Photochromic properties of perhydro- and perfluorodithienylcyclopentene molecular switches

Article abstract of DOI:10.1002/ejoc.200200719

Various substituted phenylthienyl perhydro- and perfluorocyclopentenes have been synthesized in order to compare their spectroscopic and photochromic properties. The difference in the electron densities of the central cyclopentene moieties in the perhydrocyclopentene and perfluorocyclopentene molecular switches has only a small effect on the absorption maxima of the electronic spectra, but causes some subtle changes in substituent and solvatochromic effects. The photochromic behaviour is remarkably similar, and both type of switches combine excellent quantum yields (0.6) with high degrees of photoconversion (> 0.85). The main difference is the lower photochemical and thermal stability of the perhydrocyclopentene molecular switches. It is concluded that in most studies the perhydrocyclopentenes are an excellent alternative for the perfluorocyclopentenes, while the perfluorocyclopentenes might be better suited for applications such as data storage, which depend critically on fatigue resistance and thermal stability. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200200719

FULL PAPER
Photochromic Properties of Perhydro- and Perfluorodithienylcyclopentene
Molecular Switches
Jaap J. D. de Jong,[a] Linda N. Lucas,[a] Ralph Hania,[b] Audrius Pugzlys,[b]
Richard M. Kellogg,[a] Ben L. Feringa,*[a] Koos Duppen,[b] and Jan H. van Esch*[a]
Keywords: Cross-coupling / Electrocyclic reactions / Photochromism / UV/Vis spectroscopy
Various substituted phenylthienyl perhydro- and perfluoro- degrees of photoconversion (Ͼ 0.85). The main difference is
cyclopentenes have been synthesized in order to compare
their spectroscopic and photochromic properties. The differ-
ence in the electron densities of the central cyclopentene
the lower photochemical and thermal stability of the perhy-
drocyclopentene molecular switches. It is concluded that in
most studies the perhydrocyclopentenes are an excellent al-
moieties in the perhydrocyclopentene and perfluorocyclo- ternative for the perfluorocyclopentenes, while the perfluoro-
pentene molecular switches has only a small effect on the
absorption maxima of the electronic spectra, but causes some
subtle changes in substituent and solvatochromic effects. The
photochromic behaviour is remarkably similar, and both type
of switches combine excellent quantum yields (0.6) with high
cyclopentenes might be better suited for applications such as
data storage, which depend critically on fatigue resistance
and thermal stability.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
talline phases,[7] colour pattern formation,[8] electron trans-
fer,[9] and single-molecule fluorescence[10] have been demon-
strated. The photochemical switching process involves the
thermally irreversible ring-closed product (Scheme 1) and
both the open and closed compounds have high fatigue re-
sistance. In particular, the perfluoro derivatives of diary-
lethenes display these excellent properties and are therefore
most frequently used.[11,12]
In recent years there has been increasing interest in the
synthesis, properties and application of organic photo-
chromic materials as a possible basis for optoelectronic and
photooptical devices. Diarylethenes with heterocyclic aryl
groups belong to a new class of thermally irreversible pho-
tochromic switches.[1Ϫ3] For switching purposes, the most
commonly used diarylethenes are the diarylperfluorocy-
clopentenes,[4] followed by the diaryl maleic anhydrides[5]
and bisaryl maleimides.[6] Many functionalized derivatives
of these compounds have been synthesized,[1Ϫ3] and appli-
cations in, for example, reversible switching of liquid crys-
We have recently developed (dithienyl)cyclopentenes in
which the fluorine substituents on the central cyclopentene
ring are replaced by hydrogens.[13] The synthesis of these so
called ‘‘perhydrocyclopentene’ derivatives with R1 ϭ Cl can
be performed on a large scale starting from cheap chemi-
cals, and the basic switch can easily be derivatized to pro-
vide access to a wide variety of diarylcyclopentene photo-
chromic switches by a convergent synthetic route.[14] Be-
cause of their synthetic accessibility, these perhydrocyclo-
pentene molecular switches thus possess major advantages
over the perfluorocyclopentenes.
The important remaining question is whether these dithi-
enylcyclopentene-based switches are also a suitable choice
when it comes to their photochromic properties. Prelimi-
nary studies have revealed that they also exhibit excellent
photochromic switching behaviour, provided that the cor-
rect substituents (i.e., moieties that extend the conjugation
length of the switch) are present (R1 in Scheme 1).[13] De-
tailed insight into the photochemical processes was ob-
tained by time-resolved spectroscopic and theoretical
(DFT) studies on perhydrocyclopentene 5 (Scheme 2).
