Multifunctional Imidazobenzothiadiazole Probe Displaying Solvatofluorochromism and Ability To Form Ion-Pair Complexes in Solid State and in Solution

Article abstract of DOI:10.1021/acs.orglett.5b00895

Fluorescent solid 5-pyridylimidazobenzothiadiazole displays a remarkable solvatofluorochromism and with Zn(AcO)₂ and Cd(AcO)₂, either in solution or under solvent-free conditions, forms ion-pair complexes that in the solid state can be discriminated and separated by fluorescence measurements and selective extraction with diethyl ether or chloroform. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.orglett.5b00895

Letter
pubs.acs.org/OrgLett
Multifunctional Imidazobenzothiadiazole Probe Displaying
Solvatofluorochromism and Ability To Form Ion-Pair Complexes in
Solid State and in Solution
†
‡
,†
,†
María Alfonso, Israel Fernan
́
dez, Alberto Tar
́
raga,* and Pedro Molina*
†
Departamento de Química Organ
́
ica, Facultad de Química Universidad de Murcia, E-30100 Murcia, Spain
‡
Departamento de Química Organ
́
ica I, Facultad de Ciencias Químicas, Universidad Complutense, E-28040 Madrid, Spain
*
S Supporting Information
ABSTRACT: Fluorescent solid 5-pyridylimidazobenzothia-
diazole displays a remarkable solvatofluorochromism and
with Zn(AcO) and Cd(AcO) , either in solution or under
2
2
solvent-free conditions, forms ion-pair complexes that in the
solid state can be discriminated and separated by fluorescence
measurements and selective extraction with diethyl ether or
chloroform.
he development of synthetic strategies for the preparation
of receptors able to bind concurrently a cation and an
the transport of the essential Zn2+ into and out of cells. Thus,
2+/
2+
T
molecular design strategies toward strict Zn Cd discrim-
6
anion, i.e., ion-pair recognition receptors, as well as the isolation
of the formed complexes are current research topics in the area of
ination are of significant interest. Strikingly, receptor 1 is able to
2+
2+
discriminate Cd from Zn not only in solution but also in the
solid state.
1
supramolecular chemistry. In this context, it is worth noting that
extraction and solubilization of salts have been developed due to
the investigation of ion-pair recognition.
The synthesis of the target receptor 1 was accomplished with7
19% yield from reaction of 4,5-diamino-2,1,3-benzothiadiazole
and pyridine-2-carboxaldehyde as starting materials in nitro-
benzene (Figure 1). Then the ion-pair recognition properties of
receptor 1 were analyzed in solution and solid state by means of
H NMR experiments and absorption and emission spectros-
copies.
Despite the interesting photophysical properties of benzo-
thiadiazole derivatives, both in solid state and in solution, this
structural motif has not been incorporated so far into the
framework of previously reported ion-pair receptors as either a
1
2
binding site or fluorescent signaling unit. Pyridylbenzimidazole
exhibits both tunable coordination modes and supramolecular
interactions because it is a multidentate ligand having an
3
amphoteric nature which includes a N−H hydrogen-bonding
donor (imidazole ring) and a cation-binding site (aromatic N
atoms). The development of solid-emissive fluorophores has
received considerable attention due to their interesting
4
Figure 1. Imidazobenzothiadiazole receptor 1.
applications. However, organic compounds with intense
fluorescence in both solution and solid state are rare. For this
reason, the rational design of new families of this kind of
fluorophore, in particular, those based on novel frameworks,
Receptor 1 is a fluorescent yellow powder, and the solutions of
this receptor in organic solvents display solvatofluorochromism
under UV irradiation (λ = 365 nm) (Figure 2). Whereas the
UV−vis absorption spectra did not show noticeable solvato-
chromism, the wavelength of the emission maximum bath-
ochromically shifts when the solvent polarity is increased (Figure
5
remains quite challenging.
Herein, we describe the synthesis and the binding properties of
one rare ion-pair receptor 1, which combines the fluorescent
properties of the benzothiadiazole ring with the binding ability of
the pyridylbenzimidazole fragment in a highly preorganized
system. The resulting receptor 1 exhibits a remarkable
perturbation of the fluorescence emission spectrum in the
presence of Cd(AcO) or Zn(AcO) either in solution or in solid
2
). These findings suggest an excited state of 1 with a stronger
polarity than that in the ground state. Therefore, as the solvent
Received: March 27, 2015
2
2
state. The Cd2 cations causes toxicity in living cells disrupting
+
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00895
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. (a) Solvent-dependent emission spectra of 1 (λexc = 300 nm).
