Multiresponsive photopatterning organic-inorganic polymer hybrids using a caged photoluminescence compound

Article abstract of DOI:10.1021/ma047320d

The caged photoluminescence compound 2-(2′-benzoylphenyl)benzoxazole (BzPO) modified with poly(N,N-dimethylacrylamide) was synthesized. Transparent polymer hybrids with the obtained polymer were prepared by using hydrogen bond interactions between amide g

Full text of DOI:10.1021/ma047320d

Macromolecules 2005, 38, 4425-4431
4425
Multiresponsive Photopatterning Organic-Inorganic Polymer Hybrids
Using a Caged Photoluminescence Compound
Tomoki Ogoshi, Junpei Miyake, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received December 25, 2004; Revised Manuscript Received March 15, 2005
ABSTRACT: The caged photoluminescence compound 2-(2′-benzoylphenyl)benzoxazole (BzPO) modified
with poly(N,N-dimethylacrylamide) was synthesized. Transparent polymer hybrids with the obtained
polymer were prepared by using hydrogen bond interactions between amide groups of the organic polymer
and silanol groups resulting from hydrolysis of tetramethoxysilane. By UV irradiation upon the obtained
polymer hybrids, the cage-released reaction of BzPO quickly took place even in a silica gel matrix, and
the resulting polymer hybrids showed strong green photoluminescence deriving from a phototautomer of
2-(2′-hydroxyphenyl)benzoxazole (HPBO) dye. The preparation of “photoresist-like” photopatterning
polymer hybrids could be also accomplished by using a photomask during UV irradiation. Furthermore,
the photoluminescence of the polymer hybrids was reversibly quenched and emitted by the recaging of
the hydroxyl group of HPBO with adsorption and desorption of water molecules. The miscibility of polymer
hybrids depending on UV irradiation was evaluated by SEM and nitrogen adsorption porosimetry studies.
Scheme 1
Introduction
The construction of hybrid materials by bridging
organic and inorganic chemistries at the nanoscale or
molecular scale has been an important field of current
materials research in order to generate new advanced
materials.1-4 Organic polymer/inorganic hybrid materi-
als have been elaborated with various inorganic hosts
such as inorganic clay compounds,5-7 metal oxo clus-
ters,8-10 mesoporous silica (zeolite),11-13 and metallic or
inorganic nanoparticles.14-17 Among them, the sol-gel
process employing metal alkoxides is an important and
widely used technique for the preparation of organic-
inorganic polymer hybrid materials.18-25 Very conve-
nient sol-gel access to inorganic solid under ambient
conditions enables hybridization of organic polymer with
inorganic oxides.26 Silicon alkoxides such as tetrameth-
oxysilane (TMOS) and tetraethoxysilane (TEOS) are
mainly used as starting materials due to their mild
sol-gel reactivity and easy handling. However, the poor
compatibility between organic polymer and silica gel
results in aggregation of organic polymer in a silica
matrix during the formation of the silica gel, and then
the appearance of the obtained composite materials is
turbid or phase separated. To prepare homogeneous and
transparent polymer hybrids, the improvement of the
compatibility by introducing physical interaction or
covalent bond between organic polymer and silica is
necessary. Our group has elaborated novel transparent
polymer hybrids by using physical interactions such as
hydrogen bonding,27-29 π-π stacking,30,31 and ionic32
interactions between organic polymer and silica gel. For
example, the formation of a hydrogen bond between
organic polymers having hydrogen-accepting groups
such as amide groups and hydrogen-donating groups of
the residual silanol moieties resulted in the molecular
dispersion of organic polymer in a silica gel matrix. The
most striking property of the obtained polymer hybrids
is optical transparency. Scattering loss is avoided due
to nanoscale miscibility between organic polymer and
silica.33 Therefore, applications of these materials to
optics have been desired and widely studied.
Recently, our group accomplished the construction of
the photosensitive organic-inorganic polymer hybrid
materials using a biological idea of caged (photopro-
tected) compounds.34 Caged compounds are biologically
inert molecules which can release bioactive compounds
upon photolysis. The transparency of the obtained
polymer hybrids could be controlled by introducing the
caged compound into the interaction parts between
organic polymer and siliceous phases. The UV irradia-
tion resulted in the formation of the strong interaction
between organic polymer and silica and the preparation
of the highly transparent polymer hybrids, while the
obtained materials without UV were turbid and phase
separated.
