Highly selective and sensitive fluorescence detection of Zn2+ and Cd2+ ions by using an acridine sensor

Article abstract of DOI:10.1039/c6dt00557h

Fluorescence spectroscopy investigations of the new acridine derivative bis(N,N-dimethylaminemethylene)acridine (3) show remarkable selectivity and sensitivity towards Zn²⁺ and Cd²⁺ ions in methanol and for the latter even in water.

Full text of DOI:10.1039/c6dt00557h

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DOI: 10.1039/C6DT00557H
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Highly Selective and Sensitive Fluorescence Detection of Zn2+ and
Cd2+ Ions by an Acridine Sensor
A. Visscher,a S. Bachmann,a C. Schnegelsberg,b T. Teuteberg,c R. Matac and D. Stalke*a
Received 00th January 20xx,
Accepted 00th January 20xx
Fluorescence spectroscopic investigations of the new acridine derivative bis(N,N-dimethyl-
aminemethylene)acridine (3) show remarkable selectivity and sensitivity towards Zn2+ and Cd2+
ions in methanol and for the latter even in water. Through the chelation of the metal ions the
present PET effect is quenched, significantly enhancing the emission intensity of the
DOI: 10.1039/x0xx00000x
www.rsc.org/
fluorophore. In solution, the bonding situation is studied by fluorescence and NMR
spectroscopy, as well as ESI-TOF mass-spectrometry measurements. The solid state
environment is investigated by X-ray diffraction and computational calculations. Here, we can
show the complexation of the zinc and cadmium ions by the methylene bridged amine
receptors as well as by the nitrogen atom of the acridine system. To our knowledge, this
coordination motif is unprecedented in fluorescent sensor chemistry.
significantly upon addition of an analyte. To achieve this
response several different mechanisms can be employed.
Besides intramolecular charge transfer (ICT),6 metal-ligand
Introduction
The detection of metal ions in solution has always been an
important topic in analytical chemistry, environmental
protection and medicinal applications. Metal ions like
magnesium, calcium, or zinc are essential components of many
enzymes in the human body.1 Especially Zn2+ plays a prescind
role in many different areas; e.g. in the emergence of
Alzheimer’s disease.2 Moreover, its level of concentration
could help to diagnose the growing of tumour cells in the
prostate.3 Zinc deficiency increases the susceptibility to a
variety of pathogens and is of central importance for the
immune system.4
charge transfer (MLCT)7 or excimer formation,8 an extensively
investigated concept is the photoinduced electron transfer
(PET)9 effect. Here a poor or non-fluorescent ligand starts to
emit light upon coordination of an analyte under UV light
irradiation.
The great interest in molecular sensors is reflected by the
multitude of publications.10 Acridine derivatives in particular
are commonly applied in the detection of bioorganic
compounds. For example it is frequently used as
a
fluorescence dye for DNA intercalation.11 Acridine based
sensors for cationic analytes are less commonly used12 since
they often do not show a strong enhancement of the
fluorescence emssion.13 In the following, we present the
characterization in the solid state and in solution of an
interesting new acridine based sensor.
The heavier homologue Cd2+ is known to be a very toxic metal
ion. Over several decades in the first half of the 20th century,
hundreds of people in Japan were affected by the deadly itai-
itai disease due to a cadmium polluted river.5 Therefore, it is
vital to have a fast working and very sensitive method for the
detection of metal ions in e.g. blood or water supplies.
Fluorescence spectroscopy combined with suitable fluorescent
sensors is a well-recognized method for such analysis. Sensor
molecules should be able to switch instantaneously between a
fluorescent on/off state or change their emission wavelength
Results and Discussion
Synthesis
Many of the molecular sensors reported in literature contain
14
elaborated side arms working as the receptor unit.10a,
However, another requirement to be met by a sensor is the
economic aspect of feasible and cheap availability. Therefore,
we chose a short and effective synthetic route to produce
derivatives of acridine systems with small amine side arms
(Scheme 1). The desired sensor system with the concept
a.Institut für Anorganische Chemie der Universität Göttingen, Tammannstraße 4,
37077 Göttingen, Germany. E-mail: dstalke@chemie.uni-goettingen.de
b.Institut für Organische und Biomolekulare Chemie der Universität Göttingen,
Tammannstraße 2, 37077 Göttingen, Germany.
c. Institut für Physikalische Chemie der Universität Göttingen, Tammannstraße 6,
37077 Göttingen, Germany.
“fluorophore-spacer-receptor” can be obtained in a quick two-
step synthesis. The two bromomethylene units can easily be
Electronic Supplementary Information (ESI) available: Further spectroscopic data
like NMR, UV/vis, fluorescence, and mass spectra as well as crystallographic and
computational tables. See DOI: 10.1039/x0xx00000x
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The emission intensities of both ligands areVievwerAyrticlewOenalinke,
DOI: 10.1039/C6DT00557H
whereas they show a strong absorption behaviour (SI Figure
S4.1) which hints at a working PET. Interestingly, the intensity
of the methyl derivative
3 is considerably higher than the
phenyl derivative . This observation could indicate impurities
2
and will be discussed later in full detail. In Figure 1, left, the
highest peak at 398 nm in both spectra is attributed to Raman
scattering of the used solvent methanol.17
In a next step, a metal ion screening was conducted to check
the sensing ability of the ligands. Here sixteen different metal
salts were dissolved in methanol and each was titrated to the
1·10‒5
M concentrated solutions of the ligands, respectively. To
Scheme 1: Reaction pathway to 4,5-substituted acridine derivatives.
afford a good comparability, the metal ion concentrations
were increased stepwise in the same manner (1, 5, 15,
100 eq). Additionally, only bromides were used to standardize
introduced in the 4- and 5-position through bromomethyl
methyl ether (BMME) functions and a strong acid like conc.
