.33 ns; Co2+: 0.25 ns; Zn2+: 0.17 ns.
The fluorescence lifetimes of other metal complexes of PDI-1 are
too short to be measured accurately. However, the fluorescence
lifetime of a standard PDI with tetraphenoxyl groups connected
at the bay positions and hexyl groups at the imide nitrogen
atoms is measured to be 5.46 ns under identical conditions. The
The fluorescence intensity enhancements are usually described
by the enhancement factor (EF), which is calculated from the
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significantly shorter fluorescence lifetimes of the metal complexes
of PDI-1 suggest a contribution from an additional non-radiative
decay process in the metal complexes, such as the incompletely
hindered electron transfer from DPA to PDI or simply a heavy
atom effect. Consequently, the different fluorescence lifetimes of
the metal complexes provide another complementary means to
discriminate Ni2+ from other transition metal ions.
The high selectivity of PDI-1 for Ni2+ is surprising because
the DPA chelating groups are expected to exhibit high affinity
towards Zn2+, Cd2+ or Cu2+, but small affinity towards other metal
cations as suggested by earlier reports.7 Moreover, metal ions with
open shell d-orbitals such as Ni2+, Cu2+ and Fe3+ frequently act
as quenchers to the fluorescence of fluorophores via electron or
energy transfer between metal ion and fluorophore.12 In contrast,
closed d-shell transition metal ions such as Zn2+ and Cd2+ usually
do not quench the fluorescence.
The high stability of the coordination compound of DPA with
Ni2+ or Cu2+ is well known in the literature.13 However, in earlier
studies the binding properties of DPA with metal ions varied
according to the fluorophores and the linkers, as well as the
experimental conditions. In most cases, DPA has been shown to
have good affinity for Zn2+ and, consequently, was previously used
in zinc-selective sensors.7a,b,d-j,l-q In some cases, DPA showed good
selectivity toward Cd2+ in a biological environment,7c but it is not
known to show high selectivity towards Ni2+ over Cd2+, and Zn2+.
This must be related to the electron transfer mechanism in PDI-
1. The electron donor in PDI-1 is the nitrogen atom connected
directly to the phenyl linker. When this nitrogen atom coordinates
to metal ions, its electron donating ability is reduced and thus the
electron transfer from the nitrogen atom to the PDI p-system
is hindered and the fluorescence restored. Although plenty of
examples suggested that DPA forms stable complexes with Zn2+,
crystal structures of these complexes revealed that the aniline
nitrogen atom does not always participate in the coordination.14a,b
Thus, even when PDI-1 coordinates to Zn2+, the aniline nitrogen
atom might be still available for fluorescence quenching. In
contrast, crystal structures of Ni2+ complexes of different DPAs
revealed this nitrogen atom to be always coordinated to the nickel
cation.14a,15 Therefore the coordination of Ni2+ induces significant
enhancement of fluorescence.
Fig. 3 ESI mass spectra of 8 mM DMF solutions of PDI-1 with (a) 4
equiv. of NiCl2, (b) 4 equiv. of both NiCl2 and NaCl, (c) 4 equiv. of both
NiCl2 and FeCl3, and (d) 4 equiv. of both NiCl2 and Zn(NO3)2.
[PDI-1·(NiCl)(FeCl2)]2+ complex or a [PDI-1·(FeCl2)2]2+ complex.
However, this is not the case. Ni2+ obviously binds more strongly.
Finally, when ZnCl2 is added as the second transition metal salt,
a mixed-metal [PDI-1·(NiCl)(ZnCl)]2+ ion is observed with low
intensity. The two isotope patterns of this ion and the nickel
homodimer overlap. In addition, signals for anion exchanges are
observed, because two different counterions were used in this
experiment. Still, the nickel complex is by far the most abundant
species appearing in this mass spectrum and thus these MS
results nicely confirm the analysis of the fluorescence experiments
described above.
Fluorescence titration and competition experiments
To get further insight into the stoichiometry and the stability of
the PDI-1–Ni2+ complexes, a Job plot and fluorescence titration
experiments were performed at room temperature in DMF, Fig. 4a.
