Excimer Disaggregation Enhanced Emission: A Fluorescence turn-On Approach to Oxoanion Recognition

Article abstract of DOI:10.1021/jacs.8b09021

A new approach to anion sensing that involves excimer disaggregation induced emission (EDIE) is reported. It involves the anion-mediated disaggregation of the excimer formed from a cationic macrocycle. This leads to an increase in the observed fluorescence intensity. The macrocycle in question, cyclo[1]N²,N⁶-dimethyl-N²,N⁶-bis(6-(1H-imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-diamine[1]1,4-dimethylbenzene (1²⁺ prepared as its PF₆^- salt), is obtained in ca. 70% yield via a simple cyclization. X-ray diffraction analyses of single crystals revealed that, as prepared, this macrocycle exists in a supramolecular polymeric form in the solid state. Macrocycle 1²⁺ is weakly fluorescent in acetonitrile. The emission intensity is concentration dependent, with the maximum intensity being observed at [1²⁺] 0.020 mM. This finding is ascribed to formation of an excimer, followed possibly by higher order aggregates as the concentration of 1²⁺ is increased. Addition of tetrabutylammonium pyrophosphate (HP₂O₇^3-) to 1²⁺ (0.020 mM in acetonitrile) produces a ca. 200-fold enhancement in the emission intensity (_ex = 334 nm; _em = 390-650 nm). These findings are rationalized in terms of the HP₂O₇^3- serving to break up essentially non-fluorescent excited-state dimers of 1²⁺ through formation of a highly fluorescent anion-bound monomeric complex, 1²⁺·HP₂O₇^3-. A turn-on in the fluorescence intensity is also seen for H₂PO₄^- and, to a lesser extent, HCO₃^-. Little (HSO₄^-, NO₃^-) or essentially no (N₃^-, SCN^-, F^-, Cl^-, Br^- and I^-) response is seen for other anions. Solid-state structural analysis of single crystals obtained after treating 1²⁺ with HP₂O₇^3- in the presence of water revealed a salt form wherein a H₂P₂O₇^2- anion sits above the cone-like macrocycle.

Full text of DOI:10.1021/jacs.8b09021

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Article
Excimer Disaggregation Enhanced Emission: A
Fluorescence "Turn-on" Approach to Oxoanion Recognition
Jian Yang, Chao-Chen Dong, Xu-Lang Chen, Xin Sun, Jin-Yan
Wei, Junfeng Xiang, Jonathan L. Sessler, and Han-Yuan Gong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09021 • Publication Date (Web): 24 Feb 2019
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Excimer Disaggregation Enhanced Emission: A Fluorescence
"Turn-on" Approach to Oxoanion Recognition
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Jian Yang,[a] Chao-Chen Dong,[a,b] Xu-Lang Chen,[a] Xin Sun,[a] Jin-Yan Wei,[b] Jun-Feng
Xiang,[c] Jonathan L. Sessler,*[d,e] Han-Yuan Gong*[a]
a College of Chemistry, Beijing Normal University, Xinjiekouwaidajie 19, Beijing 100875, P. R. China;
b Department of Chemistry, Renmin University of China, Beijing, 100872, P. R. China;
c Institute of Chemistry, Chinese Academy of Sciences, Zhongguancunbeiyijie 2, Beijing 100190, P. R. China;
d Center for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai 200444, China;
e Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas
78712-1224, United States
Excimer Disaggregation, Imidazolium Receptors, Oxoanion Recognition, Fluorescence Turn-on Sensor
ABSTRACT: A new approach to anion sensing that involves excimer disaggregation induced emission (EDIE) is re-
ported. It involves the anion-mediated disaggregation of the excimer formed from a cationic macrocycle. This leads to
an increase in the observed fluorescence intensity. The macrocycle in question, cyclo[1]N2,N6-dimethyl-N2,N6-bis(6-(1H-
-
imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-diamine[1]1,4-dimethyl benzene (12+; prepared as its PF6 salt), is obtained in
ca. 70% yield via a simple cyclization. X-ray diffraction analyses of single crystals revealed that as prepared this mac-
rocycle exists in a supramolecular polymeric form in the solid state. Macrocycle 12+ is weakly fluorescent in acetonitrile.
The emission intensity is concentration dependent with the maximum intensity being observed at [12+] = ca. 0.020 mM.
This finding is ascribed to formation of an excimer followed possibly by higher order aggregates as the concentration of
12+ is increased. Addition of tetrabutylammonium pyrophosphate (HP2O73-) to 12+ (0.020 mM in acetonitrile) produces a
ca. 200-fold enhancement in the emission intensity (lex = 334 nm; lem = 390 - 650 nm). These findings are rationalized
in terms of the HP2O73- serving to break up essentially non-fluorescent excited state dimers of 12+ through formation of a
highly fluorescent anion-bound monomeric complex, 12+•HP2O73-. A turn on in the fluorescence intensity is also seen for
-
-
-
-
-
H2PO4 and, to a lesser extent, HCO3 . Little (HSO4 , NO3 ) or essentially no (N3 , SCN-, F-, Cl-, Br- and I-) response is
seen for other anions. Solid state structural analysis of single crystals obtained after treating 12+ with HP2O7 in the
3-
presence of water revealed a salt form wherein a H2P2O72- anion sits above the cone-like macrocycle.
Introduction
transfer (ESIPT),13 and aggregation-induced emission
(AIE).14 However, the development of new mechanisms
for sensing remains important. We believe it could con-
tribute to the generalized problem of anion sensing be-
yond the specific problem of oxoanion recognition.15
Here, we report what to the best of our knowledge is a
new approach to anion sensing that involves excimer
disaggregation induced emission (EDIE). Briefly, dis-
aggregation, mediated by anion binding in acetonitrile,
converts a weakly fluorescent species into one with con-
siderable greater emission intensity. The net result is an
easy-to-discern increase ("turn-on") in the overall fluo-
rescence signature. The present approach, summarized
graphically in Scheme 1, could provide a useful comple-
ment to strategies involving the breakup of ground state
aggregates.16 The importance of the latter mechanism
has recently been underscored in the context of sugar
recognition.17 However, with the exception of early work
from our group,18 it has rarely been exploited for anion
sensing.
Considerable effort has been dedicated to the design
and synthesis of receptors for the recognition and sens-
ing of oxoanion species.1 Particularly important oxoan-
ions include pyrophosphate (HP2O73-), hydrophosphate
-
-
-
(H2PO4 ), bicarbonate (HCO3 ), and hydrosulfate (HSO4 ).