H2SO4.15 The bromine atoms can straightforwardly be
substituted by a variety of amines. In this publication we
present two new acridine derivatives with distinct amine
moieties: 4,5-bis(N-methyl-N-phenylaminemethylene)acridine
(2) and 4,5-bis(N,N-dimethylaminemethylene)acridine (3).
Although the two molecules are very similar only the latter
shows extraordinary fluorescence properties.
possible interference of the counter ion. With
2 as ligand
system, no significant change in emission intensity is observed
for the tested metal salts (SI Figure S5.1). To check whether
the structural motif nevertheless is suitable as a sensor in
general, we used the less polar and donating solvent
dichloromethane. In this solution we observed a massive
fluorescence enhancement by adding ZnBr2 to the ligand (SI
Figure S5.2). Since this solvent is not in the focus of our
The introduction of two receptor units directly neighboured to
the aromatic nitrogen atom holds the option to coordinate a
interest, we concentrated on the dimethylamine system 3. In
Figure 2, we compare the intensity maxima of its emission
spectra during the metal ion screening. Among the tested
sixteen metal salts this ligand shows a very high selectivity
towards Zn2+ and Cd2+ ions. The emission intensity increases
enormously. The only minute emission enhancement with
AlBr3 and SnBr2 is solely due to the strong Lewis acidity of
these metal ions which leads to slight protonation of the
ligand. This can be evidenced by the different resulting
emission wavelengths. If the ligand is protonated, the
wavelength of the emission maximum is blue-shifted to
425 nm (SI Figure S5.4), whereas in the case of Zn2+ or Cd2+
coordination the maximum is observed at 457 nm and 445 nm,
respectively. In this context it should be mentioned that
although the emission shift between the two metal ions is only
12 nm this is sufficient to differentiate the metals by
fluorescence spectroscopy. A closer look at the detailed
target cation with the side arms as well as with the fluorophore.
This is a great advantage compared to its lighter congener, the
13, 14b
widely used fluorophore anthracene.
Fluorescence Studies
The fluorescence measurements were carried out in methanol
in which both the ligands and the metal salts are readily
soluble. Moreover, this protic and very polar solvent exhibits
similar properties to water, which is important for possible
future applications. The employed structural design
(fluorophore-spacer-receptor) results in a very efficient electron
transfer which is favourable for a molecular off/on switch: The
methylene units (spacer) allow the amines (receptor) to rotate
easily about their N–C and C–C bonds. This provides the
conformational freedom to interact electronically with the
π-system of the acridine (fluorophore) and results in an
effective quenching of the fluorescence.16 At the same time
the amines are able to employ their lone pair in complexation
of analytes. If this is the case, the PET is hindered and the
emission of light is facilitated. Figure 1 shows the excitation
emission spectra depicted in Figure
3
additionally
demonstrates the high sensitivity of this sensor molecule.
and emission spectra of the ligands 2 and 3 in methanol.
Figure 2: Metal ion screening with ligand
3 M). The
in methanol (c = 1·10–5
maximum emission intensity is depicted for each metal ion addition
(λexc = 357 nm).
Figure 1: Excitation (blue, λdet = 450 nm) and emission spectra (red,
λexc = 357 nm) of (left) and methanol solution.
(right) in a 10–5
2
3
M
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Figure 3: Emission spectra of 3 titrated with ZnBr2 (left) and CdBr2 (right) in methanol; both were irradiated at 357 nm.
Figure 4: Solid state structures of [(dmf)ZnBr{(Me2NCH2)2Acr}]+ (4a, left) and [CdBr2{(Me2NCH2)2Acr}] (5, right); crystallized from a THF/DMF mixture. The omitted counter anion
in the asymmetric unit of the zinc structure is a solvent-separated [(dmf)ZnBr3]‒ unit. All hydrogen atoms were omitted for clarity. Anisotropic displacement parameters are
depicted at the 50% probability level.
Table 1: Selected bond lengths [pm] and angles [°]. The angular sums arise from the C–N–C angles.
Bond [pm]
N1‒M
N2‒M
N3‒M
Br1‒M
4a (Zn)
5 (Cd)
Angle [°]
N1‒M‒N2
N1‒M‒N3
4a (Zn)
90.05(6)
89.81(6)
128.68(6)
175.31(6)
325.0(5)
324.0(5)
–114.8(2)
112.7(2)
5 (Cd)
230.39(16)
207.89(16)
209.09(17)
239.83(4)
218.82(14)
254.07(18)
232.31(19)
231.70(19)
259.13(4)
266.16(4)
82.91(6)
82.81(6)
120.87(7)
174.08(4)
327.1(6)
327.0(6)
–105.3(3)
105.9(3)
N2‒M‒N3
N1‒M‒O1/Br2
angular sum of N2
angular sum of N3
C3–C4–C14–N2
C6–C5–C17–N3
O1/Br2‒M
After adding only 0.1 eq of ZnBr2 to the ligand solution, a This experiment can be repeated several times, confirming the
significant increase of emission intensity is observed. Besides great reversibility of the complexation of the two metal ions.
the standard titration of the analytes, we checked the Moreover, the starting intensity of the pure ligand (red line) is
reversibility of the metal complexation. Therefore, we about twice as high as the intensity after adding
alternately added a specified amount of metal salt and ethylenediamine. This indicates possible traces of impurities by
ethylenediamine (en) to the ligand (CdBr2 is depicted in Figure preparing the ligand solutions. The methanol was purchased
5, for ZnBr2 see SI Figure S5.6). With 1 eq of CdBr2 in the from VWR Chemicals® (AnalaR NORMAPUR) which provides
cuvette, the emission intensity rises from 0.2 to 3.5·105 cps analytical data for their solvents. Each kilogram contains
and is afterwards quenched to 0.1·105 cps when 10 eq of the impurities of < 0.2 mg Zn2+ and < 0.01 mg Cd2+ ions.
chelating amine are added.
