Synthesis and comparative photoswitching studies of unsymmetrical 2,3-diarylcyclopent-2-en-1-ones

Article abstract of DOI:10.1021/jo500177k

Photochromic diarylethenes (DAEs) based on the unsymmetrical ethene bridge bearing heterocycles of the different nature (oxazole and thiophene) as aromatic moieties have been designed and the photoswitching properties have been studied. The comparative studies of the photochromic characteristics of unsymmetrical isomeric 2,3-diarylcyclopent-2-en-1-ones have shown that the isomers have different thermal stability, absorption maxima, and quantum yields. It was found that the unsymmetrical diarylcyclopentenone bearing at second position of the cyclopentenone ring the thiophene unit displays high thermally stability, hypsochromic shift of absorption maxima wavelengths of initial and cyclic forms, and high quantum yields of cyclization and cycloreversion reactions. The replacement of the carbonyl group with oxime leads to a reduction of the difference in the photochromic properties of these isomers and just as the reduction of the carbonyl group to the hydroxy-group negates this difference to zero. The intramolecular hydrogen bond formation in the oxime and hydroxy derivatives was confirmed by IR and ¹H NMR spectral analysis, but the increase of the quantum yields of the cyclization reaction in a nonpolar hexane is observed only in the case of hydroxy derivatives that can be explained by the formation of more rigid six-membered heterocycle in hydrogen bonding.

Full text of DOI:10.1021/jo500177k

Article
pubs.acs.org/joc
Synthesis and Comparative Photoswitching Studies of
Unsymmetrical 2,3-Diarylcyclopent-2-en-1-ones
Valerii Z. Shirinian,*,† Andrey G. Lvov,† Mikhail M. Krayushkin,† Elena D. Lubuzh,†
and Boris V. Nabatov‡
†N. D. Zelinsky Institute of Organic Chemistry, RAS, 47 Leninsky prospekt, 119991 Moscow, Russian Federation
‡A. V. Shubnikov Institute of Crystallography, RAS, 59 Leninsky prospekt, 119333 Moscow, Russian Federation
S
* Supporting Information
ABSTRACT: Photochromic diarylethenes (DAEs) based on the
unsymmetrical ethene “bridge” bearing heterocycles of the different
nature (oxazole and thiophene) as aromatic moieties have been
designed and the photoswitching properties have been studied. The
comparative studies of the photochromic characteristics of unsym-
metrical isomeric 2,3-diarylcyclopent-2-en-1-ones have shown that the
isomers have different thermal stability, absorption maxima, and
quantum yields. It was found that the unsymmetrical diarylcyclo-
pentenone bearing at second position of the cyclopentenone ring the
thiophene unit displays high thermally stability, hypsochromic shift of
absorption maxima wavelengths of initial and cyclic forms, and high
quantum yields of cyclization and cycloreversion reactions. The
replacement of the carbonyl group with oxime leads to a reduction
of the difference in the photochromic properties of these isomers and
just as the reduction of the carbonyl group to the hydroxy-group negates this difference to zero. The intramolecular hydrogen
bond formation in the oxime and hydroxy derivatives was confirmed by IR and 1H NMR spectral analysis, but the increase of the
quantum yields of the cyclization reaction in a nonpolar hexane is observed only in the case of hydroxy derivatives that can be
explained by the formation of more rigid six-membered heterocycle in hydrogen bonding.
Therefore, the development of new photochromic systems with
the wide possibilities of chemical modification that would
provide easily tunable switching properties is the actual point of
this field.
In this context, recently we have proposed a new class of
photochromic diarylethenes, 2,3-diarylcyclopent-2-ene-1-ones
(DCPs), having high abilities to the chemical modification
reactions of ethene “bridge”,18,19 and it has been obtained a
wide range of photochromic compounds, whose spectroscopic
and kinetic characteristics have been also studied (Chart
1).18,20−22
INTRODUCTION
■
The molecular devices that can be activated using various
external stimuli, e.g., chemical, electrochemical, and photo-
chemical, hold enormous promise for the advancement of
nanotechnology.1−4 Considerable recent attention has been
focused on photoswitch molecules as an active unit for
photoreversible control of various physical and chemical
properties.5−8 The different types of photochromic compounds
have been so far synthesized in an attempt to apply the
compounds to optoelectronic devices.9−12 Among the photo-
chromic compounds, diarylethene derivatives are the most
promising candidates for the applications because of their
fatigue resistant and thermally irreversible properties.12−15
The photochromic properties of diarylethenes have been
studied quite extensively, and the elucidation of relationships
between structure and spectral properties of photochromic
diarylethenes (DAEs) is at the center of ongoing investigations
which aim at developing a molecule with improvement in
switching properties that will ultimately be used in advanced
photonic devices.12−17 However, in spite of many of these
studies, there are no widely available synthetic tools for the
rational design of the DAE structure, including the directed
synthesis of photochromic molecules with two or more
improved features, and too often attempts to improve the
certain characteristics leads to deterioration of other properties.
Chart 1. Photochromic 2,3-Diarylcyclopent-2-en-1-ones
Received: January 24, 2014
Published: March 25, 2014
© 2014 American Chemical Society
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Chart 2. Classification of Diarylethenes under Their Symmetry
Table 1. Calculated Values of Ground State’s Energy Differences of Model Structures
These studies have demonstrated that 2,3-diarylcyclopent-2-
en-1-ones can be efficient to produce new photochromic
systems with easily switchable properties and the presence of
carbonyl groups in the ethene “bridge” not only greatly
facilitates the introduction of functional groups but also gives
molecule the nonsymmetry about the central ethene “bridge”
and significantly affects the spectral properties.
