C3v-symmetric anion receptors with guanidine recognition motifs for ratiometric sensing of fluoride

Article abstract of DOI:10.1039/c5ra26039f

Two new tripodal receptors 3 and 4 derived from a trindane framework having guanidine groups acting as hydrogen bond acceptors are synthesized and characterized for the selective recognition of anions. The anion recognition ability of the receptors was ev

Full text of DOI:10.1039/c5ra26039f

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C3v-Symmetric anion receptors with guanidine recognition motifs
for ratiometric sensing of fluoride
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
∗
Won Kima‡, Suban K Sahooa,b‡, Gi-Dong Kim , and Heung-Jin Choi
DOI: 10.1039/x0xx00000x
Two new tripodal receptors 3 and 4 derived from trindane framework having guanidine groups acting as hydrogen bond
acceptors are synthesized and characterized for the selective recognition of anions. The anion recognition ability of the
receptors was evaluated by UV-Vis absorption, fluorescence and 1H NMR methods. Both the receptors showed F− selective
ratiometric chromogenic and fluorogenic responses among the tested anions due to the host-guest complexation followed
by the abstraction of the amide-NH protons supported by the 1H NMR titration study. Receptor 4 with the naphthalene
fluorophore units showed high F− selectively than the receptor 3 by giving visually detectable blue fluorescent and
significant turn-on fluorescence at 410 nm which can be switched back and forth by successive addition of F− and H+.
Further, the reversible and reproducible fluorescent state of 4 can be applied to design a molecular-scale sequential
memory unit displaying “Write–Read–Erase–Read” functions in the form of binary logic.
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instrumentations.
To this library of anion selective receptors, we have
introduced many tripodal anion receptors containing urea and
thiourea binding sites using the rigid C3v-symmetric tripodal
framework ‘trindane’ (Scheme 1) expanded either from the lower
or upper feet with high recognition ability towards bioactive anions
[25-28]. In the present study, we have introduced two new tripodal
Introduction
Research on anion recognition and sensing by artificial
receptors have gained burgeoning interest among the
a
supramolecular chemists during the past decades, because of their
ubiquitous nature and potential applications in various biological,
industrial and environmental processes [1-8]. In general, the
selective recognition of target anionic guest is a combine effect of
the non-covalent interactions between the receptor (host) and
guest, size-and-shape complementarity, the geometry of guest,
anion basicity and the nature of the solvent etc. [9-12]. Various
non-covalent interactions, such as electrostatic, anion-π, hydrogen
bonding, coordinate bonding, etc. are used for the designing of
positively charged, neutral and metal-ligand complex based anion
selective receptors. Among the different approaches, neutral anion
receptors containing the directional hydrogen bonding have been
extensively used for the designing of receptors with specific shapes
to differentiate anionic guests with different geometries and also to
avoid the possible interferences by counterion during the anion
recognition. Whenever these anion selective receptors are linked
suitably with a chromogenic and/or fluorogenic signaling unit, the
sensing of target anion can be achieved through an optical
response. Recently [13-24], many anion selective optical sensors
based on colorimetric and fluorescent changes have been reported
to contain recognition groups, such as urea/thiourea, amide,
pyrrole, sulfonamide and indole coupled with different signaling
units because of the simplicity, sensitivity, low cost and online
monitor of target anion without the need of expensive
receptors
3 and 4 derived from trindane framework having
guanidine groups acting as hydrogen bond acceptors for the
selective encapsulation of anions (Scheme 1). The guanidine
derivatives 3 and 4 are expected to behave like the reported urea
based receptors with the added benefit of the conjugated aromatic
systems for some optical changes for the chromogenic/fluorogenic
detection of anion. The receptors are synthesized and
characterized, and their anion recognition ability was evaluated by
1H NMR titration, UV-Vis and fluorescence methods.
‡Contributed equally.
Scheme 1. Synthesis of new trindane-based C3v-symmetric anion
receptors 3 and 4 with guanidine recognition motif from cis,cis,cis-
2,5,8-tribenzyltrindane frameworks 1: (a) THF/EtOH/H2O (1:1:1),
KOH, reflux, 12 hr; (b) i. DMA, 1,1'-carbonyldiimidazole, rt, 12 hr; ii.
