Photocyclization of diarylethenes: The effect of imidazole on the oxidative photodegradation process

Article abstract of DOI:10.1039/c8pp00507a

We have studied the photoreaction of 1,2-diarylethenes under aerobic conditions in the presence of various amines to prevent side processes promoted by singlet oxygen. It has been found that the most amines quite effectively deactivate processes associated with singlet oxygen, but primary and secondary amines unlike tertiary ones, react with substrates resulting in various side products. Among the studied amines, the most effective additive for preventing side processes, including those associated with singlet oxygen is imidazole, which is practically not consumed in photoreaction. It was shown that imidazole can also prevents the photodegradation of organic photochromes in solutions. The results obtained can be used in various branches of science, technology and medicine to improve the photostability of photosensitive organics (dyes).

Full text of DOI:10.1039/c8pp00507a

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Yadykov, Photochem. Photobiol. Sci., 2019, DOI: 10.1039/C8PP00507A.
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DOI: 10.1039/C8PP00507A
Photochemical & Photobiological Sciences
ARTICLE
Photocyclization of diarylethenes: the effect of imidazole on the
oxidative photodegradation process
Received 00th January 20xx,
Accepted 00th January 20xx
A. V. Zakharov,a A. G. Lvov,a I. A. Rostovtseva,b A. V. Metelitsa,b A. V. Chernyshev,b M. M.
Krayushkin,a A. V. Yadykov,a V. Z. Shirinian*a
DOI: 10.1039/x0xx00000x
We have studied the photoreaction of 1,2-diarylethenes under aerobic conditions in the presence of various amines to
prevent side processes promoted by singlet oxygen. It has been found that the most amines quite effectively deactivate
processes associated with singlet oxygen, but primary and secondary amines unlike tertiary ones, react with substrates
resulting in various side products. Among the studied amines, the most effective additive for preventing side processes,
including those associated with singlet oxygen is imidazole, which is practically not consumed in photoreaction. It was
shown that imidazole can also prevents the photodegradation of organic photochromes in solutions. The results obtained
can be used in various branches of science, technology and medicine to improve the photostability of photosensitive
organics (dyes).
www.rsc.org/
Introduction
Photochemical transformations are of great interest, both
Our interest in the issue of prevention of side processes
for fundamental research and practical applications.1,2 They associated with singlet oxygen arose in connection with the
are widely investigated for the development of advanced study of the photorearrangement reaction of 1,2-
technologies and materials for various fields of science, diarylethenes I, leading to the formation of polyaromatic
technology and medicine.1,3,4 However, one of the main systems and, first of all, naphthalene derivatives II (Scheme
limiting factors in the widespread practical application of 1A).10 We have found that under photoirradiation, the
photochemical reactions remains the need to carry out these diarylethenes containing
a benzene and five-membered
processes under anaerobic conditions to prevent the heterocyclic residues undergo cyclization with the further
formation of singlet oxygen and many side processes sigmatropic rearrangement and opening of the heterocyclic
associated with it.5 Aerobic conditions promote the generation ring.11,12 However, a more detailed study of this process
of singlet oxygen under UV irradiation in the presence of showed that depending on the reaction conditions, especially
various sensitizers.6 In most cases, photosensitive substances in scaling up to the gram level (in preparative reactions), a side
involved in photochemical transformations themselves act as reaction may occur. This transformation includes the
photosensitizers to generate singlet oxygen.7,8 For this reason, interaction of the heterocyclic moiety (oxazole ring) with the
the exclusion of the formation of singlet oxygen becomes very singlet oxygen resulting to the triacylamines III after a number
complicated. One of the solutions to this issue is the exclusion of rearrangements (Scheme 1B).13
of singlet oxygen formation in photochemical processes by
Scheme 1B shows the mechanism of formation of
removing oxygen from the reaction mixture under inert triacylamine III by the reaction of singlet oxygen (formed via
conditions and it, in turn, imposes additional restrictions on photosensitization with diarylethene triplet7) with an oxazole
the carrying out of photochemical reactions.
