On the ion-pair recognition and indication features of a fluorescent heteroditopic host based on a BODIPY core

Article abstract of DOI:10.1002/ejoc.201402214

A fluorescent heteroditopic host for ion pairs and zwitterionic species has been synthesized. Its affinity towards a series of anions, cations and ion pairs in acetonitrile has been assessed, and the spectroscopic response has been evaluated. Solid-liquid extraction experiments of inorganic salts, α-amino acids and γ-aminobutyric acid (GABA) into acetonitrile solutions were performed, and the resulting complexes were analyzed by UV/Vis absorption, fluorescence and ¹H NMR spectroscopy. The discrimination patterns observed have been rationalized in terms of the molecular topologies of the host and guests. Catching two balls: A BODIPY derivative bearing two independent yet complimentary hosts for anions and cations displays distinct absorption and fluorescence signalling patterns for different alkali metal halides. The compound also recognizes zwitterionic compounds such as amino acids and γ-aminobutyric acid (GABA). Copyright

Full text of DOI:10.1002/ejoc.201402214

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FULL PAPER
DOI: 10.1002/ejoc.201402214
On the Ion-Pair Recognition and Indication Features of a Fluorescent
Heteroditopic Host Based on a BODIPY Core
Raúl Gotor,[a,b] Ana M. Costero,*[a,b] Salvador Gil,[a,b] Pablo Gaviña,[a,b] and
Knut Rurack*[c]
Keywords: Sensors / Receptors / Ion-pair recognition / Zwitterion recognition / Fluorescence
A fluorescent heteroditopic host for ion pairs and zwitterionic
species has been synthesized. Its affinity towards a series of
anions, cations and ion pairs in acetonitrile has been
assessed, and the spectroscopic response has been evalu-
ated. Solid–liquid extraction experiments of inorganic salts,
α-amino acids and γ-aminobutyric acid (GABA) into aceto-
nitrile solutions were performed, and the resulting complexes
were analyzed by UV/Vis absorption, fluorescence and 1H
NMR spectroscopy. The discrimination patterns observed
have been rationalized in terms of the molecular topologies
of the host and guests.
with the solvent employed. Thus, they are mainly used in
separation applications such as solid–liquid extractions, the
remobilization of insoluble salts into organic solution
phases[5] or as carriers in membrane-based or liquid–
liquid-type phase-transfer applications.[6] On the other
hand, if the topology of the subunits of the host and of the
host itself are adequately tailored, the selective recognition
of zwitterionic species of biological interest (e.g., amino
acids and neurotransmitters) becomes possible[7] and opens
a route to the creation of new compounds with a potentially
wide range of biomedical applications. However, although
some of these systems already operate rather efficiently as
binders, only very few examples of heteroditopic hosts as
potent probe molecules have been devised. For optical
probes, this is basically because the installation of two dif-
ferent signalling processes with a unique indication pattern
for a cation, an anion and an ion pair in a single small
molecule is not straightforward.[8]
Among the optical signalling units that can be incorpo-
rated into probes, the boron–dipyrromethene (BODIPY)
core is a particularly versatile one as virtually all of the
carbon positions on the central three-membered ring sys-
tem are available for substitution and it guarantees pro-
nounced spectroscopic responses in an advantageous wave-
length range.[9] BODIPY-based probes with a single recep-
tor have been studied extensively in recent years,[10] and the
facile functionalization of BODIPYs has led to a sizeable
library of fluorescent probes for the detection of diverse
drugs.[11] Within the last five years, several BODIPY-type
probes with two receptors have also been reported.[12] How-
ever, most of these molecules have been designed for a re-
sponse toward two cationic metal ion species. Heteroditopic
BODIPY hosts for ion-pair guests are very rare.[13]
Introduction
Supramolecular chemistry has significantly propelled ad-
vances in the field of chemical recognition for an almost
limitless number of applications.[1] Over several decades, a
multitude of receptors for cation recognition have been de-
signed, synthesized and evaluated.[2] More recently, effective
anion-binding hosts have also become a primary focus of
research, which has resulted in the development of a
sizeable library of compounds.[3] After the basic frame-
works for the complexation of simple cationic and anionic
species were rationalized and established, more sophisti-
cated systems able to coordinate an anion and a cation
simultaneously have been developed, and the increasing
number of reports reflects this growing interest in ion-pair
recognition.[4] Commonly, the simultaneous binding of cat-
ions and anions is achieved by employing combinations of
hosts of a dedicated cation- or anion-recognition nature
such as coronands, podands or calixarenes for cations and
ureas, thioureas, calix[4]pyrroles, ammonium salts, metal
complexes or guanidinium salts for anions.
Most of the reported ion-pair receptors are not selective
but bind several salts, and the recognition patterns vary
[a] Centro de Reconocimiento Molecular y Desarrollo Tecnológico
(IDM), Unidad Mixta Universidad de Valencia – Universidad
Politécnica de Valencia,
46010 Valencia, Spain
www.uv.es
[b] Departamento de Química Orgánica, Universidad de Valencia,
Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
E-mail: Ana.Costero@uv.es
[c] Division 1.9 Sensor Materials, BAM Federal Institute for
Materials Research and Testing,
Richard-Willstätter-Str. 11, 12489 Berlin, Germany
E-mail: knut.rurack@bam.de
www.bam.de
We report here the synthesis, photophysical data and re-
cognition properties towards ion pairs and zwitterionic spe-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402214.
