Photokinetics of two novel photochromic diarylethenes derived from benzothiophene

Article abstract of DOI:10.1002/kin.20727

Two new unsymmetrically substituted photochromic diarylethenes, namely 3-[3, 3,4,4,5,5-hexafluoro-2-(2-methylbenzothien-3-yl)cyclopent-1-en-1-yl]-2- methyl-6-methoxy-1- benzothiophene and 3-[3,3,4,4,5,5-hexafluoro-2-(2- methylbenzothien-3-yl)cyclopent-1-en-1-yl]-2-methyl-6-methoxy-7-nitro-1- benzothiophene, were synthesized. Their optical properties and kinetics of cyclization and cycloreversion were determined in four solvents and compared with the symmetrical diarylethene (3,3,4,4,5,5-hexafluorocyclopent-1-en-1,2- diyl)bis(2-methyl-1-benzothiophene). While the UV spectra of the three compounds are almost insensitive to solvent changes, the rates of the ring-opening and ring-closure reactions exhibit interesting kinetic behavior that differs from other diarylethenes reported in the literature. The cycloreversion for the three compounds follows first-order kinetics, whereas the cyclization cannot be described by a simple kinetic law. Different approaches were tested, and a complex exponential equation could be derived that fits the experimental data. The unusual presence of two ring-opened conformational isomers, one of them nonphotochromic, is proposed for the ring-opened compounds to explain the derived kinetic law and observed solvent effects. These results are of interest in relation to the application of the novel diarylethenes as potential optical materials.

Full text of DOI:10.1002/kin.20727

Photokinetics of Two Novel
Photochromic
Diarylethenes Derived from
Benzothiophene
´
A. VAZQUEZ, N. SBARBATI NUDELMAN
Departament of Organic Chemistry, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad
Universitaria, C1428EHA Buenos Aires, Argentina
Received 29 July 2011; revised 7 January 2012; accepted 2 February 2012
DOI 10.1002/kin.20727
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two new unsymmetrically substituted photochromic diarylethenes, namely 3-[3,
3,4,4,5,5-hexafluoro-2-(2-methylbenzothien-3-yl)cyclopent-1-en-1-yl]-2-methyl-6-methoxy-1-
benzothiophene and 3-[3,3,4,4,5,5-hexafluoro-2-(2-methylbenzothien-3-yl)cyclopent-1-en-1-
yl]-2-methyl-6-methoxy-7-nitro-1-benzothiophene, were synthesized. Their optical properties
and kinetics of cyclization and cycloreversion were determined in four solvents and compared
with the symmetrical diarylethene (3,3,4,4,5,5-hexafluorocyclopent-1-en-1,2-diyl)bis(2-methyl-
1-benzothiophene). While the UV spectra of the three compounds are almost insensitive to
solvent changes, the rates of the ring-opening and ring-closure reactions exhibit interesting
kinetic behavior that differs from other diarylethenes reported in the literature. The cyclore-
version for the three compounds follows first-order kinetics, whereas the cyclization cannot
be described by a simple kinetic law. Different approaches were tested, and a complex expo-
nential equation could be derived that fits the experimental data. The unusual presence of
two ring-opened conformational isomers, one of them nonphotochromic, is proposed for the
ring-opened compounds to explain the derived kinetic law and observed solvent effects. These
results are of interest in relation to the application of the novel diarylethenes as potential
optical materials. ꢀ 2012 Wiley Periodicals, Inc. Int J Chem Kinet 1–9, 2012
C
optical memories, molecular switches, and holographic
optical recording medium [1]. Several materials ex-
hibit a color change when they are exposed to certain
types of radiation; when the color change is reversible,
the phenomenon is called “photochromism” [2]. Di-
arylethenes are an important type of photochromic
compounds; they change the conformation by effect
of the ultraviolet (UV) light: Both isomers of the
diarylethene are different not only in their absorp-
tion spectra but also in various physical and chemi-
cal properties [3,4]. Diarylethenes-bearing heteroaryl
rings are among the most promising photochromics for
INTRODUCTION
The photochromism is an interesting phenomenon that
is receiving renewed attention because of their use in
Correspondence to: N. S. Nudelman; e-mail: nudelman@qo
.fcen.uba.ar.
