Laser photolysis study of 1-aryl-3-methyl-5-(benzothiazole-2-yl) formazan phototransformation

Article abstract of DOI:10.1134/S0018143911010036

Photochromism of 1-aryl-3-methyl-5-(benzothiazole-2-yl)formazans in toluene has been investigated by the laser flash photolysis method. It was established that the photochromic transformations involve the triplet state of the initial chelate form of formazans. The intermediate short-lived products of phototransformation were detected that could be related to one of the conformers or to the tautomers of the formazan molecule. Pleiades Publishing, Inc., 2011.

Full text of DOI:10.1134/S0018143911010036

ISSN 0018ꢀ1439, High Energy Chemistry, 2011, Vol. 45, No. 1, pp. 52–56. © Pleiades Publishing, Inc., 2011.
Original Russian Text © L.S. Atabekyan, V.A. Barachevskii, S.A. Melkozerov, G.N. Lipunova, I.G. Perova, I.N. Lipunov, G.I. Sigeikin, 2011, published in Khimiya Vysokikh
Energii, 2011, Vol. 45, No. 1, pp. 55–59.
PHOTOCHEMISTRY
Laser Photolysis Study of 1ꢀArylꢀ3ꢀMethylꢀ5ꢀ(Benzothiazoleꢀ2ꢀyl)
Formazan Phototransformation
L. S. Atabekyana, V. A. Barachevskiia, S. A. Melkozerovb, G. N. Lipunovac,
I. G. Perovab, I. N. Lipunovb, and G. I. Sigeikind
a Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7a, bldg 1, Moscow, 119421 Russia
b Ural State Forest Engineering University, Sibirskii trakt 37, Yekaterinburg, 620100 Russia
c Postovskii Institute of Organic Synthesis, Ural Branch, Russian Academy of Sciences,
ul. Kovalevskoi 22, Yekaterinburg, 620002 Russia
d Interdepartmental Center of Analytical Research, Presidium of the Russian Academy of Sciences,
ul. Vavilova 44, bldg.2, Moscow, 119333 Russia
eꢀmail: barva@photonics.ru
Received June 30, 2010
Abstract—Photochromism of 1ꢀarylꢀ3ꢀmethylꢀ5ꢀ(benzothiazoleꢀ2ꢀyl)formazans in toluene has been invesꢀ
tigated by the laser flash photolysis method. It was established that the photochromic transformations involve
the triplet state of the initial chelate form of formazans. The intermediate shortꢀlived products of phototransꢀ
formation were detected that could be related to one of the conformers or to the tautomers of the formazan
molecule.
DOI: 10.1134/S0018143911010036
It is known that triarylformazans have photochroꢀ hydrogen bonding. Triphenylformazan and its
mic properties in nonpolar solvents, which manifest
themselves by reversible color changes of these comꢀ
pounds upon the action of visible irradiation [1–3].
analogues are the most studied with the phoꢀ
totransformation mechanism involving the photoꢀ
induced proton transfer followed by cis–trans
isomerization and the formation of the B form
The initial form (A) of these compounds has
EZZꢀconfiguration stabilized by intramolecular (Scheme 1).
H
Ph
Ph
N
N
N
N
Ph
N
Ph
N
h
ν
N
H
N
Δ
T
Ph
Ph
A
B
Scheme 1.
The absorption band of the B form shifts hypsochꢀ
romically by 90 nm in comparison with the form A
absorption band located at 495 nm [4, 5]. Spectral–
kinetic investigation of these compounds by the lamp
flash photolysis technique revealed that supposedly
the form B formation proceeds through the intermediꢀ
ate product resulting from the chelate form after the
proton phototransfer due to the E–Z isomerization of
1ꢀArylꢀ3ꢀphenylꢀ5ꢀ(benzothiazoleꢀ2yl)formazans
that exist as EZZꢀconformers in solution but with a
weaker hydrogen bond undergo the similar phoꢀ
totransformations [7].
The results of the laser flash photolysis investigaꢀ
tion of the phototransformation of 1ꢀarylꢀ3ꢀmethylꢀ
(benzothiazoleꢀ2ꢀyl)formazans FRM 1–FRM 4,
investigated earlier by the steadyꢀstate excitation
technique [8], are presented in this work. The initial
the ⎯N=N bond [6].
52

