The investigation of the intermediates of spiropyran retinal analogs by laser flash photolysis techniques with different excitation wavelengths

Article abstract of DOI:10.1016/j.jphotochem.2011.12.024

Spectral-kinetic properties of intermediates of spiropyran retinal analogs (SRA), which molecules contain spiropyran and retinoid (with different polyenic chain length and aldehyde terminal group) fragments, have been investigated by nanosecond laser flash photolysis techniques using excitation wavelengths 337 and 380-450 nm in toluene. Relative quantum yields of different SRA intermediates formation relative to those of model compounds, namely triplet excited state and merocyanine form generated upon pyran cycle cleavage have been measured. It has been found that merocyanine form does not form upon photolysis of SRA with polyenic chain length of 5 and 9 using excitation wavelengths of 380-450 nm, the triplet state of retinoid fragment is observed exclusively. The yield of merocyanine (MC) form is the largest one upon SRA photolysis with excitation wavelength of 337 nm for 6′-formylsubstituted spiropyran (it exceeds that of unsubstituted analog in 8 times and forms mainly from the triplet state). The yields of MC for SRA with polyenic chain length of 3, 5 and 9 carbon atoms are 5-10 times smaller than that for unsubstituted spiropyran.

Full text of DOI:10.1016/j.jphotochem.2011.12.024

Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 41–44
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The investigation of the intermediates of spiropyran retinal analogs by laser flash
photolysis techniques with different excitation wavelengths
Peter P. Levina, Aleksandr S. Tatikolova, Alexey V. Lapteva,b, Alexey Yu. Lukinb, Nikolay E. Belikova,b
,
Olga V. Deminaa, Andrey A. Khodonova,b,∗, Vitaly I. Shvetsb, Sergey D. Varfolomeeva
a N.M. Emanuel Institute of Biochemical Physics RAS, Kosygina 4, 119334 Moscow, Russia
b M.V. Lomonosov State University of Fine Chemical Technology, Vernadskogo 86, 119571 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Spectral-kinetic properties of intermediates of spiropyran retinal analogs (SRA), which molecules contain
spiropyran and retinoid (with different polyenic chain length and aldehyde terminal group) fragments,
have been investigated by nanosecond laser flash photolysis techniques using excitation wavelengths
337 and 380–450 nm in toluene. Relative quantum yields of different SRA intermediates formation rela-
tive to those of model compounds, namely triplet excited state and merocyanine form generated upon
pyran cycle cleavage have been measured. It has been found that merocyanine form does not form upon
photolysis of SRA with polyenic chain length of 5 and 9 using excitation wavelengths of 380–450 nm,
the triplet state of retinoid fragment is observed exclusively. The yield of merocyanine (MC) form is the
largest one upon SRA photolysis with excitation wavelength of 337 nm for 6ꢀ-formylsubstituted spiropy-
ran (it exceeds that of unsubstituted analog in 8 times and forms mainly from the triplet state). The yields
of MC for SRA with polyenic chain length of 3, 5 and 9 carbon atoms are 5–10 times smaller than that for
unsubstituted spiropyran.
Received 28 October 2011
Received in revised form
23 December 2011
Accepted 30 December 2011
Available online 11 January 2012
Keywords:
Indoline spiropyrans
Retinal analogs
Photochromism
Laser flash photolysis
Polyenic fragment
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
properties. In the 6ꢀ-nitrosubstituted derivative pyran cycle cleav-
age is possible with formation of relatively long-lived colored
Indoline spirobenzopyrans are widely known class of pho-
tochromic compounds [1]. As other photochromes they are
examined as elements of optical materials and devices: sensors
(registering materials), optical switches for wide range of differ-
ent objects (including those of biological nature), data storage,
inclusions in finished products (garments, arts, cosmetics, means
of information and document protection), special materials with
harmful irradiation protective properties [2–4].
merocyanine (MC) form, absorbed light in the long-wavelength
region, in other one – the formation of short-lived triplet state
and cis-/trans-isomers, absorbed in the short-wavelength region
[7,8]. This peculiarity of SRA may prove to be useful in applica-
tions as molecular switches, especially with implementation of
selective photoinitiation of different processes, such as light-driven
proton transport by bacteriorhodopsin, membrane protein of
The simplicity of preparation and the possibility to vary the
spectral properties of these compounds directly by the introduction
of the substituents of different nature into the specified positions
of molecule or by the medium selection (solvent, polymer, crys-
tal) make the design of new photochromic spiropyran-containing
materials very attractive.
Synthetic methods for retinal analogs with trimethylcyclohex-
enic ring replaced by spiropyran fragment have been developed
by us earlier [5,6]. These spiropyran retinal analogs (SRA) pos-
sess two various structural fragments with different photochemical
1
A
4
5
0,5
1
2
3
0
300
400
500
∗
Corresponding author at: N.M. Emanuel Institute of Biochemical Physics RAS,
Kosygina 4, 119334 Moscow, Russia. Tel.: +7 495 939 7148; fax: +7 499 137 4101.
λ, nm
E-mail address: khodretinal@sky.chph.ras.ru (A.A. Khodonov).
Fig. 1. Normalized absorption spectra of compounds 1–5 in toluene.
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.12.024

