α-monoacylated and α,α′- and α,β′- diacylated dipyrrins as highly sensitive fluorescence turn-on Zn2+ probes

Article abstract of DOI:10.1021/jo400454e

With the purpose of developing readily synthesized CHEF (chelation-enhanced fluorescence) type Zn²⁺ probes with relatively simple molecular structures and excellent sensing behavior, p-anisoyl chloride was used for the acylation of 5-(pentafluorophenyl)dipyrromethane. Interestingly, the α,β′-diacylated product PS2 with a unique substitution mode was obtained in high yield in addition to the normal α-substituted mono- and diacylated products PS1 and PS3. Further oxidation of PS1-PS3 afforded dipyrrins S1-S3. Crystal structure and ¹H NMR measurements of S2 demonstrate the existence of a pure tautomer, which is consistent with DFT calculations. S1-S3 show highly Zn²⁺ selective turn-on fluorescence based on a CHEF mechanism by the formation of 2:1 (probe:metal) Zn²⁺ complexes. The emission colors can be easily tuned from green to red by changing the dipyrrin substitution modes. Furthermore, these probes demonstrate fast responses and wide applicable pH ranges. Among them, S2 shows the highest Zn²⁺ sensitivity, with a detection limit of 4.4 × 10 ^-8 M.

Full text of DOI:10.1021/jo400454e

Article
pubs.acs.org/joc
α‑Monoacylated and α,α′- and α,β′-Diacylated Dipyrrins as Highly
Sensitive Fluorescence “Turn-on” Zn2+ Probes
Yubin Ding,† Xin Li,‡ Tong Li,† Weihong Zhu,† and Yongshu Xie*,†
†Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai
200237, People’s Republic of China
‡Department of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691
Stockholm, Sweden
S
* Supporting Information
ABSTRACT: With the purpose of developing readily synthesized
CHEF (chelation-enhanced fluorescence) type Zn2+ probes with
relatively simple molecular structures and excellent sensing behavior,
p-anisoyl chloride was used for the acylation of 5-(pentafluoro-
phenyl)dipyrromethane. Interestingly, the α,β′-diacylated product
PS2 with a unique substitution mode was obtained in high yield in
addition to the normal α-substituted mono- and diacylated products
PS1 and PS3. Further oxidation of PS1−PS3 afforded dipyrrins S1−
S3. Crystal structure and 1H NMR measurements of S2 demonstrate
the existence of a pure tautomer, which is consistent with DFT
calculations. S1−S3 show highly Zn2+ selective “turn-on” fluorescence based on a CHEF mechanism by the formation of 2:1
(probe:metal) Zn2+ complexes. The emission colors can be easily tuned from green to red by changing the dipyrrin substitution
modes. Furthermore, these probes demonstrate fast responses and wide applicable pH ranges. Among them, S2 shows the
highest Zn2+ sensitivity, with a detection limit of 4.4 × 10−8 M.
complicated intermolecular interactions, and the binding
moiety may simultaneously act as the reporting moiety. Thus,
the molecular structure may be relatively simple and easy to
design and synthesize.18 In addition, chelation effects usually
induce strong binding, and the resulting probes may have high
sensitivity. However, it is still a challenging task to develop
easily synthesized CHEF type Zn2+ probes with novel
structures and a combination of excellent sensing behavior
including fast response, low detection limit, and wide applicable
pH range.
As a component of BODIPY, dipyrrins consist of two
pyrrolic units connected by an sp2 meso carbon atom, and the
conjugated π system gives rise to an intense absorption in
visible region.19 In recent years, dipyrrins have been used as
bidentate monoanionic ligands for various applications such as
the construction of interesting supermolecular structures and
syntheses of fluorescent metal complexes.19b,20 In this respect,
Cheprakov and co-workers synthesized π-extended dipyrrins
capable of forming brightly fluorescent Zn2+ complexes.21 In
addition, boron complexes of dipyrrins (BODIPY) are well-
known for their excellent fluorescence properties.22 Encouraged
by this inspiring research on highly fluorescent dipyrrin
complexes and the fact that dipyrrin itself has rather weak
fluorescence, we reported dipyrrins D1−D3 as “turn on” type
fluorescent Zn2+ probes based on a CHEF mechanism23 and
INTRODUCTION
■
Synthetic fluorescent ion probes are highly valuable tools for
the selective recognition of chemically and biologically
important ions, with the advantages of convenient utilization,
high sensitivity, rapid response, and low cost.1 Zinc ion plays
vital roles in numerous biological processes.2 Thus, Zn2+
sensing based on fluorescence quenching or enhancement has
attracted increasing interest in recent years.1i,3 Fluorescence
quenching may be caused by a number of factors other than the
target analyte, and thus the sensing behavior may be
nonspecific. Thus, “turn-on” type fluorescent Zn2+ probes are
highly desirable. For the development of such probes, usually a
binding moiety is linked by a bridge to a fluorophore, which
acts as the reporting moiety. Representative fluorophores
include fluorescein,4 anthracene,5 naphthalimide,6 coumarin,7
quinoline,8 cyanine dyes,9 rhodamine,10 and BODIPY.11 With
use of these fluorophores, a number of “turn-on” type
fluorescent Zn2+ probes have been developed on the basis of
various sensing mechanisms, including PET (photoinduced
electron transfer),12 ICT (intramolecular charge transfer),13
FRET (fluorescence resonance energy transfer),14 excimer,15
AIE (aggregation induced emission),16 and CHEF (chelation-
enhanced fluorescence).17 Among these, CHEF type probes are
based on the simple idea that intramolecular rotations will be
suppressed by chelation so that nonradiative energy loss of the
excited state will be decreased and thus the fluorescence will be
enhanced. For the design of such probes, there is no need to
modulate complicated electron or charge transfer processes or
Received: March 8, 2013
© XXXX American Chemical Society
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demonstrated that the substituents at the α positions of the
pyrrolic units greatly affect the sensing behavior.
RESULTS AND DISCUSSION
■
Syntheses and Characterization of Probes S1−S3.