LASER PHOTOLYSIS STUDY OF 1ꢀARYLꢀ3ꢀMETHYLꢀ5ꢀ(BENZOTHIAZOLEꢀ2ꢀYL)
53
A
Δ
A
0.02
2
0.6
1
0.5
0.4
0.3
0.2
0.1
0
0
–0.02
–0.04
–0.06
–0.08
300
350
400
450
500
550
λ
, nm
400
500
600
700
, nm
λ
Fig. 2. Differential absorption spectrum of the FRM 1 comꢀ
Fig. 1. Absorption spectra of FRM 1 formazan in toluene
pound in toluene after the excitation with the laser pulse at
4
–5
before (
1) and after (
2
) irradiation (
c
= 4
×
10⎯ mol/l).
λ
= 354 nm detected after
τ
= 1 × 10 s after pulse.
form of these compounds is presented as EEZꢀconꢀ reversible photoinduced isomerization of the C=N
former, and their photochromism is due to the bond (Scheme 2).
N
R
R
H
S
N
N
N
h
ν
N
N
N
Δ
T
S
N
N
H
N
CH3
CH3
A
B
FRM 1: R = H; FRM 2: R = CH3; FRM 3: R = Br; FRM 4: R = NO2
Scheme 2.
EXPERIMENTAL
energy 70 mJ,
for the measurements with the excitation
allows to change the wavelength generated by a laser in
a wide range. The system for the registration of interꢀ
λ
= 354 nm). Parametric generator was used
= 400 nm that
λ
Toluene solutions of the 3ꢀmethylbenzothiazolylꢀ
formazans with different substituents (Scheme 2) were
used. The measurements were performed at room
temperature in a standard quartz cell
compound concentrations of = (1–2)
in a 1.01ꢀmm quartz cell at concentrations of
(1 4)
10–4 mol/l. Bubbling of the solution with
nitrogen was used for the removal of air oxygen. Sevꢀ
eral measurements were performed in the presence of
benzophenone.
1
×
1
×
4 cm at mediate products included testing light source—
c
×
10–5 mol/l or
xenon lamp Osramꢀ150 with a power supply block
(Applied Biophysics), double monochromator
MDR6, emission detector—PMTꢀ84, digital oscilloꢀ
scope Tektronix 3032B (or oscilloscope on the basis of
the Bordo 211 digital board and personal computer).
c
=
⎯
×
Absorption spectra of the intermediate products of
photoexcitation and kinetics of their transformation
were detected using the laser flash photolysis instruꢀ
ment equipped with a neodymium–yttrium Alumiꢀ
RESULTS AND DISCUSSION
Typical absorption spectrum of the initial A form,
num Garnet laser (Solar, pulse duration of 10 ns, pulse detected for formazan FRM 1 is presented in Fig. 1.
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54
ATABEKYAN et al.
Δ
A
Δ
A
0.010
0.005
0
1
0
1
–0.04
–0.08
–0.12
2
–0.005
–0.010
2
0
0.00005 0.00010 0.00015 0.00020
t
, s
Fig. 4. Kinetic curves of the photoinduced optical density
decline for the FRM 4 compound in toluene at the wave
0
0.00005 0.00010 0.00015 0.00020
, s
length
free (
λ
= 450 nm for the airꢀsaturated (
) solutions. = 354 nm.
1) and oxygenꢀ
2
λ
t
ex
Fig. 3. Kinetic curves of the photoinduced optical density
duced absorption band for FRM 2 and FRM 3 was sigꢀ
nificantly lower than that for FRM 1 and FRM 4.
changes at 620 nm (
pound in toluene.
1
) and 520 nm (
2) for the FRM 1 comꢀ
λ
= 354 nm.
ex
It has been found that the discoloration of the
major absorption band (λmax = 417 nm) occurs upon
the action of the laser pulse concurrently with the rise
of the absorption band with the maximum at 600 nm.
According to the kinetic curves presented in Fig. 3, the
photoinduced increase of the optical density at 620 nm
is accompanied by a symbatic decline of the optical
density in the region of the long wavelength absorption
band of the initial A form. The process of the optical
density change has twoꢀstep kinetics—the jumplike
The similar adsorption spectra were observed for all of
the investigated 3ꢀmethylbenzothiazolyl formazans.
Absorption band maxima for the initial A form and
photoinduced B form are presented in the table.
It should be noted that the absorption band maxꢀ
ima of the photoinduced B form of the 3ꢀmethylbenꢀ
zothiazolylformazans are hypsochromically shifted
compared with the absorption bands of the initial cheꢀ
late form A, similarly to the case of triarylformazans
[1–3]. Except that the bands overlap significantly
unlike for triarylformazans [8].
and relatively slow step (
t
≈
20 s), which indicates the
µ
presence of the shortꢀlived intermediate product of the
FRM 1 phototransformation.
A differential spectrum of photoinduced absorpꢀ
tion changes in FRM 1 upon laser excitation at
354 nm is presented in Fig. 2.
It was found that the intensity of the signal
decreases significantly in the presence of air oxygen
that suggests the involvement of the molecule in triplet
state in the process of formazane phototransformation
(Fig. 4). The conclusion on the involvement of the
