Interaction studies between photochromic spiropyrans and transition metal cations: The curious case of copper

Article abstract of DOI:10.1039/c1ob06375h

A series of four spiropyrans bearing different substituents on the indolic nitrogen were synthesized and their capability of binding mono and bivalent transition metal cations in solution was assessed via UV-visible absorption spectroscopy. All the compounds responded selectively to the presence of Cu(ii) ions producing intense absorption bands in the visible region of their spectra. Bidimensional ¹H-NMR and MALDI-TOF MS spectroscopies revealed the formation of SP dimers mediated by Cu(ii). This is the first example of cross-coupling mediated by copper(ii) in mild conditions causing the symmetric dimerization of spiropyran dyes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1ob06375h

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PAPER
Interaction studies between photochromic spiropyrans and transition metal
cations: the curious case of copper†
Manuel Natali and Silvia Giordani*
Received 12th August 2011, Accepted 27th October 2011
DOI: 10.1039/c1ob06375h
A series of four spiropyrans bearing different substituents on the indolic nitrogen were synthesized and
their capability of binding mono and bivalent transition metal cations in solution was assessed via
UV-visible absorption spectroscopy. All the compounds responded selectively to the presence of Cu(II)
1
ions producing intense absorption bands in the visible region of their spectra. Bidimensional H-NMR
and MALDI-TOF MS spectroscopies revealed the formation of SP dimers mediated by Cu(II). This is
the first example of cross-coupling mediated by copper(II) in mild conditions causing the symmetric
dimerization of spiropyran dyes.
Introduction
The development and synthesis of artificial receptors tailored
to respond to a wide variety of clinically and environmentally
important species have been of great interest in the last two
1–3
decades. Spiropyrans, reported for the first time in the early
4
fifties, are members of the well-established class of photochromic
compounds. They respond to a multitude of optical and chemical
stimuli. Ultraviolet light promotes the heterolytic cleavage of
the spiro C–O bond, followed by cis-trans isomerisation with
generation of a metastable merocyanine. The reverse process of
ring-closing takes place upon irradiation with visible light or
thermal bleaching. Both solvatochromism and acidochromism
Scheme 1 Spiropyran-based selective and reversible Zn(II) sensor.
metal coordination process (Scheme 1). In more recent studies
we expanded the library of our molecules and produced new
candidates for ion sensing, retaining the same spiropyran skeleton
we chose previously but changing the functional groups. Among
these new spirochromenes, four molecules showed outstanding
selectivity towards Cu(II) ions. Thrilled by this odd result, we
delved into studying their properties in order to produce a rational
and plausible explanation. Helped by analytical techniques such as
5–7
have been reported. They also respond reversibly to metal
8
cations producing an optical signal, and occupy a dignified
niche in the vast universe of chemosensors. Due to the intriguing
nature of their molecular recognition and signal transduction
mechanisms, great interest has arisen in the scientific community
leading to the construction of spiropyran based sensors that cover
1
bidimensional H-NMR, MALDI-TOF HRMS, and UV-visible
absorption spectroscopy we have been able to shade light on
this unique case. We report herein the synthesis and the activity
evaluation of four spiropyran-based cation receptors: this is the
curious case of copper.
9
–19
20,21
a disparate range of analytes such as metal ions, aminoacids,
and nucleosides.
22,23
Fascinated by the potentiality of this class of
compounds, we recently designed new spiropyran based probes for
the detection of metal cation that showed remarkable selectivity
24,25
and sensitivity.
Indeed, the binding constant of the metal ligation process
was affected by irradiation with visible light yielding the up-
take/release of the cation fully reversible and controllable. As
we reported, the substituents decorating the spiropyran skeleton
are crucial factors in obtaining a good degree of selectivity
towards a specific analyte while retaining reversibility of the
Results and discussion
The four spiropyrans SP1, SP2, SP3, and SP4 discussed in this
article are those depicted in Scheme 2. Although these structures
have been known for quite a long time,
reported regarding their coordinative behaviour.