Compounds S1−S3 were readily synthesized by acylation of 5-
(pentafluorophenyl)dipyrromethane with p-anisoyl chloride,
followed by oxidation with DDQ, and fully characterized by
1H NMR, 13C NMR, and HRMS (Figure 1a and Figures S1−
S18 (Supporting Information)). As we know, acylated dipyrro-
methanes are important intermediates for the syntheses of
porphyrins and porphyrinoids, because they can be easily
reduced to the corresponding alcohols and then used for
further condensation with pyrrolic derivatives.25a,b Especially,
pentafluorobenzoyl-substituted dipyrromethanes have attracted
extensive interest in this area due to the fact that most
expanded porphyrins with meso-pentafluorophenyl groups have
better stability than those with other aryl substituents.27 It is
well-known that electrophilic acylation reaction of pyrrole
occurs at an α-carbon atom, the position adjacent to the
nitrogen atom, because attack of an electrophile at the α-
position leads to a more stable cationic intermediate with three
resonance forms, while reaction at a β-carbon atom gives a less
stable cation with only two resonance forms.28 Thus, the typical
acylation of a dipyrromethane through the Friedel−Crafts
reaction will afford two products: namely, the α-monoacyl
dipyrromethane and the α,α′-diacyl dipyrromethane.29 In this
work, with the aim of increasing the Zn2+ binding ability of the
resulting dipyrrins, we used p-anisoyl chloride to acylate the
dipyrromethane by a Friedel−Crafts reaction. Unexpectedly,
three main products were observed by TLC analysis. After
separation of the crude product by silica gel columns, we
obtained products PS1−PS3. 1H NMR measurements revealed
that PS1 is an α-monoacyl product with two types of pyrrolic
On the basis of our previous work on the aforementioned
Zn2+ probes and other relevant systems in this area,23,24 we
envisioned that further introduction of electron-donating
groups to the benzoyl substituent of the D3 molecule may
further enhance its coordination with Zn2+ and thus further
improve the Zn2+ detection sensitivity. Therefore, we attempted
to acylate 5-(pentafluorophenyl)dipyrromethane by Friedel−
Crafts reactions, using p-anisoyl chloride as an acylating agent.
It is well-known that the acylation of dipyrromethanes with
pentafluorobenzoyl chloride and some other acid chlorides
always afford α-monoacylated and α,α′-diacylated dipyrro-
methanes, which have been widely used for the syntheses of
porphyrins and porphyrinoids.25 In contrast to this, we
unexpectedly obtained the α,β′-diacylated product PS2 in a
yield of 48% accompanied by the normal α-substituted mono-
and diacylated products PS1 and PS3 (Figure 1a). Oxidation of
PS1−PS3 with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none) afforded acylated dipyrrins S1−S3, which show a CHEF
type “turn on” fluorescence upon addition of Zn2+. The
detection limit of S2 for Zn2+ was calculated to be 4.4 × 10−8
M, which is more sensitive than the normal α-substituted
mono- and diacylated products S1 and S3.
The control and tuning of emission wavelengths and colors
of fluorescent probes can expand their multiplexing capability in
practical applications, thus drawing much attention from the
scientific community.26 In this work, by changing the numbers
and positions of the substituents, the calculated HOMO−
LUMO energy gaps of Zn2+ complexes of S1−S3 decreased
successively, resulting in a concomitant successive increase of
the emission maxima wavelengths, with the emission colors
varying from green to red. Furthermore, competition experi-
ments demonstrated that S1−S3 can selectively detect Zn2+ in
both DMF- and water-containing systems. Thus, probes S1−S3
can be developed as a novel and promising type of highly
sensitive and selective “turn-on” fluorescent Zn2+ probes with
tunable emission colors. Impressively, S2 shows a unique α,β′-
substitution mode, good sensitivity, and orange emission color.
1
NH signals (Figure 1b). The H NMR of PS3 demonstrated a
symmetrical structure with only one type of pyrrolic NH, which
1
is consistent with an α,α′-diacyl structure (Figure 1b). The H
NMR of PS2 shows the same hydrogen number as PS3, but the
hydrogen signals are rather unsymmetrical, exhibiting two types
of pyrrolic NH signals (Figure 1b), indicating that PS2 is also a
diacyl product but the substitution mode is different from that
of PS3. HRMS of PS2 and PS3 also shows almost identical
molecular peaks at 581.1498 and 581.1497 ([M + H]+, Figures
S6 and S9 (Supporting Information)), which further indicates
that PS2 should be a diacyl product and that one of the acyl
substituents must not be attached to the α position. To clearly
1
elucidate the identity of the structure of PS2, the H−1H
COSY NMR spectrum (Figure 2) of PS2 was collected. It can
1
Figure 1. (a) Syntheses and structures of probes S1−S3. (b) Low-field region of the H NMR spectra of PS1−PS3 (CDCl3, 400 MHz, 298 K).
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Figure 2. 1H−1H COSY NMR spectrum of PS2 (500 MHz in CDCl3 at 298 K) with the coupling modes between the pyrrolic hydrogens shown for
all possible unsymmetric isomers.
be observed that the hydrogens at ∼7.8 ppm (peak 3) couple
with those at ∼6.9 ppm (peak 5) and the hydrogen at ∼6.75
ppm (peak 6) couples with that at ∼6.08 ppm (peak 8).
Considering the coupling modes, chemical shifts, and hydrogen
numbers, the peaks at ∼7.8 and ∼6.9 ppm can be assigned to
the eight phenylene hydrogens, and the peaks at ∼6.75 and
∼6.08 ppm can be assigned to two pyrrolic hydrogens, with two
uncoupled pyrrolic hydrogens left at ∼7.2 ppm (peak 4) and
∼6.5 ppm (peak 7), respectively. Only the 1,8- or the 2,7-
substituted structure (Figure 2) is consistent with these
observations. On the other hand, the α-pyrrolic hydrogens
usually appear at lower field than the β hydrogens, and in the
1H NMR spectrum of PS2, only one typical α-hydrogen can be
observed at ∼7.2 ppm, which is consistent with the 1,8-
substitution mode. This assignment is also in agreement with
the crystal structure of the corresponding oxidized product
(vide infra). The mechanism for the generation of the α,β′-
diacylated product is still unclear to us at the present stage. It is
well-known that the utilization of pentafluorobenzoyl chloride
as the acylating agent always affords the α- and α,α′-substituted
products.27b Thus, the generation of the α,β′-diacylated
product may be related to the electron-donating effect of the
methoxy substituent on the benzoyl chloride.
substitution mode of PS2 and the corresponding oxidized
product S2, we tried to grow their single crystals. Fortunately,
single crystals of S2 were obtained and analyzed by X-ray
diffraction, which clearly indicated that the p-anisoyl groups
were really attached to the α,β′-positions of the pyrrolic units
(Figure 3a), as expected from the aforementioned results. To
the best of our knowledge, this is the first example of an α,β′-
diacylated dipyrrin utilized for Zn2+ detection.