The photochromic diarylethenes can be divided into three
groups based on the symmetry of the molecule: the first type is
symmetrical DAEs when the ethene “bridge” has the symmetry
about the central double bond and aryl moieties are the same
(Chart 2, structure I), the second group includes diarylethenes
in which either the ethene “bridge” is a symmetrical and the aryl
groups are different or the ethene “bridge” is a unsymmetrical
and aryl residues are the same (Chart 2, structures II and III),
and the third type involves the compounds in which the ethene
“bridge” is unsymmetrical and the aryl moieties are not the
same (Chart 2, compounds IVa,b). Only in the latter case,
unlike the second type, there are two possible structural
isomers that provide opportunities of a comparative study of
photochromic properties of these compounds.
Most of known DAEs mainly belong to symmetrical
molecules I (perfluorocyclopentene, cyclopentene, maleic
anhydride, or maleimide derivatives) and both aryl residues
are similar (thiophene or benzothiophene derivatives). The
photochromic properties of unsymmetrical systems (structures
II and III) have been studied in many works,23−29 but such
nonsymmetry suggests only one isomer and the study of the
relationship between the nonsymmetry of the molecule and the
spectral properties impossible. However, up to now there are
no data on a comparative study of the spectral and kinetic
properties of unsymmetrical diarylethenes having two structural
isomers (structures IVa and IVb).
substituent in the ethene “bridge”, the nature of the aryl
moiety, and the position of attachment of the aryl residue to the
ethene “bridge”.18,21 Therefore, in this work we have
synthesized and studied the photochromic properties of such
unsymmetrical diarylethenes of cyclopentenone series bearing
oxazole and thiophene derivatives as aryl moieties. The goal of
the study was to perform the comparative analysis of the switch
performance of unsymmetrical 2,3-diarylcyclopent-2-en-1-ones
and to use the high abilities to the chemical modification of
these compounds for the development of the photochromic
DAEs with improved thermal stability and high quantum yields.
RESULTS AND DISCUSSION
■
Design of Diarylethenes. To study the spectral properties
of unsymmetrical diarylethenes and to develop the photo-
chromic compounds with high thermal stability and improved
quantum yields, the cyclopentenone ring was chosen as an
ethene “bridge” and the thiophene and oxazole derivatives as
aryl moieties. The diarylcyclopentenones as the main core have
been chosen because as noted above of the unique ability of the
ethene “bridge” of these compounds (cyclopentenone ring) to
the different chemical modification.
The selection of oxazole ring along with thiophene as the aryl
moiety was preceded by preliminary quantum-chemical
calculations of the aromatic stabilization energy of 1,3-azole
cycles, thiophene, and benzene rings based on density
functional theory (DFT). Irie and co-workers previously
pointed out that the aromatic stabilization energy of the aryl
groups correlates well with the ground-state energy difference
and the aromaticity is the key molecular property which
controls the thermal stability of the closed-ring iso-
mers.12,17,31,32 The performed quantum chemical calculations
are summarized in the Table 1. It was found that oxazole,
thiazole, and thiophene rings have the low aromatic
stabilization energies. However, the choice of thiophene and
Our recent studies have shown that the photochromic
features of isomeric unsymmetrical 2,3-diarylcyclopent-2-en-1-
ones are quite different and strongly dependent on the
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Scheme 1. Synthesis of 2,3-Diarylcyclopent-2-en-1-ones Bearing Thiophene and Oxazole Units
Scheme 2. Synthesis of Oxime and Hydroxy-Derivatives of 2,3-Diarylcyclopent-2-enes
Scheme 3. Photochromic Reactions of Dihetarylcyclopentene Derivatives
oxazole derivatives as aryl moieties was also dictated by the
synthetic potential of these compounds.
Synthesis. The preparation of 2,3-diarylcyclopent-2-en-1-
ones bearing thiophene and oxazole units as aryl moieties is
based on our recently developed method.18 The key stage of
this synthetic protocol is the alkylation of ketoesters 1 by
bromoketones 2 in the presence of metallic sodium in the
absolute benzene. The subsequent cyclization in aqueous
ethanol solution leads to photochromic diarylethenes 4
(Scheme 1). The presence of the acidic proton between the
carbonyl and ester groups excludes the use of other reaction
conditions than potassium hydroxide in aqueous alcoholic
medium because the cyclization reaction is possible only after
hydrolysis of the ester group.
To obtain the photochromic molecules capable of forming
intramolecular hydrogen bonds and thereby to regulate the
geometry of the molecule, oxime and hydroxy derivatives 5a−c
and 6a−c have been synthesized by simple methods,
respectively (Scheme 2). Alcohol derivatives 6a−c bearing an
In addition, an advantage in the selection of the thiophene
and oxazole derivatives as the aryl moieties have been caused by
other factors. At first, the thiophene derivatives are widely
studied and have proven themselves as a promising photo-
chromic DAEs. The presence of an sp2-hybridized nitrogen
atom in the oxazole ring is an important premise for the
development of noncovalent bonds, including hydrogen.
Noncovalent bonds have been used for the increasing of the
quantum yields of the direct photoreaction due to the
stabilization of the photoactive antiparallel conformation in
more recent years.33−37 Also the sp2-hybridized nitrogen atom
can participate in the formation of the supramolecular
structures.38−40 And finally, the azole compounds are more
resistant to photoirradiation, acids, and oxidants, which make
them more attractive as photoswitches.
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Table 2. Spectral and Kinetic Properties of Ketones 4a−d, Oximes 5a−d, and Alcohols 6a−c
a
b
c
Absorption maxima (extinction coefficients) of open-ring (A) and closed-ring (B) isomers. Quantum yields of photocyclization. Quantum yields
of cycloreversion under irradiation with 517 nm for ketones and oximes and 436 nm for alcohols.
oxazole ring have been prepared by the reduction of the
corresponding ketones 4a−c with high yields under mild
conditions. However, attempts to reduce 4d to the correspond-
ing alcohol were unsuccessful because the latter unlike oxazole
analogues was unstable. Such a difference in the stability of
these compounds can be attributed to the existence of intra- or
intermolecular interaction of the hydroxy-group with sp2-
hybridized nitrogen atom in the oxazole derivatives.