In the endeavour of designing molecules capable of acting as
efficient, sensitive and reversible metal ion detectors, we chose
these known structures as a starting point. The reason is mainly
due to the simplicity of their structures. As we mentioned before,
the functional groups decorating the spiropyran backbone are
key elements that can facilitate metal-ion binding and promote
26–28
very little has been
29
Trinity College Dublin, School of Chemistry, College Green, Dublin, 2,
Ireland. E-mail: giordans@tcd.ie; Fax: +353 (0)1 896 1422; Tel: +353
(
0)1 671 2826
†
Electronic supplementary information (ESI) available: Methods, synthe-
1
13
sis, H-NMR and C-NMR spectra. See DOI: 10.1039/c1ob06375h
1
162 | Org. Biomol. Chem., 2012, 10, 1162–1171
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variation of the spiropyran spectrum. However, addition of 1
equivalent of copper perchlorate caused a sudden colour change
from slightly pink to deeply orange in few seconds.
This event was accompanied by the appearance of an intense
absorption band localized in the visible region with maximum
intensity at 496 nm. Other two maxima of the same intensity
were visible in the UV region at 263 nm and 305 nm respectively
(
Fig. 1a).
Despite the structural difference between SP1 and the other
members of this group, the addition of the same amount of Cu
ClO caused the same colour changes in acetonitrile solutions
(
)
4 2
of SP2, SP3, and SP4. The absorption spectrum of a SP2
shows bands in the UV region at 266 nm, 301 nm, and 340 nm.
After addition of copper(II), the UV-vis spectrum of the same
solution showed an intense absorption band, flat on its top with
maximum intensity at 515 nm and a shoulder at 493 nm. Weak
absorption shoulders were observed at 410–414 nm in the spectra
of solutions containing the same molecule and either iron(II)
or zinc(II) indicating little interactions with these two cations
as well (Fig. 1b). The absorption maxima recorded at 269 nm,
299 nm, and 340 nm for a metal-free solution of compound
SP3 were similar to those of SP1 and SP2. Addition of zinc(II)
perchlorate and iron(II) perchlorate caused the appearance of
two shoulders in the spiropyran spectra at 410 nm and 400 nm
respectively (Fig. 1c). Addition of copper(II) perchlorate caused
an intense absorption band to appear in the range of 450–600
nm with maximum intensity at 517 nm (Fig. 1c). Even compound
SP4 which is functionalized with a carboxylated tether did not
show any interaction with any of the tested metal salts except
for a copper(II) perchlorate. The presence of this metal in fact
caused analogous changes in the compound absorption spectra
which resemble the changes observed for the other compounds in
this group. The absorption band risen from addition of copper
had its maximum localized at 514 nm. The response of the
Scheme 2
photoinduced control of the metal ligation. In these four com-
pounds, only two functional groups are present on each structure.
The indolic nitrogen offers a functionalizable site and several
examples of N-modified spirochromenes for metal chelation
2
9,30
applications are known.
If a group with good coordinating
properties and sufficient conformational flexibility is placed in
this position, the presence of a metal cation may induce the
spiro C–O bond cleavage and promote its ligation through a
bidentate system formed by the chelating group itself and the
merocyanine phenolate anion. With this in mind we chose three
different aliphatic tethers bearing a terminal group with binding
properties such as a carboxylic acid (SP4), an ethyl ester (SP3),
and a hydroxy group (SP2). Compound SP1 is functionalized with
a methyl group that cannot be considered a binding site. However
a bivalent metal can be coordinated by two merocyanine units
31
through their phenolate anions. It is known that the presence of
a strongly electron-withdrawing group in the 6¢ position stabilizes
the open merocyanine form whereas an electron-donating group
produces a more stable photostationary closed form. Since the
nitro group is a very strong electron withdrawing function, the
ring opening-close in these structures should be easily photocon-
trollable as well as the uptake-release of a metal by their open
merocyanines.
four spiropyrans to different concentrations of Cu (ClO
4
2
) was
investigated. In all experiments similar events were observed.
