Photochemistry of benzothiazolylformazanes in solutions

Article abstract of DOI:10.1023/A:1016093019344

Photochemical transformations of substituted benzothiazolylformazanes were studied by electronic spectroscopy under conditions of steady-state photolysis. The phototransformations were established to be photoreversible cis-trans-isomerization, unlike phot

Full text of DOI:10.1023/A:1016093019344

Russian Chemical Bulletin, International Edition, Vol. 51, No. 5, pp. 839—843, May, 2002
839
Photochemistry of benzothiazolylformazanes in solutions
a
a
a
b
c
L. I. Tkachenko,a Yu. E. Semenov, V. A. Smirnov, S. M. Aldoshin, G. N. Lipunova, and G. I. Sigeikin
ꢀ
aInstitute of Problems of Chemical Physics, Russian Academy of Sciences,
18 Institutskii prosp., 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (096) 576 4009. Eꢀmail: bineva@icp.ac.ru
bUral State Technical University,
19 ul. Mira, 620002 Ekaterinburg, Russian Federation.
Fax: +7 (343 2) 74 0458. Eꢀmail: ndv@unets.ru
cInterdepartmental Center of Analytic Investigations of the Presidium of Russian Academy of Sciences,
5 ul. D. Ul´yanova, 117333 Moscow, Russian Federation.
Eꢀmail: psytool@orc.ru
Photochemical transformations of substituted benzothiazolylformazanes were studied by
electronic spectroscopy under conditions of steadyꢀstate photolysis. The phototransformations
were established to be photoreversible cis—transꢀisomerization, unlike photoirreversible one
of triphenylformazane. Electronꢀdonor and electronꢀacceptor substituents (except for NO2)
in the paraꢀposition of the phenyl ring of one of the nitrogen atoms retard thermoisomerization.
Key words: photochromism, formazanes, benzothiazolylformazanes, dark isomerization,
cis—transꢀisomerization, quantum yield.
Formazanes (R1—N=N—C(R2)=N—NH—R3) conꢀ
tain the mobile πꢀsystem, their redox processes, isomerꢀ
izations, and tautomeric transformations occur easily,
and these compounds, especially hetarylformazanes, are
characterized by enhanced basicity and acidity. All these
properties create prerequisites for search in this series for
thermochromic and photosensitive compounds.
relative to the С=N bond. The color of 1,3,5ꢀtriarylꢀ
formazanes changes reversibly only in nonpolar solvents.
In protic and polar aprotic solvents, in the presence of
dioxygen, irradiation results in the irreversible decoloraꢀ
tion of formazanes due to oxidative destruction.2 Thereꢀ
fore, toluene was used as a solvent for photochemical
studies.
The photochromic transformations of formazanes have
first been observed for triphenylformazane (TPF).1 Unꢀ
der the visible light, a solution of its red form was transꢀ
formed into a solution of the stable yellow form, and in
the dark the initial spectrum was recovered. It was asꢀ
sumed that the appearance of the yellow form is caused
by the opening of the chelate cycle of the initial strucꢀ
ture А (Scheme 1) and the formation of the Е isomer
In this work, we present the results of steadyꢀstate
photolysis studies of the photothermochromic transforꢀ
mations of 1ꢀarylꢀ5ꢀbenzothiazolylꢀ3ꢀphenylformazanes
1—6 and 1ꢀ(4ꢀmethoxyphenyl)ꢀ3,5ꢀdiphenylformazane
(7), whose rate constant of the dark reaction (k´) and
activation energy (Еа) are known.2
Scheme 1
R = H (1), NO2 (2), Cl (3), Br (4), NMe2 (5), COOMe (6)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 776—779, May, 2002.
1066ꢀ5285/02/5105ꢀ839 $27.00 © 2002 Plenum Publishing Corporation