residue. In the first, [4+2]-cycloaddition takes place, followed
number of skeletal
Another way to prevent the occurrence of side processes by triacylamine formation after
a
promoted by singlet oxygen is its deactivation (physical rearrangements, including a Bayer-Villiger reaction. In this
quenching or chemical binding) by various substances, transformation, not only diarylethenes based on oxazole, but
including organic compounds (tertiary amines, phenols, also with other heterocyclic residues (imidazole, thiazole,
sulfides, disulfides, nitrogen heterocycles, etc.).9
furan, thiophene, etc.) may react.14 In [4+2]-cycloadditions
five-membered heterocycles act as the diene, and the singlet
oxygen as the dienophile. Further transformations are dictated
by the nature of the heterocyclic residue, in some cases the
reaction is accompanied by the opening of the heterocycle, for
example azoles,15 or without opening (furan and thiophene).16
a. N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47,
Leninsky prosp., 119991 Moscow, Russian Federation, e-mail: shir@ioc.ac.ru.
b. Institute of Physical and Organic Chemistry, Southern Federal University, 194/2
Stachka Avenue, Rostov on Don 344090, Russian Federation.
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follows: chemical shift, multiplicity (s, singlet; d, doublet; m,
View Article Online
A. Irreversible and reversible photoreactions of diarylethenes of oxazole series
multiplet; br, broad), coupling consDtaOnI:t10.i1n039h/Ce8rPtzP00(5H0z7)A,
integration, and assignment. Mass spectra were obtained on a
mass spectrometer (70 eV) with direct sample injection into
the ion source. High resolution mass spectra were obtained
from a TOF mass spectrometer with an ESI source. All
chemicals and solvents were purchased from commercial
sources and used without further purification. Silica column
chromatography was performed using silica gel 60 (70−230
mesh); TLC analysis was conducted on silica gel 60 F254 plates.
Synthesis. Diarylethenes 1a11 / 1b10 and spiropyranes 7 / 823
were prepared according previously reported methods.
Spectral studies
O
Ph
UV
UV
Ar
H
NH
Me
N
N
N
Ar
Ar
Ar
Vis
O
O
Me
I(A)
Ph
O
Me
I(B)
Ph
Me
Me
I(B)
Ph
II
irreversible (photorearrangement)
reversible (photochromism)
H
N
Ar =
Ar =
Me
H
H
Me Me
R
S
S
X
B. Singlet oxygen generation and its reaction with oxazole moiety
h
3DAE*
3DAE* + 3O2
1O2
+
1DAE
1DAE*
1DAE
1DAE*
O
O
N
Me
Ph
O
1O2
N
O
Me
O
N
O
N
O
O
Ar
O
Ar
N
Me
Ph
O
MeCN
Ph
O
O
Me
Ph
Me
O
Ph
Absorption spectra and kinetic curves were recorded using
an “Agilent 8453” diode array spectrophotometer supplied
with a thermostated cell holder. The temperature of solutions
was kept at 293 ± 0.2 K. Solutions (2 ml) were stirred in the
four-windowed 1010 cm quartz cell with a magnetic bar
driven by a speed controlled motor. Setup consisting of the
200 W Hg Research Arc Source "Newport 66902", liquid IR
filter, set of optical bandpass filters and UV-VIS Liquid Light
Guide (300-650 nm) from Newport Corp. was used as a light
source. The light was brought with Liquid Light Guide to the
cell compartment at the right angle to the probe beam of the
spectrometer. The monochromatic light intensity at 365 nm
was determined to be 7.4·10-6 mol·L-1s-1 using an "Newport
Power Meter 2903-C".
I
III
dioxazole
imino anhydride
endo-peroxide
Scheme 1 Photoreactions of diarylethenes of oxazole series.