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DOI: 10.1039/C6DT00557H
Figure 5: Reversibility of metal complexation of
3 with alternating addition of
CdBr2 and ethylenediamine (en); all substances are dissolved in methanol
(λexc = 357 nm).
Table 2: The maximum emission intensities of the metal complexes of 3 and their
relative emission intensities compared with anthracene. All measurements were done
5
with 10 M solutions in methanol.
compound

exc [nm]
intensity [104 cps]
relative intensity
3 + ZnBr2
3 + CdBr2
3 + Zn(OAc)2
anthracene
357
357
357
373
84.15
105.83
678.63
711.91
0.12
0.15
0.95
1.00
Scheme 2: Reaction pathways to the metal ion complexes 4, 5 and 7.
indeed the case here. The comparison of the crystal structures
containing zinc ions are discussed in the SI. The single crystals
of 4a and
5 were obtained from crystallization in a THF/DMF
Converted to the employed volume and concentration for the
measurements, every sample could contain an amount of
< 0.3 eq Zn2+ ions and < 0.009 eq Cd2+ ions with respect to the
ligand. This is a significant amount one has to consider when
comparing the results. But again, this underlines the sensitivity
of the synthesized sensor system which enables even to detect
traces of impurities. In addition, this could explain the higher
mixture. By X-ray diffraction experiments, we could determine
their solid state structures (Figure 4). The zinc complex 4a
crystallizes in the monoclinic space group P21/c and the
cadmium derivative in the chiral orthorhombic space group
P212121. Both complexes are almost isostructural. In the zinc
structure one bromide ion is substituted by a coordinating
solvent molecule DMF. The positively charged complex is
counterbalanced by a solvent-separated [ZnBr3]‒ anion in the
asymmetric unit (SI Figure S7.1).
excitation and emission spectrum of
(Figure 1), which is not sensitive to the mentioned metal ions.
In addition to the bromide salt, we investigated the influence
of Zn(OAc)2 and Zn(NO3)2 on the fluorescence properties of
3 in comparison to 2
The solid state structures indicate a coordination of the Zn2+
and Cd2+ ions by all three present nitrogen atoms. This binding
motif where the methylene bridged amines in 4- and 5-
position and the aromatic nitrogen atom of the acridine all are
involved in a metal ion coordination is currently not present in
the CSD. Only two examples with methylene bridged
phosphorous atoms coordinating a ruthenium ion can be
3
.
With small additions of the nitrate salt, the emission intensity
rises as usual whereas an excess quickly leads to its reduction.
However, using acetate as the counter anion, the fluorescence
is increased strongly even resulting in an eight times higher
emission maximum than with ZnBr2 (SI Figure S5.5).
For a better assessment of the measured fluorescence values,
the relative emission intensities are compared with the typical
fluorescent compound anthracene. In Table 2, the maximum
intensities of the used compounds and their relative intensities
divided by the fluorescence of anthracene are shown.
found in the database.18 Overall,
a structural search
concerning a coordination of an acridine unit to any metal ion
merely results in 50 hits.19
Investigations of the metal complexes
To gain a better understanding of the observed fluorescence
properties, it is necessary to know how the metal ions are
linked to the ligand. Therefore, we synthesized the two metal
complexes and investigated them with different analytical
methods. For the synthesis of
dissolved in THF whereas MeOH was used for the preparation
of (Scheme 2). In both cases, one equivalent of the metal
salts was added dropwise to , resulting in an immediate
precipitation of the metal complexes. The crystallization of
with zinc acetate to give was endeavoured to check whether
4, the ligand and ZnBr2 were
5
3
Figure 6: Superpositions of both metal ion structures (4a, 5), showing the
trigonal bipyramidal geometry of the coordination motif. The methyl groups at
the amines and the main part of the DMF molecule are omitted for clarity.
3
7
complexes with different zinc salts behave the same which is
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From the nitrogen-metal bond lengths in Table 1 it is obvious
that the averaged distances from the amine nitrogen atoms to
the metal ion (208.49(17) for Zn2+ and 232.01(19) pm for Cd2+)
is shorter than from the acridine nitrogen atom (230.39(16)
and 254.07(18) pm, respectively). The shorter distances are in
perfect agreement with literature values found in the CSD,
compared to general N–Zn2+/Cd2+ bonds. The longer distances
can still be found in the database quoted as bonds but less
frequently. One reason for this different bonding situation
results from the rotational ability of the amines to improve the
orbital overlap with the target ions. The C–N–C angles of the
sp3-hybridized amines are very close to the ideal tetrahedral
angle. The resulting best geometry forces the metal ion
underneath the acridine plane which reduces the dative
bonding to the ring nitrogen atom. Moreover, the lone pair of
that sp2-hybridized nitrogen atom is partly delocalized in the
π-system of the aromatic system and has therefore a reduced
donor capacity. The N–M bond incloses an relatively acute
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DOI: 10.1039/C6DT00557H
Figure 7: Computed difference electron density map of 4a at an isosurface level
of 0.012 eÅ–3
.
