Synthesis and Structural Effect of Multifunctional Photorefractive Polymers Containing Monolithic Chromophores

Article abstract of DOI:10.1021/ma034104g

Multifunctional photorefractive polymers bearing five different monolithic chromophores as pendant units were synthesized. Monolithic chromophores are composed of the electron-donating and photoconducting carbazole and the electron-withdrawing nitro group

Full text of DOI:10.1021/ma034104g

7970
Macromolecules 2003, 36, 7970-7976
Synthesis and Structural Effect of Multifunctional Photorefractive
Polymers Containing Monolithic Chromophores
J a eh oon Hw a n g, J iw on Soh n , a n d Soo You n g P a r k *
School of Materials Science and Engineering, Seoul National University, ENG445, San 56-1,
Shillim-dong, Kwanak-gu, Seoul 151-744, Korea
Received J anuary 26, 2003; Revised Manuscript Received August 11, 2003
ABSTRACT: Multifunctional photorefractive polymers bearing five different monolithic chromophores
as pendant units were synthesized. Monolithic chromophores are composed of the electron-donating and
photoconducting carbazole and the electron-withdrawing nitro group which are connected directly or via
different conjugation bridges of azobenzene, stilbene, benzoxazole, and cyanostilbene moiety. Acrylate
derivatives of these chromophores were copolymerized with butyl acrylate to give photorefractive polymers
with Tg near room temperature. The structural effects of these monolithic photorefractive chromophores
were investigated in terms of the optical nonlinearity, photoconductivity, and photorefractivity of the
acrylate copolymers. The opposite direction of asymmetric energy transfer in two-beam coupling
measurement indicated that the photorefractive polymers had the different charge-transporting species
(hole or electron) for formation of internal space-charge field according to the chromophore structures.
The correlation between NLO property and photorefractivity exhibited that the photorefractivities of the
obtained polymers were strongly dependent on the NLO property rather than the photoconductivity.
In tr od u ction
the control of photorefractivity and the molecular design
of materials are quite different from that of the multi-
component system, since the required properties for the
photorefraction should be provided solely by a single
monolithic chromophore. Although the effect of NLO
chromophore on the photorefractive performance12,13
and the guideline for the molecular design of NLO
chromophore have been investigated very extensively
in multicomponent system,14,15 the structural effect of
monolithic photorefractive chromophore has seldom
been studied to date.
Photorefractive polymers have attracted considerable
attention due to their potential applications in the fields
of digital holographic data storage, image processing,
optical pattern recognition, holographic interferometry,
wavelength demultiplexing, and medical imaging.1,2 The
photorefractive effect involves the modulation of the
refractive index in an electrooptically active materials
due to the internal space-charge field originating from
the nonlocal redistribution of photogenerated charge
carriers. Therefore, the primary requirements for a
material to exhibit the photorefraction are photo-
conductivity and nonlinear optical (NLO) property in
principle.1,2 There are two different approaches to
incorporate these properties into organic materials: one
is a multicomponent system,3-6 and the other is a
multifunctional system.7-10 In the former, photocon-
ductivity and NLO property are provided by the indi-
vidual structural component, of which the simplest and
most successful examples are the composites based on
the photoconducting polymer, poly(N-vinylcarbazole)
(PVK), doped with NLO chromophores and appropriate
plasticizer molecules for orientational enhancement
effect.11-13
Even though many of the reported photorefractive
polymers so far are based on this system due to its
easier availability and processability, they are always
accompanied by the inherent problems, such as the
unavoidable phase separation which seriously limits
concentration of dopants and also the tradeoff of pho-
toconductivity and NLO property with composition.1,2
To overcome these problems, fully functionalized ma-
terials containing monolithic photorefractive chro-
mophore have been investigated by many research
groups,7-10 although the research in this area is often
restricted by the complicated chemical synthesis and
unpredictable final properties. In this system, however,
To elucidate the structural effect of monolithic chro-
mophore on the photorefractive properties, we synthe-
sized a homologous series of monolithic dipolar chro-
mophores containing carbazole and nitro groups as
push-pull elements. Five different conjugation bridges,
i.e., direct connection, azobenzene, stilbene, benzoxazole,
and cyanostilbene, were incorporated as structure vari-
ants of the chromophore units. In our previous work,
we have reported the synthesis and properties of
potentially photorefractive carbazole-nitro-based poly-
methacrylate. These multifunctional polymers showed
an excellent electrooptic (EO) property and photocon-
ductivity as expected from their structures.16,17 It was,
however, difficult to fabricate 100 µm thick films for
photorefractive and/or holographic measurement from
these high-Tg polymers due to their brittleness. To
overcome this limitation and to impart orientational
enhancement, low-Tg photorefractive polyacrylates con-
taining carbazole-nitro-based monolithic chromophore
were prepared in this work. Acrylate derivatives of these
chromophores were copolymerized with butyl acrylate
to give multifunctional photorefractive polymer with
glass transition temperature (Tg) near room tempera-
ture. Scheme 1 shows the chemical structures of the
monolithic chromophores and polymers synthesized in
this work. Fundamental molecular electronic param-
eters of the chromophores such as dipole moment,
polarizability, anisotropy, and hyperpolarizability were
evaluated by semiempirical quantum chemical calcula-
* Corresponding author: Tel +82-2-880-8327; Fax +82-2-886-
8331, +82-2-885-1748; e-mail parksy@plaza.snu.ac.kr.