Theoretically, unsymmetrically substituted dipyrrins have
two tautomeric forms (Scheme 1), with the proton shifting very
fast between the two pyrrolic units. Interestingly, both crystal
structure and 1H NMR measurements clearly demonstrate that
S2 exists in a pure tautomeric form whose proton is bound to
the α-acylated pyrrolic nitrogen (Figure 3a and Figure S13 in
the Supporting Information). In the crystal structure, the C4−
C5 and C5−C6 bond lengths are observed to be 1.440(3) and
1.367(3) Å, respectively. These values are typical of C−C single
bonds and CC double bonds, respectively. The C1−N1−C4
bond angle has a large value of 110.8(2)°, which is typical for a
pyrrolic amino NH moiety. Also, the C9−N2−C6 bond angle
has a much smaller value of 105.2(2)°, which is in agreement
with a typical pyrrolic imino N moiety. It is noteworthy that the
imino N2 atom is involved in an intramolecular hydrogen bond
with the amino NH donor, with an H1···N2 distance of 2.09 Å.
In addition, two classes of intermolecular hydrogen bonds are
observed between the phenyl hydrogen atoms and the oxygen
atoms from the methoxy and the carbonyl moieties, with O···H
distances of 2.48 and 2.43 Å, respectively. The corresponding
C−H···O angles are 154 and 164°, respectively. Finally, a 2D
supramolecular network is formed by linkage of the
intermolecular hydrogen bonds (Figure 3b). The wide
We then tried to oxidize PS1−PS3 by DDQ in CH2Cl2. PS1
was easily oxidized to compound S1 by stirring with 2.0 equiv
of DDQ for 1 h in a yield of 61%. PS2 and PS3 were more
difficult to oxidize. More DDQ and/or longer times were
required. Thus, PS2 was successfully oxidized to compound S2
by 2.0 equiv of DDQ for 24 h in a yield of 69%, and 8 equiv of
DDQ and a longer time of 36 h were required for the oxidation
of PS3 to S3 in a high yield of 97%. To further elucidate the
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free energy of S1A is 10.3 kJ mol−1 lower than that of S1B,
resulting in a S1A:S1B ratio of 1:0.016 (Figures S19 and S20 in
the Supporting Information).30
Titration Study. Compounds S1−S3 show rather weak
fluorescence in DMF, with small quantum yield values of 0.013,
0.017, and 0.005, respectively, using rhodamine 6G as a
reference.31 Upon addition of Zn2+ to respective solutions of
probes S1−S3, significant fluorescence enhancement was
apparent to the naked eye. To investigate the Zn2+ sensing
behavior in detail, UV−vis and fluorescence spectral changes of
these probes upon gradual addition of Zn2+ were measured in
both DMF and aqueous solutions (DMF/50 mM HEPES, 4/1
v/v, pH 7.4). For S2, for example, the titration of Zn2+ to its
DMF solution induced a hypsochromic shift from 550 to 544
nm (Figure 4a). A set of isosbestic points was observed at 300,
386, 485, and 558 nm, indicating the formation of a well-
defined complex between S2 and Zn2+. This process was
accompanied by a sharp fluorescence “turn-on” behavior. As
shown in Figure 4b, the solution of S2 showed very weak
fluorescence before the addition of Zn2+. When Zn2+ was
gradually added, its fluorescence intensity was significantly
enhanced, with the emission peak centered at about 571 nm.
Similar spectral changes were observed when the solutions of
S1 and S3 were titrated with Zn2+ (Figures S21a and S22a in
the Supporting Information). Vivid fluorescence enhancement
was observed with the emission peaks centered at 549 and 588
nm for S1 and S3, respectively (Figures S21b and S22b in the
Supporting Information). These results are indicative of good
fluorescence “turn-on” type Zn2+ detection capability of
compounds S1−S3. Hence, we continued to check the sensing
mechanism in detail.
Figure 3. (a) Crystal structure of compound S2 with ellipsoids shown
at the 20% probability level. (b) 2D supramolecular network formed
by the linkage of intra- and intermolecular hydrogen bonds. The
pentafluorophenyl groups and non-hydrogen-bonded H atoms are
omitted for clarity.
Scheme 1. Interconversion between Two Tautomeric Forms
of S2
Sensing Mechanism. Before addition of Zn2+, the free
probes S1−S3 show very weak background fluorescence. This
can be ascribed to a loss of excited state energy via free rotation
of pyrrolic units in the molecules. From the aforementioned
titration results, we found that addition of 0.5 equiv of Zn2+ to
solutions of S2 almost saturated its UV−vis and fluorescence
spectral changes, suggesting that S2 forms a 2:1 complex with
Zn2+. This conclusion is consistent with the corresponding Job
plot, wherein the fluorescence intensity reaches a peak when
the ratio S2:Zn2+ is 2:1 (Figure S23 in the Supporting
Information). The 2:1 binding stoichiometry was also observed
in the Job plots for S1 and S3 (Figures S24 and S25 in the
Supporting Information). These results indicate that the
fluorescence enhancement is induced by the formation of 2:1
Zn2+ complexes of probes S1−S3. From nonlinear fitting of the
occurrence of intra- and intermolecular hydrogen bonds may be
favorable for the stabilization of the clearly fixed tautomer and
conformation. These results are in agreement with DFT
calculations, which reveal that the free energy of the tautomeric
form S2A is 13.2 kJ mol−1 lower than that of S2B (Scheme 1),
resulting in a S2A:S2B ratio of 1:0.005. Similarly, compound S1
also predominantly exists in such a tautomeric form whose
proton is bound to the α-acylated pyrrolic nitrogen, and the
Figure 4. (a) UV−vis spectral changes during a titration of S2 (10 μM) with Zn2+ in DMF. (b) Corresponding fluorescence emission spectral
changes with λex fixed at 485 nm (one of the isosbestic points).
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fluorescence data according to a 2:1 binding model,32 Zn2+
binding constants were calculated to be 5.0 × 1013, 2.2 × 1013,
and 2.1 × 1013 M−2 for S1−S3, respectively (Figures S26−S28
in the Supporting Information). To further elucidate the
coordination structure, we successfully synthesized complexes
Zn(S1)2, Zn(S2)2, and Zn(S3)2 in high yields by stirring S1−
S3 with Zn(OAc)2·2H2O in MeOH, and the complexes were
1
fully characterized by H NMR, 13C NMR, HRMS, and IR
(Figures S29−S37 in the Supporting Information). In
comparison to free probes S1−S3, 1H NMR signals of
Zn(S1)2−Zn(S3)2 were clearly shifted upfield, and in the
HRMS, molecular peaks were observed at 951.1003, 1219.1744,
and 1219.1747 ([M + H]+), respectively. These values are in
accordance with the calculated values. Fortunately, single
crystals of Zn(S1)2 were obtained by slow evaporation of a
MeOH/H2O solution of Zn(S1)2. X-ray diffraction analyses
clearly revealed a 2:1 coordination mode, which is consistent
with the aforementioned results. The carbonyl O atoms are
weakly coordinated to Zn2+, with Zn−O1 and Zn−O3 bond
lengths of 2.37(9) and 2.46(2) Å, respectively. These values are
similar to those observed for our previously reported dipyrrin
zinc complexes.23 In the molecule of Zn(S1)2, the coordinated
dipyrrin is nearly planar, showing a rather small dihedral angle
of 6.8° between the two pyrrolic units (Figure 5).
