Electrochemically induced ring-closing of photochromic 1,2-dithienylcyclopentenes

Article abstract of DOI:10.1039/b211378c

An unprecedented combination of photochromism and electrochromism is observed for two 1,2-bis(dithienyl)cyclopentene derivatives; the ring-opening reactions are photochemically driven while the ring-closing reactions can be triggered by electrochemical ox

Full text of DOI:10.1039/b211378c

Electrochemically induced ring-closing of photochromic
1,2-dithienylcyclopentenes
Andrea Peters and Neil R. Branda*
Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6.
E-mail: nbranda@sfu.ca
Received (in Columbia, MO, USA) 19th November 2002, Accepted 18th February 2003
First published as an Advance Article on the web 12th March 2003
An unprecedented combination of photochromism and
electrochromism is observed for two 1,2-bis(dithienyl)cyclo-
pentene derivatives; the ring-opening reactions are photo-
chemically driven while the ring-closing reactions can be
triggered by electrochemical oxidation.
now report the first example of a photochromic 1,2-dithieny-
lalkene derivative that exhibits redox-driven ring-closing.4
Two photochromic dithienylalkene derivatives, 1,2-bis(2-
methyl-5,2A-dithiophen-3-yl)perfluorocyclopentene 2o and
1,2-bis(2,5A-dimethyl-5,2A-dithiophen-3-yl)perfluorocyclopen-
tene 3o, differing only in the groups attached to the external
heterocycles were prepared in one step from the known
photochromic dichloride 45 via the dibromide 5 using the
coupling procedure shown in Scheme 2.‡ The methylated
version, 3o, was prepared in order to investigate the electro-
chemical properties of the ring-open forms of these photo-
chromic 1,2-dithienylethenes at the high potentials needed to
generate their radical cations. The absence of these methyl
groups has little effect on the photochemical ring-closing and
ring-opening reactions but does play a significant role in
minimizing the electrochemical polymerization that is charac-
teristic for thiophenes.6 Selective photophysical and electro-
chemical properties are summarized in Table 1.
Both photochromic compounds undergo similar photo-
chemical ring-closing and ring-opening reactions. When
CH2Cl2 or CH3CN solutions (2 3 1025 M) of the ring-open
forms of the photochromic bis(dithiophenes) 2o and 3o are
irradiated with ultraviolet light (365 nm)§ they are converted to
their deep blue ring-closed forms (2c and 3c), which have strong
absorption bands in the visible region of the UV-VIS absorption
spectrum (Table 1). After 10 minutes of irradiation, the
photostationary states are reached which are composed of
greater than 97% of the ring-closed isomers.¶ Subsequent
The interest in using photochromic 1,2-dithienylethene deriva-
tives in optical recording and switching applications has arisen
primarily because they possess appealing photochromic proper-
ties such as thermal stability of both the colorless and colored
forms, and fatigue resistance in the photochemical ring-closing
and ring-opening processes.1 We have recently described the
important discovery that the ring-opening reaction and decolor-
ization of several photochromic 1,2-dithienylcyclopentene
derivatives, such as bis(terthiophene) 1c (Scheme 1), can also
be triggered by electrochemical or chemical oxidation.2 We can
conclude, to date, that this electrochromism3 is contingent on
having non-alkyl groups attached to the carbons involved in
forming the new single bond. This preliminary conclusion is
based on the fact that when the 2-methyl groups in bis(dithio-
phene) 2 are replaced by aryl rings (thiophene or benzene, for
example) the colored ring-closed forms rapidly re-open when
they are oxidized to their thermally unstable radical cations.2
The radical cation generated by electrochemically oxidizing
the ring-closed form of bis(dithiophene) 2c appears to be
relatively stable and does not undergo oxidative ring-opening as
is shown by the reversible redox couple (E1/2 = 0.82 V) in the
cyclic voltammogram†(Fig. 1, bottom trace). On closer inspec-
tion of the redox properties of both the ring-open (2o) and ring-
closed (2c) forms, we noticed that the reduction peak in the
voltammogram of the ring-open form appears at the same
potential (0.80 V) as that for the radical cation of its
corresponding ring-closed isomer suggesting that the ring-
closing reaction is also triggered by the oxidation process.
Additional support for this suggestion is the appearance of a
new oxidation peak at the same potential (0.85 V) where the
ring-closed form (2c) is oxidized when the sample of the ring-
open form (2o) is swept through several redox cycles (Fig. 1,
inset). This new observation prompted us to reinvestigate the
electrochemical behavior of the bis(dithiophene) derivative. We
Fig. 1 Cyclic voltammograms of CH3CN solutions (1 3 1023 M) of 2o (top)
and 2c (bottom) at 200 mV s21. The inset shows multiple sweeps of 2o (5
cycles).