The 1 : 2 binding mode between PDI-1 and Ni2+ as observed by
mass spectrometry was also clearly supported by the data of Job’s
ESI mass spectrometry of PDI-1 metal complexes
The higher stability of the Ni2+ complex of PDI-1 over that of
Zn2+ and other metal ions could qualitatively be confirmed by the
ESI mass spectra obtained from competition experiments. In these
experiments, PDI-1 was mixed with 4 equiv. of two different metal
salts in DMF. The solution was afterwards used to record ESI
mass spectra. Fig. 3 shows the results. The first spectrum serves as
a reference. When 4 equiv. NiCl2 are mixed with PDI-1, the major
signal in the spectrum corresponds to a doubly charged [PDI-
1·(NiCl)2]2+ complex at m/z 858. As expected, the stoichiometry
of the complex is 1 : 2 and both coordination sites are occupied
by a Ni2+ ion, of which three binding sites are occupied by the
DPA ligand. The fourth ligand is the chloride counterion. Not
unexpectedly, the doubly charged Ni2+ complex is the dominating
signal for a mixture of nickel and sodium chloride (Fig. 3b);
no signal is observed for sodium coordination. When FeCl3 is
used as the competitor, one might have expected to find either a
plot. A plot of F versus X[Ni ([Ni2+]/([Ni2+]+[PDI-1])) showed
2+
]
that the F value arrived at its maximum at a molar fraction of ca.
0.68, Fig. 4a confirming the 1 : 2 binding stoichiometry.
Based on the 1 : 2 stoichiometry, the Benesi–Hildebrand equa-
tion can be used to evaluate the binding strength:16
F
min and F are the fluorescence intensity of PDI-1 in the absence
and presence of Ni2+, respectively. Fmax is the fluorescence intensity
obtained with a large excess of Ni2+, Ka the association constant
of the Ni2+ complex of PDI-1, and [Ni2+] was the concentration
of Ni2+. As shown in Fig. 4b and c, the plot of 1/(F - Fmin
)
against 1/[Ni2+]2 is linear. From the Benesi–Hildebrand analysis,
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Competition experiments were also conducted by recording
fluorescence spectra of 8 mM DMF solutions of PDI-1 with Ni2+
(4 equiv.) and Na+, Cr3+, Mn2+, Fe3+, Co2+, Cu2+, Zn2+, Cd2+, Hg2+,
or Pb2+(12 equiv.) ions (Figure S5, in the ESI‡). Na+, Cr3+, Mn2+,
Fe3+, Co2+ ions did not interfere significantly with the fluorescence
of the PDI-1–Ni2+ complex; Co2+,Cu2+, Zn2+, Cd2+, Hg2+, and
Pb2+ induced slight fluorescence quenching, although they were
present in 3-fold excess as compared to the Ni2+ concentration.
These results confirm the ability of PDI-1 to sense Ni2+ with high
selectivity even in the presence of other metal ions.
Paramagnetic metal ions, such as Fe3+, Cu2+ and Ni2+, usually
induce significant fluorescence quenching via electron transfer or
energy transfer from the fluorophore to the metal complexes.
As one of the important quenching mechanisms, the redox
interaction, i.e the electron transfer between the fluorophore and
metal ions, can be efficiently reduced by simply increasing the
electron affinity of the fluorophore.17 PDI is hard to oxidize so the
electron transfer from the excited state to the metal ion is difficult.
One may argue that the hindered electron transfer between PDI-1
and Ni2+ might also be attributed to the virtual decoupling between
the PDI fluorophore and the terminal receptors DPA because of
the almost perpendicular arrangement of the PDI plane relative
to the plane of the phenyl bridge, rendering electron transfer
from the PDI excited state to Ni2+ unfavorable as described in
literature.18 It is worth noting that only one example, which had
a fluorescence enhancement output in the presence of Ni2+, has
been reported so far in the literature.19 But it cannot distinguish
Ni2+ from Cu2+ and Zn2+ due to the less selective binding of the
cryptate to the transition metal ions. To the best of our knowledge,
PDI-1 represents the first example of a fluorescent probe for Ni2+
with “turn-on” output that can distinguish Ni2+ from Zn2+, Cu2+,
Fe3+ and other transition metal ions.
