Colorimetric 'naked-eye' and fluorescent sensors for anions based on amidourea functionalised 1,8-naphthalimide structures: Anion recognition via either deprotonation or hydrogen bonding in DMSO

Article abstract of DOI:10.1039/b715533f

The synthesis and the spectroscopic studies of three new amidourea-based sensors for anions, 1-3, are described. These are based on the use of 4-hydrazine-1,8-naphthalimides which upon reaction with isocyanates give rise to the formation of the desired amidoureas. 1-3 absorb strongly in the visible region, due to the internal charge transfer excited state character of the naphthalimide moieties. Large colorimetric changes were observed upon the addition of various anions such as acetate, dihydrogenphosphate and fluoride to 1-3 in DMSO, and are brought about through either hydrogen bonding to, or deprotonation of, the amidourea. These changes were clearly visible to the naked eye, changing from yellow/green to purple, and were reversed upon addition of protic solvents. Moreover, each of the three anions gave rise to unique changes in the structure of the absorption spectra which can be considered as being a 'fingerprint' identity for each of them. The fluorescence emission spectra were also affected upon anion binding, being significantly red shifted upon excitation. Non-linear regression analysis of the ground and excited state changes showed the anions were recognized in either 1: 1 or 2: 1 stoichiometry, and that the aryl-urea substituents govern the sensitivity of the binding; which in the case of acetate was in the order of 3 > 1 > 2. The anion recognition was also monitored by ¹H NMR spectroscopy in DMSO-d ₆. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b715533f

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Colorimetric ‘naked-eye’ and fluorescent sensors for anions based on
amidourea functionalised 1,8-naphthalimide structures: anion recognition
via either deprotonation or hydrogen bonding in DMSO
Haslin Dato Paduka Ali,a Paul E. Kruger*ab and Thorfinnur Gunnlaugsson*a
Received (in Durham, UK) 11th October 2007, Accepted 11th January 2008
First published as an Advance Article on the web 8th February 2008
DOI: 10.1039/b715533f
The synthesis and the spectroscopic studies of three new amidourea-based sensors for anions, 1–3,
are described. These are based on the use of 4-hydrazine-1,8-naphthalimides which upon reaction
with isocyanates give rise to the formation of the desired amidoureas. 1–3 absorb strongly in the
visible region, due to the internal charge transfer excited state character of the naphthalimide
moieties. Large colorimetric changes were observed upon the addition of various anions such as
acetate, dihydrogenphosphate and fluoride to 1–3 in DMSO, and are brought about through
either hydrogen bonding to, or deprotonation of, the amidourea. These changes were clearly
visible to the naked eye, changing from yellow/green to purple, and were reversed upon addition
of protic solvents. Moreover, each of the three anions gave rise to unique changes in the structure
of the absorption spectra which can be considered as being a ‘fingerprint’ identity for each of
them. The fluorescence emission spectra were also affected upon anion binding, being significantly
red shifted upon excitation. Non-linear regression analysis of the ground and excited state
changes showed the anions were recognized in either 1 : 1 or 2 : 1 stoichiometry, and that the
aryl-urea substituents govern the sensitivity of the binding; which in the case of acetate was in the
1
order of 3 4 1 4 2. The anion recognition was also monitored by H NMR spectroscopy in
DMSO-d6.
been extensively used for sensing cations such as Na+, K+31
Introduction
and transition metal ions.32 We15a and others19–21,33 have
Anion recognition and sensing has become a fast growing field
previously shown that such amidoureas are particularly effec-
of research in the last few years.1,2 In particular the study of
tive hydrogen bonding donors capable of anion recognition.
anion sensing using fluorescent or colorimetric sensors has
Indeed, by simply changing the 4-amino moiety for a hydra-
been actively studied.3,4 In the latter, anion recognition gives
zine and reacting the ‘free amine’ with isocyanates gave the
rise to changes in the absorption spectra with associated
desirable 4-amidourea-1,8-napthalimide based sensors. By in-
colour changes.5,6 The use of charge neutral anion receptors
corporating electron withdrawing substituents such as CF3, as
such as imidazoles,7,8 pyrroles,9 calixpyrroles,10 amides,11,12
part of the amidourea moiety we were also able to ‘tune’ both
cabamides,13 phenols,14 ureas,15,16 thioureas17,18 and amidour-
the selectivity and sensitivity of the anion recognition.34,35
eas19–21 to achieve such sensing has been demonstrated. For
These anion sensors were all strongly coloured, absorbing in
these receptors, the anion is recognized through hydrogen
bonding or by deprotonation22,23 of labile protons by the
the visible region with a lmax of 450 nm, and we demonstrate
that the ICT fluorescence of 1–3 is significantly modulated
basic anions (such as fluoride) in organic solvents. Such
upon excitation of the ICT band, as well as upon excitation of
sensors can also operate within supporting matrices24 or
the newly formed long wavelength absorption band at 540 nm,
polymers as recently demonstrated by Anzenbacher25 or by
which gives rise to long wavelength emission. These colour
simply absorbing anion sensors onto cellulose, or filter
changes were clearly visible to the naked eye even at low
paper.26 The recognition moiety often forms part of a chro-
concentrations.
