Selective [2 + 2]-cycloaddition in methacrylic stilbene polymers without interference from e / Z -Isomerization

Article abstract of DOI:10.1021/ma2015485

Stilbene is known to undergo two different reactions upon photochemical excitation. The first is an E/Z isomerization and the second is a [2 + 2]-cycloaddition of two stilbene molecules. Because both reactions occur in parallel their use is limited. Here we report on photorefractive polymers with a methacrylate backbone and covalently attached 4-hydroxy-(E)-stilbene or 3,5-dimethoxy-4-hydroxy-(E)-stilbene units in the side chain which show [2 + 2]-cycloaddition only. Both polymers, poly(4-methacryloyloxy-(E)-stilbene) (PMAES) and poly(4-methacryloyloxy-3,5-dimethoxy-(E)-stilbene) (PMADMES), show very high initial refractive indices of 1.6533 for PMAES and 1.6288 for PMADMES. The photochemical reaction upon laser irradiation with 355 nm was monitored by UV/vis, fluorescence, and IR spectroscopy. The light-induced changes of the refractive index at 633 nm measured by surface plasmon resonance (SPR) were found to be Δn > 0.05 for PMAES and Δn > 0.04 for PMADMES. The sensitivity of PMADMES is enhanced compared to PMAES due to the electron donating groups (EDG) as the direct comparison of both polymers shows. Both polymers are useful for optical devices because they do not show any absorption in the visible range and are noncrystalline as determined by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC).

Full text of DOI:10.1021/ma2015485

ARTICLE
pubs.acs.org/Macromolecules
Selective [2 + 2]-Cycloaddition in Methacrylic Stilbene Polymers
without Interference from E/Z-Isomerization
Martin Schraub,† Helen Gray,‡ and Norbert Hampp*,†,§
†Department of Chemistry, University of Marburg, Hans-Meerwein-Straße, Bld. H, 35032 Marburg, Germany
‡Imperial College London, South Kensington Campus, London SW7 2AZ, Great Britain
§Materials Science Center Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
ABSTRACT: Stilbene is known to undergo two different reac-
tions upon photochemical excitation. The first is an E/Z iso-
merization and the second is a [2 + 2]-cycloaddition of two
stilbene molecules. Because both reactions occur in parallel their
use is limited. Here we report on photorefractive polymers with a
methacrylate backbone and covalently attached 4-hydroxy-(E)-
stilbene or 3,5-dimethoxy-4-hydroxy-(E)-stilbene units in the
side chain which show [2 + 2]-cycloaddition only. Both poly-
mers, poly(4-methacryloyloxy-(E)-stilbene) (PMAES) and poly-
(4-methacryloyloxy-3,5-dimethoxy-(E)-stilbene) (PMADMES),
show very high initial refractive indices of 1.6533 for PMAES and 1.6288 for PMADMES. The photochemical reaction upon laser
irradiation with 355 nm was monitored by UV/vis, fluorescence, and IR spectroscopy. The light-induced changes of the refractive
index at 633 nm measured by surface plasmon resonance (SPR) were found to be Δn > 0.05 for PMAES and Δn > 0.04 for
PMADMES. The sensitivity of PMADMES is enhanced compared to PMAES due to the electron donating groups (EDG) as the
direct comparison of both polymers shows. Both polymers are useful for optical devices because they do not show any absorption in
the visible range and are noncrystalline as determined by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry
(DSC).
Scheme 1. Photochemical Pathways of (E)-Stilbene: (a) E/Z
Isomerization, (b) Conrotatory Electrocyclic 6-π Photoche-
mical Cyclization, (c) Oxidation, and (d) Photochemical
Cycloaddition
’ INTRODUCTION
Over the past decades the development of photoresponsive
polymers gained significant attention as may be seen from the
increasing number of publications and applications per year. The
fields of interests for photorefractive polymers range from optical
waveguides1 over switching devices2 to photo-cross-linkable
polymers.3
There are several synthetic pathways to synthesize photorespon-
sive polymers, among them integration of a light sensitive compound
into the backbone of a polymer,4 or in the side chain (pendant
group),5 or simply by using it as an additive to a polymer.6 In general
there are two types of refractive index change materials; refraction-
increase materials7,8 and refraction-decrease materials.9,10 Typical
values for refractive index changes are in the range of Δn 0.001ꢀ0.1.1
Light sensitive compounds may be coumarins,11 cinnamic acids,12
stilbenes,13 chalcones,14 and others.15 Stilbenes are one of the best
examined substance classes from the mechanistic as well as from the
preparative photochemistry point of view.
One of the photochemical pathways of stilbene, the dimeriza-
tion of E-stilbene, was first discovered by Ciamician and Silber at
the beginning of the 20th century.16 The structural analysis of the
dimer by Fulton and Dunitz followed in 1945.17 The second
photochemical pathway, the E/Z-isomerization, was described
first in 1909 by St€ormer18 and finally a third photochemical step,
the photocyclization via oxidative dehydrogenation of dihydro-
phenanthrene was found by Smakula in 193419 (Scheme 1).
The [π2s + π2s]-cycloaddition occurs between the first excited
singlet state S1 of an E-stilbene and a second E-stilbene in the
ground state S0 under formation of a cyclobutane ring. Only
5
Received: July 7, 2011
Revised:
October 4, 2011
Published: November 01, 2011
r
2011 American Chemical Society
8755
dx.doi.org/10.1021/ma2015485 Macromolecules 2011, 44, 8755–8762
|

Macromolecules
ARTICLE
Scheme 2. Structure of the Synthesized Polymersa
from a pulsed laser source and the influence of electron donating
groups (EDG) on the photodimerization were investigated.