The nonsymmetry of the molecules 5a and 6a (which are not
capable of forming an intramolecular hydrogen bond) and
compounds 5b and 6b (which can form intramolecular
hydrogen bonds) is an excellent tool for studying the effect
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Figure 1. Absorption spectra of compounds 4a,b, before irradiation and at PSS after irradiation with UV 313 nm in acetonitrile (C = 2 × 10−5 M) at
293 K.
of hydrogen bond interaction on the spectral and kinetic
properties of diarylethene including the quantum yields of the
photochromic cyclization reaction.
maximum of the colorless form A (compare λA values for
compounds 4a−d in Table 2) and the maximum shift is
observed for the bis-oxazole derivatives 4c.
Photoswitching Properties of DCPs. The mechanism of
the photochromic switching of DAEs involves the electrocyclic-
cycloreversion reactions of the hexatriene system provoked by
UV−vis light6 (Scheme 3).
The photochromic characteristics of diarylcyclopentene
derivatives have been measured in acetonitrile and hexane
solutions at 293 K in the presence of air (oxygen) upon UV
irradiation, and the colorless solutions of DCPs were converted
into colored the ones with an absorption bands in visible range
of spectrum and bleached back to colorless under visible light
(Table 2).41
The measuring and comparing of the photochromic
properties of the synthesized diarylethenes were used to
estimate the correlations between their structures and different
photoswitching characteristics such as absorption maxima,
extinction coefficients of open- and closed-ring isomers, and
thermal stability as well as the quantum yields of photo-
cyclization and photobleaching reactions (Table 2). For the
purpose of revealing the correlation between the molecular
structures of the diarylcyclopentenone derivatives and the
spectral properties, some previously published spectral proper-
ties of the diarylcyclopentenones 4d and 5d have been
used.18,21
The photoswitching features of DAEs obtained have been
studied by different methods: UV−vis, NMR spectroscopy, and
IR spectrometry. It was found that all compounds show
photochromic properties that are strongly dependent on the
substituent in the ethene “bridge”, the nature of the aryl moiety,
and the position of the aryl residue connection to the “bridge”
(the nonsymmetry of the molecule). The typical changes
observed in the absorption spectra of the photochromic
reactions of diarylethenes 4a,b in acetonitrile are shown in
Figure 1. The spectral characteristics and quantum yields of the
photoreactions are summarized in Table 2. As it can be seen
from Table 2, the replacement of the thiophene ring with
oxazole results in a hypsochromic shift of the absorption
The absorption maxima wavelengths of the photoinduced
forms of the compounds studied also depend on the attached
position of the aromatic residues. The maximum hypsochromic
shift of the absorption band (42 nm) is observed for the bis-
oxazolylcyclopentenone 4c relative to bis-thiophene 4d. But
there are not any changes in absorption maxima of photo-
induced form of 2-substituted oxazole isomer, while the
absorption maximum of the isomer having an oxazole ring on
C-3 position shifted hypsochromically compared to bis-
thiophene derivative 4d by about 23 nm. It was found that
there is no direct correlation between the extinction coefficient
and structure of these compounds. So the extinction
coefficients of cyclic isomers differ little, but for open forms
the highest value was found for photochromic bis-oxazole 4c
(4.02 × 104 M−1 cm−1) and bis-thiophene 4d has the lowest
value (0.94 × 104 M−1 cm−1).
A significant effect of the carbonyl group on the spectral
characteristics of the unsymmetrical diarylethenes have also
been observed in the NMR spectra of compounds 4a and 4b
before and after irradiation with 365 nm at 293 K in CDCl3.42
It has been found that the signal of the hydrogen atom of
thiophene ring of the compound 4a (6.54 ppm) after UV
irradiation has shifted downfield (to 6.84 ppm), whereas the
same signal of the photochrome 4b (6.64 ppm) has shifted
upfield (to 5.78 ppm). The downfield shift of the thiophene
ring proton in the 1H NMR spectrum of the closed form of 4a
is likely due to the close proximity of the carbonyl group. In the
second case, the effect of the carbonyl group is not possible,
and a shift in the strong-field direction is only a consequence of
a breach of the aromaticity of the thiophene ring. The upfield
shift of thiophene proton of the cyclic form of the compound
4b is in good agreement with the results for dithienylethenes
obtained previously.30,43
The replacement of the carbonyl group with oxime leads to a
hypsochromic shift of the maxima of cyclic forms, while the
effect on the absorption maxima of open forms with the
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Table 3. Thermal Stability of Dihetarylcyclopentenones 4a−d and Their Hydroxy Derivatives 6a,b
exception for bis-thiophene derivative is not obvious. The
difference in the absorption maxima of the open and cyclic
forms of isomers 5a and 5b as well as its extinction coefficients
is significantly less than that between the same parameters of
compounds 4a and 4b. However, similar patterns are also
observed for the oxime derivatives, and the longest wavelength
of the absorption maximum is observed for isomer 5b, where
oxazole cycle is attached to the second position of cyclo-
pentenone ring (500 nm) and the shortest wavelength of the
absorption maximum is observed for bis-oxazole isomer 5c
(477 nm).
The reduction of the carbonyl group leads to hydroxy-
derivatives 6a−c. It should be noted that the reduction of bis-
thiophene derivative 4d leads to an extremely unstable
compound and therefore there are no experimental data on
its spectral properties. The hydroxy-group effect unlike the
carbonyl or the oxime groups on the spectral properties was
found to be small, and the difference in the spectral properties
of isomeric compounds 6a−c is strongly leveled. In the
hydroxy-compounds, the oxygen atom of the hydroxy group is
completely excluded from the chromophore system so the
absorption maxima of the open and cyclic forms of these
compounds are shifted toward the blue spectral region.