Figure 6. Fluorescence spectra of probes S1−S3 (10 μM) in the
presence of 1 equiv of Zn2+ in DMF. The inset shows corresponding
photographs under 365 nm UV light.
anisoyl group at the β-position of the other pyrrolic unit, which
enlarges the π-electron conjugation system. In S3, two p-anisoyl
substituents were also introduced, with both of them attached
to the α-positions of the pyrrolic units, which results in
conjugation even better than that of S2. Thus, red shifting of
the emission colors from S1 to S3 can be rationalized by the
increasing size of π-electron conjugation systems. To gain
further insight, we performed density functional calculations on
optimized geometries of the complexes employing the hybrid
B3LYP function. The HOMO−LUMO energy gaps of
Zn(S1)2, Zn(S2)2 and Zn(S3)2 were calculated to be 2.89,
2.78, and 2.67 eV, respectively. These results indicate that the
decrease of HOMO−LUMO energy gaps results in a
concomitant successive increase of the emission maxima
wavelengths (Figure 7). Thus, it can be concluded that tuning
of the emission colors of the dipyrrins can be conveniently
realized by changing the substituent numbers and positions.
These results demonstrate that probes S1−S3 provide good
examples of a convenient designing strategy for novel emission
color tunable fluorescent probes or light-emitting organic
materials.
High Sensitivity. The detection limit is one of the most
important parameters in cation sensing. For many practical
purposes, it is very important to detect the analytes at low
concentrations. The detection limits of probes S1−S3 for Zn2+
were obtained according to the reported method.33 We take S2
as an example: first a titration of S2 (0.1 μM) with Zn2+ (0−
0.12 μM) was carried out in DMF, and fluorescence changes
during the titration were recorded and are shown in Figure
S38a (Supporting Information). The enhancement of fluo-
rescence intensity during gradual addition of Zn2+ was clearly
observed with a good signal-to-noise ratio. Then a plot of (I −
Figure 5. X-ray crystal structure of Zn(S1)2·MeOH complex with
ellipsoids shown at the 20% probability level. The methanol solvent is
omitted for clarity.
Thus, addition of Zn2+ to solutions of S1−S3 leads to the
formation of the corresponding zinc complexes with a 2:1
stoichiometry, and the resulting complexes are more rigid and
planar than corresponding free probes and loss of excited-state
energy through intramolecular rotations is suppressed, which
results in the observed chelation-enhanced fluorescence
(CHEF).23
Emission Color Modulation. Ready control and modu-
lation of emission wavelengths of fluorescent probes are
critically important for expanding their multiplexing capability
of various application requirements.26 As shown in Figure 6,
upon addition of 1 equiv of Zn2+, solutions of probes S1−S3
show emission colors varying from green to red. These
differences originate from the difference in the size of π-
electron conjugation systems caused by the substituents in the
molecules. Only one p-anisoyl substituent is attached to one of
the pyrrolic units of probe S1, which results in the smallest π-
electron conjugation system, while S2 has one additional p-
I
min)/(Imax − Imin) versus log [Zn2+] was drawn (Figure S38a,
inset, in the Supporting Information), where Imax represents the
maximum fluorescence intensity observed upon the addition of
0.12 μM Zn2+ and Imin represents the minimum fluorescence
intensity of probe S2 before the addition of Zn2+. Thus, a good
linear relationship could be obtained in the range 10 nM−0.1
μM. A linear regression of the five intermediate values (Figure
S38b in the Supporting Information) afforded an intercept of
−7.36 at the x axis, which corresponds to the detection limit of
4.4 × 10−8 M. Similarly, detection limits of S1 and S3 were
found to be 2.6 × 10−7 and 1.7 × 10−7 M, respectively (Figures
S39 and S40 in the Supporting Information). These data
indicate that all three probes show high sensitivity toward Zn2+.
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Figure 7. Frontier orbital energy diagrams of [Zn(S1)2], [Zn(S2)2], and [Zn(S3)2].
Figure 8. Relative fluorescence intensity of 10 μM S2 in DMF upon excitation at 485 nm (one of the isosbestic points): (a) spectra in the presence
of various metal ions; (b) graph with white bars representing the addition of 1 equiv of metal ions (100 equiv for Na+, K+, and Ca2+ and 10 equiv for
Mg2+) and black bars representing the addition of 1 equiv of Zn2+ mixed with 1 equiv of the indicated metal ions (100 equiv for Na+, K+, and Ca2+
and 10 equiv for Mg2+).
The detection limits at the 10−7 M level for S1 and S3 are
comparable to those for our previously reported dipyrrin-based
Zn2+ probes D1−D3.23 It can be concluded that the number of
α-substituted acyl groups does not significantly affect the
sensitivity of the probes. It is noteworthy that S2 shows the
lowest detection limit of 4.4 × 10−8 M, indicating that the α,β′-
diacylation mode affords the best sensitivity toward Zn2+. Thus,
S2 can be applied to detect Zn2+ at an extremely low
concentration level of 10−8 M, which may fully meet the
requirements in biosensing, as labile Zn2+ in the human body
was reported to exist at a millimolar concentration level.34 The
higher sensitivity of probe S2 in comparison with S1 and S3
may be rationalized by the following observations. Upon
addition of 1 equiv of Zn2+ to the respective solutions of probes
S1−S3, a 33-fold fluorescence enhancement was observed for
S2, while only 10-fold and 9-fold enhancements were observed
for S1 and S3, respectively. Therefore, to induce detectable
fluorescence changes, a smaller quantity of Zn2+ is required for
the solution of S2, in comparison with those of S1 and S3. In
other words, probe S2 demonstrates the highest sensitivity.
