Poly[bis(triphenylamine) ether]s with low glass transition temperatures as photoconductors in fast photorefractive systems

Article abstract of DOI:10.1039/b105930k

A series of photoconducting poly(tetraphenyldiaminobiphenylene alkyl ether)s in which tetraphenyldiaminobiphenyl (TPD) units are covalently linked through flexible oligomethylene glycol spacers in the main chain were synthesized and the thermal, optical a

Full text of DOI:10.1039/b105930k

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Poly[bis(triphenylamine) ether]s with low glass transition
temperatures as photoconductors in fast photorefractive systems
Jolita Ostrauskaite,a{ Haridas R. Karickal,a Andre´ Leopold,b Dietrich Haarerb and
Mukundan Thelakkat*a
aMakromolekulare Chemie I, University of Bayreuth, 95440 Bayreuth, Germany.
E-mail: mukundan.thelakkat@uni-bayreuth.de
bExperimentalphysik IV, University of Bayreuth, 95440 Bayreuth, Germany
Received 4th July 2001, Accepted 3rd October 2001
First published as an Advanced Article on the web 14th November 2001
A series of photoconducting poly(tetraphenyldiaminobiphenylene alkyl ether)s in which
tetraphenyldiaminobiphenyl (TPD) units are covalently linked through flexible oligomethylene glycol spacers in
the main chain were synthesized and the thermal, optical and electrochemical properties were studied. Due to
the introduction of flexible spacers, the polymers are highly soluble and could be obtained as film-forming
materials with appreciably high molecular weights. The polymers exhibit glass transition temperatures between
92 uC and 128 uC which is about 100 uC less than those main chain polymeric bis(triphenylamine)s without such
spacers. The HOMO value as determined from cyclic voltammetry is about 25.1 eV. The glass transition
temperature of the photorefractive composites prepared by mixing the different polymers with an electro-optic
chromophore, 1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene, EHDNPB, could be tuned over
a wide range about room temperature by just changing the photoconductor and without the need of any
additional amount of plasticizer. Degenerate four-wave mixing and two-beam coupling in composites with
the composition, photoconductor : EHDNPB : C60 (60 : 39 : 1 by wt/wt%) results in refractive index
modulations of 1023 with corresponding response time y10 ms and a photorefractive gain of C~13 cm21
for a writing beam intensity of 1 W cm22 (645 nm) under an external electric field of 60 V mm21
.
observed.9,12,13 The main problem in these composites is the
tendency of the electro-optic molecules to phase separate due to
the non-compatibility of the photoconductor and chromo-
phore.14 Another factor that has to be improved in such
systems is the charge transport mobility, for PVK possesses
relatively poor photoconductivity.15 It is also desirable to avoid
the additional component of plasticizer by suitable structural
variation of the photoconductor, to obtain low-Tg photore-
fractive systems.16,17 In order to improve the photoconductiv-
ity, composites containing low molecular weight
tetraphenyldiaminobiphenyl (TPD) derivatives,18,19 polymeric
TPDs15,16 as well as copolymers carrying TPD units20 were
examined, because TPD derivatives exhibit high hole transport
mobility.
The problem of phase separation and the resulting instability
of the system can be overcome by designing new photo-
conductors which are compatible in chemical structure with
classical electro-optic chromophores. In this paper, the design,
synthesis and characterization of a series of photoconducting
polymers are described. In these polymers, highly photocon-
ducting TPD units are covalently linked through flexible
oligomethylene glycol spacers in the main chain. The synthetic
strategy employed here allows the synthesis of low-Tg poly-
meric photoconductors with high content of the active TPD
moiety. Photorefractive composites were prepared by blend-
ing the photoconductors with a highly soluble and compati-
ble EO-chromophore carrying a branched alkoxy group,
1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene
EHDNPB.21 The Tg of the photorefractive composites could
be tuned over a wide range about room temperature by
varying the photoconductor of the mixture and without using
any additional amount of plasticizer. The advantage of this
approach is that the amount of chromophore in the composite
Introduction
Since the first report of photorefractivity in polymeric materials
in 19911 there has been an increasing amount of research
activity in this field due to potential applications like holo-
graphic data storage,2 image processing,3 optical pattern
recognition,2,4 medical imaging,5,6 novelty filtering7 etc. The
complex phenomenon of photorefractivity is defined as the
local refractive index change in the bulk of the material due
to an unsymmetrical charge distribution in the material caused
by spatial variation of light intensity.8 This is realized by a
combination of physical functions like light absorption
followed by charge separation, trapping, charge distribution
and build-up of internal space charge field under the influence
of an external electric field. This requires properties like light
sensitization, photoconduction, linear and non-linear electro-
optical effects and charge trapping in the photorefractive
system. The simple and most widely studied photorefractive
systems consist of a polymeric photoconductor mixed with
large amounts of chromophore, a very small amount of a
sensitizer and varying amounts of a plasticizer to tune the glass
transition temperature (Tg) of the composite to room tem-
perature.9,10 However if the Tg of the photorefractive system is
low, the chromophores reorient under the influence of the space
charge field resulting in a large increase of refractive index
change known as the orientational enhancement.11
The most widely used photoconductor is polyvinylcarbazole
(PVK) which has a Tg above 200 uC. In sufficiently plasticized
PVK-based photorefractive polymers, very large diffraction
efficiencies, gain coefficients and fast response times have been
{Permanent address: Department of Organic Technology, Kaunas
University of Technology, 3028 Kaunas, Lithuania.