These species play important roles in living organisms
and are widely distributed in the environment.2 Thus,
there is considerable interest in the development of sim-
ple systems that will allow for the detection of these and
related anionic species.3 A number of elegant sensors
3- 4
- 5
- 6
-7
for HP2O7
, H2PO4 , HCO3 , and HSO4 have been
reported in recent years. Many of these have relied on
signal transduction mechanisms, wherein a binding
event is translated into a change in an optical or electro-
chemical output.8 Other mechanisms that have been
exploited in the context of developing so-called “turn on”
fluorescent sensors, include photoinduced electron
transfer (PET),9 excimer/exciplex,10 fluorescence reso-
nance energy transfer (FRET),11 intramolecular charge
transfer (ICT),12 excited-state intra-/intermolecular proton
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The dimeric excimer of the present study is produced
bispyridin-2-yl)pyridine-2,6-diamine moiety (blue part of
by dissolving the bis-PF6- salt of a new pyridine imidazo-
lium macrocycle, namely cyclo[1]N2,N6-dimethyl-N2,N6-
bis(6-(1H-imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-
diamine[1]1,4-dimethylbenzene (12+), in acetonitrile at
concentrations > 0.010 mM and subjecting to photo-
illumination (λex = 334 nm or 415 nm). As discussed be-
low, addition of oxoanionic species leads to a turn-on in
12+ in Scheme 2), a subunit that has previously been
demonstrated as being an effective intermolecular hy-
drogen bond acceptor.19-21 A 1,4-bis((1H-imidazolium-1-
yl)methyl)benzene (red part of 12+ shown in Scheme 2)
makes up the lower half. This latter fragment contains
both cationic imidazolium and neutral benzene C-H po-
tential hydrogen bond donors. Macrocycle 12+ was thus
expected to function as an anion receptor under appro-
priate conditions. The two halves of 12+ are bridged by
single bonds. It was anticipated that the resulting flexibil-
ity would allow for conformational motion and permit the
overall structure to undergo the adjustments needed to
optimize guest binding.
3-
-
the fluorescence of 12+, with HP2O7 and H2PO4 anions
(both as their tetrabutylammonium (TBA+) salts) proving
particularly effective. Little interference from other anions
is seen. These results are rationalized in terms of com-
petitive binding (self-association vs. anion recognition)
and are supported by solid state structural analyses.
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Evidence for the conformational flexibility of macrocy-
1
cle 12+ came from H NMR and two-dimensional Nuclear
Ӧverhauser effect spectroscopic (NOESY) studies car-
ried out at 298K in CD3CN-d3 (cf. ESI). Only one set of
1
high-resolution signals is seen in the H NMR spectrum
of 12+. In the NOESY spectrum, corresponding signals
between H(1, 4) and H(6), also H(1, 2) and H(8) are ob-
served. We thus propose that 12+ is subject to dynamic
motion in solution, at least on the NMR time scale.
Further support for the conformational flexibility of 12+
came from single crystal X-ray diffraction analyses. Spe-
cifically, two different sets of diffraction grade single crys-
Scheme 1. Anion recognition modes and schematic
-
tals of 12+•2PF6 were obtained depending on the specific
view of the excimer disaggregation induced emission
-
(EDIE) sensing mechanism proposed for 12+•2PF6 .
crystallization conditions employed (cf. ESI). Different
symmetries, CS- vs. C1-, were seen in the resulting struc-
-
tures, which proved to be of 12+•2PF6 •2CH3CN•0.25H2O
Results and Discussion
Synthesis and Structural Analysis of 12+.
-
and 12+•2PF6 •dioxane, respectively (Figure 1). Two dif-
Macrocycle 12+ was synthesized as shown in Scheme
2. Following a procedure reported in the literature,19-20
N2,N6-bis(6-bromopyridin-2-yl)-N2,N6-dimethylpyridine-
2,6- diamine (2) was obtained in a total yield of 72%. An
Ullmann coupling was used to generate N2,N6-bis(6-1H-
imidazol)-N2,N6-dimethylpyridine-2,6-diamine (3) in 94%
yield.21 Reaction of 3 with 1,4-bis(bromomethyl)benzene
gave 12+•2Br-. Following anion exchange via treatment
with NH4PF6 in water, the target system 12+ (as its bis-
ferent bowl-like cavities were also seen in the two struc-
tures. However, in both cases, the pyridine nitrogen at-
oms and the two imidazolium C-H bonds on the (6-(1H-
imidazolium-1-yl)pyridine-2-yl moieties point to the bot-
tom of the bowl (cf. ESI). The aromatic ring on the op-
posing N2,N6-dimethylpyridine-2,6-diamine and 1,4-
dimethylbenzene fragments are also oriented toward the
bottom of the bowl (cf. ESI). The net result is a set of
structural features, including the orientation of several
putative C-H hydrogen bond donors, that were expected
to make 12+ effective as an anion receptor.
-
PF6 salt) was obtained in up to 70% yield.
-
Scheme 2. Synthesis of macrocycle 12+•2PF6 .
Macrocycle 12+ is comprised of two “halves”, both of
which were expected to impart anion recognition capabil-
ity. The top “half” consists of an N2,N6-dimethyl-N2,N6-
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Figure 1. Top (ellipsoid representation) and side (stick
representation) views of 12+ seen in the single crystal
-
-
structures of 12+•2PF6 •2CH3CN•0.25H2O and 12+•2PF6
•dioxane shown in (a) & (c), and (b) & (d), respectively.
6
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Pyrophosphate anion binding properties of 12+.
Based on considerations of size and geometry,
3-
HP2O7 (TBA+ salt) was chosen as a first oxoanionic
guest with which to test the anion binding properties of
1
12+. UV-vis and H NMR spectroscopic methods, as well
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as isothermal titration calorimetry (ITC), were then used
to probe the interactions in acetonitrile (or acetonitrile-
d3). A Job plot analysis based on the UV-vis spectro-
scopic changes observed as a function of concentration
([H] + [G] = 0.050 mM) revealed a peak value at a mole
fraction of 0.5. Clean isosbestic behavior was also seen.
On this basis, we propose that a 1:1 (host/guest) stoichi-
ometry best describes the binding interaction between
12+ and HP2O73- (cf. ESI).
Figure 2. Concentration dependent UV-vis spectra of
-
12+•2PF6 recorded at 1.00 × 10-3 mM (black line), 0.010
mM (red line), 0.020 mM (blue line), 0.040 mM (green
line) and 0.080 mM (pink line) in acetonitrile. Inset shows
the normalized spectral intensity at 319 nm (“■”) record-
ed over the 1.00 × 10-3 mM and 2.56 mM) concentration
range. The red line corresponds to an associated linear
fit (created via the linear expression Abs. (319nm) = (13.9 ±
0.2) × [12+] with adj. R-square as 0.997) assuming Beer-
Lambert behavior.
Further support for the proposal that receptor 12+ is
3-
able to interact with HP2O7 came from a mass spectro-
metric study. In particular, a peak with an m/z of
702.1749 was observed under conditions of electrospray
ionization high-resolution mass spectrometry (ESI-
HRMS) ionization from an initial aqueous solution con-
When irradiated at a λex = 334 nm, 0.020 mM solutions
-
of 12+•2PF6 in CH3CN proved essentially non-emissive.
3-
taining 12+ and HP2O7 (vide infra). Such a finding is in
However, the fluorescence emission intensity was found
to increase monotonically as a function of increasing
HP2O73- concentration (TBA+ salt; from 0 to 0.021 mM) at
298 K (cf. Figure 3). An effort was then made to quantify
the presumed interactions. Toward this end, the inte-
grated fluorescence intensity between 395 nm and 650
accord with what would be expected for a 1:1 complex
3-
formed between 12+ and HP2O7
(calculated for
3- -
C31H30N9O7P2; [12+•HP2O7
] = 702.1749]) in the gas
phase (cf. ESI). Diagnostic shifts in the H NMR spec-
1
3-
trum were also seen when HP2O7 was added into an
acetonitrile-d3 solution of 12+•2PF6 (cf. ESI).