DMA, 2-aminobenzimidazole, 70 ºC, 18 hr; (c) i. DMA, 1,1'-
carbonyldiimidazole, rt, 12 hr; ii. DMA, 1H-naphtho[2,3-d]imidazole-
2-amine, 70 ºC, 18 hr.
a
Department of Applied Chemistry, Kyungpook National University, Daegu, 702-701,
South Korea. E-mail: choihj@knu.ac.kr; +82 (53) 950-5582; Fax: +82 (53) 950-6594.
b
Department of Applied Chemistry, SV National Institute of Technology, Surat-395007,
Gujrat, India.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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6H, ArCHaHb-), 2.96 (s, 6H, PhCH2-), 2.77 (d, J = 15.6 Hz, 6H,
ArCHaHb-); 13C NMR (100 MHz, DMSO-d6) δ 177.2, 138.1, 134.9,
129.5, 128.0, 126.4, 60.1, 55.0, 43.0; MS (EI, relative intensity,
m/z): 602 (M+2, 3), 601 (M+1, 10), 600 (M+, 24), 554 (9), 509 (12),
463 (7), 417 (15), 347 (33), 191 (9), 91 (100).
Experimental
General
All chemicals and reagents of high purity were obtained
commercially and were used without any further purification.
DMSO (HPLC grade) was purchased from Duksan, South Korea. 1,1'-
Carbonyldiimidazole, 2-aminobenzimidazole, cyanogen bromide
and anhydrous N,N'-dimethylaceteamide (DMA) were purchased
from Aldrich Chemical Co., USA. 2,3-Diaminonaphthalene was
purchased from Alfa Aesar, USA. The solutions of anions were
prepared from their tetrabutylammonium (TBA) salts of analytical
grade procured from Aldrich Chemical Co., USA.
All the analytical measurements were conducted at room
temperature. Column chromatography was performed on
Yamagen MPLC equipped with a fluid metering pump using Merck
silica gel 60 (70-230 mesh) and CH2Cl2/MeOH mixed eluent. NMR
spectra were recorded on a Bruker AVANCE digital 400 (400 MHz)
Synthesis
benzo[d]imidazol-2-yl)carbamoyl]trindane (3)
mixture of cis,cis,cis-2,5,8-tribenzyltrindane-2,5,8-
of
cis,cis,cis-2,5,8-tribenzyl-2,5,8-tri[N-(1H-
A
tricarboxylic acid (100 mg, 0.17 mmol) and 1,1'-carbonyldiimidazole
(84 mg, 0.52 mmol) in anhydrous DMA (10 mL) was stirred under a
nitrogen atmosphere at room temperature for 12 hours. To this, a
solution of 2-aminobenzimidazole (72 mg, 0.54 mmol) in anhydrous
DMA (4 mL) was added by using a syringe. The mixture was stirred
further for 18 hours at 70 ºC. Then water (2 mL) was added to this
a
reaction mixture and stirred for
a while. The mixture was
concentrated to dryness by vacuum distillation. The residue was
purified by a column chromatography on silica gel using CH2Cl2 and
then MeOH/CH2Cl2 (2:98) to give the product as a slightly yellowish
solid powder (83.7 mg, 52%): IR (KBr, cm-1): 3380 (w), 3295 (w),
3062 (w), 3032 (w), 2924 (m), 2855 (w), 1674 (m), 1628 (m), 1566
(s), 1451 (m), 1427 (m), 1273 (m), 1196 (m), 741 (s), 702 (m), 602
(w); 1H NMR (400 MHz, DMSO-d6): δ 12.08 (br s, 3H, N-H), 11.76 (br
s, 3H, N-H), 7.51-7.39 (m, 6H, Ar-H), 7.33-7.06 (m, 21H, Ar-H), 3.32
(d, J = 15.8 Hz, 6H, ArCHaHb-), 3.28 (s, 6H, ArCH2), 3.08 (d, J = 15.8
Hz, 6H, ArCHaHb-); 13C NMR (100 MHz, DMSO-d6): δ 176.5 (br),
148.1 (br), 141.4, 138.9, 135.9, 133.4 (br), 130.4, 129.2, 127.5,
122.1, 117.8 (br), 112.6 (br), 58.0, 43.6 (br), 40 (overlapped with
1
and AVANCE III (500 MHz) spectrometer in DMSO-d6. H NMR and
13C NMR chemical shifts are given relative to TMS. UV-Vis
absorption spectra were obtained on an Optizen 2120-UV
spectrophotometer. Fluorescence spectra were measured on a
Shimadzu RF-5301 fluorescence spectrometer equipped with a
xenon discharge lamp using 1 cm quartz cells. IR spectra were
recorded on a Shimadzu IR Prestige-21 FTIR spectrometer. MALDI-
TOF data were obtained on a Voyager DE-STR mass spectrometer
and α-cyano-4-hydroxycinnamic acid (CHCA) was used as a matrix.
