Synthesis, characterization, photo-induced alignment, and surface orientation of poly(9,9-dioctylfluorene-alt-azobenzene)s

Article abstract of DOI:10.1002/pola.26338

Three types of bi-functionalized copolymers (P1FAz, P2FAz, and P3FAz) with different numbers of fluorene units and an azobenzene unit were synthesized and characterized using UV-vis and polarized absorption spectroanalysis. The trans-cis photoisomerization was conformed under 400 nm light irradiation for all copolymers in chloroform. However, in the film state, only the trans-cis photoisomerization occurred by mono-fluorene attached copolymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,4′-azobenzene)] (P1FAz). Photo-induced alignment was achieved using the P1FAz film after irradiation with linear polarized 400 nm light and subsequent annealing at 60 °C. Surface orientation of a spin-coating film of poly(9,9-didodecylfluorene) (F12) was achieved using the photo-induced alignment layer of the P1FAz film after annealing at 90°C. The photo-induced alignment layer of P1FAz has potential application to the surface orientation technique for appropriate polymers, which will be useful for the fabrication of optoelectronics devices.

Full text of DOI:10.1002/pola.26338

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Synthesis, Characterization, Photo-Induced Alignment, and Surface
Orientation of Poly(9,9-dioctylfluorene-alt-azobenzene)s
Kenji Kinashi,1 Yuki Kambe,2 Masahiro Misaki,2 Yasuko Koshiba,2
Kenji Ishida,2 Yasukiyo Ueda2
1Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo,
Kyoto 606-8585, Japan
2Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko,
Nada, Kobe 657-8501, Japan
Correspondence to: Y. Ueda (E-mail: yueda@kobe-u.ac.jp)
Received 8 June 2012; accepted 12 August 2012; published online
DOI: 10.1002/pola.26338
ABSTRACT: Three types of bi-functionalized copolymers (P1FAz,
P2FAz, and P3FAz) with different numbers of fluorene units
and an azobenzene unit were synthesized and characterized
using UV–vis and polarized absorption spectroanalysis. The
trans-cis photoisomerization was conformed under 400 nm
light irradiation for all copolymers in chloroform. However, in
the film state, only the trans-cis photoisomerization occurred
by mono-fluorene attached copolymer poly[(9,9-di-n-octylfluor-
60 ꢀC. Surface orientation of a spin-coating film of poly(9,9-
didodecylfluorene) (F12) was achieved using the photo-induced
alignment layer of the P1FAz film after annealing at 90 ꢀC. The
photo-induced alignment layer of P1FAz has potential applica-
tion to the surface orientation technique for appropriate poly-
mers, which will be useful for the fabrication of optoelectronics
C
V
devices.
2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
enyl-2,7-diyl)-alt-4,40-azobenzene)]
(P1FAz).
Photo-induced
alignment was achieved using the P1FAz film after irradiation
KEYWORDS: azo polymers; functionalization of polymers; orien-
with linear polarized 400 nm light and subsequent annealing at
tation; photochemistry; synthesis
INTRODUCTION The static and dynamic properties of liquid
crystals are strongly affected by the nature of the film sur-
face alignment. Mechanically rubbed polyimide films have
been commercially used as alignment films for liquid crystal
molecules.1,2 However, the rubbing process of alignment
induces several defects, such as electrostatic charges or dust
generation. Therefore, the photo-induced alignment tech-
nique has received considerable attention as a rubbing-free
and pattern-able process.3 The photo-induced alignment of
liquid crystals with the use of photosensitive materials based
on trans-to-cis photoisomerization,4–8 photocrosslinking
involving photodimerization,9–11 and photodissociation12 has
been reported. Some of the photo-induced alignment materi-
als exhibit reversible liquid crystal alignment regulation. For
example, a functional polymer based on a photochromic
compound can reversibly control liquid crystal alignment
triggered by trans-cis photoisomerization. Therefore, photo-
induced alignment based on trans-cis photoisomerization by
irradiation with linear polarized light is one of the most
attractive candidate techniques for surface control, which
can archive a surface orientation with respect to the next
upper layer. Azobenzenes are well-known as photosensitive
chromophores that undergo trans-cis photoisomerization
upon irradiation with light of an appropriate wavelength in a
solution or in an appropriate host matrix.13–16 Repetition of
such photoisomerization in a host matrix leads to perpendic-
ular alignment of the long axis of the azobenzene chromo-
phore to the electric field vector of the linear polarized light,
which eventually induces anisotropy. Therefore, azobenzene-
functionalized polymers have attracted considerable
attention with anisotropy induced by linear polarized light
irradiation below the glass-transition temperature.17,18
Poly(9,9-dioctylfluorene) is a polymer that forms good align-
ment layers that exhibit a high dichroic ratio at an absorb-
ance of ꢁ10 using a friction-transfer technique and subse-
quent annealing below the glass-transition temperature.19
Furthermore, a highly orientated 9,9-dioctylfluorene film has
been used to successfully fabricate highly polarized light-
emitting diodes.20 Thus, a bi-functionalized copolymer based
on 9,9-dioctylfluorene and azobenzene moieties would have
performance advantages with a significant promise for sur-
face orientation.
