Effect of PEG-200 and Tween-20 on photoisomerization of 1-alkyl-2-(arylazo)imidazoles in toluene

Article abstract of DOI:10.1016/j.saa.2012.08.052

The photoisomerization of 1-alkyl-2-(arylazo)imidazole, trans-to-cis, has been studied in the matrix of PEG-200 and Tween-20 in toluene medium by UV light irradiation. The trans and cis-isomers have different absorption spectra. The cis-to-trans isomeriza

Full text of DOI:10.1016/j.saa.2012.08.052

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Effect of PEG-200 and Tween-20 on photoisomerization
of 1-alkyl-2-(arylazo)imidazoles in toluene
Pallab Gayen a,b, Chittaranjan Sinha a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Raja Peary Mohan College, Uttarpara, Hooghly 712 258, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The photoisomerization of 1-alkyl-2-(arylazo)imidazole in presence of PEG-200 and Tween-20, in toluene
solution by UV light irradiation has been reported. The rate and quantum yields of trans-to-cis isomeri-
zation are decreased in presence of microemulsion. The cis-to-trans isomerization is carried out by ther-
mal route. The activation energy (Ea) follows as free state > PEG-200-phase > Tween-20-phase. The
branched polyhydroxo structure of Tween-20 may help to wrap the polar photochrome more efficiently
than major ether functionalized PEG-200 and stabilizes trans-isomer.
"
Photoisomerization of 1-alkyl-2-
(arylazo)imidazoles, trans-to-cis, in
PEG-200 and Tween-20 is reported.
The rate and quantum yields
increase in emulsion and follows
free state < PEG-200-phase < Tween-
20-phase.
The reverse, cis-to-trans has been
carried out by thermal process.
The Ea of cis-to-trans isomerization
follows: free state > PEG-200-
phase > Tween-20-phase.
"
0.5
"
"
0.5
decreasing
trans isomer
0.4
0.4
0.3
0.2
331 nm
cis isomer
0.1
0.3
0.0
300 350 400 450 500 550
Wavelength(nm)
0.2
PaiMe in presence of Tween20(1%)
428 nm
0.1
0.0
300
350
400
450
500
550
Wavelength(nm)
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2012
Received in revised form 11 August 2012
Accepted 20 August 2012
Available online 27 August 2012
The photoisomerization of 1-alkyl-2-(arylazo)imidazole, trans-to-cis, has been studied in the matrix of
PEG-200 and Tween-20 in toluene medium by UV light irradiation. The trans and cis-isomers have differ-
ent absorption spectra. The cis-to-trans isomerization proceeds slowly in visible light irradiation while it
is appreciably fast in thermal process. The rate of trans-to-cis isomerization is decreased by 30–60% in
presence of PEG-200 and Tween-20. The quantum yield of the photoisomerization is also decreased by
35–55% and follows the rate sequence: free state > PEG-200-phase > Tween-20 phase. The activation
energy (Ea) of cis ? trans, thermal backward isomerization, is reduced in PEG-200 and Tween-20 phase
following free state > PEG-200-phase > Tween-20-phase. The branched polyhydroxo structure of
Tween-20 may help to wrap the polar photochrome more efficiently than major ether functionalized
PEG-200 and stabilizes trans-isomer.
Keywords:
Photochromism
Arylazoimidazole
PEG-200
Tween-20
Ó 2012 Elsevier B.V. All rights reserved.
Activation energy
Introduction
electrochemical etc., properties of the active molecular units have
sharply been influenced in the microenvironment of molecular
The ordered molecular aggregates are of much interest because
of their special properties and possible technological applications
[1–3]. The physical, chemical, luminescence, catalytic, biological,
aggregates [4–6]. Photochromism, a reversible photo-induced
transformation between two molecular states whose absorption
spectra differ significantly [7,8], has been influenced by microenvi-
ronments in solution [9–13]. The reversible photoisomerization
has enormous applications for controlling the properties of func-
tional polymeric materials [14], useful for the photo-control of
⇑
Corresponding author. Tel.: +91 94 3362 1872; fax: +91 33 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.08.052

P. Gayen, C. Sinha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
117
reagent grade collected from SRL, India. PEG-200 (Polyethylene
glycol) and Tween-20 were collected from Sigma–Aldrich. All other
chemicals and solvents were reagent grade as received.
