Synthesis and Solvatochromism of Substituted 4-(Nitrostyryl)phenolate Dyes

Article abstract of DOI:10.1021/acs.joc.5b00983

4-(Nitrostyryl)phenols 2a-9a were synthesized, and by deprotonation in solution, the solvatochromic phenolates 2b-9b were formed. Their absorption bands in the vis region of the spectra are due to π- π? electronic transitions, of an intramolecular charge-transfer nature, from the electron-donor phenolate toward the electron-acceptor nitroarene moiety. The frontier molecular orbitals and natural bond orbitals were analyzed for the protonated and deprotonated forms. The calculated geometries are in agreement with X-ray structures observed for 4a, 6a, and 8a. The HOMO-LUMO energy gaps suggest that, after their deprotonation, an increase in the electron delocalization is observed. In the protonated compounds, the HOMO is primarily localized over the phenol ring and the C=C bridge. After deprotonation, it extends toward the entire molecule, including the NO₂ groups. The solvatochromism of each dye was studied in 28 organic solvents, and it was found that all compounds exhibit a reversal in solvatochromism, which is interpreted in terms of the ability of the media to stabilize their electronic ground and excited states to different extents. The Catalán multiparameter equation is used in the interpretation of the solvatochromic data, revealing that the most important contribution to the solute/solvent interaction is the hydrogen-bond donor acidity of the solvent.

Full text of DOI:10.1021/acs.joc.5b00983

Page 1 of 40
The Journal of Organic Chemistry
1
1
2
3
4
5
6
7
Synthesis and Solvatochromism of Substituted 4-
(Nitrostyryl)phenolate Dyes
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Rafaela I. Stock, Leandro G. Nandi, Celso R. Nicoleti, Adriana D. S. Schramm,
Sheila L. Meller, Renata S. Heying, Daniel F. Coimbra, Karla F. Andriani,
Giovanni F. Caramori, Adailton J. Bortoluzzi and Vanderlei G. Machado*
Departamento de Química, Universidade Federal de Santa Catarina, UFSC, CP 476,
Florianópolis, SC, 88040ꢀ900, Brazil
Eꢀmail: vanderlei.machado@ufsc.br
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 40
2
1
2
3
4
5
6
7
8
9
ABSTRACT: 4ꢀ(Nitrostyryl)phenols 2a–9a were synthesized and by deprotonation in
solution the solvatochromic phenolates 2b–9b were formed. Their absorption bands in
the vis region of the spectra are due to π−π* electronic transitions, of an intramolecular
chargeꢀtransfer nature, from the electronꢀdonor phenolate toward the electronꢀacceptor
nitroarene moiety. The frontier molecular orbitals and natural bond orbitals were
analyzed for the protonated and deprotonated forms. The calculated geometries are in
agreement with Xꢀray structures observed for 4a, 6a, and 8a. The HOMOꢀLUMO
energy gaps suggest that after their deprotonation an increase in the electron
delocalization is observed. In the protonated compounds, the HOMO is primarily
localized over the phenol ring and the C=C bridge. After deprotonation, it extends
toward the entire molecule, including the NO2 groups. The solvatochromism of each
dye was studied in 28 organic solvents and it was found that all compounds exhibit a
reversal in solvatochromism, which is interpreted in terms of the ability of the medium
to stabilize their electronic ground and excited states to different extents. The Catalán
multiparameter equation is used in the interpretation of the solvatochromic data,
revealing that the most important contribution to the solute/solvent interaction is the
hydrogenꢀbond donor acidity of the solvent.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Keywords: solvatochromic dyes, solvatochromic reversal, solvatochromism,
solute/solvent interactions
ACS Paragon Plus Environment

Page 3 of 40
The Journal of Organic Chemistry
3
1
2
3
4
5
INTRODUCTION
6
7
8
9
The rate and course of a large number of chemical processes is strongly dependent on
the solvent used.1ꢀ5 The choice of solvent (or a mixture of solvents) based on certain
criteria represents a variable which is fundamental for the purification and separation of
organic compounds. The presence of contaminants in the solvent in very small amounts
can strongly influence the yield in many organic reactions. It is also known that the
medium can strongly affect the spectrometric data of the dissolved compounds, such as
their emission and absorption bands.1,2,5 These effects are commonly attributed to the
solvent polarity, which is defined as the overall solvating ability of the medium.1,5 Many
solvent polarity scales have been proposed to explain the solventꢀdependent chemical
processes. Several of these polarity scales are based on the use of solvatochromic
compounds.1,2,5,6 For these probes, the position and/or intensity of the bands in the
visible region is modified when the solvent polarity is changed.1,5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The very popular ET(30) solvent polarity scale is based on the use of Reichardt’s
pyridinium Nꢀphenolate betaine dye, 2,6ꢀdiphenylꢀ4ꢀ(2,4,6ꢀtriphenylpyridiniumꢀ1ꢀ
yl)phenolate (compound 1; see Scheme 1), and its derivatives.1,5,7ꢀ9 These zwitterionic
dyes exhibit a strong negative solvatochromism due to their solventꢀdependent vis band,
with a strong chargeꢀtransfer character. A negative solvatochromism occurs when the
solvatochromic band is hypsochromically shifted with an increase in the polarity of the
solvent.1
Pyridinium Nꢀphenolate dye 1 has inspired many research groups to design other
families of solvatochromic dyes. In general, the solvatochromic dyes exhibiting
negative solvatochromism have an electronꢀdonor phenolate group in their molecular
structure and a pyridinium center as the electronꢀacceptor group.10ꢀ22 Recently, the
solvatochromism of nitroꢀsubstituted benzylideneaminophenolates was investigated by
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 40
4
1
2
3
4
5
6
7
8
9
our group.23 These compounds have phenolate donor groups and 4ꢀnitro or 2,4ꢀ
dinitrophenyl acceptor groups connected by means of a conjugated C=N bridge. These
compounds exhibit a reverse solvatochromism: a decrease in the molar transition energy
(ET) values for these dyes was found on changing the solvent from the most polar
solvent (i.e., water) to N,Nꢀdimethylacetamide (DMA). However, the ET(dye) values
increase for solvents with ET(30) values below 42.9 kcal molꢀ1 until the least polar
solvent (nꢀhexane) used in the studies. Reverse solvatochromism has been documented
over the past four decades24 in several investigations involving solvatochromic cyanines
and merocyanines.15,21,25ꢀ29 This phenomenon has been explained on the basis of (a) dye
aggregation in low polarity solvents,10,12,28 (b) solventꢀdependent cisꢀtrans (E/Z)
isomerization processes,30,31 and, most commonly, (c) the different abilities of polar and
nonpolar solvents to stabilize the electronic ground and excited states of the
probe.23,27,32,33
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The focus of the study reported herein was the design of novel solvatochromic
dyes with a conjugated C=C bridge in their molecular structure. The main interest was
to verify the role of the conjugated bridge in the level of solvatochromism observed for
these compounds, through comparison with previously prepared analogous imines.23 In
this regard, we report herein the synthesis and characterization of compounds 2aꢀ9a
(Scheme 1). The deprotonation of these compounds generate the solvatochromic dyes
2bꢀ9b, which were studied in 28 solvents. The influence of an additional nitro group on
the solvatochromic properties of the dyes was investigated. In addition, the importance
of the conjugated C=C bridge in relation to the level of the solvatochromism exhibited
by these systems is discussed. Multiparametric analysis of the experimental data was
ACS Paragon Plus Environment