The sequential addition from 0.1 to 1 equivalent of copper(II)
caused different bands to appear at different wavelengths which
resolved eventually in those reported in Fig. 1. However, in all
cases different isosbestic points were observed. These results are
shown in Fig. 2. Addition of 0.4 equivalents of copper(II) to a
solution of SP1 caused the appearance of two bands with maxima
at 481 nm and 503 nm which coalesced into a broad absorption
with maximum at 496 nm in the presence of higher concentrations
of the metal (Fig. 2a). In the same spectra two isosbestic points
are localized at 277 nm and 348 nm. Increasing the concentration
of copper(II) in a solution of SP2 caused two similar bands to
appear in the visible region with maximum intensity at 488 nm
and 515 nm. Three isosbestic points were observed at 279, 338,
and 361 nm (Fig. 2b). The same number of isosbestic points was
observed for the solutions containing either SP3 or SP4. In the
first case they were localized at 277, 335, and 360 nm (Fig. 2c) while
in the second case at 278, 332, and 365 nm (Fig. 2d). Two bands
with maxima at 481 nm and 515 nm were observed in the spectra
of compound SP3 at low concentration of copper(II). These two
bands coalesced partially at higher concentrations of the metal
evolving in two absorptions at 491 nm and 517 nm observed after
the addition of 1 equivalent of copper (Fig. 2c). Analogously,
compound SP4 responded to low concentrations of copper(II)
Spiropyrans SP1–SP4 were synthesized according to known
28,32–34
procedures.
In order to establish whether the combination
of the different functional groups were capable of offering metal-
ion recognition, systematic binding studies were performed on
our spiropyrans with eleven different metal cations. Biologically
and environmentally relevant metal ions such as Zn(II), Cu(II),
Ni(II), Co(II), Mn(II), Cd(II), Fe(II), Mg(II), Ca(II), Na(I) and
K(I) were tested. The investigations were carried out via different
spectroscopic techniques such as UV-vis absorption and emission
1
spectroscopy, H-NMR, and MALDI-TOF HRMS.
UV-vis absorption studies
The addition of the eleven monovalent and bivalent cations
to spiropyrans solutions in acetonitrile gave unexpected results.
The first compound analysed was SP1 (Fig. 1). The absorption
spectrum of this derivative shows three maxima in the ultraviolet
region at 263 nm, 293 nm, and 341 nm respectively. The presence
of 1 equivalent of ten of the eleven metals did not produce any
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Fig. 1 Absorption spectra of SP1 (a), SP2 (b), SP3 (c), and SP4 (d) before and after the addition of 1 equivalent of different metal perchlorates and
-4
chlorides (1.0 ¥ 10 M, acetonitrile, 293 K).
with two absorption maxima at 482 nm and 517 nm in the visible
region that shifted at 491 nm and 512 nm respectively (Fig. 2d).
significant intensity decrease. The maximum absorption located
at 504 nm dropped to an intensity value which was 1/3 of the
non-irradiated solution one and blueshifted to 472 nm (Fig. 3b).
Analogously, the absorption maximum at 517 nm of a solution of
Reversibility of the spiropyran-copper interactions
SP3-Cu(ClO
4
2
) decreased in intensity and blueshifted to 478 nm.
The solutions containing the four spiropyrans and 1 equivalent
of copper were irradiated with visible light for 10 min and the
corresponding UV-vis spectra were recorded. The absorption
maxima in the spectrum of SP1 underwent remarkable changes in
its features after irradiation. First the two absorption maxima in
the ultraviolet region at 263 nm and 305 nm shifted bathochromi-
cally to 271 nm and 308 nm and their intensity increased after
irradiation with visible light. The broad absorption in the visible
region decreased in intensity and shifted hypsochromically from
The peak 266 nm of non-irradiated solution moved upward after
irradiation with visible light and shifted to 271 nm while the two
smooth absorption bands at 302 nm and 329 nm coalesced in
one unique absorption band higher in intensity whose maximum
was located at 310 nm (Fig. 3c). Finally, a solution containing
SP4 and Cu(ClO
4
2
) responded to visible light irradiation with a
blue shift of the maximum at 514 nm to a new absorption band
appeared at 480 nm concomitant with a loss of intensity of the
50%. Additionally a new absorption band was observed at 391
nm while the two peaks at 264 and 306 nm redshifted to 271
and 311 nm respectively. The maximum at 271 nm was lower in
intensity with respect to that at 264 nm while the appearance of
the new peak at 311 nm was accompanied by an intensity increase
(Fig. 3d). The interaction between the members of this group of
spiropyran-based receptors and copper(II) perchlorate underwent
4
96 nm to 452 nm. A small shoulder was barely visible at 378
nm (Fig. 3a). The changes that took place in the absorption of
a solution of SP2 and copper perchlorate after irradiation with
visible light resembled those observed for SP1. The maximum
at 347 nm redshifted slightly to 352 nm retaining its intensity
while the peak at 414 nm redshifted to 417 nm and underwent a
1
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-4
Fig. 2 Absorption spectra of SP1 (a), SP2 (b), SP3 (c), and SP4 (d) (1.0 ¥ 10 M, acetonitrile, 293 K) after increasing the concentration from 0.1 to 1
eq of Cu (ClO
4 2
) .
