Fluoride recognition by a chiral urea receptor linked to a phthalimide chromophore

Article abstract of DOI:10.1039/b908433a

The anion chemosensor 1 based on a urea-activated phthalimide with a stereogenic centre was synthesized using an efficient procedure involving a Curtius rearrangement. Its photophysical properties were estimated in several solvents. Sensor 1 detected fluo

Full text of DOI:10.1039/b908433a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Fluoride recognition by a chiral urea receptor linked to a phthalimide
chromophore†
Rau´l Pe´rez-Ruiz,a Yrene D´ıaz,a Bernd Goldfuss,a Dirk Hertel,b Klaus Meerholzb and Axel G. Griesbeck*a
Received 28th April 2009, Accepted 12th June 2009
First published as an Advance Article on the web 7th July 2009
DOI: 10.1039/b908433a
The anion chemosensor 1 based on a urea-activated phthalimide with a stereogenic centre was
synthesized using an efficient procedure involving a Curtius rearrangement. Its photophysical
properties were estimated in several solvents. Sensor 1 detected fluoride with absorption as well as
fluorescence changes and was only observable for this case and not for other halides. The appearance of
a new CT complex emission at a longer wavelength and no changes in the singlet lifetime of 1 in the
presence of fluoride supported a fluorescence static quenching mechanism. 1H-NMR studies, together
with theoretical calculations based on DFT methods at the B3lYP/6–31G* level of theory confirmed
the formation of a [1–F]- complex through H-bonding interactions rather than receptor deprotonation
in the recognition process. Reversibility of this process was observed upon addition of a protic solvent.
fluorescent dye12 and fluorescent 4,5-dimethoxyphthalimide
derivatives were applied as sensors for intra- and intermolecular
PET processes.13
Introduction
Anion recognition and sensing has attracted considerable interest
due to its importance from industrial, biological/medicinal and
environmental points of view.1 In biological systems, anion recog-
nition is often achieved by H-bonding of well-defined complex
sites in the interior of proteins.2 Chemically, the design and
development of new anion receptors has emerged substantially
during the last decade.3 In general, anion-receptor interactions
take place by H-bonding and/or electrostatic interactions. Fluo-
rescent sensors appear to be the most suitable and attractive tools
for anion recognition because of high sensitivity at low analyte
concentration.4 A variety of signaling mechanisms has been de-
scribed such as ground-state charge transfer,3c,h photoinduced elec-
tron transfer (PET),5 excimer/exciplex formation,6 intramolecular
charge transfer,5e,7 and excited-state proton transfer.8
The urea moiety is a suitable anion receptor and phthal-
imide derivatives have high fluorescence quantum yields and
are well-studied classical chromophores. Recently, Fabbrizzi and
coworkers14 have used urea/thiourea-phthalimide derivatives as
chemosensors, where binding tendencies of the sensor towards
anions were investigated by UV-Vis and 1H-NMR titration. More
recently, Samanta and coworkers15 have studied the behavior of an
amido-phthalimide derivative in the absence/presence of halide
ions, suggesting F--induced deprotonation of the amido moiety of
the sensor system as a signaling mechanism. However, one could
also expect that a urea as an anion receptor may establish a H-bond
interaction and form a stable complex instead of deprotonation
(due to weak acidity of its N–H protons).16
Since the seminal papers by Wilcox and coworkers9 and Hamil-
ton and coworkers10 based on urea moieties as appropriate recep-
tors for anions, examples of chemosensors such as acyclic, cyclic
and polycyclic compounds containing urea/thiourea fragments
have been reported.5b,11 It is well-known that simple N-alkylated-
phthalimides possess extremely low fluorescence quantum yields
(UF < 0.02). Ring substitution with amino as well as alkoxy groups
has been described to enhance the fluorescence and 4-amino-
N-methylphthalimide was shown as a convenient solvatochromic
With this premise, we were particularly interested in developing
a new receptor–fluorophore system based on urea-phthalimide
conjugates in order to observe the sensing of halide ions and
to verify the occurrence of the formation of stable species
in the ground state. Therefore, we report here on detection
of a [1–F]- complex by fluorescence studies and confirmation
of the H-bonding interaction between 1 and fluoride ions by
1H-NMR titration experiments as well as computational calcu-
lations based on DFT methods at the B3lYP/6–31G* level of
theory. Furthermore, we want to establish the use of a chiral
urea-phthalimide combination as a potential detector for chiral
PET-quenchers by diastereoselective interactions.