58
J. Mater. Chem., 2002, 12, 58–64
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b105930k

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can be kept practically constant, during the variation of Tg. The
photorefractive properties of some of these composites were
studied using two-beam coupling and degenerate four-wave
mixing experiments.
Table 1 Average molecular weight and thermal properties of poly-
[bis(triphenylamine) ether]s 3a–c
Polymer
Mn a/g mol21
Mw a/g mol21
Tg b/uC
Tdec-5% c/uC
3a
3b
3c
12 200
8300
15 000
51 200
60 900
47 300
128
102
92
430
410
420
Results and discussion
aMeasured with GPC (PS standard). bObtained from DSC (heating
and cooling rate 10 K min21, under N2 atmosphere). cTemperature
for 5% wt. loss from TGA (heating rate 10 K min21, under N2 atmo-
sphere).
Synthesis and characterization
The classical Ullmann reaction between a secondary diamine
and a diiodide using copper catalysts at high temperatures of
about 200 uC is not suitable for the preparation of polymers
due to the high amount of side reactions and the precipitation
of oligomeric products due to insolubility. The modified
reaction procedure using phase transfer catalysts reported by
Frechet and Gauthier22 for low molecular weight compounds
has been successfully utilized here for the preparation of
soluble and high molecular weight triarylamine polymers.
Three new bis(triphenylamine)-based polymers were synthe-
sized as schematically shown in Fig. 1 from stoichiometric
amounts of N,N’-diphenylbenzidine (1) and the corresponding
bis(iodophenoxy)alkanes (2a–c) by Ullmann reaction using Cu
and K2CO3 in dry 1,2-dichlorobenzene in the presence of
18-crown-6 as phase transfer catalyst.23 Aryl diiodide mono-
mers were designed to incorporate the oligomethylene glycol
spacer groups. Bis(iodophenoxy)alkanes (2a–c) were prepared
by the reaction between 4-iodophenol and the corresponding
dibromoalkanes in the presence of potassium carbonate (see
Experimental section). The poly[bis(triphenylamine) ether]s
(3a–c) were subjected to Soxhlet extraction with ethanol
and repeated reprecipitation from ethanol to remove the low
molecular weight oligomeric fractions. In this way, high yields
(above 80%) of high molecular weight polymers were obtained.
The incorporation of the oligomethylene glycol spacer into
the main chain of the polymer, on the one hand, leads to
increased solubility and, on the other hand, guarantees com-
patibility and therefore miscibility with chromophores carrying
alkoxy spacers. On comparison, Ullmann polymerization with
diiodides without spacers leads only to low molecular weight
oligomeric mixture.24 The length of the oligomethylene glycol
spacer group influences the thermal properties as well. The
chromophore EHDNPB was prepared by a known procedure21
from 2,5-dimethyl-4-(4-nitrophenylazo)phenol by etherifica-
tion with ethylhexyl bromide. The compound 2,5-dimethyl-4-
(4-nitrophenylazo)phenol was prepared from 2,5-dimethylphenol
and 4-nitrobenzenediazonium salt. All the materials synthe-
sized were characterized by FTIR- and 1H-NMR spectroscopy
and mass spectrometry measurements.
completely and the characteristic C–N stretching at 1235–
1256 cm21 is observed in the products. Additionally, polymers
3a–c exhibit characteristic aromatic absorptions at 3035–
3032 cm21 (C–H stretching), at 1593–1591, 1509–1506, 1493–
1488 cm21 (C–C stretching), characteristic aliphatic absorption
at 2937–2924, 2862–2853 cm21 (C–H stretching) and C–O–C
1
asymmetric stretching at 1250¡2 cm21. The signals in the H
NMR spectra of poly[bis(triphenylamine) ether]s 3a–c could be
exactly assigned to the characteristic hydrogen atoms of these
compounds. As expected, the signals of hydrogen atoms of the
oligomethylene glycol chain are shifted gradually to the lower
field and the methyleneoxy hydrogen (OCH2-) signals are
observed at 3.95–3.90 ppm.