Another area where it is necessary to take into account the
influence of singlet oxygen is the photoswitching of
diarylethenes (photochromic properties of diarylethenes).17
This aspect for diarylethenes, unlike spiropyrans18 and
spirooxazine19 has been studied poorly and the moreover a
systematic study have not performed. Nevertheless several
groups have noted that distinct diarylethene derivatives show
different types of photochemical side reactions associated
with air oxygen. In particular, formation of the DAE oxidative
products in the presence of oxygen (in the air) including the
oxidation of thiophene ring20 and the insertion of an oxygen
atom between the ethene linker and the aryl residue21 has
been reported. In addition, Hecht and et al. have detected the
by-products with a mass (M+16)+ and (M+32)+ upon irradiation
DAE solution in the presence of oxygen by UV-light.22
Thus, the analysis of the literature has showed that in both
reversible and irreversible photochemical reactions of
diarylethenes singlet oxygen plays a negative role and favors
the formation of undesirable oxidative side products. At the
same time, the irreversible photocyclization of diarylethenes is
widely used for the synthesis of various polyaromatic
compounds including helicenes, graphenes, fluorophores, etc.,
and photochromic diarylethenes are actively explored to
develop new smart materials for optoelectronics and
photonics. In this context, it is very important to develop a
general concept for the prevention of oxidative side processes
promoted by singlet oxygen in the irreversible and
photochromic transformations of diarylethenes.
Preparative photoreactions
Irradiation of diarylethenes were carried out in 3 ml (d = 5
mm) or 10 ml (d = 10 mm) flat-bottomed vessels from
common glass (for analysis of their transparency, see
Supporting information in work11). The irradiation was carried
out by two 8 W UV lamp (365 nm).
Solvent, photosensitizer and amine effect studies
General. Glass vessel (3 ml) was charged by solution of
diarylethene 1 (40 mg) in 2 ml of appropriate solvent (if
necessary, appropriate photosensitizer or amine were added).
The reaction mixture was irradiated by UV (2 lamps, 365 nm, 8
W) without stirring at ambient temperature. Reactions were
controlled by TLC, after completion the reaction mixture was
poured into water (100 mL), and extracted with ethyl acetate
(3 × 20 mL). The combined organic phases were washed with
water (100 mL), dried with magnesium sulfate, and evaporated
in vacuum. The residues were analyzed by 1H NMR
spectroscopy in DMSO-d6 solutions (for copies of these spectra
see Sections I, III, IV, V in ESI).
The present work provides important insights on the role
of singlet oxygen in the photochemical transformation of
diarylethenes and on the protection of organic photochromes
from oxidative photodegradation.
Photosensitizer effect studies
0.1 equivalent of each photosensitizer was added (see
Table S1 in Section III of ESI).
Amine effect studies
Experimental section
General information
Diarylethene 1a was irradiated in the presence of 1 eq. (16
mg) of naphthalene (1 eq.) and 1 eq. of appropriate amine (see
Section IV in ESI).
Proton nuclear magnetic resonance spectra (1H NMR) and
carbon nuclear magnetic resonance spectra (13C NMR) were
recorded in deuterated solvents on a spectrometers working
1
at 300 MHz for H, 75 MHz for 13C. Data are represented as
2 | J. Name., 2012, 00, 1-3
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ARTICLE
Synthesis and characterization of new compounds
Synthesis of triacylamines 3
4-(2,4-Dimethyloxazol-5-yl)-3-(2,5-dimethylthiophen-3-yl)furan-
View Article Online
1
2(5H)-one (6). Pale yellow powder, mp =D1O8I:91-01.190039°/CC.8PPH00N5M07RA
(300 MHz, CDCl3): δ = 2.08 (s, 3H, CH3), 2.40 (s, 6H, CH3), 5.15
(s, 2H, CH2), 6.58 (s, 1H, Hthiophene), 7.35-7.50 (m, 3H, Harom),
7.90-8.01 (m, 2H, Harom). 13C NMR (75 MHz, CDCl3): δ = 14.9,
15.1, 16.6, 71.7, 118.0, 120.5, 124.2, 126.4 (2C), 128.0, 128.9
(2C), 130.2, 133.3, 138.4, 139.2, 153.2, 154.8, 168.4, 172.6.
HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C20H17NO2S2:
368.0773; Found: 368.0766.