In this article, we report the synthesis and photochemical
characteristics of three different poly(9,9-dioctylfluorene-alt-
C
V
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Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,40-
azobenzene)] (P1FAz)
Compound 1 (80 mg, 0.19 mmol), 9,9-dioctylfluorene-2,7-
diboronic acid bis(1,3-propanediol)ester (100 mg, 0.18
mmol), and (PPh3)4Pd(0) (2 mg) were dissolved in a mixture
of toluene (3 mL) and K2CO3 (2 M, 1.8 mL) undꢀer an argon
atmosphere. The solution was maintained at 70 C with vig-
orous stirring for 45 h and then poured into a large amount
of cold methanol. The precipitated crude product was sepa-
rated twice by filtration to remove oligomers and catalysts
residues. The polymer was obtained as a yellow solid (93
mg, 0.17 mmol, 94%).
FIGURE 1 Chemical structure of poly(9,9-dioctylfluorene)
copolymers.
azobenzene) copolymers (Fig. 1) and their potential for use
as alignment layers.
EXPERIMENTAL
mp: decomposed at 292 ꢀC. Mw: 17,000 g molꢂ1, Mw/Mn
¼
1.7. ATR-FTIR (cmꢂ1): 2924, 2851, 1598, 1464, 1406, 1340,
1158, 1010, 854, 819, 745.
The chemical structures of the synthesized compounds were
characterized by 1H nuclear magnetic spectroscopy (NMR;
Bruker Advance500), CDCl3 solvent, tetramethylsilane (inter-
nal standard), and time-of-flight mass spectrometry (PerSep-
tive Biosystems Mariner). Melting points (mp) were deter-
mined using a melting point apparatus (Yanaco MP-500P)
with a digital thermometer. UV–vis absorption and photolu-
minescence (PL) spectra were recorded on Jasco V-670 and
Hitachi F-2500 spectrophotometers, respectively. The relative
fluorescence quantum yields (Uf) were measured using a PL
spectrophotometer (Hitachi F-2500, anthracene or pyrene in
ethanol as common standards). The morphologies of the
films were characterized by atomic force microscopy (AFM;
Seiko Instruments SPI-3700). The photoisomer was obtained
by irradiation with light from a Xe lamp (Asahi spectra MAX-
301, 300 W) passed through a 400 nm band pass filter. The
irradiation intensity was 20 mW cmꢂ2. The light from a Xe
lamp was passed through a band pass filter and a polarizing
to produce a parallel beam of linear polarized light with a
wavelength in the vicinity of 400 nm (10 J cmꢂ2). Absorp-
tion and PL spectral measurements were performed in chlo-
roform using in a quartz cell with a path length of 1 cm.
Films were spin-coated from 0.025 wt % chloroform solu-
tions onto a fused silica substrate at 1500 rpm for 30 s.