mophore possessing a push–pull character based on electron-
donor–acceptors,27 which often make them non or weakly
luminescent. We have been active in developing sensors for
anions.28,29 Herein we present three new anion sensors 1–3,
based on the 4-amino-1,8-naphthalimide30 structure, which
has an internal charge transfer excited state, and which has
a School of Chemistry, Centre of Synthesis and Chemical Biology,
University of Dublin, Trinity College Dublin, Dublin 2, Ireland.
E-mail: gunnlaut@tct.ie
b Department of Chemistry, College of Science, University of
Canterbury, Christchurch, 8020, New Zealand
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Ground state investigations of anion recognition
The anion sensors 1–3 are soluble in a variety of solvents such
as EtOH, DMSO, CH3CN and 50 : 50 H2O–EtOH mixtures.
However, preliminary studies of these amidourea sensors
showed that they were only able to successfully bind anions
in aprotic solvents.w Therefore, anion recognition of 1–3 was
carried out in DMSO at room temperature. In DMSO their
extinction coefficients were determined to be 14 586, 14 507
and 13 748 MÀ1 cmÀ1, respectively, for the ICT transition,
which for 3 had a lmax at 420 nm. Upon the addition of acetate
(AcOÀ), dihydrogenphosphate (H2PO4À) or fluoride (FÀ) to 3,
the 420 nm band decreased in intensity in response to each
anion (Fig. 2 for AcOÀ). Indeed, binding of AcOÀ also gave
rise to the concomitant formation of two new absorption
bands, one at longer wavelength (ca. 569 nm) and one at
shorter wavelength (ca. 360 nm). These changes were also
accompanied by the formation of two clear isosbestic points at
385 and 470 nm, respectively. The ICT band was significantly
reduced in absorbance after the titration, appearing as a broad
shoulder at ca. 430 nm. These overall changes are consistent
with anion binding, through hydrogen bonding at the ami-
dourea moiety, which enhances the ICT character of the
chromophore by placing a highly electron rich moiety adjacent
to the donating amine. This gives rise to a stronger ‘push–pull’
effect and consequently, shifts the long wavelength absorption
towards the red. This feature was characteristic of all the anion
titrations carried out on 1–3.
Scheme 1 Synthesis of anion sensors 1–3.
Results and discussion
Synthesis and characterisation
The three anion sensors differ only in the nature of the ‘urea’
part of their receptors. Where 1 is a simple aryl urea, 2 has an
electron donating substituent making the receptor more elec-
tron rich whilst 3, in contrast, has a strongly electron with-
drawing substituent, making its urea protons better hydrogen
bonding donors.34 It was hoped that these simple structural
differences could be used to fine-tune the sensitivity of the
anion binding.
Further analysis of these spectral changes (Fig. 2) clearly
shows that the long wavelength absorption band has the
appearance of ‘fine structure’, while the short wavelength
absorption increased in intensity to ca. 80% of the band for
the free (unbound) sensor at 420 nm. Similar features were
also observed for the titrations against 1 or 2. For 1 a lmax at
415 nm was recorded for the free sensor and upon titration
with AcOÀ two new bands arose centred at 353 and 556 nm,
respectively, with associated isosbestic points at 384 and
464 nm, respectively. Similarly for 2, a lmax at 419 nm was
observed for the free sensor whilst after titration with AcOÀ
two new bands appeared centred at 353 and 558 nm, respec-
tively. Again, two clear isosbestic points were observed at 382
and 466 nm, respectively. The reversibility of this sensing
action was also confirmed by adding a polar protic solvent
(EtOH) to the above sample. This fully reversed the changes
seen in the absorption spectra in Fig. 2. Such changes were not
seen in the thiourea analogue of 1,6 which allowed for the
sensing of AcOÀ in aqueous media.