The NPA charges for the two compounds are 1.49 (
4) and 1.60
(
4a), so that the charge difference is not very significant. The
second possibility, hence the interaction with a second
bromide to weaken the coordination to the acridine seems to
be the most likely explanation for this effect. Furthermore, we
conducted calculations on the hypothetical complexes formed
angle of 44.5 °(4a) and 38.5 °(5), to the plane of the central
acridine ring. A closer look at the bonding situation and the
angle values in Table 1 reveals the coordination polyhedron of
a trigonal bipyramid at each metal atom. This geometry is
depicted in Figure 6.
in
solution,
[(MeOH)2Zn{(Me2NCH2)2Acr}]2+
and
[(MeOH)2Cd{(Me2NCH2)2Acr}]2+. Since we are interested to
replicate the conditions in solution as close as possible the
COSMO continuum model25 was applied (for standard
methanol solution conditions). The NBO results show that the
coordination to the acridine nitrogen is strengthened (21.9 and
15.0 kcal·mol–1 for Zn2+ and Cd2+, respectively). This is in line
Although the longer N–M2+ distances can still be found in the
CSD, the solid state investigations are not a sufficient evidence
for the existence of a bonding situation.20 In order to judge on
the main binding forces of the metal ion coordination, and to
relate the findings to the chemistry in solution, we conducted
a series of electronic structure calculations on the Zn2+ and
Cd2+ compounds. All structures were optimized at the B3LYP-
D3/def2-TZVPP21 level of theory. Included were the complexes
with the observations made when comparing
4 and 4a. The
complexes formed in the solid state have a weaker binding to
the acridine moiety due to the crystallization with bromide
anions. Our computed structures show smaller N1–Zn/Cd
distances in agreement with this observation. The values are
216 and 237 pm, for Zn2+ and Cd2+ respectively. In methanol
solution, sizeable interactions between the metal ion and the
N1 nitrogen atom can be confirmed. The interactions with the
side arm nitrogens are relatively constant, just slightly
enhanced in solution. In Figure 7, the computed difference
electron density map of 4a is illustrated. The electron densities
4a and
hypothetical complex
5
, identified in the solid state structures, as well as the
. The structures obtained are in good
4
agreement with the crystal data (SI). A natural bond orbital
(NBO) analysis22 was carried out to gain insight into the
coordination of the metals to the acridine derivative 3. The
data in Table 3 give the second order perturbation theory
energies23 for the interaction between the lone pairs of the
different nitrogen atoms of
3 and the zinc or cadmium ion. As
of the individual parts (3
, Zn2+, Br–, and DMF) were computed
expected, in these structures the metal has a weaker
coordination to N1, comparing to the other nitrogen atoms.24
Substitution of one bromide anion by a DMF solvent molecule
and subtracted from the electron density of 4a; all with the
same geometry. This facilitates us to illustrate the regions
where the electron density is enhanced due to the interaction
of the relevant atoms.
increases this value from 5.6 to 17.0 kcal·mol–1
(4 in
comparison to 4a). Another possibility would be that the bulky
bromide anion pulls the metal away from the acridine ring
nitrogen atom.
Table 3: NBO second-order perturbation theory analysis of the metal ion (Zn/Cd)
coordination to 3 at the B3LYP-D3/def2-TZVPP level of theory.
E(PT2) [kcal·mol–1]
N1
N2
N3
[(dmf)ZnBr{(Me2NCH2)2Acr}]+ (4a)
[ZnBr2{(Me2NCH2)2Acr}] (4)
[CdBr2{(Me2NCH2)2Acr}] (5)
[(MeOH)2Zn{(Me2NCH2)2Acr}]2+
(in MeOH)
17.0
5.6
4.7
25.4
24.7
22.8
25.4
25.1
25.4
22.7
26.5
21.9
Figure 8: 1H NMR spectra of compounds
temperature.
3–5 in DMSO-d6 at ambient
[(MeOH)2Cd{(Me2NCH2)2Acr}]2+
(in MeOH)
15.0
32.1
31.8
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are no longer broadened after the addition of NaBr. That is
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why the dynamic processes can beDOaI:t1t0r.i1b0u3t9e/Cd6DtTo005t5h7iHs
solvent/bromide exchange.
At the coalescence temperature of nearly 303 K, the
diastereotopicity of the CH2 protons becomes visible. The
signal of the four methyl groups at the amines splits into two
signals. The velocity of their hindered rotation caused by the
coordination of the zinc ion lies underneath the NMR
timescale at lower temperatures and can be monitored. Zinc
ions are spectroscopically silent which makes it difficult to
detect a ligand-metal interaction by NMR spectroscopy.
However, we measured the 15N NMR chemical shifts of the
free ligand and of the zinc complex. The comparison of the
resulting shifts shows a small but significant deshielding of
Δδ = 8.6 ppm.27
Figure 9: Temperature-dependent 1H NMR spectra of
signals are marked at 233 K.
4 in DMF-d7. The discussed
In principle, the 1H NMR spectra of
5 show similar depen-
dencies upon temperature variation (SI Figure S3.17). The
coalescence temperature in DMF-d7 is around 313 K where the
diastereotopicity of the CH2 protons becomes visible as well as
the mentioned splitting of the signal of the methyl groups. But
in this case no second set of signals appears and the spectrum
is less broadened. The advantage of this complex is the NMR
The green cloud represents the accumulated electron density.
