Novel photochromic compounds based on the 1-thienyl-2-vinylcyclopentene backbone

Article abstract of DOI:10.1021/ol027487w

(Matrix presented) A new class of photochromic compounds based on the hexatriene backbone has been developed. The backbone, composed of a thiophene ring, a perfluorocyclopentene ring, and a trisubstituted olefin, undergoes reversible ring-opening and ring

Full text of DOI:10.1021/ol027487w

ORGANIC
LETTERS
2003
Vol. 5, No. 8
1183-1186
Novel Photochromic Compounds Based
on the 1-Thienyl-2-vinylcyclopentene
Backbone
,†
Andrea Peters,† Colin Vitols,‡ Robert McDonald,‡ and Neil R. Branda*
Department of Chemistry, 8888 UniVersity DriVe, Simon Fraser UniVersity,
Burnaby, British Columbia, Canada, V5A 1S6, and Department of Chemistry,
UniVersity of Alberta, Edmonton, Alberta, Canada, T6G 2G2
nbranda@sfu.ca
Received December 18, 2002
ABSTRACT
A new class of photochromic compounds based on the hexatriene backbone has been developed. The backbone, composed of a thiophene
ring, a perfluorocyclopentene ring, and a trisubstituted olefin, undergoes reversible ring-opening and ring-closing photoisomerization reactions
when irradiated with ultraviolet and visible light, respectively.
If organic photochromic compounds are to find widespread
use in photonic device applications such as erasable memory
media and optical switching, several mandatory properties
must be met. Foremost are thermal irreversibility, photo-
fatigue resistance, and high efficiency in a rapid photocon-
version processes.1 An additional factor that cannot be
downplayed is the ability to tailor the physical and chemical
properties of the photochromic backbone in a facile, flexible,
and modular manner. This can only be accomplished by
designing photochromic compounds that possess many
different locations on their molecular skeletons where
functional groups can be anchored.
that undergo reversible 6π-electrocyclization reactions. Both
classes of compounds, however, have drawbacks that limit
the scope of their use in optical device applications.
The most promising organic photochromic compounds are
based on the fulgide2 (eq 1) and 1,2-dithienylcyclopentene3
(eq 2) backbones, which contain 1,3,5-hexatriene skeletons
Fulgides present many sites for potential synthetic modi-
fication as well as flexibility in the choice of the heterocycle.
Their synthesis, however, most often involves the Stobbe
condensation, which does not always provide the desired
product in high yield, particularly when the components are
† Simon Fraser University.
‡ University of Alberta.
(1) (a) Organic Photochromic and Thermochromic Compounds; Crano,
J. C., Gugliemetti, R. J., Eds.; Plenum: New York, 1999; Vols. 1 and 2.
(b) Irie, M., Ed. Chem. ReV. 2000, 100, 1685-1890.
(2) (a) Fan, M. G.; Yu, L.; Zhao, W. In Organic Photochromic and
Thermochromic Compounds; Crano, J. C., Gugliemetti, R. J., Eds.;
Plenum: New York, 1999; Vol. 1, pp 141-206. (b) Yokoyama, Y. In
Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, Germany,
2001; pp 107-121. (c) Yokoyama, Y. Chem. ReV. 2000, 100, 1717-1739.
(3) (a) Irie M. In Organic Photochromic and Thermochromic Com-
pounds; Crano, J. C., Gugliemetti, R. J., Eds.; Plenum: New York, 1999;
Vol. 1, pp 207-222. (b) Irie M. In Molecular Switches; Feringa, B. L.,
Ed.; Wiley-VCH: Weinheim, Germany, 2001; pp 37-62.
10.1021/ol027487w CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/15/2003

