Fluoride-selective optical sensor based on the dipyrrolyl- tetrathiafulvalene chromophore

Article abstract of DOI:10.1039/c1ob06459b

A chemosensor bearing dipyrrolyl motifs as recognition sites and a tetrathiafulvalene redox tag has been evaluated as an optical and redox sensor for a series of anions (F^-, Cl^-, Br^-, HSO ₄^-, CH₃COO^-, and H ₂PO₄^-) in DCM solution. The receptor shows specific optical signaling for fluoride but little electrochemical effect in solution. The solid-state performance of the sensor leads to measurable changes in water. Design implications towards better systems based on these results and other examples are discussed.

Full text of DOI:10.1039/c1ob06459b

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Organic &
Biomolecular
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Fluoride-selective optical sensor based on the dipyrrolyl-tetrathiafulvalene
chromophore†
Shadi Rivadehi,a Ellen F. Reid,b Conor F. Hogan,b Sheshanath V. Bhosalea and Steven J. Langford*a
Received 25th August 2011, Accepted 27th October 2011
DOI: 10.1039/c1ob06459b
A chemosensor bearing dipyrrolyl motifs as recognition sites
and a tetrathiafulvalene redox tag has been evaluated as
an optical and redox sensor for a series of anions (F-, Cl-,
Br-, HSO4 , CH3COO-, and H2PO4 ) in DCM solution.
The receptor shows specific optical signaling for fluoride
but little electrochemical effect in solution. The solid-state
performance of the sensor leads to measurable changes in
water. Design implications towards better systems based on
these results and other examples are discussed.
several dual-mode F- probes having being proposed so far.16
Indeed, the colorimetric sensor based on the deprotonation of
the binding moiety by F- is very efficient and highly sensitive.
Unfortunately, only a few colorimetric anion sensors are able
to differentiate selectively between anionic substrates of similar
basicity and surface charge density.17 An additional problem is
the compatibility of most of the anion sensors with water and
electrolytes. Despite considerable effort, the need for good anion
sensors remains.
-
-
Tetrathiafulvalene (TTF) and its derivatives have recently
appeared as key constituents for new applications, exploiting the
remarkably well behaved redox properties and high p-donating
ability.18 There are however only a few reports of TTF-based
Luminescent and colorimetric sensors for various chemically,
biologically, medically, and environmentally significant anions
is currently an area of great interest.1 The ubiquity of anions
and their importance in biology, as agricultural fertilizers and
industrial raw materials necessitates the development of highly
sensitive anion sensors.2 Nature provides us with many great
examples of anion recognition motifs.3 Chemistry has developed
a number of alternative synthetic chemosensors, generally in-
volving the covalent linking of an optical-signaling fragment to
an anion receptor.4–8 A widely used approach to the synthesis
of colorimetric anion sensors utilizes chromophores such as
indoles,9 carbazole,10 quinone,11 and naphthalene diimide,12 and
other electron-withdrawing moieties covalently or non-covalently
attached to an anion receptor.13 Two approaches to anion bind-
ing are plausible. The first relates to a reversible non-covalent
interaction between the sensor and anion, and this method has
led to a significant amount of literature. The second is to induce
an irreversible, or pseudo reversible chemical event that leads to
the formation of a permanent or semi-permanent change. Both
methods are pragmatic methods.
anion sensors, primarily towards H2PO4 and Cl- anions utilizing
-
anthracene disulfonamide-TTF, calix[4]arene-TTF platform and
amide TTF derivatives, respectively.19 Recently, a diindolylquinox-
aline receptor bearing a TTF redox probe was prepared and
found to be selective for dihydrogen phosphate over other small
anions in dichloromethane using electrochemical and fluorescence
quenching methods.20 Here, we report on a very close analogue 1
(Scheme 1) bearing pyrrole groups and report on how this minor
change affects both the selectivity as well as the nature of the
optical readout in both absorption and emission.
Fluoride recognition is an area of immense research interest
to the scientific community due to its role in chemical, industrial,
food, hygiene and toxicity.14 Fluoride is one of the most challenging
targets for anion recognition because of its high hydration
enthalpy. For practical application, probes that can detect F- by
both colorimetric and fluorometric15 analyses are favored, with
aSchool of Chemistry, Monash University, Clayton, Vic-3800, Australia.
