Solvatochromic behavior of dyes with dimethylamino electron-donor and nitro electron-acceptor groups in their molecular structure

Article abstract of DOI:10.1002/poc.3402

Six dyes with N,N-dimethylaminophenyl and 4-nitrophenyl or 2,4-dinitrophenyl groups in their molecular structures were prepared and characterized. These compounds have different conjugated bridges (C-C, C-N, and N-N) connecting the electron-donor and the electron-acceptor groups. All compounds are solvatochromic, with reverse solvatochromism occurring. The solvatochromic band observed in each spectrum for the dyes is due to a π rarr π? transition, of an intramolecular charge transfer nature, which occurs from the electron-donor N,N-dimethylaminophenyl group to the electron-acceptor group in the molecules, which is reinforced by the structures of the compounds optimized by applying density functional theory, which exhibit high planarity. The reverse solvatochromism was explained considering two resonance structures. The benzenoid form is better stabilized in less polar solvents and characterizes the region displaying positive solvatochromism, while the dipolar form is better stabilized in more polar solvents, in the region of negative solvatochromism. The Catalán multiparametric approach was used to study the contribution of solvent acidity, basicity, dipolarity, and polarizability to the solvatochromism exhibited by the compounds. These compounds are good candidates for the investigation of the polarizability and, to a lesser extent, the dipolarity of the medium, with very little interference from specific interactions of the solvent through hydrogen bonding.

Full text of DOI:10.1002/poc.3402

Research Article
Received: 10 September 2014,
Revised: 22 October 2014,
Accepted: 9 November 2014,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3402
Solvatochromic behavior of dyes with
dimethylamino electron-donor and nitro
electron-acceptor groups in their molecular
structure
Carlos E. A. de Meloa, Leandro G. Nandia, Moisés Domínguezb,
Marcos C. Rezendeb and Vanderlei G. Machadoa*
Six dyes with N,N-dimethylaminophenyl and 4-nitrophenyl or 2,4-dinitrophenyl groups in their molecular structures were
prepared and characterized. These compounds have different conjugated bridges (C¼C, C¼N, and N¼N) connecting the
electron-donor and the electron-acceptor groups. All compounds are solvatochromic, with reverse solvatochromism
occurring. The solvatochromic band observed in each spectrum for the dyes is due to a π ➔ π* transition, of an intramolecular
charge transfer nature, which occurs from the electron-donor N,N-dimethylaminophenyl group to the electron-acceptor
group in the molecules, which is reinforced by the structures of the compounds optimized by applying density functional
theory, which exhibit high planarity. The reverse solvatochromism was explained considering two resonance structures.
The benzenoid form is better stabilized in less polar solvents and characterizes the region displaying positive
solvatochromism, while the dipolar form is better stabilized in more polar solvents, in the region of negative
solvatochromism. The Catalán multiparametric approach was used to study the contribution of solvent acidity, basicity,
dipolarity, and polarizability to the solvatochromism exhibited by the compounds. These compounds are good candidates
for the investigation of the polarizability and, to a lesser extent, the dipolarity of the medium, with very little interference
from specific interactions of the solvent through hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: dipolarity/polarizability; reverse solvatochromism; solvation; solvatochromic dyes; solvatochromism
and a phenolate as the electron-donor group.[10–22] Recently,
INTRODUCTION
our group studied the solvatochromism of nitro-substituted
benzylideneaminophenolates,[23] which have phenolate donor
It is well known that the choice of a particular solvent or a
groups and 4-nitro or 2,4-dinitrophenyl acceptor groups
connected through a C¼N conjugated bridge. The compounds
studied exhibited a reversal in their solvatochromism: first a
decrease in the molar transition energy (ET) values of the dyes
occurred on changing from water to DMA, but for solvents with
ET(30) values below 42.9 kcal molꢀ1 the ET(dye) values increase
until the least polar solvent studied (n-hexane). The concept of
mixture of solvents is crucial in terms of the rate and course of
chemical processes[1–5] as well as the success of the purification
and separation of organic compounds in the laboratory, as in the
case of recrystallization and chromatographic techniques. In ad-
dition, the medium can strongly influence the spectrometric
data of compounds, such as the maximum absorption and
emission bands of organic compounds.[1,2,5]These effects
are usually explained in terms of the solvent polarity, which
represents the overall solvating ability of the medium.[1,5] In
order to investigate the polarity of the medium, many solvent
polarity scales have been created, and several are based on the
use of solvatochromic probes.[1,2,5,6] These compounds have
bands in the visible region, and their position and/or intensity
change when the polarity of the medium is altered.[1,5] One of
the most popular solvent polarity scales is ET(30), which is based
on the use of Reichardt’s pyridinium N-phenolate betaine, 2,6-
diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (1), and its
derivatives.[1,5,7–9] The negative solvatochromism exhibited by
these probes is based on its solvent-dependent vis band, with
a charge-transfer character (from the donor phenolate group to
the pyridinium acceptor moiety in the molecule).
a
reversal in the solvatochromism, observed for various
solvatochromic cyanines and merocyanines,[15,24–29] has been
studied over the past four decades[30] and interpreted in terms
of: dye aggregation in low polarity solvents[12,27,31]; solvent-
dependent cis-trans (E/Z) isomerization processes[32,33]; and,
most commonly, on the basis of the different abilities of polar
* Correspondence to: Vanderlei G. Machado, Departamento de Química,
Universidade Federal de Santa Catarina, UFSC, Florianópolis, SC 88040-900,
Brazil.
E-mail: vanderlei.machado@ufsc.br
a C. E. A. Melo, L. G. Nandi, V. G. Machado
Departamento de Química, Universidade Federal de Santa Catarina, UFSC,
Florianópolis, SC, Brazil
Many research groups have studied other probes inspired by
the molecular structure of compound 1. In general, the
solvatochromic dyes exhibiting a pronounced negative solva-
tochromism have a pyridinium group as the electron acceptor
b M. Domínguez, M. C. Rezende
Facultad de Química y Biología, Universidad de Santiago, Av. B. O’Higgins,
Santiago, Chile
J. Phys. Org. Chem. 2014
Copyright © 2014 John Wiley & Sons, Ltd.

C. E. A. DE MELO ET AL.
and non-polar solvents to stabilize the ground and excited states
of the dye.