highly substituted. In addition, as fulgides contain a reactive
anhydride moiety, further modifications on the photochromic
backbone are limited. Fulgide derivatives are also plagued
by photoinduced cis-trans isomerization of the double bond
connecting the heterocycle to the rest of the photochromic
backbone. This process competes with photocyclization
reaction as only the isomer where the three conjugated double
bonds are lying in the s-cis-cis-s-cis manner are capable
of forming the new C-C single bond. To overcome this
energy-wasting drawback, large groups must be attached to
the remaining free site on the alkene, where their steric bulk
can influence the E-Z isomerization and populate the active
(Z)-isomer.4 This comes with a price, however, because the
syntheses of these modified compounds tend to be more
difficult.
Scheme 1
Dithienylcyclopentene derivatives, on the other hand, do
not undergo competing E-Z isomerization reactions. Two
rotomers, parallel and antiparallel, coexist in solution, and
these rotomers, unless there are substitutents attached onto
the 4-position of the heterocycle or benzothiophenes are used
in place of thiophene, are rapidly interconverting. Dithienyl-
alkenes are relatively easy to synthesize due to the manner
in which the thiophene rings are connected onto the cyclo-
pentene ring at their 2- or 3-positions. However, there is a
limited number of sites where functional groups may be
attached (primarily the 2- and 5-positions of the hetero-
cycles), and in many cases, introducing particular functional
groups at these locations results in thermal reversibility5 or
poor efficiency of the photoreactions.6
One of our research goals is to prepare novel photochromic
backbones that take advantage of the appealing properties
of both the fulgides and dithienylalkenes without being
hampered by their limitations. Here we describe three new
hexatriene systems 1a-c that show good photochromic
behavior. They are similar in structure to 1,2-dithienylalkenes
except that one of the thiophene rings has been replaced with
a substituted olefin. It is this olefin that provides the
functional group versatility of fulgides without sacrificing
the synthetic ease at which the 1,2-dithienylethenes can be
prepared.7
compound 4, which can be further treated with the anion
generated from 3-bromo-2-methyl-5-phenylthiophene9 to
yield hexatriene 1a (Figure 1).10 Compounds 1b and 1c are
better prepared by an alternate route in which the heterocyle
is attached to the octafluorocyclopentene prior to addition
of the substitutued olefin portion. This is a preferred route
because the reaction of the appropriate vinylbromide to the
octafluorocyclopentene affords oils, which are much less
Photochromic compounds 1a-c are prepared as shown
in Scheme 1 using the same approach as used for preparing
1,2-dithienylalkene derivatives except that one of the anionic
heterocycles has been replaced with a lithiated olefin.
Lithiation of the known triphenylvinylbromide8 3 followed
by addition of excess octafluorocyclopentene generates
(4) Yokoyama, Y.; Goto, T.; Inoue, T.; Yokoyama, M.; Kurita, Y. Chem.
Lett. 1988, 1049. Yokoyama, Y.; Inoue, T.; Yokoyama, M.; Goto, T.; Iwai,
T.; Kera, N.; Hitomi, I.; Kurita, Y. Bull. Chem. Soc. Jpn. 1994, 67, 3297.
Yokoyama, Y.; Kurita, Y. Mol. Cryst. Liq. Cryst., Sect. A 1994, 246, 87.
(5) Uchida, K.; Tsuchida, E.; Aoi, Y.; Nakamura, S.; Irie, M. Chem.
Lett. 1999, 63. Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Lett.
2000, 1340.
(6) Takami, S.; Kawai, T.; Irie, M. Eur. J. Org. Chem. 2002, 3796.
Morimitsu, K.; Shibata, K.; Kobatake, S.; Irie, M. J. Org. Chem. 2002, 67,
4574.
(7) After acceptance of this manuscript for publication, the following
closely related paper appeared: Shrestha, S. M.; Nagashima, H.; Yokoyama,
Y.; Yokoyama, Y. Bull. Chem. Soc. Jpn. 2003, 76, 363-367.
(8) Gilman, H.; Fothergill, R. E. J. Am. Chem. Soc. 1929, 51, 3149.
Lomas, J. S.; Fain, D.; Briand, S. J. Org. Chem. 1990, 55, 1052. Koelsch,
C. F. J. Am. Chem. Soc. 1932, 54, 2045.
Figure 1. Molecular structure of 1a in the crystal. Thermal
ellipsoids are drawn at the 20% probability level.
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Org. Lett., Vol. 5, No. 8, 2003