The synthesis of 1–3 is shown in Scheme 1. 6-Bromo-2-
ethyl-benzo[de]isoquinoline-1,3-dione 4 was formed in 89%
yield using a literature procedure6 from the corresponding
4-bromo-1,8-naphthalic anhydride, which was then reacted
with hydrazine monohydrate at 130 1C for one hour.6 Upon
cooling to room temperature, the solution was added to H2O,
which gave a yellow precipitate which was collected and
washed with H2O, giving 5 in 95% yield. Finally, the anion
sensors 1–3 were formed by reacting 5 with phenyl isocyanate
(6), para-tolyl isocyanate (7) and 4-trifluoromethylphenyl
isocyanate (8), respectively, under reflux in anhydrous
MeCN. This gave rise to the formation of precipitates after
ca. 30 min, which were collected and purified by washing with
hot CH2Cl2 : MeOH (1 : 4) solvent mixture. This gave 1–3 in
41, 52 and 45% yields, respectively, with no further purifica-
tion needed. All the compounds were fully characterised (see
Experimental); by, for instance, their characteristic urea pro-
tons as observed in the 1H NMR spectrum (400 MHz, DMSO-
d6). For instance, for 3, the characteristic resonances for the
urea protons were observed in the 1H NMR (400 MHz,
DMSO-d6) at 9.64 and 9.45 ppm, respectively (Fig. 1). The
4-amino proton occurred at 8.90 ppm, and the five naphtha-
limide protons as well as the two resonances for the aryl urea
based receptors were all clearly visible. All the compounds
were highly coloured and gave rise to strong green fluorescence
typical for such naphthalimide derivatives.
À
When titrations were carried out on 3 using either H2PO4
or FÀ, similar overall results were observed as seen in Fig. 3
and Fig. 4, respectively. However, the absorption spectra of
each displayed some subtle differences from that observed for
the titration with AcOÀ. In the case of H2PO4À, the formation
of a long wavelength absorption band at 565 nm was accom-
panied by the formation of a short wavelength absorption
band at 353 nm and two isosbestic points at 356 and 459 nm,
respectively. However, the latter of these was shifted slightly to
w This is in contrast to their thioureaamido analogues that we have
also investigated and shown to sense anions in both organic and
aqueous solutions (see ref. 6).
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Fig. 1 1H NMR spectrum (400 MHz, DMSO-d6) of 3 with assignments.
the red at high anion concentration. We also noted that the
short wavelength absorption band was not as ‘broad’ as that
seen for AcOÀ in Fig. 2, and that the long wavelength
absorption band was slightly more structured. Again, we
assign these changes to the recognition of the anion at the
amidourea moiety, occurring through hydrogen bonding.
These differences were even more pronounced for FÀ where
‘collapse’ of the lmax band at 420 nm was initially replaced by
the formation of aÀshoulder similar to that observed for both
AcOÀ and H2PO4 above, and eventually by an absorption
band centred at ca. 419 nm. Moreover, the long wavelength
absorption was more structured than that seen for either
AcOÀ or H2PO4À, with significantly sharper ‘tailing off’
towards the red, and a lmax at 559 nm. Furthermore, looking
at the ratio between the 559 nm and the short wavelength
353 nm transitions, yields an almost 1 : 1 ratio between these
two wavelengths, whereas in Fig. 2 and Fig. 3, the short
wavelength band was ca. 50% and 40% stronger than the
long wavelength absorptions, for AcOÀ and H2PO4À, respec-
tively.
Upon carrying out anion titrations on 1 and 2, similar
behaviour was observed for each anion–sensor combination.
The changes in the absorption spectra for 1 upon titration with
AcOÀ are shown in Fig. 5 and subtly demonstrate a difference
to the absorption spectra of 3. Hence, we propose that these
changes are due to the direct interaction of the anion with the
receptors in a specific and unique manner, and not due to one
common mechanism such as deprotonation of the thiourea
protons, as such mechanism would have lead to the formation
of the same ‘product’ for all of these anions, and hence an
identical absorption spectrum. These titrations were also
carried out on 1–3 using ether ClÀ or BrÀ, but these titrations
did not lead to measurable changes in the absorption spectra.
By plotting the growth of the new long wavelength band of
these colorimetric sensors as a function of Àlog [anion],
Fig. 2 The changes observed in the absorption spectrum of urea 3
(1.5 Â 10À5 M) upon addition of AcOÀ (0 M - 3 Â 10À3 M) in
DMSO.
Fig. 3 The changes observed in the absorption spectrum of urea 3
(1.5 Â 10À5 M) upon addition of H2PO4 in DMSO at 25 1C
À
(0 M - 3 Â 10À3 M).
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Fig. 5 The changes observed in the absorption spectrum of urea 1
(1.5 Â 10À5 M) upon addition of AcOÀ (0 M - 3 Â 10À3 M) in
DMSO.
Fig. 4 Changes observed in the absorption spectrum of urea 3 (1.5 Â
10À5 M) upon addition of FÀ in DMSO at 25 1C (0 M - 3 Â 10À3 M).
