The role of metal ions and counterions in the switching behavior of a carboxylic acid functionalized spiropyran

Article abstract of DOI:10.1039/c0dt00242a

The ability of a carboxylic acid functionalized spiropyran, SP, to act as a reversible and selective receptor for metal ions has been systematically examined. A series of spectroscopic measurements, combined with single-crystal X-ray diffraction have been employed in order to determine metal-ion sensitivity and selectivity, binding constants, solution compositions, binding kinetics as well as solid-state structure of a metal-complex of this ligand. This molecule acts as a chelating ligand and binds to metal ions using a triad of functionalities and will release the metal ion by application of visible light. Altering the complex from ZnCl₂ and Zn(ClO₄)₂, leads to only a small change in the complex absorption maxima from 494 to 492 nm, but produced a drastic increase in the sensitivity for the zinc(ii) ion. Fluorescence spectroscopy was employed to establish that the detection limit for Zn(ii) is approximately 3 × 10^-7 M for Zn(ClO ₄)₂. The counterion also influences binding constants and the equilibrium constants for Cu(ClO₄)₂ and CuCl ₂ are K = 3.37 × 10⁴ M^-1 to K = 2.04 × 10³ M^-1, respectively. Additionally, in the presence of perchlorate, the spiropyran-Cu complex formation is fast enough to allow for real-time naked eye detection of this metal ion in less than three minutes; the formation of other metal complexes takes between 50 minutes to 2 hours. Finally, the single-crystal structure determination of the SP-zinc chloride complex shows that the Zn(ii) ion displays a distorted square-pyramidal arrangement, and neighbouring metal ions are bound together into a 1-D coordination polymer with a hydrophilic core, surrounded by hydrophobic 'cladding'.

Full text of DOI:10.1039/c0dt00242a

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www.rsc.org/dalton | Dalton Transactions
The role of metal ions and counterions in the switching behavior of a
carboxylic acid functionalized spiropyran†
Manuel Natali,a Christer Aakero¨y,b John Desperb and Silvia Giordani*a
Received 31st March 2010, Accepted 22nd June 2010
First published as an Advance Article on the web 6th August 2010
DOI: 10.1039/c0dt00242a
The ability of a carboxylic acid functionalized spiropyran, SP, to act as a reversible and selective
receptor for metal ions has been systematically examined. A series of spectroscopic measurements,
combined with single-crystal X-ray diffraction have been employed in order to determine metal-ion
sensitivity and selectivity, binding constants, solution compositions, binding kinetics as well as
solid-state structure of a metal-complex of this ligand. This molecule acts as a chelating ligand and
binds to metal ions using a triad of functionalities and will release the metal ion by application of
visible light. Altering the complex from ZnCl2 and Zn(ClO4)2, leads to only a small change in the
complex absorption maxima from 494 to 492 nm, but produced a drastic increase in the sensitivity for
the zinc(II) ion. Fluorescence spectroscopy was employed to establish that the detection limit for Zn(II)
is approximately 3 ¥ 10-7 M for Zn(ClO4)2. The counterion also influences binding constants and the
equilibrium constants for Cu(ClO4)2 and CuCl2 are K = 3.37 ¥ 104 M-1 to K = 2.04 ¥ 103 M-1,
respectively. Additionally, in the presence of perchlorate, the spiropyran-Cu complex formation is fast
enough to allow for real-time naked eye detection of this metal ion in less than three minutes; the
formation of other metal complexes takes between 50 minutes to 2 hours. Finally, the single-crystal
structure determination of the SP-zinc chloride complex shows that the Zn(II) ion displays a distorted
square-pyramidal arrangement, and neighbouring metal ions are bound together into a 1-D
coordination polymer with a hydrophilic core, surrounded by hydrophobic ‘cladding’.
upon visible light irradiation. So far, several spiropyran-based
Introduction
receptors capable of reversibly detecting a variety of metal ions
in solution have been reported.5 Although most of the known
spiropyran-based receptors are tailored to bind metal ions, very
little has been reported about the importance of counterions on the
coordination process and on the overall photochemical behaviour
of spiropyrans.6 Long term we are interestedindevelopingselective
and rapid metal-ion sensors that can function in complex solutions
with a range of potentially competing ligands. In this paper,
we examine the complexation behaviour of a photoregenerable
spiropyran derivative in the presence of Zn(II) and Cu(II). In
particular, we analyse the effects that two different counterions,
(Cl-, [ClO4]-) have on complex stability, sensitivity, and kinetics
of binding, and we also employ both solution- and solid-state
methods to establish composition of the resulting spiropyran-
metal complexes.
