Novel synthesis of photochromic polymers via ROMP

Article abstract of DOI:10.1021/ol006040p

Ring-opening metathesis polymerization (ROMP) of a photochromic 1,2-bis(3-thienyl)cyclopentene monomer generated a series of novel polymers. All polymers exhibit reversible light-activated interconversion between their colorless-open and their colored-closed forms. ? University of Alberta. ? University of Calgary.

Full text of DOI:10.1021/ol006040p

ORGANIC
LETTERS
2000
Vol. 2, No. 18
2749-2751
Novel Synthesis of Photochromic
Polymers via ROMP
,†
Andrew J. Myles,† Zengrong Zhang,‡ Guojun Liu,‡ and Neil R. Branda*
Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, Canada, T6G 2G2, and Department of Chemistry,
UniVersity of Calgary, Calgary, Alberta, Canada, T2N 1N4
neil.branda@ualberta.ca
Received May 10, 2000
ABSTRACT
Ring-opening metathesis polymerization (ROMP) of a photochromic 1,2-bis(3-thienyl)cyclopentene monomer generated a series of novel polymers.
All polymers exhibit reversible light-activated interconversion between their colorless-open and their colored-closed forms.
The photochromism of 1,2-bis(3-thienyl)cyclopentene de-
rivatives involves the reversible photoinduced cyclization
between the colorless-open and the colored-closed forms of
the chromophore (Figure 1).1 This phenomenon is interesting
materials such as films, sheets, fibers, or beads dictates the
use of polymeric rather than monomeric photochromes.3,4
Homopolymers are more desirable than copolymers as they
will have an increased density of the photochromic unit
within the material. This translates into a greater amount of
information expressed or stored per unit volume or surface.
Not only do 1,2-bis(3-thienyl)cyclopentene derivatives
possess optimal photochromic properties, including thermal
irreversibility and fatigue resistance, but the wavelength of
light expressed by the colored forms can be readily tuned by
tailoring the electronic distribution in the conjugated pathway
created upon cyclization.1 This is most conveniently ac-
complished by modifying pendant functional groups located
on the heterocycles. To fully exploit this structure-property
relationship in polymers, a mild and universal polymerization
technique is required so that the pool of pendant groups is
not limited by functional group incompatibility.
Figure 1. Photoinduced interconversion between the colorless-
open and colored-closed forms of the 1,2-bis(3-thienyl)cyclopentene
photochrome used in this study.
for its potential role in optical materials and photonic devices
such as variable-transmission filters, optical information
storage systems, and photoregulated molecular switches.2 The
need for practical handling of definite forms of photochromic
(2) Wakashima, H.; Irie, M. Polym. J. 1998, 30, 985 and references
therein.
(3) Ichimura, K. In Organic Photochromic and Thermochromic Com-
pounds; Crano, J. C., Gugliemetti, R. J., Eds.; Plenum Press: New York,
1999; Vol. 2, Chapter 1, pp 9-63.
(4) For other examples of photochromic polymers, see: (a) Kumar, G.
S.; Neckers, D. C. Chem. ReV. 1989, 89, 1915. (b) Warshawsky, A.; Kahana,
N.; Buchholtz, F.; Zelichonok, A.; Ratner, J.; Krongauz, V. Ind. Eng. Chem.
Res. 1995, 34, 2825. (c) Buchholtz, F.; Zelichenok, A.; Krongauz, V.
Macromolecules 1993, 26, 906. (d) Lyubimov, A. V.; Zaichenko, N. C.;
Marevtsev, V. S. J. Photochem. Photobiol. A: Chem. 1999, 120, 55.
† University of Alberta.
‡ University of Calgary.
(1) (a) Irie, M. In Organic Photochromic and Thermochromic Com-
pounds; Crano, J. C., Gugliemetti, R. J., Eds.; Plenum Press: New York,
1999; Vol. 1, Chapter 5, pp 207-222. (b) Irie, M.; Mohri, M. J. Org. Chem.
1988, 53, 803. (c) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem Eur. J.
1995, 1, 275.
10.1021/ol006040p CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/16/2000