These investigation revealed that the ring-closure reaction
Scheme 1. Photochromism in dithienylcyclopentene molecular
switches
[a]
Department of Organic and Molecular Inorganic Chemistry,
Stratingh Institute, Materials Science Center, University of
Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Fax: (internat.) ϩ31-(0)503634296
Esch@chem.rug.nl
[b]
Ultrafast Laser and Spectroscopy Laboratory, Materials
Science Center, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Eur. J. Org. Chem. 2003, 1887Ϫ1893
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FULL PAPER
Scheme 2. Synthesis of (arylthienyl)perhydro- and perfluorocyclopentene photochromic switches; reagents and conditions: a) nBuLi, THF,
room temperature, b) B(OBu)3, THF, room temperature, c) ArBr, [Pd(PPh3)4], 2  Na2CO3(aq), ethylene glycol, THF, reflux
takes place with a time constant of 4.2 ps.[15] Although this higher yields, even when Lehn’s conditions were applied.[24]
time constant compares well with that for the ring-closure The bis(arylthienyl)cyclopentene switches 5؊12 were puri-
reaction of perfluorodithienyl switches (1Ϫ3 ps, depending fied by chromatography, and the symmetric nature of the
on the chemical structure of the switch),[16Ϫ18] direct com- products due to the double Suzuki coupling was confirmed
1
parison between the photochromic properties of perhydro- by H NMR, 13C NMR, and HRMS.
cyclopentenes and perfluorocyclopentenes can still not be
made because of the different structures (substituent R1 in
Scheme 1) of the compounds used throughout these studies.
It was therefore decided to synthesize a series of similarly
substituted perhydro- and perfluorocyclopentenes[19Ϫ23]
(Scheme 2) and to compare their spectroscopic and pho-
tochromic properties. The first results from these studies are
reported here.
The photochromic behaviour of the synthesized com-
pounds was first studied by irradiation with a high-pressure
mercury lamp with band-pass filters. The photochemical re-
actions were monitored by UV/Vis and 1H NMR spec-
troscopy. Before irradiation, compounds 5؊12 have strong
absorptions in the UV region (Table 1), and the chemical
shifts of the thienyl protons appear at positions character-
istic for the aromatic thiophene group. These data show that
compounds 5؊12 are initially present as the ring-open iso-
mers, as depicted in Scheme 1. The open form of the per-
hydro switch 5 and the corresponding perfluoro switch 10
have nearly the same absorption maxima, at 277 nm and
279 nm, respectively, in n-hexane (see Table 1, Figure 1), al-
though the central double bond in 5 is relatively more elec-
tron-rich than in 10. The presence of a cyano substituent,
however, causes a bathochromic shift of this absorption in
the case of the perhydrocyclopentene derivative 9 and a
hypsochromic shift in that of the perfluoro derivative 12.
These opposite effects must be directly related to the differ-
ent electron densities at the central double bond, and indi-
cate that there is a significant electronic interaction between
the phenylthienyl moiety and the central cyclopentene
group.
Results and Discussion
It has recently been shown that 2,2Ј-(dichlorodithienyl)-
cyclopentenes 1 and 2 are easily accessible and versatile
starting compounds for the synthesis of a wide range of
functionalized (dithienyl)perhydro- and perfluorocyclopen-
tenes.[14] In order to obtain the desired phenyl derivatives
of the two diarylethenes, 1 and 2 were first converted into
the bis(boronic) esters 3 and 4, respectively (Scheme 2).
These compounds were used directly in the Suzuki reaction
without any workup because it was found that the
bis(boronic) esters 3 and 4 easily deboronize during iso-
lation. Suzuki coupling with various aryl bromides gave the
corresponding bis(arylthienyl)cyclopentenes in yields up to
Irradiation of solutions of compounds 5؊12 at λ ϭ
76% after purification by column chromatography. It 313 nm resulted in the appearance of a broad absorption
should be noted that application of switch 1 in place of the band in the visible region, with λmax ranging from 529 nm
perfluoro switch 2 in the Suzuki coupling resulted in much to 603 nm (Figure 1, Table 1). Upon irradiation, the 1H
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Perhydro- and Perfluorodithienylcyclopentene Molecular Switches
FULL PAPER
Table 1. Absorption spectroscopic data and photochemical conversions of compounds 5؊12
Compound[a]
λmax Open (ε)[b]
n-Hexane
λmax Open (ε)[b]
Toluene
λmax Closed (ε)[b]
n-Hexane
λmax Closed (ε)[b]
Toluene
Conv.[c]
Toluene
[d]
[d]
[d]
[d]
5 (R3 ϭ H, R2 ϭ H)
6 (R3 ϭ Cl, R2 ϭ H)
7 (R3 ϭ Br, R2 ϭ H)
8 (R3 ϭ OMe, R2 ϭ H)
9 (R3 ϭ CN, R2 ϭ H)
10 (R3 ϭ H, R2 ϭ F)
11 (R3 ϭ OMe, R2 ϭ F)
12 (R3 ϭ CN, R2 ϭ F)
277
529
533
535
529
570
569
580
588
531 (16.8)
535 (16.7)
540 (16.4)
528 (17.5)
569 (11.0)
592 (14.5)
592 (17.5)
603 (18.5)
0.95
0.98
0.91
0.86
0.99
0.76
0.91
0.85
281, 312
284, 314
279, 309
295, 332
279
290, 316
271, 313
299 (17.7) 355 (11.3)
[d]
296 (37.2)
318 (49.7)
[a]
[b]
At room temperature before and after photoirradiation with 313 nm light. λmax (nm), ε (103 cmϪ1 Ϫ1). The extinction coefficients

of the closed form are obtained from the extinction coefficients of the PSS divided by the photoconversion. [c] Photochemical conversion
given as molar fraction of the closed form [Xclosed/(Xclosed ϩ Xopen)] in the PSS (λirradiation ϭ 313 nm) as determined by 1H NMR in
[d]
toluene solutions.
of the solvent.
