Influence of solvent polarity and medium acidity on the UV-Vis spectral behavior of 1-methyl-4-[4-amino-styryl] pyridinum iodide

Article abstract of DOI:10.1016/S1386-1425(02)00182-8

Electronic absorption, and excitation spectra of 1-methyl-4-[4-aminostyryl] pyridinum iodide (M-NH₂) were measured in solvents of different polarity. The (M-NH₂) dye exhibits negative solvatochromism, i.e. a hypsochromic band shift as the solvent polarity increases. The fluorescence quantum yield is also sensitive to the polarity and viscosity of the medium. The ground and excited state protonation constants were calculated and amount to 3.35 and 0.62, respectively. The effect of micellization on the emission spectrum of (M-NH₂) are also studied in sodium dodecyl sulphate (SDS). The fluorescence intensity increases as the concentration of SDS increases with an abrupt change at cmc. The quantum yield of the cis trans photoisomerization is also determined in aqueous buffer solution of pH 1.1.

Full text of DOI:10.1016/S1386-1425(02)00182-8

Spectrochimica Acta Part A 59 (2003) 405Á411
/
www.elsevier.com/locate/saa
Influence of solvent polarity and medium acidity on the
UVÁ
/
Vis spectral behavior of 1-methyl-4-[4-amino-styryl]
pyridinum iodide
Samy A. El-Daly ꢀ, Mahmoud H. Abdel-Kader, Raafat M. Issa,
El-Sayed A. El-Sherbini
Chemistry Department, Faculty of Science, Tanta University, Tanta Egypt
Received 10 October 2001; received in revised form 30 April 2002; accepted 2 May 2002
Abstract
Electronic absorption, and excitation spectra of 1-methyl-4-[4-aminostyryl] pyridinum iodide (MÁ
/NH ) were
2
measured in solvents of different polarity. The (MÁNH ) dye exhibits negative solvatochromism, i.e. a hypsochromic
/
2
band shift as the solvent polarity increases. The fluorescence quantum yield is also sensitive to the polarity and viscosity
of the medium. The ground and excited state protonation constants were calculated and amount to 3.35 and 0.62,
/
2
respectively. The effect of micellization on the emission spectrum of (MÁNH ) are also studied in sodium dodecyl
sulphate (SDS). The fluorescence intensity increases as the concentration of SDS increases with an abrupt change at
cmc. The quantum yield of the cis trans photoisomerization is also determined in aqueous buffer solution of pH 1.1.
#
2002 Elsevier Science B.V. All rights reserved.
Keywords: Influence of solvent on spectra; Spectral studies of 1-methyl-4-[4-aminostyryl] pyridinum iodide
1
. Introduction
In the present article, it is attempted to study the
effect of medium polarity, acidity, and viscosity on
Merocyanine dyes are well known as solvato-
the UVÁ
yield of a new merocyanine dye, namely 1-methyl-
4-[4-aminostyryl] pyridinum iodide (MÁNH ).
/
Vis spectra and fluorescence quantum
chromic and photoreactive compounds which are
also sensitive to the medium acidity [1]. Due to
these properties they are potentially useful in many
areas such as solar energy conversion [2,3], photo-
sensitizers [4], phototherapy [5], in laser doublers
and as liquid crystals [6]. Merocyanine dyes are
also useful as electrochromic compounds for
membrane potentials and high-voltage sensitivity
/
2
[
7,8].
ꢀ
Corresponding author. Tel.: ꢁ
/
20-4075-2734
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 8 2 - 8