Metal-sensing properties of PDI-2
The response of PDI-2 to the presence of various transition metal
ions is different from that of PDI-1 (Fig. 5). The fluorescence
intensity enhanced significantly in the presence of Fe3+ while only
Fig. 4 (a) Job’s plot for determining the stoichiometry of PDI-1 and
Ni2+ in DMF, where the integrated fluorescence was plotted against the
mole fraction of Ni2+ ([Ni2+]/([Ni2+]+[PDI-1])). (b) Fluorescence titration
spectra of PDI-1 (7 mM) upon increasing the concentration of Ni2+ in DMF.
The excitation wavelength was 440 nm. (c) Relative fluorescence intensities
(F - Fmin) as a function of the concentration of Ni2+ in DMF. The inset
shows the Benesi–Hildebrand plot of 1/(F - Fmin) against 1/[Ni2+]2.
Fig. 5 Fluorescence spectra of PDI-2 in the presence of different metal
ions including Na+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
and Pb2+ in DMF solutions (lex = 440 nm, [PDI-2] = 5 mM, [Mn+] =
20 mM, (4 equiv.)).
the association constant Ka is determined to be 2.7 ¥ 109 M-2, thus
confirming the high nickel affinity of PDI-1.
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a slight increase in the fluorescence intensity was observed in the
presence of Co2+, Zn2+ and Cu2+. Other metal ions resulted in
negligible changes of the fluorescence spectra as compared to free
PDI-2. The EFs of PDI-2 in the presence of different metal ions
are compared in Fig. 6. The largest EF is that of Fe3+, which
amounts to 138, since the fluorescence quantum yield, U, increased
from ~0.0004 to 0.04. The EFs of Cu2+, Co2+, and Zn2+ are 18
(U = 0.0076), 10 (U = 0.0038) and 7 (U = 0.0034), respectively.
This result suggests that PDI-2 afforded remarkable “turn-on”
fluorescence, and showed high selectivity towards Fe3+ among the
various ions being investigated, including Zn2+ for which again a
significant increase would have been expected.
Fig. 7 1H NMR spectra of PDI-2 and PDI-2·Zn2+ in DMSO-d6.
linkage do not change much, which indicates that the aniline
nitrogen atoms do not participate in the coordination. The efficient
binding of the PDI-2 with Zn2+ was further confirmed by the 1H
NMR titration experiment as shown in Figure S10 (see the ESI‡).
We do not find a gradual shift of the signal positions, but instead
see some new peaks after the addition of Zn2+. This suggests that
the complex between PDI-2 and Zn2+ is very stable. This is also
supported by the fact that the 1H NMR spectra of PDI-2 change
dramatically when the mole ratio (Zn2+/PDI-2) is in the range
of 0.5–2, but do not change any further after the mole ratio is in
excess of 2. On the one hand this verified the 1 : 2 stoichiometry for
the PDI-2–Zn2+ complex, as well as on the other hand indicating
that the complex between PDI-2 and Zn2+ has very high stability
This has proved that PDI-2 can bind to Zn2+ with high stability.
However, because the electron donor, the amino group connected
directly at the phenyl ring, does not participate in the coordination,
the presence of Zn2+ does not restore the fluorescence of PDI-2. We
cannot record the 1H NMR spectrum of PDI-2 in the presence of
Fe3+ because of the paramagnetic nature of the complex. Therefore,
we do not know how the Fe3+ ions coordinate with the receptor
EPDA at this stage, but the ESI mass spectra of the 1 : 2 mixture of
PDI-2 and FeCl3 show clearly one peak, which corresponds to a
complex with a 1 : 2 stoichiometry, [(PDI-2)(Fe2O(Cl2)]2+ (Fig. 8).
