Reverse Solvatochromism of Imine Dyes Comprised of 5-Nitrofuran-2-yl or 5-Nitrothiophen-2-yl as Electron Acceptor and Phenolate as Electron Donor

Article abstract of DOI:10.1002/chem.201800613

Eight compounds with phenols as electron-donating groups and 5-nitrothiophen-2-yl or 5-nitrofuran-2-yl acceptor moieties in their molecular structures were synthesized. The crystalline structures of six compounds were obtained. Their corresponding phenola

Full text of DOI:10.1002/chem.201800613

A Journal of
Accepted Article
Title: Reverse solvatochromism of imine dyes comprised of 5-
nitrofuran-2-yl or 5-nitrothiophen-2-yl as electron acceptor and
phenolate as electron donor
Authors: Carlos Eduardo Albino Melo, Celso Rodrigo Nicoleti, Marcos
Caroli Rezende, Adailton João Bortoluzzi, Renata Silva
Heying, Robson Silva Oliboni, Giovanni Finoto Caramori, and
Vanderlei Gageiro Machado
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To be cited as: Chem. Eur. J. 10.1002/chem.201800613
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Reverse solvatochromism of imine dyes comprised of 5–
nitrofuran–2–yl or 5–nitrothiophen–2–yl as electron acceptor and
phenolate as electron donor
Carlos E. A. de Melo,[a] Celso R. Nicoleti,[a] Marcos C. Rezende,[b] Adailton J. Bortoluzzi,[a] Renata
da S. Heying,[a] Robson da S. Oliboni,[c] Giovanni F. Caramori,[a] Vanderlei G. Machado*[a]
Compound 1 exhibits a negative solvatochromic behavior,
meaning that the solvatochromic band of the probe is shifted
toward a lower maximum wavelength (max) values when the
polarity of the solvent is increased. The opposite behavior is
found for compound 2, introduced by Effenberger and
coworkers.[4] This compound is known for its very pronounced
positive solvatochromism, with its vis solvatochromic band being
displaced to increasing max values when the polarity of the
medium is increased. It has also been verified that some families
of solvatochromic dyes, for instance the classical Brooker’s
merocyanine (dye 3)[5] or compound 4,[6] interestingly present a
reversal in their solvatochromic behavior. For these dyes, a
negative solvatochromism is verified for the most polar solvents
through to solvents exhibiting intermediate polarity, followed by a
reversal in the solvatochromism, from negative to positive,
occurring with a further reduction in the polarity of the solvent.
Abstract: Eight compounds with phenols as electron–donating
groups and 5–nitrothiophen–2–yl or 5–nitrofuran–2–yl acceptor
moieties in their molecular structures were synthesized. The
crystalline structures of six compounds were obtained. Their
corresponding phenolate dyes were studied in 29 solvents and the
data showed that in all cases a reverse solvatochromism occurred.
The results were explained in terms of the ability of the medium to
stabilize the electronic ground and excited states of the probes to
different extents. The frontier molecular orbitals were analyzed for
the protonated and deprotonated forms of the compounds. The
calculated geometries are in agreement with X–ray structures
determined for the compounds and it was verified that after their
deprotonation an increase in the electron delocalization occurs.
Radial distribution functions were calculated for the dyes in water
and n–hexane to analyze different solvation patterns resulting from
the interaction of the solvents with the dyes. Data obtained using the
Catalán multiparameter equation revealed that the medium acidity is
responsible for hypsochromic shifts, while the solvent basicity,
polarizability, and dipolarity contributed to bathochromic shifts of the
solvatochromic band of these dyes. Two model “hybrid cyanine”
dyes were used in the design of simple experiments to demonstrate
that the solvatochromic behavior of these dyes in solution can be
tuned with careful consideration of the properties of the medium.
Introduction
Solvatochromic dyes have been used in recent decades as
probes to measure the polarity of solvents.[1] These compounds
exhibit bands in the visible region and their position and/or
intensity is dependent on the nature of the medium. The best
known example of a solvatochromic dye is the Reichardt
pyridinium–N–phenolate betaine 1 (Scheme 1), whose visible
charge transfer (CT) solvatochromic band was the basis for the
development of the widely employed ET(30) empirical polarity
solvent scale.[1a, 2, 3]
Scheme 1. Examples of class I and class II solvatochromic dyes.
Solvatochromic merocyanines (and other dyes with related
molecular structures) are molecules composed of conjugated
electron–donor and acceptor fragments, capable of an internal
CT, which gives rise to a solvent–dependent transition. The
relationship between this transition energy (ET) and the nature of
the two fragments can be rationalized with the aid of a cyanine–
limit model.[7] This model postulates a hyperbolic dependence of
ET on the solvatochromic strength (S) of the molecule, that is,
the difference between the tendencies to donate and accept
charge, by the donor and acceptor fragments, respectively. The
model distinguishes two classes of merocyanines (or
[a]
Departamento de Química, Universidade Federal de Santa
Catarina, UFSC, CP 476, Florianꢀpolis, SC, 88040–900, Brazil
E–mail: vanderlei.machado@ufsc.br
[b]
Facultad de Química y Biología, Universidad de Santiago, Av.
B. O’Higgins 3363, Santiago, Chile
[c]
Centro de Ciências Químicas, Farmacêuticas e de Alimentos,
Universidade Federal de Pelotas, UFPEL, Pelotas, RS, 96010–900,
Brazil

Supporting information for this article can be found at
http://dx.doi.org/
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merocyanine–like dyes): class I, constituted by charged donor
and acceptor moieties, and class II, composed of two neutral
fragments.[8] Thus, solvatochromic dyes 1 and 3 are examples of
class I, while compounds 2 and 4 are class II dyes. The two
classes exhibit an opposite dependence of ET on the S value of
the molecule. The value of ET increases with increased S for
class I compounds, and decreases for class II dyes.
We have termed these compounds “hybrid cyanine” dyes,
as their molecular structure may be considered a hybrid of class
I and class II dyes.[10c] As a consequence, solvatochromic plots
employing a class I dye as a reference, like the commonly
employed ET(30) scale based on pyridinium N–phenolate
betaine 1, or a class II dye, such as probe 2, inevitably yield
poor correlations with the solvatochromic shifts of these “hybrid”
compounds. We have therefore proposed the reference “hybrid”
dye 5 (X = CH, R1 = R2 = H) to evaluate the effect of different
structural features on the solvatochromic behavior of this family
of compounds. ET values for all of these derivatives correlate
We have recently proposed a generalized reversal model
for the interpretation of the behavior of solvatochromic dyes, as
an alternative to the cyanine–limit model.[9] Starting from the
observation that the environment changes the solvatochromic
strength of a dye, it is postulated that a minimum value of the
transition energy should always exist for any merocyanine, as a
consequence of the hyperbolic dependence of ET on S. This
minimum value is a point of solvatochromic reversal and thus all
merocyanines should exhibit reverse or inverted solvatochromic
behavior at some polarity value. This polarity value is frequently
“virtual”: if an extended polarity scale is employed, it occurs
beyond the limits of the range of experimental solvent polarities,
leading to the particular cases of a positive or a negative
solvatochromism. “Real” reversal is verified only when this
minimum occurs at some point within the range of experimental
solvent polarities.[9]
ref
linearly with the ET values of the unsubstituted reference
stilbene, in around 30 solvents with widely different polarities.[10c]
The simple two–parameter Eqn (1) yields values for the slope n
and intercept m for any derivative 5, thus allowing a quantitative
comparison of these compounds, and revealing the effect of the
variables R1, R2 and X on their solvatochromic behavior.
ref
ET = m + n ET
Eqn (1)
Searching for more sensitive derivatives in this family, we
decided to prepare novel solvatochromic dyes by replacing the
homoaromatic acceptor fragment with heterocyclic analogs (5–
nitrothiophen–2–yl or 5–nitrofuran–2–yl groups). Compounds
6a–13a (Scheme 3) were completely characterized and the
crystalline structures of six compounds are presented. Like their
homoaromatic analogs 5, all of these phenolate dyes were
generated in situ through the treatment of their phenolic
precursors 6a–13a with tetra–n–butylammonium hydroxide
(TBAOH). Their solvatochromic behavior was studied in 29
solvents and compared with that of their homoaromatic analogs,
with the aid of Eqn (1).
