Nitro-substituted 4-[(phenylmethylene)imino]phenolates: Solvatochromism and their use as solvatochromic switches and as probes for the investigation of preferential solvation in solvent mixtures

Article abstract of DOI:10.1021/jo301890r

Four 4-[[(4-nitrophenyl)methylene]imino]phenols (2aa-d) were synthesized. After deprotonation in solution, they formed the solvatochromic phenolates 3aa-d, which revealed a reversal in solvatochromism. Their UVa-vis spectroscopic behavior was explained on

Full text of DOI:10.1021/jo301890r

Article
pubs.acs.org/joc
Nitro-Substituted 4‑[(Phenylmethylene)imino]phenolates:
Solvatochromism and Their Use as Solvatochromic Switches and as
Probes for the Investigation of Preferential Solvation in Solvent
Mixtures
Leandro G. Nandi, Felipe Facin, Vanderleia G. Marini, Lizandra M. Zimmermann, Luciano A. Giusti,
́
Robson da Silva, Giovanni F. Caramori, and Vanderlei G. Machado*
́
Departamento de Química, Universidade Federal de Santa Catarina, UFSC, CP 476, Florianopolis, Santa Catarina 88040-900, Brazil
S
* Supporting Information
ABSTRACT: Four 4-[[(4-nitrophenyl)methylene]imino]phenols (2a−d) were synthesized. After deprotonation in solution,
they formed the solvatochromic phenolates 3a−d, which revealed a reversal in solvatochromism. Their UV−vis spectroscopic
behavior was explained on the basis of the interaction of the dyes with the medium through combined effects, such as nonspecific
solute−solvent interactions and hydrogen bonding between the solvents and the nitro and phenolate groups. Dyes 3a−c were
used as probes to investigate binary solvent mixtures, and the synergistic behavior observed was attributed to solvent−solvent
and solute−solvent interactions. A very unusual UV−vis spectroscopic behavior occurred with dye 3d, which has in its molecular
structure two nitro substituents as acceptor groups and two phenyl groups on the phenolate moiety. In alcohol/water mixtures,
the ET(3d) values increase from pure alcohol (methanol, ethanol, and propan-2-ol) until the addition of up to 80−96% water.
Subsequently, the addition of a small amount of water causes a very sharp reduction in the ET(3d) value (for methanol, this
corresponds to a bathochromic shift from 543 to 732 nm). This represents the first example of a solvatochromic switch triggered
by a subtle change in the polarity of the medium, the color of the solutions being easily reversed by adding small amounts of the
required cosolvent.
medium.1,5 The solvent polarity causes changes in the
INTRODUCTION
■
absorption and/or emission spectra of many organic dyes.1,2
This phenomenon is known as solvatochromism, and the
spectral changes can be potentially used in the investigation of
Various physicochemical events, such as the rate, course, and
position of the equilibrium of processes and the spectroscopic
data of compounds, are influenced by the solvents in which
they are carried out.1−5 These solvent effects are commonly
interpreted as arising from changes in the polarity, a general
term that is related to the overall solvating capability of the
Received: September 10, 2012
Published: October 22, 2012
© 2012 American Chemical Society
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solvent properties.1,2,5 Many solvatochromic probes are known,
the most popular example being Reichardt’s betaine dye, 2,6-
diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (1). The
maximum absorption of 1 in different solvents is the basis for
the very commonly employed ET(30) polarity scale.1,2,5 The
perichromic properties of this compound and others with
related molecular structure have allowed their use in a large
number of applications.1,5,6
of a small amount of acetic acid (Scheme 1). The compounds
were purified by recrystallization, and the characterizations
showed that their purity was adequate for the spectroscopic
studies.
Scheme 1. Preparation of Compounds 2a−d
In the research carried out applying solvatochromic probes to
understand medium effects, much attention has been given to
the chemistry of solvent mixtures.7−36 The behavior of solutes
in these systems is more complex than in pure solvents due to
the fact that in many cases preferential solvation (PS) occurs,
leading to a solute microenvironment having more of one
solvent than the other, in comparison to the bulk composition.
Therefore, this is a very important subject in terms of analyzing
kinetic, spectroscopic, and equilibrium data in solution. It is
also important to observe that mixed solvents are used in many
industrial applications.8,37
Compounds 2a−d are almost colorless in solution, but their
solutions turn colored when they are deprotonated to give 3a−
d. Importantly, as can be observed in Figure 1, 3a−d are
The consideration that structurally different probes should be
able to detect different aspects of the physical chemistry of pure
and mixed solvents is important for the design of different
systems with the potential to act as solvatochromic compounds.
In this paper, we describe the synthesis of a series of 4-[[(4-
nitrophenyl)methylene]imino]phenols (2a−d) and the behav-
ior of their conjugated bases (3a−d) as solvatochromic probes.
The motivation for this study came from the results of our
previous research involving compounds able to act as
chromogenic chemosensors for anions in solution. We studied
4-[[(4-nitrophenyl)methylene]imino]-2,6-diphenylphenol (2c)
in acetonitrile and in acetonitrile/water mixtures38 and
discovered that this compound, when deprotonated, colors
solutions and that the color changes depend on the percentage
of water in the medium.38 Thus, herein, we describe the UV−
vis spectral behavior of 3a−d in pure solvents and potential
applications of these compounds as probes to investigate binary
solvent mixtures and as solvatochromic switches. The data
obtained are discussed on the basis of dye−solvent, dye−dye,
and solvent−solvent interactions.