-
3-
The charged nature of 12+ led us to consider that, in
analogy to what proved true for protonated sapphyrin18
and europium-adjusted carbon dots,22 this flexible mac-
rocyclic receptor might prove effective as a disaggrega-
tion-based sensor for the four test oxoanions for which
spectroscopic evidence of binding was obtained (vide
infra). As a predicate to these studies, we sought to ex-
plore whether 12+ was appreciable aggregated in ace-
tonitrile solution. Initial tests involved Beer-Lambert plots.
Over a wide range of concentrations, no appreciable
differences in the extinction coefficients were seen (Fig-
ure 2 and S10-11). Likewise, no charge transfer (CT)
band corresponding to ground state aggregation was
seen at higher concentrations. On this basis, we con-
clude that the ground state form of 12+ is not appreciably
aggregated. It was thus not expected to function as a
fluorescent turn on sensor based on the simple breakup
of ground state aggregates.
nm data was plotted as a function of HP2O7 concentra-
-
tion with [12+•2PF6 ] held constant at 0.020 mM (Figure 3
inset; “●”).
Importantly, the fluorescent features of any given set of
3-
solutions produced by adding HP2O7 to acetonitrile so-
lutions of 12+•2PF6- proved highly reproducible. Moreover,
no appreciable changes in the emission values were
observed even after subjecting the solutions to 30 inde-
pendent analyses (i.e., repeated scans under conditions
of fluorescence analysis; cf. Figure S33 in the Support-
ing Information).
Figure 3. Fluorescent emission spectra of a solution
-
containing 12+•2PF6 (0.020 mM) in CH3CN as a function
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of increasing HP2O73- concentration (TBA+ salt; from 0 to
to the maximum emission intensity at 415 nm of the
0.026 mM) at 298 K. λex = 334 nm. Voltage = 400 V, en-
trance slit width = 5 nm, exit slit width = 10 nm. Inset
shows the corresponding change in the integrated fluo-
rescence intensity between 390 nm and 650 nm (“●”).
monomeric form, was used. For a well-behaved system,
the intensity was expected to increase linearly with con-
centration. This was not seen (Figure 5). In fact, the
maximum fluorescence intensity for the monomer (inte-
grated over the 390 - 650 nm spectral region) was seen
-
-
Since 12+•2PF6 on its own displays little in the way of
when the concentration of 12+•2PF6 was 0.020 mM. The
fluorescence, at least as a 0.020 mM solution in CH3CN,
and the pyrophosphate salt itself is likewise non-
emissive, the above quantitative analysis leaves unan-
swered the specific origin of the increase in fluorescence
fluorescence intensity then decreases as the concentra-
-
tion of 12+•2PF6 is raised. Meanwhile, a new shoulder
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appears at around 500 nm when the concentration of
12+•2PF6 is higher than 0.030 mM, a finding consistent
with the formation of an excimer (cf. ESI).
3-
emission intensity observed as HP2O7 is added to
-
0.020 mM solutions of 12+•2PF6 in CH3CN. As a first
The above findings lead us to suggest that individual
units of 12+ dimerize to form excimers, which might pos-
sibly self-associate further to form higher order aggre-
gates. Normalization of the data across the full concen-
tration range reveals that it is the monomeric form that is
most fluorescent (highest quantum yield) on a per mole
basis. Therefore, it was tentatively concluded that addi-
tion of pyrophosphate serves to break up the aggregated
forms of 12+, particularly the dimeric excimer that is ex-
pected to dominate over most of the 2.50 × 10-4 mM to
0.100 mM concentration range associated with this study.
Efforts were thus made to study this proposed phenom-
enon in detail.
step in an effort to understand the underlying determi-
nants, the packing diagrams of the structures of
-
[12+•2PF6 •2CH3CN•0.25H2O] and [12+•2PF-•dioxane]
discussed above were analyzed. This analysis revealed
interactions between neighboring macrocycles (Figure
4a-d). Single crystals of the pyrophosphate salt were
then grown. The resulting structure [12+•H2P2O72-•4H2O]
revealed a sitting-top anion complex and reduced inter-
actions between the individual macrocycles (Figure 4e
and ESI). Although not a proof, such a finding lends
support to the suggestion that break up of an excimer
contributes to the pyrophosphate anion-induced increase
in fluorescence intensity seen in dilute acetonitrile solu-
tion.
More direct evidence for excimer formation in the case
-
of 12+•2PF6 came from experiments involving excitation
at 334 and 415 nm, respectively. Excitation at the 334
nm led to a monomer-like emission at 415 nm, as well as
weak excimer emission at around 500 nm when the con-
-
centration of 12+•2PF6 is high (larger than 0.030 mM). In
contrast, excitation at 415 nm gave rise to emission at
480 nm over a wide range of concentrations (Figure 6).
-
When the concentration of 12+•2PF6 is higher than 0.32
mM, a linear relationship between the fluorescence in-
-
tensity at 480 nm and the concentration of 12+•2PF6 is
observed.
Additional support for the proposed excimer formation
in the case of 12+ came from excitation spectral studies.
When scans were carried out with monitoring at λem
=
415 nm and 480 nm (cf. Figures S14-S15), the excitation
profile proved similar to that of the UV-vis spectrum of
-
-
12+•2PF6 provided the concentration of 12+•2PF6 was
kept lower than 0.100 mM. Increasing the concentration
-
of 12+•2PF6 from 0.001 mM to 2.56 mM led to a de-
Figure 4. Single crystal X-ray diffraction structures of
[12+•2PF6＿•2CH3CN•0.25H2O] shown in top (a) and front
crease in the intensity of the peaks at 270 nm and 300
-
nm seen in the excitation scans recorded at [12+•2PF6 ] <
(c) views, [12+•2PF6＿•dioxane] shown in top (b) and front
(d) views, and (e) [12+•H2P2O72-•4H2O]. Close contacts
between individual units neighboring macrocycles 12+
units (shown with orange double headed arrows) are
observed in the first two of these three structures.
0.100 mM. These latter peaks were essentially absent at
the highest concentrations. Meanwhile, the longest
wavelength feature at 372 nm was seen to undergo a
red shift to 390 nm as the concentration increased, with
a commensurate increase in spectral intensity being ob-
served (cf. Figure S14-15 in ESI). Taken in concert, the-
se findings are consistent with a dimeric excimer being
stabilized at relatively high concentrations. This species
displays a maximal fluorescence intensity at 480 nm
when photoexcitation is affected at 415 nm (cf. Table 1).
Importantly, it is substantially less emissive than the
monomeric form as noted above.
Monomer-Dimeric Excimer Equilibrium Model
-
The fluorescence emission features of 12+•2PF6 were
then analyzed in detail over a wide range of relatively
low concentrations, namely from 2.50 × 10-4 mM to 0.100
mM in acetonitrile. At the lowest of these concentrations
a non-aggregated form was expected to dominate. Thus,
an excitation wavelength (λex) of 334 nm, corresponding
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To test whether a simple monomer-dimeric excimer
Table 1. Excitation wavelengths (λex) and maximum fluo-
rescence intensity wavelengths of 12+ (λem
model could be used to describe the experimental ob-
servations seen in acetonitrile at relatively low concen-
)
-
trations ([12+•2PF6 ] = [u]0 = 2.5 × 10-4 mM to 0.020 mM),
Species
12+
λex (nm)
λem (nm)
the following elementary reactions were considered in
the context of a global kinetic analysis:
334
415
~500*
480
334
415
(12+)2
*
3-
12+•HP2O7
334
445
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*A shoulder located around 500 nm is observed when
-
the concentration of 12+•2PF6 is larger than 0.030 mM.