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FULL PAPER
cies of the BODIPY-based heteroditopic receptor 1.[14] benzaldehyde in the presence of NaH with the ditosylate
Compound 1 (Figure 1) is composed of an 18-azacrown-6 derivative of tetraethylene glycol under high-dilution condi-
moiety (A18C6) as a cation host and a meso-octamethyl- tions.
calix[4]pyrrole (C[4]P) as anion host, both of which are elec-
Compound 2 was condensed with aldehyde 3 in the pres-
tronically connected through the boron–dipyrromethene ence of piperidine and p-toluenesulfonic acid (TsOH) by
core, which, at the same time, acts as chromophore/fluoro- using a Dean–Stark apparatus to afford 5 in 34% yield after
phore and electronic mediator/spacer between the two purification by column chromatography. The resulting
hosts.
product was then condensed with aldehyde 4 under similar
conditions to yield the final heteroditopic BODIPY recep-
tor 1 as a green solid in 26% yield (Scheme 2). The new
ligand was characterized by 1H and 13C NMR spectroscopy
and mass spectrometry.
Figure 1. Heteroditopic receptor 1.
Results and Discussion
Scheme 2. Synthesis of the heteroditopic probe 1.
Synthetic Procedure
Compound 1 was synthesized by two consecutive
Knoevenagel condensations between BODIPY 2 and the
corresponding aldehyde derivative of the host moieties by
using procedures described in the literature.[15] The synthe-
sis of the aldehyde derivatives 3 and 4 is depicted in
Scheme 1. The anion receptor 3 was obtained by the for-
mylation of C[4]P with N,N-dimethylformamide (DMF)
and phosphorus oxychloride. The cation receptor 4 was
synthesized by the reaction of p-[bis(2-hydroxyethyl)amino]-
Spectroscopic Properties
Compound 1 (1ϫ10–5 m in MeCN) shows an absorption
maximum at 685 nm with a molar absorption coefficient of
74600 cm–1 m–1. This band corresponds to the S1ǟS0
(πǞπ*) (0Ǟ0) transition of the extended BODIPY dye and
possesses a shoulder at 635 nm, which corresponds to the
vibronic (0Ǟ1) transition of the same electronic transition.
Weaker bands at 328 and 416 nm with ε = 27750 and
26630 cm–1 m–1, respectively, are ascribed to S2ǟS0 and
S3ǟS0 transitions in analogy to assignments by Qin et al.
in related alkenyl–BODIPY derivatives.[16] The emission
spectrum has a maximum at 743 nm, which appears broad-
ened compared with the typically narrow BODIPY core
emission, presumably because of an admixture of an intra-
molecular charge transfer (ICT) character involving the
donor aniline and the acceptor BODIPY (Figure 2).[15a]
In addition, the significant Stokes shift observed between
the absorption (λabs) and emission bands (λem) compared
with that of the parent BODIPY 2 (λabs = 497 nm, λem
=
505 nm) is consistent with the behaviour observed in other
π-extended BODIPYs.[9a] The fluorescence quantum yield
Φf of 1 in MeCN is 0.34, which is comparable to the values
reported earlier by some of us for such red-absorbing
BODIPY dyes.[17] The fluorescence decay is biexponential
with an almost equal contribution of a short-lived (τ1
=
0.28 ns, a1,rel = 0.49; ax,rel = relative amplitude of species x)
and a long-lived component (τ2 = 1.98 ns, a2,rel = 0.51). In
Scheme 1. Synthesis of the anion and cation receptor moieties 3
and 4.
2
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A Fluorescent Heteroditopic Host Based on a BODIPY Core
stants for Li+, Na+, K+, and NH4 cations and F–, Cl–,
Br– and AcO– anions were obtained by UV/Vis titrations
(Table 1). The values determined were consistent with the
well-established complexation properties of both azacoron-
ands and calixpyrroles.[21]
+
Table 1. Complexation constants (logKa) of 1 with various cations
and anions in acetonitrile. Values were obtained after least-squares
fits of UV/Vis titrations with the SpecFit© software[22] for a 1:1
binding model.
1 + K+
3.3Ϯ0.1
1 + AcO–
5.0Ϯ0.1
1 + Na+
3.0Ϯ0.2
1 + Cl–
1 + Li+
2.0Ϯ0.3
1 + Br–
Ͻ1
+
1 + NH4
3.2Ϯ0.1
1 + F–
Figure 2. UV/Vis absorption and fluorescence spectrum (λexc
685 nm) of 1 (1ϫ10–5 m in CH3CN).
=
logKa
logKa
contrast to dyes such as 6 (Figure 3), which show a single
exponential decay (e.g., τf = 0.13 ns for 6 in MeCN),[15a]
biexponential decays are often found for crown ether ap-
pended BODIPYs such as 7 and 8 (Figure 3).[15b,18] Such
behaviour is related to the flexibility of the crown ether unit
and its ability to adopt more than one preferential confor-
mation in the ground and/or excited state. For instance,
6.3Ϯ0.2
5.1Ϯ0.1
The optical properties of 1 in acetonitrile (1ϫ10–5 m) in
the presence of different cations and anions are listed in
Table 2. To achieve full complexation, an excess of the men-
tioned salts was added.
fluorescence lifetimes of 0.22 (arel = 0.54) and 1.42 ns (arel Table 2. Optical properties of 1 (1ϫ10–5 m in CH3CN) in the pres-
= 0.46) have been found for 7 in MeCN.[19]
ence of an excess of different ions. Cations and anions were added
–
as ClO4 and TBA+ salts, respectively (10 mg).