(
3
b) Visual changes observed in the fluorescence under UV lamp (λ =
65 nm) in the solid state and in solution in different solvent.
polarity increases a solvatofluorochromic red shift is observed as
a consequence of the stronger dipole−dipole interaction
−6
Figure 3. (a) Changes in the emission spectra of 1 in CH CN (2 × 10
3
M) in the presence of the indicated species, added in aqueous solution
8
between the polar solvent molecules and the excited state of 1.
−3
(
1.2 × 10 M); (b) visual changes observed.
The sensing properties of receptor 1 toward cations and
9
anions in acetonitrile solution were studied first. The UV−vis
To seek more detailed information on the capability of
receptor 1 to form ion-pair complexes, a H NMR spectroscopic
1
spectrum of receptor 1 exhibits two bands at λ = 301 and 360 nm.
2
+
2+
It is worth noting that the addition of Cd and Zn to 1
promoted the most significant changes in its absorption
spectrum consisting of a progressive appearance of a new low
energy (LE) band blue-shifted (λ = 325 nm). Titration studies
analysis was also performed in DMSO-d . While gradual addition
6
of an aqueous solution of the Cd2+ or Zn cations did not
2+
11
promote any change in the spectrum of the free receptor
−
(
Figure 4b), addition of AcO anion in water induced an upfield
−
toward anions revealed that only AcO anion induced detectable
−
−
3−
changes, (Δλ = 10 nm), whereas CN , F and HP O anions
2
7
clearly induced deprotonation (Δλ = 57 nm) (see Figures S11
and S12, Supporting Information).
−6
Receptor 1 showed a strong fluorescence in CH CN (2 × 10
3
M) displaying an intense band at λ = 466 nm (Φ = 0.262) when
excited at λ = 300 nm, which was also affected in the presence
exc
of the above-mentioned ions. Therefore, only after the addition
2
+
2+
of Cd (I /I = 2.9; Δλ = 34 nm; Φ = 0.146) and Zn (I /I =
0
F
0
F
4
.7; Δλ = 29 nm; Φ = 0.106) cations was a significant decrease in
the fluorescence intensity with concomitant red-shift of the
wavelength of the emission band observed, whereas addition of
−
AcO anions (I /I = 1.1; Δλ = 7 nm) promoted just a slight
0
F
decrease in intensity along with a negligible shift in the emission
band (Figure S14).
Ion-pair complex formations in solution have been studied by
using absorption and emission techniques. Stepwise addition of
−
2
aqueous solutions of Cd(AcO) or Zn(AcO) (c = 2.5 × 10 M)
2
2
−5
to a solution of receptor 1 in CH CN (5 × 10 M) elicited the
same optical response in the absorption spectrum; namely the
3
1
−3
Figure 4. Evolution of H NMR of 1 in DMSO-d (2 × 10 M) in the
6
progressive appearance of a new high energy band located at λ =
presence of the indicated species.
317 nm (Δλ = 16 nm), together with a blue-shifted low energy
band at λ = 350 nm (Δλ = −10 nm). A well-defined isosbestic
point at λ = 310 nm indicated the occurrence of a neat
interconversion between the uncomplexed and complexed
species. Binding assays using the method of continuous
shift of all signals (Figure 4c). The detection of the ion-pair
formation was evidenced by addition of Cd(AcO) or Zn(AcO)
either as an aqueous solution or in the solid state. In both cases, a
strong perturbation in the spectrum of the free receptor was
observed (Figures 4d,e).
2
2
́
variations (Jobs plot) suggest a 2:1 (receptor/Cd(AcO) ) and
2
1
:1 (receptor/Zn(AcO) ) binding model.