Contract grant sponsor: University of Buenos Aires.
Contract grant sponsor: Agencia Nacional de Promocio´n
Cient´ıfica y Tecnolo´gica (ANPCYT).
Supporting Information is available in the online issue at
wileyonlinelibrary.com.
c
ꢀ
2012 Wiley Periodicals, Inc.

´
VAZQUEZ AND NUDELMAN
2
Scheme 1
various photoelectronic applications [5,6], because of
their fatigue-resistant and their thermally irreversible
properties [7–9].
4,4,5,5-hexafluoro-2-(2-methylbenzothien-3-yl)
cyclopent-1-en-1-yl]-2-methyl-6-methoxy-7-nitro-
1-benzothiophene,3 (Scheme 1). We selected a perflu-
orocyclopentene as the central bridging structure be-
cause of its demonstrated high fatigue resistance [15].
To determine the relevance of the substitution in one
of the phenyl rings, we selected two substituents of
opposite electronic effects and H-bonding character-
istics. The spectroscopic properties of the three com-
pounds in solvents of different properties (toluene, hex-
ane, acetonitrile, and methanol), as well as the kinetics
of the photochemical cyclization and cycloreversion
processes. The purpose of this study was to examine
the photokinetic properties of the three compounds as
potentially suitable materials for memory storage and
other applications.
These attractive features turn the study of the
physicochemical properties of diarylethenes, and
particularly their photokinetic behavior in a very
relevant question. Because of its importance, some
kinetic studies of diarylethenes are appearing in the
recent literature [10]. Wang and coworkers [11] have
described a study of an unsymmetrical diarylethene,
1-(2,5-dimethyl-3-thienyl)-2-(2-methyl-5-(2,4-
difluorophenyl))-perfluorocyclopentene and found
a kinetics of zero order for the UV-induced pho-
tocyclization process, whereas the visible-induced
photoreversion process exhibits first-order kinetics.
Likewise, Liu and coworkers [12] have determined
similar kinetic behavior for the thienyl diarylethene 1-
[2-methyl-5-(4-cyanophenyl)-3-thienyl]-2-[2-methyl-
5-(4-chlorophenyl)-3-thienyl]-perfluorocyclopentene.
Kobeleva et al. [13] have described the photochromism
of some diarylethenes with sulfur-containing sub-
stituents in DMSO with and without silver nanoparti-
cles. The photobleaching of solutions is well described
by straight lines, whereas the photocoloration kinetics
required complex equations. On the other hand,
Nagakawa and coworkers [14] have investigated
the thermal bleaching of the ring-closed isomer of
an angular terthiazole photochromic diarylethene
induced by acid in acetonitrile and have determined a
sigmoid shape in the absorbance versus time plot.
Owing to their potential applications, the search
for new photochromics is a very active area. In this
work, we describe the synthesis of three diarylethenes
derived from benzothiophene, namely (3,3,4,4,5,5-
hexafluorocyclopent-1-en-1,2-diyl)bis(2-methyl-1-
benzothiophene), 1; 3-[3,3,4,4,5,5-hexafluoro-2-(2-
methylbenzothien - 3 - yl)cyclopent - 1 - en - 1 - yl] - 2 -
methyl-6-methoxy-1-benzothiophene, 2; and 3-[3,3,
EXPERIMENTAL
General. All solvents used were of spectroscopic
grade and were purified by distillation before use. The
synthesis of photochromic compounds 1–3 was car-
ried out by the synthetic route as shown in Scheme 2,
adapting procedures that were previously applied
to the synthesis of related compounds [16,17]. Es-
sentially, 2-methyl-3-iodobenzothiophene, 4 (obtained
from iodination of 2-methyl-benzothiophene), was
lithiated at −78◦C and coupled with C5F8 render-
ing symmetric compound 1o (throughout this study,
“o” is used to indicate the “ring-opened” isomer).