2-Bromo-9,9-di-n-octylfluorene (2)
1-Octylbromide (3.5 g, 18.0 mmol) were slowly added to a
suspension containing 2-bromofluorene (2.0 g, 8.2 mmol),
benzyltriethylammonium chloride (96 mg, 0.42 mmol), aque-
ous NaOH solution (3.5 mL, 50 wt %), and sieve-dried di-
methyl sulfoxide (DMSO; 16 mL). The mixture was vigo-
rously stirred at 35 ꢀC for 7.5 h and then extracted with
diethyl ether. The organic layer was washed with water,
dilute hydrochloric acid, and brine, and the resulting organic
layer was dried over anhydrous MgSO4 and concentrated in
vacuo. The resulting crude red oil was purified by column
chromatography using hexane as an eluent to give 2 as a
pale yellow oil (3.1 g, 6.6 mmol, 80%). Rf: 0.7 (for hexane).
1H NMR (500 MHz, CDCl3): d (ppm) 7.65 (m, 1H), 7.54 (d,
2H), 7.43 (m, 2H), 7.32 (m, 3H), 1.95ꢂ1.84 (m, 4H),
1.45ꢂ1.04 (m, 20H), 0.88ꢂ0.80 (m, 6H), 0.60ꢂ0.58 (m, 4H).
9,9,90,90-Tetra-n-octyl-2,20-bifluorene (3)
n-Butyllithium (n-BuLi) in hexane (1.6 M, 2.2 mL) was added
slowly to a solution of 2 (1.5 g, 3.3 mmol) in tetrahydrofuran
(THF; 10 mL) at ꢂ78 ꢀC under an argon atmosphere and
stirred for 1 h. Trimethyl borate (1 mL) was added to the
ꢀ
mixture at ꢂ78 C, which was then warmed slowly to room
SYNTHESIS
temperature with continuous stirring for 5 h. The mixture
4,40-Diiodoazobenzene (1)
ꢀ
was then cooled slowly to 0 C, and dilute hydrochloric acid
A solution of KOH (3.7 g, 66 mmol) in methanol was slowly
added to a solution of aluminum powder (0.51 g, 18.8
mmol) and 4-iodonitirobenzene (2.45 g, 9.8 mmol) in metha-
nol (6 mL), and the mixture was refluxed by stirring for 4 h
under an argon atmosphere. The reaction mixture was
cooled to room temperature and the precipitated crude
product was separated by filtration, as well as then washed
with dichloromethane and dilute hydrochloric acid to main-
tain the pH at 4. The filtered solution was extracted with
water, and the organic layer was dried over anhydrous
MgSO4 and concentrated in vacuo. The crude product was
sublimed onto a cold finger to give 1 as orange plate-like
crystals (0.72 g, 1.66 mmol, 17%).
(3 mL) was added to the mixture. The mixture was extracted
with diethyl ether and the resulting organic layer was dried
over anhydrous MgSO4. Concentration in vacuo gave the cor-
responding 9,9-di-n-octylfluorene-2-boronic acid, which with-
out further purification due to instability, was mixed with 2
(1.5 g, 3.3 mmol) in toluene (10 mL) under a nitrogen
atmosphere. The mixture was then added to a solution of
aqueous K2CO3 (2.0 M, 5 mL) and Pd(PPh3)4(0) (13 mg),
which was stirred under reflux at 75 ꢀC for 40 h. The mix-
ture was extracted with diethyl ether and washed with dilute
hydrochloric acid, and the resulting organic layer was dried
over anhydrous MgSO4 and concentrated in vacuo. The crude
product was purified by column chromatography using hex-
ane/dichloromethane (10/1) as an eluent to give 3 as a pale
blue oil (1.0 g, 1.3 mmol, 39%). Rf: 0.2 (for hexane).
mp: 236ꢂ240 C. Rf: 0.9 (for chloroform). ATR-FTIR (cmꢂ1):
ꢀ
1573, 1558, 1468, 1392, 1296, 1096, 1049, 1000, 832, 713.
DI-MS: m/z 433 [Mþ]. 1H NMR (500 MHz, CDCl3): d (ppm)
7.64 (d, 4H, J ¼ 8.5 Hz), 7.86 (d, 4H, J ¼ 8.5 Hz).