10.1021/ma034104g CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/16/2003

Macromolecules, Vol. 36, No. 21, 2003
Multifunctional Photorefractive Polymers 7971
Sch em e 1. Ch em ica l Str u ctu r es of Mu ltifu n ction a l
Ch r om op h or es a n d P olym er s
Sch em e 2. Syn th esis of Mu ltifu n ction a l
Ch r om op h or es a n d Mon om er s
tion, and the photorefractive figure-of-merits of chro-
mophores were then estimated by the obtained param-
eters. The photoconductivity, NLO properties, and the
photorefractive properties of these polymers were mea-
sured and interpreted in terms of the structure-
property correlation in this paper. The nature of the
charge carrier transport in the multifunctional photo-
refractive polymer was also investigated by two-beam
coupling and time-of-flight techniques.
Exp er im en ta l Section
Ma ter ia ls. All chemicals were purchased from commercial
suppliers and used as received unless otherwise specified. The
initiator, 2,2′-azobis(isobutyronitrile) (AIBN) (Aldrich Chemi-
cal Co., 98%), and 2-amino-5-nitrophenol (Aldrich Chemical
Co., 90%) were recrystallized from methanol and ethanol,
respectively. Dichloromethane (J unsei Chemical Co.) used as
reaction solvent was dried over P2O5.
Syn th esis of Mon om er s a n d P olym er s. Synthetic path-
ways to the photorefractive chromophores and monomers are
outlined in Scheme 2. Synthesis of aldehyde derivatives 3, 4,
and compound 5 was reported earlier.7,18
6-(3-Nitr oca r ba zol-9-yl)-h exa n -1-ol (CzN). To a magneti-
cally stirred solution of compound 2 (1 g, 4.6 mmol) in
dichloromethane (50 mL) was added HNO3 (0.18 mL, 4.6
mmol) in dichloromethane (20 mL) dropwise at 0 °C. The
reaction mixture was stirred at room temperature for over-
night. After the solvent was evaporated under reduced pres-
sure, the product was washed with water and recrystallized
from ethanol to yield 0.93 g of yellow solid (Y ) 61%). 1H NMR
(CDCl3): δ 9.03 (d, 1H), 8.39 (q, 1H), 8.17 (d, 1H), 7.58 (m,
1H), 7.48 (m, 1H), 7.42 (m, 1H), 7.36 (m, 1H), 4.37 (t, 2H),
3.62 (t, 2H), 1.93 (m, 2H) 1.54 (m, 2H), 1.41 (m, 4H). IR (KBr
pellet, cm-1): 3304 (νOH), 1524, and 1316 (νnitro).
sium tert-butoxide (0.67 g, 5.98 mmol) at room temperature.
To this solution was added compound 3 (1.5 g, 4.27 mmol),
which was dissolved in 10 mL of THF. And then the reaction
mixture was heated to 80 °C and stirred for 1 h. To remove
the protecting acetyl group, potassium hydroxide (0.47 g, 8.54
mmol) dissolved in ethanol/water (10 mL/10 mL) was added,
and this mixture was stirred for another 1 h at 80 °C. After
the solution was cooled and the solvent was removed under
reduced pressure, the residue was poured onto the ice-cold
water and neutralized with 1 N HCl solution. The product was
extracted with dichloromethane and washed with plenty of
brine and water. After being dried over MgSO4, the solvent
was removed under reduced pressure. The crude product was
purified by column chromatography on silica gel with ethyl
acetate/n-hexane (vol ratio 1/1) as the eluent to obtain 1 g of
6-[3-(4-Nitrophenylazo)carbazol-9-yl]hexan-1-ol(CzAN).19
4-Nitroaniline (1.55 g, 11.2 mmol) was dissolved in a solution
of concentrated HCl (8 mL) in water (50 mL). The mixture
was cooled in an ice bath until temperature was below 4 °C.
Then a solution containing sodium nitrite (0.9 g, 13.44 mmol)
in water (10 mL) was added slowly to the above solution. The
mixture was allowed to stir in the ice bath for 1 h. While the
mixture was kept in the ice bath, the phase transfer catalyst,
sodium dodecyl sulfate (60 mg), was added. To this solution
was added compound 2 (1.5 g, 5.6 mmol) in dichloromethane
(50 mL), and then the resultant mixture was stirred vigorously
at room temperature for 48 h. The mixture was heated to
remove the dichloromethane layer. The red precipitate was
filtered, washed with water, and air-dried. The solid was
recrystallized from ethanol to yield 1.4 g of red solid (Y ) 60%).