The average molecular weight and their distribution were
detected by gel permeation chromatography (GPC) using
polystyrene as standard. All the polymers exhibit appreciably
high average molecular weights, Mn varying from about 8000
to 15 000 and Mw values between 50 000 and 60 000 g mol21
(see Table 1). In spite of the achieved high molecular weight, all
the polymers are soluble in common solvents like THF,
chloroform etc. It should be noted that these polymers form
thin and stable amorphous films of high optical clarity from
solution casting.
The thermal properties of polymers were examined by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) measurements. TGA measurement reveals that
the polymers are highly thermally stable and the onset of
decomposition occurs only above 410 uC (Fig. 2). The polymers
exhibit only a glass transition; 3a at 128 uC, 3b at 102 uC and 3c
at 92 uC and show no melting and no crystallization behavior
on further cooling and heating cycles between 250 uC and
250 uC at 10 K min21. The second heating curves of polymers
are shown in Fig. 3. The polymeric bis(triphenylamine)s with-
out any spacer show Tg above 200 uC,23,25 and the introduction
of a long oligomethylene glycol chain as spacer lowers the Tg
value by more than 100 uC (see Table 1). Thus, these photo-
conductors are suitable for the preparation of composites with
In the synthesis of polymers 3a–c, the strong IR absorption
at 3380 cm21 due to N–H stretching in compound 1 disappears
Fig. 2 Thermogravimetric analysis (TGA) curves of poly[bis(triphenyl-
amine) ether]s 3a–c at heating rate of 10 K min21 under N2 atmo-
sphere.
Fig. 1 Scheme of synthesis of poly[bis(triphenylamine) ether]s 3a–c by
Ullmann reaction.
J. Mater. Chem., 2002, 12, 58–64
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Table 2 Cyclic voltammetric data and HOMO–LUMO values of
poly[bis(triphenylamine) ether]s 3a–c and EHDNPB
Compound Eox1 vs.Fc/V HOMO/eV Ered vs.Fc/V LUMO/eV
a
3a
3b
0.28
0.29
0.29
25.08
25.07
25.09
—
—
—
—
—
—
—
3.39
a
a
3c
EHDNPB
b
—
21.41
aNo reduction observed up to 22.3 V vs. Fc. bNo oxidation observed
up to z1.2 V vs. Fc.
range of those for bis(triphenylamine) derivatives:23,24 the
polymers do not show any reduction behaviour in the
measurement range from z1.2 V to 22.3 V with respect to
Fc. On the other hand, the EO-chromophore EHDNPB
exhibits only a reversible reduction at 21.4 V vs. Fc corres-
ponding to a LUMO value of 23.4 eV with respect to zero
vacuum level. The measured LUMO-value is in agreement with
the reported value in the literature28 and also with those for
similar azo compounds.18
Fig. 3 Differential scanning calorimetry (DSC) curves of poly[bis(tri-
phenylamine) ether]s 3a–c (2nd heating cycle at 10 K min21).
Tg close to room temperature in order to exploit the orientation
enhancement phenomenon.11
The optical properties of the polymers and photorefractive
composites were investigated by measuring the UV–vis spectra
of their solutions in CHCl3. It is very important to know the
absorption spectra of polymers and photorefractive composites
because it determines the wavelength region in which they
can be used. It is important that the composite does not absorb
in the photorefractive measurement region, for it can lead
to competing non-photorefractive gratings. As expected, the
polymers 3a–c exhibit very similar absorption spectra with two
vibronic bands at 308 nm and 355 nm. The spectra are similar
to the UV–vis absorption of the model compound, TPD. A
comparison of the absorption spectra of the photoconductor
3a, composite (3a : 3c : EHDNPB : C60~30 : 30 : 39 : 1 wt/
wt%) and the NLO-chromophore, EHDNPB, in CHCl3 is
shown in Fig. 5. The composite shows a long-wavelength
cut-off at 550 nm, making it possible to consider this material
for photorefractive applications using laser light of 633 nm
or 645 nm.