Diarylethene 1a or 1b (100 mg, 0.32 mmol) was dissolved
in 5 ml of acetone and naphthalene (41 mg, 0.32 mmol) was
added. The solution was irradiated until completion of reaction
(TLC control). The resulting mixture was purified by column
chromatography on silica gel to give target triacylamines 3 as
labile amorphous powder.
N-Acetyl-N-benzoyl-5-oxo-4-phenyl-2,5-dihydrofuran-3-
carboxamide (3a). Yield 11%. Yellow amorphous powder. 1H
NMR (300 MHz, CDCl3): δ = 2.36 (s, 3H, CH3), 5.14 (s, 2H, CH2),
7.18 (d, J = 7.9 Hz, 2H, Harom), 7.23 (d, J = 7.7 Hz, 2H, Harom),
7.26-7.29 (m, 1H, Harom), 7.39-7.44 (m, 3H, Harom), 7.47-7.53 (m,
2H, Harom). 1H NMR (300 MHz, DMSO-d6): δ = 2.35 (s, 3H, CH3),
5.23 (s, 2H, CH2), 7.38-7.53 (m, 9H, Harom), 7.62-7.71 (m, 1H,
Harom). 13C NMR (75 MHz, DMSO-d6): δ = 25.7, 70.2, 128.4,
129.0 (2C), 129.3 (2C), 129.6 (2C), 129.8 (2C), 129.9, 130.5,
131.9, 135.3, 151.6, 166.0, 171.2, 172.0, 172.3. HRMS (ESI-
TOF) m/z: [M + H]+ Calcd for C20H15NO5: 350.1023; Found:
350.1018. MS (EI): m/z (%) = 349 (16) [M]+, 307 (69), 105
(100), 77 (27), 43 (19), 18 (15).
Results and Discussion
Solvent effect
Studies on competition between photorearrangement and
interaction with singlet oxygen were carried out on model
diarylethenes based on furanone and cyclopentenone
derivatives 1a11 and 1b,10 which were obtained previously in
our group (Scheme 2). The choice of these model compounds
was dictated by their electron-withdrawing bridges, which
accelerate hydrogen migration during photorearrangement.25
The irradiation of 1 in various solvents by UV light (λ = 365 nm)
leads in most cases to the formation of two products: a major
product from photoinduced rearrangement 2a,b10,11 and a
minor product from [4+2]-cycloaddition of an oxazole moiety
with singlet oxygen 3a,b (Scheme 2 and Table 1). The
N-Acetyl-N-(3-oxo-2-phenylcyclopent-1-enecarbonyl)benzamide
(3b). Yield 27%. Yellow amorphous powder. 1H NMR (300 MHz,
CDCl3): δ = 2.40 (s, 3H, CH3), 2.66-2.69 (m, 2H, CH2), 3.04-3.05
(m, 2H, CH2), 7.08 (d, J = 7.6 Hz, 2H, Harom), 7.19-7.24 (m, 3H,
structures of compounds 3a,b have been proved by H, 13C
1
1
Harom), 7.35-7.51 (m, 5H, Harom). H NMR (300 MHz, DMSO-d6):
NMR spectroscopy and mass spectrometry, and they are well
correlated with literature data.13 As can be seen from Table 1,
in the majority of the solvents studied, the
photorearrangement reaction of furanone derivative 1a
proceeds in good yields, while for diarylcyclopentenone 1b,
the yields of triacylamine 3b are in many cases significantly
higher (for copies of NMR spectra see section I in ESI). As
noted above, in these reactions diarylethenes act as a
photosensitizer, generating singlet oxygen, consequently the
δ = 2.39 (s, 3H, CH3), 2.60-2.65 (m, 2H, CH2), 2.87-2.95 (m, 2H,
CH2), 7.18-7.25 (m, 2H, Harom), 7.28-7.46 (m, 7H, Harom), 7.60-
7.67 (m, 1H, Harom). 13C NMR (75 MHz, CDCl3): δ = 25.7, 27.9,
34.6, 128.5, 128.7 (4C), 128.8 (2C), 129.2 (2C), 129.8, 131.9,
134.0, 140.4, 161.5, 170.4, 172.0, 172.7, 205.9. HRMS (ESI-
TOF) m/z: [M + H]+ Calcd for C21H17NO4: 348.1230; Found:
348.1217. MS (EI): m/z (%) = 305 (13) [M-CH3C(O)+H], 128 (25),
115 (42), 105 (100), 77 (61), 43 (59).
