Photochromic cross-linked copolymer containing thermally stable fluorescing 2-indolylfulgimide

Article abstract of DOI:10.1039/b002335n

A new photochromic cross-linked copolymer composed of PMMA and fluorescing 2-indolylfulgimide is synthesized which can be converted photochemically into two forms which are found to be thermally stable; the spectroscopic properties and quantum yields of photochemistry and fluorescence are also measured.

Full text of DOI:10.1039/b002335n

Photochromic cross-linked copolymer containing thermally stable fluorescing
2-indolylfulgimide
Yongchao Liang, A. S. Dvornikov and P. M. Rentzepis*
Department of Chemistry, University of California, Irvine, CA 92697, USA. E-mail: pmrentze@uni.edu
Received (in Irvine, CA, USA) 20th March 2000, Accepted 30th June 2000
Published on the Web 10th August 2000
A new photochromic cross-linked copolymer composed of
PMMA and fluorescing 2-indolylfulgimide is synthesized
which can be converted photochemically into two forms
which are found to be thermally stable; the spectroscopic
properties and quantum yields of photochemistry and
fluorescence are also measured.
of this copolymer was determined by immersing it into
1,2-dichloroethane solvent for one month, in which both
PMMA and fulgimide E-8 are very soluble. We found, as
expected, for cross-linked copolymers, that this immersed
material was not dissolved, at all, in 1,2-dichloroethane. This
indicates, strongly, that this material is a cross-linked copoly-
mer of MMA and E-8. After the solid, nondissolved polymer
was removed from the solution, the absorption spectra of the
remaining phase were measured. The spectra showed that not
The increasing fast developments in new technologies asso-
ciated with optoelectronics, waveguides, molecular switches,
optical computer memories among others, require the design
and synthesis of novel materials with properties which satisfy
the specific needs of each new device.1 Organic photochromic
materials have attracted significant attention, in recent years,
because of their potential application to optical devices, and in
particular to switches and 3D optical storage.2 One of the most
promising classes of photochromic materials are fulgides
because they possess excellent fatigue resistance and thermal
stability in both isomeric forms.3
In previous papers we have reported the synthesis, photo-
chromic and spectroscopic properties of several new thermally
stable fluorescing photochromic 2-indolylfulgides and fulgi-
mides which may be potential materials for use as media in
rewritable 3D optical memory devices.4–6 Here we describe the
synthesis and properties of a new fluorescing photochromic
2-indolylfulgimide which is attached to methyl methacrylate
monomer, to form a cross-linked photochromic fluorescing
polymer.
Photochromic molecules must be uniformly dispersed in a
polymer matrix at relatively high concentration to be suitable
for use in optical devices. The photochromic materials which
have been used in 3D optical memory devices, based on two-
photon absorption, require concentrations of 1021 M and higher
in order to record a sufficient number of bits of information.2
The photochromic cross-linked copolymer described here is
formed by attaching the photochromic molecule to the polymer
chain by copolymerization with the corresponding monomer.
We were able to copolymerize the methyl methacrylate
monomer and the photochromic 2-indolylfulgimide (E-8),
which we synthesized, to form an optically clear photochromic
cross-linked copolymer.
Fulgimide E-8 was readily prepared by the process shown in
Scheme 1. Commercially available 3-methylindole (1) was
converted to 2 by reacting it with vinyl benzyl chloride in
potassium hydroxide DMSO solution (78% yield). Product 2
was then converted to 3 using the Vilsmeier-Haack formylation
reaction (42% yield).7 Diethyl isopropylidenesuccinate (6) was
prepared by Stobbe condensation (86% yield).8 Stobbe con-
densation of indole-2-carbaldehyde (3) with diethyl iso-
propylidenesuccinate (6) followed by hydrolysis and intra-
molecular acid anhydride formation yields 2-indolylfulgide
(E-7 and Z-7) with 23% yield from 3 to 7, where E-7 was
separated as the main product. Fulgimide E-8 was synthesized
from E-7 by Lewis acid and hexamethyldisilazane-promoted
one pot reaction (42% yield).9,10
The fulgimide E-8 was subsequently dissolved in methyl
methacrylate and the initiator (AIBN) was added forming a
homogenous mixture. This mixture was transferred to a glass
cell and sealed under vacuum. After 24 h of polymerization, at
50 °C, a rigid optically clear polymer was formed. The structure
Scheme 1 Reagents and conditions: i, KOH, DMSO, 4-vinyl benzyl
chloride, 0 °C to r.t., 78%; ii, POCl3, DMF, 0 °C to r.t., 42%; iii, acetone,
ButOK, ButOH, then HCl; iv, EtOH, H2SO4, 86% (from 4 to 6); v, NaH,
benzene; vi, KOH, EtOH, then HCl; vii, DCC, THF, 23% (from 3 to 7); viii,
4-vinylaniline, HMDS, ZnCl2, benzene, 43%; ix, MMA, AIBN, 50 °C.
DOI: 10.1039/b002335n
Chem. Commun., 2000, 1641–1642
This journal is © The Royal Society of Chemistry 2000
1641

