Fluoride ion sensing by an anion-π interaction

Article abstract of DOI:10.1021/ja107382x

We report the discovery of a supramolecular interaction (anion-π and charge/electron transfer, CT/ET) involving fluoride ion and π-electron deficient colorless naphthalene diimide (NDI) receptors. Strong electronic interactions between lone-pair electrons

Full text of DOI:10.1021/ja107382x

Published on Web 11/29/2010
Fluoride Ion Sensing by an Anion-π Interaction
Samit Guha and Sourav Saha*
Department of Chemistry and Biochemistry, Florida State UniVersity, 95 Chieftan Way,
Tallahassee, Florida 32306, United States
Received August 16, 2010; E-mail: saha@chem.fsu.edu
Abstract: We report the discovery of a supramolecular interaction
(anion-π and charge/electron transfer, CT/ET) involving fluoride
ion and π-electron deficient colorless naphthalene diimide (NDI)
receptors. Strong electronic interactions between lone-pair elec-
trons of F- ion and π*-orbitals of the NDI unit lead to an
unprecedented F-fNDI ET event, which produces an orange
colored NDI•- radical anion. Further reduction of NDI•- by another
F- ion produces a pink colored NDI2- dianion, rendering NDI a
colorimetric F- sensor. Preorganization of two NDI units in
overlapping positions using folded linkers improves their selectivity
and sensitivity for the F- ion significantly, allowing F- detection
at nM concentration in 85:15 DMSO/H2O solutions.
Figure 1. Graphical illustrations of (a) anion-π and CT interactions
between F- and NDI receptor, generating fluorochromogenic response via
F-fNDI ET event, (b) stepwise F- recognition by preorganized receptors
(PR) through (i) π-anion-π and (ii) H-bonding interactions.
The recent discovery of anion-π interaction1 s a noncovalent
interaction between an anion and an electron deficient organic
π-system with a strong positive quadrupole moment s has added
new dimensions to recognition, sensing,2 and transmembrane
passage3 of anions. Myriads of anions play critical roles in chemical
and biological processes, demanding continuous research in the field
of anion recognition.4 Dunbar et al.2b recently reported chromogenic
Iverson and others demonstrated that CT and π-π-stacking
interactions between a colorless NDI unit and electron rich aromatic
rings produce colored donor-acceptor CT complexes.5 We envi-
sioned that NDI units could also bind with appropriate anions
through anion-π interactions and report the binding event by
generating optical signals on account of anionfNDI CT interactions
(Figure 1). To test our hypothesis, we surveyed interactions of NDI
receptors with F-, Cl-, Br-, I-, NO2-, NO3-, N3-, PF6-, AcO-,
anion-π and charge transfer interactions involving halides (Cl-
>
-
and H2PO4 anions as tetra-n-butylammonium (TBA) salts.
Br- > I-) and electron deficient aromatic rings in organic solvents.
Herein, we present an unprecedented example of chromogenic
anion-π and CT interactions involving the F- ion and π-electron
deficient colorless NDI receptors.3,5 A plethora of experimental
evidence, supported by computational models, indicate that (Figure
1) NDI/F- interactions facilitate an unprecedented F-fNDI electron
transfer (ET) event, which generates an orange NDI•- radical anion.
Excess amounts of F- chemically reduces NDI•- further to a pink
NDI2- dianion. Thus, NDI/F- interactions and ET events produce
a two-step optical response, offering a new strategy for toxic F-
ion sensing.
A fluoride deficiency causes osteoporosis and poor dental health.6
Overexposure to F- is blamed for fluorosis and osteosarcoma.7 The
EPA-recommended F- level in drinking water is 1 ppm, and over
2 ppm is considered a health-risk.8 The low level of F- tolerance
demands for a selective and sensitive F- sensor. Gabba¨ı and others
developed borane, lanthanide, and other Lewis acid based efficient,
albeit disposable, F- sensors.9 Conventional hydrogen bonded F-
receptors have also been reported.10 Because of the nonchromogenic
nature of most Y-H· · ·X- interactions, H-bonded receptors10 rely
on either adjacent chromophore units or deprotonation followed
by electron delocalization to display a colorimetric response.
Therefore, they cannot differentiate F- from other strongly basic
Figure 2. Molecular structures of receptors 1, SR, LR, and CR.
