Synthesis and photoswitching studies of difurylperfluorocyclopentenes with extended π-systems

Article abstract of DOI:10.1002/chem.201003716

In an attempt to design molecular optoelectronic switches functioning in molecular junctions between two metal tips, we synthesized a set of photochromic compounds by extending the π-system of 1,2-bis-(2-methyl-5-formylfuran-3-yl) perfluorocyclopentene th

Full text of DOI:10.1002/chem.201003716

FULL PAPER
DOI: 10.1002/chem.201003716
Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes with
Extended p-Systems
Dmytro Sysoiev,[a] Artem Fedoseev,[a] Youngsang Kim,[b] Thomas E. Exner,[a]
Johannes Boneberg,[b] Thomas Huhn,[a] Paul Leiderer,[b] Elke Scheer,*[b]
Ulrich Groth,*[a] and Ulrich E. Steiner*[a]
Abstract: In an attempt to design mo-
lecular optoelectronic switches func-
tioning in molecular junctions between
two metal tips, we synthesized a set of
photochromic compounds by extending
the p-system of 1,2-bis-(2-methyl-5-for-
mylfuran-3-yl)perfluorocyclopentene
through suitable coupling reactions in-
volving the formyl functions, thereby
also introducing terminal groups with a
binding capacity to gold. Avoiding the
presence of gold-binding sulphur atoms
in the photoreactive centre, as they are
present in the frequently used analo-
gous thienyl compounds, the newly syn-
thesized compounds should be more
suitable for the purpose indicated. The
kinetics of reversible photoswitching of
the new compounds by UV and visible
light was quantitatively investigated in
solution. The role of conformational
flexibility of the p-system for the width
of the UV/Vis spectra was clarified by
using quantum chemical calculations
with time-dependent (TD)-DFT. As a
preliminary test of the potential of the
new compounds to serve as optoelec-
tronic molecular switches, monolayer
formation and photochemical switching
on gold surfaces was observed by using
surface plasmon resonance.
Keywords: electrocyclic reactions ·
molecular electronics · photochro-
mism
· photoswitches · surface
plasmon resonance
Introduction
Molecular switches have been vastly investigated in the last
two decades due to promising applications in molecular
electronics, in particular, optoelectronics, optical data stor-
age or self-assembling polymeric systems.[1–2] Of particular
interest for molecular electronics applications are molecules
that can be reversibly switched between two well-defined
states through optical activation. These can be contacted in
a two-wire geometry and do not require a third (control)
electrode. Among a great variety of types of molecular
switches,[3–4] diarylethenes, in particular, substituted bis-
Scheme 1. Switching process of diheteroarylethenes induced by light of
different wavelengths.
A
cause the switching process goes along with only minor
length change, they are suitable as a molecular bridge be-
tween two metal contacts,[6] thus providing a break-junction
molecular unit, the conductance of which can be optically
controlled.[7] Also interesting is the immobilization of the
functionalized switches on a gold surface: reversible switch-
ing by light of appropriate wavelengths has been demon-
strated for an asymmetrically substituted dithienylcyclopen-
tene with one anchoring group under such conditions.[8]
Molecular junctions of the diarylethene type have been
investigated before. However, their switching properties
were reported to be non-reversible when closely embedded
between two metal electrodes.[9] The molecules switched re-
versibly when attached to only one electrode though.[8] It
has been argued that photoisomerization did not occur be-
cause the photoexcited state was quenched in the presence
of the metal. An alternative explanation of the irreversibility
could be undesired bonding of the sulphur atoms in the thio-
recent years. They exhibit moderate to good quantum yields
of photocyclization–photocycloreversion reactions (e.g. “on–
off” switching, Scheme 1) and high fatigue resistance. Be-
[a] D. Sysoiev, A. Fedoseev, Dr. T. E. Exner, Dr. T. Huhn,
Prof. Dr. U. Groth, Prof. Dr. U. E. Steiner
Fachbereich Chemie, Universitꢀt Konstanz
78457 Konstanz (Germany)
Fax : (+49)7531-883014
E-mail: ulrich.groth@uni-konstanz.de
ulrich.steiner@uni-konstanz.de
[b] Y. Kim, Prof. Dr. J. Boneberg, Prof. Dr. P. Leiderer,
Prof. Dr. E. Scheer
Fachbereich Physik, Universitꢀt Konstanz
78457 Konstanz (Germany)
Fax : (+49)7531-883090
E-mail: elke.scheer@uni-konstanz.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003716.
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phene rings to the metal surface (gold is the most commonly
used electrode metal in molecular electronics).
To avoid this undesired mode of adsorption, we synthe-
sized a series of molecular switches of the difurylethene
type. These are sulphur-free in their switching unit but have
heteroatom-modified side chains, hence allowing to bridge
metal contacts. Another reason for the choice of furans is
that their diene character is more pronounced than that of
thiophenes. Therefore, an enhanced reactivity in pericyclic
reactions may be expected.
So far, the photoswitching properties of only two difuryl-
ACHTUNGTRENNUNGethenes have been thoroughly studied by Yamaguchi and
Irie.[10] They compared the properties of 1,2-diarylperfluoro-
cyclopentene derivatives with either aryl=thienyl or furyl
(X=S or O) and R=methyl or phenyl. In the short-hand
notation that will be used for our compounds in the remain-
der of the paper, the furyl compounds would be assigned
the codes C5F-Me and C5F-Ph, respectively, C5F represent-
ing the central bridging perfluorinated five-membered ring,
Me coding for methyl and Ph for phenyl substituents at the
5-position of the furan rings. As was described in refer-
ence [10], the thienyl and furyl derivatives exhibit very simi-
lar switching properties. The closed forms of the phenyl de-
rivatives of both types of photochromic molecules have long
thermal half lives and stand more than 150 switching cycles
before deteriorating to a level below 80%. These results en-
couraged us to go ahead with the synthesis of photochromic
furyl compounds with more extended p-systems, including
terminal groups capable of bonding to a gold surface, thus
opening a pathway to the controlled molecular design and
tuning of the photochemical properties.