2
Next, titrations with these salts were studied by fluorescence3
The solvent-free synthesis of the solid ion-pair complexes was
spectroscopy. Addition of an aqueous solution (c = 1.2 × 10−
M) of the corresponding acetate to a solution of receptor 1 in
achieved by grinding the receptor 1 with Cd(AcO) (2:1) or with
2
Zn(AcO) (1:1) in a mortar using a pestle for 30 min at constant
2
−
6
CH CN (c = 2 × 10 M) (λexc = 320 nm) resulted in the
temperature. No purification of the resulting powder was
3
appearance of a red-shifted broad emission band at λ = 515 nm
performed. Strikingly, no examples toward the formation of
12
(
=
Δλ = 49 nm; (Φ = 0.349) for Cd(AcO) and at λ = 500 nm (Δλ
ion-pair complexes in the solid state have been reported so far.
2
34 nm; Φ = 0.317) for Zn(AcO). In addition, a well-defined
The ion-pair complex formation through this viable alternative to
conventional methods was investigated by X-ray powder
diffraction (PXRD) and spectroscopic techniques.
The presence of several peaks in the XRPD of the resulting
material, which are absent both in the acetate and in the receptor
patterns, together with the fact that neither the fingerprint d-
isoemissive point appeared in the region of 490 nm. Nonlinear
1
0
regression global analysis on fluorogenic titrations gave
1
2
−2
apparent association constants of β = 9.53 × 10
M
for
7
−1
Cd(AcO) and K = 1.51 × 10 M for Zn(AcO) (Figure 3).
The stoichiometries proposed from the above spectroscopic data
2
2
have been further confirmed by ESI-MS experiments.
spacing-intensity pattern of the Cd(AcO) nor the receptor 1 can
2
B
DOI: 10.1021/acs.orglett.5b00895
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Organic Letters
Letter
be recognized in the XRPD, are indicative of the formation of a
new compound. The resulting diffraction patterns are broadened
which may be ascribed to the small size of the corresponding
crystallites. The formation of the ion pair was assigned to be [1 ·
2
1
Cd(AcO) ] by comparing the IR, HNMR, absorption, and
2
emission spectra of the starting materials with those of the pure
complexes. A similar result was achieved when Zn(AcO) was
2
used as the inorganic salt partner. IR spectra of the free receptor
1
, Zn(AcO) , and Cd(AcO) , as well as the corresponding ion-
2 2
pair complexes, were obtained. The latter show the expected
carbonyl band associated with the acetate anion present in these
complexes together with a fingerprint which does not match with
any of the other species (Figures S38 and S39).
Figure 6. (a) Emission spectra of 1 (5 × 10− M) in CHCl after stirring
6
1
Interestingly, the H NMR spectra of the solid ion-pair
3
with the solid salts. (b) Visual changes observed in the fluorescence
complexes prepared, [1 ·Cd(AcO) ] and [1·Zn(AcO) ], in
2
2
2
under the UV lamp (λ = 365 nm) of (i) 1; (ii) 1 + Cd(AcO) ; (iii) 1 +
2
DMSO-d , were found to be identical to those of the
6
Zn(AcO) ; (iv) 1 + Cd(AcO) + Zn(AcO) .
2
2
2
corresponding complexes formed in solution (Figures 4d,e).
The UV−vis spectrum of the receptor 1 in the solid state
displayed an absorption band at λ = 313 nm, appearing at nearly
1
for Zn(AcO)2 arises from 1H NMR studies. Indeed,
2
+
simultaneous addition of both metal acetates to a solution of 1
the same position as in the Zn complex (λ = 317 nm), whereas
2+
in DMSO-d , resulted in the same spectrum as that obtained by
the Cd complex showed a red-shifted band at λ = 323 nm (Δλ =
6
adding only Zn(AcO) (Figure S45).
1
0 nm) (Figures S40 and S41). Fluorescence studies revealed
2
Unfortunately, all attempts to obtain good quality crystals of
the ion-pair complexes for their X-ray study were unsuccessful.
Therefore, density functional theory (DFT) calculations were
finally carried out at the RI-BP86-D3/def2-TZVPP//RI-BP86-
D3/def2-SVP level to gain more insight into the nature of the
that both complexes are emissive in the solid state and also
displayed solvatofluorochromism (Table S11). As shown in
Figure 5 the emission spectra of the receptor 1 showed a broad
ion-pair complexes formed upon addition of Cd(AcO) or
2
Zn(AcO) to receptor 1. According to the above experimental
2
findings, 1:1 and 2:1 binding models were found for Zn(AcO)2
and Cd(AcO) , respectively. Screening of the possible modes of
2
interaction between these acetates and 1 (Figures S1 and S2)
reveals that species [1·Zn(AcO) ] and [1 ·Cd(AcO) ] are the
2
2
2
more stable species for 1:1 and 2:1 stoichiometries, respectively
Figure 7).