aDepartment of Chemistry, Organic Division, University of Cologne,
Greinstr. 4, D-50939, Ko¨ln, Germany. E-mail: griesbeck@uni-koeln.de;
Fax: (+0049)-221-4705057
bDepartment of Chemistry, Physical Division, University of Cologne,
Luxemburgerstr. 116, D-50939, Ko¨ln, Germany
† Electronic supplementary information (ESI) available: Additional ex-
perimental details; H, 13C and H-1H correlation spectra of compound
1 in acetonitrile and DMSO. Color changes observed in the emission
on addition of F- in the absence/presence of methanol (10%) to an
acetonitrile solution of 1 Normalized emission and excitation spectra of
1 in the presence of F-. Emission spectra of 1 in the presence of the
corresponding amines as well as alcohols. Normalized fluorescence decays
traces of 1 in different solvents and in the presence of F-. Optimized
geometry coordinates of model systems. See DOI: 10.1039/b908433a
1
1
Chart 1
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Results and discussion
The synthesis of the urea-phthalimide sensor 1 was achieved
following
a
recently reported procedure.17 N-Benzyl-1,3-
dioxoisoindoline-5-carboxylic acid 2 (1 eq.) was reacted with
phenylchloroformate (1.5 eq.) and sodium azide (1.7 eq.) to
give the corresponding acyl azide that undergoes a Curtius
rearrangement to the isocyanate derivative. After reaction with
(R)-methylbenzylamine (1.5 eq.) compound 1 was obtained in
analytically pure form as a colourless solid (Scheme 1).
Fig. 1 Absorption spectra of 1 (10-4 M) in the presence of increasing
amounts of F- (0, 0.033 → 0.3 mM) in acetonitrile. Inset: Difference
UV-spectra of [1 + F-]–1 in the long wavelength region.
Scheme 1 Synthesis of sensor 1 from precursor 2.
To examine the formation of a charge transfer (CT) complex,
difference spectra ([1 + F-]–1) were obtained (Fig. 1, inset). A new
band was clearly observed at ca. 380 nm which was attributed to
the CT complex absorption maximum. The formation constant
of a CT complex (KCT) was estimated spectro-photometrically by
the Benesi–Hildebrand procedure (eqn 1).18 A concentration plot
is shown in Fig. 2.
Sensor 1 is soluble in polar solvents such as DMSO, MeOH
1
and MeCN. In the H-NMR significant differences appeared for
the signals corresponding to the urea protons. In DMSO-d6 these
resonances were observed at 6.97 ppm (H1) and 9.21 ppm (H2). In
CD3CN they were strongly shifted to 5.89 ppm (H1) and 7.81 ppm
(H2). (see ESI†). In both cases, these signals appeared as a doublet
(due to the coupling with the proton in the stereogenic centre)
and a singlet, respectively. The photophysical properties of sensor
1 were determined in different solvents and are summarized in
Table 1.
[1]/AbsCT = [1/(KCT eCT[F-])] + (1/eCT
)
(1)
No significant changes were observed with increasing solvent
polarity, however, the substantial red-shift in emission in the
protic solvent methanol and the strongly increased Stokes shift
account for the formation of an internal charge transfer state after
excitation to the first excited singlet state. Singlet state deactivation
by non-radiative pathways is also increased in the protic solvent
methanol; in comparison with acetonitrile, the fluorescence quan-
tum yield drops to one-third and the fluorescence lifetime increases
to 17 ns.