Figure 1. Solutions of 3a−d in ethyl acetate (A), DMA (B),
acetophenone (C), and methanol (D).
solvatochromic, exhibiting a very large range of colors in
solution. Figure 2 shows the UV−vis spectra for 3a−d in
selected solvents, which reinforces the solvatochromic behavior
of these species. It can be observed that, for instance, the
solvatochromic band of 3a has a λmax at 446 nm in methanol
(εmax = 1.80 × 104 L mol−1 cm−1), which is bathochromically
shifted to 508 nm in 1,2-dichloroethane (εmax = 1.42 × 104 L
mol−1 cm−1): Δλmax = +62 nm. The deprotonation of
compounds 2a−d was performed using tetra-n-butylammo-
nium hydroxide. The presence of the bulky ammonium ion
inhibits the formation of ion pairs with the anion in solution.
This was confirmed by the fact that, with the addition of tetra-
n-butylammonium tetrafluoroborate at a concentration of up to
−1
2.8 mol ×10−4
L
to the solutions of 3a−d (5.0 × 10−5 mol
L−1) in methanol, the UV−vis spectrum of the dyes remained
unaltered, while only a very small decrease (4%) in the
maximum absorbance value was detected in ethyl acetate. The
solvatochromic band observed in the UV−vis spectra of 3a−d
is due to a π−π* transition, of an intramolecular charge-transfer
nature, from the phenolate donor moiety to the nitrophenyl
acceptor group, which is reinforced by the molecular structures
of 3a−d, optimized by applying density functional theory, DFT,
which exhibit a high planarity in comparison with the molecular
structures of 2a−d. In addition, the frontier molecular orbitals
in 3a−d are more delocalized through the π-aromatic system
than those in 2a−d. As an example, the structure of 3d is
planar, whereas that of 2d is twisted. According to Figure 3, the
HOMO electron density in 2d is mainly located in the
phenolate donor moiety, whereas the LUMO electron density
is mainly located on the nitrophenyl acceptor group, indicating
the donor and acceptor moieties. In the case of 3d the situation
is different. Both HOMO and LUMO electron densities are
RESULTS AND DISCUSSION
■
Synthesis of the Dyes and Influence of Solvents on
Their UV−vis Spectra. Compounds 2a−d were synthesized
by condensation of the corresponding 4-aminophenols with 4-
nitrobenzaldehyde or 2,4-dinitrobenzaldehyde in the presence
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Figure 2. UV−vis spectra of 3a−d, measured in methanol (gray), ethanol (red), propan-1-ol (blue), octan-1-ol (blue-green), ethyl acetate (green),
and 1,2-dichloroethane (pink) at 25 °C.
hydrogen bond donor acidity), β (solvent hydrogen bond
acceptor basicity), and π* (solvent dipolarity/polarizability)
parameters,39,40 which are commonly used to describe solvent
polarity and solvent hydrogen-bonding properties. The data
show very large shifts of the solvatochromic absorption band of
the dyes depending on the polarity of the medium. For dye 3a,
a change in the medium from water to N,N-dimethylacetamide
(DMA) leads to a shift of Δλmax = 149.5 nm (λmax(DMA) =
580.0 nm and λmax(water) = 430.5 nm), while for dyes 3b and
3c the Δλmax values obtained are 244.0 nm (λmax(DMA) =
708.0 nm and λmax(water) = 464.0 nm) and 232.0 nm
(λmax(DMA) = 654.0 nm and λmax(water) = 422.0 nm),
respectively. In comparison, the same solvents cause a shift of
Δλmax = 213.0 nm (λmax(DMA) = 666.0 nm and λmax(water) =
453.0 nm) in the solvatochromic band of pyridiniophenolate 1.
For dye 3d the shift obtained is relatively low due to the fact
that the ET(3d) value obtained in water is small (see the
discussion below), but its solvatochromic shift, compared with
Figure 3. Frontier orbitals of compounds 2d and 3d.
those in methanol (λmax = 573.0 nm) and DMA (λmax = 758.5
delocalized through the phenolate and nitrophenyl moieties,
exhibiting an energy gap which is 2.00 eV smaller than that in
2d. These findings (as expected) lead to the characteristic color
of dyes 3a−d when excited.
The UV−vis spectra of these compounds were measured in
23 solvents. The absorbance maxima in each spectrum were
obtained with a precision of 0.5 nm, and the reproducibility of
the λmax values was verified by the determination of five spectra
for each dye in each pure solvent. All data were transformed
into ET(dye) values, given in kilocalories per mole with a
precision of 0.1 kcal mol−1, which are shown in Table 1. Table
1 also shows, for the solvents used in this study, the Reichardt
polarity scale ET(30)1,5,36 and the Kamlet−Taft α (solvent
nm), is very significant (Δλmax = 185.5 nm).