This result indicates that the complex formed between PDI-2 and
FeCl3 again has a 1 : 2 stoichiometry.
Fig. 6 Fluorescence response of PDI-2 (5 mM) to various metal cations
(4 equiv.) in DMF. Bars represent EF (enhancement factor F/Fo), lex
440 nm.
=
The high selectively of PDI-2 towards Fe3+ was further con-
firmed by the competition experiment (Figure S6 in the ESI‡).
When the mixed solution of Fe3+ (4 equiv.) with Na+, Cr3+,
Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, or Pb2+(8 equiv.) was
added to the solution of PDI-2 (5 mM), the Na+, Cr3+, Mn2+,
Cd2+ ions showed no disturbance to the fluorescence of PDI-2;
Co2+, Ni2+, Hg2+, Pb2+, Cu2+ and Zn2+ induced small fluorescence
quenching. Therefore, these tested metal cations had no significant
interference to the sensing by PDI-2 of iron(III) ions, and PDI-2
exhibited high selectivity towards Fe3+ while coexisting with other
metal ions.
In PDI-2, the DPA group was connected to the phenyl
group by an ethylene diamine bridge, which represents another
popular ionophore, N-ethyldipicolylamine aniline (EDPA). The
ethylene diamine linker provides an additional nitrogen atom
for coordination to the metal ions. EDPA has been employed
widely in various fluorescence sensors with different fluorophores
based on PET.7b,e,g,i,n These sensors show exclusively high selectivity
towards Zn2+. The high selectivity of PDI-2 towards Fe3+ over
Zn2+ is thus unexpected. Furthermore, the structural change from
DPA to EDPA changes the binding behavior of our PDI sensors
significantly by shifting the selectivity from Ni2+ to Fe3+. The
1H NMR spectra of PDI-2 in the absence and presence of 2
equivalents of Zn2+ in DMSO-d6 were recorded (Fig. 7). The
significant shifts of the EDPA signals relative to those observed in
the absence of Zn2+ suggest the efficient coordination of EDPA
with Zn2+. However, the signals of the protons on the phenyl
Fig. 8 ESI mass spectrum of a 1 : 2 mixture of PDI-2 and FeCl3·6H2O in
DMF. (The only major signal corresponds to a 2 : 1 iron complex of PDI-2
whose charges are counterbalanced by one oxo and two chloride anions.
While the source of the oxo ligand is likely the water from the metal salt, it
is not fully clear why no other combinations of counterions are observed.)
The fluorescence enhancements of PDI-2 in the presence of Fe3+
at different concentrations are shown in Fig. 9. With the increase
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Paramagnetic Fe3+ is a well-known efficient fluorescence
quencher like Ni2+. Consequently, developing novel probes for
Fe3+ with fluorescence enhancement is still a challenging task.
Up to now, molecular probes that selectively show amplified
fluorescence in the presence of Fe3+ are still rare. Only a few
examples have been reported in the literature.21 For instance,
the probes based on receptor 1-oxa-4,10-dithia-7-aza macro cycle
developed by Bricks and co-workers show high selectivity towards
Fe(III) with turn-on output in methanol.21a Some rhodamine-based
spirolactam probes exhibit significant fluorescence enhancement
in the presence of Fe3+ because of the exchange between spirocyclic
and ring-open forms.21b-d Bru¨ckner and colleagues developed
a squarate hydroxamate-coumarin-based chemosensor for Fe3+
with turn-on output based on oxidation reactions.21e Recently,
a phenanthroimidazole-based probe developed by Lin and co-
workers showed ratiometric fluorescence response to Fe3+.21f PDI-
2 represents another example of the small collection of Fe3+ probes
with turn-on fluorescence output.
Fig. 9 Relative fluorescence intensities (F/Fo) of PDI-2 (3 mM) as a
function of the concentration of Fe3+ in DMF.
of the concentration of Fe3+, the fluorescence intensities of PDI-
2 increase significantly at the early stage (<3 equivalents), but
decrease again after the concentration of Fe3+ exceeds this ratio.