1H NMR (500 MHz, CDCl3): d (ppm) 7.76 (d, 2H, J ¼ 7.5 Hz),
7.73 (d, 2H, J ¼ 7.5 Hz), 7.65ꢂ7.61 (m, 4H), 7.37ꢂ7.31 (m,
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6H), 2.04ꢂ2.00 (m, 8H), 1.27ꢂ1.07 (m, 40H), 0.80 (t, 12H),
0.72ꢂ0.70 (m, 8H).
1H NMR (500 MHz, CDCl3): d (ppm) 7.82ꢂ7.62 (m, 14H),
7.37ꢂ7.31 (m, 4H), 7.18ꢂ7.16 (m, 2H), 2.11ꢂ2.01 (m, 12H),
1.30ꢂ1.08 (m, 72H), 0.90ꢂ0.71 (m, 18H).
7,70-Dibromo-9,9,90,90-tetra-n-octyl-2,20-bifluorene (4)
A solution of 3 (1.0 g, 1.3 mmol) in dichloromethane (1 mL)
was added to a solution of bromine (1.0 g, 6.3 mmol) in
dichloromethane (3 mL), and the mixture was refluxed with
stirring for 1.5 h under an argon atmosphere. The mixture
was poured into water and washed with sodium thiosulfate
until the red color disappeared. The combined organic layer
was dried over anhydrous MgSO4 and concentrated in vacuo
to give 4 as a pale yellow oil (1.2 g, 1.3 mmol, ca. 100%). Rf:
0.8 (for hexane).
7,700-Dibromo-9,9,90,90,900,900-hexa-n-octyl-2,20-70,
200-terfluorene (6)
A solution of 5 (0.7 g, 0.6 mmol) in dichloromethane (1 mL)
was added to a solution of bromine (0.25 g, 1.6 mmol) in
dichloromethane (2 mL) and the mixture was refluxed with
stirring for 1.5 h under an argon atmosphere. The mixture
was poured into water and washed with sodium thiosulfate
until the red color disappeared. The combined organic layer
was dried over anhydrous MgSO4 and concentrated in vacuo
to give 6 as a pale yellow oil (0.8 g, 0.6 mmol, ca. 100%). Rf:
0.5 (for hexane).
1H NMR (500 MHz, CDCl3): d (ppm) 7.57ꢂ7.43 (m, 12H),
2.04ꢂ1.89 (m, 8H), 1.26ꢂ1.07 (m, 40H), 0.83 (t, 12H),
0.69ꢂ0.61 (m, 8H).
1H NMR (500 MHz, CDCl3): d (ppm) 8.00ꢂ7.35 (m, 18H),
2.05ꢂ1.95 (m, 12H), 1.25ꢂ1.09 (m, 72H), 0.85ꢂ0.81 (m, 18H).