Figure 4 shows plots of ET(dye) as a function of the ET(30)
values for 3a−d in the pure solvents. The plots for 3a−c
indicate a reversal in solvatochromism: the ET values first
decrease from water to DMA; i.e., a hypsochromic shift of the
solvatochromic band of the dyes occurs with an increase in the
polarity of the medium. However, for solvents with ET(30)
values below 42.9 kcal mol−1, the ET(dye) values increase until
the least polar solvent studied (n-hexane), a bathochromic shift
occurring in the solvatochromic band of the dyes when the
polarity of the medium is increased. The concept of the reversal
in solvatochromism has been discussed over the past four
decades,41 being observed for various solvatochromic cyanines
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Table 1. ET(dye) Values for Compounds 3a−d in 23 Pure Solvents at 25 °C and the Corresponding “Polarity” ET(30) and
Kamlet−Taft Parameters
a
b
b
b
c
c
c
c
solvent
ET(30)
α
β
π*
ET(3a)
ET(3b)
ET(3c)
ET(3d)
water
63.1
56.3
55.4
51.9
50.7
50.4
49.7
48.4
48.1
47.7
47.1
45.6
45.1
43.7
43.2
42.9
42.2
41.3
40.7
40.6
39.1
38.1
31.0
1.17
0.90
0.98
0.86
0.84
0.60
0.79
0.76
0.77
0.70
0.69
0.19
0.00
0.42
0.00
0.00
0.08
0.0
0.47
0.52
0.66
0.75
0.90
0.52
0.88
0.84
0.81
0.82
0.80
0.40
0.76
0.93
0.69
0.76
0.43
0.0
1.09
0.92
0.60
0.54
0.52
0.98
0.47
0.48
0.40
0.45
0.40
0.75
1.00
0.41
0.88
0.88
0.71
0.81
0.82
0.90
0.58
0.55
−0.04
66.4
64.4
64.1
61.0
53.4
59.3
59.3
58.3
58.6
58.8
56.9
55.8
49.3
54.1
49.5
49.3
54.3
56.2
56.0
50.0
57.6
56.0
58.8
61.6
57.6
57.7
54.1
52.7
52.3
51.8
49.8
49.5
49.5
48.9
49.3
47.8
44.9
38.81
40.4
49.3
46.5
49.3
49.1
48.5
44.9
58.3
67.7
56.9
57.3
54.25
53.1
52.2
53.1
51.7
51.8
52.2
49.3
49.3
44.9
49.8
43.7
43.7
49.1
48.5
49.3
43.5
49.3
47.9
53.1
38.1
49.1
49.9
46.5
46.0
45.6
44.1
43.7
43.8
44.1
43.7
41.0
44.1
40.9
37.7
37.7
37.7
39.9
41.0
38.3
41.5
40.4
46.0
ethane-1,2-diol
methanol
ethanol
propan-1-ol
benzyl alcohol
butan-1-ol
propan-2-ol
octan-1-ol
decan-1-ol
butan-2-ol
acetonitrile
DMSO
2-methylpropan-2-ol
DMF
DMA
acetone
1,2-dichloroethane
dichloromethane
acetophenone
trichloromethane
ethyl acetate
n-hexane
0.13
0.04
0.20
0.00
0.0
0.10
0.49
0.10
0.45
0.0
Values obtained from the literature,1,5 given in kilocalories per mole. Values obtained from the literature.39,40 This study, values given in
a
b
c
kilocalories per mole.
polarity solvents46,49,50 and of solvent-dependent cis−trans
isomerization processes,51,52 the most common explanation
given is related to the different abilities of polar and nonpolar
solvents to stabilize the ground and excited states of the dye.
Scheme 2 shows two resonance structures for dye 3a, one
Scheme 2. Benzenoid and Quinonoid Resonance Structures
for the Anion of Dye 3a
benzenoid (more dipolar) and the other quinonoid (less
dipolar), both representative of the other dyes studied. More
polar solvents are able to stabilize the benzenoid form, which
contributes to the ground state to a greater extent than the
quinonoid form, being responsible for the negative solvato-
chromism observed (the higher the polarity of the medium, the
higher the HOMO−LUMO gap). In less polar solvents,
however, the quinonoid structure makes a greater contribution
to the ground state, causing the reversal in solvatochromism;
i.e., on increasing the polarity of the medium, the difference
between the ground and excited states is reduced. For dyes 3a−
c a roughly linear correlation between the ET(30) and ET(dye)
values for the region corresponding to the hydroxylic solvents
was observed. Compound 1, the basis for the ET(30) scale, has
a phenolate donor group in its molecular structure and is
therefore very susceptible to the acidity of the medium, being
capable of interacting through hydrogen bonding with
hydroxylic solvents.53 Dyes 3a−d also have a phenolate
group in their molecular structure, which makes them also
Figure 4. ET(dye) values for 3a−d in various solvents as a function of
Reichardt’s ET(30) parameter.