For the methoxy-substituted derivatives, a more com-
plex synthetic route was designed. Starting from m-
methoxy-benzenethiol, 6-methoxybenzothiophene, 5,
was obtained and was then methylated (6), iodinated
(7), lithiated at −78◦C, and coupled with 3-(3,3,
4,4,5,5-heptafluorocyclopent-1-en-1-yl)-2-methyl-1-
benzothiophene, 8, rendering asymmetric compound
International Journal of Chemical Kinetics DOI 10.1002/kin.20727

PHOTOKINETICS OF TWO NOVEL PHOTOCHROMIC DIARYLETHENES
3
Scheme 2
2o. By nitration of compound 2 at 8◦C compound
3o was obtained. A more detailed description for the
synthesis of each one of the compounds presented in
Scheme 2 is available in the Supporting Information.
The identity of compounds1–3 was fully confirmed
by 1H and 13C NMR and by HRMS spectroscopic de-
terminations. NMR spectra were recorded on a Brucker
were measured at room temperature (ca. 24◦C) using
a Varian Cary 50 diode array spectrophotometer. Pho-
toirradiation was carried out using a Lumatec SUV-DC
lamp, with a monochromator. The required wavelength
was monitored by the use of the appropriate filters.
For UV irradiations, a 1-mm passband filter Schott
UG1 of 290–300 nm was used. For visible irradia-
tions, a 3-mm-long pass filter Schott OG of 515 nm
was used. Upon irradiation, a clear shift to the red was
1
200 spectrometer, with a H frequency of 200 MHz,
and a 13C frequency of 50 MHz. The absorption spectra
International Journal of Chemical Kinetics DOI 10.1002/kin.20727

´
VAZQUEZ AND NUDELMAN
4
3H), 2.15 (s, CH3 ap isomer, 3H); 13C NMR (CDCl3,
50 MHz): 157.3, 157.2, 142.6, 142.2, 139.8, 139.6,
139.4, 139.3, 138.3, 138.1, 138.1, 132.2, 124.6, 124.5,
124.3, 122.8, 122.6, 122.2, 122.1, 122.0, 121.8, 119.4,
119.2, 118.9, 118.7, 114.4, 114.2, 104.7, 104.4, 55.6,
55.5, 15.3, 15.2, 15.1, 15.0; HR-ESIMS: Calcd. for
C24H16F6NaOS2 (M + Na)+ = 521.04390. Found =
521.04553.
observed in all cases and two new bands appeared in
the UV–visible (UV–vis) spectra. For the kinetic de-
terminations, an aliquot of 2.5 mL of a solution of the
compound in the desired solvent was placed in a quartz
cell and irradiated during 10-s cycle periods, alternat-
ing with spectrophotometric determinations, until the
stationary state was achieved.
Spectroscopic Data
3-[3,3,4,4,5,5-Hexafluoro-2-(2-methylbenzothien-3-
yl)cyclopent-1-en-1-yl]-2-methyl-6-methoxy-7-nitro-
1-benzothiophene, 3o. H NMR (CDCl3, 200 MHz):
NMR and high resolution-electrospray ionization mass
spectra (HR-ESIMS) of the three synthesized com-
pounds are given below. The HR-ESIMS are fully con-
sistent with the structures of 1o–3o as shown in Scheme
1 as well as the NMR data. The full spectroscopic data
for each compound follow.