Determination of the absorption maxima was not possible because the bands were partly obscured by absorption
hydro derivatives (5, 8, 9) on the one hand and the anal-
ogous perfluoro derivatives (10؊12) on the other hand are
more pronounced than the differences in the open forms, as
is evident from the clear bathochromic shift of 30Ϫ60 nm
found upon substitution of the hydrogens of the central
cyclopentene ring by fluorine atoms. On comparison of the
closed forms of the perhydro compounds 5؊9, it is evident
that substituents at the para position of the phenyl group
do not cause major changes of the absorption maxima in
the visible region, and only the strongly electron-with-
drawing cyano group induces a large bathochromic shift, of
40 nm. In the case of the closed perfluoro compounds, the
changes due to substituent effects are only moderate and
both the electron-donating methoxy and the electron-with-
drawing cyano groups cause a bathochromic shift, suggest-
ing that the ground state is stabilized though resonance with
the methoxy or cyano groups. Comparison of the absorp-
tion maxima in n-hexane and toluene shows that the closed
forms of the perfluoro derivatives 10؊12 experience signifi-
cant positive solvatochromic shifts, whereas with the per-
hydro derivatives 5؊10 only small positive or negative sol-
vatochromic shifts are observed. Apparently, the transition
dipole moments differ significantly between the closed
forms of the perhydro- and perfluorocyclopentene deriva-
tives.[25]
Figure 1. Absorption spectra of (a) perhydro switch 5 and (b) the
corresponding perfluoro switch 10 in the open form (—) and in the
photostationary state (PSS, ----) (4.9 ϫ 10Ϫ5  (5) and 3.7 ϫ 10Ϫ5
 (10) solutions in n-hexane at room temperature, PSS obtained by
irradiation at 313 nm)
Irradiation of the closed forms of compounds 5؊12 at
λ Ͼ 500 nm in all cases resulted in the disappearance of the
absorption band in the visible region and the restoration of
absorption spectra of the open form, which establishes that
the photochemical ring-opening/ring-closing process is fully
NMR spectra each showed conversion into a single new reversible. Because of the non-zero absorption of the closed
product with C2 symmetry. The upfield shift of the thienyl form in the UV spectral region, both ring closure and ring
hydrogens by 0.4Ϫ0.8 ppm is most pronounced, and indi- opening take place after photoexcitation in the UV region,
cates that the aromatic character of the thiophene moiety producing a photostationary state (PSS), the position of
is lost. These spectral changes are characteristic for the for- which is determined by the quantum yields of ring closure
mation of the closed forms (Scheme 1).[1] The UV/Vis data and ring opening and the extinction coefficients of the open
for the closed forms of 5؊12 are also summarized in and closed forms at the wavelength of irradiation. The equi-
Table 1. From the Table it is clear that the visible region librium ratios in the PSS (λirradiation ϭ 313 nm) could easily
1
spectral differences between the closed forms of the per- be obtained from the H NMR spectra after the PSS had
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B. L. Feringa, J. H. van Esch et al.
FULL PAPER
been reached, and these values (as molar fractions of the
NMR data[26,27] on diarylcyclopentene compounds in the
closed form) are given in Table 1. For both the perhydro- open form in solution indicate a close to 1:1 ratio between
cyclopentenes 5؊9 and the perfluorocyclopentenes 11 and the conformers of Cs symmetry and those of C2 symmetry
12 the molar fraction of the closed form is higher than 0.85 (Figure 4). Only the conformer of C2 symmetry can un-
in the PSS upon irradiation at 313 nm.
dergo the ring-closure reaction in a conrotatory fashion, as
A favourable property of the (dithienyl)perfluorocyclo- dictated by the WoodwardϪHoffman rules.[28] For most di-
pentenes is their excellent photostability (fatigue resist- arylcyclopentene switches, interconversion between the two
ance[1]). Repeated irradiation with UV and visible light re- forms by rotation of the thienyl moieties is a fast process
vealed that the perhydrocyclopentenes are also very stable on the NMR timescale at room temperature (milliseconds),
towards photochemical decomposition (Figure 2). The pho- but a slow process on the timescale of switching.[26,27] If one
tostability of the photochromic switches was studied in takes into account an approximate ratio of the two con-
more detail by continuous irradiation of solutions of 5 or 10 formers of about 1:1, the observed quantum yield of 0.6
with a laser beam, while the build-up of the lowest-energy would mean that practically every excited molecule of C2
absorption band of the closed form was monitored with a symmetry undergoes the ring-closure reaction.