The emphasis put on the solvatochromic reversal of
merocyanines naturally stimulates research on new dyes that
exhibit a clear inverted behavior in solvents of medium polarity.
We have been interested in a particular family of dyes of general
structure 5 (Scheme 2),[10, 11] with a phenolate donor conjugated
with a polynitroaryl acceptor moiety, which, besides being
readily accessible, satisfy the above requirement and also the
need for good solvent sensitivity.
Scheme 3. Compounds 6a–13a and their deprotonation to generate 6b–13b.
Scheme 2. General structure of a “hybrid cyanine” dye 5 with a phenolate
donor and polynitrophenyl acceptor fragments.
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Besides
rationalizing
their
solvatochromism
by
characterizing their internal charge–transfer in terms of their
frontier molecular orbitals, the solvation of these dyes in water
and in n–hexane was investigated by molecular simulations with
two model cyanine–like dyes (compounds 6b and 10b).
Multiparametric regression analysis allowed a comparison of the
effect of specific solvent properties on their solvatochromic
behavior. The trends observed were confirmed experimentally
by studying the spectral response of some dyes, 6b and 14[10c]
(Scheme 4), to specific environmental changes (using an
organic solvent like dichloromethane), brought about by the
addition of a co–solvent or of a cationic species. Thus, besides
their full characterization, these novel compounds provided
additional experimental and theoretical information that helped
us to understand and rationalize the solvatochromic inversion of
this interesting family of “hybrid cyanine” dyes.
Scheme 5. Formation of compounds 6a–13a by condensation of 5–nitro–2–
thiophenecarboxyaldehyde or 5–nitro–2–furaldehyde with anilines.
The X–ray structure of compounds 6a, 7a, 9a, 10a, 12a, and
13a
Monocrystals of compounds 6a, 7a, 9a, 10a, 12a, and 13a were
obtained, allowing their structural characterization through X–ray
diffraction (Figure 1). Compounds 6a, 9a, 10a, 12a, and 13a
crystallize in the monoclinic system (space groups P–21/n for 6a,
10a, 12a, and 13a and Cc for 9a) while compound 7a
crystallizes in the triclinic mode (space groups Pī). The main
bond distances and torsion angles for the compounds are
reported in Tables S1–S17 (Supporting Information). The
crystallographic data show that the iminophenols have an (E)–
configuration.
Figure 2 reproduces supramolecular crystalline arrays for
compounds 12a and 13a. In these arrays, a single water
molecule interacts through hydrogen–bonds with both the 2–
nitrofuryl ring (2–nitro substituent and the oxygen of the furyl
group) of one molecule and the phenolic hydroxyl group of
another, binding together the molecules of the furyliminophenol.
The fact that these supramolecular arrays mediated by water
molecules occur only with compounds 12a and 13a (of the
compounds analyzed) shows that this is a result of the
combination of two features in the same molecule, i.e., the
presence of (a) oxygen as a heteroatom, providing an atom able
to act as a hydrogen–bonding acceptor species, and (b) halogen
groups, which makes the hydroxyl in the phenolic moiety
sufficiently acidic to act as a strong hydrogen–bonding donor
group.
Scheme 4. Molecular structure of model “hybrid cyanine” solvatochromic dye
14.
Results and Discussion
Synthesis of the compounds
The synthesis of compounds 6a–13a was performed as shown
in Scheme 5, using a methodology described by Stock et al.,[10c]
through
the
condensation
of
5–nitro–2–
thiophenecarboxyaldehyde or 5–nitro–2–furaldehyde with the
corresponding aniline in the presence of acetic acid as the
catalyst and ethanol as the solvent. The 2,6–substituted
aminophenols were prepared according to a methodology
described by Johnson et al.[12] No purification technique was
required for any of the compounds. The products of the
reactions were obtained in yields of 50–88%. Compounds 7a, 8a,
9a, 10a, 11a, 12a, and 13a are novel compounds and were
characterized using IR, 1H NMR, 13C NMR, HRMS, and DSC
techniques (Figures S1–S40 in Supporting Information).
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Figure 1. ORTEP projections of iminophenols 6a, 7a, 9a, 10a, 12a, and 13a.
Figure 2. Supramolecular arrays of 12a (A) and 13a (B) crystals, with the inclusion of water molecules hydrogen–bonding with both the phenolic hydroxyl and
the nitro substituents.
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Table 1 lists, for all compounds, the C=N bond distances
and dihedral angles between the phenolic and the 2–nitrothienyl
or 2–nitrofuryl rings, as well as between the ring and the 2–nitro
substituent. The length of the C=N bond in compounds 6a, 7a,
and 9a (with a 2–nitrothienyl ring) had a mean value of 1.276 Å
while for compounds 10a, 12a, and 13a (with a 2–nitrofuryl ring)
a mean value of 1.281 Å was obtained.
Compounds 6b–13b are salts that, in principle, could form
ion pairs in solution, which would influence the absorption
spectra for the dyes, especially in low polarity solvents. Some
experiments were performed in order to verify this possibility
(see Figures S43 and S44). The results are similar to those
obtained previously for
a series of (nitrostyryl)phenolate
solvatochromic dyes.[10b] Therefore, based on these experiments,
the possibility of the formation of ion pairs in non–polar media
was discarded.
Table 1. Some structural features of iminophenols 6a, 7a, 9a, 10a, 12a, and
13a.
Dihedral angle
Dihedral angle
C=N bond
distance (Å)
Compound
α (o)[a]
β (o)[b]
6a
7a
2.09
4.25
6.57
10.08
3.55
3.41
1.68
1.38
3.72
11.15
3.04
2.49
1.2763(12)
1.2759(14)
1.2750(3)
1.2830(12)
1.2790(16
1.2810(2)
9a
10a
12a
13a
[a] Dihedral angle between the phenolic and the 2–nitrothienyl or 2–nitrofuryl
rings. [b] Dihedral angle between the heterocyclic ring and the 2–nitro group.
Figure 3. (A) Solutions and (B) UV–vis spectra for dye 10b in (a) water, (b)
methanol, (c) dimethoxyethane, and (d) acetone.
Nearly complete coplanarity between the phenolic ring and
the nitro–substituted heterocyclic fragment was verified for all
compounds, with small dihedral angles between the two rings
(α) or between the heterocyclic ring and the nitro substituent (β).
The dihedral angles for compound 10a were slightly larger than
the corresponding angles for the other iminophenols. Inspection
of the ORTEP projections of Figure 1 suggests that this
difference might be due to the fact that 10a crystallized with a
conformation that differed from that of all other analogs.
The spectra for dyes 6b–13b were recorded in 29 solvents.