The capability of 1 to sense anions such as halides was tested
in acetonitrile solution using the anions of the corresponding
tetrabutylammonium salts (TBA+). The absorption spectrum of
1 in the absence of anions showed bands centred at 241, 253 and
340 (log e = 2.95) nm. Upon titration of F-, the ground state was
affected and the absorption was weakly shifted to the red because
of recognition of the anion and three distinctive isosbestic points
were observed at 258, 317 and 342 nm, respectively (Fig. 1).
Fig. 2 Benesi–Hildebrand plot to obtain the formation constant from the
absorbance of the CT complex (lmax = 380 nm) at different concentrations
of F-.
Table 1 Photophysical data of sensor 1
AbsCT describes the absorbance due to the CT band at 380 nm
at different concentrations of F-, and eCT represents the molar
absorption coefficient. The eCT value in acetonitrile was calculated
from the intercept and found to be 1000 M-1cm-1 (log eCT = 3).
The corresponding KCT value, as determined from the slope, was
6966 M-1. The high value of KCT indicates a strong intermolecular
interaction between 1 and F- in the ground state. This behavior
was not observed in the presence of Cl-, Br- and I-.
a
a
c
d
e
lexc
lem
Stokesb
ES
tF
UF
kF x107f
DMSO
MeCN
MeOH
DCM
353
339
337
337
443
430
465
428
5755
6242
8168
6309
72
75
72
75
11.8
13.0
16.9
14.8
0.31
0.37
0.11
0.22
2.6
2.8
0.6
1.4
a In nm b In cm-1 c Singlet energy in kcal/mol d In ns e From comparison
with quinine sulfate reference (see Experimental) f Fluorescence rate
constant (kF = (UF/tF)) in s-1.
In order to detect changes in the excited state, the fluorescence
of 1 was studied in the presence of increasing amounts of anions.
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In contrast to the small variations observed in the ground state,
the emission of 1 was dramatically affected in the presence of
F- (Fig. 3) where it was fully quenched. A weak band at ca.
520–550 nm was detected with the formation of an isoemissive
point at 515 nm. Concerning to Cl-, Br- and I- no changes in the
maximum fluorescence intensity of 1 were observed (Fig. 3; inset)
clearly supporting that sensor 1 was suitable for F- recognition in
the family of halides. Besides, color changes in the emission of 1
upon addition of F- were perceptible to the naked eye whereas no
variation in the fluorescence was observed in the rest of halides.
Fig. 4 Stern–Volmer plots for the fluorescence quenching of 1 upon F-
(᭹) and DBU (ꢀ) titration (0, 0.033 → 0.3 mM).
Fig. 3 Emission spectra of 1 (lexc = 340 nm) in the presence of increasing
amounts of F- (0, 0.033 → 0.3 mM) in acetonitrile. Inset: Changes in the
emission at 430 nm upon titration with F-, Cl-, Br- and I- with 1.
To detect a possible fluorescence dynamic quenching, the
singlet lifetime, tF of 1 (3.3 ¥ 10-6 M) was determined in the
absence and in the presence of F- (3 ¥ 10-3 M) under the same
conditions. The tF values were found to be 13 ns and 12.5 ns,
respectively (Fig. S1, ESI†). The Stern–Volmer plot showed a
non-linear behavior at high amounts of F- enhancing the idea
that a static quenching of the fluorescence was occurring (Fig. 4).
For comparison, the emission of 1 was recorded in the presence
of increasing amounts of a strong non-nucleophilic base such as
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) which is also depicted
in Fig. 4. In this case, fluorescence quenching followed a clearly
fitted-linearity in contrast than that found for F-. In addition, new
fluorescence bands at longer wavelengths were not observed (see
Fig. S2, ESI†). Therefore, the different effects obtained for the
emission of 1 in the presence of DBU and fluoride, appeared to
support CT complex formation between sensor 1 and F-.