The popularity of ring-opening metathesis polymerization
(ROMP) of strained bicyclic olefins is rising due to the mild
reaction conditions needed, their compatibility with a wide
range of functional groups, and the ease of generating well-
ordered homopolymers from a wide range of strained olefin
monomers.5 These features make ROMP an attractive choice
to complement the photochromic versatility of the 1,2-bis-
(3-thienyl)cyclopentene photochrome. Also, by varying the
catalyst-to-substrate stoichiometry, ROMP allows the poly-
mer chain length to be readily tailored. This report represents
the first example of photochromic homopolymers synthesized
using ROMP technology.6
Simultaneously, the recently reported dichloride 38 was
treated with tert-butyllithium to convert it into its monoanion,
which was quenched with carbon dioxide to afford the
photochromic carboxylic acid 4. It is interesting to note that
the yield of the monoanion in the lithiation step is greater
than would be expected from a statistically governed product
distribution. We attribute this to the unfavorable charge
buildup that would exist if both chlorine atoms underwent
metal-halogen exchange reactions. This charge buildup is
still significant despite the fact that the two thiophene rings
are cross-conjugated instead of directly conjugated, suggest-
ing the charge-charge repulsion between two thiophene
heterocycles would be felt through space.
The monomeric precursor 5 was prepared as outlined in
Scheme 1. Condensation of 7-oxabicyclo[2.2.1]hept-5-ene-
Coupling the acid chloride of 4 with the strained olefin 2
completed the preparation of monomer 5. All new com-
pounds were characterized by NMR spectroscopy, UV-vis
spectroscopy, and mass spectrometry.
Scheme 1
ROMP reactions of monomer 5 were performed under
rigorously inert conditions in a Schlenk tube. The polym-
erization process was initiated with commercially available
bis(tricyclohexylphosphine)benzylidine ruthenium(IV)dichlo-
ride (Grubbs’ catalyst).9 The polymerizations were then
terminated by quenching the reaction mixtures with excess
ethyl vinyl ether. The homopolymers were conveniently
isolated in pure forms by precipitating them from cold ether
and then washing with the same solvent to remove the
catalyst and any unreacted monomer. Homopolymers with
varying molecular weights were synthesized in a systematic
fashion by changing the molar amount (1-4 mol %) of
Grubbs’s catalyst used to initiate the reaction. In all cases,
the ROMP reactions were reproducible, affording polymers
1a-c as off-white powders in good yields (∼75%).
The 1H NMR spectra of the precipitates from the ROMP
reactions lack the signals corresponding to the double bond
(6.56 ppm) and bridgehead protons (5.38 ppm) of the strained
bicyclic olefin 5, showing that any unreacted monomer was
washed away in the isolation process. These signals are
replaced by characteristically broadened peaks at 6.1 and 4.6
ppm for the polymers.
The polymeric products were all readily soluble in
common organic solvents such as chloroform, dichlo-
romethane, tetrahydrofuran, and benzene. They are air-stable
solids of reasonable number-average molecular weights (Mn)
and relatively narrow polydispersities (Mw/Mn) as determined
by gel permeation chromatographic analyses calibrated by
polystyrene (Table 1).
Table 1 also summarizes the UV-vis properties of THF
solutions of the novel polymers along with those for
monomer 5. All polymers show typical absorbances for the
colorless-open forms of the 1,2-bis(3-thienyl)cyclopentene
photochrome at 248 nm. Photoinduced isomerization studies
were carried out by irradiating the THF solutions of 1a-c
2,3-dicarboxylic anhydride7 with p-aminophenol in acetic
acid generated the requisite strained olefin fragment 2.
(5) (a) Grubbs, R. H.; Tumas, W. Science 1989, 243, 907. (b) Wu, Z.;
Benedicto, A. D.; Grubbs, R. H. Macromolecules 1993, 26, 4975. (c)
Robson, D. A.; Gibson, V. C.; Davies, R. G.; North, M. Macromolecules
1999, 32, 6371 and references therein.
(6) For other examples of polymers using dithienyl and diarylalkenes,
see: (a) Wakashima, H.; Irie, M. Polym. J. 1998, 30, 985. (b) Kawai, T.;
Kunitake, T.; Irie, M. Chem. Lett. 1999, 905. (c) Stellacci, F.; Bertarelli,
C.; Toscano, F.; Gallazzi, M. C.; Zotti, G.; Zerbi, G. AdV. Mater. 1999, 64,
292. (d) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T.; Maekawa, M.;
Suenaga, Y.; Furuichi, K. J. Am. Chem. Soc. 1996, 118, 3305.
(7) Bolm, C.; Dinter, C. L.; Seger, A.; Ho¨cker, H.; Brozio, J. J. Org.
Chem. 1999, 64, 5730.
(8) Lucas, L. N.; van Esch, J.; Kellog, R. M.; Feringa, B. L. Chem.
Commun. 1998, 2313.
(9) For recent examples of the use of this catalyst, see ref 6 and
Montalban, A. G.; Steinke, J. H. G.; Anderson, M. E.; Barrett, A. G. M.;
Hoffman, B. M. Tetrahedron Lett. 1999, 40, 8151.
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Org. Lett., Vol. 2, No. 18, 2000