406
S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á411
/
2
. Experimental
-Methyl-4-[4-aminostyryl] pyridinium iodide
1
was synthesized by condensation of 4-aminoben-
zaldehyde with 1-methyl-picolinium iodide for 2 h.
The brown precipitate was filtered and recrysta-
lized several times from water containing traces of
KOH. The purity of the dye was checked by thin-
layer chromatography on silica gel 60 and by
elemental analysis.
Organic solvents (Fluka, puriss) were of spectro-
scopic grade and used without further purification.
For the pKa determinations, KH PO buffer
2
4
Fig. 1. UVÁ
/
Vis absorption spectra of MÁ
/
NH
2
dye (cꢂ
/
3ꢃ
/
(
Merck) and NaOH were used to prepare titrisol
buffer according to Britton [9].
UVÁVis absorption spectra were recorded on a
PerkinÁElmer Lambda 17 Spectrophotometer
ꢄ5
ꢄ3
1
0
mol dm ) in nine different solvents.
/
/
a concentration of cꢂ
/
3ꢃ
l0ꢄ5 mol dmꢄ3 in 11
/
using matched 1 cm quartz cells; the cell holder
was equipped for variable temperature thermo-
state by using Julbo Thermo-Bath u3. Steady-state
emission spectra were recorded on a Shimadzu
RF-5000 Spectrofluorometer using 1 cm quartz
cells. Fluorescence intensities were measured at
right angle to the exciting light. Fluorescence
solvents of different polarity. As shown in Figs. 1
and 2, the solvent polarity influences strongly the
position of the electronic absorption and emission
maxima. The hypsochromic band shifts in the
electronic absorption and emission spectra of MÁ
/
quantum yields (f ) were measured relative to
f
the merocyanine dye [10] in basic media, using, an
absorbance of less than 0.06 to avoid reabsorption
of the emitted light. The following relation has
been applied to calculate the fluorescence quantum
yields relative to those of the merocyanine dye in
basic media [11]:
Is( n¯ )d n¯
Ir( n¯ )d n¯
2
f (s)
g
g
Ar ns
ꢃ
f
ꢂ
ꢃ
2
r
f (r)
As
n
f
The integral represents the corrected fluores-
cence peak areas, A is the absorbance at the
excitation wavelength, n is the solvent refractive
index, the subscripts s and r refer to the sample
and reference, respectively.
3
. Results and discussion
3
.1. Influence of medium polarity and viscosity
Fig. 2. Emission spectra of MÁ
/
NH
2
dye (cꢂ
/
3ꢃ
10ꢄ5 mol
dm ) in nine different solvents (excited at the corresponding
/
ꢄ3
The UVÁ/Vis absorption, emission, and excita-
tion spectra of the (MÁ/NH ) dye were measured at
2
visible absorption maximum lmax of the dye in each solvent).

S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á
/411
407
Table 1
2
UVÁVis spectral data of MÁNH , measured in 11 solvents and some solvent properties
/
/
Solvent
Z
b
pꢀ
DE
T
l
max (Ab)
lmax (Em)
l
exc.
HBD-A
1
2
3
4
. H
2
O
94.6
85.1
83.6
76.3
0.14
0.51
0.62
0.92
1.09
71.10
64.67
63.53
62.15
402
440
450
460
558
565
565
566
402
440
450
460
. 1,2-ethandiol
. MeOH
. 2-propanol
0.932
0.586
0.505
Aprotic HBA-solvents
5
6
7
. DMSO
. DMF
. Acetone
71.1
68.5
65.7
0.75
0.61
0.50
1.000
0.875
0.683
62.69
63.11
63.82
456
453
448
576
571
566
456
453
448
HBDA and HBD-solvents
8
9
1
1
. CH
. Pyridine
0. CH CI
1. CHCI
3
CN
71.3
64.0
64.2
63.2
0.40
0.66
0.10
0.10
0.713
0.867
0.820
0.58
65.72
61.22
62.69
64.53
435
467
456
443
563
578
552
523
435
476
456
443
2
2
3
NH2 with increasing solvent polarity of the
medium is expected and reflects a decrease of the
ground state dipole moment of the dye molecule
upon excitation and an increase of its ground-state
dipole moment as a result of solvent polarization
such as water and alcohol, the interaction stabi-
lizes the benzenoind structure with a large charge
separation. It is expected that the enhancement
of the negative solvatochromism of MÁ
dye results from increasing acidity with decreasing
electron donating ability in the order of pyridinB
2-propanolBmethanolB1,2-ethandiolBwater.
/
NH2
[
12Á
data of MÁ
HBA) basicity of solvents can be described by the
/
14]. Table 1 summarizes the UVÁ
/
Vis spectral
/
/
NH . The hydrogen bond acceptor
/
/
/
2
(
In the presence of the more polar solvent the
Kamlet and Taft’s b-scale, whereas the hydrogen
bond donor solvents acidity can be empirically
described by the coresponding a-scale [13,14].
The plot of the empirical solvent polarity
parameter (z) of Kosower [15,16] against the
electronic transition energy (E ) of the long-
T
wavelength Vis absorption band is shown in Fig.
3; the electronic transition energy increases with
increasing solvent polarity. The dramatic shifts in
electronic absorption spectra observed on going
ꢄ1
from pyridine ( max
ꢂ
/
467 nm, zꢂ
/
64 kcal mol
)
)
ꢄ1
to water ( max 402 nm, zꢂ
ꢂ
/
/
94.6 kcal mol
results from the solute solvent interaction; the
HBD solvents can reduce the delocalization of the
amino group lone pair of electrons and hence the
bond order of the central bond. It is proposed that
one of the main factors causing the solvatochromic
behavior is the hydrogen-bonding interaction
Fig. 3. Plot of the transition energy DEmax vs. Kosower’s Z
parameter for MÁ/NH2 in ten different solvents of variable
between the MÁ
/
NH lone pair of electrons and
2
the HBD solvent molecules. In the HBD solvents
polarity.