Diarylethenes 4-6 were prepared according to a previously
reported method24 from corresponding hetarylacetic acid and
bromoketone.
formation of triacylamine
3
also depends on the
photochemical characteristics of the substrate.
In the case of the cyclopentenone derivative, not only an
increase in the formation of triacylamine (ethyl acetate,
acetonitrile, toluene, acetone and methylene chloride) is
observed, but the prolonged irradiation is required for
complete conversion. Especially high yields of 3b were
observed in toluene and acetonitrile, 25 and 31%, respectively.
3-(2,5-Dimethylthiophen-3-yl)-4-(5-methyl-2-phenyloxazol-4-
yl)furan-2(5H)-one (4). Pale yellow powder, mp = 146-148 °C. 1H
NMR (300 MHz, CDCl3): δ = 1.92 (s, 3H, CH3), 2.17 (s, 3H, CH3),
2.44 (s, 3H, CH3), 5.27 (s, 3H, CH2), 6.71 (s, 1H, Hthiophene), 7.43-
7.55 (m, 3H, Harom), 7.94-8.10 (m, 2H, Harom). 13C NMR (75 MHz,
CDCl3): δ = 11.2, 14.3, 15.2, 71.0, 121.3, 126.2 (2C), 126.6 (2C),
127.1, 128.9 (2C), 129.3, 130.7, 136.7, 136.9, 149.8 (2C), 160.8,
173.4. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C20H17NO3S:
352.1002; Found: 352.0999.
Scheme 1 Photochemical transformations of diarylethenes bearing benzene and
oxazole units.
X
X
X
O
O
O
O
O
N
4-(2,4-Dimethyloxazol-5-yl)-3-(2,5-dimethylthiophen-3-yl)furan-
2(5H)-one (5). Gray powder, mp = 98-100 °C. 1H NMR (300 MHz,
CDCl3): δ = 1.85 (s, 3H, CH3), 2.16 (s, 3H, CH3), 2.43 (s, 3H, CH3),
2.45 (s, 3H, CH3), 5.14 (s, 2H, CH2), 6.65 (s, 1H, Hthiophene). 13C
NMR (75 MHz, CDCl3): δ = 12.5, 13.9, 14.1, 15.1, 69.1, 119.7,
126.6, 136.4, 137.1, 139.0, 140.6, 143.7, 163.0, 172.6, 174.1.
HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C15H15NO3S: 290.0845;
Found: 290.0838.
Ph
O
UV
Ph
+
N
N
H
O
O
Me
Ph
Me
Me
2a,b
1a,b
3a,b
a: X = O; b: X = CH2
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Table 1 Solvent optimization of the photorearrangement reaction.a
Yieldsb for furan-2(5H)-
View Article Online
DOI: 10.1039/C8PP00507A
Yieldsb for
ones
cyclopentenones
Time
(h)
Time
(h)
Solvent
Other by-
products
Other by-
products
2a
3a
2b
3b
1
2
3
4
5
6
7
MeCN
MeCN + imidazole
DMF
10
5
3
13
7
10
13
87
99
97
99
95
93
86
12
0
3
0
5
<1
0
0
0
0
17
25
5
5
16
5
56
99
81
99
80
76
72
31
0
19
0
0
10
15
13
0
0
NMP
CH2Cl2
CHCl3
0
20
14
13
7
13
0
<1
Acetone
Acetone +
naphthalene
EtOH
17
8
17
70
30
0
13
35
37
28
9
9
6
96
88
3
<1
<1
17
8
65
34
9
26
43
10
EtOAc
10
23
11
12
13
EtOAc + imidazole
CH3NO2
-
-
-
-
25
17
27
99
67
55
0
0
17
26
80
75
5
8
15
17
16
25
17
20
Toluene
a Typical procedure: solution of 2 (40 mg) in 2 ml of solvent was irradiated by 2 UV lamps (365 nm, 8W) until full conversion of 2. After common work-up procedure, the residues were analyzed by 1H NMR spectroscopy.
b Yields were determined by 1H NMR spectroscopy.