840
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
Tkachenko et al.
Table 1. Spectral and kinetic characteristics of benzothiazolylꢀ
formazanes 1—6 and triarylformazane 7
Experimental
Formazanes 1—7 were synthesized as described previously.3
b
Comꢀ Ca
pound
λ
max/nm
ε •10–4
k´•104
Purity of the synthesized compounds were monitored by TLC.
Absorption spectra were recorded on an Specord Mꢀ40 specꢀ
trophotometer. Samples were irradiated with the light from a
DRShꢀ1000 mercury lamp using a set of light filters in a stanꢀ
dard quartz cell. The duration of irradiation was varied from
1 s to 1 h.
Toluene was treated with H2SO4 three times, washed with
water, a 10% solution of NaOH, and water again, and dried for
several days above sieves 4А. The anhydrous toluene was disꢀ
tilled above sodium. Before experiments, toluene was frozen
and defrosted above LiAlH4. The presence or absence of air
oxygen in samples had no effect on photochemical transformaꢀ
tions under the experimental conditions. The quantum yield of
trans—cisꢀisomerization (ϕt—c) was determined relative to
stylbene isomerization in EtOH (standard) for which ϕt—c = 0.5
under irradiation with the light with λ = 313 nm at room
temperature. The k´ values were estimated from the time of
exponential decay of absorbance of the photoproduct under
dark conditions.
/s–1
Ic
IId
Ic
IId
1
2
3
4
5
6
7
8
7
6
4
—
6
484, 609
560, 595
490, 625
490, 625
434, 500, 529 434, 488
550
495, 526
465
458
476
476
1.2
1.2
1.8
2.1
—
>2.4
>3.0
>4.0
>5.3
—
6.7
57.0
3.2
4.6
2.0
4.8
1.6
455
404
1.9
2.4
>3.8
>4.8
5
a Concentration of a toluene solution, C•105/mol L–1
b Molar absorption coefficient/L mol–1 cm–1
c Initial form.
.
.
d Photoinduced form.
the proton transfer in the excited state can be the priꢀ
mary act in TAF photochromism.10 The formation of
the yellow form of nonsymmetric TAF occurs through
the oneꢀstage photoisomerization of the initial chelate
form relative to the C=N bond accompanied by the proꢀ
ton transfer in the excited state.2 The photochromic propꢀ
erties of diꢀ, triarylformazanes, and their derivatives found
practical use11 but the problem concerning the mechaꢀ
nism of phototransformations remains disputable.
Results and Discussion
Based on the spectral and Xꢀray structure studies of
TPF,2 the author showed that the chelate form А (see
Scheme 1) has the Е1,2Z2,3 3,4ꢀconfiguration of the
Z
azohydrazone chain stabilized by the intramolecular hyꢀ
drogen bond (IMHB). The electronic spectra of TPF
contain a longꢀwave band at λ = 495 nm from the
π—π*ꢀtransition polarized along the long axis of the molꢀ
ecule in the conjugated azohydrazone system including
the phenyl fragment bound to the azo group.4 This band
is sensitive to the conformation of the azohydrazone
chain. With the chelate ring opening, its position is
hypsochromically shifted by 90 nm and the intensity
increases.5 The longꢀwave band overlaps the lowꢀintenꢀ
sity band from the n—π*ꢀtransition caused by the fragꢀ
ment of the —N= type, which results in the appearance
of a shoulder at 546 nm.4
The study of TPF photoisomerization is impeded by
fast thermal reactions between intermediate isomerizaꢀ
tion products and a strong influence of traces of acids or
bases on the rate of thermal reactions. The study of TPF
photochromism in thoroughly purified solvents at low
temperatures showed6 that photoisomerization occurs
only at the N=N bond, whereas isomerization at the
C=N bond is a thermal reaction. These conclusions were
confirmed7,8 by the Raman spectroscopic data. The reꢀ
sults of studying the phototransformations of triarylꢀ
formazanes (TAF) under flash irradiation indicate9 that
at the first stage, along with cis—transꢀisomerization at
the N=N bond, sꢀcis—transꢀisomerization also occurs
by the turn at 180° about the simple N(2)—C(3) bond of
the formazane chain. It has lately been established that
The spectral and photochemical properties of benzoꢀ
thiazolylformazanes 1—6 are presented in Table 1. It has
previously12 been shown that compound 1 in crystals
(according to the Xꢀray structure analysis data) exists in
the E1,2Z2,3 3,4ꢀconformation stabilized by the IMHB
Z
(similarly to TPF) with hydrogen localization at the niꢀ
trogen atom bound to the heterocycle. In compounds
1—6 the IMHB is weaker than that in TPF, and they
exist in a solution as an equilibrium mixture of the
E
1,2Z2,3 3,4ꢀ and E1,2E2,3 3,4ꢀforms with prevailing of
Z Z
the first one.12 The electronic absorption spectra of freshly
prepared solutions of compound 7 in thoroughly purified
toluene exhibit two bands at λmax = 495 and 526 nm
(Fig. 1). Under irradiation with the light with λ = 546 nm,
the red form is isomerized to the open yellow form with
λ = 404 nm, and the inverse process occurs in the dark.
Such a photochemical process is analogous to the beꢀ
havior of TPF and associated with the chelate cycle openꢀ
ing and isomeric transformations, which results in the
hypsochromic shift of the absorption band of the initial
form by 100 nm. The rate constant of the dark reaction
(k´) is equal to 1.6•10–4 s–1 at 18 °С. This compound is
characterized by the previously13 found k´ = 0.78•10–4 s–1
and Еа = 199.31 kJ mol–1 at 45 °С, whereas for TPF k´ is
equal to 1.27•10–4 and 4.1•10–4 s–1 at 21 and 43 °С,
respectively.
The replacement of the phenyl fragment in TPF by
benzothiazolyl results in the bathochromic shift of the