E-mail: sheshanath.bhosale@monash.edu, steven.langford@monash.edu;
Fax: +61(03)99054597; Tel: +61399055980
Scheme
1
(a) General synthetic route for the synthesis of 2,3-
dipyrrol-2¢-yl-TTF-quinoxaline 1, and (b) change in colour observed
for 1 only upon the addition of 3 equiv of fluoride. All anions were
introduced as their tetrabutylammonium salts respectively. [1] = 1.0 ¥
10-5 M.
bDepartment of Chemistry, La Trobe Institute for Molecular Sciences, La
Trobe University, VIC, 3086, Australia
† Electronic supplementary information (ESI) available: Synthetic details
of compound 1 and characterisation. See DOI: 10.1039/c1ob06459b
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The target compound 2,3-dipyrrol-2¢-yl-TTF-quinoxaline 1 was
obtained via the direct condensation reaction of diamino-TTF
221 with the 2,3-dipyrrol-2¢-yl ethanedione 317c in the presence of
acetic acid under an argon atmosphere at reflux, leading to the
formation of the yellow crystalline solid in excellent yield (Scheme
1). Among the six anions tested in solution (DCM), namely, F-,
H2PO4 , AcO-, Cl-, Br-, and HSO4 , 1 responded positively to
only F-, resulting in a marked color change (Scheme 1b). This
qualitative analysis was further explored using UV–vis absorption
and fluorescence spectroscopy.
-
-
The UV–visible spectrum of 1 in CH2Cl2 (Fig. 1) shows three
strong absorption bands at 288, 311 and 484 nm with a shoulder
at 374 nm. The addition of TBAF to 1 causes significant changes
in the absorption spectra with bands at 288, 311 and 484 nm
decreasing in intensity while three new bands at 290, 323 and 503
nm appeared with a shoulder at 384 nm. Three clear isosbestic
points at 326, 418, and 492 nm indicated a clean conversion
throughout the titration process. The resultant Job plot indicated
a 1 : 1 interaction stoichiometry. No significant changes in the
absorption spectra of 1 were observed upon addition of AcO-,
H2PO4 , Cl-, Br-, and HSO4 (at 0–50 equiv) in the same solvent
(see ESI†) indicating very weak binding at best. Association
constants were obtained by fitting the changes in the emission
to a 1 : 1 binding stoichiometry.22 The association constants for
1 : 1 complexes between 1 and F- were calculated to be 6 ¥
104 M-1.‡
-
-
Fig. 2 (a) Fluorescence emission spectral changes of complex 1 upon the
addition of TBAF; [1] = 1.0 ¥ 10-5 M, [TBAF] = 0–5 equivalents in DCM
at 25 ◦C (lex = 446 nm). (b) The changes observed at emission wavelength
at 663 nm of 1 upon titration with TBAF.
plexation with the fluoride anion. The TTF unit is a strong p-
electron donor and has two sequential, and reversible oxidation
∑
couples corresponding to the radical cation TTF+ and dication
TTF2+.23 Table 1 in the ESI† displays the changes in electrochem-
ical behaviour with the progressive addition of F- ions to a 1 mM
solution of the dipyrrole in DCM at 298 K. The results are shown
graphically in Fig. 3.
Fig. 1 UV–vis spectral changes of complex 1 upon the addition of fluoride
anion; [1] = 1.0 ¥ 10-5 M, [TBAF] = 0–5 ¥ 10-4 M, DCM, 25 ◦C. The inset
shows the fit of the experimental data to a 1 : 1 binding profile.
Fig. 3 Cyclic voltammogram of 1mM TTF-dipyrrole obtained by adding
increasing quantities of 0.5 M TBAF solution (0 to 1.0 equiv). (Reference
electrode: Fc+/Fc, supporting electrolyte, n-Bu4NPF6 (0.1 M) in DCM.)
The emission spectrum of 1 in DCM at room temperature shows
a strong band at 662 nm with a shoulder 703 nm (lex = 446 nm) with
a calculated quantum yield Uem = 0.22 against Rhodamine (Fig.
2). Importantly, an emission intensity enhancement was observed
with slight red shift (~4 nm) for 1 only upon addition of F- (0–5
equivalents). Under the same conditions, no significant spectra
changes were observed in the presence of large excesses (10–100
equiv) of other anions (AcO-, H2PO4 , Cl-, Br-, and HSO4 , as
their tetrabutylammonium anion salts). The results suggest that
sensor 1 has a high selectivity for the fluoride anion over the anions
under common forms of spectroscopy.
Overall, changes were consistent for both redox couples. Al-
though a slight positive shift of the first and second oxidation
ox
potentials was observed, the characteristic potential E1/2 (= Ep
+
Epred/2) did not change significantly. An increase in current is
also evident, however this proved not to be associated with F-
concentration. Control experiments showed the same effect with
successive scans in the absence of the fluoride anion.
-
-
This increase in peak current, in combination with the somewhat
Gaussian nature of the first peak couple, suggested surface effects.
