Spectral kinetic characteristics of the photoisomerization products of naphthylmethylideneiminospironaphthopyran induced by photolysis at different wavelengths

Article abstract of DOI:10.1007/s11172-012-0078-6

The spectral kinetic characteristics of intermediates generated by photolysis with light at the wavelengths 337 and 430 or 470 nm of the photobifunctional compound (PBC), 1,3-dihydro-5-(2-hydroxy-1- naphthylmethylideneimino)-1,3,3-trimethylspiro[2H-indole

Full text of DOI:10.1007/s11172-012-0078-6

532
Russian Chemical Bulletin, International Edition, Vol. 61, No. 3, pp. 532—538, March, 2012
Spectral kinetic characteristics of the photoisomerization products
of naphthylmethylideneiminospironaphthopyran induced by photolysis
at different wavelengths
P. P. Levin,a N. L. Zaichenko, A. I. Shienok, L. S. Kol´tsova, I. R. Mardaleishvili, and A. S. Tatikolov
b
b
b
b
a
aN. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences,
4
ul. Kosygina, 119334 Moscow, Russian Federation.
Fax: +7 (499) 137 4101. Eꢀmail: levinp@sky.chph.ras.ru
N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences,
b
4
ul. Kosygina, 119991 Moscow, Russian Federation.
Fax: +7 (499) 137 8284. Eꢀmail: zaina@polymer.chph.ras.ru
The spectral kinetic characteristics of intermediates generated by photolysis with light at
the wavelengths 337 and 430 or 470 nm of the photobifunctional compound (PBC), 1,3ꢀdihydroꢀ
5
ꢀ(2ꢀhydroxyꢀ1ꢀnaphthylmethylideneimino)ꢀ1,3,3ꢀtrimethylspiro[2Hꢀindoleꢀ2,3ꢀ[3H]ꢀnaphꢀ
tho[2,1ꢀb]pyran], whose molecule combines the spironaphthopyran and hydroxyazomethine
fragments, and also parameters of model compounds, viz., naphthylmethylideneimine and
spironaphthopyran, were studied in methanol and toluene. The relative quantum yields of
formation of different intermediates of PBC were measured relatively to model compounds,
namely, transꢀketo isomer formed due to cis—transꢀisomerization and prototropic equiliration
in the azomethine fragment, and in the merocyanine form generated by spiro bond opening. It was
found that the photolysis of the PBC with light at the wavelengths  = 430 or 470 nm nearly no
produces the merocyanine form, whereas the relative yield of the transꢀketo tautomer is ~0.6.
For PBC photolysis at  = 337 nm, the yield of the merocyanine form is ~0.2 and the yield of the
transꢀketo isomer decreases substantially (0.2). The solvent nature affects the kinetic behavior
of the system. The consistency of the isomerization and proton transfer processes is discussed.
Key words: photobifunctional compounds, spiropyran, azomethine, photochromism,
proton transfer, cis—transꢀisomerization, laser photolysis.
Investigations in the field of design and research for
ent processes by the variation of the exciting light waveꢀ
length and the medium.
the properties of photocontrolled hybrid photochromic
systems are intensively developed at present. On the one
hand, photochromic probes and labels are of interest that
make it possible to monitor the properties of a modified
In this work the spectral kinetic characteristics of the
intermediate products (IPs) formed upon photolysis of the
5
earlier synthesized 1,3ꢀdihydroꢀ5ꢀ(2ꢀhydroxyꢀ1ꢀnaphthylꢀ
1
object using light. On the other hand, these compounds
methylideneimino)ꢀ1,3,3ꢀtrimethylspiro[2Hꢀindoleꢀ2,3ꢀ
[3H]ꢀnaphtho[2,1ꢀb]pyran] (1) with light at the waveꢀ
lengths  = 337 and 430 or 470 nm in methanol and toluꢀ
ene were studied by nanosecond laser photolysis. The model
compounds, viz., 1ꢀ(phenyliminomethyl)ꢀ2ꢀnaphthol (2)
and 1,3ꢀdihydroꢀ1,3,3ꢀtrimethylspiro[2Hꢀindoleꢀ2,3´ꢀ
[3H]ꢀnaphtho[2,1ꢀb]pyran] (3), were also studied.
provide the basis for the development of materials for moꢀ
lecular electronics, namely, optical switches, logical deꢀ
vices, etc.2
,3
We developed the methods for synthesis of the soꢀcalled
photobifunctional compounds (PBCs) containing two
structural fragments different in photochemical properꢀ
4
,5
ties. Spiro cycle opening with the formation of the relaꢀ
tively longꢀlived colored merocyanine form absorbing in
the longꢀwavelength region is possible in one of the fragꢀ
ments, whereas in another the intramolecular proton transꢀ
fer and cis—transꢀisomerization occur resulting in the colꢀ
ored shorterꢀlived isomer absorbing in a shorterꢀwaveꢀ
Compound 3 was chosen as a fragment of the PBC,
because the initial closed form А of compound 3 has an
intense absorption band at 337 nm and shows nearly
no absorption in the region of 400—500 nm (Fig. 1).
Photoexcitation of compound 3 with light at the waveꢀ
length  = 337 nm affords the open merocyanine form В
6
7,8
length region. This specific feature of PBCs can be fruitꢀ
(Scheme 1) with a rather high quantum yield (~0.2).
ful for their possible application as molecular switches,
especially in the case of selective photoinitiation of differꢀ
An important property of model naphthylmethylideneꢀ
imine 2 is the fact that the compound exists in a МеОН
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 531—537, March, 2012.
066ꢀ5285/12/6103ꢀ532 © 2012 Springer Science+Business Media, Inc.
1