visible changes upon irradiation with visible light. If a complex was
actually formed between the spirochromenes and copper(II), these
results would indicate only a partial release of the metal guest from
the receptors sites as the original spectra of metal free solutions of
spiropyrans were not recovered after irradiation with visible light.
Overall, a very selective response to copper(II) was observed with
all the members of this group regardless to the substituent present
on the indolic half of the molecules.
by the coordination between two phenolate and the metal while
the substituent on N is completely extraneous to the binding. In
order to confirm this hypothesis we carried out MALDI-TOF MS
studies on the solutions containing SP1, SP2, SP3, and SP4 in
the presence of an equimolar amount of Cu(ClO
4
)
2
. Calculation
of the theoretical mass of spiropyran-copper complex 1 : 1 affords
2
+ +
a value of 385 m/z for the molecular ion [SP1-Cu ] . If two
molecules of compound SP1 form a sandwich like complex with
2
+
2+ +
one Cu cation the resulting molecular ion [SP1–SP1-Cu ]
mass would be 707 m/z. In Fig. 4a the spectrum of SP1 in
the presence of copper shows a peak whose value of 643 m/z
corresponds exactly to a [SP1–SP1] dimer with no copper in
Mass spectroscopy analysis
+
As mentioned above, spiropyrans can form complexes with
transition metals whose composition can vary depending on
the structure of the switchable molecule. Complexes where the
it and a weight loss of one proton. Peaks at 642 and 644 m/z
were also detected indicating different protonation states of the
molecule. The small peak at 322 m/z correspond to monomer
2
+
ratio SP-M is either 2 : 1 or 1 : 1 are often observed. When the
ratio is 2 : 1, two open merocyanines usually cooperate in the
coordination of a metal cation by means of their phenolate anions.
Thus, sandwich-like complexes are formed and the metal guest
is trapped between the two molecules. Considering the unusual
results observed with these spiropyrans, a similar explanation
would be plausible. Since all compounds responded to the presence
of copper(II) perchlorate in the same manner regardless to the
substituent on the indolic moiety, we thought that sandwich like
complexes formed between to molecular switches and the metal
guests where the interaction between the three parts was only due
+
+
[SP1] . A peak indicating the molecular ion [SP2–SP2] at 702
m/z was found in mass spectroscopy analysis carried out on the
SP2 solution containing copper (Fig. 4b). This peak indicates the
mass of two SP2 molecules with a weight loss of one proton. In this
experiment other peaks were found at 330, 352, 423, 702, and 704
+
m/z. That at 352 corresponds to the molecular ion [SP2] . Also in
this case it seems that the metal does not form a complex while the
peaks at 702 and 704 m/z may indicate different protonation states
of the same molecule. In the analysis of the solution containing
SP3 and the same cation, the presence of a spiropyran-spiropyran
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-4
Fig. 3 Absorption spectra of SP1 (a), SP2 (b), SP3(c), and SP4 (d) (1.0 ¥ 10 M, acetonitrile, 293 K) before (black curves), after the addition of 1 eq
of copper perchlorates (grey curves), and irradiation with visible light for 10 min (dotted curves).