1H NMR (CDCl3): δ 8.75 (d, 1H), 8.39 (d, 2H), 8.18 (m, 2H),
8.04 (d, 2H), 7.51 (m, 3H), 7.32 (m, 1H), 4.36 (t, 2H), 3.62 (t,
2H), 1.94 (m, 2H) 1.55 (m, 2H), 1.44 (m, 4H). IR (KBr pellet,
cm-1): 3339 (νOH), 1535, and 1327 (νnitro).
1
orange viscous liquid (Y ) 56%). H NMR (CDCl3): δ 8.28 (d,
1H), 8.23 (d, 2H), 8.15 (t, 1H), 7.68 (m, 3H), 7.52-7.40 (m,
4H), 7.28 (m, 1H), 7.19 (d, 1H, J ) 16), 4.33 (t, 2H), 3.63 (t,
2H), 1.91 (m, 2H) 1.65 (m, 2H), 1.43 (m, 4H). IR (KBr pellet,
cm-1): 3416 (νOH), 1504, and 1343 (νnitro).
6-[3-(6-Nit r ob en zooxa zol-2-yl)ca r b a zol-9-yl]h exa n -1-
ol (CzBN). To a boiling solution of compound 3 (2 g, 5.9 mmol)
in 1,2-dichlorobenzene (50 mL) was added 2-amino-5-nitro-
phenol (1.12 g, 6.5 mmol) under magnetic stirring. The reaction
mixture was boiled for 2 h, and then the solvent was distilled
off in a vacuum. The viscous residue was dissolved in 30 mL
of dichloromethane. Under magnetic stirring, lead(IV) acetate
(3.16 g, 7.13 mmol) was added in portions. The mixture was
6-{3-[2-(4-Nitr op h en yl)vin yl]ca r ba zol-9-yl}h exa n -1-ol
(CzSN). To a magnetically stirred solution of 5 (1.14 g, 4.27
mmol) in 10 mL of tetrahydrofuran (THF) was added potas-

7972 Hwang et al.
Macromolecules, Vol. 36, No. 21, 2003
Ta ble 1. P h ysica l P r op er ties of Mu ltifu n ction a l P h otor efr a ctive P olym er s
feed mole
molar ratio
in polymerb
absorption coeff
d
ratioa
yield (%)
Mn
PDI
Tg (°C)
λ
max (nm)c
(cm-1
)
PCzN
1:3
1:3
1:3
1:3
1:3
1:1.9
1:1.8
1:1.7
1:1.9
1:2.2
59
50
58
47
45
7900
7700
6300
4000
5800
1.9
1.7
2.8
1.5
1.2
10
24
28
24
23
374
384
414
418
434
15
12
16
14
29
PCzBN
PCzSN
PCzCSN
PCzAN
a
PR monomer: butyl acrylate. b Determined by element analysis. c Wavelength of absorption maximum estimated in chloroform solution.
d
Determined using polymer films containing 1 wt % of TNF at 633 nm.
stirred at room temperature for 1 h to give a solution with a
Acr ylic Acid 6-[3-(6-Nitr oben zooxa zol-2-yl)ca r ba zol-9-
yl]h exyl Ester (AcCzBN). AcCzBN was prepared from 0.8 g
(1.86 mmol) of CzBN, 0.23 mL (2.8 mmol) of acryloyl chloride,
and 0.79 mL (5.6 mmol) of triethylamine by the same proce-
dure as described for AcCzN (0.6 g, Y ) 67%). 1H NMR
(CDCl3): δ 9.04 (d, 1H), 8.49 (d, 1H), 8.35 (m, 2H), 8.22 (d,
1H), 7.81 (d, 1H), 7.55 (m, 2H), 7.47 (d, 1H), 7.35 (t, 1H), 6.38
(q, 1H), 6.11 (q, 1H), 5.81 (q, 1H), 4.37 (t, 2H), 4.13 (t, 2H),
1.95 (m, 2H), 1.66 (m, 2H), 1.45 (m, 4H). IR (KBr pellet, cm-1):
1715 (νCdO of ester), 1515, and 1343 (νnitro).
small amount of suspended solids. After filtration, the solution
was washed with water, and the solvent was removed under
reduced pressure. This crude product was used in the succes-
sive hydrolysis step. It was dissolved in ethanol (48 mL), and
potassium hydroxide (0.78 g, 13.9 mmol) dissolved in water
(18 mL) was added. After being stirred at reflux for 2 h, the
solution was cooled and the solvent was removed under
reduced pressure. The residue was poured onto the ice-cold
water and neutralized with 1 N HCl solution. Precipitate was
filtered and recrystallized from ethanol to yield 0.94 g of yellow
solid (37%). 1H NMR (CDCl3): δ 9.05 (d, 1H), 8.51 (d, 1H),
8.37 (m, 2H), 8.22 (d, 1H), 7.82 (d, 1H), 7.55 (m, 2H), 7.47 (d,
1H), 7.35 (t, 1H), 4.36 (t, 2H), 3.61 (t, 2H), 1.93 (m, 2H), 1.57
(m, 2H), 1.47 (m, 4H). IR (KBr pellet, cm-1): 3387 (νOH), 1506,
and 1334 (νnitro).