Herein this research describes the other type of novel
photosensitive organic-inorganic polymer hybrid ma-
terials utilizing a caged photoluminescent compound.
Caged photoluminescent dyes usually show no photo-
luminescence (nonactive). However, upon photoirradia-
tion, the cage moiety is released and the original photo-
luminescence is recovered (activated). Thus, caged pho-
toluminescence compounds are mainly used for a bio-
medical indicator conjugated covalently with drugs.35-38
Kocher et al. discovered that 2-(2′-benzoylphenyl)ben-
zoxazole (BzPO), which exhibited no photoluminescence,
was changed to a well-known PL compound of 2-(2′-
hydroxyphenyl)benzoxazole (HPBO) by UV irradiation
with a cleavage of the ether bond of BzPO (Scheme 1).35
The observation of no photoluminescence of BzPO
derived from the inhibition of the intramolecular hy-
* Corresponding author: Tel +81-75-383-2604; Fax +81-75-383-
2605; e-mail chujo@chujo.synchem.kyoto-u.ac.jp.
10.1021/ma047320d CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/15/2005

4426 Ogoshi et al.
Macromolecules, Vol. 38, No. 10, 2005
Scheme 2
Scheme 4
Scheme 3
distilled under reduced pressure and stored under nitro-
gen. Phenyltrimethoxysilane (PhTMOS), tetramethoxysilane
(TMOS), and methyltrimethoxysilane (MeTMOS) were dis-
tilled and stored under nitrogen. The other solvents and
reagents were used as supplied.
1
Measurements. The H NMR spectra were recorded on a
400 MHz JEOL EX-400 spectrometer. Thermogravimetric
analysis (TGA) was performed using a TG/DTA6200, SEIKO
Instruments, Inc., with heating rate of 10 °C min-1 in air.
Scanning electron microscopy (SEM) measurements were
conducted using a JEOL JNM-5310/LV system. The FT-IR
spectra were obtained using a Perkin-Elmer 1600 infrared
spectrometer. Nitrogen adsorption porosimetry was conducted
with a BEL Japan Inc. Fluorescence emission spectra were
recorded on a Perkin-Elmer LS50B luminescence spectrom-
eter. Gel permeation chromatography (GPC) analysis was
carried out on TSK gel R-3000 by using DMF as an eluent at
40 °C after calibration with the standard polystyrene samples.
Absorption spectra were obtained on a JASCO V-530 spec-
trometer.
Synthesis of 2-(2′-Benzoylphenyl)benzoxazole (Model
Compound, BzPO). The model compound BzPO was pre-
pared according to the detailed experimental procedure de-
scribed in a previous literature.35
Synthesis of 2-(2′-p-Vinylbenzoylphenyl)benzoxazole
(Vinyl-BzPO). Novel monomer having a BzPO moiety (vinyl-
BzPO) was prepared as shown in Scheme 4.
drogen bond between the nitrogen atom and hydroxyl
group of HPBO. To show strong PL of HPBO, formation
of the intramolecular hydrogen bond of HPBO is neces-
sary.39 The benzoic acid group of BzPO acted as a cage
of the hydroxyl group of HPBO. In this study, the
preparation of high transparent and homogeneous
polymer hybrids with novel BzPO-modified polymer
could be accomplished (Scheme 2). The obtained poly-
mer hybrid showed strong photoluminescence with UV
irradiation due to the generation of HPBO in a silica
gel matrix. By using a photomask during UV irradia-
tion, the synthesis of the photoresist-like polymer
hybrids, which showed strong photoluminescence within
UV-irradiated parts, could also be succeeded. Further-
more, the photoluminescence of the polymer hybrid was
reversibly quenched and recovered by adsorption and
desorption of water molecules (Scheme 3). Thus, the
obtained polymer hybrid materials can memorize data
by UV irradiation and reversibly erase-reload by
adsorption-desorption of water molecules. Therefore,
they are promisingly applied for optical memory devices,
humidity sensors, and resist materials. The advantages
of hybridizing BzPO-modified polymer with silica gel at
a nanometer level are as follows: (1) The obtained
polymer hybrids are perfectly transparent, which easily
makes it a good candidate for optics. (2) The high
transparency of polymer hybrids is maintained after
hard treatments such as UV irradiation and water
adsorption-desorption processes because of the rein-
forcement of organic polymer with silica gel at a
nanometer level. (3) The photodurability of the PL
compound should also increase since the destruction of
the PL compound with UV irradiation in air should be
impeded by hard silica gel matrix. To our best knowl-
edge, it is the first example that the biological concept
of caged photoluminescence compounds is applied for
nanocomposite materials science.