We further postulated that preorganization of two overlapping
NDI units by connecting them with folded linkers11 should improve
the anion binding affinity, selectivity, and sensitivity. To investigate
the effect of preorganization we designed and synthesized (Sup-
porting Information (SI), Scheme S1) NDI receptor 1, a short
receptor (SR) containing a bisamide linker connecting two NDI
units, a long receptor (LR) containing a tetraamide linker between
two NDI units, and a control tetraamide receptor (CR) carrying no
NDI unit (Figure 2). Bifurcated intramolecular H-bonds involving
10e
anions, such as AcO- and H2PO4
.
In contrast, the selectivity
-
and reusability of NDI-based F- sensors that exploit reversible
anion-π and CT/ET interactions distinguish them from the existing
F- sensors.
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C O M M U N I C A T I O N S
the pivotal pyridine N atom and adjacent amide protons should
render the bis- and tetraamide linkers folded conformations that
bring two ends of receptor molecules into close proximity.11
Hartree-Fock global energy minimization shows that while the
short linker in SR brings two NDI units into a parallel overlapping
orientation, the longer linker in LR projects two NDI units at an
angle (SI, Figure S1). In addition to properly orienting NDI units,
amide linkers of SR and LR provide additional anion binding sites
in their cavities via H-bonding interaction.
and LR display characteristic NDI absorption peaks at 343, 361,
and 381 nm. Titration of 1 with 0-5 equiv of F- gradually bleached
NDI absorption peaks and concurrently produced new peaks at 475,
605, 711, and 791 nm, establishing a clear isosbestic point at 394
nm (Figure 3c), as the solution turned orange. The absorption
spectrum of orange species generated by F- (Figure 3c) matches
exactly with that of an electrochemically generated NDI•- radical
anion (-450 mV vs Ag/AgCl in DMF) produced in the absence of
F- (Figure 3d: orange trace). Manifestations of identical spectro-
scopic changes with the same isosbestic point (394 nm) during F-
titration (Figure 3c) and during spectroelectrochemical (SEC)
analysis of 1 in the absence of F- (SI, Figure S3a) strongly suggest
that the F-fNDI ET event takes place in the NDI/F- complex. A
nucleophilic attack of F- on NDI forming a covalent C-F bond
should have produced spectroscopic transitions different from SEC.
The EPR spectrum (SI, Figure S4) of the F--induced orange solution
of 1 further confirms the formation of a delocalized NDI•- radical
anion (g ) 2.0030). These results indicate that at first F- binds
with NDI through anion-π and CT interactions that facilitate
F-fNDI electron transfer and generate NDI•- (Figure 1a).
As the solution of 1 turned from orange to pink during the
titration with 5-30 equiv of the F- ion, NDI•- absorption peaks
gradually disappeared concomitantly with the emergence of a broad
absorption band at 542 nm. This transition at higher F- equivalents
can be attributed to one of the following possibilities: (a) NDI•- is
further reduced to NDI2- dianion by another F- (E1/2 ) -2.87 V)
or (b) NDI•- is attacked by the F- ion forming a C-F bond, which
would be an extremely high-energy process because of electrostatic
repulsions. Strong similarities between absorption spectra of the
pink solution of 1 produced by excess F- (Figure 3c) and
electrochemically generated NDI2- at -900 mV vs Ag/AgCl, DMF
in the absence of F- (Figure 3d, SEC, pink trace) strongly support
the first scenario. A higher relative intensity of the 542 nm band
of the F--induced quickly generated pink solution than that of
slowly (diffusion controlled) electrochemically reduced NDI2- may
be attributed to degradation of reduced NDI species under the UV/
Vis light during prolonged SEC experiments. Consistent with the
formation of NDI2- by excess F-, the pink solution of 1 became
EPR silent (SI, Figure S4).
The presence of the [1·F-] complex at e1 equiv of F- suggests
that NDI•- may exist in the electron delocalized NDI/F- T NDI•-/
F• anion-π and CT complex. In contrast, ESI-MS in the presence
of excess F- reveals only the [1]2- dianion at m/z 210.40 but does
not show any signal representing [1/Fn]n- (n g 1) complexes (SI,
Figure S2c). Therefore, only the first F-fNDI ET event that
produces NDI•- is facilitated by NDI/F- anion-π binding, whereas
the formation of NDI2- with an additional F- ion takes place
through the direct chemical reduction of NDI•- and not via a
physical binding of F- with NDI•- due to electrostatic repulsions
(Figure 1a). ESI-MS also confirms that once the NDI2- dianion is
formed it does not associate with F- anymore.