and merocyanines.42−48 Although some authors have inter-
preted the phenomenon in terms of dye aggregation in low-
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capable of interacting with hydrogen bond donating (HBD)
solvents. According to Zhao and Han,54 a hypsochromic shift
observed in the solvatochromic band of a compound with an
increase in the hydrogen bond (HB) donor ability of the
solvent indicates a weakening of the dye−solvent intermo-
lecular HB in the corresponding electronic excited state. This
rationalization can be applied to the spectral behavior of dyes
3a−d. Han and co-workers have observed that intermolecular
HB weakening and strengthening in the electronic excited
states facilitates many nonradiative processes,54 such as solute−
solvent photoinduced electron transfer55 and intramolecular
charge transfer.56
dye(S1)2 + 2S2 ⇄ dye(S2)2 + 2S1
dye(S1)2 + S2 ⇄ dye(S12)2 + S1
This model has been used successfully to describe the PS of dye
1 2 7 , 5 8 − 6 0 a n d o t h e r s o l v a t o c h r o m i c
probes13−15,17,19−21,23,25,27,28,30,31 in various binary solvent
mixtures. S1 and S2 represent the two solvents in the mixture,
and the subscript 2 refers to the more polar component. The
interaction of S1 and S2 by means of hydrogen bonding forms a
common structure, S12. Solvation of the probe by S1, S2, and S12
is represented by dye(S1)2, dye(S2)2, and dye(S12)2, respec-
tively. The two solvent-exchange processes shown above are
defined by f 2/1 and f12/1, which are two PS parameters that
measure the tendency of the probe to be solvated by S2 and S12
in relation to solvent S1, according to
However, the data show that, in the case of 3d, the presence
of two nitro groups associated with pending phenyl groups
develops a fundamental role in giving the compound its unique
solvatochromic properties. This dye has a very similar behavior
in relation to the other dyes studied, but a comparison between
methanol and water reveals that, in contrast to the behavior of
the other dyes studied, a notable lack of linearity is observed,
with an ET(3d) value decreasing from 49.9 kcal mol−1 in
L
f2/1 = (X2 /X1L)/(X2/X1)2
(1)
f12/1 = (X12L/X1L)/(X2/X1)
(2)
methanol (λmax = 572 nm) to 38.1 kcal mol−1 in water (λmax
=
L
L
L
where X1 , X2 , and X12 are the S1, S2, and S12 mole fractions,
respectively, in the microenvironment of the dye. X1 and X2 are
the mole fractions of S1 and S2 in the bulk binary mixture. The
ET(dye) for a given mixture is equal to the average of the
ET(dye) values of S1, S2, and S12 in the cybotactic region of the
dye, resulting in the following equation:
750 nm), corresponding to a Δλ max of 178 nm. On the basis of
studies performed with this system following the methodology
proposed by El Seoud and co-workers,46 the possibility of
aggregation or cis−trans isomerization for 3d in methanol and
water was discarded. The data suggest that water is able to
stabilize the quinonoid form of the dye through specific
interactions, as shown in Scheme 3. An analysis of the α
L
ET(dye) = X1LET(dye)1 + X2 ET(dye)2 + X12LET(dye)12
(3)
Scheme 3. Stabilization of the Quinonoid Form of the Anion
of 3d by Water
Substituting eqs 1 and 2 into eq 3 leads to the following
equation, which relates ET(dye) in the mixed system to the
ET(dye) values of each pure component:58−60
ET(dye)
ET(dye)1(1 − X2)2 + ET(dye)2f X2 + ET(dye)12f (1 − X2)X2
2
2/1
2
12/1
=
(1 − X2)2 + f X2 + f12/1(1 − X2)X2
2/1
(4)
Plots of the ET(dye) values for dyes 3a−c as a function of X2
(water mole fraction) are presented in Figure 5, and nonlinear
regression fits of the experimental data were performed using
eq 4. The results are displayed in Table 2 and show a standard
deviation (SD) of <4.4 × 10−3 for all binary solvent mixtures
studied. Table 2 also shows the PS parameter f12/2, which is a
measure of the tendency of the probe to be preferentially
solvated by the solvent S12 in comparison with S2. This
Kamlet−Taft parameter reveals a larger value for water (1.17)
in comparison with methanol (0.98). In addition, considering
the molecular structure of the dyes, this anomalous effect
occurs only with dye 3d, which has both two acceptor nitro
substituents and two phenyl groups on the phenolate donor
moiety. In principle, water would interact with both the nitro
and phenolate groups through hydrogen bonding, but the data
indicate that because of the molecular design of the dye, the
water preferentially solvates the nitro groups, probably due to
the fact that the presence of nitro groups on one side and
phenyl groups on the other makes the acceptor 2,4-
dinitrophenyl group more hydrophilic. These observations
suggest that dye 3d has the potential to be used as a
solvatochromic switch (see below).
parameter is calculated through the expression f12/2 = f12/1/f 2/1
.
The calculated ET(dye) values in Table 2 for the pure solvents
are in good agreement with the experimental values shown in
Table 1.
Figure 5 shows plots of the ET(dye) values of compounds
3a−c as a function of the water mole fraction for its mixtures
with methanol. A very strong synergistic effect is observed for
these mixtures, and the ET values are lower than those for the
pure solvents. Synergism is often observed in the analysis of
solvatochromic probes in solvent mixtures,13,14,17,20,27,28,58−63
and this term was first employed by Reichardt and co-
workers.61 It occurs due to the presence of a hydrogen bond
accepting (HBA) solvent and a HBD partner in the binary
solvent mixture, which form 1:1 S12 complexes through
hydrogen bonding. Data from the literature have demonstrated
that these species solvate the solute in different manners.
Compound 127,58,59,61 and Brooker’s merocyanine14,17,27 are
Preferential Solvation (PS) of the Dyes in the Mixed
Solvents. Dyes 3a−d can be used to investigate binary solvent
mixtures. To study the PS of the dyes, the two-step solvent-
exchange model shown below was used, taken from a study by
Skwierczynski and Connors:57
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subsequent addition of water leads to a mild reduction in the
ET(3d) value to finally 38.1 kcal mol−1 in pure water (d). The
four selected UV−vis spectra presented in Figure 6B for this
system show the spectral changes observed in the four regions
of the system. The very large spectral shifts cause very different
colors for solutions of 3d in the solvent mixtures (see the inset
in Figure 6A). This represents the first example of a
solvatochromic switch triggered by a subtle change in the
polarity of the medium, the color of the solutions being easily
reversed by adding small amounts of the required cosolvent
(Figure 6C).
