Fabrication of photoactive self-assembled ultra-thin films from diazoresin and poly(4-vinylphenol) via H-bonding

Article abstract of DOI:10.1039/b006972h

Using a self-assembly technique, a type of photoactive multilayer film was successfully fabricated from diazoresin (DR) and poly(4-vinylphenol) (PVPh) followed by UV irradiation. The driving force of the self-assembly was confirmed to be the H-bonding interaction between the diazonium group (-N₂ ⁺) of DR and the phenolic hydroxy group (-Ph-OH) of PVPh. A linkage conversion from an H-bond to a covalent bond takes place following the decomposition of the -N₂ ⁺ group when the multilayer film undergoes UV irradiation. As a result, the stability of the film towards etching by polar solvents increases dramatically.

Full text of DOI:10.1039/b006972h

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Fabrication of photoactive self-assembled ultra-thin Ðlms from
diazoresin and poly(4-vinylphenol) via H-bonding
Tingbing Cao, Jinyu Chen, Chaohui Yang and Weixiao Cao*
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
E-mail: wxcao=263.net
Received (in Montpellier, France) 24th August 2000, Accepted 31st October 2000
First published as an Advance Article on the web 11th January 2001
Using a self-assembly technique, a type of photoactive multilayer Ðlm was successfully fabricated from diazoresin
(
DR) and poly(4-vinylphenol) (PVPh) followed by UV irradiation. The driving force of the self-assembly was
conÐrmed to be the H-bonding interaction between the diazonium group (ÈN `) of DR and the phenolic hydroxy
2
group (ÈPhÈOH) of PVPh. A linkage conversion from an H-bond to a covalent bond takes place following the
decomposition of the ÈN ` group when the multilayer Ðlm undergoes UV irradiation. As a result, the stability of
2
the Ðlm towards etching by polar solvents increases dramatically.
Since DecherÏs pioneering work, the layer-by-layer self-
assembly technique used to form organized ultra-thin Ðlms
has been investigated extensively and reviewed recently in
several publications.1 This technique, which depends on the
Coulombic attraction between oppositely charged polymers
or multi-ionic components, has attracted more and more
attention,2h11 and has been applied to prepare unique mem-
branes with desired components such as biomolecules,6
chromophoric molecules,7 dendritic molecules,8 polymer
dyes,9 conductive polymers10 and so on.11
Preparation of MCA. Anisaldehyde (40.8 g, 0.3 mol),
malonic acid (60 g, 0.58 mol), dry pyridine (120 ml) and piperi-
dine (3 ml) were mixed in a 500-ml three-necked bottle
equipped with a thermometer and a condenser. The mixture
was heated for 2 h at 100 ¡C, then reÑuxed gently for 15 min
at 120 ¡C. The mixture, while still hot, was poured into 175 ml
of concentrated hydrochloric acid and 300 g of chopped ice
under stirring. The precipitate was Ðltered, washed with 25 ml
of 10% hydrochloric acid and water, then dried. Yield: 52.0 g
(97.4%); mp: 183È185 ¡C.
The mechanism of Ðlm formation and the inner structure of
the Ðlms have also been developed.12 Furthermore, the weak
attraction of the H-bond has also been used to fabricate
molecular-level multilayer Ðlms. Stockton and Rubner13
reported a number of mutilayer ultra-thin Ðlms between poly-
aniline and various water-soluble polymers including
poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA),
poly(acrylamide) (PA) and poly(ethylene oxide) (PEO) via H-
bonding interactions, and Zhang et al.,14 almost at the same
time, reported a polyacrylic acid (PAA)ÈPVP multilayer Ðlm
formed via an H-bonding interaction.
However, both the Coulombic and the H-bonding attrac-
tions are too weak to prevent the etching of the Ðlm by polar
solvents. Recently, we reported the formation of photoactive
multilayer Ðlms from DR and anionic polyelectrolytes or
phenolÈformaldehyde resin, and veriÐed that the Ðlms are very
stable towards polar solvents after UV irradiation.15
Poly(4-vinylphenol) (PVPh) has long been an attractive
polymer due to its availability in polymer blends16 and its
ability to form H-bonds with many kinds of proton-donating
polymers. In this article we report a photoactive ultra-thin
Ðlm from DR and PVPh and conÐrmed that the Ðlm will
change its ionic bond to a covalent bond when it undergoes
UV irradiation. As a result, the stability of the Ðlm towards
polar solvents increases dramatically.
Preparation of MS. MCA (35.6 g, 0.2 mol), anhydrous
copper sulfate (3.6 g), hydroquinone (2 g) and dry quinoline
(
140 g) were charged into a 200-ml three-necked bottle
equipped with a thermometer and a stirrer. The mixture was
heated to reÑux for 30 min; the fraction between 190 and
10 ¡C was collected by distillation, washed with 150 ml of
0% sulfuric acid and then with water, then dried with anhy-
2
1
drous magnesium sulfate. It was then puriÐed chromato-
graphically on a 200 mesh silica gel column using CH Cl as
2
2
elution agent. The colorless part of the eluent was collected
and the CH Cl was evaporated, leaving MS as a colorless
2
2
liquid. Yield: 7.8 g (29.1%). 1HNMR (200 MHz, CDCl ) d:
3
3
(
(
.49 (3H, OCH ); 6.6È7.4 (4H, aromatic); 5.65 (1H, 2CH); 5.56
2H, 2CH ). Anal. found (calc.) for C H
80.59); H 7.68 (7.46); O 12.43 (11.94).
3
9 10O (%): C 79.36
2
Preparation of PVPh. PVPh was prepared by elimination of
the methoxy group of PMS by treating with trimethylsilyl