’ EXPERIMENTAL SECTION
The thermal characterization of the polymers was done using thermal
gravimetric analysis (TGA) run on a thermo-balance TGA/SDTA 851e
(Mettler Toledo) in open corundum crucibles under nitrogen atmo-
sphere. Heating was at a rate of 10 K minꢀ1 in the range from 25 to
800 °C. Differential scanning calorimetry (DSC) measurements were
performed on a DSC 821e (Mettler Toledo) under nitrogen atmosphere
in a temperature range from 25 to 230 °C at a rate of 10 K minꢀ1
.
1H NMR and 13C NMR spectra were recorded on Bruker AV-300 A
(300 MHz) and on Bruker AV-300 B (75 MHz) spectrometers using
CDCl3 as a solvent. The δ chemical shift scale was expressed in ppm and
calibrated using the residual solvent peak.
WAXS measurements were performed on a Siemens D5000 goni-
ometer with nickel filtered CuKα-radiation (1.5404 Å) at room
temperature.
a Key: (left) poly(4-methacryloyloxy-(E)-stilbene) (PMAES), (middle)
poly(4-methacryloyloxy-3,5-dimethoxy-(E)-stilbene) (PMADMES), and
(right) poly(8-methacryloyl-octyl-4-oxy-(E)-stilbene) (PMAC8ES).
Density measurements were performed using a 2 cm3 pycnometer.
Samples were heated to 150 °C under vacuum for 6 h before use for
density measurement.
E-stilbene shows dimerization because the lifetime of the S1 state
of Z-stilbene is too short to contribute effectively to the reaction.
Neither does Z-stilbene take part in the ground state as part of the
exiplex.20 It is noteworthy that photochemical cycloaddition does
not occur in highly diluted solutions. Because the S1-lifetime of
the molecules is too short to find a dimerization partner by
diffusion5 only isomerization is observed. If Z-stilbene is exposed
to light of appropriate wavelength it undergoes isomerization as
well as conrotatory, electrocyclic 6-π photochemical cyclization
to form dihydrophenanthrene (DHP). DHP accumulates over
time and in the presence of mild oxidizing agents, e.g., oxygen or
iodine, it undergoes an irreversible oxidation to phenanthrene.21
Polymers containing stilbene either as part of the backbone or in
the side chain, were studied and physical and mechanical property
changes in dependence on the E/Z isomerization in condensed
matter, e.g. conductivity,22 solubility,23 change in volume,24 and
shape,25 were analyzed. Only little attention has been payed to the
irradiation dependent refractive index changes so far.
Exposure of tetraphenylcyclobutane, the dimerization product
of E-stilbene, to wavelengths shorter than 300 nm leads to
cycloreversion.26ꢀ28 With different wavelengths this reaction
may be driven either toward cyclization or cycloreversion. The
refractive index n of a material is depending on physical proper-
ties like density, molecular weight, molar refraction and relative
permittivity. Indeed if one or more of these properties are
changed the materials refractive index is changed too. The
relation between the refractive index n and the relative permit-
tivity εr is given by the Maxwell equation.
Mass spectra were measured after electron impact ionization using
either a CH7 (Varian) or a MAT95 (Finnigan) for high resolution mass
spectra (HRMS).
UV/vis spectra were measured on a Perkin-Elmer Lambda 35 with the
UV-WinLab software. IR-measurements were performed on a Perkin-
Elmer 1600 Series FTIR. Fluorescence spectra were taken on a
Shimadzu spectrofluorophotometer model RF-1502.
Polymer films were cast onto quartz windows for UV/vis and fluores-
cence analysis, on NaCl plates for IR measurements, and on silicon wafers
for refractive index measurements. Solutions of identical concentrations of
the polymers in chloroform (HPLC grade) were prepared using a Spin 150-
v3 spin coater from Semiconducter Production Systems.
Polymer films were irradiated using a diode-pumped, Q-switched,
frequency-tripled Nd:YAG laser (Avia) from Coherent at a wavelength
of 355 nm and a pulse length of 25 ns. For the described experiments the
pulse rate was set to 10 kHz and a pulse energy of approximately 265 μJ.
The refractive indices as well as the thickness of the polymer films
were determined using a Metricon Model 2010 prism coupler. The
refractive indices were measured at 632.8 nm with an accuracy (0.001.
For each refractive index value 4 randomly selected positions on a
polymer sample were chosen.
Gel permeation chromatography (GPC) was performed on a Knauer
System equipped with a PSS-SDV (10 μm) 300 ꢁ 8 mm2 column and
two columns 600 ꢁ 8 mm2 at 25 °C with CHCl3 as the eluent at a flow
rate of 1.0 mL/min equipped with a differential refractometer (Knauer)
and a UV detector (Knauer). Polystyrene (PS) standards were used for
calibration.
Methacryloyl chloride (Lancaster) was distilled under reduced pressure
prior to use. Benzyltriphenylphosphonium bromide (Acros, 97%), 8-bro-
mooctan-1-ol (ABCR, 90%), n-butyl lithium (n-BuLi, Aldrich, 2 M solution
in cyclohexane), 3,5-dimethoxy-4-hydroxybenzaldehyde (Acros, 98%),
(E)-4-hydroxystilbene (Acros, 98%), Triethylamine (Acros, 99%) and
solvents were used as received. Where necessary, the drying of solvents
was performed according to standard protocols.29
pffiffiffiffi
n ¼ εr
ð1Þ
During dimerization and cyclobutane formation the carbonꢀ
carbon double bond in stilbene is replaced by a carbonꢀcarbon
single bond and, caused by the loss of conjugation, the polarizability
of the molecule, its relative permittivity εr, and in turn its refractive
index are decreased.