Electrochemical and optical properties
The electrochemical stability and the reversibility of the redox
process of the polymers 3a–c and the EO-chromophore were
studied using cyclic voltammetry (CV). The measurements
were carried out at a glassy carbon electrode in a solution of
carefully dried acetonitrile containing 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) at room temperature.
The potentials were measured against Ag/AgNO3 as reference
electrode and each measurement was calibrated as usual with
the standard ferrocene/ferrocenium (Fc) redox system.26 The
HOMO and LUMO energy values of the compounds were
determined from oxidation and reduction potentials respec-
tively by taking the value of 24.8 eV as HOMO energy level for
the Fc with respect to zero vacuum level as described by Daub
et al. in the literature.27 The polymers are electrochemically
stable, for the same redox potentials are observed for repeated
cycles of redox processes. As expected for the bis(triphenyl-
amine) unit, polymers 3a–c exhibit two oxidation steps
corresponding to monocation and dication radicals. The first
and second oxidation steps are completely reversible for the
Photorefractive properties
entire scan rate of 50 mV s21 to 500 mV s21
.
Three photorefractive composites containing 60% of different
photoconductors, 3a, 3a : 3c (1 : 1) and 3c respectively, 39%
EHNDPB and 1% C60 as light sensitizer were prepared without
adding any plasticizer. The thermal properties of these
composites were characterized by DSC. All the composites
show only a broad glass transition without any melting and
recrystallization peaks, even though EHDNPB is a crystalline
compound showing recrystallization and melting peaks in the
second heating cycle at 27 uC and 49 uC respectively. The Tg
values of the different composites with 60% of photoconduc-
tors, 3a, 3a : 3c (1 : 1) and 3c are 42 uC, 28 uC and 12 uC
The cyclic voltammograms of the polymers measured at
a scan rate of 50 mV s21 are given in Fig. 4. Oxidation poten-
tials and HOMO values determined from the first oxidation
potentials with respect to ferrocene/ferrocenium as internal
standard are presented in Table 2. These values (Eox1 vs.
Fc~0.3 V and HOMO~25.1 eV) for 3a–c are in the typical
Fig. 4 Cyclic voltammetry curves of poly[bis(triphenylamine) ether]s
3a–c measured at 25 uC at a scan rate of 50 mV s21 vs.Ag/AgNO3 using
ferrocene/ferrocenium as standard.
Fig. 5 UV–vis spectra of 3a, EHDNPB and the composite containing
60% 3a : 3c (1 : 1)–39% EHDNPB–1% C60 in CHCl3 solution
60
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respectively. In this way we were able to tune the Tg of the
composites over range of temperatures about room
cycles if its transition dipole moment has a component parallel
to the polarization of the light. During these cycles, the
chromophore reorients itself into a random position. Thus,
over time, in the bright regions of the light intensity grating, the
number of chromophores with an orientation perpendicular
to the polarization of the light will increase. This process,
however, creates a zero-phase grating, which adds to the modu-
lation of the refractive index as well, thus being counter-
productive to a large phase shift W which is desired in
photorefractive gratings. Bolink et al. obtained similar low
gain values in composites prepared from polymeric bis-
(triphenylamine) photoconductor 4-(N,N-diethylamino)-b-
nitrostyrene (DEANST) as NLO-chromophore and C60 as
sensitizer.16 But the diffraction decay time and the rise-time
observed in gain experiments in the latter was also rather slow
and about 0.5 s was the photorefractive response time in this
system.
a
temperature just by varying the composition of photoconduc-
tor and keeping the amount of EO-chromophore constant. Due
to the difference in the length of the oligomethylene glycol
spacers in the polymers, the effective wt% values of the TPD
content in these three composites are different and are
calculated to be 48.4%, 45.5% and 42.5% respectively. This
fact should be taken into consideration along with the
difference in Tg values in the discussion of photorefractive
properties of the various composites. All these composites
prepared with the poly[bis(triphenylamine) ether]s form
optically clear films on filling cells of thickness between 25
and 80 mm and are stable for the last one year. We reported
earlier the applicability of low molecular weight triarylamine
derivatives as photoconductors to obtain fast photorefractive
systems having low Tg and containing no plasticizer.19 But the
low molecular weight photoconductor system does not possess
any such long-term stability due to phase separation of the
components. The addition of plasticizer to lower the Tg and to
stabilize the system has its own influence on the photorefractive
effect.29 In order to understand the interplay of electrical
and optical processes in photorefractive composites and the
dynamics of the grating build-up, it is preferable to avoid the
additional component of plasticizer and to have the minimum
number of components in the composite. Thus the new
polymers are very promising, for they can be blended with
compatible NLO-chromophores to obtain low Tg composites
which do not show any phase separation.