To analyze the different solvation patterns of the observed
trends in the electronic excitation of 3d, molecular mechanics
(MM) simulations were performed for this compound in
different solvent environments, such as water, methanol, and
two different methanol/water mixtures, with X2(H2O) = 0.62
(i) and X2(H2O) = 0.88 (ii), as shown in Figure 7. The two
mixtures were chosen not only to investigate the drastic change
in the absorption spectrum via the radial distribution function
(RDF), but also to observe the difference in the solvation shells
and the solute/solvent interactions in the system. The atoms
chosen for the analysis were the phenolate oxygen atom in the
donor moiety (named O25 in the MM simulations) and the
nitro oxygen in the acceptor group (named O33). The
representation of the solvent molecules in MM is depicted in
Scheme 4. For the sake of clarity, only one nitro oxygen is
considered in Figure 7, but all nitro oxygen atoms exhibit the
same pattern. On analyzing the graphs, it is possible to observe
distinct patterns for mixture i, which contains more methanol,
and mixture ii, which is rich in water. The theoretical RDFs
reveal that the solvation environment around the phenolate
donor and nitro acceptor moieties, in particular for the first
solvation shell, is distinct for each group depending on the
solvent mixture employed. According to Figure 7, changes in
the methanol concentration cause only minor variations in the
RDFs for mixtures i and ii related to the nitro oxygen (Figure
7a,c). However, the same changes cause a considerable
modification in the RDFs for the phenolate oxygen (Figure
7b,d). The RDFs make clear that, on increasing the methanol
concentration, more hydrogen-bonding interactions between
the methanol and phenolate groups are established, changing
the solvent microenvironment of the dye and consequently
modifying the absorption spectrum of compound 3d.
Figure 5. Influence of water on the ET(dye) values of 3a−c in
methanol/water mixtures: (---) theoretical linear dependence for no
■
PS; () curve fitted with eq 4; ( ) experimental data.
solvated by the more polar moiety of the S12 species, while
pyrene13 and the dye 4-[4-(dimethylamino)styryl]-1-methyl-
pyridinium iodide20,27 are preferentially solvated by the less
polar moiety of these complexes. For dyes 3a−c in methanol/
water mixtures, PS by the less polar moiety of S12 complexes
occurs, as observed through the lower ET values for the
mixtures, which can be related to a less polar microenviron-
ment.
Thus, the data show that in pure water 3d is solvated through
hydrogen bonding as shown in Scheme 3. With the addition of
methanol to the medium, the alcohol molecules interact with
water, forming S12 complexes and disrupting the HBs of the
latter with the dye. The data suggest that the design of these S12
species enables them to interact with the 2,6-diphenylphenolate
donor group of the dye, the less polar moiety of the species
interacting via a hydrophobic effect, whereas the most polar
moiety of the S12 species interacts with the oxygen atom of the
donor group through hydrogen bonding. Therefore, the results
suggest that the observed increase in the ET(3d) values on
Solvatochromic Switch Using 3d in Alcohol/Water
Mixtures. Figure 6A shows a plot of the molar transition
energies of 3d as a function of the mole fraction of water,
X2(H2O), for water/methanol mixtures. A very unusual
spectroscopic behavior occurs with this dye in these mixtures:
the ET(3d) value increases from 50.0 kcal mol−1 in pure
methanol (a) to 52.7 kcal mol−1 after the addition of 80%
(mol/mol) water (b). Subsequently, the addition of only
another 5% (mol/mol) water causes a very sharp reduction in
the ET(3d) value to 39.1 kcal mol−1 (c), corresponding to a
bathochromic shift from 543 to 732 nm (Δλ = 189 nm). The
a
Table 2. Parameters of the Binary Methanol/Water Mixtures at 25 °C
dye
ET(dye)
ET(dye)2
ET(dye)12
f 2/1
f12/1
f12/2
SD
1
3a
3b
3c
64.2
57.6
57.2
66.9
61.5
61.1
63.5
54.1
56.2
0.0688
0.276
0.416
0.318
0.172
0.737
4.62
0.623
1.77
1.1 × 10−3
2.2 × 10−3
4.4 × 10−3
a
Calculated from eq 4 (see the text).
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■
Figure 6. (A) Influence of water on the ET(dye) values of 3d in methanol/water mixtures: ( ) experimental data. (B) Selected UV−vis spectra of 3d
in (a) pure methanol, (b) methanol with 80% (mol/mol) water, (c) methanol with 85% (mol/mol) water, and (d) pure water (the inset in (A)
shows the corresponding solutions of 3d). (C) Effect of the alternate addition of small volumes of methanol and water on the A732 nm/A538 nm ratio in
the region between 0.82 and 0.90.
analysis of the Kamlet−Taft parameters (Table 1) for the
acidity and basicity of hydroxylic solvents shows that the α
values decrease in the order water > methanol > ethanol >
propan-1-ol, whereas the β values decrease in the reverse order.
Thus, to verify whether solvent−solvent interactions are in fact
responsible for the phenomenon observed, the spectroscopic
behavior of 3d was compared in mixtures of water with the
three alcohols methanol (β = 0.66), ethanol (β = 0.75), and
propan-1-ol (β = 0.90). Figure 8 shows plots of the molar
Figure 7. Radial distribution functions for methanol/water mixtures
with X2(H2O) = 0.62 (i) and X2(H2O) = 0.88 (ii) for the nitro oxygen
(named O33) and phenolate oxygen (named O25) atoms. (a) mixture
i, atom O33; (b) mixture i, atom O25; (c) mixture ii, atom O33; (d)
mixture ii, atom O25.
Figure 8. Variation in the ET(3d) values according to binary mixtures
of water with ( ) methanol, ( ) ethanol, and ( ) propan-1-ol.