Acr ylic Acid 6-{3-[2-Cya n o-2-(4-n itr op h en yl)vin yl]ca r -
ba zol-9-yl}h exyl Ester (AcCzCSN). AcCzCSN was prepared
from 1.1 g (2.5 mmol) of CzCSN, 0.40 mL (5.0 mmol) of acryloyl
chloride, and 1.04 mL (7.5 mmol) of triethylamine by the same
1
procedure as described for AcCzN (0.85 g, Y ) 69%). H NMR
(CDCl3): δ 8.71 (d, 1H), 8.30 (d, 2H), 8.19 (t, 2H), 7.86 (m,
3H), 7.57-7.41 (m, 3H), 7.32 (m, 1H), 6.38 (q, 1H), 6.11 (q,
1H), 5.81 (q, 1H), 4.35 (t, 2H), 4.13 (t, 2H), 1.93 (m, 2H), 1.65
(m, 2H), 1.45 (m, 4H). IR (KBr pellet, cm-1): 2198 (νCN), 1715
3-[9-(6-H yd r oxyh e xyl)-9H -ca r b a zol-3-yl]-2-(4-n it r o-
p h en yl)a cr ylon itr ile (CzCSN). To the solution of compound
4 (1.2 g, 4.06 mmol) and 4-nitrophenylacetonitrile (0.72 g, 4.47
mmol) in 50 mL of ethanol was added piperidine (0.8 mL, 8.12
mmol) at 60 °C. The mixture was stirred at reflux for 2 h and
poured into water. The precipitate was collected by filtration
and washed with water. The product was purified by recrys-
tallization from ethanol to yield 1.1 g of orange solid (Y ) 62%).
1H NMR (CDCl3): δ 8.71 (d, 1H), 8.31 (d, 2H), 8.18 (t, 2H),
7.88 (m, 3H), 7.57-7.44 (m, 3H), 7.32 (m, 1H), 4.36 (t, 2H),
3.61 (t, 2H), 1.93 (m, 2H) 1.57 (m, 2H), 1.43 (m, 4H). IR (KBr
pellet, cm-1): 3359 (νOH), 2198 (νCN), 1515, and 1343 (νnitro).
Acr ylic Acid 6-(3-Nitr oca r ba zol-9-yl)h exyl Ester (Ac-
CzN). To a magnetically stirred solution of CzN (1.3 g, 4.2
mmol) and triethylamine (1.74 mL, 12.49 mmol) in dichlo-
romethane (20 mL), acryloyl chloride (0.66 mL, 8.3 mmol) was
added dropwise at 0 °C. After stirring for 1 h, the resulting
solution was allowed to warm to room temperature. After
another 1 h, the solution was washed with brine and water
and dried over MgSO4, and the solvent was removed under
reduced pressure. The yellow residue was purified by silica
gel column chromatography (ethyl acetate/dichloromethane )
(νCdO ester), 1515, and 1334 (νnitro).
of
P olym er iza tion . Photorefractive polymers were obtained
by free radical copolymerization of monolithic photorefractive
monomers and butyl acrylate with feed mole ratio of 1:3. A
typical polymerization procedure is as follows: 1.0 g (2.72
mmol) of AcCzN, 1.05 g (8.16 mmol) of butyl acrylate, and
0.089 g (0.55 mmol) of AIBN were dissolved in 10 mL of
N-methylpyrrolidinone (NMP). The solution was degassed by
standard vacuum-freeze-thaw technique. After sealing the
degassed ampule, the reaction mixture was heated at 65 °C
for 48 h. After cooling, the resulting solution was diluted to
twice its original volume with THF and poured into cold
methanol to precipitate the polymer PCzN, which was purified
by silica gel column chromatography (dichloromethane and
THF) (1.2 g, 59%). Compositional mole ratios in obtained
polymers were determined by element analysis and sum-
marized in Table 1.
Ch a r a cter iza tion . 1H NMR spectra were recorded with the
use of a J EOL J NM-LA300 spectrometer. IR spectra were
measured with KBr pellet or KBr windows on a Bomen FT-
IR spectrophotometer. A HP 8452-A spectrophotometer was
used for the UV-vis absorption spectra. Gel permeation
chromatography (GPC) was performed at a flow rate of 1.0
mL/min in THF at 30 °C with a Waters HPLC component
system equipped with five Ultra-µ-styragel columns (2 × 105,
105, 104, 103, 500 Å), which was calibrated with polystyrene
standards. Differential scanning calorimetry (DSC) was carried
out under a nitrogen atmosphere at a heating rate of 20 °C/
min on a Perkin-Elmer DSC7.