All reactions were carried out in vacuum-dried glassware under argon
atmosphere.
In this study we present three photorefractive homopolymers,
poly(4-methacryloyloxy-(E)-stilbene) (PMAES), poly(4-metha-
cryloyloxy-3,5-dimethoxy-(E)-stilbene) (PMADMES) and poly-
(8-methacryloyl-octyl-4-oxy-(E)-stilbene) (PMAC8ES) which
were synthesized by free radical polymerization (Scheme 2).
Their photochemical behavior upon irradiation with 355 nm
Syntheses. Synthesis of 3,5-dimethoxy-4-hydroxystilbene. Ben-
zyltriphenylphosphonium bromide (6.579 g, 15 mmol, 1 equiv) was
suspended in 100 mL of dry THF. Once dissolved, the reaction mixture
was cooled to 0 °C and n-BuLi (18.75 mL, 30.1 mmol, 2.1 equiv) was
added dropwise to the stirred solution. After 0.5 h, 3,5-dimethoxy-4-
hydroxybenzaldehyde (2.734 g, 15 mmol, 1 equiv) was added in aliquots
8756
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
to the deep red solution. The reaction mixture was stirred at room
temperature for another 16 h. The yellow suspension was quenched with
saturated NH4Cl solution (100 mL), extracted with dichloromethane (3
ꢁ 100 mL) and the combined organic phases were dried over Na2SO4.
Evaporation of the solvent under reduced pressure yielded the crude
product, which was purified by column chromatography (200:1 CHCl3:
MeOH). A pale yellow powder was obtained (2.98 g, 73% yield). Rf: 0.33
(200:1 CHCl3:MeOH). 1H NMR (300 MHz, CDCl3), δ/ppm: 7.43 (d,
2H, 3J = 7.1 Hz, Harom), 7.29 (m, 3H, Harom), 6.97 (d, 1H, 3J = 16.3 Hz,
Holef), 6.89 (d, 1H, 3J = 16.1 Hz, Holef), 6.70 (s, 1H, Harom, ꢁ2), 3.89 (s,
3H, OꢀCH3, ꢁ2), 3.61 (s, 1H, OH). 13C NMR (75 MHz, CDCl3), δ/
ppm: 147.2 (sp2, Cq, CꢀOH), 137.4 (sp2, Cq), 134.9 (sp2, Cq), 129.0
(sp2, CH, Carom), 128.8 (sp2, CH, Carom), 128.7 (sp2, C-OMe, Carom, ꢁ2),
127.3 (sp2, Colef), 126.9 (sp2, Colef), 126.3 (sp2, CH, Carom), 103.4 (sp2,
CH, Carom), 56.3 (sp3, OꢀCH3, ꢁ2). MS (EI, 70 eV), m/z (%): 256
(100) [M+], 209 (9), 195 (12), 181 (22), 165 (8), 152 (12), 141 (8), 69
(9). HRMS (EI): calcd for C16H16O3 (M+), 256.1099; found, 256.1094.
Synthesis of (E)-8-(4-styrylphenoxy)octan-1-ol. 4-Hydroxystilbene
(3 g, 15 mmol, 1 equiv), 8-bromooctan-1-ol (4.06 g, 19.5 mmol, 1.3
equiv) and potassium carbonate (8.25 g, 60 mmol, 4 equiv) were
refluxed under stirring for 24 h in acetone. The solid was filtered off,
washed with hot acetone and the solvent of the combined organic phases
was evaporated under reduced pressure. The crude product was purified
by column chromatography (100:1 CHCl3:MeOH). A colorless powder
was obtained (4.17 g, 84% yield), (E)-8-(4-styrylphenoxy)octan-1-ol
(C8ES). Rf: 0.30 (100:1 CHCl3:MeOH). 1H NMR (300 MHz, CDCl3),
δ/ppm: 7.47 (dd, 4H, 3J = 8.0 Hz, 3J = 13.9 Hz, Harom), 7.34 (t, 2H, 3J =
7.5 Hz, Harom), 7.22 (td, 1H, 4J = 1.6 Hz, 4J = 2.4 Hz), 7.02 (q, 2H, 3J =
16.3 Hz, Holef), 6.89 (d, 2H, 3J = 8.8 Hz, Harom), 3.97 (t, 2H, 3J = 6.5 Hz,
ꢀOꢀCH2), 3.65 (t, 2H, 3J = 6.6 Hz, ꢀOꢀCH2), 1.79 (m, 2H, Haliph),
1.57 (m, 2H, Haliph), 1.44 (m, 8H, Haliph). 13C NMR (75 MHz, CDCl3),
δ/ppm: 158.8 (sp2, Cq), 137.7 (sp2, Cq), 129.9 (sp2, CH, Carom), 128.6
(sp2, CH, Carom), 128.2 (sp2, CH, Carom), 127.6 (sp2, Colef), 127.1 (sp2, Colef),
126.4 (sp2, CH, Carom), 126.2 (sp2, CH, Carom), 114.7 (sp2, CH, Carom),
68.0 (sp3,ꢀOꢀCH2ꢀ, Caliph), 63.0 (sp3,ꢀOꢀCH2ꢀ, Caliph), 32.7 (sp3,
ꢀCH2ꢀ, Caliph), 29.3 (sp3, ꢀCH2ꢀ, Caliph, ꢁ2), 29.2 (sp3, ꢀCH2ꢀ,
Caliph, ꢁ2), 25.9 (sp3, ꢀCH2ꢀ, Caliph, ꢁ2), 25.6 (sp3, ꢀCH2ꢀ, Caliph).
MS (EI, 70 eV), m/z (%): 324 (78) [M+], 196 (47). 165 (9), 130 (17),
69 (78).