Scheme 4. Representation of the Solvent Molecules H2O and
H3COH in the MM Simulations
■
●
▲
transition energy values for the aqueous mixtures, where it can
be observed that a change in the alcohol has a very important
influence on the position of the maximum ET(3d) value:
methanol < ethanol < propan-1-ol, which corresponds to the
increasing order of basicity of the alcohol. The more basic the
alcohol, the stronger its interaction with water, causing the
disruption of the HBs of the latter with the nitro groups of 3d,
changing from pure methanol to 80% water reflects the PS of
3d by the S12 species through these combined interactions. An
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Figure 9. (A) Influence of water on the ET(dye) values for 3d in propan-1-ol/water mixtures. (B) Selected UV−vis spectra for 3d in (a) propan-1-ol,
(b) propan-1-ol with 96% (mol/mol) water, and (c) pure water. The inset in (A) shows the corresponding solutions of 3d.
a
Table 3. Correlation Coefficients a, b, and s Obtained from the Catalan Multiparametric Analysis through the Treatment of
ET(dye) Values for Compounds 3a−d in Various Solvents
́
dye
ET(dye)0
a
b
s
N
F
r
SD
3a
3b
3c
3d
72.6 4.6
71.1 5.2
64.2 3.2
35.4 8.0
15.2 1.8
16.3 2.1
18.6 1.3
19.9 3.2
−2.4 1.8
−6.1 2.0
−3.4 1.2
−2.6 3.1
−21.1 5.4
−25.2 6.1
−18.5 3.7
6.2 9.3
23
23
23
23
<8 × 10−7
<6 × 10−7
<9 × 10−11
<3 × 10−5
0.89
0.89
0.96
0.84
2.3
2.6
1.6
3.9
a
Equation 7 was used (see the text).
and leading to the observed effects. Whereas in the methanol/
water mixtures the ET(3d) value has a maximum at 52.7 kcal
mol−1 (80% water, mol/mol), in the ethanol mixtures (52.2
kcal mol−1) this maximum occurs at 91% water (mol/mol),
which leads to a band shift of Δλ max = 203.0 nm in comparison
with the value for pure water.
A very impressive effect was observed for the propan-1-ol/
water mixtures, as shown in Figure 9A. The addition of water to
this alcohol leads to a hypsochromic shift of 70 nm up to
ET(3d) = 51.05 kcal mol−1 for a water mole fraction of 0.96.
Above this value a strong decrease in the ET(3d) value occurs.
On the addition of only 4% (mol/mol) propan-1-ol to water,
the λmax shifts from 751.0 to 560.0 nm (Figure 9B), resulting in
Δλmax = 191 nm, which can be monitored visually because the
solution's change from green to violet (inset in Figure 9A).
ET(dye) = ET(dye)0 + aα + bβ + s(π* + dδ)
(6)
where δ is a polarizability correction term of the solvent.
Another multiparametric polarity scale was developed by
Catalan
́
et al.,67−69 which utilizes the following equation:
ET(dye) = ET(dye)0 + a(SA) + b(SB) + s(SPP)
(7)
where SPP, SA, and SB are parameters used to describe the
solvent’s dipolarity/polarizability, the HBD acidity, and the
HBA basicity, respectively.
The contributions of the solvent properties to the ET(dye)
values obtained for 3a−d were ascertained with the use of eqs 6
and 7, and the results of the multiple square correlation analysis
are shown in Tables 3 and S2 and S3 (Supporting
Information). A comparison between the data shows that,
although the treatment with eq 6 gives reasonable SD and r
values, the b values are very large and the s values are very small
for dyes 3a−c. These values are not consistent with the
molecular structure of these dyes, which lack appreciably acidic
sites to interact with the medium, and it is feasible to expect
them to be appreciably sensitive to the dipolarity/polarizability
́
Kamlet−Taft and Catalan Studies of the Solvato-
chromism of 3a−d. Two different analyses of the
experimental ET(dye) values in pure solvents for compounds
3a−d were carried out with the use of the Kamlet−
Taft39,40,64−66 and Catalan66−69
multiparameter approaches.
́
These strategies consider a linear correlation between the
measured spectroscopic parameter, ET(dye), and several
solvent parameters, as shown in the following equation:
of the solvent. The correlation analysis by means of the Catalan
́
approach shows a more comprehensive picture for compounds
3a−c, since the contribution of the HBA basicity of the solvent
is small (small b coefficients), with very important contribu-
tions observed for the a (HBD acidity of the solvent) and s
(dipolarity/polarizability) coefficients.
Figure 10 shows four plots that allow a comparison between
the calculated and measured ET(dye) values for 3c and 3d using
ET(dye) = ET(dye)0 + aA + bB + cC + ...
(5)
where ET(dye)0 represents the ET(dye) value relating to an
inert solvent and a, b, and c are coefficients that reflect the
importance of the solvent parameters A, B, and C, respectively,
in terms of ET(dye). The Kamlet−Taft strategy39,40 makes use
of the following equation:
́
the Kamlet−Taft and the Catalan parametrical analyses. These
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Figure 10. Relationships between calculated and measured ET(dye) values for 3c and 3d in various solvents, considering the ET(dye) values
calculated using the Kamlet−Taft and Catalan approaches.
́
plots show that the Catalan
the data. For compound 3d the r value obtained using the
Catalan analysis is higher, but it is lower than those for the
́
parameters offer a better fitting of
differently solvated in pure water or in the presence of small
amounts of an alcoholic component, because in alcohol/water
mixtures the two solvents interact with each other to form a
third species (S12) which is able to interact with the phenolate
moiety. The MM simulations reinforce the observation that
very small changes in the composition of the medium cause
alterations in the microenvironment of the dye, leading to the
modifications observed in its UV−vis absorption spectrum.