Poly[(9,9,90,90-tetra-n-octylfluorenyl-2,70-diyl)-
alt-4,40-azobenzene)] (P2FAz)
Poly[(9,9,90,90,900,900-hexa-n-octylfluorenyl-2,70-diyl)-alt-
4,40-azobenzene)] (P3FAz)
n-BuLi in hexane (1.6 M, 2.2 mL) was added slowly to a
stirred solution of 4 (0.2 g, 0.5 mmol) in THF (10 mL) at
ꢂ78 ꢀC under an argon atmosphere, and the mixture was
stirred for 1 h. Trimethyl borate (9 mL) was slowly added at
ꢂ78 ꢀC, and the mixture was then warmed slowly to room
temperature and stirred continuously for 10 h. The mixture
was slowly cooled to 0 ꢀC and then dilute hydrochloric acid
(6 mL) was added. The mixture was extracted with diethyl
ether and the resulting organic layer was dried over anhy-
drous MgSO4. Concentration in vacuo gave the corresponding
n-BuLi in hexane (1.6 M, 2 mL) was added slowly to a
stirred solution of 6 (0.8 g, 0.6 mmol) in THF (5 mL) at ꢂ78
ꢀC under an argon atmosphere and the mixture was stirred
for 1 h at ꢂ78 ꢀC. Trimethyl borate (1.5 mL) was slowly
added at ꢂ78 ꢀC, and the mixture was then warmed slowly
to room temperature and stirred continuous for 20 h. The
mixture was then slowly cooled to 0 ꢀC and then dilute hy-
drochloric acid (6 mL) was added. The mixture was
extracted with diethyl ether and the resulting organic layer
was dried over anhydrous MgSO4. Concentration in vacuo
gave the corresponding 9,9,90,90,900,900-hexa-n-octyl-7,20-70,700-
terfluorene-2,200-diboronic acid (0.7 mg, 0.5 mmol). The
resulting boronic compound (0.21 g, 0.17 mmol) was mixed
with 1 (77 mg, 0.2 mmol) in toluene (3 mL) under a nitro-
gen atmosphere. This mixture was added to a solution of
aqueous K2CO3 (2.0 M, 1.8 mꢀL) and Pd(PPh3)4(0) (3 mg)
and stirred under reflux at 70 C for 42 h. The mixture was
poured into cold methanol and washed with dilute hydro-
chloric acid, and the polymer precipitate was collected on a
filter. The polymer precipitate was resolved with toluene and
poured into cold methanol three times. The polymer powder
was collected by filtration to give PF3Az as an orange pow-
der (86 mg, 0.07 mmol, 12%).
9,9,90,90-tetra-n-octyl-7,70-bifluorene-2,20-diboronic
acid,
which without further purification due to instability, was
mixed with 1 (0.2 g, 0.5 mmol) in toluene (10 mL) under a
nitrogen atmosphere. The mixture was then added to a solu-
tion of aqueous K2CO3 (2.0 M, 1.8 mL) and Pd(PPh3)4(0) (6
ꢀ
mg) and stirred under reflux at 70 C for 45 h. The mixture
was extracted with diethyl ether and washed with dilute hy-
drochloric acid, and the resulting organic layer was dried
over anhydrous MgSO4 and concentrated in vacuo. The crude
product was filtrated and washed with toluene to give
PF2Az as an orange powder (135 mg, 0.15 mmol, 30%).
ꢂ1
ꢀ
ꢀ
mp: 133ꢂ150 C, decomposed at 235 C. Mw: 7300 g mol
,
Mw/Mn (ppm)
¼
1.8. 1H NMR (500 MHz, CDCl3):
d
8.10ꢂ8.05 (br), 7.88ꢂ7.83 (br), 7.71ꢂ7.67 (br), 2.17ꢂ2.05
(br), 1.12 (br), 0.80 (br). ATR-FT-IR (cmꢂ1): 2924, 2852,
1598, 1462, 1157, 1168, 1004, 850, 821, 741.
mp: 129ꢂ150 ꢀC, decomposed at 277 ꢀC. Mw: 21,000 g molꢂ1
,
1
Mw/Mn ¼ 3.2. H NMR (500 MHz, CDCl3): d (ppm) 7.90–7.72
(br), 2.16–2.13 (br), 1.22–1.00 (br). ATR-FTIR (cmꢂ1): 2923,
2849, 1599, 1465, 1230, 1220, 1156, 895, 819.
9,9,90,90,900,900-Hexa-n-octyl-2,20,70,200-terfluorene (5)
Compound 2 (1.3 g, 2.9 mmol), 9,9-dioctylfluorene-2,7-dibor-
onic acid bis(1,3-propanediol)ester (0.8 g, 1.4 mmol), and
(PPh3)4Pd(0) (16 mg) were dissolved in a mixture of toluene
(10 mL) and K2CO3 (2 M, 6 mL) under an argon atmosphere.
RESULTS AND DISCUSSION
The poly(9,9-dioctylfluorene-alt-azobenzene)s (PFAz)s with
different numbers of fluorene units and an azobenzene unit
(P1FAz, P2FAz, and P3FAz) were synthesized by palladium-
catalyzed Suzuki polymerization. A 0.2-lm diameter PTFE
syringe filter was used to collect the monodisperse polymers.