Thermal Stability of DCPs. The thermal stability (stability
of open- and closed-ring isomers of DAEs for a long time in the
dark) is a main property of photochromic compounds essential
for their various applications.12,16 Less aromatic stabilization
energies of thiophene and oxazole as noted above are
responsible for the stability of closed-ring isomers of diary-
lethenes by reducing the degree of destabilization of closed-ring
isomers to the open-ring form. The half-lifetimes (τ1/2) of the
closed-ring isomer of some photochromic compounds, ketones
4a−d and alcohols 6a,b, were measured in acetonitrile solutions
(C = 2 × 10−5 mol L−1) at 293 K in the presence of air (Table
3) and the dependence of this parameter on the structures of
the photochromic compounds has been established. The
thermal stability of oximes 5a−d was not studied, so recently
it was shown that the replacement with oxime group of
carbonyl function results in a significant decrease of this
photoswitching characteristic.21
The half-lifetime (τ1/2) of closed-ring isomer of bis-
thiophene derivative 4d was previously estimated to be 940 h
at 293 K.20 The replacement of the thiophene ring with
oxazole, the latter has a smaller stabilization energy than
thiophene (entries 2 and 3 in Table 1) leads to the
improvement of the τ1/2 values to 1900 h for 4b, to 3200 h
for 4a, and to 9600 h for bis-oxazole compound 4c (entries 1, 2,
and 3 in Table 3). The contribution to increase the half-lifetime
(τ1/2) of closed-ring isomer of these compounds brings not
only the low aromaticity of oxazole unit but also the
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nonsymmetricity of diarylethene molecule. For structurally
isomeric DAEs 4a and 4b, the very different values of the half-
lifetime (3200 h vs 1900 h, entries 1 and 2 in Table 3) were
obtained. The reduction of the carbonyl group resulted in an
unexpected results; the half-lifetime (τ1/2) of the closed-ring
isomer for two hydroxy group bearing diarylethenes 6a,b was
estimated to be 14200 and 15700 h, respectively, and it is an
order of magnitude more than that for the starting diary-
lcyclopentenones (entries 5 and 6 in Table 3).
Quantum Yields of Photocyclization and Photo-
bleaching Reactions of DCPs. The efficiency of photo-
chromic reactions of diarylethenes obtained has been estimated
by quantum yield values. The efficient utilization of light energy
in photochromic processes (high quantum yield) is an
important performance, for example to reduce the time of
light exposure on materials and objects, which is especially
important for biological systems. In some work previously, the
influence of the different substituted of diarylethenes on
photochromic properties has been studied.44−47 However, in
fact, the quantum yield is the most difficult predictable property
and to predict up to the actual synthesis of the molecule is
almost impossible.44
Our earlier studies have also shown that it is very difficult to
bring out the accurate correlation between the structure of
diarylcyclopentenones and quantum yields of their photo-
chromic reactions, however, some correlation between these
parameters has been estimated.18,21,48 The results of the
measurements of quantum yields of cyclization and cyclo-
reversion reactions of compounds obtained are given in Table
2. The quantum yields measurements were performed in
acetonitrile and hexane solutions (C ≈ 3 × 10−5 M) in the
sequential irradiation of the filtered light (313 and 517 nm). As
can be seen from Table 2, there are certain regularities between
the structure and quantum yields of cyclization reaction of
diarylethenes obtained depending on the polarity of the used
solvent. First of all, it should be noted that in acetonitrile
solution the quantum yields of the cyclization reaction of the
ketones 4a−d have the same order 20−27%, while in a
nonpolar hexane ones depend strongly on the symmetry of the
molecules. The quantum yields of the compounds bearing a
thiophene unit at second position (compounds 4a and 4d)
were found to be in two times higher values than ones of the
compounds having on C-2 position an oxazole ring and were
34% and 38%, respectively. Such a feature is likely related to a
presence of carbonyl group in these unsymmetrical diary-
lethenes, but in order to evaluate the accurate mechanism of
this phenomenon, more detailed studies are needed.
c) have been prepared and the quantum yields have been
measured.
The difference in quantum yields of oximes 5a−d in various
solvents (acetonitrile and hexane) is not observed, although it
was expected that owing to the formation of intramolecular
hydrogen bonds between the atoms of the oxime group and the
nitrogen atom oxazole ring the quantum yields in hexane
should be higher than in acetonitrile. The quantum yields of
unsymmetrical oximes 5a and 5b in both solvents is
significantly higher than the ones of their ketone analogues
4a and 4b (Table 2). The reduction of carbonyl group to a
hydroxy group leads to increasing of the quantum yields of the
latter in both solvents (acetonitrile and hexane). The best
results of quantum yields of the cyclization reaction (>50%)
were achieved for alcohols 6a−c in hexane solution. Moreover,
the quantum yields of 6b,c in hexane solution are considerably
higher than that in acetonitrile, whereas for the compound 6a
the one in hexane conversely decreases. Such patterns can be
explained by the presence of a hydrogen bond between the
hydrogen atom of the hydroxy group and the nitrogen atom of
the oxazole ring. To evaluate intramolecular interactions in
these compounds in solution, we have performed the IR study
of the two pairs of isomeric photochromes 5a,b and 6a,b. It was
found that the compounds 5b and 6b in contrast to 5a and 6a
form intramolecular hydrogen bonds, as evidenced by the
presence of the broad signals at 3440 cm−1 for compound 5b
and at 3270 cm−1 for 6b (Figure 2).54
In a solution the DAEs exist in two forms, parallel and
antiparallel conformers.31,49 According to the Woodward−
Hoffmann rule,50 the cyclization reaction occurs photochemi-
cally in a conrotatory mode and the conrotatory photo-
cyclization reaction can only proceed from the antiparallel
open-ring conformer. Because the interconversion between the
conformers occurs in a time scale larger than the excited-state
lifetime, only the light absorbed by the antiparallel form can
induce photocyclization.49 To achieve the large quantum yields,
there were attempts to fix a conformation capable to
photochemical cyclization.51−53 Therefore, in addition to
diarylcyclopentenones 4a−d, in order to achieve the large
quantum yields of the cyclization reaction, some diarylethenes
bearing different functional groups (oxime and hydroxy groups)
capable of forming intermolecular hydrogen bonds with the
nitrogen atom of the oxazole ring (compounds 5a−d and 6a−
Figure 2. IR spectra of diarylethenes 5a,b and 6a,b in chloroform
solutions (C = 0.005 M).