The corresponding transition energies (ET), in kcal mol–1, are
listed in Table 2. Figure 4 shows a plot of the transition energies
obtained as a function of the ET(30) solvent polarity values for
dye 13b, these being similar to those obtained for the other dyes
(Figure S45). In all cases, an inverted behavior was observed,
as previously described for analogous dyes 5.[10, 11] As a typical
example, for dye 7b, the solvatochromic band has a maximum
at 573 nm [ET(7b) = 49.9 kcal mol–1] in n–hexane (the least polar
solvent studied), while in DMSO max = 798 nm [ET(7b) = 35.8
kcal mol–1]. Thus, a reduction in the ET(7b) value occurs with an
increase in the polarity of the medium, leading to a bathochromic
shift of Δmax = +225 nm, which is characteristic for a positive
solvatochromism. The region of the plot corresponding to the
solvent DMSO is where the reverse solvatochromism starts to
occur. A further increase in the solvent polarity leads to an
increase in the ET(7b) value, which is typical for a negative
solvatochromism. The vis band of 7b in water has a maximum at
519 nm [ET(7b) = 55.1 kcal mol–1], which, in comparison to the
max value in DMSO, corresponds to a strong hypsochromic
band shift of Δmax = –279 nm.
Solvatochromic behavior of dyes 6b–13b
Solutions of 6a–13a are yellow, but after deprotonation using
very small aliquots of TBAOH dyes 6b–13b are generated,
which exhibit different colors (Figure 3A and Figure S41).
Figure 3B and Figure S42 show the UV–vis spectra for dyes
6b–13b in some selected solvents, confirming that the
compounds are solvatochromic species. For instance, dye 11b
in water exhibits an absorption band with a maximum at 503 nm.
This band is shifted to max = 735 nm in dimethylacetamide while
in dimethylsulfoxide, trichloromethane, and n–hexane the band
maximum is displaced to 733, 620, and 510 nm, respectively.
The solvatochromic bands in the vis region of the spectra are a
result of –* electronic transitions, with an intramolecular CT
from the electron–donor phenolate moiety towards the electron–
acceptor 2–nitrothienyl or 2–nitrofuryl moiety.
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Table 2. ET(30) and ET(dye) values for compounds 6b–13b in 29 solvents.
Solvent
ET(30)[a]
ET(6b)[b]
ET(7b)[b]
ET(8b)[b]
ET(9b)[b]
ET(10b)[b]
ET(11b)[b]
ET(12b)[b]
ET(13b)[b]
Cyclohexane
n–Hexane
30.9
31.0
33.9
34.5
37.4
38.1
38.2
39.1
40.6
40.7
41.3
42.2
42.9
43.2
43.3
45.1
45.6
47.1
47.7
48.1
48.4
49.1
49.7
50.4
50.7
51.9
55.4
56.3
63.1
55.4
56.5
51.9
49.5
46.0
47.0
47.1
49.4
47.1
48.4
48.3
45.0
44.5
41.4
47.3
40.8
47.0
49.7
52.7
51.4
50.9
52.6
52.8
52.7
53.6
54.5
57.9
57.3
58.9
49.6
49.9
46.8
44.5
39.6
40.5
40.7
44.6
41.4
42.5
42.4
39.1
38.3
36.3
42.3
35.8
41.2
43.6
47.1
46.2
44.8
47.0
46.8
46.5
47.6
48.4
51.6
51.4
55.1
55.8
56.8
52.9
50.5
47.7
49.4
48.4
50.8
46.4
50.1
50.1
49.0
44.2
45.7
50.2
45.5
49.5
50.6
53.6
52.9
51.5
53.4
54.0
53.7
54.0
55.5
58.0
57.5
60.3
54.3
56.2
52.1
50.4
47.0
49.2
47.9
49.7
45.8
49.2
49.1
46.8
44.6
45.8
50.1
45.9
49.1
50.6
53.0
52.3
51.6
52.9
53.1
52.4
53.9
54.6
57.0
57.1
60.0
58.0
58.5
55.4
54.1
49.1
50.7
51.2
51.7
49.9
51.0
51.2
48.6
43.5
44.6
52.7
43.7
49.7
52.7
55.9
54.6
54.2
56.1
56.2
55.7
56.7
57.3
60.1
61.4
61.6
55.6
56.0
50.0
48.3
43.8
45.4
44.7
46.1
43.1
45.2
45.1
42.7
38.9
39.6
44.9
39.0
43.7
46.4
50.0
48.8
47.9
49.8
50.0
49.1
50.5
51.3
55.5
56.1
56.9
58.1
58.4
55.5
53.9
51.4
52.3
50.9
52.5
49.3
52.1
51.8
50.5
47.7
48.6
52.8
48.7
52.0
53.7
56.1
55.5
54.7
56.1
56.1
55.8
56.8
57.3
60.4
59.9
62.5
58.0
58.5
55.4
54.1
50.9
52.2
51.4
52.2
49.3
51.4
51.6
50.6
48.1
48.9
53.4
48.8
51.9
53.7
56.0
55.1
54.7
55.8
55.9
55.4
56.5
57.3
59.8
59.9
62.2
Toluene
Ethyl ether
THF
Ethyl acetate
Dimethoxyethane
Trichloromethane
Acetophenone
Dichloromethane
1,2–Dichloroethane
Acetone
DMA
DMF
2–Methylpropan–2–ol
DMSO
Acetonitrile
Butan–2–ol
Decan–1–ol
Octan–1–ol
Propan–2–ol
Pentan–1–ol
Butan–1–ol
Benzyl alcohol
Propan–1–ol
Ethanol
Methanol
Ethane–1,2–diol
Water
[a] Values obtained from Reichardt.[1a] [b] In kcal mol–1
.
If a given 2–nitrothienyl dye is compared to its analogous
2–nitrofuryl dye (for instance 6b with 10b, 7b with 11b, and so
on) the solvatochromism is more expressive for the thiophenyl
dyes. In addition, in the same comparison, the max values for the
increases the electronic density in the electron–donating moiety
of the dye, increasing the probability of the occurrence of CT.
On the other hand, the presence of chlorine or bromine as
substituents causes the opposite effect, lowering the efficiency
of the CT and resulting in a relatively weak solvatochromic
behavior.
thiophenyl dyes are higher, suggesting that sulfur as
a
heteroatom facilitates the CT electronic transition. With respect
to the substituents in the phenolate moiety, the strongest
solvatochromic behavior occurs with the methyl group, which
.
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also reduced the dye sensitivity to the medium polarity, with
63
lower n values.
water
60
Table 4. Values for the slope n and intercept m in linear plots of ET(dye) as a
function of ET(5) values, for compounds 6b–13b.
n-hexane
57
Dye
6b
m
n
r2
–5.48 ± 2.60
–11.55 ± 2.93
4.81 ± 2.64
6.32 ± 2.80
–4.43 ± 2.30
–13.37 ± 2.60
10.02 ± 2.35
11.47 ± 2.42
0.94 ± 0.04
0.95 ± 0.05
0.79 ± 0.04
0.76 ± 0.04
0.98 ± 0.04
1.03 ± 0.04
0.75 ± 0.04
0.72 ± 0.04
0.94
0.93
0.92
0.90
0.96
0.95
0.92
0.91
54
51
48
7b
8b
9b
10b
11b
12b
13b
30
35
40
45
50
55
60
65
ET(30) [kcal mol-1]
Figure 4. ET(30) value as a function of ET(dye) for dye 13b.
Table 3. Values of max for compounds 6b–13b in the most polar, least polar
and intermediate polarity solvents, and the Δmax values obtained considering
the regions of positive and negative solvatochromism.