Laser photolysis of photobifunctional compounds
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
533
the azomethine group in the stable transꢀconfiguration)
with a noticeable content of Кcis.9
—11
The Кcis form preꢀ
dominantly absorbs in the wavelength region 400 nm,
providing the possibility of its selective photoexcitation,
for example, with light at the wavelength  = 430 or 470 nm
(
see Fig. 1).
Scheme 2
Scheme 1
It is assumed that, as in the well studied salicylideneꢀ
anilines,1
1—16
the photoexcitation of compound 2 at both
3
37 and 430 or 470 nm can induce cis—transꢀisomerizaꢀ
tion and proton phototransfer with the formation of the
corresponding transꢀforms.
Experimental
Compounds 1 (see Ref. 5), 2 (see Ref. 17), and 3 (see Ref. 18)
were synthesized by earlier described procedures. Purity of the
compounds was monitored by TLC, measuring melting points,
NMR spectroscopy, and elemental analysis.
Methanol and toluene (for UV spectroscopy) were used to
prepare solutions.
Absorption spectra in the visible and UV regions were reꢀ
corded on a MultiSpecꢀ1501 spectrophotometer.
Absorption spectra and the kinetics of formation and decay
of IPs were recorded in the time interval 10 nm on an nanoꢀ
second laser photolysis technique with electronic absorption deꢀ
1
9,20
tection.
An N laser (PRA LN 1000, pulse duration 1 ns,
2
solution predominantly as cisꢀketone Кcis (Scheme 2) and
in toluene as cisꢀenol form Еcis (we mean the cisꢀrotamer
relatively to the hydroxy group of the naphthyl residue and
pulse energy 1 mJ, radiation wavelength 337 nm) working in the
frequency mode (10 Hz) or a dye laser (PRA LN, radiation
wavelength 102, 430, or 470 nm, pulse duration 0.5 ns, pulse
energy 0.2 mJ) with an N laser pump were used as excitation
2
sources. The kinetic curves were averaged (over 4—128 laser
pulses) by a UF.258 highꢀperformance analogꢀtoꢀdigital conꢀ
verter (Sweden) connected with a personal computer based on
a Pentium 4 processor. The data presented in the work represent
the average values obtained by processing at least ten kinetic
curves under the indicated conditions.
The relative yields of IP I were determined by comparison of
the absorption intensities of the IPs at the maxima of bands
measured at the same absorption of the initial solutions at the
excitation wavelength assuming that the molar absorption coefꢀ
ficients of the corresponding IPs are equal.
A
0
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
.1
2
1
3
All measurements were performed at ambient temperature.
4
00
500 /nm
Results and Discussion
Fig. 1. Absorption spectra of solutions (5•10–5 mol L–1) of modꢀ
el compound 2 in methanol (1) and toluene (2) and compound 3
in toluene (3).
Intermediate products of photolysis of model 3. Pulse
photoexcitation of form А of compound 3 in methanol and