Page 5 of 40
The Journal of Organic Chemistry
5
1
2
3
4
5
6
7
also carried out in order to verify the contribution of various solvent parameters to the
solvatochromism exhibited by the probes.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2a, b 3a, b 4a, b 5a, b 6a, b 7a, b 8a, b 9a, b
R1
R2
NO2
H
H
H
NO2
Br
H
NO2
Cl
H
NO2
CH3
H
Br
Cl
CH3
Scheme 1. Molecular structures of pyridinium Nꢀphenolate betaine dye 1 and
compounds 2a-9a, as well as their corresponding phenolates 2b-9b.
RESULTS AND DISCUSSION
Synthesis of the compounds. Compounds 2aꢀ8a were synthesized as shown in Scheme
2A through condensation of 4ꢀnitrotoluene or 2,4ꢀdinitrotoluene with the corresponding
aldehyde in the presence of pyrrolidine. Pyrrolidine acts as a base, abstracting a proton
from the nitroarene and providing the carbanion which carries out a nucleophilic attack
on the aldehyde carbonyl generating a tetrahedral intermediate. The latter undergoes
water elimination to form the substituted stilbene. The products of the reactions were
obtained in yields of 19ꢀ51 %. Low yields obtained in reactions to afford stilbenes
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 40
6
1
2
3
4
5
6
7
8
9
through condensation reactions have been reported in the literature.34,35 In addition, the
highest yields were obtained for the products containing 2,4ꢀdinitrophenyl groups, due
to the fact that 2,4ꢀdinitrotoluene is appreciably more acidic than 4ꢀnitrotoluene.
Compound 9a could not be obtained through this methodology. Therefore, this stilbene
was synthesized by means of the Wittig reaction (Scheme 2B), reacting (4ꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
nitrobenzyl)triphenylphosphonium
bromide36
with
3,5ꢀdimethylꢀ4ꢀ
hydroxybenzaldehyde in the presence of sodium hydrogen carbonate.
The compounds were purified through column chromatography and/or
recrystallization and the characterizations showed that their purity was adequate for the
spectroscopic studies. Stilbenes 4a, 6a, 7a, and 8a are novel compounds and were
1
characterized using IR, H NMR and HRMS techniques. Only the 13C NMR spectrum
of 7a was obtained, due to the low solubility of the other compounds. Monocrystals of
compounds 4a, 6a, and 8a were obtained, allowing their structural characterization
through Xꢀray diffraction.
Scheme 2. Preparation of compounds 2aꢀ9a.
ACS Paragon Plus Environment

Page 7 of 40
The Journal of Organic Chemistry
7
1
2
3
4
5
6
7
8
9
The Xꢀray structure of compounds 4a, 6a, and 8a. Figure 1 shows the molecular
structures for 4a, 6a, and 8a. Compounds 4a and 6a crystallize in the monoclinic system
(space group P 21/c) while compound 8a crystallize in the triclinic mode, space group
Pꢀ1. Their main bond distances and torsion angles are reported in Tables S1ꢀS6
(Supporting Information). The crystallographic data show that all compounds have an
(E)ꢀconfiguration. It can be observed in Table 1 that the length of the C=C bond in the
three compounds analyzed is compatible with those of conjugated systems where
aromatic groups are connected via a C=C bridge. The interplanar angles between donor
and acceptor groups varies depending on the groups in the 2,6 positions of the phenol
moieties. Compounds 4a and 6a, which have bromine and chlorine atoms as
substituents, exhibit interplanar angles between the mean plane of the aromatic rings of
25.02o and 36.6o, respectively. For compound 8a, with methyl groups, this angle is very
small (0.82o). The interplanar angles between acceptor ring and nitro group at position 4
are smaller than that at position 2 for 4a and 8a, suggesting a considerable steric
hindrance at the ortho position.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Interplanar angles and C=C bond lengths for compounds 4a, 6a, and 8a
obtained from the crystallographic data.
Compound
C=C bond
length [Å]
Interplanar
angle between
donor and
Interplanar
angle of the
nitro group at
position 2 [º]
Interplanar
angle of the
nitro group at
position 4 [º]
acceptor ring [º]
1.3350 (2)
1.3379 (13)
1.3414 (15)
25.02 (3)
36.60 (2)
0.82 (3)
17.7 (1)
ꢀ
5.80 (9)
5.40 (7)
2.14 (13)
4a
6a
8a
67.04 (6)
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 40
8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Molecular structures of compounds 4a, 6a, and 8a. Displacement ellipsoids
are drawn at the 40 % probability level.
Solvatochromism of dyes 2bꢀ9b and theoretical calculations. Solutions of 2a-
9a are pale yellow, but after deprotonation dyes 2bꢀ9b are generated, which exhibit
different colors, as shown in Figure 2. For instance, solutions of dye 6b are yellow in
water, rose in toluene, redꢀviolet in benzyl alcohol, violet in acetonitrile, lilac in
acetone, and blue in DMF. The solvatochromism was confirmed through the changes
observed in the UVꢀvis spectra of the dyes when the polarity of the medium was altered.
Figure 3 shows the UVꢀvis spectra for dyes 2bꢀ9b in some selected solvents. For
instance, dye 5b in water exhibits an absorption band with a maximum at 424 nm. This
band is shifted to
λ
max = 454 nm in ethanol while in dichloromethane, diethyl ether, and
ACS Paragon Plus Environment