The stock solutions of host and anions (1 mM) were prepared in
DMSO. These solutions were appropriately diluted further for
different spectroscopic experiments to study the anion recognition
and sensing ability of the hosts 3 and 4. The spectral titrations of 3
and 4 were performed by taking 2 mL of host solution with
appropriate concentration directly into cuvette and the spectra
were recorded after each aliquot addition of TBAF (1 mM). All
fluorescence emission spectra were recorded at a fixed excitation
wavelength (λex of receptor 3: 299 nm and receptor 4: 345 nm). The
quantum yield (Ф) of the receptors before and after the addition of
DMSO-d6),
referenced
to
cis,cis,cis-2,5,8-tribenzyl-2,5,8-
tri(carbamoyl)trindane (δ 178.3, 139.3, 135.9, 130.3, 128.4, 126.6,
56.4, 44.0, 40.2); MALDI-TOF-MS, m/z (rel intensity): 946.7956
(100), 947.7957 (74), 948.7990 (23), Calcd for C60H51N9O3: m/z
946.4193 (M + H+, 100), 947.4227 (68.2), 948.4260 (22.9).
Synthesis of cis,cis,cis-2,5,8-tribenzyl-2,5,8-tri[N-(1H-naphtho[2,3-
d]imidazol-2-yl)carbamoyl]trindane (4)
Synthesis of 1H-naphtho[2,3-d]imidazol-2-amine [30]: In a 100
mL round bottom flask, 2,3-diaminonaphthalene (158 mg, 1 mmol)
was dissolved in a mixture of MeOH/water (1:1, 20 mL). The
reaction mixture was treated with cyanogen bromide (165 mg, 1.55
mmol) and heated at 50 ºC for 1 hour. After cooling to room
temperature, the solvent was removed in vacuo, and the residue
was basified with 1 M KOH (to pH 10) and extracted with EtOAc (30
mL X 3). The combined organic fractions were dried with MgSO4,
filtered and concentrated in vacuo. The reaction mixture was
purified by recrystallization from n-hexane/THF to yield greenish
brown powder (180 mg, 98%): 1H NMR (400 MHz, DMSO-d6) δ
10.88 (br s, 1H, N-H), 7.81 (m, 2H, Ar-H), 7.51 (s, 2H, Ar-H), 7.25 (m,
2H, Ar-H), 6.67 (br s, 2H, -NH2).
F- was calculated by applying the relationship [29]:
2
Ф = Фref (I/Iref) (Aref/A) (η/ηref
)
where, Ф is the radiative quantum yield of the receptor, Фref is the
quantum yield of quinine sulfate in 0.1 M aqueous H2SO4 (Фref
=
0.54), I is the integrated emission, A is the absorbance at the
excitation wavelength, and η is the refractive index of the solvent.
For 1H NMR titration, the receptor solution (0.5 mL, 4.0 mM, DMSO-
d6) was taken in NMR tube and then the spectra were recorded
after each stepwise addition of fluoride anion as their TBA salt
prepared in DMSO-d6.