λabs [nm] Δλabs [nm] λem [nm] Δλem [nm]
Φ
+
1 + NH4
1 + K+
1 + Na+
1 + Li+
1
653
660
678
684
685
696
692
688
687
–32
–25
–7
–1
–
+11
+7
+3
+2
708
717
728
729
743
748
746
744
742
–35
–26
–15
–4
0.53
0.59
0.52
0.55
0.34
0.006
0.034
0.18
0.23
–
1 + F–
+5
+3
+1
–1
1 + AcO–
1 + Cl–
1 + Br–
As can be observed for the example of the F– anion in
Figure 4, the anionic guests produce bathochromic dis-
placements of the main absorption as well as the emission
band of up to ca. 5 nm accompanied by a quenching of the
fluorescence. The bathochromic shift is consistent with an
increase in the electron-donating strength of the C[4]P arm
through coordination of the anion, and the absorption ap-
proximates that of bis-aminostyryl-substituted dyes such as
9 (Figure 3), which absorbs at 711 nm.[17] However, the
comparatively strong reduction in the fluorescence quan-
tum yield in 1–F–, cannot be explained within the simple
framework of the colour rules of BODIPYs, that is, an en-
hanced internal conversion as the energy gap between the
ground and excited states decreases (according to the en-
ergy-gap rule),[15a] but is tentatively attributed to the pop-
ulation of an only weakly emissive ICT state involving the
complexed calix[4]pyrrole[23a–23d] and possibly possessing a
Figure 3. Previously reported BODIPY discussed in the text.
Complexation Studies
The affinity of 1 towards several ions in acetonitrile solu- certain charge-shift character.[23e] The involvement of the
tion[20] was tested by UV/Vis spectroscopy. To examine the latter is known to result in nonfluorescent BODIPY
ion-binding properties of each coordination site indepen- dyes.[23f] The magnitude of these anion-induced shifts as
–
+
dently, the noninterfering counterions ClO4 and NBu4
well as the decrease in emission intensity is additionally re-
(tetrabutylammonium, TBA) were employed for titrations lated to the affinity of the C[4]P moiety towards the tested
with cations and anions, respectively. The interactions of anion and reflects an order F– ϾAcO– ≈Cl– ϾBr–. Regard-
these counterions with the hosts were negligible and ne- ing the fluorescence decay behaviour, at a fixed anion–1 ra-
glected in the further studies. In this way, the affinity con- tio of 500:1, the quenching effect of the anions is ac-
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companied by the appearance of a third fluorescence life- spectroscopic changes upon saturation with several cations
time component, which decays even faster than the short- and anions are summarized in Table 2. The changes in
lived component of free 1, for example, in 78 ps for F– and fluorescence quantum yield upon the binding of 1 to the
83 ps for Br– (the uncertainty in the measurement is Ϯ3 ps). various cations are in line with other reports on BODIPYs
Moreover, the relative amplitude of the shortest decay com- carrying 3- or 5-styryl-appended amino crowns,[12,13] that is,
ponent for the F– complex is more than twice that for the excited-state decoordination[26a] does not play a role in such
Br– complex. This clearly indicates that once an anion is BODIPY-type ICT probes.
bound in the C[4]P unit its quenching ability is rather sim-
After proving the ability of 1 to bind discrete cations and
ilar (at least for the monoatomic halides) and that the mag- anions, we tested its ability to coordinate and sense a cation
nitude of the quenching in a titration series depends pri- and an anion at the same time and, thus, respond to an ion
marily on logKa. The latter is supported by the fact that pair. For this purpose, the halide series (F–, Cl–, Br–) of the
the fluorescence lifetimes of 1–F– and 1–Br– are rather sim- first three alkali metal ions (Li+, Na+, K+) was chosen. To
ilar (78 vs. 83 ps).
overcome the problem of the low solubility of some of these
salts in acetonitrile and even in water, [cf. LiF: 0.13 g in
100 mL at 25 °C], we opted for a modified phase-transfer
protocol in the sense of solid–liquid extraction experiments,
especially for the NMR experiments that required signifi-
cantly higher concentrations.
For the NMR investigations in the solid–liquid extrac-
tion experiments, 5ϫ10–3 m solutions of 1 in CD3CN
(1 mL) in a vial were saturated with the corresponding salt,
and the mixture was mechanically stirred for 24 h. After
1
that time, the solution was filtered, and the H NMR spec-
trum was recorded. Simultaneous complexation of both cat-
ion and anion was clearly demonstrated because a shift of
the NMR signals of both the azacrown and the calixpyrrole
moieties was observed (Table 3).
Table 3. Change in 1H NMR chemical shifts [ppm] of selected pro-
tons after complexation of 1 with different salts.