Continuous cycling experiments confirmed layer formation at the
Cyclic voltammetry was used to probe the changes in redox
properties of the TTF containing dipyrrole receptor upon com-
706 | Org. Biomol. Chem., 2012, 10, 705–709
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Table 1 EC data—addition of NaF to aqueous solution contacting glassy carbon electrode containing electrodeposited TFF-dipyrrole layer
PEAK COUPLE I
Equiv. F- added
ip ox (uA)
ip red (uA)
E1/2/mV
Ep ox (mV)
Ep red (mV)
DEp (mV)
0
0.2
0.4
0.6
0.8
1.0
26.7
28.5
27.5
26.7
27.7
27.0
NO TREND
-18.5
581
591
609
634
637
638
664
686
705
722
725
735
498
496
513
546
549
541
166
190
192
176
176
194
-15.2
-12.6
-10.3
-9.2
-8.3
Overall change
DECREASE
Positive SHIFT
Positive SHIFT
surface of the working electrode by electrochemical deposition
(see ESI†). When the switching potential was altered so that the
scan was reversed after the first oxidation, deposition ceased and
the voltametric peak no longer grew in magnitude. This suggests
that layer deposition is associated only with the doubly oxidised
TTF species. As scanning to the potential of the second oxidation
process resulted in electrochemical deposition, further solution
phase electrochemical experiments were conducted without scan-
ning past the first oxidation peak couple. In organic media, the
first oxidation peak, corresponding to the radical cation TTF∑+,
was stable over >50 successive scans with no change in Ep or ip,
and no evidence of deposition or other surface effects. The effect
of adding increasing quantities of TBAF solution (0 to 1.0 equiv)
leads to only minor changes to the voltammogram, certainly not
significant enough to ascribe to anion binding.
Modified electrodes are interesting for possible technological
applications, therefore the coated electrode was tested in blank
aqueous 0.1 M LiClO4 solution. The electrodeposited layer main-
tained its electrochemical properties when scanned between 0 and
0.8 V. Scanning to the second oxidation potential however, led to
desorption of the material (see ESI†). Table 1 displays the changes
in electrochemical behaviour with the progressive additions of
F- ions (in the form of NaF), to the blank aqueous electrolyte
contacting the coated electrode at 298 K. The corresponding
voltammograms can be seen in Fig. 4. The most significant trends
were a positive shift (73 mV) in E1/2 and a decrease in the magnitude
of ipred as the concentration of fluoride was increased. The decrease
in peak height of the reductive peak occurred concomitantly with
the appearance of a new broad reduction wave at more negative
potentials indicating the potential for 1 to be used as a solid-state
sensor.
To elucidate the nature of the intermolecular interactions
between 1 and fluoride anion, 1H NMR titration experiments were
also carried out in CD2Cl2, primarily by monitoring the changes
in signal of pyrrole NH, C–H pyrrole (Fig. 5). Upon the addition
1
of 3 equiv. fluoride anion, dramatic changes occurred in the H
NMR spectrum of 1. In particular the multiplicity and splitting
of the pyrrole C–H signals and disappearance of N–H signals
consistent with the 1 : 1 stoichiometry of the complex by UV–vis
and fluorescence. No change in signal intensity or chemical shift
was observed for the TTF protons throughout the addition of
anions whereas the proton signal of the pyrrole moiety became
broad and shifted upfield. The reason for the upfield shifts is the
enhanced resonance of TTF electrons from the anionic character
of pyrrole nitrogen upon bifurcation and twisting.24 Based on these
results we suggest a deprotonation mechanism consistent with
other sulfonamide and dipyrrole receptors.17,21 As a consequence,
there is an increase in the rigidity of 1 causing the enhancement of
fluorescence upon addition of fluoride anion.
Fig. 5 Changes in the 1H NMR (400 MHz) spectra of 1 in CD2Cl2 upon
addition of 3 equiv of F-. * = TBA peaks.
Crystals of 1 suitable for X-ray analysis were grown by vapour
diffusion of MeOH into a CHCl3 solution of 1 (Fig. 6).§ The
dark colored crystals have a rod-shaped morphology, with 1
crystallising in the Pbca space group.25 As expected, the crystal
Fig. 4 Glassy Carbon WE drop coated with TTF-dipyrrole, in 0.1 M
LiClO4, 0–0.8 V, 0.2 V s-1 with varying additions of NaF (0–1.0 equiv).
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molecular axes gives rise to a distance of 3.48 A between each plane.
˚
is stabilised by p–p interactions between TTF units as evidenced
˚
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there is a self-organisation of the TTF direction on account of
˚
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of one molecule with the quinoxaline group of another. The
dispersive interactions of the propyl groups on one side of the
molecular stack with the same set on an adjacent column lead
ultimately to a herringbone structure.
In summary, we report a novel dipyrrolic based fluorescent
neutral sensor for fluoride displaying optical changes for both
UV/vis-absorption and fluorescence. Although the addition of
fluorine causes an intense change in the electronic absorption
spectrum of the compound, the electrochemical response remains
largely unaffected. This suggests that while the molecular orbital
responsible for the colour is close to the anion binding site,
the electrochemical orbitals associated with the TTF moiety are
sufficiently remote not to be influenced by changes in electron
distribution caused by binding. The material may be easily
electrodeposited from solution by scanning the potential to
the second oxidation process. This produces a stable modified
electrode which is electrochemically active in contact with aqueous
electrolyte solution. While a polymerization process involving the
pyrrole groups can also yield an explanation, we are reluctant
to fully discount a mechanism involving TTF2+ alone because of
the surface effects where no pyrrole groups were present. Further
work is clearly required to elucidate the mechanism involved in
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