Synthesis of trindane receptor 4: A mixture of cis,cis,cis-2,5,8-
tribenzyltrindane-2,5,8-tricarboxylic acid (100 mg, 0.17 mmol) and
1,1'-carbonyldiimidazole (84 mg, 0.52 mmol) in anhydrous DMA (10
mL) was stirred under a nitrogen atmosphere at room temperature
for 12 hours. To this solution was added a solution of 1H-
naphtho[2,3-d]imidazole-2-amine (100 mg, 0.54 mmol) in
anhydrous DMA (5 mL) via a syringe. The mixture was stirred
further for 18 hours at 70 ºC. After cooling to room temperature, to
this reaction mixture was added water (5 mL) and stirred for a
while. The mixture was concentrated by vacuum distillation. The
residue was purified by a column chromatography on silica gel using
CH2Cl2 and then MeOH/CH2Cl2 (2:98) to give the product (97 mg,
52%): IR (KBr, cm-1): 3378 (m), 3055 (w), 2921 (w), 2847 (w), 1677
(m), 1648 (m), 1567 (s), 1504 (m), 1424 (s), 1263 (m), 1160 (m), 858
(m), 732 (m), 700 (m); 1H NMR (400 MHz, DMSO-d6): δ 12.17 (br s,
Synthesis of cis,cis,cis-2,5,8-tribenzyltrindane-2,5,8-tricarboxylic
acid (2)
The mixture of triethyl cis,cis,cis-2,5,8-tribenzyltrindane-2,5,8-
tricarboxylate (70 mg, 0.11 mmol) and KOH (33 mg, 0.50 mmol) in
THF (5 mL), water (5 mL) and ethanol (5 mL) medium was heated at
reflux condition for 12 hours. The clear solution was concentrated
under reduced pressure. The residue was acidified with conc. HCl
(6 mL) and cooled in an ice bath. The precipitate was filtered and
washed with 3N HCl (10 mL). The white mixture was dried. The
dried mixture was taken up in THF (10 mL), and filtered to remove
insoluble inorganic salt. The filtrate was concentrated and dried
under high vacuum to give white solid (65 mg, 98%): IR (KBr, cm-1):
(KBr) 3062, 3027, 2924, 1702, 1201; 1H NMR (400 MHz, DMSO-d6) δ
12.47 (s, 3H, -CO2H), 7.28 (t, J = 7.30 Hz, 6H, Ar-H), 7.21 (t, J = 7.30
Hz, 3H, Ar-H), 7.15 (d, J = 7.32 Hz, 6H, Ar-H), 3.07 (d, J = 15.6 Hz,
2 | J. Name., 2012, 00, 1-3
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3H, N-H), 12.02 (br s, 3H, N-H), 8.04-7.75 (m, 12H, Ar-H), 7.40-7.16 receptors 3 and 4 (10 μM) was next performed by the incremental
(m, 21H, Ar-H), 3.37 (d, J = 14.6 Hz, 6H, ArCHaHb-), 3.32 (s, 6H, addition of TBAF from 1 to 100 equivalents (Fig. 1c-d) to determine
ArCH2), 3.10 (d, J = 14.6 Hz, 6H, ArCHaHb-); 13C NMR (100 MHz, the anion binding constant. Addition of just 1 equivalent of F− i.e. 10
DMSO-d6): δ 176.8 (br), 151.7 (br), 139.0, 135.9, 130.4, 129.1, μM to the receptors 3 and 4 solutions resulted the appearance of
128.4, 127.4, 124.0, 113.1 (br), 107.6 (br), 58.3, 43.5 (br), 41.4 the new charge-transfer band at 320 nm and 372 nm, respectively.
