Photorefractive triphenylamine-based glass: A multifunctional low molecular weight compound with fast holographic response

Article abstract of DOI:10.1039/a902562f

A novel organic photorefractive composite system is presented consisting of the amorphous multifunctional low-molar mass compound DR1-DCTA, doped with C₆₀ as sensitizer and diisooctyl phthalate (DOP) as plasticizer. DR1-DCTA, a carbazole substi

Full text of DOI:10.1039/a902562f

J O U R N A L O F
Photorefractive triphenylamine-based glass: a multifunctional low
molecular weight compound with fast holographic response
C. Hohle,a U. Hofmann,b S. Schloter,b M. Thelakkat,a P. Strohriegl,*a D. Haarerb and
S. J. Zilkerb
C H E M I S T R Y
aMakromolekulare Chemie I and Bayreuther Institut fur Makromolekulforschung (BIMF),
¨
¨
bExperimentalphysik IV, University of Bayreuth, D-95440 Bayreuth, Germany
Received 30th March 1999, Accepted 10th May 1999
A novel organic photorefractive composite system is presented consisting of the amorphous multifunctional low-
molar mass compound DR1-DCTA, doped with C as sensitizer and diisooctyl phthalate (DOP) as plasticizer.
60
DR1-DCTA, a carbazole substituted triphenylamine with a covalently attached nonlinear optical azo chromophore
unit has been synthesized in a 4 step reaction. The compound forms a stable glass. Its thermal, optical and
electrochemical properties are discussed. Holographic experiments (2BC, DFWM) of the photorefractive composite
exhibit a fast response time in the range of only 30 ms and a maximum gain coefficient of 90 cm−1.
and characterization of one representative of a group of
multifunctional low-molecular weight glasses11 which com-
bines shelf lifetimes of more than one year with an excellent
photorefractive performance.
Introduction
Organic photorefractive (PR) materials which combine two
well known properties, photoconductivity and optical nonlin-
earity, have attracted much attention because of their possible
application in optical computing, optical switching and holo-
graphic data storage. The PR-effect is defined as a light-
induced modulation of the refractive index involving the
generation of space charges.1,2 Since the first observation of
this unique electro-optic effect in organic crystals in 19903 and
in polymers in 1992,4 it became obvious that these materials
are able to compete with commonly used inorganic crystals
such as LiNbO .5 Since then, several organic systems have
been found which show higher refractive index modulations
(Dn) and two-beam-coupling gain coefficients (C) than their
inorganic counterparts.6–8
Photorefractivity in an organic material can be achieved by
incorporating a nonlinear optical (NLO) chromophore into a
photoconducting matrix. Small amounts of sensitizing agents
Experimental
Instrumentation
NMR-spectra were measured with
a Bruker AC 250
(250 MHz), UV–VIS spectra with a Perkin Elmer Lambda 19,
and FTIR spectra with a Bio-Rad Digilab FTS-40 spec-
trometer. DSC measurements were performed with a Perkin
Elmer DSC-7 and cyclic voltammetry (CV) measurements
with a three electrode and potentiostat configuration from
EG&G Princeton Applied Research.
3
Photorefractive measurements were performed with
a
holographic setup. We determined the gain coefficient C from
two-beam coupling measurements (2BC) and the refractive
index modulation Dn from degenerate four-wave mixing
experiments (DFWM). The experiment is based on a diode
laser operating at 670 nm, which is split into two writing
beams of equal intensity (1.2 W cm−2). They overlap in the
sample at an angle of 17°; the sample is tilted by 60° with
respect to the bisector of the writing beams. This results in a
grating period of 4.0 mm. 2BC experiments are performed by
a rapid sample translation using a piezoelectric element.