Theoretical calculations
The frontier molecular orbital plots for 6a–6b and 10a–10b as
well as their eigenvalues and HOMO−LUMO gaps indicate that,
after deprotonation, the electron density delocalization increases
significantly, as revealed by the decrease in the HOMO–LUMO
gap (Figure 5).
Least polar
Most polar
Intermediate
[a]
[b]
solvent
solvent
(water)
Δmax
Δmax
solvent
Dye
(n–hexane)
(nm)
(nm)

max (nm)

max (nm)

max (nm)
6b
7b
485
519
474
476
464
503
457
460
702 (DMSO)
798 (DMSO)
647 (DMA)
641 (DMA)
657 (DMA)
735 (DMA)
599 (DMA)
595 (DMA)
506
573
503
509
489
510
490
489
+196
+225
+144
+132
+168
+225
+109
+106
–221
–279
–173
–165
–193
–232
–142
–135
In order to investigate the solvation of the dyes, molecular
dynamics simulations of compounds 6b and 10b were
performed with water and n–hexane as solvents. These two
dyes were chosen as prototypes to analyze different solvation
patterns that can arise depending on the acceptor group, and
also to investigate the behavior of two solvents that are at the
extremes of the solvatochromic scale. Radial distribution
functions (RDFs) were calculated from equilibrated trajectories
generated for the combination of the two dyes and solvents. The
analysis was focused on the phenolate and nitro substituents of
the donor and acceptor groups, respectively, on the nitrogen
atom of the imino bridge, and on the furyl and thienyl rings in the
compounds.
8b
9b
10b
11b
12b
13b
[a] Δmax = max (intermediate solvent) – max (n–hexane).
[b] Δmax = max (water) – max (intermediate solvent).
A comparison of the heteroaromatic dyes 6b–13b with
their homoaromatic analogs 5 could be made with the aid of Eqn
(1). The intercept m in this equation is a measure of the average
shift of the solvatochromic max value for a given dye with respect
to the stilbene reference 5 (X = CH, R1 = R2 = H) of Scheme 2.
The slope n compares the solvatochromic sensitivity of the dye
to polarity changes, with that of the same stilbene reference.
Table 4 summarizes these values for dyes 6b–13b (Figure S46).
In all cases, the linear correlations were good. Dyes with
electron–donating substituents on the phenolate moiety had
negative m values, indicating bathochromic shifts in their
solvatochromic band with respect to the stilbene reference.
Electron–accepting substituents, like Cl and Br in compounds 8b,
9b, 12b, and 13b, led to hypsochromic shifts in their
solvatochromic band with respect to the same reference. They
The two dyes yielded very similar RDFs around the
phenolate oxygen (Figure 6A) with a first solvation shell
centered at 1.8 Å with one water molecule, and a second more
diffuse shell at 3.2 Å with four water molecules (Figure 7). The
nitro oxygens of 6b and 10b also formed hydrogen bonds with
the solvent, as shown by the RDFs of Figure 6B, giving rise to a
first solvation shell centered at 2.1 Å with one water molecule. In
addition, a second solvation shell, less intense as compared to
the donor group, at 3.5 Å can be identified. A solvation shell is
also verified around the nitro group, but this is less intense
compared to the donor group. The heteroatoms of the thienyl
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(compound 6b) and furyl (compound 10b) rings of the two dyes
also showed a small, specific interaction with the surrounding
water molecules, as indicated by the corresponding RDFs of
Figure 6C, with small, diffuse solvation shells centered at 3.1 Å
around the sulfur atom of 6b, and at 2.5 Å around the furyl
oxygen atom of 10b. The shorter distance obtained for
compound 11b (O—HW) may be due to the more
electronegative heteroatom in the bridging group. Finally, the
imino nitrogen atom of both dyes showed a first solvation shell
centered at 2.2 Å with one water molecule and a second shell at
3.5 Å with three water molecules (Figure 6D). The comparison
of the solvation results for the two dyes with n–hexane (see
Figure S47) reveals that the solvation patterns are very similar.
The data show the hydrogens of n–hexane interacting more
strongly with the electronegative atoms of the dyes for both
compounds. However, as would be expected, the interactions
are much weaker than those verified in the intense and strongly
packed first solvation shell with water.
(A)
(B)
Figure 5. Contour plots and energies (eV) of the frontier orbitals (HOMO and LUMO) for compounds (A) 6a, 6b and (B) 10a, 10b.
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Figure 6. Radial distribution functions (RDFs) for the solvation of (A) phenolate oxygen, (B) nitro oxygens, (C) heteroatoms of the thienyl and furyl rings and (D)
imino nitrogen bridge of 6b and 10b in water. A similar pattern was verified for the second oxygen of the nitro group. Atom types (HW) and (OW) denote the
hydrogen and oxygen atoms of water, respectively, (OH) the oxygen of the phenolate group, (ON) the oxygen of the acceptor nitro group, and (NC) the nitrogen of
the imino bridge, while (S) and (OS) represent the heteroatoms of thienyl and furyl rings of dyes 6b and 10b, respectively.
Figure 7. Snapshots of the molecular dynamics simulation emphasizing the two solvation shells of water molecules around 10b. The water molecules highlighted
are 6 Å from the oxygen of the phenolate group, and within 3 Å of the other groups.
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Multiparametric regression analysis
data points obtained in water from the regressions led to
The data in Table 2 were subjected to a multiparametric
regression analysis, according to Eqn (2), to compare the effects
of different solvent properties, like acidity (SA), basicity (SB),
dipolarity (SdP) and polarizability (SP), on the transition energies
of dyes 6b–13b.
improved correlations. Table 5 lists the values for the
coefficients a, b, c, and d, obtained for dyes 6b–13b. As verified
for dyes 5 (Scheme 2),[10] the medium acidity was responsible
for hypsochromic shifts in the solvatochromic band of the
heteroaromatic analogs 6b–13b, with positive values for
coefficient a. In contrast, the solvent basicity, polarizability, and
dipolarity, all with negative values for the corresponding
coefficients, contributed to a decrease in the transition energies,
or to bathochromic shifts in the solvatochromic band of these
dyes.
ET(dye) = ET(dye)0 + aSA+ bSB + cSP + dSdP
Eqn (2)
The Catalán parameters[13] have proved to be superior to
14]
the Kamlet–Abboud–Taft parameters[13a,
in previous
regressions with hybrid dyes 5.[10] In general, the exclusion of
Table 5. Correlation coefficients a, b, c, and d obtained from the Catalán multiparametric (Eqn 2) analysis through the treatment of ET(dye) values for compounds
6b–13b in various solvents.
Dye
6b
6b[d]
ET(dye)0
61.94
64.70
55.09
57.07
63.43
65.42
62.19
63.76
65.35
68.19
63.52
66.25
64.72
66.29
65.51
66.76
a
b
c
d
N[a]
29
28
29
28
29
28
29
28
29
28
29
28
29
28
29
28
R[b]
0.95
0.98
0.97
0.98
0.96
0.98
0.97
0.99
0.94
0.98
0.96
0.99
0.97
0.99
0.98
0.99
S.D.[c]
1.58
0.93
1.24
0.84
1.12
0.64
0.92
0.57
1.68
1.05
1.37
0.51
0.85
0.45
0.74
0.46
16.28
20.87
17.03
20.33
14.09
17.40
13.90
16.51
16.71
21.43
17.00
21.54
13.56
16.17
12.75
14.83
–3.11
–5.94
–3.42
–5.45
–3.10
–5.14
–2.40
–4.00
–2.49
–5.40
–3.91
–6.71
–2.09
–3.70
–1.91
–3.20
–9.02
–10.14
–9.28
–10.48
–9.46
–7.80
–7.18
–7.25
–6.76
–10.93
–10.05
–12.23
–11.38
–7.55
–7.07
–7.30
–6.91
–12.47
–7.48
7b
7b[d]
8b
8b[d]
–9.96
–10.79
–13.27
–10.85
–12.81
–9.48
9b
9b[d]
10b
10b[d]
11b
11b[d]
12b
12b[d]
13b
13b[d]
–13.02
–12.42
–15.83
–9.89
–11.85
–11.33
–12.90
[a] Number of solvents. [b] Linear correlation. [c] Standard deviation. [d] Excluding water from the studies.