Taking into account the formation of a [1–F]- complex, a steady-
state fluorescence measurement was carried out with a solution of
1 (10-4 M) and F- (3.4 ¥ 10-3 M) in acetonitrile. Upon selective CT
complex excitation (380 nm) its emission was actually observed
with a maximum at 520 nm (Fig. 5). The corresponding excitation
spectrum is also shown in Fig. 5. The excitation maximum
appears at 380 nm, in good agreement with the UV-absorption
measurement.
Fig. 5 Normalized absorption band of 1 (ꢀ), excitation (᭹, lem = 520 nm)
and emission (ꢀ, lexc = 380 nm) spectra of 1 (10-4 M) and F- (3.4 ¥ 10-3 M)
in acetonitrile. All measurements were made under aerated conditions.
7.81 ppm (H2) (Fig. 6, top spectrum). In the presence of increasing
equivalents of F-, the urea resonances were gradually shifted to
downfield by 2 ppm and 4.5–5 ppm, respectively, reflecting a
H-bond between the receptor and anion (Fig. 6).
More information about the nature of the [1–F]- complex
could be drawn from aryl-protons Ha and Hb. It is worth
considering that H-bonding interaction between urea subunit
and anion could induce some effects on aromatic substituents,
i.e. polarization-induced shift of the C-H bonds via a through-
space effect, producing downfield shifts due to deshielding effect
by a partial positive charge formed on the proton.14 In fact, this
electrostatic effect was observed in aromatic protons Ha and Hb
as indicated by the weak downfield shift upon addition of F-
equivalents (Fig. 6), being in agreement with previous studies.14
Besides, proton Hc was too far away from the N–H protons to
undergo any electrostatic effect. On the other hand, the H1 proton
did not vanish even at a high concentration of anion concluding
that although deprotonation was not taking place, the bond-length
of N–H amide may increase sufficiently.
To further confirm the formation of a [1–F]- complex through
a H-bonding interaction between F- and the urea moiety, we
The nature of the H-bonding interaction was supported by the
effect of protic solvents on the emission of the [1–F]- complex.
Addition of methanol to a mixture of 1 and F- led to recovery of
1
also performed H-NMR titration experiments in CD3CN. As
stated above, the urea protons appeared at 5.89 ppm (H1) and
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Fig. 7 Geometries of the free (left) and fluoride bonded (right) model
system.
7.7 ppm (DdN–H = 3.7 ppm) and 10.7 ppm (DdN–H = 5 ppm),
respectively, after complexation with F-. Overall, these compu-
tational results are in agreement with experimental observations
where formation of a complex between sensor 1 and F- in the
ground state prevails over a possible deprotonation of the urea
moiety.
Finally, taking into account the stereogenic centre of sensor
1, we were interested in studying enantiodifferentiation of the
fluorescence quenching in the presence of enantiomerically pure
amines (R- and S-methylbenzylamine) as well as alcohols (R-
and S-1-phenylethanol). Fluorescence investigations showed that
amines efficiently quenched the emission of 1 whereas no changes
were detected in the presence of the alcohols (Fig. S5 and S6, ESI†).
No enantiodifferentiation was however observed in the quenching
process with chiral amines as quenchers and the chiral model
sensor 1 (Fig. 8). Synthesis and investigation of more complex
chiral sensors with additional binding sites are in progress.
Fig. 6 Changes in the 1H NMR (300 MHz) spectra of 1 in CD3CN upon
addition of F-.
the blue emission, i.e. the [1–F]- interaction was presented (Fig. S3,
ESI†). The fluorescence spectrum of 1 in methanol became red-
shifted in comparison with acetonitrile as the solvent (see Table 1).
Subsequently, addition of F- to a 1 acetonitrile solution containing
10% of methanol led to no change in the emission, thus, clearly
supporting the interaction between the urea moiety and the protic
solvent.