408
S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á
/411
Scheme 1.
probability of the conjugation increases and it may
decrease with decreasing the polarity [1]. This is
indicated by Scheme 1.
For the HBD solvent the dye acts as a proton
acceptor at the amino nitrogen group, this is
on the orientation of the dye within the solvation
cage which is much effective in hindering mole-
cular twisting [20] in 1,2-ethandiol than that in
acetone.
represented by the acidÁ
tures in Scheme 1. Fig. 4 shows the effect of the
medium basicity on the behavior of (MÁNH ) dye.
A negative solvatochromic behavior was found on
decreasing the basicity of the medium. This blue
shift increases with increasing solvent polarity and
/
base equilibrium struc-
3
.2. pK and pK determination
ꢀ
a a
/
2
The variation of the electronic absorption
ꢄ3
ꢄ5
spectra (cꢂ 10
with pH is represented in Fig. 5a; these spectra
exhibit a well-defined isosbestic point, indicating
the existence of an acidÁ
two species of MÁNH ; the first one in which the
lone-pair of electrons is free ( max
the second one is the protonated MÁ
/
3ꢃ
/
mol dm ) of MÁNH2
/
decreasing the basicity indicating that the S 0
/
S1
0
/base equilibrium between
transition of MÁ
/
NH dye is of p-pꢀ character [17].
2
/
2
The effect of solvent polarity and viscosity on
ꢂ402 nm) and
/
the fluorescence quantum yield (f ) of the MÁ
/
ꢁ
f
/
N H in
3
NH dye was studied. As gathered from Table 2
2
the fluorescence quantum yield (f ) of the MÁ
/
f
NH dye is strongly quenched in solvent of greater
polarity and/or low viscosity. The dramatic de-
2
crease in (f ) of MÁNH as the solvent polarity
/
f
2
increases, is observed on going from pyridine
f ꢂ0.24) to water (f ꢂ0.04). This indicates
(
/
/
f
f
clearly that a very efficient non-radiative decay
channel opens which is related to the charge-
transfer nature of the emitting state and which
must involve a further increase in the excited
solute dipolarity during the decay [18,19], On the
other hand, the fluorescence quantum yield (ff)
increases with increasing viscosity. For example, a
f value as high as 0.747 is obtained in 1,2-
f
Fig. 4. Linear relationship between DE
NH in different solvents 1-2-porpanol. 2-DMSO, 3-Pyridine,
-MeOH, 5-DMF, 6- l,2-ethandiol, 7-Acetone, 8-CH CN and
9-water.
T
and b-scal for MÁ
/
ethandiol (hꢂ
is obtained in acetone (hꢂ
explained by the effect of the solvent bulk viscosity
/19.9 cp.) but a low value of 0.155
2
/
0.316 cp.). This can be
4
3