(overlapped with DMSO-d6); MALDI-TOF-MS, m/z (rel intensity): With the further addition of F−, the intensity of the charge transfer
1096.7208 (100), 1097.7255 (86), 1098.7271 (36), 1099.7305 (10), band attributed to new receptor-anion complex species formed in
Calcd for C60H51N9O3: m/z 1096.4663 (M + H+, 100), 1097.4696 solution was enhanced continuously with the formation of
(81.2), 1098.4730 (32.5), 1099.4763 (7.5).
isosbestic points at 300 nm for 3, and at 271 and 340 nm for 4. The
spectral changes shown by the receptors clear indicate the similar
F− recognition modes. Analysis of the UV-Vis titration data with the
least-square fitting equation [31] showed best fit for 1:1 binding
stoichiometry with the binding constant of 4.5X103 M-1 and 6.5X103
M-1 for 3.F− and 4.F−, respectively (Fig. S3). The higher F− binding
ability of receptor 4 can be linked with the enhanced aromatic
platform due to the naphthalene group that generated a more
preorganized deep hollow cavity to accommodate fluoride ions.
Results and discussion
The synthesis of receptors 3 and 4 were achieved by following
the steps shown in Scheme 1 and characterized by various spectral
techniques before embarking for further anion recognition and
sensing. The trindane tricarboxylic ester scaffold (1) which was used
as
a starting material of trindane tricarboxylic acid (2) was
synthesized from mesitylene in seven steps by following our
reported methods with an overall yield of 10 % (Scheme S1) [26].
The trindane tricarboxylic acid 2 was synthesized in 98% yield from
hydrolysis of trindane tricarboxylic ester
1 with potassium
hydroxide in THF/ethanol/water (1:1:1) medium at reflux condition.
Anion receptor 3 was synthesized from the reaction of 2 and 2-
aminobenzimidazole
via
carbonyl
activation
with
carbonyldiimidazole in 52% yield. Receptor 3 was purified by a
column chromatography to remove of excess of 2-
aminobenzimidazole and mono/di-substituted compounds.
Similarly, anion receptor 4 with naphthoimidazole recognition motif
was synthesized and purified by a column chromatography to
remove the excess of starting reagent and mono/di-substituted
compounds. Both the receptors are soluble in common organic
solvents such as DMSO, THF, CH2Cl2, CH3CN, but insoluble in water.
The molecular structure of receptors 3 and 4 were characterized by
various spectral (IR, 1H NMR, 13C NMR) and MALDI-TOF (Fig. S1a-
S2a) data. The 1H NMR spectrum of 3 showed two peaks at δ 12.08
and 11.76 ppm attributed to the imidazole-NH and amide-NH
protons, respectively (Fig. S1b). The C3v-symmetrical form of 3 was
ascertained from the appearance of two sets of doublets (δ 3.32
and 3.08 ppm, 2J = 15.8 Hz) for the two diastereotopic benzylic
protons and a singlet at δ 3.28 ppm for the methylene protons of
benzyl groups. Similarly, the imidazole-NH and amide-NH protons
peaks of receptor 4 were appeared at δ 12.17 and 12.02 ppm,
respectively (Fig. S2b). Also, the two diastereotopic benzylic protons
showed two sets of doublets (δ 3.37 and 3.10 ppm, 2J = 14.6 Hz) and
a singlet appeared at δ 3.32 ppm for the methylene protons of
benzyl groups that supported the C3v-symmetrical structure of
receptor 4.
Fig. 1. Changes in the UV–Vis absorption spectra of receptors (10
μM) 3 (a) and 4 (b) in absence and presence of 100 equiv of
selected anions in DMSO. The UV–Vis spectral changes of 3 (c) and
4 (d) upon incremental addition of TBAF from 1, 2, 3, 5, 7, 10, 13,
16, 20, 30, 40, 60, 80 to100 equiv.
The fluoride recognition behavior of the receptors was
1
examined by recording the H NMR spectra of the receptors 3 (Fig.