Meanwhile, the intensities of both writing beams are monitored
with photodiodes. In the case of DFWM experiments, a weak
beam from a second diode laser (670 nm) counterpropagates
one of the writing beams. Its transmitted and diffracted part
are detected by photodiodes.
such as 2,4,7-trinitrofluoren-9-one (TNF) or fullerene C
have to be added to form charge-transfer complexes sensitive
60
to the wavelength of the laser irradiation. Many of these so-
called guest–host systems based on molecularly doped poly-
mers (e.g. polyvinylcarbazole (PVK)9 or carbazole-substituted
polysiloxanes10) have been described in the past. However,
the maximum dye concentration in such materials is usually
limited by the onset of crystallization.
A possible solution of this problem is the design of a
multifunctional low-molecular weight system consisting of an
amorphous hole transporting molecule covalently bound to
an NLO-moiety.11–14
A morphologically stable amorphous state in a low molar
mass compound can be achieved by a structurally rigid and
bulky substitution of a non-planar core, e.g. triphenylamine.
If such a material is heated to the melt followed by quick
cooling (quenching), a homogeneous amorphous glass is
obtained. Several glass forming hole transport materials based
on substituted triphenylamines such as N,N∞-diphenyl-N,N∞-
bis(3-methylphenyl)biphenyl-4,4∞diamine (TPD),15 tris(4-car-
bazol-9-ylphenyl)amine (TCTA)16 and tris[4-(diphenyl-
amino)phenyl]amine (TDATA)17 have been designed within
the last few years. Because of their excellent hole transport
properties they are commonly used in xerography and in
organic electroluminescent (EL) devices.18
Synthesis
All reagents and solvents were of commercial quality and were
purified or dried and stored under argon using standard
procedures. All reactions were performed in argon atmosphere.
4-Methoxytriphenylamine 1 was synthesized as described by
Gauthier and Fre`chet.19
4,4∞-Diiodo-4◊-methoxytriphenylamine, 2. 8.77 g (32 mmol)
of 4-methoxytriphenylamine 1 and 7.05 g (43 mmol) of potass-
ium iodide in 130 ml of acetic acid and 13 ml of water were
heated to 80 °C. 6.83 g (32 mmol) of potassium iodate were
added and the mixture was reacted for 3 h, allowed to cool
overnight and neutralized with aqueous sodium hydrogen
Our aim was to combine the well known properties of such
a material with those of an NLO-chromophore via chemical
bonding. The current contribution focuses on the chemistry
J. Mater. Chem., 1999, 9, 2205–2210
2205

carbonate. The black residue was filtered off, dissolved in
toluene, washed with aqueous sodium hydrogen carbonate
and dried. Further purification by column chromatography
using silica gel with toluene as eluent gave 13.71 g (yield: 82%)
(C H N O /888.04). Found C: 77.9; H: 5.06; N: 10.3%.
58 45 7 3
Requires C: 78.45; H: 5.11; N: 11.04%. IR (n/cm−1): 3046, 2921,
2850, 1600 (CLC, ar.), 1507 (NLO), 1452, 1337 (NLO), 1231
(C–O–C), 1134, 750. 1H-NMR (CDCl ) d (ppm): 1.31 (3H,
3
of a transparent glass (C H I NO/527.14). IR (n/cm−1): 2821,
CH ), 3.63, 3.87, 4.22 (3×2H, CH ), 6.81, 6.95 (2×2H, ar.),
19 15 2
3
2
1573, 1507, 1483 (CLC ar.), 1315, 1287 (C–N), 1242 (C–O–C),
7.2–7.5 (22H, ar.), 7.87, 7.9 (2×2H, ar.), 8.13 (4H, ar.), 8.27
(2H, ar.). 13C-NMR (CDCl ) d (ppm): 12.3 (CH ), 46.1, 49.8,
65.6 (3×CH ), 109.7, 111.4, 115.7, 119.8, 120.3, 122.5, 123.3,
123.8, 124.6, 125.8, 126.2, 127.9, 131.5, 140.4, 141.0, 143.7, 146.8,
817. 1H-NMR (CDCl ) d (ppm): 3.78 (3H, methoxy), 6.76,
3
3
3
6.83, 7.01, 7.46 (6×2H, ar.). 13C-NMR (CDCl ) d (ppm): 55.4,
84.7, 115.0, 124.6, 127.5, 138.0, 139.4, 147.3, 156.8. MS [m/z]:
527, 512, 400, 273, 229.