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Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
Levin et al.
toluene solutions with light at the wavelength  = 337 nm
results in the formation of a single photoproduct: meroꢀ
cyanine form В with the characteristic absorption specꢀ
A
0
.05
1
2
trum with maxima at 560 and 550 nm, respectively
(
Fig. 2).7 The kinetics of disappearance of form В is
,8
described by the firstꢀorder equation with rate constants of 14
–
1
(
in MeOH) and 42 s (in toluene). Form А of this comꢀ
pound does not absorb in the region of   380 nm (see
Fig. 1, spectrum 3) and, therefore, no measurements were
performed for photoexcitation with long wavelengths of light.
Intermediates of photolysis of model compound 2. Pulse
photoexcitation of compound 2 in methanol and toluene
solutions with light at the wavelengths  = 337 and 430 or
0
4
00
500
/nm
Fig. 3. Absorption spectra of the intermediates obtained by laser
photolysis of compound 2 in toluene (1) and methanol (2) with
light at the wavelength  = 337 nm (the absorbance of the initial
solutions at  = 337 nm is 0.4) immediately after a laser pulse.
4
70 nm, i.e., at the absorption bands of both tautomers
9—11
Еcis and Кcis,
generates only one IP with the absorpꢀ
tion spectrum (in methanol a maximum in the differential
spectrum at ~475 nm and a band at 450 nm; in toluene
a maximum in the differential spectrum at ~430 nm and
a shoulder at 470 nm (Fig. 3)), which is substantially difꢀ
ferent from the spectrum of the Кcis form (two bands at
The obtained kinetic characteristics of the IP and speꢀ
cific features of its absorption spectrum are similar to the
known characteristics for the IPs of photochemical reacꢀ
6,11—16
tions of other salicylideneanilines.
These IPs were
4
55 and 435 nm with a close intensity, see Fig. 1, spectra 1
identified earlier as the transꢀketo form (Ktrans) of saliꢀ
cylideneanilines. The solvation of Ktrans in protonꢀdonor
solvents and the proton exchange with the initial moleꢀ
and 2) by the intensity ratio of two absorption bands.
In toluene at the low initial concentration of the IP,
the kinetics of its decay obeys the monoexponential
6
,15
cules and with other protons favor the decay of Ktrans
.
–
1
law with the rate constant in dilute solutions ~25 s
.
It can be assumed that the IP observed 10 ns after
photoexcitation is also the Кtrans form of compound 2. In
this case, at   400 nm, where the absorption of the initial
solution is mainly caused by the Ксis form of 2, no bleachꢀ
ing (absorption decrease) of the solution is observed even
upon photolysis with the longꢀwavelength light. The deꢀ
cay kinetics of the induced absorption is independent of
the observation wavelength. It is difficult to substantiate
the assumption about the fact that the molar absorption
coefficient of the Кtrans form of 2 is substantially higher
than the analogous characteristic of the Ксis form of 2 in
the whole range of  > 400 nm. It is most likely that the
initial concentration of the Кcis form of 2 is appreciably
recovered after 10 ns due to prototropic equilibrium,
and in toluene (where the equilibrium is shifted to Есis)
the concentration is recovered almost completely, whereꢀ
as in methanol (where the equilibrium is shifted to Ксis)
the recovery is only partial, which results in a substanꢀ
tial difference between the observed differential specꢀ
tra of the Кtrans form of 2 in toluene and methanol
At rather high initial concentrations of the IP, the IP
disappears according to a bimolecular law with a high
8
–1 –1
rate constant of ~6•10 L mol
s
(in the calculation
the molar absorption coefficient at the maximum of
the differential absorption spectrum was accepted to be
4
–1
–1 9—11
1
•10 L mol cm ).
The IP in methanol disappears considerably more rapꢀ
idly, and its decay kinetics obeys the monoexponential
law with the rate constant in dilute solutions equal to
3
–1
~
9.9•10 s . An increase in the concentration of comꢀ
pound 2 accelerates the decay of the IP. The correspondꢀ
ing linear dependence on the concentration of compound 2
gives the value of the rate constant of IP "quenching" by
7
–1 –1
the ground state of 2 equal to ~3•10 L mol
s .
A
0
0
.2
.1
1
2
(
see Fig. 3).
The data obtained do not allow one to draw a concluꢀ
sion about the detailed mechanism of formation and deꢀ
cay of the Ktrans form, in particular, to reveal whether the
processes of isomerization and proton transfer are conꢀ
secutive or occur correlatively through one transition state
5
00
600
/nm
(
see further). However, the result of photolysis indepenꢀ
dent of the wavelength of the exciting light and of the
solvent can indicate that proton transfer and isomerizaꢀ
tion occur in the same electronꢀexcited state, for example,
Fig. 2. Absorption spectra of the intermediates obtained by laser
photolysis of compound 3 in methanol (1) and toluene (2) with
light at the wavelength  = 337 nm (the absorbance of the initial
solutions at  = 337 nm is 0.4) immediately after a laser pulse.
2
1
in the twisted chargeꢀtransfer state.