interconversion to the triplet state is supported by the
observed significant increase of the signal in the presꢀ
ence of the triplet energy donor benzophenone as well
The similar differential spectrum was observed for
the all investigated formazans FRM 1–FRM 4. Howꢀ
ever, the intensity of the longꢀwavelength photoinꢀ
Absorption band maxima locations for the initial and photoꢀ as by the established absence of oxidationꢀreduction
induced forms of the 3ꢀmethylbenzothiazolyl formazans
reactions of benzothiazolylformazans in nonpolar solꢀ
vents [7].
FRM 1–FRM 4 in toluene (c = 4 × 10–4 mol/l)
Compound
Designation
λ
, nm
The gradual disappearance of the absorption band
at 600 nm together with the recovery of the absorption
band intensity in the short wavelength spectral region
is observed with time.
max
R
Form
A
Form B
FRM
FRM
FRM
FRM
1
2
3
4
H
417
423
419
415
412
412
400
402
Br
The kinetics of dark recovery of intensity at the
absorption band of FRM 2 in toluene at the different
wavelengths has a complex twoꢀstep character (Fig. 5).
CH
3
NO
2
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LASER PHOTOLYSIS STUDY OF 1ꢀARYLꢀ3ꢀMETHYLꢀ5ꢀ(BENZOTHIAZOLEꢀ2ꢀYL)
55
The spectrum of photoinduced changes in the optiꢀ
cal density for the solution of the nitrophenyl FRM 4
compound is presented in Fig. 6. As follows from the
presented spectrum, the rate of the dark optical denꢀ
sity recovery at 400 nm is notably lower than for the
Δ
A
0
(а)
–0.01
–0.02
–0.03
process at = 480 nm (insert in Fig. 6), which is indicꢀ
λ
ative of the existence of several forms of the compound
with the overlapping spectra. These forms could be
assigned to different tautomers of the FRM 4 [9].
Some differences in the differential spectra of the
photoinduced absorption changes were observed upon
excitation of the photochromic transformation by the
0
0.001
0.002
, s
0.003
0.004
laser pulse with = 400 nm (Fig. 7). The existence of
λ
t
several intermediate forms during the dark recovery of
the initial absorption of the solution is clearly visible in
the presented spectrum.
Δ
A
(b)
0.005
0.004
0.003
0.002
0.001
0
A comparison of the spectrum of the photoinduced
absorption changes for the 3ꢀmethyl formazans with
their absorption spectrum in the ground state (Figs. 1,
2, table) indicates the shift of the maximum of the disꢀ
coloration band relative to the maximum of the
absorption band. In can be explained much as in [7] by
the existence of several forms in the toluene solution of
3ꢀmethyl formazans that are present as a mixture of
different conformers or tautomer forms at equilibꢀ
rium. This is also supported by the dependence of the
FRM 4 discoloration band maximum on the excitaꢀ
tion wave length (Figs. 6, 7). Twoꢀstage character of
the photoinduced absorption changes supports this
conclusion as well (Figs. 3 and 5).
–0.001
0
0.001 0.002 0.003 0.004
, s
t
Fig. 5. Kinetics of the dark processes of the optical density
changes for the FRM 2 compound in toluene at 580 nm (a)
and 510 nm (b).
λ
= 354 nm.
ex
Δ
A
0.02
1
2
3
0.01
0
Δ
A
–0.01
–0.02
–0.03
–0.04
–0.05
–0.06
0
–0.02
–0.04
–0.06
–0.08
1
2
0
0.001 0.002 0.003 0.004 0.005
, s
t
350
400
450
500
550
600
650
700
750
, nm
λ
–4
–3
Fig. 6. Differential absorption spectra of the FRM 4 compound in toluene measured at the
τ
= 3
×
10 s (
1
), 1.5
×
10 s (
2), and
λ
–3
4
×
10 s (
3
) after laser pulse. In the insert–kinetic curves of the optical density changes at
λ
= 480 ( ) and 400 nm (
1
2).
=
ex
354 nm.
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56
ATABEKYAN et al.
that the process of the 1ꢀarylꢀ3ꢀmethylꢀ5ꢀ(benzothiaꢀ
zoleꢀ2ꢀyl)formazans phototransformation in toluene
involves the ꢀstate of the initial chelate form.
Δ
A
0.005
0
3
2
Т
1
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–0.005
–0.010
–0.015
–0.020
–0.025
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Hence it was established that the laserꢀinduced
phototransformation processes observed for the 3ꢀ
methylbenzothiazolylformazans solutions indicate the
presence of the intermediate products of the photoinꢀ
duced transformation related to the possible existence
of several isomers and tautomeric forms of the benꢀ
zothiazole formazan molecules. Contrary to the data
[6, 7, 9, 10] the influence of oxygen on the intensity of
the photoinduced absorption and discoloration bands
in toluene was first detected. On this basis we suggest
HIGH ENERGY CHEMISTRY
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2011