Photochemistry of benzothiazolylformazanes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
841
D (rel. units)
D (rel. units)
400
500
λ/nm
400
500
λ/nm
0.75
1
5
1.5
1.0
0.5
0.50
0.25
2
2
1
1
25
21
λ•10–3/cm–1
25
21
λ•10–3/cm–1
D (rel. units)
D (rel. units)
400
500
λ/nm
400
500
λ/nm
2
6
0.6
3
1.5
1.0
0.5
2
0.4
0.2
2
1
1
25
21
λ•10–3/cm–1
λ/nm
25
21
λ•10–3/cm–1
λ/nm
D (rel. units)
D (rel. units)
400
500
400
500
3, 4
7
2
0.75
1.5
1.0
0.5
0.50
0.25
1
2
1
25
21
λ•10–3/cm–1
25
21
λ•10–3/cm–1
Fig. 1. Absorption spectra of toluene solutions of benzothiazolylformazanes 1—6 and substituted triphenylformazane 7 at 18 °С
before (1) and after (2, 3) irradiation with the light with λ = 578 (1—6) and 546 nm (7). Irradiation time: 1.5 (1), 0.5 (2) and
1.0 (3) (2), 10 (3 and 4), 40 (5), 8 (6), and 0.5 min (7).

842
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
Tkachenko et al.
absorption bands of a nonꢀirradiated toluene solution of
compound 1 (see Fig. 1). The electronꢀdonor (NMe2)
and electronꢀacceptor (especially NO2 and COOMe) subꢀ
stituents in the paraꢀposition of the aryl fragment still
more enhance this shift (see Fig. 1). When the Cl and Br
atoms are introduced as substituents into the benzoꢀ
thiazolylformazane molecule, an insignificant bathoꢀ
chromic shift of absorption bands of the initial form is
observed. The spectra of compounds 3 and 4 are indisꢀ
cernible by the position of the absorption bands before
irradiation (see Fig. 1).
Irradiation of toluene solutions of benzothiazolylꢀ
formazanes 1—6 with the light with λ = 578 nm changes
the color from red (compounds 1, 3, and 4) or blue
(compounds 2 and 6) to yellow. The photoinduced form
returns to the initial state due to both the dark thermal
reaction and irradiation with the light with λ = 436 nm.
However, the authors11,14 reported that a solution
of 5ꢀbenzothiazolylꢀ1,3ꢀdiphenylformazane (1) under
steadyꢀstate photolysis conditions (PRKꢀ4 and PRKꢀ7
lamps) is photochemically stable but reversible changes
in the absorption spectrum were detected under flash
photolysis. We believe that this is due to experimental
conditions (irradiation of the sample with the nonꢀfilꢀ
tered light and, perhaps, the presence of trace amounts
of admixtures in the solvent).
For compound 1 k´ = 6.7•10–4 s–1 at 18 °С. In
benzothiazolylformazane 1 the symmetric distribution of
the electronic density in the formazane chain is distorted
compared to TPF, which changes the acidꢀbase properꢀ
ties of the NH and N=N groups and, hence, violates the
tautomeric equilibrium. As a result, virtually one tauꢀ
tomer is observed in which a proton is bound to the
nitrogen atom at the more withdrawing benzothiazolyl.
The intramolecular hydrogen bond is weakened comꢀ
pared to that in TPF. This fixation should increase the
localization of the electron density on the N=N and
C=N bonds in the chelate form of the initial formazane
and, as a consequence, decreases the double bonding of
the simple N(2)—С(3) bond, which accelerates thermoꢀ
isomerization.