The ability of photochromic compounds to recognize and respond
to a range of external stimuli has resulted in an increasing
number of studies that focus on their design and synthesis. The
photoinduced switching characteristics of such compounds give
them tremendous potential in molecular electronics and sensor
device fabrication.1
The photochromism of spiropyrans has been known for ap-
proximately fifty years,2 and their switchability is due to reversible
heterolytic cleavage of the spiro C–O bond upon UV irradiation,
followed by cis-trans isomerization which generates a metastable
merocyanine. The reverse process of ring-closing takes place upon
visible light irradiation or thermal bleaching.3
The negatively charged phenolic oxygen in the zwitterionic
open form also produces a moiety that can convert the neutral
spiropyran into an efficient ligand for protons and metal ions.4
In addition, it may be possible to engineer sensitive and efficient
receptors for metal ions through careful functionalization of the
spiropyran scaffolding using suitable metal-binding sites localized
in strategic positions that facilitate the interaction of the different
chelating moieties with the metal. Additionally, the complexation
equilibrium is photocontrollable as the cation is readily released
Results and discussion
Design
In order to design molecules capable of acting as efficient, sensitive
and reversible metal ion detectors, a range of challenges need to be
addressed. Our design brings together elements that can facilitate
metal-ion binding as well as promote photoinduced control of
the metal ligation (compound SP, Scheme 1 and Fig. 1). First,
a methoxy group in position 8¢ on the benzopyran moiety has
previously been used in several spirochromenes aimed at metal
coordination.7 This functional group can cooperate with the phe-
nolate anion of the open merocyanine in the chelation of the ion.
Second, the indolic nitrogen offers another functionalizable site on
aSchool of Chemistry/CRANN, Trinity College, Dublin, Ireland.
E-mail: giordans@tcd.ie.
bDepartment of Chemistry, Kansas State University, Manhattan, KS 66506.
E-mail: aakeroy@ksu.edu
† Electronic supplementary information (ESI) available: Kinetic plots,
binding plots, crystallography, MS, and NMR data. CCDC reference
number 771869. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0dt00242a
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Scheme 1 Synthesis of SP.
the molecule and several examples of N-modified spirochromenes
for metal chelation applications are known.8 In order to combine
sufficient conformational flexibility with a strong coordinating
moiety we chose a carboxylated chain as an additional binding site.
The fact that the coordinating ability (and charge) of the carboxylic
acid moiety can be modulated means that we have an additional
tool for further fine-tuning the coordination chemistry of our
ligand. The phenoxy-methoxy-carboxy triad produces a ligand
that should be capable of forming strong, yet reversible, metal-ion
binding even in the presence of competing ligands such as solvent
molecules and other counterions. Third, the introduction of a nitro
group in position 6¢ is aimed at creating a reversible cation receptor.
It is known that the presence of a strongly electron-withdrawing
group in position 6¢ stabilizes the open form whereas an electron-
donating group produces a more stable photostationary closed
form.3 Thus, an ionochromic receptor where the cation release is
photo-controllable is obtained. Once this ligand was in place, we
wanted to examine the coordination chemistry and photochemical
behaviour of SP with respect to two different metal ions, Cu(II)
and Zn(II) in the presence of a strongly coordinating and a weakly
coordinating counterion, chloride and perchlorate, respectively.
Fig. 1 Reversible interconversion between spiropyran SP and the open
merocyanine-metal complex ME–M (the complex structure is hypotheti-
cal). A Absorption spectra of (black) SP (1.0 ¥ 10-4 M, CH3CN, 293 K),
after the addition of 1 equivalent of ZnCl2 (red) and after the irradiation
with visible light for 60 s (blue). B Absorption spectra of (black) SP (1.0 ¥
10-4 M, CH3CN, 293 K), after the addition of 1 equivalent of CuCl2 (red)
and after the irradiation with visible light for 60 s (blue); (Insets A and
B) Sequential cycles of conversion between SP and ME–M controlled by
visible light in the presence of ZnCl2 and CuCl2 respectively.
Synthesis
The targeted spiropyran SP was synthesized in three steps. Com-
mercially available 2,3,3-trimethyl-3H-indole was alkylated with 4-
bromobutyrate. The resulting indolium salt 1 was condensed with
3-methoxy-5-nitrosalicylaldehyde to afford the spirochromene 2,
previously reported9 which was then hydrolyzed with NaOH to
give the desired product (Scheme 1).
of the peak at 588 nm is due to the presence of a small amount of
the open merocyanine in solution produced by thermal conversion
of the spiropyran (black curves in Fig. 1A and 1B).
After the addition of 1 equivalent of ZnCl2, the solution turned
orange and a high intensity peak, hypsochromically shifted with
respect to the merocyanine peak, appeared at 494 nm (red curve in
Fig. 1A). A similar response was observed after the addition of 1
equivalent of CuCl2 as the solution turned yellow and a shoulder
in the absorption spectrum appeared at 447 nm (red curve in
Fig. 1B). The new absorption peaks observed in the visible region
in the presence of the metal cations are due to the formation of
spiropyran-metal complexes where three different binding sites, a
carboxy group, a methoxy and a phenolate anion may be involved.
The complex formation reversibility was confirmed by additional
Reversibility of the complex formation
The binding interactions of SP with ZnCl2 and CuCl2 were studied
by UV-visible spectroscopy. The absorption spectra of a solution
of SP in acetonitrile (1 ¥ 10-4 M) before and after the addition of
1 equivalent of Zn2+ and Cu2+ are featured in Fig. 1A and 1B. A
solution containing only the spiropyran is pale blue and it absorbs
mainly in the UV region with a peak at 358 nm. The low intensity
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experiments where the solutions containing the spiropyran and
the metals were irradiated with visible light. Upon irradiation
the metals are almost fully released as the typical peaks of the
complexes disappear (blue curves in Fig. 1A and 1B). All the
complexes are fully reversible as the peaks in the visible region
re-appear after storage in the dark. Thus the metal release is
fully controllable and modulated by light, and several cycles of
irradiation are possible and reproducible (insets Fig. 1A and 1B).
complex can not be determined from the data in hand, but it is
likely that several ligands act as bridges between the two zinc ions.