To determine the response times of the material, we used a
slight modification of the conventional biexponential fit, by
replacing the fit to the slower component with a stretched
exponential function, which provides more realistic data for
chromophore orientation processes [eqn. (3)]:31
ꢀ
ꢁ
{t
{(t=
b
=
)
t1
t2
Dn~A0 A1e
z(1{A1)e
(3)
where b has a value between 0 and 1. The latter case would be
a normal monoexponential decay. The smaller the value of
b, the more stretched out is the decay. Using the four-wave
mixing experiments, we determined the decay times of the
materials. Two distinct processes could be observed for the
decay process. Fig. 7 shows a sample curve of a photorefractive
erasing process. The time scale of the slow one, being in the
The photorefractive gain was measured using two-wave
mixing experiments with p-polarized writing beams and
calculated using the formula given in eqn. (1)
range of
1 to 10 seconds, matches with ellipsometric
1
C~
½ln (c0b){ ln (bz1{c0)ꢀ
(1)
measurements. Therefore, we attribute this to the orientation
of the chromophores, whereas the fast one is interpreted as
Pockels contribution. The material thus shows a fast Pockels
response; its dependence on the electric field is depicted in
Fig. 8. The contribution of the Pockels effect to the overall
modulation of the refractive index is about 40% at moderate
electric fields (60 V mm21).
On comparing the different photorefractive composites,
there is a clear difference in the response times as well as the
achieved modulation of the refractive index. There is an order
of magnitude difference between the fastest component of the
two response times; the composite using polymer 3a being the
fastest one (t1~1 ms) and the composite using pure 3c being an
order of magnitude slower (t1~12 ms). The achieved refractive
index modulations behave the opposite way: the material
using 3c creates the highest Dn, which is explainable by the
considerably lower Tg of the material, compared to the other
composites29 (see Table 3).
d= cos H
where b is the ratio of the power of the writing beams in front
of the sample, and c0 is given by [power of beam 1 (beam 2 on)]/
[power of beam 1 (beam 2 off)] behind the sample. By moving
the sample with a piezo control after reaching steady state
conditions, the phase shift was measured using the translating
grating technique.30 In our experiment, we translate the sample
along the direction of the Dn grating much faster than the
response time of the material. Additionally, we determined the
photorefractive gain C from the phase shift W. As an example,
the photorefractive gain and phase dependence on the applied
electric field for the composite, 3a : 3c (1 : 1)–39% EHDNPB–
1% C60 is given in Fig. 6. The latter one is small, and
subsequently, the gain, obtained from eqn. (2)
C! sin W
(2)
is also small. We attribute this to the presence of a cis–trans
isomerization grating. Under illumination, the azo group in the
chromophore EHDNPB performs cis–trans isomerization
Ogino et al. have examined the photorefractive properties of
Fig. 7 Example for an erasing process in degenerate four-wave mixing
(DFWM) measurement with the composite, 60% 3a–39% EHDNPB–
1% C60.
Fig. 6 Photorefractive gain and phase dependence on the applied
electric field for the composite, 3a : 3c (1 : 1)–39% EHDNPB–1% C60.
J. Mater. Chem., 2002, 12, 58–64
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solution, a cold solution of 5.92 g (25 mmol) of 4-nitrobenze-
nediazonium tetrafluoroborate dissolved in a mixture of 30 ml
acetic acid and 30 ml water was added dropwise. The reaction
mixture was stirred at 5 uC for 1 h and it was kept alkaline
throughout the reaction time by adding additional amounts of
cold 10% NaOH solution if needed. The completion of reaction
was tested by TLC (eluent: cyclohexane–ethyl acetate~3 : 1).
After the reaction was completed, acetic acid was added to
make the mixture acidic and the mixture was kept overnight in
the fridge. The precipitated 2,5-dimethyl-4-(4-nitrophenylazo)-
phenol was filtered, washed with water and dried. Yield~6.4 g
(94.4%) orange–red powder .
Mp: 222–223 uC (C14H13N3O3 ~ 271.15 g mol21). IR (in
KBr) n/cm21: 3206 (O–H), 3069, 3013 (ar. C–H), 2923 (alk.