Scheme 1
Scheme 2
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Table 1 Selective electrochemical and photophysical data for CH3CN
solutions of 1,2-dithienylethenes 2o and 3o and their ring-closed forms in
the photostationary states
AgCl (in a saturated NaCl solution) reference electrode and tetra-
butylammonium hexafluorophosphate (0.1 M) as the electrolyte. All results
were referenced against ferrocene (0.40 V vs. SCE).
‡
Preparation of 5: A solution of 1,2-bis(5A-chloro-2A-methylthien-3A-
Compound
Eox/V
Ered/V
lmax/nm
yl)perfluorocyclopentene (500 mg, 1.14 mmol) in Et2O (40 mL) cooled to
278 °C was treated with tert-butyllithium (1.34 mL of a 1.7 M solution in
pentane) dropwise under argon. After stirring for 15 min, a solution of
bromine (0.117 mL, 2.28 mmol) in Et2O (10 mL) was added dropwise. The
reaction was stirred at 278 °C for 20 min, warmed slowly to room
temperature and quenched with H2O. The organic layer was separated,
washed with H2O (2 3 10 mL), followed by brine (10 mL), dried (Na2SO4)
and filtered. The solvent was evaporated under reduced pressure and the
resulting crude product was purified using column chromatography through
silica (hexanes) to afford 0.452 g (75%) of the pure product as a white solid.
mp 146–148°C. 1H NMR (300 MHz, CDCl3) d/ppm: 6.99 (s, 2H), 1.87 (s,
6H). 13C NMR (100 MHz; CDCl3) d/ppm: 143.3, 135.6, 129.1, 125.2,
115.8, 110.0, 14.4. Anal. Calcd for C15H8Br2F6S2: C, 34.24, H, 1.53. Found:
C, 34.07, H, 1.56%. Preparation of bis(dithiophene)s 2o and 3o: A solution
of 2-bromothiophene or 2-bromo-5-methylthiophene (1 mmol) in Et2O (10
mL) was treated with magnesium turnings (30 mg, 1.2 mmol) and heated at
reflux for 45 min. The heating source was removed, the reaction mixture
was allowed to cool to room temperature and it was added to a mixture of
dibromide 5 (263 mg, 0.5 mmol), a catalytic amount of Pd(dppf)Cl2 ( ~ 4
mol%) and Et2O (10 mL) at 0 °C dropwise via a cannula. The reaction was
stirred at this temperature for 1 h. It was allowed to come to room
temperature, stirred overnight and quenched with 5% HCl (5 mL). The
aqueous layer was separated and extracted with Et2O (3 3 10 mL). The
combined organic layers were washed with H2O (3 3 10 mL), followed by
brine (10 mL), dried (Na2SO4) and filtered. The solvent was removed under
reduced pressure and the crude product was purified using column
chromatography through silica (hexanes) to afford the pure product as a
white solid. Selected data for 2o: mp 127–128 °C. 1H NMR (600 MHz;
CD2Cl2) d/ppm: 7.28 (dd, J = 5.1, 1.2 Hz, 2H), 7.16 (dd, J = 3.6, 1.2 Hz,
2H), 7.15 (s, 2H), 7.03 (dd, J = 5.1, 3.6 Hz, 2H), 1.97 (s, 6H). 13C NMR
(125 MHz, CDCl3) d/ppm 140.8, 136.2, 135.5, 127.9, 125.5, 124.9, 124.1,
122.8, 14.4. Anal. Calcd for C23H14F6S4: C, 51.87, H, 2.65. Found: C,
52.05, H, 2.59%. Selected data for 3o: mp 124–125 °C. 1H NMR (400 MHz,
CD2Cl2) d/ppm 7.04 (s, 2H), 6.94 (d, J = 3.6 Hz, 2H), 6.69 (dq, J = 3.4,
1.0 Hz, 2H), 2.47 (d, J = 0.8 Hz, 6H), 1.94 (s, 6H). 13C NMR (125 MHz,
CDCl3) d/ppm: 140.2, 139.7, 135.9, 133.9, 126.0, 125.4, 123.9, 122.0, 15.3,
14.4. Anal. Calcd for C25H18F6S4: C, 53.56; H, 3.24. Found: C, 53.47; H,
3.26%.
§ Standard lamps used for visualizing TLC plates (Spectroline E-series, 470
mW cm22) were used to carry out the ring-closing reactions of 2o and 3o.
The ring-opening reactions were carried out using the light of a 150 W
tungsten source that was passed through a 490 nm cutoff filter to eliminate
higher energy light.
¶ The compositions of all photostationary states were measured using 1H
NMR spectroscopy.