To ensure whether the recognition process involves a CT
complex formation through a H-bonding interaction between
1 and F-, computational calculations based on B3lYP/6–31G*
level of theory using the CPCM method (acetonitrile as solvent)
were carried out. Model system geometries were optimized in the
absence and presence of the fluoride ion (Fig. 7) and corresponding
N–H bond distances were calculated. Hence, N–H1 and N–H2
˚
bond lengths in the absence of an anion were found to be 1.022 A
Fig. 8 Stern–Volmer plot for the fluorescence quenching of 1 upon amine
˚
and 1.024 A, respectively, whereas values of these bond distances
titration.
˚
after fluoride binding complex optimization were 1.035 A for the
1
2
˚
N–H bond and 1.051 A for the N–H bond. Although bond-
elongation was observed in both cases (+0.013 A for N–H and
+0.025 A for N–H ), it appeared that it was not sufficient for
hydrogen abstraction by F-, in contrast to previous observations.15
1
˚
Conclusions
2
˚
In summary, we have synthesized chemosensor 1 based on a chiral
urea receptor linked to a phthalimide chromophore using a recent
procedure involving a Curtius rearrangement. Sensor 1 was found
to selectively detect F- since absorption as well as fluorescence
changes were only observed for this halide anion.20 In this context,
a fluorescence static quenching was proposed for the signaling
mechanism: the overall picture is depicted in Scheme 2.
Upon recognition of the anion, absorption studies revealed a
typical band of a CT complex at longer wavelengths with a large
formation constant. Moreover, not only were there no changes
˚
The H–F distances were also estimated and found to be 1.710 A
1
2
˚
(H –F) and 1.602 A (H –F) which were close to that experimentally
found in the literature for complexation of isophthalimide-like
compounds with fluoride ions.19
Moreover, both N–H1 and N–H2 GIAO-NMR shifts (d) were
also theoretically calculated in the absence/presence of a fluoride
ion. These data were in line with that obtained experimentally
where dN–H values for H1 and H2 in the absence of F- were 4.0 ppm
and 5.7 ppm, respectively whereas they shifted to downfield to
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Scheme 2
in the singlet lifetime of 1 found in the presence of F- but also
a non-linear dependence on the Stern–Volmer plot was observed,
ruling out the possibility of a dynamic fluorescence quenching. The
formation of the [1–F]- complex through H-bonding interactions
was proven by 1H-NMR studies and theoretical calculations.
concentrations were fixed as indicated. The excitation and emis-
sion slit widths were 2.5 nm. The fluorescence quantum yields in
different solvents were measured with reference to quinine sulfate
(UF = 0.546 in 0.5 M H2SO4) by comparing the area of fluorescence
and absorbance at the excitation wavelength of 340 nm, using the
formula22
Experimental
Usample = (asample/astd) (Astd/Asample) (nsample/nstd) Ustd
Materials
where Usample and Ustd, asample and astd, nsample and nstd and Asample and
A
std are the quantum yield, area under emission spectra, refractive
The starting materials phenylchloroformate, sodium azide and
potassium tert-butoxide as well as (R)-phenylethanamine were
commercially available. Compound 2 was synthesized follow-
ing a literature procedure.21 Spectroscopic grade acetonitrile
(MeCN), dimethylsulfoxide (DMSO), methanol (MeOH) and
dichloromethane (DCM) were used as solvents.
index and the absorbance of the sample under study (sensor 1)
and the standard (quinine sulfate), respectively.
Time-resolved fluorescence spectroscopy
Fluorescence lifetimes were measured using a gated intensified
CCD equipped monochromator. The spectral resolution has been
set to 2 nm. The samples were excited with the third harmonic
(355 nm) of a Nd–YAG laser. The overall instrument response
function is 1.5 ns. The samples were placed into quartz cells of 1 cm
path length. Compound concentrations were fixed as indicated.
1H-NMR
The 1H-NMR spectra were recorded on a Bruker AC 300
spectrometer operating at 300 MHz or on a Bruker AV 600
spectrometer instrument operating at 600 MHz. Chemical shifts
are reported as d in ppm and the coupling constant, J, in Hz. In all
spectra solvent peaks were used as the internal standard. Solvents
used were DMSO-d6 (d = 2.49 ppm) and CD3CN (d = 1.94 ppm).