Effect of different counterions on absorption
Although we designed this SP to act as an efficient chelating
ligand for metal ions, it is also important (for real-life applications)
to establish what role, if any, different counterions may play in
the coordination chemistry or photochemical behaviour of the
ligand. To explore this, we carried out a series of experiment
on chloride/perchlorate salts of Zn(II) and Cu(II), respectively,
in order to establish how counterion characteristics relate to the
sensitivity and complexation rate of the SP, as well as to the
composition of the ME–M complex.
Binding constants K for the ME–M complexes were determined
from the absorbance intensities recorded at different metal salts
concentrations following the modified Benesi–Hildebrand equa-
tion 1/DA = 1DAmax +(1/K[C])(1/DAmax).12 Here DA = Ax - A0
and DAmax = A• - A0 where A0, Ax and A• are the intensities of the
ME–M complex at the nm considered in the absence of metals,
at a certain concentration of metals and at a concentration of
complete interaction respectively; K is the binding constant and
[C] is the concentration of metal. The K values were extrapolated
from the slope of the plot (A• - Ax)/(Ax - A0) against 1/[C]. The
most stable complex was obtained with copper perchlorate with
a binding constant K = 3.37 ¥ 104 M-1. Changing the counterion
from perchlorate to chloride the affinity of the molecule to copper
decreases of an order of magnitude with a value K = 2.04 ¥ 103 M-1.
The same effect was observed by changing the counterion of zinc
but it was not as dramatic as it was for copper. The binding values
of 2.68 ¥ 104 M-1 and 1.29 ¥ 104 M-1 were found for the ME–
Zn(ClO4)2 and ME–ZnCl2 complexes respectively.
Complexes’ composition studies
The spectroscopic determination of the complexes’ composition
was investigated by means of the isomolar solutions technique
(Job method).5e,10 The plots of the Job analysis of a the spiropyran-
zinc(II)chloride and spiropyran-copper(II)chloride complexes are
featured in Fig. 2. The absorbance values were taken at the
complexes absorption maxima, 494 nm for zinc(II) and 447 nm
for copper(II), and they were correlated to the composition of the
isomolar solutions. As shown in the plots, in both cases the maxima
correspond to the solutions of composition metal/spiropyran 2 : 1.
In Fig. 3 the absorption spectra of solutions of SP containing
different concentrations of ZnCl2 are shown. The inset of Fig. 3
is the plot of the Benesi–Hildebrand equation for the spiropyran-
ZnCl2 complex.
Fig. 2 Job’s analysis of (A) SP–Zn2+ complex ([SP]+[Zn2+] = 3.0 ¥ 10-5 M,
CH3CN, 293 K) and (B) SP–Cu2+ complex ([SP]+[Cu2+] = 3.0 ¥ 10-5 M,
CH3CN, 293 K). Each analysis was carried out three times and the data
were plotted as the average of the different series.
The stoichiometry observed herein is different to those reported
previously for overall charge neutral SP-based ligands. With
such ligands, 1 : 2 or 1 : 1 stoichiometries (metal ion/SP) are
observed.5a,11 The 2 : 1 stoichiometry that we obtain indicates
that the dominating species in solution is most likely a dinuclear
species which must therefore also incorporate three chloride ions
to achieve overall charge neutrality (the carboxylic acid group on
the SP is expected to be protonated). The exact geometry of this
Fig. 3 The absorption spectra of solutions of SP (1.0 ¥ 10-4 M, CH3CN,
293 K) after 20 min from the addition of 0.2, 0.4, 0.6, 0.8 and 1 equivalent
of ZnCl2; (Inset) Plot of (A• - Ax)/(Ax - A0) against 1/[C].
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Although the binding constant was higher for copper perchlo-
rate, this SP showed a surprisingly high sensitivity to the presence
of Zn(ClO4)2. For all the complexes, the maximum value of
absorption intensity was reached after the addition of 1 equivalent
of metal cation while only 0.3 equivalents of zinc perchlorate are
sufficient to produce the maximum response of the spiropyran
(Fig. 4).
Fig. 5 The increase in absorption at 422 nm of a solution of SP (1.0 ¥
10-4 M, CH3CN, 293 K) after the addition of 1 equivalent of Cu(ClO4)2;
(Inset) Plot of (A• - At)/(A• - A0) versus time.
spirochromene and copper(II) turns from colourless to orange-
yellow immediately upon addition of the cation.
By analyzing and comparing the different effects of cations and
counterions on the behaviour of SP, a curious result was obtained
when the concentration of copper perchlorate was higher than
1 equivalent. As shown in Fig. 6, a shoulder at 422 nm proved
the formation of the spiropyran-metal complex. Its corresponding
intensity reached the maximum as the concentration of copper was
equimolar to the molecule concentration. If the concentration of
metal rises above the equimolarity, a new shoulder appears at
486 nm, indicating probably the formation of a different complex.