Physical measurements
Spectroscopic data were obtained using the following instru-
ments: UV–Vis spectra from Perkin Elmer Lambda 25 spectropho-
tometer; photoexcitation has been carried out using Perkin Elmer
LS-55 spectrofluorimeter.
Scheme 1. Isomerization of 1-alkyl-2-(arylazo)imidazole (m > m
/).
Photometric measurements
biomolecules [15] and for novel applications such as data storage
[16,17] and self-assembling systems [18,19]. We have been inves-
tigating the trans-to-cis photoisomerization of 1-alkyl-2-(ary-
lazo)imidazoles and their metal complexes [20–27]. The
influence of microenvironment on the photoisomerization has
been studied from different aspects and has mainly been focused
to explain the mechanism of the trans-to-cis conversion [28]. The
solvent properties (density, polarity, viscosity, hydrogen bonding
ability, van der Waals, electrostatic activity etc.) and the presence
of noninteracting (innocent) and interacting (noninnocent) foreign
molecules have controlled the isomerization rate and quantum
yields [9–13,29,30]. In this work we report the effect of microenvi-
ronment originated by adding PEG-200 (polyethylene glycol) and
Tween-20 on the photoisomerization of four different 1-alkyl-2-
(arylazo)imidazoles. Only a few reports have been published to
count the effect of micelle on the photochromic activity of active
molecules [11–13] while the photochromic molecules solubilized
in micelles have been suggested as probes for the study of the
hydrodynamics of fluids [31].
Absorption spectra were taken with a Perkin Elmer Lambda 25
UV/VIS Spectrophotometer in a 1 ꢀ 1 cm quartz optical cell main-
tained at 25 °C. The light source of a Perkin Elmer LS 55 spectroflu-
orimeter was used as an excitation light, with a slit width of 10 nm.
An optical filter was used to cut off overtones when necessary. The
absorption spectra of the cis isomers were obtained by extrapola-
tion of the absorption spectra of a cis-rich mixture for which the
composition is known from 1H NMR integration. Quantum yields
(/) were obtained by measuring initial trans-to-cis isomerization
rates (
using the equation,
v) in a well-stirred solution within the above instrument
m
¼ ð/I0=VÞð1 ꢁ 10ꢁAbs
Þ
where I0 is the photon flux at the front of the cell, V is the volume of
the solution, and Abs is the initial absorbance at the azobenzene (/
= 0.11 for
condition.
p–
p⁄ excitation [33]) under the same irradiation
Experimental
Results and discussions
Reagents/materials
The compounds
1-Alkyl-2-(arylazo)imidazoles were synthesized by reported
The photoisomerization (Scheme 1) of 1-methyl-2-(pheny-
procedure [32] Imidazole, aniline, p-toluidine were of Analytical
lazo)imidazole (Pai-Me, 1a), 1-methyl-2-(p-tolylazo)imidazole
N
N
N
N
N
N
N
N
Me
Et
Et
Me
N
N
N
N
N
N
N
N
Me
Tai-Me
Me
Tai-Et
Pai-Me
Pai-Et
HO
O
H
n
PEG-200 (Polyethylene glycol 200)
O
O
O
W
O
OH
O
X
OH
OH
O
O
W+X+Y+Z=20
Z
Y
Polyoxyethylene (20) sorbitan monolaurate (Tween 20)
Scheme 2. Ligands, PEG-200 and Tween-20 used in this work.

118
P. Gayen, C. Sinha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
decreasing
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
trans isomer
cis isomer
300
350
400
450
500
550
Wavelength(nm)
330 nm
TaiMe in presence of PEG(1%)
442 nm
300
350
400
450
500
550
Wavelength(nm)
Fig. 1. Photochromism of Tai-Me in toluene upon UV light irradiation at 363 nm at
3 min interval at 25 °C.
Fig. 2. Spectral changes of Tai-Me in toluene solution in presence of PEG-200 (1%)
upon repeated irradiation at 364 nm at 3 min interval at 25 °C.
Table 1
trans-to-cis photoisomerization are determined using azobenzene
as a standard and the results are given in Table 1. The trans-to-
cis photoisomerization has proceeded and the cis molar ratio is
reached to >75%. The absorption spectra of the trans-ligands chan-
ged with isosbestic points upon excitation (Figs. 2 and 3) into the
cis-isomer. The quantum yields were measured for the trans-to-
cis (/t?c) photoisomerization of these compounds in toluene on
irradiation of UV light (Table 1). The quantum yields, /t?c, follow
the sequence of Pai-Me > Tai-Me; Pai-Et > Tai-Et; Pai-Me > Pai-Et
and Tai-Me > Tai-Et. In general, the increase in mass of the mole-
cule reduces the rate of photoisomerization, trans-to-cis (Table 1).