Thus despite the fact that the oxime and hydroxy derivatives
form hydrogen bonds, but only in the latter case there is a
relationship between the molecular “geometry” and the
quantum yields. This is probably due to the fact that the
intramolecular hydrogen bond between the oxime group and
sp2-nitrogen atom of oxazole ring leads to a seven-membered
cycle formation, whereas the hydroxy-derivatives gives the six-
membered ring (Chart 3). And as it is known the six-membered
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UV irradiation of the solution of compound 6b also leads to
the formation of two diastereomeric products, but the
proportion is another, 76:24 (Figure 4). This process is also
reversible. The high selectivity of a photoreaction product 6b′
(76%) indicates the presence of a hydrogen bond between
hydroxy-group and nitrogen atom of oxazole ring, which can
control the molecule conformation, and this in turn leads to the
formation of predominantly one diastereomer pairs. It should
be noted that the results obtained are in good agreement with
literature data.34,55
Chart 3. The Intramolecular Hydrogen Bond of Compounds
9b and 10b
ring has a more rigid ‘geometry’ (conformation) than the
seven-membered ring, and this in turn leads to an increase in
the quantum yield of the direct reaction of hydroxy-substituted
compounds 6b,c in an apolar environment (hexane).
We also evaluated the effectiveness of the cycloreversion
reaction of diarylcyclopentenones synthesized. To date, there
are no effective ways to improve the cycloreversion reaction
efficiency (>50%). In many works,44,56,57 it was attempted to
study the effect of various factors on the quantum yields of
cycloreversion, but the effective tool has not been found yet.
However, some patterns are promising to improve this
parameter. In particular, in the work44 it has been shown that
the introduction of various substituents at the reactive carbons
leads to a significant change of the cycloreversion quantum
yields. So the introduction of methoxy group leads to a strong
suppression of the cycloreversion quantum yield to less than
10−4, while cyano or a π-conjugated groups enhanced the
reactivity to 41%.44
Additional confirmation of the existence of an intramolecular
hydrogen bond between the hydroxy group and the nitrogen
atom of the oxazole ring in the compound 6b was obtained in
the comparative analysis of 1H NMR spectra of cyclic forms of
1
isomeric alcohols 6a and 6b. The H NMR spectrum of the
sample of the cyclic form obtained by irradiation of
diarylethene 6a indicates the formation of two different
photoproducts in the ratio 56:44 (Figure 3). The subsequent
irradiation with visible light restores the initial ring-opened
form that is clearly seen in the NMR spectrum. This fact, as
well as an approximately equal ratio of the photoproducts,
indicates that they are diastereomers 6a′ and 6a″. A slight
excess of one of them appears to be explained by the greater
content of the conformers 6a′ that does not contain steric
hindrance.
As it can be seen from Table 2, the cycloreversion reactivity
of diarylcyclopentenones prepared appeared also strongly
dependent on the symmetry of the diarylethene systems but
does not depend on the solvent polarity. The quantum yields of
the cycloreversion reaction of the isomeric ketones in
Figure 3. Structures of photogenerated diastereomers for 6a and NMR spectra of 6a in the field of protons of the thiophene ring after irradiation of
benzene solution by UV light (254 nm). For convenience, the structure of only one stereoisomer of 6a is given.
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Figure 4. Structures of photogenerated diastereomers for 6b and NMR spectra of 6b in the field of protons of the thiophene ring after irradiation of
benzene solution by UV light (254 nm). For convenience, the structure of only one stereoisomer of 6b is given.
acetonitrile 4a and 4b were estimated to be 0.166 and 0.012,
respectively. Interestingly, that similar tendency has been
observed for the oxime (5a, 0.144, 5b, <0.01, respectively)
and hydroxy derivatives (6a, 0.170, 6b, 0.055, respectively).
Probably in each case the nature of these patterns is different
because it is difficult to suggest that the influence of the
carbonyl and hydroxy groups on the photochromic properties
will be the same. The effect of solvent polarity on the efficiency
of the cycloreversion reaction of the diarylethenes obtained has
not been quite clear, but it should be noted that in most cases
in hexane solution the quantum yields are higher than that in
acetonitrile, although there are contradicting results. The results
obtained on the influence of the solvent polarity on the
quantum yields of the cyclization and cycloreversion reactions
are in good agreement with literature data.58
structural isomers has allowed us to perform a comparative
analysis of the spectral and kinetic properties of these
compounds. The study of the spectral properties of such
unsymmetrical 2,3-diarylcyclopent-2-en-1-ones has shown that
the isomers have different thermal stability, absorption maxima,
and quantum yields. The replacement of the carbonyl group
with oxime leads to a decrease of the difference in the
photochromic properties of these isomers, and the reduction of
the carbonyl function to the hydroxy-group absolutely removes
this difference. It was demonstrated that by simple
modifications of these diarylethenes photochromic compounds
with improved performance (the thermal stability of photo-
induced isomer and the quantum yields of the cyclization
reaction) can be obtained. It has been found that DAEs bearing
at the second position of the cyclopentene ring the oxazole unit
as aryl moiety and an oxime or hydroxy group as the ethene
“bridge” substituent are able to form the intramolecular
hydrogen bond that in the case of the hydroxy-diarylethene
leads to an increase of the quantum yield to 61% and the
improvement of the diastereoselectivity (up to 76%).