7.06 (s, 2H, Harom), 6.78 (s, 2H, Holef), 6.70(s, 1H, Harom, ꢁ2), 6.41 (s, 1H,
Holef) 5.76 (s, 1H, Holef) 3.87 (s, 3H, OꢀCH3, ꢁ2), 2.10 (s, 3H, CH3). 13
C
NMR: (75 MHz, CDCl3), δ/ppm: 165.2 (sp2, CdO), 152.4 (sp2, Cq,
Carom, ꢁ2), 137.0 (sp2, Cq, Carom), 135.7 (sp2, Cq, Carom), 135.5 (sp2, CH,
Colef), 129.0 (sp2, Cq, Carom, CꢀOCO), 128.7 (sp2, CH, Carom, ꢁ2), 128.6
(sp2, CH, Carom), 128.5 (sp2, CH, Carom), 127.7 (sp2, CH, Colef), 127.1 (sp2,
CH2, Carom, ꢁ2), 126.5 (sp2, CH2, Carom), 103.3 (sp2, CH, Carom, ꢁ2),
56.2 (sp3, OꢀCH3, ꢁ2), 18.5 (sp3, CH3). MS (EI, 70 eV), m/z (%): 324
(92) [M+], 255 (98), 195 (16), 181 (14), 167 (12), 152 (9), 141 (8), 113
(6), 86 (36), 69 (92), 41 (52). HRMS (EI): calcd for C20H20O4 (M+),
324.1362; found, 324.1369. A similar procedure yielded (E)-8-(4-styryl-
phenoxy)octyl methacrylate (MAC8ES). Rf: 0.38 (pentane:diethyl ether
3:1). 1H NMR (300 MHz, CDCl3), δ/ppm: 7.47 (dd, 4H, 3J = 8.0 Hz,
J=13.9Hz, Harom), 7.34 (t, 2H, 3J=7.5Hz,Harom), 7.22(td, 1H,4J=1.6Hz,
4J=2.4Hz. Harom), 7.02(q, 2H, 3J=16.3Hz, Holef), 6.89 (d, 2H, 3J=8.8Hz,
4
4
H
arom), 6.10 (dd, 1H, J = 0.9 Hz, J = 1.7 Hz, Holef), 5.55 (dd, 1H,
4J = 0.9 Hz, 4J = 1.6 Hz, Holef), 4.15 (t, 2H, 3J = 6.7 Hz, ꢀOꢀCH2), 3.97 (t,
2H, 3J = 6.5 Hz, ꢀOꢀCH2), 1.95 (dd, 3H, 4J = 1.0 Hz, 4J = 1.4 Hz, ꢀCH3),
1.78 (m, 2H, Haliph), 1.67 (m, 2H, Haliph), 1.45 (m, 8H, Haliph). 13C NMR
(75 MHz, CDCl3), δ/ppm: 167.5 (sp2, CdO), 158.8 (sp2, Cq, Carom),
137.7 (sp2, Cq), 129.9 (sp2, CH, Carom), 128.6 (sp2, CH, Carom), 128.2 (sp2,
CH, Carom), 127.6 (sp2, Colef), 127.1 (sp2, Colef), 126.4 (sp2, Colef), 126.2
(sp2, Colef), 125.1 (sp2, CH, Carom), 114.7 (sp2, CH, Carom), 68.0 (sp3,
ꢀOꢀCH2ꢀ, Caliph), 64.7 (sp3,ꢀOꢀCH2ꢀ, Caliph), 29.2 (sp3, ꢀCH2ꢀ,
Caliph, ꢁ2), 29.1 (sp3, ꢀCH2ꢀ, Caliph, ꢁ2), 28.6 (sp3, ꢀCH2ꢀ, Caliph),
25.9 (sp3, ꢀCH2ꢀ, Caliph), 25.8 (sp3, ꢀCH2ꢀ, Caliph), 25.7 (sp3, ꢀCH2ꢀ,
Caliph). MS (EI, 70 eV), m/z (%): 392 (47) [M+], 324 (34), 264 (36), 196
(63), 152 (7), 130 (12). 69 (82), 44 (15), 41 (27).
Polymerization in Solution. For a typical reaction, 0.658 g of
MADMES (2.25 mmol, 1 equiv) or 0.710 g of MAES (2.69 mmol, 1
equiv) was dissolved in 7 mL of DMF. AIBN (2.4 mg, 8 μmol, 0.01
equiv) was added and the solutions were degassed by three freezeꢀ
pumpꢀthaw cycles each, flushing with argon after each cycle. The
reaction mixtures were stirred under heating at 70 °C for 72 h. After
cooling, the reaction mixtures were added dropwise into ice cold
methanol (1.25 L), upon which precipitation of the polymers as color-
less threads occurred. After filtration, the polymers were redissolved in
DMF and once again precipitated into ice cold methanol. After filtration,
the colorless polymers were left to dry under vacuum for 48 h, yielding
poly(4-methacryloyloxy-(E)-stilbene) (PMAES): GPC: Mn 75 690 g
molꢀ1; Mw 32 500 g molꢀ1. TGA: T5%: 288.9 °C. DSC: Tg: 143.0 °C.
IR: 3026, 2950, 1950, 1892, 1748, 1674, 1596, 1506, 1448, 1414, 1388,
1266, 1198, 1164, 1102, 1014, 960, 940, 884, 806, 750, 710, 690, 666,
534. A similar procedure yielded poly(4-methacryloyloxy-3,5-dimethoxy-(E)-
Esterification of 4-Hydroxystilbene Compounds. The stilbene com-
pound (15 mmol, 1 equiv) was dissolved in 60 mL of dry THF.
