Dual responsive chemosensors for anions: The combination of fluorescent PET (Photoinduced Electron Transfer) and colorimetric chemosensors in a single molecule

Article abstract of DOI:10.1016/S0040-4039(03)01699-X

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of 'fluorophore-spacer-(anion)receptor'. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F^- and AcO^- in DMSO, the fluorescence emission of 1 and 2 was 'switched off', with no significant changes in the UV-vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F^-, further additions of the anion, gave rise to large changes in the UV-vis spectra, where the λ_max at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.

Full text of DOI:10.1016/S0040-4039(03)01699-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6575–6578
Dual responsive chemosensors for anions: the combination of
fluorescent PET (Photoinduced Electron Transfer) and
colorimetric chemosensors in a single molecule
Thorfinnur Gunnlaugsson,a,* Paul E. Kruger,a T. Clive Lee,b Raman Parkesh,a Frederick M. Pfeffera
and Gillian M. Husseya
aDepartment of Chemistry, Trinity College Dublin, Dublin 2, Ireland
bDepartment of Anatomy, Royal College of Surgeons in Ireland, St. Stephen’s Green, Dublin 2, Ireland
Received 15 May 2003; revised 8 July 2003; accepted 8 July 2003
Abstract—The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of
‘fluorophore-spacer-(anion)receptor’. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the
fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition
of anions such as F− and AcO− in DMSO, the fluorescence emission of 1 and 2 was ‘switched off’, with no significant changes
in the UV–vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F−, further
additions of the anion, gave rise to large changes in the UV–vis spectra, where the umax at 455 nm was shifted to 550 nm. These
changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact
found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where
at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.
© 2003 Elsevier Ltd. All rights reserved.
There is considerable interest within the field of supra-
molecular chemistry in the design of chemosensors for
cation and anion recognition.1 In this regard several
excellent examples of fluorescent and colorimetric sen-
sors for cations have been developed.1,2 However, the
area of anion recognition and sensing is much less
explored.3,4 We are particularly interested in anion sens-
ing and have designed several Tb(III) cyclen complexes
as chemosensors for aromatic carboxylates such as
salicylic acid.5 We recently also developed the first
examples of fluorescent anion PET chemosensors using
simple charge neutral receptors.6 Here the recognition
event occurred through hydrogen bonding between
urea or thiourea receptors and anions with concomitant
quenching of fluorescence via a PET mechanism.7
Unfortunately, a drawback to this design was the short
emission wavelength of the fluorophore (anthracene),
which suffers from the affects of background auto-
fluorescence and light scattering.8 Accordingly, we set
out to design new PET chemosensors that emit at
longer wavelengths and chose 4-amino-1,8-naphthal-
540–550 nm) with high quantum yields (FF).9
Moreover, we aimed to improve our receptor design by
incorporating a second aromatic unit within the
thiourea scaffold with the aim of further enhancing the
sensitivity of anion recognition. Compounds 1 and 2
best exemplify our current design strategy. Addition-
ally, due to the presence of a large dipole moment in
the naphthalimide, which originates from an internal
charge transfer excited state (ICT),10 the ground state,
and therefore the color of the compounds, may be
modulated via hydrogen bonding or deprotonation of
the 4-amino moiety by small anions possessing high
charge density such as F− and HO−. The dual action of
1 and 2 would thus yield a combined fluorescence and
colorimetric based sensor in a single molecule. To the
best of our knowledge, this would be the first such
example of a combined fluorescent and colorimetric
system.
The two chemosensors are designed on the classic
fluorophore-spacer-receptor principle developed by de
Silva et. al.11 The synthesis of 1 and 2 is shown in
Scheme 1. Both sensors are made from readily available
starting materials. The precursor to both was 4, made
in two steps in 90% yield form 4-bromo-1,8-naphthalic
imide as a fluorophore, as it emits in the green (umax
ꢀ
* Corresponding author. Tel.: +00 353 1 608 3459; fax: +00 353 1 671
2826; e-mail: gunnlaut@tcd.ie
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01699-X

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T. Gunnlaugsson et al. / Tetrahedron Letters 44 (2003) 6575–6578
Figure 1. The changes in the fluorescence emission spectra of
+
1 (6 mM) upon titration with solutions of F−N(C4H9)4 in
DMSO, showing the fluorescence emission being ‘switched
off’ upon F− recognition. All measurements were repeated
several times.