Figure 3. Colorimetric changes of 1 (a) from colorless (no F-) to orange
(0-5 equiv of F-) to pink (5-30 equiv of F-) and (b) by other anions (30
equiv) in DMSO. (c) UV/Vis titration of 1 (10 µM/DMSO) with F- and
(d) spectroscopic changes of 1 (0.5 mM in 0.1 M TBAPF6/DMF) upon
direct electrochemical reduction in the absence of F-. Black trace: neutral
NDI 1 (Eap ) 0 mV); orange trace: NDI•- (Eap ) -450 mV); and pink
trace: NDI2- (Eap ) -900 mV). Inset: cyclic voltammogram of 1 (vs Ag/
AgCl in 0.1 M TBAPF6/DMF) in the absence of F-. (e) Optical response
of SR to F- (orange and pink) in the presence of excess Cl- but no change
only with Cl- (green vs black). (f) Fluorescence amplifications of SR (1
nM/DMSO); inset: 1 (10 µM/DMSO) by F- ion (0-30 equiv F-, λexcitation
) 381 nm).
The first indication of selective F- ion sensing by 1, SR, and
LR came from visible color changes (Figure 3). While Cl-, Br-,
I-, NO2-, NO3-, N3-, AcO-, and H2PO4- even at 30 equiv did not
affect the colorless solutions of NDI-based receptors, titrations with
F- in aqueous DMSO, DMF, DMAc, MeCN, Me2CO, and THF,
containing up to 15% of H2O, immediately changed the color in
two steps (Figure 3a,b). At first, the colorless NDI solutions turned
orange at lower F- equivalents (e5 equiv) and then turned pink at
higher F- equivalents (>5 equiv). NDI-free CR did not change color
in response to any anion.
Oxidation of orange (1•-) and pink (12-) solutions with NOBF4
decolorized them. Because of strong absorptions of NOBF4 in
350-400 nm regions, regeneration of 1 could not be confirmed by
UV/Vis spectroscopy. However, 1H NMR spectroscopy confirmed
complete recovery of 1 after NOBF4 oxidation (Vide infra). The
reversibility of NDI/F- interactions further proves that these are
noncovalent interactions.
Preorganized NDI receptor SR displayed (SI, Figure S5a) similar
two-step spectroscopic changes with F-. In addition to binding a
F- ion between two terminal NDI units, forming an NDI/F-/NDI
sandwich complex, SR and LR can potentially bind a second F-
ion in the amide cavities via H-bonding interaction (Figure 1b).
ESI-MS confirmed (SI, Figure S2) the formation of [1·F-],
[1·F- ·1], [SR·F-], and [LR·F-] complexes by revealing the
corresponding peaks at m/z 439.06, 859.15, 1018.28, and 1256.27,
respectively, as well as signals associated with 1, 1·1 dimer, SR,
and LR at m/z 420.23, 840.23, 999.25, and 1237.29, respectively,
at F- ion concentrations up to 1 equiv. At 2 equiv of F- [SR·2F-]
and [LR·2F-] complexes were found at m/z 518.40 and 637.70,
respectively.
UV/Vis titration experiments were conducted to quantify F--
induced colorimetric transitions of NDI receptors. Receptors 1, SR,
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C O M M U N I C A T I O N S
This interaction, however, did not produce an additional optical
signal. NDI-free CR showed (SI, Figure S5b) a modest change in
the UV region owing to NH· · · F- interaction and possible depro-
tonation of amide protons.
Interestingly, NDI receptors did not show any significant
spectroscopic change with other anions (SI, Figure S5c), although
the [1·Cl-] complex was found in ESI-MS (m/z 455.07).3 To
investigate the selectivity and sensitivity of SR toward F-, it was
titrated with F- in the presence of 30 equiv of Cl- (Figure 3e).
While SR did not optically respond to Cl-, it showed the
characteristic two-step color change with F- even in the presence
of Cl-, demonstrating the desired selectivity for the F- ion. The
selective colorimetric response of NDI receptors for F- recognition
compared to nonchromogenic binding of more polarized anions may
be attributed to the smaller ionic radius (1.33 Å) and 2p orbital of
the F- ion.1d These factors allow F- to come into closer proximity
of the NDI π-surface and engage in efficient anion-π and CT
interactions through better orbital interactions with NDI π*-
orbitals.2b Such interactions facilitate an electron transfer from F-
to the NDI π-system that produces the orange NDI•- radical anion.