weak probe beam, at 550 nm for 5 and at 615 nm for 10
(Figure 3, part a). The quantum yield of the ring-closure
reaction upon excitation at 300 nm could then be obtained
from the initial slopes of the absorption curves. Figure 3 (b)
shows the initial part of the curves from Figure 3 (a) after
conversion of the absorbance and time axes (Exp. Sect.). It
can immediately be concluded that the quantum yields of 5
and 10 are nearly identical and equal to 0.6 (Ϯ 0.1). The
absolute error is determined mainly by the accuracy with
which the output power of the pump laser is measured.
However, the value of 0.6 is in excellent agreement with the
Figure 4. Two possible conformations of the open form of phenyl-
value of 0.59 reported previously for 10.[26]
substituted dithienyl cyclopentene switch 5
From Figure 3 (a) it can be seen that the photostability
of the switches is limited. According to Irie et al.[29] the
main pathway for decomposition takes place from the ex-
cited state of the closed form, which produces the scheme
for photochromic reactions as shown in Equation (1), where
O, C and D represent the concentrations of molecules in
the open form (5o, 10o), closed form (5c, 10c) and de-
composition products of 5 and 10, and k1, k2 and k3 are
the rate constants associated with the probability of ring-
closure, ring-opening and photodecomposition, respec-
Figure 2. Cycling between the open and closed forms of the bis(p-
tively.
cyanophenyl)-substituted cyclopentene switch 9 by repetitive ir-
radiation at λ ϭ 313 nm and λ Ͼ 520 nm at 25 °C in n-hexane. The
ordinate shows the formation of the closed form as monitored by
the absorbance at 570 nm
(1)
Figure 3. Photoconversion of 5 (•) and 10 (᭺) in cyclohexane at 25 °C by continuous irradiation with a laser at 300 nm (a); the formation
of the closed form was monitored at 550 nm (5) and 615 nm (10); the conversion traces in (b) are calculated from the first minute of the
curves shown in Figure 3 (a)
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Perhydro- and Perfluorodithienylcyclopentene Molecular Switches
FULL PAPER
Since both open and closed forms absorb in the UV spec- pentene molecular switches have only a small effect on the
tral region, the formation of 5c (10c) from 5o (10o) upon absorption maxima of the electronic spectra, but cause
irradiation by UV light is a continuous process resulting in subtle changes in substituent and solvatochromic effects.
an equilibrium, the position of which is determined by the The photochromic behaviour is remarkably similar, and
ratio between the rate constants k1 and k2, which are depen- both types of switch combine excellent quantum yields (0.6)
dent on the extinction coefficients of O and C at the with high levels of photoconversion (Ͼ 0.85). The main dif-
excitation wavelength. The influence of rate constant k3 on ference is the lower photochemical and thermal stabilities
the equilibrium is negligible because k3 is much smaller of the perhydrocyclopentene molecular switches, but they
than k1 and k2.
can still undergo multiple switching cycles without notable
Equation (1) predicts a biexponential evolution of the photodecomposition, whereas the half lives for thermal in-
concentrations C and D with the rate constants ka and kb, terconversion between the open and closed forms are in all
where ka and kb represent the growth and decay of the ab- cases exceeding 3 h at 100 °C. It is therefore concluded that
sorption of C at visible wavelengths,[30] respectively, and ful- the perhydrocyclopentenes should be excellent alternatives
fil the relations ka ϩ kb ϭ k1 ϩ k2 ϩ k3 and ka· kb ϭ k1·k3. for the perfluorocyclopentenes in most studies on diaryle-
Biexponential fits of the absorption curves presented in Fig- thene photochromic switches, but the perfluorocyclopen-
ure 3 yield rate constants ka ϭ 5.5 ϫ 10Ϫ3 sϪ1 and kb ϭ 3.1 tenes might be better suited for applications such as data
ϫ 10Ϫ5 sϪ1 for 5, and ka ϭ 3.9 ϫ 10Ϫ3 sϪ1 and kb ϭ 8.1 storage, which depend critically on fatigue resistance and
ϫ 10Ϫ5 sϪ1 for 10. The larger value of ka in the case of long-term thermal stability.
perhydrocyclopentene 5 reveals that the equilibrium situ-
ation is reached more rapidly than in the case of the per-
fluorocyclopentene 10, while the ratio between the kb values
obtained for the two analogues indicates that the photost-
ability of the perfluorinated analogue is roughly 2Ϫ3 times
higher than that of the nonfluorinated compound.