3
2
147.3, 151.2, 155.7, 156.6. MS [m/z]: 887, 671, 591, 296, 241,
146, 92, 57, 44.
4,4∞-Di(carbazol-9-yl)-4◊-methoxytriphenylamine, 3. 12.60 g
(75 mmol) of carbazole, 6.38 g (0.1 mol) of copper powder,
27.75 g (0.2 mol) of potassium carbonate and 1.33 g (5 mmol)
of 18-crown-6 in 150 ml of 1,2-dichlorobenzene were heated
to reflux. 13.24 g (25 mmol) of 2 in 20 ml of 1,2-dichloro-
benzene were added slowly and the mixture was reacted at
190 °C for 24 h. After the inorganic components were filtered
off while hot, the product was precipitated into methanol and
further purified by column chromatography with silica gel
using toluene as eluent to yield 14.32 g (yield: 94%) of a
colorless solid (C H N O/605.74). IR (n/cm−1): 3043, 1625,
Sample preparation
Samples of the photorefractive composite of DR1-DCTA 6,
diisooctyl phthalate (DOP) and C (7152851 wt%) were
obtained by filling commercial ITO cells (10–100 mm, obtained
from E.H.C. Co. Ltd., Tokyo) with the melted material at
180 °C. The mixture was previously dissolved in benzene,
filtered (0.2 mm) and freeze-dried over night to remove
residual solvent.
60
43 31
3
1599, 1507, 1480 (CLC ar.), 1452, 1318, 1288 (C–N), 1242
Results and discussion
(C–O–C), 1231, 830. 1H-NMR (CDCl ) d (ppm): 3.78 (3H,
methoxy), 6.93 (2H, ar.), 7.25–7.45 (22H, ar.), 8.11 (4H, ar.).
3
Synthetic approach
13C-NMR (CDCl ) d (ppm): 55.5 (O-CH ), 109.8, 115.2, 119.7,
120.3, 123.2, 123.5, 125.8, 127.9, 128.1, 131.4, 139.9, 141.0, 147.0,
157.0. MS [m/z]: 605, 590, 303, 302, 281, 241.
DR1-DCTA 6 was synthesized as described in Scheme 1 in a
four-step reaction. We started from the monoprotected tri-
phenylamine core, 4-methoxytriphenylamine 1,19 which was
iodinated in the two remaining para-positions with potassium
iodide–potassium iodate in acetic acid. The resulting
compound 2 was coupled with carbazole in an Ullman reaction
to yield 4,4∞-dicarbazol-9-yl-4◊-methoxytriphenylamine 3.
3
3
4,4∞-Di(carbazol-9-yl)-4◊-hydroxytriphenylamine, 4. A solu-
tion of BBr (14 mmol) in 14 ml of dry CH Cl was added
3
2 2
dropwise to a solution of 6.0 g (10 mmol) of 3 in 50 ml of
CH Cl at −78 °C. The mixture was stirred and allowed to
Subsequently the methyl ether was cleaved with BBr in
2
2
3
warm up to room temperature overnight. 60 ml of H O were
CH Cl and thus the phenol 4 was obtained. In the last step,
2
2
2
added and the phenolic product was collected by extraction
compound 4 was converted to the phenolate by KOtBu in
THF and reacted with the tosylated NLO-dye Disperse Red
1 5 to give the final glass DR1-DCTA 6. Purification of this
compound as well as some of the intermediates cannot be
achieved by recrystallization as these materials are obtained
as glasses. Column chromatography using silica gel and toluene
or methylene chloride as eluents is the best method to obtain
pure and well defined compounds.
with CH Cl . Column chromatography using silica gel and
2
2
toluene–ethyl acetate (1551) as eluent afforded 4.97 g (yield:
85%) of a colorless solid (C H N O/591.71). IR (n/cm−1):
42 29
3
3600–3400 (O–H), 3042, 1622, 1597, 1506, 1469, 1446, 1312,
1270, 1230, 1134, 1056, 834, 798. 1H-NMR (CDCl ) d (ppm):
3
4.58 (OH), 6.9 (2H, ar.), 7.2–7.5 (22H, ar.), 8.15 (4H, ar.). 13C-
NMR (CDCl ) d (ppm): 109.8, 116.7, 119.8, 120.3, 123.2, 123.5,
3
125.9, 128.3, 128.5, 131.5, 140.4, 141.1, 147.0, 153.0. MS [m/z]:
591, 504, 332, 295, 241, 166.