Laser photolysis of photobifunctional compounds
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
A
535
Intermediates of photolysis of the PBCs in МеОН.
A comparison of the absorption spectra of compounds 1
and 2 in a methanol solution (Figs 1 and 4) suggests that
molecules 1 in a methanol solution exist predominantly in
the cisꢀketo form AKcis. It is expected that the introduction
0.005
1
0
–
0.005
–0.010
0.015
2
of electronꢀdonor substituents into the phenyl fragment
2
of molecule 2 would shift the prototropic equilibrium
1
1
to К .
сis
–
Kcis
Pulse photoexcitation of A
in methanol solutions
3
with light at the wavelength  = 430 or 470 nm results in
the reversible bleaching (absorption decrease) at   500 nm
and in a shorterꢀlived and less intense induced absorpꢀ
tion (Figs 5 and 6). The bleaching spectrum (see Fig. 5,
10
Fig. 6. Kinetics of the change in the absorption of the solution at
30 (1), 500 (2), and 450 nm (3) upon laser photolysis of comꢀ
pound 1 in methanol (0.05 mmol L ) with light at the waveꢀ
length  = 430 nm.
20 t/ms
5
K
–1
spectrum 2) corresponds to the absorption of A cis (see
Fig. 4, spectrum 1), whereas the spectrum of induced abꢀ
sorption (calculated by the subtraction of the spectra obꢀ
served immediately after a laser pulse and after the disapꢀ
pearance of the induced absorption) contains a maximum
at ~490 nm (see Fig. 5, spectrum 3).
Scheme 3
By analogy to 2, it can be assumed that this IP is the
K
trans
transꢀketo form A
in which no opening of the С—О
spiro bond occurred (Scheme 3).
A
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
2
1
4
00
500
600 /nm
Fig. 4. Absorption spectra of solutions (5•10–5 mol L–1) of comꢀ
pound 1 in methanol (1) and toluene (2).
The decay kinetics of this tautomer obeys the firstꢀ
A
3 –1
order law with the rate constant 2.8•10 s (see Fig. 6),
which is somewhat lower than a similar value for K of
3
0
.01
0
trans
K
model compound 2. The relative yield of A trans as comꢀ
pared to Ktrans of compound 2 upon photolysis at  = 430 nm
is 0.64.
Unlike the Ktrans form of 2, the disappearance of AKtrans
1
2
is not accompanied by the recovery of the absorption of
the initial compound A cis at   400 nm. The recovery of
–
0.01
K
the initial absorption at   400 nm is rather slow (see Fig. 6).
The kinetics of the process also obeys the firstꢀorder law
–
1
with a rate constant of 92 s . It is difficult to substantiate
such a low rate constant for the prototropic equilibration
of cisꢀenol—cisꢀketone (the process of backward proton
transfer). It can be assumed that the rateꢀdetermining step
of recovery of the initial mixture of tautomers 1 is the
trans—cisꢀisomerization of enol.
5
00
550
/nm
Fig. 5. Differential absorption spectra of the intermediates obꢀ
tained by laser photolysis of compound 1 with light at the waveꢀ
length  = 430 nm in methanol (0.03 mmol L–1) 0.01 s (1) and
ms (2) after a laser pulse. Spectrum 3 is the difference spectrum
resulted from spectra 2 and 3.
1

536
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
Levin et al.
Thus, it can be concluded that AKtrans is transformed
A
K
first not into A cis, but, most likely, into transꢀenol tauꢀ
0
0
.04
.02
0
E
trans
tomer A
absorbing at   400 nm (Scheme 4).
1
2
Scheme 4
3
5
00
600
/nm
Fig. 7. Differential absorption spectra of the intermediates obꢀ
tained by laser photolysis of compound 1 with light at the waveꢀ
length  = 337 nm in methanol (the absorbance of the initial
solutions at  = 337 nm is 0.4) 0.01 s (1), 1 ms (2), and 20 ms (3)
after a laser pulse.