Excited state spectroscopic investigation of anion recognition
sigmoidal binding curves were observed, which changed ovÀer
In parallel with the absorption binding experiments discussed
above, the changes in the fluorescence emission spectra of 1–3
were also monitored in DMSO upon anion sensing. The
changes in the fluorescence emission of 1 upon excitation lmax
at 420 nm, is shown in Fig. 7. The emission spectrum of the
free sensor gave rise to the formation of a broad band with a
maximum intensity at 520 nm upon excitation of the ICT band
at 420 nm. Following titration with AcOÀ fluorescence
quenching was observed for this band and the emission was
significantly ‘‘switched off’’. However, it is clear from Fig. 7
that the lFmax was also shifted to a longer wavelength. When
the changes in emission intensity at the 520 nm band are
plotted as a function of Àlog [AcOÀ] (Fig. 7, insert) a
sigmoidal profile results, indicative of 1 : 1 binding between
the anion and the sensor. From these changes, we estimated
the binding constant log b to be B5.0 (Æ0.2), which correlates
well with that observed in the ground state (Table 1). The
emission spectra was also monitored by exciting at the two
isosbestic points shown in Fig. 1, as well as at the anion
induced long wavelength absorption band. Upon excitation of
the isosbestic point at 360 nm, similar changes were observed,
with lmax at 510 nm. However, here the emission was fully
quenched, or fully ‘‘switched off’’, with concomitant forma-
tion of a new weakly emitting band at longer wavelength
centred at ca. 625 nm. On the other hand, when exciting at the
470 nm isosbestic point, an emission band with lmax at 545 nm
was observed that tailed into the 760 nm region. This emission
was much less modulated upon titration with AcOÀ, with only
approximately two log units for both AcOÀ and H2PO4
,
which is indicative of 1 : 1 binding. This stoichiometry was also
confirmed by fitting the changes in the long wavelength band
using the nonlinear regression analysis program SPECFIT.
Using a number of different binding models with host-to-guest
stoichiometries such as 1 : 1, 1 : 2 and 1 : 3, gave log b values as
shown in Table 1. However, only the 1 : 1 binding gave good
fits to the experimental data. In contrast to these results it was
not possible to accurately calculate the log b values for FÀ
binding, as the resulting log b values carried significant errors.
This is most likely due to the nature of the binding interactions
of FÀ with the amidourea protons. For AcOÀ and H2PO4À the
sensing is due to hydrogen bonding interactions between the
anion and the amidourea protons, however for FÀ, initial
hydrogen bonding interaction is most likely followed by the
À
formation of the ion pair complex (bifluoride ion) HF2
,
because of the ability of this anion to deprotonate acidic
moieties. Similar deprotonation events have been demon-
strated previously by our own group as well as those of
Gale,19,23 Fabbrizzi18 and Pfeffer.7,16
Colorimetic responses: naked eye detection of anions
As is clear from the above ground state changes, the absorp-
tion spectra of 1–3 were dramatically affected upon interaction
with AcOÀ, H2PO4À or FÀ, and each one of these anions gives
rise to unique change in the structure of the absorption
spectra. However, the ‘colour’ changes seen in the absorption
spectra were also clearly visible to the naked eye as demon-
strated in Fig. 6, which shows the chromogenic response of 3
(1 Â 10À3 M) in DMSO upon the addition of these anions
(1 eq.). In general, the colour change was from a bright
fluorescent colour to blue or deep purple. However, in the
case of FÀ, the purple colour change was only observed at low
guest concentrations, as at high concentrations the initial
colour changed, via purple, to pale orange. This may be
ascribed to the deprotonation of the N–H protons of the
urea/thiourea moiety of the sensor, resulting in the formation
of the HF2À. No colour change was seen in the presence of
either ClÀ or BrÀ.
Fig. 6 Chromogenic response of solutions of sensor 3 (1 Â 10À3 M)
upon interaction with various anions: (a) Free host 3; (b) 3 + 1 eq.
AcOÀ; (c) 3 + 1 eq. H2PO4À; (d) 3 + 1 eq. FÀ; (e) 3 + FÀ (excess).
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Fig. 8 Changes observed in the fluorescence of 1 (1.5 Â 10À5 M) upon
À
addition of H2PO4 in DMSO at 25 1C (excited at 420 nm; 0 M -
Fig. 7 Changes observed (quenching) in the fluorescence of 1 (1.5 Â
10À5 M) upon addition of AcOÀ in DMSO at 25 1C (excited at 420 nm;
0 M - 3 Â 10À3 M). Insert: the changes at 520 nm as a function of
Àlog [AcOÀ]. See Fig. 5 for the corresponding ground state changes.
3 Â 10À3 M.
systems, which, in our case could result in full proton trans-
fer.16 In the case of FÀ it is possible that initially the ion is
bound through hydrogen bonding to the urea moiety, fol-
lowed by either the binding of a second FÀ at the 4-amino
moiety or, by deprotonation of a urea NH, or the 4-amino
moiety protons, in either case to yield HF2À. We have recently
demonstrated that such deprotonation occurs for similar
4-amino-1,8-naphthlimides at high concentrations of FÀ will
deprotonate the most acidic proton to form HF2À. The
changes shown in Fig. 9, suggest that such deprotonation is
probably occurring here. Analysis of the emission spectra by
plotting the changes at 440, 510 and 615 nm, respectively, as a
function of Àlog [FÀ], clearly revealed the complexity of the
anion recognition–deprotonation process for both H2PO4À or
FÀ, supporting our explanation. Unfortunately, we were un-
able to determine accurate binding constants from these
changes by fitting the emission changes by using the nonlinear
regression analysis program SPECFIT.