CONCLUSION
■
Thus, novel unsymmetrical photochromic dihetarylcyclopente-
nones bearing thiophene and oxazole rings as aryl moieties have
been developed. The relationship between structure of
photochromic molecule and different switching properties
(absorption maxima, thermal stability, and quantum yields)
has been estimated. It was found that the spectral characteristics
of these compounds depend on the substituents in cyclo-
pentene ring, nature of aryl residues, and the positions of the
attachment of the aryl groups to the ethene “bridge”. The
synthesis of the unsymmetrical DAEs consisting of two
EXPERIMENTAL SECTION AND METHODS
■
General Experimental Procedures. Proton nuclear magnetic
resonance spectra (1H NMR) and carbon nuclear magnetic resonance
spectra (13C NMR) were recorded in deuterated solvents on a
spectrometers working at 300 MHz for 1H and 75 MHz for 13C. Both
1H and 13C NMR chemical shifts are referenced relative to the solvents
3448
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The Journal of Organic Chemistry
Article
1
residual signals (CHCl3: δ 7.27 for H NMR and δ 77.16 for 13C
absorption spectroscopic measurement. The thermal bleaching process
was determined as a first-order reaction (linearity for relationships
between the logarithms of absorbance and exposal time) that is a well-
known fact.63
Synthetic Procedures and Characterization Data. The
ketoester 1b was prepared from corresponding hetarylacetic acid64
by the method used in ref 65. The ketoester 1a,65 bromoketones 2a66
and 2b,67 ketone 4d,18 and oxime 5d21 were synthesized according to
known procedures.
NMR) and reported in parts per million (ppm) at 293 K. Data are
represented as follows: chemical shift, multiplicity (s, singlet; d,
doublet; m, multiplet; br, broad), coupling constants in hertz (Hz),
integration, and assignment. Infrared spectra were measured on a FT-
IR spectrometer in KBr pellets or abs CHCl3 (C = 0.005 M) solutions
and are reported in terms of frequency of absorption (cm−1). Melting
points (mp) were recorded using an apparatus and not corrected. 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.
Microanalyses were obtained using an automatic CHNS/O elemental
analyzer. All chemicals and anhydrous 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.
Theoretical Calculations. Ground-state energies of open- and
closed-ring isomers were calculated by the B3LYP1 hybrid density
functional method (basis set 6-31G(d)). The computations were
performed with Firefly QC package,59 which is partially based on the
GAMESS (US)60 source code.
Ethyl 4-(5-Methyl-2-phenyl-1,3-oxazol-4-yl)-3-oxobutanoate
(1b). Yield 1.87 g (65%), light-brown oil. IR (KBr, thin film), cm−1:
1
2984, 2926, 1745, 1722, 1640, 1321, 1028, 715, 694. H NMR (300
MHz, CDCl3): δ = 1.25 (t, J = 7.1 Hz, 3H, CH3), 2.33 (s, 3H, CH3),
3.59 (s, 2H, CH2), 3.72 (s, 2H, CH2), 4.17 (q, J = 7.1 Hz, 2H, CH2),
7.39−7.44 (m, 3H, Hphenyl), 7.94−7.99 (m, 3H, Hphenyl). 13C NMR (75
MHz, CDCl3): δ = 10.3, 14.1, 40.6, 48.4, 61.4, 126.1, 127.4, 128.7,
128.9, 130.1, 146.3, 160.0, 167.2, 199.4. MS (EI, 70 eV): m/z (%) =
287 (30), [M]+, 241 (55), [M − EtOH]+, 215 (70), [M − EtOCO]+,
199 (35), [M − MeCOOEt]+, 173 (90), 172 (100). HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C16H18NO4, 288.1230; found,
288.1217.
Photochemical Studies. UV−vis spectra were recorded in 1.0 cm
quartz cuvettes. The experimental measurements were performed at
293 K in the presence of air in solutions of acetonitrile and hexane,
respectively. Photocoloration and photobleaching reactions were
carried out using a high-pressure mercury lamp as the exciting light
source. The required wavelengths (313, 436, and 517 nm) were
isolated by the use of the appropriate filters. Molar extinction
coefficients of the photogenerated isomers were determined as follows:
a diarylethene (10 mg) was dissolved in dichloromethane (3 mL), and
the solution was irradiated with UV light (313 nm) for 30−45 min in
1.0 cm quartz cuvette. The obtained deeply colored solution was
evaporated in the dark under vacuum, and the residue was used in
preparation of samples for NMR and UV/vis spectroscopy studies.
Molar extinction coefficients (εC) were calculated by eq 1:
2,3-Diarylcyclopent-2-en-1-ones (General Procedure). To a
solution of ketoester 1 (10 mmol) in abs benzene (30 mL), sodium
(0.23 g, 10 mmol) was added. The reaction mixture was stirred for 2 h,
and bromoketone 2 (10 mmol) was added portionwise. The mixture
was kept overnight, then poured into water (150 mL) and extracted
with ethyl acetate (3 × 50 mL). The combined organic phases were
washed with water (100 mL), dried with magnesium sulfate, and
evaporated in vacuum. The residue was dissolved in ethanol (37 mL),
and a solution of KOH (2.80 g, 50 mmol) in water (37 mL) was
added. The reaction mixture was refluxed until completion of the
reaction (monitored by TLC), then cooled, poured into water (100
mL), and extracted with ethyl acetate (3 × 50 mL). The combined
organic phases were washed with water (100 mL), dried with
magnesium sulfate, and evaporated in vacuum. The residue was
purified by column chromatography by petroleum ester/ethyl acetate
3:1 and recrystallized from ethanol.