(
Figure 5. (a) Emission spectra in the solid state of receptor 1 (black line)
(
Φ < 0.01), and ion pair complexes [1·Zn(AcO) ] (Φ < 0.01) and [1 ·
2
2
Cd(AcO) ] (Φ < 0.01). (b) Visual changes observed in the fluorescence
(
2
UV lamp, λ = 365 nm) of solid KBr pellets of the free receptor (A) and
after grinding with Cd(ClO) (B), Zn(OTf) (C), [(n-Bu) N]OAc (D),
4
2
4
Cd(AcO) (E), and Zn(AcO) (F).
2
2
emission band centered at λ = 470 nm, while the [1·Zn(AcO)2]
and [1 ·Cd(AcO) ] complexes displayed a red-shifted emission
2
2
band at λ = 500 (Δλ = 30 nm) and λ = 522 nm (Δλ = 52 nm),
respectively.
Figure 7. Computed most stable structures for complexes resulting
upon complexation of 1 with Zn(OAc) (left) or Cd(OAc) (right).
Bond distances are given angstroms. All data have been computed at the
RI-BP86-D3/def2-SVP level.
One direct consequence of this behavior is that the receptor
allows the otherwise insoluble Cd(AcO) and Zn(AcO) salts to
dissolve in organic solvents. Extraction experiments demon-
strated that the host is capable of solubilizing both acetates in
acetonitrile and chloroform. Interestingly, a selective extraction
of Zn(AcO) in the presence of Cd(AcO) was successfully
2
2
2
2
As seen in Figure 7, the Zn2+ cation in [1·Zn(AcO) ] is
2
2
2
−
achieved when an equimolecular mixture of these salts was
extracted with a solution of receptor 1 (c = 10 M) in diethyl
coordinated by one bidentate AcO ligand and the nitrogen
−6
atoms lone-pairs of the pyridine and benzimidazole moieties of 1.
2
+
2+
−
ether: the Zn salt was soluble in diethyl ether, whereas the Cd
salt remained insoluble in the form of the ion-pair complex
Figure S43). Likewise, a selective extraction of Zn(AcO) in the
The other AcO is bound to 1 through a hydrogen bond via the
NH group of the imidazole. Therefore, this species can be viewed
(
as a ligand-separated AcO−1−Zn(OAc) ion-pair complex.
2
2
+
presence of Cd(AcO) was achieved when an equimolecular
Differently, the Cd cation in [1 ·Cd(AcO) ] is surrounded
2
2 2
−
mixture of these salts was extracted with a solution of receptor 1
by both AcO ligands and the nitrogen atoms of the pyridine
moieties of two different molecules of 1 in a highly distorted
octahedral coordination. Moreover, two clear hydrogen bonds
between the acetates and the NH of the benzimidazoles
(computed NH···OAc bond lengths of 1.637 and 1.697 Å)
6
(
c = 10− M) in chloroform. After filtration, the resulting solution
displayed an emission spectrum identical to that registered for
the ion-pair [1·Zn(AcO) ] complex in this solvent (Figure 6).
Further evidence that confirms the higher affinity of the receptor
2
C
DOI: 10.1021/acs.orglett.5b00895
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
contribute further to the stabilization of this species, which can be
ACKNOWLEDGMENTS
■
viewed as a contact 1−Cd(OAc) −1 ion-pair complex.
2
This work was funded by MINECO-Spain (Project CTQ 2011/
27175, CTQ-2013-44303-P) and European FEDER. M.A.
thanks MICINN for a predoctoral grant.
The nature of the interactions in [1·Zn(AcO) ] and [1 ·
2
2
Cd(AcO) ] was analyzed in detail with the help of the atoms and
2
−
molecules (AIM) method. As seen in Figure S3, the AcO in [1·
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2
■
3
with the H7 and H3′ hydrogen atoms of the receptor, as revealed
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(
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corresponding signals in the H NMR spectrum (Figure 4).