Scheme 2. Synthetic pathway to the key intermediate, dialdehyde 5:
a) Br2, AlCl3, neat; b) ethylene glycol, p-toluensulfonic acid, toluene,
reflux; c) tBuLi, THF, ꢀ788C, then C5F8; d) HClaq., acetone/THF 1:1,
24 h, RT; e) LiAlH4, Et2O, reflux.
trosymmetric space group C2/c with half a molecule in the
asymmetric unit with carbon atom C7 located on a two-fold
rotation axis. In the crystalline state, the molecule adopts
the antiparallel conformation with both furan rings pointing
away from the central hexafluorocyclopentene. With a value
of 11.468 the torsion angle (C4-C5-C4i-C5i) between the
furan rings is much larger than the corresponding angle in
the two other structurally related furan derivatives known
so far (7.21 for C5F-Ph and 6.958 for C5F-Me).[17] However,
in the crystal a distance of 3.59 ꢂ between the reactive
carbon atoms C8 and C8i in 4 (Figure 1) is in surprisingly
good accord with the data from reference [17] (3.54 for
C5F-Ph and 3.74 ꢂ for C5F-Me), for which a value of 4.2 ꢂ
Results and Discussion
Synthesis: Several routes to difuryl-substituted ethenes have
been described in the literature. Among these are reductive
coupling of carbonyl precursors according to the McMurry
reaction[11] with TiCl4/Zn as the reducing system,[12–14] Stille
coupling,[15] and Suzuki cross-coupling.[13] For our purposes,
the coupling of brominated aryl moieties to the double bond
in perfluorinated cyclopentene with nBuLi as used in refer-
ence [10] was the method of choice. In general, furans are
known to be labile molecules relative to the corresponding
thiophene derivatives since the furan ring shows more
diene-like rather than aromatic properties. Moreover, acidic
conditions may lead to protonation and further destruction
of the furan rings, especially in the case of electron-rich
ones. To avoid this, the electron-poor furaldehyde C5F-
CHO (5) was synthesized as a common starting compound
(Scheme 2), for which further extension of the p-system can
be easily achieved.
The structure of the protected aldehyde 4 was character-
ized by X-ray crystallographic structure determination. Suit-
able single crystals were grown by slow diffusion of n-
hexane into a saturated solution of 4 in diethyl ether.[16] The
compound crystallizes in the monoclinic system in the cen-
Figure 1. X-ray crystal structure of the protected aldehyde 4. Thermal el-
lipsoids are drawn at the 50% probability level. Main occupancy is
shown for disordered fluorine atoms; hydrogen atoms are omitted for
clarity.
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Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes
FULL PAPER
is given as a condition sufficient for photochromic ring clo-
sure in the crystal.
ylide accessible by sodium methoxide induced deprotona-
tion of [p-(methylthio)benzyl]triphenylphosphonium bro-
mide.
The reactive aldehyde groups of compound 5 offer vari-
ous pathways of modification of the switching unit to vary
its coupling strength to the metalꢃs electronic system. There-
by a wide range of possibilities is opened up for modifica-
tions (extending the p-system, prolonging an aliphatic side
chain to increase solubility, connecting with chelating li-
gands with further metal coordination, etc.). In this work,
we synthesized the thiosemicarbazones 7 and 8 and the com-
pounds 9–12 carrying vinylogous p-systems attached to the
furan rings (Scheme 3). All the N and S atom containing
endgroups bear the affinity to bind to gold, thereby provid-
ing suitable anchoring of the switches between two gold
contacts.
UV/Vis spectra: For solubility reasons, all spectral and pho-
tokinetic investigations were carried out in MeOH except
for the aldehyde C5F-CHO (5), for which the formation of
the acetal was observed on irradiation. Therefore CDCl3
was used in that case, which also allowed us to observe the
NMR spectrum of irradiated solutions in the same solvent.
The general picture of the UV/Vis absorption spectra of the
new molecular switches appears as expected for this class of
compounds. In the open form, the first absorption band is in
the UV to blue spectral range, whereas the first absorption
band of the closed form is in the visible region. The posi-
tions of these bands move to
longer wavelength as the length
of the conjugated p-system in-
creases. The spectra of two ex-
amples (6, C5F-OH and 9, C5F-
RN) demonstrating this effect
are shown in Figure 2. The
wavelengths and absorption co-
efficients of all synthesized new
photochromic compounds are
summarized in Table 1 (for the
UV/Vis spectra see the Sup-
porting Information).
To account for the influence
of the substituents on the ab-
sorption spectra and to test if a
rational design of the substitu-
ents for specific applications is
possible, theoretical spectra
were calculated by using TD-
DFT
(B3LYP/6-31++g-
AHCTUNGTRENNUNG
parison with the experiment,
the obtained absorption ener-
gies were broadened by a Gaus-
sian distribution (full-width
half-maximum: 3000 cmꢀ1). The
influence of solvents on the
transition energies was tested
by an implicit solvent model
(IEF-PCM[21–22]). The polariza-
tion due to different solvents
Scheme 3. Synthesis of functionalized molecular switches based on either the formation of an azomethine (7–
8), a Knoevenagel-approach (9, 11 and 12) or a Wittig-reaction (10): a) thiosemicarbazide, MeOH, RT, 24 h;
b) methylthiosemicarbazide, MeOH, RT, 24 h; c) rhodanine, dichloromethane, piperidine, RT, 12 h; d) [4-
(methylthio)benzyl]triphenylphosphonium bromide, MeOH, MeONa, RT, 36 h; e) 2-(pyridin-4-yl)acetonitrile
hydrochloride, K2CO3, MeOH, RT, 48 h; f) malodinitrile, cat. piperidine, benzene, RT, 4 h.
The synthesis of thiosemicarbazone C5-TSC (7) and meth-
ylthiosemicarbazone C5F-MTSC (8) was achieved in good
yield by azomethine formation with aldehyde 5 and the re-
spective hydrazine derivatives in methanolic solution at
room temperature (Scheme 3). Switches 9, 11 and 12 were
synthesized by Knoevenagel condensation of the rather CH-
active rhodanine (9), 2-(pyridin-4-yl)acetonitrile (11) and
malodinitrile (12) with aldehyde 4 under mild basic condi-
tions. The 4-thioanisyl derivative C5F-pSMe (10) was pre-
pared by a classical Wittig approach by starting from the
yielded a shift of the absorption frequencies to larger wave-
lengths of less than 50 nm (data not shown). Since we were
only interested in a qualitative description of the influences
of the substituents, we restricted the discussion to calcula-
tions in vacuo.
The results for C5F-OH (6) and C5F-RN (9) are shown in
Figure 2. For the smaller conjugated system C5F-OH, rea-
sonable agreement between experiment and calculations is
found. In particular, the most important lowest-energy tran-
sition is nicely reproduced. The shift to longer wavelengths
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U. E. Steiner, E. Scheer, U. Groth et al.
with increasing conjugated system is also well predicted. But
for the largest systems, such as C5F-RN, the very flat and
broad absorption at larger wavelengths cannot be repro-
duced. One possible explanation for this broadening is the
steric freedom of the extended conjugated chain. For exam-
ple, a change from the transoid to the cisoid configuration at
the formal single bonds designated a and a’ in Figure 2
brings about considerable changes of spectral intensity in
the region between 500 and 1000 nm. Spectral broadening
may not only be due to different configurations for fully
conjugated, planar forms of the central part and the sub-
stituents. Due to thermal fluctuations, the substituents could
rotate relative to the central part, partially disrupting the
conjugated system. Thereby shifts of the lowest-energy tran-
sition to shorter wavelengths are expected. To test this as-
sumption, we rotated one of the substituents of C5F-RN
around the formal single bond, optimized all other variables
and calculated the absorption spectra according to the
method described above. The dependence of the lowest-
energy absorption with the torsion angle is shown in
Figure 3. It can be clearly seen that for 0 and 1808, which
Figure 2. UV/Vis spectra of open and closed forms in MeOH: a) C5F-
OH (6) (c: exptl. closed; c: exptl. open; a: theor. closed; a:
theor. open), b) C5F-RN (9) (c: exptl. closed; c: exptl. open; c:
theor. open; b: theor. closed conf. 1; a: theor. closed conf. 2). Con-
formation 1 is as shown in the graph, conformation 2 is rotated by 180 de-
grees around bonds a and a’. There is only a minor effect of this rotation
in the open form.