The microenvironment of photochrome controls the rate of
photoisomerization and quantum yield of the progress. The cova-
lent and non-covalent interaction amongst the molecules may
influence physical, photophysical, catalytic, electrochemical prop-
erties. Molecular and supramolecular interactions may regulate
photochromic activity of these molecules. In this work we use
PEG-200 (polyethylene glycol 200, Mwt. 190–210) and Tween-20
as emulsifier in toluene. PEG in different molecular weights have
been used in different physical, biological and chemical processes
such as the molecular repairing of nerve membranes, repair of mo-
tor neurons damaged in crush or laceration incidence in vivo and
in vitro, and also have been used to concentrate viruses. They pro-
duce different types of interaction with the incoming species, viz.,
electrostatic, hydrophobic, and charge transfer. These interactions
can alter the rate, quantum yield, mechanism, and the reaction sit-
uations. Photochromic substances are encapsulated in the hydro-
phobic phase of the microemulsion (PEG-200 and Tween-20) that
has changed the potential energy of the analyte (photochrome).
It is assumed that the analyte in this phase may have higher sus-
ceptibility to the structural change, dipole moment, energy of
molecular orbitals etc. due to reduction in molecular association
that is feasible in the bulk [34,35].
Excitation wavelength ðkꢂ Þ, rate of trans (t) ? cis (c) conversion and quantum yield
p;p
(/t?c) of photoisomerization of RaaiR/ in presence of PEG-200 and Tween-20 toluene.
Compd Microemulsion kꢂ ðnmÞ Isobestic
Rate of t ? c
/
t?c
p;p
point
(nm)
conversion ꢀ 108
(sꢁ1
)
Pai-Me None
Tai-Me
Pai-Et
Tai-Et
Pai-Me PEG-200 (1%)
Tai-Me
363
365
364
365
360
360
358
360
333,431
331,443
333,434
328,444
334,433
330,442
330,436
331,444
331,428
332,440
341,409
325,441
5.06
3.70
4.83
4.59
2.91
2.58
2.83
2.40
2.69
2.17
2.53
1.87
0.250
0.218
0.241
0.206
0.157
0.129
0.150
0.119
0.133
0.110
0.122
0.096
Pai-Et
Tai-Et
Pai-Me Tween-20 (1%) 357
Tai-Me
Pai-Et
Tai-Et
355
356
357
(Tai-Me, 1b), 1-ethyl-2-(phenylazo)imidazole (Pai-Et, 2a) and
1-ethyl-2-(p-tolylazo)imidazole (Tai-Et, 2b) in the matrices of
PEG-200 and Tween-20 are studied in this work (Scheme 2).
Photoisomerization, trans ? cis conversion
The absorption spectra are recorded in toluene solution in 200–
900 nm. The spectra show an absorption band around 370–390 nm
with a molar extinction coefficient on the order of 104 Mꢁ1 cmꢁ1
and a tail extending to 450 nm. It is likely that the high energy
intense band corresponds to
p–
p⁄ transitions, while the tail is
defined to n–p⁄ transition. The assignment is supported by theo-
retical calculations, as described elsewhere [20].
The irradiation of UV light to the toluene solution of the com-
pound changes the absorption spectrum as shown in Fig. 1 and re-
peated up to 15 cycles the absorption modulation remains
unchanged that signifies photo-stability of the molecule (Supple-
mentary materials, Fig. S1). The intense peak at kmax decreases,
which is accompanied by a slight increase at the tail portion of
the spectrum around 490 nm until a stationary state is reached.