iodide at room temperature.18 PMS was prepared by free
radical polymerization of MS using AIBN as the initiator in
bulk. The number- and weight-average molecular weights of
PMS were determined by gel permeation chromatography to
be 7.7 ] 104 and 10.8 ] 104 g mol~1. From the IR spectrum
and the elemental analysis of PMS it is believed that there are
no methoxy groups in PVPh.
Preparation of DR. Diazoresin was prepared in our labor-
atory from diphenylamine-4-diazonium salt and paraformal-
dehyde according to ref. 19: g \ 0.12 dl g~1, M D 2.1
Experimental
p@c
n
Syntheses
]
103 g mol~1.
Film preparation and characterization
4-Methoxystyrene (MS) was synthesized with 4-methoxy-
cinnamic acid (MCA) according to ref. 17 with some modiÐ-
cations.
DR was dissolved in methanol at a concentration of 0.2 mg
ml~1, PVPh was dissolved in acetone at 0.5 mg ml~1. A
DOI: 10.1039/b006972h
New J. Chem., 2001, 25, 305È307
305
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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quartz wafer, which was treated with 30% H O and concen-
2
2
trated sulfuric acid (3 : 7 in volume) at 100 ¡C for 0.5 h, was
used as the substrate. The substrate was immersed Ðrst in the
PVPh acetone solution for 5 min, then rinsed with acetone
and dried, then immersed in the methanol solution of DR for
5
min, followed by rinsing with methanol and drying. In each
immersion cycle, a bilayer of PVPhÈDR was deposited on
both sides of the substrate.
UV-vis spectra recorded on a Shimadzu 2100 spectropho-
tometer were used to monitor the layer-by-layer deposition.
The photoreaction occurring in the Ðlm was also determined
spectrometrically after UV irradiation. IR spectra were
obtained on a Bruker Vector 22 FTIR spectrometer of a
Scheme 1 PVPh and DR form a complex via H-bonding interaction.
DRÈPVPh Ðlm fabricated on a CaF wafer.
2
The multilayer Ðlm from PVPh and DR is photoactive.
With decomposition of the diazoresin group under UV irra-
diation, the absorbance at 380 nm decreases gradually. Fig. 3
shows the change in the UV-vis spectra of a 12-bilayer Ðlm of
PVPhÈDR with irradiation time. It shows that the diazonium
group decomposes quickly and almost completely in 5 min
under the experimental conditions. The insert plot of Fig. 3
shows that the decomposition follows Ðrst-order reaction
kinetics.
Results and discussion
Fig. 1 shows the absorbance of the PVPhÈDR multilayer Ðlm
increasing with bilayer number. The continuous increase in
absorbance at 380 and 240 nm, which are the characteristic
absorptions of the ÈN ` and phenyl groups, respectively, indi-
2
cates that DR and PVPh were successfully deposited on the
quartz wafer.
The driving force for layer-by-layer deposition should be
ascribed to a hydrogen bond rather than an electrostatic
attraction because neither PVPh nor DR is dissociated in
acetone or in methanol solution. Although the diazonium
The photoreaction taking place in the PVPhÈDR Ðlm
under exposure to UV light may be similar to that occurring
in a Ðlm of DR and poly(sodium acrylate) (PSA),21 in which
the electrostatic ÈCÈN `OOC~ bond converts to an ester
group (ÈN `) is a cation, its two nitrogen atoms are not
2
2
equivalent. One (connected to the phenyl moiety) is charge
deÐcient and in equilibrium with the counter-ion HSO ~, the
4
other is charge rich. Therefore, it is reasonable to consider
that the diazonium group can form a hydrogen bond with a
strong H-donor such as the phenolic hydroxy group present
in PVPh, as shown in Scheme 1.
In order to verify the H-bonding nature in the Ðlm struc-
ture, a 50-bilayer Ðlm from PVPh and DR was fabricated on a
CaF wafer to measure the IR spectrum of the Ðlm directly.
2
The stretching vibration of the ÈN ` group of the DRÈPVPh
2
Ðlm is at 2157 cm~1 (Fig. 2, curve a); it is 9 wavenumbers
lower than in DR itself, which has the stretching vibration of
the ÈN ` group at 2166 cm~1.15a
2
Fig. 2 shows the IR spectra of the PVPhÈDR Ðlm before
and after UV irradiation. The 1584 cm~1 bond should be
attributed to vibration of the phenyl nucleus conjugated with
an unsaturated group; it shifts to 1605 cm~1, the normal
absorption of the phenyl group,20 after UV irradiation. The
Fig. 2 FT-IR spectra of PVPhÈDR multiplayer Ðlm: (a) before irra-
diation; (b) after irradiation. Irradiation intensity (at 360 nm): 230
lW cm~2; irradiation time: 10 min.
1
369 cm~1 band is assigned to the in-plane bending vibration
of OÈH after formation of the H-bond;20 it disappears after
UV irradiation. This is further evidence of formation of an
H-bond between ÈN ` and HOÈPh.
2
Fig. 1 The UV-vis spectra of PVPhÈDR multilayer Ðlms with di†er-
Fig. 3 The UV-vis spectra of a PVPhÈDR multilayer (12 bilayers)
Ðlm with di†erent irradiation times (from top to bottom at 380 nm):
0, 10, 30, 60, 120, 300 s. Irradiation intensity (at 360 nm): 230 lW
cm~2.
ent numbers of bilayers. Bilayer number (bottom to top): 0, 2, 4, 6, 8,
1
0, 12. The inset plot indicates the linear relationship between absorb-
ance and bilayer number: (=) at 240 nm; (…) at 380 nm.
306
New J. Chem., 2001, 25, 305È307

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The authors are grateful to the NSFC (Contract Nos: 297-,
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298-74001 and 59633110) for Ðnancial support of this work.
New J. Chem., 2001, 25, 305È307
307