The polymer films with 1 wt % 2,4,7-trinitrofluorenone
(TNF) sandwiched between two indium-tin oxide (ITO) cov-
ered glasses were prepared by casting from 20 wt % dichlo-
romethane solution. The thickness of the films was maintained
at 12 µm (for TOF), 25 µm (for photoconductivity measure-
ment), and 100 µm (for NLO response and photorefractivity
measurement) with the polyimide or Teflon spacer.
The dark and photoconductivity of the polymer films were
evaluated by measuring a current through the polymer films
between ITO glasses using the Keithley 6517 electrometer and
633 nm of He-Ne laser as light source.
Charge carrier mobilities of polymers were measured via
the conventional time-of-flight (TOF) technique. The current
induced by the laser pulse was transformed to a voltage, which
was subsequently amplified and monitored with an oscil-
1
1/15) to yield 1.6 g of yellow solid (70%). H NMR (CDCl3): δ
9.03 (d, 1H), 8.39 (q, 1H), 8.17 (d, 1H), 7.58 (m, 1H), 7.48 (m,
1H), 7.42 (m, 1H), 7.36 (m, 1H), 6,39 (q, 1H), 6.09 (q, 1H), 5.82
(q, 1H), 4.37 (t, 2H), 4.12 (t, 2H), 1.90 (m, 2H) 1.65 (m, 2H),
1.41 (m, 4H). IR (KBr pellet, cm-1): 1724 (νCdO ester), 1504,
of
and 1316 (νnitro).
Acr ylic Acid 6-[3-(4-Nit r op h en yla zo)ca r b a zol-9-yl]-
h exyl Ester (AcCzAN). AcCzAN was prepared from 0.9 g (2.2
mmol) of CzAN, 0.34 mL (4.3 mmol) of acryloyl chloride, and
0.9 mL (6.5 mmol) of triethylamine by the same procedure as
described for AcCzN (0.8 g, Y ) 77%). 1H NMR (CDCl3): δ
8.76 (d, 1H), 8.39 (d, 2H), 8.18 (m, 2H), 8.05 (d, 2H), 7.51 (m,
3H), 7.33 (m, 1H), 6.38 (q, 1H), 6.12 (q, 1H), 5.81 (q, 1H), 4.37
(t, 2H), 4.13 (t, 2H), 1.95 (m, 2H) 1.63 (m, 2H), 1.45 (m, 4H).
IR (KBr pellet, cm-1): 1718 (νCdO of ester), 1524, and 1339 (νnitro).
Acr ylic Acid 6-{3-[2-(4-Nitr op h en yl)vin yl]ca r ba zol-9-
yl}h exyl Ester (AcCzSN). AcCzSN was prepared from 1.0 g
(2.4 mmol) of CzSN, 0.38 mL (4.8 mmol) of acryloyl chloride,
and 1.0 mL (7.2 mmol) of triethylamine by the same procedure
1
as described for AcCzN (0.8 g, Y ) 71%). H NMR (CDCl3): δ
8.28 (d, 1H), 8.23 (d, 2H), 8.14 (d, 1H), 7.68 (m, 3H), 7.52-
7.40 (m, 4H), 7.28 (m, 1H), 7.19 (d, 1H, J ) 16), 6.38 (q, 1H),
6.12 (q, 1H), 5.80 (q, 1H), 4.33 (t, 2H), 4.12 (t, 2H), 1.91 (m,
2H) 1.65 (m, 2H), 1.43 (m, 4H). IR (KBr pellet, cm-1): 1713
(νC of ester), 1504, and 1332 (νnitro).
dO

Macromolecules, Vol. 36, No. 21, 2003
Multifunctional Photorefractive Polymers 7973
Ta ble 2. Ca lcu la ted Op tica l P a r a m eter s a n d F igu r e-of-Mer its of Mon olith ic Ch r om op h or es
µ
∆R
â
M
(g/mol)
Fbr (10-77 C2 m4
Feo (10-77 C2 m4
(10-30 C m)
(10-40 C m2 V-1
)
(10-50 C m3 V-2
)
mol V-2 kg-1
)
mol V-2 kg-1
)
CzN
CzBN
CzSN
CzCSN-twisted
CzCSN-planar
CzAN
25.12
25.21
25.64
23.16
24.48
25.66
45.24
89.66
101.95
84.83
98.81
87.34
2.42
10.32
12.52
9.69
14.99
9.37
236.23
343.34
328.37
353.38
353.38
330.34
594.0
815.4
1003
632.6
823.2
856.6
2.43
6.82
8.82
5.76
9.34
6.56
loscope. Then, the mobility was calculated from a transit time
ized with butyl acrylate by free radical polymerization.
Because of the higher extent of chain transfer in the
radical polymerization of acrylate monomer,24 the ob-
tained polymers exhibited a relatively lower molecular
weight compared to that of the polymethacrylate ana-
logues.16,17 However, polyacrylates were much softer
than polymethacrylate due to the absence of the methyl
groups on alternating carbons of the polymer chains.25
Therefore, all of the obtained polyacylates showed the
glass transition temperature around room temperature,
and the transparent thick films with high optical quality
could be fabricated for the measurements despite their
low molecular weight. Molecular weight and physical
properties of obtained polymers are summarized in
Table 1.
t
T through the relation µh,e ) d2/(tTV), where d is the thickness
of the sample and V is the voltage applied.20 A nanosecond
single pulse of 355 nm light was used to optically generate
the charge carriers at the applied voltage of 400 V.