Thermal characterization
4-[N-(2-Tosyloxyethyl)-N-ethylamino]-4∞-nitroazobenzene, 5.
8.0 g (25 mmol) of 4-[N-(2-hydroxyethyl)-N-ethylamino]-4∞-
nitroazobenzene (Disperse Red 1) were dissolved in 40 ml of
pyridine, cooled to 0 °C and subsequently reacted with 5.98 g
(31 mmol) of toluene-4-sulfonyl chloride for 16 h at that
temperature. The product was isolated by adding water,
extraction with CH Cl , washing with diluted HCl, aqueous
The DSC-traces of DR1-DCTA 6 are depicted in Fig. 1. Only
a glass transition is found at 120 °C within the first heating
cycle. Upon cooling with 10 K min−1, no recrystallization
occurs. In order to check the stability of the glass, we carried
out annealing experiments, but we never found evidence for
recrystallization. After annealing at 160 °C for 24 h,
DR1-DCTA is still fully amorphous.
To prove the fully amorphous character of 6, its WAXS
spectrum20 is displayed in Fig. 2. No crystalline peak is visible
but only a broad amorphous halo.
Transparent films of high optical quality with a thickness
up to 1 mm can be obtained by spin-coating from solution.
Thicker films, which are necessary for photorefractive measure-
ments, are extremely brittle, making device preparation rather
difficult. In addition, to obtain a net electrooptic gain, a certain
orientational mobility of the multifunctional compounds has
to be guaranteed to allow for an alignment of the polar
chromophores under the influence of the space-charge field.
Moerner et al.21 have shown that this orientational enhance-
ment of the chromophores increases the photorefractive per-
formance drastically. Thus, we lowered the glass transition
temperature of our material to room temperature where
sufficient flexibility and orientational freedom is guaranteed.
For this we used 28 wt% of diisooctyl phthalate (DOP) as
2
2
Na CO and H O. Red crystals of 5 (5.55 g, 47%) were
2
3
2
obtained by recrystallization from isopropyl alcohol.
(C H N O S/468.54). IR (n/cm−1): 2921, 2850, 1600 (CLC,
23 24 4 5
ar.), 1517 (NLO), 1391 (SLO), 1337 (NLO), 1135, 1001, 858.
1H-NMR (CDCl ) d (ppm): 1.18 (3H, CH ), 2.38 (3H, CH ),
3
3
3
3.42, 3.69, 4.29 (3×2H, CH ), 6.61, 7.25, 7.71, 7.82, 7.9, 8.31
(6×2H, ar.).
2
4,4∞-Di(carbazol-9-yl)-4◊-(2-{N-ethyl-N-[4-(4-nitrophenylazo)
phenyl]amine}ethoxy)triphenylamine, DR1-DCTA, 6. To a solu-
tion of 2.55 g (4.3 mmol) of 4 in 75 ml of dry THF, 4.7 mmol
of KOtBu (1 M solution in THF) were added and heated to
reflux for 30 min. Afterwards, 2.06 g (4.4 mmol) of 5 in 60 ml
of dry THF were added dropwise and reacted for 19 h at
60 °C. After evaporation of the solvent crude 6 was redissolved
in CH Cl washed with H O and purified by column chroma-
2
2
2
tography using silica gel and CH Cl as eluent. Freeze drying
2
2
from benzene gave 2.6 g (yield: 68%) of a red powder
plasticizer, lowering the T of the composite to 21 °C (Fig. 1)
g
2206 J. Mater. Chem., 1999, 9, 2205–2210

I
N
Cu / K2CO3
[18-crown-6]
KI / KIO3
AcOH
1,2-dichlorobenzene
N
OCH3
N
OCH3
2
N
OCH3
H
N
I
N
1
2
3
BBr3
CH2Cl2
N
N
KOtBu
THF
N
N
NO2
Tos
N
O
N
N
OH
N
N
NO2
O
N
5
N
N
DR1-DCTA
6
4
Scheme 1 Synthesis of DR1-DCTA 6.