2) and 4 (Fig. S4) after adding different equivalents of F−. Upon
addition of 0.5 equivalent of F−, the imidazole-NH and amide-NH
protons (Hb) peaks at δ 12.08 and 11.76 ppm were broadened
significantly due to the possible hydrogen bonding interactions
occurred between 3 and F−. At higher equivalents of F−, the amide-
NH protons peak disappeared completely and a new peak appeared
at 15.5 ppm for the formation of bifluoride ions i.e. HF2− [32]. Other
peaks, including the characteristic peaks due to the C3v-symmetry of
3 were not shifted throughout the titrations. The receptor 4 also
The anion recognition and sensing ability of the receptors 3
−
−
−
and 4 towards F−, Cl−, Br−, HSO4 , H2PO4 , NO3−, CH3SO3 and
C7H7SO3− anions was investigated by the 1H NMR, colorimetric and
fluorometric methods in DMSO. As shown in Fig. 1a-b, the receptor
3 showed absorption bands at 265, 290 and 299 nm, whereas 4 at
265, 331, 338 and 345 nm due to multiple π-π* transitions. Upon
addition of F−, the absorption bands of 3 and 4 were selectively red-
shifted to 320 nm and 372 nm, respectively. The appearance new
red-shifted band indicates the formation of an anion-receptor
complex through intramolecular hydrogen bonds and/or the strong
basicity of F− triggered the abstraction of –NH protons which
accelerated the internal charge transfer (ICT) between the receptor
and added F−. No noticeable spectral changes of the receptors 3
and 4 were observed with other tested anions. Also, the F−-induced
spectral changes of the receptors were not interfered under
competitive environment. The UV-Vis absorption titration of
1
showed similar changes in H NMR titration experiment with TBAF
(Fig. S4). Therefore, it can be proposed that the receptors first
forming the hydrogen bonded host-guest complex with F− and then
the deprotonation of most acidic amide-NH protons occurred when
F− added in excess.
The anion recognition ability of the receptors 3 and 4 was next
investigated by fluorescence spectroscopy in DMSO. The receptor 3
(10 μM) showed a weak fluorescence at 328 nm (Ф = 0.003), when
excited at 299 nm (Fig. S5). Addition of F− in excess (100 equiv.)
caused a slight red-shift in fluorescence band from 328 nm to 336
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nm (Ф = 0.004). With naphthalene fluorophore, the receptor 4 (5 fluorescence turn-on [33]. Further to complement the proposed
μM) showed a broad fluorescence between 350 nm to 450 nm with deprotonation mechanism, the fluorescence spectra of 4 was
emission maxima at 368 nm (Ф = 0.004). Upon addition of 50 equiv recorded in the presence of strong base NaOH that showed similar
of F− resulted quenching in the fluorescence of the receptor 4 and a turn-on fluorescence at 410 nm as observed with TBAF (Fig. S6).
new fluorescence band was appeared in the visible region with Further, the fluorescence of receptor 4 was reversed back upon
emission maxima at 410 nm (Ф = 0.01) (Fig. 3). The ratiometric subsequent addition of HCl.
fluorescence response of 4 is highly selective and specific, occurred
only in the presence of F−, and not interfered in the presence of
other competing anions. In addition,
a selective naked-eye
detectable fluorescent color change of 4 was observed upon
addition of F− (inset Fig. 3).
Fig. 4. The fluorescence spectral changes of
4 (5μM) upon
incremental addition of TBAF in DMSO. Inset showing the change in
fluorescence intensity of 4 at 368 nm and 410 nm at different
concentration of TBAF.
Fig. 2. Partial 1H NMR spectral changes of 3 (4 mM) upon addition
of TBAF in DMSO-d6.
The ‘Off-On’ fluorescent state of 4 and increase in the emission
intensity ratio i.e. I410/I368 with the increase in the [F−] linearly from
49.8 μM to 361 μM indicates the possible application of 4 for the
ratiometric fluorescent sensing of F− ions. Using the fluorescence
titration data, the F− detection limit of 208 nM was estimated using
the slope of the calibration curve (Fig. S7) and the equation
3σ/slope (where σ represents the standard deviation of the blank).
The United States Environmental Protection Agency (USEPA)
mandates a drinking water standard for F− of 100 ꢀM and 200 ꢀM
respectively to prevent dental fluorosis and osteofluorosis [34], but
a recent review demonstrated that the intake of F− above 250 ꢀM
can caused bone damage and mottled teeth [35]. The estimated
detection limit of 4 for the ratiometric detection of F− supported its
analytical novelty.