Thionyl chloride (SOCl2) (10.3 mL, 140 mmol) was added
to p-vinylbenzoic acid (5.00 g, 33.9 mmol) at 0 °C under a
nitrogen atmosphere. The temperature of the mixture was
allowed to rise at room temperature, and the reaction mixture
was stirred for 4 h. Then, the reaction mixture was stirred at
40 °C for 1 h to proceed the reaction completely. Excess thionyl
chloride was evaporated under reduced pressure at 40 °C for
2 h. To the residual mixture, a solution of 2-(2′-hydroxyphenyl)-
benzoxazole (HPBO, 4.49 g, 21.3 mmol) in 80 mL of chloroform
was added at 0 °C. Then, 5.25 mL (15.8 mmol) of solution of
pyridine was carefully added dropwise using a syringe at 0
°C, and the mixture was stirred at 0 °C for 1 h. Then, the
reaction mixture was heated at 40 °C for 20 h. The resulting
mixture was washed with water several times to remove
pyridine HCl salt. The organic portion was dried over MgSO4.
The resulting solution was concentrated by evaporation and
dried under reduced pressure. After the residual solid was
recrystallized several times with methanol, a slightly yellow
solid was obtained (vinyl-BzPO; 1.15 g, 3.36 mmol, yield
1
40.0%). H NMR (CDCl3): δ ) 8.38 (m, 1H, BzPO), 8.25 (m,
2H, BzPO) 7.60-7.55 (m, 6H, BzPO), 7.39 (m, 2H, BzPO), 7.29
(m, 1H, BzPO), 6.87 (q, 1H, vinyl), 5.97 (d, 1H, vinyl), 5.47 (d,
1H, vinyl). IR (KBr, cm-1): 1738 cm-1 (the stretching band of
ester-carbonyl group), 1635 cm-1 (the stretching band of amide-
carbonyl group). UV/vis (CHCl3): 294 nm.
Synthesis of BzPO-Modified Poly(N,N-dimethyl-
acrylamide) (Polymers 1a and 1b). Vinyl-BzPO/N,N-di-
methylacrylamide copolymers were synthesized by a conven-
tional free radical polymerization with changing the ratio of
N,N-dimethylacrylamide to vinyl-BzPO (Scheme 4). A typical
Experimental Section
Materials. Tetrahydrofuran (THF) was dried and distilled
over sodium under nitrogen. N,N-Dimethylacrylamide was

Macromolecules, Vol. 38, No. 10, 2005
Photopatterning Organic-Inorganic Polymer Hybrids 4427
preparation procedure for 5 mol % BzPO-modified poly(N,N-
dimethylacrylamide) (polymer 1a) is as follows. N,N-Dimeth-
ylacrylamide (0.706 mL, 6.87 mmol), vinyl-BzPO (0.295 g,
0.865 mmol), and AIBN (0.0280 g, 0.01 equiv molar ratio to
monomer) were dissolved in 15 mL of THF solution under a
nitrogen atmosphere. The mixture was refluxed for 24 h. The
resulting solution was poured into hexane. The white solid was
obtained (polymer 1a: 1.81 g, yield 93.9%). 1H NMR (CDCl3):
δ ) 8.43-8.10 (m, BzPO), 7.60-7.00 (m, BzPO), 3.11-2.20 (m,
N-C(CH3)2), 2.00-1.10 (m, Ar-CHCH2 and -OCOCHCH2).
IR (KBr, cm-1): 1740 cm-1 (the stretching band of ester-
carbonyl group), 1635 cm-1 (the stretching band of amide-
carbonyl group).
Preparation of Organic-Inorganic Polymer Hybrids.