Whereas the photochemical conversion between the open
and closed forms proceeds with high quantum yields, the
thermal interconversion is very slow for most perfluorocyc-
lopentene switches. It has been reported that the thermal
ring opening of 10c in the crystalline phase has a half-life
time of approximately 1900 years at 30 °C and 3.3 h at 150
°C.[20] Because the lifetimes could have been influenced by
Experimental Section
General Procedures: Melting points were determined with a Büchi
melting point apparatus or a Mettler FP1 melting point apparatus
1
and are uncorrected. H NMR spectra were recorded at 200 MHz,
300 MHz or 500 MHz, 13C NMR spectra were recorded at
50.3 MHz, 75.4 MHz or 125.7 MHz, and 19F NMR spectra were
recorded at 188.2 MHz or 470.3 MHz. All spectra were taken at
ambient temperature. The residual protons from the solvent are
taken as an internal reference and the chemical shifts are reported
relative to TMS. Mass spectroscopy was performed by CIϩ, DEI
or EIϩ ionization procedures. The dithienylethenes are sometimes
crystal packing forces, the thermal stabilities of the closed hard to sublime and only by means of DEI (desorption electron
forms of 5 and 8؊12 were studied in toluene solution by 1H
ionization) it was possible to obtain the mass spectra in these cases.
Derivatives synthesized starting from compounds 1 and 2 are light-
NMR spectroscopy. For the perfluorocyclopentenes 10؊12,
sensitive and so were exclusively handled in the dark, in brown
glassware, with column chromatography being performed under
yellow light.
the thermal conversion into the open form was found to be
negligible (Ͻ 2%) after 14 h at 100 °C, indicating that the
perfluoropentenes are also thermally very stable in solution.
The closed forms of perhydrocyclopentenes 5 and 9 also
showed excellent thermal stabilities, with half lives of 66 h
and 40 h, respectively, for ring opening at 100 °C. Only the
closed form of 8 is significant less stable at elevated tem-
peratures, but has still a half-life time of 3 h at 100 °C. It
should be noted that the thermal conversion of the closed
form to the open form is a clean process in all cases, with
no sign of any decomposition products in the 1H NMR
spectra.
In conclusion, various derivatives of perhydro- and per-
fluorocyclopentene photochromic switches have success-
fully been prepared by a Suzuki coupling of boronylated
dithienyl perhydro- (3) and perfluorocyclopentene (4) with
aryl bromides bearing electron-withdrawing or -donating
substituents. It is to be expected that this route should be
well suited for the convergent synthesis of other func-
tionalized perhydrocyclopentene molecular switches, but
less suitable for the corresponding perfluorocyclopentenes
because of the lower yields of the Suzuki coupling. The dif-
ferences in the electron densities of the central cyclopentene
Materials: Reagents and starting materials were used as supplied.
The solvents were distilled and dried before use, if necessary, by
standard methods. Aldrich silica gel grade 9385 (230Ϫ400 mesh)
was used for column chromatography.
General Procedure for the Suzuki Reactions Starting from 1
1,2-Bis(5Ј-dibutoxyboryl-2Ј-methylthien-3Ј-yl)cyclopentene
(3):
Compound 1 (1.75 g, 5.3 mmol) was dissolved in anhydrous THF
(12 mL) under nitrogen, and nBuLi (4.5 mL of 2.5  solution in
hexane, 11 mmol) was added at once by syringe. This solution was
stirred at room temperature for 30 min, and B(nBuO)3 (4.3 mL,
16 mmol) was added at once. The resulting solution was stirred at
room temperature for 1 h and used directly in the Suzuki cross-
coupling reaction without any workup because boronic acid 3 was
found to be hydrolysed during isolation.
1,2-Bis(2Ј-methyl-5Ј-phenylthien-3Ј-yl)cyclopentene (5):[14] Bromo-
benzene (1.1 mL, 9.3 mmol) was dissolved in THF (12 mL), and
after addition of [Pd(PPh3)4] (0.37 g, 0.3 mmol), the solution was
stirred at room temperature for 15 min. Aqueous Na2CO3 (23 mL,
2 ) and 6 drops of ethylene glycol were then added, and the re-
sulting two-phase system was heated in an oil bath until reflux (60
moieties of the perhydrocyclopentene and perfluorocyclo- °C). The solution of 3 was added dropwise by syringe over a few
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B. L. Feringa, J. H. van Esch et al.
FULL PAPER
min. After the addition was complete, the reaction mixture was
heated at reflux for 2 h, and then allowed to cool to room tempera-
ture. Diethyl ether (50 mL) and H2O (50 mL) were added, and the
organic layer was collected and dried (Na2SO4). After evaporation
of the solvent, the product was purified by column chromatography
(SiO2, hexane) to give a brown/yellowish solid (1.80 g, 70%). M.p.
Na2CO3 (15 mL, 2 ) and 6 drops of ethylene glycol. Purification
of the product by column chromatography (SiO2, hexane/CH2Cl2,
1:4) gave a brown/yellowish solid (0.37 g, 35%). M.p. 126Ϫ127 °C.