Analysis of the influence of the solvent properties on the
reverse solvatochromism of the dyes
shifts of the dye in such media must then reflect variations in
that specific interaction.
As discussed in the previous section, medium acidity on the one
hand and solvent basicity, polarizability and dipolarity on the
other hand represent opposite trends which are responsible for
the minima verified in all plots of Figure 4, and are a
consequence of the hybrid nature of these dyes. In order to
highlight this through direct observation, we recorded the
solvatochromic shifts of some of these compounds in solvent
mixtures composed of two solvents or of a solvent and an
electrolyte. The rationale behind these experiments is that, with
the use of an appropriate “co–solvent”, a specific interaction can
be varied in a solvent mixture without affecting significantly the
other properties of the medium. The resulting solvatochromic
Compounds 6b and 14[10c] were chosen as model “hybrid
cyanine” solvatochromic dyes. Dichloromethane was chosen as
a reference solvent, capable of mixing in all ratios with a more
polar solvent and of solubilizing an organic electrolyte. Its acidity
(SA = 0.040) and basicity (SB = 0.178) are negligible.[13e] The
polarizability (SP = 0.761) and dipolarity (SdP = 0.769) of this
solvent are reasonably high, but still allow room for increased
values of basicity and dipolarity, in mixtures with solvents like
DMF (SA = 0.031, SB = 0.613, SP = 0.759, SdP = 0.977) or
DMSO (SA = 0.072, SB = 0.647, SP = 0.830, SdP = 1.00).[13e]
Thus, binary solutions of CH2Cl2/DMF or of CH2Cl2/DMSO allow
a controlled and specific increase in the dipolarity and basicity
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values, as they become richer in the more polar component
DMF or DMSO.
the medium acidity, or its EBD strength, without affecting
significantly its dipolarity or basicity.
The addition of a soluble organic electrolyte like sodium
tetraphenylborate (NaPh4B) to dichloromethane or DMF should
result in mixtures with negligible variations in basicity, dipolarity
or polarizability, but with strongly increased acidity values. This
results from the strong tendency of the sodium cation to
associate with the phenolate anion of the model dyes. As an
54
52
50
(in DMF)
48
46
44
electrophile–donor “co–solvent”, NaPh4B behaves as
a
hydrogen–bond donor, increasing the transition energy of the
phenolate dye and blue–shifting its solvatochromic band.
42
(in CH2Cl2)
40
0.0
0.2
0.4
0.6
0.8
1.0
Figure 8A shows the variations in the ET value of dye 6b
in binary mixtures of CH2Cl2/DMF. In all cases, decreased
transition energies result from the addition of the more dipolar
and basic co–solvent. The opposite trend is verified when DMF
is replaced by the polar NaPh4B “co–solvent” (Figure 8B).
102 x c (salt) [mol L-1]
Figure 9. Variations in the ET value of the solvatochromic band of dye 14 in
(A) dichloromethane (●) and (B) DMF (■), through the addition of increasing
concentrations of NaPh4B.
Addition of this sodium salt to
a
solution of 6b in
dichloromethane leads to increased ET values and
Comparison of the plots for the solutions of 14 containing
increasing concentrations of NaPh4B in DMF and in CH2Cl2
(Figure 9) reveals a notable difference in the dye sensitivity to
the addition of salt. The halochromism observed is more
pronounced in DMF than in CH2Cl2. This can be rationalized by
the greater dissociation of the NaPh4B salt in DMF than in
CH2Cl2, enhancing the availability of the sodium cation to
associate with the phenolate dye. NaPh4B in CH2Cl2 behaves as
a covalent species, unable to affect the dye transition energy
through cation complexation with the phenolate group. Much
larger salt concentrations are thus required to induce
halochromic shifts by the sodium salt.
hypsochromic shifts in the solvatochromic band of the dye.
% DMF [v/v]
0
20
40
60
80
100
56
56
52
48
44
40
52
48
44
40
0
1
2
3
4
The above arguments and evidences can be used to
interpret the observed solvatochromic inversion of these “hybrid
cyanine” dyes in solutions of medium polarity. In spite of their
102 x c (salt) [mol L-1]
Figure 8. Opposite effects of the addition of a more polar “co–solvent” to
solutions of the nitrothienyliminophenolate 6b in CH2Cl2: (A) bathochromic
shift (transition–energy decrease) caused by addition of DMF (●); (B)
hypsochromic shift (transition–energy increase) caused by addition of sodium
tetraphenylborate (■).
structural variations, this family of compounds exhibit
a
solvatochromic reversion in a narrow region of solvent polarities,
N
characterized by intermediate ET
values (0.4–0.5).[10,11]
Minimum ET values are verified with the same group of solvents,
all of them showing negligible acidity, and significant dipolarity–
polarizability and basicity. Medium acidity clearly increases the
internal CT energy, by partially protonating the phenolate
oxygen donor (in the case of hydrogen–bond donating solvents),
or by reducing its charge through association with a cationic
The negative halochromic shifts caused by the sodium salt
reflect the association of the phenolate dye with the Na+ cation.
Similar effects are verified in solutions of dye 14 (Scheme 5) in
CH2Cl2 and in DMF. In both cases, the addition of increasing
concentrations of NaPh4B leads to increased ET values for the
solvatochromic band of the dye (Figure 9). NaPh4B acts as a
polar electrophile–bond–donor (EBD) “co–solvent”, increasing
electrophile, as occurs with
a “co–solvent” like NaPh4B.
Nevertheless, the CT energy also increases in non–polar, non–
hydrogen–bond donating solvents, which have difficulty
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promoting the phenol ionization necessary to generate a
solvatochromic species, even in the presence of an added base
(like TBAOH). Increasing the medium dipolarity–polarizability
and, more importantly, the medium basicity, has the effect of
increasing the effective charge on the phenolate oxygen, thus
reducing the corresponding CT energy. This is evident from a
comparison of the effect of two solvents with similar average
values of dipolarity–polarizability, that is, dichloromethane (SdP
= 0.769, SP = 0.761) and acetone (SdP = 0.907, SP = 0.651),
but that differ appreciably in terms of basicity (SB values = 0.178
of the electronic ground and excited states of the dyes and,
consequently, in the energy related to the charge–transfer
electronic transition.
The frontier molecular orbitals were analyzed for the
protonated and deprotonated forms of the compounds. The
calculated geometries are in agreement with X–ray structures
verified for the compounds studied, it being verified that an
increase in the electron delocalization occurs for the phenolate
dyes in comparison with their corresponding phenols. RDFs
were calculated for the dyes in water, showing that although
solvent molecules are found in the solvation shell of the dye
around the phenolate oxygen, nitro oxygen, and imino nitrogen
atoms, the most intense solvation is on the donor phenolate
group. A study was also performed using n–hexane as the
solvent. Although the solvation pattern is similar (hydrogens of
n–hexane interact more strongly with the electronegative atoms
of the compounds) the interactions are much weaker than those
occurring with water as a solvent, due to the ability of the latter
to form strong hydrogen bonds with the electronegative atoms of
the dyes.