Where, u is 12+ in ground state; u* is 12+ in excited state
under the irritation (λex = 334 nm); u2 is the non-
fluorescent
Quantitative analyses of the concentration dependent
fluorescence spectral features were then carried out in
an effort to determine the contributions of the monomer
*
-
and excimer of 12+•2PF6 , respectively, to the overall
emission intensity.
Figure 5. Selected concentration dependent fluorescent emission spectra for 12+•2PF6 are shown in (a) and (b).
Shown in (c) is a plot of the corresponding change in the value of the integrated emission intensity between 390 nm
and 650 nm as the concentration was varied from 2.50 × 10-4 mM to 0.100 mM in acetonitrile (“●”). As discussed in the
text, this emission intensity is ascribed to the monomeric form (12+). lex = 334 nm, voltage = 900 V, entrance slit width =
-
5 nm, exit slit width = 10 nm. Note: The fluorescence spectra of 12+•2PF6 at concentrations of 0.001, 0.010 and 0.020
-
mM were independently remeasured (scanned) 30 times. No appreciable differences were seen upon repeat scanning.
This leads us to suggest that the system is stable under the study conditions.
tion of concentration. The red line was created using the
linear expression If(480nm) = (88.0 ± 1.0) × [12+] + (32.7 ±
0.7) giving an adjusted R-square value of 0.998.
dimeric excimer of 12+; M is media which induced the
relaxation of the species in the excited state; hv1 is the
light intensity at the excitation wavelength of 334 nm
interacting with 12+ and reflects the absorbance (A) value
at that wavelength; hv2 is the integrated emission intensi-
ty over the 390 nm - 650 nm spectral region. Parameters
j1, j2, k1, k2 and k3 are rate constants of the individual (but
inter-related) elementary reactions. In this model, the
effects of media (including solvent, acetonitrile, and
Figure 6. Concentration dependent fluorescence
-
-
emission spectra of 12+•2PF6 in acetonitrile recorded
counter anion, PF6 ) are ignored, as are the potential
from 2.00 × 10-4 mM to 2.56 mM using a λex = 415 nm.
Voltage = 700 V, entrance slit width = 5 nm, exit slit
width = 10 nm. Inset shows the corresponding change
(“■”) in the emission intensity value at 480 nm as a func-
effects of higher order aggregates. Thus, it is assumed
that the sole aggregation product of 12+ (u) is the non-
*
fluorescent dimeric excimer u2 .
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At the limit where the total concentration of 12+ (u0 ) is
zero ([u]0 →0), elementary reactions involving u2 (name-
ly reactions 4 and 5) may be ignored. Thus, the theoreti-
cal quantum yield of the emitting monomeric species 12+
(i.e., ϕu(monomer)) can be calculated using elementary reac-
tions 1-3. Accordingly, only monomeric species were
considered in the following equilibrium expression (eq. 1):
A quasi-steady state assumption was then made re-
sulting in eq. 7:
*
This allowed a relationship between ϕu(monomer) and the
parameters j2 and k1 to be expressed as eq. 8.
9
Here, [u]0 is the total concentration of 12+; [u]monomer is
the concentration of the monomeric forms of 12+ and is
the sum of 12+ in the ground state (i.e., [u]) and in the
excited state (i.e., [u*]).
With the analysis at [u]0 →0 complete, the monomer-
dimeric excimer model was used to treat the experi-
mental data for 0 ≤ [u]0 ≤ 0.020 mM. Towards this end,
eqs. 9-10 were derived based on the elementary reac-
tions (1)-(5) as follows:
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12
13
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17
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With [u]0 →0, the observed emission intensity (Fobs
=
F[u] →0) can be expressed as eq. 2:
0
In eq. 2, A[u] →0 is the absorbance of 12+ at 334 nm, a
0
value that obeys the Beer-Lambert law (vide supra and
eq. 3). As noted above, Fs = 4.87 × 106, As = 0.0104,
and ϕs = 0.556 are, respectively, the detected emission
intensity between 390 and 650 nm, the absorbance val-
ue at 334 nm, and the quantum yield of a reference
standard (i.e., a solution of quinine bisulfate in 0.1 N sul-
furic acid).
As above, a quasi-steady state assumption was made
(eq. 11):
Combining eqs. 9-11, gave eq. 12 as follows:
Combining eqs. 2 and 3, gives the equilibrium expres-
sion governing ϕu(monomer) as eq. 4:
It was further assumed that the relationship between u,
u* and u2* can be treated as a chemical equilibrium:
Meanwhile, eqs. 13-14 were defined (note: here [u]all is
equal to [u]0):
A plot of Fobs /[u]0 vs [u]0 was then used to determine
ϕu(monomer). The linear analysis of Fobs /[u]0 at the low con-
centration (2.50 × 10-4 mM to 6.00 × 10-4 mM) gave the
value of Fobs /[u]0 under [u]0 →0 as the intercept. Then eq.
4 was used to give the value of ϕu(monomer) as eq. 5.
Linear fitting in Figure 7b and eq. 4 thus gave:
*
Since u2 is defined as non-fluorescent in this model
per experimental observation (vide supra), the observed
emission is from umonomer (i.e., ground and excited state
forms of 12+; see definitions above). Accordingly, eq. 15
could be set up as follows:
Accordingly, a value of ϕu(monomer) = 3.42 × 10-4 was
used for the ensuing analyses. The error in the curve
fitting is ca. 10% and was ignored in these latter calcula-
tions.
Eq. 15 was then used to produce eq. 16 as follows:
Finally, the total concentration of monomeric 12+
([u]monomer) could be related to Fobs per eq. 17:
Eq. 17 was used to generate eq. 18:
Figure 7. Plot of Fobs /[u]0 vs [u]0 (a) and expanded
view of the plot for the data recorded at low concentra-
tions showing the linear relationship (b).
Eqs. 11 and 13 were then used to derive eq. 19:
Considering elementary reactions 1-3, the following re-
lationship could be derived as eq. 6:
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Eqs. 9, 10, 13, and. 19 were then considered simulta-
neously to obtain a relation between [u2 ] and [u]monomer in
*
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11
12
13
14
15
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17
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the form of eq. 20:
*
Figure 8. Plot of [u2 ] vs [u]monomer (black points).
Shown in red is a non-linear fitting of the data per eq. 25.
From these parameters and using the relationship em-
bodied in eq. 17, the simulated emission intensity (Fsim
could be calculated. This was then compared with the
experimental Fobs values as shown in Figure 9.
)
The parameters A, B and C were set as follows:
Eq. 31 could then be expressed as eq. 24:
Figure 9. Plot of Fobs and Fsim vs [u]0. Experimental
points are shown in black, whereas the red line is a
simulation made per eq. 17.