Anion recognition and sensing in protic solvents, such as water,
ethanol is challenging because of the competing nature with anions
for the receptor binding sites that disturbed the hydrogen bonding
interactions between the receptor and the anionic guest [36].
Among the various anions, the F− is known to show high hydration
energy due to the considerably high bond strength (569 kJ/mol) of
its conjugated acid (HF) and therefore its detection from aqueous
medium is very challenging. In order to detect F− from aqueous
medium, the fluorescence spectra of 4 were recorded in the
presence of 50 equiv of F− in DMSO containing different
percentages of water (Fig. S8). We have observed that the F−-
induced turn-on fluorescence at 410 nm can be observed
distinguishably in DMSO containing water not more than 7.5%. The
fluorescence titration of 4 was next performed at the optimized
condition of DMSO:H2O (95:5, v/v). As shown in Fig. 5, the
incremental addition of F− resulted fluorescence enhancement at
390 and 410 nm linearly from 123 μM to 909 μM and concomitantly
quenched at 368 nm with the formation of an isoemission point at
378 nm.
Fig. 3. Changes in the fluorescence spectra of receptor 4 (5 μM)
after addition of 50 equiv of selected anions in DMSO (λex = 345
nm). Inset shown a color change of vials under UV light (365 nm).
The fluorescence titration of
incremental addition of TBAF in DMSO (Fig. 4). The typical
naphthalene fluorescence of decreases gradually and the
4 was performed by the
4
emission at 410 nm was appeared with the formation of a well-
defined isoemission point at 378 nm. The structural modification of
4 at the excited state upon addition of F− that led to the significant
enhancement of fluorescence at 410 nm was most likely due to the
hydrogen bonding interactions to the polar-NH groups followed by
the deprotonation of most acidic amide-NH protons. It is worthy to
mention here that fluoride binding resulted fluorescence turn-off
for most of the reported sensors with only a few exhibiting
4 | J. Name., 2012, 00, 1-3
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As discussed above, the receptor 4 showed an instantaneous of 4 could be repeated for many times, suggesting “Write-Read-
color change under UV light along with a drastic red-shift in the Erase-Read” cycles could be conducted. In other words, this system
fluorescence from 368 nm to 410 nm in DMSO upon F− binding. The exhibits “Multi-write” ability without obvious degradation in its
F−-induced fluorogenic process of 4 was reversed with the addition optical output at 410 nm.
of H+, results in disappearance of the 4.F− emission bands at 410 nm
and reappearance of the fluorescence at 368 nm. Especially, the
reversible fluorescence changes could be repeated for several times
by alternating addition of F− and H+ (Fig. 6), mimicking the behavior
of an optical switch.
Fig. 6. Change in fluorescence intensity of 4 (5 μM) upon an
alternate addition of F− and H+ in DMSO (λex= 345 nm).
Fig. 5. The fluorescence spectral changes of 4 (5 μM) upon
incremental addition of TBAF in DMSO containing 5% H2O. Inset
showing the change in fluorescence intensity of 4 at 368 nm, 390
nm and 410 nm at different concentration of TBAF.
Optical switches have great interest for molecular-level
information processing and for the designing of Boolean type logic
gates at the molecular level [37-39]. As depicted in Fig. 7a-b, the
fluorescence switching process of 4 may be represented by a
molecular “INHIBIT/IMPLICATION” type logic gates by employing
F− (Inp1) and H+ (Inp2) as the chemical inputs and the fluorescence
intensity at 410 nm and 368 nm as the outputs. When the
fluorescence at 368 nm was used as output, an “IMPLICATION” (a
Fig. 7. The (a) truth table and (b) logic gates for the IMPLICATION
combination of NOT and OR logic gates) logic gate can be
and INHIBIT logic gates. (c) Feedback loop showing reversible logic
fabricated. However, an “INHIBIT” (a combination of AND and NOT
operations mimicking “Read-Erase-Write-Read” functions and (d)
logic gates) logic gate can be constructed if the fluorescence at 410
the sequential logic circuit for memory element with two inputs and
nm was used as an output. In this way, a complementary IMP/INH
one output.