Fig. 4 Absorption spectra of SP (1.0 ¥ 10-4 M, CH3CN, 293 K) after
increasing the concentration of Zn(ClO4)2; (Inset) Absorption intensities
at 492 nm of the same solution of SP. The plateau of the maximum
absorption intensity at 492 nm is reached after the addition of 0.3 eq of
the zinc perchlorate.
In order to characterize the spiropyran-metal complexes, the
complexation kinetics were studied in acetonitrile after addition
of 1 eq of zinc and copper chlorides and perchlorates. The
coordination of the different cations follows first order kinetic
and the observed rate constant can be determined using eqn (1),13
where A•, At and A0 are the absorbance of the complex at time
infinity, t and 0 respectively, kob is the observed rate constant and
t is the time.
(A∞− At)
(A∞− Ao)
ln
= kobt
(1)
The rate constants of each complex can be extrapolated from the
slope of the plot of (A• - At)/(A• - A0) versus t as its falloff is linear
(inset Fig. 5). Thus the kob values of all complexes were calculated
and they are 1.3 ¥ 10-2 s-1 for copper perchlorate, 1.5 ¥ 10-1 s-1 for
copper chloride while, values of 1.8 ¥ 10-2 s-1 and 6.4 ¥ 10-2 s-1
were obtained for zinc perchlorate and zinc chloride, respectively.
In the presence of Zn(ClO4)2, ZnCl2 and CuCl2 the coordination
process is slow. The maximum absorptions of the three complexes
were reached after the solutions containing SP and metal salts had
been stored in the dark for 120, 60 and 50 min respectively. A very
fast response was instead observed upon addition of Cu(ClO4)2
with a full complexation time that ranges between 2.5–3 min. The
absorbance increase at 422 nm versus time upon addition of 1 eq
of Cu(ClO4)2 to a spiropyran solution (10-4 M in acetonitrile) is
displayed in Fig. 5. The fast coloration rate allows a naked eye
real-time detection of the cation as the solution containing the
Fig. 6 Absorption spectra of a solution of SP (1.0 ¥ 10-4 M, CH3CN,
293 K) after the addition of more than 1 equivalent of Cu(ClO4)2; the
formation of a new species is observable at 486 nm as the concentration of
copper is higher than 1 equivalent.
The reason of this phenomenon is currently under investigation.
Emission measurements and detection limit
The ring opening of spiropyrans by means of UV irradiation
produces the open merocyanines. The latter are known to be
emitting species due to the conjugation through the molecule
skeleton, which is instead disrupted in the closed form.14 Upon
excitation of the solutions of our spiropyran in the presence of
different cations at lexc = 481 nm, the emission spectra revealed
high intensity bands in the ranging between 633 and 644 nm only
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in the presence of zinc. No response was observed in the presence
of copper(II) (Fig. 7).
1H NMR analysis on the zinc complex
1H NMR analysis of the Zn2+ complex revealed precious informa-
tion about the chelation mechanism. Fig. 9 shows the comparison
between the partial 1H NMR spectra of a solution of SP prior to
(SP) and after the addition of 1 eq of ZnCl2 (ME–Zn).
Fig. 7 Emission spectra of a SP solution (1.0 ¥ 10-5 M, CH3CN, 293
K) before (black), after the addition of 1 equivalent of ZnCl2 (red) and
after the addition of 1 equivalent of CuCl2 (blue), excitation l = 481 nm;
(Picture) coloration of a SP solution (1.0 ¥ 10-5 M, CH3CN, 293 K) after
addition of ZnCl2.
The effect of the counterion was observed in the emission
experiment as the addition of either zinc chloride or perchlorate
produced two slightly different responses. The emission intensity
of the SP–M complex after the addition of zinc perchlorate
was approximately double the emission in the presence of zinc
chloride (inset Fig. 8). Additionally, going from perchlorate to
chloride produced a bathochromic shift from 633 nm to 644 nm.
The dramatic response of SP to the addition of zinc perchlorate
allowed the investigation of the corresponding detection limit. As
shown in Fig. 8, an increased concentration of zinc perchlorate
produced an increase in the emission at 633 nm. The maximum
intensity was reached after the addition of 1 eq of Zn(ClO4)2. The
LOD method was used to find the detection limit. The lowest
concentration of zinc detected was 3 ¥ 10-7 M.
Fig. 9 1H NMR partial spectra of 1 (2 ¥ 10-2 M in CD3CN) prior to (SP)
and after the addition of 1eq of ZnCl2 (ME–Zn).
Once the thermal equilibrium was reached, the concentration
of the closed form was negligible with respect to the open one.
The presence of the free carboxylic group in this ligand on the
indolic nitrogen was decisive in stabilizing the metal complex. The
signals of the vinylic (b and c) and aromatic protons disappeared
and two distinguishable group of signals appeared in the range of
7.61–7.82 ppm and 8.30–8.42 ppm. The typical shifts of the gem
methyl groups (a) coalesced in a singlet at 1.83 ppm (a’) due to the
symmetry of the quasi planar merocyanine. Moreover, all the peaks
corresponding to the aliphatic protons of the carboxylated chain
moved downfield. Among them, the widest shift was observed for
protons adjacent to the indolic nitrogen from 3.21 ppm (m) to
4.53 ppm (m’) because of the effect of the positively charged N in
the open form. The shifts to a lower field observed for the protons
of the methox¢y group (f-f¢) and the aliphatic protons adjacent to
the acid group (n, n¢, o, o¢) confirmed the formation of the complex
involving the triad carboxy, methoxy and phenolate.