 400 nm followed by recovery (Fig. 7). The kinetic
parameters of these processes coincide with analogous valꢀ
ues obtained by photolysis with light at the wavelength

= 430 or 470 nm.
However, a relatively longꢀlived induced absorption is
K
observed in addition to that of A trans. This absorption is
characterized by the spectrum (a maximum at 590 nm)
similar to the spectrum of form В of model compound 3
but shifted to the longꢀwavelength region (see Fig. 7). The
relative yield of form В of PBC 1 as compared to the yield
of form В of compound 3 is 0.17.
Hereinafter the values of k are given in s–1
.
This process is comparatively fast due to proton exꢀ
change with molecules of the solvent МеОН.
Then the relatively slow trans—cisꢀisomerization ocꢀ
curs accompanied by the prototropic equilibration
The decay kinetics of form В of PBC 1 obeys the
monoexponential law with a rate constant of 37 s–
1
.
This value is noticeably higher than the correspondꢀ
ing value for model compound 3, which is due to the
weak electronꢀacceptor character of the cisꢀketo form
of the salicylideneimine substituent in the indoline
fragment.
(
Scheme 5).
Pulse photoexcitation of AKcis in methanol solutions
with light at the wavelength  = 337 nm is also accompaꢀ
nied by the formation and decay of a small amount of
K
A
trans (the relative yield is 0.091) and the disappearance
Thus, it can be assumed that the observed isomer of
K
of the absorption of the initial mixture of tautomers 1 at
PBC 1 is the B cis form (Scheme 6).
Scheme 5

Laser photolysis of photobifunctional compounds
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
A
537
Scheme 6
0
.01
0
1
–
–
–
0.01
0.02
0.03
2
1
0
20 t/ms
Fig. 9. Kinetics of the change in the absorption of the solution at
70 (1) and 450 nm (2) upon laser photolysis of compound 1
4
(
0.05 mmol L– ) with light at the wavelength  = 430 nm.
1
trum 1) correlates with the absorption of compound 1 (see
Fig. 4, spectrum 2). However, the recovery kinetics of the
initial absorption is described by the biexponential law
–
1
with rate constants of 970 and 220 s (see Fig. 9). So, at
–
1
It seem hardly possible that the observed isomer of the
first (the rate constant is 970 s ) the IP is formed characꢀ
terized by the absorption spectrum with maxima at ~470
and 450 nm (see Fig. 8, spectrum 3). By analogy with the
data on photolysis of compound 1 in methanol and phoꢀ
tolysis of compound 2 in methanol and toluene, it can be
open form was form BKtrans, since the recovery of the initial
absorption of compound 1 at   500 nm was observed
before the disappearance of form В of the PBC (see Fig. 7).
Intermediates of photolysis of the PBCs in toluene. An
analysis of the state of the model compounds in toluene
solutions and a comparison of their absorption spectra
with the absorption spectrum of compound 1 in toluene
K
assumed that this IP is the A trans form. Its relative yield in
toluene as compared to the yield of form Ktrans of model
compound 2 is 0.57.
The decay kinetics of AKtrans in toluene obeys the firstꢀ
(
see Figs 1 and 4) suggest that molecules 1 exist in toluene
E
K
–1
mainly as form A trans, but the fraction of keto form A cis
order law with a rate constant of 220 s and results in the
remains rather noticeable.
recovery of the initial absorption (see Fig. 9). Thus, in toluꢀ
ene, unlike the proton solvent methanol, the rateꢀdeterꢀ
mining step of the process of returning to the initial state
of tautomeric equilibrium is not the trans—cisꢀisomerization
Pulse photoexcitation of compound 1 in toluene with
light at the wavelength  = 430 or 470 nm results, as in the
case of methanol solutions, in the reversible bleaching of
the solution at 400 nm    500 nm, but no induced
absorption is observed immediately after a laser pulse
K
K
K
of enol but the isomerization of A trans to A cis, and A trans is
E
formed, most likely, from A trans, which is the primary
(
Figs 8 and 9). The bleaching spectrum (see Fig. 8, specꢀ
photolysis product (appears less than within 10 ns).
The processes that occur in a toluene solution upon
the excitation of compound 1 with light at  = 430 and
A
4
70 nm can presumably be presented as Scheme 7.
0
.02
0
3
Scheme 7
2
1
–
0.02
5
00
550 /nm
Fig. 8. Differential absorption spectra of the intermediates obꢀ
tained by laser photolysis of compound 1 with light at the waveꢀ
length  = 430 nm in toluene (0.05 mmol L–1) 0.01 s (1) and
2
ms (2) after a laser pulse. Spectrum 3 is the difference spectrum
Pulse photoexcitation of compound 1 in a toluene soꢀ
resulted from spectra 2 and 1.
lution with light at  = 337 nm is also accompanied by the