1H NMR (CDCl3, 300 MHz): δ ϭ 2.01 (s, 6 H), 2.05Ϫ2.15 (m, 2
H), 2.84 (t, J ϭ 7.5 Hz, 4 H), 7.12 (s, 2 H), 7.54 (d, J ϭ 8.4 Hz, 4
H), 7.59 (d, J ϭ 8.4 Hz, 4 H) ppm. 13C NMR (CDCl3, 75.4 MHz):
84Ϫ85 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 1.98 (s, 6 H), δ ϭ 14.6 (q), 23.0 (t), 38.4 (t), 110.0 (s), 118.9 (s), 125.3 (d), 126.0
2.03Ϫ2.13 (m, 2 H), 2.84 (t, J ϭ 7.5 Hz, 4 H), 7.03 (s, 2 H), (d), 132.6 (d), 134.8 (s), 137.1 (s), 137.1 (s), 137.6 (s), 138.5 (s) ppm.
7.19Ϫ7.25 (m, 2 H), 7.32 (dd, J ϭ 6.9, J ϭ 7.5 Hz, 4 H), 7.49 (d, HRMS calcd. for C29H22N2S2 462.121, found 462.122.
J ϭ 7.2 Hz, 4 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 14.4
(q), 23.0 (t), 38.5 (t), 123.9 (s), 125.2 (d), 126.8 (d), 128.7 (d), 134.4
General Procedure for the Suzuki Reactions Starting from 2
(s), 134.5 (s), 136.6 (s), 139.5 (s) ppm. C27H24S2 (412.6): calcd. C
78.60, H 5.86; found C 78.65, H 5.90.
1,2-Bis(5Ј-dibutoxyboryl-2Ј-methylthien-3Ј-yl)perfluorocyclopentene
(4): The same procedure as described for 3 was used, starting from
2 (0.20 g, 0.5 mmol) and nBuLi (0.66 mL of 1.6  solution in hex-
ane, 1.1 mmol), and B(nBuO)3 (0.41 mL, 1.5 mmol). The resulting
solution was also used directly in the Suzuki cross-coupling reac-
tion without any workup, because boronic acid 4 is hydrolysed dur-
ing isolation.
1,2-Bis[5Ј-(4ЈЈ-chlorophenyl)-2Ј-methylthien-3Ј-yl]cyclopentene (6):
The same procedure as described for 5 was followed, starting from
1 (1.00 g, 3.04 mmol), and nBuLi (5.0 mL of 1.6  solution in hex-
ane, 8 mmol), B(nBuO)3 (2.3 mL, 8.3 mmol), 4-bromochloroben-
zene (2.20 g, 12 mmol), [Pd(PPh3)4] (0.40 g, 0.3 mmol), aqueous
Na2CO3 (17 mL, 2 ) and 6 drops of ethylene glycol. Purification
of the product by column chromatography (SiO2, hexane) gave a
1,2-Bis(2Ј-methyl-5Ј-phenylthien-3Ј-yl)perfluorocyclopentene (10):[19]
Bromobenzene (0.10 mL, 1.0 mmol) was dissolved in THF (8 mL)
and, after addition of [Pd(PPh3)4] (35 mg, 0.03 mmol), the solution
was stirred at room temperature for 15 min. Aqueous Na2CO3
(1 mL, 2 ) and 6 drops of ethylene glycol were then added, and
the resulting two-phase system was heated to reflux (60 °C) in an
oil bath. The solution of 4 was added dropwise by syringe over a
period of several min. After addition was complete, the reaction
mixture was heated at reflux for 2 h, and was then allowed to cool
to room temperature. Diethyl ether (50 mL) and H2O (50 mL) were
added, and the organic layer was collected and dried (Na2SO4).
After evaporation of the solvent the product was purified by col-
umn chromatography (SiO2, hexane) to gave a greenish solid
1
white solid (0.94 g, 68%). M.p. 113Ϫ114 °C. H NMR (300 MHz,
CDCl3): δ ϭ 1.97 (s, 6 H), 2.05Ϫ2.10 (m, 2 H), 2.81 (t, J ϭ 7.5 Hz,
4 H), 6.97 (s, 2 H), 7.27 (d, J ϭ 8.7 Hz, 4 H), 7.38 (d, J ϭ 8.7 Hz,
4 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 14.4 (q), 23.0 (t),
38.4 (t), 124.3 (d), 126.4 (d), 128.9 (d), 132.6 (s), 133.0 (s), 134.7
(s), 134.9 (s), 136.7 (s), 138.4 (s) ppm. HRMS calcd. for
C27H22Cl2S2 480.054, found 480.050.