∑ Electrolysis experiments were performed under similar conditions as
described for the cyclic voltammetry experiments except the platinum
working electrode was replaced by a platinum coil. A potential 15–20 mV
more positive than the half-wave potential for 3o was used for the
electrolysis experiment. This typically was in the range 1.35–1.39 V
depending on the particular Ag/AgCl reference electrode used. In a typical
experiment, a 1 3 1023 M solution of 3o in CH3CN was electrolyzed, 40 mL
aliquot amounts were periodically removed and diluted to 2 3 1025 M with
fresh CH3CN containing 0.1 M NBu4PF6 for analysis by UV-VIS
absorption spectroscopy.
2o
2c
3o
3c
1.41a
0.85
1.26a
0.76
0.80b
0.80
0.70b
0.70
312
605
320
612
a Irreversible oxidation. b The small peaks observed are assigned to the
reduction of the radical cation of the ring-closed form.
Fig. 2 Changes in the UV-VIS absorption spectra of 3o upon electrolysis at
1.35 V. Electrolysis periods are every 20 seconds until a 160 second period
was reached. The dotted trace shows the spectrum of the photostationary
state containing > 97% 3c. The photostationary state was obtained by
irradiating a solution of 3o with 365 nm light until no spectral changes were
observed ( ~ 60 seconds). The concentrations of all solutions are 2 3 1025
M.
irradiation of these blue solutions at wavelengths greater than
490 nm§ for 15 minutes results in complete decoloration of the
solutions and the restoration of the absorption spectra to their
original traces by regenerating the ring-open isomers quanti-
tatively.
UV-VIS absorption spectroscopy can also be used to monitor
the progress of the electrochemical ring-closing reactions.
When a colorless CH3CN solution of the ring-open form of
bis(dithiophene) 3o containing tetrabutylammonium hexa-
fluorophosphate (Fig. 2) is electrolyzed at 1.35 V,∑ it im-
mediately turns the same deep blue color and generates the same
absorption spectrum as is observed for the photochemical ring-
closing event. After 160 seconds of electrolysis, the growth in
the intensity of the absorption at 600 nm levels off. The
resulting absorption profile is the same as that recorded when 3o
is photochemically ring-closed, except that the absorption peak
in the visible region is only half as intense. At this point,
electrochemical degradation is minimized and the resulting
electrochemically generated deep blue solution can be photo-
bleached by exposing it to light of wavelength greater than 490
nm (15 minutes). If the electrolysis is continued beyond this
point, the electrochemical degradation becomes more sig-
nificant as is attested by both the loss of any isobestic points and
the inability to regenerate a spectrum that overlaps with that for
the ring-open isomer 3o. We attribute this to electrochemical
polymerization at the remaining 3A-position of the terminal
thiophene heterocycles, although these polymers have yet to be
isolated and characterized.
1 M. Irie, in Molecular Switches, ed. B. L. Feringa, Wiley-VHC, New
York, 2001, p. 37; M. Irie, Chem. Rev., 2000, 100, 1685.
2 A. Peters and N. R. Branda, J. Am. Chem. Soc., in press.
3 For a useful description of electrochromism and its applications, see: P.
M. S. Monk, R. J. Mortimer and D. R. Rosseinky, Electrochromism:
Fundamentals and Applications, VCH, New York, 1995; C. Bechinger,
S. Ferrere, A. Zaban, J. Sprague and B. A. Gregg, Nature, 1996, 383,
608.
4 For an example of a reductive ring-closing reaction of fulgides, see: M.
A. Fox and J. R. Hurst, J. Am. Chem. Soc., 1984, 106, 7626.
5 L. N. Lucas, J. van Esch, R. M. Kellogg and B. L. Feringa, Tetrahedron
Lett., 1999, 40, 1775.
6 For the electrochemical polymerization of oligothiophenes, see: J.
Roncali, Chem. Rev., 1992, 92, 711; P. Audebert, J.-M. Catel, G. Le
Coustumer, V. Duchenet and P. Hapiot, J. Phys. Chem., 1995, 99, 11923;
L. Laguren, C. Van Pham, H. Zimmer and H. B. Mark Jr., J. Electrochem.
Soc., Electrochem. Sci. Technol., 135, 1406; F. Martínez, J. Retuert and
G. Neculqueo, Int. J. Polym. Mater., 1995, 28, 51; B. Krische, J. Hellberg
and C. Lilja, J. Chem. Soc., Chem. Commun., 1987, 1476.
We thank the Natural Sciences and Engineering Research
Council of Canada, the Canada Research Chairs Program and
Simon Fraser University for financial support of this research.
We thank Nippon Zeon Corporation for supplying the octa-
fluorocyclopentene that is needed to prepare compound 4.
Notes and references
† Electrochemical cyclic voltammetry experiments were performed using a
platinum disk working electrode, a platinum wire counter electrode, an Ag/
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