A typical preparation method of polymer hybrids using ob-
tained polymers is as follows. The synthesized polymer was
dissolved in a solvent. To the mixture, a solution of alkoxysi-
lane and 0.1 or 1 M aqueous hydrochloric acid (as a catalyst
for sol-gel reaction) was added. The resulting mixture was
stirred at room temperature for certain time in a sealed bottle.
Then, the resulting mixture was placed in a container covered
with a paper towel and kept at 60 °C to evaporate the solvent.
After evaporating the solvent completely, the polymer hybrids
were obtained as a glassy material.
Photoinduced Cage-Released Reaction. The cage-
released reaction of BzPO was induced by the irradiation of
UV light using 175 W Xe lamp filtered with a Toshiba UV-
D33S glass filter. The cage-released reaction was monitored
by UV and PL spectra during photoirradiation of the polymer
hybrids. Efficiencies of the cage-released reaction of the
polymer hybrids were calculated by UV absorption at 332 nm
by utilizing molar absorption coefficients of model compounds
of BzPO and HPBO. Molar absorption coefficients of BzPO and
HPBO at 332 nm were 1500 and 15 700 M-1 cm-1, respectively.
Reversible Quenching and Recovering of Photolumi-
nescence of Polymer Hybrids. To quench the photolumi-
nescence of the polymer hybrids after UV irradiation, which
showed strong photoluminescence of HPBO moieties due to
removal of the cage groups by UV irradiation, the samples
were left in a closed desiccator containing the solvent (water,
methanol, and chloroform). Furthermore, after quenching the
photoluminescence of the polymer hybrids, the hybrid materi-
als were kept at 60 °C to remove the adsorbed solvent. The
photoluminescence of the polymer hybrids depending on the
processes of adsorption and desorption of the solvent was
monitored by fluorescence emission measurement.
Nitrogen Adsorption Porosimetry. The powder of the
polymer hybrid was heated at 600 °C in an ambient atmo-
sphere for 24 h to remove organic polymer. The samples dried
at 200 °C for 2 h at reduced pressure before porosimetry
measurements. The surface area and pore volume were
calculated by the Brunauer-Emmet-Teller (BET)40 equation
in the range of 0.05-0.30 (p/p0), and the pore size distribution
was estimated by the Barrett-Joyner-Halenda (BJH) method.41
Figure 1. 1H NMR spectrum of polymer 1a (in CDCl3).
Table 1. Synthesis of BzPO-Modified
Poly(N,N-dimethylacrylamide) (Polymers 1a and 1b)
degree of BzPO
(mol %)
feed ratio
(mol %)
yield
(%)
polymer
UV
1H NMR
Mn
Mw/Mn
1a
1b
5
20
4.5
15
3.6
17
4600
6800
3.1
2.7
94
90
group. (The molar absorption coefficient of BzPO at 294
nm was 19 600 M-1 cm-1.) The introduction efficiency
1
of BzPO moieties was calculated by H NMR and UV
measurements (Table 1). The introduction efficiency of
BzPO in both polymers 1a and 1b was slightly small
compared with the feed ratio, but BzPO moieties were
successfully modified with polymer side chain. The
obtained polymers should not be block but quite random
polymers due to the alternating reactivity between
vinyl-BzPO and styrene.
Synthesis of Organic-Inorganic Polymer Hy-
brids. Organic-inorganic polymer hybrids with the
polymers 1a and 1b were prepared (Table 2). The
obtained polymer hybrids using the polymer 1b and
TMOS brought about phase separation (runs 1-3). On
the other hand, transparent polymer hybrids with the
polymer 1a were prepared (runs 4-6). These results are
different from the previous study with poly(N,N-di-
methylacrylamide), which is dispersed in a silica gel
matrix at a nanometer level in a wide range without
any difficulties due to the strong hydrogen bond between
amide groups of poly(N,N-dimethylacrylamide) and
residual silanol moieties of silica. The hydrogen bonds
between poly(N,N-dimethylacrylamide) and silanol moi-
eties from sol-gel reaction of tetramethoxysilane (TMOS)
were confirmed by FT-IR measurement in previous
papers.27-29 The amide-carbonyl stretching band of
poly(N,N-dimethylacrylamide) at 1638 cm-1 was shifted
to 1625 cm-1 on hybridization with silica gel from
TMOS, which is strong evidence of the hydrogen bonds.