Salt
Ha
Hc
Hi
Hj
LiF
–0.01
–0.23
–0.25
–0.02
–0.21
–0.26
–0.22
–0.20
–0.20
–0.01
–0.16
–0.18
–0.02
–0.14
–0.18
–0.11
–0.13
–0.13
0.01
0.08
0.05
0.01
0.10
0.03
0.26
0.22
0.28
0
LiCl
LiBr
NaF
NaCl
NaBr
KF
0.01
0.13
0.01
0.05
0.10
–0.25
–0.19
–0.24
Figure 4. UV/Vis (top) and fluorescence emission (bottom) spec-
trum (λexc = 685 nm) of 1 (1ϫ10–5 and 1ϫ10–6 m in MeCN) in
the absence (red) and presence of 500 equiv. of KClO4 (green) and
500 equiv. of TBAF (blue). For a representative set of UV/Vis ti-
tration spectra, see Figures S14 and S15.
KCl
KBr
–
On the other hand, the addition of cations (as ClO4
In general, anion coordination induced a shift in all of
salts) to MeCN solutions of 1 induced a hypsochromic shift the proton signals related to the calixpyrrole moiety. Thus,
of both the absorption and the emission band. This the β-pyrrolic protons (Ha and others, Figure 1) and the
phenomenon is attributed to the coordination of the cation vinylic proton Hc are shielded upon anion coordination
by the azacrown moiety.[12c,18,24] For instance, the binding (consistent with an electronic density injection of the anion
of a Na+ cation to the tetraoxa–monoaza crown of 7 leads to the conjugated system), whereas the N–H and meso-
to analogous hypsochromic shifts.[18] The positively charged methyl groups and the Hb vinylic proton become
cation monopolizes the lone electron pair of the crown deshielded.
ether nitrogen atom and weakens the electron donor
On the other hand, cation coordination has a strong ef-
strength of this unit and, hence, the ICT character, and fect on the chemical shifts of the protons related to the
there is a concomitant increase of the fluorescence emis- A18C6 moiety. For instance, protons Hi and Hh (see Sup-
sion. The magnitude of the observed shift follows the order porting Information) are deshielded upon cation coordina-
NH4+ ≈K+ ϾNa+ ϾLi+, which is in agreement with the co- tion, along with the aliphatic protons of the coronand. This
ordination preference of A18C6.[2] In addition, Li+ and observation is consistent with the electronic demand of the
Na+ ions are also known to prefer oxygen over nitrogen cationic species.
coordination in such crowned dyes in acetonitrile, which
Among all the salts studied, KF and KBr produced the
leads to smaller spectral shifts that are frequently even ac- largest shifts in the signals of Hc, Hi and Hj, indicating
companied by higher complexation constants.[25,26a] The strong complexation. In contrast, the changes observed for
4
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A Fluorescent Heteroditopic Host Based on a BODIPY Core
LiF and NaF were small, which indicates a low extent of formational changes in the ligand that would favour the
complexation, presumably because of the high lattice ener- cation complexation (see Supporting Information). Ad-
gies of these salts (1036 and 923 kJmol–1, respectively). No- ditionally, the ion-pair size and its association constant
tably, the complex with KBr shows sharp signals for the could play a role in the entire process.
NH protons in contrast with the broad signals found for
the other potassium salts (Figure 5). This behaviour can be
related to the size of the bromide ion, which results in a
perfect fit of the ion pair into the cavity and, thus, the rigid-
ity of the ternary complex is enhanced. Another interesting
observation was related to the signal of Hj when the Li+,
Na+ and K+ salts were compared. Whereas the Li+ and
Na+ salts lead to a slight deshielding of Hj, the opposite
behaviour was observed for K+ salts regardless of the
counteranion (Figure 5, cf. NaBr and KBr; see also Sup-
porting Information). Thus, this behaviour seems to be re-
lated to the cation size. The small cations are preferably
complexed by the oxygen atoms of the crown moiety and
Figure 6. UV/Vis spectrum of 1 (1ϫ10–5 m in CH3CN) as a free
ligand and after 24 h of mechanical agitation in the presence of
10 mg of NaF, NaCl and NaBr.
are most likely buried in the cavity. On the other hand, the
larger K+ cation completely fills the coronand cavity and
The fluorescence behaviour followed the same pattern as
induces a conformational change that inhibits the conjuga-
tion between the lone electron pair of the nitrogen atom
and the styryl extension of the BODIPY core. This confor-
mational modification puts Hj under the influence of the
aromatic ring. The lack of conjugation is also supported by
the larger shift of Hi in the potassium salts than in the other
compounds studied.
the absorption bands. All fluorescence emission spectra are
slightly hypsochromically shifted, presumably because of
the interaction of the cation with the azacrown moiety.
Less-coordinating anions (Br– or Cl–) did not compensate
the cation effect, whereas anions with a higher affinity for
the C[4]P unit such as F– reduced this blueshift. However,
the distinct effect of anion binding, fluorescence quenching,
was also found in all of the complexes formed (Figure 7).
Figure 7. Fluorescence spectrum of 1 (1ϫ10–5 m in CH3CN) as a
free ligand and after 24 h of mechanical agitation in the presence
of 10 mg of KF, KCl and KBr.
Figure 5. 1H NMR spectra of 1 as free ligand (top) and in the
presence of NaBr (middle) and KBr (bottom) in CD3CN.
