A new class of photochromic 1,2-diarylethenes; synthesis and switching
properties of bis(3-thienyl)cyclopentenes
Linda N. Lucas, Jan van Esch, Richard M. Kellogg* and Ben L. Feringa*
Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands. E-mail: r.m.kellogg@chem.rug.nl; b.l.feringa@chem.rug.nl
Received (in Cambridge, UK) 8th September 1998, Accepted 17th September 1998
A novel synthetic route to diarylethenes fixed in a cyclo-
pentene based on titanium-mediated carbonyl coupling
leads to bis(3-thienyl)cyclopentenes that show photochromic
behaviour similar to that of known diarylethenes.
Table 1 shows the UV–VIS data for the open and closed forms
of the new photochromic compounds. Compared to the known
diarylethene derivatives the wavelengths at the absorption
maxima of the closed forms show a blue shift; only 6b measured
in benzene showed the same absorption maximum in the closed
form as was reported earlier for the perfluorocyclopentene
analog.10 The diarylethenes 1a, 2a and 3a turn yellow upon UV
irradiation, which is unusual; no diarylethene compounds
showing this behaviour have been described in literature. The
Diarylethenes constitute an important class of photochromic
molecules, as they are thermally irreversible and show high
fatigue resistance,1 which are promising features for application
in optical data storage2 and as molecular switches.3,4 The aryl
groups are bound to a cycloalkene to prevent cis–trans
isomerization, which might compete with photocyclization.
1,2-Bisaryl-substituted maleic anhydride,5 maleimide6 and
perfluorocyclopentene7 moieties have been employed so far.
Each bridging unit has its advantages and disadvantages.
Diarylmaleic anhydrides are readily accessible, but are sensitive
to acidic conditions and need certain types of aryl derivatives to
maintain their photofatigue resistance in the presence of air.8
Diarylmaleimides are also sensitive to acidic conditions and
furthermore show some degradation in the presence of oxygen.9
Diarylperfluorocyclopentenes exhibit excellent photochromic
behaviour, since up to 80 °C they are stable in the presence of
air during cyclization, unless strong electron-donating and
2withdrawing substituents are introduced at the 5-position of
the thiophene ring.10 Despite these highly attractive properties
the expensive and rather volatile starting material octa-
fluorocyclopentene and the low yields commonly found in
double substitution reactions of octafluorocyclopentene with
lithiated thiophenes are major disadvantages.
λ1
λ2
R
R
S
S
S
S
R
R
a (open)
b (closed)
1 R = H
2 R = Cl
3 R = CH3
4 R = CHO
5 R = PhMeCHN CH
6 R = (CN)2C CH
O
O
iii
i
ii
Cl
Cl
Cl
S
S
S
S
Herewith we present a new class of diarylethenes, the
dithienylcyclopentenes, and a novel synthetic route to diaryle-
thenes based on titanium-mediated carbonyl coupling. The
synthesis can be performed on a large scale from rather cheap
starting materials (Scheme 1). We envisioned that an intra-
molecular McMurry coupling of a bisthienyl substituted
1,3-dicarbonyl compound would be a feasible route to 1,2-di-
thienylcyclopentenes. Compounds 1–6† were synthesized start-
ing from 2-methylthiophene, which was chlorinated at the
5-position with NCS in AcOH and benzene, followed by a
Friedel–Crafts reaction with AlCl3 and glutaryl chloride at 0 °C.
The resulting 1,5-bis(5-chloro-2-methyl-3-thienyl)pentadione
was used in a McMurry reaction11 with TiCl3(THF)3 and Zn in
THF at 40 °C to provide 2a. Starting from 2,5-dimethylth-
iophene, 3a was obtained via the same procedure as described
above. By using Mg instead of Zn in the McMurry reaction with
1,5-bis(5-chloro-2-methyl-3-thienyl)pentane-1,5-dione, 1a was
obtained.‡ Dialdehyde 4a was synthesized by double lithiation
of 1a or 2a followed by quenching with DMF. The conversion
of 4a to 5a and 6a was performed according to published
procedures.10,12 Enantiomerically pure (+)-(R)-1-phenylethyla-
mine was used in the synthesis of compound 5.12 Although the
yields in the low-valent titanium coupling step are still modest,
this new route provides ready access to a variety of dithie-
nylcyclopentenes.
iv
H
H
Cl
Cl
S
S
S
S
O
O
2a
4a
Scheme 1 Reagents and conditions: i, NCS, AcOH, benzene, reflux, 80%;
ii, AlCl3, glutaryl chloride, CS2, 0 °C, 94%; iii, TiCl3(THF)3, Zn, THF,
40 °C, 44%; iv, BunLi, DMF, 39%.
The photochromic behaviour was followed by both 1H NMR
and UV spectroscopy. Fig. 1 illustrates the change in absorption
of a solution of 4a in benzene upon irradiation at 313 nm. A new
absorption band appeared at 583 nm due to formation of the
closed form, which has an extended conjugated structure.
Fig. 1 UV–VIS spectra of 4a (a) before and (b) after irradiation for 5 min
at 313 nm (2.05 3 1025
M in benzene).
Chem. Commun., 1998, 2313–2314
2313
Lucas, Linda N.