C–H), 1591,1508 (ar. CLC). MS (m/z): 271, 254, 241, 225, 195,
181, 149, 121, 92, 91, 77, 75. 1H-NMR (CDCl3), d (ppm): 2.31
(s, 3H, methyl), 2.72 (s, 3H, methyl), 5.12 (s, 1H, hydroxy), 6.78
(s, 1H, ar.), 7.65 (s, 1H, ar.), 7.98 (d, 2H, ar.), 8.38 (d, 2H, ar.).
4.07 g (15 mmol) of 2,5-dimethyl-4-(4-nitrophenylazo)phe-
nol, 4.83 g (25 mmol) of ethylhexyl bromide, 3.45 g K2CO3 and
a pinch of KI were added together in 100 ml of dry acetone and
refluxed for 5 days. The mixture was filtered hot to remove the
salts and the solvent was removed by rotovapor. The dark red
oil obtained was dissolved in diethyl ether, washed with water
to remove any inorganic impurities, dried over Na2SO4 and the
ether was removed by rotovapor. The residue was recrystallized
from methanol to obtain red needles of EHDNPB. Yield:
3.73 g (65%). Mp: 65 uC (C22H29N3O3 ~ 383 g mol21). IR (in
KBr) n/cm21: 2925, 1606, 1516, 1338, 1244, 1089, 859. 1H-
NMR (CDCl3), d (ppm): 1.0 (m, 6H, CH3), 1.50 (m, 8H, CH2),
1.80 (m, 1H, CH), 2.30 (3H, CH3), 2.71 (3H, CH3), 3.92 (2H,
OCH2), 6.75 (s, 1H, ar.), 7.62 (s, 1H, ar.), 7.94 (d, 2H, ar), 8.32
(d, 2H, ar). MS (m/z): 383 [Mz], 271, 254, 149, 135, 121, 43.
Fig. 8 Dependence of photorefractive response times (fast component)
on the applied electric field for the composite, 60% 3c–39% EHDNPB–
1% C60
.
Table 3 Composition and photorefractive properties of different
composites with 60% photoconductor–39% EHDNPB–1% C60 at a
writing beam intensity of 1 W cm22 (645 nm)
Photoconductor in
composite/wt%
TPD
content/wt%
Tg/uC
t1/ms
Dn
3a (60%)
3a : 3c (30% : 30%)
3c (60%)
48.4
45.5
42.5
42
28
12
1
7
12
261024
661024
161023
composites prepared from copolymers carrying side chains of
bis(triphenylamine) as well as butyl acrylate moieties and
an EO-chromophore, DEANST, with C60 as sensitizer.15
Response times less than 10 ms were obtained at 50 V mm21
for writing intensity of 250 mW cm22. Therefore, on compar-
ison the newly synthesized polymeric triarylamine ethers show
very good photorefractive properties in composites with
compatible NLO-dye.
1,6-Bis(4-iodophenoxy)hexane 2a. 5.61 g (23 mmol) of 1,6-
dibromohexane was added to a solution of 10.12 g (46 mmol)
4-iodophenol in 100 ml dry methyl ethyl ketone containing
9.54 g (69 mmol) of potassium carbonate and a bit of
potassium iodide. The mixture was refluxed under argon for
20 h under stirring. The inorganic salts were removed by
filtration of the hot reaction mixture and the product was
obtained after removal of solvent and purification by repeated
recrystallizations from ethanol.
Conclusion
We synthesized a series of polymeric bis(triphenylamine)s
with ether linkages which are compatible with the NLO-dye,
EHDNPB, so that different stable and low-Tg photorefractive
composites could be prepared. The shelf-lives of these samples
without any plasticizer are excellent. Two-beam coupling and
four-wave mixing experiments were carried out to determine
the photorefractive properties. The response time analysis
indicates the two distinct processes due to the slow chromo-
phore orientation and fast Pockels contribution. Fast response
time of 1 ms was obtained for a composite with the photo-
conductor 3a. The low values of phase shift and gain exhibited
by these composites are attributed to the presence of a cis–trans
isomerization grating in this system.
Yield 9.6 g (80%) of 2a (C18H20I2O2 ~ 522.12 g mol21). Mp:
123–125 uC. IR (in KBr) n/cm21: 3086 (ar. C–H), 2937, 2862
(alk. C–H), 1584, 1490 (ar. CLC), 1255 (C–O–C), 1063, 628
1
(C–I). H-NMR (CDCl3), d (ppm): 1.53 (m, 4H, alk.), 1.80 (t,
4H, alk.), 3.92 (t, 4H, alkoxy), 6.65 (m, 4H, ar.), 7.53 (m, 4H,
ar.). MS (m/z): 522, 396, 303, 220, 203, 83.
