gies are reported to be in agreement with the experimental
2. Results and Discussion
[
8,9]
data in solution.
Depending on the nature of the substitu-
ents and the medium, the electronic distribution of ME varies
from a zwitterionic form to a nonionic quinoidal structure. ME
2
.1. Synthesis
[10,11]
can be stabilized by highly polar solvents
and solid matri-
Protonated merocyanines can be transiently generated in solu-
tion upon irradiation of the corresponding spiropyrans under
acidic conditions (Figure 1). Classical synthesis of the spiropyr-
ans relies on the condensation of salicylaldehyde derivatives
with 1,3,3-trimethyl-2-methyleneindoline (Fischer base). Reflux
in solvents and long reaction times (2 h to 7 d) are typically re-
quired. The synthesis of spiropyrans with ultrasounds has been
[12–14]
[15–17]
ces
or through coordination to metal ions.
Encapsula-
tion can selectively stabilize one of the isomeric forms, which
enables the direct synthesis of functional and tunable solid
[
18]
materials. Formation of salts with the protonated form (i.e.,
+
[19,20]
MEH ) is induced in the presence of acids.
A three-state molecular switch based on SP_NO has been
2
[21]
[36]
designed and investigated. It combines light and chemical
stimuli that transduce into optical outputs through a sequence
proposed as an interesting alternative.
Recently, protonated merocyanines were directly synthesized
by Knoevenagel condensation between substituted benzalde-
+
of logic operations involving the SP, ME, and MEH forms
[
37]
(
(
Figure 1).
hydes and 1,2,3,3-tetramethyl-3H-indolium iodide in ethanol.
This three-state molecular switch detects three input signals
This procedure required long reaction times (typically 15–20 h)
and was performed at a high temperature (reflux) to reach
yields in the 60–85% range.
+
ultraviolet light, visible light, and H ) and generates two
+
output signals (absorption bands at l=400 nm for MEH and
[21,22]
l=563 nm for ME), leading to logic gates.
Direct access to nitro- and bromo-substituted protonated
+
Protonated ring-opened isomers of SP (protonated merocya-
merocyanines [MEH , X=NO , Br (Figure 1)] by mechano- and
2
+
nine form, MEH ) are thermodynamically stable in acidic aque-
sonosynthesis is reported here.
+
ous solutions in the dark. The effect of substituents on the spi-
Nitro-substituted merocyanine (MEH , X=NO ) was ob-
2
[
23]
robenzopyrans on ring opening was previously studied. Sub-
stitution by an electron-withdrawing nitro group decreased
the rate of ring opening, an effect that was explained by
changes in the electron density of the oxygen atom of the spi-
robenzopyran.
tained by solid-state grinding of 1,2,3,3-tetramethyl-3H-indoli-
um iodide and 5-nitrosalicylaldehyde for 90 min at 90 Hz
(Scheme 1, see the Supporting Information for experimental
Recently, the protonated open-ring merocyanine forms were
obtained by recrystallization of spiropyrans in the presence of
inorganic acids. In the resulting crystal structures, the counter-
À
À
2À
À
anions (Br , Cl , SO , and NO ) stabilized, by formation of
4
3
an hydrogen bond in the crystal packing, the primary proton-
ated oxygen atom resulting from ring-opening isomeriza-
[
24]
tion. The counteranions further compensated the charge of
Scheme 1. Mechanosynthesis of the iodide salt of the protonated nitromero-
cyanine derivative.
+
the MEH ion.
In the present work, we directly synthesized the protonated
+
merocyanine forms of nitro- and bromo-substituted SP (MEH ,
X=NO , Br) by mechano- and sonosynthesis. Mechanosynthe-
details). Liquid-assisted grinding proved most effective with
ethanol. This reaction led to the corresponding nitro-substitut-
ed merocyanine, as confirmed by powder X-ray diffraction
analysis (Figure 2). Pure compound was obtained by recrystalli-
zation from ethanol. Less-reactive 4-bromosalicylaldehyde did
not yield the desired product by mechanochemistry under the
2
sis consists in the use of mechanical energy to trigger chemical
[
25–27]
reactions between solids.
Solid-state reactions are there-
[28]
fore performed in mixer mills. Among the advantages of me-
chanosynthesis are: large quantities of starting materials can
be used, timesaving properties, and the possibility to reduce
side reactions, a factor that can lead to higher yields and
applied conditions.
[
28,29]
+
better conversions.
Furthermore, mechanosynthesis has
Both protonated merocyanines (MEH , X=NO , Br) could be
2
[30–32]
been successfully employed in co-crystal synthesis
and
obtained by using sonochemistry (Scheme 2, see the Support-
ing Information for experimental details). Saturated solutions
of the reactants (i.e., 5-nitrosalicylaldehyde or 5-bromosalicylal-
dehyde and 1,2,3,3-tetramethyl-3H-indolium iodide) in EtOH
were placed in an ultrasound bath for 20 or 40 min, respective-
[33]
polymorph selection, allowing the generation of solid forms
that are otherwise not accessible. Sonosynthesis (also called ul-
trasound-assisted synthesis) has been widely used in the syn-
thesis of graphene-based materials and in the catalyst-free syn-
[34,35]
thesis of a variety of organic compounds.
ly, and this yielded the desired product in high yield (over
1
The two four-state molecular switch systems (involving the
90%, as judged by H NMR spectroscopy in [D ]DMSO).
6
+
two MEH compounds) were systematically analyzed by re-
Pure crystals were obtained by slow evaporation of concen-
trated ethanol solutions. For each product, one proton signal
was observed around d=1.8 ppm for the two homotopic
methyl groups on C3 of the indolinium ring, and this reso-
nance confirmed formation of the open merocyanine form. In
cording UV/Vis absorption spectra in solution.
ChemistryOpen 2018, 7, 520 – 526
521
ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim