Reversal in solvatochromism in some novel styrylpyridinium dyes having a hydrophobic cleft

Article abstract of DOI:10.1016/j.saa.2006.12.057

The influence of solvent polarity on the electronic transition of four different N-hexadecyl styrylpyridinium dyes has been investigated in 15 solvents. The E_T(30) scale has been used to propose a quantitative approach towards the relative stab

Full text of DOI:10.1016/j.saa.2006.12.057

Spectrochimica Acta Part A 68 (2007) 757–762
Reversal in solvatochromism in some novel styrylpyridinium
dyes having a hydrophobic cleft
Mallika Panigrahia, Sukalyan Dashb, Sabita Patela,
P.K. Beheraa, B.K. Mishraa,∗
a Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur University,
Jyoti Vihar 768019, India
b Department of Chemistry, University College of Engineering, Burla 768018, India
Received 30 September 2006; received in revised form 22 December 2006; accepted 22 December 2006
Abstract
The influence of solvent polarity on the electronic transition of four different N-hexadecyl styrylpyridinium dyes has been investigated in 15
solvents. The ET(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state
species. The extents of contribution of dipolar aprotic solvents towards the solvation of the excited species have been determined to be 42–48% for
some of the dyes. Instead of a steady solvatochromism, all the dyes suffer a reversal in solvatochromism. The transitions of the solvatochromism,
referred to as solvatochromic switches, are found to be at ET(30) values of ∼50 for methyl and N,N-dimethylamino substituted dyes while at 37.6
for hydroxyl substituted dye and ∼45 for 4-(1-methyl-2-phenylethenyl) pyridinium dye. A reversal in the trend of solvent effect in the later dye
corresponding to 4-(4-methyl styryl)pyridinium dye has been attributed to an analogy of series and parallel electron flow.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Styrylpyridinium dyes; Solvatochromism; ET(30) switch; Dipolar aprotic solvents; Nonpolar solvents; Hydroxylic solvents
1. Introduction
contribution of both the dipolar zwitterionic benzenoid and the
neutral quinoid form [1–3]. The solvatochromism is explained
through differential solvation of the ground and Franck–Condon
excited state [4]. If the ground state is more stabilized than the
excited state due to solvation by solvents of increasing polarity,
a negative solvatochromism is exhibited and vice versa. The
influence of solvent polarity on the preferential solvation of
the solute due to differential solute–solvent interaction has
also been proposed for diazoaminobenzene [5] and substituted
naphthalimide [6].
Brooker et al. [7] carried out pioneering work on sol-
vatochromic merocyanine dyes. Interestingly, some of the
merocyanine dyes such as 1, show an inverted solvatochromism
[8], i.e., their long-wavelength absorption band first shows
a bathochromic shift followed by a hypsochromic shift with
increasing solvent polarity. This reversal in solvatochromism
has been rationalized by semiempirical MO calculations [8,9]
and the results were confirmed by Jacques [10]. From the
transition energy calculations of 1, Luzhkov and Warshel [11]
have reported negative solvatochromism, whereas Morley [12]
has shown a positive solvatochromism for it by the CNDOVS
method.
Solvatochromism represents the solvent-induced spectral
band shift, which is due to changes in the ␲ → ␲ and n → ␲
*
*
transition energy because of differential solvation of ground
and excited states of the probe. With increasing polarity of the
solvent the spectral band can undergo a hypsochromic or a
bathochromic shift depending on the electronic structure of the
probe and its interaction with the environment given by the sol-
vent shell. Accordingly, a negative or positive solvatochromism
can be found, which gives qualitative and quantitative conclu-
sions about the energetics of the dye solvation. Solvatochromic
dyes generally exhibit steady bathochromic (positive solva-
tochromism) or hypsochromic (negative solvatochromism) band
shifts in solvents with a change in polarity. Hydroxy styrylpyri-
dinium dyes having solvatochromic characteristics exhibit a
largechangeindipolemomentuponexcitationduetotherelative
∗
Corresponding author. Tel.: +91 6632430093; fax: +91 6632430158.
E-mail address: bijaym@hotmail.com (B.K. Mishra).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.12.057

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M. Panigrahi et al. / Spectrochimica Acta Part A 68 (2007) 757–762
2. Experimental
2.1. Materials
The melting points of the compounds were recorded in open
capillary in an acid bath and are uncorrected. All the solvents
were analytical grade obtained from SRL and Merck, Mumbai.
The solvents were distilled before use.
Recently, Masternak et al. [13] have reported about the
behaviour of a betaine dye derived from purine and predicted a
lower-energy absorption band of charge-transfer character that
is highly solvatochromic, and a higher-energy absorption band
with ␲–␲* character which is not solvatochromic. Cavalli et al.
[14] studied the fluorosolvatochromism of Brooker’s merocya-
nine in pure and mixed solvents and proposed a model based
on solvent-exchange equilibria, which allowed a separation of
the different contributions of the solvent species in the solvation
shell of the dye.
2.2. Synthesis of 1-n-hexadecyl-4-(4-methylstyryl)
pyridinium bromide (2)
An ethanolic solution (20 mL) of N-hexadecyl-4-
methylpyridinium bromide (0.01 mol, 0.4 g), 4-tolualdehyde
(0.01 mol, 0.09 mL), and two drops of piperidine was refluxed
for 5 h. The volume of the solution was reduced under vacuum
to a pasty mass, which was then washed several times with
anhydrous diethyl ether, and the solid thus obtained was
recrystallized from ethanol to give a yellow crystalline solid.
Yield 78%, mp 141 ◦C. 1H NMR (CDCl3): δ = 0.87 (t, 3H
of hexadecyl) 1.24 (distorted s, 26H of hexadecyl) 1.98 (q,
␤–CH2– of hexadecyl) 4.77 (t, ␣–CH2– of hexadecyl) 7.26
(d, Bz H), 7.53 (d, Bz H), 7.1 (d, –CH ), 7.68 (d, CH–,
J = 15.9 Hz), 8.02 (d, Py H), 9.12 (d, Py H), 2.401 (s, 3H on Bz).
Using this procedure the following compounds were synthe-
sized.
Earlier, we have reported a reversal in the sign of solva-
tochromism for a series of bischromophoric styrylpyridinium
dyes with hydroxy group in the styryl unit. The reversal has been
ascribedtoachangeinthehydrogen-bondingpropertyofthesol-
vents. Hydrogen-bond donor (HBD) solvents exhibit a negative
solvatochromism, while the hydrogen-bond acceptor (HBA) sol-
vents show a positive solvatochromism with increasing solvent
polarity[15]. Thereversionpointhasbeenconsideredasaswitch
in the ET(30) scale to identify the HBD and HBA characteristics
of the solvents. To generalize the phenomenon, herein, we have
made an attempt to synthesize and to analyze the influence of
solvent of different polarity on the electronic transition of some
N-hexadecyl (substituted styryl)pyridinium bromides (2–5). The
absorption behaviour of 3 in aqueous and micellar media has
been studied extensively and aggregations of the dye have been
observed in aqueous and surfactant systems [16–22]. In aqueous
medium3absorbsaround390–400 nmduetoH-typeofaggrega-
tion [17]. In cetyltrimenthylammonium bromide (CTAB) [18],
sodium dodecyl sulphate (SDS) [19] and triton-X-100 (TX100)
[20] micellar system it exists as monomer. However, in linear
alkylbenzene sulfonate (LABS) the compound stacks with a
slippage of one chromophoric unit over other leading towards
J-aggregate and so exhibits bathochromic shift [21,22]. Sim-
ilar phenomena has also been observed in CTAB-chloroform
reversed micelle, wherein 3 exists as J-aggregates at the inter-
face of water core and mesophase. In nonaqueous solvents the
compound exists as monomer [17].
2.2.1. 1-n-Hexadecyl-4-[4-N,N-(dimethylamino)styryl]
pyridinium bromide (3)
1
Yield 78%, mp 220 ◦C. H NMR (CDCl3): δ = 0.87 (t, 3H
of hexadecyl), 1.24 (distorted s, 26H of hexadecyl), 4.63 (q,
␤–CH2– of hexadecyl), 3.06 (s, 6H), 4.63 (t, ␣–CH2– of hex-
adecyl, J = 7.2 Hz), 6.85 (d, –CH of methane, J = 15.9 Hz), 7.52
(d, CH– of methane, J = 15.9 Hz), 6.68 (d, Bz H), 7.52 (d, Bz
H, J = 8.8 Hz), 7.88 (d, Py H), 8.94 (d, Py H, J = 6.6 Hz).
2.2.2. 1-n-Hexadecyl-4-(2-hydroxystyryl)pyridinium
bromide (4)
1
Yield 70%, mp 151 ◦C. H NMR (CDCl3): δ = 0.87 (t, 3H
of hexadecyl), 1.24 (distorted s, 26H of hexadecyl), 4.63 (q,
␤–CH2– of hexadecyl), 3.06 (s, 6H), 4.63 (t, ␣–CH2– of hex-
adecyl, J = 7.2 Hz), 6.85 (d, –CH of methane), 7.52 (d, CH– of
methane, J = 15.9 Hz), 6.68 (d, Bz H), 7.52 (d, Bz H, J = 8.8 Hz),
7.88 (d, Py H), 8.94 (d, Py H, J = 6.6 Hz).
2.2.3. 1-n-Hexadecyl-4-[(2ꢀ-methyl)styryl]pyridinium
bromide (5)
1
Yield 70%, mp 153 ◦C. H NMR (CDCl3): δ = 0.87 (t, 3H
of hexadecyl), 1.24 (distorted s, 26H of hexadecyl), 2.00 (q,
␤–CH2– of hexadecyl), 3.14 (s, 6H), 4.89 (t, ␣–CH2– of hex-
adecyl), 7.28 (s, 1H, –CH ), 2.68 (s, 3H on C–), 9.17 (d, 4H,
J = 6.3) 7.91 (d, 4H J = 5.7) 7.40 (s, 1H).
2.3. Methods
The UV–vis absorption spectra were recorded on a Lambda-
25 Perkin-Elmer spectrophotometer in a matched pair of quartz

M. Panigrahi et al. / Spectrochimica Acta Part A 68 (2007) 757–762
759
Table 1
cells of 1 cm thickness with thermostatic facilities to maintain
Absorption maxima (λmax) of dyes 2–5 in different solvents
the solution temperature at 300 0.1 K by using a Fourtech
cryostat.
Solvents
λmax (nm)
2
3
4
5
3. Results and discussion
Hexane
Carbon tetrachloride
Toluene
Benzene
Dioxane
Chloroform
Dichloromethane
Acetone
DMF
DMSO
355.4
359.1
363.2
360.2
354.0
370.0
376.0
360.0
360.6
361.1
363.8
364.8
364.7
364.0
360.0
454.6
467.2
474.8
476.0
463.0
493.0
515.2
477.0
475.3
479.0
481.2
487.1
485.6
484.9
480.2
530.1
548.4
554.2
553.7
558.2
554.2
553.8
547.0
549.1
523.4
520.4
523.4
518.9
518.9
503.8
398.5
399.5
393.5
386.8
387.4
384.0
386.4
383.2
384.4
384.6
385.4
390.5
398.5
399.5
400.6
3.1. Substituent-effect on the electronic transition of the
dyes
The styrylpyridinium dyes (2–5) have three different sub-
stituents, which can influence the electronic transition of the
dyes. Each dye has a quaternary nitrogen acceptor at one ter-
minus of the chromophoric group and thus assumes a vectorial
flow of electron from the substituent present in the styryl unit.
The absorption maxima have been obtained in 15 different sol-
vents having HBD, HBA abilities and rest with non-hydrogen
bonding characteristics (Table 1). In these solvents dye 2 with
a methyl substituent absorbs within a solvent dependent range
of λmax = 354–365 nm, where as for dye 3 with a N,N-dimethyl
amino substituent the variation in absorption maxima between
the highest and lowest value of absorption maxima due to change
in solvent (ꢀλmax) is within 33 nm. However, dye 4 with a
hydroxy group shows a variation of ꢀλmax = 55 nm. The trend
in the variation of the absorption maxima is in accordance with
the electron-releasing ability of the substituent, R2 of the linear
plot of ꢀλmax versus σI (Taft’s inductive parameter [23]) being
0.96 and those of plots of λmax versus σI in each solvent being
0.92 0.2.
2-Propanol
1-Butanol
1-Propanol
Ethanol
Methanol
constant. The presence of a methyl group in between the electron
delocalisation path of one terminus to the other in dye 5 has a
significant effect on the λmax value. The compound suffers a
bathochromic shift to an extent of ꢀλmax = 20–40 nm, which
may be due to the proximity of methyl group (with +I effect) to
the acceptor group.
To have an overall picture of the effect of substituent on the
solvent effect, the absorption maxima of all the four compounds
were compared through scattergrams (Figs. 1–3). The linearity
of the plot in Fig. 1 reveals that with the change of solvents
neither the solvation pattern nor the solvated structure changes
For (3) with a NMe2 substituent the λmax is higher for HBD
solvents when compared to HBA solvents, whereas for (4) with
a hydroxy group, the reverse is the case. This may be attributed
to the differential stabilization of the ground state and Frank
Condon excited state of the hydrogen bonded structures (6) and
(7) of dyes 3 and 4, respectively.
Fig. 1. Plot of UV–vis absorption maxima of 2 vs. those of 3.
Further, the polarity change in each class of solvent has also
an adverse effect. With increasing polarity the λmax decreases,
which envisages the stability of the ground state species in polar
medium. Dye 2 with a methyl group does not experience such
type of solvent effect. The influence of substituent on the λmax
values of 2–5 maintains a trend with that of Hammett substituent
Fig. 2. Plot of UV–vis absorption maxima of 2 vs. those of 4.