The syntheses were conducted according to procedures in
the literature.21–24 The procedure for synthesis of the PFAzs
is shown in Scheme 1. The structures of the products were
ꢀ
The solution was maintained at 70 C with vigorous stirring
for 24 h. The mixture was then extracted with diethyl ether
and washed with dilute hydrochloric acid, and the resulting
organic layer was dried over anhydrous MgSO4 and concen-
trated in vacuo. The crude product was purified by column
chromatography using hexane/dichloromethane (10/1) as an
eluent to give 5 as a pale blue oil (0.7 g, 0.6 mmol, 43%). Rf:
0.3 (for hexane/dichloromethane 10/1).
1
confirmed by MS, H NMR, and IR spectroscopy.
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SCHEME 1 Synthesis of poly(9,9-dioctylfluorene) copolymers: (i) Al, KOH, MeOH, rt; (ii) 9,9-dioctylfluorene-2,7-diboronic acid
bis(1,3-propanediol)ester, Pd(PPh3)4, K2CO3, toluene, 70 ꢀC; (iii) CH3(CH2)7Br, DMSO, 50% NaOH aq., 35 ꢀC; (iv) n-BuLi, B(OMe)3,
THF ꢂ78 ꢀC; (v) K2CO3, Pd(PPh3)4, toluene, 75 ꢀC; (vi) Br2, CH2Cl2, reflux temperature; (vii) 1, K2CO3, Pd(PPh3)4, toluene, 70 ꢀC.
UV–vis absorption spectra of PFAzs in chloroform are shown
in Figure 2(a–c). The PFAzs have a broad absorption that
extends from 300 to 570 nm with a maxima at 417 nm for
P1FAz, 390 nm for P2FAz, and 356 nm for F3FAz, respec-
tively, which are attributed to the pꢂp* transitions of the flu-
orene backbone and azobenzene chromophore. In particular,
the absorption bands appearing around 375–390 nm, which
are clearly evident in the P2FAz and P3FAz spectra, are
attributed to the pꢂp* transition of the conjugated polyfluor-
ene backbone,25,26 whereas the absorption bands greater
than 400 nm shown in P1FAz are due to the pꢂp* transition
of the azobenzene chromophore. This indicates that the
copolymers do not have extended conjugation along the
polymer main-chain; therefore, the conjugation could be
twisted, due to steric hindrance around the 9-position dioc-
tyl substituent in the fluorene unit, which prevents a system
with fluorene-to-fluorene conjugation. The intensity of this
UVꢂvis absorption band decreases for the PFAzs under 400
nm light irradiation [Fig. 2(a–c), dotted lines]. Simultane-
ously, the absorption bands at 500–550 nm increase slightly
[inset in Fig. 2(a–c)], which indicate the trans-cis photoiso-
merization of the azobenzene moiety in the copolymers.
The increase in these absorption bands is related to the
absorption of the cis-isomer of azobenzene.27,28 The initial
intensities of the short-wave absorption band recover after
exposure to visible light or spontaneous relaxation in the
dark. After irradiation with 400 nm light, the differences in
the absorption intensities of the PFAzs are dependent on the
respective fluorine structures. The fluorescence spectra of
PFAzs in chloroform were also measured; however, the in-
tensity of all fluorescence spectra was extremely weak (data
not shown). It is likely that intramolecular energy transfer
from the photoexcited fluorine units to the azobenzene
chromophore could occur, which would result in quenched
fluorescence. Fluorescence quantum yield measurements
produced extremely small Uf values: 6.7 ꢃ 10ꢂ4 (P1FAz),
7.9 ꢃ 10ꢂ4 (P2FAz), and about 0 (P3FAz).
