Synthesis and characterization of a group of new medium responsive non-symmetric viologens. Chromotropism and structural effects

Article abstract of DOI:10.1016/j.dyepig.2012.06.013

A group of new non-symmetric viologens was synthesized and characterized spectrally using various techniques. These viologens contain active methylene moieties being capable to act as hydrogen bond donors (HBD) and interact in solution with various solven

Full text of DOI:10.1016/j.dyepig.2012.06.013

Dyes and Pigments 95 (2012) 478e484
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of a group of new medium responsive non-
symmetric viologens. Chromotropism and structural effects
,
b
,
b
b
Raffaello Papadakis a , Ioanna Deligkiozi , Athanase Tsolomitis
*
a Aix-Marseille Université, iSm2/BiosCiences, UMR CNRS 7313, 13397, Marseille, France
b School of Chemical Engineering, National Technical University of Athens (NTUA), Laboratory of Organic Chemistry, 15780 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of new non-symmetric viologens was synthesized and characterized spectrally using various
techniques. These viologens contain active methylene moieties being capable to act as hydrogen bond
donors (HBD) and interact in solution with various solvents, the latter acting as hydrogen bond acceptors
(HBA). These specific solute-solvent interactions result in intense coloration of the resulting solutions.
The chromotropic behavior of these compounds is triggered by basic solvents, and no significant change
in color is observed in poorer HBA-solvents, such as several alcohols. The dependence of the color of the
solutions on the polarity parameters of solvents (solvatochromism) was investigated. Solvent polarity
parameters expressing both dipolarity/polarizability and basicity of solvents have been used. Finally the
correlation of the chromotropic behavior of these compounds with their structure is examined, using
both experimental data and Hammett equation.
Received 24 April 2012
Received in revised form
14 June 2012
Accepted 21 June 2012
Available online 28 June 2012
Keywords:
Viologens
Betaines
Solvatochromism
Substituent effects
Chromotropism
Dipole moments
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
of transition metals like RuII and FeII have been recently reported. In
these cases 4,40-bipyridine exhibits important dual functionality:
Viologens are charged aromatic heterocyclic compounds which
for many years attract the interest of chemical and materials
scientists. They structurally are N,N0-disubstituted 4,40-bipyridines.
In most of the cases their importance is aligned with their strong
electron withdrawing character. Besides their biological activity [1]
they have been used as building redox active functional blocks of
switchable interlocked molecules such as rotaxanes and catenanes
[2e5], as well as molecular sensors of some compounds of bio-
logical importance such as NAD [6] and glucose. [7e9] Their
reversible one-electron reduction leading in colorful relatively
stable radical cations (blue or violet depending on the substituents)
has been proposed for use in new hi-tech applications such as in
electrochromic displays and rear view windows [10]. The phor-
ophysics and photochemistry of viologens has also been investi-
gated in the past [11], as well as their photochromic properties,
with very interesting results. [12] The nonlinear optical properties
and solvatochromism of molecules possessing N-monoquaternized
4,40-bipyridine (the so-called monoquats) as ligands of complexes
aromatic and electron withdrawing system [13e15]. Sol-
vatochromism is an additional interesting aspect of the chromo-
tropic behavior of viologens which to the best of our knowledge has
not been intensely investigated. Recently we described the medium
responsive character of a new N-(2,4-dinitrophenyl), N0- phenacyl
viologen [16].
In this work a group of new non-symmetric N-aryl,N0-phena-
cetyl viologens has been synthesized and fully characterized. Their
interesting chromotropic behavior strongly dependent on solvent
polarity (solvatochromism) was thoroughly investigated. This
unique behavior which was proved to be triggered by basic enough
solvents, was connected to their structure using experimental data
as well as empirical parameters (i.e. Hammett parameter).
2. Experimental section
2.1. Physical measurements
NMR spectra were obtained using a Varian Gemini 300 spec-
trometer (300 MHz 1H, 75 MHz 13C). Both 1H and 13C-NMR spectra
were recorded in [D6]DMSO or [D6]Me2CO at 25 ꢀ1 ꢁC. The 1H NMR
spectra were calibrated by using residual undeuterated solvent as
an internal reference (4.79 ppm) and 13C-NMR spectra were
* Corresponding author. School of Chemical Engineering, National Technical
University of Athens (NTUA), Laboratory of Organic Chemistry, 15780 Athens,
Greece. Tel.: þ30 2107723081; fax: þ30 2107723072.
E-mail address: rafpapadakis@gmail.com (R. Papadakis).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.06.013

R. Papadakis et al. / Dyes and Pigments 95 (2012) 478e484
479
calibrated using the [D6]DMSO signal at 39.52 ppm or the [D6]
Me2CO at 29.84 ppm [17]. Abbreviations used for multiplicity in the
text: s ¼ single; d ¼ doublet; m ¼ multiplet; arom ¼ aromatic.
(t, J ¼ 7.2 Hz, 1H, PhAc), 7.70 (m, 2H: arom.), 7.35 (d, J ¼ 8.7 Hz, 2H:
arom.), 6.76 (s, 2H: >CH2).13C-NMR (75 MHz, [D6]AcMe): 190.32,
163.30, 151.68, 151.13, 148.56, 146.69, 135.85, 135.39, 134.56, 130.12,
129.24, 128.27, 127.93, 126.95, 116.51, 66.51 (>CH2). Elemental
analysis calcd for C24H19N2OClP2F12: C 42.59, H 2.83, N 4.14, found:
UVeVisible spectra were recorded using
a Varian CARY 1E
UVeVisible spectrophotometer at 25 ꢀ1 ꢁC. The concentrations of
the solutions used, were about 50 ppm and they were prepared
right before each measurement. Electrospray ionization (ESI) HRMS
spectra were obtained on a Waters, Inc. Q-TOF Premier Mass
Spectrometer. The solvent used was MeOH. IR spectra were recor-
ded on a PerkineElmer Spectrum 1 FTIR spectrophotometer in the
solid state (without any preparation of the samples) using the
attenuated total reflectance technique (ATR) in the region
600e4000 cmꢂ1. Melting points were determined in open capillary
tubes using a Gallenkamp HFB-595 melting point apparatus and are
uncorrected. Finally elemental analyses were carried out using
a PerkineElmer Elemental Analyzer 2400 CHN.
C 42.82, H 2.34, N 4.23, IR (ATR): n .
(C]O): 1739, 1513, 1367 cmꢂ1
2.2.1.4. Viologen 6d (4-CNPh). Beige powder, 536 mg (0.744 mmol)
62%, mp ¼ 207e208 ꢁC. 1H NMR (300 MHz, [D6]AcMe): 9.76 (d,
J ¼ 6.6 Hz, 2H: C5H4N), 9.31 (d, J ¼ 5.1 Hz, 2H: C5H4N), 9.00 (m, 4H:
arom.), 8.37 (d, J ¼ 8.4 Hz, 2H: Ph), 8.22 (d, J ¼ 8.1 Hz, 2H: Ph), 7.84
(pt, J ¼ 7.2 Hz, 1H: PhAc), 7.71 (pd, 2H: Ph), 6.58 (s, 2H, >CH2). 13C-
NMR (75 MHz, [D6] AcMe): 191.42, 150.98, 150.26, 149.87, 148.63,
147.21, 147.34, 139.90, 137.09, 136.62, 134.72, 131.31, 129.97, 127.42,
126.94, 65.41 (>CH2). HRMS-ESI (m/z): [M-2PFꢂ6 ], 100%, calc. for
C25H19N3O, 377.1528; found, 377.1524; IR (ATR):
n(C]O): 1739,
1602, 1500, 1367 cmꢂ1
.
2.2. Synthetic procedures and characterization details
Viologen 6e (2,4-dinitrophenyl): This compound was synthe-
sized according to an already published procedure [16].
Synthetic procedures and characterization of compounds 1,
3aed and 5aee are included in the Supplementary data file.
3. Calculations
2.2.1. General synthetic procedure (6aed)
The Van der Waals radii of the cationic parts of the viologens
6aed studied, were calculated using the 3V: Voss Volume Vox-
alator, available on-line. The results for each compound are listed in
Table S1 (Supplementary data). The difference of dipole moments
in ground and excited state were determined graphically using the
LipperteMataga equation according to literature [18,19]. All
correlations and linear regression analyses were performed using
the program QtiPlot.
A total of 1.20 mmol of the product 5(aed) were dissolved in
H2O by sonication and mild heating (at about w50 ꢁC). To the
resulting solution 5 mL of a 4 M NH4PF6 aqueous solution was
added. Immediately a beige or pale-yellow solid was formed
(6aed). The solid was washed thoroughly with water in order to
remove the NHþ4 salts, and then with EtOH and finally with Et2O.
The as formed products are cleaned as follows. The hexa-
fluorophosphate viologen salt 6(aed) was dissolved in MeCN and
a solution of tetraethylammonium bromide (Et4NBr) in MeCN
(containing approximately 10 times the equimolar quantity of
Et4NBr in the minimum volume of MeCN) was added. The resulting
bromide salt was filtered and washed with MeCN and Et2O. Finally
the bromide salt was transformed to the desired product 6aed by
ion exchange using NH4PF6 as described before [16].
4. Results and discussion
4.1. Synthesis and characterization of the viologens
For the synthesis of the solvatochromic non-symmetric viol-
ogens the synthetic procedure depicted in Scheme 1 was followed.
The compounds 3aed (monoquats) were synthesized through
Zincke reactions between suitable substituted anilines (2aed) and
the precursor compound 1, as described before [20]. These mono-
quats further reacted with phenacetyliodide (4) in refluxing EtOH
to spontaneously result in microcrystalline precipitates of viol-
ogens with mixed anions (Clꢂ and Iꢂ, compounds 5aee). These
compounds (5aed) were transformed to the desired hexa-
fluorophosphate corresponding salts (soluble in various organic
solvents) by ion exchange to give compounds 6aee in high yields.
The compounds 5,6e have been reported in an earlier publication of
our group [16] and their intense solvatochromic behavior and HBD
activity has been discussed (see also synthetic details in
Supplementary data file). The compounds 5,6aed were character-
ized using NMR (1H and 13C) spectroscopy, UVeVis and FTIR
spectrophotometry as well as ESI-HRMS (Figs. S3eS6 of
Supplementary data file). As mentioned compounds 5aed were
isolated as microcrystalline solids. Representative scanning elec-
tron microphotographies and powder-XRD pattern of the
compound 5a, are depicted in Figs. S1 and S2 of Supplementary
data file, respectively. Attempts to grow bigger crystals suitable
for single-crystal XRD in order to solve their structure failed.
Nevertheless these compounds were also proved to be sol-
vatochromic as well. It is though of great interest to examine the
unique chromotropic character of compounds 5aed separately
since it is known that iodide salts of N-substituted pyridinium
(especially those with electron withdrawing moieties at position 4)
behave as solvatochromic compounds as well, Kosower’s salt being
one of the most popular solvatochromic analogue [21]. Besides
2.2.1.1. Viologen 6a (4-MePh). Beige powder, 629 mg (0.864 mmol)
72% mp ¼ 228e230 ꢁC. 1H NMR (300 MHz, [D6] AcMe): 9.69 (d,
J ¼ 6.0 Hz, 2H: C5H4N), 9.30 (d, J ¼ 5.7 Hz, 2H: C5H4N), 8.99 (m, 4H:
arom.), 8.14 (d, J ¼ 7.5 Hz, 2H: arom.), 7.88 (m, 3H: PhAc.), 7.71 (t,
J ¼ 7.2 Hz, 2H: PhAc), 7.63 (d, J ¼ 8.1 Hz, 2H: PhAc), 6.57 (s, 2H,
>CH2), 2.50 (s, 3H, CH3). 13C-NMR (75 MHz, [D6]AcMe): 190.25,
149.17, 149.40, 147.26, 145.67, 141.87, 139.90, 134.90, 133.39, 130.61,
129.23, 128.27, 126.67, 126.61, 124.48, 66.38 (>CH2), 20.73 (Me).
HRMS-ESI (m/z): [M-2PF6ꢂ], 100%, calc. for C25H22N2O, 366.1732;
found, 366.1727; IR (ATR):
n(C]O): 1739, 1641, 1541, 1438,
1369 cmꢂ1
.
2.2.1.2. Viologen 6b (4-FPh). Beige powder, 591 mg (0.828 mmol)
69%, mp ¼ 225e226 ꢁC. 1H NMR (300 MHz, [D6]AcMe): 9.71 (d,
J ¼ 6.0 Hz, 2H: C5H4N), 9.42 (d, J ¼ 6.3 Hz, 2H: C5H4N), 9.07 (d,
J ¼ 6.6 Hz, 2H: C5H4N), 9.03 (d, J ¼ 6.3 Hz, 2H: C5H4N), 8.17 (m,
4H,arom.), 7.81 (t, J ¼ 7.5 Hz, 1H: PhAc) 7.70 (m, 4H: arom.), 6.77 (s,
2H, >CH2). 13C-NMR (75 MHz, [D6]AcMe): 190.52, 151.91, 151.57,
148.58, 147.12, 140.11, 137.30, 135.82, 134.55, 130.10, 129.23, 128.33,
127.97, 118.52, 118.20, 67.71 (>CH2). HRMS-ESI (m/z): [M-2PFꢂ6 ],
100%, calc. for C24H19N2OF, 370.1481; found, 370.1489; IR (ATR):
n
(C]O): 1739, 1504, 1367 cmꢂ1
.
2.2.1.3. Viologen 6c (4-ClPh). Beige powder, 584 mg (0.780 mmol)
65%, mp ¼ 232e234 ꢁC. 1H NMR (300 MHz, [D6]AcMe): 9.65 (d,
J ¼ 6.6 Hz, 2H: C5H4N), 9.41 (d, J ¼ 6.0 Hz, 2H: C5H4N), 9.02 (m, 4H:
C5H4N), 8.18 (d, J ¼ 7.8 Hz, 2H: Ph), 8.00 (d, J ¼ 8.7 Hz, 2H, Ph), 7.81

480
R. Papadakis et al. / Dyes and Pigments 95 (2012) 478e484
Scheme 1. Synthetic procedure followed for the synthesis of the solvatochromic viologens 6aed.
their red-brown color in the solid state reflects this behavior. In this
work we focus on the solvatochromism of the hexafluorophosphate
salts.
methylene of the phenacetyl moiety (formed in basic solutions) to
the strong electron withdrawing quaternized nitrogen atom of the
viologen, through the aromatic system of the 4,40-bipyridyl bridge.
This band is strongly affected by solvent polarity and substituents
of different nature attached to 4,40-bipyridyl (see structure of
compounds 5,6aed of Scheme 1). The latter appears at about
450e550 nm, and presents a molecular extinction coefficient of
about 850 Mꢂ1 cmꢂ1. What is interesting with this band is that it
only exists in solvents presenting basic character, whereas in
solvents of low or negligible HBA-basicity there is no visible
contribution in the electronic absorption spectra of the studied
compounds. This is a strong indication that proton donation or even
deprotonation can occur only in basic solvents, as expected. Thus in
It was also attempted to synthesize compounds 6aed via Zincke
reactions of the product 5e, using the arylamines 2aed respec-
tively. Unfortunately these reactions did not yield in the formation
of the desired viologens 6aed. This fact was attributed to the
“acidic” character of the product 5e, described also before. [16] Thus
upon addition of a substituted aniline, the solution in any case
turned dark and the resulting product was not the desired product
6aed.
4.2. UVeVis spectrophotometry
Compounds 6aed show an intense solvatochromic behavior. It
is noteworthy though, that they express this behavior upon solva-
tion in solvents which can rather act as Lewis bases, or more
specifically as HBA-bases. In Fig. 1 the visible spectra of the
compound 6d are depicted, in five solvents capable of acting as
HBA-bases: dimethyl-acetamide (DMAC), formamide (FA), N-
methyl-formamide (NMF), hexamethylphosphoramide (HMPA),
and pyridine (PY). Nevertheless it was observed that the solvation
of the pale-yellow to beige colored solids 6aed, in solvents acting
as better HBD-acids rather than HBA-bases like several alcohols,
e.g. MeOH and EtOH resulted in yellow colored solutions, whereas
when these compounds were dissolved in HBA-solvents (like the
aforementioned) they yielded in blue-purple solutions strongly
depending on the polarity of solvents.
Compounds 6aed, present two main features in their UVeVis
spectra. One high absorbance sharp Gaussian type band attrib-
uted to the
pep* excitations, appearing at about 230 nm of the
viologen aromatic system, which is generally not intensely affected
by solvent polarity, temperature, or substituents. The molecular
extinction coefficient of this band is high, reaching values of about
7500 Mꢂ1 cmꢂ1. The second band is a low energy visible band,
assigned as a charge transfer band (CT) from the carbene of the
Fig. 1. Normalized visible spectra of 6d (p-CN) in different solvents at 298 K.

R. Papadakis et al. / Dyes and Pigments 95 (2012) 478e484
481
cases of solutions of 6aed in MeOH and other alcohols acting as
poorer bases, the color observed is pale yellow or beige (Figs. S7 and
S8 of Supplementary data file). The aforementioned visible band
does not exist either in the solid state. This fact reflects the strong
solute-solvent interaction (solute being compounds 6aed) which is
pronounced in basic media, giving rise to solvatochromism. The
proposed limiting resonance structures of compound 6d, and the
corresponding structures in ground and excited states are depicted
in Scheme 2.
What is interesting is that successful correlations were obtained
as well, between the CT maxima absorption energies (JCT) and the
solvent normalized donor numbers (DNN) or the KamleteTaft
b
parameters, both expressing the donicity of the solvent (Lewis
basicity and HBA-basicity respectively). The results of these corre-
lations are depicted in Figs. S13 and S14 of Supplementary data file.
This observations have the meaning that the solvatochromic
phenomena in all cases are controlled by both dipolarity/polariz-
ability (expressed by the function f(
3
) þ g(n)) and by the basicity of
solvents (expressed by the parameter DNN or
b
). Basic enough
solvents are needed to interact strongly with the “active methy-
lenes” of 6aed and thus generate a betaine of the type of Scheme 2,
which is stabilized by the solvent with lower dipolarity/polariz-
ability. This fact is also reflected by the negative slopes of the
4.3. Solvent effects
Solvent polarity has a significant effect on the visible spectra of
the viologens studied in this work. As shown in Fig. 1 and Table 1,
bathochromic shifts of the CT bands of the viologens 6aed, are
induced upon decreasing solvent polarity (following the sequence
FA, NMF, HMPA, DMAC, PY). This behavior corresponds to negative
solvatochromism. As shown in Fig. 2 (see also Figs. S9eS12 of
Supplementary data file), the measured CT absorption maxima
wavenumbers are correlated very well with the polarity function
correlations JCTfDNN (or
lations JCTf½f ð
b) and the positive slopes of the corre-
3
Þ þ gðnÞꢃ. Thus increasing solvent basicity induces
bathochromic shifts on the CT absorption band of 6aed, whereas
increasing of solvent dipolarity/polarizability induces hyp-
sochromic shifts on the CT absorption band of 6aed.
Interestingly there are several analogies of the as formed beta-
ines and some pentacyanoferrate complexes with N-aryl-
substituted monoquats already published by our group in the past
[20]. As shown in Scheme S1 of Supplementary data file, in both
cases, upon excitation with visible light charge transfer from the
electron donating moiety (in one case FeII(CN)5 and PhCO-HC in the
other) to the quaternary electron withdrawing nitrogen atom,
connected with the aryl substituent, occurs. The colors of the
solutions obtained in both cases were blue-purple indicating that
the charge transfer in both cases demands relatively low energy
(visible light). The main difference is that the betaine “sol-
vatochromically active” form of the viologens studied here, is
formed in solution. In the solid state the viologens are stable and
their colors as already mentioned are beige or pale yellow. Whereas
in the case of the corresponding pentacyanoferrate complexes, the
color of the substances is blue even in the solid state. The behavior
of the viologens 6aed, is a special case of chromotropism, triggered
only by solvents presenting HBA-basicity.
f(
3
) þ g(n), describing both solvent polarity (dipolarity through the
term f(
3
) and polarizability through the term g(n), see Eqs. (ES1)
and (ES2) of Supplementary data file). This is attributed to the fact
that the excited state in all cases is less polar than the ground state,
thus the excited state (e.g. Scheme 2) is stabilized by solvents of
lower polarity (i.e. pyridine) and this corresponds to a classical
negative solvatochromic behavior.
The results of the correlations between the CT maxima
absorption energies (JCT) and the polarity function f(
listed in Table 2. From the graphically obtained slopes of the family
3 ) þ g(n), are
of these linear equations it is possible to determine the term
2
Dm
k
= a is the Van der Waals
a3 with energy dimensions, where
k
radius of the solvatochromic compound (6aed), and kDmk is the
difference kmexk ꢂ kmgk (where kmexk is the norm of the vector of
the dipole moment of the excited state and kmgk that of the ground
state). By calculating the Van der Waals radii of 6aed (see Section
3), the kDmk were determined for each compound, and they are
listed in Table 2 as well. As expected the difference kDmk was higher
for the case of 6d which is the compound possessing the most
electron withdrawing moiety (p-CN) amongst 6aed, as it is
analyzed thoroughly in next paragraph. Increasing the electron
withdrawing character of the aryl substituent, an increase of the
dipole moment difference between the ground and excited state is
observed, implying a more intense susceptibility on the solvent
polarity for compound 6d, possessing the electron deficient group
p-CN-phenyl.
4.4. Structural effects
The electron donating or withdrawing nature of the aryl
substituent, readily affects the UVeVis spectra (mainly the visible
CT band) of 6aed. The plots of the experimentally obtained CT
maxima absorption wavelengths in all solvents, versus the Ham-
mett constants are depicted in Fig. 3. In all cases of solvents,
~
negative slopes were determined for the equations of the type yx
¼
~
~
yo
þ
rsx (where yx is the maximum CT absorption wavelength for
Scheme 2. Limiting resonance structures and ground and excited states of compound 6d. S: symbolizes an HBA-solvent.

482
R. Papadakis et al. / Dyes and Pigments 95 (2012) 478e484
Table 1
~
Wavenumbers ðyCTÞ of the CT Vis absorption maxima of 6aed, measured in five solvents of different polarity and polarity parameters of these solvents.
Solvent
yCT 6a (103 cmꢂ1
)
yCT 6b (103 cmꢂ1
)
yCT 6c (103 cmꢂ1
)
yCT 6d (103 cmꢂ1
)
3
(e) [21,22]
n (e) [21,22]
DNN (e) [23]
b (e) [25]
~
~
~
~
DMAC
FA
HMPA
NMF
Py
21.692
22.523
21.368
22.222
20.747
21.692
22.422
21.413
22.124
20.833
21.692
22.573
21.368
22.222
20.833
21.598
21.786
21.322
21.834
20.161
37.8
109.5
29.3
182.4
12.9
1.4384
1.4475
1.4588
1.4319
1.5102
0.72a [23,24]
0.64
1.00
0.70
0.85
0.69
0.48
1.00
0.80
0.64
a
The relationship between the donor numbers and the parameter b, has been discussed in detail in the past [26e28].
~
a compound (6aed) having the aryl substituent x, yo the maximum
CT absorption wavelength when x ¼ H, sx is the Hammett substit-
character becomes higher in solvents of high dipolarity/polariz-
ability, i.e. in the cases of higher energy difference between the
ground and excited states of 6aed.
uent constant and
r is the susceptibility of the CT absorption
maxima wavenumbers of a compound 6aed on the substituent
changes). The results of the corresponding correlations are listed in
Table 3. These negative slopes in all cases reflect the tense of
electron withdrawing substituents to decrease the energy needed
to cause the excitation of the viologen betaines, possibly by effi-
ciently decreasing the energy of excited state (depicted in Scheme
2). The substituent p-CN, in all solvents, is the one which causes the
highest stabilization than the less electron withdrawing substitu-
ents p-Cl and p-F, and by far the electron donating p-Me
substituent.
There is also an interesting correlation between the 13C-NMR
shifts of the compounds 6aed, and their CT maxima absorption
wavelengths. As shown in Fig. 4, hypsochromic UVeVis shifts keep
up with the increasing low-field 13C chemical shifts attributed to
the resonance of the methylene carbon nuclei (measured in
deuterated acetone) which follow the sequence: p-CN < p-Me < p-
Cl < p-F. This general tense reflects the importance of the structure
on the observed solvatochromic behavior. The deshielding of the
carbon nuclei of the methylenes of compounds 6aed in NMR, is
a strong indication of the electron withdrawing character of the
cationic part of the viologens, and of the HBD-acidic character of
the compound. The compounds demonstrating the most acidic
character are those which present the highest energy gap between
the ground and excited state, since the ground state in these cases is
more polar because of the intense charge separation. Thus in those
cases the wavenumber of the CT visible maxima is higher. A linear
plot is obtained for HMPA, whereas for the other four solvents
parabolic plots were obtained. This can be attributed to the fact that
HMPA is the best HBA-base of the examined group of solvents
(HMPA, FA, NMF, DMAC PY). For a strong base like HMPA small
It was also observed that as the polarity of the solvent increases,
the susceptibilities expressed by the obtained regression values of
r
(slopes of Hammett equations, Table 3 and Fig. 3), tend to linearly
increase, as shown in Fig. S16 of Supplementary data file. The
physical meaning of this observation is that in more polar solvents
the substituent effects are more pronounced, or in other words, the
contribution of the substituent electron donating or withdrawing
Fig. 3. Correlation plots between Hammett’s substituent constants, sx, and the
Fig. 2. Plot of the experimentally obtained wavenumbers of the CT absorption maxima
versus the calculated ones for 6a-d in all five solvents used.