Therefore, it is considered that the modification of the
BzPO moiety at the side chain of poly(N,N-dimethyl-
acrylamide) largely effects the transparency of the
obtained polymer hybrids. The BzPO moiety has a
rather poor miscibility in the reaction mixture and high
crystallinity; thus, BzPO groups tend to aggregate
during the formation of the polymer hybrids. The
polymer 1b was modified with high molar percentages
of BzPO moiety (20 mol %) compared to the polymer 1a
(5 mol %) so that the polymer hybrids using the polymer
1b easily form aggregation of BzPO moieties and
Results and Discussion
Synthesis of BzPO-Modified Poly(N,N-dimeth-
ylacrylamide) (Polymers 1a and 1b). The novel
BzPO monomer (vinyl-BzPO) was synthesized as il-
lustrated in Scheme 4. The structure was characterized
by 1H NMR, UV, and FT-IR. BzPO-modified poly(N,N-
dimethylacrylamide) was prepared with vinyl-BzPO and
N,N-dimethylacrylamide monomer by free radical po-
lymerization. Figure 1 shows the 1H NMR spectrum of
polymer 1a, which has 5 mol % of BzPO groups at the
PDMAAm polymer side chain. After radical polymeri-
zation, the vinyl peaks (5.00-6.90 ppm) from vinyl-
BzPO and N,N-dimethylacrylamide disappeared com-
pletely, and the new BzPO peaks were observed around
8.43-8.10 and 7.60-7.00 ppm. The adsorption maxi-
mum of the polymers 1a and 1b was shown at 294 nm
in chloroform, which was identified to that of BzPO

4428 Ogoshi et al.
Table 2. Synthesis of Organic-Inorganic Polymer Hybrids with Polymers 1a and 1ba
ceramic yield (wt %)
Macromolecules, Vol. 38, No. 10, 2005
run
polymer (mg)
TMOS (mg)
ratiob
solvent (mL)
HClaqc (mL)
appearance
calcd
obsdd
1
2
3
4
5
6
7
8
1b 50
1b 50
1b 50
1a 25
1a 25
1a 25
1a 50
1a 50
125
250
500
1000
500
250
250
250
1
2
THF 2.5
0.1 M
0.1 M
0.1 M
0.1 M
0.1 M
0.1 M
0.1 M
1.0 M
phase separated
phase separated
phase separated
transparent
transparent
transparent
turbid
THF 2.5
4
THF 5.0
16
8
MeOH 0.25
MeOH 0.25
MeOH 0.25
MeOH 0.50
MeOH 0.50
94.1
88.9
80.0
84.9
80.9
72.7
4
2
2
transparent
66.7
60.2
a Stirring time was 1 h, and the solvent was evaporated at 60 °C. b Feed ratio of silica/polymer (w/w). c 4 equiv to TMOS. d Calculated
by TGA.
Table 3. Synthesis of Organic-Inorganic Polymer Hybrids with Polymer 1ba
run
polymer 1b (mg)
BzPO (mg)
PDMAAmb (mg)
RSi(OMe3)3
ratioc
appearance
9
10
11
12
13
14
15
16
17
25
25
25
25
25
25
25
Ph
OMe
Me
Ph
Ph
Ph
Ph
Ph
Ph
16
16
16
8
transparent
turbid
turbid
transparent
transparent
transparent
transparent
phase separated
phase separated
4
2
1
8
8
17
1
a 2.0 mL of THF was used as a solvent. The amount of 0.1 M HCl(aq) used was 4 equiv to TMOS and 3 equiv to MeTMOS and PhTMOS.
Stirring time was 1 h, and the solvent was evaporated at 60 °C. b Poly(N,N-dimethylacrylamide). c Feed ratio of silica/polymer (w/w).
become phase separation. And with 1 M aqueous
hydrochloric acid, transparent polymer hybrids using
polymer 1a content as mush as 33% could be obtained
(runs 7 and 8). These observations mean that the
problem of aggregation of BzPO moieties in the case of
polymer 1a could be overcome by controlling the acid
concentration. Formation of siliceous oligomers is ac-
celerated at high acid concentration.26 That is, the
gelation of TMOS using 1 M aqueous acid catalyst takes
place faster than that using 0.1 M aqueous acid.