Page 9 of 40
The Journal of Organic Chemistry
9
1
2
3
4
5
6
7
8
9
DMA the band maximum is displaced to 484, 476, and 534 nm, respectively. The bands
in the vis region of the spectra are due to π−π* electronic transitions, with an
intramolecular chargeꢀtransfer from the electronꢀdonor phenolate moiety towards the
electronꢀacceptor 4ꢀnitrophenyl (or 2,4ꢀdinitrophenyl) moiety.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Color of solutions of 2bꢀ9b in water (a), toluene (b), benzyl alcohol (c),
acetonitrile (d), acetone (e), and DMF (f).
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 40
10
1
2
3
4
5
6
7
8
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(e)
(d)
(b)
(c)
2b
3b
(c)
(d)
(a)
(e)
(b)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
300 350 400 450 500 550 600 650 700 750 800
350 400 450 500 550 600 650 700 750 800
λ
/nm
λ
/nm
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(c)
(d)
4b
5b
(a)
(b)
(e)
(e)
(d)
(b)
(a)
350 400 450 500 550 600 650 700 750 800
350 400 450 500 550 600 650 700 750
λ
/nm
λ
/nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
6b
7b
(b)
(a)
(c)
(d)
(a)
(b)
(e)
(c)
(e)
(d)
350 400 450 500 550 600 650 700 750 800
350
400
450
500
550
600
650
700
λ
/nm
λ
/nm
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
8b
9b
(c)
(d)
(e)
(d)
(c)
(e)
(b)
(b)
(a)
(a)
350 400 450 500 550 600 650 700 750 800
400
500
600
700
800
λ
/nm
λ
/nm
Figure 3. UVꢀvis spectra of 2bꢀ9b, dissolved in methanol (a), ethanol (b),
dichloromethane (c), diethyl ether (d), and DMA (e).
ACS Paragon Plus Environment

Page 11 of 40
The Journal of Organic Chemistry
11
1
2
3
4
5
6
7
8
9
The frontier molecular orbitals and natural bond orbitals (NBOs), including
secondꢀorder perturbation analysis of the Fock matrix to estimate donorꢀacceptor
interactions, were analyzed for the protonated (2aꢀ9a) and deprotonated analogues (2bꢀ
9b). The same analysis was extended to analogues of 2aꢀ3a and 2bꢀ3b containing C=N
conjugated bridges in order to provide information on the differences in the electronic
structures of the two types of bridges. The geometries calculated were validated by
comparison with Xꢀray structures available for 4a, 6a, and 8a. The geometries obtained
are in excellent agreement with the Xꢀray data, as indicated by selected parameters
reported in Figures S49 and S50 (Supporting Information). For instance, the C=C bond
lengths calculated (and obtained from the crystallographic data) for 4a, 6a, and 8a are
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1.356 (1.335) Å, 1.356 (1.338) Å, and 1.360 (1.341) Å, respectively. In general,
calculated bond lengths are slightly longer when compared with Xꢀray data, which can
be attributed to the crystalꢀpacking effect. The geometries calculated indicate that the
chemical bonds connecting donor and acceptor rings, C(1)ꢀC(7)=C(8)ꢀC(9) (Figure 1),
present a very particular behavior. In compounds 2aꢀ9a, the doubleꢀbond character of
C(7)=C(8) is very pronounced. However, after deprotonation to give 2bꢀ9b, the
C(7)=C(8) bond is longer, while the C(1)ꢀC(7) and C(8)ꢀC(9) are shorter, indicating an
increase in the electron delocalization between the two aromatic moieties after the
deprotonation (Figures S49ꢀS50 – Supporting Information). The results also show that
the C(1)ꢀC(7)=C(8)ꢀC(9) bond lengths in 2aꢀ9a or in 2bꢀ9b are not dependent on the
number of NO2 groups or on the nature of substituents employed. A similar trend, in
terms of C(1)ꢀC(7)=C(8)ꢀC(9) bond lengths, was observed for the corresponding
analogs of 2aꢀ3a and 2bꢀ3b, named 2a-im, 3a-im, 2b-im, and 3b-im, which contain the
C=N (instead of C=C) bridge. The C(1)ꢀC(7)=N(8)ꢀC(9) bond lengths reveal that, as
expected, the bond distances for C=N are shorter than those for C=C. The comparison
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 40
12
1
2
3
4
5
6
7
8
9
of 2a-im with 2b-im and of 3a-im with 3b-im verifies that the C(7)=N(8) bond lengths
become longer, while in the cases of C(1)ꢀC(7) and N(8)ꢀC(9) they become shorter
(Figure S51 – Supporting Information).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4 shows the frontier molecular orbital plots for 2aꢀ3b as well as the
eigenvalues and HOMOꢀLUMO gaps for all compounds (2aꢀ9b). According to Figure
4, the HOMOꢀLUMO energy gaps suggests that after deprotonation there is an increase
in the electron delocalization, which is entirely in line with the calculated geometrical
parameters. It is evident that the energy gaps for 2aꢀ9a are larger than for 2bꢀ9b. For
instance, moving from 2a to 2b the gap decreases from 1.88 to 1.21 eV. This is
consistent with the orbital plots, indicating that in 2a and 3a the HOMO is primarily
localized over the phenol ring and the C=C bridge. On the other hand, after
deprotonation it extends toward the entire molecule, including the NO2 groups. A
similar qualitative trend is observed for the analogues 2a-im – 3b-im (Figure 5). In
order to quantitatively evaluate the electron delocalization over the C=C bridges, the
secondꢀorder perturbation analysis of the Fock matrix was performed. The results show
that the electron delocalization over the C=C bridge provides significant energy
stabilization for both sets of compounds, that is, 2aꢀ9a and 2bꢀ9b (Tables S16ꢀS31,
Figure 6). After the deprotonation to 2bꢀ9b, an increase in the quantity and magnitude
of donorꢀacceptor stabilizing interactions is observed for all compounds. In fact,
electron donation is observed from occupied NBOs located at the C=C bond towards
unoccupied NBOs in both rings. For instance, the interactions πC7ꢀC8 → π*C1ꢀC2 and πC7ꢀ
→
π*C9ꢀC10 provide stabilizations not only for protonated (2aꢀ9a) but also for
C8
deprotonated (2bꢀ9b) species, ranging from 12.71 to 15.05 kcal molꢀ1 and from 11.33 to
11.76 kcal molꢀ1, respectively (Tables S16ꢀ31 – Supporting Information and Figure 6).
ACS Paragon Plus Environment