The irradiation at the newly appeared longer wavelength peak re-
verses the course of the reaction and the original spectrum is
recovered but needs very long time. The quantum yields of the
The UV light is irradiated to solution of 1-alkyl-2-(arylazo)imid-
azole in toluene in presence of PEG-200/Tween-20 (1% solution)
and the rate of photoisomerization changes are recorded in Table 1
and Figs. 2 and 3. PEG-200/Tween-20 affects the ground and ex-
cited state of the photochrome [35]. The
chrome has been shifted to shorter wavelength (
emulsion phase compared to free state. This signifies the better
stabilization of ground
-state than excited state (p⁄) in the
emulsion phase. The change in solvent polarity upon addition of
p
–
p⁄ transition of photo-
k = 3 – 9 nm) in
D
p

P. Gayen, C. Sinha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
119
0.5
0.4
0.3
0.2
0.1
0.0
0.35
0.35
trans isomer
cis isomer
0.5
0.4
0.3
0.2
0.1
0.0
decreasing
0.30
0.25
0.20
0.15
0.10
0.05
0.00
trans isomer
0.30
0.25
0.20
0.15
0.10
0.05
0.00
328 nm
331 nm
cis isomer
300 350 400 450 500 550
Wavelength(nm)
300 350 400 450 500 550
Wavelength(nm)
increasing
PaiMe in presence of Tween20(1%)
428 nm
452 nm
PaiMe in presence of PEG(1%)
300
350
400
450
500
550
Wavelength(nm)
300
350
400
450
500
550
Wavelength(nm)
Fig. 4. Spectral changes of Pai-Me during thermal process of cis (c) ? trans (t) in
toluene solution in presence of PEG-200 (1%) upon repeated irradiation at 362 nm
at 3 min interval at 30 °C.
Fig. 3. Spectral changes of Pai-Me in toluene solution in presence of Tween 20 (1%)
upon repeated irradiation at 362 nm at 3 min interval at 25 °C.
Table 2
Rate and activation parameters for cis (c) ? trans (t) thermal isomerization.
c
Compd
Pai-Me
Co-factor
None
Temp. (K)
Rate of thermal c ? t conversion ꢀ 105 (sꢁ1
)
Ea (kJ molꢁ1
)
D
H⁄ (kJ molꢁ1
)
D
S⁄ (J molꢁ1 Kꢁ1
)
D
G⁄ (kJ molꢁ1
)
298
303
313
323
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
298
303
308
313
0.22
0.40
0.88
2.75
0.73
1.20
2.60
3.70
0.33
0.50
0.97
1.80
0.64
0.98
1.87
3.40
2.17
3.07
4.11
5.19
2.68
3.42
4.40
5.33
2.41
3.19
4.20
5.27
2.71
3.47
4.28
5.24
2.59
3.38
4.36
5.31
2.87
3.49
4.29
5.31
2.72
3.42
4.41
5.29
79.0
77.05
85.03
84.33
87.08
42.60
33.38
38.15
31.41
34.84
30.26
32.37
ꢁ77.1
100.00
96.60
98.58
97.06
93.81
93.34
93.60
93.30
93.41
93.21
93.30
Tai-Me
Pai-Et
Tai-Et
Pai-Me
Tai-Me
Pai-Et
Tai-Et
Pai-Me
Tai-Me
Pai-Et
87.57
86.87
87.63
45.14
35.92
40.69
33.95
37.38
32.80
34.91
ꢁ38.84
ꢁ47.83
ꢁ40.20
PEG-200 (1%)
ꢁ171.86
ꢁ201.22
ꢁ186.08
ꢁ207.71
ꢁ196.55
ꢁ211.25
ꢁ204.49
Tween 20 (1%)
(continued on next page)

120
P. Gayen, C. Sinha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
Table 2 (continued)
c
Compd
Tai-Et
Co-factor
Temp. (K)
Rate of thermal c ? t conversion ꢀ 105 (sꢁ1
2.96
3.51
4.33
5.29
)
Ea (kJ molꢁ1
)
D
H⁄ (kJ molꢁ1
27.71
)
D
S⁄ (J molꢁ1 Kꢁ1
ꢁ219.48
)
D
G⁄ (kJ molꢁ1
)
298
303
308
313
30.25
93.11
emulsion is exhibited by the shifting of
p
–
p⁄ transition to shorter
composition of the photochrome and the solution structure. The
rate of photoisomerization, trans-to-cis follows the order free sta-
te > PEG-200-phase > Tween-20 phase while thermal isomeriza-
tion cis-to-trans shows reverse sequence. The activation energy
calculated for thermal isomerization supports the sequence, free
state > PEG-200-phase > Tween-20 phase.
wavelength and is defined the solvatochromism [36]. The results
(Table 1) show that the rate of trans-to-cis photoisomerization is
decreased by 30–60% and the quantum yields are reduced by 35–
55% compare to the free ligand data (Table 1). This suggests that
the photochrome in emulsion phase become rigid and trans-isomer
is more stable than free ligand phase towards the light irradiation.