Larger size, orbital mismatch,1d,2b and weaker binding of Cl- and
other anions3b with NDI π-systems could rationalize why they do
not induce visible color change.
Effects of preorganization on the sensitivity of NDI receptors
were probed by monitoring the F--induced fluorescence changes
at the minimum receptor concentrations. The titration of SR (1 nM
in DMSO) with F- (30 nM), probed by 381 nm excitation, displayed
a 4.5-fold amplification of the original 430 nm emission peak of
the NDI unit and a 20-fold amplification of a new peak at 465 nm
(Figure 3f). NDI 1 (10 µM in DMSO) showed a similar fluorescence
profile and 5.5-fold increase of the 465 nm emission peak (Figure
3f, inset), albeit at 104 times higher concentrations than SR. The
excellent nM sensitivity of SR vs weaker µM sensitivity of 1
supports our hypothesis that preorganization of two NDI units
should improve the F- affinity and sensitivity through stronger NDI/
F- interactions. The high F- ion sensitivity of preorganized
receptors (PR) bode well for their potential applications as F- ion
sensors.
Figure 4. 1H NMR titration of (a) 1 and (b) SR with F-, and (c) 19F NMR
titration of 1 with F- showing NDI/F- interaction (DMSO-d6, 298 K). Blue
arrows: signal disappearance; dotted lines: signal shift.
NDI core protons in SR (Figure 4b) and LR (SI, Figure S7a) did
not split before disappearing, potentially indicating the formation
of NDI/F-/NDI sandwich complexes in which both NDI units
interact evenly with the F- ion. A significant downfield shift of
the HX signal at 1-2 equiv of F- indicates subsequent NH · · ·F-
H-bonding interaction with amide protons. These NMR results are
consistent with ESI-MS data. In addition to showing [SR·F-] and
[LR·F-] complexes at e1 equiv of F-, ESI-MS of SR and LR in
the presence of 2 equiv of F- revealed signals that represent
[SR·2F-] and [LR ·2F-] complexes, respectively (SI, Figure
S2d-g). These results confirm that PRs can bind up to two F-
ions. The binding of two F- ions within one receptor molecule
possessing two recognition sites has been reported previously.10a
Thus, 1H NMR titrations of SR and LR showing the disappear-
ance of NDI signals before the onset of significant downfield shifts
of amide signals, as well as ESI-MS showing the presence of
[PR·2F-] species, suggest that the first F- ion binds through NDI/
F-/NDI interaction and then a second F- binds within the amide
cavities (Figure 1b). The first PR/F- interaction is fully reversible
by NOBF4 oxidation (SI, Figure S6b), indicating that it involves
NDI/F- interaction. In the presence of excess F-, amide protons
may be deprotonated, as their NMR signals became broad and
finally disappeared and NDI units were further reduced to NDI2-
as the solutions turned pink. No F- ion should be bound with the
receptors at this stage because of strong electrostatic repulsions.
1H and 19F NMR titration experiments were conducted to gain a
1
better insight into NDI/F- interaction (Figure 4). The H NMR
spectrum of receptor 1 reveals a singlet at 8.75 ppm corresponding
to four identical NDI core protons (Ha) and two doublets at 7.58
and 8.81 ppm corresponding to Hb and Hc of the pyridine ring,
respectively (Figure 4a). During the titration of 1 with F- all signals
became broad but none shifted at all, virtually ruling out the
possibility of a CH · · · F- H-bond formation. Consistent with UV/
Vis results, only the Ha signal gradually disappeared as F- reached
1 equiv, indicating the formation of the NDI•- radical anion. The
EPR spectrum of this species (SI, Figure S4) confirmed the presence
of the NDI•- radical anion. NOBF4 oxidation of the 1•- radical
anion completely regenerated 1, as the original NMR spectrum
reappeared, showing the Ha signal at 8.75 ppm (SI, Figure S6).