Scheme 1. The synthesis of 1 and 2 from the starting material
3 in two steps: (i) CH3CH2NH2, 1,4-dioxane, reflux; (ii)
4-aminobenzylamine (neat) 130°C.
anhydride 3, by first reacting it with ethylamine (70%)
in 1,4-dioxane; the desired product being precipitated
upon addition to water. This was followed by the
nucleophilic displacement using an excess of 4-
aminobenzylamine (neat) at 130o C, followed by recrys-
tallization from ethanol. Consequently, the two anion
sensors were formed by reacting 4 with phenyl- or
4-(trifluoromethyl)phenyl isothiocyanate at room tem-
perature in dry CHCl3 giving 1 and 2 in 60 and 58%
yields, respectively (Scheme 1). All intermediates and
sensors were characterized using conventional methods
(See Reference Section). Both sensors showed the pres-
Figure 2. Changes in the emission spectra of 1 at 525 nm as
a function of −log[anion]: F− (green triangle); AcO− (purple
circle); H2PO4 (blue diamond).
−
minor extent, quenched by electron transfer from the
receptor to the fluorophore. However, after the addi-
tion of the anion, and the formation of the anion-recep-
tor hydrogen bonding complex, the reduction potential
of the receptor is increased, making the electron trans-
fer more feasible. This subsequently gives rise to
enhanced fluorescence quenching. This is clearly
demonstrated in Figure 1 for the addition of F− to 1 in
DMSO. Here the fluorescence emission of 1 is effec-
tively quenched or completely ‘switched off’ after the
addition of 40 mM of F−. Concurrently, only minor
changes were seen in the absorption spectra of 1 at this
concentration of F− (see later). Addition of MeOH (ca.
10% v/v) to this solution ‘re-switched on’ the emission,
demonstrating that the process was fully reversible, i.e.
the hydrogen bonding interactions were broken.
1
ence of two thiourea protons in their H NMR spectra
in DMSO-d6. For 1 these appeared as broad resonance
at 9.72 ppm, whereas the amino proton appeared as a
broad singlet at 8.43 ppm. Similar results were observed
for 2, with resonances at 10.02 ppm and 8.45 ppm,
respectively.
The ability of 1 and 2 to recognize several anions was
investigated using fluorescence and UV–vis spec-
troscopy in DMSO. Both sensors were highly colored
with umax at ca. 444 nm (log mꢀ4.20), which was
assigned to the ICT excited state. Excitation at umax
gave rise to strong emission between 450 and 700 nm
(green to the naked eye) with FF=0.60 for 1 and 0.71
for 2. Upon addition of several anions such as AcO−,
H2PO4 and F− the emission of 1 and 2 (ꢀ6 mM) was
−
Similar effects were observed in the emission spectra of
substantially reduced in intensity. The mechanism for
this quenching is via PET, which takes place between
the receptor and the fluorophore. Unlike many PET
sensors for cations, the fluorescence of 1 and 2 is
‘switched off’ rather then ‘switched on’ upon ion recog-
nition. We propose that this quenching process is due
to the following; prior to the recognition process, the
excited state of the fluorophore is not, or only to a
1 upon addition of AcO− and H2PO4 , but the quench-
−
ing factors were somewhat smaller. The addition of Cl−
and Br− did not lead to any significant changes in the
emission of 1, possibly due to their large size and low
charge density. Plotting the changes at 525 nm (uFmax
)
as a function of −log [anion] gave in all cases, sigmoidal
curves that ‘switched off’ over two log units (Fig. 2).

T. Gunnlaugsson et al. / Tetrahedron Letters 44 (2003) 6575–6578
6577
Figure 4. The changes in the absorption spectra (uncorrected)
of 1 upon addition of F−. Insert: the changes at 525 nm as a
function of [F−].
Figure 3. The changes 1H NMR (400 MHz, DMSO-d6) in
one of the aromatic protons of 1 upon titration of several
−
anions: F− (blue diamond), AcO− (red diamond); H2PO4
(orange triangle); Cl− (pink square).
These changes were fitted using a non-linear least
squares regression algorithm, giving the binding con-
stants log i=3.8( 0.1), 3.9( 0.1) and 2.9( 0.1) for F−,
AcO− and H2PO4 respectively. Even though the bind-
−
ing of F− and AcO− was quite similar more effective
quenching was seen for the former. We attribute this to
stronger hydrogen bonding between the F− and the
thiourea protons, since F− is smaller and has higher
charge density then AcO−. In the case of 2, the rather
simple modification of the receptor site, the presence of
the aryl-CF3 group, further increased the acidity of the
thiourea hydrogens giving rise to slightly stronger bind-
ing with log i=4.4 ( 0.1) and 3.7 ( 0.1) for F− and
Scheme 2. The one-pot synthesis of 6 from the anhydride 5.
increases the charge density on the amino nitrogen with
concurrent enhancement in the push–pull character of
the ICT.
The concomitant changes in the UV–vis spectra for 1
upon addition of F− can be seen in Figure 4. As stated
previously no major changes were seen with up to 30
mM of F−. However, upon further addition, the
absorption spectra were dramatically affected. In the
first instance, the absorption at 444 nm was reduced
with the formation of a new band centered at 536 nm,
tailing to 620 nm, with an isosbestic point at 475 nm.