ca. 15% changes being observed in the emission intensity. This
long wavelength emission was also clearly observed when
exciting at the newly formed band at 569 nm, where a lmax
was observed at ca. 610 nm, with a smaller shoulder at 690 nm,
which was enhanced upon increasing the concentration of
AcOÀ. Both 2 and 3 also gave rise to long wavelength emission
bands upon excitation at the ICT bands and at their isosbestic
points, which was again modulated upon titration with AcOÀ
in a similar manner to that described above.
These titrations were also carried out using H2PO4À and FÀ.
The changes in the fluorescence emission of 1 (upon excitation
max at 420 nm) following the addition of H2PO4À, are shown
l
in Fig. 8. However, in contrast to the behaviour noted for
AcOÀ, initially only a slight quenching of the fluorescence
emission by these anions was observed. These changes were
however, followed by a significant ‘‘switching on’’ effect (after
the addition ca. 50 equivalents of the anion) which also gave
rise to a red shift from a maximum intensity at 520 nm to
530 nm, Fig. 8. Similarly, the effect of the addition of either
1H NMR spectroscopic investigation of anion recognition
Having established above that anion recognition was either
through ‘pure’ hydrogen bonding interactions, or as a combi-
nation of hydrogen bonding and/or deprotonation, we carried
out detailed 1H NMR spectroscopic titrations on 1–3 in
DMSO-d6 using 1.0 Â 10À2 M of the above anions at 25 1C.
The naked eye effect discussed above was also quite striking
in these studies due to the relatively high concentration of the
sensors, and the yellow to purple colour change began to occur
immediately after the addition of 0.1 equivalent of anion. The
naphthalimide protons of these sensors were primarily moni-
tored as the chemical shifts of the NH protons of the urea
moiety broadened significantly early on in the titration. Never-
theless, it was also possible to observe the changes in the
chemical shift of the 4-amino-naphthamide proton in 1 and 2,
while that of sensor 3 became too broad to monitor upon
anion interaction, most likely due to the electron-withdrawing
effect of the 4-(trifluoromethyl)-phenyl substituents which
would be expected to increase in the acidity of the urea N–H
protons.
H2PO4 or FÀ following excitation at 360 nm gave rise to an
À
initial decrease in the fluorescence emission (lmax 510 nm) by
ca. 25%, followed by an increase in the same band with the
appearance of two new bands at 440 and 615 nm, respectively,
Fig. 9 shows these changes for 1 in the presence of FÀ. Similar
behaviour was also observed for 2 and 3 when titrated with
À
H2PO4 or FÀ.
One rationale for these changes is that the hydrazine proton
directly attached to the 4-position of the naphthalimide ring is
also involved in the binding of these anions. In the case of
H2PO4À it may be due to hydrogen-bonding interactions with
this moiety as demonstrated by Pfeffer et al. for related
Table 1 Stability constants (log b) obtained from fitting the changes
in the absorption spectra of 1–3 using the nonlinear regression analysis
program SPECFIT and fitting the data to 1 : 1 bindinga
À
H2PO4
Sensor
AcOÀ
FÀb
1
1
2
3
4.19 Æ 0.03
3.99 Æ 0.06
4.26 Æ 0.06
2.86 Æ 0.04
—
—
—
Fig. 10 and Fig. 11 show the changes observed in the H
b
—
3.49 Æ 0.02
NMR spectrum of 3 upon the addition of AcOÀ and FÀ,
respectively. The data obtained from these titrations was
plotted as the cumulative changes in chemical shift (Dd)
against the equivalents of anion added and the resulting plots
a
b
All measured in DMSO. The binding profile was too complicated
to allow for accurate determination of log b.
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Fig. 9 Changes observed in the fluorescence of 1 (1.5 Â 10À5 M) upon
addition of FÀ (0 - 3 Â 10À3 M) in DMSO upon excitation at
360 nm; light grey arrow: first significant change; dark grey arrow:
second significant change).
were analysed using WinEQN MR. As can be seen from
Fig. 10, the amide protons disappeared after ca. 0.1 equivalent
of the anion, consequently it should be noted that since the
changes were comparably small in magnitude, hence carried
larger errors. For all the three sensors, the binding curves
obtained for their complex formation with AcOÀ reached a
plateau after ca. one equivalent, indicating a 1 : 1 host : guest
stoichiometry. This supports our finding from the ground and
the excited state measurements discussed above that the bind-
ing is solely due to hydrogen bonding. On the other hand, the
binding of either H2PO4À required the addition of two equiva-
lents of the anion in order for the binding process to reach
completion. In the case of FÀ, Fig. 11, the deprotonation
process was clearly visible, with the formation of the char-
acteristic triplet for the formation of HF2À, at ca. 16 ppm.