compared to the solution, probably due to segment rotation of
the polymer chain.
The colored form of the fulgimide–MMA copolymer emits
red fluorescence when promoted to its first excited state. The
fluorescence spectra observed are similar to the colored forms
of the pure fulgimides in solution. To confirm that the recorded
fluorescence is due to the colored forms of the organic
photochrome rather than any impurities or other species, we
measured the excitation spectra and fluorescence emission
spectra intensity change as a function of bleaching/coloration
cycles. The data show that the fluorescence intensity and the
excitation spectra of the colored from decrease proportionally
with the decrease of the absorption of the fulgimide colored
form. When the material was completely bleached, i.e. the
absorption band of the C form disappeared no fluorescence was
detected. When the fluorescence spectrum appeared again, its
intensity increased with the same rate as the rate of growth of
the C form concentration. The fluorescence quantum yield of
the colored form was found to be 5%. The method that we have
used to measure the quantum efficiencies of these forms has
been described previously by us.5
Fig. 1 Absorption and fluorescence spectra of cross-linked copolymer E-9.
(a) absorption spectrum of the E form; (b) and (c) absorption and
fluorescence spectra of the C form.
even traces of fulgimide E-8 were present in the solution and
only the spectrum of 1,2-dichloroethane solvent was recorded.
To the best of our knowledge, this is the first cross-linked
photochromic fulgimide copolymer ever synthesized.
The copolymer was found to possess photochromism, i.e. can
be reversibly colored and bleached by irradiation with light of
the appropriate wavelength. In addition the colored form (C)
also fluoresces when excited with visible light which is rather
seldom found in fulgimides. This is, also, the first time that a
copolymer of this class has been observed. Fig. 1 shows the
absorption spectra of the copolymer in its colorless and colored
forms and the fluorescence spectrum of the colored form. The
colored form was photoinduced by irradiating the copolymer
with 400 nm light. When irradiated with visible light, l = 530
nm, the colored form can be easily bleached back to the original
colorless form, as shown in Scheme 2. The quantum efficiency
of the coloration and bleaching processes were measured to be
0.13 and 0.17 respectively. These numbers are the same as the
quantum efficiencies measured for pure E-8 in ethyl acetate
solution. We thought, previously, that in rigid cross-linked co-
polymer matrices, where the moieties of the fulgimide molecule
are attached to the polymer chains, the efficiency of E–Z
isomerization would be decreased, because of stereo constrains.
In addition we had expected that because the E–Z isomerization
competes with the ring-closure reaction, the cyclization reaction
yield will increase. However, our data show that in a copolymer
composed of fulgimide E-8 and methyl methacrylate the
efficiency of cyclization yield does not change. This suggests
that the E–Z isomerization be not significantly reduced
This work was supported in part by the United States Air
Force, Rome Laboratory, under contract number F 30603-97-C-
0029.
Notes and references
1 R. Piyaket, I. Cokgor, F. B. McCormick, S. Esener, A. S. Dvornikov and
P. M. Rentzepis, Opt. Lett., 1996, 21, 1032.
2 A. S. Dvornikov and P. M. Rentzepis, Advances in Chemistry Series
240, ed. Robert R. Birge, ACS, Washington DC, 1994, ch. 7, pp.
161–177.
3 H. G. Heller, in CRC Handbook of Organic Photochemistry and
Photobiology, ed. W. M. Horpool and P. S. Song, CRC Press, FL, 1995,
p. 174.
4 Y. C. Liang, A. S. Dvornikov and P. M. Rentzepis, Res. Chem.
Intermed., 1998, 24, 905.
5 Y. C. Liang, A. S. Dvornkov and P. M. Rentzepis, J. Photochem.
Photobiol. A: Chem., 1999, 125, 79.
6 Y. C. Liang, A. S. Dvornikov and P. M. Rentzepis, Tetrahedron Lett.,
1999, 40, 8067.
7 C. Bastianelli, C. Cipiciani and G. Giulietti, J. Heterocycl. Chem., 1981,
18, 1275.
8 C. G. Overberger and C. W. Roberts, J. Am. Chem. Soc., 1949, 71,
3618.
9 P. Y. Reddr, S. Konodo, T. Toru and Y. Ueno, J. Org. Chem., 1997, 62,
2652.
10 E-8: mp: 149.5–150.5 °C, 1H NMR (500 MHz, CDCl3 TMS) d 1.17 (s,
3H), 1.96 (s, 3H), 2.50 (s, 3H), 5.21 (d, J = 10.9 Hz, H), 5.31 (d, J =
10.9 Hz, H), 5.41 (m, 2H); 5.68 (d, J = 17.6 Hz, H), 5.79 (d, J = 17.6
Hz, H), 6.63 (dd, J = 17.6, 10.9 Hz, H), 6.74 (dd, J = 17.6, 10.9 Hz, H),
7.02 (d, J = 8.15 Hz, H), 7.16–7.71 (m, 10H); 13C NMR (500 MHz,
CDCl3, TMS) d 11.7, 22.8, 27.2, 47.3, 109.6, 114.1, 114.9, 119.9, 124.0,
126.5, 126.7, 126.8, 136.0, 136.2, 137.4, 154.5, 167.5, 168.3; HRMS
(CI) m/z calcd for C34H30N2O2 498.2307 (M+), found 498.2317; Anal.
Calcd for C34H30N2O2: C, 81.90; H, 6.06; N, 5.62. Found: C, 81.73; H,
6.04; N, 5.60%.
Scheme 2
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Chem. Commun., 2000, 1641–1642