Triethylamine (75 mmol, 5 equiv) was added at once and the reaction
mixture was left to stir at room temperature for 1 h. Methacryloyl chloride
(18 mmol, 1.2 equiv) was added dropwise over a period of 15 min and the
reaction mixture was left to stir at room temperature for additional 16 h. The
precipitated ammonium chloride salts were filtered off and after evaporation
of the solvent under reduced pressure the crude product was purified by
column chromatography (3:1 pentane:diethyl ether). A colorless powder
was obtained (75% yield), 4-methacryloyloxy-(E)-stilbene (MAES). Rf:
0.27 (3:1 pentane:diethyl ether). 1H NMR (300 MHz, CDCl3), δ/ppm:
7.45 (d, 2H, 3J = 7.2 Hz, Harom), 7.30 (t, 2H, 3J = 7.4 Hz, Harom), 7.21 (d,
2H, 3J = 10.2 Hz, Harom), 7.19 (s, 2H, Harom), 6.99 (s,1H, Harom), 6.70 (s,
stilbene) (PMADMES). GPC: Mn 47 690 g molꢀ1; Mw 23 510 g molꢀ1
.
TGA: T5%: 284.9 °C. DSC: Tg: 140 °C. IR: 3058, 3026, 2998, 2938, 2840,
1960, 1800, 1754, 1674, 1592, 1504, 1456, 1418, 1348, 1326, 1260, 1240,
1204, 1130, 1084, 1022, 988, 956, 864, 812, 750, 692, 666, 622, 570, 520. A
similar procedure yielded poly(8-methacryloyl-octyl-4-oxy-(E)-stilbene)
(PMAC8ES). GPC: Mn 27 390 g molꢀ1; Mw: 12540 g molꢀ1. TGA: T5%
= 283.5 °C. DSC: Tg not detectable.
2H, Holef), 6.29 (s, 1H, Harom), 5.69 (s, 1H, Harom), 2.02 (s 3H, CH3). 13
C
’ RESULTS AND DISCUSSION
NMR (75 MHz, CDCl3), δ/ppm: 165.76 (sp2, CdO), 150.36 (sp2, Cq,
Synthesis of (E)-3,5-Dimethoxy-4-hydroxystilbene. It is
known that stilbene synthesis is not restricted to a single reaction
type.30 In this study, we have chosen the Wittig reaction to
achieve the desired stilbene derivative. The aldehyde component
has a phenolic hydroxyl group, which, if added to the ylide, would
reprotonate the ylide and no reaction would take part. We choose
to use two equivalents of base to prevent the protonation of the
ylide instead of using a protectionꢀdeprotection procedure for
the phenolic hydroxyl group which would increase the number of
synthesis steps by two. The desired (E)-3,5-dimethoxy-4-
Carom), 136.93 (sp2, CH, Colef), 135.81 (sp2, CH, Colef), 129.2 (sp2, Cq,
Carom, CꢀOCO), 128.69 (sp2, CH, Carom, x4), 127.74 (sp2, CH, Carom
,
ꢁ2), 127.68 (sp2, CH, Carom, ꢁ2), 127.40 (sp2, CH, Carom, ꢁ2), 123.50
(sp2, CH2, Carom, ꢁ2), 18.38 (sp3, CH3). MS (EI, 70 eV), m/z (%): 264
(68) [M+], 196 (52), 177 (8), 167 (8), 165 (12), 152 (9), 119 (5), 69 (98),
44 (16), 41 (32). HRMS (EI): calcd for C18H16O2 (M+), 264.1150; found,
264.1145.
A
similar procedure yielded 4-methacryloyloxy-3,5-
dimethoxy-(E)-stilbene (MADMES). Rf: 0.23 (pentane:diethyl ether
1
3
3:1). H NMR: (300 MHz, CDCl3), δ/ppm: 7.52 (d, 2H, J = 7.1 Hz,
Harom), 7.37 (t, 2H, 3J = 7.4 Hz, Harom), 7.28 (d, 2H, 3J = 7.3 Hz, Harom),
8757
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
The extinction coefficients of the monomers MAES and
MADMES were determined from chloroform solutions (0.01ꢀ
0.05 mmol Lꢀ1) and for MAES an extinction coefficient of ε 301
=
24 454 L molꢀ1 cmꢀ1 and for MADMES of ε
= 31 977 L
308
molꢀ1 cmꢀ1 was found. These finds correlate with the electron
donating effect of the methoxy groups on the conjugated π-system.
Synthesis of the Polymers. Poly(4-methacryloyloxy-(E)-
stilbene) (PMAES), poly(4-methacryloyloxy-3,5-dimethoxy-(E)-
stilbene)
(PMADMES), and poly(8-methacryloyl-octyl-4-oxy-(E)-stilbene)
(PMAC8ES) were obtained by free radical polymerization of the
monomers using AIBN as an initiator in DMF at 60 °C for 72 h.
PMAES was obtained as a colorless powder with Mn = 75 690 g
molꢀ1 in 90% yield, PMADMES as a colorless powder with Mn =
47 690 g molꢀ1 in 88% yield and PMAC8ES as a colorless powder
with Mn = 27 390 g molꢀ1 in 90% yield.
Photochemical Dimerization with UV Light. The photo-
chemical properties of the synthesized polymers under UV
radiation were investigated. Stilbene and its derivatives are
known to undergo several photochemical reactions when irra-
diated with ultraviolet light.20 In a previous study it was shown
that a copolymer containing MAES shows photoisomerization
when irradiated with ultraviolet light.2 In this study we report on
an exclusive photodimerization when thin films of PMAES and
PMADMES were irradiated with ultraviolet light from a fre-
quency tripled Nd:YAG laser (355 nm). In contrast to PMAES
and PMADMES the third polymer PMAC8ES shows both,
photodimerization as well as photo induced E/Z isomerization
(Figure 1, bottom).