Single-crystal X-ray diffraction
Fig. 8 Emission spectra of a SP solution (1.0 ¥ 10-5 M, CH3CN, 293 K)
after varying the concentration of Zn(ClO4)2 excitation l = 481 nm; (Inset)
counterion effect on the emission response of a solution of SP.
Numerous attempts were made to grow crystals of a ME–M
complex and we were finally successful when thin plates
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(dec. 270 ◦C) of (E)-4-(2-(3-methoxy-5-nitro-2-oxidostyryl)-3,3-
dimethyl-3H-indolium-1-yl)butanoate zinc(II) chloride were ob-
tained in an NMR tube (see experimental 1H-NMR section). The
crystal structure determination of Zn(II)–ME–Cl shows that the
merocyanine is a monovalent anion, as the carboxylic acid moiety
has been deprotonated The Zn(II) displays a somewhat unusual
distorted square-pyramidal arrangement, where each metal ion
is coordinated to one chloride ion and three different ME’s
simultaneously. The four Zn ◊ ◊ ◊ O distances within each complex
but we did not detect any signal due to the presence of coordinated
counterions. However, peaks corresponding to SP–M complexes
with coordinated solvent molecules were found.
In the SP–Zn(ClO4)2 solution, two peaks indicating the presence
of a 2SP–Zn and 3SP–2Zn complexes were found at 911 m/z
and 1401 m/z respectively (Fig. 11a). Three additional peaks
can be assigned to SP–M complexes with varying number of
coordinated solvent molecules; 568 m/z for the molecular ion
[SP+2CH3CN+Zn]+, 675 m/z for [SP+3CH3CN+2Zn]+ and 1013
m/z for [2SP+H2O+2 CH3CN+Zn]+.
˚
ion are in the range of 2.01–2.83 A, with the longest distance
involving the methoxy oxygen atom.
The structure is split into a distinctly hydrophilic core, an
extended 1-D chain composed of metal ions, chloride ions and
the coordinating sites of the ligand, surrounded by a hydrophobic
‘cladding’ composed of the relatively non-polar and aromatic
components of the merocyanine ligand, Fig. 10.
Fig. 10 An extended core of metal ions (Zn ions in pink, chloride ions in
green) surrounded by a hydrophobic cladding.
The segregated and low-dimensional nature of the crystal
structure of this compound is likely a strong contributor to the
poor crystallinity of the sample. It is also likely that the reversible
nature of the molecular geometry of the ligand produces a solution
with both the closed and the open form of the SP from which
crystal growth is exceedingly difficult. Nevertheless, this is an
extremely rare example of a crystal structure of a metal complex
of a functionalised spiropyran.
A comparison of the results obtained from 1NMR and single-
crystal X-ray diffraction on the SP–Zn(II) chloride complex shows
that the triad of binding sites that was intended to interact directly
with the metal ion was indeed fully engaged in both solution and
in the solid state.
Fig. 11 MALDI-TOF mass spectrum of an acetonitrile solution of SP
containing (a) [Zn(ClO4)2] = [SP] = 1 ¥ 10-3 M and (b) [ZnCl2] = [SP] = 1 ¥
10-3 M.
MALDI-TOF Mass Spectrometry
A similar situation was observed in the SP–ZnCl2 solution
(Fig. 11b). The peaks at 568 m/z, 911 m/z and 1401 m/z are
analogous to the ones observed in the presence of zinc perchlorate.
Additionally the signal at 668 m/z likely corresponds to the
molecular ion [SP+2CH3CN+3H2O+2Zn]+ which, incidentally,
is the same stoichiometry as found in the solid-state structure, as
well as in the Job’s analysis. The peak at 1011 m/z was attributed
to [2SP+H2O+2CH3CN+Zn]+. The difference between the values
of 1013 m/z in the presence of zinc perchlorate and 1011 m/z
in the presence of zinc chloride are due to the different state
of protonation of the carboxy group of SP. The addition of
CuCl2 to a solution of SP produced the formation of similar
complexes and the contribution of the solvents in the coordination
was confirmed also in this instance. The following peaks were
Matrix-assisted laser desorption/ionization or MALDI is a soft
ionization technique that allows the detection of molecules that
tend to be fragile when ionized with other techniques. Therefore
we decided to employ this tool in order to gather additional
information about spiropyran-metal ion complexation in solution.
Acetonitrile solutions of SP containing equimolar amounts
of Zn(ClO4)2, ZnCl2, CuCl2 and Cu(ClO4)2 were equilibrated
overnight and the corresponding spectra were recorded.