1,10-Bis-(4-iodophenoxy)decane 2b. 0.48 g (20.0 mmol) NaH
was added stepwise to a solution of 4.40 g (20.0 mmol)
4-iodophenol in 20 ml dry DMF at 0 uC. The mixture was
allowed to warm to room temperature and was stirred until
there was no more H2 evolution. Then 3.30 g (11.0 mmol) of
1,10-dibromodecane was added slowly. The mixture was
heated to 120 uC and stirred for 18 h, then cooled to room
temperature. The solvent was removed by distillation. The
crude product was dissolved in 100 ml CH2Cl2 and washed
with 10% NaOH solution and then with water. The organic
phase was dried over anhydrous Na2SO4, and the solvent
was removed by rotary distillator. The product was purified
by recrystallization from isopropyl alcohol and then from
n-hexane.
Yield: 3.25 g (51.1 %) of 2b (C22H28I2O2 ~ 578.23 g mol21).
Mp: 109–110 uC. IR (in KBr) n/cm21: 3063 (ar. C–H), 2928,
2851 (alk. C–H), 1570, 1488 (ar. CLC), 1248 (C–O–C). MS
(m/z): 578, 452, 220. 1H-NMR (CDCl3), d (ppm): 1.39–1.45
(m, 12H, alk.), 1.77 (q, 4H, alk.), 3.89 (t, 4H, alkoxy), 6.66 (d,
4H, ar.), 7.53 (d, 4H, ar.).
Experimental
Materials
All the chemicals were purchased from Aldrich as reagent
grade and used as received without further purification,
except N,N’-diphenylbenzidine (1), which was sublimed twice
in vacuum before use. 1-(2-Ethylhexyloxy)-2,5-dimethyl-4-(p-
nitrophenylazo)benzene (EHDNPB) was synthesized by a
similar procedure to that described in literature.21 Solvents
were distilled and dried as usual.
Synthesis
1-(2-Ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo)benzene
(EHDNPB). To an ice-cooled (0 to 5 uC) solution of 3.05 g
(25 mmol) 2,5-dimethylphenol dissolved in 25 ml 10% NaOH
62
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1,12-Bis(4-iodophenoxy)dodecane 2c. This was prepared by a
similar procedure to that for compound 2a. Yield 8.4 g (61%)
of 2c (C24H32I2O2 ~ 606.32 g mol21). Mp: 101–103 uC IR (in
KBr) n/cm21: 3081 (ar. C–H), 2938, 2850 (alk. C–H), 1586,
1487 (ar. CLC), 1253 (C–O–C), 1056, 624 (C–I). MS (m/z): 606,
480, 386, 220, 186, 69, 55. 1H-NMR (CDCl3), d (ppm): 1.29 (s,
16H, alk.), 1.75 (s, 4H, alk.), 3.90 (s, 4H, alkoxy), 6.65 (s, 4H,
ar.), 7.51 (s, 4H, ar.).
permeation chromatography (GPC) using Waters HPLC (PS
standard).
Sample preparation
The photorefractive measurements were performed on films
with thicknesses between 20 and 80 mm. The composites were
prepared by mixing solutions containing 60 wt% of the
photoconductor, 39 wt% of the chromophore EHDNPB and
1 wt% of C60 in benzene followed by filtration through a 0.2 mm
filter and finally freeze-drying overnight under vacuum. For
preparation of the photorefractive samples, the composite
material was put onto a glass plate partially covered with
indium tin oxide (ITO) and subsequently heated to 160 uC. At
this temperature, a second ITO covered glass plate was put
on top of the first one. Spacers with a definite thickness were
put in between the two glass plates to maintain the desired
thickness of the photorefractive samples.
Poly[1,4-phenylene(phenylimino)biphenyl-4,4’-ylene(phenyl-
imino)-1,4-phenyleneoxyhexamethyleneoxy] 3a. 3.36 g (10 mmol)
of N,N’-diphenylbenzidine (1), 5.22 g (10 mmol) of 1,6-bis(4-
iodophenoxy)hexane 2a, powdered anhydrous potassium carbo-
nate (11,06 g, 80 mmol), electrolytic copper powder (2.54 g,
40 mmol) and 18-crown-6 (0.53 g, 2 mmol) were refluxed in 35 ml
o-dichlorobenzene under argon for 3 days. The copper and
inorganic salts were removed by centrifugation. The product
was reprecipitated from cyclohexane, extracted in a Soxhlet
apparatus with ethanol, dissolved in small amount of THF and
reprecipitated from ethanol.