Table 1. Absorption maxima and photoconversion quantum yields.[a]
Compound lmax
(open) (open)
[nm] [mꢀ1 cmꢀ1] [nm]
emax
lmax
(closed) (closed) (l1)
[mꢀ1 cmꢀ1
emax
fo!c
fc!o
(l2)
]
C5F-CHO
289
2.40ꢄ104 533
0.95ꢄ104 0.40[b,c] 0.11[d]
(5)
C5F-OH (6) 299
0.59ꢄ104 455
0.61ꢄ104 449
3.87ꢄ104 525
5.46ꢄ104 576
5.98ꢄ104 575
0.75ꢄ104 0.42[b]
0.87ꢄ104 0.35
1.65ꢄ104 0.53
1.54ꢄ104 0.28[b]
1.79ꢄ104 0.38[b]
0.42[d]
0.32
C5F-Me[10]
C5F-Ph[10]
302
285
0.077
C5F-TSC (7) 327
C5F-MTSC 329
(8)
4.9ꢄ10ꢀ3[f]
3.2ꢄ10ꢀ3[f]
Figure 3. Theoretical dependence of wavelength of lowest energy transi-
tion on internal rotation coordinate for C5F-RN (9).
C5F-RN (9) 388[h] 3.80ꢄ104 667
0.50ꢄ104 0.0015[e] 7.2ꢄ10ꢀ5[g]
4.72ꢄ104 592
1.29ꢄ104 0.69[b]
1.6ꢄ10ꢀ3[f]
correspond to full conjugation, the longest wavelengths are
obtained and with 908 the conjugate system is divided into
two parts without the possibility of overlapping p-orbitals
resulting in shorter wavelengths. In the calculations only one
of the two substituents is rotated whilst the other one is
kept in the optimal planar arrangement, so that an even
broader range is expected for the experiment in which both
substituents are flexible. Thus we can conclude that the cal-
culations are able to qualitatively reproduce the absorption
spectra and that the broad peaks of the larger conjugated
systems are caused by conformational averaging of the flexi-
ble substituents. Similar calculations can now be used to
C5F-4SMe
(10)
C5F-4Py
(11)
C5F-MN
(12)
341
360
361
2.87ꢄ104 612
0.62ꢄ104
–
–
–
[i]
[i]
[j]
[j]
[j]
[j]
3.67ꢄ104
–
–
–
[a] All data for MeOH as the solvent, except for 5·C5F-CHO, in which
CDCl3 was used. [b] Irradiation wavelength: 313 nm, [c] For the aldehyde
5, the switching reaction is photoreversible at 313 nm with f
c!oACHTUNGERTN(NUNG 313)=
0.12. [d] 438 nm. [e] 366 nm. [f] 576 nm. [g] 633 nm (He/Ne-Laser).
[h] This compound shows a double maximum at 388/409 nm. [i] More-
step photoreaction, quantum yield not determined. [j] Photoirradiation of
the open form does not yield characteristic single maximum in the visible
region.
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Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes
FULL PAPER
design substituents with desired properties as, for example,
absorption in a specific frequency range, but the introduc-
tion of more rigid connections to the central part should
also be considered to obtain more narrow spectra. More
rigid molecules may probably also be of advantage for ob-
taining higher photochemical quantum yields due to fewer
routes of radiationless deactivation.
Photokinetic behaviour in solution: All synthesized com-
pounds undergo reversible photoswitching in solution. The
switching process goes along with a pronounced colour
change (cf. Table 1). The transformations from the open to
the closed form and back can be nicely followed and quanti-
fied by UV/Vis spectrophotometry. An example of a spec-
trokinetic series for C5F-MTSC (8) is given in Figure 4.
A similar behaviour was found for compounds 6–10 (for
the spectra see the Supporting Information). Isosbestic
points and linear absorption/absorption correlation diagrams
for pairwise combinations of different wavelengths (not
shown) indicate the uniformity of the process, that is, the ab-
sence of side reactions during the switching. Within an irra-
diation series, the spectra converge to a photostationary
state after long times. In case of the ring-closing reaction it
is not possible from the UV/Vis spectra alone to determine
the open/closed ratio in this situation. Therefore, the photo-
reactions were also followed by NMR spectroscopy. An ex-
ample is shown in Figure 5. It was found that, except for the
aldehyde C5F-CHO (5), the photostationary state for short-
wave illumination of the open form resulted in complete
switching to the closed form. In MeOH, the aldehyde 5
showed a hypsochromic shift of the newly developing band
in the visible region during irradiation. This can be assigned
to the formation of the acetal of 5. To avoid this reaction
and to be able to measure the
Figure 4. Spectrokinetic series for irradiation of C5F-MTSC (8) in MeOH
at 313 nm. Time intervals of irradiation are given in the figure. Concen-
tration c0 =2.57ꢄ10ꢀ5 m, photon irradiance I0 =7.9ꢄ10ꢀ9 Einsteincmꢀ2 sꢀ1
.
Compounds C5F-4Py (11) and C5F-4MN (12) showed
somewhat irregular behaviour (see the Supporting Informa-
tion). In both cases, even the open form shows a weak, long-
tailed absorption in the visible region. For compound 11, the
photochemical switching to the coloured form and back did
not occur in a uniform reaction (no clear isosbestic points,
non-monotonic shift of maximum in the UV region), so that
an evaluation of the quantum yield was difficult. In case of
compound 12, the absorption in the visible region increases,
but does not develop a clear single maximum.
composition of the photosta-
tionary state in the same sol-
vent by NMR spectroscopy, we
used CDCl3 as a solvent in this
case. An open/closed ratio of
2:5 corresponding to 71.4%
closed form could be assessed
from the integral values of the
pertinent proton signals. When
irradiating the open forms in
the visible region, a complete
reversion to the closed form
was achieved in all cases. It is
of interest to note that for com-
pounds C5F-Me and C5F-Ph,
derivatives investigated in the
Irie group,[10] all the data for
C5F-Me resemble closely those
of C5F-OH (6) and the data for
C5F-Ph those of C5F-CHO (5),
Figure 5. 1H NMR spectra of 8 (400 MHz, [D4]MeOD, 10ꢀ3 m) recorded at different time intervals of irradia-
tion with 366 nm. Spectra are shown top down: 0 (c), 45 (c) and 220 min (c). As a reference, the
bottom spectrum (c) shows the completely closed form, isolated by column chromatography from a reac-
tion mixture. The spectral region from d=3.3 to 6.1 ppm has been omitted for clarity.
which is very gratifying in view
of the electronic similarity of
these pairs of compounds.
