4
F.B. Miguez et al. / Journal of Molecular Structure 1211 (2020) 128105
molecules arranged in a distorted tetrahedral geometry (ZnMC (1)).
The tetrahedral complex presents a high abundance relative to the
hexacoordinated one.
3.3. Nuclear magnetic resonance (NMR)
SPCOOH and ZnMC were also analyzed by means of 1H NMR
spectroscopy (Figure SI-3 and chemical shifts (d) for hydrogen in
1
Tables SIe3) in solutions of acetonitrile-d3, and the spectra of H can
be seen in Figure SI-3b. For ZnMC, signals equivalent to the free
ligand can be found (Figure SI-3a), which will be approached below.
1
1
Besides, homonuclear correlations H/ H were used to assign
ZnMC’s hydrogens (Figure SI-3c and SI-3d). The signal observed in
1.75 for ZnMC was attributed to the methyl hydrogens, which were
utilized as a reference for the integration of the other signals and
fixed as equivalent to six hydrogens. Both signals in the region
between 3.7 and 3.4 can be attributed to the two groups of meth-
ylene hydrogens of SPCOOH in its free form. However, ZnMC’s
methylene hydrogens can be assigned to the broad signal at 2.9 and
the second signal in 4.6; the latter could be integrated with a value
of two. The signals relating to the aromatic hydrogen’s region of the
spiropyran molecule coordinated are seen in the region between
ꢀ
5
Fig. 3. UVevis absorption spectra of ZnMC (2.1 ꢂ 10
mol/L) (I) in acetonitrile.
ꢀ
5
Fluorescence emission spectra of ZnMC (2.5 ꢂ 10 mol/L) at the excitation wave-
lengths of 490 nm (II) and 350 nm (III), and SPCOOH (5.5 ꢂ 10ꢀ mol/L) (IV) in MeCN.
5
its fluorescence was highly intensified after forming a complex
with the zinc ion. This increase in the fluorescence intensity may be
attributed to the complex mainly due to zinc, as it has been shown
in other reports that not every spiropyran complex exhibits this
behavior [28]. The emission spectra were obtained by scanning the
maximum excitation wavelength and fixing those wavelengths for
the complex, using MeCN as the solvent. It is worth noting that
SPCOOH was previously exposed to 5 min of UV radiation before
the spectrum was obtained.
A kinetics experiment of the complex dissociation was per-
formed (Figure SI-5) to evaluate ZnMC stability in solution, which is
also a property of interest when dealing with spiropyrans. ZnMC
solutions using tetrahydrofuran (THF) and acetonitrile (MeCN)
were compared. Using THF, a sharp decline in the absorbance was
observed in the first hour of analysis, followed by stabilization at,
approximately, 400 min; the total time of analysis was 16 h
6
.5 and 8.6; the sum of their integrations is compatible with the
expected number.
The remaining signals present in this same region can be
assigned to the free ligand SPCOOH. This result can be confirmed by
the chemical shift compatible with those observed in Figure SI-3b;
moreover, the signals close to 8 may be covered by the ZnMC signal.
The signal at 5.9 was also integrated from the ZnMC spectrum,
which is relative to one hydrogen (H-14) of the ligand, resulting in
an approximate value of 0.1. When comparing its integration with
that of signals equivalent to one hydrogen, it can be inferred that
the proportion of SPCOOH in the ZnMC sample is approximately
10%. The presence of the ligand may be correlated with the equi-
librium between the coordinate and the free forms that can coexist
in a solution equilibrium, even when maintained in the absence of
radiation.
(
960 min). In MeCN, however, the absorbance presented a minor
3.4. UVeVis absorption spectroscopy
decline (of about 5%) in the first hour and then remained stable
throughout the rest of the analysis, which lasted for a total of 48 h
Among the information UVevis absorption spectroscopy can
(2880 min), showing that ZnMC is stable in MeCN if there is no
provide, the occurrence of binding events is one of its most useful.
Spiropyrans, as previously mentioned, can be found in its colorless
closed-ring form, labeled SP, and its colored open-ring form, named
MC. When in SP form, they are known to display two absorption
influence of radiation in the system.
3.5. Theoretical calculations
bands in the UV region of the spectrum, which are attributed to
* electronic transitions between the two halves of the molecule, in
the regions of indoline and benzospyran [51]. After exposure to UV
radiation (
which presents a broad band at 563 nm in MeCN (Figure SI-4), for
SPCOOH. This band in the visible region of the spectrum detected
p-
p
The geometrical parameters of the coordination site for both
complexes, obtained through the methodology presented in sec-
tion 2.4 (Fig. 4 and Tables SIe6), presents the ZnMC (1) complex
with a tetrahedral geometry, although slightly distorted with an-
gles between the Zn atoms and the oxygen atoms of the ligands
l
¼ 365 nm), the isomerization gives rise to the MC form,
ꢃ
ꢃ
for MC refers to the extensive
p-electron conjugation present in the
varying from 90.6 up to 127.8 in the coordination sphere
molecule after the ring-opening isomerization. A comparison be-
tween ZnMC (curve I, Fig. 3) and MC’s absorption spectra obtained
in solution of MeCN reveals a blue shift of 73 nm for the bands in
the visible region of the spectrum, with the maximum absorbance
for MC at 563 nm and 490 nm for the ZnMC complex; this is
another indicator of the occurrence of the complexation reaction.
In Fig. 3, the emission spectra for ZnMC in two different exci-
tation wavelengths (curves II and III, Fig. 3) can be observed, and it
was found that the intensity of the fluorescence is higher at 490 nm.
The MC isomer of spiropyrans is also known to present fluorescent
properties, for the same reason mentioned above. However, at
similar molar concentrations in comparison with the complex, the
MC fluorescence was not observed (curve IV, Fig. 3), meaning that
(Tables SIe6). The ZnMC (2) presents a distorted octahedral ge-
ometry with the sharpest angle of 58.8 located between Zn and
ꢃ
the oxygens of coordinated nitrate ion in the equatorial position,
while the oxygens from the MC ligands coordinated to zinc atom
ꢃ
form angles of approximately 87 . In the axial position, the coor-
ꢃ
dinated atoms describe an angle of 164.7 with the metal center.
The results also revealed that the complex is more stable when
coordinated by two oxygens of a single nitrate instead of one ox-
ygen of each nitrate anion. Both structures are also presented in
high resolution in Figure SI-7.
Comparing the bond lengths of the C]O bond for both the
phenolate carbonyl group and the carboxyl acid carbonyl of the
complexes with the free MC ligand (Table 1), a small increase in the
De Sousa, Frederico B.