Page 13 of 40
The Journal of Organic Chemistry
13
1
2
3
4
5
6
7
8
9
Donorꢀacceptor interactions are also observed in the opposite direction, that is, from
occupied NBOs in both rings to the π*C7ꢀC8 orbital at the C=C bridge (πC1ꢀC2 → π*C7ꢀC8
and πC9ꢀC10 → π*C7ꢀC8). In this case, the most stabilizing interactions come from the
phenol and phenolate rings towards the bridges (πC9ꢀC10 → π*C7ꢀC8), presenting
stabilization energy values ranging from 14.84 to 21.85 kcal molꢀ1 (Tables S16ꢀ31),
while the donorꢀacceptor interactions involving occupied NBOs located in the rings
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
containing NO2 groups and unoccupied NBOs of the C=C bridges (πC1ꢀC2 → π*C7ꢀC8
)
provide energy stabilization values ranging from 9.74 to 14.58 kcal molꢀ1. A comparison
between the nitro and dinitro groups for 2a-9a compounds shows that the additional
nitro substituent decreases the magnitude of the πC7ꢀC8 → π*C1ꢀC2 and πC9ꢀC10 → π*C7ꢀC8
interactions, while an increase in the πC7ꢀC8 → π*C9ꢀC10 and πC1ꢀC2 → π*C7ꢀC8 interactions
is observed. On the other hand, for the deprotonated species 2b-9b this trend is not
observed, the energy stabilizations that stem from πC7ꢀC8 → π*C1ꢀC2 and πC7ꢀC8 → π*C9ꢀC10
donorꢀacceptor interactions decrease while the πC1ꢀC2 → π*C7ꢀC8 and πC9ꢀC10 → π*C7ꢀC8
interactions are more stabilizing. The same type of donorꢀacceptor interactions is less
stabilizing in analogues containing C=N, 2a-im – 3b-im. The NBO results also show
that the presence of electronꢀdonor or electronꢀwithdrawing substituents in the
phenol/phenolate ring does not cause a significant change in the magnitude of the πC7ꢀC8
→ π*C1ꢀC2, πC7ꢀC8 → π*C9ꢀC10, πC1ꢀC2 → π*C7ꢀC8, and πC9ꢀC10→ π*C7ꢀC8 donorꢀacceptor
interactions.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 40
14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Contour plots for compounds 2a, 3a, 2b, 3b and energies of the frontier
orbitals (HOMO and LUMO) for compounds 4aꢀ9a and 4bꢀ9b. Contour plots for 4aꢀ9b
are shown in Figures S52 and S53 (Supporting Information).
ACS Paragon Plus Environment

Page 15 of 40
The Journal of Organic Chemistry
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Contour plots and energies of the frontier orbitals (HOMO and LUMO) for
compounds 2a-im, 3a-im, 2b-im, and 3b-im.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 40
16
1
2
3
4
5
6
7
8
πC7─C8→π*C1─C2
πC7─C8→π*C9─C10
πC1─C2→π*C7─C8
πC9─C10→π*C7─C8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
πC7─C8→π*C1─C2
πC7─C8→π*C9─C10
πC1─C2→π*C7─C8
πC9─C10→π*C7─C8
πC9─C10→π*C7─C8
πC9─C10→π*C7─C8
πC7─C8→π*C1─C2
πC7─C8→π*C9─C10
πC1─C2→π*C7─C8
πC7─C8→π*C1─C2
πC7─C8→π*C9─C10
πC1─C2→π*C7─C8
Figure 6. Plots for selected donorꢀacceptor interactions between NBOs of 2a,b and
3a,b.
The solvatochromism of dyes 2bꢀ9b was investigated in 28 solvents of
different polarity. Table 2 shows the [ET(dye)] values for each solvent, calculated from
the λmax values obtained from the UVꢀvis spectra, and also the corresponding ET(30)
values of betaine dye 1. Figure 7 shows the plot of ET(dye) as a function of ET(30) for
dye 8b, which is representative for all dyes studied (see Supporting Information),
indicating the presence of two regions, one in less polar solvents and the other in more
polar solvents. Starting with water, the most polar solvent studied, the ET values for
ACS Paragon Plus Environment