Here, the interaction of the lone pairs of all three nitrogen
atoms with the metal ion is accentuated. When comparing the
results of the computational studies with the fluorescence
properties, it is apparent that the emission wavelength does
not change significantly by adding the metal salts to the sensor
solutions, although the aromatic system is involved in the
complexation of the metal ions. A possible explanation would
be that the coordination does not influence the π-system
significantly. Considering the geometry optimisations for the
complex in methanol solution, both metal ion interactions with
the aromatic nitrogen atom become stronger.
active 113Cd nucleus. Employing
a
2D 113Cd,1H-HMBC
experiment at low temperature, we could identify vicinal
couplings of the methyl and methylene protons to the metal
ion (SI Figure S3.18). The chemical shift of the cadmium
isotope is –345 ppm, referenced to Me2Cd. Interestingly, a
coupling to the 113Cd nucleus was only observed for one
proton of each methylene group. The coupling to the other
proton is likely not to be observed due to an unfavourable
angle between the related atoms.28 From the NMR
spectroscopic experiments it is clear that the complexes adopt
the same contact ion pairs in solution as observed in the solid
state. Another analytical method which allows investigating
the transferability of the solid state structure to solution is
This should also be the case for the titration experiment of the
fluorescence measurements. For a deeper insight into the
1
liquid phase, we recorded several H NMR spectra depicted in
Figure 8. In comparison to the spectrum of the pure ligand, the
complexes show significantly broadened signals. This is often
caused by dynamic processes. Consequently we measured the
mass spectrometry. We chose
a
time-of-flight (TOF)
spectrum of
4 at different temperatures (Figure 9). Increase of
spectrometer in combination with the mild electrospray
ionisation method because of the poor solubility in most of the
the temperature sharpens the broad signals until one definite
set of signals is obtained (353 K). At lower temperatures, the
spectrum splits into two different sets of signals with a
maximum intensity at around 253 K. At this temperature they
exhibit an intensity ratio of around 2:1 which can be
monitored by the singlet of the H-9 proton at 9.57 ppm and
9.17 ppm, respectively. This splitting is most likely to be
attributed to the exchange of the bromine atoms with the
solvent DMF. To prove this statement, we carried out a pseudo
2D 1H DOSY experiment at low temperature and added an
excess of NaBr to the NMR sample. After the addition, a third
set of signals appears whereby the intensity ratio of the other
two sets of signals is reversed (SI Figure S3.13). The diffusion
coefficient of the rising signals decreases slightly which can be
explained by the smaller radius/mass ratio of a bromine atom
compared to a DMF molecule.26 Consequently, the latter
should represent the dibrominated species (4). The new
signals have a diffusion coefficient which lies between the two
others and are therefore assigned to the monobrominated
compound (4a). Furthermore, the signals at room temperature
Figure 10: Extract from the mass spectrum of the cationic
simulated (blue) and the experimental (black) isotope pattern of the positive
charged complex.
4, showing the
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common solvents (e.g. THF, MeOH, MeCN). The To achieve an enhancement of the emission inteVnieswitAyr,ticalelOanrlginee
measurements were performed in pure THF for the zinc excess of hydrochloric acid was needed, DwOhIe: 1r0e.a10s3f9o/rC6DT00557H
, already
3
complex and in THF/H2O for the cadmium complex due to the two equivalents of the acid resulted in a notable change of the
even lower solubility. However, both complexes were spectrum (SI Figure S5.3-4). The latter was expected for this
sufficiently present in solution for the TOF spectrometer. compound since with regard to the lone pair the bonding to a
Figure 10 depicts a comparison of two mass patterns. The proton is equivalent to the coordination of a metal ion. In both
black lines represent the experimental data from the zinc cases the PET is quenched and the emission of light is
complex, whereas the blue lines illustrate the simulated facilitated. According to these results, measurements in water
spectrum for the proposed structural motif (in the positive ion were only reasonable with a buffer system in a high pH region
mode one bromide anion was omitted for the calculated (> 11). However, we could prevent these difficulties by solving
spectrum). The comparison demonstrates a perfect match of the ligand in methanol and the analyte in water. In Figure 11,
the predicted and observed species. Furthermore, the full the good sensitivity of the sensor system towards Cd2+ ions is
mass spectrum is free of any ionic fragments because only still present even in the presence of water.
signals generated by the acridine derivative, with and without
metal ion, and related fragments can be detected. The same is
valid for the Cd2+ complex (SI Figure S6.5). Both methods, NMR
spectroscopy and mass spectrometry, emphasize that the solid
state structure is maintained in solution. As mentioned in the
introduction, molecular sensors play a key role in a wide range
of areas, especially in medicinal or analytical applications. In
this publication, we have mainly shown the possibility to use
our sensor molecules in very polar solvents (fluorescence
measurements in MeOH; NMR in DMF; ESI-MS in THF).
However, using water as a solvent frequently evokes serious
challenges. Despite the poor solubility of organic molecules,
the self-ionization of water mainly results in the protonation of
the amine side-arm receptor units. This dramatically hampers
the coordination of analytes. Additionally, the pKa value of
acridine rises from 5.45 to 10.7 when getting excited in the
fluorescence spectrometer,29 an even higher value than the
aliphatic amine (~9.7, value of the related N,N-
dimethylbenzylamine).30 The formation of the protonated
species can easily be monitored by fluorescence and NMR
spectroscopy. In the laboratory, the addition of hydrochloric
Conclusions
The fluorescence spectra of ligand
ion selectivity and increase in fluorescence emission upon
titration with dissolved ZnBr2 and CdBr2 in methanol/water.
We could demonstrate the high sensitivity of the acridine
based sensor molecule and the good stability of the systems.