perhaps, by the catalytic influence of protons appeared
by the partial dissociation of the forms present in a soluꢀ
tion according to the equilibrium RH
This assumption is based on the previously15 published
results, according to which this is precisely the nitro
group which affects the acidic properties of hetarylꢀ
formazanes.
R– + H+.
The absorption spectra of the initial and final forms
of compound 5 differ slightly (see Fig. 1). The introducꢀ
tion of the electronꢀdonor NMe2 substituent into the
phenyl ring at the nitrogen atom hinders both photoꢀ
chemical and thermal transitions. In compounds with
electronꢀacceptor substituents (Cl, Br, COOMe) these
processes also slow down. The introduction of the NO2
group increases k´ by an order of magnitude (see Table 1).
The study of the kinetics of the photoꢀ and thermoꢀ
transformations of compounds 2, 6, and 7 showed
isosbestic points in their absorption spectra. This indiꢀ
cates that the lifetimes of the intermediate forms are
much shorter than those of the initial compounds and
target products. Photothermochromic transformations in
compounds 1, 3, 4, and 5 occur with the formation of
longꢀlived intermediates. The quantum yield of transforꢀ
mation of the blue form into the yellow one under the
light with λ = 578 nm for compound 6 is (7 1)•10–3
.
For other compounds the superposition of the absorpꢀ
tion spectra of the initial and final forms did not allow us
to determine the quantum yields of isomerization but we
can assume, according to the irradiation time, that the
quantum yields are of the same order.
We did not observe photochemical transformations
under steadyꢀstate conditions for benzothiazolylꢀ
formazanes with the alkyl substituent at the mesoꢀcarꢀ
bon atom.
Thus, benzothiazolylformazanes in liquid toluene soꢀ
lutions under steadyꢀstate photolysis undergo reversible
photochemical transformations. The main photoreaction
is trans—cisꢀisomerization relative to the C=N bond
(Scheme 2). The system returns to the initial state under
both dark conditions and irradiation.
The electronic absorption spectrum of a freshly preꢀ
pared toluene solution of compound 2 contains two bands:
at 560 and 595 nm (see Fig. 1). When this solution is
irradiated with the visible light with λ = 578 nm, the
absorbance in the spectrum of the initial form decreases
at 560 and 595 nm and the peak of the yellow form at
λmax = 458 nm increases in the spectrum of the initial
Scheme 2
form. The yellow form decays with k´ = 57•10–4 s–1
.
The difference in the wavelengths of the initial and phoꢀ
toinduced forms is 100 nm as for compound 7. In the
presence of trace amount of acids in toluene, no photoꢀ
chemical transformations occur under steadyꢀstate phoꢀ
tolysis because of the substantial acceleration of the therꢀ
mal reaction in the presence of protons. This is explained,

Photochemistry of benzothiazolylformazanes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
843
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Received June 21, 2001;
in revised form January 31, 2002