[13e]
and 0.475 for dichloromethane and acetone, respectively).
The CT energies are consistently lower in acetone than in
dichloromethane for all members of these “hybrid cyanine”
dyes,[10, 11] an observation that can be ascribed to the greater
basicity of acetone. Increasing this basicity even further, using
DMF, DMA or DMSO, leads in all cases to a consistent
bathochromic shift of the solvatochromic band of these
phenolate dyes. These shifts are readily explained by the
reduction in the stability of the ground–state charged phenolate
caused by a solvating, basic solvent. Internal CT and charge–
dispersion are therefore promoted by such a solvent, leading to
a more stabilized excited state.
The Catalán multiparameter approach provided a good fit
with the experimental data, showing that the medium acidity was
responsible for hypsochromic shifts, while the solvent basicity,
polarizability, and dipolarity contributed to the bathochromic
shifts of the solvatochromic band of these dyes. These opposite
contributions could be responsible for the reversal in the
solvatochromism verified in these systems. In order to
investigate this supposition, simple experiments were performed
using the model “hybrid cyanine” solvatochromic dyes 6b and 14,
based on the use of carefully choisen solvent mixtures or of a
solvent and an electrolyte, so that a specific interaction could be
varied in the system without affecting the other medium
properties significantly. The experiments performed indicated
that the solvatochromic behavior of these dyes in solution can
be tuned with careful consideration regarding the properties of
the medium.
Conclusions
Iminophenols 6a–13a, which present heterocyclic (5–
nitrothiophen–2–yl or 5–nitrofuran–2–yl) acceptor moieties in
their molecular structures, were synthesized and characterized.
Their deprotonation in solution led to the formation of their
corresponding phenolate dyes 6b–13b. The absorption band in
the vis region of the spectra for each dye is related to a –*
electronic transition, of an intramolecular charge–transfer nature,
from the electron–donor phenolate toward the electron–acceptor
heterocyclic group. A study was performed in 29 different
solvents, showing in all cases a reversal in solvatochromism.
These “hybrid cyanine” dyes exhibit a solvatochromic behavior
10]
similar to that of their homoaromatic analogs,[9,
with the
solvatochromic reversion occurring in a narrow region of solvent
polarities, characterized by intermediate ETN values (0.4–0.5).
The reverse solvatochromism of the dyes studied was
discussed in the light of the fact that changes in the polarity of
the solvent can modify the contribution weight of the quinonoid
(less dipolar) and benzenoid (more dipolar) forms to the
resonance hybrid of the dyes, causing changes in the energies
Experimental Section
All reagents and solvents were analytically pure and they were
purchased from commercial sources (Sigma–Aldrich and Vetec).
The solvents were HPLC grade and were purified following the
methodology described in the literature.[15] Deionized water was
used for all measurements and it was boiled and bubbled with
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nitrogen and kept in a nitrogen atmosphere to avoid the
presence of carbon dioxide. Dye 14 was obtained according with
a previously described procedure.[10c]
149.80; 148.84; 140.34; 130.38; 130.29; 124.99; 121.96; 16.51.
HRMS m/z: 275.0488 [M–H]–, calculated for C13H11N2O3S,
275.0485.
NMR spectra were recorded with 200 MHz Bruker AC–
200F and with 400 MHz Bruker Avance 400 spectrometers.
Chemical shifts were recorded in ppm with the solvent
resonance as the internal standard and data are reported as
follows: chemical shift, multiplicity (s = singlet, d = doublet),
coupling constants (Hz), and integration. IR spectra were
obtained with an FT Varian 3100 spectrometer, using KBr
pellets. High–resolution mass spectra were collected using a
Bruker OTOF–Q II 10243 equipment. UV–Vis spectra were
obtained with an Agilent Technologies Cary 60 spectrometer.
The melting points were determined by means of DSC analysis,
using a Perkin Elmer Jade–DSC apparatus.
(E)–2,6–dichloro–4–(((5–nitrothiophen–2–yl)methylene)
amino)phenol (8a). This compound was prepared from 2,6–
dichloro–4–aminophenol
and
5–nitro–2–
thiophenecarboxyaldehyde as an yellow solid (yield: 50%). m.p.:
194.86 ºC. IR (KBr, 휐max
̅
/cm–1): 3412 (OH); 1617 (C=N); 1533
1
and 1482 (N=O); 1205 (C–S); 1131 and 1034 (C–Cl). H NMR
(400 MHz, acetone–d6) δ/ppm: 8.91 (s, 1H); 8.07 (d, 1H, J = 4.1
Hz); 7.68 (d, 1H, J = 4.1 Hz); 7.45 (s, 1H). 13C NMR (100 MHz,
DMSO–d6) δ/ppm: 152.91; 152.40; 148.87; 148.29; 141.27;
131.70; 130.23; 122.73; 122.17. HRMS m/z: 314.9393 [M–H]–,
calculated for C11H5Cl2N2O3S, 314.9392.
(E)–2,6–dibromo–4–(((5–nitrothiophen–2–yl)methylene)
amino)phenol (9a). This compound was prepared from 2,6–
General procedure for the preparation of iminophenols. In a
round–bottom flask, 5–nitro–2–thiophenecarboxyaldehyde or 5–
dibromo–4–aminophenol
and
5–nitro–2–
nitro–2–furaldehyde (0.7 mmol),
1
equivalent of the
thiophenecarboxyaldehyde as dark yellow crystals (yield: 88%).
corresponding amine, 3 mL of ethanol previously dried with
molecular sieves (4 Å), and 3 drops of acetic acid were mixed
together. The mixture was maintained under reflux at 85 ºC for
90 min and the precipitate was then filtered and washed with
ice–cold ethanol. No purification technique was required for any
of the compounds.
m.p.: 219.51 ºC. IR (KBr, 휐max
̅
/cm–1): 3487 (OH); 1607 (C=N);
1
1552 and 1497 (N=O); 1221 (C–S); 1042 (C–Br). H NMR (400
MHz, acetone–d6) δ/ppm: 8.90 (s, 1H); 8.07 (d, 1H, J = 4.3 Hz);
7.68 (d, 1H, J = 4.3 Hz); 7.64 (s, 1H). 13C NMR (100 MHz,
DMSO–d6) δ/ppm: 153.30; 152.40; 150.18; 148.17; 142.74;
131.82; 130.22; 125.69; 112.13. HRMS m/z: 404.8364 [M–H]–,
calculated for C11H5Br2N2O3S, 404.8362.
(E)–4–(((5–nitrothiophen–2–yl)methylene)amino)phenol (6a).
This compound was prepared from 4–aminophenol and 5–nitro–
2–thiophenecarboxyaldehyde as red crystals (yield: 82%). m.p.:
(E)–4–(((5–nitrofuran–2–yl)methylene)amino)phenol
(10a).
This compound was prepared from 4–aminophenol and 5–nitro–
2–furaldehyde as brown crystals (yield: 72%). m.p.: 183.60 ºC
227.57 ºC. [m.p. lit.[16] = 219 – 220 ºC]. IR (KBr, 휐max
̅
/cm–1): 3404
1
(decomposes). IR (KBr, 휐max
/cm–1): 3436 (OH); 1623 (C=N);
̅
(OH); 1615 (C=N); 1570 and 1493 (N=O); 1203 (C–S). H NMR
(400 MHz, DMSO–d6) δ/ppm: 9.80 (s, 1H); 8.84 (s,1H); 8.14 (d,
1H, J = 4.0 Hz); 7.61 (d, 1H, J = 4.0 Hz); 7.29 (d, 2H, J = 8.4
Hz); 6.81 (d, 2H, J = 8.4 Hz). 13C NMR (100 MHz, DMSO–d6)
δ/ppm: 157.85; 151.61; 149.61; 149.00; 140.44; 130.42; 130.42;
130.32; 123.33; 115.85. HRMS m/z: 247.01722 [M–H]–,
calculated for C11H7N2O3S, 247.01719.