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U. E. Steiner, E. Scheer, U. Groth et al.
From the photokinetics of compounds 5–10 under contin-
uous irradiation, the quantum yields of closing and opening
the central ring were evaluated according to the method de-
scribed by Gauglitz (see the Supporting Information).[23–24]
The resulting values are listed in Table 1. In general, the
quantum yields of closing are much higher than for re-open-
ing. This observation is in line with what was reported in
Irieꢃs review,[5] in which it was also pointed out that the
quantum yield of ring closure seems to be limited by the
fraction of molecules that are present in the “antiparallel”
conformation (i.e. with C2 symmetry of the photoreactive
group), whereas the “parallel” conformation (with sv sym-
metry) cannot undergo this reaction. Thus a value of 0.5 for
the ring-closing reaction seems to be a natural limit. In fact,
for the compounds listed in Table 1, most of the quantum
yields fo!c are below although rather near 0.5. As excep-
tions we note C5F-4SMe (10), which is clearly above 0.5 and
C5F-RN (9), with a very low quantum yield of 0.0015. With-
out further studies it cannot be decided whether this is due
to the conformational distribution or to photophysical rea-
sons.
With the exceptions of C5F-OH (6) and C5F-CHO (5),
the quantum yields of the cycloreversion, that is, re-opening
the switch by irradiation in the visible region, are quite
small. As has already been remarked by Irie,[5] the quantum
yields of this reaction are generally found to be much small-
er than those of the closing reaction, and that the cyclore-
version quantum yields were dependent on the p-conjuga-
tion length of the aryl groups. As an explanation, it is sug-
gested that in the excited singlet state the antibonding
nature of the central photogenerated carbon–carbon bond
decreases with the extension of the p-conjugation. We add
that by the inclusion of rotational flexibility in the p-system
the tendency for radiationless deactivation may be also in-
creased with the effect of lowering the quantum yield of
photoinduced cycloreversion. In line with this reasoning is
the finding of a very high cycloreversion quantum yield for
the aldehyde 5 and still higher for the alcohol 6, the latter
approaching a value of 1. If the internal flexibility of the
conjugated p-system is primarily responsible for the low
quantum yields of cycloreversion, one may hope that in a
situation in which the ends of the p-system are anchored to
gold contacts, the flexibility of the chain might be reduced
and the quantum yield thereby increased. Alternatively, one
might choose to design photochemical switches with a short
aliphatic interruption of the p-system to approach the fa-
vourable properties of the alcohol 5.
tional to the optical path length of the adlayer given as nd,
the product of its refraction index n and thickness d.
Adlayers of the closed forms of several of the new com-
pounds were prepared by adsorbing them from a droplet of
MeOH solutions of the samples (4.2–9.5ꢄ10ꢀ5 m), pre-irradi-
ated at 365 nm. After an incubation time of 24 h, washing
away of the unbound molecules and further illuminating
with 365 nm light to make sure that complete conversion to
the closed form is achieved, the SPR resonance curves
shown in Figure 6a were obtained. In all cases, a shift of the
resonance angle a on the order of 0.058 to higher values
with reference to a clean gold surface was observed. To
assess the thickness of the adsorbed adlayers, we calculated
the theoretical value of the resonance angle a as a function
of the thickness d of a layer with a refractive index of 1.58,[5]
a value applying to thin films of some thiazolyl analogues.
They should be reasonably applicable also to the present
furanyl derivatives. The results of the theoretical simulations
yield a linear correlation between Da and d with a slope of
about 0.048nmꢀ1 (cf. Figure 6b). Placing the experimental
data points with their observed Da values on this line (cf.
c), would indicate that the adlayers formed correspond to
a dense packing in a layer of an apparent thickness of 1–
1.5 nm. For an interpretation of the molecular ordering
within the adlayers, these values should be compared to
some characteristic molecular length parameter. As such,
we chose the distance between the terminal heteroatoms of
compounds 7, 9, 10 and 11. The geometries were derived
from molecular mechanics calculations based on the Univer-
sal Force Field.[28] The starting geometries were chosen such
that in the closed forms double bonds of the terminal p-
chains in direct conjugation with the furan rings were orient-
ed in a trans configuration. The distance of the terminal het-
eroatoms was measured after geometry optimization. The
theoretical length parameters thereby obtained are repre-
sented by the horizontal positions of the data points in Fig-
ure 6b. It appears that the experimental data points for com-
pounds 7 and 9 are close to the theoretical Da (d) line. This
means that the molecules seem to form a fairly dense mono-
layer (the refraction index corresponds to that of a compact
film) with the main extension of the molecule directed per-
pendicular to the surface. For 10 and 11, the data points fall
somewhat below the theoretical line, which could mean that
the monolayer is less dense. It may be less well ordered in
these cases, perhaps because a certain fraction of the mole-
cules is attached with both termini on the gold surface.
For demonstrating the switching functionality, we repeat-
edly irradiated the molecular film with light suitable for in-
ducing the ring-opening or -closing reaction, respectively. In
Figure 6c) we show how the SPR signal of the closed form
of C5F-4Py (11) (as already presented in Figure 6a) changes
after irradiation with visible light (l=633 nm). A clear shift
of the SPR angle Daswitch =0.0208 is observed.[29] In Fig-
ure 6d) we plot the reflectance at a fixed angle of a=42.28
as a function of time for compound C5F-4Py (11). The
shaded areas indicate the irradiation periods with UV and
visible light, respectively. A small, but recurrent variation of
Surface plasmon resonance (SPR): Preliminary experiments
for testing the photoswitching potential of the newly synthe-
sized molecules in contact with gold were performed by
using SPR.[25–26] In this method, the resonance angle a de-
tectable at the minimum of the resonance curve when scan-
ning the reflectivity of a gold layer on a glass prism under
conditions of total reflectance is very sensitive to adlayers of
molecules chemisorbed to the gold surface.[27] Such an adlay-
er causes a shift Da of the resonance angle that is propor-
6668
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6663 – 6672

Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes
FULL PAPER
Figure 6. a) SPR curves obtained for a clean Au surface and Au with adlayers of closed forms of 11 (C5F-4Py), 10 (C5F-pSMe), 9 (C5F-RN) and 7 (C5F-
TSC), respectively. b) Shift Da of resonance angle as a function of layer thickness. Black squares: theoretical simulation, red dots: experimental Da
values from (a) versus geometrical length parameter of the respective molecule (for details see text). c) Effect of illumination by visible light (633 nm)
on the SPR curve for C5F-4Py. Blue: curve from a), Red: curve after illumination with visible light. The angular shift between the SPR curves is 0.02o.
d) Differential reflectance of 11, C5F-4Py, (red dots) recorded at 42.2o as a function of irradiation time. The reflectance changes of clean Au (black
squares) are small and independent of the irradiation wavelength, which indicates that temperature or humidity changes do not dominate the SPR shift
of the molecular layer.
the reflectance with maximal amplitude on the order of
3.5% reflectance change is observed, which indicates that
the molecules switch reversibly while chemically bonded to
the metal surface. The amplitude of the signal becomes
smaller for further repetitions signalling ageing effects in the
film. For estimating the influence of potential artefacts, such
as the effects of temperature or humidity changes on the re-
flectance, we also show the behaviour of a clean Au surface
under the same irradiation conditions (c). The reflectance
change of the pure Au layer is in the order of 0.6% and has
no systematic dependence on the irradiation wavelength or
time. Its stochastic variation amplitude represents the noise
of the measurement.