42
P.P. Levin et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 41–44
Halobacterium salinarum [6,9], by varying excitation light wave-
length and medium.
In the present study spectral-kinetic properties of intermedi-
ates formed upon photolysis of compounds synthesized earlier [6]
have been investigated using the nanosecond laser flash photolysis
techniques:
high speed digitizer (Sweden) interfaced to a personal computer.
The data presented in the paper are the average values obtained
by processing of at least ten kinetic curves under the indicated
conditions.
N
O
CHO
N
O
CHO
N
O
(1)
(2)
(3)
CHO
CHO
N
O
N
O
(4)
(5)
The selection of 1 as SRA fragment is due to the fact that the
initial closed form A of compound 1 absorbs at 337 nm, but almost
does not absorb at ꢀ ≥ 360 nm (Fig. 1). The photoexcitation of 1 by
337 nm light results in formation of open merocyanine form B with
relatively large quantum yield [10,11].
A
B
R
N
O
N
O
R
merocyanine (MC)
spiropyran (SP)
Relatively large yield of triplet states is an important property
of retinal and its analogs [12–14]. At the same time the presence
of absorption at the region ꢀ ≥ 380 nm provides the possibility of
selective photoexcitation by light with ꢀ ≥ 380 nm (Fig. 1).
The yields of the intermediates were determined relative to
the yields of corresponding intermediate of 1 (merocyanine form)
and 2 (triplet states) by comparison of the corresponding absorp-
tion intensities at the absorption maxima, measured at same
absorbance of initial solutions at excitation wavelength, assuming
similar absorption coefficients of the corresponding intermediates.
All measurements were carried out at room temperature.
2. Experimental
The retinal analogs (SRA) 3–5 were synthesized with the use of
a universal experimental procedure that had been proposed by us
earlier [6,9]. Model spiropyrans 1, 2 were prepared as previously
described in the literature [15,16]. Purity (≥95%) of all deriva-
tives 1–5 was characterized by TLC or HPLC. Their structures were
approved by 300 MHz NMR spectroscopy and mass-spectrometry
[6].
3. Results and discussion
3.1. Intermediates of model compound 1
The appearance of only one intermediate – the merocyanine
form B (MC) is observed after flash photoexcitation of compound 1
(spiroform A) with 337 nm light. MC has the characteristic absorp-
Methanol and toluene spectral grade were used in UV spec-
troscopy for preparing solutions.
Electronic spectra were measured in 10-mm quartz cuvettes on
Shimadzu UV-2140PC (Japan) spectrophotometer.
tion spectrum with maximum at about 600 nm (ꢀ in Table 1)
В
[10,11]. The decay kinetics of MC obeys the first order law (rate con-
Absorption spectra and the kinetics of formation and decay
of intermediates were recorded in the time range ≥ 10 ns at
400 ≤ ꢀ ≤ 800 nm spectral region by the nanosecond laser flash
photolysis techniques with registration of electronic absorption
[17,18]. A nitrogen laser (PRA LN 1000, with 1 ns pulse duration and
337 nm radiation wavelength), operating in the ≤10 Hz frequency
regime, or a dye laser (PRA LN 102, with 0.5 ns pulse duration and
430–470 nm radiation wavelengths) pumped by a N2 laser were
used as an excitation source. Acquisition and averaging of kinetic
curves (from 4 to 128 laser pulses) were performed by a UF.258
stant, k , is presented in Table 1). Spiroform A of this compound
В
does not absorb at ꢀ ≥ 380 nm range (Fig. 1, spectrum 5), there-
fore the measurements upon photoexcitation with light of longer
wavelengths were not performed.
3.2. Intermediates of model compound 2
The photoexcitation of 2 in toluene solution with 337 nm light
results in the formation of 2 intermediates (Fig. 2): (1) relatively