S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á
/411
409
Table 2
3.3. Effect of micelle
Fluorescence quantum yield (f
some different solvents
f
/
2
) of (MÁNH ) dye measured in
The emission spectrum of MÁ
/
NH2 (cꢂ
/
3ꢃ
/
ꢄ5
ꢄ3
1
0
mol dm ) has been measured at different
Solvent
h
(CP)
f
f
concentrations of SDS (anionic micelle). The
fluorescence intensity increases substantially as
the surfactant concentration increases, with a
Water
0.890
0.345
0.547
0.316
0.920
0.940
2.200
1.770
19.900
0.043
0.110
0.120
0.155
0.193
0.246
0.272
0.445
0.747
3
CH CN
MeOH
Acetone
DMF
ꢄ3
break at a SDS concentration of cꢂ
/
8.1ꢃ10
/
ꢄ3
mol dm , that coincides well with the critical
micelle concentration (CMC) of SDS [23]. The role
of SDS micelles in enhancing fluorescence emis-
sion is explained in terms of the increased medium
Pyridine
DMSO
2
1
-propanol
,2-ethandiol
which the lone pair of electron is localized with a
second positive charge present on the other
heterocyclic nitrogen atom ( max
The pKa value has been calculated from the
ꢂ330 nm).
/
following equation [21].
E ꢄ Emin
pHꢂpK ꢁlog
a
Emax ꢄ E
where Emax is the maximum absorbance of the
ꢁ
protonated form MÁ
/
N H or MÁNH at a given
/
3
2
wavelength, Emin is the minimum absorption, from
the plot of log(EꢄEmin/Emax E) versus pH as
/
ꢄ
/
shown in Fig. 5b the pK was calculated as 3.35.
a
The effect of medium acidity on the emission
spectrum has been studied in aqueous buffer
solutions. As shown in Fig. 6, upon increasing
the pH value the fluorescence intensity at lꢂ
nm decreases and the spectral pattern also
changes. An isoemissive point is obtained at lꢂ
00 nm indicating an acidÁbase equilibrium in the
excited state, the protonation constant of the
excited state pKa was calculated as 0.62 by
/
440
/
4
/
ꢀ
applying the F o¨ rster cycle using the following
relation [22].
pK ꢄpKꢀꢂ2:1ꢃ10ꢄ3(n¯
ꢄ n¯ B)
ꢁ
a
a
BH
Fig. 5. (a) UVÁ
10
/
Vis absorption spectra of MÁ
/
NH
mol dm ) in aqueous buffer solutions (handled only in
dark). (b) Determination of pK value of MÁNH dye (cꢂ3ꢃ
mol dm ) in aqueous buffer solutions (lexc.ꢂ330 nm).
2
dye (cꢂ
/
3ꢃ
/
The quantities n¯ BH
ꢁ
and n¯ represent the wave-
numbers of pure electronic transitions of the acid
B
ꢄ5
ꢄ3
a
/
2
/
/
ꢄ5
ꢄ3
and conjugate base, respectively.
10
/