1,2-Bis[5Ј-(4ЈЈ-bromophenyl)-2Ј-methylthien-3Ј-yl]cyclopentene (7):
The same procedure as described for 5 was followed, starting from
1 (1.00 g, 3.04 mmol), and nBuLi (5.0 mL of 1.6  solution in hex-
ane, 8 mmol), B(nBuO)3 (2.3 mL, 8.3 mmol), 1,4-dibromobenzene
(3.40 g, 14.4 mmol), [Pd(PPh3)4] (0.40 g, 0.3 mmol), aqueous
Na2CO3 (17 mL, 2 ) and 6 drops of ethylene glycol. Purification
of the product by column chromatography (SiO2, hexane) gave a
yellowish solid (1.30 g, 76%). 1H NMR (300 MHz, CDCl3): δ ϭ
1.97 (s, 6 H), 2.02Ϫ2.10 (m, 2 H), 2.81 (t, J ϭ 7.5 Hz, 4 H), 6.98
(s, 2 H), 7.32 (d, J ϭ 8.4 Hz, 4 H), 7.42 (d, J ϭ 8.4 Hz, 4 H). 13C
NMR (75.4 MHz, CDCl3): δC ϭ 14.4 (q), 23.0 (t), 38.4 (t), 124.4
(d), 126.8 (d), 131.8 (d), 133.4 (s), 133.6 (s), 134.7 (s), 135.0 (s),
136.8 (s), 138.4 (s) ppm. HRMS calcd. for C27H22Br2S2 567.953,
found 567.951.
1
(16 mg, 7%). H NMR (300 MHz, CDCl3): δ ϭ 1.96 (s, 6 H), 7.28
(s, 2 H), 7.29 (d, J ϭ 6.9 Hz, 2 H), 7.38 (dd, J ϭ 7.2, J ϭ 7.5 Hz,
4 H), 7.53 (d, J ϭ 7.5 Hz, 4 H) ppm. 13C NMR (75.4 MHz,
CDCl3): δ ϭ 14.5 (q), 120.5 (d), 122.4 (d), 125.6 (d), 127.9 (d),
129.0 (d), 133.3 (s), 141.3 (s), 142.2 (s) ppm. 19F NMR (188.2 MHz,
CDCl3): δ ϭ Ϫ111.26 (t, J ϭ 6 Hz, 4 F), Ϫ133.03 (m, 2 F) ppm.
HRMS calcd. for C27H18F6S2 520.075, found 520.075.
1,2-Bis[5Ј-(4ЈЈ-methoxyphenyl)-2Ј-methylthien-3Ј-yl]perfluorocyclo-
pentene (11):[20] The same procedure as described for 10 was fol-
lowed, starting from 2 (0.10 g, 0.32 mmol), and nBuLi (0.32 mL of
1.6
 solution in hexane, 0.51 mmol), B(nBuO)3 (0.22 mL,
1,2-Bis[5Ј-(4ЈЈ-methoxyphenyl)-2Ј-methylthien-3Ј-yl]cyclopentene
(8): The same procedure as described for 5 was followed, starting
from 1 (0.74 g, 2.3 mmol), and nBuLi (1.9 mL of 2.5  solution in
hexane, 4.7 mmol), B(nBuO)3 (3.1 mL, 12 mmol), 4-bromoanisole
(0.56 mL, 4.5 mmol), [Pd(PPh3)4] (0.31 g, 0.25 mmol), aqueous
Na2CO3 (15 mL, 2 ) and 6 drops of ethylene glycol. Purification
of the product by column chromatography (SiO2, hexane/CH2Cl2,
1.1:1) gave a brown/yellowish solid (0.43 g, 40%). 1H NMR
(CDCl3, 300 MHz): δ ϭ 1.97 (s, 6 H), 2.01Ϫ2.11 (m, 2 H), 2.82 (t,
J ϭ 7.5 Hz, 4 H), 3.81 (s, 6 H), 6.90 (s, 2 H), 6.86 (d, J ϭ 8.9 Hz,
4 H), 7.41 (d, J ϭ 8.9 Hz, 4 H) ppm. 13C NMR (CDCl3,
75.4 MHz): δ ϭ 14.4 (q), 23.0 (t), 38.5 (t), 55.4 (q), 114.2 (d), 123.0
(d), 126.6 (d), 127.5 (s), 133.4 (s), 134.6 (s), 136.6 (s), 139.5 (s),
158.8 (s) ppm. HRMS calcd. for C29H28O2S2 472.153, found
472.153.
0.78 mmol), 4-bromoanisole (0.08 mL, 0.6 mmol), [Pd(PPh3)4]
(40 mg, 0.04 mmol), aqueous Na2CO3 (10 mL, 2 ) and 6 drops of
ethylene glycol. Purification of the product by column chromatog-
raphy (SiO2, CH2Cl2/hexane, 1:2) gave a blue/grey solid (31 mg,
22%). 1H NMR (CDCl3, 200 MHz): δ ϭ 1.93 (s, 6 H), 3.82 (s, 6
H), 6.90 (d, J ϭ 9.2 Hz, 4 H), 7.14 (s, 2 H), 7.45 (J ϭ 9.0 Hz, 4 H)
ppm. 13C NMR (CDCl3, 125.7 MHz): δ ϭ 14.5 (q), 55.4 (q), 114.4
(d), 116.2 (s), 118.2 (s), 121.3 (d), 125.7 (s), 126.2 (s), 126.9 (d),
136.0 (s), 140.3 (s), 142.1 (s), 159.4 (s) ppm. 19F NMR (CDCl3,
188.2 MHz): δ ϭ Ϫ111.16 (t, J ϭ 5.6 Hz, 4 F), Ϫ132.96 (m, 2 F)
ppm. HRMS calcd. for C29H22F6O2S2 580.097, found 580.095.