without causing any phase separation or crystallinity. A simple
method to obtain thick and homogeneous samples free of
solvent or gas cavities is to fill commercial ITO-cells with the
melted composite by means of capillary forces. The photoref-
ractive mixture of 6, DOP (71528 wt%) as well as 1 wt% of
to 300 °C in air, weight loss starts at 195 °C within the
composite material which is caused by evaporation of the
plasticizer DOP. In spite of its high boiling temperature of
384 °C, the liquid plasticizer is volatile above 195 °C.
Nevertheless, decomposition of the composite under the con-
ditions applied for device preparation (180 °C) can be
excluded.
fullerene C (allowing for a more efficient photogeneration at
60
the laser wavelength of 670 nm) is shown in Fig. 3. Due to its
low melt viscosity, this low molar mass composite is easily
drawn between the glass layers of the ITO-cell yielding homo-
geneous films of defined thickness.
The thermal stability of our composite has been checked by
TGA measurements. The TG-traces of pure DR1-DCTA and
the photorefractive mixture are depicted in Fig. 4. Compared
to 6, which is characterized by a high thermal stability of up
Fig. 5 shows the absorption spectrum of 6 in dioxane
solution. The bands at l=310 nm and 340 nm belong to the
absorption of the aromatic triphenylamine core. The maximum
absorption of the azo chromophore is at 465 nm. The insert
shows the tail of the absorption coefficient a of a 25 mm film
of the composite. Only a small absorption of approximately
30 cm−1 is found at the laser wavelength of 670 nm.
Fig. 1 DSC-traces of DR1-DCTA 6 (——), and the mixture of
6/DOP/C (7152851 wt%), (----), 1st heating (above) and cooling
60
(below) at a rate of 10 K min−1.
Fig. 2 WAXS-spectrum of DR1-DCTA 6.
J. Mater. Chem., 1999, 9, 2205–2210
2207

O
OC8H17
OC8H17
O
DOP
C60
N
N
NO2
N
O
N
N
Fig. 6 Cyclic voltammograms of DR1-DCTA 6 for the first oxidation
step (——) and the reduction steps (----) measured in
N
acetonitrile–TBAPF at a scan rate of 200 mV s−1.
6
DR1-DCTA
6
Ag/AgNO as reference electrode and each measurement was
calibrated as usual with the standard ferrocene/ferrocenium
Fig. 3 Components of the photorefractive composite material.
3
(Fc) redox system.22 The HOMO and LUMO energy values
of the compound were determined from the oxidation and
reduction potentials respectively by taking the value of −4.8 eV
as HOMO energy level for Fc with respect to zero vacuum
level as described by Daub et al. in the literature.23 The
compound is electrochemically stable, for the same redox
potentials are observed for repeated cycles of redox processes.
As expected, DR1-DCTA exhibits multi-oxidation steps corre-
sponding to mono-, di-, and tri-cation radicals similar to its
parent photoconductor, TCTA.16 The first oxidation stage and
the reduction curves obtained at a scan rate of 200 mV s−1 are
given in Fig. 6. The average of the peak potentials is 0.45 V vs.