Page 17 of 40
The Journal of Organic Chemistry
17
1
2
3
4
5
6
7
8
9
each dye decrease until solvents of intermediate polarities, after which this behavior
changed, i.e., the ET values increase with a further reduction in solvent polarity, with a
reverse solvatochromism occurring. For dye 8b, the solvatochromic band has a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
maximum at 572 nm [ET(8b) = 50.0 kcal molꢀ1] in cyclohexane, while in DMSO λmax
=
718 nm [ET(8b) = 39.8 kcal molꢀ1] is obtained. Thus, a reduction in the ET(8b) value
occurs with an increase in the polarity of the medium, leading to a bathochromic shift of
ꢀλmax = +146 nm, which is characteristic for a positive solvatochromism. The region in
the plot corresponding to DMSO as solvent is where the reversal in the
solvatochromism starts to occur. Above this solvent polarity, a further increase in the
solvent polarity leads to an increase in the ET(8b) value, which is typical for a negative
solvatochromism. The vis band of 8b in water has a maximum at 496 nm [ET(8b) =
57.6 kcal molꢀ1], which, in comparison with the λmax value in DMSO, corresponds to a
strong hypsochromic band shift of ꢀλmax = ꢀ222 nm. The other dyes studied reveal a
similar behavior, with a reversal in the solvatochromism occurring in DMA as solvent.
Table 3 lists the λmax values for dyes 2bꢀ9b in the least polar, intermediate
polar, and most polar solvent, as well as the ꢁλmax values for the bathochromic and
hypsochromic band shifts corresponding to each dye. The data show that the
compounds with two nitro groups in their molecular structure (2b, 4b, 6b, and 8b)
exhibit larger ꢁλmax values than their counterparts containing only one nitro group (3b,
5b, 7b, and 9b). Of the compounds with two nitro groups, compound 8b exhibits the
highest ꢁλmax value, followed by dyes 2b, 6b, and 4b. These results suggest that the
positive inductive effect of the methyl groups in dye 8b represents an important
contribution to the charge transfer, while for dyes 6b and 4b the chloro and bromo
substituents have positive mesomeric effects counterbalanced by the negative inductive
effect due to their pronounced electronegativity. Concerning the dyes with only one
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 40
18
1
2
3
4
5
6
7
8
9
nitro group, 7b and 5b exhibit lower ꢁλmax values in comparison with 3b, which
suggests that the electronꢀwithdrawing effect of the halogens is responsible for the
reduced probability of chargeꢀtransfer from the phenolate to the nitrophenyl group in
these dyes.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. ET(dye) values for compounds 2bꢀ9b in 28 pure solvents at 25 °C and the
ET(30) values in kcal molꢀ1.
SOLVENT
ET(30)a ET(2b) ET(3b) ET(4b) ET(5b) ET(6b) ET(7b) ET(8b) ET(9b)
Water
63.1
56.3
55.4
51.9
50.7
50.4
49.7
49.1
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
37.4
34.5
33.9
31.0
61.4
58.1
59.1
55.4
54.1
53.3
53.5
53.1
51.4
52.4
52.7
50.0
49.3
42.7
47.5
43.2
41.9
47.5
49.8
49.5
46.7
50.5
48.8
47.0
50.5
53.1
56.5
66.8
69.4
67.1
62.7
61.6
60.1
60.8
60.8
59.1
59.8
60.8
57.9
55.8
50.0
55.4
51.0
49.8
53.9
57.0
57.0
55.4
57.9
55.8
55.8
58.8
61.1
63.5
62.7
59.3
59.3
55.8
55.0
53.9
54.4
54.1
52.7
53.5
53.9
52.0
52.0
47.6
51.2
48.8
46.9
50.0
51.2
50.5
49.1
51.4
50.5
50.2
52.6
54.1
55.8
67.4
65.3
65.6
63.0
62.4
60.6
61.9
61.6
60.3
61.1
61.6
59.6
59.1
54.4
59.1
54.8
53.5
56.7
59.1
59.1
57.4
59.3
58.3
57.9
60.1
61.4
64.1
63.0
59.8
60.1
56.5
55.4
54.4
55.0
54.6
52.7
53.9
54.4
52.0
52.4
48.0
51.0
47.6
46.5
50.0
51.8
50.5
49.8
52.4
50.5
50.0
52.0
54.4
57.9
67.7
65.6
66.2
63.5
62.4
61.1
62.4
62.2
60.6
61.4
62.2
59.8
59.1
54.1
59.1
54.8
53.1
57.0
59.3
59.3
57,9
60.1
58.3
57.6
60.1
62.2
65.3
57.6
53.3
53.1
49.0
48.1
47.5
47.5
46.9
45.1
47.5
47.5
44.0
43.0
39.8
42.7
40.3
40.0
41.6
43.4
43.6
41.0
45.7
42.7
41.9
45.0
48.3
50.0
63.0
60.8
61.1
57.4
56.7
55.4
56.0
55.8
55.5
55.8
55.8
52.6
52.0
45.0
51.6
46.3
47.5
49.8
54.1
54.1
50.7
55.2
52.6
50.7
54.8
57.9
59.1
Ethaneꢀ1,2ꢀdiol
Methanol
Ethanol
Propanꢀ1ꢀol
Benzyl alcohol
Butanꢀ1ꢀol
Pentanꢀ1ꢀol
Propanꢀ2ꢀol
Octanꢀ1ꢀol
Decanꢀ1ꢀol
Butanꢀ2ꢀol
Acetonitrile
DMSO
2ꢀMethylpropanꢀ2ꢀol
DMF
DMA
Acetone
1,2ꢀDichloroethane
Dichloromethane
Acetophenone
Trichloromethane
Ethyl acetate
THF
Diethyl ether
Toluene
nꢀHexane
ACS Paragon Plus Environment

Page 19 of 40
The Journal of Organic Chemistry
19
1
2
3
4
5
6
Cyclohexane
30.9
55.0
64.1
55.8
63.5
57.2
64.1
50.0
59.1
a Values obtained from the literature.1
7
8
9
Table 3. Values of λmax for compounds 2-7 in the most polar, intermediate polarity, and
least polar solvents, as well as the ꢁλmax values obtained considering the regions of
positive and negative solvatochromism.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Most polar solvent
(water)
Intermediate polar Least polar solvent
solvent (cyclohexane)
ꢁ
/nm
λmax
¹
ꢁλmax²
/nm
Dye
λ
max/nm
λ
max/nm
λ
max/nm
466
682 (DMA)
574 (DMA)
610 (DMA)
534 (DMA)
614 (DMA)
538 (DMA)
718 (DMSO)
636 (DMSO)
520
+162
+128
+98
ꢀ216
ꢀ146
ꢀ154
ꢀ110
ꢀ160
ꢀ118
ꢀ222
ꢀ182
2b
3b
4b
5b
6b
7b
8b
9b
428
446
456
512
424
450
+84
454
500
+114
+92
420
446
496
572
+146
+152
454
484
1ꢁλmax = λmax (intermediate solvent) ꢀ λmax (least polar solvent)
2ꢁλmax = λmax (water) ꢀ λmax (intermediate solvent)
60
8b
55
50
45
40
30
35
40
45
50
55
60
65
ET(30) / (kcal molꢀ1)
Figure 7. ET(8b) values for 28 solvents as a function of Reichardt’s ET(30) parameter.
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 40
20
1
2
3
4
5
El Seoud et al.28 proposed a methodology to evaluate the different factors that
can contribute to the reverse solvatochromism of dyes. Thus, studies were performed
for probes 2bꢀ9b in three solvents (of small, intermediate, and high polarity), in which
the possibility of aggregation in the concentrations used in the spectrophotometric
experiments or cisꢀtrans isomerization (see Supporting Information) was discarded.
Compounds 2b-9b are salts, which can form, in principle, ion pairs in solution,
especially in low polarity solvents. The formation of ion pairs would influence the
absorption spectra of the dyes in these solvents. In order to verify this, UVꢀvis
absorption spectra of the dye in toluene were collected in the presence of increasing
concentrations of tetraꢀnꢀbutylammonium iodide. It was verified that the addition of this
salt up to 3.5×10ꢀ3 mol Lꢀ1 did not change the position of the maximum on the UVꢀvis
spectra of 2b-9b (see Supporting Information). In other experiment, compound 6a in
toluene was deprotonated after the addition of potassium hydroxide (see Supporting
Information). The UVꢀvis spectrum was measured and the solvatochromic band of 6b
has a maximum at 468 nm, which is very different from the value obtained if tetraꢀnꢀ
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
butylammonium hydroxide is used (λmax = 526 nm). The results suggest an electrostatic
interaction of potassium cation with the phenolate moiety of the dye. 18ꢀCrownꢀ6, a
receptor highly selective for potassium ion, was added to the solution and the UVꢀvis
spectrum was made. The spectrum obtained is exactly the same as that measured in the
presence of tetraꢀnꢀbutylammonium hydroxide. The results showed that the
solvatochromism observed is not influenced by the presence of tetraꢀnꢀbutylammonium
cation as a counteranion in the concentrations used in the experiment.
ACS Paragon Plus Environment