Considering the supposedly small difference in refraction
index of open and closed form and the overall geometric
similarity of both forms, the observed size of the photoeffect
on the resonance angle of SPR appears quite pronounced.
Further studies to understand and model these effects quan-
titatively are in progress.
Conclusion
By starting with the dialdehyde C5F-CHO (5), a systematic
synthetic access to photoswitchable difurylethenes with ex-
tended p-systems is opened up. Whereas switching of the
compounds from the open, less conductive form (off-state)
to the closed, better conductive form (on-state) by UV light
is achieved with high quantum yields, the quantum yields
for re-opening the switches by the reverse photoreaction
when irradiating with visible light are very low for the fully
conjugated extended p-systems. Interrupting the conjugation
by an sp3 centre adjacent to the furan ring might be a way
to improve the quantum yield for the reverse switching pro-
cess without losing too much contrast in conductance be-
tween the on- and off-state.
As a proof of principle, SPR experiments have demon-
strated that the new molecules do have the potential to act
as photoreversible optoelectronic switches in contact with
gold.
Chem. Eur. J. 2011, 17, 6663 – 6672
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6669

U. E. Steiner, E. Scheer, U. Groth et al.
13C NMR (100 MHz, CDCl3, 258C): d=16.18 (CH3), 93.76 (CCH3),
111.24 (CH (furyl)), 164.94 (CCHO), 181.71 ppm (CHO); 19F NMR
(376 MHz, CDCl3, 258C): d=ꢀ114.51 (m, 4F; CF2CF2CF2), ꢀ133.61 ppm
(m, 2F; CF2CF2CF2).
Experimental Section
Syntheses: The compounds used were purchased from Sigma–Aldrich,
Fluka, Acros, ABCR and Fluorochem. Compound 3 was synthesized as
described in the literature.[30] TLC was performed on Polygram Sil G/
UV254 plates. A UV lamp (254 nm) was used for detection. Elemental
analyses were performed on a CHN-analyzer Heraeus (CHN-O-RAPID)
by the Microanalysis laboratory of Konstanz University. IR spectra were
recorded on a Perkin–Elmer 100 Series FTIR spectrometer. GCMS was
performed on an Agilent GCMS 7890A/5975C instrument (EI, 70 eV).
HRMS ESI/FTICR spectra were recorded on a Bruker APEX II FTICR
instrument. FABMS were performed on a Finnigan MAT 8200 instru-
ment. MALDI-TOF spectra were recorded on a Bruker Biflex III instru-
ment with pulsed nitrogen-laser (337 nm). UV/Vis spectra were recorded
on a Cary 50 spectrophotometer. NMR spectra were recorded on a
1,2-Bis(2-methyl-5-hydroxymethylfuran-3-yl)perfluorocyclopentene
(6,
C5F-OH): Compound 5 (0.21 g, 0.54 mmol) was dissolved in dry diethyl
ether (100 mL) and the solution was cooled to 08C. LiAlH4 (0.2 g,
5.36 mmol) was added in portions while keeping the temperature at 08C
and the reaction mixture was brought to RT. After refluxing for 5 h (re-
action monitored by TLC analysis) the mixture was cooled down,
quenched with water and the organic phase was separated. After drying
with MgSO4, the organic solvent was removed in vacuo and 0.13 g of ana-
lytically pure yellow oil was obtained (product solidified over several
weeks into brown crystals). Yield: 61%; m. p. 97–998C; 1H NMR
(600 MHz, CDCl3, 258C): d=1.96 (s, 6H; CH3), 2.21 (brs, 2H; OH), 4.52
(s, 4H; CH2), 6.27 ppm (s, 2H; Ar-H (furyl)); 13C NMR (150 MHz,
CDCl3, 258C): d=13.58 (CH3), 57.3 (CH2), 108.52 (C-4), 109.86 (C-3),
Bruker Avance DRX600 (600 MHz) and
(400 MHz).
a Jeol ECP-Eclipse 400
111.03 (m, CF2CF2CF2), 116.37 (tt, 1J
CF2CF2CF2), 132.46 (t, 2J
N
ACHTUNGTRENUN(NG C,F)=24 Hz;
1,2-Bis[5-(1,3-dioxolan-2-yl)-2-methylfuran-3-yl]perfluorocyclopentene
(4): tBuLi (52.5 mL, 79 mmol, 1.5m/hexane) was added dropwise to a so-
lution of 3 (7.58 g , 32.7 mmol) in dry THF (50 mL) at ꢀ788C under N2
and the reaction mixture was stirred for 30 min at ꢀ788C. A solution of
octafluorocyclopentene (2.2 mL, 16.4 mmol) in dry THF (5 mL) was
added dropwise over a period of 5 min and the reaction mixture was
brought to RT and stirred overnight. After quenching with aqueous
NH4Cl, the organic solvent was removed in vacuo, water (100 mL) was
added and the oily residue was extracted with diethyl ether (3ꢄ100 mL).
The combined organic phases were dried with MgSO4 and the solvent
was removed in vacuo yielding a brown oil. After column chromatogra-
phy, crude 4 (2.84 g) was obtained as a yellow oil and was used for the
next step. The compound was obtained in an analytically pure form as
light-brown crystals after recrystallization from n-hexane. Yield: 36%;
Rf =0.31 (silica, hexanes/EtOAc 2:1); m.p. 98–998C; 1H NMR (400 MHz,
CDCl3, 258C): d=1.96 (s, 6H; CH3), 3.93–4.01 (m, 4H; CH2), 4.04–4.11
(m, 4H; CH2), 5.80 (s, 2H; CH (acetal)), 6,42 ppm (s, 2H; Ar-H (furyl));
13C NMR (100 MHz, CDCl3, 258C): d=13.51 (CH3), 65.25 (CH2), 97.27
(CH (acetal)), 109.30 (C-4), 109.49 (C-3), 110.9 (m, CF2CF2CF2), 116.25
AHCTUNGTRENNUNG
1,2-Bis{2-methyl-5-[(E)-(2-thiocarbamoylhydrazono)methyl]furan-3-yl}-
perfluorocyclopentene (7, C5F-TSC): Compound 5 (50 mg, 0.13 mmol)
and thiosemicarbazide (23 mg, 0.26 mmol) were dissolved in hot EtOH
(10 mL) with a few drops of pure acetic acid. The reaction mixture was
stirred for 24 h at RT and then poured into water (30 mL) and filtered.