Synthesis
N,N-Dimethyl-4-[(E)-2-(4-nitrophenyl)vinyl]aniline (2)
An aspect to be considered in the design of new dye systems
involves the use of aminophenyl groups instead of phenolate
groups as the donor moiety in the molecular structure of
solvatochromic compounds. Although the literature reports some
studies on solvatochromic dyes with amino donor groups in their
molecular structures, they often have pyridinium centers as accep-
tor groups.[34–39] Whitten et al.[40,41] reported the synthesis of
donor–acceptor-substituted azobenzenes with one nitro
electron-acceptor group and one di-alkylamino electron-donor
group in their molecular structures. These compounds exhibited
a strong solvatochromic behavior. The solvatochromism of some
nitrostyryl dyes has also been studied.[42–45] Effenberger and
Würthner have synthesized the dye 5-dimethylamino-5′-nitro-
4-Nitrotoluene (1.00 g; 7.20 mmol), 4-dimethylaminobenzaldehyde (0.99 g;
6.60 mmol), and pyrrolidine (1.20 mL; 14.5 mmol) were mixed in a round-
bottomed flask, and the mixture was refluxed for 24 h. After this period a
red solid was obtained. Diethyl ether (10 mL) was added to the flask, and
the solid was filtered and washed with water, yielding 0.48 g (27%) of the
product; m.p. obtained: 251.8 °C (m.p. lit.[49]: 255 °C). IR (KBr, ν max/cmꢀ1):
1442 (C―N); 1504 (N¼O); 1365, 1223 (N¼O); 1365 (C―H); 1524 (C¼C). 1H
NMR (400 MHz, DMSO-d6) δ/ppm: 8.17 (d, 2H, J = 8.6 Hz), 7.75 (d, 2H,
J = 8.4 Hz), 7.50 (d, 2H, J = 8.6 Hz), 7.41 (d, 1H, J = 16.4 Hz), 7.11 (d, 1H,
J = 16.4 Hz), 6.73 (d, 2H, J = 8.4 Hz), 2.95 (s, 6H). HRMS (ESI, TOF): m/z calcd.
for C16H17N2O2 [M + H]+ 269.1285, found 269.1283.
4-[(E)-2-(2,4-Dinitrophenyl)vinyl]-N,N-dimethylaniline (3)
2,2′-bithiophene
and
demonstrated
its
positive
2,4-Dinitrotoluene (0.55 g; 3.00 mmol), 4-dimethylaminobenzaldehyde
(0.67 g; 4.49 mmol), and pyrrolidine (0.25 g; 3.50 mmol) were stirred in a
round-bottomed flask, and the mixture was refluxed in an oil bath (90 °C)
for 30 min. The solid product formed was filtered, washed with iced water,
and recrystallized from ethanol, yielding 0.14 g (14.9%) of a dark red prod-
uct; m.p. obtained: 176 °C (m.p. lit.[50]: 181 °C). IR (KBr, ν max/cmꢀ1): 1444
(C―N); 1367, 1270 (N¼O); 1331 (C―H); 1523 (C¼C). 1H NMR (400 MHz,
DMSO-d6) δ/ppm: 8.67 (s, 1H, J = 2.3 Hz); 8.38 (dd, 1H, J = 9.0 and 2.3 Hz),
8.21 (d, 1H, J = 9.0 Hz), 7.56 (d, 1H, J = 16.0 Hz), 7.49 (d, 2H, J = 8.6 Hz), 7.21
(d, 1H, J = 16.0 Hz), 6.73 (d, 2H, J = 8.6 Hz), 2.98 (s, 6H). HRMS (ESI, TOF):
m/z calcd. for C16H16N3O4 [M + H]+ 314.1135, found 314.1132.
solvatochromism.[46] Beckert et al. have recently reported the neg-
ative solvatochromism of an anionic thiazole-based dye.[47]
Herein, we report the synthesis of the dyes 2–7 and their
application as solvatochromic probes for the investigation of
24 solvents. The influence of an additional nitro group on the
solvatochromic properties of the dyes was investigated. In addi-
tion, the importance of three conjugated bridges, C¼C, C¼N,
and N¼N, on the type and level of the solvatochromism of the
dyes was studied. Multiparametric analysis of the experimental
data was also carried out in order to verify the contribution of each
parameter to the solvatochromism exhibited by the dyes.
N,N-Dimethyl-4-[(1E)-(4-nitrobenzylidene)amine]aniline (4)[51]
4-Amino-N,N-dimethylaniline (0.21 g; 1.46 mmol) and 4-nitrobenzaldehyde
(0.23 g; 1.46 mmol) were dissolved in methanol (3 mL) in a closed round-
bottomed flask. One drop of glacial acetic acid was added, and the reaction
mixture was stirred for 3 h. The dark solid formed was filtered and washed
with cold methanol, yielding 0.14 g (35.8%); m.p. obtained: 224 °C (m.p.
lit.[51]: 228 °C). IR (KBr, ν max/cmꢀ1): 1595 (C¼N); 1444 (C–N); 1514, 1367
(N¼O); 1341 (C―H); 1514 (C¼C). 1H NMR (400 MHz, CDCl3) δ/ppm: 8.58
(s, 1H), 8.27 (d, 2H, J = 9.2 Hz); 8.00 (d, 2H, J = 9.2 Hz); 7.33 (d, 2H,
2
4
3
5
7
EXPERIMENTAL
Materials and methods
All solvents were HPLC grade, purified following the methodology
described in the literature and stored on molecular sieves (Sigma-Al-
drich). 4-Amino-N,N-dimethylaniline was synthesized in two steps,[48]
first by nitrosation of N,N-dimethylaniline in the presence of sodium
nitrite and concentrated HCl. The product obtained was reduced
using granulated tin, concentrated HCl, and ethanol, under reflux
conditions.
UV–vis measurements were performed with an HP 8452A spectropho-
tometer equipped with thermostated cell compartments at 0.1 °C, using
1-cm square quartz cuvettes. The NMR spectra were recorded on a Varian
AS-400 spectrometer. Chemical shifts were recorded in ppm with the
solvent resonance as the internal standard. Data are reported as follows:
chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, and
dd = double doublet), integration, and coupling constants (Hz). Infrared
(IR) spectra were obtained on a Shimadzu model Prestige-21 spectropho-
tometer, with KBr pellets. High-resolution mass spectra were obtained
with an electrospray ionization-quadrupole time-of-flight mass spectrom-
eter (HR ESI-MS QTOF).