The intensity of the quenching did not follow the trend
For the UV/Vis and fluorescence studies, an excess of the observed with the corresponding anions as tetrabutyl-
diverse salts (10 mg), previously dried, was added to 10–5
m
ammonium salts, most likely because ion association is only
CH3CN solutions (3 mL) of 1, and the mixtures were me- considerably moderate for all of the TBA salts (e.g., Ka =
chanically stirred for 24 h in the dark. After this time, the 27.45 for TBABr and 24.71 for TBAClO4).[26b,26c] However,
solutions were filtered and their spectroscopic properties the K+ salts follow a comparable trend to that mentioned
were measured. The complexation of 1 with different inor- for Li+ salts above (the only data available for K+ salts is
ganic salts generated different macroscopic responses in the Ka = 32.68 for KClO4).[26b,26c] The anion coordination of
two optical channels (absorption and emission).
C[4]P seems to be modulated by the presence of the potas-
All the alkali halides showed a hypochromic effect and a sium ion. The strong interaction between K+ and F– ions
hypsochromic displacement of the main absorption band, displaces the anion to a certain degree from the C[4]P
which suggests that the electronic properties of the com- cavity; therefore, the extent of coordination is diminished
plexes are mainly governed by the interaction of the cation compared with that of KBr, for example, because the Br–
with the azacrown moiety and modulated by the presence ion is larger and undergoes weaker electrostatic interac-
of the anions (Figure 6). However, this is not the only effect tions. These factors should allow the bromide ion to fit bet-
to be taken into account, as the interaction of the anion ter into the C[4]P cavity and still fulfil the electrostatic
with the C[4]P moiety could induce electrostatic and con- needs of the K+ cation. As a consequence, as compared to
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the TBA salts, the bromide ion modulates the fluorescence also be suitable for the binding of zwitterionic guests of a
intensity of 1 more efficiently in the presence of K+.
certain topology, for instance, amino acids. For these ex-
However, the most dramatic effect besides spectral shifts periments, glycine, tyrosine, arginine, cysteine, aspartic acid,
and fluorescence quenching is the strong hypochromic glutamic acid and γ-aminobutyric acid (GABA) were con-
change of the main BODIPY-centred absorption transi- sidered. The amino acids were dried and added in an excess
tions. Such a pronounced reduction in oscillator strength (10 mg) to 10–5 m CH3CN solutions (3 mL) of 1 followed
cannot only be related to a decoupling of the nitrogen atom by mechanical stirring of the mixtures for 24 h in the dark.
of the crown from the π system through cation complex- After filtration, the absorption and emission spectra of the
ation, for instance, protonated 6 (Figure 3) does not show solutions were recorded.
such a behaviour,[15a] but is most probably related to a
Glycine, tyrosine, cysteine and glutamic acid showed a
twisting of the appended pyrrole ring out of the BODIPY very similar behaviour in absorption with a main band at
plane upon the formation of the ion-pair complex. The ca. 677 nm and a shoulder at 631 nm (see the examples for
complexation of the ion pair, especially in the case of the Gly and GABA in Figure 9; for the other amino acids, see
larger and strongly electrostatically attracted ions such as Supporting Information). These spectra are slightly hypso-
K+ and Br–, can only occur if the crown unit adopts an “L” chromically shifted by ca. 10 nm compared with that of free
shape with the cavity pointing toward the void of the 1. This behaviour can be explained by the complexation of
tweezers and if the entire C[4]P unit adopts an almost per- the ammonium group of the amino acid in analogy to the
pendicular conformation with regard to the π plane (see cation complexation features described above.
Figure 10).
Combining the observations obtained in the studies with
different inorganic salts, we can span a matrix that allows
the discrimination between different ion pairs on the basis
of their absorption and fluorescence behaviour. In Figure 8,
a 3D plot of the ratio between the absorption at 680 and
625 nm (x coordinate), the emission intensity (z coordinate)
and the emission maximum wavelength (y coordinate) is
presented. Upon closer inspection, it is evident that the dif-
ferent complexes show a particular clustering, which
permits the identification of the different salts or ion pairs
by comparatively simple means. The differences in effects
for the weakly soluble salts (LiF, NaF, etc.) and the more
soluble salts (e.g., KBr) in particular in the NMR and op-
tical spectroscopic studies is presumably related to the effec-
tive ratios of the partners in the final measurement solu-
tions. At NMR concentrations, too little of the weakly solu-
ble salts are dissolved to achieve full complexation.
Figure 9. UV/Vis spectra of 1, 1 + GABA and 1 + Gly (top) and
emission spectra (λexc = 685 nm) of 1, 1 + GABA and 1 + Gly
(bottom).
However, when the UV/Vis spectra of 1 with sodium
halides (Figure 6) are compared with the spectrum of 1 +
Gly (Figure 9, top), it is clear that the interaction of the
ammonium group with the azacrown ether is less pro-
nounced for 1 + Gly. In addition, the binding of the ammo-
nium group in the cavity of the crown seems to dominate
over the binding of the carboxylate group to C[4]P. On the
other hand, the UV/Vis spectra of the complexes of 1 with
Arg and especially GABA show a shape that is very similar
to that of the ion-pair complexes (Figure 6).