Gratifyingly, a good match between the calculated
curve and the experimental points was seen. Therefore,
we believe is that at least up to [12+] = 0.020 mM contri-
butions from possible higher order aggregated species
(e.g., trimers or tetramers) can be largely ignored. There-
fore, on the basis of eqs. 9, 10, 13, 14, and 25, the rela-
*
From the observed emission intensity values (Fobs) of
acetonitrile solutions containing different [u]all, the
[u]monomer term can be calculated from eq. 17. This allows
tionship between [u], [u*], [u2 ] and [u]0 could be plotted in
the form of a speciation diagram (cf. Figure 10 and ESI).
*
*
[u2 ] to be deduced through eq. 18. A plot of [u2 ] vs
[u]monomer could then be fitted in a nonlinear fashion using
eq. 24. Fixed values of the parameters of A = 4.47 × 107
M-1s-1, B = 1.68 s-1, and C = 59.9 s-1 allowed the nonline-
ar fitting to be optimized (cf. Figure 8). This gave a calcu-
lated Kdimer value of (5.48 ± 0.12) × 106 M-1 where the
errors are the curve fitting errors.
From the result of the nonlinear fitting, a series of rela-
tive parameter values j1 = 59.9 s-1, j2 = 1.56 s-1, k1 = 4.55
× 103 s-1, k2 = 4.47 × 107 M-1s-1, and k3 = 0.12 s-1 could be
derived. Likewise, a relationship between [u2 ] and
[u]monomer could be established per eq. 25:
Figure 10. Plot of [u], [u*] and [u2 ] vs [u]0
*
*
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Analysis of the Effect of Pyrophosphate Binding on the
Change in absorbance at 317 nm seen under conditions
-
Fluorescence Features of 12+•2PF6 and Fit to a 1:1
of concentration dependent UV-vis spectral analysis of
-
1:1 (“●”) and 1:10 (“●”) mixtures of 12+•2PF6 and
Model
3TBA+•HP2O73- in acetonitrile where the concentration of
12+ is varied from 1.00 × 10-3 mM to 0.020 mM. The black
line corresponds to a linear fit with an adj. R-square as
0.998 to a linear expression that assumes Beer-Lambert
behavior, namely Abs. (319nm) = (21.0 ± 0.2) × [12+].
The key inference to be drawn from the above fitting
efforts and the underlying experiments is that a dis-
aggregation of the excimer form (or forms) will lead to an
enhancement in the emission intensity. We thus consid-
ered it likely that species, such as oxoanions, that in-
duced such disaggregation could be detected via
9
-
changes in the emission intensity of 12+•2PF6 . Accord-
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Although it was appreciated that the above 1:1 mono-
mer-dimeric excimer equilibrium model would not ac-
count for the full fluorescence spectral response when
ingly, we set out to explore further the nature and the
extent of the pyrophosphate-induced turn on fluorescent
response noted above (cf. Figure 3 and accompanying
discussion).
-
mixtures of 12+•2PF6 were treated with 3TBA+•HP2O73- in
acetonitrile, it was considered a good starting point for a
possible quantitative analysis. The following elementary
reactions were thus considered in the context of an initial
global kinetic analysis of the emission intensities ob-
served at varying [HP2O73-]/[12+] ratios:
Concentration dependent fluorescence studies of 1:1
3-
or 1:10 mixtures containing 12+•2PF6- and 3TBA+•HP2O7
were carried out over the concentration range included
in the analysis of the anion-free receptor solutions,
-
namely [12+•2PF6 ] as 1.00 × 10-3 mM to 0.020 mM (λex
=
334 nm in acetonitrile for emission). In the case of 1:10
macrocycle to pyrophosphate ratio, the emission intensi-
-
ty shows good linear relationship with [12+•2PF6 ]. This
was also true for the 1:1 mixture with the same emission
-
-
intensity at every [12+•2PF6 ] provided that the [12+•2PF6 ]
concentration was ≥0.010 mM.
The deviation seen for the 1:1 macrocycle to pyro-
-
phosphate samples at lower 12+•2PF6 concentrations
could reflect a lack of full complexation, which would be
expected to be essentially complete under the other
conditions associated with this set of experiments (Fig-
ure 11a and ESI). Supporting concentration dependent
*
where, u, u*, u2 , M, hv1, hv2 are defined as before, PPi
is HP2O73-, u•PPi is the thermostable 1:1 complex be-
tween 12+ and PPi in the ground state, (u•PPi) * is the
excited state of u•PPi under conditions of irradiation (λex
= 334 nm), and parameters j1, j2, k1, k2, k3, k4, k5, j3, j4,
and k6 are rate constants corresponding to the individual
(but inter-related) elementary reactions. Here, the effects
of media (including solvent, acetonitrile, and counter
-
UV-vis spectral studies of these 1:1 or 1:10 ([12+•2PF6
]/[3TBA+•HP2O73-]) mixtures revealed a good fit to the
Beer-Lambert relationship (Figure 11b and ESI). These
findings are most easily interpreted in terms of the for-
mation of a 1:1 thermostable complex, 12+•HP2O73-, over
a limited concentration regime (i.e., ≥0.010 [12+•2PF6 ]
-
≤0.020 mM) as long as the pyrophosphate anion is pre-
sent at least 1 molar equivalent.
-
anion, PF6 ) are ignored, as are the potential contribu-
tions from higher order aggregates. It is thus assumed
that the sole aggregation product of 12+ (u) is the non-
*
fluorescent dimeric excimer u2 ; PPi is also considered
as non-emissive.
The 1:10 (“●”) dataset corresponding to the plot of Fobs
vs [u]0 in Figure 11a was used to determine ϕu•PPi. The
value of ϕu•PPi could then be expressed as eq. M1:
The concentration dependent UV-vis spectra of 1:10
Figure 11. (a) The corresponding change between
390 nm and 650 nm in the emission intensity of concen-
tration dependent fluorescent emission spectra of 1:1
-
(“●”) mixtures containing 12+•2PF6 and 3TBA+•HP2O73- in
acetonitrile were used to determine the molar absorption
coefficient of u•PPi (εu•PPi) (cf. ESI). The value of εu•PPi at
334 nm could then be expressed as eq. M2:
-
(“●”) and 1:10 (“●”) mixtures of 12+•2PF6 and
3TBA+•HP2O73- in acetonitrile where the concentration of
12+ is varied from 1.00 × 10-3 mM to 0.020 mM. The exci-
tation wavelength (λex ) was 334 nm. The black line cor-
responds to an associated linear fit (created via the line-
Considering elementary reactions M1-M10 and assum-
ing quasi-steady state kinetics, the following expression
were deduced:
ar expression Fobs
.
= (1.3 ± 0.1) × 109 [12+] with
(390-650nm)
an adj. R-square value of 0.999) between the emission
intensity and the concentration of 12+. Voltage = 400 V,
entrance slit width = 5 nm, exit slit width = 10 nm. (b)
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Combining with eq. M13 gives eq. M17:
3
4
5
6
7
Eq. M17 implies that the relationship between u, PPi,
and (u•PPi)all may also be considered as a chemical
equilibrium:
8
9
It is noted that K*u•PPi is larger than Ku•PPi
.