The extent of trans-to-cis photoisomerization under UV-irradia-
tion has been controlled by the compartmentalization of the chro-
mophore in the host. PEG-200 and, Tween-20 have large number of
–OH groups and may associate about polar centre of photochrome
that leads to stabilize the entrapped azoimidazole [37]. Because of
branching of Tween-20 on comparing with straight structure of
PEG the photochrome is encapsulated more rigidly by former and
the rate and quantum yields of photoisomerization follows the
decrement order: free state > PEG-200-phase > Tween-20 phase
(Table 1).
Acknowledgements
Financial support from Department of Science & Technology
New Delhi is thankfully acknowledged. We thank Mr. Mukul Bik-
ash Maity for his support to carry out some preliminary work dur-
ing M.Sc. Project work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.08.052.
Thermal isomerization, cis ? trans
References
1-Alkyl-2-(arylazo)imidazoles are stable at trans geometry at
ambient condition. Upon UV light irradiation they have been isom-
erised to cis configuration. The cis-isomer may reversibly trans-
formed, trans ? cis-isomer (>75%) when the solution is irradiated
with visible light at 455 nm. The process is very slow. While the
thermal change at dark is relatively fast (Fig. 4) and the reaction
rate is characterized by a single exponential function. The unimo-
lecular rate constant of the thermal backward isomerization (k) has
been calculated by the analysis of the measured absorbance, Abs(t),
Abs(t) = (AbstꢁAbsE)eꢁkt + Absc where Absc and Abst are absor-
bances corresponding to pure samples of c (cis) and t (trans) forms,
respectively. The thermal cis-to-trans isomerization rates were
obtained by monitoring absorption changes intermittently for a
cis-rich solution kept in the dark at constant temperature (T) in
the range 298–313 K. The activation energy (Ea) and the frequency
factor (A) were obtained from ln k = ln A–Ea/RT, where k is the mea-
sured rate constant, R is the gas constant, and T is temperature. The
[1] A.S. Tatikolov, G. Ponterini, Zh.A. Krasnaya, Int. J. Photoenergy 2 (2000) 17.
[2] M. Mishra, G.B. Behra, M.M.G. Krishna, N. Periasamy, J. Lumin. 92 (2001) 175.
[3] N.C. Maiti, S. Mazumdar, N. Periasamy, J. Phys. Chem. B 102 (1998) 1528.
[4] X.-G. Lei, G.-H. Zhao, Y.-C. Liu, N.J. Turro, Langmuir 8 (1992) 475.
[5] S. Mazumdar, S. Mitra, Struct. Bonding 81 (1993) 115.
[6] T.A. Fayed, S. El-Din, H. Etaiw, N.Z. Saleh, J. Lumin. 121 (2006) 431.
[7] H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems,
Elsevier, Amsterdam, 2003.
[8] B. Feringa, Molecular Switches, Wiley-VCH, 2001.
[9] E. Hadjoudis, V. Verganelakis, C. Trapalis, G. Kordas, Mol. Eng. 8 (1999) 459–
469.
[10] R. Gomes, A.J. Parola, C.A.T. Laia, F. Pina, Photochem. Photobiol. Sci. 6 (2007)
1003.
[11] M.A. Zayed, G.G. Mohamed, S.A.M. Abdullah, Spectrochim. Acta, Part A: Mol.
Biomol. Spectrosc. 78 (2011) 1027.
[12] Z. Liang, Z. Liu, Y. Gao, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 68
(2007) 1231.
[13] N. Nijegorodov, V. Zvolinski, P.V.C. Luhanga, R. Mabbs, J. Ahmad, Spectrochim.
Acta, Part A: Mol. Biomol. Spectrosc. 65 (2006) 196.
[14] M. Irie, Y. Hirano, S. Hashimoto, K. Hayashi, Macromolecules 14 (1981) 262.
[15] A.A. Beharry, G.A. Woolley, Chem. Soc. Rev. 40 (2011) 4422–4437.
[16] M. Kamenjicki, I.K. Lednev, S.A. Asher, J. Phys. Chem. B 108 (2004) 12637.
[17] A. Archut, F. Vogtle, L. De Cola, G.C. Azzellini, V. Balzani, P.S. Ramanujam, R.H.