6
(a) (b) (c)
(d)
(a) (b)
(c) (d)
Figure 1. Dyes 2–7 in: a) n-hexane, b) diethyl ether, c) propan-2-ol, and
d) DMSO
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J. Phys. Org. Chem. 2014

SOLVATOCHROMIC BEHAVIOR OF N,N-DIMETHYLANILINE DYES
1,5
solution was poured into another solution of
N,N-dimethylaniline (1.25 mL; 10 mmol) in
methanol (50 mL). The final orange solution
was alkalinized using KOH pellets, and the pre-
cipitate obtained was filtered and washed with
methanol, yielding 0.81 g (30.2%) of a dark red
(3)
(2)
0,6
0,4
0,2
0,0
1,0
0,5
0,0
solid; m.p. obtained: 220 °C (m.p. lit.[53]
:
225–228 °C). IR (KBr, ν
̅
max/cmꢀ1): 1423 (N¼N);
1602 (N¼O); 1511 (N―H); 1333 (C―H); 1587
(C¼C); 1136 (C―N). 1H NMR (400 MHz,
DMSO-d6) δ/ppm: 8.35 (d, 2H, J = 8.6 Hz); 7.93
(d, 2H, J = 8.6 Hz); 7.85 (d, 2H, J = 8.6 Hz); 6.86
(d, 2H, J = 8.6 Hz); 3.10 (s, 6H). HRMS (ESI, TOF):
m/z calcd. for C14H15N4O2 [M + H]+ 271.1190,
found 271.1194.
400
500
600
350
400
450
500
wavelength [nm]
wavelength [nm]
(5)
(4)
1,0
0,5
0,0
0,2
0,1
0,0
4-[(E)-2,4-Dinitrophenyl)diazenyl)-N,N-
dimethylaniline (7)[53]
2,4-Dinitroaniline (1.83 g; 10 mmol) was
dissolved in a mixture of glacial acetic acid
(50 mL) and concentrated sulfuric acid
(20 mL). The reaction mixture was placed in
an ice bath at 0–5 °C. A solution of sodium
nitrite (0.70 g; 10.1 mmol) in water (2.5 mL)
was added dropwise under magnetic stirring,
and after 30 min the resulting solution of the
diazonium salt was mixed with another
solution of N,N-dimethylaniline (1.25 mL;
9.8 mmol) in methanol (50 mL). The resulting
red solution was diluted with 200 mL of
water, yielding a dark precipitate, which was
filtered and washed with methanol. The yield
was 1.30 g (41.2%); m.p. obtained: 206.7 °C
400
500
600
350
400
450
500
550
wavelength [nm]
wavelength [nm]
1,5
1,0
0,5
0,0
0,6
0,4
0,2
0,0
(7)
(6)
(m.p. lit.[53]: 211 °C). IR (KBr,
ν
max/cmꢀ1):
1423 (N¼N); 1594 (N¼O); 1364 (C–N); 1332
(C―H); 1521 (C¼C). 1H NMR (400 MHz,
DMSO-d6) δ/ppm: 8.83 (d, 1H, J = 1.9 Hz),
8.48 (dd, 1H, J = 9.0 Hz, J = 1.9 Hz), 7.88 (d,
1H, J = 9.0 Hz), 7.76 (d, 2H, J = 9.0 Hz), 6.87 (d,
2H, J = 9.0 Hz), 3.31 (s, 6H). HRMS (ESI, TOF):
m/z calcd. for C14H14N4O4 [M + H]+ 316.1040,
found 316.1045.
400
500
600
400
450
500
550
wavelength [nm]
wavelength [nm]
Figure 2. UV–vis spectra for dyes 2–7 in n-hexane ( ), diethyl ether (―), propan-2-ol ( ), and
DMSO (
)
J = 9.2 Hz), 6.74 (d, 2H, J = 9.2 Hz), 3.01 (s, 6H). HRMS (ESI, TOF): m/z calcd. for
C15H16N3O2 [M + H]+ 270.1237, found 270.1245.
UV–vis measurements
The following procedure was typical for all measurements performed. A
1.0 × 10ꢀ2 mol Lꢀ1 stock solution of compounds 2–7 was prepared in an-
hydrous trichloromethane. From this stock solution 5 μL was transferred
to 5-mL volumetric flasks. After evaporation of the trichloromethane the
compounds were dissolved in the pure solvent, yielding a solution with a
final dye concentration of 1.0 × 10ꢀ5 mol Lꢀ1. The UV–vis spectra were
recorded at 25 °C using a 1-cm square cuvette. The maxima of the
UV–vis spectra (λmax) were calculated from the first derivative of the
absorption spectrum. The λmax values thus obtained were transformed
4-[(E)-(2,4-Dinitrobenzylidene)amine]-N,N-dimethylaniline (5)
2,4-Dinitrobenzaldehyde (0.15 g; 0.76 mmol) and 4-amino-N,N-dimethy-
laniline (0.11 g; 0.76 mmol) were mixed with absolute ethanol (5 mL),
propan-2-ol (5 mL), and one drop of glacial acetic acid. The reactants
were dissolved under magnetic stirring at room temperature, and the
reaction mixture was stirred for 4 h. The resulting dark solid was filtered
and recrystallized from propan-2-ol, yielding 0.05 g (21.0%) of a dark red
product; m.p. obtained: 206 °C (m.p. lit.[52]: 211 °C). IR (KBr, ν max/cmꢀ1):
1604 (C¼N); 1517 (C¼C); 1345 (N¼O); 1160 (C―N). 1H NMR (400 MHz,
CDCl3) δ/ppm: 9.04 (s, 1H); 8.84 (d, 1H, J = 2.3 Hz); 8.63 (d, 1H, J = 8.6 Hz);
8.44 (dd, 1H, J = 8.6 and 2.3 Hz); 7.38 (d, 2H, J = 9.0 Hz); 6.73 (d, 2H,
J = 9.0 Hz); 3.04 (s, 6H). HRMS (ESI, TOF): m/z calcd. for C15H15N4O4
[M + H]+ 315.1088, found 315.1087.
[1,5]
into ET(dye) values, according to the expression ET(dye) = 28591/λmax.