1,2-Bis[5Ј-(4ЈЈ-cyanophenyl)-2Ј-methylthien-3Ј-yl]perfluorocyclo-
pentene (12): The same procedure as described for 10 was used,
starting from 2 (0.10 g, 0.32 mmol), and nBuLi (0.32 mL of 1.6 
solution in hexane, 0.51 mmol), B(nBuO)3 (0.22 mL, 0.78 mmol),
1,2-Bis[5Ј-(4ЈЈ-cyanophenyl)-2Ј-methylthien-3Ј-yl]cyclopentene (9): 4-bromobenzonitrile (1.1 mL, 0.6 mmol), [Pd(PPh3)4] (40 mg,
The same procedure as described for 5 was followed, starting from
1 (0.74 g, 2.3 mmol), and nBuLi (1.9 mL of 2.5  solution in hex-
0.04 mmol), aqueous Na2CO3 (10 mL, 2 ) and 6 drops of ethylene
glycol. Purification of the product by column chromatography
ane, 4.7 mmol), B(nBuO)3 (3.1 mL, 12 mmol), 4-bromobenzonitrile (SiO2, first EtOAc/hexane, 1:2, then CH2Cl2/hexane, 1:2) gave a
(0.81 mL, 4.5 mmol), [Pd(PPh3)4] (0.31 g, 0.25 mmol), aqueous
blue/grey solid (30 mg, 23%). M.p. 189Ϫ191 °C. 1H NMR (CDCl3,
1892
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 1887Ϫ1893

Perhydro- and Perfluorodithienylcyclopentene Molecular Switches
FULL PAPER
[1]
[2]
[3]
200 MHz): δ ϭ 1.97(s, 6 H), 7.37 (s, 2 H), 7.58Ϫ7.68 (m, 8 H) ppm.
13C NMR (CDCl3, 125.7 MHz): δ ϭ 14.7 (q), 111.2 (s), 116.0 (s),
118.5 (s), 124.3 (d), 125.3 (s), 125.8 (d), 126.2 (s), 131.84 (s), 132.8
(d), 137.3 (s), 140.1 (s), 143.4 (s) ppm. 19F NMR (CDCl3,
188.2 MHz): δ ϭ Ϫ111.23 (t, J ϭ 6 Hz, 4 F), Ϫ132.90 (m, 2 F)
ppm. HRMS calcd. for C29H16F6N2S2 570.066, found 570.066.
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[5]
[6]
Photochromic Behaviour: Solutions of switches 5؊12 of known con-
centration were irradiated in a 1-cm quartz cuvette for UV/Vis
measurements or in 5-mm Pyrex tubes for NMR experiments, using
a 200 W mercury lamp (Oriel) with the appropriate filters (Andover
Corporation Optical Filters). 1H NMR experiments were per-
formed on a Varian VXR-300 spectrometer. All measurements were
carried out at room temperature except for the thermal stability
tests of the closed forms.
[7]
[8]
[9]
[10]
[11]
[12]
Real Time Dynamics: The quantum efficiency of the ring-closure
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measured by irradiation of a 1-mm cell, containing a solution of
known concentration of 5 or 10 dissolved in cyclohexane, with UV
light of power 285 µW at wavelength 300 nm. Simultaneously a
weak probe (at 550 nm for 5 and at 615 nm for 10) monitored the
build-up of the lowest-energy absorption band of the closed form.
Direct readout of the quantum yield is possible after conversion of
the time and absorbance axes into numbers of absorbed photons
and numbers of closed-form molecules, respectively. The time axis
is converted by insertion of the known irradiation intensity I0 and
the absorbance of the open-form molecules at 300 nm into the re-
lation:
[13]
[14]
[15]
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[17]
[18]
[19]
[20]
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Received December 23, 2002
where t is the time in seconds, Nphotons the number of photons, h
the Planck constant and c the speed of light. The absorbance of
the open-form molecules at 300 nm is known exactly only at time
t ϭ 0. The absorbance axis is converted into numbers of closed-
form molecules by the relation:
[25]
[26]
with εclosed(λ) the extinction coefficient of the closed-form mol-
ecules at wavelength λ, l the length of the sample in cm, Nclosed the
number of closed-form molecules and V the volume of the sample.
[27]
[28]
[29]
[30]
Acknowledgments
The Royal Netherlands Academy of Sciences is gratefully acknowl-
edged for a fellowship for Dr. J. van Esch.
Eur. J. Org. Chem. 2003, 1887Ϫ1893
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1893