For GABA in particular, the distance between the an-
ionic and cationic moieties seems to be much better suited
to satisfy the coordination needs of 1, presumably through
Figure 8. 3D representation of significant changes in the spectro-
scopic properties of 1 in the presence of different inorganic salts.
1:1 complexation by simultaneous binding of the guest in
Considering other guests that express two differently both the cation and the anion receptor.[27] Tentative struc-
charged entities, the tweezerlike architecture of 1 should tures for the possible complexes between 1 and GABA and
6
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A Fluorescent Heteroditopic Host Based on a BODIPY Core
Figure 10. Schematic illustration of 1–GABA (left) and 1–Gly (right) complexes. The distance is adequate for the complexation of GABA
with the two hosts, whereas glycine can only be complexed by either the azacrown or the calixpyrrole moiety. The angles shown are
approximations based on results obtained after computation of a model compound.
Gly are shown in Figure 10. The truncation of the extended quenching seems to be related to the strength of the com-
π system by twisting of the C[4]P moiety is also illustrated plexation. By following a similar approach to that pre-
in Figure 10; this also leads to a reduction in oscillator viously described (using data from UV and fluorescence
strength and a hypochromic change in absorption in the spectra), the studied amino acids can be discriminated (Fig-
case of the zwitterionic guests and supports the interpret- ure 11).
ation given above for the good fit of ion pairs such as K+/
Br–.
Conclusions
The fluorescence spectra show a similar behaviour to
those of the alkaline salts; the magnitude of fluorescence
We have synthesized a heteroditopic receptor for the si-
multaneous recognition of anions and cations. In the pres-
ence of cations with inactive counterions, hypsochromic
shifts of both the absorption and emission bands were ob-
served. By contrast, in the presence of anions with inactive
counterions, bathochromic shifts of both the absorption
and the emission bands along with fluorescence quenching
were observed. In the presence of diverse alkali halides,
clear changes in the UV/Vis, fluorescence and 1H NMR
spectra were observed. The combination of optical changes
induced after complexation of different alkali halides is dif-
ferent in each case and can be used to discriminate between
the salts. It is important to note that the single effects are
retained in these cases and an additional hypochromic syn-
ergistic effect occurs, that is, the anion and cation do not
annihilate each other like they do in various other reported
systems. Finally, compound 1 also gives rise to different re-
sponses in the presence of zwitterionic amino acids. The
distance between the ammonium and the carboxylate group
is a crucial factor for the optical behaviour of the com-
plexes.
Experimental Section
Materials and Methods: Tetrahydrofuran was distilled from Na
prior to use. All other materials were purchased and used as re-
ceived. The 1H and 13C NMR spectra were recorded by using
Bruker DRX-500 and Bruker Avance 400 MHz spectrometers.
HRMS spectra were recorded with an AB SCIEX QTOF mass
spectrometer. UV/Vis measurements were performed by using 1 cm
path length quartz cuvettes with a Shimadzu UV-2101PC spectro-
photometer. Fluorescence spectra were recorded with Varian Cary
Figure 11. Top: 3D representation of significant changes in the op-
tical properties of 1 in the presence of several amino acids. Bottom:
structures of these amino acids.
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R. Gotor, A. M. Costero, S. Gil, P. Gaviña, K. Rurack
FULL PAPER
Eclipse and Horiba Jobin–Yvon FluoroMax-4P fluorescence spec-
trophotometers. Fluorescence experiments were performed with a
90° standard geometry with polarizers set at 54.7° for emission and
0° for excitation.
two-neck round-bottomed flask connected to a Dean–Stark appa-
ratus. Then, piperidine (2.8 mL), a small amount of 3 Å molecular
sieves, and trace amounts of pTsOH were added to the mixture.
The reaction was kept under argon at the reflux temperature of
toluene for 2 h. After this time, the reaction mixture was allowed
to reach room temperature, and then the solvent was evaporated.
The residue was redissolved in EtOAc and washed with 10%
NH4Cl three times. The aqueous layer was reextracted with EtOAc,
and the combined layers were dried with NaCl (satd.) and MgSO4.