D
εC =
conv × c0
(1)
2-(2,5-Dimethylthiophen-3-yl)-3-(5-methyl-2-phenyl-1,3-oxazol-
4-yl)cyclopent-2-en-1-one (4a). Yield 1.40 g (40%), white powder,
mp 174−176 °C. IR (KBr), cm−1: 3061, 2920, 2859, 1696, 1630, 1580,
1561, 1482, 1447, 1404, 1380, 1263, 1188, 1143. 1H NMR (300 MHz,
CDCl3): δ = 1.85 (s, 3H, CH3), 2.07 (s, 3H, CH3), 2.41 (s, 3H, CH3),
2.63−2.70 (m, 2H, CH2), 3.10−3.18 (m, 2H, CH2), 6.54 (s, 1H,
where D is the absorption at band maximum of photogenerated
isomer, conv is the conversion of diarylethene, and c0 is the total
concentration of the compound.
Quantum yields of ring-closure and ring-opening processes were
calculated by eqs 2 and 3, respectively:61
H
thioph), 7.43−7.52 (m, 3H, Hphenyl), 7.98−8.07 (m, 2H, Hphenyl). 13C
NMR (75 MHz, CDCl3): δ = 11.2, 14.4, 15.3, 29.3, 34.5, 126.2, 126.8,
127.1, 128.8, 129.0, 130.4, 133.3, 135.2, 135.5, 136.3, 149.0, 160.4,
161.4, 207.8. MS (EI, 70 eV): m/z (%) = 349 (70) [M]+, 334 (20), [M
− CH3]+. Anal. Calcd for C21H19NO2S: C, 72.18; H, 5.48; N, 4.01.
Found: C, 72.03; H, 5.36; N, 4.06.
D(∞) − D(0)
ln
= 2.303 × 103 × I0 × ε0 × φ
× t
O−C
D(∞) − D(t)
c0
cB(∞)
×
(2)
(3)
3-(2,5-Dimethylthiophen-3-yl)-2-(5-methyl-2-phenyl-1,3-oxazol-
4-yl)cyclopent-2-en-1-one (4b). Yield 1.12 g (32%), gray crystals, mp
176−178 °C. IR (KBr), cm−1: 2960, 2908, 2853, 1701, 1639, 1595,
1556, 1483, 1434, 1323, 1260, 1120, 1063. 1H NMR (300 MHz,
CDCl3): δ = 2.07 (s, 3H, CH3), 2.09 (s, 3H, CH3), 2.38 (s, 3H, CH3),
2.62−2.71 (m, 2H, CH2), 2.94−3.04 (m, 2H, CH2), 6.64 (s, 1H,
D(0)
D(t)
ln
= 2.303 × 103 × I0 × εC × φ
× t
C−O
where D(∞) is the absorption at band maximum of photogenerated
isomer in photostationary state (PSS), D(t) is the absorption at time t,
and D(0) is the initial absorption, I0 is the irradiation light intensity, ε0
and εC are molar extinction coefficients of initial and photogenerated
isomers at the irradiation wavelength, respectively, c0 and cB(∞) are
the total concentration and the concentration of photogenerated
isomer in PSS, respectively, φO−C and φC−O are cyclization and ring-
opening quantum yield, respectively, and ε0 × c0 and εC × cB(∞) are
lower than 0.2.
1,2-Bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene in
hexane solution45 and azobenzene in methanol solution62 were used
as a chemical actinometer.
Kinetic Studies. The half-lifes of thermal bleaching for the
diarylethenes were determined as follows. The solution of open-ring
form in acetonitrile (C ≈ 2 × 10−5 M) was irradiated with UV light (λ
= 313 nm) until reaching the photostationary state and was kept in
dark at 293 K. The reaction yields were periodically determined by
H
thioph), 7.34−7.44 (m, 3H, Hphenyl), 7.90−8.00 (m, 2H, Hphenyl). 13C
NMR (75 MHz, CDCl3): δ = 11.1, 15.0, 15.1, 31.5, 35.0, 125.3, 126.3,
127.7, 128.6, 129.0, 129.9, 131.9, 133.4, 136.8, 137.3, 147.5, 160.2,
167.8, 206.8. MS (EI, 70 eV): m/z (%) = 349 (25) [M]+, 244 (100).
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H20NO2S, 350.1209;
found, 350.1207.
2,3-Bis(5-methyl-2-phenyl-1,3-oxazol-4-yl)cyclopent-2-en-1-one
(4c). Yield 1.31 g (33%), red powder, mp 142−144 °C. IR (KBr),
cm−1: 3066, 2912, 2852, 1697,1650, 1601,1559, 1487, 1333, 1203,
1065, 1023. 1H NMR (300 MHz, CDCl3): δ = 2.12 (s, 3H, CH3), 2.30
(s, 3H, CH3), 2.67−2.74 (m, 2H, CH2), 3.16−3.23 (m, 2H, CH2),
7.38−7.49 (m, 6H, Hphenyl), 7.94−8.03 (m, 4H, Hphenyl). 13C NMR (75
MHz, CDCl3): δ = 11.5, 12.3, 29.7, 34.6, 126.2, 126.3, 127.1, 127.6,
128.7, 128.8, 128.8, 130.0, 130.4, 131.2, 133.1, 147.7, 149.3, 160.1,
160.3, 163.3, 206.9. MS (EI, 70 eV): m/z (%) = 396 (100), [M]+, 381
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Article
(30), [M − CH3]+. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C25H21N2O3, 397.1547; found, 397.1541.
(EI, 70 eV): m/z (%) = 351 (10), [M]+, 333 (30), [M − H2O]+.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H22NO2S, 352.1366;
found, 352.1363.
Oximes 5 (General Procedure). A mixture of ketone 4 (2.0
mmol), hydroxylamine hydrochloride (0.40 g, 6.0 mmol), and
anhydrous sodium acetate (0.64 g, 6.0 mmol) in ethanol (7 mL)
was refluxed for 3 h and poured into water (70 mL). The residue was
filtered off, washed with water (2 × 30 mL), and recrystallized from
ethanol.
2,3-Bis(5-methyl-2-phenyl-1,3-oxazol-4-yl)cyclopent-2-en-1-ol
(6c). Yield 0.19 g (95%), yellow powder, mp 180−181 °C. IR (KBr),
cm−1: 3307, 2912, 2852, 1625, 1553, 1484, 1449, 1086, 1025, 721, 693.