Computational details
All theoretical calculations were made at the DFT level of theory using the
hybrid functional B3LYP and the basis set 6–31 + G(d,p) with the aid of
the GAUSSIAN09 package.[54] The geometries were optimized in the
gas-phase and re-optimized in the presence of the polarized/continuum
solvent model (SCRF-PCM)[55] in n-hexane (ε = 1.88), tetrahydrofuran
(ε = 7.43), dichloromethane (ε = 8.93), acetone (ε = 20.49), dimethylsulfoxide
(ε = 46.83), ethanol (ε = 24.85), and methanol (ε = 32.61). All optimized ge-
ometries correspond to a minimum on the potential energy surface, since
no imaginary values were obtained in the Hessian matrix. The spectra were
N,N-Dimethyl-4-[(E)-(4-nitrophenyl)diazenyl]aniline (6)[53]
4-Nitroaniline (1.38 g; 10 mmol) was placed in a mixture of acetic acid
(50 mL) and concentrated sulfuric acid (20 mL). The reaction mixture
was placed in an ice bath at 0–5 °C. Subsequently, a solution of sodium ni-
trite (0.70 g; 10 mmol) in water (2.5 mL) was added dropwise under mag-
netic stirring. After a period of 30 min the resulting diazonium salt
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C. E. A. DE MELO ET AL.
calculated at the same level of theory and the solvent
model using the time depending (TD) method.
Calculations
All constants were calculated through the fitting of
least-squares regression curves using the program
ORIGIN® 6.1.
RESULTS AND DISCUSSION
Figure 1 shows the colors of the solutions of
compounds 2–7 in solvents of different
polarity, and it can be observed that all
compounds are solvatochromic. For instance,
solutions of compound 7 are yellow in
n-hexane, orange in diethyl ether, red in
propan-2-ol, and purple in DMSO.
The solvatochromic behavior could be
quantified through the variations observed
in the λmax values obtained from the
solvatochromic band of each dye in solution.
Figure 2 shows the UV–vis spectra for the
probes 2–7 in selected solvents. By way of
example, it can be observed for compound
3 that in diethyl ether its solvatochromic
band occurs at λmax = 475 nm. In n-hexane,
the solvatochromic band shifts to 466 nm,
while in DMSO the shift is to 496 nm. The
solvatochromic band observed in each spec-
trum for the dyes is due to a π ➔ π* transition,
of an intramolecular charge transfer nature,
which occurs from the electron-donor N,N-
dimethylaminophenyl group to the electron-
acceptor group, 4-nitrophenyl (compounds
2, 4, and 6) or 2,4-dinitrophenyl (compounds
3, 5, and 7), in the molecules, which is
reinforced by the structures of 2–7 optimized
by applying density functional theory, DFT,
which exhibit high planarity (Fig. 3). Ac-
cording to Fig. 4, the HOMO density in
compound 2 is mainly located in the N,N-
dimethylaminophenyl donor group, while the
Figure 3. Optimized molecular geometry of compounds 2–7 obtained in gas phase at B3LYP/
6-31 + G(d,p) level of theory
LUMO is mainly located in the nitro acceptor groups, indicating
the donor and acceptor moieties. A similar scenario was verified
for compounds 3–7 (see Figs. S22–S26).
The spectrophotometric studies were performed for the six
dyes in 24 solvents of different polarity, the λmax values being
obtained from the UV–vis spectra. The measurements could
not be taken in water since the compounds are not suffi-
ciently soluble in this solvent. Table 1 shows the [ET(dye)]
values for each solvent and also the corresponding ET(30)
values. These data were used to make plots of ET(dye) as a
function of ET(30) for each dye (Fig. 5). The presence of two
regions, one in less polar solvents and the other in more
polar solvents, is evident in all plots. More specifically,
starting with less polar solvents, the ET values for the dyes
decreased until solvents of intermediate polarities, after
which the ET values increased with an increase in the polarity
of the medium, with a reversal in the solvatochromism
occurring.
HOMO
LUMO
Figure 4. HOMO and LUMO of compound 2 in gas phase obtained at
B3LYP/6-31 + G(d,p) TD level of theory
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SOLVATOCHROMIC BEHAVIOR OF N,N-DIMETHYLANILINE DYES
Table 1. ET(30) and ET(dye) values for compounds 2–7 in 24 solvents
Solvent
ET(30)a
ET(2)b
ET(3)b
ET(4)b
ET(5)b
ET(6)b
ET(7)b
Methanol
Ethanol
Diethyl ether
1-Butanol
Propan-2-ol
1-Octanol
1-Decanol
2-Butanol
Acetonitrile
DMSO
2-Methylpropan-2-ol
DMF
DMA
55.4
51.9
34.5
49.7
48.4
48.1
47.7
47.1
45.6
45.1
43.3
43.2
42.9
42.2
41.3
40.7
40.6
39.1
38.1
49.1
37.4
33.9
31.0
30.9
66.5
66.2
67.2
65.9
66.0
65.7
65.9
66.0
65.6
62.8
66.6
63.9
63.8
65.6
64.5
64.5
63.0
65.1
66.5
65.9
65.7
66.0
70.1
69.0
60.0
59.5
60.2
58.9
59.5
58.9
58.6
59.5
60.0
57.6
59.5
58.1
58.1
59.3
58.0
58.1
57.4
58.3
59.7
59.2
59.5
58.8
61.3
61.5
63.2
63.5
64.1
63.2
63.3
62.2
63.0
63.3
63.3
61.4
63.0
61.5
62.2
63.8
62.2
62.0
60.1
62.6
63.7
63.0
62.3
63.5
65.6
65.0
58.1
57.6
58.1
56.5
56.8
55.9
56.0
56.5
57.7
56.6
57.0
56.7
56.9
58.1
55.6
55.4
54.5
55.2
57.9
56.7
57.4
56.4
58.4
57.7
59.9
60.2
62.4
59.0
60.3
60.0
60.2
60.3
59.3
56.5
60.9
57.6
57.6
59.0
58.3
59.2
57.0
58.8
59.9
59.8
60.2
61.1
64.2
63.0
55.5
55.4
57.3
55.7
55.6
55.2
55.7
55.6
54.6
53.0
56.0
53.6
53.5
55.7
54.0
54.3
52.8
54.6
56.1
55.4
55.6
56.3
59.9
58.8
Acetone
1,2-Dichloroethane
Dichloromethane
Acetophenone
Trichloromethane
Ethyl acetate
1-Pentanol
THF
Toluene
n-Hexane
Cyclohexane
aValues obtained from Reichardt, 1994.
bIn kcal molꢀ1
.
In the case of compound 7, the λmax for the solvatochromic band
in n-hexane occurs at 477 nm [ET(7)=59.9kcalmolꢀ1], while in
acetophenone the λmax occurs at 541 nm [ET(7) = 52.8 kcal molꢀ1].