538
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 3, March, 2012
Levin et al.
A
This work was financially supported by the Russian
Academy of Sciences (Program of the Presidium of the
Russian Academy of Sciences No. 7).
0
.02
1
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0
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5
00
600
/nm
Fig. 10. Differential absorption spectra of the intermediates obꢀ
tained by laser photolysis of compound 1 with light at the waveꢀ
length  = 337 nm in toluene the absorbance of the initial soluꢀ
tions at  = 337 nm is 0.4) 10 ms (1), 2 ms (2), and 0.01 s (3)
after a laser pulse.
5
6
formation and decay of a small amount of AKtrans (the
relative yield of 0.2) followed by the recovery of the absorpꢀ
tion of the initial mixture of PBC tautomers at   500 nm
7
(
Fig. 10). The kinetic parameters of these processes coinꢀ
8. A. Kellmann, F. Tfibel, E. Pottier, R. Guglielmetti, A. Saꢀ
cide with analogous values obtained by photolysis with
light at  = 430 or 470 nm.
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7
6, 77.
9
. G. O. Dudek, E. P. Dudek, J. Am. Chem. Soc., 1966, 88, 2407.
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1
1
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form B with a maximum at 570 nm is observed along with
K
A
trans. The absorption spectrum of form B of compound 1
is similar to that of form В of model compound 3 but is
slightly shifted to the longꢀwavelength region (see Fig. 10).
The relative yield of form В of PBC 1 as compared to form
В of compound 3 is 0.23. The decay kinetics of form В of
1
13. P. Pita, E. Lusina, Cz. Radzewicz, A. Grabowska, J. Chem.
Phys., 2006, 125, 184508.
1
4. F. E. Gostev, A. N. Petrukhin, A. A. Titov, V. A. Nadtoꢀ
chenko, O. M. Sarkisov, A. V. Metelitsa, V. I. Minkin, Khim.
Fiz. [Chem. Phys.], 2004, 23, 3 (in Russian).
5. J. S. Stephan, A. Mordzinski, C. R. Rodriguez, K. H. Grellꢀ
mann, Chem. Phys. Lett., 1994, 229, 541.
6. A. Grabowska, K. Kownacki, J. Karpiuk, S. Dobrin, L. Kacꢀ
zmarek, Chem. Phys. Lett., 1997, 267, 132.
7. R. W. Wizinger, H. W. Wenning, Helv. Chim. Acta, 1940,
23, 247.
PBC 1 obeys the monoexponential law with a rate conꢀ
stant of 34 s–1. This value, unlike analogous data obtained
for experiments in methanol, is even somewhat lower than
the corresponding value for compound 3, which is due,
most likely, to the weak electronꢀdonor character of the
enol form of the salicylideneimine substituent in the indoꢀ
1
1
1
line fragment. It can be concluded that the isomer obꢀ
E
served is form В cis
.
Thus, three photochromic photoprocesses can occur
in parallel in the studied PBC 1: spiro bond opening folꢀ
lowed by cis—transꢀisomerization leading to the formaꢀ
tion of the merocyanine form in the spironaphthopyran
fragment and the proton transfer and photoisomerization
in the azomethine fragment. During these processes an
induced absorption is formed in the longꢀwavelength reꢀ
gion, whereas the reversible bleaching of the system ocꢀ
curs in the shortꢀwavelength region. The ratio of efficienꢀ
cies of different processes depends substantially on the
wavelength of the exciting light. The initial and intermeꢀ
diate absorption spectra and the kinetic behavior of the
system depend substantially on the solvent nature, which
gives additional possibilities of controlling photochromism
along with the wavelength of the photolyzing light.
18. P. Fita, E. Luzina, T. Dziembowska, D. Kopec, P. Piatꢀ
kowski, Cz. Radzewicz, A. Grabowska, Chem. Phys. Lett.,
2
005, 416, 305.
1
2
9. P. P. Levin, N. B. Sul´timova, O. N. Chaikovskaya, Izv.
Akad. Nauk, Ser. Khim., 2005, 1391 [Russ. Chem. Bull., Int.
Ed., 2005, 54, 1433].
0. N. B. Sul´timova, P. P. Levin, O. N. Chaikovskaya, Izv.
Akad. Nauk, Ser. Khim., 2005, 1397 [Russ. Chem. Bull., Int.
Ed., 2005, 54, 1439].
21. M. Z. Zgierski, A. Grabowska, J. Chem. Phys., 2000,
112, 6329.
Received December 8, 2010;
in revised form October 13, 2011