From these results, it can be concluded that the substitution
position has a great influence on the sensing sensitivity.
ions should not disturb the detection of the target ion. To our
delight, probes S1−S3 meet these two requirements for
selective detection of Zn2+. In the case of S2 in DMF (Figure
8) or in a HEPES buffer (Figure S41 in the Supporting
Information), when 1 equiv of various cations was added, only
Zn2+ significantly enhanced the fluorescence by about 33-fold,
whereas other divalent metals (Cd2+, Hg2+, Co2+, Ni2+, Cu2+,
Pb2+, Mg2+) and in vivo abundant alkali-metal cations (for Na+,
K+, and Ca2+, 100 equiv was added, since they are abundant in
living systems) only induced slight fluorescence changes
(Figure 8a). As is well-known, fluorescent Zn2+ probes may
encounter interference by other cations, especially Cd2+ and
Cu2+. Thus, competition behavior was checked to further
elucidate whether the coexistence of competing metal cations
interferes with the detection of Zn2+. In the solutions of probe
S2, upon addition of Zn2+ together with other competing
cations (Figure 8b and Figure S38d in the Supporting
Information), similar fluorescence enhancement was clearly
observed. Only the presence of Cu2+ slightly quenched the
fluorescence. Fortunately, Cu2+ is not typically present at high
concentrations in biochemical systems, and the interference of
Cu2+ in potential biochemical applications of S2 can be
neglected.35 Thus, it can be concluded that the detection of
Zn2+ with S2 is not seriously affected by the presence of
competing metal cations. Similar measurements for S1 and S3
also showed that these probes have good selectivity toward
Zn2+ Selectivity. In addition to sensitivity, selectivity is
another major issue in the field of ion sensing. For an ideal
fluorescent ion probe, only the target ion can induce drastic
fluorescence changes, and the coexistence of other competing
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Zn2+ (Figures S42 and S43 in the Supporting Information).
Accordingly, S1−S3 can be established as highly sensitive and
selective “turn-on” fluorescent Zn2+ probes.
Response Time and pH Influences. Response time is also
an important practical parameter for fluorescent probes. After
addition of 1 equiv of Zn2+ to the respective solutions of the
probes, fluorescence changes were tracked as a function of time
(Figure 9), which indicated that, for S1 and S2, the
Information). At lower or higher pH values, fluorescence of the
Zn2+-containing solutions is sharply decreased, which may be
attributed to the decomposition of the Zn2+ complexes. Under
strongly acidic conditions, deprotonation of the probe molecule
will be suppressed, and thus it tends to exist in the neutral and
noncoordinated form. On the other hand, under strongly basic
conditions, some basic species such as OH− will displace the
coordinated probe molecules from the complexes and thus
decompose the fluorescent Zn2+ complexes, which will
inactivate the probes. Therefore, the probes work effectively
around the neutral pH range, which may fully meet the
requirements for practical applications, such as fluorescent
imaging in living cells.
CONCLUSIONS
■
In conclusion, with the purpose of developing readily
synthesized CHEF type Zn2+ probes with relatively simple
molecular structures and excellent sensing behavior, the three
mono- and diacylated dipyrrins S1−S3 were readily synthesized
and developed as highly sensitive fluorescent Zn2+ probes.
Among these compounds, S2 shows a unique α,β′-diacylation
mode. In addition, its crystal structure demonstrates the
existence of a pure tautomer. Addition of Zn2+ to solutions of
S1−S3 induced drastic fluorescence enhancement, whereas
other divalent or in vivo abundant alkali-metal cations only
induced slight fluorescence changes. Competition experiments
further demonstrated that S1−S3 can selectively detect Zn2+
with little disturbance from other metal ions. A combination of
1H NMR, HRMS, Job plots, and X-ray crystal structure analyses
indicated that the fluorescence enhancement is induced by the
formation of 2:1 Zn2+ complexes of S1−S3. These complexes
have better planarity and rigidity than the corresponding probe
molecules, and thus intramolecular rotation is suppressed,
resulting in the “CHEF” type fluorescence enhancement. DFT
calculations indicated that, from S1 to S3, the HOMO−LUMO
energy gaps of the corresponding Zn2+ complexes are decreased
successively, resulting in a concomitant successive increase of
the emission maxima wavelengths. Thus, the emission colors
can be easily tuned from green to red by changing the
substitution numbers and positions. In addition, these probes
demonstrated fast response toward Zn2+, and they work
effectively around the neutral pH range.
In summary, novel acylated dipyrrins were synthesized and
applied as fluorescence “turn-on” Zn2+ probes with the
advantages of high sensitivity, high selectivity, fast response,
wide applicable pH range, and tunable emission colors. In
comparison with most porphyrin-based zinc probes,37 the
dipyrrin-derived probes have the advantages of easy synthesis
and modification, and they demonstrated higher sensitivity and
faster response to Zn2+, because porphyrins have rigid
macrocyclic structures, which results in relatively slow
coordination reactions and weak CHEF effects due to the
negligible rigidity enhancement upon coordination. The
detection limit of S2 reached a low value at the 10−8 M level,
which is comparable to the limits reported for the best
fluorescent Zn2+ probes.1i,3a The importance of substitution at
the pyrrolic α- and β′-carbons has been well elucidated, and
these results offer a good strategy for the development of
relevant Zn2+ probes. On the other hand, the meso substituents
have also been demonstrated by us to have important effects on
the oxidation and subsequent Zn2+ sensing behavior.24e More
systematic work on the effects of the meso and the acyl
substituents is now under way in our laboratory. In addition,
Figure 9. Time course of the fluorescence response of S1−S3 (10
μM) upon addition of 1 equiv of Zn2+ in DMF.
fluorescence intensity reached 90% of the saturation value
within 25 s. In addition, only 5 s was required for S3 to reach
90% of the saturation fluorescence. This response time is
shorter than those for most reported fluorescent probes.7a,36 In
addition to fast response, the applicable pH range is also
important for practical applications. In the case of biochemical
systems, it is important for the probes to work well in the
neutral pH range. Thus, the influence of pH on the
fluorescence of S1−S3 was checked in DMF/water (4/1)
solutions. The weak emission bands of S1−S3 remained
unaffected with increasing acidity from pH 12 to 5.0 (Figure 10
and Figures S44 and S45 in the Supporting Information). Upon
addition of 1 equiv of Zn2+, S1 and S2 exhibited drastic
fluorescence enhancement, with the intensity only slightly
affected by the pH value in the pH range 6.5−9.0, indicating
that S1 and S2 work well over this wide pH range. Also for S3,
7.0−9.0 is a suitable pH range (Figure S45 in the Supporting
Figure 10. Relative fluorescence emission intensity of S2 (10 μM,
black squares) and S2 + 1 equiv of Zn2+ (red circles) over the pH
range 5−12 in DMF/water (4/1) solutions.
G
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The Journal of Organic Chemistry
Article
Anisotropic thermal parameters were applied to all non-hydrogen
atoms. All of the hydrogen atoms in these structures were located from
differential electron density maps and constrained to ideal positions in
the refinement procedure. All calculations were performed using the
SHELX-97 software package.40
the unique α,β′-diacylated dipyrromethane PS2 may be utilized
for the synthesis of porphyrinoids, such as N-confused
porphyrins.25d
EXPERIMENTAL SECTION
■
Crystal data for S2: C31H19F5N2O4, Mw = 578.48, 0.31 × 0.21 ×
General Considerations. Commercially available solvents and
reagents were used as received. Water was used after redistillation.