Therefore, an increase in the polarity of the solvent shifts the
solvatochromic band of the dye to larger λmax values (bathochromic
shift), with Δλmax = +64 nm. In the region of ET(7) value corres-
ponding to acetophenone, at 52.8 kcal molꢀ1, a reversal in the
solvatochromism occurs. From this region onward, a gradual in-
crease in the polarity of the medium causes a hypsochromic shift
in the solvatochromic band, which characterizes negative
solvatochromism. For the solvatochromic band in methanol
acetophenone [ET(3) = 57.4 kcal molꢀ1], after which a reversal in
the solvatochromism occurs, with negative solvatochromism
being observed until methanol [ET(3) = 60.0 kcal molꢀ1].
Table 2 lists the λmax values for the dyes in the most polar,
least polar, and intermediate polarity solvents, as well as the
Δλmax values that characterize the bathochromic and
hypsochromic shifts for each system. The data show that the
azo (N¼N) conjugated bridge, in general, leads to the dye
exhibiting greater variations in the wavelengths and conse-
quently a more accentuated solvatochromism. Except for the
azocompounds, the dyes with only one nitro group exhibit a
more extensive solvatochromism, suggesting that a greater
planarity of the systems is an important factor regarding the
contribution of the mesomeric effect in the studied systems.
Another aspect to be observed involves the suggestion that
the charge transfer is strongest in the systems with only one
nitro group.
With the application of the B3LYP/6-31 + G(d,p) SCRF = PCM
TD level of theory to the previously optimized structures of dyes
2–7, on comparing HOMO and LUMO molecular orbitals, it was
verified that the transition is strongly dependent on the
surrounding solvent. Table 3 shows the calculated ET values in
kcal molꢀ1 for the dyes in the gas phase and in seven different
solvents. Although PCM calculations do not provide good results
for solvents with medium to high polarity[56] and TD-DFT calcula-
tions tend to overestimate the internal charge-transfer
transition-energy of solvatochromic dyes,[57] the results are
good. The data show a reversal in the solvatochromism for all
dyes, as shown in Fig. 6 for compound 7, this being a represen-
tative example of the results obtained for compounds 2–6.
λ
λ
max = 515 nm (ET(dye) = 55.5 kcal molꢀ1), which in comparison with
max = 541 nm in acetophenone provides Δλmax = ꢀ26 nm, charac-
terizing the hypsochromic shift. The plots of ET(30) vs ET(dye) for
compounds 4 and 5 show that the positive solvatochromism
begins with the dyes in n-hexane [4: ET(4)=65.6kcalmolꢀ1; 5:
ET(5) = 58.4 kcal molꢀ1], and continues until polarities ap-
proaching that of acetophenone [4: ET(4) = 60.1 kcal molꢀ1; 5:
ET(5)= 58.4 kcal molꢀ1], and from this region the negative
solvatochromism occurs with an increase in the solvent polarity,
values obtained in methanol being ET(4) = 63.2 kcal molꢀ1 and
ET(5) = 58.1 kcal molꢀ1. For compounds 2 and 6, the positive
solvatochromism starts with the dye in n-hexane [2: ET(2)=
70.1 kcal molꢀ1; 6: ET(6) = 64.2 kcal molꢀ1], and the region char-
acterizing the reversal in the solvatochromism occurs for solvents
with polarities close to that of DMSO [6: ET(6) = 56.5kcal molꢀ1; 1:
ET(2) = 62.8 kcal molꢀ1]. The most polar solvent studied using
the dyes was methanol [6: ET(6)=57.0kcal molꢀ1; 2: ET(2)=
66.5 kcal molꢀ1]. For compound 3, the positive solvatochromism
starts in cyclohexane [ET(3) = 61.5 kcal molꢀ1] and continues until
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C. E. A. DE MELO ET AL.
62
61
60
59
58
57
solvatochromism, while the dipolar form is
better stabilized in more polar solvents, in
the region of negative solvatochromism.
Thus, the reversal in the solvatochromism,
from positive to negative, is probably a re-
sult of the fact that with the increase in the
polarity of the medium the weight of the
contribution of the dipolar form to the res-
onance hybrid of the dye is gradually
increased.
(3)
70
68
66
64
62
(2)
30
35
40
45
50
55
30
35
40
45
50
55
ET(30) [kcal mol-1]
ET(30) [kcal mol-1]
Therefore, the solvatochromism of the
dyes studied herein can be summarized
by the diagram for compound 2 in Fig. 7,
which can be applied to the other dyes
investigated. According to this proposal,
the resonance forms with a greater contri-
bution are strongly dependent on the
solvent. In non-polar solvents, such as n-
hexane, the resonance form with the
greatest contribution is the benzenoid
form. With a gradual increase in the polar-
ity of the medium the solvent is less able
to solvate the ground state of the dye, but
in compensation it effectively stabilizes
the excited state, due to the fact that the
latter is more polar. This explains the posi-
tive solvatochromism observed for dye 2
in solvents with polarity between n-hexane
and DMSO. Above a certain polarity of the
medium, the dipolar resonance form
provides the largest contribution for the
resonance hybrid. In this case, since the
excited state is now less polar, negative
solvatochromism occurs with an increase
in the polarity of the medium.
66
65
64
63
62
61
60
59
58
57
56
55
54
(5)
(4)
30
35
40
45
50
55
60
30
35
40
45
50
55
ET(30) [kcal mol-1]
ET(30) [kcal mol-1]
66
64
62
60
58
56
(7)
60
58
56
54
52
(6)
30
35
40
45
50
55
30
35
40
45
50
55
60
ET(30) [kcal mol-1]
ET(30) [kcal mol-1]
Studies on the influence of the dye con-
centration on the absorbance values were
carried out to verify that there is no
aggregation of the compounds in the con-
Figure 5. ET(30) values as a function of ET(dye) for dyes 2–7
All compounds studied have the features expected according
to their molecular structure. The reverse solvatochromism of the
compounds studied can be explained on the basis of Scheme 1,
considering two resonance structures, the benzenoid and dipo-
lar forms. The benzenoid form is better stabilized in less polar
solvents and characterizes the region displaying positive
centration range in which the absorbance values were collected.