3 show a remarkable metal
The solid state contact ion pairs
4 and 5 determined by X-ray
structure analyses were confirmed to be present both in the
gas phase by computational chemistry and in solution by NMR
spectroscopy and mass spectrometry. This features the 4,5-
bis(N,N-dimethylaminemethylene)acridine a valuable ligand in
the medical and environmental important photochemical
detection of zinc and cadmium ions. Compared to the
numerous publications concerning zinc sensors, only very few
provide structural evidence of the metal ion coordination,
although this is inevitable for the accurate understanding of
fluorescence processes.
acid to a clear solution of
3 in toluene resulted in a yellow
Experimental Section
Materials and Measurements
precipitate. The solubility in this solvent is reduced due to the
induced charge in the product. After purification, a 1H NMR
spectrum in DMSO-d6 shows the attached proton and the
related coupling constants. Furthermore, the solid state crystal
structure of this salt could be determined (SI Figures S3.19 and
All materials were purchased from SIGMA
-ALDRICH, VWR INTER-
NATIONAL
,
and ABCR CHEMICALS and used without further
purification – except water which was purified first through a
Millipore water purification system Milli-RO 3 plus and finally
with a Millipore ultrapure water system Milli-Q plus 185. The
NMR measurements were performed on a BRUKER Avance III
300 and BRUKER Avance III HD 400 spectrometer. The chemical
shifts (δ) are reported in parts per milion (ppm) relative to the
residual proton signals of the incompletely deuterated
solvents. The fluorescence measurements were carried out on
S7.2). With 2, the protonation experiment was not feasible. On
account of the aniline like structure, the lone pair of the
nitrogen atom is partly delocalized in the phenyl ring which
results in a lower basicity (+M effect).31
a
HORIBA JOBIN-YVON Fluoromax-4 spectrometer. All emission
spectra were recorded with an excitation wavelength of
357 nm. For eletrospray ionization mass spectrometric studies,
sample solutions of c ≈ 5 mm were continuously administered
into the ESI source of a micrOTOF-Q II mass spectrometer
(
BRUKER DALTONIK) by a step motor-driven gas-tight syringe (flow
rate: 0.5 ml/h). This instrument combines a quadrupole mass
filter with a time-of-flight analyzer. The simulated isotope
patterns were calculated using the COMPASS® software package
Figure 11: Emission spectra of
dissolved in purified water (λexc = 357 nm).
3 in methanol for various concentrations of CdBr2
from BRUKER DALTONIK
.
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ARTICLE
Journal Name
Single-Crystal Structural Analysis
MgSO4, filtered and the solvent was removed under reduced
pressure. For purification, the crude dark yellow product was
View Article Online
DOI: 10.1039/C6DT00557H
Suitable single crystals for X-ray structural analysis were
selected from a Schlenk flask under an argon atmosphere and
covered with perfluorated polyether oil on a microscope slide,
recrystallized from chloroform two times to obtain a light
1
yellow powder (2.54 g, 6.96 mmol, 30.7%). H NMR (300 MHz,
CDCl3): δ 8.78 (s, 1 H, H9), 7.99 (d, 3J = 8.5 Hz, 2 H, H1,8), 7.94 (d,
3J = 6.8 Hz, 2 H, H3,6), 7.51 (dd, 3J = 8.5 Hz, 6.9 Hz, 2 H, H2,7),
5.43 (s, 4 H, CH2). 13C{1H} NMR (75.5 MHz, CDCl3): δ 145.8 (2 C,
C4a,10a), 136.8 (1 C, C9), 136.5 (2 C, C4,5), 131.3 (2 C, C3,6), 129.2
(2 C, C1,8), 126.9 (2 C, C8a,9a), 126.0 (2 C, C2,7), 30.3 (2 C, CH2). EI-
MS: m/z (%) 365 (34) [M]+•, 286/284 (76/76) [M–Br]+•, 205
(100) [M–Br2]+•. Anal. Calcd for C15H11Br2N: C, 49.35; H, 3.04;
N, 3.84; Br, 43.78. Found: C, 48.86; H, 3.06; N, 3.78; Br, 43.30.
Synthesis of 4,5-bis(N-methyl-N-phenylaminemethylene)-
which was cooled with a nitrogen gas flow using the X-TEMP2
device.32 An appropriate crystal was selected using a polarizing
microscope, mounted on the tip of a MiTeGen MicroMount,
fixed to a goniometer head and shock-cooled by the crystal
cooling device. The data for 4a were collected on an INCOATEC
Mo microsource33 with mirror optics and a Mo-Kα radiation
with λ = 71.073 pm. The data for
5 were collected at an
INCOATEC Ag microfocus source with INCOATEC Quazar mirror
optics and an Ag radiation with λ = 56.086 pm. Both
diffractometers were equipped with an APEX II detector with a
D8 goniometer and a low-temperature device. The data for 4a
acridine (2): To a suspension of
1 (0.50 g, 1.37 mmol, 1.0 eq)
and K2CO3 (0.66 g, 4.78 mmol, 3.5 eq) in MeCN (30 ml)
methylphenylamine (0.30 ml, 2.76 mmol, 2.0 eq) was added.
After stirring the reaction mixture for 4 h at rt, the volatile
components were removed under reduced pressure. The
crude product was dissolved in ethyl acetate and aqueous NaCl
solution and washed three times with brine. The organic layer
was dried over MgSO4, filtrated and the solvent was removed
in vacuo. For further purification, a recrystallization from
methanol was implemented. Thereby, the impurities were
and
5
were integrated with SAINT34 and an empirical absorption
35
correction (SADABS
)
was applied. The structures were solved
36
by direct methods using SHELXT and refined by full-matrix
least-squares methods against F2 (SHELXL
)
within the SHELXLE
37
GUI.38 The hydrogen atoms were refined isotropically on
calculated positions using a riding model with their Uiso values
constrained to equal 1.5 times the Ueq of their pivot atoms for
terminal sp3 carbon atoms and 1.2 times for all other carbon
atoms. Disordered moieties were refined using bond length
restraints and anisotropic displacement parameter restraints.39
Crystallographic data for the structures reported in this paper
have been deposited within the Cambridge Crystallographic
Data Centre. The CCDC numbers, crystal data, and
experimental details for the X-ray measurements are listed in
Table S7.2 of the SI. Copies of the data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or from the correspon-
ding author.
dissolved and filtrated off.