Table 1. Polymer Yields and Characterization
equivalents
λmax (nm)
open form
λmax (nm)
closed forma
photochrome
of monomer
yield, %
Mn
Mw
Mw/Mn
1a
1b
1c
5
25
50
100
76
78
75
7920
16771
28007
11917
24577
43820
1.50
1.47
1.56
248
248
248
248
512
512
512
512
a After a 40 s irradiation period (λ ) 254 nm).
at 254 nm with a short-wavelength hand-held lamp. Spectral
changes were monitored in the UV-vis region (Figure 2).
chrome centered at 512 nm. The similarity of the absorption
spectrum for monomer 5 and those of the polymers in both
open and closed forms illustrates that the intimacy of the
photochromes covalently linked to the polymer backbone
affects neither the ground-state nor the excited-state proper-
ties of the photochrome.
The polymers can be easily decolorized by subsequent
irradiation for 1 min with light of wavelength greater than
434 nm using a high power lamp with an appropriate cutoff
filter. The photoisomerization process is thermally irrevers-
ible, and solutions of the closed forms kept in the dark
showed no changes in their absorption spectra.
The polymers were very robust, and their thin films were
easily cast by layering toluene solutions of 1a-c onto water.
After slow evaporation of the toluene, the resulting transpar-
ent films were transferred to slides. Transmission electron
microscopy of the films revealed a smooth surface possessing
no topological features. These films immediately turned from
colorless to pink when exposed for brief periods (60 s) at
254 nm. As with the polymers in solution, the films could
be decolorized with light at a wavelength greater than 434
nm.
The polymers described in this study lend support to the
mildness of the ROMP process. The selective ring-opening
of the strained olefin in the monomer should be emphasized.
Figure 2. Changes in the UV-vis absorption spectra of THF
solutions of (a) monomer 5 (3 × 10-5 M) and (b) polymer 1a
(approximately 1.2 × 10-6 M, corresponding to an equivalent
concentration of the photochromic unit as for solutions of monomer
5) upon irradiation with 254 nm light. Irradiation periods are 2, 4,
6, 8, 10, 15, 20, 25, 30, and 40 s.
Acknowledgment. We are grateful to Greg Popowich and
Marek Malak for performing the TEM analysis. Financial
support for this work was provided by the Natural Sciences
and Engineering Research Council of Canada and the
University of Alberta.
Supporting Information Available: Experimental pro-
cedures for preparation of compounds 2, 4, and 5 and
polymers 1a-c. This material is available free of charge via
the Internet at http://pubs.acs.org.
In all cases, irradiation produced an immediate decrease in
the absorbances corresponding to the open form of the
dithienylethene photochrome at 248 nm. The decrease in
these absorbances was accompanied by the appearance of
broad absorbances for the pink closed form of the photo-
OL006040P
Org. Lett., Vol. 2, No. 18, 2000
2751