The studies were performed with the dyes in two solvents of
very different polarity, methanol, and ethyl acetate. For all com-
pounds studied the plots of the absorbance values (λmax) as a
function of the dye concentration were linear, indicating the ab-
sence of solute–solute aggregation under the experimental
Table 2. Values of λmax for compounds 2–7 in the most polar, least polar, and intermediate polarity solvents, as well as the Δλmax
values obtained considering the regions of positive and negative solvatochromism
b
Dye
Most polar solvent (methanol)
Intermediate solvent
Least polar solvent
Δλmaxa (nm)
Δλmax (nm)
λmax (nm)
λmax (nm)
λmax (nm)
2
3
4
5
6
7
430
476
452
492
477
515
455 (DMSO)
408 (n-hexane)
465 (cyclohexane)
435 (n-hexane)
489 (n-hexane)
445 (n-hexane)
477 (n-hexane)
+47
+33
+40
+35
+61
+64
ꢀ25
ꢀ22
ꢀ23
ꢀ32
ꢀ29
ꢀ26
498 (acetophenone)
475 (acetophenone)
524 (acetophenone)
506 (DMSO)
541 (acetophenone)
aΔλmax = λmax (intermediate solvent) ꢀ λmax (least polar solvent)
bΔλmax = λmax (methanol) ꢀ λmax (intermediate solvent)
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SOLVATOCHROMIC BEHAVIOR OF N,N-DIMETHYLANILINE DYES
Table 3. ET values (in kcal molꢀ1) for dyes 2–7 calculated ap-
plying B3LYP/6-31 + G(d,p) SCRF = PCM TD level of theory to
previously optimized structures
Medium
2
3
4
5
6
7
Gas phase
n-Hexane
Tetrahydrofuran
Dichloromethane
Acetone
DMSO
Ethanol
Methanol
59.6 53.7 55.0 51.0 56.3 55.3
53.5 49.2 50.3 46.8 55.5 53.9
49.3 46.2 46.8 44.1 52.4 50.9
49.0 45.9 46.5 43.9 52.1 50.6
48.5 45.4 46.1 43.7 51.7 50.4
47.8 45.0 45.6 43.2 51.1 49.8
48.3 45.3 45.9 43.6 51.6 50.2
48.4 45.3 46.0 43.7 51.7 50.3
Scheme 1. Benzenoid and dipolar forms of the compounds
structure is very important, suggesting less interaction through
resonance structure between electron-donor and electron-
acceptor groups due to steric hindrance caused by nitro groups
at ortho position.
Interpretation of the solvatochromism using the multipara-
metric approaches of Kamlet–Abboud–Taft and Catalán
54
53
52
51
50
Two different approaches to the analyses of the experimental
ET(dye) values in pure solvents for compounds 2–7 were
carried out with the use of Kamlet–Abboud–Taft[59–64] and
Catalán[63–68] multiparameter approaches. These strategies con-
sider a linear correlation between the measured spectroscopic
parameter, ET(dye), and several solvent parameters, as shown
in Equation (1):
ETðdyeÞ ¼ ETðdyeÞ0 þ aA þ bB þ cC þ …
(1)
where ET(dye)0 represents the ET(dye) value relating to an inert
solvent, and a, b, and c are coefficients that reflect the impor-
tance of the solvent parameters A, B, and C, respectively, in
terms of ET(dye). The Kamlet–Abboud–Taft strategy[59,60] re-
quires the use of Equation (2),
49
30
35
40
45
50
55
ET (30) [kcal mol-1]
Figure 6. Variation in the calculated ET values for compound 7 in media
with different ET(30) values
ETðdyeÞ ¼ ETðdyeÞ0 þ aα þ bβ þ sðπ* þ dδÞ
(2)
where α, β, and π* are parameters that represent the solvent
hydrogen-bond donor (HBD) acidity, hydrogen-bond acceptor
(HBA) basicity, and solvent dipolarity/polarizability, respectively,
while δ is a polarizability correction term of the solvent. An-
other multiparametric polarity scale was developed by Catalán
et al.,[68] employing Equation (3),
conditions of the studies performed. Therefore, the reverse
solvatochromism could not be attributed to the aggregation
phenomenon, but may be the result of the differential solvation
of the medium on the resonance structures of the dyes. This dif-
ferential solvation of the two forms, dipolar and benzenoid,
changes the extent of the contribution of different species and
is responsible for the reverse solvatochromism observed
experimentally.
The values for the molar absorptivity of the dyes were ob-
tained in methanol and ethyl acetate (see supplementary data).
Azocompound 7 had the highest molar absorptivity value, due
to the fact that the azo (N¼N) bridge is a strong electron-
acceptor group,[58] increasing the rigidity of the conjugated
system, which leads to high light absorption, as well as a great
variety of colors in solution. Comparing data in methanol,
compound 4 had the lowest molar absorptivity value, since the
C¼N bridge is not as effective as electron-acceptor group com-
pared with the azo bridge. Apart from the conjugate bridge,
the compounds with two nitro groups (compounds 3, 5, and 7)
had the highest absorptivity values, because the presence of
these groups enhances the mesomeric effect, leading to more
resonance contributors. For compounds 3 and 5 in ethyl acetate,
the molar absorptivity values were lower than those for dyes 2
and 4. These data reinforce the previous proposal that in less
polar solvents the contribution of the benzenoid resonance
ETðdyeÞ ¼ ETðdyeÞ0 þ aSA þ bSB þ cSP þ dSdP
(3)
where SA (solvent HBD acidity) and SB (solvent HBA basicity)
are specific and SP (solvent polarizability) and SdP (solvent
dipolarity) are nonspecific solvent parameters.
The contributions of the solvent properties to the ET(dye)
values obtained for dyes 2–7 were ascertained with the use
of Equations (2) and (3). The results of the multiple square
correlation analysis are shown in Tables 4 and 5 for the
Kamlet–Abboud–Taft and Catalán approaches, respectively.
Some general trends can be observed through a comparison
of the correlation data given in the tables. A notable aspect
is related to the analysis of the coefficients associated with
the acidity and basicity of the medium. Considering the mo-
lecular structure of dyes 2–7 they would be expected to be
sensitive to the acidity of the medium, exhibiting very low
sensitivity to the basicity of the medium. This is because
acidic hydrogens, which could interact through hydrogen
bonding with the basic groups of HBA solvents, are not pres-
ent in the molecular structure of these compounds. On the
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C. E. A. DE MELO ET AL.