Photorefractive measurements
Yield: 6.6 g (90%). IR (in KBr) n/cm21: 3035 (ar. C–H), 2937,
2862 (alk. C–H), 1593, 1507, 1494 (ar. CLC), 1272 (C–N), 1256
(C–O–C). UV (in CHCl3) l/nm: 308, 355. 1H-NMR (CDCl3), d
(ppm): 1.55 (s, 4H, alk.), 1.81 (s, 4H, alk.), 3.95 (s, 4H, alkoxy),
6.6–7.6 (m, 26H, ar.).
The photorefractive measurements were performed using four-
wave mixing or two-beam coupling experiments. A coherent
899 dye laser, operating at a wavelength of 645 nm was used
for the writing beams, meeting the sample with an external
angle of 20u in between them. The sample itself was tilted by
60u. For the four-wave mixing (FWM) experiments, a second,
p-polarized laser beam at 670 nm, generated by a laser diode,
counterpropagated one of the s-polarized writing beams.
Photodiodes (Thor Labs PDA55EC) attached to lock-in
amplifiers (Stanford SR830) were used to detect the diffracted
and transmitted reading laser. FWM writing processes were
performed switching on the second writing beam using a fast
(400 ms) mechanical shutter, while the first one illuminated the
sample previously. The erasure of the photorefractive grating
was done by blocking the writing beam again, leaving the
second one illuminating the sample.
Poly[1,4-phenylene(phenylimino)biphenyl-4,4’-ylene(phenyl-
imino)-1,4-phenyleneoxydecamethyleneoxy] 3b. This was pre-
pared by a similar procedure to that for compound 3a by
reacting 2.019 g (6 mmol) of N,N’-diphenylbenzidine 1, 3.47 g
(6 mmol) of 1,10-bis(4-iodophenoxy)decane 2b, powdered
anhydrous potassium carbonate (4.97 g, 36 mmol), electrolytic
copper powder (1.54 g, 24 mmol) and 18-crown-6 (0.63 g,
2.4 mmol) in 30 ml o-dichlorobenzene. Yield: 3.48 g (87.5%).
IR (in KBr) n/cm21: 3032 (ar. C–H), 2924, 2854 (alk. C–H),
1591, 1509, 1488 (ar. CLC), 1272 (C–N), 1235 (C–O–C). UV
1
(in CHCl3) l/nm : 308, 355. H-NMR (CDCl3), d (ppm): 1.3–
1.67 (m, 12H, alk.), 1.78 (s, 4H, alk.), 3.90 (s, 4H, alkoxy), 6.7–
7.6 (m, 26H, ar.).
Acknowledgement
We thank Professor J. V. Grazulevicius (Kaunas University of
Technology, Lithuania) and Professor H.-W. Schmidt (Uni-
versity of Bayreuth, Germany) for providing JO with a stipend
for the period of stay at Bayreuth, Mrs A. Lang and Mrs
H. Wietasch for their help in sample preparation and synthesis.
Financial support from Bayerischer Forschungsstiftung, Pro-
jekt: ‘‘Organische optische Funktionsmaterialien’’ is kindly
acknowledged.
Poly[1,4-phenylene(phenylimino)biphenyl-4,4’-ylene(phenyl-
imino)-1,4-phenyleneoxydodecamethyleneoxy] 3c. This was also
prepared by the general procedure as for compound 3a by
reacting 3.36 g (10 mmol) of N,N’-diphenylbenzidine 1, 6.06 g
(10 mmol) of 1,12-bis(4-iodophenoxy)dodecane 2c, powdered
anhydrous potassium carbonate (11.06 g, 80 mmol), electro-
lytic copper powder (2.54 g, 40 mmol) and 18-crown-6 (0.53 g,
2 mmol) in 35 ml o-dichlorobenzene under argon. Yield: 6.7 g
(90%). IR (in KBr) n/cm21: 3034 (ar. C–H), 2925, 2853 (alk.
C–H), 1593, 1506, 1493 (ar. CLC), 1276 (C–N), 1238 (C–O–C).
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UV (in CHCl3) l/nm: 308, 355. H-NMR (CDCl3), d (ppm):
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