The synthesized compounds have the potential to be applied
as probes in the investigation of binary solvent mixtures and
other systems, such as surfactants in aqueous medium and the
micropolarity of cyclodextrins, besides their possible applica-
tions as anionic chromogenic chemosensors.38,70 In addition,
the data presented herein indicate the possibility to synthesize
programmed perichromic dyes (as shown by dye 3d) that can be
designed with stored information to be read by its solvation
shell, which is translated into a sharp change in the optical
signal with very subtle changes in the composition of the
medium.71 The authentic solvatochromic switch demonstrated
by the behavior of 3d in alcohol/water mixtures is of interest in
relation to the synthesis of other systems, aimed at the design
of smart materials that undergo changes in their optical
properties, which are triggered by alterations in their micro-
environment.
́
other compounds, and the s value is very different from those of
the other dyes, suggesting a more complex solvation pattern.
Following the procedure of El Seoud and co-workers,65 and
considering the fact that 3c and 3d have phenyl groups in their
molecular structures, which would increase the lipophilicity of
the dyes, an attempt was made to improve the correlations by
adding, to the equations, the lipophilicity parameter log P.
However, this parameter did not have any effect on the fittings.
Thus, the results verify that the solvation of 3a−d is mainly due
to specific interactions through hydrogen bonding between the
HB acceptor solvents and the nitro and/or the phenolate
groups in the dyes combined with nonspecific solute−solvent
interactions dictated by the dipolarity/polarizability of the
solvents.
CONCLUSIONS
■
Dyes 3a−d exhibit a very expressive solvatochromic behavior,
with a reversal in solvatochromism observed for solvents with
an ET(30) lower than ca. 43 kcal mol−1. In addition, compound
3d exhibits a remarkable behavior in hydroxylic solvents, since
the ET(3d) value increases with an increase in the ET(30) value
of the alcohols, whereas in water an abrupt decrease occurs.
The solvatochromism of the dyes is explained on the basis of
the interaction of the dyes with the medium through combined
solute/solvent effects, such as hydrogen bonding between the
solvents and the nitro and phenolate groups and nonspecific
solute/solvent interactions. Dye 3d is specially tailored to be
EXPERIMENTAL SECTION
■
Materials and Methods. All solvents were HPLC grade and were
purified following the methodology described in the literature.72,73
Deionized water was used in all measurements. It was boiled and
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bubbled with nitrogen and kept in a nitrogen atmosphere to avoid the
presence of carbon dioxide.
bulky tetra-n-butylammonium ion has no influence on the UV−vis
spectrum of the anionic dye. The UV−vis spectra were recorded at 25
°C, using a 1 cm square cuvette. The maxima of the UV−vis spectra
were calculated from the first derivative of the absorption spectrum.
The melting points were uncorrected. The NMR spectra were
recorded in DMSO-d6 with a 400 MHz spectrometer. Chemical shifts
were recorded in parts per million with the solvent resonance as the
internal standard. Data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet), coupling constants
(Hz), and integration. IR spectra were obtained with KBr pellets.
Microanalyses are given as the average values of two determinations.
High-resolution (HR) mass spectra were obtained with an electrospray
ionization (ESI) quadrupole time-of-flight (QTOF) mass spectrom-
eter.
Synthesis of the Compounds. Compound 2a was synthesized by
the reaction of 4-nitrobenzaldehyde with 4-aminophenol in ethanol at
room temperature, and comparison of the characterization data with
those in the literature74 confirmed that 2a was obtained.
The λmax values thus obtained were transformed into ET(dye) values,
1,5
according to the expression ET(dye) = 28590/λmax
.
Mixed Solvents. Binary mixtures were prepared by weighing, in a
quartz cuvette, the solutions of each dye prepared as described above,
and the final values are expressed in terms of the mole fraction X2. The
cuvette was kept hermetically closed with a rubber stopper to
minimize problems with solvent evaporation. The maxima of the
emission spectra were determined, and the λmax values thus obtained
were transformed into ET(dye) values (see above). All experiments
with the mixed solvent systems were carried out in duplicate, and the
reproducibility of the curve features was confirmed.
Computational Details. The structural and electronic properties
of the dyes were calculated with the Gaussian 03 package,76 using the
DFT level of theory, with the B3LYP77−80 functional and 6-31G**
basis set.81,82 All calculated molecular structures correspond to a
minimum on the potential energy surface, since no imaginary values
were obtained in the Hessian matrix. The MM simulations were
obtained via the GROMACS package,83 using the OPLS-AA84,85 force
field and the water model SPC/E.86 Simulation boxes containing the
pure solvents (water and ethanol) and mixtures (mixture i, X2 = 0.62;
mixture ii, X2 = 0.88), containing approximately 780 solvent molecules,
were thermalized at the NVT ensemble (temperature 298 K, τ = 0.1
ps) for 500 ps and equilibrated at the NPT ensemble (pressure 1 bar,
τP = 0.2 ps) for 2 ns. To describe the anionic compound 3d via the
MM method, the molecular structure of compound 2d was used as the
basis and the charges for the molecule were obtained via the CHelpG
method,87 using the Gaussian 03 program.76
Compound 2b was prepared by means of the following procedure:
2,4-dinitrobenzaldehyde (0.100 g, 0.51 mmol), 4-aminophenol (0.055
g, 0.51 mmol), and methanol (10 mL) were added to an Erlenmeyer
flask. The mixture was stirred until complete solubilization of the
reactants. One drop of acetic acid was added, and the reaction mixture
was stirred for 4 h. The mixture was left to stand in a freezer overnight.