10
11
12
13
14
15
16
More expressions that relate to experiments are:
Thus:
*
The relationship between [u], [u*], [u]monomer, [u2 ] can be
In this model, the fluorescent response only come from
the emission of u(monomer) and [u•PPi]all:
expressed in terms of [u]monomer
:
17
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19
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From concentration dependent fluorescence and UV-
vis spectral studies of 1:10 ([u]all/[PPi]all) mixtures (Figure
11 and ESI), eq. M22 was obtained;
The previously described monomer-dimeric excimer
equilibrium model without PPi gives eq. M23:
The combination of eq. M10 and eqs. M20-M23 gives
eq. M24:
Eq. M11 may then be derived from eq. M5 and eq. M7:
Combining eqs. M5-M7 gives eq. M12:
Eq. M12 may be rewritten as eq. M13:
Eq. M13 implies that the relationship between u, PPi,
and u •PPi can be expressed as a chemical equilibrium:
Here, Ku•PPi can be considered as the binding constant
corresponding to the interaction between u and PPi.
From mass balance and eq. M11, eqs. M14-M15 may
be defined:
With [u]all and [PPi]all known, [u]monomer can be solved
using the fluorescence spectral data (eq. M24). Then,
*
[u2 ] can be calculated from [u]monomer and eq. M10, while
[u•PPi]all may be further calculated via eq. M20. Thus, the
value of [PPi] may be obtained using eq. M25:
Eq. M14 can be expressed as eq. M16:
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Finally, K*u•PPi could be calculated using eq. M26.
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7
8
9
The titration data from Figure 3 were then used as a
check of eq. M26. If the model is valid, the values of
K*u•PPi should remain constant during the full course of
the titration. However, it was found that the calculated
values of K*u•PPi based on this model were not constant.
Rather, they a peak value (2.18 × 108) was observed
when [PPi]all/[u]all = 1.
*
where, u, u*, u2 , M, hv1, hv2, hv3, PPi, u•PPi and
(u•PPi)* are defined as before. u2•PPi is the thermostable
*
*
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1:1 complex between u2 and PPi. u2 , PPi are consid-
ered as non-emission, as is u2•PPi. Parameters j1, j2, k1,
k2, k3, k4, k5, j3, j4, k6, k7 and k8 are rate constants for the
individual (but inter-related) elementary reactions. As in
the case of the initial simplified 1:1 model above, the
effects of media (M) are ignored, as are the potential
effects of higher order aggregates.
Considering elementary reactions D1-D12 with a qua-
si-steady state assumption, the following expression may
be deduced:
It is possible that when [PPi]all / [u]all is larger than 1,
aggregation of PPi causes a decrease in [u•PPi]all, which
is reflected in a smaller K*u•PPi. Conversely, when the
[PPi]all / [u]all ratio is less than 1, PPi could have a direct
effect on the dimeric excimer. When [PPi]all / [u]all is
around 1 the effect of these competing influences is min-
imized. To the extent such rationales are correct, the
peak value (2.18 × 108) may approximate the true K*u•PPi
value. For ease, this is summarized in the form of eq.
M27:
Figure 12. Values of K*u•PPi calculated using a simple
1:1 complexation analysis. A peak value of 2.18 × 108 M-
1 was obtained when [PPi]all / [u]all = 1.
Mixed 2:1 and 1:1 (PPi / u) Complexation Analysis of
Pyrophosphate Binding and its Effect on the Fluores-
-
cence Features of 12+•2PF6
Although the above analyses highlight the fact that a
"regime of reliability" can likely be found where a simple
1:1 complexation model can be made to fit the data, it is
clear that such a treatment cannot account well for the
fluorescence response seen over the full course of the
titration process involving treating acetonitrile solutions
-
of 12+•2PF6 with 3TBA+•HP2O73-. Thus, a mixed 2:1 and
*
Here, the relationship between [u], [u*], [u]monomer, [u2 ]
1:1 (PPi / u) model was employed. For this global analy-
is defined via eqs. D7-D11:
sis, additional elemental reactions representing the puta-
*
tive interactions between u2 and PPi were considered:
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Eq. D11 may be rewritten as eq. D12:
*
Eq. D12 implies that the relationship between u2 , PPi,
and u2•PPi can be treated as a chemical equilibrium:
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More expressions related to the experiments include:
In this model, the fluorescent response comes only
from the emission contributions of [u]monomer and [u•PPi]all:
*
If we do not consider the dissociation of PPi from u2 ,
[u2 ]’ can be expressed as eq. D28:
Where, Ф1 and Ф2 are defined as per eqs. M22-M23.
Assuming that PPi is completely bound to u during the
titration process, the maximum fluorescence response
may be expressed per eq. D16:
*
The following inequality expression could then be used
to judge the reasonableness of the resulting calculated
values:
Finally, Ku •PPi and k7 was calculated via eq. D30-D31:
2
The minimum fluorescence response when no binding
between PPi and u occurs is expressed by eq. D17:
Where, [u]monomer,0 is defined as the concentration of
[u]monomer before adding PPi. The range of fluorescence
intensity for [u•PPi]all is thus:
For each titration point, the values of [u]all and [PPi]all
are known. Therefore, Fmax and Fmin can be calculated
using eqs. D16 and D17, respectively. Meaningful values
of [u•PPi]all were then obtained using the limitations set
by eq. D21. The full range of [u•PPi]all values was trun-
cated into 400 equal regimes, and [u•PPi]all was consid-
ered for each subset. For each, the relative concentra-
tion of the constituent species was calculated according
to eqs. D22-D27. Eq. D29 was then used as criterion to
evaluate which of the resulting calculated concentration
values were meaningful. Using the resulting subset of
calculated concentrations, the corresponding Ku •PPi and
k7 values were obtained from eqs. D30 and D31. At each
titration point, a range of Ku •PPi (the black line shown in
Figure 13a) and k7 values (the black line shown in Figure
13b) were calculated using the meaningful set of
[u•PPi]all values and corresponding concentration values
(cf. Figure 13 and ESI).
This gives eqs. D18-D19 for the lower bound of [u•PPi]all
and the upper bound of [u•PPi]all as:
with a range for [u•PPi]all of:
2
For mixtures of [u]all and [PPi]all we can calculate the
relative concentration of the constituent species with
definite value of [u•PPi]all according to the following
equation:
2
The above analysis is predicated on the assumption
that the values of Ku2•PPi and k7 are constant and do not
only during a given titration, or even for titrations carried
out using different initial 12+•2PF6 concentrations. This
-
implies that the calculated values (or range of meaning-
ful values) for Ku •PPi and k7 should be the same or nearly
2
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the same at all or, at least, most titration points in any
In order to test the validity of the above analysis, a se-
given titration. Per this caveat, a set of Ku •PPi and k7 val-
ries of concentration dependent fluorescent spectral titra-
tions were carried out. Calculated K*u•PPi = 2.18 × 108 M-1,
Ku2•PPi = 4.08 × 106 M-1, k7= 4.91 × 108 s-1 values were
used for simulated emission intensity calculation. For
every given concentration of [u]all and [PPi]all, solving eqs.
D34-D38 and eqs. D13-D14 provided simulated concen-
tration values corresponding to a given specific titration
datum point:
2
ues that provides the best fit for the entire process was
selected. These values fall between the blue line (lower
bound) and the red line (upper bound) shown in Figures
13a and 13b, respectively. It should be noted that results
from two limiting cases, namely the titration points with
[PPi]all / [u]all → 0 and [PPi]all / [u]all → 1, were not taken
into consideration. The concentration values of the spe-
cies containing PPi (i.e., [PPi], [u•PPi]all and [u2•PPi]) are
small such that the calculation of Ku2•PPi and k7 is unrelia-
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ble when [PPi]all / [u]all → 0. In the case of [PPi]all/[u]all
1, the small values of [u2•PPi] likewise give rise to values
for Ku2•PPi and k7 that are unreliable.