Therefore, this fast reaction allows the reaction mixture
to be frozen before the microscopic phase separation
derived from aggregation of BzPO moieties and gives
transparent polymer hybrids. The same trend was
observed in the case of the preparation of polymer
hybrids using poly(2-methyl-2-oxazoline) having func-
tional groups.43,44 The ceramic yields calculated by TGA
measurement were slightly small compared with the
theoretical ceramic yield because the small amounts of
TMOS were evaporated before gelation of TMOS.
The high homogeneous polymer hybrids with polymer
1b could be accomplished by using alkoxysilane having
organic residues (Table 3). In the case of the sam-
ples using polymer 1b and phenyltrimethoxysilane
(PhTMOS), the obtained polymer hybrids were trans-
parent in a wide range (runs 9 and 12-15). On the other
hand, the appearance of the polymer hybrids using
TMOS (run 10) or methyltrimethoxysilane (MeTMOS;
run 11) was turbid. These observations strongly indicate
that strong π-π interaction between BzPO moieties of
polymer 1b and phenyl groups of PhTMOS efficiently
inhibited the aggregation of BzPO groups during the
preparation of the polymer hybrids. In the samples
using BzPO (model compound) with PDMAAm (run 16)
or without PDMAAm (run 17), the obtained materials
showed phase separation. The preparation of high
transparent polymer hybrids having BzPO moieties
could be only achieved by introducing BzPO moieties
into the polymer side chain. It is because the physical
entrapment of the polymer chain by silica matrix deters
BzPO aggregations.
Figure 2. Changes of UV absorption of polymer 1a during
photoirradiation (in CHCl3).
Photoinduced Cage-Released Reaction of Poly-
mer Hybrids. The cage-released reaction of polymers
1a and 1b in solution was monitored by UV measure-
ment. Figure 2 shows the change of UV absorption of
polymer 1a upon photoirradiation. The UV absorption
peak at 332 nm derived from HPBO moieties gradually
increased, indicating the progress of the cage-released
reaction of BzPO moieties of polymer 1a. Photoinduced
cage-released reaction in a silica gel matrix was moni-
tored by UV and PL measurements (Figure 3). The films
of the polymer hybrid with polymer 1a or 1b were
prepared on the quartz substrates. The changes of UV
absorption and photoluminescence intensity during
photoirradiation were checked. As shown in Figure 3a,
the absorbance around 332 nm, which resulted from
HPBO absorption, increased depending on the time of
UV irradiation. The photoluminescence intensity around
500 nm was also increasing by UV irradiation (Figure
3b). From these observations, the cage-released reaction
of the BzPO moiety in a silica gel matrix could be clearly
confirmed.

Macromolecules, Vol. 38, No. 10, 2005
Photopatterning Organic-Inorganic Polymer Hybrids 4429
Figure 3. (a) UV and (b) PL spectra of the polymer hybrid
film (run 6) during photoirradiation.
Table 4. Efficiency of Cage-Released Reaction of BzPO
run polymer RSi(OMe)3 ratioa appearance efficiency (%)
Figure 4. PL spectra of (a) BzPO in methanol, (b) BzPO in
chloroform, (c) polymer 1a in methanol, (d) polymer 1a in
chloroform, and (e) the polymer hybrid film (run 5) (excitation
at 350 nm). All samples were measured after UV irradiation
for 30 min.
4
1a
1a
1a
1a
2a
2a
OMe
OMe
OMe
OMe
Ph
16
8
transparent
transparent
transparent
transparent
transparent
transparent
16.2
10.1
7.5
5
6
8
12
14
4
2
8.2
8
15.5
21.0
84.9
66.8
Ph
2
polymer 1a (in CHCl3 solution)
polymer 2a (in CHCl3 solution)
a Feed ratio of silica/polymer (w/w).