The best yields of the desired photorearrangement products triacylamine were 30% (see section III in ESI). In addition, the
for both substrates (compounds 2a,b) have been obtained in choice of naphthalene was dictated by another factor. Signals
N-methyl-2-pyrrolidone (entry 4 in Table 1), where the of protons of naphthalene in the 1H NMR spectrum are
formation of side products, including triacylamine was not observed in the form of two multiplets in the range of 7.51 and
observed. The nature of the solvent also affects the 7.91 ppm and they are not overlapped by signals of the
photoreaction time. However, the reaction time is not a photoreaction product. It should be noted that, without
relevant parameter for this process, since it also depends on naphthalene, the yields of triacylamine in acetone did not
many other factors such as concentration, loading, solubility, exceed 13% (Table 1, entry 6).
etc. Nevertheless, we have found that in nonpolar solvents the
For the neutralization of singlet oxygen and prevent the
reaction slows down significantly. In particular, in DMF, the formation of a side process - [4+2]-cycloaddition, we have
reaction requires 3 hours, whereas in toluene under otherwise investigated the effect of various commercially available
identical conditions requires 26 h for competition. In addition, amines on the photorearrangment process (Table 2). In all
in the case of the cyclopentenone, the formation of other by- cases, equimolar amounts of amines were used to prevent the
products is observed in most solvents. We were unable to formation of triacylamine. As can be seen from Table 2,
establish the structure of these compounds due to the practically all amines except pyridine, in equimolar amounts,
formation of
a complex mixture of hard-to-separate completely prevent the formation of triacylamine. And only in
substances. In ethyl acetate and ethanol, the yields of the reaction with pyridine is the formation of triacylamine
unestablished by-products are significantly higher than those (12%), although to a significant degree less, as compared with
of triacylamine 3b. For this reason, further studies on the the reaction without amines in the presence of naphthalene
prevention of side processes associated with singlet oxygen by (30%, entry 7 in Table 1).
amines were carried out only for the furanone derivative 1a.
The use of secondary and primary amines (entries 1-5 in
Table 2) leads to the formation of a mixture of hardly
separable products, including the desired product (see 1H NMR
spectra of the reaction mixtures in ESI). However, the product
of the [4+2]-cycloaddition according to 1H NMR was not
observed. Along with quenching of singlet oxygen and the
photorearrangement reaction, there are probably other
chemical transformations involving amines, which result in its
Amine effect.
The study to prevent the formation of by-products
associated with singlet oxygen was carried out in acetone in
the presence of various amines. To more accurately assess the
efficiency of amines, the photocyclization of diarylethene 1a in
the presence of an additional sensitizer (naphthalene) has
been studied as a model reaction. It is well known that
consumption.28,29,30
In
the
case
of
DBU
the
aromatic
hydrocarbons
(naphthalene,
anthracene,
photorearrangement product, as well as triacylamine were not
separable and a complex mixture is formed. Perhaps DBU
contributes to the formation of other additional side
processes. In the case of triethylamine and, along with the
target product, the formation of corresponding N-oxide is
observed31 (see table S2 in ESI), which, in particular, is
indicative of the chemical quenching of singlet oxygen.
phenanthrene, biphenyl, etc.) are good photosensitizers and
are capable of generating singlet oxygen upon UV
irradiation.26,27 We have studied the photocyclization of
diarylethene 1a in the presence of various photosensitizers
and found that among them, naphthalene more effectively
sensitized the generation of singlet oxygen and the yields of
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Table 2 The effect of amines on the yields of photoproducts.a
the formation of the triplet state of diarylethenes in this
process is the generation of singlet oxygen. In addition,
View Article Online
DOI: 10.1039/C8PP00507A
Yields
Other by-products
Amines
2a
83
99b
94c
73
3a
0
0
recently, we have demonstrated the formation of triplet state
of diarylethenes in photorearrangement reaction.25 In the
future, a more detailed study by flash photolysis of the excited
singlet and triplet states of diarylethenes, as well as their
transformations without or in the presence of imidazole under
aerobic conditions is planned.