There are many reports of a 1 : 2 host : guest stoichiometry for
FÀ binding, mostly due to such anion induced protonation.
However, there are to the best of our knowledgeÀno known
cases of the binding of two equivalents of H2PO4 to such a
simple mono-urea (or thiourea) receptor. We were however,
unable to determine the binding constant for this second
interaction accurately (cf. Table 2), which supports that
Fig. 11 Stack plot of 1H NMR spectra of 3 (1 Â 10À2 M) upon
addition of FÀ in DMSO-d6 at 25 1C. (Note: The chemical shifts were
greatly perturbed; hence, the 1H NMR spectra had to be magnified
significantly after the first addition of FÀ.)
theory. The binding constants (log b) for the binding of 1–3
(determined by the WinEQN MR program) are listed in
Table 2. On all occasions a good fit was observed. Unfortu-
nately, we were unable to determined binding constants for FÀ
due to the deprotonation of the urea N–H. However, the
results demonstrate the same trend as observed for the ground
state changes, and highlight the effect the substituents has on
the anion affinity for these sensors.
Conclusions
A family of colorimetric sensors based on the use of 4-amino-
1,8-napthlimide as a chromophore (fluorophore) were devel-
oped for the sensing of anions through hydrogen bonding with
a few steps synthesis. All the three sensors showed significant
changes in the absÀorption spectra upon binding to anions such
as AcOÀ, H2PO4 and FÀ in DMSO, where the ICT band of
the naphthalimide moiety was shifted to longer wavelength,
and a new band was formed at shorter wavelengths and with
the formation of several isosbestic points. These changes were
fully reversible upon addition of a polar protic solvent. Only
minor changes were seen for ClÀ and BrÀ. The sensitivity of
the anion binding could also be tuned by changing the nature
of the aryl urea moiety. Importantly, unique differences were
seen in the fine structure of the resulting absorption bands as a
consequence of the AcOÀ, H2PO4À and FÀ binding, which can
be considered as being a ‘fingerprint’ identity for each of these
anions. Moreover, the anion binding and the changes seen in
the absorption spectra were also clearly visible to the
naked eye.
Fig. 10 Stack plot of 1H NMR spectra of 3 (1 Â 10À2 M) upon
addition of AcOÀ in DMSO-d6 at 25 1C.
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Table 2 Stability constants (log b) obtained from fitting the changes
was collected by filtration, washed with water and dried to
yield 4 as a cream-coloured solid (1.96 g, 89%). m.p. 160–
162 1C (lit., 163 1C); calculated for C14H10NO2Br: C, 55.3; H,
3.3; N, 4.6%; found: C, 55.2; H, 3.3; N, 4.7%.
1
in the H NMR spectra of 1–3 upon titration with various anionsa
À
Sensor
AcOÀ
H2PO4
2.21c
—
FÀb
1
2
3
3.58
2.85
3.75
—
—
—
a
2-Ethyl-6-hydrazino-benzo[de]isoquinoline-1,3-dione (5)
2.33c
a
b
All measured in DMSO-d6. The binding process is too complicated
Hydrazine monohydrate (excess) was added to 4 (0.20 g, 0.66
mmol). The neat reaction mixture was heated at 130 1C and
left stirring for 1 h. It was then poured into water, forming a
precipitate, which was collected by filtration, washed with
water and dried to yield 5 as a yellow solid (0.16 g, 95%).
m.p. 255–257 1C; HRMS (MeOH, ES+): calculated for
C14H14N3O2: 256.1086 (M + H)+; found: 256.1077; dH (400
MHz, (CD3)2SO) 9.13 (1H, br s, Naph-NH), 8.61 (1H, d, J =
8.5 Hz, Naph-H7), 8.41 (1H, d, J = 7.0 Hz, Naph-H5), 8.28
(1H, d, J = 7.5 Hz, Naph-H2), 7.64 (1H, dd, J = 7.8 Hz, 8.0
Hz, Naph-H6), 7.24 (1H, d, J = 8.6 Hz, Naph-H3), 4.69 (2H,
d, J = 10.6 Hz, Naph-NH-NH2), 4.05 (2H, q, J = 7.0 Hz,
CH2), 1.18 (3H, t, J = 7.0 Hz, CH3); dC (100 MHz, (CD3)2SO)
163.6, 162.7, 153.2, 134.2, 130.5, 128.2, 127.3, 124.1, 121.7,
118.4, 107.4, 104.0, 34.2, 13.3; IR (KBr) umax (cmÀ1) 3450,
3366, 3316, 1672, 1636, 1614, 1578, 1540, 1439, 1389, 1366,
1346, 1310, 1251, 1114, 1069, 950, 772.
c
for analysis by WinEQN MR.. For the 1 : 1 binding mode. Esti-
mated error: 10%
The fluorescence of these sensors was also modulated upon
anion recognition, where the ICT band was either quenched or
shifted to longer wavelengths. From these changes we were
able to determine binding constants which showed that AcOÀ
À
was bound more strongly than H2PO4 and that the binding
of FÀ was most likely due to initial hydrogen bonding to the
anion receptor followed by deprotonation and the formation
of HF2À. The formation of this species in solution was also
confirmed by 1H NMR measurements; which demonstrated
that AcOÀ was recognised through ‘pure’ hydrogen bonding.