A similar phenomenon has earlier been found for a rhodamine
based Cu2+ sensor and was attributed to the formation of H
aggregates of the metal complexes with different stoichiometry.20
But in our case, no significant changes in the absorption spectra
were observed. We ascribe this fluorescence quenching in the
presence of excess Fe3+ to the dynamic quenching of free Fe3+
to the fluorescent [(PDI-2)(Fe2O(Cl2)]2+ based on the following
two reasons. The first is the Stern–Volmer plot (I0/I vs. [Fe3+],
see the ESI‡, Figure S7) in the concentration range of [Fe3+] >
4[PDI-2], which gives a linear relationship between I0/I and
[Fe3+] and suggests no interactions between the fluorescent [(PDI-
2)(Fe2O(Cl2)]2+ and quencher Fe3+. The second reason is that
the lifetimes of [(PDI-2)(Fe2O(Cl2)]2+ in the presence of excess
Fe3+ show mono-exponential decay (see the ESI‡ Figures S8–
S9). This result suggests that [(PDI-2)(Fe2O(Cl2)]2+ is the only
fluorescent component and excludes the presence of new fluo-
rescent species in the presence of excess Fe3+. Both the Stern–
Volmer plot and the fluorescence lifetime measurements reveal
that the excess Fe3+ does not induce new fluorescent species in
the reaction mixture. Therefore, the quenching caused by excess
Fe3+ might be induced simply by the dynamic collision between
Fe3+ and [(PDI-2)(Fe2O(Cl2)]2+. To verify this hypothesis, the
fluorescence titration experiment was conducted with a PDI ana-
log, N,N¢-di-n-butyl-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-
3,4:9,10-tetracarboxylic diimide, which does not have metal
binding sites. The dynamic fluorescence quenching was assuredly
observed with the increasing of the concentration of Fe3+ in DMF
(Figure S11 in the ESI‡). This result indicates that the suggested
dynamic fluorescence quenching mechanism for the PDI-2–Fe3+
complex by excess of Fe3+ is reasonable.
Conclusion
In summary, two novel “turn-on” fluorescent probes bearing PDI
as the fluorophore were successfully prepared and their sensing ca-
pabilities for transition metal ions were examined. PDI-1 exhibited
high selectivity for Ni2+ in the presence of various metal cations
including Zn2+, Cd2+ and Cu2+ which were expected to interfere. A
1 : 2 stoichiometry was obtained from mass spectrometry as well
as a Job’s plot and non-linear fitting of the fluorescence titration
curves. To the best of our knowledge, this is the first example
of a fluorescent probe for Ni2+ that demonstrated significant
fluorescent enhancement. PDI-2 presented high selectivity toward
Fe3+ with a fluorescence enhancement factor as high as 138.
Most interestingly, both Ni2+ and Fe3+ are paramagnetic metal
ions, which are normally known as fluorescence quenchers. Thus,
it is challenging to develop “turn-on” fluorescent probes for
them. The result of our research suggests that PDIs are favorable
fluorophores for “turn-on” fluorescence probes for paramagnetic
transition metal ions. Considering the multitude of choices for
the structure modification of PDI, we are confident that the
design strategy presented here will help to develop a new class of
excellent PDI-based chemosensors with practical application in
many sensing fields, such as environmental research and biology.
Experimental section
General methods
1H and 13C NMR spectra were recorded on a Bruker 400 MHz
NMR spectrometer with chemical shifts reported in ppm (in
CDCl3, TMS as internal standard). MALDI-TOF mass spectra
were taken on a Bruker/ultraflexinstrument. ESI mass spectra
were recorded on a Varian/Ionspec QFT-7 Fourier-transform
ion-cyclotron-resonance (FT-ICR) mass spectrometer with a
Micromass Z-spray ESI ion source. Absorption spectra were
measured on HITACHI U-4100 spectrophotometer. Fluorescence
emission spectra and fluorescence lifetime were measured on an
ISS K2 system. IR spectra were recorded on a Nicolet Impact
400D infrared spectrophotometer.