After washing with cold EtOH (3 mL) and drying dark-brown crystals
(61 mg) were obtained. Yield: 89%; m.p. 2038C; 1H NMR (600 MHz,
[D6]DMSO, 258C): d=1.95 (s, 6H; CH3), 7.07 (s, 2H; Ar-H (furyl)), 7.74
(brs, 2H), 7.89 (s, 2H; HC-6), 8.25 (brs, 2H), 11.50 ppm (brs, 2H;
HNN); 13C NMR (150 MHz, [D6]DMSO, 258C): d=13.58 (CH3), 110.68
(C-3), 110.84 (m; CF2CF2CF2), 112.11 (C-4), 116.06 (tt, 1J
(C,F)=24 Hz; CF2CF2CF2), 131.80 (C-6), 132.16 (t, 2J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
R
CCF2), 149.62 (C-5), 155.14 (C-2), 177.83 ppm (C=S); 19F NMR
(376 MHz, DMSO, 258C): d=ꢀ109.37 (br, 4F; CF2CF2CF2),
ꢀ130.78 ppm (br, 2F; CF2CF2CF2); IR: n˜ =3422, 3526, 3144, 2962, 1590,
1504, 1275 cmꢀ1; FAB-MS: m/z: 539 [M+1H]+, 462, 389; HRMS: m/z:
calcd for C19H16F6N6O2S2: 539.0753 [M+1H]+; found: 539.0750; elemen-
tal analysis calcd (%) for C19H20F6N6O4S2 [M+2H2O]+: C 39.72, H 3.51,
S 11.16; found: C 39.58, H 3.36, S 11.23; closed form isomer of 7: dark-
blue crystals; 1H NMR (400 MHz, CDCl3, 258C): d=1.64 (s, 6H; CH3),
6.44 (s, 2H; CH (furyl)), 7.70 ppm (s, 2H; HC-6); 13C NMR (100 MHz,
CDCl3, 258C): d=16.55 (CH3), 94.11 (CCH3), 107.10 (CH (furyl)), 132.33
(C=N), 148.39 (C-3), 167.28 (C-5), 180.98 ppm (C=S); 19F NMR
(376 MHz, CDCl3, 258C): d=ꢀ115.43 (m, 4F; CF2CF2CF2), ꢀ135.13 ppm
(m, 2F; CF2CF2CF2).
1
2
2
(tt, JACHTUNGTRENNUNG(C,F)=257, JACHTUNGTRENNUNG(C,F)=24 Hz; CF2CF2CF2), 132.37 (t, JACTHNUGTREN(GUNN C,F)=24 Hz;
CCF2), 150.60 (C-5), 153.88 ppm (C-2); 19F NMR (376 MHz, CDCl3,
258C): d=ꢀ110.10 (t, 3J
(F,F)=5.2 Hz, 4F; CF2CF2CF2), ꢀ131.63 ppm
G
(m, 2F; CF2CF2CF2); IR: n˜ =2896, 1685, 1612, 1292, 1279 cmꢀ1; MS: m/z:
480 [M]+, 435, 73; HRMS: m/z: calcd for C21H19F6O6: 481.1080
[M+1H]+; found: 481.1069; elemental analysis calcd (%) for C21H18F6O6:
C 52.51, H 3.78; found: C 52.48, H 3.95.
1,2-Bis(2-methyl-5-formylfuran-3-yl)perfluorocyclopentene (5): Com-
pound 4 (0.78 g, 1.63 mmol) was dissolved in THF (50 mL). Acetone
(50 mL) and conc. HCl (3 mL) were added and the solution was stirred
overnight at RT. The reaction was monitored by TLC analysis. After
complete deprotection, the organic solvents were removed in vacuo and
the residue was brought to pH 9 by adding aqueous NaHCO3. After ex-
traction with diethyl ether (3ꢄ50 mL) the combined organic fractions
were dried with MgSO4 and the solvent was removed in vacuo. The oily
residue was purified by column chromatography thus giving of 5 (0.55 g).
Analytically pure 5 was obtained after recrystallization from diethyl
ether as light-brown crystals. Yield: 87%; Rf =0.26 (hexanes/EtOAc 5:2);
m.p. 116–1188C; 1H NMR (600 MHz, CDCl3, 258C): d=2.11 (s, 6H;
CH3), 7.23 (s, 2H; Ar-H), 9.59 ppm (s, 2H; CHO); 13C NMR (150 MHz,
CDCl3, 258C): d=13.96 (CH3), 110.55 (m, CF2CF2CF2), 111.70 (C-3),
1,2-Bis{2-methyl-5-[(E)-(1-methylthio-1-iminomethylhydrazono)methyl]-
furan-3-yl}perfluorocyclopentene (8, C5F-MTSC): Compound 5 (50 mg,
0.13 mmol) and methylthiosemicarbazide (27 mg, 0.26 mmol) were dis-
solved in hot EtOH (10 mL) with a few drops of pure acetic acid. The re-
action mixture was stirred for 24 h at RT and was poured into water
(30 mL) and filtered. After washing with cold EtOH (3 mL) and drying
blue crystals (68 mg) were obtained. Yield: 97%; m.p. 1578C; 1H NMR
(600 MHz, [D6]DMSO, 258C): d=1.97 (s, 6H; CH3), 2.98 (s, 6H; SCH3),
7.04 (s, 2H; Ar-H (furyl)), 7.90 (s, 2H; HC-6), 8.29 (brs, 2H; C=NH),
11.51 ppm (brs, 2H; HNN); 13C NMR (150 MHz, [D6]DMSO, 258C): d=
13.42 (CH3), 30.87 (SCH3), 110.51 (C-3), 110.69 (m; CF2CF2CF2), 111.62
115.80 (tt, 1J
ACHTUNGTRENNUNG HCAUTNGTRENN(GUN C,F)=24 Hz; CF2CF2CF2), 120.71 (C-4),
(C,F)=256, 2J
(C-4), 115.79 (tt, 1J
(C,F)=255, 2J
ACHTUTGNRENUNG HCAUTNGTNER(NUGN C,F)=24 Hz; CF2CF2CF2), 131.05 (C-
132.80 (t, 2J
ACHTUNGTRENNUNG(C,F)=24 Hz; CCF2), 152.15 (C-2), 158.95 (C-5), 176.93 ppm
6), 132.07 (t, 2J
(C,F)=24 Hz; CCF2), 149.63 (C-5), 154.85 (C-2),
(CHO); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ110.10 (t, 3J