À
In contrast, the changes observed upon binding of H2PO4
and FÀ to 1–3 suggest that the initial recognition is more likely
due to initial hydrogen bonding interactions, followed by full
or partial deprotonation. Hence, we conclude that if the anion
is basic enough in the solvent media deprotonation will occur
on the most acidic protons of the sensors. However, since the
absorption spectrum does not give rise to identical spectra at
General synthesis of 1–3
The appropriate isocyanate (1.1 eq.) was added to the amine
and this reaction mixture was stirred with heating at reflux for
3 days in dry CH3CN. A precipitate formed, which was
collected and washed with CH3CN. The solid was then dried,
yielding a yellow solid. Purification of the crude products was
carried out by hot filtration using a mixture of DCM : MeOH
(1 : 4).
the endpoint, it strongly suggests that each of AcOÀ, H2PO4
À
and FÀ binds to 1–3 in a unique manner, which can be
potentially used to aid in the distinction of these anions. We
are currently investigating these and related systems as anion
sensors for real time monitoring and sensing of anions in
competitive media.
2-Ethyl-6-[(phenylcarbamoyl)hydrazino]benzo[de]iso-quinoline-
1,3-dione (1)
Experimental
1 was synthesised according to above procedure using 5 (0.21
g, 0.82 mmol) and phenyl isocyanate 6 (0.11 g, 0.90 mmol) in
dry MeCN (30 mL). The desired product was obtained as a
dark yellow solid (0.12 g, 41%). m.p. 252–254 1C; calculated
for C21H18N4O3Á(H2O): C, 64.3; H, 5.1; N, 14.3%; found: C,
64.0; H, 4.5; N, 14.5%; dH (400 MHz, (CD3)2SO) 9.60 (1H, br
s, urea-NH), 9.03 (1H, br s, urea-NH), 8.73 (1H, d, J = 8.8
Hz, Naph-H7), 8.67 (1H, br s, Naph-NH), 8.50 (1H, d, J =
7.0 Hz, Naph-H5), 8.37 (1H, d, J = 7.9 Hz, Naph-H2), 7.78
(1H, t, J = 7.9 Hz, Naph-H6), 7.51 (2H, d, J = 7.9 Hz, Ar-
H2, Ar-H6), 7.26 (2H, t, J = 7.4 Hz, Ar-H3, Ar-H5), 7.05
(1H, d, J = 8.8 Hz, Naph-H3), 6.97 (1H, t, J = 7.0 Hz, Ar-
H4), 4.07 (2H, q, J = 6.7 Hz, CH2), 1.20 (3H, t, J = 7.0 Hz,
CH3); dC (100 MHz, (CD3)2SO) 163.4, 162.7, 155.8, 152.6,
151.3, 139.7, 139.6, 133.7, 130.7, 128.8, 128.6, 124.8, 122.1,
121.8, 119.0, 118.1, 110.7, 105.1, 34.4, 13.3; IR (KBr) umax
(cmÀ1) 3285, 1641, 1545, 1498, 1444, 1388, 1369, 1349, 1239,
1141, 1066, 914, 876, 839, 773, 742, 691.
Starting materials were obtained from Sigma Aldrich and
Fluka. Solvents used were HPLC grade unless otherwise
1
stated. H NMR spectra were recorded at 400 MHz using a
Bruker Spectrospin DPX-400, with chemical shifts expressed
in parts per million (ppm or d) downfield from the standard.
13C NMR spectra were recorded at 100 MHz using a Bruker
Spectrospin DPX-400 instrument. Infrared spectra were re-
corded on a Mattson Genesis II FTIR spectrophotometer
equipped with a Gateway 2000 4DX2-66 workstation. Mass
spectroscopy was carried out using HPLC grade solvents.
Electrospray Mass spectra were recorded on a Micromass
LCT spectrometer. The system was controlled by MassLynx
3.5 on a Compaq Deskpro workstation. UV-Vis spectroscopic
analysis was carried out on a Varian Cary 100 UV-Vis
spectrophotometer. Luminescence measurements were carried
out on Varian Carey Eclipse spectrophotometer.