+
dimer was confirmed by its molecular ion peak [SP3–SP3] at
8
added to each spiropyran solution which was stirred at room
temperature for 24 h. The solvent was evaporated and each
crude was redissolved in dichloromethane and washed with an
43 m/z along with other two peaks at 842 and 844 m/z smaller
in intensity (Fig. 4c), while a peak at 422 m/z indicated the a
single molecule of SP3. The carboxylated compound SP4 formed
apparently a dimer as well, whose molecular ion [SP4–SP4] peak
is the one shown in Fig. 4d at 787 m/z. Two peaks at 786 and
88 m/z were detected as well. Also in this case a small peak at
93 m/z revealed the presence of the monomer SP4. All these
experiments showed the formation of new species that apparently
were not complexes.
aqueous solution of NaHCO containing the 5% of EDTA. The
3
+
latter is a well known chelating agent that should bind all the
copper(II) cations in solution leaving any spiropyran derivative
as metal-free compounds. In each solution a red precipitate was
observed and filtered off. Each organic phase was collected and the
solvent was evaporated under reduced pressure. A first thin layer
chromatography analysis of compounds SP1 and SP3 reaction
mixtures revealed after 24 h the appearance of one new spot
each, whose retention factor (0.4 in hexane/ethylacetate 6 : 4) was
lower than that of the corresponding spiropyran derivative (0.8 in
hexane/ethylacetate 6 : 4). In both cases, these new species were
purified via flash chromatography on silica gel and successively
7
3
Purification and identification of SP-dimers
The MALDI-TOF mass spectra suggested the possible formation
of dimers with no trace of copper(II). In order to investigate
if copper(II) could be involved in a process different from a
chelation, namely a redox reaction, and to characterize the dimers
observed via mass spectroscopy, we scaled up the process by
reacting amounts of the four spiropyrans with equimolar amount
of copper perchlorate in the same condition used in the UV-vis
experiments. Approximately 10 mmol of each spirochromene were
dissolved in 5 ml of acetonitrile and an equimolar amount of
copper perchlorate dissolved in 100 ml of deionised water was
1
characterized by means of both mass spectroscopy and H-NMR
analysis. Reaction yields were calculated to be 46% and 43%
for SP1 and SP3 respectively. MALDI-TOF MS analysis of the
compound purified from the reaction of SP1 revealed the same
peak observed in the previous experiment whose value correspond
to a molecular ion of mass equal to 742 Da. Analogously, the
purification of the compound obtained during the reaction of SP3
with copper perchlorate had a mass of 842 Da. In both cases these
1
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-3
4 2
Fig. 4 MALDI-TOF mass spectrum of acetonitrile solutions of SP1 (a), SP2 (b), SP3 (c), and SP4 (d) containing [Cu(ClO ) ] = [SP] = 1 ¥ 10 M.
molecular masses are double of the corresponding spiropyrans
mass, less two protons each. This implies that two spiropyrans
reacted with each other forming probably a C–C bond with the
doublet at 6.82 ppm while protons e and d are shifted to a lower
field at 8.05 (double doublet) and 8.13 (doublet) ppm respectively.
The fact that these two signals are not a doublet for e, and a
singlet for d, indicates a long range coupling between the two
protons, confirmed by their coupling constant of 2.8 Hz. Protons
g give a doublet at 6.68 ppm whose coupling constant is equal to
8.0 Hz. The same value is observed for the double doublet located
at 7.45 ppm which corresponds to protons h. A non-symmetric
doublet at 7.42 ppm indicates protons i. Also in this case long range
coupling between h and i was observed. The coupling between d–
1
loss of two protons. If this hypothesis was true, the H-NMR
of each spiropyran should lack of two protons. To confirm this,
1
both mono and bidimensional H-NMR analysis were carried
out on the two compounds obtained from these purifications.
1
The H-NMR spectrum of SP1–SP1 is shown in Fig. 5a. It is
very interesting to observe that this spectrum resembles perfectly
that of a single molecule of SP. Seventeen protons are counted
in this spectrum while a single molecule of SP1 has 18 protons.