1580 and 1482 (N=O); 1272 (C–O). 1H NMR (200 MHz,
acetone–d6) δ/ppm: 8.66 (s, 1H); 8.59 (s, 1H); 7.64 (d, 1H, J =
3.8 Hz); 7.38 (d, 2H, J = 8.8 Hz); 7.29 (d, 1H, J = 3.8 Hz); 6.93
(d, 2H, J = 8.8 Hz). 13C NMR (100 MHz, DMSO–d6) δ/ppm:
157.84; 153.59; 152.02; 143.60; 140.99; 123.35; 116.42; 115.89;
114.38. HRMS m/z: 231.0402 [M–H]–, calculated for C11H7N2O4,
231.0400.
(E)–2,6–dimethyl–4–(((5–nitrothiophen–2–yl)methylene)
amino)phenol (7a). This compound was prepared from 2,6–
(E)–2,6–dimethyl–4–(((5–nitrofuran–2–yl)methylene)amino)
dimethyl–4–aminophenol
and
5–nitro–2–
phenol (11a). This compound was
dimethyl–4–aminophenol and 5–nitro–2–furaldehyde as a dark
red solid (yield: 65%). m.p.: 176.64 ºC. IR (KBr, 휐max
/cm–1): 3528
prepared from 2,6–
thiophenecarboxyaldehyde as red crystals (yield: 65%). m.p.:
185.24 ºC. IR (KBr, 휐max
̅
/cm–1): 3395 (OH); 2914 (C–H); 1599
̅
1
(C=N); 1533 and 1482 (N=O); 1207 (C–S). H NMR (200 MHz,
acetone–d6) δ/ppm: 8.82 (s, 1H); 8.04 (d, 1H, J = 4.3 Hz); 7.58
(d, 1H, J = 4.3 Hz); 7.08 (d, 2H, J = 0.7 Hz); 2.26 (d, 2H, J = 0.7
Hz). 13C NMR (100 MHz, DMSO–d6) δ/ppm: 153.58; 151.54;
(OH); 2912 (C–H); 1578 (C=N) 1515 and 1488 (N=O); 1229 (C–
O). 1H NMR (200 MHz, acetone–d6) δ/ppm: 8.60 (s, 1H); 8.56 (s,
1H); 7.79 (d, 1H, J = 3.9 Hz); 7.29 (d, 1H, J = 3.9 Hz); 7.09 (s,
1H); 2.18 (s, 6H). 13C NMR (100 MHz, DMSO–d6) δ/ppm:
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153.76; 153.65; 151.98; 143.43; 140.86; 124.98; 121.90; 116.33;
114.39; 16.62. HRMS m/z: 259.0716 [M–H]–, calculated for
C13H11N2O4, 259.0713.
Calculation methods. The multiparametric analysis was
performed from non–linear regressions using ORIGIN 8.5
software.
(E)–2,6–dichloro–4–(((5–nitrofuran–2–yl)methylene)amino)
phenol (12a). This compound was prepared from 2,6–dichloro–
4–aminophenol and 5–nitro–2–furaldehyde as brown crystals
Single–crystal X–ray structure determinations. X–ray
diffraction data were recorded on a Bruker APX–II DUO
diffractometer equipped with an APEX II CCD area detector
using graphite–monochromated Mo Kα ( = 0.71073 Å). The
temperature was set at 200(2) K. The frames were recorded
from ω and ϕ scans using APEX2[17] and the integration
procedure was performed using the SAINT and SADABS
programs.[17] The structures were solved and refined with
(yield: 84%). m.p.: 147.41 ºC. IR (KBr, 휐max
/cm–1): 3595 (OH);
̅
1650 (C=N); 1519 and 1495 (N=O); 1231 (C–O); 1180 and 1025
(C–Cl). 1H NMR (200 MHz, acetone–d6) δ/ppm: 8.65 (s, 1H);
7.65 (d, 1H, J = 3.8 Hz); 7.48 (s, 1H); 7.38 (d, 1H, J = 3.8 Hz).
13C NMR (100 MHz, DMSO–d6) δ/ppm: 153.63; 153.35; 148.68;
147.70; 142.10; 122.73; 122.16; 118.11; 114.13. HRMS m/z:
298.9618 [M–H]–, calculated for C11H5Cl2N2O4, 298.9621.
SHELXS97
and
All
SHELXL2013
software
atoms
programs,[18]
respectively.
non–hydrogen
were refined
(E)–2,6–dibromo–4–(((5–nitrofuran–2–yl)methylene)amino)
phenol (13a). This compound was prepared from 2,6–dibromo–
4–aminophenol and 5–nitro–2–furaldehyde as dark yellow
anisotropically. Hydrogen atoms attached to C atoms were
placed at their idealized positions using standard geometric
criteria. Hydrogen atoms of the hydroxyl groups were located
from the Fourier difference map and treated with riding model
approximation. The ORTEP plots and partial packing diagram
were drawn with PLATON[19a] and MERCURY[19b] software,
respectively. Further crystallographic information is given in
Table 6. Full tables containing the crystallographic data (except
structural factors) have been deposited at the Cambridge
Structural Database, as supplementary publication numbers
CCDC 1813014–1813019. Copies of the data can be obtained
free of charge at www.ccdc.cam.ac.uk.
crystals (yield: 65%). m.p.: 155.90 ºC. IR (KBr, 휐max
/cm–1): 3602
̅
(OH); 1648 (C=N); 1515 and 1490 (N=O); 1229 (C–O); 1025 (C–
Br). 1H NMR (200 MHz, acetone–d6) δ/ppm: 8.66 (s, 1H); 7.80 (d,
1H, J = 3.9 Hz); 7.67 (s, 1H); 7.35 (d, 1H, J = 3.9 Hz). 13C NMR
(100 MHz, DMSO–d6) δ/ppm: 152.59; 152.34; 150.39; 147.76;
143.25; 125.69; 118.15; 114.11; 112.25. HRMS m/z: 388.8594
[M–H]–, calculated for C11H5Br2N2O4, 388.8591.
UV–vis measurements. The following procedure was typical for
all measurements performed.