Page 21 of 40
The Journal of Organic Chemistry
21
1
2
3
4
5
6
7
8
9
Therefore, the solvatochromism of the stilbenes should be interpreted in terms of
the ability of the medium to stabilize the electronic ground and excited states of the
dyes. According to Figure 8A, two resonance structures make a very important
contribution to the resonance hybrid in compounds 2bꢀ9b, the quinonoid (less dipolar)
and the benzenoid (more dipolar) forms. The resonance form that contributes mostly to
the resonance hybrid is dependent on the solvent. Figure 8B represents schematically
the reverse solvatochromism exhibited by dye 5b, and this is representative of all of the
other dyes studied herein. In less polar solvents, such as cyclohexane, the quinonoid
form represents the most important contribution to the ground state. However, with a
gradual increase in the solvent polarity the difference between the energies for the
ground and excited states is reduced, due to the fact that the solvent is less able to
solvate the ground state of the dye, but more able to stabilize the more dipolar excited
state. This explains the region of positive solvatochromism verified for these dyes in
solvents with polarities between those of cyclohexane and DMA. In more polar
solvents, such as water, the stabilization of the more dipolar benzenoid form is better in
comparison with the less dipolar quinonoid form, and thus it makes a more important
contribution under the conditions to the resonance hybrid, and, consequently, to the
ground state of the dye. In this polarity range, an increase in the polarity of the medium
leads to an increase in the difference, in terms of the energy, between the ground and
excited states, with negative solvatochromism being observed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 40
22
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8. (A) Benzenoid and quinonoid resonance structures of compounds 2bꢀ9b. (B)
Reverse solvatochromism exhibited by compound 5b.
Multiparametric studies on the solvatochromism of 2b-9b. The KamletꢀAbboudꢀ
Taft20,37ꢀ41 and Catalán41ꢀ46 multiparameter equations were used in the analysis of the
experimental ET(dye) values for compounds 2bꢀ9b in pure solvents. These approaches
consider a linear correlation between the experimentally obtained spectrometric
parameter, ET(dye), and several solvent parameters according to Eq. (1). ET(dye)0
represents the ET(dye) value related to an inert solvent or the gas phase and a, b, and c
are coefficients that reflect the relative importance of the solvent parameters A, B, and
C, respectively, in terms of ET(dye).
ACS Paragon Plus Environment

Page 23 of 40
The Journal of Organic Chemistry
23
1
2
3
4
5
6
7
ET(dye) = ET(dye)0 + aA + bB + cC + ...
(1)
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The KamletꢀAbboudꢀTaft equation37,38 requires the use of Eq. (2), where
* are parameters that represent the solvent hydrogenꢀbond donor (HBD) acidity,
hydrogenꢀbond acceptor (HBA) basicity, and solvent dipolarity/polarizability,
respectively, while is a polarizability correction term for the solvent. In the
α, β,
and
π
δ
multiparameter equation developed by Catalán et al.45 Eq. (3) is used, where SP (solvent
polarizability) and SdP (solvent dipolarity) are nonspecific and SA (solvent HBD
acidity) and SB (solvent HBA basicity) are specific solvent parameters.
ET(dye) = ET(dye)0 + aα + bβ + s(π* + dδ)
(2)
(3)
ET(dye) = ET(dye)0 + aSA+ bSB + cSP + dSdP
The contributions of the solvent properties to the ET(dye) values determined for
probes 2bꢀ9b were ascertained with the use of Eqs. (2) and (3). The data obtained from
the multiple square correlation analysis are shown in Tables 4, as well as S2ꢀS5, and S6
(Supporting Information). The use of Eq. (2) gives reasonable S.D. and r values;
however, the b values are very large and the s values are very small for all dyes studied.
These data is not consistent with the molecular structure of the probes, since they do not
have hydrogenꢀbond donating groups to interact with HBA solvents. In addition, these
compounds are expected to be considerably sensitive to the dipolarity/polarizability of
the medium. The application of Eq. (3) to compounds 2bꢀ9b provide more coherent
results: (a) the r values are > 0.95 for all solvents studied; (b) the contribution of the
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 40
24
1
2
3
4
5
6
7
8
9
HBA basicity of the solvent is small (small b coefficients); (c) the most important
contributions were verified for the HBD acidity of the solvent (large a coefficients; for
instance, for probe 2b a/c = 16.37/12.62 = 1.30 and a/d = 16.37/8.64 = 1.89); and (d) the
coefficients related to the polarizability and dipolarity of the medium represent
important contributions to the solvatochromism exhibited by the dyes. The a/c ratio for
the acidity and polarizability coefficients for probe 2b is a/c = 16.37/12.62 = 1.30, while
a comparison between the contribution of the polarizability and dipolarity terms give
the ratio c/d = 12.62/8.64 = 1.46. For compounds 2bꢀ7b the contribution of the
polarizability term is more important than the dipolarity term. However, for dyes 8b and
9b, which have methyl groups in their molecular structures, the dipolarity term has a
larger contribution than the polarizability term.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4. Regression coefficients a, b, c, and d obtained from Catalán multiparametric
analysis, significance (F), correlation coefficient (r) and standard deviation (S.D.)
obtained from the treatment of ET(dye) values for compounds 2bꢀ9b in 28 solvents.
Dye constant
A
b
c
d
F
r
S.D.
2b
3b
4b
5b
6b
7b
8b
9b
64.94±2.98 16.37±1.18
71.46±3.46 15.48±1.29
62.78±1.62 13.12±0.64
70.90±1.90 11.37±0.75
64.02±2.00 13.90±0.79
71.67±2.10 12.09±0.83
54.60±1.79 16.30±0.70
66.65±3.04 14.24±1.20
ꢀ4.33±1.00
ꢀ12.62±4.13 ꢀ8.64±1.17 < 3.30x10ꢀ12
ꢀ10.54±4.52 ꢀ9.79±1.28 < 5.14x10ꢀ11
ꢀ10.19±2.24 ꢀ5.24±0.63 < 1.66x10ꢀ15
ꢀ10.14±2.62 ꢀ6.40±0.74 < 3.57x10ꢀ13
ꢀ10.11±2.76 ꢀ6.41±0.78 < 3.15x10ꢀ14
0.96
1.46
ꢀ3.33±1.10
ꢀ3.20±0.54
ꢀ2.70±0.64
ꢀ3.86±0.67
ꢀ3.12±0.70
ꢀ4.09±0.60
ꢀ4.65±1.02
0.95
0.98
0.97
0.97
0.96
0.98
0.95
1.60
0.80
0.93
0.98
1.03
0.88
1.49
ꢀ9.92±2.91
ꢀ6.53±2.47
ꢀ9.14±4.21
ꢀ7.21±0.82 < 7.22x10ꢀ13
ꢀ7.73±0.70 < 1.11x10ꢀ16
ꢀ9.15±1.19 < 3.47x10ꢀ11
ACS Paragon Plus Environment