In previously reported ESI-MS experiments, spiropyran-metal
complexes with stoichiometry 1 : 1 and 2 : 1 were identified,15 and
counterions were often included in the pseudomolecular ions
signals confirming their contribution to the coordination. In our
case, we did observe several SP–M complexes with different ratios,
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observed and assigned to their corresponding molecular ions
(Fig. 12a): 563 m/z for [SP+2H2O+CH3CN+Cu]+, 911 m/z
[2SP+Cu]+, 1013 m/z [2SP+H2O+2CH3CN+Cu]+, 1401 m/z
[3SP+2Cu]+. Also the coordination of copper perchlorate followed
the same patterns and the peaks at 563, 911, 1011 and 1401
m/z were assigned to [SP+2H2O+CH3CN+Cu]+, [2SP+Cu]+,
[2SP+H2O+2CH3CN+Cu]+ and [3SP+2Cu]+ respectively. An
unusual signal at 846 m/z was noticed and it corresponds to
a dimer [2SP]+ with no copper (Fig. 12b). We are currently
investigating this result as we found this unexpected species only
in the presence of Cu(ClO4)2.
fluorescence spectroscopy which is a more sensitive technique with
respect to UV-vis absorption spectroscopy, in order to estimate the
detection limit which was found to be 3 ¥ 10-7 M for Zn(ClO4)2.
Going from Cu(ClO4)2 to CuCl2 a dramatic drop in the affinity
for the cation was observed as the binding constant falls from K =
3.37 ¥ 104 M-1 to K = 2.04 ¥ 103 M-1 respectively. Additionally,
in the presence of perchlorate, the SP–Cu complex formation is
the fastest of the series, allowing the real-time naked eye detection
of the cation in less than 3 min from the addition while the other
complexes are fully formed in a time ranging between 50 min to
2 h.
1H NMR and UV-vis data on the zinc chloride complex demon-
strate that, importantly, all three binding sites on the ligand are
actively coordinating to the metal ion, and the dominating species
in solutions seems to be, in this case, a discrete dinuclear 1 : 2
SP/Zn(II) complex. However, it is reasonable to assume that, upon
nucleation, a coordination polymer would be favoured kinetically,
and this is shown by single-crystal X-ray diffraction; the crystal
structure of the SP-zinc chloride complex contains a coordination
polymer with a stoichiometry of 1 : 1 : 1 (SP/Zn/Cl). Finally, we
were able to detect 2 : 1 and 1 : 1 spiropyran-metal complexes in
the presence of both copper and zinc with different counterions
using MALDI-TOF MS. Additionally, a peak indicating a 1 : 2
SP : Zn complex was detected in the presence of ZnCl2 and this
result matches very well with the data obtained from spectroscopic
and crystallographic data.
Experimental
General remarks
All chemicals were purchased from Acros Organics or Aldrich
and were used as received. Proton Nuclear Magnetic Resonance
spectra were recorded on a Bruker DPX 400 spectrometer in
CD3CN and CDCl3. All the solvents were standardised with
respect to TMS. Chemical shifts are reported in ppm and coupling
constants in Hertz. Carbon NMR spectra were recorded on the
same instrument with total proton decoupling. All melting points
were determined with a Stuart SMP3. Flash chromatography
was carried out using silica gel, particle size 0.04–0.063 mm.
TLC analysis was performed on precoated 60F254 slides, and
visualised by either UV irradiation or KMnO4 staining. Infrared
spectra were obtained on a Perkin Elmer Paragon 1000 Fourier
Transform spectrometer. Emission measurements were carried
out with a Perkin Elmer LS 55 Fluorimeter. UV-Vis absorption
measurements were carried out with a Perkin Elmer Lambda 35
UV-Vis Spectrometer. Quartz cuvettes of 1 cm path length were
used. The solutions were irradiated by means of a visible lamp
Schott KL 1500 LCD.
Fig. 12 MALDI-TOF mass spectrum of acetonitrile solutions of SP
containing (a) [CuCl2] = [SP] = 1 ¥ 10-3 M and (b) [Cu(ClO4)2] = [SP] = 1 ¥
10-3 M.
Conclusions
In summary, we have developed a fully reversible spiropyran-based
cation receptor capable of chelating Zn(II) and Cu(II) that can
release its ionic guests by application of visible light. Additionally
we studied extensively the effect of two different counterions,
chloride and perchlorate, on the coordination process. Changing
counterion from ZnCl2 and Zn(ClO4)2, only a slight shift in the
complex absorption maxima from 494 to 492 nm was observed
while a drastic increase in the sensitivity for the bivalent cation was
noticed in the presence of perchlorate. Since the zinc complexes
are highly emitting, this last result was further investigated by
Synthesis
Preparation of 1-(4-ethoxy-4-oxobutyl)-2,3,3-trimethyl-3H-
indolium bromide (1). A solution of 2,3,3-trimethylindolenine
(8.49 mmol, 1.36 ml) and 4-bromobutyrate (12.7 mmol, 1.81 ml)
in 20 ml of chloroform was stirred under reflux for 24 h. The
solution was cooled to room temperature and the solvent was
then evaporated. To the purple residue 1 ml of methanol was
added and the product was crystallized from 20–30 ml of diethyl
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ether affording a pink powder (1.22 g, 43%). 1H NMR (400 MHz,
CDCl3): d 1.19 (t, 3H, J = 7.04 Hz, CH3), 1.62 (s, 6H, (CH3)2),
2.21–2.27 (m, 2H, CH2), 2.70 (t, 2H, J = 5.84 Hz, CH2), 3.17 (s,
3H, CH3), 4.02 (q, 2H, J = 7.6 Hz and 14.6 Hz, CH2), 4.86 (t,
2H, J = 8 Hz, CH2), 7.53–7.60 (m, 3H, arom.), 8.01–8.10 (m,
1H, arom.). 13C-NMR (100 MHz, CDCl3): d 13.6, 15.7, 22.2,
22.4, 22.6, 29.9, 48.0, 54.1, 60.5, 115.4, 122.6, 129.1, 129.5, 140.7,
141.1, 172.3, 195.9.