ACHTUNGTRNE(NUNG F,F)=
177.51 ppm (C=N); 19F NMR (376 MHz, DMSO, 258C): d=ꢀ109.33 (br,
4F; CF2CF2CF2), ꢀ130.77 ppm (br, 2F; CF2CF2CF2); IR: n˜ =3138, 2940,
1540, 1277, 1243 cmꢀ1; HRMS: m/z: calcd for C21H20F6N6O2S2: 567.1066
[M+1H]+; found: 567.0986; closed form isomer of 8: dark-blue crystals;
1H NMR (400 MHz, CDCl3, 258C): d=1.63 (s, 6H; CH3), 3.14 (s, 6H;
SCH3), 6.43 (s, 2H; CH (furyl)), 7.66 ppm (s, 2H; HC-6); 13C NMR
(100 MHz, CDCl3, 258C): d=16.59 (CH3), 94.05 (CCH3), 106.41 (CH
(furyl)), 131.29 (HC=N), 148.50 (C-3), 150.18 (C=NH), 167.64 ppm (C-5);
4.9 Hz, 4F; CF2CF2CF2), ꢀ131.63 ppm (m, 2F; CF2CF2CF2); correlations
confirmed by 1H-13C-HSQC, 1H-13C-HMBC, 19F-13C-HSQC and 19F-13C-
HMBC experiments; IR: n˜ =3136, 2880, 1672 (C=O), 1651, 1590,
1537 cmꢀ1
;
MS: m/z: 392 [M]+, 377, 307; HRMS: m/z: calcd for
C17H11F6O4: 393.0556 [M+1H]+; found: 393.0529; elemental analysis
calcd (%) for C17H10F6O4: C 52.05, H 2.57; found: C 52.39, H 2.93; closed
form isomer of 5: dark-pink crystals; 1H NMR (400 MHz, CDCl3, 258C):
d=1.71 (s, 6H; CH3), 6.61 (s, 2H; CH (furyl)), 9.71 ppm (s, 2H; CHO);
6670
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6663 – 6672

Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes
FULL PAPER
19F NMR (376 MHz, CDCl3, 258C): d=ꢀ115.37 (m, 4F; CF2CF2CF2),
112.32 (CN (trans to HC-6)), 113.20 (C-3), 113.35 (CN (cis to HC-6)),
ꢀ135.07 ppm (m, 2F; CF2CF2CF2).
115.66 (tt, 1J
(C,F)=256, 2J
ACHTUTGNRENUNG CAHUTNGTRENN(UNG C,F)=24 Hz; CF2CF2CF2), 122.27 (C-4),
133.14 (t, 2J
ACTHNUGTRNEN(UG C,F)=24 Hz; CCF2), 141.89 (C-6), 147.69 (C-5), 160.43 ppm
1,2-Bis{2-methyl-5-[(E)-4-oxo-2-thioxothiazolidin-5-ylidenemethyl]lfuran-
3-yl}perfluorocyclopentene (9, C5F-RN): Piperidine (0.2 mL) was added
to a solution of 5 (0.2 g, 0.51 mmol) and rhodanine (0.13 g, 1 mmol) in
CH2Cl2 (30 mL). The reaction was stirred for 24 h at RT, the organic sol-
vent was removed in vacuo and the residue was purified by column chro-
matography on silica thus giving gold-brown crystals (0.21 g). Yield:
67%; Rf =0.43 (hexanes/EtOAc 1:1); m.p. 1068C; 1H NMR (600 MHz,
CDCl3, 258C): d=2.12 (s, 6H; CH3), 4.30 (brs, 2H; NH), 6.67 (s, 2H;
Ar-H (furyl)), 7.20 ppm (s, 2H; C-6); 13C NMR (150 MHz, CDCl3, 258C):
d=14.19 (CH3), 110.85 (m; CF2CF2CF2), 112.52 (C-3), 115.35 (C-6),
(C-2); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ109.99 (t, 3J
ACHTUNGTRENNUNG(F,F)=
4.8 Hz, 4F; CF2CF2CF2), ꢀ131.47 ppm (m, 2F; CF2CF2CF2); correlations
confirmed by 1H-13C-HSQC and 1H-13C-HMBC (cis-3J
ACTHNUGRTENUNG(H,CN)=7.2,
trans-3J
(H,CN)=13 Hz) experiments; IR: n˜ =3059, 2237 (CꢁN), 2225
(CꢁN), 1615 cmꢀ1; FAB-MS: m/z: 511 [M+Na]+, 489 [M+H]+.
For further experimental details see the Supporting Information.
116.00 (tt, 1J
128.39 (C-7), 132.78 (t, 2J
AHCTUNGTRENNUNG
N
ACHTUNGTERN(NUNG C,F)=24 Hz; CF2CF2CF2), 116.82 (C-4),
Acknowledgements
d=ꢀ109.91 (t, 3J
G
Financial support from DFG through SFB767 is gratefully acknowledged.
Furthermore we have to thank T. Geldhauser, B. Briechle, A. Erbe and
K. Drexler for their contributions to the work.
CF2CF2CF2); correlations confirmed by H-13C-HSQC and H-13C-HMBC
1
1
(3J
1421 cmꢀ1
ACHTUNGTRENNUNG(H, C=O)=21 Hz) experiments; IR: n˜ =3101, 2849, 1686 (C=O), 1607,
;
EIMS: m/z: 622 [M]+, 535; HRMS: m/z: calcd for
C23H12N2S4F6O4 [M+1H]+: 622.9657; found: 622.9660; elemental analysis
calcd (%) for C23H12N2S4F6O4: C 44.37, H 1.94, N 4.50; found: C 44.28, H
2.93, N 4.48.
[1] For a selection of review articles, see the special issue on “Photo-
chromism: Memories and Switches”, Chem. Rev. (Ed.: M. Irie)
2000, 100, Issue 5.
[2] M. Irie, Chem. Rev. 2000, 100, 1683–1684, Editorial to special issue.
[3] Photochromism: Molecules and Systems (Eds.: H. Dꢅrr, H. Bouas-
Laurent), Elsevier, Amsterdam, 2003.
[4] Dynamic Studies in Biology: Phototriggers, Photoswitches, and
Caged Biomolecules (Eds.: M. Goeldner, R. Givens), Wiley-VCH,
Weinheim, 2005.