→
Using the calculated values of [u•PPi]all and [u]monomer, a
series of emission intensity values that match the titra-
tion process could be found via eq. D15; these were
then compared with the measured values. Fluorescent
-
emission spectra of a solution containing 12+•2PF6
3-
(0.020 mM) in CH3CN treated with increasing HP2O7
concentrations (TBA+ salt; from 0 to 0.026 mM) as per
Figure 3, as well as titrations at [12+•2PF6 ] = 0.005 mM,
-
were used to verify the model and are shown in Figure
-
14. Data from a number of other titrations ([12+•2PF6 ] ≤
0.020 mM) were also used to verify the model (cf. ESI).
All of the experimental titration data sets proved con-
sistent with the simulated values (the adjusted R-square
values were found to lie between 0.999 and 0.997). On
this basis, we suggest that the mixed 2:1 and 1:1 (PPi / u)
binding model proposed above provides a reasonable
basis for quantifying the EDIE effects seen when macro-
cycle 12+ is treated with the pyrophosphate anion under
conditions of photoexcitation.
Figure 13. Range of Ku •PPi (a) and k7 values (b) calcu-
2
lated from a mixed 2:1 and 1:1 (PPi / u) complexation
analysis. On this basis, log Ku •PPi values between 6.58–
2
6.64, combined with log k7 values between 8.66-8.72,
are considered as providing the best overall match to the
data. The black lines show the calculated range of Ku •PPi
2
(a) and k7 (b) values corresponding to each titration point.
The values between the blue (lower bound) and red (up-
per bound) lines show the meaningful value ranges for
Ku •PPi (a) and k7 (b) derived by fitting the titration data to
2
a mixed 2:1 and 1:1 (PPi / u) model as detailed more
fully in the text.
On this basis, the following Ku2•PPi and k7 values were
selected:
Figure 14. Experimental data obtained from EDIE ex-
periments carried out at (a) 0.020 mM and (b) 0.005 mM
in 12+ are given as black points while the simulated val-
ues calculated per eq. D15 are shown as red crosses.
Other simulated Fobs values were also calculated; they
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are tabulated in the Supporting Information. The adjust-
ed R-square values were 0.999 for (a) and 0.998 for (b).
4
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EDIE-based Anion Sensing and Underlying Assump-
tions
To explore the selectivity of 12+ as a turn on fluores-
cence probe for other anions, analogous fluorescence
-
titrations of 12+•2PF6 (0.020 mM) in acetonitrile (λex
=
334 nm) were carried out with three other oxoanions
-
(H2PO4- and HSO4- as their respective TBA+ salts; HCO3
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as its TEA+ salt). A strong response (fluorescence turn-
on) was seen in the case of H2PO4 (cf. Figure S30). A
-
less effective, but still notable, response was seen in the
-
case of HCO3 (cf. Figure S31). In contrast to what was
seen in the case of HP2O73-, where a 1:1 complex domi-
nates in the presence of excess guest, curve fits to the
titration results were consistent with a 1:2 (H:G) binding
Figure 15. Anion induced immediate fluorescence re-
-
sponse seen immediately after mixing 12+•2PF6 (0.020
-
stoichiometry in the case of H2PO42- and HCO3 . Support
mM) in CH3CN (λex = 334 nm) with the indicated anion.
-
Note: The use of 100 molar equiv of HCO3 (2.00 mM)
for this suggestion came from mass spectrometric stud-
ies (cf. Table S3), as well as from solid state structural
analyses (vide infra).
was found to induce precipitation. Therefore, only 20
-
equiv were used in these studies. Except for HCO3
(used as its tetraethylammonium salt, TEA+), all anions
were studied as their respective TBA+ salts.
Little, if any, increase in the fluorescence intensity was
seen when analogous titrations were carried out using a
From the UV-vis titrations with [12+] = 0.005 mM (for
HP2O73-) and 0.020 mM (for the other tested anions), Ka1
values corresponding to the formation of 1:1 complex
could be derived for several of the oxoanions and were
-
-
variety of other anions (i.e., HSO4 SCN-, N3 ,F-, Cl-, Br-,
and I-; all as their respective TBA+ salts). In the case of
TBANO3, little immediate response was seen upon
treatment with 100 molar equiv. However, when an initial
-
-
found to follow the order HP2O73- > H2PO4 ≫ HCO3
>
-
0.020 mM acetonitrile solution of 12+•2PF6 was treated
HSO4 . In the case of NO3 , SCN-, N3 , F-, Cl-, Br-, and I-
the binding thermodynamics proved too small to quantify.
For the 1:2 (receptor:anion) complexes, the Ka2 order
-
-
-
-
with a large excess of NO3 (100 mM), the fluorescence
intensity was seen to increase over the course of roughly
90 minutes (Figure S49). This finding is ascribed to the
nitrate anion being a weak competitive inhibitor that is
able to disaggregate the excimer of 12+. That this pro-
cess occurs slowly could reflect what are presumably
poor binding thermodynamics. In fact, the Ka corre-
sponding to the interaction between 12+ and NO3- proved
too small to measure reliably (see discussion below).
A series of competition studies were then undertaken
-
-
-
was found to be H2PO4 ≫ HCO3 > HSO4 (see Table 2
and the ESI).
Table 2. Interactions between 12+ and anionic species in
acetonitrile at 298Ka, f
Ka
Ka
Ka
-
with [12+•2PF6 ] = 0.020 mM in CH3CN and λex = 334 nm
Guest
[H]/[G]
(Fluores-
cence)
(UV-vis)
(ITC)a
(cf. Figure 15 and the ESI). These revealed little effect
on the maximal fluorescence intensity when titrations
3-
-
with HP2O7 or H2PO4 were carried out in in the pres-
---
---
(4.1 ± 0.2) ×
106 M-1
-
ence of a large excess of SCN-, N3 , F-, Cl-, Br-, I-, or
2:1
1:1
3-
-
NO3 ; again, 12+ was found to function as a good "turn-
HP2O7
(4.7 ± 0.1) ×
106 M-1
(1.3 ± 0.1)
× 106 M-1
(2.2 ± 0.2) ×
108 M-1
3-
-
on" fluorescence sensor for HP2O7 (and H2PO4 ), the
presence of these potential interferants notwithstanding.
3-
However, it is important to note that the HP2O7 and
(2.3 ± 0.1) ×
106 M-1
(2.0 ± 0.2)
× 105 M-1
(2.0 ± 0.2)
× 1010 M-2
1:1b
1:2 c
-
---g
H2PO4 were cross-competitive and acted as interferants
-
H2PO4
(4.7 ± 0.1) ×
--- g
for one another under these standard titration conditions
(cf. ESI).