1
7.82 (d), 7.67 (m), 7.62 (m), 7.35 (t), 7.25 (d), 6.96
(d), 4.09 (s, OCH3 ap isomer, 3H), 3.99 (s, OCH3 p
isomer, 2.1H), 2.50 (s, CH3 p isomer, 2.2H), 2.47 (s,
CH3 p isomer, 2.2H), 2.21 (s, CH3 ap isomer, 3H),
2.19 (s, CH3 ap isomer, 3H); 13C NMR (CDCl3, 50
MHz): 138.3, 128.1, 128.0, 124.8, 124.6, 122.2, 121.5,
111.8, 111.5, 77.7, 77.0, 29.7, 26.8, 15.0, 14.5; HR-
(3,3,4,4,5,5-Hexafluorocyclopent-1-en-1,2-diyl)bis
(2-methyl-1-benzothiophene), 1o. H NMR (CDCl3,
1
ESIMS: Calcd. for C24H15F6NNaO3S2 (M + Na)+
566.02898. Found = 566.02709.
=
200 MHz): 7.68 (m), 7.34 (m), 7.19 (m), 2.50 (s, CH3 p
isomer, 3H), 2.22 (s, ap isomer, 6H); 13C NMR (CDCl3,
50 MHz): 142.6, 141.1, 138.3, 138.2, 138.0, 124.6,
124.4, 124.3, 122.1, 121.8, 119.4, 119.1, 15.2, 15.1;
HR-ESIMS: Calcd. for C23H14F6NaS2 (M + Na)+
491.03333. Found = 491.03292.
=
RESULTS AND DISCUSSION
The UV–vis spectra of diarylethenes 1–3 were reg-
istered selecting four solvents, of different properties
and polarity, namely n-hexane (aliphatic), toluene (aro-
matic), acetonitrile (dipolar, aprotic), and methanol
(dipolar, proton donor). Figure 1 shows the spectra
of the ring-opened isomer of compound 2 (the com-
plete set of spectra for compounds 1–3 is given in the
Supporting Information). It could be observed that no
3-[3,3,4,4,5,5-Hexafluoro-2-(2-metilbenzothien-
3-yl)cyclopent-1-en-1-yl]-2-methyl-6-methoxy-1-
1
benzothiophene, 2o. H NMR (CDCl3, 200 MHz):
7.69 (t), 7.54 (d), 7.34 (m), 7.22 (m), 7.07(d), 7.00
(dd), 6.80 (dd), 3.85 (s, OCH3 ap isomer, 2.8H), 3.77
(s, OCH3p isomer, 1.3H), 2.49 (s, CH3 p isomer, 1.7H),
2.44 (s, CH3 p isomer, 1.7H), 2.21 (s, CH3 ap isomer,
Figure 1 UV–Vis spectra of compound 2, before irradiation. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
International Journal of Chemical Kinetics DOI 10.1002/kin.20727

PHOTOKINETICS OF TWO NOVEL PHOTOCHROMIC DIARYLETHENES
5
Figure 2 Kinetics of cyclization of compound 2 in toluene. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
Table I Absorption Maxima of Photochromic
Compounds
absorption peaks are found in the visible region (400–
700 nm), as expected for colorless solutions. However,
upon exposure at UV radiation (∼ 300 nm) the solu-
tions turn to red and the compounds exhibit marked
absorptions in the visible region. Figure 2 shows the
spectra of compounds 2 in toluene, at different times
of irradiation. Two new maxima could be observed,
the first one near to 430 nm and the second at approx-
imately 520 nm. Table I shows the observed maxima
for the three compounds in the four solvents. While
examining the substitution pattern of compounds 1–3,
little substituent effect is observed: Thus in the case
of the presence of the methoxy group (compound 2),
a slight bathochromic effect is observed for the first
maxima, whereas for compound 3 the introduction of
the electron-withdrawing nitro group balances the ef-
fect. On the other hand, it could be observed that, for
the same compounds, the effect associated with the
change of solvent is very small.