Because of the drop in the fluorescence intensity after the
maximum at [Fe3+] = 3 equivalents of PDI-2, the fitting of the
titration curve following the Benesi–Hildebrand equation does
not give any reliable results. Therefore the stoichiometry as well as
the association constant can neither be determined quantitatively
by the fluorescence titration nor by a Job’s plot.
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Fluorescent response experiments
N,N¢-Bis-(N,N-di-(2-pyridylmethyl)-aniline)-1,6,7,12-tetra-
(4-tert-butylphenoxy)-perylene-3,4:9,10-tetracarboxylic-diimide
Stock solutions (1 mM) of each metal salt, PDI-1 and PDI-2
(0.1 mM) in DMF were prepared. Test solutions were prepared
by placing 0.3–1 ml of the probe’s stock solution into a test tube,
adding an appropriate aliquot of each metal stock solution, and
then diluting the solution to 10 ml with DMF to give the final
concentration. After complete mixing, measurements of UV–vis
absorption and fluorescent emission were carried out on above
mentioned spectrophotometers with a 1 cm standard quartz cell.
(PDI-1).
perylene-3,4:9,10-tetracarboxylic
A
mixture of 1,6:7,12-tetra(4-tert-butylphenoxy)-
dianhydride (820 mg,
0.83 mmol), compound 3 (830 mg, 2.86 mmol) and imidazole
(3.00 g, 44.05 mmol) in toluene (100 mL) was refluxed under
N2 for 3 h. After the solvent was evaporated, the residue was
dissolved in chloroform and washed with water to remove the
imidazole. The collected organic phase was dried over anhydrous
MgSO4 for overnight. After evaporation of the chloroform, the
crude product was purified by column chromatography on silica
gel (200–300 mesh) using 100 : 3 (v/v) CHCl3–MeOH as the
eluent. A subsequent recrystallization from a mixture of CHCl3
and MeOH gave pure product. PDI-1 was collected as a dark
purplish solid (598.6 mg, 0.391 mmol, 47%); 1H NMR (400 MHz,
CDCl3, 25 ◦C, TMS): d = 8.58 (d, 4H; pyridyl), 8.20 (s, 4H;
perylene), 7.64 (tri, 4H; pyridyl), 7.30 (d, 4H; pyridyl), 7.22 (d,
8H; phenyl), 7.15 (tri, 4H; pyridyl), 7.01 (d, 4H; phenyl), 6.84 (d,
8H; phenyl), 6.78 (d, 4H; phenyl), 4.85 (s, 8H; NCH2), 1.26 (s,
36H; C(CH3)3); 13C NMR (400 MHz, CDCl3, 25 ◦C, TMS): d =
163.9, 156.4, 156.0, 152.9, 149.7, 148.2, 147.3, 136.9, 133.1, 129.0,
126.6, 124.5, 122.7, 122.1, 120.9, 120.6, 120.2, 119.7, 119.3, 112.9,
57.4, 34.3, 31.4; MS (MALDI-TOF): m/z: 1529.8 [M+]; Calcd for
C100H88N8O8 (m/z): 1529.8; IR (KBr) [cm-1] 2958 (C–H), 2866
Materials
The salts used in stock solutions of metal ions were
NaCl, CrCl3·6H2O, MnCl2·4H2O, FeCl3·6H2O, CoCl2·6H2O,
NiCl2·6H2O, Cu(OAc)2·H2O, Zn(NO3)2·6H2O, CdCl2·2.5H2O,
Hg(NO3)2,
Pb(NO3)2.
1,6,7,12-Tetra(4-tert-butylphenoxy)-
perylene-3,4:9,10- tetracarboxylic dianhydride,22 compounds 1,7c
423 and 523 were synthesized according to literature methods.
Other chemicals were purchased from commercial sources.
Solvents were of analytical grade and purified by standard
methods.
4-Nitro-N,N-di-(2-pyridylmethyl)-aniline (2). To the HNO3
solution (100 ml, 65%), SiO2 (30 g, 200-300 mesh) was rapidly
added with vigorous stirring at room temperature over 5 min.