The product was purified by column chromatography with neutral-
ized silica (Et3N) as the stationary phase and a hexane/EtOAc mix-
ture (95:5Ǟ85:15) as eluent. A deep blue solid (225 mg, 34%) was
obtained along with the disubstitution isomer (80 mg, 4%). Rƒ =
The fluorescence quantum yields (Φf) were determined relative to
that of 3 in acetonitrile (Φf = 0.10Ϯ0.01).[17]
The synthetic procedure for 2 can be found in ref.[15] The synthesis
of 3 was performed as described in ref.[28]
The fluorescence lifetimes (τf) were determined by a unique custo-
mized laser impulse fluorometer with picosecond time resolution,
which we described in an earlier publication.[29] The fluorescence
was collected at a right angle (polarizer set at 54.7°), and the fluo-
rescence decays were recorded with a modular single-photon timing
unit.[17] Typical instrument response functions [full width at half-
maximum (fwhm) ca. 40 ps] and a time division of 1.2 ps per chan-
nel allowed us to obtain an experimental accuracy of Ϯ3 ps. Typi-
cal excitation energies were in the nanowatt to microwatt range
(average laser power). The fluorescence lifetime profiles were ana-
lyzed with a PC by using the Global Unlimited V2.2 software pack-
age (Laboratory for Fluorescence Dynamics, University of Illinois),
0.55 (EtOAc/Hex, 2:8). IR: ν = 3418, 2971, 2922, 2869, 1696, 1602,
˜
1
1540, 1478, 1425, 1297, 1191, 1155, 1102, 1044, 982, 765 cm–1. H
NMR (500 MHz, CD2Cl2): δ = 7.70 (d, J = 15.7 Hz, 1 H), 7.57–
7.52 (m, 3 H), 7.40–7.36 (m, 2 H), 7.31 (s, 1 H), 7.29 (s, 1 H), 7.20
(dd, J = 15.9, 1.9 Hz, 1 H), 7.02 (s, 1 H), 6.95 (s, 1 H), 6.60 (s, 1
H), 6.39 (d, J = 2.9 Hz, 1 H), 6.04 (s, 1 H), 6.01–5.95 (m, 4 H),
5.91 (t, J = 3.0 Hz, 1 H), 5.87 (t, J = 3.0 Hz, 1 H), 2.58 (s, 3 H),
1.72 (s, 6 H), 1.57 (s, 6 H), 1.56 (s, 6 H), 1.55 (s, 6 H), 1.48 (s, 3
H), 1.44 (s, 3 H) ppm. 13C NMR (126 MHz, CD2Cl2): δ = 155.26,
152.42, 143.26, 140.83, 139.38, 139.07, 138.92, 138.67, 138.63,
138.27, 137.50, 137.47, 137.44, 135.21, 132.93, 132.31, 130.97,
129.01, 128.77, 128.43, 120.25, 117.42, 117.32, 114.12, 103.68,
103.66, 103.52, 102.83, 102.78, 102.47, 101.85, 37.45, 35.15, 35.08,
35.01, 30.57, 29.50, 28.95, 28.41, 28.35, 14.38, 14.27, 13.94 ppm.
HRMS (ESI): calcd. for BC48F2H54N6 [M + H]+ 763.4466; found
763.4439.
2
and the goodness of the fit was calculated from the reduced χR
and autocorrelation function C(j) of the residuals.
UV/Vis quantitative analysis was performed by adding aliquots of
–
different cations or anions (as ClO4 or TBA+ salts) to 1ϫ10–5
m
CH3CN solutions of 1.
Solid–liquid phase extraction experiments were performed by add-
ing the different salts (10 mg) to 1ϫ10–5 m CH3CN solutions
(3 mL) of 1. Mixtures were mechanically stirred for 24 h. After that
time, the solutions were filtered, and the UV/Vis and fluorescence
spectra were recorded.
Compound 1: Compound 5 (160 mg, 0.21 mmol) and aldehyde 4
(77 mg, 0.21 mmol) were dissolved in dry toluene (30 mL) in a two-
neck round-bottomed flask connected to a Dean–Stark apparatus.
Then, piperidine (0.2 mL), a small amount of 3 Å molecular sieves,
and trace amounts of pTsOH were added to the mixture. The reac-
tion mixture was heated to reflux under argon for 2 h. After this
time, the mixture was allowed to reach room temperature, and then
the solvent was evaporated. The residue was redissolved in EtOAc
and washed with NH4Cl 10% three times. The aqueous layer was
reextracted with EtOAc, and the combined layers were dried with
satd. NaCl and MgSO4. The product was purified by column
chromatography with neutralized silica (Et3N) as the stationary
phase and hexane/EtOAc (95:5Ǟ85:15) as the eluent. A dark green
Full complexation experiments of 1 with TBA+ and ClO4 salts
–
were achieved by adding an excess (ca. 10 mg) of the salts to
1ϫ10–5 m CH3CN solutions (3 mL) of 1, which were then stirred
for 24 h. The solutions were then filtered to remove the remaining
solid (if any).