The efficiencies of the cage-released reaction were
calculated (Table 4). In the case of the polymer hybrids,
the efficiencies were about 10-20%. In contrast, the
cage-released reaction of polymers 1a and 1b efficiently
proceeded in solution. The efficiencies of the cage-
released reaction of polymers 1a and 1b in chloroform
were 84.9% and 66.8%, respectively. As previously
reported, the cage-released reaction of the model com-
pound (BzPO) took place completely.35 These observa-
tions result from the mechanism of the cage-released
reaction. The UV irradiation leads to homolytic cleavage
of the O-C(O) bond of BzPO moieties, yielding an
HPBO and a benzoyl radical groups. Therefore, the
recombination of both radical pairs should affect the
cage-released efficiency. The recombination of radical
pairs between HPBO and benzoyl radical moieties
more easily occurred by incorporating the BzPO moi-
eties into the polymer chain due to the steric hindrance.
Furthermore, the recombination of radical pairs
should more easily proceed in the silica gel because of
the poor mobility of the radical pairs in hard silica gel
matrix.
Figure 5. Picture of the transparent hybrid film (run 8) after
UV (300 nm) irradiation with photomask.
Figure 6. Changes of PL intensity of (a) the polymer hybrid
film (run 6) after UV irradiation in saturated vapor pressure
of water and (b) the polymer hybrid film (run 6) after annealing
at 60 °C.
The photoluminescence of HPBO generated from the
cage-released reaction of BzPO was also measured
(Figure 4). As previously reported, the color of photo-
luminescence of HPBO dye largely depended on the
environment;39 HPBO had the ability to undergo an
excited-state intramolecular proton transfer (ESIPT).
Upon photoirradiation, the hydroxyl proton of HPBO
is transferred to the adjacent nitrogen atom (ESIPT),
and a “phototautomer” is generated as a result. The
phototautomer of HPBO shows strong green photolu-
minescence at 490 nm. In nonpolar solvents such as
chloroform and THF, the formation of the phototau-
tomer of HPBO formed efficiently. The green strong
emission of HPBO (Figure 4b; λem ) 490 nm) was
observed in chloroform. On the other hand, in polar
solvents such as water and methanol, HPBO can form
an intermolecular hydrogen bond with the solvent,
which leads to inhibition of the ESIPT and shows
normal blue PL of HPBO. HPBO showed an intense blue
emission (Figure 4a; λem ) 430 nm) in methanol. The
trend was almost the same in the case of the polymer
1a. The photoluminescence resulting from polymer 1a
after UV irradiation showed a peak around 430 nm in
methanol (Figure 4c), while the green emission (λem
)
490 nm) was observed in chloroform (Figure 4d). In the
case of the polymer hybrid after cage-released reaction,
the photoluminescence of HPBO in a silica gel matrix
from TMOS was observed at 490 nm (Figure 4e). These
observations indicate that the excited HPBO molecules
with UV irradiation efficiently form the phototautomer
of HPBO through ESIPT in a silica gel matrix.
Photoresist-like Photoluminescent Organic-
Inorganic Polymer Hybrids. By using a photoresist
mask, the photoresist like organic-inorganic polymer
hybrid could be prepared. The UV light was irradiated
upon the photoresist mask. After UV irradiation, the
appearance of the polymer hybrid was not changed
compared to that before UV irradiation. The part with
UV irradiation was fully colorless and transparent.

4430 Ogoshi et al.
Macromolecules, Vol. 38, No. 10, 2005
Figure 7. SEM images of (a) the phase-separated polymer hybrid (run 7), (b) the transparent polymer hybrid (run 8) before UV
irradiation, and (c) the transparent polymer hybrid (run 8) after UV irradiation.
Scheme 5
But, the part with photoirradiation showed strong
green photoluminescence, and the characters “polymer
hybrids” clearly appeared by excitation at 365 nm
(Figure 5).
Reversible PL Properties of Organic-Inorganic
Polymer Hybrids. The photoluminescence of HPBO
was affected by the intramolecular hydrogen bond
between the hydroxyl group and nitrogen atom of
HPBO. The PL of HPBO was quenched by inhibition of
the intramolecular hydrogen bond of HPBO. Therefore,
the intensity of the photoluminescence of HPBO should
be quenched and recovered by reversible formation of
the intramolecular hydrogen bond of HPBO.39 In this
study, the photoluminescence of HPBO in a silica matrix
Figure 8. Adsorption isotherm curve of the transparent
polymer hybrid (run 6) before UV irradiation.