410
S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á411
/
Fig. 7. UVÁ
/
Vis absorption spectra of MÁ
/
NH
mol dm ) in buffer solution pH 1.1 at different times of
irradiation (lirr 366 nm).
2
dye (cꢂ
/
3ꢃ
/
ꢄ5
ꢄ3
1
0
ꢂ/
as ft0cꢂ
/
0.61 and fc0tꢂ0.49. At the photosta-
/
Fig. 6. Fluorescence spectra of MÁ
dm ) in aqueous buffer solutions lexc.
/
NH
2
dye (cꢂ
/
3ꢃ
/
10ꢄ5 mol
tionary state the proportion of cis to trans was
found to be 64:36, Steiner [25] has noticed that
there is an analogy in the structure of the proto-
nated merocyanine dye and stilbene in their
ꢄ3
ꢂ
/
330 nm).
viscosity accompanying micellization [23] as well
as the increased molecular dispersion of the
fluorescer with subsequent minimization of mole-
cular aggregation that causes emission quenching.
photochemical trans0
/
cis isomerization reaction
and in quantum yield. It was suggested that a
phantom state is present in the protonated form at
a 908 twisted conformation of the excited state by
which the deactivation process can occur to either
trans or cis ground state.
3
.4. Photoisomerization of MÁNH2
/
Upon irradiation of the MÁ
/
NH dye (cꢂ
/
3ꢃ
mol dm ) in aqueous buffer solution of pH
.1 at room temperature ( irrꢂ366 nm, I ꢂ5.3ꢃ
E cm s), the absorption spectrum changes
/
2
ꢄ5
ꢄ3
1
1
1
0
References
/
/
/
0
1
0
ꢄ2
0
in
[1] S.T. Abdel-Halim, M.K. Awad, J. Phys. Chem. 97 (1993)
3160 (and references there in).
and shows an isosbestic point, finally reaching a
photostationary state. The long-wave length ab-
sorption maximum of the photoproduct is dis-
placed to shorter wavelengths than that of the
trans-isomer. The photoproduct is assigned to the
cis-isomer in accordance with the results of a
[
[
[
2] W. Arden, P. Fromherz, J. Electrochem. Soc. 127 (1980)
72.
3
3] O. Inacker, H. Kuhun, D. M o¨ bius, G. Debuch, Z. Phys.
Chem. 101 (1976) 337.
4] K. Othmer, Encycl. Chem. Technol. 6 (1985) 122.
[5] F. Sieber, J.L. Spivak, A. Sutctiffe, Proc. Natl. Acad. Sci.
USA 81 (1984) 7584.
previous work [1], UVÁ
the trans and cis isomers in aqueous buffer, pH
.1, are shown in Fig. 7. By using the modified
Blance and Ross method [24], the quantum yield
of the trans0cis photoisomerization is calculated
/Vis absorption spectra of
[
[
6] D.J. Williams (Ed.), Nonlinear Optical Properties of
Organic and Polymeric Materials. ACS Symposium Series
1
233, American Chemical Society, Washington DC, 1983.
7] A. Grinwald, R.D. Frosting, E. Lieke, R. Hildesheim,
/
Phys. Rev. 8 (1985) 263.

S.A. El-Daly et al. / Spectrochimica Acta Part A 59 (2003) 405Á
/411
411
[
[
8] L.M. Loew, S. Saully, L. Simpson, A.S. Waggoner, Nature
81 (1979) 497.
[17] C. Reichardt (Ed.), Solvents and Solvent Effects in
Organic Chemistry, Second ed., VCH, Verlagsgesellschatt,
MBH, Weinheim, 1988, p. 285.
2
9] H.T.S. Britton, Hydrogen Ions, Second ed., Chapman &
Hall, London, 1952.
[18] J.D. Cowley, I. Pasha, J. Chem. Soc. Perkin Trans. 2
(1981) 918.
[
10] M.H. Abdel-Kader, U. Steiner, J. Chem. Educ. 60 (1983)
60.
1
[19] J.D. Cowley, E. Okame, R.S.T. Todd, J. Chem. Soc.
Perkin Trans. 2 (1991) 1495.
[
[
11] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 1001.
12] E. Lippert, F. Moll, Ber. Bunsenges. Phys Chem. 58 (1954)
[20] E.M. Ebeid, M.H. Abdel-Kader, R.M. Issa, S.A. El-Daly,
Chem. Phys. Lett. 146 (3.4) (1988) 331.
7
18.
13] M.J. Kamlet, J.L. Abbo u¨ d, R.W. Taft, J. Am. Chem. Soc.
00 (1978) 377.
14] M.J. Kamlet, J.L. Abbo u¨ d, R.W. Taft, J. Am. Chem. Soc.
9 (1977) 6027.
[
[
[21] J. Blance, D.L. Ross, J. Phys. Chem. 72 (1968) 2819.
[22] S.G. Schulman, in: E.L. Wehry (Ed.), Modern Fluores-
cence Spectroscopy, vol. 2, Plenum Press, New York, 1976.
[23] C.A. Bunton, Progr. Solid State Chem. 8 (1973) 293.
[24] J. Blance, D.L. Ross, J. Phys. Chem. 72 (1968) 2617.
[25] U. Steiner, M.H. Abdel-Kader, P. Fisher, H.E. Kramer, J.
Am. Chem. Soc. 100 (1978) 3190.
1
9
[
[
15] E.M. Kosower, J. Am. Chem. Soc 80 (1958) 3253.
16] E.M. Kosower, An Introduction to physical organic
chemisty, Wiley, NY, 1968.