1013 M-2 e
(1.2 ± 0.1) ×
103 M-1
1:1 b
1:2 c
--- g
--- g
-
HCO3
---
---
(9.9 ± 0.1) ×
106 M-2
-
HSO4
1:2
---
---
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parameters, is stabilized by O-H hydrogen bonds be-
tween neighbor anions. Charged and neutral C-H hydro-
Other
tested
anionsd
---
---
---
---
gen bonds between the imidazolium and benzene ring(s)
of 12+, respectively, and O atoms present in the anion
cluster, as well as anion-π interactions, may also play a
role in stabilizing the overall structure. A key point is that
in this complex, as true for the pyrophosphate anion
complex, little evidence of close receptor-receptor con-
tact is seen. The contrast with the anion-free forms (cf.
Figures 4a-d) is noteworthy and provides support for the
suggestion that the solution phase binding of HP2O73- will
serve to break up the excimer forms of 12+.
aITC titrations were carried out under higher concentra-
tions, and yielded smaller Ka2 values compared with the
results from UV-vis and fluorescence titrations. This is
ascribed to the effect of aggregation.
b,c
Equations governing the relevant equilibria:
9
b
c
;
.
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dOther tested anions include NO3 , SCN-, N3 , F-, Cl-,
-
-
Br-, I-, all as their TBA+ salts.
eIt is noteworthy that a higher value for Ka.2/Ka.1 larger
-
-
than Ka.1 is seen in the case of H2PO4 and HCO3 under
standard titration conditions ([12+] = 0.020 mM) as de-
termined from both UV-vis spectroscopic titrations. This
finding provides additional support for the proposed 1:2
binding mode, which may benefit from positive homo-
tropic allostery.24
f All errors are fitting errors.
g Fluorescence spectroscopic titrations of H2PO4 and
-
-
HCO3 appear to involve more complicated processes.
Currently, we have unable to calculate Ka.1 and Ka.2 val-
ues reliably from the emission data.
-
Figure 16. Single crystal structure of 12+•2H2PO4
•7.5H2O showing (a) the interactions between 12+ and the
bound H2PO4- anions (b) a side view shown in stick form
showing the anion cluster that serves to separate indi-
vidual units of 12+..
The Ka values calculated on the basis of UV-vis spec-
troscopic titrations assuming a 1:1 (H/G) binding model
for HP2O73- and mixed 1:1 and 1:2 model for H2PO4- and
-
3-
-
HCO3 (cf. ESI). In the case of HP2O7 and H2PO4 it
proved possible to obtain concordant Ka (for HP2O73-) or
-
Ka1 (for H2PO4 ) values via ITC analyses even though
higher concentrations were used and the effects of high-
er order aggregation could not be discounted. The val-
ues from these independent studies are included in Ta-
ble 2. It is important to note that these measurements
are probing ground state interactions not EDIE per se.
Interactions between 12+ and Anionic Guests in the
Solid State
To obtain further insights into the presumed interac-
tions between 12+ and various anions, efforts were made
to obtain a series of single crystals suitable for X-ray
diffraction analysis. Towards this end, solutions of anion
exchange products in a mixture of water/acetonitrile (1:1,
v/v), water/(N,N-dimethylformamide (DMF; 1:1, v/v), or
water/dioxane (1:1, v/v) were subject to slow evaporation.
This allowed crystals structures of [12+•H2P2O72-•4H2O],
[12+•2H2PO4 •7.5H2O], [12+•2HCO3 •dioxane•2H2O], and
-
-
[12+•2HSO4 •1.5CH3CN] to be solved (see below and the
-
ESI). The structure of the pyrophosphate complex was
presented previously and taken as initial evidence that
-
Figure 17. (a) Single crystal structure of 12+•2HCO3
•dioxane•2H2O showing the binding interactions involv-
treatment of 12+ with H2P2O7 would induce disaggrega-
2-
-
-
ing 12+ and the HCO3 anions; (b) top view of 12+•2HCO3
tion of excimer (cf. Figure 4e and accompanying discus-
sion). The other anion-containing structures are dis-
cussed below.
•dioxane•2H2O shown in stick form; (c) single crystal
-
structure of 12+•2HSO4 •1.5CH3CN showing the interac-
-
tions involving 12+ and the HSO4 anions; (d) a top view
The solid state structure of 12+•2H2PO4 •7.5H2O re-
-
-
of 12+•2HSO4 •1.5CH3CN shown in stick form.
vealed a 1:2 receptor: anion stoichiometry consistent
with what was inferred from the solution state studies
discussed above (Figure 16). The phosphate anions
exist in the form of a cluster that, based on the metric
-
The structure of the [12+•2HCO3 •dioxane•2H2O] com-
plex revealed a 1:2 receptor: anion stoichiometry, a find-
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ing corresponding to the result of a solution phase Job
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AUTHOR INFORMATION
-
plot (cf. ESI). In the solid state, two HCO3 molecular
anions interact to form a dimeric structure (HCO3 )2. This
subunit is stabilized via two hydrogen bonds involving
-
Corresponding Authors
sessler@cm.utexas.edu
hanyuangong@bnu.edu.cn
-
-
neighboring HCO3 units. One of the HCO3 anions is
complexed directly with 12+ and is held via C-H interac-
tions involving 12+ and an O atom of the bound HCO3
-
ORCIDS
anion, even though this substrate fills only part of the
core (cf. Figures 17a-b; ESI). Interestingly, examples of
Jonathan L. Sessler: 0000-0002-9576-1325
Han-Yuan Gong: 0000-0003-4168-7657
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structurally characterized imidazolium-based HCO3
complexes are limited.25 Thus, this structure may con-
tribute to our understanding of the design features need-
ed to recognize this all-important anion
Notes
The authors declare no competing financial interests.
The structure of 12+•2HSO4 •1.5CH3CN was also
solved. It revealed binding modes similar to those seen
in the case of the HCO3 complex; again, one of two
-
-
ACKNOWLEDGMENT
-
-
HSO4 anions present in a dimeric anion pair (i.e., (HSO4
)2) also interacts with 12+ via what are presumed to be
strong hydrogen bonding interactions (cf. Figures 17c-d).
As above, the presence of the hydrogen bonded anions
may reflect a reduced propensity to stabilize a dimer
form capable of forming an excimer under solution phase
conditions.
H.-Y.G. is grateful to the National Natural Science Foun-
dation of China (21472014 and 21672025), National Basic
Research Program of China (973 Program 2015CB856502),
the Young One-Thousand-Talents Scheme, the Fundamen-
tal Research Funds for the Central Universities, the Beijing
Municipal Commission of Education, the Beijing National
Laboratory for Molecular Science (BNLMS) and Beijing
Normal University for financial support. Support from
Shanghai University and the Robert A. Welch Foundation
(F-0018) is also gratefully acknowledged.
Conclusion
In summary, we describe here the use of an excimer
disaggregation induced emission (EDIE) strategy for the
"turn-on" solution phase fluorescence-based detection of
the HP2O73- and H2PO4- oxoanions. The system relies on
the use of a pyridine imidazolium-based anion receptor
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-
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excimer in acetonitrile solution. The HP2O73-, H2PO4 ,
-
-
and to a lesser extent HCO3 , anions are effective at
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ASSOCIATED CONTENT
Supporting Information
Supporting information is provided that includes experi-
mental details, UV-vis and NMR spectroscopic analyses,
ESI-MS results, data fitting, and single crystal X-ray diffrac-
tion studies. This material is available free of charge via the
Internet at http://pubs.acs.org.
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