A stock solution of each
compound (1.0×10–3–1.0×10–2 mol L–1) was prepared in
acetone. From this stock solution, 7–40 μL was transferred to 5–
mL volumetric flasks. After evaporation of the acetone the
compound was dissolved in the pure solvent, resulting in a
solution presenting a final dye concentration of 2.0×10–5–
5.0×10–5 mol L–1. In order to generate the deprotonated
compounds, 5–15 μL of a methanolic 0.1 mol L–1 TBAOH
solution was added to each flask. The addition of this very small
amount of methanol did not change the position of the UV–vis
band of the dye. The bulky tetra–n–butylammonium ion has no
influence on the UV–vis spectrum for the anionic dye. The UV–
vis spectra were recorded at 25.0 °C. The maxima of the UV–vis
spectra were calculated from the first derivative of the absorption
spectrum, with a precision of ±0.5 nm, and the reproducibility of
the max values was verified through the determination of two
spectra for each dye in each pure solvent. The max values thus
Computational details. The molecular dynamics simulations
were performed via the GROMACS package,[20] using the
OPLS–AA force field[21] and the water model SPC/E.[22]
Simulation boxes containing the solvents (water and n–hexane)
were thermalized at the NVT ensemble (temperature 298 K,  =
0.1 ps) for 500 ps and equilibrated at the NPT ensemble
(pressure 1 bar,  = 0.5 ps) for 2 ns. To describe the anionic
compounds via the molecular mechanics (MM) method, charges
for the molecule were obtained via the CHelpG method[23] using
the Gaussian 09 package,[24] at the DFT level of theory with the
B3LYP functional[25] and 6–31G** basis set.[26]
Acknowledgements
The authors are grateful for the financial support of the Brazilian
governmental agencies Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq), Fundação de Amparo à
Pesquisa e Inovação do Estado de Santa Catarina (FAPESC),
Financiadora de Estudos e Projetos (FINEP), and Coordenação
obtained were transformed into ET(dye) values, according to the
[1]
expression ET(dye) = 28590/max
,
given in kcal mol–1 with a
precision of ±0.1 kcal mol–1.
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10.1002/chem.201800613
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de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), as
well as the Laboratório Central de Biologia Molecular
(CEBIME/UFSC) and UFSC. M.C.R. also acknowledges
Fondecyt (project 1140212, by Dicyt/USACH) and CNPq (PVE
2014, process 400322/2014–5).
Keywords: solvatochromic dyes • solvatochromic reversal •
solvatochromism • solute–solvent interactions
Table 6. Crystal data for compounds 6a, 7a, 9a, 10a, 12a, and 13a.
Compound
6a
7a
9a
10a
12a
13a
Empirical formula
C11H8N2O3S
248.25
C13H12N2O3S
276.31
C11H6Br2N2O3S
406.06
C11H8N2O4
232.19
C11H8Cl2N2O5
319.09
C11H8Br2N2O5
408.01
Formula weight
Temperature/K
200(2)
200(2)
200(2)
200(2)
200(2)
200(2)
Wavelength/Å
0.71073
Monoclinic
P21/n
0.7103
0.7103
0.71073
Monoclinic
P21/n
0.7103
0.7103
Crystal system
Triclinic
Monoclinic
Cc
Monoclinic
P21/n
Monoclinic
P21/n
Space group
Pī
a/Å
9.0148(4)
9.8884(5)
12.2217(6)
90º
7.9214(5)
7.9456(7)
12.3506(7)
72.086(2)º
76.878(2)º
60.1790(10)º
639.13(8)
2
14.0802(13)
13.3380(12)
6.9691(6)
90º
8.9143(4)
9.9006(4)
11.7516(5)
90º
6.98180(10)
13.9271(3)
13.2089(3)
90º
7.16070(10)
14.0854(3)
13.0554(3)
90º
b/Å
c/Å
α/Å
β/Å
102.9210(10)º
90º
94.548(2)º
90º
104.5550(10)º
90º
92.9950(10)º
90º
90.9230(10)º
90º
γ/Å
Volume/Å3
1061.88(9)
4
1304.60(2)
4
1003.87(7)
4
1282.63(4)
4
1316.61(5)
4
Z
Density (calculated)/mg
1.553
1.436
2.067
1.536
1.652
2.058
m–3
Absorption coefficient/
mm–1
0.301
512
0.258
288
6.377
784
0.120
480
0.527
648
6.177
792
F(000)
Crystal size/ mm3
0.400 × 0.200 ×
0.160
0.400 × 0.200 ×
0.100
0.400 × 0.220 ×
0.120
0.400 × 0.380 ×
0.200
0.340 × 0.160 ×
0.120
0.220 × 0.120 ×
0.120
Theta range for data
collection
2.677 to 32.787º
1.740 to 30.525º
2.106 to 33.232º
2.580 to 30.651º
2.126 to 30.133º
2.127 to 30.144º
−13 ≤ h ≤ 7
−15 ≤ k ≤ 15
−18 ≤ l ≤ 18
−11 ≤ h ≤ 11
−11 ≤ k ≤ 11
−17 ≤ l ≤ 17
−21 ≤ h ≤ 21
−20 ≤ k ≤ 20
−10 ≤ l ≤ 10
−11 ≤ h ≤ 12
−14 ≤ k ≤ 14
−16 ≤ l ≤ 13
−6 ≤ h ≤ 9
−19 ≤ k ≤ 18
−16 ≤ l ≤ 18
−10 ≤ h ≤ 6
−19 ≤ k ≤ 19
−18 ≤ l ≤ 18
Index ranges
Reflections collected
15027
24192
13904
10614
13748
15810
3934 [R(int) =
0.0156]
3903 [R(int) =
0.0219]
4777 [R(int) =
0.0289]
3097 [R(int) =
0.0167]
3777 [R(int) =
0.0206]
3871 [R(int) =
0.0233]
Independent reflections
Completeness to theta =
25.242º
99.9%
100.0%
100.0%
100.0%
100.0%
99.9%
Absorption correction
Refinement method
Data / restraints /
parameters
Semi−empirical from equivalents
Full−matrix least−square on F2
3934 / 0 / 154
1.067
3903 / 0 / 172
4777 / 2 / 173
0.936
3097 / 0 / 154
1.043
3777 / 0 / 181
1.057
3871 / 0 / 181
1.023
2
Goodness–of–fit on F
1.051
Final R indices
[I>2sigma(I)]
R1 = 0.0327
wR2= 0.0909
R1 = 0.0315
wR2= 0.0873
R1 = 0.0246
wR2= 0.0506
R1 = 0.0391
wR2= 0.1066
R1 = 0.0328
wR2= 0.0831
R1 = 0.0257
wR2= 0.0563
R1 = 0.0376
wR2= 0.0962
R1 = 0.0355
wR2= 0.0917
R1 = 0.0364
wR2= 0.0551
R1 = 0.0457
wR2= 0.1127
R1 = 0.0443
wR2= 0.0901
R1 = 0.0373
wR2= 0.0597
R indices (all data)
Extinction coefficient
Largest diff. peak and
hole/e Å–3
0.680 and −0.235
0.436 and −0.264
0.390 and −0.574
0.407 and −0.256
0.357 and −0.334
0.933 and −0.348
C11H8N2O3S
C13H12N2O3S
C11H6Br2N2O3S
C11H8N2O4
C11H8Cl2N2O5
C11H8Br2N2O5
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10.1002/chem.201800613
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Compounds with phenols as electron-
donating groups and 5-nitrothiophen-
2-yl or 5-nitrofuran-2-yl acceptor
moieties in their molecular structures
were synthesized. The crystalline
structures of six compounds were
Carlos E. A. de Melo,[a] Celso R.
Nicoleti,[a]
Adailton
Marcos
J. Bortoluzzi,[a] Renata
S. Heying,[a] Robson da S. Oliboni,[c]
Giovanni F. Caramori,[a] Vanderlei G.
Machado[a,*]
C. Rezende,[b]
da
obtained.
Their
corresponding
((Insert TOC Graphic here: max.
phenolate dyes were studied in 29
solvents and the data showed that in
all cases a reverse solvatochromism
occurred. The results were explained
in terms of the ability of the medium to
stabilize the electronic ground and
excited states of the probes to
different extents.
Page No. – Page No.
width: 5.5 cm; max. height: 5.0 cm))
Reverse solvatochromism of imine
dyes comprised of 5–nitrofuran–2–yl
or 5–nitrothiophen–2–yl as electron
acceptor and phenolate as electron
donor
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