logic functions can be realized based on the fluorescence switching
of receptor 4. Overall, the fluorescent changes of 4 in DMSO are
The quantum mechanically calculated structure of the receptors
controlled by the chemical inputs of F− and H+: F− switches ON the
3 and 4, and their host-guest complexes with F− was obtained
optical output, while H+ switches OFF the optical output.
through density functional theory (DFT) method. The equilibrium
The reversible and reproducible fluorescence switching process
conformers of the receptors and their F− complexes were first
of 4 can also be used to design a useful sequential logic circuit
searched by applying the MMFF force field and the computational
displaying “Write-Read-Erase-Read” behavior in the form of binary
program Spartan'14. The least strain structure obtained by the
logic for molecular-level information processing (Fig. 7c-d). The ON
conformational search was next optimized by the DFT method at
state (Output 2 = 1) is defined as the strong fluorescence at 410 nm,
B3LYP/6-31G* level in the gas phase. As shown in Fig. 8, the three
whereas the OFF state (Output 2 = 0) corresponds to the
appended arms of the receptors were oriented in the same
significantly weak fluorescence at 410 nm. The inputs are
direction to maintain the C3v-symmetry with the help of multiple
constituted by F− (Inp1) and H+ (Inp2) for the ‘set’ and ‘reset’,
intramolecular/intrastand hydrogen bonding which gave a perfect
respectively. The operation of this memory unit is as follows:
geometry to accommodate incoming anionic guest within the
whenever the set input is high (S = 1), the system writes and
cavity. Compare to 3, the receptor 4 with longer aromatic platform
memorizes the binary state 1; on the other hand, when the reset
showed better tripodal symmetry with a deep hollow cavity that
input is high (R = 1), the 1 state is erased and the 0 state is written
resulted higher binding ability towards F−. Similar to urea-based
and memorized. The reversible and reconfigurable sequences of
tripodal anion receptors, the F− was encapsulated within the cavity
fluorescence output at λem = 410 nm for the set/reset logic
of the receptors to form 1:1 caviplex by forming six hydrogen bonds
operations in the feedback loop demonstrated the memory feature
with the imidazole-NH and amide-NH groups. The calculated
with “Write-Read-Erase-Read” functions. Also, the “ON–OFF” states
average bond lengths for the amide-NH….F− and imidazole-NH….F−
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bonds in 3•F− complex were 2.135 Å and 1.678 Å, whereas of 2.286 7. R.M. Duke, E.B. Veale, F.M. Pfeffer, P.E. Kruger, T.
Å and 1.669 Å in 4•F− complex, respectively.
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3, (b) receptor 4, (c) 3•F− and 4•F−.
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In conclusion, we have introduced two new C3v-symmetrical
receptors 3 and 4 with guanidine recognition motif for the selective
recognition and sensing of bioactive anions. The UV-Vis absorption
and fluorescence spectra recorded for the receptors 3 and 4
showed selectivity towards fluoride due to the selective formation
of hydrogen bonded host-guest complex followed by the
deprotonation of amide-NH protons. The spectral changes were
depends on the substituent used. In compared to 3, the receptor
with naphthoimidazole group showed 40 nm red-shift in the UV-Vis
spectral study and a significant fluorescence enhancement at 410
nm in the presence of fluoride. The receptor 4 also showed a F−-
induced visually detectable color change under UV light and
potential to detect F− ratiometrically with the detection limit down
to 208 nM. The fluorescence changes triggered by F− cab be
reversed with the addition of H+, which allowed to construct
different logic gates and sequential logic operations mimicking
“Read-Erase-Write-Read” functions at the molecular level.
Acknowledgments
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This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (NRF-2010-
0008583).
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6 | J. Name., 2012, 00, 1-3
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39. S. Erdemir, O. Kocyigit, Sensors and Actuators B, 2015, 221,
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Graphical Abstract