UV−vis absorption and fluorescence spectra were recorded with a
0.18 mm3, triclinic, P1, a = 7.2931(9) Å, b = 13.1303(14) Å, c =
̅
13.7726(15) Å, α = 92.7100(10)°, β = 96.3670(10)°, γ = 98.725(2)°,
V = 1292.8(3) Å3, F(000) = 592, ρcalcd = 1.486 Mg m−3, μ(Mo Kα) =
0.123 mm−1, T = 298(2) K, 6822 data measured on a Bruker SMART
Apex diffractometer, 4517 of which were unique (Rint = 0.0370), 381
parameters refined against Fo2 (all data), final wR2 = 0.0964, S = 1.044,
R1 (I > 2σ(I)) = 0.0528, largest final difference peak/hole +0.173/−
0.208 e Å−3, structure solution by direct methods and full-matrix least-
squares refinement against F2 (all data) using SHELXTL.
1
quartz cuvette (path length 1 cm). H NMR and 13C NMR spectra
were measured on a 400 MHz spectrometer (1H, 400 MHz; 13C, 100
MHz) at 298 K with tetramethylsilane (TMS) as the internal standard.
The electronic spray ionization (ESI) mass spectra were tested on a
TOF mass spectrometer. 5-(Pentafluorophenyl)dipyrromethane was
synthesized according to a reported procedure.38
UV/Vis Absorption Spectrum Measurements. The absorption
spectra of S1−S3 (10 μM) were measured at 25 °C in DMF and
aqueous solution (DMF/50 mM HEPES, 4/1 v/v, pH 7.4). The tested
ions Zn2+, Na+, K+, Mg2+, Ca2+, Cd2+, Hg2+, Co2+, Ni2+, Cu2+, and Pb2+
were added as acetate salts dissolved in DMF.
Crystal data for [Zn(S1)2]·MeOH: C47H28F10N4O5Zn, Mw = 984.10,
0.40 × 0.20 × 0.13 mm3, triclinic, P1, a = 12.8900(13) Å, b =
̅
16.4959(15) Å, c = 21.712(2) Å, α = 92.2310(10)°, β = 90.2100(10)°,
γ = 112.136(2)°, V = 4272.3(7) Å3, F(000) = 1992, ρcalcd = 1.530 Mg
m−3, μ(Mo Kα) = 0.672 mm−1, T = 298(2) K, 21390 data measured
on a Bruker SMART Apex diffractometer, 14733 of which were unique
Fluorescence Emission Spectral Measurements. The fluo-
rescence emission spectra of S2 (10 μM) were measured at 25 °C in
DMF and aqueous solution (DMF/50 mM HEPES, 4/1 v/v, pH 7.4),
with excitation at λ 485 nm in DMF and 498 nm in HEPES buffer.
The fluorescence spectra of S1 and S3 (10 μM) were measured in
DMF with excitation at 462 nm for S1 and 498 nm for S3 (one of the
isosbestic points). The slit width was 5 nm, and the PMT voltage was
600 V. The tested ions Zn2+, Na+, K+, Mg2+, Ca2+, Cd2+, Hg2+, Co2+,
Ni2+, Cu2+, and Pb2+ were added as acetate salts dissolved in DMF.
Quantum Yield Measurements. The fluorescence quantum
yields of S1−S3 in DMF were measured using rhodamine 6G (Φstandard
= 0.94 in EtOH31) as a reference. The quantum yield Φsample was
calculated using the equation39
2
(Rint = 0.0661), 1213 parameters refined against Fo (all data), final
wR2 = 0.1576, S = 1.032, R1 (I > 2σ(I)) = 0.0732, largest final
difference peak/hole +0.535/−0.629 e Å−3, structure solution by direct
methods and full-matrix least-squares refinement against F2 (all data)
using SHELXTL.
CCDC 914151 (S2) and 914152 ([Zn(S1)2]·MeOH) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
DFT Calculations. All calculations were carried out using the
Gaussian09 program package.30 The geometries of all compounds
were optimized by density functional calculations employing the
hybrid B3LYP function. The LANL2DZ basis set was used for the Zn
atom, while the 6-31G* basis set was adopted to describe other atoms.
Subsequent single-point calculations were performed with the same
method and basis sets but in a DMF solvent environment represented
by the polarizable continuum model.
General Procedure for the Synthesis of Compounds PS1−
PS3. Aluminum chloride (693 mg, 5.2 mmol) and p-anisoyl chloride
(0.7 mL, 5.2 mmol) were added to a three-necked round-bottom flask,
CH2Cl2 (50 mL) was added, and the solution was stirred at room
temperature under N2. After 3 h, a solution of 5-(pentafluorophenyl)-
dipyrromethane (625 mg, 2.0 mmol) in 12.5 mL of CH2Cl2 was added
dropwise at room temperature. After the mixture was stirred for
another 2 h, water was added. The aqueous layer was extracted with
CH2Cl2, and the combined extracts were washed with brine and dried
over anhydrous Na2SO4. After removal of the solvent in vacuo, the
crude product was separated by a silica gel column (eluted with 50/1
CH2Cl2/EA). PS1 was obtained in 12% yield (107 mg), PS2 was
obtained in 48% yield (561 mg), and PS3 was obtained in 21% yield
(244 mg).
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
F
nsample
Absstandard
Abssample
sample
⎜
⎜
⎟
⎟
Φ
= Φ
×
×
×
sample
standard
F
n
standard
standard
where Φ is the quantum yield, Abs is the absorbance of the solution at
the excitation wavelength, F is the corrected emission intensity, and n
is the average refractive index of the solution. The compounds were
excited at the same wavelength (λex 470 nm). The sample absorbance
at the excitation wavelength was kept as low as possible (Absex < 0.1).
The slit width was 5 nm, and the PMT voltage was 550 V for both
excitation and emission.
pH Influence Measurements. pH influence measurements were
carried out in DMF mixed with the following buffers (4/1 v/v): MES
buffer (50 mM, pH 5.7, 6.1 and 6.5), HEPES buffer (50 mM, pH 7.1
and 7.8), and CHES buffer (50 mM, pH 8.3, 8.7, 9.3 and 10.1). For
pH 11.6, a mixture of CHES and NaOH was used as the buffer system.