1,2-Bis[2-methyl-5-(E)-(4-methylthiostyryl)furan-3-yl]perfluorocyclopen-
tene (10, C5F-4SMe) (mixture of diastereomers): [4-(Methylthio)benzyl]-
triphenylphosphonium bromide (0.67 g, 1.4 mmol) was dissolved in dry
MeOH (50 mL) containing MeONa (0.17 g, 3.15 mmol). After stirring for
15 min, compound 5 (0.25 g, 0.64 mmol) was added and the whole stirred
overnight. The reaction mixture was neutralized with aqueous NH4Cl
and concentrated in vacuo. The remaining dark-blue oily residue was
mixed with dichloromethane (150 mL) and water (50 mL). The organic
phase was separated, dried with MgSO4 and the solvent removed in
vacuo. Dark-blue crystals (0.17 g) were obtained. Yield: 41%; 1H NMR
(600 MHz, CDCl3, 258C): d=2.03 (m, 6H; CH3), 2.50 (m, 6H; SCH3),
6.23 (m, 1H), 6.31 (m, 2H), 6.43 (m, 1H), 6.77 (m, 1H), 6.99 (m, 1H),
7.20 (m, 4H), 7.36 ppm (m, 4H); 19F NMR (376 MHz, DMSO, 258C): d=
ꢀ109.857 (m, 4F; CF2CF2CF2), ꢀ131.49 ppm (m, 2F; CF2CF2CF2);
MALDI-TOF-MS: m/z: 634, 633, 632.
[5] M. Irie, Chem. Rev. 2000, 100, 1685–1716.
[6] A. Staykov, K. Yoshizawa, J. Phys. Chem. C 2009, 113, 3826–3834.
[7] F. L. E. Jakobsson, P. Marsal, S. Braun, M. Fahlmann, M. Berggren,
J. Cornil, X. Crispin, J. Phys. Chem. C 2009, 113, 18396–18405.
[8] N. Katsonis, T. Kudernac, M. Walko, S. J. van der Molen, B. J.
van Wees, B. L. Feringa, Adv. Mater. 2006, 18, 1397–1400.
´
[9] D. Dulic, S. J. van der Molen, T. Kudernac, H. T. Jonkman, J. J. D.
de Jong, T. N. Bowden, J. van Esch, B. L. Feringa, B. J. van Wees,
Phys. Rev. Lett. 2003, 91, 207402.
1,2-Bis(2-methyl-5-{(Z)-[2-cyano-2-(pyridin-4-yl)vinyl]}furan-3-yl)per-
fluorocyclopentene (11, C5F-4Py): Compound 5 (0.1 g, 0.25 mmol) and
pyridine-4-acetonitrile hydrochloride (0.08 g, 0.5 mmol) were dissolved in
dry MeOH (10 mL). After the addition of K2CO3 (0.176 g, 1.27 mmol),
the reaction mixture was stirred for 48 h at RT. After addition of diethyl
ether (50 mL) the mixture obtained was washed consecutively with water
(30 mL), aqueous K2CO3 (30 mL) and again with water (30 mL). The or-
ganic fraction was dried with MgSO4, the solvent removed in vacuo and
the dark residue purified by filtering through a silica column by using
hexanes/EtOAc 1:1 as the eluent, thus giving brown crystals (0.12 g).
Yield: 77%; m.p. 1828C; 1H NMR (600 MHz, CDCl3, 258C): d=2.24 (s,
6H; CH3), 7.24 (s, 2H; Ar-H (furyl)), 7.47 (s, 2H; C-6), 7.51 (dd, 3J-
[10] T. Yamaguchi, M. Irie, J. Mater. Chem. 2006, 16, 4690–4694.
[11] J. E. McMurry, Acc. Chem. Res. 1983, 16, 405–411.
[12] M. Irie, M. Mohri, J. Org. Chem. 1988, 53, 803–808.
[13] M. Dubernet, V. Caubert, J. Guillard, M.-C. Viaud-Massuard, Tetra-
hedron 2005, 61, 4585–4593.
[14] Z.-N. Huang, B.-A. Xu, S. Jin, M.-G. Fan, Synthesis 1998, 1092–
1094.
[15] X. Deng, L. S. Liebeskind, J. Am. chem. Soc. 2001, 123, 7703–7704.
[16] Crystal data for 4: C21H18F6O6; M=480.35; T=173(2) K; monoclin-
ic; space group C2/c; a=21.390(4), b=8.9727(10), c=10.6721(18) ꢂ;
b=93.767(13)8;
V=2043.8(5) ꢂ3;
Z=4;
1calcd =1.561 gcmꢀ3
;
mMoKa =0.148 mmꢀ1; 2117 reflections measured; 1998 independent
reflections (Rint =0.0182); R1 (I>2s(I))=0.0512, wR2 (all data)=
0.1534. CCDC-801309 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
A
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(H,H)=1.7 Hz), 4H; C-9), 8.69 ppm (dd, 3J(H,H)=4.6, 4J-
ACHTUNGTRENNUNG
14.02 (CH3), 106.63 (C-7), 110.82 (m; CF2CF2CF2), 112.30 (C-3), 115.90
(m; CF2CF2CF2), 116.31 (CN), 117.23 (C-4), 119.60 (C-9), 128.91 (C-6),
132.90 (t, 2J
ACHTUNGTRENNUNG(C,F)=24 Hz; CCF2), 140.75 (C-8), 149.07 (C-5), 150.78 (C-
10), 157.01 ppm (C-2); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ109.94
(m, 4F; CF2CF2CF2), ꢀ131.46 ppm (m, 2F; CF2CF2CF2); IR: n˜ =3048,
2216 (CꢁN), 1591, 1415, 1275 cmꢀ1; FAB-MS: m/z: 592 [M]+, 506;
HRMS: m/z: calcd for C31H19F6N4O2: 593.1407 [M+1H]+; found:
593.1394.
[17] The relevant structures of the open forms described in reference [10]
have been retrieved from the Cambridge Structural Database. Iden-
tifier: CESWED for C5F-Ph and CESWON for C5F-Me.
[18] R. E. Stratmann, G. E. Scuseria, M. J. Frisch, J. Chem. Phys. 1998,
109, 8218–8224.
1,2-Bis[2-methyl-5-(2,2-dicyanovinyl)furan-3-yl]perfluorocyclopentene
(12, C5F-MN): Piperidine (2 drops) was added to a solution of 5 (50 mg,
0.13 mmol) and malodinitrile (18 mg, 0.28 mmol) in benzene (5 mL) and
the mixture was stirred for 48 h. The organic solvent was removed in
vacuo and the oily residue was filtered through a silica column by using
hexanes/EtOAc 1:1 as the eluent. Violet crystals (13 mg) were obtained.
Yield: 21%; 1H NMR (400 MHz, CDCl3, 258C): d=2.25 (s, 6H; CH3),
7.25 (s, 2H; Ar-H (furyl)), 7.41 ppm (s, 2H; C-6); 13C NMR (100 MHz,
CDCl3, 258C): d=14.40 (CH3), 79.94 (C-7), 110.60 (m, CF2CF2CF2),
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Chem. Eur. J. 2011, 17, 6663 – 6672
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[29] The repeated switching experiments shown in Figure c have been
performed in a somewhat different optical setup than the measure-
ments shown in Figure a. This results in a slightly different shape of
the SPR resonance but has no influence on the value of the reso-
nance angle.
[30] M. Gorzynski, D. Rewicki, Liebigs Ann. Chem. 1986, 625–637.
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http://www.arguslab.com.
Received: December 23, 2010
Published online: May 5, 2011
6672
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6663 – 6672