Stilbene and its derivates show characteristic UV/vis absorp-
tion spectra (Figure 1). Absorption maxima around 300 nm
correspond to the πꢀπ* electronic transition of the E-stilbene
moiety, while absorption maxima around 280 nm correspond to
Z-stilbene.5 Absorption maxima found for both PMAES and
PMAC8ES were 301 and 308 nm for PMADMES. The slight
bathochromic shift in absorption maximum arises from the
electron donating methoxy groups which expand the conjugated
π-system and therefore lower the energy needed for the πꢀπ*
transition. The absorption band seen at around 230 nm is due to a
n-π* transition occurring in the PMA-backbone ester linkage.
Obviously there is no absorption in the visible (400 nm and higher)
as it is required for a polymer to be applied in optical devices.
The polymers were dissolved at a concentration of 1% (w/w)
in chloroform solution and spin-coated onto quartz plates. When
the stilbene unit in the polymer is photochemically excited and
undergoes [2 + 2] cycloaddition the πꢀπ* absorption band
decreases with increasing energy and irradiation time, respec-
tively. Figure 1 shows the decreasing absorbance of PMAES and
of PMADMES as a function of total applied energy at 355 nm. In
both cases an isosbestic point is observed indicating that only two
states are involved. Like mentioned earlier PMAC8ES shows
photodimerization as well as photo induced E/Z isomerization.
This is due to the alkyl spacer, which increases the space between
two stilbene molecules and in turn lowers the probability that
two stilbene molecules react with each other. As the photo-
chemistry of PMAC8ES is a mixture of isomerization and
photodimerization, this material is not useful for optical applica-
tions. It serves as a reference to proof that PMAES and
PMADMES show exclusive dimerization.
Figure 1. Photoinduced absorption changes of the polymers PMAES,
PMADMES and PMAC8ES induced by excitation with 355 nm light.
The π f π* absorption at about 300 nm decreases as a result of
photochemical [2π + 2π] cycloaddition. Identical energy doses cause
larger absorption changes in PMADMES than in PMAES. In PMAC8ES,
both photodimerization as well as photoinduced E/Z isomerization
takes place. This is indicated by the appearance of the new absorption at
282 nm.
hydroxystilbene was received in good yields (73%). The product
was clearly defined by 1H and 13C NMR, MS, and HRMS.
Synthesis of (E)-8-(4-styrylphenoxy)octan-1-ol. A typical
Williamson ether synthesis was performed to obtain the desired
alkylated stilbene derivate. The yield of 81% is in the typical range
of such reactions. The product (E)-8-(4-styrylphenoxy)octan-1-
ol was clearly identified by 1H and 13C NMR and MS.
Synthesis of Methacrylates Containing Stilbene Units.
The monomers were synthesized according to a standard
esterification reaction starting from (E)-4-hydroxystilbene and
(E)-3,5-dimethoxy-4-hydroxystilbene and (E)-8-(4-styrylphenoxy)
octan-1-ol, respectively. Under alkaline conditions esterification with
methacyrloyl chloride takes place and 4-methacryloyloxy-(E)-stil-
bene (MAES) was obtained with 81% yield, 4-methacryloyloxy-3,5-
dimethoxy-(E)-stilbene (MADMES) with 78% yield, and (E)-8-(4-
styrylphenoxy)octyl methacrylate (MAC8ES) with 80% yield,
As the number of monomeric stilbene units decreases with
increasing irradiation time the photochemical dimerization de-
celerates and finally stops (Figure 2). The decrease in absorbance
respectively. The products were clearly defined by 1H NMR, 13
NMR, MS, and HRMS.
C
8758
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
Figure 2. Optical density changes (ΔOD) in dependence on cumula-
tive energy exposure for PMAES and PMADMES. The initially fast
dimerzation reaction slows down as the number of reactive monomeric
stilbene units decreases and reaches an equilibrium at about 2 J mmꢀ2
,
Figure 3. Absence of phenanthrene formation. (Top) UV/vis spectrum
of a PMAES film in the vicinity of 345 nm, which is a characteristic peak
for phenanthrene (see absorption spectrum, dashed line), as a function
of energy dose. No increase in absorbance is detectable which rules out
phenanthrene formation. (Bottom) Fluorescence spectra of a PMAES
film prepared on a quartz window plate (excitation wavelength is
345 nm) after certain irradiation times at 355 nm. Typical fluorescence
maxima for phenanthrene (see fluorescence spectrum, dashed line) were
not observed.2
but at significantly different levels.
of PMADMES progresses faster and with a greater change than
the decrease observed with PMAES. This observation is due to
the fact that the methoxy groups increase the conjugated system
and therefore the probability that a light quantum is absorbed is
increased.
Because of the about 30% higher extinction coefficient of the
disubstituted MADMES over the “reference” MAES, PMADMES
will have more excited molecules in the S1 state compared to
PMAES at comparable light intensities. This in turn enhances the
dimerization rate and PMADMES appears to be more light sensitive
than PMAES and at the same exposure a faster refractive index
change occurs.