Considering a mass that is equal to the double of that of SP1
less two protons, the structure of SP1–SP1 must have a plane of
symmetry. Overall, the spectrum found corresponds to molecule
SP1–SP1 (Fig. 5) which is the dimer of two spiropyrans linked
to each other through a C–C bond located in 5-position of their
indolic system. The typical peaks of the gem methyl groups are
visible as two singlets that integrate for 6 protons each at 1.25
and 1.36 ppm (a in Fig. 5a). These values indicate that the two
methyl groups are not magnetically equivalent and the spiropyran
is in its closed form. This hypothesis is confirmed by the shift of
the indolic methyl groups j, which are observed as a singlet that
integrates for six protons at 2.79 ppm, and by the coupling constant
of the two vinylic protons b and c, located at 6.02 ppm and 7.14
ppm respectively, that is equal to 10.4 Hz. This value is typical for
a cis configuration. The aromatic protons f are observable as a
1
1
e–f and h–i–j are underlined in the H- H COSY of the compound
aromatic region depicted in Fig. 5b. Purification by column flash
chromatography of the crude obtained from reacting compound
SP3 with Cu(ClO
H-NMR and H- H COSY NMR spectra are reported in Fig. S1a
4
1
) afforded the pure compound SP3–SP3 whose
2
1
1
and S1b (ESI†), respectively. The presence of a symmetry plane
1
is deduced from its H-NMR that shows the proton shifts of one
single molecule, minus one aromatic proton. Thus the spectra
obtained correspond to a spiropyran dimer like the one depicted
in Scheme 3 with two dyes attached to each other through a
C–C bond located in para position with respect to the indolic
nitrogen. Reaction of spiropyran SP2 with equimolar amount
of Cu(ClO
4
2
) in larger scale produced several by-products. The
purification of these compounds by flash chromatography resulted
to be particularly difficult. Several attempts were made changing
conditions and eluent polarity but they were all unsuccessful.
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Scheme 3
1
1
1
Fig. 5 (a) H-NMR and (b) H- H COSY NMR partial spectra of
-2
SP1–SP1 (1 ¥ 10 M in acetonitrile, 293 K).
However, a sufficient amount of compound SP2–SP2, depicted in
Scheme 3, was purified via preparative thin layer chromatography
by using a mixture 1 : 1 of ethyl acetate and hexane as eluent
1
(
R
f
0.4), and eventually characterized. The H-NMR spectra of
SP2–SP2 are reported in Fig. S2 (ESI†). Although it was not
possible to carry out any UV-visible experiment on this molecule
1
since it was not pure, we identified in its H-NMR very similar
patterns to those observed for SP2–SP2 and SP3–SP3, and both
magnetic resonance and mass spectroscopy analysis suggested that
a dimerization took place. Reaction of Cu(ClO
4
2
) with SP4 was
carried out but the purification of any side product failed. The
MALDI-TOF MS analysis reported above is the only data we have
and no speculations are allowed on the hypothetical structures of
compounds generated during the reaction.
Possible mechanisms of dimerization
The dimerization observed consists of an aryl carbon-carbon bond
formation via a regioselective oxidative coupling between two
identical molecules. The reaction is likely to be induced by the
stoichiometric amount of copper(II) which is probably reduced to
copper(0) (the red precipitate we observed) during the process.
We suggest a mechanistic explanation to such a process by
following a reaction pathway that involves the formation of
Scheme 4 Mechanism of dimerization via radical formation.
radical species (Scheme 4). In this case the Cu(II) is reduced
to Cu(0) triggering the formation of two SP radicals. The two
radical SPs react with each other forming a symmetric dimer
1
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-6
-5
Fig. 6 (a)and (c) UV-vis spectra of a solution of SP1–SP1 and SP3–SP3 (2.5 ¥ 10 M and 2.5 ¥ 10 M in acetonitrile, 293 K) before (black curves)
-6
-5
and after (dotted curves) irradiation with UV light; (b) and (d) UV-vis absorption spectra of SP1–SP1 and SP3–SP3 (2.5 ¥ 10 M and 2.5 ¥ 10 M,
acetonitrile, 293 K) after increasing the concentration of Cu(ClO
4 2
) .
with no conjugation between the two halves. Abstraction of two
protons from the indolic aromatic regions allows the conjugation
to be fully restored and the final dimer to be formed. The
ability of copper(II) to trigger radical reactions on similar systems
investigated by UV-vis absorption spectroscopy. Spectra of colour-
less acetonitrile solutions containing SP1–SP1 and SP3–SP3 are
shown in Fig. 6. Spiropyran SP1–SP1 absorbs in the UV region
with two distinctive bands with maxima at 275 nm and 303 nm.