Compound 4: 4-[Bis(2-hydroxyethyl)amino]benzaldehyde (729 mg,
3.49 mmol) was dissolved in dry THF (220 mL) under an argon
atmosphere. Then, 60% sodium hydride (860 mg, 21.11 mmol) was
carefully added to the reaction mixture. The reaction was then
heated to reflux for 2 h. After this time, tetraethylene glycol di-p-
tosylate (1.83 g, 3.64 mmol) was added dropwise over 2 h with an
automatic syringe. The macrocycle formation reaction was heated
to reflux for 96 h. Then, water (10 mL) was added, and the mixture
was stirred for 5 min. The organic solvent was evaporated, and the
mixture was redissolved in EtOAc and washed three times with
10% NaHCO3. The aqueous layer was reextracted with EtOAc,
and the combined organic layers were washed with brine and dried
with MgSO4. The remnant oil was purified by flash chromatog-
raphy with dichloromethane/MeOH (DCM/MeOH 95:5Ǟ80:20)
as eluent to yield a yellowish oil (642 mg, 50%). Rƒ = 0.47 (DCM/
solid was obtained (62 mg, 27%). IR: ν = 3406, 3308, 3104, 3063,
˜
2965, 2925, 2863, 1732, 1656, 1594, 1514, 1483, 1429, 1376, 1287,
1166, 1104, 1046, 984, 948, 770, 730 cm–1. 1H NMR (500 MHz,
[D2]dichloromethane): δ = 7.65 (d, J = 15.8 Hz, 1 H), 7.55–7.46
(m, 5 H), 7.48 (d, J = 16.8 Hz, 1 H), 7.39–7.33 (m, 3 H), 7.30 (s, 1
H), 7.21 (d, J = 15.8 Hz, 1 H), 7.20 (d, J = 16.8 Hz, 1 H), 6.99 (s,
1 H), 6.94 (s, 1 H), 6.72 (d, J = 8.9 Hz, 2 H), 6.63 (s, 1 H), 6.56 (s,
1 H), 6.42 (d, J = 2.8 Hz, 1 H), 5.99–5.91 (m, 4 H), 5.87 (t, J =
3.1 Hz, 1 H), 5.84 (t, J = 3.0 Hz, 1 H), 3.79–3.50 (m, 24 H), 1.70
(s, 6 H), 1.58 (s, 6 H), 1.53 (s, 6 H), 1.51 (s, 6 H), 1.45 (s, 3 H),
1.45 (s, 3 H) ppm. 13C NMR (126 MHz, CD2Cl2): δ = 153.85,
152.95, 149.47, 141.94, 141.69, 139.91, 139.47, 139.12, 138.88,
138.82, 138.22, 138.16, 138.06, 137.10, 136.83, 136.04, 133.38,
133.33, 131.64, 129.62, 129.49, 129.42, 129.26, 125.05, 118.12,
117.57, 117.52, 115.14, 114.64, 112.31, 104.23, 104.18, 104.07,
103.42, 103.33, 103.09, 102.47, 71.30, 71.28, 71.19, 71.15, 69.07,
51.80, 38.04, 35.78, 35.68, 35.59, 30.09, 29.64, 29.02, 28.94, 14.88,
14.87 ppm. HRMS (ESI): calcd. for BC67F2H80N7O5 1112.6341;
found 1112.6358.
MeOH, 95:5). IR: ν = 3330, 2971, 2927, 2887, 1656, 1452, 1385,
˜
1
1098, 1049, 880 cm–1. H NMR (500 MHz, CDCl3): δ = 9.73 (s, 1
H), 7.72 (d, J = 8.8 Hz, 2 H), 6.75 (d, J = 8.8 Hz, 2 H), 3.79–3.57
(m, 24 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 190.10, 152.69,
132.13, 125.27, 111.05, 70.88, 70.84, 70.79, 70.77, 68.36, 51.47 ppm.
HRMS (EI): calcd. for C19H30NO6 [M + H]+ 368.2068; found
368.2073.
Compound 5: BODIPY 2 (429 mg, 1.30 mmol) and aldehyde 3
(400 mg, 0.88 mmol) were dissolved in dry toluene (30 mL) in a
Supporting Information (see footnote on the first page of this arti-
cle): IR, NMR and HRMS spectra of 1, NMR spectra of 1 in the
8
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A Fluorescent Heteroditopic Host Based on a BODIPY Core
Selcuk, S. Kolemen, G. Gulseren, T. Nalbantoglu, H. Boyaci,
presence of different salts, emission and absorption spectra of 1
free and in the presence of different salts and amino acids, titration
of 1 with TBAF and KClO4 in acetonitrile.
E. U. Akkaya, J. Am. Chem. Soc. 2010, 132, 8029–8036.
[13] Z. Yin, A. Y.-Y. Tam, K. M.-C. Wong, C.-H. Tao, B. Li, C.-T.
Poon, L. Wu, V. W.-W. Yam, Dalton Trans. 2012, 41, 11340.
[14] In a very recent communication, we have reported the use of 1
as a molecular logic gate with K+ and F– ions as inputs, see:
R. Gotor, A. M. Costero, S. Gil, M. Parra, P. Gaviña, K. Rur-
ack, Chem. Commun. 2013, 49, 11056–11058.
Acknowledgments
[15] a) K. Rurack, M. Kollmannsberger, J. Daub, Angew. Chem. Int.
Ed. 2001, 40, 385–387; Angew. Chem. 2001, 113, 396; b) M.
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Financial support from the Spanish Government (project
MAT2012-38429-C04-02) is gratefully acknowledged. R. G. is
grateful to the Spanish Ministry of Education for an FPU grant.
The Universidad de Valencia (SCSIE) is also gratefully acknowl-
edged for support.
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Received: March 6, 2014
Published Online: ᭿
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www.eurjoc.org
9

Job/Unit: O42214
/KAP1
Date: 07-05-14 18:56:29
Pages: 10
R. Gotor, A. M. Costero, S. Gil, P. Gaviña, K. Rurack
FULL PAPER
Sensors
Catching two balls: A BODIPY derivative
bearing two independent yet complimen-
tary hosts for anions and cations displays
distinct absorption and fluorescence signal-
ling patterns for different alkali metal hal-
ides. The compound also recognizes zwit-
terionic compounds such as amino acids
and γ-aminobutyric acid (GABA).
R. Gotor, A. M. Costero,* S. Gil,
P. Gaviña, K. Rurack* .................... 1–10
On the Ion-Pair Recognition and Indi-
cation Features of a Fluorescent Heterodi-
topic Host Based on a BODIPY Core
Keywords: Sensors / Receptors / Ion-pair
recognition
/ Zwitterion recognition /
Fluorescence
10
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Eur. J. Org. Chem. 0000, 0–0