Figure 7. Reverse solvatochromism exhibited by dye 2
other hand, all compounds have basic dimethylamino
groups, which are potentially able to interact through hydro-
gen bonding with HBD solvents. An inspection of the coeffi-
cients in Table 4 reveals that the contribution of the b
coefficients, related to the sensitivity of the dye to the basic-
ity of the medium, is larger than the contribution of the a co-
efficients, related to the sensitivity of the dye to the medium
acidity. On the other hand, the results obtained using the
Catalán approach offered a more coherent picture, since
the sensitivity of the dyes with respect to the basicity of
the medium was negligible, while the contribution of the coef-
ficient related to the sensitivity of the dyes to the solvent acidity
was more important. Figure 8 shows a comparison between the
measured and calculated transition energy values, using the
multiparametric strategies studied, for compound 2. It can be
verified that the Catalán strategy is more appropriate for
describing the system.
The inclusion of another term in the multiparametric
equations, log P, which is a parameter associated with
the lipophilicity of the medium, yielded Equations (4) and (5)
for the Kamlet–Abboud–Taft and Catalán approaches, respec-
tively.[20,69] The use of this term in the Kamlet–Abboud–Taft
analysis [Equation (4)] generally led to an improvement in
the correlations (see Supporting Information), except for
Table 4. Correlation coefficients a, b, and s obtained from the Kamlet–Abboud–Taft multiparametric analysis a through the treat-
ment of ET(dye) values for compounds 2–7 in various solvents
Dye
Constant
a
b
s
N
F
R
S.D.
2
3
4
5
6
7
69.07
60.99
65.10
57.27
63.51
59.93
0.57
ꢀ0.10
0.32
ꢀ0.87
ꢀ0.15
ꢀ0.68
1.02
ꢀ5.28
ꢀ3.13
ꢀ3.25
ꢀ2.07
ꢀ5.90
ꢀ5.52
24
24
24
24
24
24
<1.4 × 10ꢀ8
<5.3 × 10ꢀ5
<5.6 × 10ꢀ5
<0.12
0.92
0.81
0.81
0.49
0.92
0.93
0.66
0.64
0.71
0.96
0.74
0.65
ꢀ0.80
0.26
ꢀ0.79
<1.9 × 10ꢀ8
<8.5 × 10ꢀ9
ꢀ0.12
ꢀ0.53
aEquation (2) was used (see text).
Table 5. Correlation coefficients a, b, c, and d obtained from the Catalán multiparametric analysis a through the treatment of ET
(dye) values for compounds 2–7 in various solvents
Constant
a
b
c
d
N
F
R
S.D.
2
3
4
5
6
7
79.11
68.89
73.15
66.27
72.99
67.71
0.12
ꢀ0.51
ꢀ0.32
ꢀ1.79
ꢀ0.66
ꢀ0.84
ꢀ0.88
ꢀ0.69
ꢀ0.92
ꢀ0.42
ꢀ0.50
ꢀ0.58
ꢀ15.22
ꢀ12.06
ꢀ12.20
ꢀ13.10
ꢀ14.33
ꢀ13.28
ꢀ3.41
ꢀ1.27
ꢀ1.67
0.35
24
24
24
24
24
24
<4.1 × 10ꢀ15
<3.2 × 10ꢀ9
<2.3 × 10ꢀ8
<4.8 × 10ꢀ5
<5.8 × 10ꢀ12
<1.9 × 10ꢀ13
0.98
0.94
0.93
0.84
0.97
0.98
0.27
0.36
0.44
0.60
0.44
0.34
ꢀ4.27
ꢀ3.86
aEquation (3) was used (see text).
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SOLVATOCHROMIC BEHAVIOR OF N,N-DIMETHYLANILINE DYES
Kamlet - Abboud - Taft
70
fact that for all dyes studied the most important contribu-
tion from the solvent is associated with the polarizability
of the medium. Considering the data in Table 5, the c/a
ratio for the polarizability and acidity coefficients shows
that, for probe 2, c/a = 15.22/0.120 = 127, while a compari-
son between the contribution of polarizability and
dipolarity terms shows c/d = 15.22/3.41 = 4.46. The highest
c/d ratios were obtained for dyes 3 and 5, 9.49 and
37.4, respectively. This observation, which is valid for all
other dyes studied, confirms the importance of the contri-
bution of the polarizability term to the solvatochromism
reported for these probes.
Table 6 shows the influence of each parameter using the
Catalán multiparametric analysis [Equation (3)] on the fitting
of the data for dyes 2 and 3 (see Supporting Information
for the other dyes). The use of the equation with the
suppression of the polarizability term to fit the data led to
a decrease in the R value from 0.98 to 0.78 for dye 2, while
the influence of the acidity and basicity parameters on this
relationship was negligible. The suppression of the dipolarity
term resulted in a decrease in the R value (0.82). However,
the most important effect was observed with the suppression
of the combined dipolarity and polarizability terms in Equa-
tion (3), with a decrease in the R value from 0.98 to 0.19
for dye 2. It was also verified that suppressing both the
acidity and basicity terms in Equation (3) resulted in an R
value of 0.97, which is practically the same as the value
obtained when all solvent terms were used. Similar results
were obtained for dye 3, with a reduction in the R value
from 0.94 to 0.58 and 0.89 with the suppression of
polarizability and dipolarity terms in Equation (3), respectively,
and to 0.13 with the removal of both polarizability and
dipolarity terms. For all other dyes the suppression of
the acidity and the basicity terms did not significantly alter
the R values. For instance, for dyes 6 and 7, the R values
remained almost the same with the suppression of the acidity
and basicity terms (see Supporting Information). On the other
hand, the suppression of the polarizability and dipolarity
terms resulted in a reduction in the R value from 0.97 to 0.82
68
66
R = 0.80
64
62
60
65
70
Catalán
70
68
66
64
62
R = 0.98
62
64
66
68
70
ET(2) calculated [kcal mol-1]
Figure 8. Relationship between calculated and measured ET for dye 2
considering Kamlet–Abboud–Taft and Catalán strategies
compounds 6 and 7, but the inconsistencies observed with
the use of Equation (2) remained. No improvements were ob-
tained with the use of Equation (5), the only exception being
in the case of dye 5 (from R = 0.84 to 0.91). Thus, the results
offered by the Catalán approach suggest that the contribution
of the lipophilic methyl groups to the solvatochromism of the
compounds can be neglected.
ETðdyeÞ ¼ ETðdyeÞ0 þ aα þ bβ þ sðπ* þ dδÞ þ pðlogPÞ (4)
ETðdyeÞ ¼ ETðdyeÞ0 þ aSA þ bSB þ cSP þ dSdP þ pðlogPÞ
(5)
Another important aspect to be analyzed through
comparison of the data shown in Table 5 concerns the
a
Table 6. Influence of each parameter in Catalán multiparametric analysisa on the fitting of data for dyes 2 and 3
Constant
a
b
c
d
N
F
R
S.D.