The fact that the Ha signal never splits as a result of NDI/F-
interactions, a sign that would have indicated a loss of symmetry
of the NDI core had a covalent C-F bond formed, rule out this
possibility. These results support our hypothesis that NDI/F-
interactions facilitate the F-fNDI ET event that generates the
NDI•- radical anion (Figure 1a).
1H NMR titrations of receptor 1 with Cl- and other anions did
not display any change (SI, Figure S8a), confirming that NDI/Cl-
anion-π interaction is weak.3b It can be attributed to weaker
electronic interactions of the NDI unit with larger anions compared
to a stronger electronic interaction with F-. Titrations of receptor
1, SR, and LR with Cl-, Br-, and I- did not affect NDI protons.
Only the 1H NMR signals of amide protons in SR and LR shifted
In SR, NDI core protons (HA) and the bisamide linker (HX)
appeared at 8.73 and 11.25 ppm, respectively (Figure 4b). During
the titration of SR with F- the HA signal gradually disappeared as
the F- ion concentration reached 1 equiv, while the HX signal shifted
slightly downfield, indicating that at first F- binds with NDI units.
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C O M M U N I C A T I O N S
downfield in the presence of Cl-, showing that Cl- preferentially
binds inside the cavities of amide linkers via stronger N-H· · · Cl-
H-bonding interaction (SI, Figure S8b-c).
be exploited for the detection of various levels of F- ion concentra-
tions in drinking water, consumer products, as well as bone and
muscle tissues for the early detection and prevention of F- ion
related diseases.
Fluoride ion recognition by NDI receptors was also observed
from 19F NMR spectroscopy. The 19F NMR spectrum of
TBAF ·3H2O in DMSO-d6 shows (Figure 4c) a strong singlet at
-102 ppm corresponding to the F- ion and a weak doublet at
Acknowledgment. S.S. thanks FSU for financial support and
Profs. Naresh Dalal and Igor Alabugin for assistance with EPR
studies and B3LYP/6-31+G** calculations, respectively.
12
-
-142.5 ppm for HF2
.
Titrations of TBAF with 1 caused an
upfield shift of the -102 ppm signal (Figure 4c), which indicates
shielding of F- by the NDI receptor. The disappearance of the F-
signal at 1:1 TBAF/1 may be attributed to an oxidation of F- to F•
as a result of the F-fNDI ET process that produces the NDI•-
radical anion. Although we previously considered a possibility of
C-F bond formation as one of the modes of NDI/F- interaction,
it was not supported by any evidence, including 19F NMR, as no
new signal corresponding to a covalent C-F bond was observed.
The fact that F- induced reductions of NDI to NDI•- and NDI2-
can be fully reversed by oxidizing them back to neutral NDI with
NOBF4 and that the process can be repeated (SI, Figure S6a)
confirm that F- or the resulting F• never reacts with any NDI species
covalently. Whether the transient F• reacts ultimately with solvent
molecules, TBA counterions, homocouples to emanate F2 gas, or
generates HF acid remains unclear after extensive analyses of NDI/
F- mixtures. Nevertheless, F• is produced as a result of NDI/F-
interaction and ET events. Therefore, the lack of precise information
on the fate of F• is inconsequential, as it does not impede the clear
understanding of NDI/F- anion-π interaction that leads to an
unprecedented F-fNDI ET event.
To demonstrate a potential application of NDI/F- interaction,
SR was treated individually with aqueous DMSO extracts of an
anticavity toothpaste containing 0.24% (w/v) NaF and F--free
toothpaste. To our delight, colorless SR turned light orange and
displayed the absorption spectrum of the NDI•- radical anion with
the F- containing toothpaste but did not show any optical changes
with the F--free one (SI, Figure S9).
For the first time, a strong NDI/F- interaction was identified and
fully characterized by experimental results, as well as validated by
computational models (SI, B3LYP/6-31+G**). Supramolecular
NDI/F- (anion-π and CT) interactions promote an unprecedented
electron transfer process from the F- ion to electron deficient NDI
receptors. NDI receptors are highly selective toward F- over other
anions because of better orbital interactions. They display nM range
F- sensitivity in preorganized systems, in which two NDI units
perfectly overlap with each other. The reversibility of the colori-
metric response and reproducibility of NDI receptors render them
excellent reusable F- ion sensors. Therefore, NDI derivatives may
Supporting Information Available: Energy minimized structures;
synthesis and characterizations; additional NMR, EPR, SEC, and ESI-
MS data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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