Apparent Association Constant (Kass) Measurements. The
association constants were calculated on the basis of the titration of the
probes S1−S3 with Zn2+. From Job plots, HRMS, and single-crystal
structure diffraction analysis, we can conclude that the binding
stoichiometry of probes S1−S3 with Zn2+ is 2:1. Thus, the
corresponding association constants were determined by a nonlinear
least-squares fit of the titration data with the following equation for a
2:1 binding model:32
Improved Synthesis of PS1. Aluminum chloride (400 mg, 3
mmol) and p-anisoyl chloride (0.4 mL, 3 mmol) were added to a
three-necked round-bottom flask, CH2Cl2 (50 mL) was added, and the
solution was stirred at room temperature under N2. After 3 h, a
solution of 5-(pentafluorophenyl)dipyrromethane (625 mg, 2.0 mmol)
in 12.5 mL of CH2Cl2 was added dropwise at room temperature. After
the mixture was stirred for another 2 h, water was added. The aqueous
layer was extracted with CH2Cl2, and the combined extracts were
washed with brine and dried over anhydrous Na2SO4. After removal of
the solvent in vacuo, the crude product was separated by a silica gel
column (eluted with 50/1 CH2Cl2/EA). PS1 was obtained in 45%
yield (402 mg).
x
x × [L]
[Zn2+] =
+
2Kass × (1 − x)2 × [L]
2
where x is the fluorescence intensity ratio (I − I0)/(Imax − I0) observed
at the corresponding emission wavelength during addition of Zn2+,
[Zn2+] is the concentration of total added Zn2+, Kass is the association
constant, and [L] is the concentration of the probe.
Crystallography. Single crystals suitable for X-ray analyses of S2
and [Zn(S1)2]·MeOH were obtained by slow evaporation of the
MeOH/H2O solutions of the corresponding compounds at room
temperature. X-ray diffraction data were collected utilizing Mo Kα
radiation (λ = 0.71073 Å). The structures were solved by direct
methods and refined with full matrix least-squares techniques.
1
PS1. H NMR (CDCl3, 400 MHz, 298 K): δ 9.64 (s, 1H), 8.32 (s,
1H), 7.88 (d, J = 8.8 Hz, 2H), 6.97 (d, J = 8.8 Hz, 2H), 6.81−6.79 (br
s, 1H), 6.77−6.76 (br s, 1H), 6.17 (t, 1H), 6.14 (s, 1H), 6.07 (t, 1H),
5.93 (s, 1H), 3.88 (s, 1H). 13C NMR (CDCl3, Bruker 100 MHz, 298
K): δ 183.6, 162.9, 136.7, 131.3, 131.1, 130.5, 126.6, 119.5, 118.7,
H
dx.doi.org/10.1021/jo400454e | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
113.7, 110.0, 108.8, 108.4, 55.5. HRMS: obsd 447.1132, calcd for
C23H16N2O2F5 ([M + H]+) 447.1132.
840 (m), 752 (m), 640 (w), 619 (w). HRMS: obsd 951.1003, calcd for
C46H25N4O4F10Zn ([M + H]+) 951.1008.
PS2. 1H NMR (CDCl3, 400 MHz, 298 K): δ 10.46 (s, 1H), 9.33 (s,
1H), 7.83 (d, J = 8.8 Hz, 2H), 7.78 (d, J = 8.7 Hz, 2H), 7.20−7.19 (br
s, 1H), 6.95−6.91 (t, 4H), 6.75−6.74 (q, 1H), 6.52 (s, 1H), 6.08−6.07
(br s, 1H), 5.93 (s, 1H), 3.87 (s, 3H), 3.86 (s, 3H). 13C NMR (CDCl3,
Bruker 100 MHz, 298 K): δ 189.6, 183.9, 163.1, 162.5, 136.4, 132.1,
131.4, 131.1, 130.2, 129.4, 125.8, 124.7, 120.1, 113.7, 113.4, 110.5,
109.4, 55.5, 55.4. HRMS: obsd 581.1498, calcd for C31H22N2O4F5 ([M
+ H]+) 581.1500.
Synthesis of Zn(S2)2. This complex was synthesized as a deep red
solid (49 mg, yield 80%) by a procedure similar to that for Zn(S1)2.
UV−vis (DMF) λmax (nm): 544. 1H NMR (CDCl3, 400 MHz, 298 K):
δ 7.92 (d, J = 8.88 Hz, 4H), 7.71 (t, 4H), 7.68 (s, 2H), 7.09 (s, 2H),
7.00 (d, J = 4.2 Hz, 2H), 6.92 (d, J = 8.88 Hz, 4H), 6.88 (d, J = 8.84
Hz, 4H), 6.75 (d, J = 4.2 Hz, 2H), 3.85 (s, 6H), 3.84 (s, 6H). 13C
NMR (CDCl3, 100 MHz, 298 K): δ 187.7, 185.9, 162.9, 162.6, 156.0,
151.4, 143.6, 141.2, 135.8, 134.3, 134.1, 131.4, 130.8, 129.5, 129.4,
120.6, 113.24, 113.16, 54.96, 54.91. IR (KBr pellet, cm−1): 3357 (w),
2917 (m), 2847 (m), 1635 (w), 1599 (m), 1552 (m), 1494 (w), 1453
(w), 1424 (m), 1300 (s), 1281 (s), 1249 (s), 1167 (s), 1067 (w), 1021
(m), 994 (s), 955 (s), 878 (s), 851 (m), 793 (w), 744 (m), 648 (w),
619 (w). HRMS: obsd 1219.1744, calcd for C62H37N4O8F10Zn ([M +
H]+) 1219.1743.
PS3. 1H NMR (CDCl3, 400 MHz, 298 K): δ 10.98 (s, 2H), 7.82 (d,
J = 8.4 Hz, 4H), 6.93 (d, J = 8.8 Hz, 4H), 6.63 (q, 2H), 6.12 (s, 1H),
6.07 (t, 2H), 3.86 (s, 6H). 13C NMR (CDCl3, Bruker 100 MHz, 298
K): δ 183.3, 162.8, 135.5, 131.7, 131.3, 130.6, 119.5, 113.4, 110.3, 55.4.
HRMS: obsd 581.1497, calcd for C31H22N2O4F5 ([M + H]+)
581.1500.
Synthesis of Zn(S3)2. This complex was synthesized as a deep red
solid (44 mg, yield 72%) by a procedure similar to that for Zn(S1)2.
UV−vis (DMF) λmax (nm): 535. 1H NMR (CDCl3, 400 MHz, 298 K):
δ 7.69 (d, J = 8.84 Hz, 8H), 6.71 (d, J = 8.84 Hz, 8H), 6.54 (t, 8H),
3.76 (s, 12H). 13C NMR (CDCl3, 100 MHz, 298 K): δ 186.5, 184.6,
163.2, 162.7, 156.2, 153.5, 143.2, 143.1, 142.4, 131.7, 131.4, 130.6,
129.5, 129.1, 127.1, 123.1, 121.1, 113.5, 112.7, 54.9, 54.8, 29.2. IR
(KBr pellet, cm−1): 3361 (w), 2921 (m), 2851 (w), 1652 (m), 1600
(s), 1561 (m), 1500 (m), 1415 (w), 1341 (w), 1250 (s), 1165 (s),
1065 (m), 992 (s), 943 (m), 889 (w), 845 (s), 748 (m), 644 (w), 615
(w). HRMS: obsd 1219.1747, calcd for C62H37N4O8F10Zn ([M + H]+)
1219.1743.