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M. Panigrahi et al. / Spectrochimica Acta Part A 68 (2007) 757–762
Fig. 3. Plot of UV–vis absorption maxima of 2 vs. those of 5.
for dyes 2 and 3. The colonies of points in Fig. 2 may propose
a difference in structure of 4 in presence of solvents. Dye 4
having a hydroxyl substituent, assumes a structural change due
to the presence of hydrogen bond donor–acceptor solvents. In
the presence of these solvents the dye can have some extent of
merocyanine characteristics and thus the trend in the absorption
maxima changes with respect to dye 2. However, Fig. 3 produces
a scattergram of the relationship of absorption maxima of 2 and
5 in different solvents. This result may be attributed to a differen-
tial solvation of the dye 5. The dye 5 with two electron-releasing
groups (phenyl and methyl) connected in parallel does not have
similar characteristics, when those are connected serially (2).
Thus the vectorial electron flow from both the electron-releasing
groups may have different type of solvation effect on the absorp-
tion maxima. Further, the chromophoric group may deviate from
planarity due to the steric interaction between the methyl group
and 3-H of pyridinium nucleus (8).
Fig. 4. Plot of ET(30) vs. ν¯max of dye 2.
Fig. 5. Plot of ET(30) vs. ν¯max of dye 3.
3.2. Quantitative structure-solvent-spectra relationship
To have a quantitative picture of the above observations the
wave number (ν¯max) values at λmax of the dyes in various sol-
vents have been correlated with solvent parameters. The ET(30)
parameter developed by Reichardt [24] is reported to be the
most appropriate to explain the electronic transition of various
organic compounds. The parameter has been generated from a
pyridinium N-phenolate betaine dye. Because of the analogy
of functional group of the dyes (2–5) with the betaine, ET(30)
parameter has been considered to explain the effect of solvents.
For all the compounds, ν¯max observed in nonpolar solvents
exhibit a linear relationship with the polarity scale (Figs. 4–7).
Similar is the trend in the relationship for hydrogen acceptor
solvents. However, the increasing trend with polarity is less sig-
nificant for the later class of solvents when compared to the
former. This trend may be ascribed to the differential contri-
bution of the two mesomeric structures (as example 3 and 9)
to the ground state and excited state. During excitation, differ-
ential intramolecular charge transfer steers the population of
Fig. 6. Plot of ET(30) vs. ν¯max of dye 4.
Fig. 7. Plot of ET(30) vs. ν¯max of dye 5.