After stirring at room temperature for 3 days, the nitrated dry
SiO2 was obtained. To a stirred suspension of the nitrated SiO2
(7 g) in CH2Cl2 (200 ml), compound 1 (1.7 g, 6.18 mmol) was
added. After being stirred vigorously at room temperature for
10 min, a dark-green suspension formed, which was further
neutralized to pH 7.5–8 with triethylamine (TEA). The resulting
brownish suspension was stirred for another 10 min, after which
the SiO2 was filtered and washed several times with CH2Cl2. After
evaporation of the solvent on a rotary evaporator, compound 2
(1.4 g, 4.38 mmol) was obtained as yellow oil in 71% yield through
column chromatography (silica 200–300 mesh, CH2Cl2–CH3OH
100/2, v/v); 1H NMR (400 MHz, CDCl3, 25 ◦C, TMS): d = 8.58
(d, 2H), 8.00 (d, 2H), 7.66 (tri, 2H; pyridyl), 7.24 (d, 2H), 7.19 (tri,
2H; pyridyl), 6.75 (d, 2H), 4.94 (s, 4H; NCH2); MS (ESI): m/z:
321.36 [M+H+]; Calcd for C18H16N4O2: 320.35.
=
=
=
(C–H), 1707 (C O), 1671 (C O), 1589 (C C, perylene ring).
N-(4-Amino-phenyl)-N¢,N¢-[di-(2-pyridylmethyl)]-ethylenedi-
amine (6)25. Compound 5 (2.62 g, 7.22 mmol), NH2NH2·H2O
(85%, 10 ml), and graphite (6 g) were heated in refluxing EtOH
(150 mL) for 24 h under N2 protection. After being cooled to
room temperature, the reaction mixture was diluted with CH2Cl2
(150 mL). Graphite was separated from the reaction mixture by
filtration. After evaporation of the solvent, compound 6 (1.56 g,
4.70 mmol) was collected as light brown oil in 65% yield via
column chromatography (silica 200–300mesh, CH2Cl2–CH3OH
100/6, v/v). 1H NMR (300 MHz, CDCl3, 25 ◦C, TMS): d = 8.54
(d, 2H), 7.65 (tri, 2H), 7.59 (d, 2H), 7.13 (tri, 2H), 6.60 (d, 2H),
6.52 (d, 2H), 3.87 (s, 4H; NCH2C), 3.48 (br, 2H, NH2), 3.13 (tri,
2H), 2.87 (tri, 2H).
N,N¢-Bis-(N¢¢-2-(N¢¢¢,N¢¢¢-di-(2-pyridylmethyl)-amino-ethyl-
ene)-aniline)-1,6,7,12-tetra-(4-tert-butylphenoxy)-perylene-3,4:9,
10-tetracarboxylic-diimide (PDI-2).
A mixture of 1,6:7,12-
4-Amino-N,N-di-(2-pyridylmethyl)-aniline (3)24. To a solution
of 2 (2.23 g, 6.96 mmol) in ethanol (100 ml), a mixture of
concentrated HCl (50 ml) and SnCl2·2H2O (9.5 g, 42 mmol) was
added at room temperature. The resulting yellow solution was
refluxed for 24 h. After being cooled to room temperature and
diluted with H2O, the resulting mixture was carefully alkalized
to pH 7.5–8 with a 30% NH4OH aqueous solution. A white
suspension formed in the reaction mixture. The reaction mixture
was extracted with ethyl acetate, and the combined organic phases
were dried over anhydrous MgSO4 overnight. After evaporation
of the solvent, compound 3 (0.82 g, 2.83 mmol) was obtained as
white solid in 41% yield via column chromatography (silica 200–
tetra-(4-tert-butylphenoxy)-perylene-3,4:9,10-tetracarboxylic di-
anhydride (800 mg, 0.825 mmol), compound 6 (1.24 g, 3.72 mmol),
imidazole (3.00 g, 44.05 mmol) and toluene (120 mL) was refluxed
under N2 for 3 h. After the solvent was evaporated under reduced
pressure, the residue was dissolved in chloroform and washed
with water to remove the imidazole. The collected organic phase
was dried over anhydrous MgSO4 and evaporated to dryness. By
recrystallization from a mixture of CHCl3 and MeOH, excess
compound 6 was removed. After a second recrystallization from a
mixture of CHCl3 and n-hexane, the pure product was obtained.