1
2
n-Butylamine
17
0
0
Aniline
6
3
4
5
6
Morpholine
Piperidine
TEMP-H
DBU
0
27
40
0
99
0
0
0
0
0
0
0
0
0
60
99
0
0
0
0
0
5
0
0
12
0
Role of imidazole in the photodegradation process of
photochromic compounds.
99b
95c
99b
99c
88
99b
99c
99b
89c
7
Et3N
Having in hand the results on the effectiveness of imidazole
in preventing oxidative processes in the photocyclization of
diarylethenes we have tested it for other processes. It has
been studied the photodegradation of photochromic
compounds in the presence of imidazole under aerobic
conditions (in the presence of air oxygen). Photochromic
compounds belong to a relatively small number of families of
organic compounds that undergo photoinduced reversible
molecular rearrangements between two states characterized
by different spectral parameters.35 These molecules have a
few real and many potential applications.36 One of the main
obstacles to the use of photochromic compounds is still the
photodegradation.37
8
9
DABCO
Pyridine
Imidazole
10
0
0
10
11
1-Methylimidazole
a
Typical procedure: solution of 2 (40 mg), naphthalene (1 eq.) and amine (1 eq.)
in 2 ml of acetone was irradiated by 2 UV lamps (365 nm, 8W) until full
conversion of 2. After common work-up procedure, the residues were analyzed
by 1H NMR spectroscopy.
b 1 eq. of amine;
c 0.1 eq. of amine.
Unfortunately, isolation of the triethylamine N-oxide in its
pure form failed. However, in the 1H NMR spectrum, the
proton signals are shifted to a weak field, which indicates the
formation of the N-oxide.32 A similar pattern is observed for
the DABCO. The observed weak-field shift of NMR signals of
the hydrogen atoms of DABCO agrees very well with the
literature data.33 And only in the case of imidazole, the use of
even catalytic amounts (0.1 equiv) ensures complete
deactivation of singlet oxygen. Thus, it has been found that
imidazole is an effective additive to prevent by-products
associated with singlet oxygen in the diarylethene synthetic
photocyclization reactions. Despite the literature data on the
oxidative degradation of imidazole and its derivatives by
singlet oxygen,34 we have failed to fix of the imidazole
oxidation products, even when scaling the reaction.
It is very interesting to note that the use of N-
methylimidazole does not have the same effect as in the case
of imidazole. A decrease in the amount of N-methylimidazole
leads to the appearance of triacylamine 3b. In addition, there
is also a shift in the NMR signals of the hydrogen atom of N-
methylimidazole in a weak field, like triethylamine and DABCO
(see Table S2 in ESI).
There are two main photodegradation pathways for the
photochromic compounds (Scheme 3). The first of these is
related to the structural features of the photochromic
molecule, leading to different rearrangements and, as a result,
to a decrease in fatigue resistance. For example, photochromic
diarylethenes undergo
a
monomolecular 1,2-dyotropic
rearrangement.22,38 The second channel includes the
interaction of the photochromic molecule with singlet oxygen,
which also leads to photodegradation.
Scheme 3 Photochromism of model diarylethenes and spiropyrans.