Page 25 of 40
The Journal of Organic Chemistry
25
1
2
3
4
5
6
7
8
9
Figure 9(A) shows plots of the experimentally obtained ET(dye) values as a
function of the ET(dye) values calculated for dyes 4b and 5b, using the data from Table
4. Figure 9(B) shows the same plots using Eq. (3), but without the acidity term,
verifying a decrease in the r value from 0.98 to 0.44 and from 0.97 to 0.51 for dyes 4b
and 5b, respectively. The suppression of the basicity term in Eq. (3) has practically no
influence on the r values (Figure 9(C)). Figure 9(D) shows the plots for the calculated
data in which both the polarizability and dipolarity terms of Eq. (3) were suppressed. In
this case the r values decreased to 0.85 for dye 4b and 0.76 for 5b.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CONCLUSIONS
4ꢀ(Nitrostyryl)phenols 2aꢀ9a were synthesized and characterized and their
deprotonation in solution led to the formation of the corresponding phenolates (2bꢀ9b).
All compounds studied exhibited a reversal in solvatochromism. The change in the
nature of the solvatochromism was verified, from positive in solvents of low polarity to
negative in more polar solvents, in the region corresponding to DMA (ET(30) = 42.9
kcal molꢀ1) for dyes 2bꢀ7b. These results are, in general, similar to those obtained by
Nandi et al23 for related dyes with an imino group as the conjugate bridge. For dyes 8b
and 9b, the change observed from negative to positive solvatochromism occurs with
DMSO (ET(30) = 45.1 kcal molꢀ1) as the solvent. Through a comparison between the
dyes it was verified that the phenolates with an additional nitro group in their molecular
structure exhibit a more accentuated solvatochromism.
The solvatochromic behavior of the compounds is interpreted in terms of the
interaction of the solvent with the electronic ground and excited states of the dyes,
considering two resonance structures, i.e. quinonoid (less dipolar) and benzenoid (more
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 26 of 40
26
1
2
3
4
5
6
7
8
9
dipolar) forms, whose contribution to the resonance hybrid is dependent on the solvent
polarity. The Catalán multiparameter equation works very well for the fitting of the
experimental data and shows that the dyes have, as expected, poor sensitivity to the
basicity of the medium and high sensitivity to the acidity of the medium, which can be
understood in terms of the ability of HBD solvents to interact through hydrogen
bonding with the phenolate moiety in the dyes. In addition, the polarizability and
dipolarity of the medium make important contributions to the solvatochromism reported
for these dyes.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In conclusion, the strong solvatochromic behavior of dyes 2bꢀ9b should
motivate the synthesis of other related compounds aimed at not only understanding the
reverse solvatochromism in these and other systems but also obtaining increasingly
more effective solvatochromic systems. The strong solvatochromism exhibited by this
class of compounds is of interest in relation to the design of systems aimed at the
investigation of their perichromic properties, such as in the investigation of the polarity
of solvent mixtures, cyclodextrins, and surfactants in water, and also in the development
of optical devices, which can be used for the detection of analytes and in organic
electronics.
ACS Paragon Plus Environment

Page 27 of 40
The Journal of Organic Chemistry
27
1
2
3
4
5
6
7
8
9
64
62
60
58
56
54
52
50
48
46
70
(A)
(A)
68
66
64
62
60
58
56
54
52
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
54 56 58 60 62 64 66 68 70 72
ET(5b) calculated / (kcal molꢀ1)
(B)
46 48 50 52 54 56 58 60 62 64 66
ET(4b) calculated / (kcal molꢀ1)
64
70
68
66
64
62
60
58
56
54
52
(B)
62
60
58
56
54
52
50
48
46
56 57 58 59 60 61 62 63 64 65 66
50
51
52
53
54
55
56
57
58
ET(5b) calculated / (kcal molꢀ1)
ET(4b) calculated / (kcal molꢀ1)
64
62
60
58
56
54
52
50
48
46
70
(C)
(C)
68
66
64
62
60
58
56
54
52
48
50
52
54
56
58
60
62
64
54
56
58
60
62
64
66
68
70
ET(4b) calculated / (kcal molꢀ1)
ET(5b) calculated / (kcal molꢀ1)
64
62
60
58
56
54
52
50
48
46
70
(D)
(D)
68
66
64
62
60
58
56
54
52
50
52
54
56
58
60
62
64
66
58
60
62
64
66
68
70
72
ET(4b) calculated / (kcal molꢀ1)
ET(5b) calculated / (kcal molꢀ1)
Figure 9. Relationship between experimental and calculated ET values for the
compounds 4b and 5b with all terms (A), without the acidity term (B), without the
basicity term (C), and without polarizability and dipolarity terms (D).
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 28 of 40
28
1
2
3
4
5
6
7
8
EXPERIMENTAL SECTION
9
Materials and methods. All solvents were HPLC grade and were purified following
the methodology described in the literature.47,48 Deionized water was used in all
measurements, and it was boiled and bubbled with nitrogen and kept in a nitrogen
atmosphere to avoid the presence of carbon dioxide.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The melting points were uncorrected. IR spectra were obtained with KBr pellets.
The NMR spectra were recorded in CDCl3, DMSOꢀd6, or acetoneꢀd6 with 200 and 400
MHz 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, t = triplet, dd = double doublet), coupling constants (Hz), and
integration. Highꢀresolution mass spectra were obtained with an electrospray ionizationꢀ
quadrupole timeꢀofꢀflight mass spectrometer (HR ESIꢀMS QTOF).
Synthesis of the compounds. 2,4ꢀDinitrotoluene (10) was prepared according to the
procedure described in the literature,47 the melting point obtained being 69ꢀ70 ºC
(literature melting point of 70 ºC).47
3,5ꢀDibromoꢀ4ꢀhydroxybenzaldehyde (11) was obtained based on the
methodology described by Mahajan and coworkers49 with some modifications: 60 mL
of water was heated to 40 °C and 4ꢀhydroxybenzaldehyde (6.00 g, 49.1 mmol),
potassium bromide (17.53 g, 147.3 mmol), 4ꢀtoluenesulfonic acid (16.90 g, 98.2 mmol)
and finally Nꢀbromosuccinimide (17.48 g, 98.2 mmol) were added and reacted for 2 h.
The compound was filtered off, washed with a solution of sodium metabisulfite
ACS Paragon Plus Environment