Compound
Solvent
Hexane
Maxima (nm) Maxima (nm)
1
418
425
420
419
436
438
436
436
421
425a
518
525
525
523
515
525
526
523
515
525
516
517
Toluene
Acetonitrile
Methanol
Hexane
2
3
Toluene
Acetonitrile
Methanol
Hexane
Toluene
Acetonitrile Not observed
Methanol Not observed
aA third maxima at ca. 400 nm is observed.
solutions in a quartz cell at UV = 290–300 nm using
a filter, in periods of 10 s of irradiation, followed by
determinations of the UV–vis spectra of the solutions
using a diode array spectrophotometer (see Fig. 2). The
cycloreversion (aperture) cycle was achieved by expo-
sure at visible (nearly green) light (using a high-pass
filter), in periods of 10 s of irradiation, followed by
spectrophotometric determinations (see Fig. 3).
Figure 3 shows the spectra of compound 2 in
toluene, for different exposure times at visible light
(near 500 nm). It could be observed that the cyclization
process is completely reversible; in less than 2 min the
spectra are similar to that before UV irradiation, and a
complete decoloration of the solutions is observed.
The plot of the absorbance at 525 nm versus time
for the cyclization process of compounds 1–3 does
not agree with a simple order. This result is unusual,
since in most of the kinetic studies on diarylethenes
recently reported, linear trends are observed in
Kinetics of Closure and Reversion Cycles
The kinetics of cyclization and cycloreversion was
studied. Cyclization was achieved by irradiation of the
International Journal of Chemical Kinetics DOI 10.1002/kin.20727

´
VAZQUEZ AND NUDELMAN
6
Figure 3 Kinetics of ring opening of compound 2 in toluene. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
Table II Fitting Values for the Cyclization of
Compounds 1o, 2o, and 3oa
the plot of (ln(A/A0)) versus time. Thus, the
cyclization of [1-(2-methyl-5-(3-methoxyphenyl)-3-
thienyl) and 2-(2-methyl-3-benzothienyl)] perfluo-
rocyclopentene exhibits first-order kinetics [18];
1-(2-methyl-benzothiophenyl)-2-(2-methyl-5-(4-
chlorophenyl))-perfluorocyclopentene shows similar
kinetic trends [19]; and similarly, the kinetics
of cyclization of 1-[(2-methyl-5-(2-cyanophenyl)-
3-thienyl)]-2-[(2-trifluoromethylphenyl)] perfluorocy-
clopentene is also of first order [20].
However, in the present case, the data for the cy-
clization can be well fitted by using an exponential
function of the type of Eq. (1), where A is the ab-
sorbance at the maxima wavelength, t is the time, and
Y0, D, and kꢁ are fitting constants (see Fig. 2, inset).
Compound
Solvent
102 kꢁ (s−1
)
Y0
−D
1
Hexane
Toluene
Acetonitrile
Methanol
Hexane
6.24
6.23
4.61
5.35
7.15
5.08
5.49
5.36
5.44
6.79
1.84
5.93
0.243 0.241
0.305 0.301
0.487 0.481
0.193 0.180
0.063 0.054
0.186 0.174
0.163 0.163
0.050 0.056
0.213 0.210
0.183 0.175
0.554 0.543
0.217 0.214
2
3
Toluene
Acetonitrile
Methanol
Hexane
Toluene
Acetonitrile
Methanol
aR2 > 0.99 in all the cases. Concentrations in the range of (0.35–
3.85) × 10−4 M.
ꢁ
A = Y0 + D ek t
(1)
It could be observed that in all the cases Y0 ≈ −D;
thus Eq. (1) could be simplified to Eq. (2).
On the contrary, for the case of the cyclorever-
sion process, the data could be fitted into the function
(ln(A/A0)) versus time (see Fig. 3, inset). A straight
line is obtained, showing that the reversion process
exhibits first-order kinetics, following Eq. (3):
ꢁ
A ≈ Y0(1 − ek t )
(2)
Table II shows the fitting parameters for 1–3 in the
four solvents; the correlation coefficients are good in
all the cases. A mechanistic proposal is given below to
explain the exponential kinetic law.
v = −d[C] / dt = kꢁꢁA
(3)
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PHOTOKINETICS OF TWO NOVEL PHOTOCHROMIC DIARYLETHENES
7
Table III Linear Regression for the Reversion Process
of 1c, 2c, and 3ca
where C0 is the analytical concentration of pho-
tochromic compound.