A
Photodegradation of
h
h
B
A
singlet oxygen
N
structural features
O
O
O
O
4
5
6
: Het =
Me
Me
Me
Ph
O
UV
Vis
O
Me
Me
X
X
: Het =
: Het =
Me
HetY
HetY
N
S
Z
S
Z
Me
Me
Me
Me
S
4(A) / 5(A) / 6(A)
4(B) / 5(B) / 6(B)
Ph
N
The effectiveness of imidazole was also demonstrated
using photocyclization of diarylethene 1b in acetonitrile and
ethyl acetate solutions. In these solvents without imidazole,
the formation of triacylamine 3b is observed with yields of 31
and 23%, respectively (entries 1 and 10 in Table1) whereas the
addition of imidazole leads to the complete disappearance of
this product (entries 2 and 11 in Table 1). Moreover, as seen in
the Table, imidazole prevents not only the formation of
triacylamine 3b, but also other by-products (entries 2 and 11 in
Table 1). Such behavior of imidazole can be explained by the
fact that it quenches not only singlet oxygen, but also (or only)
triplet (excited) state of diarylethenes. Indirect confirmation of
NO2
Me
Me
R1
Me Me
UV
R1
N
O
NO2
N+
Me
-O
R2
Vis  
Me
R2
7(B) 8(B)
/
: R1 = R2 = H;
: R1 = Br; R2
7(A)
8(A)
N
O
=
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View Article Online
DOI: 10.1039/C8PP00507A
Figure 1.
Dependence of the normalized optical density at the absorption maximum
Dependence of the normalized optical density at the absorption maximum
of photoinduced isomer B of DAE 4 on the irradiation time (solvent – CH3CN,
C = 1.4•10-5 M, C(imidazole) = 1.03·10-3 M, λirr = 365 nm, T = 293 K).
557 nm of merocyanine form B of spiropyran 7 on the irradiation time
(solvent – CH3CN, C = 1.4•10-5 M, λirr = 365 nm, T = 293 K).
A
B
To compare the photodegradation of the photochromic
compounds in the presence of imidazole, their acetonitrile
solutions were investigated under prolonged continuous
irradiation similar to the earlier studies.39 During attainment of
the photostationary state of the system its coloration gradually
faded and the absorbance at the long wavelength band
belonging to the photoinduced isomers decreased. Such
spectral behavior is due to irreversible photodestruction of the
photochromic system. We have investigated two classes of
photochromic compounds: diarylethenes 4 – 6 and spiropyrans
Conclusions
Thus, the photocyclization of 1,2-diarylethenes under aerobic
conditions in the presence of various amines has been studied
to prevent side processes promoted by singlet oxygen. It has
been found that most amines effectively quench singlet
oxygen, but primary and secondary amines, unlike tertiary
ones, react with substrates leading to various by-products.
Comprehensive studies have shown that the most effective
quencher of singlet oxygen is imidazole, which is practically
not consumed in the process of photoreaction. It was shown
that imidazole also prevents the photodegradation of
photochromic diarylethenes and spiropyrans in solution. An
important achievement of this work is the possibility of
carrying out photochemical transformations in the presence of
oxygen (in the air) without fear of the harmful effect of singlet
oxygen.
7
and 8 (Scheme 3). The photochromic switching of
diarylethenes is based on electrocyclic reaction of a hexatriene
system (form A) followed by ring-opening of cyclohexadiene
system (form B).17a The photochromism of spiropyrans
includes the reversible heterolytic Cspiro–O bond cleavage in
the cyclic isomer A followed by cis–trans-isomerization into
the metastable merocyanine form B.40
The photodegradation of photochromic diarylethene 4
(Figure 1A) and spiropyran 7 (Figure 1B) has been studied in
solution in the presence of oxygen (in the air) with and without
imidazole. For the spiropyran 7 has also been performed
additional experiment: the photodegradation has been carried
out under inert conditions (argon). The results obtained
indicate that imidazole very effectively prevents
photodegradation of these photochromic compounds. It was
found that the stability of the photochromes depends on the
imidazole concentration, the higher it is, the higher the
stability. Similar results have been obtained for diarylethenes
5, 6 and spiropyran 8 (see section VII in ESI). The partial
photodegradation of these compounds is likely due to the
structural features of these photochromic molecules.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Russian Science Foundation (RSF
grant 18-13-00308).
Notes and references
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6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8PP00507A
It was found that imidazole prevents the side process of diarylethenes photocyclization
and the photodegradation of photochromic compounds.