Upon further increasing the concentration the band at
ca. 410 nm also increased, which is possibly due to
aggregation. Similar results were observed using tetra-
butyl ammonium hydroxide, indicating that deprotona-
tion had indeed occurred. Upon excitation of the newly
formed 535 nm band, a new weak and broad fluores-
cence emission was observed between 580 and 800 nm,
which was [F−] dependent. Addition of MeOH reversed
these changes. Similar results were also seen for 2,
indicating that 1 and 2 can both detect F− at two
different [F−] ranges with two different sensing modes.
To the best of our knowledge these are the first exam-
ples of such dual fluorescent-colorimetric sensors for
anions, e.g. the naphthalimide emission is quenched by
the anion recognition at the thiourea receptor site,
whereas hydrogen bonding/deprotonation at the 4-
amino moiety gave rise to large color changes resulting
in the formation of dual responsive chemosensors for
anions such as F−.
H2PO4 and 4.0 ( 0.2) for AcO−.† We have recently
−
developed several families of such bis-aromatic ureas
and thioureas as anion receptors, and measured their
binding interactions with several ‘simple’ anions and
1
amino acids using H NMR. These clearly show that
the binding affinity and the selectivity of these kinds of
receptors can be tuned.12
These binding interactions were also monitored using
1H NMR in DMSO-d6, by monitoring the aromatic
protons as the thiourea protons became too broad
upon addition of F−, AcO− and H2PO4 . For AcO− and
−
−
H2PO4 the plot of Dl versus equivalents added indi-
cated 1:1 binding by 1 and 2, whereas for F− 1:2
binding was determined for both sensors. No binding
was seen for Cl− or Br−. These results are shown in
Figure 3. The 1:2 binding was also visible to the naked
eye after the addition of ca. 2–2.5 equiv. of F− as the
color of the solution changed from light yellow to deep
purple. These color changes signify the interaction of
F− with the 4-amino moiety which is either through
strong hydrogen bonding between NꢀH···F, or more
likely by complete deprotonation. This significantly
† In the case of AcO−, the emission was quenched, and log i reported
was determined from those changes. However, at higher concentra-
tions, the emission was enhanced again. We did not see this effect
for any of the other anions. We are currently investigating this in
greater detail.
To investigate this phenomena further, we synthesized
the reference compound 6, by reacting in a one-pot
synthesis the starting material 4-chloro-1,8-naphthalic

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T. Gunnlaugsson et al. / Tetrahedron Letters 44 (2003) 6575–6578
anhydride, 5, in neat n-butylamine at 80°C (Scheme 2).9
The resulting solution was then poured over ice-water
to give 6 as a yellow solid in 72% yield. The addition of
F− to a solution of 6 in DMSO shifted the absorption
spectra of 6 to the red, in a similar manner as shown
for 1 above. These changes mirrored that of 1 at high
F− concentrations, but no PET quenching was observed
since the molecule lacks the thiourea receptor. We are
currently investigating molecules such as 6 to act as
anion (pH) sensors in organic solutions.12
Perreault, D. M.; Monahan, M. K.; Anslyn, E. V. J. Chem.
Soc., Perkin Trans. 2 2001, 315; (g) Cooper, C. R.; Spencer,
N.; James, T. D. Chem. Commun. 1998, 1365.
5. Gunnlaugsson, T.; Harte, A. J.; Leonard, J. P.; Nieuwen-
huyzen, M. Chem. Commun. 2002, 2134.
6. Gunnlaugsson, T.; Davis, A. P.; Glynn, M. Chem. Commun.
2001, 2556.
7. Gunnlaugsson, T.; Davis, A. P.; O’Brien, J. E.; Glynn, M.
Org. Lett. 2002, 4, 2449.
8. (a) Gunnlaugsson, T.; MacDo´naill, D. A.; Parker, D. J. Am.
Chem. Soc. 2001, 123, 12866; (b) Gunnlaugsson, T. Tetra-
hedronLett. 2001, 42, 8901;(c)Gunnlaugsson, T.;MacDo´n-
aill, D. A.; Parker, D. Chem. Commun. 2000, 93; (d)
Gunnlaugsson, T.; Parker, D. Chem. Commun 1998, 511.
9. (a) de Silva, A. P.; Rice, T. E. Chem. Commun. 1999, 163;
(b) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson,
T. Tetrahedron Lett. 1998, 39, 5077.
In summary, we have developed new, combined PET
and colorimetric chemosensors for anions, which give
rise to large changes in the fluorescence at long wave-
lengths, and naked eye detectable color changes from
green to purple for 1 and 2 within two concentration
ranges. We are currently modifying the design of 1 and
2 with the aim of enhancing the use of such fluorescent–
colorimetric dual anion sensors.