Being the rate of cyclization given by Eq. (7):
Compound
Solvent
Hexane
102 k (s−1
)
R2
1
5.30
2.77
4.38
3.27
2.80
3.13
2.89
1.98
4.90
2.86
3.09
4.11
1.00
0.98
0.99
0.97
0.89
1.00
1.00
0.95
1.00
1.00
1.00
0.99
v = d[C]/dt = k1[Ap] − k−1[C]
(7)
Toluene
Acetonitrile
Methanol
Hexane
replacing Ap with the expression of Eq. (6) and ap-
plying the Lambert–Beer law in Eq. (7), Eq. (8) is
obtained:
2
3
Toluene
Acetonitrile
Methanol
Hexane
Toluene
Acetonitrile
Methanol
1/εl dA/dt = k1(C0 − A/εl)/(1 + Keq) − k−1A/εl
= k1C0(1 + Keq) − 1/el [k1/(1 + Keq)
+k−1]A
(8)
ꢀ
ꢀ
aConcentrations in the range of (0.35–3.85) × 10−4 M.
dA/p − uA = dt
where [C] is the concentration of the closed isomer
and kꢁꢁ is the pseudo–first-order rate coefficient. In fact,
under the conditions used to carry out the cyclorever-
sion experiments, that is, by irradiation with a light of
>500 nm, without UV components, only the (closed)
where
p = εlk1C0/(1 + Keq) u = k1/(1 + Keq) + k
Integration of the preceding equation renders
−1/u ln[(p − uA)/p] = t
−1
C isomer has significant absorbance. Thus, k >> k1.
−1
Under these conditions, considering that the Lambert–
Beer law is obeyed Eq. (4) could be obtained, where ε
is the extinction coefficient and l is the cell width.
A = p/u(1 − e−ut
)
(9)
−d[C] / dt = k−1 [C] = k−1 1 / εl A = kꢁꢁA (4)
This expression has the form of Eq. (2), where Y0 =
p/u and u = kꢁ.
Table III shows the first-order parameters for com-
pounds 1–3; in all the cases, good correlation coeffi-
cients were obtained. The linear trend is consistent with
some recent reports of first-order kinetics for other di-
arylethenes.
To explain the anomalous kinetic behavior of the
cyclization process, a more complex mechanism is
proposed, where the ring-opened isomer exhibits a
two-conformer equilibrium, parallel (P) and antipar-
allel (Ap), as shown in Scheme 3. This scheme would
lead to a complex rate equation; however, some condi-
tions can be considered to simplify the system leading
to a more simple kinetic law, as shown by Eq. (2).
To justify Eqs. (1) and (2), it is necessary to assume
that, under the conditions of the experiment, the iso-
mer equilibration, namely the P–Ap interconversion, is
faster than the cyclization/ring-opening process. Thus,
Therefore, according to the overall experimental re-
sults, if the proposal of the two conformers applies,
the fast component could be assigned to the direct
ring closure reaction, whereas the slow component is
assigned to the reaction through a previous confor-
mational change. Thus, the present results are con-
sistent with a conformational inhomogeneity model,
in which a broad distribution of conformations of the
ring-opened isomers in the ground state is projected
into two minima in the excited electronic potential sur-
face leading to the slow and the fast reaction pathways.
To the best of our knowledge, this is the first
time that two conformational isomers are proposed
for the ring-opened structure of aprotic diarylethenes;
nevertheless, it is worthwhile mentioning that it
has been previously proposed for the symmetrical
(3,3,4,4,5,5-hexafluorocyclopent-1-en-1,2-diyl)bis(2-
methyl-6-(ethylcarboxylic)-1-benzothiophene) [21].