To ensure the decrease in absorbance is due to photodimer-
ization and not due to photoisomerization, like it was reported in
a previous study,2 we analyzed the photoreaction in the polymer
with several complementary methods. In the case that E/Z-
isomerization would be contributing to the overall changes
observed, phenanthrene formation should be detectable, caused
by oxidation through air. Thin polymer films were irradiated with
a 355 nm laser and the fluorescence was analyzed. Phenanthrene
formation was neither monitored by UV/vis nor by the more
sensitive fluorescence spectroscopy in PMAES and PMADES. In
no case an increase of the phenanthrene typical absorption at
345 nm could be detected and the fluorescence spectra did not
show any incidence for the typical emission spectrum of phenan-
threne (Figure 3).
typical CꢀH deformation vibration at 950ꢀ970 cmꢀ1 is decreas-
ing and concomitantly no other absorption increase in the
fingerprint region was observed. At 2931 cmꢀ1 an absorbance
peak is detected, which refers to tetracyclobutane.31
Photo-Induced Refractive Index Changes. One of the key
parameters for photorefractive polymers are the refractive index
changes obtained upon irradiation. Polymer films were prepared
by spin coating solutions of the polymer in chloroform at a
concentration of 6% (w/w) onto silicon wafers. The refractive
index of PMAES was determined to be n633 = 1.6533 and the
thickness of a typical film to be 1.5231 μm. The corresponding
values for PMADMES films were a refractive index of n633
=
1.6288 and a typical film thickness of 1.5116 μm and for
PMAC8ES films n633 = 1.5930 and a typical film thickness of
2.2985 μm, respectively.
The huge and somewhat unexpected difference in initial
refractive index between PMAES and PMADMES is astonishing
as the disubstituted polymer should have, according to Maxwell's
eq 1, a higher refractive index than the nonsubstituted. However,
the refractive index is also linearly related to the density of a
material according to the LorentzꢀLorenz equation. The density
of both polymers was measured by a pycnometer and found to be
1.283 for PMAES and 1.253 for PMADMES respectively. On one
hand the electron donating methoxy groups increase the relative
permittivity εr and therefore the refractive index n is increased,
FT-IR measurements were used to trace the photochemistry
of PMAES and PMADMES. Polymer films were cast on NaCl
plates and dried in an oven at 60 °C for 48 h. The FT-IR spectra
(Figure 4) show the typical absorptions for stilbenes and for the
methacrylate backbone. With progressing irradiation time, the
8759
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
Figure 4. Molecular changes resulting from light exposure at 355 nm in PMAES and PMADMES. (a and d) FT-IR spectra of PMAES/PMADMES after
various energy doses of irradiation. (b and e) CꢀH vibration regime enlarged. (c and f) cyclobutane regime enlarged. Decrease of CꢀH and appearance
of cyclobutane vibration indicates the dimerzation process.
but on the other hand, the methoxy groups require space and
therefore the number of molecules per unit volume is decreased
resulting in a lower density which finally becomes the over-
whelming effect that decreases the refractive index. The lower
initial refractive index of PMAC8ES compared to PMAES and
PMADMES is in accordance with the theory, because the
number of stilbene molecules per unit volume is the lowest of
the three synthesized polymers.
lower the better. A refractive index change of Δn = 0.02 is
obtained at an energy of 0.26 J mmꢀ2 for PMAES and 0.03 J
mmꢀ2 for PMADMES. This clearly shows that the disubstituted
polymer is significantly more photosensitive than the unsubsti-
tuted polymer as less than one-eighth of energy is required only.
As shown in the UV/vis spectra PMAC8ES shows a different
behavior than PMAES and PMADMES. At the very beginning of
the irradiation the decrease in refractive index of PMAC8ES up
to a refractive index change of Δn = 0.02 is almost the same as for
PMAES. After an energy dose of 0.25 J mmꢀ2 the change in
refractive index becomes very small. This behavior is typical also
for other refractive index change polymers which show
photoisomerization.1 This result indicates, that in PMAC8ES
photodimerization takes place before photoisomerization hap-
pens. A more detailed description of the kinetics of PMAC8ES
will be part of future work.
The refractive index was measured as an absolute value and after
each irradiation dose the value of the polymer sample was analyzed
on four randomly chosen positions. The refractive index change
Δn633 compared to the nonirradiated polymer is plotted in Figure 5
for PMAES, PMADMES, and PMAC8ES. The change in absorp-
tion (Figures 1 and 2) is directly related to the refractive index
change. The gradient of the curve is remarkably steep in the
beginning. In fact for all polymers, almost the full refractive index
change Δnis observed after only 0.4 J mmꢀ2 have been applied. The
Thickness changes of the polymer films related to irradiation
and the resulting dimerization reaction were not observed
(Figure 6). This is important in all cases where the polymers
are used for optical devices. In this case the refractive index is
decoupled from any change in density potentially induced by the
dimerization process.
reaction is almost completed after application of 2.5 J mmꢀ2
.
Whereas the maximal refractive index change obtainable is
important from a fundamental point of view, in an application the
full range probably never will be used. From a more practical
point of view there is a figure-of-merit for the required refractive
index change. In many cases a Δn = 0.02 is such a value. The
question is, how much energy is needed to obtain this value, the
Finally a clear material free of any opaqueness etc. is desired.
We used WAXS (Figure 7) as well as DSC to prove that the
8760
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
polymer films of PMAES and PMADMES no side reactions like E/Z
isomerization or phenanthrene formation are observed during
photoinduced dimerization with 355 nm light from a pulsed laser
source. This finding is analytical supported by UV/vis, fluorescence,
and FT-IR spectroscopy. The polymer PMAC8ES, which was
synthetized as a reference, shows photodimerization as well as
photoisomerization, which was shown by UV/vis and refractive
index measurements. The two polymers PMAES and PMADMES
with a well-defined photochemistry are noncrystalline, are highly
transparent in the visible wavelength regime, show a high initial
refractive index (n > 1.6) and high photoinduced refractive index
changes of up to Δn = 0.05. The combination of these attractive
properties make these photorefractive polymers potentially interest-
ing for optical devices and applications in optical data storage. Future
work will deal with 2-photon cleavage of the tetraphenylcyclobutane
to show the reversibility of the photorefractive polymers PMAES
and PMADMES.