PDI-2 was collected as a purplish-red solid (1.16 g, 0.716 mmol,
87%). H NMR (400 MHz, CDCl3, 25 ◦C, TMS): d = 8.55 (d,
1
1
300 mesh, CH2Cl2–CH3OH 100/5, v/v); H NMR (400 MHz,
4H; pyridyl), 8.23 (s, 4H; perylene), 7.62 (tri, 4H; pyridyl), 7.43
(d, 4H; pyridyl), 7.21 (d, 8H; phenyl), 7.14 (tri, 4H; pyridyl), 6.97
(d, 4H; phenyl), 6.86 (d, 8H; phenyl), 6.64 (d, 4H; phenyl), 3.90 (s,
8H; NCH2C), 3.18 (tri, 4H; NHCH2), 2.90 (tri, 4H; NHCH2CH2),
1.26 (s, 36H; C(CH3)3); 13C NMR (400 MHz, CDCl3, 25 ◦C, TMS):
CDCl3, 25 ◦C, TMS): d = 8.55 (d, 2H), 7.58 (tri, 2H), 7.28 (d,
2H), 7.12 (t, 2H), 6.58–6.50 (m, 4H), 4.72 (s, 4H; NCH2), 3.37 (s,
2H; NH2); MS (ESI): m/z: 291.17 [M+H+]; Calcd for C18H18N4:
290.37.
1024 | Org. Biomol. Chem., 2010, 8, 1017–1026
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d = 163.9, 159.1, 156.0, 152.9, 149.1, 148.7, 147.3, 136.5, 133.1,
128.9, 126.6, 123.9, 123.2, 122.8, 122.2, 120.6, 120.2, 119.8, 119.3,
112.9, 60.4, 52.8, 41.4, 34.3, 31.4; MS (MALDI-TOF): m/z: 1615.5
[M+]; Calcd for C104H98N10O8 (m/z): 1615.9; IR (KBr) [cm-1] 3418
(q) N. C. Lim, S. V. Pavlova and C. Bruckner, Inorg. Chem., 2009, 48,
1173–1182.
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Rowan, Adv. Mater., 2006, 18, 1251–1266; (c) H. Langhals, Helv. Chim.
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1564–1579.
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4437–4440; (b) C. Zhao, Y. Zhang, R. Li, X. Li and J. Jiang, J. Org.
Chem., 2007, 72, 2402–2410; (c) W. Xu, H. Chen, Y. Wang, C. Zhao,
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(d) B. A. Jones, M. J. Ahrens, M.-H. Yoon, A. Facchetti, T. J. Marks and
M. R. Wasielewski, Angew. Chem., Int. Ed., 2004, 43, 6363–6366; (e) K.
Muthukumaran, R. S. Loewe, C. Kirmaier, E. Hindin, J. K. Schwartz,
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=
(N–H), 3056 (C–H), 2958 (C–H), 2866 (C–H), 1707 (C O), 1671
=
=
(C O), 1589 (C C, perylene ring).
Acknowledgements
Financial support from the Natural Science Foundation of China
(Grant No. 20640420476, 20771066), Ministry of Education of
China, and Shandong University are gratefully acknowledged. We
also thank the Deutsche Forschungsgemeinschaft and the Fond
der Chemischen Industrie for funding.
6 (a) X. Guo, D. Zhang and D. Zhu, Adv. Mater., 2004, 16, 125–130;
(b) Y. Li, N. Wang, H. Gan, H. Liu, H. Li, Y. Li, X. He, C. Huang, S.
Cui, S. Wang and D. Zhu, J. Org. Chem., 2005, 70, 9686–9692; (c) Y.
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