was reversibly quenched and recovered by adsorption
and desorption of water molecules into the polymer
hybrid film after cage-released reaction of BzPO. At
first, the changes of the intensity of the photolumines-
cence of HPBO in silica gel were monitored during the
adsorption of water molecules. As shown in Figure 6a,
the PL intensity at 490 nm of HPBO declined gradually
with increasing of the keeping time under the saturated
vapor pressure of water at 25 °C. In contrast, the
quenching of the photoluminescence was not observed
under the saturated vapor pressure of methanol and
chloroform. These observations mean the formation of
an intermolecular hydrogen bond between the water
molecule and hydroxyl group of HPBO, as shown in
Scheme 3. The intermolecular hydrogen bond between
the water molecule and hydroxyl group of HPBO
inhibited ESIPT and photoluminescence. Then, the
sample was left in an oven at 60 °C. As shown in Figure
6b, the intensity of photoluminescence was recovered
perfectly by keeping the hybrid at 60 °C, suggesting the
removal of water molecules and re-formation of the
intramolecular hydrogen bond between the nitrogen
atom and hydroxyl group of HPBO.
tions (Figure 7a). White parts indicate silica phase. On
the other hand, in the transparent polymer hybrid
(Table 1, run 8), bright domains could not be observed
even at 10 000 magnifications (Figure 7b). No (black)
image in Figure 7b at 10 000 magnifications indicates
high homogeneity of the transparent polymer hybrid
compared with the turbid composite. The nanolevel
dispersion of some hybrid materials showed the same
results in the SEM as previously reported in our
group.30-32,34,45 The SEM image of the polymer hybrid
was not changed after UV irradiation (Figure 7c). It
means that the polymer hybrids keep high homogeneity
with UV irradiation.
The homogeneity of the polymer hybrids was also
quantitatively evaluated on the nanometer scale by
nitrogen adsorption porosimetry study. The porous silica
was prepared by burning out the organic polymer of the
polymer hybrids at 600 °C for 24 h (Scheme 5). The silica
gel matrix is so rigid that the size of the pores obtained
by calcination of the polymer hybrids is found to be the
domain of the organic polymer. The previously reported
paper actually confirmed that the size of the porous
silica reflected the organic polymer domain. For ex-
ample, the pore size of the porous silica prepared by
calcination of the hybrids with dendrimer was almost
the same as the size of the dendrimer used.28 The pore
size of the obtained porous silica became larger accord-
ing to the increasing of the generation of the used
dendrimer. The porous silica obtained by charring the
SEM and Nitrogen Adsorption Studies To In-
vestigate Homogeneity of Organic-Inorganic Poly-
mer Hybrids. The miscibility between polymer 1a and
silica phases was examined by SEM and nitrogen
adsorption porosimetry studies. Figure 7 showed SEM
images of the obtained polymer hybrids with polymer
1a. In the case of the turbid sample (Table 1, run 7),
bright domains were clearly found at 10 000 magnifica-

Macromolecules, Vol. 38, No. 10, 2005
Photopatterning Organic-Inorganic Polymer Hybrids 4431
Table 5. Porosity of the Silica Obtained by Calcinating
Polymer Hybrids
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pore vola
(mL/g)
surf. areaa
pore radiusb
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117
206
119
145
1.1
1.7
6c
a Calculated by BET. b Calculated by the BJH method. c The
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Figure 9. Pore size distribution plots of porous silica obtained
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The adsorption isotherm curve obtained by the nitro-
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the polymer hybrid (run 6) is shown in Figure 8. The
shape of the curve was a type IV curve, indicating that
the porous silica had nanopores. The porous silica
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area and pore volume (Table 5). The pore size distribu-
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method (Figure 9). The size of the pore was below 2.0
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Conclusions
High transparent homogeneous polymer hybrids were
obtained with novel BzPO-modified polymer. The cage-
released reaction of BzPO upon UV irradiation pro-
ceeded in a silica gel matrix. The photopatterning
polymer hybrids, which showed strong photolumines-
cence within UV irradiated area, were also prepared
using a photomask during UV irradiation. The PL of
the polymer hybrids was reversibly quenched and
emitted by adsorption-desorption of water molecules.
Thus, the obtained polymer hybrids can be applied for
memory devices by writing data using UV light, erasing,
and reloading data using adsorption-desorption of
water molecules.
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References and Notes
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