Synthesis of S1. PS1 (446 mg, 1 mmol) was dissolved in 250 mL
of CH2Cl2, then DDQ (454 mg, 2 mmol) was added. The mixture was
stirred at room temperature for 1 h and then directly purified by a
silica gel column (eluent CH2Cl2) to afford the crude product S1,
which was recrystallized from CH2Cl2 and n-hexane to afford a brown
1
solid (271 mg, yield 61%). H NMR (CDCl3, 400 MHz, 298 K): δ
12.86 (s, 1H), 8.15 (s, 1H), 7.98 (d, J = 8.8 Hz, 2H), 7.00 (d, J = 8.8
Hz, 2H), 6.81 (d, J = 4.4 Hz, 1H), 6.70 (d, J = 4.8 Hz, 1H), 6.64 (d, J
= 4.8 Hz, 1H), 6.27 (d, J = 4.4 Hz, 1H), 3.90 (s, 3H). 13C NMR
(CDCl3, Bruker 100 MHz, 298 K): δ 183.9, 163.2, 162.6, 152.1, 139.3,
135.1, 133.2, 131.5, 130.5, 127.7, 122.5, 119.9, 118.8, 113.8, 55.5.
HRMS: obsd 445.0975, calcd for C23H14N2O2F5 ([M + H]+)
445.0975.
Synthesis of S2. PS2 (580.5 mg, 1 mmol) was dissolved in 250
mL of CH2Cl2, and then DDQ (454 mg, 2 mmol) was added. The
mixture was stirred at room temperature for 24 h and then directly
purified by a silica gel column (eluent CH2Cl2/MeOH 100/1) to
afford the crude product S2, which was recrystallized from CH2Cl2 and
n-hexane to afford a red solid (396 mg, yield: 69%). 1H NMR (CDCl3,
400 MHz, 298 K): δ 12.85 (s, 1H), 8.29 (d, J = 0.84 Hz, 1H), 8.07 (d,
J = 8.8 Hz, 2H), 7.83 (d, J = 8.8 Hz, 2H), 7.03−7.01 (m, 3H), 6.98 (d,
J = 1.8 Hz, 2H), 6.93 (d, J = 4.3 Hz, 1H), 6.52 (d, J = 4.3 Hz, 1H),
3.92 (s, 3H), 3.89 (s, 3H). 13C NMR (CDCl3, Bruker 100 MHz, 298
K): δ 188.3, 184.9, 163.7, 163.4, 154.4, 147.9, 146.1, 139.6, 135.8,
132.8, 131.9, 131.3, 130.9, 129.9, 127.3, 125.3, 121.3, 113.9, 55.6, 55.5.
HRMS: obsd 579.1343, calcd for C31H20N2O4F5 ([M + H]+)
579.1343.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving the H/13C NMR and
1
HRMS spectra of compounds PS1−PS3 and S1−S3, calculated
structures of S1 and S2, UV−vis and fluorescence spectra, Job
plots, apparent association constant (Kass) determination, and
1
detection limits of S1−S3, H NMR and HRMS spectra of
complexes Zn(S1)2, Zn(S22) and Zn(S3)2, full crystal data of
S2 and Zn(S1)2·MeOH containing complete details of data
collection, crystal and unit-cell parameters, structure solution
and refinement, and tables of atomic coordinates and bond
lengths, and calculated absolute energy levels of S1−S3 and
their corresponding zinc complexes. This material is available
free of charge via the Internet at http://pubs.acs.org.
Synthesis of S3. PS3 (299 mg, 0.52 mmol) was dissolved in 175
mL of CH2Cl2, and then DDQ (945 mg, 4.2 mmol) was added. The
mixture was stirred at room temperature for 36 h and then directly
purified by a silica gel column (eluent CH2Cl2/MeOH 100/1) to
afford the crude product S3, which was recrystallized from CH2Cl2 and
n-hexane to afford a red solid (290 mg, yield: 97%). 1H NMR (CDCl3,
400 MHz, 298 K): δ 12.62 (s, 1H), 8.21 (d, J = 8.9 Hz, 4H), 7.06 (d, J
= 4.4 Hz, 2H), 7.03 (d, J = 8.9 Hz, 4H), 6.60 (d, J = 4.4 Hz, 2H), 3.91
(s, 6H). 13C NMR (CDCl3, Bruker 100 MHz, 298 K): δ 185.1, 163.7,
153.9, 143.6, 132.2, 129.5, 127.7, 126.7, 123.7, 114.0, 55.5. HRMS:
obsd 579.1336, calcd for C31H20N2O4F5 ([M + H]+) 579.1343.
Synthesis of Zn(S1)2. A mixture of S1 (44 mg, 0.1 mmol) and
Zn(OAc)2·2H2O (22 mg, 0.1 mmol) was dissolved in 50 mL of
MeOH. The mixture was stirred at room temperature for 30 min and
then directly purified by a silica gel column to afford the crude product
Zn(S1)2, which was recrystallized from CH2Cl2 and n-hexane to afford
a dark green solid (39 mg, yield 83%). UV−vis (DMF) λmax (nm):
519. 1H NMR (CDCl3, 400 MHz, 298 K): δ 7.92 (d, J = 8.88 Hz, 4H),
7.28 (s, 2H), 6.95 (d, J = 4.04 Hz, 2H), 6.90 (d, J = 8.84 Hz, 4H), 6.67
(d, J = 4.48 Hz, 2H), 6.55 (d, J = 4.0 Hz, 2H), 6.39 (q, 2H), 3.84 (s,
6H). 13C NMR (CDCl3, 100 MHz, 298 K): δ 185.9, 162.4, 158.4,
147.8, 145.29, 145.27, 145.2, 143.3, 140.2, 133.8, 131.2, 130.2, 129.7,
125.4, 123.1, 119.3, 112.9, 54.9, 52.9. IR (KBr pellet, cm−1): 3359 (w),
2923 (s), 2852 (m), 1658 (m), 1632 (m), 1596 (m), 1569 (m), 1521
(m), 1500 (m), 1455 (m), 1411 (m), 1366 (w), 1321 (s), 1306 (s),
1252 (s), 1169 (s), 1083 (w), 1019 (s), 989 (s), 941 (m), 875 (w),
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.X.: yshxie@ecust.edu.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the NSFC (21072060),
the Program for Professor of Special Appointment (Eastern
Scholar) at Shanghai Institutions of Higher Learning, Program
for New Century Excellent Talents in University (NCET), the
Innovation Program of Shanghai Municipal Education
Commission, the Fundamental Research Funds for the Central
Universities (WK1013002), and the SRFDP
(20100074110015).
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