M. Panigrahi et al. / Spectrochimica Acta Part A 68 (2007) 757–762
761
Scheme 1. Qualitative representation of the solvent effect on the stability of ground and excited states of the styrylpyridinium dyes (compound 3 is reported as a
representative dye).
(−2.97 0.39) × 103 while for the dipolar aprotic solvent and
both the mesomers. Accordingly the differential solvation of the
species results in shift of absorption maxima. In polar solvents
hydroxylic solvents the values come to (−1.2 0.34) × 103 and
(0.5 0.04) × 103, respectively. Thus, the extent of contribution
the increasing trend suffers a reversal at an ET(30) value of 49.4.
The decreasing trend is mostly due to hydroxylic solvents. The
hypsochromic shift with increasing polarity may be ascribed to
the stability of the ground state of the dye (Scheme 1).
Assuming (i) the contribution of nonpolar solvent to the sta-
bility of excited state and (ii) that of hydroxylic solvent to the
stability of ground state are 100 percent each, the extents of con-
tribution of dipolar aprotic solvents to both the states have been
calculated from the sensitivity (α) of ET(30) towards ν¯max (Eq.
(1)). In Eq. (1), β is the ν¯max in a hypothetical environment with
‘0’ ET(30) value.
of the dipolar aprotic solvent being 49.2%. The reversal in sol-
vatochromism is observed at an ET(30) value of 50.0 with a
transition from dipolar aprotic solvents to hydroxylic solvents.
3.3. Reversal of solvatochromism
Earlier we have reported a reversal of solvatochromism for a
series of bischromophoric styrylpyridinium dyes with hydroxyl
substituents in the styryl unit at an ET(30) scale of 49.7 kcal/mol
with a transition from hydrogen-bond acceptor to donor system
[15]. The result was ascribed to the interaction of the hydroxyl
group of the dyes with the hydrogen-bonding group of the sol-
vent. Interestingly the trend of solvatochromism in the dye
4 suffers a reversal at ET(30) of 37.6 with a transition from
nonpolar to polar solvents. Further the sensitivity of solvent
polarity is less for hydroxylic solvents than for dipolar aprotic
solvents.
ν¯max = αET(30) + β
(1)
For three nonpolar solvents (hexane, tetrachloromethane and
toluene) the ‘α’ and ‘β’ are found to be (−2.07 0.001) × 103
and (345.5 0.003) × 103, respectively. Similarly for hydrox-
ylic solvents the values are found to be (0.68 0.1) × 103 and
(239.8 5.8) and for dipolar aprotic solvents the values being
(−0.96 0.26) × 103 and (319.6 11.2) × 103, respectively.
By considering the sensitivity of the ET(30) scale towards
the ν¯max, the extent of contribution of the dipolar aprotic
solvent to the stability of the excited species can be calculated
All the above observation may lead to the following propo-
sition for solvation.
• In a nonpolar solvent-cage the effect of change in polarity of
the solvent is more when compared to that in a polar sol-
vent cage. The polar solvents having p-orbitals may have
secondary interactions with the p-orbitals of the dye, which
probably masks the sensitivity of the polar effect of the solvent
on the electronic transition of the dye.
• The dipolar aprotic and hydroxylic solvents have solvent
cages with almost similar polarity. However the reversal may
be attributed to the interaction of polar functional group of the
solvent with the quaternary nitrogen of the pyridinium ring.
• For hydroxylic dye, the hydrogen bond acceptor characteristic
of dipolar aprotic solvent on the hydroxy group makes the
as = 100 × (αdipolaraprotic − αhydroxylic)/(αnonpolar − αhydroxylic
)
= 59.8%.
Though the nonlinearity in the solvatochromic plots ques-
tions the generality of the ET(30) scale, the parameter can
successfully classify the solvents into three aforesaid classes
with respect to the solvation. The absolute sensitivity in nonpo-
lar solvent is more than hydroxylic solvent, which has almost
same sensitivity as that in dipolar aprotic solvents.
The absorption maxima of dye 3 in different solvents also
behave similarly as those of dye 2. The sensitivity of polar-
ity in nonpolar solvent towards ν¯max increases significantly

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M. Panigrahi et al. / Spectrochimica Acta Part A 68 (2007) 757–762
ground state more stable due to a partial negative charge on
the hydroxyl group (7). With increasing hydrogen-bonding
ability of the solvent, the ground state stabilizes more leading
to a hypsochromism. Thus the reversal in this case is observed
at ET(30) value of 37.6.
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• The three distinctly grouped solvents due to their polarity
stabilize the ground and excited species of the dyes 2 and 3.
Thus the groups of solvents provide specific solvent cages,
wherein the changes in polarity of the medium have effect on
the electronic transition to different extent.
• In dye 4, the polar solvents cause a structural change of the
dye (7) for which there is a difference in the polarity switch.
• The electron-releasing vector in dye 5 provides a parallel
electron flow from methyl group and phenyl group to the
pyridinium unit whereas dye 2 experiences an electron flow
in series connection of the phenyl and methyl groups. In anal-
ogy to inverse relationship of electron flow in parallel and
series connection, the ν¯max experience a reversal trend with
respect to ET(30) scale in 2 and 5.
[23] M. Charton, Prog. Phys. Org. Chem. 16 (1987) 287.
[24] C. Reichardt, Chem. Rev. 94 (1994) 2319;
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The authors thank the Department of Science and Technology
and the University Grants Commission, New Delhi, for financial
support through FIST and DRS programs, respectively.