The precipitate formed was filtered off under vacuum, washed with
iced methanol, and recrystallized three times from methanol. The
product, after drying, was an amorphous yellow solid (0.079 g, yield
54%), with a melting point of 156.0−157.2 °C. IR (KBr, νmax/cm−1):
̅
3460 (O−H), 1598 (CN), 1439 (CC), 1525, and 1342 (NO).
1H NMR (400 MHz, DMSO-d6): δ/ppm 9.84 (1H, s) 8.92 (1H, s),
8.77 (1H, s), 8.60 (1H, d, J = 8.4 Hz), 8.40 (1H, d, J = 8.4 Hz), 7.31
(2H, d, J = 7.8 Hz), 6.85 (2H, d, J = 7.8 Hz). 13C NMR (100 MHz,
DMSO-d6): δ/ppm 158.4, 151.2, 149.1, 148.2, 141.6, 135.8, 131.2,
128.1, 124.0, 120.6, 116.4. Anal. Calcd for C13H9N3O5: C, 54.36; H,
3.16; N, 14.63. Found: C, 54.33; H, 3.18; N, 14.61. HRMS (ESI,
TOF): m/z calcd for C13H10N3O5 [M + H]+ 288.0620, found
288.0615.
Calculation Methods. The parameters ET(dye)1, ET(dye)2,
ET(dye)12, f 2/1, and f12/1 in the studies of the solvent mixtures
containing 2b−4b were calculated, and the multiparametric analysis
was performed from nonlinear regressions using ORIGIN 8.5 software.
For the synthesis of compounds 2c and 2d, 4-amino-2,6-
diphenylphenol was obtained according to the procedure described
in the literature:75 2,6-diphenylphenol was stirred with an excess of
sodium nitrite, followed by reduction of the product with granulated
tin and concentrated HCl.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
IR, LRMS, HRMS, and H and 13C NMR data for compounds
2b and 2d, experiments used to discard aggregation of 3d, UV−
vis spectra of 3d in water and methanol, “polarity” parameters
for the pure solvents, tables containing the correlation
coefficients obtained from the multiparametric analyses,
correlation of experimental and calculated ET values for 3a
and 3b, and tables of atom coordinates and absolute energies
for compounds 2d and 3d. This material is available free of
charge via the Internet at http://pubs.acs.org.
Compound 2c was prepared by means of a procedure described
recently.38
Compound 2d was prepared by a procedure similar to that
described for 2b, but using ethanol (5 mL) as the solvent, as well as
2,4-dinitrobenzaldehyde (0.100 g, 0.51 mmol) and 4-amino-2,6-
diphenylphenol (0.133 g, 0.51 mmol). The precipitate formed after
freezing overnight was filtered off under vacuum, washed with iced
ethanol, and recrystallized three times from ethanol. The dried product
was an amorphous orange solid (0.094 g, yield 42%), with a melting
point of 126.1−126.9 °C. IR (KBr, νmax/cm−1): 3438 (O−H), 1598
̅
AUTHOR INFORMATION
Corresponding Author
*E-mail: vanderlei.machado@ufsc.br.
Notes
(CN), 1463, 1427 (CC), 1524, 1346 (NO), and 1226 (C−O).
1H NMR (400 MHz, DMSO-d6): δ/ppm 9.07 (1H, s) 8.79 (1H, d, J =
2.4 Hz), 8.71 (1H, s), 8.63 (1H, dd, J = 8.6 and 2.4 Hz), 8.42 (1H, d, J
= 8.6 Hz), 7.61 (4H, d, J = 8.4 Hz), 7.49 (4H, t, J = 7.6 Hz), 7.39 (2H,
t, J = 7.6 Hz), 7.31 (2H, s). 13C NMR (100 MHz, DMSO-d6): δ/ppm
150.5, 150.0, 148.9, 148.3, 142.7, 136.7, 136.1, 131.1, 129.8, 129.3,
129.0, 128.2, 127.4, 123.5, 120.4. Anal. Calcd for C25H17N3O5: C,
68.33; H, 3.90; N, 9.56. Found: C, 68.56; H, 3.92; N, 9.54. HRMS
(ESI, TOF): m/z calcd for C25H18N3O5 [M + H]+ 440.1246, found
440.1278.
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of the Brazilian Conselho Nacional de
́
Desenvolvimento Cientifico e Tecnolog
́
ico (CNPq), Coor-
UV−Vis Measurements. The following procedure was typical for
all measurements performed. A 1.0 × 10−2 mol L−1 stock solution of
compounds 2a−d was prepared in anhydrous dichloromethane. From
this stock solution 5 μL was transferred to two 5 mL volumetric flasks.
After evaporation of the dichloromethane the probe was dissolved in
the pure solvent, resulting in a solution presenting a final dye
concentration of 1.0 × 10−5 mol L−1. To generate the deprotonated
compounds 3a−d, 2 μL of a 0.2 mol L−1 tetra-n-butylammonium
hydroxide aqueous solution was added to each flask. This very small
amount of added water did not shift the UV−vis band of the dye. The
́
denaca̧ o de Aperfeico̧ amento de Pessoal de Nivel Superior
̃
(Capes), and UFSC is gratefully acknowledged.
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