light. Spectral features similar to those of the PFAzs in chlo-
roform were observed before irradiation. Upon irradiation
with 400 nm light, only the P1FAz film showed a decrease
and slight increase in the intensity of the absorption band
due to the trans-isomer [Fig. 2(d), dotted lines] and cis-iso-
mer, respectively [inset in Fig. 2(d)]. The spectral changes of
the P1FAz film slowly recovered to the initial state in the
dark. For the P2FAz and P3FAz films, the occurrence of
trans-cis photoisomerization did not contribute to spectral
changes at 500–550 nm, which indicates that the P2FAz and
P3FAz films are less light sensitive. The decreases of absorb-
ance at 400 nm for the P2FAz as well as that for the P3FAz
are attributed to photodecomposition of the azobenzene
chromophore. The intense decreases did not recover to the
initial state in the dark. The trans-cis photoisomerization
rates of azobenzene chromophores tend to be strongly
dependent on the matrix environment.29–32 Therefore, the
efficient light sensitivity of the P1FAz film suggests that the
process of trans-cis photoisomerization is strongly dependent
on the mobility of photosensitive azobenzene moieties in the
polymer main-chain and on the glass-transition temperature.
Thus, the most efficient trans-cis photoisomerization was for
the P1FAz film, because the fluorene and azobenzene units
are alternately bonded in the periodic polymer, which pro-
vides the polymer main-chain with flexibility. The fluores-
cence characteristics of the PFAz films could not be clearly
detected, as for the chloroform solutions of the PFAzs. This
fluorescence behavior had been reported from research on
azobenzene liquid crystalline polymers, where the light
energy absorbed during alignment changes both the molecu-
lar potential energy and the heat energy.33
Figure 3 shows the angular dependence of absorbance (kmax
¼ 430 nm) for the P1FAz film. Absorbance was measured af-
ter irradiation with linear polarized 400 nm light under
ꢀ
annealing at 60 C for 2 h, where 0ꢀ and 90ꢀ are the absorb-
ance parallel to and perpendicular to the direction of the
polarized light, respectively. The annealing temperature of
ꢀ
Figure 2(d–f) shows UV–vis absorption spectra for the spin-
60 C used in this work was determined from a study of the
coated PFAz films before and after irradiation with 400 nm
dichroic ratio dependence on the temperature (Fig. 4).The
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FIGURE 2 UV–vis absorption spectra before and after irradiation with 400 nm light for (a) P1FAz, (b) P2FAz, (c) P3FAz in chloro-
form, and (d) P1FAz, (e) P2FAz, (f) P3FAz films. The insets show the respective spectra in more detail.
as-spun (before polarized light irradiation) P1FAz film shows
no angular dependence and similar spectral features to the
aforementioned spin-coated film—the orientational state of
P1FAz film remains optically isotropic. Upon irradiation with
linear polarized 400 nm light, the absorbance at 90ꢀ exhibits
higher intensity compared to that at 0ꢀ, which results in a
high dichroic ratio of 5.3. (The dichroic ratio is A90ꢀ/A0ꢀ
where A90ꢀ is the absorbance with respect to light polarized
perpendicularly to the polarizing plane and A0ꢀ is the ab-
sorbance with respect to light polarized parallel to the polar-
izing plane.) For the last few decades, three principal proc-
esses have been proposed to induce anisotropy: angular hole
burning, angular redistribution, and the rotational Brownian
diffusion of the molecules.34–36 According to the angular
redistribution process, the azobenzene chromophore accu-
mulates in a direction with the transition dipole axis perpen-
dicular to the light polarization to minimize the absorption.