conversion between SP and ME–Zn controlled by visible light the
solutions were irradiated by means of a visible lamp Schott KL
1500 LCD.
In the emission studies standard solution of SP (1.0 ¥ 10-5 M)
in spectroscopic grade acetonitrile were prepared and equilibrated
overnight at 293 K in the dark. The spiropyran solution (3 ml) was
placed into a 1 cm path length cuvette and the emission spectrum
was recorded; 1 equivalent (3 ml) of standard solution of metal
perchlorates (1.0 ¥ 10-2 M) was added and the resulting solutions
were equilibrated in the dark for 20 min and the emission spectra
were taken.
Preparation of ethyl 4-(8-methoxy-3¢,3¢-dimethyl-6-nitrospiro-
[chromene-2,2¢-indoline]-1¢-yl)butanoate (2). A solution contain-
ing 0.783 g (2.21 mmol) of 1 and 0.653 g (3.32 mmol) of 3-methoxy-
5-nitrosalicylaldehyde in 20 ml of ethanol was stirred at reflux
for 24 h. The solution was then concentrated and kept in the
freezer for 24h. A yellow powder was filtered off from the chilled
solution and washed with cold ethanol. It was then dissolved in
30 ml of dichloromethane, washed with a 10% aqueous solution
of Na2CO3 , dried over Na2SO4. The solvent was then distilled
at reduced pressure affording a blue solid (351 mg, 35%). 1H
NMR (400 MHz, CDCl3): d 1.19 (s, 3H, CH3), 1.24 (t, 3H, J =
4.32 Hz, CH3), 1.29, (s, 3H, CH3), 1.90–2.00 (m, 2H, CH2), 2.34–
2.38 (m, 2H, CH2), 3.25–3.27 (m, 2H, CH2), 3.77 (s, 3H, OCH3),
4.08–4.13 (m, 2H, CH2), 5.85 (d, 1H, J = 10.32 Hz, CH), 6.64
(d, 1H, J = 7.76 Hz, arom.), 6.85 (d, 1H, , J = 10.32 Hz, CH),
6.88 (m, H, arom.), 7.08 (d, 1H, J = 7.2 Hz, arom.), 7.20–7.28
(m, 1H, arom.), 7.63–7.70 (m, 2H, arom.). 13C-NMR (100 MHz,
CDCl3): d 13.7, 19.5, 23.5, 25.5, 31.3, 42.5, 52.2, 55.8, 59.9, 106,
106.3, 107.4, 114.9, 117.7, 118.9,121.2, 121.5, 127.2, 127.7, 135.5,
139.9, 146.5, 146.9, 148.9, 172.7. HRMS (m/z -ES) : Found:
453.2023 (M+ , C25H28N2O6 Requires: 452.5071), 475.1927 (M+ +
Na, C25H28N2O6Na, Requires: 475.4969). n/cm-1 : 2932 (m), 1733
(s), 1519 (m), 1479 (m), 1333 (m), 1270 (s). mp : 123 ◦C.
1H NMR studies
Standard solutions of zinc chloride (2 M) were prepared in
D2O. An aliquot of this solution (10 ml) was added to 800 ml
of spiropyran solution 2 ¥ 10-2 M in CD3CN. The samples were
stored in the dark for 24 h and the spectra were collected.
X-ray crystallography
Data were collected on a Bruker SMART APEX II systems using
Mo-Ka radiation.16 Data were collected using APEX2 software.17
Initial cell constants were found by small widely separated
“matrix” runs. Data collection strategies were determined using
COSMO. Scan speed and scan width were chosen based on
scattering powe◦r and peak rocking curves. Temperature was main-
tained at -153 C using an Oxford Croystream low-temperature
device. Unit cell constants and orientation matrix were improved
by least-squares refinement of reflections thresholded from the
entire dataset. Integration was performed with SAINT,18 using this
improved unit cell as a starting point. Precise unit cell constants
were calculated in SAINT from the final merged dataset. Lorenz
and polarization corrections were applied. Absorption correction
was made with the multi-scan procedure. Data were reduced with
SHELXTL.19 The structure was solved by direct methods without
incident. All hydrogen atoms were assigned to idealized positions
and were allowed to ride. All structures were fully ordered and
none contained solvent or water of hydration. Refinement of
the structure required geometry restraints and constraints. Bond
distances in the C22-C25 and C27-C28 chains were restrained with
DFIX commands. In addition, ISOR and SIMU commands were
used for the C22-C25 chain to restrain the thermal parameters.
The geometry of the C14-C19 benzene ring was idealized with the
“AFIX 66” command. Similar constraints on the C31-C36 ring
introduced instability to the refinement, and this ring was neither
constrained nor restrained. All atoms were given anisotropic
thermal parameters, with the exception of C28, which required
a isotropic thermal parameter in order to remain positive definite.