1H NMR (300 MHz, CDCl3): δ = 1.94 (s, 3H, CH3), 1.98−2.09 (m,
4H), 2.45−2.54 (m, 1H), 2.94−3.02 (m, 2H, CH2), 5.15−5.25 (m,
1H, CH), 7.41−7.49 (m, 6H, Hphenyl), 7.97−8.05 (m, 4H, Hphenyl). 13
C
2-(2,5-Dimethylthiophen-3-yl)-3-(5-methyl-2-phenyl-1,3-oxazol-
4-yl)cyclopent-2-en-1-one Oxime (5a). Yield 0.65 g (90%), yellow
powder, mp 95−97 °C. IR (KBr), cm−1: 3273, 2917, 1701, 1628, 1488,
NMR (75 MHz, CDCl3): δ = 11.5, 29.8, 31.2, 34.8, 80.4, 126.2, 126.2,
127.3, 127.5, 128.8, 130.2, 130.3, 131.9, 132.7, 133.5, 145.6, 145.7,
159.78, 160.0. MS (EI, 70 eV): m/z (%) = 398 (15), [M]+, 380 (60),
[M − H2O]+. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C25H23N2O3, 399.1703; found, 399.1696.
1
1448, 1193, 867, 692. H NMR (300 MHz, CDCl3): δ = 1.83 (s, 3H,
CH3), 2.06 (s, 3H, CH3), 2.41 (s, 3H, CH3), 1.85−1.97 (m, 2H, CH2),
3.00−3.17 (m, 2H, CH2), 6.59 (s, 1H, Hthioph), 7.40−7.53 (m, 3H,
H
phenyl), 7.93−8.06 (m, 2H, Hphenyl). 13C NMR (75 MHz, CDCl3): δ =
10.9, 14.3, 15.3, 24.6, 32.5, 126.2, 127.0, 127.4, 128.8, 130.0, 130.1,
131.7, 133.3, 134.5, 136.2, 145.7, 146.9, 159.8, 168.4. MS (EI, 70 eV):
m/z (%) = 364 (25), [M]+, 347 (100), [M − OH]+. HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C21H21N2O2S, 365.1318; found,
365.1299.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C NMR spectra of compounds, UV−vis spectra of
diarylethenes 4−6, and IR spectra of diarylethenes 5a,b and
6a,b. This material is available free of charge via the Internet at
http://pubs.acs.org.
3-(2,5-Dimethylthiophen-3-yl)-2-(5-methyl-2-phenyl-1,3-oxazol-
4-yl)cyclopent-2-en-1-one Oxime (5b). Yield 0.62 g (85%), yellow
powder, mp 165−166 °C. IR (KBr), cm−1: 3262, 2917, 1643, 1555,
1488, 1448, 1334, 1143, 963, 775, 693. 1H NMR (300 MHz, CDCl3):
δ = 1.85 (s, 3H, CH3), 2.03 (s, 3H, CH3), 2.38 (s, 3H, CH3), 2.82−
2.99 (m, 4H, CH2), 6.52 (s, 1H, Hphenyl), 7.34−7.51 (m, 3H, Hphenyl),
7.99 (br s, 1H, OH), 8.05−8.15 (m, 2H, Harom). 13C NMR (75 MHz,
CDCl3): δ = 10.6, 14.7, 12.2, 25.0, 34.3, 125.6, 126.6, 127.3, 127.7,
128.5, 129.8, 130.0, 133.8, 134.5, 136.5, 146.8, 151.2, 160.1, 166.6. MS
(EI, 70 eV): m/z (%) = 364 (15), [M]+, 347 (30), [M − OH]+.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H21N2O2S, 365.1318;
found, 365.1315.
AUTHOR INFORMATION
Corresponding Author
*E-mail: shir@ioc.ac.ru.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by Russian Foundation for Basic Research
(RFBR grant 14-03-31871) is gratefully acknowledged. The
authors are grateful to Anna Bogacheva (I. Ya. Postovsky
Institute of Organic Chemistry, Ural Branch of RAS) for kindly
providing samples of 1,2-bis(2-methyl-1-benzothiophen-3-yl)
perfluorocyclopentene.
2,3-Bis(5-methyl-2-phenyl-1,3-oxazol-4-yl)cyclopent-2-en-1-one
Oxime (5c). Yield 0.72 g (87%), yellow powder, mp 190−192 °C. IR
(KBr), cm−1: 3283, 3055, 2914, 1615, 1488, 1449, 1334, 1196, 991,
1
867, 690. H NMR (300 MHz, CDCl3): δ = 2.02 (s, 3H, CH3), 2.07
(s, 3H, CH3), 2.90−3.02 (m, 2H, CH2), 3.04−3.14 (m, 2H, CH2),
7.34−7.48 (m, 6H, Hphenyl), 7.93−8.00 (m, 2H, Hphenyl), 8.05−8.12
(m, 2H, Hphenyl). 13C NMR (75 MHz, CDCl3): δ = 11.2, 11.7, 24.7,
32.8, 126.2, 126.5, 127.4, 127.6, 128.6, 128.8, 130.0, 130.2, 133.1,
146.7, 146.8, 160.1, 160.3, 167.0. MS (EI, 70 eV): m/z (%) = 411 (55)
[M]+, 394 (100), [M − OH]+. HRMS (ESI-TOF) m/z: [M + H]+
calcd for C25H22N3O3, 412.1656; found, 412.1644.
Alcohols 6 (General Procedure). A ketone 4 (0.5 mmol) was
dissolved in methanol (3 mL), and sodium borohydride (0.095 g, 2.5
mmol) was added slowly. The solution was stirred for 2 h, 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.
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3.09−3.23 (m, 1H), 5.04−5.13 (m, 1H, CH), 6.63 (s, 1H, Hthioph),
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1
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