In that case, the H bond that can be established be-
tween the carboxylic acid units at both 6-positions of
the benzothiophene rings when the compound is in
cyclohexane or decalin, firmly maintains the P form
and the cyclization is completely inhibited. Adding
EtOH disrupts the intramolecular H bonds and allows
[P] = Keq [Ap]
(5)
Likewise,
[Ap] + [P] + [C] = C0
[Ap] = C0 − A/εl − Keq[Ap]
[Ap] = (C0 − A/εl)/(1 + Keq)
(6)
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VAZQUEZ AND NUDELMAN
8
Scheme 3
the conrotatory photocyclization from the Ap form to
occur, since the Ap conformer is able to undergo the cy-
clization process, whereas the P conformer is not [22].
This property could be used in the so-called “gated
photochromism,” a special type of photochromism in
which one or both forms of the photochromic system
are transformed (chemically or electrochemically) re-
versibly into a nonphotochromic form [23].
a nitro group is present at the ortho position, the rate
of cyclization is very slow in the dipolar acetonitrile
but the rate spectacularly increases in methanol. Since
MeOH is a very good H donor, it is probably that the
steric requirements of the intramolecular H bonds with
the nitro group hinder the P conformation and prefer-
ably move the equilibrium to the Ap isomer, which is
ready to react.
To find out whether the ring-opened isomers of the
present compounds actually exist in two conforma-
tions, two additional evidences were looked for. A
All of the above results indicate that the novel di-
arylethenes exhibit interesting properties, their UV–vis
spectra are almost insensitive to solvent effects making
them suitable to be used in different media; the cycliza-
tion/ring opening cycles could be repeated more than
50 times in the four solvents keeping adequate pho-
tochromic performance; the ring-opened structures ex-
ist in two conformations differing as to whether the two
rings are in mirror symmetry (P) or in C2 symmetry
(Ap), a phenomenon that could be used in “gated pho-
tochromism”; the kinetic results show that the rate of
the process can be tuned up by an appropriate choice
of the solvent.
1
careful examination of the H NMR spectra of the
three ring-opened compounds shows some interesting
features. For the symmetric compound 1o, two signals
of different area could be recognized corresponding to
the methyl protons. Similarly, for compounds 2o and
3o, two pairs of methyl signals of different area can be
observed and likewise two signals for the methoxy pro-
tons. This fact can be interpreted as due to the presence
of two conformers of the ring-opened diarylethenes,
that we call P and Ap isomers. In base to the in-
tegration areas of methyl protons of each compound
(areap/areaap), it is possible to estimate the percentage
of each conformer of the ring-opened isomer. This af-
fords the following results: compound 1o: 33% P, 67%
Ap; compound 2o: 36% P, 64% Ap; compound 3o:
42% P, 58% Ap. The second evidence is related to the
kinetic parameters presented in Table II. The cycliza-
tion process for compounds 1 and 2 is slightly faster
in hexane than in polar solvents. Nevertheless, when
CONCLUSIONS
The photochemical properties and the kinetics of
photochromic compounds 1–3 were examined; these
novel diarylethenes exhibit interesting photochromic
properties, and they can be activated and deactivated
reversibly by switching on and off the closed form
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PHOTOKINETICS OF TWO NOVEL PHOTOCHROMIC DIARYLETHENES
9
as demonstrated by the synchronous change induced
by the UV–vis irradiation cycles. The cyclization/ring
opening cycles could be repeated more than 50 times
in the four solvents keeping adequate photochromic
performance. A complex exponential kinetics for the
cyclization was derived, which is consistent with a
model that considers the existence of a P and an Ap
isomer of the ring-opened compound. This P–Ap equi-
librium is reported for the first time in aprotic di-
arylethenes, though it has been previously reported in
carboxylic–benzothiophene diarylethenes where the P
form is maintained by intramolecular H bonds. The
sensitivity of the cyclization rate to solvent effects for
the nitro-substituted compound makes it possible to
tune up the photochromic process by suitably chosen
substituent and solvent.
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AJV is the grateful recipient of a post-doctoral fellowship of
the National Research Council (CONICET).
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International Journal of Chemical Kinetics DOI 10.1002/kin.20727