Besides the above commented well-defined NH···OAc
hydrogen bonds in [1 ·Cd(AcO) ], this species is further
1
2, 2303. (c) Chen, Y.-H.; Lin, L.-Y.; Lu, C.-W.; Lin, F.; Huang, Z.-Y.;
2
2
Lin, H.-W.; Wang, P.-H.; Liu, Y.-H.; Wong, K.-T.; Wen, J.; Miller, D. J.;
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stabilized by intramolecular π−π interactions between the
pyridine fragment of one molecule of 1 and the benzimidazole
moiety of the adjacent receptor as confirmed again by the
presence of BCPs and BCs (Figure S3) running between both
aryl groups (computed C···C bond distances ranging from 3.172
(
(
3) Molina, P.; Tar
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1
3
to 3.264 Å). Again, the observed shift of the signals of the
1
̈
llen, K.
involved aromatic hydrogen atoms in the corresponding H
NMR spectrum may be ascribed to these intramolecular
interactions.
(
In conclusion, fluorescent solid 5-pyridylimidazobenzothia-
diazole receptor 1, which displays a noticeable solvatofluor-
ochromism, behaves as an ion-pair receptor either in solution or
in the solid state. The solvent-free prepared solid [1 ·Cd(AcO) ]
(
2
2
and [1·Zn(AcO) ] complexes show a remarkable different
2
́
raga, A.; Molina, P. Dalton
fluorescent behavior both in solution and solid state, which
allows the discrimination between them. In addition, formation
of the ion-pair complexes in solution, which also displayed
solvatofluorochromism, allows the selective extraction of the
(
2
+
2+
Zn salt in the presence of the Cd salt either by a chloroform or
2
+
diethyl ether solution of the receptor. Whereas the Zn complex
can be viewed as a ligand-separated AcO−1−Zn(OAc) ion-pair
2
+
(
f) Tong, J.; Wang, Y.; Mei, J.; Wang, J.; Qin, A.; Sun, J. Z.; Tang, B. Z.
complex, our calculations showed that the Cd counterpart can
Chem.Eur. J. 2014, 20, 4661.
be considered as a contact 1−Cd(OAc) −1 ion-pair complex.
2
+
+
2+
2+
2+
2+
(
9) The cations tested were Li , K , Mg , Ni , Cd , and Pb (added
Moreover, within this structural motif, significant features such as
strong fluorescence in solution and in solid state and unusually
large Stoke shifts are observed. These properties qualify these
ion-pair complexes as multifunctional fluorophores that can also
be employed for the future development of luminescent displays
and light sources.
+
2+
2+
2+
2+
as perchlorate salts) and Na , Ca , Cu , Zn , and Hg (added as
−
−
−
−
−
−
triflate salts). The anions tested were F , Cl , Br , AcO , NO , HSO ,
3
4
−
3−
H PO , and HP O , added as tetrabutylammonium salts.
2
4
2
7
(10) Specfit/32 Global Analysis System, version 3.0.36 for 32-bit
Windows Systems, 1999−2004 Spectrum Software Associates
SpecSoft@compuserve.com) acquired from Bio-logic, SA (www.bio-
logic.info) in January 2005.
11) When the experiments were carried in CD CN, the addition of
these cations promoted a remarkable downfield shift of all the signals,
and the ion-pair complexes displayed a set of signals similar to those
found in DMSO-d6.
(
(
3
ASSOCIATED CONTENT
■
*
S
Supporting Information
Experimental section. Synthesis, NMR, and titration data.
Cartesian coordinates for all computed structures. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.orglett.5b00895.
(12) James, S. L.; Adams, C. J.; Bolm, C.; Braga, D.; Collier, P.; Friscic,
T.; Greponi, F.; Harris, K. D. M.; Hyett, G.; Jones, W.; Krebs, A.; Mack,
J.; Maini, L.; Orpen, A. G.; Parkin, I. P.; Shearouse, W. C.; Steed, J. W.;
Waddell, D. C. Chem. Soc. Rev. 2012, 41, 413.
(
13) Similar C···C bond distances have been reported for different
systems featuring intramolecular π−π interactions. See, for instance:
Sarotti, A. M.; Fernandez, I.; Spanevello, R. A.; Sierra, M. A.; Suarez, A.
G. Org. Lett. 2008, 10, 3389 and references therein.
́
́
AUTHOR INFORMATION
■
Corresponding Authors
*
E-mail: atarraga@um.es.
E-mail: pmolina@um.es.
*
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.5b00895
Org. Lett. XXXX, XXX, XXX−XXX