10. de Silva, A. P.; Gunaratne, H. Q. N.; Habib-Jiwan, J.-L.;
McCoy, C. P.; Rice, T. E.; Soumillion, J.-P. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1728.
11. deSilva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice,
T. E. Chem. Rev. 1997, 97, 1515.
Acknowledgements
12. Manuscript in preparation.
We thank the National Pharmaceutical Biotechnology
Centre, BioResearch Ireland, HRB, Enterprise Ireland,
IRCSET, Kinerton Ltd, TCD and Wexford County
Council for financial support. We also thank Drs.
Hazel M. Moncrieff, Anthea C. Lees, Julie Tierney and
John E. O’Brien for their help. We thank Kinerton
Ltd., Enterprise Ireland (Basic Research Grant), and
TCD for financial support.
Synthesis:
4 was obtained as light fluffy yellow needles (1.53 g, 90%).
1H NMR (400 MHz, DMSO-d6): 1.17 (t, 3H, J=7.0 Hz),
4.04 (q, 2H, J=7.0 Hz), 4.54 (d, 2H, J=5.2 Hz) 4.95 (bs,
-NH2), 6.54 (d, 2H, J=8.0 Hz), 6.70 (d, 1H, J=8.54 Hz),
7.07 (d, 2H, J=8.0 Hz), 7.68 (t, 1H, J=7.5 Hz), 8.18 (d,
1H, J=8.5), 8.42 (bs, -NH), 8.43 (d, 1H, J=8.5 Hz), 8.74
(d, 1H, J=8.5 Hz); 13C NMR (400 MHz, DMSO-d6); 13
C
NMR (400 MHz, DMSO-d6): 163.53, 162.67, 150.54,
147.70, 133.92, 130.54, 129.37, 128.52, 127.93, 124.93,
124.27, 121.95, 120.26, 113.93, 107.81, 104.51, 45.86, 34.23,
13.29; m/z (ESMS) 346 (M+H+). Anal. calcd for
C21H19N3O2: C, 73.03; H, 5.54; N, 12.17. Found: C, 72.76;
H, 5.63; N, 12.06.
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1
1 was obtained as a light yellow solid (0.191 g, 58%). H
NMR (400 MHz, DMSO-d6): 1.17 (t, 3H, J=7.0 Hz), 4.05
(q, 2H, J=7.0 Hz), 4.64 (d, 2H, J=5.2 Hz), 6.71 (d, 2H,
J=8.5 Hz), 7.11 (t, 1H, J=7.5), 7.29–7.38 (m, 4H),
7.44–7.48 (m, 4H), 7.72 (t, 1H, J=7.5), 8.20 (d, 1H, J=8.5),
8.43 (bs, -NH), 8.47 (d, 1H, J=8.5 Hz), 8.78 (d, 1H, J=8.5
Hz), 9.72 (bs, -NH); 13C NMR (100 MHz, DMSO-d6):
179.62, 163.53, 162.70, 150.38, 143.41, 138.08, 134.90,
133.92, 130.67, 129.38, 128.49, 127.14, 125.56, 125.52,
124.51, 123.93, 122.75, 122.06, 120.33, 113.02, 108.28,
104.58, 45.57, 34.25; m/z 481 (M+H+). Anal. calcd for
C28H24N4O2S.CH2Cl2.CHCl3: C, 52.61; H, 3.97; N, 8.18.
Found: C, 52.59; H, 4.00; N, 8.39.
1
2 was obtained as a light yellow solid (0.245 g, 60%). H
NMR (400 MHz, DMSO-d6): 1.18 (t, 3H, J=7.0 Hz), 4.05
(q, 2H, J=7.0 Hz), 4.66 (d, 2H, J=5.2 Hz), 6.72 (d, 1H,
J=8.5 Hz), 7.38–7.47 (m, 4H), 7.65–7.76 (m, 4H), 8.29 (d,
1H, J=8.5), 8.45 (bs, NH), 8.47 (d, 2H, J=7.5 Hz), 8.74
(d, 1H, J=8.5 Hz), 10.02 (bd, -NH). 13C NMR (100 MHz,
DMSO-d6): 179.62, 163.53, 162.70, 150.38, 143.41, 138.08,
134.90, 133.92, 130.67, 129.38, 128.49, 127.14, 125.56,
125.52, 124.51, 123.93, 122.75, 122.06, 120.33, 113.02,
108.28, 104.58, 45.57, 34.25, 13.30; m/z (ESMS) 549
(M+H+). Anal. calcd for C29H23F3N4O2S CHCl3: C,
53.94; H, 3.62; N, 8.39. Found: C, 53.89; H, 3.83; N, 8.40.
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