It is interesting to observe that its molar absorption coefficient
35
has been reported in the literature. In 1985 Berti and his
collaborators reported the dimerization of indoxyls mediated
5
-1
-1
of 3.5 ¥ 10 M cm is an order of magnitude higher than
3
6
4
-1
-1
by FeCl
3
.
More recently, Li and co-workers investigated the
in
1.1 ¥ 10 M cm observed for the corresponding monomer SP1.
This increased capability of absorbing light of compound SP1–
SP1 is probably due to its extended conjugation between the two
indolic moieties. Irradiation with UV light at 254 nm caused the
appearance of a new band in the visible region at 579 nm (Fig.
6a) accompanied by blue coloration of the solution. This band
indicates more likely the photoisomerization of the spiropyran
units in their corresponding merocyanines. At this stage it is not
clear if one or both units are involved in the photoisomerization.
The band in the visible region disappeared after 10 min of storage
in the dark and the spiropyran spectrum was fully recovered.
oxidative homocoupling of naphthylamines mediated by FeCl
3
37
extremely mild conditions. While Berti claimed the formation of
radical intermediates, Li suggested a mechanistic explanation with
two plausible routes, involving the formation of either radical or
organometallic intermediates. Although the oxidant in these cases
is Fe(III) and the substrates are different from our spiropyrans,
we believe that an alternative pathway for the homocoupling of
our molecules that produces an organometallic intermediate may
resemble that suggested by Li. We are currently evaluating this
hypothesis.
Addition from 0.4 to 3 equivalent of Cu(ClO
4
2
) to the same
spiropyran solution produced the appearance of new bands in
the visible region with the highest intensity peak at 475 nm that
reached its maximum value after the addition of 3 equivalent of
salt. Concomitantly, the two absorption maxima in the UV region
SP dimers’ photoswitchability and response to Cu(II)
The binding capability of the spiropyran dimers obtained by
addition of Cu(ClO
4
2
) and their corresponding monomers were
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decreased asymmetrically and two isosbestic points were noticed
at 264 nm and 368 nm (Fig. 6b). No response was observed with
concentrations of salt below 0.4 equivalents and the fact that 3
of the metal cations produced any changes in the dimers UV-vis
absorption spectra (Fig. S3, ESI†). We are currently evaluating if
this apparent selectivity towards copper(II) is retained also in the
presence of different counter ions.
equivalents of Cu(ClO
4
2
) were needed to produce the maximum
effect may reflect a low association constant between the dimer
and the salt. The appearance of the new bands is probably due to
ring opening of both spiropyran units and the following formation
of a copper complex where the metal is trapped between the two
phenolates. Compound SP3–SP3 responded to photostimulation
with UV light as well. Its spectra before and after irradiation
are shown in Fig. 6c. Analogously to compound SP1–SP1, an
acetonitrile solution of SP3–SP3 is colourless and absorbs in
the UV region with maxima at 275 nm and 308 nm. Its molar
The reversibility of copper complexation was investigated by
irradiating with visible light the spiropyran solutions SP1–SP1
and SP3–SP3 containing the metal. Interestingly, the phenomena
observed were very similar to those observed after shining visible
light on the solutions containing spiropyrans SP1 and SP3 in
the presence of copper(II). In both cases the bands in the visible
region of their spectra decreased in intensity and blueshifted but
the spectra of the spirochromenes in the absence of copper(II)
was not recovered (Fig. 7a and 7b). This indicates probably just a
partial release of the metal cation from the photochromic system.
Additionally these changes were not reversible; neither storage
in the dark nor irradiation with UV light recovered the spectra
observed before irradiation with visible light. These unexpected
results need to be further investigated.