could be obtained as a yellow solid. H NMR (300 MHz, DMSO-
2 (0.49 mg, 1.17 mmol, 85.4%)
1
3
d6): δ 9.15 (s, 1 H, H9), 8.07 (d, J = 8.0 Hz, 2 H, H1,8), 7.55 (dd,
3
3
3J = 8.0 Hz, J = 6.9 Hz, 2 H, H2,7), 7.47 (d, J = 6.9 Hz, 2 H, H3,6),
7.12 (dd, 3J = 8.6 Hz, 3J = 7.2 Hz, 4 H, metaHPh), 6.73 (d,
3J = 8.6 Hz, 4 H, orthoHPh), 6.58 (dd, J = 7.2 Hz, 3J = 7.2 Hz, 2 H,
3
paraHPh), 5.38 (s, 4 H, CH2), 3.19 (s, 6 H, CH3). 13C{1H} NMR
(75.5 MHz, DMSO-d6): δ 149.2 (2 C, ipsoCPh), 145.8 (2 C, C4a,10a),
136.7 (1 C, C9), 135.7 (2 C, C4,5), 129.0 (4 C, metaCPh), 127.0
(2 C, C1,8), 126.8 (2 C, C3,6), 126.1 (2 C, C8a,9a), 125.6 (2 C, C2,7),
115.6 (2 C, paraCPh), 111.7 (4 C, orthoCPh), 52.6 (2 C, CH2), 38.9
(2 C, CH3). EI-MS: m/z (%) 417.2 (14) [M]+•, 310.1 (100) [M–
NMePh]+•, 205.1 (84) [M–(NMePh)2]+•. Anal. Calcd for
C29H27N3: C, 83.42; H, 6.52; N, 10.06. Found: C, 82.31; H, 6.33;
N, 9.92.
Computational studies
All geometry optimizations were carried out at the B3LYP-
D3/def2-TZVPP21, 40 level of theory (including Becke-Jones type
damping41 of the dispersion correction21f). In the case of Cd,
the Stuttgart/Dresden ECP28MDF42 was used. The electronic
densities and the corresponding NBO analysis22-23 were
computed at the same level. All calculations were carried out
with the ORCA43 program package. The B3LYP calculations
were performed under the RIJCOSX approximation44.
Synthesis of 4,5-bis(N,N-dimethylaminemethylene)acridine
(3): The synthesis procedure for
3 is the same as for 2; with 1
(1.55 g, 4.25 mmol, 1.0 eq), K2CO3 (1.76 g, 12.75 mmol, 3.0 eq),
a solution of dimethylamine in THF (2.0 m, 4.5 ml, 9.0 mmol,
2.1 eq), and MeCN (40 ml). The desired product (0.78 g,
2.66 mmol, 62.6%) could be obtained as a yellow-brown solid.
1H NMR (300 MHz, DMSO-d6): δ 9.09 (s, 1 H, H9), 8.08 (dd,
3J = 8.5 Hz, 4J = 1.6 Hz, 2 H, H1,8), 7.86 (dd, 3J = 6.8 Hz,
Synthesis of 4,5-bis(bromomethylene)acridine (1): A solution
of acridine (4.06 g, 22.6 mmol, 1.0 eq) in conc. H2SO4 (50 ml)
was heated up to 50 °C when BMME (7.4 ml, 90.6 mmol,
4.0 eq) was added. After stirring for 17 h at this temperature
the reaction mixture was cooled to rt and the formed excess of
bromine gas was discharged through an aqueous solution of
thiosulfate. The crude product precipitated by pouring into ice
(400 g) and stirring for 1 h. The yellow precipitate was filtered,
washed several times with dem. water and dissolved in
chloroform (400 ml). The solution was washed with brine (3x
200 ml) and the gathered aqueous phases were extracted with
chloroform (100 ml). The organic phases were dried with
3
4J = 1.3 Hz, 2 H, H3,6), 7.61 (dd, J = 8.5, 6.8 Hz, 2 H, H2,7), 4.28
(s, 4 H, CH2), 2.35 (s, 12 H, CH3). 13C{1H} NMR (75.5 MHz,
DMSO-d6): δ 146.2 (2 C, C4a,10a), 136.6 (2 C, C4,5), 135.9 (1 C, C9),
130.0 (2 C, C3,6), 127.4 (2 C, C1,8), 125.9 (2 C, C8a,9a), 125.6 (2 C,
C2,7), 58.2 (2 C, CH2), 45.4 (4 C, CH3). 15N-HMBC (40.6 MHz,
DMF-d7): δ –81.2 (Narom), –355.3 (NMe2). EI-MS: m/z (%) 293
(4) [M]+•, 250 (64) [M–NMe2]+•, 205 (100) [M–(NMe2)2]+•. Anal.
Calcd for C19H23N3: C, 77.78; H, 7.90; N, 14.32. Found: C, 76.45;
H, 7.77; N, 13.94.
Synthesis of 4,5-bis(N,N-dimethylaminemethylene)acridine
dibromido zinc(II) (4): 2 (175 mg, 596 μmol, 1.0 eq) was
8 | J. Name., 2012, 00, 1-3
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ARTICLE
3
4
5
(a) M. Cortesi, E. Fridman, A. Volkov, S. S. Shilstein, R. Chechik,
dissolved in THF (1 ml) and a solution of ZnBr2 (134 mg,
596 μmol, 1 eq) in THF (0.2 ml) was added dropwise. Thereby,
the zinc complex precipitated immediately. The suspension
was filtrated and washed several times with THF. The zinc
complex (145 mg, 280 μmol, 47.0%) was isolated as a yellow
powder. Suitable crystals for X-ray analysis could be grown
after recrystallization from a THF/DMF (9:1) mixture. The
A. Breskin, D. Vartsky, N. Kleinman, G. Kogan, VEie.w MArtoicrleieOl,nliVne.
DOI: 10.1039/C6DT00557H
Gladysh, M. Huszar, J. Ramon and G. Raviv, The Prostate, 2008,
68, 994-1006; (b) Z. Medarova, S. K. Ghosh, M. Vangel, R. Drake
and A. Moore, Am. J. Cancer Res., 2014, 4, 385-393.