Page 29 of 40
The Journal of Organic Chemistry
29
1
2
3
4
5
6
7
(Na2S2O5, 10 %) and recrystallized twice from a mixture of water/ethanol. The product
is a colorless solid (8.25 g, 60 % yield) with a melting point of 181 ºC.
8
9
3,5ꢀDichloroꢀ4ꢀhydroxybenzaldehyde (12) was obtained according to the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
procedure described in the literature49 with a melting point of 154ꢀ157 ºC.
3,5ꢀDimethylꢀ4ꢀhydroxybenzaldehyde (13) was synthesized according to the
literature50 but the purification was performed on a chromatographic column with nꢀ
hexane/ethyl acetate (7:3 v/v). The product has a melting point of 112ꢀ114 ºC.
4ꢀNitrobenzylꢀtriphenylphosphonium bromide (14) was prepared based on the
methodology described by Sá and coworkers.36 1ꢀ(Bromomethyl)ꢀ4ꢀnitrobenzene (0.22
g, 1 mmol) and triphenylphosphane (0.27 g, 1 mmol) were added to 3 mL of acetonitrile
and the solution was stirred at 25 ºC for 24 h. The product is colorless (0.38 g, 80 %
yield) and has a melting point of 273ꢀ274 ºC.
(E)-4-(2,4-Dinitrostyryl)phenol (2a): this compound was prepared based on the
methodology described by Saravanan and Srinivasan34 with some modifications: 2,4ꢀ
dinitrotoluene (1.00 g, 5.5 mmol), 4ꢀhydroxybenzaldehyde (0.67 g, 5.5 mmol), and
pyrrolidine (360 ꢂL, 4.4 mmol) were refluxed at 90 °C for 4 h. The compound was
purified by column chromatography using methanol/trichloromethane (1:1 v/v) as the
eluent and then by recrystallization from methanol. The product is an orange solid (0.50
g, 32 % yield) with a melting point of 177ꢀ179 ºC. IR (KBr, max/cmꢀ1): 3408 (O‒H),
3073 (=C‒H), 1587, 1513, and 1440 (C=C), 1334 (N=O). 1H NMR (400 MHz, DMSOꢀ
d6) δ/ppm: 9.95 (1H, s), 8.70 (1H, d, J = 2.0 Hz), 8.43 (1H, dd, J = 2.0 Hz and J = 8.0
Hz), 8.21 (1H, d, J = 8.0 Hz), 7.54 (1H, d, J = 16.0 Hz), 7.51 (2H, d, J = 8.0 Hz), 7.27
(1H, d, J = 16.0 Hz), 6.81 (2H, d, J = 8.0 Hz).
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 30 of 40
30
1
2
3
4
5
(E)-4-(4-Nitrostyryl)phenol (3a): this compound was synthesized according to
6
7
8
9
the literature51 with a melting point of 204ꢀ206 ºC. IR (KBr, max/cmꢀ1): 3418 (O‒H),
1585, 1501, and 1438 (C=C), 1338 (N=O). 1H NMR (400 MHz, DMSOꢀd6) δ/ppm: 9.70
(1H, s), 8.18 (2H, d, J = 8.0 Hz), 7.77 (2H, d, J = 8.0 Hz), 7.50 (2H, d, J = 8.0 Hz), 7.42
(1H, d, J = 16.0 Hz), 7.16 (1H, d, J = 16.0 Hz), 6.80 (2H, d, J = 8.0 Hz).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(E)-2,6-Dibromo-4-(2,4-dinitrostyryl)phenol (4a): 2,4ꢀdinitrotoluene (0.27
g, 1.5 mmol), 3,5ꢀdibromoꢀ4ꢀhydroxybenzaldehyde (0.41 g, 1.5 mmol), and pyrrolidine
(98 µL, 1.2 mmol) were refluxed at 80ꢀ85 °C for 6 h. Purification was performed by
column chromatography eluting with trichloromethane/nꢀhexane (9:1 v/v). A yellow
solid is obtained (0.27 g, 41 % yield) with a melting point of 273ꢀ275 °C; IR (KBr,
1
max/cmꢀ1): 3408 (OꢀH), 3098 (=CꢀH), 1593, 1521, and 1475 (C=C), 1346 (C=O); H
NMR (400 MHz, DMSOꢀd6) δ/ppm: 10.39 (1H, s), 8.71 (1H, d, J = 2.0 Hz,), 8.51 (1H,
dd, J = 2.0 Hz and J = 9.0 Hz), 8.14 (1H, d, J = 9.0 Hz), 7.91 (2H, s), 7.46 (2H, s);
HRMS (ESI/TOFꢀQ) m/z: [M]ꢀ Calcd for C14H7N2O5Br2 440.8716; Found 440.8712.
(E)-2,6-Dibromo-4-(4-nitrostyryl)phenol (5a): pꢀnitrotoluene (0.50 g, 3.6
mmol), 3,5ꢀdibromoꢀ4ꢀhydroxybenzaldehyde (1.02 g, 3.6 mmol), and pyrrolidine (750
µL, 9.1 mmol) were refluxed at 70ꢀ75 °C for 72 h. In the purification column
chromatography was used eluting with nꢀhexane/ethyl acetate (1:1 v/v). A yellow solid
(0.39 g, 27 % yield) is obtained with a melting point of 235ꢀ237 °C; IR (KBr, max/cmꢀ
1): 3467 (OꢀH), 3067 (=CꢀH), 1595, 1513, and 1472 (C=C), 1336 (C=O); 1H NMR (400
MHz, DMSOꢀd6) δ/ppm: 10.24 (1H, s), 8.22 (2H, d, J = 8.0 Hz), 7.88 (2H, s), 7.78 (2H,
d, J = 8.0 Hz), 7.39 (2H, s); HRMS (ESI/TOFꢀQ) m/z: [M]ꢀ Calcd for C14H8NO3Br2
395.8865; Found 395.8867.
ACS Paragon Plus Environment