2
79.11
79.15
78.20
78.06
68.49
80.47
77.66
66.19
68.89
68.71
68.18
67.72
60.48
69.40
67.55
59.37
0.12
–
ꢀ0.88
ꢀ0.86
–
ꢀ15.22
ꢀ15.28
ꢀ14.24
ꢀ1.06
–
ꢀ19.40
ꢀ16.98
–
ꢀ12.06
ꢀ11.82
ꢀ11.29
ꢀ10.72
–
ꢀ13.62
ꢀ11.98
–
ꢀ3.41
ꢀ3.40
ꢀ3.70
ꢀ3.73
ꢀ4.58
–
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
<4.1 × 10ꢀ15
<2.2 × 10ꢀ16
<1.0 × 10ꢀ13
<7.1 × 10ꢀ15
<2.2 × 10ꢀ4
<2.8 × 10ꢀ5
<3.5 × 10ꢀ5
<0.68
0.98
0.98
0.97
0.97
0.78
0.82
0.74
0.19
0.94
0.94
0.93
0.92
0.58
0.89
0.81
0.13
0.27
0.27
0.36
0.36
1.08
0.97
1.11
1.67
0.36
0.36
0.40
0.42
0.90
0.50
0.61
1.08
ꢀ0.26
–
–
1.20
ꢀ0.38
–
0.89
ꢀ0.51
–
0.13
ꢀ1.99
–
ꢀ1.08
ꢀ0.69
ꢀ0.78
–
–
–
3
ꢀ1.27
ꢀ1.30
ꢀ1.51
ꢀ1.61
ꢀ2.20
–
<3.2 × 10ꢀ9
<7.1 × 10ꢀ10
<5.3 × 10ꢀ9
<2.1 × 10ꢀ9
<3.4 × 10ꢀ2
<3.4 × 10ꢀ7
<1.1 × 10ꢀ6
<0.83
ꢀ0.82
–
–
0.34
ꢀ0.70
–
0.10
ꢀ1.11
–
ꢀ0.47
–
–
0.18
aEquation (3) was used (see text).
J. Phys. Org. Chem. 2014
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C. E. A. DE MELO ET AL.
62
61
60
59
58
57
62
61
60
59
58
57
(A)
CONCLUSIONS
(A)
70
68
66
64
62
Compounds 2–7 are solvatochromic and
can be used as probes in the investigation
of the physico-chemical properties of the
solvents. The conjugated bridge influ-
ences the solvatochromic properties of
the dyes, compounds 6 and 7, with the
azo bridge, having a more pronounced
solvatochromism. The data suggest that the
reverse solvatochromism observed for all
compounds studied is due to the differential
solvation of the medium on the resonance
structures of the compounds. This differential
solvation of the two forms, dipolar and ben-
zenoid, changes the extent of the contribu-
tion of each species and is responsible for
the type of solvatochromism observed.
R = 0.94
R = 0.98
62
64
66
68
70
57
58
59
60
61
62
(B)
70 (B)
68
66
64
R = 0.58
R = 0.78
62
64
The multiparametric approaches ap-
65
66
67
68
69
58
61
59
60
plied to the data obtained for the six com-
pounds revealed that the Catalán equation
provided a better fit with the experimental
data and that the use of log P improved
only slightly the relationships. In addition,
this study showed that for these dyes the
ability to probe the basicity of the medium
is negligible, which was expected based
on the analysis of their molecular structure.
However, the multiparametric analysis re-
vealed that the contribution of the acidity
parameter to the overall solvatochromism
of the dyes studied is small, which is
somewhat surprising considering that all
compounds have a dimethylamino group
in their molecular structure, which is, in
principle, able to interact, as a HBA group,
with the medium. Therefore, the data
suggest that the electronic density of the
amino group is delocalized due to the
presence of the electron withdrawing nitro
groups in the acceptor moiety of the
molecules. Finally, the multiparametric
analysis of the data showed that the main
factor contributing to the solvatochromism
exhibited by compounds 2–7 is the
polarizability parameter and, to a lesser extent, the dipolarity pa-
rameter, which makes these compounds good candidates for the
investigation of these types of physical properties of the medium,
the interference of specific interactions of the solvent through hy-
drogen bonding being negligible.
62
60
58
(C)
(C)
70
68
66
64
R = 0.89
R = 0.82
62
63
64
65
66
67
68
69
57
58
59
60
61
62
60
58
(D)
(D)
70
68
66
64
62
R = 0.13
R = 0.18
65,4
65,7
66,0
66,3
66,6
59,0
59,2
59,4
ET(2) calculated [kcal mol-1]
ET(3) calculated [kcal mol-1]
Figure 9. Relationship between calculated and measured ET for dyes 3 and 7 considering Catalán
strategy [Equation (3)] using (A) all parameters, (B) without SP parameter, (C) without SdP parame-
ter, and (D) without both SP and SdP parameters
and 0.75, respectively, for dye 6 and from 0.98 to 0.97 and 0.77,
respectively, for dye 7. The simultaneous removal of the two
terms lowered the correlation coefficients, to R = 0.16 for dye 6
and to R = 0.19 for dye 7.
Figure 9 shows plots of the experimentally obtained ET
(dye) values as a function of the ET(dye) value calculated for
dyes 2 and 3, using the data from Table 5. The plots verify
the contribution of the polarizability and, to a lesser extent,
the dipolarity to the solvatochromism of the dyes studied.
In addition, it is clear from the plots that the suppression of
both the polarizability and dipolarity contributions weakened
considerably the correlation between the experimental and
calculated data. The results suggest that dyes 2–7 are good
candidates for use as probes to measure, with high efficiency,
the dipolarity/polarizability of the solvents, with negligible
ability to report the influence of the acidity and basicity of
the medium.
Acknowledgements
The financial support of the Brazilian Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq), Coordenação
de Aperfeiçoamento de Pessoal de Nível Superior (CAPES),
Laboratório Central de Biologia Molecular (CEBIME/UFSC), and
UFSC is gratefully acknowledged. Also, M.C.R. acknowledges the
FONDECYT project 1140212 and M.D. a grant from the CONICYT
program PBCT/PDA-23. We thank also Renata Heying for labora-
tory facilities.
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Copyright © 2014 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014

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