Figure 5. Change of refractive index Δn in dependence on the
cumulative energy applied at 355 nm for PMAES, PMADMES, and
PMAC8ES films on silicon wafer. Analogue to the decrease in optical
density the change of refractive index slows down and finally stops as the
number of monomeric stilbene units decreases.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hampp@staff.uni-marburg.de.
’ ACKNOWLEDGMENT
This work was financially supported by the Federal Ministry of
Education and Research (BMBF) under Grant FKZ 13N8978.
’ REFERENCES
(1) Kudo, H.; Yamamoto, M.; Nishikubo, T. Macromolecules 2006,
39, 1759.
(2) Tsai, F.-J.; Torkelson, J. M.; Lewis, F. D.; Holman, B. Macro-
molecules 1990, 23, 1487.
Figure 6. Thickness changes of PMAES, PMADMES and PMAC8ES
polymer films upon irradiation with 355 nm light. The thicknesses of the
samples were 1.5231 μm for PMAES, 1.5116 μm for PMADMES, and
2.2985 μm for PMAC8ES.
(3) Tr€ager, J.; Heinzer, J.; Kim, H.-C.; Hampp, N. Macromol. Biosci.
2008, 8, 177.
(4) Osado, Y.; Katsumura, E.; Inoue, K. Makromol. Chem. Rapid
Commun. 1981, 2, 411.
(5) Meier, H. Angew. Chem. 1992, 104, 1425.
(6) Stuber, F. A.; Ulrich, H.; Rao, D. V.; Sayigh, A. A. R. J. Appl.
Polym. Sci. 1969, 13, 2247.
(7) H€ofler, T.; Grießer, T.; Gstrein, X.; Trimmel, G.; Jakopic, G.;
Kern, W. Polymer 2007, 48, 1930.
(8) Zhang, Y.; Burzynski, R.; Ghosal, S.; Casstevens, M. K. Adv.
Mater. 1995, 8, 111.
(9) Horie, K.; Murase, S.; Takahashi, S.; Teramoto, M.; Furukawa, K.
Macromol. Symp. 2003, 195, 201.
(10) Zhao, C.; Park, C.-K.; Prasad, P. N. Chem. Mater. 1995, 7, 1237.
(11) Kim, H.-C.; Kreiling, S.; Greiner, A.; Hampp, N. Chem. Phys.
Lett. 2003, 372, 899.
(12) Bernstein, H. I.; Quimby, W. C. J. Am. Chem. Soc. 1943, 65, 1845.
(13) Ulrich, H.; Rao, D. V.; Stuber, F. A.; Sayigh, A. A. R. J. Org.
Chem. 1970, 35, 1121.
Figure 7. WAXS spectra of PMAES and PMADMES proving that both
polymers do not show microcrystallinity.
(14) Stobbe, H.; Hensel, A. Chem. Ber. 1926, 59, 2254.
(15) Sharma, L.; Matsuoka, T.; Kimura, T.; Matsuda, H. Polym. Adv.
Technol. 2002, 13, 481.
polymers prepared are amorphous and do not show any phase
transitions at ambient temperatures. No Bragg reflexes indicating
crystalline domains were observed, but there are signs for near
field effects arising from π-stacking of the stilbene units in the
polymer. This probably supports the dimerization and is respon-
sible for the suppression of E/Z-isomerization.
(16) Ciamician, G.; Silber, P. Chem. Ber. 1902, 35, 4128.
(17) Fulton, J. D.; Dunitz, J. D. Nature 1947, 160, 161.
(18) St€ormer, R. Ber. Dtsch. Chem. Ges. 1909, 42, 4865.
(19) Smakula, A. Z. Phys. Chem. Abt. B 1934, 25, 90.
(20) Lewis, F. D.; Wu, T.; Burch, E. L.; Bassani, D. M.; Yang, J.-S.;
Schneider, S.; J€ager, R. L. J. Am. Chem. Soc. 1995, 117, 8785.
(21) Maeda, H.; Nishimura, K.; Mizuno, K.; Yamaji, M.; Oshima, J.;
Tobita, S. J. Org. Chem. 2005, 70, 9693.
’ CONCLUSION
We reported on refractive index change materials consisting of
a PMA backbone and (E)-stilbene or (E)-3,5-dimethoxystilbene
units covalently attached in the side chain of the polymer. In
(22) Irie, M.; Hirano, Y.; Hashimoto, S.; Hayashi, K. Macromolecules
1981, 14, 262.
8761
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762

Macromolecules
ARTICLE
(23) Fissi, A.; Pieroni, O. Macromolecules 1989, 22, 1115.
(24) Blair, H. S.; Law, T. K. Polymer 1980, 21, 1475.
(25) Stille, J. K.; Zimmermann., E. K. Macromolecules 1985, 18, 321.
(26) Frings, R. B.; Schnabel, W. J. Photochem. 1983, 23, 265.
(27) Buckup, T.; Dorn, J.; Hauer, J.; H€artner, S.; Hampp, N.;
Motzkus, M. Chem. Phys. Lett. 2007, 439, 308.
(28) H€artner, S.; Kim, H.-C.; Hampp, N. J. Polym. Sci. A: Polym.
Chem. 2007, 45, 2443.
(29) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 2000.
(30) Hilt, G. J. Org. Chem. 2007, 72, 7337.
(31) L€uttke, W. Eur. J. Org. Chem. 1963, 668, 184.
8762
dx.doi.org/10.1021/ma2015485 |Macromolecules 2011, 44, 8755–8762