P.P. Levin et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 41–44
43
Table 1
0,2
Absorption bands maxima, relative yields and decay rate constants of merocyanine
1
form B (ꢀ , ϕ and k , respectively) and triplet state (ꢀT, ϕT and kT, respectively)
in deoxygenated toluene.
A
В
В
В
Compound
1
2
3
4
5
0,1
0
ꢀ
ϕ
k
, nm
, rel. units
, s−1
580
1.0
2.0
–
–
–
600
8.2
0.058
420
620
0.18
0.19
–
–
–
620
600
В
В
В
2
0.093
0.22
430
0.22
0.27
490
0.77
7.4 × 104
7.4
ꢀT, nm
ϕT, rel. units
1.0
1.5
3
kT, s−1
8.4 × 104
4.6
2.8 × 106
7.4
kq, 109 l/mol sa
–
–
400
500
600
700
a
λ, nm
O2 solubility in air saturated toluene was taken equal to 1.11 mM [20].
Fig. 4. Absorption spectra of the intermediates produced by laser flash photolysis
with 337 nm light of 4 immediately after the laser pulse (1) and 5 20 ns (2) and
100 ␮s (3) after the laser pulse (optical density of initial solutions at 337 nm equals
to 0.4) in toluene.
long-lived MC form B (corresponding values of ꢀ and k , as well as
В
В
quantum yield ϕ , as compared to similar value for 1, are presented
В
in Table 1); (2) relatively short-lived triplet excited state T2, which
absorbs in 400–500 nm range (position of ꢀmax, ꢀT, is presented in
Table 1) (Fig. 2) [19].
intersystem crossing in the singlet excited state, which is provided
by the presence of carbonyl or nitro groups as substituents in aro-
matic rings [21–24]. As a rule, the substituents of this group possess
electronegative properties, which lead to significant deceleration of
The decay kinetics of T2 obeys the monoexponential law (rate
constant in deoxygenated toluene, kT, is presented in Table 1).
The disappearance of T2 is accompanied by simultaneous forma-
tion of MC B2 (Figs. 2 and 3). The introduction of the air oxygen
into solutions results in the significant decay acceleration of T2
(corresponding quenching rate constant, kq, calculated based on
molecular oxygen concentration in air saturated toluene being
equal to 1.11 × 10−3 mol/l [20], is presented in Table 1) and in the
decrease of B2 yield. The experimental data analysis allows us to
conclude that about 50% of MC B2 is formed from T2. MC B2 quan-
tum yield is significantly higher than that of B1 (Table 1). Such
behavior is typical for spiropyrans which have possibility of fast
(which is able to compete with the spiro-bond cleavage process)
MC B form decay [see k values for compounds 1 and 2 in Table 1)
В
[19,21–25].
Spiroform A of compound 2 does not absorb at ꢀ ≥ 380 region
(Fig. 1, spectrum 5), therefore the measurements upon photoexci-
tation with longer wavelengths light were not performed.
3.3. Intermediates of compounds 3–5
The main intermediate of compounds 4 and 5 observed under
the photolysis with 337 nm light is the relatively short-living triplet
excited state (see Fig. 4, corresponding values of ꢀT, kT and kq, as
well as T yields as compared to T2, are presented in Table 1). Con-
trary to T2, the disappearance of T4 and T5 is not accompanied
by the formation of any longer-lived intermediates absorbed in the
registration spectral range. The spectral-kinetic properties of T5 are
quite similar to the respective parameters of the retinal triplet state
[12–14].
Decrease of the polyenic substituent length (transition from 5
to 4) is accompanied by the short-wave length shift of T absorption
band and by the acceleration of its decay (Table 1). Apparently, the
compound 3 with short polyenic substituent behaves similarly, but
temporal and/or spectral resolution of our device did not allow to
observe it.
0,3
A
1
2
0,2
0,1
0
3
400
500
600
700
λ, nm
The formation of small amounts of respective MC B forms is
observed in addition to the triplet states upon photolysis of 3, 4
and 5 with 337 nm light (relative yields are presented in Table 1).
MC form B is barely formed upon photolysis of compounds 4 and
5 with ꢀ ≥ 380 nm light, that is due to the occupation of S1 state.
Thereby the opening of spirocycle in spiropyrans with polyenic sub-
stituents containing terminal aldehyde group proceeds with low
efficiency (as a result of the presence of the most important com-
petitive internal conversion process) in an excited state higher than
S1.
Fig. 2. Absorption spectra of the intermediates produced by laser flash photolysis
of 2 in toluene with 337 nm light (optical density of solution at 337 nm equals to
0.4) in 20 ns (3), 50 ␮s (2) and 20 ms (1) after laser pulse.
0,3
1
A
0,2
−
The characteristic time of the internal conversion from the Ag
2
*
S2 state of all trans retinal to low-lying n␲ state that decays with
0,1
a 33 ps time constant to form the retinal triplet state is near 0.44 ps
[26]. This time is comparable with the typical duration of MC for-
mation from the S1 state of indolinospiropyran (0.9 ps) [27]. The
coloring reaction of spirocompounds proceeds via some interme-
diate X, which corresponds to the molecular electronic state with
the practically initial structure, but broken C–O bond. The forma-
tion of X form can be quite fast, up to 0.25 ps [28] and it can easily
compete with the internal conversion from S2 state of compounds
3, 4 and 5 to S1 state localized mainly on the retinoid fragment.
0
0
50
100
Time, μs
Fig. 3. Decay kinetics of the intermediates, observed at 440 (2) and 600 (1) nm under
laser flash photolysis of 2 with 337 nm light (optical density of solution at 337 nm
equals to 0.4) in toluene.

44
P.P. Levin et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 41–44
The values of MC decay rate constants in the range of studied
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removing of the polar group away from indole core through a sys-
tem of double bonds leads to the increase of electron density at
spiropyran fragment. The introduction of one double bond results
in the order of magnitude increase of MC decay rate constant which
is then grows in some arithmetic progression with the increase of
the polyenic chain length.
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