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Wellinghausen, M. Martin and L. Rink, Eur. J. Immunol., 1997,
27, 2529-2535; (c) M. J. Salgueiro, M. Zubillaga, A. Lysionek, M.
I. Sarabia, R. Caro, T. De Paoli, A. Hager, R. Weill and J. Boccio,
Nutr. Res., 2000, 20, 737-755.
(a) K. Nogawa, A. Ishizaki and E. Kobayashi, Environ. Res., 1979,
18, 397-409; (b) C. Tohyama, Z. A. Shaikh, K. Nogawa, E.
Kobayashi and R. Honda, Arch. Toxicol., 1982, 50, 159-166.
Y. Li, T. Liu, H. Liu, M.-Z. Tian and Y. Li, Acc. Chem. Res., 2014,
47, 1186-1198.
1
crystals were formed at rt after two weeks. H NMR (400 MHz,
DMF-d7, 353 K): δ 9.25 (s, 1 H, H9), 8.22 (d, 3J = 8.3 Hz, 2 H,
3
3
H1,8), 8.01 (d, J = 7.1 Hz, 2 H, H3,6), 7.69 (dd, J = 8.3, 7.1 Hz,
2 H, H2,7), 4.52 (4 H, CH2), 2.62 (s, 12 H, CH3). 13C{1H} NMR
(100 MHz, DMF-d7, 243 K): δ 148.7 (2 C, C4a,10a), 142.0 (1 C, C9),
135.5 (2 C, C3,6), 130.5 (2 C, C1,8), 127.7 (2 C, C8a,9a), 126.4 (2 C,
C2,7), 63.1 (2 C, CH2), 48.9 (2 C, CH3), 46.4 (2 C, CH3). 15N NMR
(40.6 MHz, DMF-d7, 243 K): δ –346.7 (NMe2). ESI-TOF: m/z:
813.15 [(C19H23N3)2ZnBr2H]+, 438.05 [(C19H23N3)ZnBr]+, 294.20
[(C19H23N3)H]+. Anal. Calcd for C19H23N3ZnBr2: C, 44.00; H, 4.47;
N, 8.10; Br, 30.81. Found: C, 44.00; H, 4.50; N, 8.10; Br, 30.00.
Synthesis of 4,5-bis(N,N-dimethylaminemethylene)acridine
dibromido cadmium(II) (5): 2 (141 mg, 480 μmol, 1.0 eq) was
dissolved in methanol (0.5 ml) and a solution of CdBr2 (131 mg,
480 μmol, 1.0 eq) in methanol (1.5 ml) was poured into. Upon
addition, the cadmium complex precipitated immediately.
Another 5 ml of methanol were added and the suspension was
filtered. The residue was washed several times with methanol
(3x 5 ml) and dried under reduced pressure. The pure product
(245 mg, 433 μmol, 90.2%) was obtained as a yellow powder.
The product was dissolved in a THF/DMF (3:2) mixture, heated
up to the boiling point of THF and then cooled to rt overnight.
The crystals thus formed were suitable for X-ray diffraction. 1H
NMR (400 MHz, DMF-d7, 353 K): δ 9.32 (s, 1 H, H9), 8.28 (d,
6
7
8
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de Silva, J. Phys. Chem. Lett., 2011, 2, 2865-2871.
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Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
Rev., 1997, 97, 1515-1566; (b) Z. Fei, N. Kocher, C. J.
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Ajayaghosh, P. Carol and S. Sreejith, J. Am. Chem. Soc., 2005,
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K. Sekiguchi, T. Tahara and M. Fujita, J. Am. Chem. Soc., 2009,
131, 12526-12527; (e) Z. Xu, J. Yoon and D. R. Spring, Chem. Soc.
Rev., 2010, 39, 1996-2006; (f) H. Woo, S. Cho, Y. Han, W.-S.
Chae, D.-R. Ahn, Y. You and W. Nam, J. Am. Chem. Soc., 2013,
135, 4771-4787; (g) M. Yamashina, M. M. Sartin, Y. Sei, M.
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3
3J = 8.5 Hz, 2 H, H1,8), 8.08-7.99 (m, 2 H, H3,6), 7.71 (dd, J = 8.5,
6.8 Hz, 2 H, H2,7), 4.56 (s, 4 H, CH2), 2.74 (s, 12 H, CH3).
113Cd,1H-HMBC (66.6 MHz, DMF-d7, 243 K): δ –345.5. ESI-TOF:
m/z: 861.15 [(C19H23N3)2CdBr2H]+, 486.02 [(C19H23N3)CdBr]+,
294.21 [(C19H23N3)H]+. Anal. Calcd for C19H23N3CdBr2: C, 40.35;
H, 4.10; N, 7.43; Cd, 19.87. Found: C, 40.28; H, 4.31; N, 7.20;
Cd, 19.00.
11 (a) J. Joseph, E. Kuruvilla, A. T. Achuthan, D. Ramaiah and G. B.
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Acknowledgements
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Thanks to the Danish National Research Foundation (DNRF93)
funded Center for Materials Crystallography (CMC) for partial
support and the Land Niedersachsen for providing a fellowship
in the GAUSS PhD program. We also want to thank Timo
Schillmöller for the support of some fluorescence
measurements.
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DOI: 10.1039/C6DT00557H