Therefore, the result of angular dependency indicates that
the polymer backbone of P1FAz involves the azobenzene
unit aligned perpendicularly to the electric field vector of
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hexane was considered to be the most suitable solvent in
this system. A surface oriented film on the photo-induced
alignment layer of the P1FAz film was obtained by spin-coat-
ing of a 0.025 wt % hexane solution of F12, with subsequent
ꢀ
annealing at 90 C for 20 min followed by quenching (here-
inafter referred to as ‘‘F12/P1FAz film’’). The RMS roughness
of the F12/P1FAz film was 1ꢂ2 nm. Figure 5 shows polar-
ized absorption spectra of the F12/P1FAz film, where 0ꢀ
and 90ꢀ are the absorption and fluorescence spectra parallel
to and perpendicular to the direction of the polarized 400
nm light, respectively. The F12/P1FAz film has a broad
absorption peak centered at 398 nm, which is attributed to
the F12 and P1FAz layers. The absorption spectra have a
strong peak for 90ꢀ and a weak peak for 0ꢀ. The dichroic
ratio of the F12/P1FAz film is estimated to be 2.1 from the
ratio of absorption intensities. The resulting dichroic ratio
indicates that the F12 film has parallel orientation to the
P1FAz film. Note that the polarized fluorescence spectra of
the F12/P1FAz film, also shown in Figure 5, have a similar
tendency to the absorption spectra characteristics. The
dichroic ratio, estimated from the peak tops at 435 nm in
the fluorescence spectra, was 1.6. Accordingly, orientation of
the F12 film was achieved by a combination of photochemi-
cal- and thermal-based processes. The most important feature
of the surface orientation technique using the photo-induced
alignment layer of the P1FAz film is that the arrangement of
other polymers can be controlled, which would be useful for
the fabrication of optoelectronics devices.
FIGURE 3 Angular dependence of the absorbance for the
P1FAz film (h) before polarized light irradiation and (*) after
polarized light irradiation.
polarized light, and photo-induced alignment can be
achieved during the repeated trans-to-cis-to-trans-… photoi-
somerization. To characterize the surface morphological
effect of photo-induced alignment, the P1FAz film was
observed by AFM (data not shown). However, no significant
morphological changes were obtained and the root mean
square (RMS) roughness was extremely small at 2ꢂ3 nm.
AFM observation showed characteristics similar to those pre-
viously reported.37 The results can also be explained briefly
as follows. The polymer backbone is aligned perpendicularly
to the electric field vector of polarized light after irradiation
with linear polarized 400 nm light and is eventually crystal-
lized in an extremely small area, thereby yielding a photo-
alignment film. Thus, an as-spun P1FAz film surface could
be assigned amorphous with random orientation.
A schematic illustration of the overall technique presented
here is summarized in Figure 6 and is described as follows:
(i) Initial state: the surfaces of all polymer films have ran-
dom ordering and the most stable conformation, the trans-
isomer. (ii) Photoisomerization: upon irradiation with 400
nm light, the reversible trans-cis photoisomerization occurs
only for the P1FAz film, and the polymer backbone of P1FAz
aligns randomly. (iii) Photo-induced alignment: irradiation
The photo-induced alignment layer of the P1FAz film was
applied to a poly(9,9-didodecylfluorene) (F12, Mw: 30,000 g
molꢂ1, Mw/Mn ¼ 2.0) spin-coated film. The F12 film was
spin-coated on top of the photo-induced alignment layer of
the P1FAz film from appropriate solutions. As the solvent
for the surface oriented film cannot dissolve P1FAz, no sig-
nificant intermixing of the two polymers is required. Thus,
FIGURE 5 Polarized UV–vis absorption spectra of the F12/
P1FAz film after annealing at 90 ꢀC for 20 min. A0ꢀ and A90ꢀ
donate absorption parallel and perpendicular to the direction
of polarized light, respectively. F0ꢀ and F90ꢀ denote fluores-
cence intensity parallel and perpendicular to the direction of
polarized light, respectively.
FIGURE 4 Dichroic ratios of the P1FAz film as a function of
irradiation energy.
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FIGURE 6 Schematic representation of the overall mechanism for the surface orientation of PFAz films.
ꢀ
with polarized 400 nm light under annealing at 60 C results
in an accumulation of trans-isomers oriented perpendicular
to the direction of polarized light for the P1FAz film after
repeated periodic trans-cis photoisomerization. (iv) Surface
orientation: the spin-coated F12 film aligns parallel to the
direction of the photo-induced alignment layer of the P1FAz
film.
functionalized polymers and related structures in the design
of optoelectronic devices.
ACKNOWLEDGMENTS
This research was funded by a Grant-in-Aid for Challenging Ex-
ploratory Research (No. 23655186) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan.
CONCLUSIONS
REFERENCES AND NOTES
Three types of bi-functionalized copolymers (PFAz) with azo-
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