Figure 2. Changes in the UV-vis absorption spectra of 1a-c upon irradiation with 365 nm light. Irradiation periods are 10, 30, 60, and
120 s. All spectra are of CH2Cl2 solutions at 2 × 10-5 M.
convenient to handle than the solid diene 5. Monothiophene
5 reacts with the anions of 1,1-diphenylpropene and 3-meth-
yl-2-butene11 to produce hexatrienes 1b and 1c, respectively.
All three compounds 1a-c exhibit good photochromic
properties and can be toggled between their colorless ring-
open (1a-c) and colored ring-closed (2a-c) forms by
alternate irradiation with ultraviolet and visible light12 as
monitored using UV-vis absorption and 1H NMR spectros-
copy. When CH2Cl2 solutions (2 × 10-5 M) of the colorless
ring-open forms are irradiated with 365 nm light, absorption
bands in the visible region appear and the solutions turn
orange as a result of the ring-closing reactions to produce
2a-c (Figure 2 and Table 1). 1H NMR spectroscopic studies
and 64% for 2c, respectively. All the solutions of 2a-c can
be decolorized by irradiating them with visible light of
wavelengths greater than 434 nm to induce the ring-opening
reactions and regenerate 1a-c. In the absence of visible light,
the color persisted, attesting to the thermal stability of the
photochromic compounds.
1H NMR spectroscopy studies also show that the conver-
sion of 1a to 2a results in the generation of a photo side-
product, which has yet to be isolated or characterized. We
suggest that this side-product could possibly be due to the
cyclization followed by oxidation of the stilbene fragment
to generate a phenanthrene system.13 The side-product
formation can also be monitored by luminescence spectros-
copy. Solutions of the ring-open isomer 1a exhibit no
observable emission when they are excited with ultraviolet
light (λmax ) 300 nm). Solutions of the ring-closed isomer
2a, however, did produce intense emission at 400 nm when
excited at the same wavelength (300 nm). Photobleaching
of the solutions using visible light (>434 nm) resulted in
the disappearance of the band centered at 473 nm in the
absorption spectrum; however, the emission at 400 nm
remained. Subsequent irradiation of the resulting solution
with 365 nm light triggered a further increase of the band in
the luminescence spectrum, indicating that some photodeg-
radation was occurring upon cyclization of 1a to 2a.
Compounds 1b and 1c showed clean conversion to com-
Table 1. UV-vis Absorption Data for All Photochromic New
Compounds in Their Ring-Open and Ring-Closed Forms
compound
λmax (nm)
ꢀ (L mol-1 cm-1
)
1a
279
473
294
460
288
450
27 600
4470
16 200
7860
18 200
7580
2a a
1b
2ba
1c
2ca
a Measured at the photostationary states obtained by irradiating CH2Cl2
solutions (2 × 10-5 M) of the ring-open forms with 365 nm light for 140
s (2a), 180 s (2b), and 120 s (2c).
1
pounds 2b and 2c, respectively, as monitored by H NMR
spectroscopy.
We have succeeded in preparing several photochromic
compounds based on a modified hexatriene skeleton. This
novel molecular scaffold offers the chance to decorate
performed on CD2Cl2 (1 × 10-3 M) solutions of 1a-c reveal
that the photostationary states are 47% for 2a, 75% for 2b,
(9) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000,
122, 4871.
(12) Standard lamps used for visualizing TLC plates (Spectroline E-series,
365 nm, 470 mW/cm2) were used to carry out the ring-closing reaction of
all photochromic compounds in this study. The ring-opening reactions were
carried out using the light of a 150 W tungsten source that was passed
through a 434 nm cutoff filter to eliminate higher energy light.
(13) Irie has reported that bis(thienyl)ethenes lacking substituents on the
C-4 positions of the thiophene rings produce photo byproducts after
prolonged irradiation with UV light (Irie, M. Bull. Chem. Soc. Jpn. 2000,
73, 2389). In the present cases, a side-product was only observed after short
irradiation periods, and only for 1a. We, therefore, believe that the side-
product is more likely the phenanthroline derivative, although the other
possibility cannot be ruled out.
(10) X-ray-quality single crystals were grown by slow evaporation from
a hexane solution. Crystal data for 1a: C36H24F6S, M ) 602.61, triclinic,
space group P1h (No. 2), a ) 10.8285(14), b ) 11.9726(16), c ) 12.5721-
(16) Å, R ) 98.094(3), â ) 115.285(3), γ ) 95.371(2), V ) 1437.4(3) Å3,
T ) -80 °C, Z ) 2, µ ) 0.177 mm-1, 7193 reflections measured, 5779
unique, R1(F) ) 0.0431, wR2 (F2) ) 0.1190 (all data).
(11) These anions are prepared from their respective bromides. 2-Bro-
mo-1,1-diphenylpropene: Koelsch, C. F.; White, R. V. J. Org. Chem. 1941,
6, 602. 2-Bromo-3-methyl-2-butene: Patel, B. A.; Kim, J.-I. I.; Bender, D.
D.; Kao, L.-C.; Heck, R. F. J. Org. Chem. 1981, 46, 1061.
Org. Lett., Vol. 5, No. 8, 2003
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photoresponsive systems with a wide range of functional
groups without sacrificing photochromic behavior. The use
of this backbone in materials applications as well as the
precise nature of the photo side-product in the case of 1a is
currently underway and will be reported in due time.
of Alberta, and Simon Fraser University. Special thanks go
to Nippon Zeon Corporation for supplying the octafluoro-
cyclopentene.
Supporting Information Available: Experimental pro-
cedures for preparation of compounds 1a-c. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada, the Canada Research Chair Program, the University
OL027487W
1186
Org. Lett., Vol. 5, No. 8, 2003