Ag/AgNO which corresponds to 0.40 V vs. Fc. The HOMO
energy value determined from the first oxidation potential is
3
−5.20 eV. The first oxidation step is completely reversible for
the entire scan rate of 50 mV s−1 to 500 mV s−1. The anodic
and cathodic peak currents are 11.3 mA and 11.6 mA respect-
ively and the peak separation potential approaches 60 mV (at
a scan rate of 50 mV s−1) as expected for a fast reversible
single electron transfer. The compound shows two reduction
steps, at −1.35 V and −1.79 V, respectively. The reversibility
of the reduction processes could not be confirmed under the
experimental conditions used. The LUMO energy value which
results from the first reduction potential is −3.40 eV.
Fig. 4 TGA-measurements of DR1-DCTA 6 (——) and the mixture
of 6/DOP/C (7152851 wt%) (---), heating rates: 10 K min−1 (air).
60
Holographic characterization
With DFWM experiments, we determined the absolute value
and the dynamics of the diffraction efficiency g. At an applied
electric field of 80 V mm−1, diffraction efficiencies for p-
polarized reading of up to 27% are reached in samples of
40 mm thickness. This corresponds to refractive index modu-
lations (see Fig. 7) of about 3.5×10−3.
The response time of the material was determined from the
decay of the diffraction efficiency after one writing beam was
blocked. The decay was fitted with a biexponential func-
tion24–27 [eqn. (1)].
Fig. 5 UV–VIS-spectrum of DR1-DCTA 6 (10−4 mol l−1 dioxane
solution). The insert shows the absorption coefficient a of a 25 mm
film of 6/DOP/C
.
g(t)=g (a exp[−t/t ]+(1−a) exp [−t/t ])2.
(1)
60
0
1
2
The latter is motivated by the occurrence of two processes.
First, homogeneous light-induced generation of charges will
Cyclic voltammetry
The electrochemical stability and the reversibility of the redox
process of DR1-DCTA 6 was studied using cyclic voltammetry
(CV). The measurements were carried out at a glassy carbon
electrode in a solution of carefully dried acetonitrile containing
erase the internal space charge field (first decay constant t ),
then the chromophores—which perform an orientation during
writing—will subsequently exhibit orientational relaxation
1
(second time constant t ). Fig. 8 shows the fitted first time
2
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF )
constants which describe the decay of the space charge field.
6
at room temperature. The potentials were measured against
The material reaches response times down to 30 ms, which is
2208
J. Mater. Chem., 1999, 9, 2205–2210

Fig. 9 Two-beam coupling gain C for s-polarized (%) and p-polarized
(&) beams as a function of the electric field.
Fig. 7 Refractive index modulation Dn for a p-polarized reading beam
as a function of the applied electric field E.
is amorphous and exhibits a shelf lifetime of more than a year
is obtained. The photorefractive properties were characterized
by 2BC and DFWM experiments. The material shows a fast
holographic response of 30 ms and a gain coefficient of
90 cm−1. High refractive index modulations are readily
obtained.
The flexibility of the synthetic concept allows for further
optimization, e.g. by incorporating sophisticated NLO
chromophores.
Acknowledgements
Financial support from the ‘Bayerische Forschungsstiftung’ in
the framework of the FOROPTO II program is gratefully
acknowledged. The authors thank D. Hanft, I. Otto, W. Joy
for technical assistance and R. Resel for performing the
WAXS-measurements.
Fig. 8 Fast time constant t of the holographic grating decay as
1
determined by a fit with eqn. (1). Insert: Erasure dynamics of the
diffraction efficiency at an applied electric field of 80 V mm−1.
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depicts a close-up of the grating decay at 80 V mm−1.
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nature of the grating, i.e. the phase shift between the light
intensity and the refractive index grating. This gives rise to
the two-beam coupling (2BC) effect, an energy exchange
between the two writing beams. The latter is characterized by
the gain constant C [see eqn. (2)].
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Conclusion
We have shown the synthesis and chemical properties of a
novel organic photorefractive material in which a nonlinear
optical chromphore is covalently bound to hole transporting
triphenylamine and carbazole moieties. With suitable plas-
ticizer and sensitizer, a stable photorefractive composite which
20 WAXS-measurements were performed by R. Resel, Institut fur
¨
Festkorperphysik, Technische Universitat Graz, Austria.
¨
¨
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