There is a considerable residual peak which is likely due to the fact
that the diffraction spots were very broad, which produced a weak
dataset with poor signal-to-nose, leading to imprecise structural
parameters. There is one fairly large peak “Q1” somewhat close
to the CH CH unit. Although such peaks sometimes represent
unaccounted-for electron density, in this case it was not possible to
find any meaningful explanation. It is most likely that the peak is
an artifact of the poor fit. Because of its location directly on top of
a C–C bond it is unlikely to be the result of a mis-identified atom
or solvent. In any case, it’s sufficiently below the average electron
Preparation
of
4-(8-methoxy-3¢,3¢-dimethyl-6-nitrospiro-
solution
[chromene-2,2¢-indoline]-1¢-yl)butanoic acid (SP).
A
containing 180 mg (0.398 mmol) of 2 was stirred in a solution
2 : 1 of THF and aqueous NaOH 10% for 24 h. The reaction was
quenched with 40 ml of an aqueous solution of citric acid 10%,
the product was then extracted with chloroform, washed with an
aqueous solution of NaHSO3 15% and dried over Na2SO4. The
solvent was evaporated to afford a blue powder (150 mg, 88%.
1H NMR (400 MHz, CD3CN): d 1.19 (s, 3H, CH3), 1.29, (s, 3H,
CH3), 1.72-1.78 (m, 2H, CH2), 2.24-2.26 (m, 2H, CH2), 3.09-3.16
(m, 2H, CH2), 3.77 (s, 3H, OCH3), 5.76 (d, 1H, J = 10.32 Hz,
CH), 6.53 (d, 1H, J = 7.76 Hz, arom.), 6.73 (d, 1H, J = 10.32 Hz,
CH), 6.82 (m, 1H, arom.), 6.98-7.10 (m, 2H, arom.), 7.52-7.63 (m,
2H, arom.), 9.95 (s, 1H, COOH). 13C-NMR (100 MHz, CD3CN):
d 18.7, 23.5, 25.3, 31.3, 42.5, 52.2, 55.9, 106.0, 106.3, 107.2,
115.2, 117.7, 118.5, 121.1, 121.5, 127.2, 127.7, 135.5, 130.0, 147.1,
147.1, 148.9, 172.7. HRMS (m/z -ES) : Found: 425.1706 (M+,
C23H24N2O6 Requires: 424.4534). n/cm-1 : 2962 (m), 1707 (m),
1583 (m), 1513 (m), 1232 (s), 1166 (m), 1088 (m). mp : 178 ◦C.
UV-vis absorption and emission spectroscopy
In the UV-Vis. absorption studies, standard solutions of spiropy-
rans with concentration of 1 ¥ 10-4 M in acetonitrile and standard
solutions of metal perchlorates and chlorides in water with
concentration of 1 ¥ 10-1 M were used. Quartz cuvettes of 1 cm
path length were used. In the experiment of sequential cycles of
8276 | Dalton Trans., 2010, 39, 8269–8277
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density for this crystal to trigger only a “C” level warning in
checkcif. We collected data on numerous crystals, but the reported
structure is the best we could obtain.
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Mass Spectrometry
MALDI-TOFMS spectra were acquired with a Waters MALDI-
Q
TOF Premier spectrometer. The instrument was op-
erated in positive reflectron mode. The matrix used in
the experiments was trans-2[3(4-tert-Butylphenyl)-2-methyl-2-
propenylidene]malononitrile dissolved in DCM (2 mg ml-1).
Acetonitrile solutions of the complexes were prepared by adding
10 ml of a 1 ¥ 10-1 M solution of the desired metal chloride or
perchlorate to a 1 ¥ 10-3M solution of SP. The resulting mixtures
were equilibrated overnight at 20 ◦C in the dark. Matrix solutions
were mixed with complex solutions with ratio 1 : 1 and 1 ml of
the resulting mixture was spotted on the MALDI plate. The
compound [Glu1] Fibrinopeptide B, 1570,6774 m/z (M+H)+ was
used as reference. All the data were collected in the range of 500–
1550 Da.
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Acknowledgements
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This work was supported by Science Foundation Ireland (PIYRA
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Dr John O’Brien for NMR, Dr Martin Feeney and Dr Bernard
Jean-Denis for MS, and Trinity College Dublin for support (Trinity
Award to MN).
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16 Crystal data for Zn-ME-Cl: C23H23ClN2O6Zn, M = 524.25, monoclinic,
a = 9.000(10) b = 7.729(9), c = 37.55(4), b = 94.774(8), U = 2603(5), T =
120 K, space group P21/c, Z = 4, 61286 reflections measured, 6509
independent reflections, R1(observed) = 0.2237, wR2 (all) = 0.4679.
ꢀ
C
17 APEXII v1.27,
2005, Bruker Analytical X-ray Systems,
Madison, WI.
ꢀ
C
18 SAINT v6.02, 1997–1999, Bruker Analytical X-ray Systems, Madi-
son, WI.
19 SHELXTL v5.10, 1997, Bruker Analytical X-ray Systems, Madison,
WI.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 8269–8277 | 8277
©