Berg, Chem. Eur. J. 4 (1998) 699.
values of the activation free energy (
D
G⁄) and the activation entro-
py (D
S⁄) were obtained through the relationships,
D
Gꢂ ¼ Ea ꢁ RT ꢁ T
D
Sꢂ and
D
Sꢂ ¼ ½ln A ꢁ 1 ꢁ lnðkBT=hÞ=R
[18] H. Sawada, T. Tsuzuki-ishi, T. Kijima, J. Kawakami, M. Iizuka, M. Yoshida, J.
Colloid Interface Sci. 359 (2011) 461.
[19] B.V. Shankar, A. Patnaik, Langmuir 22 (2006) 4758.
[20] J. Otsuki, K. Suwa, K. Narutaki, C. Sinha, I. Yoshikawa, K. Araki, J. Phys. Chem. A
109 (2005) 8064.
[21] J. Otsuki, K. Suwa, K.K. Sarker, C. Sinha, J. Phys. Chem. A 111 (2007) 1403.
[22] K.K. Sarker, S. Saha Halder, D. Banerjee, T.K. Mondal, A.R. Paital, P.K. Nanda, P.
Raghavaiah, C. Sinha, Inorg. Chim. Acta 363 (2010) 2955.
[23] K.K. Sarker, D. Sardar, K. Suwa, J. Otsuki, C. Sinha, Inorg. Chem. 46 (2007) 8291.
[24] K.K. Sarker, B.G. Chand, K. Suwa, J. Cheng, T.-H. Lu, J. Otsuki, C. Sinha, Inorg.
Chem. 46 (2007) 670.
where kB and h are Boltzmann’s and Plank’s constants, respectively.
The activation energy, Ea, of thermal isomerization was deter-
mined by fitting the temperature dependent k values (Supplemen-
tary material, Fig. S2; Table 2). Data reveal that the activation
energy (Ea) decreases in the emulsion phase. The thermal cis-to-
trans isomerization rate of the ligands are increased even by one
fold and the rate follows free ligand state < PEG-200-phase < Tw-
een-20 phase.
[25] P. Pratihar, T.K. Mondal, A.K. Patra, C. Sinha, Inorg. Chem. 48 (2009) 2760.
[26] G. Saha, P. Datta, K.K. Sarkar, R. Saha, G. Mostafa, C. Sinha, Polyhedron 30
(2011) 614.
[27] D. Mallick, A. Nandi, S. Datta, K.K. Sarker, T.K. Mondal, C. Sinha, Polyhedron 31
(2012) 506.
[28] T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T.J. Martınez, S. Lochbrunner, M.
Conclusion
´
Schmitt, J.P. Shaffer, M.Z. Zgierski, A. Stolow, J. Am. Chem. Soc. 125 (2003)
8098.
[29] L. Biczók, A. Cser, K. Nagy, J. Photochem. Photobiol. A: Chem. 146 (2001) 59.
[30] S. Kobatake, Y. Terakawa, H. Imagawa, Tetrahedron 65 (2009) 6104.
[31] A. d’Arco, J. Charmet, M. Cloitre, Rev. Phys. Appl. 17 (1982) 89.
The trans-to-cis isomerization of 1-alkyl-2-(arylazo)imidazoles
on UV-light irradiation is examined in presence of PEG-200 and
Tween-20 emulsifier. The reverse, cis-to-trans, is very slow by light
and susceptible to heat. The isomerization is dependent on the

P. Gayen, C. Sinha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 116–121
121
[32] T.K. Misra, D. Das, C. Sinha, P.K. Ghosh, C.K. Pal, Inorg. Chem. 37 (1998) 1672.
[33] G. Zimmerman, L. Chow, U. Paik, J. Am. Chem. Soc. 80 (1958) 3528.
[34] J. Schwemer, I.M. Pepe, R. Paulsen, C. Cugnoli, J. Comp. Physiol. A. 154 (1984)
549.
[36] P. Suppan, N. Choneim, Solvatochromism, Royal Chemical Society, Cambridge,
1997. p. 25.
[37] H. Sakai, H. Ebana, K. Sakai, K. Tsuchiya, T. Ohkubo, M. Abe, J. Colloid Interface
Sci. 316 (2007) 1027.
[35] F.M. Serra, M. Terentjev, Macromolecules 41 (2008) 981.