wavenumbers of 6a-d, measured in different solvents at room temperature.
Table 3
Table 2
Results of the linear regression analysis between the wavenumbers of the CT
absorption maxima of 6aed and the Hammett’s substituent constants, sx
Results of the linear regression analysis between the wavenumbers of the CT
.
absorption maxima of 6aed and the solvent polarity function [f(
3
) þ g(n)].
a
a
yo (103 cmꢂ1
)
r )
(103 cmꢂ1
Rb
~
Solvent
aLM (10ꢂ19 Joule)a
yo (10ꢂ19 Joule)a
Rb
k
Dmk (D)
~
Compound
FA
22.499 ꢀ 0.139
22.189 ꢀ 0.066
21.692 ꢀ 0.016
21.382 ꢀ 0.020
20.792 ꢀ 0.133
ꢂ0.889 ꢀ 0.385
ꢂ0.455 ꢀ 0.184
ꢂ0.118 ꢀ 0.040
ꢂ0.074 ꢀ 0.054
ꢂ0.764 ꢀ 0.370
0.85
0.87
0.89
0.70
0.83
6a: p-Me
6b: p-F
6c: p-Cl
6d: p-CN
2.566 ꢀ 0.568
2.247 ꢀ 0.494
2.460 ꢀ 0.633
2.676 ꢀ 0.195
2.410 ꢀ 0.422
2.643 ꢀ 0.367
2.494 ꢀ 0.470
2.253 ꢀ 0.145
0.93
0.94
0.91
0.99
10.986
10.201
10.619
11.535
NMF
DMAC
HMPA
Py
a
a
~
~
yo
~
y
~
yo
Slope and intercept of the family of equations:
Correlation coefficient.
y
¼
þ
aLM½f ð
3 Þ þ gðnÞꢃ.
Intercepts and slopes of the linear Hammett equations:
¼
þ
rsx
.
b
b
Correlation coefficient.

R. Papadakis et al. / Dyes and Pigments 95 (2012) 478e484
483
aryl substituents of 6aed, do not control the observed phenom-
enon. The main impact is the HBD-acidity of 6aed (namely the
HBD-acidity of the methylenes of 6aed). Thus linear correlations of
Dm with the 13C-NMR shifts are obtained, in contrast to the
k
k
correlations of kDmk with the Hammet parameter. This observation
is aligned with our findings regarding the correlation dC-13 ¼ f(sx
(Fig. S15a, of Supplementary data file).
)
5. Concluding remarks
A group of new solvatochromic viologens was synthesized. Their
chromotropic behavior was induced only by solvents showing
significant HBA-basic character. The impact of solvent dipolarity
and polarizability as well as solvent basicity was proved to play an
important role on the solvatochromic behavior of the studied non-
symmetric viologens. Structural effects were also investigated and
it was proved that the 13C shifts of the methylenes of compounds
6aed, correlate well with the dipolarity of these compounds in
their ground and exited states.
Fig. 4. Correlation plots between the wavenumbers of 6aed, measured in different
solvents at room temperature, and the 13C chemical shifts of the methylenes of 6aed
measured in [D6]AcMe.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.dyepig.
2012.06.013.
changes in HBD-acidity amongst 6aed (reflected by downfield
chemical shifts as mentioned) do not demonstrate an important
role on the absorption maxima of the formed betaines. Thus linear
dependence between dC-13 and nmax in this case is observed, as
depicted in Fig. 4.
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viologens decreases, following the sequence p-CN < p-Me < p-
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