The NLO response was investigated by the ellipsometric
technique described in the literature21,22 to determine the total
contribution of the birefringence effect, Pockels effect, and Kerr
effect to the overall electric field-induced refractive index
changes generated in a low-Tg polymer composite. The voltage
applied to the sample was the superposition of a dc component
VB and a modulated ac component V(Ω) ) VM sin Ω. In this
experiment, VB was 4 kV, and VM was 600 V whose modulation
frequency Ω was 1000 Hz, at which the NLO response was
governed by both the Pockels effect and birefringence contri-
bution.23
Photorefractivity measurements were conducted with a
conventional two-beam coupling and degenerated four-wave
mixing technique with a 633 nm He-Ne laser. In the two-
beam coupling, p-polarized beams with the same intensity of
60.7 mW/cm2 were intersected in the samples. For the four-
wave mixing, two s-polarized beams with the intensity of 60.7
mW/cm2 were used as writing beams, and a p-polarized
reading beam with the intensity of 1.27 mW/cm2 counter-
propagated to one of writing beams. The normal of the sample
surface was tilted 60° with respect to the symmetric axis of
the two intersected beams, and the external interbeam angle
was 11°.
Non lin ea r Op tica l P r op er ties a n d P h otocon d u c-
tivity. To predict NLO properties of the monolithic
photorefractive chromophores, the dipole moment, po-
larizability anisotropy (∆R), and hyperpolarizability (â)
were calculated by the semiempirical method, MOPAC
97 with PM3 procedure for geometry optimization in the
ground state. For simplicity of calculation, the hydroxy-
hexyl group in chromophore was substituted by the
methyl group.26 The figure-of-merit of photorefractive
chromophore is estimated by the following equation:1
9µâ 2µ∆R
Resu lts a n d Discu ssion
F ) Feo + Fbr
)
+
(1)
M
kTM
Syn th esis of Ch r om oph or es an d P olym er s. Mono-
lithic photorefractive chromophores and monomers were
successfully synthesized via a synthetic route shown in
Scheme 2. CzN and CzAN were obtained by nitration
and diazo coupling reaction of compound 2, respectively.
The general procedure of diazotization and diazo cou-
pling are progressed in the aqueous phase, but CzAN
was synthesized in the heterogeneous phase with phase
transfer catalyst, sodium dodecyl sulfate, since the
carbazole derivative 2 is insoluble to aqueous phase.19
CzSN, CzBN, and CzCSN were obtained by the Wads-
worth-Emmons reaction, oxidative ring closing, and the
Knoevenagel reaction from aldehyde derivatives 3 and
4. Their chemical structures were identified by FT-IR
where M is the molecular weight of the chromophore.
The first part of the numerator of eq 1 corresponds to
the electrooptic (EO) contribution, and the second part
corresponds to the birefringence contribution. The cal-
culated molecular electronic parameters (µ, ∆R, and â)
and photorefractive figure-of-merits are summarized in
Table 2.
The values of Fbr are 2 orders of magnitude larger
than those of Feo, which means that the contribution of
birefringence will play a major role in the refractive
index modulation via the orientation enhancement
effect, since the glass transition temperatures of the
obtained polymers are around room temperature. When
a cyano group is introduced into the stilbene, planarity
of the conjugated architecture in the optimized geometry
is remarkably reduced due to the steric repulsion, and
this distortion from planarity causes to reduce the
extent of π-orbital conjugation and thus reduced µ, ∆R,
and â values compared to those of CzSN. However, it is
well-known that the twisted structure due to the steric
repulsion in isolated gas phase recovers its planar
structure when it is solidified, and this planarization
leads to extend the effective conjugation length and
change the optical properties.27 In this aspect, we
assumed the planar structure of CzCSN and calculated
its electric parameters and figure-of-merit value which
was comparable to that of the other chromophores.
When the stilbene moiety was replaced by the het-
eroaromatic benzoxazole group (CzSN f CzBN), the
1
and H NMR spectroscopy. In all the chromophores, the
stretching vibration of nitro group was observed at
∼1500 and ∼1300 cm-1, and the stretching band of
nitrile group in CzCSN was identified at 2198 cm-1 in
FT-IR spectra. The trans structure of the stilbene group
in CzSN was also identified by the peaks at 7.2 ppm
with the coupling constant of 16 Hz in the 1H NMR
spectrum. Acrylate monomers were easily prepared by
the reaction of chromophores and acryloyl chloride,
which were confirmed by three vinyl protons at ∼6.3,
∼6.1, and ∼5.8 ppm in 1H NMR spectra and the
absorption peaks of carbonyl group at ∼1720 cm-1 in
FT-IR spectra.