Splitting patterns are designated as follows: s, singlet; d, doublet;
dd, double doublet; p, quintet; m, multiplet.
Computational Calculations
The structures were optimized with GAUSSIAN03,23 using
B3LYP/6–31*24 and the CPCM-SCRF method, solvent =
acetonitrile.25 NMR shifts were computed of the optimized
structures using the GIAO method.26
13C-NMR
General procedure for the synthesis of chemosensor 1
The 13C-NMR spectra were recorded either on a Bruker AC 300
spectrometer instrument operating at 75 MHz or on a Bruker AV
600 spectrometer instrument operating at 126 MHz.
To a solution of sodium azide (110 mg, 1.70 mmol), potassium tert-
butoxide (17 mg, 0.150 mmol), and 2 (281 mg, 1.00 mmol) in DME
(10.0 mL) at 25 ◦C, was added the phenylchloroformate (235 mL,
1.5 mmol). The resulting mixture was stirred at 75 ◦C overnight.
Then, the mixture was slowly cooled down to room temperature
and R-methylbenzylamine (200 mL, 1.50 mmol) was added and the
reaction mixture was stirred for 16 h. Afterwards, it was diluted
with hexane (40 mL) and the resulting solution was poured into
ice-cold water with continuous stirring, 10 mL water was added
and the stirring was maintained during 20 min. The white solid
was filtered and washed several times with cold chloroform. The
urea-activated phthalimide 1 (253 mg, 63% yield) was obtained
as a white solid after filtration. Rf 0.3 (cyclohexane/ethyl acetate
60/40 v/v). 1H-NMR (600 MHz, DMSO-d6): 1.42 (d, J = 7.0 Hz,
Absorption spectroscopy
Absorption spectra were recorded using a Beckman Coulter UV-
DU800 spectrometer. The samples were placed into quartz cells
of 1 cm path length. Compound concentrations were fixed as
indicated.
Steady-state fluorescence spectroscopy
Fluorescence and excitation spectra were carried out using a
Perkin-Elmer LS-50B luminescence spectrometer. The samples
were placed into quartz cells of 1 cm path length. Compound
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3H, CH3), 4.72 (s, 2H, CH2), 4.85 (p, J = 7.0 Hz, 1H, CH),
6.98 (d, J = 7.7, 1H, NH1), 7.25–7.36 (m, 10H, CHaromatic), 7.58
(dd, J1 = 1.7 Hz and J2 = 8.2 Hz, 1H, CHaromatic), 7.73 (d,
J = 8.2 Hz, 1H, CHaromatic), 8.06 (s, 1H, CHaromatic), 9.21 (s, 1H,
NH2); 13C NMR (126 MHz, DMSO-d6): 22.7 (CH3), 40.6 (CH2),
48.7 (CH), 111.14 (CHaromatic), 121.48(CHaromatic), 122.8 (Caromatic),
124.37 (CHaromatic), 125.82 (2-CHaromatic), 126.74 (CHaromatic), 127.27
(2-CHaromatic), 127.31 (CHaromatic), 128.32 (2-CHaromatic), 128.53 (2-
CHaromatic), 133.22 (Caromatic), 136.81 (Caromatic), 144.65 (Caromatic),
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=
=
146.34 (Caromatic), 153.77 ((NH)2C O), 167.41 (C O), 167.62
=
(C O). MS (m/z (%)): 399 (12), 278 (25), 260 (11), 252 (100),
234 (19), 120 (23), 105 (79), 91 (32), 77 (39). Exact mass (EI):
required for C24H21N3O3: 399.1583 (M+): found 399.159 Melting
point: 212–213◦.
Acknowledgements
This work was generously financed by the Alexander von Hum-
boldt Foundation (fellowship to R. P.-R.) and DAAD (fellowship
to Y. D.).
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3504 | Org. Biomol. Chem., 2009, 7, 3499–3504
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