6-Bromo-2-ethyl-benzo[de]isoquinoline-1,3-dione (4)
2-Ethyl-6-[(para-tolylcarbamoyl)hydrazino]-benzo[de]iso-
4-Bromo-1,8-naphthalic anhydride (2.00 g, 7.2 mmol) and
ethylamine (70% solution in water) (0.69 mL, 8.66 mmol)
were refluxed in 1,4-dioxane (100 mL) for 7 h. The solution
was then poured into water. A precipitate is formed, which
quinoline-1,3-dione (2)
2 was synthesised according to above procedure using 5 (0.20,
0.78 mmol) and para-tolyl isocyanate 7 (0.11 g, 0.86 mmol) in
ꢀc
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dry MeCN (30 mL). The desired product was obtained as a
dark yellow solid (0.16 g, 51%). m.p. decomposed at 310 1C;
calculated for C22H20N4O3Á1/2H2O: C, 66.5; H, 5.3; N, 14.1%;
found: C, 66.5; H, 5.0; N, 13.5%; dH (400 MHz, (CD3)2SO)
9.59 (1H, br s, urea-NH), 8.92 (1H, br s, urea-NH), 8.73 (1H,
d, J = 7.9 Hz, Naph-H7), 8.61 (1H, br s, Naph-NH), 8.50
(1H, d, J = 7.0 Hz, Naph-H5), 8.37 (1H, d, J = 7.9 Hz, Naph-
H2), 7.78 (1H, t, J = 7.9 Hz, Naph-H6), 7.39 (2H, d, J = 8.8
Hz, Ar-H2, Ar-H6), 7.05 (3H, m, Ar-H3, Ar-H5, Naph-H3),
4.07 (2H, q, J = 7.0 Hz, CH2), 2.23 (3H, s, Ar-CH3), 1.20 (3H,
t, J = 7.0 Hz, CH3); dC (100 MHz, (CD3)2SO) 163.5, 162.8,
151.4, 136.9, 133.8, 130.9, 130.8, 129.2, 129.0, 125.8, 124.8,
121.9, 119.1, 110.6, 105.1, 34.4, 20.3, 13.3; IR (KBr) umax
(cmÀ1) 3295, 1641, 1580, 1543, 1434, 1390, 1370, 1350, 1310,
1243, 1143, 1066, 917, 877, 832, 774, 757.
G. M. Hussey, F. M. Pfeffer, C. M. G. dos Santos and J. Tierney,
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J. Fluoresc., 2005, 15, 585; (n) R. Martı
F. Sancenon, Chem. Rev., 2003, 103, 4419.
nez-Manez and
´ ´
´
3. Examples include: (a) R. Nishiyabu and P. Anzenbacher, Jr,
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E.-K. Sim, C.-K. Lee, W.-S. Cho, J. L. Sessler and C.-H. Lee,
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´ ´
4. Examples include: (a) C. Suksai and T. Tuntulani, Chem. Soc.
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benzo[de]iso-quinoline-1,3-dione (3)
3 was synthesised according to above procedure using 5 (0.20
g, 0.80 mmol) and 4-trifluoromethyl-phenyl isocyanate 8 (0.16
g, 0.88 mmol) in dry MeCN (30 mL). The desired product was
obtained as a dark yellow solid (0.15 g, 45%). m.p. decom-
posed at 250 1C; calculated for C22H17N4F3O3ÁH2O: C, 57.4;
H, 4.2; N, 12.2%; found: C, 57.7; H, 3.8; N, 12.0%; dH (400
MHz, (CD3)2SO) 9.64 (1H, br s, urea-NH), 9.45 (1H, br s,
urea-NH), 8.90 (1H, br s, Naph-NH), 8.73 (1H, d, J = 7.9 Hz,
Naph-H7), 8.51 (1H, d, J = 7.9 Hz, Naph-H5), 8.38 (1H, d, J
= 8.8 Hz, Naph-H2), 7.78 (3H, m, Ar-H3, Ar-H5, Naph-H6),
7.62 (2H, d, J = 8.8 Hz, Ar-H2, Ar-H6), 7.06 (1H, d, J = 7.0
Hz, Naph-H3), 4.07 (2H, q, J = 6.7 Hz, CH2), 1.20 (3H, t, J =
7.0 Hz, CH3); dC (100 MHz, (CD3)2SO) 163.4, 162.7, 155.5,
151.1, 149.7, 143.4, 133.7, 130.8, 128.9, 125.8, 124.9, 123.2,
121.9, 120.7, 118.6, 113.0, 110.9, 105.1, 34.4, 13.3; dF (376
MHz, (CD3)2SO) À58.59 (Ar-CF3); IR (KBr) umax (cmÀ1
)
3317, 1641, 1548, 1434, 1389, 1371, 1327, 1234, 1162, 1111,
1066, 1016, 916, 875, 841, 773, 756, 705.
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´
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Acknowledgements
The authors would like to thank Trinity College Dublin for
financial support (Postgraduate Award to Dr Ali) and IRC-
SET BASIC research grant awarded to PEK and TG for this
project.
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