To obtain the photorefractive polymers with the low
glass transition temperature, acrylate monomers con-
taining these monolithic chromophores were copolymer-

7974 Hwang et al.
Macromolecules, Vol. 36, No. 21, 2003
enhanced aromaticity in the conjugation bridge leads
to decrease ∆R and â. This can be explained by the fact
that additional aromatic delocalization energy limits the
excitation, i.e., the intermolecular charge separation and
then excitation of the benzoxazole chromophore require
a higher energy than that of stilbene chromophore
which was confirmed by the blue-shifted absorption
maximum (λmax) of CzBN compared to that of CzSN.
According to the two-level model, ∆R and â values
decease as λmax deceases; therefore, the blue-shifted λmax
of CzBN reduced its figure-of-merit value.14,26
It is noted that, in the multifunctional photorefractive
materials bearing the monolithic chromophore, the
figure-of-merit of photorefractive chromophore could not
be considered as the actual degree of photorefractivity,
since the formation of space-charge field and the refrac-
tive index modulation are influenced by the chromo-
phore structure in this system. In other words, the
photoconductive properties of monolithic chromophore,
i.e., photogeneration, transport, and trap of charge
carrier, were also important factors to determine the
photorefractivity. In addition, the birefringence effect
originating from the reorientation of chromophore which
is the major contribution to figure-of-merit is strongly
dependent on the free volume of polymer matrix and
Tg. So in this work, the figure-of-merit was just one of
the guidelines to predict the NLO properties of chro-
mophores, and the obtained results confirmed that the
monolithic chromophores in this work have sufficient
NLO properties compared to the reported NLO chro-
mophores1 and thus good candidates for the photore-
fractive molecular materials.
F igu r e 1. Effective EO coefficients as a function of applied
field.
The macroscopic NLO properties of photorefractive
polymers were measured by the ellipsometric technique.
In this technique, the total electric field applied across
the sample is composed of a dc and ac electric field. The
dc field induced a permanent chromophore orientation
and consequently establishes the noncentrosymmetry
required for the second-order NLO properties, and the
ac electric field was imposed to probe the magnitude of
NLO response. Since the photorefractive polymers
studied in this work showed the glass transition around
the room temperature, these NLO responses were
affected by the combined contribution of birefringence
effect and the simple electrooptic effect (Pockels effect)
according to the modulated ac voltage. The effective EO
coefficient of the samples was estimated by following
equation:21,22
F igu r e 2. Photoconductivity of the multifunctional photore-
fractive polymers with 1 wt % TNF as a function of applied
field. The inset shows the dark currents.
The photoconductive properties of the polymer films
were estimated by measuring photocurrent and dark
current through the polymer films, and the electric field
dependence of the photoconductivity is shown in Figure
2. As expected, PCzBN containing the fused heteroaro-
matic benzoxazole moiety, which is a well-known photo-
conductor, as conjugation bridge exhibited the highest
photoconductivity,28 and the strong absorption of PCz-
AN at 633 nm helped efficient charge carrier generation
and led to the higher photoconductivity.
P h otor efr a ctivity. To determine the photorefrac-
tivities of the polymers, the two-beam coupling and four-
wave mixing experiments were performed. The two-
beam coupling gain was calculated from the asymmetric
energy transfer between the two incident beams by the
following equation:
2
2
3λIm
πn3IiVm
x
n n - sin θ
reff
)
(2)
sin2 θ
where Ii is half the maximum intensity, Im is the
modulated intensity, n is the refractive index of materi-
als, Vm is the applied ac voltage, λ is the wavelength of
laser, and θ is the incident angle between laser beam
and the sample. As shown in Figure 1, the effective EO
coefficients were on the order of PCzAN > PCzCSN >
PCzN > PCzSN > PCzBN, although the calculated
figure-of-merit values were on the order of CzSN >
CzAN > CzCSN(planar) > CzBN > CzN. This different
tendency could be ascribed to the different Tg of poly-
mers, since the motion of chromophore and the contri-
bution of birefringence are strongly dependent on Tg;
i.e., the higher Tg of PCzSN and the lower Tg of PCzN
led to decrease and increase the birefringence contribu-
tion to NLO response of the samples, respectively.23
1
Γ )
[ln(γ0δ) - ln(δ + 1 - γ0)]
(3)
L/cos θ
where L/cos θ (L ) sample thickness) is the beam path
length, δ is the ratio of beam intensities, and γ0 ) P/P0
is the beam coupling ratio (P0 is the signal beam
intensity without the pump beam, and P is the signal
beam intensity with the pump beam).
The calculated gain coefficients are shown in Figure
3. The positive and negative values of gain represent
the opposite direction of energy transfer; i.e., in the
samples with the positive gain, the energy of beam 2

Macromolecules, Vol. 36, No. 21, 2003
Multifunctional Photorefractive Polymers 7975
F igu r e 4. Internal diffraction efficiency as a function of
applied field. Solid lines are guides to the eyes
F igu r e 3. Two-beam coupling gain coefficients as a function
of applied field. The positive and negative sign of gain
coefficients represent the opposite direction of asymmetric
energy transfer (see text). Solid lines are guides to the eyes.