4
-1
-1
absorption coefficient of 4.2 ¥ 10 M cm four times higher than
4
-1
-1
1
.0 ¥ 10 M cm observed for the corresponding monomer SP3.
Irradiation with UV light triggers the photoisomerization of the
spiropyran subunits to the open merocyanines accompanied by the
appearance of a visible band with maximum intensity at 586 nm.
This new absorption band disappeared shortly after storage in
the dark. The addition of Cu(ClO
4
2
) caused dramatic changes
in the molecule absorption profile, similar to those observed for
SP1–SP1. The absorption maximum at 275 nm blueshifted to
2
61 nm while that at 308 nm decreased in intensity (Fig. 6d). A
strong intensity band evolved in the visible region reaching its
maximum absorption intensity at 515 nm after the addition of 1
equivalent of Cu(II). This band indicates probably the formation
of a complex with a copper ion coordinated by the phenolate units
of an isomer with two merocyanines. The two tethers present on
each SP subunits can participate at the chelation as well; this
offers alternative coordination geometries that still need more
comprehensive investigations. However, we exclude the possibility
of formation of only one type of complex where the metal is
coordinated by the two tethers with the SP units being in their
closed form as the conjugation through the two spiropyrans would
remain unaltered. Thus no absorption would be expected in the
visible region. By comparing the UV-vis spectra of compounds
SP1, SP3, and their corresponding dimers SP1–SP1 and SP3–
SP3 (Fig. 1 and Fig. 6) at their ground state in absence of
Cu(II), it is noticed that all SPs show negligible absorptions in
the visible region. Addition of Cu(II) to the dimers’ solutions
causes the appearance of absorption bands in the visible region
that are comparable to those observed when Cu(II) is added to
the monomer solutions. If the dimerization mechanism involving
the formation of an organometallic intermediate is correct, the
absorption bands in the visible region observed after the addition
of Cu(II) to SP1 and SP3 solutions (Fig. 1) may be due by
the contribution of more than one species. It is plausible to
2
+
expect a first Cu cation to trigger the formation of a transient
merocyanine-metal complex (the first species absorbing in the
visible region) with the cation coordinated by the phenolate.
This open form, for the reasons mentioned in the mechanistic
explanation, would be susceptible to homocoupling with an
analogous species that would lead to a dimer formation. The same
dimer would not be able to react any further but could coordinate
Cu cations thus forming a complex with absorption in the visible
region.
The response of both dimers to the presence of mono and
bivalent metal cations was also evaluated. The same cations used
before with the monomers were utilized also in this instance. None
-
6
Fig. 7 (a) UV-vis spectra of a solution of SP1–SP1 (2.5 ¥ 10 M in
acetonitrile, 293 K) in the presence of 3 equivalents of Cu(ClO before
black curve) and after (dotted curve) irradiation with visible light; (b)
2
+
4 2
)
(
-5
UV-vis spectra of a solution of SP3–SP3 (2.5 ¥ 10 M in acetonitrile, 293
K) in the presence of 1 equivalent of Cu(ClO ) before (black curve) and
4 2
after (dotted curve) irradiation with visible light.
1
170 | Org. Biomol. Chem., 2012, 10, 1162–1171
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Conclusions
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This group of spiropyrans bearing different substituents only on
the indolic part of their skeleton showed selective responses only
to the presence of copper(II) ions. However the interaction with
copper(II) appeared to be only partially reversible as irradiation
with visible light was not capable of producing the complete release
of the metal cation. By deepening our studies on these unusual
interactions with copper(II) we discovered that the binding of this
metal is accompanied by a dimerization of our molecules. Indeed,
we purified two compounds, namely SP1–SP1 and SP3–SP3,
that are the products of a regioselective oxidative cross-coupling
mediated by copper(II). A third compound, SP2–SP2, was iden-
4
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pure dimers SP1–SP1 and SP3–SP3 retained their capability of
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with visible light produced irreversible changes that are currently
under investigation. It is interesting to observe that compounds
SP1–SP1 and SP3–SP3 were produced in very mild conditions
with yields ranging around 45%. To the best of our knowledge
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This journal is © The Royal Society of Chemistry 2012
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