Ta ble 3. Hole a n d Electr on Mobility Mea su r ed a t 33.3
V/µm of Ap p lied Electr ic F ield
PCzN PCzBN PCzSN PCzCSN PCzAN
electron mobility,
1.55
1.49
0.06
1.46
1.47
1.38
0.09
1.35
1.19
0.16
1.49
1.42
0.07
µe (10-5 cm2/(V s))
hole mobility,
2.06
µh (10-5 cm2/(V s))
µe - µh
-0.5
(10-5 cm2/(V s))
was transferred to beam 1, while the energy of beam 1
was transferred to beam 2 in the sample with negative
gain in our experiment setup. It is well-known that this
different direction of energy transfer originates from
either the nature of chromophore29 or the characteristics
of charge carrier.30 First, to investigate the effect of the
nature of chromophore, the sign of the field induced
refractive index anisotropy was determined by compar-
ing the retardations induced by a Soleil-Babinet com-
pensator in the dc ellipsometric technique as described
in ref 29. All of our polymers showed identical signs of
birefringence with the compensator modulation, which
indicated that the direction of energy transfer was
governed solely by the characteristics of charge carriers,
i.e., whether the major charge transporting species for
the internal space-charge field buildup is hole or elec-
tron.30 To determine which is the major charge trans-
porting species, the electron and hole mobilities of the
polymer were measured by the TOF technique, and the
obtained results and the difference of electron and hole
mobility are summarized in Table 3. In the case of
PCzN, PCzSN, PCzAN, and PCzCSN which show the
positive gain, the electron mobility was higher than the
hole mobility. On the other hand, PCzBN with the
negative gain exhibited the faster hole mobility than
electron mobility. Consequently, the electron is the
major charge transporting species in the materials with
the positive gain, and the hole is the major one in the
materials with the negative gain in our experiment
setup. From these results, it could be said that when
the strong electron-withdrawing group was incorporated
into the monolithic photorefractive chromophore, the
electron mobility exceeds the hole mobility and the
electron plays a major role in the formation of internal
space-charge field, even though the monolithic chro-
mophores consist of the carbazole moiety which is well-
known as the hole transporting material. Refractive
index grating induced by hole transporting was observed
F igu r e 5. Effective EO coefficients (external electric field E0
) 45 V/µm) vs diffraction efficiency (E0 ) 50 V/µm) and gain
coefficients (E0 ) 50 V/µm).
only in PCzBN, since its hole mobility was strongly
enhanced due to the fused heteroaromatic benzoxazole
moiety,28 and it was consistent with the result from the
photoconductivity measurement. This result means that
the nature of the charge carrier transporting for the
formation of internal space-charge field in monolithic
chromophore system was dependent on the chromo-
phore structure and could be controlled by its modifica-
tion.
The diffraction efficiency obtained by the following
equation is shown in Figure 4:
ηint ) intensity of the diffracted beam/
(intensity of the transmitted beam +
intensity of the diffracted beam) (4)
Since the ηint is the ratio of intensity of the diffracted
beam to the sum of intensity of the transmitted beam
and the diffracted beam, the loss originating from the
absorption and reflection in surface of the glass and
polymer film was excluded. The photorefractivities, i.e.,
gain coefficients and diffraction efficiency, were on the
order of PCzAN > PCzCSN > PCzN > PCzSN > PCzBN
as the NLO response was, while there was no direct
correlation between the photorefractivity and the pho-
toconductivity. This correlation between the photore-
fractivity and the NLO property is shown in Figure 5.
In the other words, it could be said that the photore-
fractivities of the obtained polymers were strongly
dependent on the NLO property rather than photocon-
ductivity within the result of our experiments. Deviant

7976 Hwang et al.
Macromolecules, Vol. 36, No. 21, 2003
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to the lower Tg compared to that of the other polymers.
It was reported that Tg below the measurement tem-
perature causes the thermal induced change of the
conformational trap, and the charge carriers trapped in
such a conformational trap can be released more easily
due to the thermal motion. Therefore, the increase of
ratio of dark conductivity to photoconductivity leads to
reduce internal space-charge field and consequently the
decrease of the photorefractivity. It was confirmed by
the large dark current of PCzN (inset of Figure 2).31,32
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Con clu sion
In this work, the multifunctional photorefractive
polyacrylates with the monolithic chromophores were
synthesized. These photorefractive chromophores con-
sist of the carbazole unit as an electron-donating moiety
connected with the electron-withdrawing nitro group via
the different conjugation bridges. The correlation be-
tween the photorefractivity and the NLO property
showed that the photorefractivity of the obtained poly-
mers was strongly dependent on the NLO property
rather than photoconductivity within the result of our
experiments. The opposite direction of asymmetric
energy transfer in the two-beam coupling measurement
indicated that the charge transporting species in mul-
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